Regulation of membrane and secretory mu synthesis was examined in human lymphoblastoid cell lines representing various stages of differentiation. Immunoglobulin phenotype was determined by surface and cytoplasmic staining with fluorochrome-conjugated antibodies and by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis of anti-mu precipitable cellular products. The thymidine analogue, 5-bromo-2'-deoxyuridine (BUdR), which inhibits differentiation-specific proteins in a variety of systems, was used to examine regulation of immunoglobulin synthesis. We found that BUdR had a differential effect on membrane (mum) and secretory (mus) type mu heavy chains. Ig production in pre-B and plasma cell-like lines, which make mus, was unaffected by BUdR. However, surface expression of IgM (mum) in B cell lines was drastically inhibited at similar doses of BUdR without diminishing total Ig or protein synthesis. Examination of labeled mu chains from control and BUdR-treated B cell lines by SDS-PAGE revealed the production of two sizes of mu (mum and mus) in control cells and only the smaller size (mus) in BUdR-treated cells. This size difference could not be attributed to alterations in glycosylation of the molecules. These data show that BUdR inhibits the production of membrane mu chains without diminishing secretory mu chain synthesis in the same cell. Our findings suggest that thymidine-rich regions of the genome are involved in the regulation of mum vs. mus during B cell differentiation.

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