免疫学（英文版）：Chapter 06

chapter 6 Antibody Affinity Is a Quantitative Measure of Binding Strength The combined strength of the noncovalent interactions be- tween a single antigen-binding site on an antibody and a sin- gle epitope is the affinity of the antibody for that epitope. Low-affinity antibodies bind antigen weakly and tend to dis- sociate readily, whereas high-affinity antibodies bind antigen more tightly and remain bound longer. The association be- tween a binding site on an antibody (Ab) with a monovalent antigen (Ag) can be described by the equation k 1 Ag H11001 Ab 34Ag-Ab k H110021 a73 Strength of Antigen-Antibody Interactions a73 Cross-Reactivity a73 Precipitation Reactions a73 Agglutination Reactions a73 Radioimmunoassay a73 Enzyme-Linked Immunosorbent Assay a73 Western Blotting a73 Immunoprecipitation a73 Immunofluorescence a73 Flow Cytometry and Fluorescence a73 Alternatives to Antigen-Antibody Reactions a73 Immunoelectron Microscopy Antigen-Antibody Interactions: Principles and Applications T ?? ???????-???????? ??????????? ?? ? ????- lecular association similar to an enzyme-substrate interaction, with an important distinction: it does not lead to an irreversible chemical alteration in either the antibody or the antigen. The association between an anti- body and an antigen involves various noncovalent interac- tions between the antigenic determinant, or epitope, of the antigen and the variable-region (V H /V L ) domain of the an- tibody molecule, particularly the hypervariable regions, or complementarity-determining regions (CDRs). The exquis- ite specificity of antigen-antibody interactions has led to the development of a variety of immunologic assays, which can be used to detect the presence of either antibody or antigen. Immunoassays have played vital roles in diagnosing diseases, monitoring the level of the humoral immune response, and identifying molecules of biological or medical interest. These assays differ in their speed and sensitivity; some are strictly qualitative, others are quantitative. This chapter ex- amines the nature of the antigen-antibody interaction, and it describes various immunologic assays that measure or ex- ploit this interaction. Strength of Antigen-Antibody Interactions The noncovalent interactions that form the basis of antigen- antibody (Ag-Ab) binding include hydrogen bonds, ionic bonds, hydrophobic interactions, and van der Waals interac- tions (Figure 6-1). Because these interactions are individu- ally weak (compared with a covalent bond), a large number of such interactions are required to form a strong Ag-Ab in- teraction. Furthermore, each of these noncovalent interac- tions operates over a very short distance, generally about 1 H11003 10 H110027 mm (1 angstrom, ?); consequently, a strong Ag- Ab interaction depends on a very close fit between the anti- gen and antibody. Such fits require a high degree of complementarity between antigen and antibody, a require- ment that underlies the exquisite specificity that character- izes antigen-antibody interactions. Fluorescent Antibody Staining Reveals Intracellular Immunoglobin 8536d_ch06_137-160 8/1/02 12:41 PM Page 137 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: where k 1 is the forward (association) rate constant and k H110021 is the reverse (dissociation) rate constant. The ratio k 1 /k H110021 is the association constant K a (i.e., k 1 /k H110021 H11005 K a ), a measure of affinity. Because K a is the equilibrium constant for the above reaction, it can be calculated from the ratio of the molar con- centration of bound Ag-Ab complex to the molar concentra- tions of unbound antigen and antibody at equilibrium as follows: K a H11005 The value of K a varies for different Ag-Ab complexes and depends upon both k 1 , which is expressed in units of liters/mole/second (L/mol/s), and k H110021 , which is expressed in units of 1/second. For small haptens, the forward rate con- stant can be extremely high; in some cases, k 1 can be as high as 4 H11003 10 8 L/mol/s, approaching the theoretical upper limit of diffusion-limited reactions (10 9 L/mol/s). For larger pro- tein antigens, however, k 1 is smaller, with values in the range of 10 5 L/mol/s. The rate at which bound antigen leaves an antibody’s binding site (i.e., the dissociation rate constant, k H110021 ) plays a major role in determining the antibody’s affinity for an antigen. Table 6-1 illustrates the role of k H110021 in determining [Ag-Ab] H5007 [Ab][Ag] the association constant K a for several Ag-Ab interactions. For example, the k 1 for the DNP-L-lysine system is about one fifth that for the fluorescein system, but its k H110021 is 200 times greater; consequently, the affinity of the antifluores- cein antibody K a for the fluorescein system is about 1000- fold higher than that of anti-DNP antibody. Low-affinity Ag-Ab complexes have K a values between 10 4 and 10 5 L/mol; high-affinity complexes can have K a values as high as 10 11 L/mol. For some purposes, the dissociation of the antigen-anti- body complex is of interest: Ag-Ab 334Ab H11001 Ag The equilibrium constant for that reaction is K d , the recipro- cal of K a K d H11005 [Ab][Ag]H11408[Ab-Ag] H11005 1H11408K a and is a quantitative indicator of the stability of an Ag-Ab complex; very stable complexes have very low values of K d , and less stable ones have higher values. The affinity constant, K a , can be determined by equilib- rium dialysis or by various newer methods. Because equilib- rium dialysis remains for many the standard against which 138 PART II Generation of B-Cell and T-Cell Responses VISUALIZING CONCEPTS FIGURE 6-1 The interaction between an antibody and an anti- gen depends on four types of noncovalent forces: (1) hydrogen bonds, in which a hydrogen atom is shared between two elec- tronegative atoms; (2) ionic bonds between oppositely charged residues; (3) hydrophobic interactions, in which water forces hy- drophobic groups together; and (4) van der Waals interactions between the outer electron clouds of two or more atoms. In an aqueous environment, noncovalent interactions are extremely weak and depend upon close complementarity of the shapes of antibody and antigen. ANTIGEN CH 2 ANTIBODY OH ??? O CCH 2 CH 2 NH 2 Hydrogen bond CH 2 CH 2 NH 3 + – O CH 2 CH 2 C Ionic bond O CH 2 CH 3 CH CH 3 CH 3 + H 3 N CH 2 CHCH 3 van der Waals interactionsCH CHCH 3 CH CH 3 O O – CH 2 C CH 2 Ionic bond CH 3 Hydrophobic interactions 8536d_ch06_137-160 8/1/02 12:41 PM Page 138 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: other methods are evaluated, it is described here. This proce- dure uses a dialysis chamber containing two equal compart- ments separated by a semipermeable membrane. Antibody is placed in one compartment, and a radioactively labeled lig- and that is small enough to pass through the semipermeable membrane is placed in the other compartment (Figure 6-2). Suitable ligands include haptens, oligosaccharides, and oligo- peptides. In the absence of antibody, ligand added to com- partment B will equilibrate on both sides of the membrane (Figure 6-2a). In the presence of antibody, however, part of the labeled ligand will be bound to the antibody at equi- librium, trapping the ligand on the antibody side of the ves- sel, whereas unbound ligand will be equally distributed in both compartments. Thus the total concentration of ligand will be greater in the compartment containing antibody (Fig- ure 6-2b). The difference in the ligand concentration in the two compartments represents the concentration of ligand bound to the antibody (i.e., the concentration of Ag-Ab com- plex). The higher the affinity of the antibody, the more ligand is bound. Antigen-Antibody Interactions: Principles and Applications CHAPTER 6 139 TABLE 6-1 Forward and reverse rate constants (k 1 and k H115461 ) and association and dissociation constants (K a and K d ) for three ligand-antibody interactions Antibody Ligand k 1 k H110021 K a K d Anti-DNP H9280-DNP-L-lysine 8 H11003 10 7 11H11003 10 8 1 H11003 10 H110028 Anti-fluorescein Fluorescein 4 H11003 10 8 5 H11003 10 H110023 1 H11003 10 11 1 H11003 10 H1100211 Anti-bovine serum albumin (BSA) Dansyl-BSA 3 H11003 10 5 2 H11003 10 H110023 1.7 H11003 10 8 5.9 H11003 10 H110029 SOURCE: Adapted from H. N. Eisen, 1990, Immunology, 3rd ed., Harper & Row Publishers. FIGURE 6-2 Determination of antibody affinity by equilibrium dial- ysis. (a) The dialysis chamber contains two compartments (A and B) separated by a semipermeable membrane. Antibody is added to one compartment and a radiolabeled ligand to another. At equilibrium, the concentration of radioactivity in both compartments is mea- sured. (b) Plot of concentration of ligand in each compartment with time. At equilibrium, the difference in the concentration of radioac- tive ligand in the two compartments represents the amount of ligand bound to antibody. (a) Radiolabeled ligand AB AB (b) Concentration of ligand, M 100 50 100 50 ControlControl: No antibody present (ligand equilibrates on both sides equally) ExperimentalExperimental: Antibody in A (at equilibrium more ligand in A due to Ab binding) Ligand bound by antibody 2468 Time, h Initial state Equilibrium AB AB Initial state Equilibrium D Antibody A B A B 8536d_ch06_137-160 8/1/02 12:41 PM Page 139 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: Since the total concentration of antibody in the equilib- rium dialysis chamber is known, the equilibrium equation can be rewritten as: K a H11005 [Ab-Ag]H11408[Ab][Ag] H11005H5007 c(n r H11002 r) H5007 where r equals the ratio of the concentration of bound ligand to total antibody concentration, c is the concentration of free ligand, and n is the number of binding sites per antibody molecule. This expression can be rearranged to give the Scatchard equation: H5007 c r H5007 H11005 K a n H11002 K a r Values for r and c can be obtained by repeating the equi- librium dialysis with the same concentration of antibody but with different concentrations of ligand. If K a is a constant, that is, if all the antibodies within the dialysis chamber have the same affinity for the ligand, then a Scatchard plot of r/c versus r will yield a straight line with a slope of H11002K a (Figure 6-3a). As the concentration of unbound ligand c increases, r/c approaches 0, and r approaches n, the valency, equal to the number of binding sites per antibody molecule. Most antibody preparations are polyclonal, and K a is therefore not a constant because a heterogeneous mixture of antibodies with a range of affinities is present. A Scatchard plot of heterogeneous antibody yields a curved line whose slope is constantly changing, reflecting this antibody hetero- geneity (Figure 6-3b). With this type of Scatchard plot, it is possible to determine the average affinity constant, K 0 ,by de- termining the value of K a when half of the antigen-binding sites are filled. This is conveniently done by determining the slope of the curve at the point where half of the antigen bind- ing sites are filled. Antibody Avidity Incorporates Affinity of Multiple Binding Sites The affinity at one binding site does not always reflect the true strength of the antibody-antigen interaction. When complex antigens containing multiple, repeating antigenic determinants are mixed with antibodies containing multiple binding sites, the interaction of an antibody molecule with an antigen molecule at one site will increase the probability of reaction between those two molecules at a second site. The strength of such multiple interactions between a multivalent antibody and antigen is called the avidity. The avidity of an antibody is a better measure of its binding