Treatment of small resting B cells with soluble F(ab')2 fragments of anti-IgM, an analogue of T-independent type 2 antigens, induced activation characterized by proliferation and the expression of surface CD5. In

The expression of CD5 can be induced on murine B-2 cells by anti-IgM, a recognized analog of thymus-independent 2 type (TI-2) antigen. Given that cyclosporin A (CsA) sensitivity is a distinguishing feature of TI-2 type B cell activation, we asked whether the in vitro induction of CD5 on B cells by anti-mu is CsA sensitive. We report that anti-mu induced CD5 expression on B-2 cells was inhibited by CsA as well as FK-520 and rapamycin. When L-685,818, a FK-520 and rapamycin antagonist, was added to anti-mu stimulated B cell cultures containing FK-520 or rapamycin, but not CsA, suppression was abrogated and complete induction of CD5 was seen. When we used either CD4+CD8+ thymocytes or peripheral T cells activated by phorbol ester and ionomycin, the cell surface induction of CD5 was also partially blocked by CsA, FK-520 and rapamycin. Moreover, in both B and T cells, the same immunosuppressive drugs did not affect constitutive CD5 expression but only blocked de novo induction. To determine the level of CD5 regulation, we activated T cells using phorbol myristate acetate (PMA)/ionomycin and report that CD5 induction was sensitive to actinomycin D (AcD). Similarly, the induction of CD5 on anti-mu activated B cells was blocked by AcD. In addition, T cells that were activated by PMA/ionomycin expressed more abundant CD5 mRNA than CsA or FK-520 treated cells. Based on the CsA-sensitive regulation of CD5 we thought that the CsA-sensitive nuclear factor of activated T cells (NFAT) might be involved in CD5 regulation. We report evidence by Western blot analysis that NFATp is expressed by both resting and TI activated B cells but apparently not CD4+CD8+CD5+ thymocytes. We conclude that in both B and T cells the induction of CD5 requires transcriptional regulation, and that the inhibition of CD5 expression by the immunosuppressive drugs CsA, FK-520 and rapamycin requires drug-immunophilin complex formation.