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Limited repertoire of utilized VH gene segments in a VHa3-allotype-suppressed rabbit.

Between 70 and 90% of serum Ig molecules of normal laboratory rabbits bear one of the serologically defined VHa allotypic specificities, a1, a2, or a3, and are termed VHa+ (a-positive) molecules; the remaining 10-30% of Ig molecules that do not have VHa allotypic specificities are designated VHa- (a-negative). The repertoire of utilized VHa(+)-encoding gene segments has been examined extensively, but only limited studies of the repertoire of utilized VHa(-)-encoding gene segments in normal rabbits have been done. To examine the repertoire of utilized VHa- gene segments, we analyzed VH-encoding cDNA clones from mRNA of a VHa-allotype-suppressed rabbit whose serum Ig was primarily VHa-. For VHa suppression a newborn a3/a3 rabbit was injected periodically with anti-VHa3 antiserum; when it was 2 months of age and its serum Ig was greater than 94% VHa-, cDNA clones were generated from splenic RNA. The nucleotide sequences of eight putative VHa(-)-encoding cDNA clones were compared to those of eight cDNA clones generated from RNA of non-suppressed a3/a3 rabbits. The presumed VHa3-encoding cDNA clones from the non-suppressed rabbits appeared to derive from VH1-a3, the 3'-most germline VH gene segment. In contrast, the VHa(-)-encoding cDNA clones from the suppressed rabbit were distinctly different from VH1 and were most probably derived from germline VH segments other than VH1. Because the expressed VHa- gene repertoire was highly restricted, we propose that VHa- molecules in a3/a3 rabbits may be derived from as few as three germline VH gene segments.