capacity within bi- ological systems (e.g., the reaction of an antibody with anti- genic determinants on a virus or bacterial cell) than the affinity of its individual binding sites. High avidity can com- pensate for low affinity. For example, secreted pentameric 140 PART II Generation of B-Cell and T-Cell Responses FIGURE 6-3 Scatchard plots are based on repeated equilibrium dialyses with a constant concentration of antibody and varying con- centration of ligand. In these plots, r equals moles of bound lig- and/mole antibody and c is the concentration of free ligand. From a Scatchard plot, both the equilibrium constant (K a ) and the number of binding sites per antibody molecule (n), or its valency, can be ob- tained. (a) If all antibodies have the same affinity, then a Scatchard plot yields a straight line with a slope of H11002K a . The x intercept is n, the valency of the antibody, which is 2 for IgG and other divalent Igs. For IgM, which is pentameric, n H11005 10, and for dimeric IgA, n H11005 4. In this graph, antibody #1 has a higher affinity than antibody #2. (b) If the antibody preparation is polyclonal and has a range of affinities, a Scatchard plot yields a curved line whose slope is constantly chang- ing. The average affinity constant K 0 can be calculated by determin- ing the value of K a when half of the binding sites are occupied (i.e., when r H11005 1 in this example). In this graph, antiserum #3 has a higher affinity (K 0 H11005 2.4 H11003 10 8 ) than antiserum #4 (K 0 H11005 1.25 H11003 10 8 ). Note that the curves shown in (a) and (b) are for divalent antibodies such as IgG. 1.0 2.0 r (a) Homogeneous antibody — × 10 8 r c 2.0 3.0 4.0 #1 #2 Slope = –K a Intercept = n (b) Heterogeneous antibody 1.0 — × 10 8 r c 2.0 3.0 4.0 2.0 r 1.0 Slope at r of 1/2 n = –K 0 Intercept = n #3 #4 8536d_ch06_137-160 8/1/02 9:01 AM Page 140 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: IgM often has a lower affinity than IgG, but the high avidity of IgM, resulting from its higher valence, enables it to bind antigen effectively. Cross-Reactivity Although Ag-Ab reactions are highly specific, in some cases antibody elicited by one antigen can cross-react with an un- related antigen. Such cross-reactivity occurs if two different antigens share an identical or very similar epitope. In the lat- ter case, the antibody’s affinity for the cross-reacting epitope is usually less than that for the original epitope. Cross-reactivity is often observed among polysaccharide antigens that contain similar oligosaccharide residues. The ABO blood-group antigens, for example, are glycoproteins expressed on red blood cells. Subtle differences in the termi- nal residues of the sugars attached to these surface proteins distinguish the A and B blood-group antigens. An individual lacking one or both of these antigens will have serum anti- bodies to the missing antigen(s). The antibodies are induced not by exposure to red blood cell antigens but by exposure to cross-reacting microbial antigens present on common in- testinal bacteria. These microbial antigens induce the for- mation of antibodies in individuals lacking the similar blood-group antigens on their red blood cells. (In individu- als possessing these antigens, complementary antibodies would be eliminated during the developmental stage in which antibodies that recognize self epitopes are weeded out.) The blood-group antibodies, although elicited by mi- crobial antigens, will cross-react with similar oligosaccha- rides on foreign red blood cells, providing the basis for blood typing tests and accounting for the necessity of com- patible blood types during blood transfusions. A type A in- dividual has anti-B antibodies; a type B individual has anti-A; and a type O individual thus has anti-A and anti-B (Table 6-2). A number of viruses and bacteria have antigenic determi- nants identical or similar to normal host-cell components. In some cases, these microbial antigens have been shown to elicit antibody that cross-reacts with the host-cell compo- nents, resulting in a tissue-damaging autoimmune reaction. The bacterium Streptococcus pyogenes, for example, expresses cell-wall proteins called M antigens. Antibodies produced to streptococcal M antigens have been shown to cross-react with several myocardial and skeletal muscle proteins and have been implicated in heart and kidney damage following streptococcal infections. The role of other cross-reacting antigens in the development of autoimmune diseases is dis- cussed in Chapter 20. Some vaccines also exhibit cross-reactivity. For instance, vaccinia virus, which causes cowpox, expresses cross-reacting epitopes with variola virus, the causative agent of smallpox. This cross-reactivity was the basis of Jenner’s method of us- ing vaccinia virus to induce immunity to smallpox, as men- tioned in Chapter 1. Precipitation Reactions Antibody and soluble antigen interacting in aqueous solu- tion form a lattice that eventually develops into a visible pre- cipitate. Antibodies that aggregate soluble antigens are called precipitins. Although formation of the soluble Ag-Ab com- plex occurs within minutes, formation of the visible precipi- tate occurs more slowly and often takes a day or two to reach completion. Formation of an Ag-Ab lattice depends on the valency of both the antibody and antigen: a73 The antibody must be bivalent; a precipitate will not form with monovalent Fab fragments. a73 The antigen must be either bivalent or polyvalent; that is, it must have at least two copies of the same epitope, or have different epitopes that react with different antibodies present in polyclonal antisera. Experiments with myoglobin illustrate the requirement that protein antigens be bivalent or polyvalent for a precip- itin reaction to occur. Myoglobin precipitates well with spe- cific polyclonal antisera but fails to precipitate with a specific monoclonal antibody because it contains multiple, distinct epitopes but only a single copy of each epitope (Figure 6-4a). Myoglobin thus can form a crosslinked lattice structure with polyclonal antisera but not with monoclonal antisera. The principles that underlie precipitation reactions are presented because they are essential for an understanding of commonly used immunological assays. Although various modifications of the precipitation reaction were at one time the major types of assay used in immunology, they have been largely replaced by methods that are faster and, because they are far more sen- sitive, require only very small quantities of antigen or anti- body. Also, these modern assay methods are not limited to antigen-antibody reactions that produce a precipitate. Table 6-3 presents a comparison of the sensitivity, or minimum amount of antibody detectable, by a number of immunoas- says. Antigen-Antibody Interactions: Principles and Applications CHAPTER 6 141 TABLE 6-2 ABO blood types Blood type Antigens on RBCs Serum antibodies A A Anti-B B B Anti-A AB A and B Neither O Neither Anti-A and anti-B 8536d_ch06_137-160 8/1/02 9:01 AM Page 141 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: Precipitation Reactions in Fluids Yield a Precipitin Curve A quantitative precipitation reaction can be performed by placing a constant amount of antibody in a series of tubes and adding increasing amounts of antigen to the tubes. At one time this method was used to measure the amount of antigen or antibody present in a sample of interest. After the precipitate forms, each tube is centrifuged to pellet the pre- cipitate, the supernatant is poured off, and the amount of precipitate is measured. Plotting the amount of precipitate against increasing antigen concentrations yields a precipitin curve. As Figure 6-4b shows, excess of either antibody or antigen interferes with maximal precipitation, which occurs in the so-called equivalence zone, within which the ratio of antibody to antigen is optimal. As a large multimolecular lattice is formed at equivalence, the complex increases in size and precipitates out of solution. As shown in Figure 6-4, un- der conditions of antibody excess or antigen excess, extensive lattices do not form and precipitation is inhibited. Although the quantitative precipitation reaction is seldom used exper- imentally today, the principles of antigen excess, antibody ex- cess, and equivalence apply to many Ag-Ab reactions. Precipitation Reactions in Gels Yield Visible Precipitin Lines Immune precipitates can form not only in solution but also in an agar matrix. When antigen and antibody diffuse toward one another in agar, or when antibody is incorporated into the agar and antigen diffuses into the antibody-containing matrix, a visible line of precipitation will form. As in a precipitation re- action in fluid, visible precipitation occurs in the region of equivalence, whereas no visible precipitate forms in regions of antibody or antigen excess. Two types of immunodiffusion re- actions can be used to determine relative concentrations of an- tibodies or antigens, to compare antigens, or to determine the relative purity of an antigen preparation. They are radial im- munodiffusion (the Mancini method) and double immun- odiffusion (the Ouchterlony method); both are carried out in a semisolid medium such as agar. In radial immunodiffusion, an antigen sample is placed in a well and allowed to diffuse into 142 PART II Generation of B-Cell and T-Cell Responses FIGURE 6-4 Precipitation reactions. (a) Polyclonal antibodies can form lattices, or large aggregates, that precipitate out of solution. However, if each antigen molecule contains only a single epitope rec- ognized by a given monoclonal antibody, the antibody can link only two molecules of antigen and no precipitate is formed. (b) A precip- itation curve for a system of one antigen and its antibodies. This plot of the amount of antibody precipitated versus increasing antigen concentrations (at constant total antibody) reveals three zones: a zone of antibody excess, in which precipitation is inhibited and anti- body not bound to antigen can be detected in the supernatant; an equivalence zone of maximal precipitation in which antibody and antigen form large insoluble complexes and neither antibody nor antigen can be detected in the supernatant; and a zone of antigen ex- cess in which precipitation is inhibited and antigen not bound to antibody can be detected in the supernatant. ++++ + _ __ __ __ _______ ++ ++ Antigen added Equivalence zone Antibody-excess zone POLYCLONAL ANTISERUM MONOCLONAL ANTIBODY Myoglobin Antigen-excess zone Supernatants excess Ab excess Ag Antibody precipitated (b)(a) 8536d_ch06_137-160 8/1/02 9:01 AM Page 142 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: agar containing a suitable dilution of an antiserum. As the antigen diffuses into the agar, the region of equivalence is es- tablished and a ring of precipitation, a precipitin ring, forms around the well (Figure 6-5, upper panel). The area of the pre- cipitin ring is proportional to the concentration of antigen. By comparing the area of the precipitin ring with a standard curve (obtained by measuring the precipitin areas of known concen- trations of the antigen), the concentration of the antigen sam- ple can be determined. In the Ouchterlony method, both antigen and antibody diffuse radially from wells toward each other, thereby establishing a concentration gradient. As equiv- alence is reached, a visible line of precipitation, a precipitin line, forms (Figure 6-5, lower panel). Immunoelectrophoresis Combines Electrophoresis and Double Immunodiffusion In immunoelectrophoresis, the antigen mixture is first elec- trophoresed to separate its