The structural and genetic basis for expression of normal and latent VHa allotypes of the rabbit.

The immunoglobulin heavy chain variable regions of the rabbit are unusual in having genetically controlled, serologically detectable alternative forms, the VHa allotypes, as well as minor VH allotypes of the x, y and w groups. New insights into the probable structural basis for the VHa allotypes have come from re-examination of earlier protein sequence data in the light of newly deduced protein sequences derived from sequencing cloned cDNAs and genomic DNAs encoding VH regions. Here we review this sequence information, and define the allotype-correlated differences at seven positions in framework region 1 and 10 positions in framework region 3 that may lead to the serologically detectable allotypic determinants (allotopes). Most alternative amino acids at allotype-correlated positions can be derived from each other by single-base changes. Thus somatic mutations and/or gene conversion-like events must be considered along with other serological and genetic explanations for various reported observations of the production of latent VHa allotypes. The proximity of rabbit VH genes (approximately 3 kb apart) might enhance the likelihood of conversion-like events in both germline and somatic cells.