Southern analysis has previously shown that the rabbit genome contains multiple genes coding for the constant regions of IgA heavy chains. In the present study, clones containing these C alpha genes have been isolated from cosmid and phage libraries. Restriction mapping and Southern analysis of the clones identified 13 non‐allelic C alpha genes; 11 of the genes were clustered in individual or overlapping clones. The clustered genes are separated by 8‐18 kb, and in total, the C alpha genes span a minimum of 160 kb of DNA. Southern analysis has shown that all genes within a cluster have the same transcriptional orientation, and that switch sequences are present 5′ of at least 12 of the 13 genes. The nucleotide sequence of each C alpha gene was determined, and it appears that all genes are functional; thus, rabbit may have as many as 13 IgA isotypes. Comparisons of the protein sequences encoded by the 13 C alpha genes showed that the CH2 and CH3 domains of the alpha‐chains are highly conserved, whereas the CH1 and hinge regions are highly diverse. Southern analysis of genomic DNA samples from other species within the order Lagomorpha showed that all samples had multiple C alpha hybridizing fragments. Thus, it is likely that all lagomorphs have multiple IgA isotypes and hence complex secretory immune systems.

Transgenic rabbits with the rabbit c-myc oncogene fused with the rabbit immunoglobulin heavy chain enhancer region (E,,) DNA were developed by microinjecting pronuclei of single cell zygotes with the gene construct and implanting the microinjected eggs into pseudopregnant females. At age 17-20 days, 3 of 21 offspring born to seven females were found to have peripheral blood leukocyte counts of >100,000 per mm3. Histology analyses showed extensive lymphocytic infiltration in the liver and kidney, indicating that these rabbits had a malignant lymphocytic leukemia. Genomic DNA analyses of thymus and peripheral blood lymphocytes revealed that the leukemic rabbits were transgenic and that both immunoglobulin heavy and K light chain genes were rearranged in the leukemic cells; thus, the leukemic cells are of B-cell lineage. One to four heavy and light chain gene rearrangements were observed, suggesting that the B-cell tumors were oligoclonal. Stable tissue culture cell lines from the bone marrow and peripheral blood of one of the transgenic rabbits have been developed. The development of B-cell leukemias in rabbits with the E,,-myc transgene contrasts with the development of B-cell lymphomas in mice carrying a similar transgene. The lymphomas in mice develop in adults and are monoclonal in origin. The leukemias in rabbits develop in juveniles, less than 3 weeks after birth, and appear oligoclonal in origin. The leukemias seem to develop in rabbit at a specific stage of development, and we suggest that a normal developmental signal may be involved in the oncogenesis.

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SUMMARY Several class II α and β chain genes of the rabbit MHC have been cloned and classified into three distinct subregions, R‐DP, R‐DQ and R‐DR, based on their homology to the corresponding HLA‐DP, ‐DQ and ‐DR genes. The organization of the rabbit MHC class II genes has now been studied in greater detail by analysing genomic DNA of an inbred III/J strain and several other RLA‐D homozygous rabbits, with DNA probes derived from cloned R‐DRβ genes. Eight previously cloned R‐DRβ genes were shown to be allelic forms of five R‐DRβ loci. Genomic blot analyses of DNA from seven rabbits homozygous for different RLA haplotypes revealed that the germline contains a total of approximately seven class II β genes, one DQβ, one DPβ and five DRβ. Extensive allelic polymorphism was identified by RFLP analysis using DQ and DR probes; limited RFLP was observed with DP probes. RFLP analyses allowed us to distinguish two haplotypes which had not been previously distinguished by MLR. Such RFLP analyses will be useful for identifying MHC ‘compatible’ rabbits for various immunobiological studies, including transplantation.