components by charge. Troughs are then cut into the agar gel parallel to the direction of the electric field, and antiserum is added to the troughs. Antibody and antigen then diffuse toward each other and produce lines of precipitation where they meet in appropri- ate proportions (Figure 6-6a). Immunoelectrophoresis is used in clinical laboratories to detect the presence or absence of proteins in the serum. A sample of serum is elec- trophoresed, and the individual serum components are identified with antisera specific for a given protein or im- munoglobulin class (Figure 6-6b). This technique is useful in determining whether a patient produces abnormally low amounts of one or more isotypes, characteristic of certain immunodeficiency diseases. It can also show whether a pa- tient overproduces some serum protein, such as albumin, immunoglobulin, or transferrin. The immunoelectropho- retic pattern of serum from patients with multiple myeloma, for example, shows a heavy distorted arc caused by the large amount of myeloma protein, which is monoclonal Ig and therefore uniformly charged (Figure 6-6b). Because immu- noelectrophoresis is a strictly qualitative technique that only detects relatively high antibody concentrations (greater than several hundred H9262g/ml), it utility is limited to the detection Antigen-Antibody Interactions: Principles and Applications CHAPTER 6 143 TABLE 6-3 Sensitivity of various immunoassays Sensitivity ? Assay (H9262g antibody/ml) Precipitation reaction in fluids 20–200 Precipitation reactions in gels Mancini radial immunodiffusion 10–50 Ouchterlony double immunodiffusion 20–200 Immunoelectrophoresis 20–200 Rocket electrophoresis 2 Agglutination reactions Direct 0.3 Passive agglutination 0.006–0.06 Agglutination inhibition 0.006–0.06 Radioimmunoassay 0.0006–0.006 Enzyme-linked immunosorbent assay (ELISA) H110210.0001–0.01 ELISA using chemiluminescence H110210.0001–0.01 ? Immunofluorescence 1.0 Flow cytometry 0.06–0.006 ? The sensitivity depends upon the affinity of the antibody as well as the epi- tope density and distribution. ? Note that the sensitivity of chemiluminescence-based ELISA assays can be made to match that of RIA. SOURCE: Adapted from N. R. Rose et al., eds., 1997, Manual of Clinical Laboratory Immunology, 5th ed., American Society for Microbiology, Washington, D.C. RADIAL IMMUNODIFFUSION Antibody incorporated in agar Antigen Antigen diffusion Precipitate forms ring DOUBLE IMMUNODIFFUSION PrecipitateAgar matrix Antibody Antigen FIGURE 6-5 Diagrammatic representation of radial immunodiffu- sion (Mancini method) and double immunodiffusion (Ouchterlony method) in a gel. In both cases, large insoluble complexes form in the agar in the zone of equivalence, visible as lines of precipitation (purple regions). Only the antigen (red) diffuses in radial immuno- diffusion, whereas both the antibody (blue) and antigen (red) diffuse in double immunodiffusion. 8536d_ch06_137-160 8/1/02 9:01 AM Page 143 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: of quantitative abnormalities only when the departure from normal is striking, as in immunodeficiency states and im- munoproliferative disorders. A related quantitative technique, rocket electrophore- sis, does permit measurement of antigen levels. In rocket electrophoresis, a negatively charged antigen is elec- trophoresed in a gel containing antibody. The precipitate formed between antigen and antibody has the shape of a rocket, the height of which is proportional to the concen- tration of antigen in the well. One limitation of rocket electrophoresis is the need for the antigen to be negatively charged for electrophoretic movement within the agar matrix. Some proteins, immunoglobulins for example, are not sufficiently charged to be quantitatively analyzed by rocket electrophoresis; nor is it possible to measure the amounts of several antigens in a mixture at the same time. Agglutination Reactions The interaction between antibody and a particulate antigen re- sults in visible clumping called agglutination. Antibodies that produce such reactions are called agglutinins. Agglutination reactions are similar in principle to precipitation reactions; they depend on the crosslinking of polyvalent antigens. Just as 144 PART II Generation of B-Cell and T-Cell Responses Antigens (a) Antibody FIGURE 6-6 Immunoelectrophoresis of an antigen mixture. (a) An antigen preparation (orange) is first electrophoresed, which separates the component antigens on the basis of charge. Antiserum (blue) is then added to troughs on one or both sides of the separated antigens and allowed to diffuse; in time, lines of precipitation (col- ored arcs) form where specific antibody and antigen interact. (b) Im- munoelectrophoretic patterns of human serum from a patient with myeloma. The patient produces a large amount of a monoclonal IgG (H9261-light-chain-bearing) antibody. A sample of serum from the patient was placed in the well of the slide and electrophoresed. Then anti- serum specific for the indicated antibody class or light chain type was placed in the top trough of each slide. At the concentrations of pa- tient’s serum used, only anti-IgG and anti-H9261 antibodies produced lines of precipitation. [Part(b), Robert A. Kyle and Terry A. Katzman, Manual of Clinical Immunology, 1997, N. Rose, ed., ASM Press, Wash- ington, D.C., p. 164.] Ig A Ig G Ig M κ λ (b) 8536d_ch06_137-160 8/1/02 9:01 AM Page 144 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: an excess of antibody inhibits precipitation reactions, such excess can also inhibit agglutination reactions; this inhibition is called the prozone effect. Because prozone effects can be en- countered in many types of immunoassays, understanding the basis of this phenomenon is of general importance. Several mechanisms can cause the prozone effect. First, at high antibody concentrations, the number of antibody bind- ing sites may greatly exceed the number of epitopes. As a re- sult, most antibodies bind antigen only univalently instead of multivalently. Antibodies that bind univalently cannot crosslink one antigen to another. Prozone effects are readily diagnosed by performing the assay at a variety of antibody (or antigen) concentrations. As one dilutes to an optimum antibody concentration, one sees higher levels of agglutina- tion or whatever parameter is measured in the assay being used. When one is using polyclonal antibodies, the prozone effect can also occur for another reason. The antiserum may contain high concentrations of antibodies that bind to the antigen but do not induce agglutination; these antibodies, called incomplete antibodies, are often of the IgG class. At high concentrations of IgG, incomplete antibodies may oc- cupy most of the antigenic sites, thus blocking access by IgM, which is a good agglutinin. This effect is not seen with agglu- tinating monoclonal antibodies. The lack of agglutinating activity of an incomplete antibody may be due to restricted flexibility in the hinge region, making it difficult for the anti- body to assume the required angle for optimal cross-linking of epitopes on two or more particulate antigens. Alterna- tively, the density of epitope distribution or the location of some epitopes in deep pockets of a particulate antigen may make it difficult for the antibodies specific for these epitopes to agglutinate certain particulate antigens. When feasible, the solution to both of these problems is to try different antibod- ies that may react with other epitopes of the antigen that do not present these limitations. Hemagglutination Is Used in Blood Typing Agglutination reactions (Figure 6-7) are routinely performed to type red blood cells (RBCs). In typing for the ABO antigens, RBCs are mixed on a slide with antisera to the A or B blood-group antigens. If the antigen is present on the cells, they agglutinate, forming a visible clump on the slide. Determination of which antigens are present on donor and recipient RBCs is the basis for matching blood types for transfusions. Bacterial Agglutination Is Used To Diagnose Infection A bacterial infection often elicits the production of serum antibodies specific for surface antigens on the bacterial cells. The presence of such antibodies can be detected by bacterial agglutination reactions. Serum from a patient thought to be infected with a given bacterium is serially diluted in an array of tubes to which the bacteria is added. The last tube showing visible agglutination will reflect the serum antibody titer of the patient. The agglutinin titer is defined as the reciprocal of the greatest serum dilution that elicits a positive agglutina- tion reaction. For example, if serial twofold dilutions of serum are prepared and if the dilution of 1/640 shows agglu- tination but the dilution of 1/1280 does not, then the agglu- tination titer of the patient’s serum is 640. In some cases serum can be diluted up to 1/50,000 and still show agglutina- tion of bacteria. The agglutinin titer of an antiserum can be used to diag- nose a bacterial infection. Patients with typhoid fever, for ex- ample, show a significant rise in the agglutination titer to Salmonella typhi. Agglutination reactions also provide a way to type bacteria. For instance, different species of the bac- terium Salmonella can be distinguished by agglutination re- actions with a panel of typing antisera. Passive Agglutination Is Useful with Soluble Antigens The sensitivity and simplicity of agglutination reactions can be extended to soluble antigens by the technique of passive hemagglutination. In this technique, antigen-coated red blood cells are prepared by mixing a soluble antigen with red blood cells that have been treated with tannic acid or chromium chloride, both of which promote adsorption of the antigen to the surface of the cells. Serum containing anti- body is serially diluted into microtiter plate wells, and the antigen-coated red blood cells are then added to each well; agglutination is assessed by the size of the characteristic spread pattern of agglutinated red blood cells on the bottom of the well, like the pattern seen in agglutination reactions (see Figure 6-7). Over the past several years, there has been a shift away from red blood cells to synthetic particles, such as latex beads, as matrices for agglutination reactions. Once the anti- gen has been coupled to the latex beads, the preparation can either be used immediately or stored for later use. The use of synthetic beads offers the advantages of consistency, Antigen-Antibody Interactions: Principles and Applications CHAPTER 6 145 FIGURE 6-7 Demonstration of hemagglutination using antibodies against sheep red blood cells (SRBCs). The control tube (10) con- tains only SRBCs, which settle into a solid “button.” The experimen- tal tubes 1–9 contain a constant number of SRBCs plus serial two-fold dilutions of anti-SRBC serum. The spread pattern in the ex- perimental series indicates positive hemagglutination through tube 3. [Louisiana State University Medical Center/MIP. Courtesy of Harriet C. W. Thompson.] 