Relative affinity of antisera for myelin basic protein (MBP) and degree of affinity heterogeneity

Abstract

A method based essentially on that of Steward and Petty (1972) was used to evaluate the relative binding affiinities of several syngeneic Lewis-rat antisera to myelin basic protein (MBP)†. The syngeneic immune reactions to this ‘self’ antigen were found to contain only populations of relatively high affinity antibodies, ranging from a K 0 of 7 × 10 7 to 2.6 × 10 8 M −1 when measured at 100 nM MBP; and from 4 × 10 8 to 9 × 10 9 M −1 at 1 n M. The biphasic nature of the plots of antigen binding capacity (ABC) vs log MBP concentration suggested that MBP binding was usually distributed among two or more major discontinuous antibody populations. Advantage was taken of the dual-dilution phenomenon to obtain constants interpreted as degrees of relative affinity heterogeneity of syngeneic antisera. It was found that the dual- dilution phenomenon (the decrease of ABC with increasing antigen P dilution) could be expressed mathematically in the form of a power curve ABC = bP 1−α with b (the intercept at 1 n M P) related to the total number of antibody molecules in the system and α, the degree of affinity heterogeneity of antibodies against the multideterminant antigen. The more heterogeneous the system the higher the number of antibody molecules required to maintain a constant ABC at a given P and the more rapid the decrease of ABC with decreasing P. The degree of affinity heterogeneity of the syngeneic Lewis rat antisera ranged from pronounced (α = 0.14) to moderately restricted ( α = 0.8). The heterogeneity index of a rabbit anti-MBP serum was 0.821. An antiserum refractory to the dual-dilution effect was also found for which α was 0.98.