Subpopulations of antibodies to phosphocholine in human serum

We investigated the heterogeneity of anti-phosphocholine (PC) antibodies present in human serum taken from individuals before and after immunization with a multivalent pneumococcal vaccine. The fine specificity of IgM, IgG, and IgA anti-PC antibodies was determined in an ELISA by using phosphocholine or p-nitrophenyl phosphocholine (NPPC) to inhibit binding of antibody to PC-histone. We identified two populations specific for PC that differed in their binding properties. One population is inhibited by NPPC much better than by PC and is most evident in IgG antibodies. The second population has similar avidity for PC and NPPC and is consistently associated with the IgM and IgA isotypes as well as with IgG. The IgG antibodies in both populations were predominantly of the IgG2 subclass. Both populations were found in serum samples taken before immunization with pneumococcal vaccine, suggesting that they had been stimulated through prior environmental contacts with PC-containing antigens. Previously, we found populations with similar fine specificity patterns in the murine response to PC. The two murine antibody populations have been shown to derive from different immunoglobulin variable region genes. The presence of comparable antibody populations in the human suggests the possibility that these two fine specificity families have been conserved in evolution.