8536d_ch06_137-160 8/1/02 9:01 AM Page 145 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: uniformity, and stability. Furthermore, agglutination reac- tions employing synthetic beads can be read rapidly, often within 3 to 5 minutes of mixing the beads with the test sam- ple. Whether based on red blood cells or the more convenient and versatile synthetic beads, agglutination reactions are simple to perform, do not require expensive equipment, and can detect small amounts of antibody (concentrations as low as nanograms per milliliter). In Agglutination Inhibition, Absence of Agglutination Is Diagnostic of Antigen A modification of the agglutination reaction, called agglu- tination inhibition, provides a highly sensitive assay for small quantities of an antigen. For example, one of the early types of home pregnancy test kits included latex particles coated with human chorionic gonadotropin (HCG) and antibody to HCG (Figure 6-8). The addition of urine from a pregnant woman, which contained HCG, inhibited agglu- tination of the latex particles when the anti-HCG antibody was added; thus the absence of agglutination indicated pregnancy. Agglutination inhibition assays can also be used to deter- mine whether an individual is using certain types of illegal drugs, such as cocaine or heroin. A urine or blood sample is first incubated with antibody specific for the suspected drug. Then red blood cells (or other particles) coated with the drug are added. If the red blood cells are not agglutinated by the antibody, it indicates the sample contained an antigen recog- nized by the antibody, suggesting that the individual was 146 PART II Generation of B-Cell and T-Cell Responses FIGURE 6-8 The original home pregnancy test kit employed hap- ten inhibition to determine the presence or absence of human chori- onic gonadotropin (HCG). The original test kits used the presence or absence of visible clumping to determine whether HCG was present. If a woman was not pregnant, her urine would not contain HCG; in this case, the anti-HCG antibodies and HCG-carrier conjugate in the kit would react, producing visible clumping. If a woman was preg- nant, the HCG in her urine would bind to the anti-HCG antibodies, thus inhibiting the subsequent binding of the antibody to the HCG- carrier conjugate. Because of this inhibition, no visible clumping oc- curred if a woman was pregnant. The kits currently on the market use ELISA-based assays (see Figure 6-10). KIT REAGENTS Hapten carrier–conjugate Anti–HCG antibody TEST PROCEDURE Urine Incubate HCG carrier conjugate Observe for visible clumping Visible clumping+ reaction: pregnant No visible clumping HCG in urine + POSSIBLE REACTIONS reaction: not pregnant HCG and +Anti–HCG+ – + + + 8536d_ch06_137-160 8/1/02 9:01 AM Page 146 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: using the illicit drug. One problem with these tests is that some legal drugs have chemical structures similar to those of illicit drugs, and these legal drugs may cross-react with the antibody, giving a false-positive reaction. For this reason a positive reaction must be confirmed by a nonimmunologic method. Agglutination inhibition assays are widely used in clinical laboratories to determine whether an individual has been exposed to certain types of viruses that cause agglutination of red blood cells. If an individual’s serum contains specific an- tiviral antibodies, then the antibodies will bind to the virus and interfere with hemagglutination by the virus. This tech- nique is commonly used in premarital testing to determine the immune status of women with respect to rubella virus. The reciprocal of the last serum dilution to show inhibition of rubella hemagglutination is the titer of the serum. A titer greater than 10 (1:10 dilution) indicates that a woman is im- mune to rubella, whereas a titer of less than 10 is indicative of a lack of immunity and the need for immunization with the rubella vaccine. Radioimmunoassay One of the most sensitive techniques for detecting antigen or antibody is radioimmunoassay (RIA). The technique was first developed in 1960 by two endocrinologists, S. A. Berson and Rosalyn Yalow, to determine levels of insulin–anti-in- sulin complexes in diabetics. Although their technique en- countered some skepticism, it soon proved its value for measuring hormones, serum proteins, drugs, and vitamins at concentrations of 0.001 micrograms per milliliter or less. In 1977, some years after Berson’s death, the significance of the technique was acknowledged by the award of a Nobel Prize to Yalow. The principle of RIA involves competitive binding of ra- diolabeled antigen and unlabeled antigen to a high-affinity antibody. The labeled antigen is mixed with antibody at a concentration that saturates the antigen-binding sites of the antibody. Then test samples of unlabeled antigen of un- known concentration are added in progressively larger amounts. The antibody does not distinguish labeled from unlabeled antigen, so the two kinds of antigen compete for available binding sites on the antibody. As the concentration of unlabeled antigen increases, more labeled antigen will be displaced from the binding sites. The decrease in the amount of radiolabeled antigen bound to specific antibody in the presence of the test sample is measured in order to determine the amount of antigen present in the test sample. The antigen is generally labeled with a gamma-emitting isotope such as 125 I, but beta-emitting isotopes such as tri- tium ( 3 H) are also routinely used as labels. The radiola- beled antigen is part of the assay mixture; the test sample may be a complex mixture, such as serum or other body fluids, that contains the unlabeled antigen. The first step in Antigen-Antibody Interactions: Principles and Applications CHAPTER 6 147 setting up an RIA is to determine the amount of antibody needed to bind 50%–70% of a fixed quantity of radioactive antigen (Ag ? ) in the assay mixture. This ratio of antibody to Ag ? is chosen to ensure that the number of epitopes presented by the labeled antigen always exceeds the total number of antibody binding sites. Consequently, unlabeled antigen added to the sample mixture will compete with ra- diolabeled antigen for the limited supply of antibody. Even a small amount of unlabeled antigen added to the assay mixture of labeled antigen and antibody will cause a de- crease in the amount of radioactive antigen bound, and this decrease will be proportional to the amount of unlabeled antigen added. To determine the amount of labeled antigen bound, the Ag-Ab complex is precipitated to separate it from free antigen (antigen not bound to Ab), and the ra- dioactivity in the precipitate is measured. A standard curve can be generated using unlabeled antigen samples of known concentration (in place of the test sample), and from this plot the amount of antigen in the test mixture may be precisely determined. Several methods have been developed for separating the bound antigen from the free antigen in RIA. One method in- volves precipitating the Ag-Ab complex with a secondary anti-isotype antiserum. For example, if the Ag-Ab complex contains rabbit IgG antibody, then goat anti-rabbit IgG will bind to the rabbit IgG and precipitate the complex. Another method makes use of the fact that protein A of Staphylococcus aureus has high affinity for IgG. If the Ag-Ab complex con- tains an IgG antibody, the complex can be precipitated by mixing with formalin-killed S. aureus. After removal of the complex by either of these methods, the amount of free la- beled antigen remaining in the supernatant can be measured in a radiation counter; subtracting this value from the total amount of labeled antigen added yields the amount of la- beled antigen bound. Various solid-phase RIAs have been developed that make it easier to separate the Ag-Ab complex from the unbound antigen. In some cases, the antibody is covalently cross- linked to Sepharose beads. The amount of radiolabeled anti- gen bound to the beads can be measured after the beads have been centrifuged and washed. Alternatively, the antibody can be immobilized on polystyrene or polyvinylchloride wells and the amount of free labeled antigen in the supernatant can be determined in a radiation counter. In another ap- proach, the antibody is immobilized on the walls of mi- crotiter wells and the amount of bound antigen determined. Because the procedure requires only small amounts of sam- ple and can be conducted in small 96-well microtiter plates (slightly larger than a 3 H11003 5 card), this procedure is well suited for determining the concentration of a particular anti- gen in large numbers of samples. For example, a microtiter RIA has been widely used to screen for the presence of the he- patitis B virus (Figure 6-9). RIA screening of donor blood has sharply reduced the incidence of hepatitis B infections in re- cipients of blood transfusions. 8536d_ch06_137-160 8/1/02 9:01 AM Page 147 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: Enzyme-Linked Immunosorbent Assay Enzyme-linked immunosorbent assay, commonly known as ELISA (or EIA), is similar in principle to RIA but depends on an enzyme rather than a radioactive label. An enzyme conjugated with an antibody reacts with a colorless substrate to generate a colored reaction product. Such a substrate is called a chromogenic substrate. A number of enzymes have been employed for ELISA, including alkaline phosphatase, horseradish peroxidase, and H9252-galactosidase. These assays approach the sensitivity of RIAs and have the advantage of being safer and less costly. There Are Numerous Variants of ELISA A number of variations of ELISA have been developed, al- lowing qualitative detection or quantitative measurement of either antigen or antibody. Each type of ELISA can be used qualitatively to detect the presence of antibody or antigen. Alternatively, a standard curve based on known concentrations of antibody or antigen is prepared, from which the unknown concentration of a sample can be determined. INDIRECT ELISA Antibody can be detected or quantitatively determined with an indirect ELISA (Figure 6-10a). Serum or some other sam- ple containing primary antibody (Ab 1 ) is added to an anti- gen-coated microtiter well and allowed to react with the antigen attached to the well. After any free Ab 1 is washed away, the presence of antibody bound to the antigen is de- tected by adding an enzyme-conjugated secondary anti-iso- type antibody (Ab 2 ), which binds to the primary antibody. Any free Ab 2 then is washed away, and a substrate for the en- zyme is added. The amount of colored reaction product that forms is measured by specialized spectrophotometric plate readers, which can measure the absorbance of all of the wells of a 96-well plate in seconds. Indirect ELISA is the method of choice to detect the pres- ence of serum antibodies against human immunodeficiency virus (HIV), the causative agent of AIDS. In this assay, re- combinant envelope and core proteins of HIV are adsorbed 148 PART II Generation of B-Cell and T-Cell Responses FIGURE 6-9 A solid-phase radioimmunoassay (RIA) to detect hepatitis B virus in blood samples. (a) Microtiter wells are coated with a constant amount of antibody specific for HBsAg, the surface antigen on hepatitis B virions. A serum sample and [ 125 I]HBsAg are then added. After incubation, the supernatant is removed and the radioactivity of the antigen-antibody complexes is measured. If the sample is infected, the amount of label bound will be less than in controls with uninfected serum. (b) A standard curve is obtained by adding increasing concentrations of unlabeled HBsAg to a fixed quantity of [ 125 I]HBsAg and specific antibody. From the plot of the percentage of labeled antigen bound versus the concentration of unlabeled antigen, the concentration of HBsAg in unknown serum samples can be determined by using the linear part of the curve. (a) Infected serum [ 125 I] HBsAg Uninfected serum [ 125 I] HBsAg Unlabeled HBsAg 125 I bound 125 I bound Anti-HBsAg (b) Concentration of unlabeled HBsAg, ng/ml [ 125 I] HBsAg bound to anti-HBsAg, % 70 60 50 40 30 20 10 0 123456 Approximately linear part of curve 8536d_ch06_137-160 8/1/02 9:01 AM Page 148 