Distinct functions of monoclonal IgG antibody depend on antigen-site specificities.

Abstract

Intraveneous hyperimmunization of selectivity bred rabbits with streptococcal group A-variant vaccines elicits antibody responses of restricted heterogeneity at high antibody levels. All antisera contain two functionally distinct antibody populations, which can be isolated in single-band purity upon analytical isoelectric focusing. Typical examples of these two kinds of single-band antibodies were investigated in great detail for several parameters by a variety of methods. 85--99% of the streptococcal group A-variant polysaccharide (Av-CHO)-specific antibody in the antisera does not precipitate the isolated 5,000 daltons poly-L-rhamnose antigen, neither agglutinates nor lyses in the presence of complement Av-CHO-coated sheep erythrocytes (SRBC), binds the radio-labeled Av-CHO with an association constant in the ragne of 10(5)--10(6) M-1, and is of terminal specificity (nonreducing end) for the linear Av-CHO. In contrast, the minor fraction of Av-CHO-specific antibody (1--15%) does precipitate the linear Av-CHO, both agglutinates and lyses Av-CHO-coated SRBC in the presence of complement, has an affinity range of 10(8)--10(9) M-1, and is of internal specificity for the Av-CHO. The antigenic determinants of the Av-CHO for the antibodies are nonoverlapping, only one Fab of the low affinity antibody can be bound whereas four Fab of the high affinity antibody are accommodated. Hence, the determinant specificity explains the functional differences observed, for there is no indication of subclass differences. A mechanistic model of the A-variant carbohydrate presentation on the vaccine appears to account best for the unbalanced levels of low and high affinity antibody.

The Use of Streptococcal Antigens to Probe the Mechanisms of Immunity

“…I). From recent experimental data there is no evidence that the side chain branches of the groups A and C polysaccharides would contain L-rhamnose moieties as previouslyassumed (12,21,26,31,39).…”

“…Would the reactivity patterns of elicited homogeneous antibodies give any clues to the surface configurations of the polysaccharides on whole heat killed, extensively washed, and pepsinized streptococcal cells, called vaccine (27)? The answer to these questions was provided by the studies of Schalch et al (39) which were later on fully supported by the antigen shape inferred from high resolution IH and 13C nuclear magnetic resonance spectra and hard-sphere calculations (I). This is shown in Figs.…”

“…However, a minority of antibodies, which accounts for 10% and less, does precipitate the isolated antigen in solution, because these are directed against the repeating determinants along the homopolymer chain (Figs. I, 2), and about four antibody molecules can bind to every linear homopolymer (39). These internally specific anti-A-variant polysaccharide antibodies have high intrinsic affinities (K A '"'-'10 9 M-I) while the terminally specific antibodies have intrinsic antigen binding affinities of K A '"'-'10 5 M-I (39).…”

“…A detailed knowledge of the structure of the streptococcal group polysaccharide antigens, as it emerged in recent years (12), has been instrumental in designing the immune response systems to be studied, and to subsequently interpret the elicited antibodies in functional and in structural terms (39). The previously proposed structures of the streptococcal groups A, A-variant and C polysaccharides (12) depicted in Fig.…”

“…In the elucidation of this picture the antibody response to the A-variant polysaccharide was of key importance. Rabbit hyperimmune sera to this antigen regularly contain in a ratio of 9: I nonprecipitating and precipitating polysaccharide specific antibodies (39). All of the non-precipitating antibodies -which agglutinate streptococcal A-variant cells -are directed against the non-reducing end of the rhamnose homopolymer (Figs.…”