Heterogeneity of the BALB/c IgM anti-phosphorylcholine antibody response

Abstract

Although recent advances in the study of the molecular genetics of the antibody system have greatly increased our understanding of the generation of immunological diversity^1–7, the state of B-cell differentiation at which diversification and specificity commitment occur has yet to be determined. In particular, it is unclear whether the entire repertoire of antibody specificities in an individual is expressed in the absence of selective forces, such as antigenic stimulation or network regulation. It has recently been suggested that some elements of variable region diversification, particularly somatic mutations, may accompany the shift from the IgM to IgG isotype^8–10. These findings support the view that much of B-cell repertoire diversity is the result of antigen-driven somatic events, because isotype shifts normally occur after antigenic stimulation^11–13. As these experiments used antibodies obtained after at least one in vivo antigenic stimulation, they do not address the possibility that the selection occurring during clonal expansion and subsequent IgG synthesis may favour the use of clones expressing unique antibody specificities (clonotypes) far removed from the innate or frequently occurring germ-line sequences. To test whether B-cell diversification may precede IgM expression, we have now determined the range of diversity available in the primary IgM antibody repertoire by studying the clonal heterogeneity of monoclonal IgM antibodies expressed by BALB/c mice in response to phosphorylcholine (PC). Our results indicate that the repertoire of IgM antibodies in an inbred murine strain exceeds 10⁷ clonotypes. This diversity is sufficiently large to preclude meaningful comparisons between the IgM and IgG repertoires and suggests that antibody diversification may occur early in B-cell differentiation.