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: as solid-phase antigens to microtiter wells. Individuals in- fected with HIV will produce serum antibodies to epitopes on these viral proteins. Generally, serum antibodies to HIV can be detected by indirect ELISA within 6 weeks of infection. SANDWICH ELISA Antigen can be detected or measured by a sandwich ELISA (Figure 6-10b). In this technique, the antibody (rather than the antigen) is immobilized on a microtiter well. A sample containing antigen is added and allowed to react with the immobilized antibody. After the well is washed, a second en- zyme-linked antibody specific for a different epitope on the antigen is added and allowed to react with the bound anti- gen. After any free second antibody is removed by washing, substrate is added, and the colored reaction product is measured. COMPETITIVE ELISA Another variation for measuring amounts of antigen is com- petitive ELISA (Figure 6-10c). In this technique, antibody is first incubated in solution with a sample containing antigen. The antigen-antibody mixture is then added to an antigen- coated microtiter well. The more antigen present in the sam- ple, the less free antibody will be available to bind to the antigen-coated well. Addition of an enzyme-conjugated sec- ondary antibody (Ab 2 ) specific for the isotype of the primary antibody can be used to determine the amount of primary antibody bound to the well as in an indirect ELISA. In the Antigen-Antibody Interactions: Principles and Applications CHAPTER 6 149 FIGURE 6-10 Variations in the enzyme-linked immunosorbent as- say (ELISA) technique allow determination of antibody or antigen. Each assay can be used qualitatively, or quantitatively by comparison with standard curves prepared with known concentrations of anti- body or antigen. Antibody can be determined with an indirect ELISA (a), whereas antigen can be determined with a sandwich ELISA (b) or competitive ELISA (c). In the competitive ELISA, which is an inhibi- tion-type assay, the concentration of antigen is inversely proportional to the color produced. (a) Indirect ELISA (b) Sandwich ELISA (c) Competitive ELISA Antigen- coated well wash wash wash Add specific antibody to be measured Add enzyme- conjugated secondary antibody Add substrate (S) and measure color E E E E S S Antibody- coated well wash wash wash Add antigen to be measured Add enzyme- conjugated secondary antibody Add substrate and measure color S S EEE E E Incubate antibody with antigen to be measured wash wash Add Ag-Ab mixture to antigen-coated well Add enzyme- conjugated secondary antibody Add substrate and measure color S S E 8536d_ch06_137-160 8/1/02 9:01 AM Page 149 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: competitive assay, however, the higher the concentration of antigen in the original sample, the lower the absorbance. CHEMILUMINESCENCE Measurement of light produced by chemiluminescence dur- ing certain chemical reactions provides a convenient and highly sensitive alternative to absorbance measurements in ELISA assays. In versions of the ELISA using chemilumines- cence, a luxogenic (light-generating) substrate takes the place of the chromogenic substrate in conventional ELISA reac- tions. For example, oxidation of the compound luminol by H 2 O 2 and the enzyme horseradish peroxidase (HRP) pro- duces light: luminol H11001 H 2 O 2 Ab-HRP H11001 Ag ?→ Ab-HRP-Ag ?????→ light The advantage of chemiluminescence assays over chro- mogenic ones is enhanced sensitivity. In general, the detec- tion limit can be increased at least ten-fold by switching from a chromogenic to a luxogenic substrate, and with the addi- tion of enhancing agents, more than 200-fold. In fact, under ideal conditions, as little as 5 H11003 10 H1100218 moles (5 attomoles) of target antigen have been detected. ELISPOT ASSAY A modification of the ELISA assay called the ELISPOT assay allows the quantitative determination of the number of cells in a population that are producing antibodies specific for a given antigen or an antigen for which one has a specific anti- body (Figure 6-11). In this approach, the plates are coated with the antigen (capture antigen) recognized by the anti- body of interest or with the antibody (capture antibody) spe- cific for the antigen whose production is being assayed. A suspension of the cell population under investigation is then added to the coated plates and incubated. The cells settle onto the surface of the plate, and secreted molecules reactive with the capture molecules are bound by the capture mole- cules in the vicinity of the secreting cells, producing a ring of antigen-antibody complexes around each cell that is produc- ing the molecule of interest. The plate is then washed and an enzyme-linked antibody specific for the secreted antigen or specific for the species (e.g., goat anti-rabbit) of the secreted antibody is added and allowed to bind. Subsequent develop- ment of the assay by addition of a suitable chromogenic or chemiluminescence-producing substrate reveals the position of each antibody- or antigen-producing cell as a point of color or light. Western Blotting Identification of a specific protein in a complex mixture of proteins can be accomplished by a technique known as West- ern blotting, named for its similarity to Southern blotting, 150 PART II Generation of B-Cell and T-Cell Responses Incubate at 37°C Add enzyme-linked anti-cytokine antibody Well coated with anti-cytokine Side view E H11005 enzyme CS H11005 chromogeni substrate CP H11005 colored product S NS Non-secretor Secretor Add test cell population Discard cells Wash plate NSS EE CS Top view CS CPCP E Site of secreting cell FIGURE 6-11 In the ELISPOT assay, a well is coated with antibody against the antigen of interest, a cytokine in this example, and then a suspension of a cell population thought to contain some members syn- thesizing and secreting the cytokine are layered onto the bottom of the well and incubated. Most of the cytokine molecules secreted by a par- ticular cell react with nearby well-bound antibodies. After the incubation period, the well is washed and an enzyme-labeled anti-cytokine antibody is added. After washing away unbound antibody, a chromogenic sub- strate that forms an insoluble colored product is added. The colored product (purple) precipitates and forms a spot only on the areas of the well where cytokine-secreting cells had been deposited. By counting the number of colored spots, it is possible to determine how many cytokine-secreting cells were present in the added cell suspension. 8536d_ch06_137-160 8/1/02 12:41 PM Page 150 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: which detects DNA fragments, and Northern blotting, which detects mRNAs. In Western blotting, a protein mixture is electrophoretically separated on an SDS-polyacrylamide gel (SDS-PAGE), a slab gel infused with sodium dodecyl sulfate (SDS), a dissociating agent (Figure 6-12). The protein bands are transferred to a nylon membrane by electrophoresis and the individual protein bands are identified by flooding the nitrocellulose membrane with radiolabeled or enzyme- linked polyclonal or monoclonal antibody specific for the protein of interest. The Ag-Ab complexes that form on the band containing the protein recognized by the antibody can be visualized in a variety of ways. If the protein of interest was bound by a radioactive antibody, its position on the blot can be determined by exposing the membrane to a sheet of x-ray film, a procedure called autoradiography. However, the most generally used detection procedures employ enzyme-linked antibodies against the protein. After binding of the enzyme- antibody conjugate, addition of a chromogenic substrate that produces a highly colored and insoluble product causes the appearance of a colored band at the site of the target antigen. The site of the protein of interest can be determined with much higher sensitivity if a chemiluminescent compound along with suitable enhancing agents is used to produce light at the antigen site. Western blotting can also identify a specific antibody in a mixture. In this case, known antigens of well-defined molec- ular weight are separated by SDS-PAGE and blotted onto ni- trocellulose. The separated bands of known antigens are then probed with the sample suspected of containing antibody specific for one or more of these antigens. Reaction of an an- tibody with a band is detected by using either radiolabeled or enzyme-linked secondary antibody that is specific for the species of the antibodies in the test sample. The most widely used application of this procedure is in confirmatory testing for HIV, where Western blotting is used to determine whether the patient has antibodies that react with one or more viral proteins. Antigen-Antibody Interactions: Principles and Applications CHAPTER 6 151 – Protein antigens denatured in SDS Direction of migration Add SDS-treated protein mixture to well of gel (a) Electrophorese in SDS-polyacrylamide gel (b) Remove gel and perform electrotransfer Electric current Porous membrane sheet (c) Bind antigen of interest with enzyme-linked antibodies (d) Add substrate to activate color reaction (e) + FIGURE 6-12 In Western blotting, a protein mixture is (a) treated with SDS, a strong denaturing detergent, (b) then separated by elec- trophoresis in an SDS polyacrylamide gel (SDS-PAGE) which sepa- rates the components according to their molecular weight; lower molecular weight components migrate farther than higher molecular weight ones. (c) The gel is removed from the apparatus and applied to a protein-binding sheet of nitrocellulose or nylon and the proteins in the gel are transferred to the sheet by the passage of an electric current. (d) Addition of enzyme-linked antibodies detects the antigen of interest, and (e) the position of the antibodies is visualized by means of an ELISA reaction that generates a highly colored insoluble product that is deposited at the site of the reaction. Alternatively, a chemiluminescent ELISA can be used to generate light that is readily detected by exposure of the blot to a piece of photographic film. 8536d_ch06_137-160 8/1/02 12:41 PM Page 151 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: Immunoprecipitation The immunoprecipitation technique has the advantage of al- lowing the isolation of the antigen of interest for further analysis. It also provides a sensitive assay for the presence of a particular antigen in a given cell or tissue type. An extract pro- duced by disruption of cells or tissues is mixed with an anti- body against the antigen of interest in order to form an antigen-antibody complex that will precipitate. However, if the antigen concentration is low (often the case in cell and tis- sue extracts), the assembly of antigen-antibody complexes into precipitates can take hours, even days, and it is difficult to isolate the small amount of immunoprecipitate that forms. Fortunately, there are a number of ways to avoid these limitations. One is to attach the antibody to a solid support, such as a synthetic bead, which allows the antigen-antibody complex to be collected by centrifugation. Another is to add a secondary antibody specific for the primary antibody to bind the antigen-antibody complexes. If the secondary anti- body is attached to a bead, the immune complexes can be collected by centrifugation. A particularly ingenious version of this procedure involves the coupling of the secondary an- tibody to magnetic beads. After the secondary antibody binds to the primary antibody, immunoprecipitates are collected by placing a magnet against the side of the tube (Figure 6-13). When used in