Molecular cloning techniques have recently demonstrated that rabbit has 13 different IgA C alpha H chain genes. This is in contrast to human and mouse that have only two and one C alpha heavy chain genes, respectively. In previous studies, nucleotide sequence analysis indicated that the 13 rabbit C alpha genes were potentially functional, and in vitro expression experiments showed that at least 12 of these genes were expressible. To understand the role of these multiple IgA isotypes we analyzed RNA of various lymphoid tissues for the presence of mRNA representing each of the multiple C alpha genes. We used the RNase protection assay with probes that are specific for the 13 different C alpha genes and we consistently found that at least 10 of the C alpha genes are expressed, albeit at different levels, in gut (small intestines), appendix, mesenteric lymph node, and mammary tissue. However, in salivary gland (submandibular), only seven of these genes are expressed at significant levels and in lung and tonsil only one C alpha gene, C alpha 4, is expressed at a level comparable to its expression in other tissues. Analysis of RNA of Peyer's patch showed differences in the level of C alpha gene expression between different animals and between different Peyer's patches of the same rabbit; in some cases, most of the C alpha genes were expressed, but in some cases only C alpha 4 was expressed at a significant level. Inasmuch as C alpha 4 is the 5' most C alpha gene we propose that IgA-producing cells are derived from B cells that have initially undergone isotype switching to C alpha 4 and we discuss various mechanisms that could explain switching to the more 3' C alpha genes.

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