Mechanisms that limit the diversity of antibody: three sequentially acting mechanisms that favor the spontaneous production of germline encoded anti-phosphatidyl choline

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Abstract
Antibody to phosphatidyl choline (PtC) is produced spontaneously in mice, by —2—10% of naturally occurring CD5+ (B1) B cells in the peritoneum. Much of this antibody is encoded by the VH11 gene associated with a specific Vx9 gene. Constraints on the size and structure of the H chain CDR3 have been defined from nucleotide sequences of genes expressed by hybridomas and lymphomas derived from adult mice. All employ JH1 and all encode tyrosine as the first amino acid in CDR3, which is either nine or 10 amino acids long; the last six are always the same, start with tyrosine, and are rich in aromatic amino acids. Those with nine amino acids in CDR3 have glycine or serine in the second position and asparagine, serine or proline in the third; those with 10 have an additional aspartate or glycine inserted after the first tyrosine. DSP2 genes are used by 80% and DFL16 by 20%. Productively rearranged VH11 genes in neonates and in 18 day fetal liver display a greater, but still limited degree of diversity. All four JH genes are used and the length of CDR3 varies from three to 12 amino acids, but the first is tyrosine in 58 of 61 and DSP2 genes are used by 80% of these productive VDJ assemblies. Non-productive rearrangements of VH11 in fetal liver show a different pattern; 41% use DFL16 genes and only 40% have a TAx codon in the first position of CDR3. All rearrangements show evidence of a bias in favor of joining the VH11 gene to D genes at positions of matching nucleotide overlap. The data are interpreted as indicating the existence of three sequentially acting mechanisms that lead to the spontaneous appearance of anti-PtC antibodies; a biased rearrangement of germ line elements, selective survival of pre-B cells, and classical, antigen-driven, clonal selection.

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