conjunction with biosynthetic radioisotope labeling, immunoprecipitation can also be used to determine whether a particular antigen is actually synthesized by a cell or tissue. Radiolabeling of proteins synthesized by cells of in- terest can be done by growing the cells in cell-culture medium containing one or more radiolabeled amino acids. Generally, the amino acids used for this application are those most resistant to metabolic modification, such as leucine, cysteine, or methionine. After growth in the radioactive medium, the cells are lysed and subjected to a primary anti- body specific for the antigen of interest. The Ag-Ab complex is collected by immunoprecipitation, washed free of unin- corporated radiolabeled amino acid and other impurities, and then analyzed. The complex can be counted in a scintil- lation counter to obtain a quantitative determination of the amount of the protein synthesized. Further analysis often in- volves disruption of the complex, usually by use of SDS and heat, so that the identity of the immunoprecipitated antigen can be confirmed by checking that its molecular weight is that expected for the antigen of interest. This is done by sep- aration of the disrupted complex by SDS-PAGE and subse- quent autoradiography to determine the position of the radiolabeled antigen on the gel. Immunofluorescence In 1944, Albert Coons showed that antibodies could be la- beled with molecules that have the property of fluorescence. Fluorescent molecules absorb light of one wavelength 152 PART II Generation of B-Cell and T-Cell Responses Specific antibody Antigen A Add specific antibody to cell extract Add secondary antibody coupled to magnetic beads Apply magnet and rinse to remove unbound material Magnetic bead (a) (b) (c) FIGURE 6-13 Immunoprecipitates can be collected using mag- netic beads coupled to a secondary antibody. (a) Treatment of a cell extract containing antigen A (red) with a mouse anti-A antibody (blue) results in the formation of antigen-antibody complexes. (b) Addition of magnetic beads to which a rabbit anti-mouse anti- body is linked binds the antigen-antibody complexes (and any unre- acted mouse Ig). (c) Placing a magnet against the side of the tube allows the rapid collection of the antigen-antibody complexes. After rinsing to remove any unbound material, the antigen-antibody com- plexes can be dissociated and the antigen studied. (d) An electron micrograph showing a cell with magnetic beads attached to its sur- face via antibodies. [Part (d), P. Groscurth, Institute of Anatomy, Uni- versity of Zurich-Irchel.] (d) 8536d_ch06_137-160 8/1/02 12:41 PM Page 152 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: (excitation) and emit light of another wavelength (emission). If antibody molecules are tagged with a fluorescent dye, or fluorochrome, immune complexes containing these fluores- cently labeled antibodies (FA) can be detected by colored light emission when excited by light of the appropriate wave- length. Antibody molecules bound to antigens in cells or tis- sue sections can similarly be visualized. The emitted light can be viewed with a fluorescence microscope, which is equipped with a UV light source. In this technique, known as im- munofluorescence, fluorescent compounds such as fluores- cein and rhodamine are in common use, but other highly fluorescent substances are also routinely used, such as phyco- erythrin, an intensely colored and highly fluorescent pig- ment obtained from algae. These molecules can be conjugated to the Fc region of an antibody molecule without affecting the specificity of the antibody. Each of the fluo- rochromes below absorbs light at one wavelength and emits light at a longer wavelength: a73 Fluorescein, an organic dye that is the most widely used label for immunofluorescence procedures, absorbs blue light (490 nm) and emits an intense yellow-green fluorescence (517 nm). a73 Rhodamine, another organic dye, absorbs in the yellow-green range (515 nm) and emits a deep red fluorescence (546 nm). Because it emits fluorescence at a longer wavelength than fluorescein, it can be used in two-color immunofluorescence assays. An antibody specific to one determinant is labeled with fluorescein, and an antibody recognizing a different antigen is labeled with rhodamine. The location of the fluorescein-tagged antibody will be visible by its yellow- green color, easy to distinguish from the red color emitted where the rhodamine-tagged antibody has bound. By conjugating fluorescein to one antibody and rhodamine to another antibody, one can, for example, visualize simultaneously two different cell-membrane antigens on the same cell. a73 Phycoerythrin is an efficient absorber of light (~30-fold greater than fluorescein) and a brilliant emitter of red fluorescence, stimulating its wide use as a label for immunofluorescence. Fluorescent-antibody staining of cell membrane mole- cules or tissue sections can be direct or indirect (Figure 6-14). In direct staining, the specific antibody (the primary anti- body) is directly conjugated with fluorescein; in indirect staining, the primary antibody is unlabeled and is detected with an additional fluorochrome-labeled reagent. A num- ber of reagents have been developed for indirect staining. Antigen-Antibody Interactions: Principles and Applications CHAPTER 6 153 FIGURE 6-14 Direct and indirect immunofluorescence staining of membrane antigen (mAg). Cells are affixed to a microscope slide. In the direct method (a), cells are stained with anti-mAg anti- body that is labeled with a fluorochrome (Fl). In the indirect meth- ods (b and c), cells are first incubated with unlabeled anti-mAg antibody and then stained with a fluorochrome-labeled secondary reagent that binds to the primary antibody. Cells are viewed under a fluorescence microscope to see if they have been stained. (d) In this micrograph, antibody molecules bearing H9262 heavy chains are detected by indirect staining of cells with rhodamine-conjugated second antibody. [Part(d), H. A. Schreuder et al., 1997, Nature 386:196, courtesy H. Schreuder, Hoechst Marion Roussel.] Cells with membrane antigens (mAg) (a) Direct method with fluorochrome– labeled antibody to mAg (b) Indirect method with fluorochrome– labeled anti–isotype antibody (c) Indirect method with fluorochrome– labeled protein A Primary antibody Fl Fl Fl Fl Fl Fl Secondary anti–isotype antibody Fl Fl Fl Fl Fl Fl Fl Protein A Fl Fl Fl Primary antibody to mAg (d) 8536d_ch06_137-160 8/1/02 9:01 AM Page 153 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: The most common is a fluorochrome-labeled secondary antibody raised in one species against antibodies of another species, such as fluorescein-labeled goat anti-mouse im- munoglobulin. Indirect immunofluorescence staining has two advan- tages over direct staining. First, the primary antibody does not need to be conjugated with a fluorochrome. Because the supply of primary antibody is often a limiting factor, indirect methods avoid the loss of antibody that usually occurs dur- ing the conjugation reaction. Second, indirect methods in- crease the sensitivity of staining because multiple molecules of the fluorochrome reagent bind to each primary antibody molecule, increasing the amount of light emitted at the loca- tion of each primary antibody molecule. Immunofluorescence has been applied to identify a num- ber of subpopulations of lymphocytes, notably the CD4 H11001 and CD8 H11001 T-cell subpopulations. The technique is also suit- able for identifying bacterial species, detecting Ag-Ab com- plexes in autoimmune disease, detecting complement components in tissues, and localizing hormones and other cellular products stained in situ. Indeed, a major application of the fluorescent-antibody technique is the localization of antigens in tissue sections or in subcellular compartments. Because it can be used to map the actual location of target antigens, fluorescence microscopy is a powerful tool for relat- ing the molecular architecture of tissues and organs to their overall gross anatomy. Flow Cytometry and Fluorescence The fluorescent antibody techniques described are ex- tremely valuable qualitative tools, but they do not give quantitative data. This shortcoming was remedied by development of the flow cytometer, which was designed to automate the analysis and separation of cells stained with fluorescent antibody. The flow cytometer uses a laser beam and light detector to count single intact cells in suspension (Figure 6-15). Every time a cell passes the laser beam, light is deflected from the detector, and this interruption of the laser signal is recorded. Those cells having a fluorescently tagged antibody bound to their cell surface antigens are ex- cited by the laser and emit light that is recorded by a second detector system located at a right angle to the laser beam. The simplest form of the instrument counts each cell as it passes the laser beam and records the level of fluorescence the cell emits; an attached computer generates plots of the number of cells as the ordinate and their fluorescence inten- sity as the abscissa. More sophisticated versions of the in- strument are capable of sorting populations of cells into different containers according to their fluorescence profile. Use of the instrument to determine which and how many members of a cell population bind fluorescently labeled an- tibodies is called analysis; use of the instrument to place cells having different patterns of reactivity into different contain- ers is called cell sorting. The flow cytometer has multiple applications to clinical and research problems. A common clinical use is to deter- mine the kind and number of white blood cells in blood samples. By treating appropriately processed blood sam- ples with a fluorescently labeled antibody and performing flow cytometric analysis, one can obtain the following information: a73 How many cells express the target antigen as an absolute number and also as a percentage of cells passing the beam. For example, if one uses a fluorescent antibody specific for an antigen present on all T cells, it would be possible to determine the percentage of T cells in the total white blood cell population. Then, using the cell-sorting capabilities of the flow cytometer, it would be possible to isolate the T-cell fraction of the leukocyte population. a73 The distribution of cells in a sample population according to antigen densities as determined by fluorescence intensity. It is thus possible to obtain a measure of the distribution of antigen density within the population of cells that possess the antigen. This is a powerful feature of the instrument, since the same type of cell may express different levels of antigen depending upon its developmental or physiological state. a73 The size of cells. This information is derived from analysis of the light-scattering properties of members of the cell population under examination. Flow cytometry also makes it possible to analyze cell pop- ulations that have been labeled with two or even three differ- ent fluorescent antibodies. For example, if a blood sample is reacted with a fluorescein-tagged antibody specific for T cells, and also with a phycoerythrin-tagged antibody specific for B cells, the percentages of B and T cells may be deter- mined simultaneously with a single analysis. Numerous vari- ations of such “two-color” analyses are routinely carried out, and “three-color” experiments are common. Aided by appro- priate software, highly sophisticated versions of the flow cy- tometer can even perform “five-color” analyses. Flow cytometry now occupies a key position in im- munology and cell biology, and it has become an indispens- able clinical tool as well. In many medical centers, the flow cytometer is one of the essential tools for the detection and classification of leukemias (see the Clinical Focus). The choice of treatment for leukemia depends heavily on the cell types involved, making precise identification of the neoplas- tic cells an essential part of clinical practice. Likewise, the rapid measurement of T-cell subpopulations, an important prognostic indicator in AIDS, is routinely done by flow- cytometric analysis. In this procedure, labeled monoclonal antibodies against the major T-cell subtypes bearing the CD4 and CD8 antigens are used to determine their ratios in the patient’s blood. When the number of CD4 T cells falls below a certain level, the patient is at high risk for oppor- tunistic infections. 