Regulation of germline promoters by the two human Ig heavy chain 3′ α enhancers

The human IgH 3' enhancers, located downstream of each of the two Ca genes, modulate germline (GL) transcription of the IgH genes By influencing the activity of promoter-enhancer complexes upstream of the switch and intervening (I) regions, The regulation of GL alpha 1 and alpha 2 promoters by different human 3' enhancer fragments was investigated in cell lines representing various developmental stages. Both alpha 1HS1,2 and alpha 2HS1,2 fragments show equally strong enhancer activity on the GL alpha 1 and alpha 2 promoters in both orientations when transiently transfected into a number of mature B cell line (DG75, CL-01, and HS Sultan). However, there is no activity in a human pre-B cell line (NALM-6) nor a human T cell line (Jurkat), HS3 shows no enhancer activity by itself in any of the cell lines, whereas a modest effect is noted using HS4 in the three mature B cell lines. However, the combination of the alpha 2HS3-HS1,2-HS4 fragments, which together form a potential locus control region, displays a markedly stronger enhancer activity than the individual fragments with a differential effect on the alpha 1 and alpha 2 promoters as compared with the gamma 3 promoter. Our results suggest that the human GL alpha promoter may be regulated by two independent pathways. One pathway is induced by TGF-beta(1) which directs IgA isotype switch through activation of the GL alpha promoter and no TGB-beta(1)-responsive elements are present in the different 3' enhancer fragments. The other route is through the human 3' enhancer regions that cis-up-regulate the GL alpha promoter activity in mature B cells.