ABBREVIATED JUNCTIONAL SEQUENCES IMPOVERISH ANTIBODY DIVERSITY IN URODELE AMPHIBIANS

Of the six complementarity-determining regions (CDR) forming the struc ture of the Ab combining site, CDR3 of heavy chain is the most variabl e in length and sequence, Diversity of this loop is determined by the number of gene segments involved, extent of addition to or deletion fr om the joining genes, and imprecision of the site of recombination. In neonatal mice and Xenopus tadpoles, the last two factors occur less f requently than in adults, which in tadpoles result in low affinity Ab responses that do not mature. In contrast, adult urodele amphibians ma ke larval-like responses and are notorious for lifelong poor immunocom petence. The mechanism for this is not known, and in this study we clo ned germline V-H genes from the axolotl and obtained rearrangements to these V-H gene segments by reverse-transcriptase PCR, These sequences were analyzed for heavy chain junctional diversity and found to be ev en less variable than that in newborn mouse or Xenopus tadpoles, altho ugh for different reasons, Only 29% of the CDR3 loop in the axolotl co nsisted of somatically generated sequences, compared with 44% in tadpo le, 39% in newborn mice, and 57% in both adult mice and Xenopus. This distinguishing feature of axolotl CDR3 results not only from shorter j unctional sequences, but also unusually extensive integration of germl ine J(H) sequence. As the CDR3 loop is the most important portion of t he Ig sequence for determining Ab combining site diversity, our data p rovide the molecular basis for a contributing factor in the deficient urodele amphibian Ab responses.