154 PART II Generation of B-Cell and T-Cell Responses 8536d_ch06_137-160 8/1/02 9:01 AM Page 154 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: Antigen-Antibody Interactions: Principles and Applications CHAPTER 6 155 FIGURE 6-15 Separation of fluorochrome-labeled cells with the flow cytometer. In the example shown, a mixed cell population is stained with two antibodies, one specific for surface antigen A and the other specific for surface antigen B. The anti-A antibodies are labeled with fluorescein (green) and the anti-B antibodies with rho- damine (red). The stained cells are loaded into the sample cham- ber of the cytometer. The cells are expelled, one at a time, from a small vibrating nozzle that generates microdroplets, each contain- ing no more than a single cell. As it leaves the nozzle, each droplet receives a small electrical charge, and the computer that controls the flow cytometer can detect exactly when a drop generated by the nozzle passes through the beam of laser light that excites the fluo- rochrome. The intensity of the fluorescence emitted by each droplet that contains a cell is monitored by a detector and dis- played on a computer screen. Because the computer tracks the position of each droplet, it is possible to determine when a partic- ular droplet will arrive between the deflection plates. By applying a momentary charge to the deflection plates when a droplet is pass- ing between them, it is possible to deflect the path of a particular droplet into one or another collecting vessel. This allows the sort- ing of a population of cells into subpopulations having different profiles of surface markers. In the computer display, each dot represents a cell. Cells that fall into the lower left-hand panel have background levels of fluorescence and are judged not to have reacted with either antibody anti-A or anti-B. Those that appear in the upper left panel reacted with anti-B but not anti-A, and those in the lower right panel reacted with anti-A but not anti-B. The upper right panel contains cells that react with both anti-A and anti-B. In the example shown here, the A H11002 B H11002 —and the A H11001 B H11001 — subpopulations have each been sorted into a separate tube. Staining with anti-A and anti-B fluorescent antibodies allows four subpopula- tions to be distinguished: A H11002 B H11002 , A H11001 B H11001 , A H11002 B H11001 , and A H11001 B H11002 . Laser Deflection plates Fluorescence + – Computer screen Anti-A antibody fluorescence Anti-B antibody fluorescence A – B + cells A – B – cells A + B + cells A + B – cells Ultrasonic nozzle vibrator A ? B ? cells A ? B + and A + B ? cells A + B + cells Anti-A + Anti-B antibody Anti-A antibody Anti-B antibody Unstained Cells stained with: 8536d_ch06_137-160 8/1/02 12:41 PM Page 155 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: Alternatives to Antigen-Antibody Reactions As a defense against host antibodies, some bacteria have evolved the ability to make proteins that bind to the Fc region of IgG molecules with high affinity (K a ~ 10 8 ). One such molecule, known as protein A, is found in the cell walls of some strains of Staphylococcus aureus, and another, protein G, appears in the walls of group C and G Streptococcus. By cloning the genes for protein A and protein G and generating a hybrid of both, one can make a recombinant protein, known as protein A/G, that combines some of the best fea- tures of both. These molecules are useful because they bind IgG from many different species. Thus they can be labeled with flourochromes, radioactivity, or biotin and used to de- tect IgG molecules in the antigen-antibody complexes formed during ELISA, RIA, or such fluorescence-based as- says as flow cytometry or fluorescence microscopy. These bacterial IgG-binding proteins can also be used to make affinity columns for the isolation of IgG. Egg whites contain a protein called avidin that binds biotin, a vitamin that is essential for fat synthesis. Avidin is believed to have evolved as a defense against marauding rodents that rob nests and eat the stolen eggs. The binding between avidin and biotin is extremely specific and of much higher affinity (K a ~ 10 15 ) than any known antigen-antibody reaction. A bacterial protein called streptavidin, made by streptomyces avidinii, has similarly high affinity and specificity. The extraordinary affin- ity and exquisite specificity of the interaction of these proteins with biotin is widely used in many immunological procedures. The primary or secondary antibody is labeled with biotin and allowed to react with the target antigen, and the unbound an- tibody is then washed away. Subsequently, streptavidin or avidin conjugated with an enzyme, flourochrome, or radioac- tive label is used to detect the bound antibody. Immunoelectron Microscopy The fine specificity of antibodies has made them powerful tools for visualizing specific intracellular tissue components 156 PART II Generation of B-Cell and T-Cell Responses adults than in children; and chronic lym- phocytic leukemia (CLL), which is rarely seen in children but is the most com- mon form of adult leukemia in the Western world. A fourth type, chronic myelogenous leukemia (CML), occurs much more often in older adults than in children. The diagnosis of leukemia is made on the basis of two findings. One is the detection of abnormal cells in the blood- stream, and the other is observation of abnormal cells in the bone marrow. Clin- ical experience has shown that designing the most appropriate therapy for the pa- tient requires knowing which type of leukemia is present. In this regard, two of the important questions are: (1) What is the lineage of the abnormal cells and (2) What is their maturational stage? A variety of approaches, including cyto- logic examination of cell morphology and staining characteristics, immuno- phenotyping, and, in some cases, an analysis of gene rearrangements, are useful in answering these questions. One of the most powerful of these ap- proaches is immunophenotyping, the determination of the profile of selected cell-surface markers displayed by the leukemic cell. Although leukemia-spe- cific antigens have not yet been found, profiles of expressed surface antigens of- ten can establish cell lineage, and they are frequently helpful in determining the maturational stages present in leukemic cell populations. For example, an abnor- mal cell that displays surface immuno- globulin would be assigned to the B-cell lineage and its maturational stage would be that of a mature B cell. On the other hand, a cell that had cytoplasmic H9262heavy chains, but no surface immuno-globulin, would be a B-lineage leukemic cell but at the maturational stage of a pre-B cell. The most efficient and precise technol- ogy for immunophenotyping uses flow cytometry and monoclonal antibodies. The availability of monoclonal antibod- ies specific for each of the scores of anti- gens found on various types and sub- types of hematopoietic cells has made it possible to identify patterns of antigen Leukemia is the un- checked proliferation of an abnormal clone of hematopoietic cells. Typically, leukemic cells respond poorly or inap- propriately to regulatory signals, display aberrant patterns of differentiation, or even fail to differentiate. Furthermore, they sometimes suppress the growth of normal lymphoid and myeloid cells. Leukemia can arise at any maturational stage of any one of the hematopoietic lineages. Lymphocytic leukemias display many characteristics of cells of the lym- phoid lineage; another broad group, myelogenous leukemias, have attributes of members of the myeloid lineage. Aside from lineage, many leukemias can be classified as acute or chronic. Some examples are acute lymphocytic leukemia (ALL), the most common childhood leukemia; acute myelogenous leukemia (AML), found more often in CLINICAL FOCUS Flow Cytometry and Leukemia Typing 8536d_ch06_137-160 8/1/02 12:41 PM Page 156 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: Antigen-Antibody Interactions: Principles and Applications CHAPTER 6 157 files of surface markers on tumor cell populations. Flow cytometric determina- tion of immuno-phenotypes allows: a73 Confirmation of diagnosis a73 Diagnosis when no clear judgment can be made based on morphology or patterns of cytochemical staining a73 Identification of aberrant antigen pro- files that can help identify the return of leukemia during remission a73 Improved prediction of the course of the disease expression that are typical of cell lin- eages, maturational stages, and a num- ber of different types of leukemia. Most cancer centers are equipped with flow cy- tometers that are capable of performing and interpreting the multiparameter analyses necessary to provide useful pro- An ALL of the pre-B lineage (the most commonly occurring ALL) ALL of the T lineage A B-lineage CLL CD10 (a metalloproteinase) CD19 (a B-cell coreceptor) CD34 (marker of hematopoietic precursors) CD4 (coreceptor for MHC II) CD2 (an adhesion molecule) CD44 ( adhesion molecule) CD7 (marker of some T cells, thymocytes and pluripotent hematopoietic cells) CD5 (a T-cell marker) CD8 (coreceptor for MHC I) CD1 (an MHC class I-like molecule) CD23 (low-affinity IgE receptor) CD5 CD34 MHC II MHC II CD19 Ig CD20 (B-cell marker) Distribution of selected markers on some leukemic cell types. Shown are typical surface antigen profiles found on many, but not all, ALLs and CLLs. by immunoelectron microscopy. In this technique, an elec- tron-dense label is either conjugated to the Fc portion of a specific antibody for direct staining or conjugated to an anti- immunoglobulin reagent for indirect staining. A number of electron-dense labels have been employed, including ferritin and colloidal gold. Because the electron-dense label absorbs electrons, it can be visualized with the electron microscope as small black dots. In the case of immunogold labeling, dif- ferent antibodies can be conjugated with gold particles of different sizes, allowing identification of several antigens within a cell by the different sizes of the electron-dense gold particles attached to the antibodies (Figure 6-16). FIGURE 6-16 An immunoelectronmicrograph of the surface of a B-cell lymphoma was stained with two antibodies: one against class II MHC molecules labeled with 30-nm gold particles, and another against MHC class I molecules labeled with 15-nm gold particles. The density of class I molecules exceeds that of class II on this cell. Bar H11005 500 nm. [From A. Jenei et al., 1997, PNAS 94:7269–7274; cour- tesy of A. Jenei and S. Damjanovich, University Medical School of De- brecen, Hungary.] 