IGH DIVERSITY IN AN INDIVIDUAL WITH ONLY ONE MILLION B-LYMPHOCYTES

Immunoglobulin sequences from an individual Xenopus laevis froglet wer e analyzed for combinatorial and junctional diversity. In an animal wi th about 10(6) B lymphocytes, at least 26 out of the 56 V(H)1 genes av ailable in a diploid genome were expressed, as were all JH segments. J unctional diversity was similar to that observed in Xenopus tadpole se quences, that is, little or no N diversification was found and the rec ombination site sometimes occurred in a region of V/D or D/J homology. The froglet IgH diversity is further restricted by the elimination of D-gene participation through direct V to J joining. Of the six comple mentary-determining regions (CDR) contributing to the structure of the antigen-combining site, CDR3 is the most variable in sequence and str ucture. Froglet IgH CDR3 are restricted to both aspects. Compared to I gH sequences isolated from a 5-month-old adult, froglet CDR3 were, on the average, two codons shorter; overall, 58% of the froglet Ig sequen ces isolated carried CDR3 of less-than-or-equal-to 7 codons, compared to 30% of the adult sequences. In addition to being shorter, the tadpo le/froglet CDR3 are less variable in sequence, as the absence of N reg ions also results in the V/D and D/J junctions being derived from germ line elements. We therefore suggest that latent anti-adult specificiti es are not eliminated in situ, in the tadpole, but rather that such ge rmline gene segments, singly or in their combinations thereof, that ca n potentially react to adult self-epitopes after metamorphosis have be en counterselected during the course of evolution.