Late acquisition of a germ line antibody specificity

THE humoral immune response to most antigenic determinants is characterised by a highly heterogeneous population of antibody molecules derived from a large array of precursor cell clonotypes^1-3. The response to certain antigenic determinants, however, is relatively restricted and may be dominated by a single homogeneous antibody species. In inbred strains of mice, such dominant antibody specificities are present in all individuals of a strain ^4-9, are characterised by identical idiotypic determinants ^4-8, and are linked to heavy chain allotype markers^7-10. Thus, these clonotypes are presumed to be a direct reflection of genetic information present in the germ line. A good example of an antibody specificity considered to be 'germ line' is the major clonotype responsive to phosphorylcholine in BALB/c mice, which is identical in its combining region to the TEPC 15 and SI07 plasmacytoma proteins^4-5. Representatives of the cell clone producing this phosphoryilcholme-specific antibody have been found in high frequency (19±6 per 10⁶ bone marrow-derived antibody-forming cell precursors (B cells)) in both conventionally reared and germ-free BALB/c mice¹¹.