Effect of cytokines on switching to IgA and alpha germline transcripts in the B lymphoma I.29 mu. Transforming growth factor-beta activates transcription of the unrearranged C alpha gene
Shockett, P.; Stavnezer, J.
Journal of Immunology 147(12): 4374-4383
1991
ISSN/ISBN: 0022-1767 PMID: 1753105 Document Number: 385297
H chain isotype switch recombination is preceded by the appearance of RNA initiating 5' of the specific switch region that will undergo recombination. In an effort to understand the potential function of germline transcripts in switch recombination and whether the regulation of germline transcripts correlates with the regulation of switching, we are studying this process in the murine B lymphoma cell line 1.29 .mu., which switches, after treatment with bacterial LPS, primarily to IgA and less frequently to IgE. Levels of .alpha. germline transcripts initiating upstream of .alpha. switch (S.alpha.) sequences are elevated in clones of this line that switch well, compared with clones that switch less frequently. Transforming growth factor-.beta. (TGF-.beta.) has been shown to increase .alpha. germline transcripts and switching to IgA expression in LPS-stimulated murine splenic B cells. We now demonstrate that TGF-.beta. increases LPS-induced switching to IgA by 10-fold at optimal doses and increases the level of .alpha. germline transcripts 5- to 9-fold in 1.29 .mu. cells. Nuclear run-on analysis shows that this increase is at the level of transcription. Thus, TGF-.beta. appears to direct switching to IgA by inducing transcription from the unrearranged S.alpha.-C.alpha. DNA segment. Germline .alpha. RNA is quite stable in 1.29.mu. cells, having a half-life of about 5 h, and we find no evidence for further stabilization in the presence of TGF-.beta. Levels of .epsilon. germline trnascripts are decreased by TGF-.beta. treatment. IL-4, which modestly increases switching in 1.29 .mu. cells, slightly increases transcription of .alpha. germline RNA. IFN-.gamma., which reduces switching to IgA in these cells, also reduces the level of .alpha. germline transcripts. IFN-.gamma. also reduces the level of .epsilon. germline transcripts induced by IL-4. Our results support the hypothesis that the regulation of transcription of particular switch sequences by cytokines requlates the specificity of recombination. We also present evidence that IL-4 may provide other signals, distinct from transcriptional targeting, that increase LPS-induced switching to IgA.