Rabbit immunoglobulins

The complement of immunoglobulin (Ig) genes in the rabbit includes IgM, and IgE and is unusual in having only one IgG gene and 13 IgA genes; no IgD gene has been reported. A major distinguishing feature of the rabbit Ig family is the magnitude of structural differences between allelically determined immunoglobulin molecules. Several groups of allotypes have been described for the rabbit: the most important are the group a on the VH region (al, a2 and a3), groups d and e in the hinge region (dll, dl2) and the C,,2 portion (el4, el5) of the y chain, and the group b on the constant part of the k light (L) chains (b4, b5, b6 and b9). It should be pointed out that by extending the study of allotypes to include feral rabbits the number of alleles increases dramatically. This demonstrates that the domesticated pool represents only a fraction of the genetic diversity of the rabbit population.

The group a and b allotypes are distinguished by large differences in primary structure that charac terize the allelic forms. There is as much as 20% difference in amino acid sequence between two alleles of the b types and these differences reflect the strong serologic reactivity of the alloantisera used to detect them.

The k/\ ratio is about 95/5, but this may vary (based on the allotype of the k chain) with observations of X chain as high as 15% in rabbits with certain b allotypes.

Molecular studies of the k locus have indicated that there are two CK genes in the rabbit and that six of the seven J region genes of the b4 and the b5 types are not functional. Most of the k light chains in the rabbit result from rearrangement of the CK1 gene. Light chains derived from the CK1 gene are of the K1 isotype, the five allelic forms of the K1 isotype determine the group b allotypes described above. The K2 isotype is encoded by the CK2 gene and is expressed infrequently. There are six to seven \ chain constant region genes but expression seems to be restricted to only the Cx5 and Cx6 genes.

Antibody diversity in the rabbit is generated by somatic gene conversion. Rabbits have many germ-line VH genes but only one is productively rearranged in most B lymphocytes. These VH genes are members of one large VH gene family and can function as donors in gene conversion. It has also been shown that all B lymphocytes in the rabbit are CD5 and thus may be self-renewing with little or no B lymphopoiesis occurring in mature rabbits.

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