Multiple Gene Segments Encode Ant i - 4 - ( hydroxy - 3 - n i t ro - phenyl ) acetyl Idiotypic Ant ibodies

The antigen binding specificity of an antibody molecule is determined by the amino acid sequences of the variable (V) 1 regions of the heavy (H) and light (L) chains. The antibody repertoire of an individual is large and complex. The studies of antibody polypeptides and genes in mice have suggested three sources of diversity: multiple germline V genes (1-4), rearrangement of three sets of H chain (VH, D ]diversity], and JH [joining region of the H chain]) and two sets of L chain (VL andJL) germline segments (5-7); and two types of somatic alterations (7-9 and 10-12). Several questions, however, remain to be answered: (a) What are the relative contributions of the germline, combinatorial, and somatic variation mechanisms to the functional repertoire of antibody molecules? (b) Is somatic diversification a random process happening at the base somatic mutation rate, or is it a consequence of a V gene-specific mutation mechanism (13, 14)? (c) At what stage of B cell differentiation does somatic mutation take place? A better understanding of this problem requires a more detailed study of the heterogeneity of the response to specific antigens. As amino acid sequences of antibodies (15, 16) and nucleotide sequences (2, 11, 17, 18) of VH genes of restricted heterogeneity (idiotypes) have been obtained, the notion has emerged that somatic point mutation and combinatorial diversity of a single germline VH gene are responsible for the diversity observed among idiotype-crossreactive-antibody-producing cells. We have undertaken a systematic analysis of the immune response to the hapten 4-(hydroxy-3-nitrophenyl)acetyl (NP) at early stages of the response, using nucleotide sequence analysis of expressed V region genes.