FK 506 and cyclosporin each block antigen-induced T cell receptor signalling that is dependent on CD4 co-receptor and operates in the absence of detectable cytoplasmic calcium fluxes.

The T cell hybridoma "171", which lacks CD4 but expresses T cell receptor (TCR) for hen egg white lysozyme, requires introduction of wild-type CD4 for antigen-mediated induction and secretion of interleukin-2 (IL-2). Mutant CD4, which fails to associate with the tyrosine kinase p56lck does not support IL-2 secretion, suggesting that a role of CD4 is to bring cytoplasmic p56lck into alignment for signal transduction to the IL-2 promotor. Using 171, 171-CD4 (wild-type) and 171-CD4 (mutant), we found that IL-2 secretion was inhibited by FK 506 and cyclosporin but not by rapamycin. However, this inhibition was not associated with calcium fluxes since no change in cytoplasmic free calcium levels ([Ca]i; resting level 80 nM) was detectable during antigen stimulation of the 171 or 171-CD4 cells. Thus, although FK 506 and cyclosporin inhibited calcium-dependent signalling to the IL-2 promoter via inhibition of the protein phosphatase calcineurin, it is possible that IL-2 induction via TCR/CD4 requires an FK 506 (and cyclosporin) sensitive step which is independent of cytoplasmic calcium changes.