8536d_ch06_137-160 8/1/02 12:41 PM Page 157 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: SUMMARY a73 Antigen-antibody interactions depend on four types of noncovalent interactions: hydrogen bonds, ionic bonds, hydrophobic interactions, and van der Waals interactions. a73 The affinity constant, which can be determined by Scatchard analysis, provides a quantitative measure of the strength of the interaction between an epitope of the anti- gen and a single binding site of an antibody. The avidity re- flects the overall strength of the interactions between a multivalent antibody molecule and a multivalent antigen molecule at multiple sites. a73 The interaction of a soluble antigen and precipitating anti- body in a liquid or gel medium forms an Ag-Ab precipi- tate. Electrophoresis can be combined with precipitation in gels in a technique called immunoelectrophoresis. a73 The interaction between a particulate antigen and aggluti- nating antibody (agglutinin) produces visible clumping, or agglutination that forms the basis of simple, rapid, and sensitive immunoassays. a73 Radioimmunoassay (RIA) is a highly sensitive and quanti- tative procedure that utilizes radioactively labeled antigen or antibody. a73 The enzyme-linked immunosorbent assay (ELISA) de- pends on an enzyme-substrate reaction that generates a colored reaction product. ELISA assays that employ chemiluminescence instead of a chromogenic reaction are the most sensitive immunoassays available. a73 In Western blotting, a protein mixture is separated by elec- trophoresis; then the protein bands are electrophoretically transferred onto nitrocellulose and identified with labeled antibody or labeled antigen. a73 Fluorescence microscopy using antibodies labeled with fluorescent molecules can be used to visualize antigen on or within cells. a73 Flow cytometry provides an unusually powerful technol- ogy for the quantitative analysis and sorting of cell popula- tions labeled with one or more fluorescent antibodies. References Berzofsky, J. A., I. J. Berkower, and S. L. Epstein. 1991. Antigen- antibody interactions and monoclonal antibodies. In Funda- mental Immunology, 3rd ed., W. E. Paul, ed. Raven Press, New Yo r k . Coligan, J. E., A. M. Kruisbeek, D. H. Margulies, E. M. Shevach, and W. Strober. 1997. Current Protocols in Immunology. Wiley, New York. Harlow, E., and D. Lane. 1999. Using Antibodies: A laboratory manual. Cold Spring Harbor Laboratory Press. Herzenberg, L. A., ed. 1996. Weir’s Handbook of Experimental Immunology, 5th ed. Oxford, Blackwell Scientific Publications. Rose, N. R., E. C. de Macario, J. D. Folds, C. H. Lane, and R. M. Nakamura. 1997. Manual of Clinical Laboratory Immunology. American Society of Microbiology, Washington, D.C. Stites, D. P., C. Rodgers, J. D. Folds, and J. Schmitz. 1997. Clinical laboratory detection of antigens and antibodies. In Medical Immunology, 9th ed., D. P. Stites, A. I. Terr, and T. G. Parslow, eds., Appelton and Lange, Stamford, CT. Wild, D., ed. 2001. The Immunoassay Handbook. Nature Publish- ing Group, NY. USEFUL WEB SITES http://pathlabsofark.com/flowcyttests.html Explore the Pathology Laboratories of Arkansas to see what kinds of samples are taken from patients and what markers are used to evaluate lymphocyte populations by flow cy- tometry. http://jcsmr.anu.edu.au/facslab/AFCG/standards.html At the highly informative Australian Flow Cytometry Group Web site, one can find a carefully detailed and illustrated guide to the interpretation of flow cytometric analyses of clin- ical samples. http://www.kpl.com The Kirkegaard & Perry Laboratories Web site contains a sub- site, http://www.kpl.com/support/immun/pds/50datasht/54- 12-10.html, which allows one to follow a step-by-step procedure for using a chemiluminescent substrate in a sensi- tive immunoassay. Study Questions CLINICAL FOCUS QUESTION Flow-cytometric analysis for the de- tection and measurement of subpopulations of leukocytes, in- cluding those of leukemia, is usually performed using mono- clonal antibodies. Why is this the case? 1. Indicate whether each of the following statements is true or false. If you think a statement is false, explain why. a. Indirect immunofluorescence is a more sensitive tech- nique than direct immunofluorescence. b. Most antigens induce a polyclonal response. c. A papain digest of anti-SRBC antibodies can agglutinate sheep red blood cells (SRBCs). d. A pepsin digest of anti-SRBC antibodies can agglutinate SRBCs. e. Indirect immunofluorescence can be performed using a Fab fragment as the primary, nonlabeled antibody. f. For precipitation to occur, both antigen and antibody must be multivalent. g. Analysis of a cell population by flow cytometry can simultaneously provide information on both the size distribution and antigen profile of cell populations containing several different cell types. 158 PART II Generation of B-Cell and T-Cell Responses 8536d_ch06_137-160 8/1/02 12:41 PM Page 158 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: h. ELISA tests using chemiluminescence are more sensitive than chromogenic ones and precipitation tests are more sensitive than agglutination tests. i. Western blotting and immunoprecipitation assays are useful quantitative assays for measuring the levels of pro- teins in cells or tissues. j. Assume antibody A and antibody B both react with an epitope C. Furthermore, assume that antibody A has a K a 5 times greater than that of antibody B. The strength of the monovalent reaction of antibody A with epitope C will always be greater than the avidity of antibody B for an antigen with multiple copies of epitope C. 2. You have obtained a preparation of purified bovine serum albumin (BSA) from normal bovine serum. To determine whether any other serum proteins remain in this prepara- tion of BSA, you decide to use immunoelectrophoresis. a. What antigen would you use to prepare the antiserum needed to detect impurities in the BSA preparation? b. Assuming that the BSA preparation is pure, draw the im- munoelectrophoretic pattern you would expect if the assay was performed with bovine serum in a well above a trough containing the antiserum you prepared in (a) and the BSA sample in a well below the trough as shown below: a. How could you produce isotype-specific antibodies that could be used to determine the isotype of myeloma pro- tein, X? b. How could you use this anti-isotype antibody to measure the level of myeloma protein X in normal serum? 6. For each antigen or antibody listed below, indicate an appro- priate assay method and the necessary test reagents. Keep in mind the sensitivity of the assay and the expected concentra- tion of each protein. a. IgG in serum b. Insulin in serum c. IgE in serum d. Complement component C3 on glomerular basement membrane e. Anti-A antibodies to blood-group antigen A in serum f. Horsemeat contamination of hamburger g. Syphilis spirochete in a smear from a chancre 7. Which of the following does not participate in the formation of antigen-antibody complexes? a. Hydrophobic bonds b. Covalent bonds c. Electrostatic interactions d. Hydrogen bonds e. Van der Waals forces 8. Explain the difference between antibody affinity and anti- body avidity. Which of these properties of an antibody better reflects its ability to contribute to the humoral immune re- sponse to invading bacteria? 9. You want to develop a sensitive immunoassay for a hor- mone that occurs in the blood at concentrations near 10 H110027 M. You are offered a choice of three different antisera whose affinities for the hormone have been determined by equilibrium dialysis. The results are shown in the Scatchard plots. Antigen-Antibody Interactions: Principles and Applications CHAPTER 6 159 BSA preparation Bovine serum 2 r 1 60 40 20 r / c × 10 4 ( ) r / c × 10 5 ( ) ( ) 2 1 3 3. The labels from four bottles (A, B, C, and D) of hapten- carrier conjugates were accidentally removed. However, it was known that each bottle contained either 1) hapten 1–carrier 1 (H1-C1), 2) hapten 1–carrier 2 (H1-C2), 3) hapten 2–carrier 1 (H2-C1), or 4) hapten 2–carrier 2 (H2-C2). Carrier 1 has a molecular weight of 60,000 dal- tons and carrier 2 has a molecular weight of over 120,000 daltons. Assume you have an anti-H1 antibody and an anti- H-2 antibody and a molecular-weight marker that is 100,000 daltons. Use Western blotting to determine the contents of each bottle and show the Western blots you would expect from 1, 2, 3, and 4. Your answer should also tell which anti- body or combination of antibodies was used to obtain each blot. 4. The concentration of a small amount (250 nanograms/ml) of hapten can be determined by which of the following as- says: (a) ELISA (chromogenic), (b) Ouchterlony method, (c) RIA, (d) fluorescence microscopy, (e) flow cytometry, (f) immunoprecipitation, (g) immunoelectron microscopy, (h) ELISPOT assay, (i) chemiluminescent ELISA. 5. You have a myeloma protein, X, whose isotype is unknown and several other myeloma proteins of all known isotypes (e.g., IgG, IgM, IgA, and IgE). 8536d_ch06_137-160 8/1/02 9:01 AM Page 159 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: a. What is the value of K 0 for each antiserum? b. What is the valence of each of the antibodies? c. Which of the antisera might be a monoclonal antibody? d. Which of the antisera would you use for your assay? Why? 10. In preparing a demonstration for her immunology class, an instructor purified IgG antibodies to sheep red blood cells (SRBCs) and digested some of the antibodies into Fab, Fc, and F(ab.) 2 fragments. She placed each prepara- tion in a separate tube, labeled the tubes with a water- soluble marker, and left them in an ice bucket. When the instructor returned for her class period, she discovered that the labels had smeared and were unreadable. Deter- mined to salvage the demonstration, she relabeled the tubes 1, 2, 3, and 4 and proceeded. Based on the test results described below, indicate which preparation was con- tained in each tube and explain how you identified the contents. a. The preparation in tube 1 agglutinated SRBCs but did not lyse them in the presence of complement. b. The preparation in tube 2 did not agglutinate SRBCs or lyse them in the presence of complement. However, when this preparation was added to SRBCs before the addition of whole anti-SRBC, it prevented agglutination of the cells by the whole anti-SRBC antiserum. c. The preparation in tube 3 agglutinated SRBCs and also lysed the cells in the presence of complement. d. The preparation in tube 4 did not agglutinate or lyse SR- BCs and did not inhibit agglutination of SRBCs by whole anti-SRBC antiserum. 11. You are given two solutions, one containing protein X and the other containing antibody to protein X. When you add 1 ml of anti-X to 1 ml of protein X, a precipitate forms. But when you dilute the antibody solution 100-fold and then mix 1 ml of the diluted anti-X with 1 ml of protein X, no pre- cipitate forms. a. Explain why no precipitate formed with the diluted anti- body. b. Which species (protein X or anti-X) would likely be pre- sent in the supernatant of the antibody-antigen mixture in each case? 12. Consider equation 1 and derive the form of the Scatchard equation that appears in equation 2. 1. S H11001 L H11005 SL 2. B/F H11005 K a ([S] t H11002 B) Where: S H11005 antibody binding sites; [S] H11005 molar concentration of antibody binding sites; L H11005 ligand (monovalent antigen); [L] H11005 molar concentration of ligand; SL H11005 site-ligand complex; [SL] H11005 molar concentration of site ligand complex; B is substi- tuted for [SL] and F for [L]. Hint: It will be helpful to begin by writing the law of mass action for the reaction shown in equa- tion 1. 160 PART II Generation of B-Cell and T-Cell Responses 8536d_ch06_137-160 8/1/02 9:01 AM Page 160 mac79 Mac 79:45_BW:Goldsby et al. / Immunology 5e: