Immunologic memory to phosphorylcholine. II. PC-KLH induces two antibody populations that dominate different isotypes

We examined the fine specificity of the memory response to phosphorylcholine (PC) using an enzyme-linked immunosorbent assay (ELISA) that measures the relative abilities of PC analogs to inhibit the binding of antibody to antigen. Along with the usual haptens (phosphorylcholine, L-alpha-glycerophosphorylcholine, and choline) these studies included as an inhibitor p-nitrophenyl phosphorylcholine, a compound that is homologous to the structure of the PC antigen in a protein conjugate. Evaluation of the fine specificity profiles revealed two populations of antibody elicited by PC-KLH, which were unequally dominant among the various immunoglobulin classes and IgG subclasses. Group I antibodies exhibited a similar or slightly greater avidity for PC than for NPPC and constituted the majority of IgM, IgA, and IgG3 antibodies. Group II antibodies, which were expressed primarily by IgG1, IgG2a, and IgG2b antibodies, possessed significant avidity only for NPPC. Possible mechanisms that would link IgM, IgA, and IgG3 to similar variable regions while excluding IgG1, IgG2a, and IgG2b from the same grouping are discussed.