In vitro induction of the expression of multiple IgA isotype genes in rabbit B cells by TGF-beta and IL-2.

The rabbit genome has 13 different C-alpha genes that are expressed at different levels in mucosal tissues, To analyze the factors involved in the differential expression of these C-alpha genes, we cloned and sequenced the promoters of the I-alpha regions that control the expression of sterile mRNA. We found that all C-alpha genes, including C(alpha)3 and C(alpha)8. which are not expressed, and C(alpha)4, which is expressed at high levels, hare similar nucleotide sequences in the I, region, and all contain the recognition elements for TGF-beta in the promoter. B lymphocytes from popliteal lymph nodes or Peyer's patch activated in vitro could he induced by TGF-beta to express sterile IgA transcripts of all IgA isotypes, except C(alpha)2, C(alpha)3, and C(alpha)8. Many single B lymphocytes transcribed sterile mRNA of more than one IgA, isotype, which demonstrates that transcription of sterile mRNA atone does not regulate the IgA isotype switch, The addition of IL-2 led to the expression of transcripts of mature IgA of all isotypes, except C(alpha)2, C(alpha)3, and C(alpha)8, The predominantly expressed isotype in these experiments was C(alpha)4. With the use of an IgA4-specific mAb we found that IgA4(+) plasma cells are unevenly distributed throughout the small intestine such that many of the IgA(+) plasma cells in the duodenum-jejunum produced IgA4, whereas in the lower part of the iIeum IgA4-producing cells were almost absent. Because the microbial flora varies throughout the intestine, He suggest that the microbial Bora creates different local environments and thus affects either isotype switching or homing of IgA-expressing cells.