CYCLOSPORINE-INSENSITIVE PARTIAL SIGNALING AND MULTIPLE ROLES OF CA2-INDUCED LYSIS( IN FAS LIGAND)

The induction of Fas ligand (FasL) mRNA expression and FasL-mediated c ytotoxicity in CD8(+) CTL is a rapid and transient response to activat ion via the TCR, This response fan also be initiated by pharmacologic activation of two major TCR signaling pathways using phorbol ester (PM A) and calcium ionophore (ionomycin). In these experiments using CD8() alloreactive cell lines, we demonstrate that induction of FasL mRNA can occur in response Co either PMA or ionomycin independently, Howeve r, only the ionomycin pathway is sensitive to inhibition by cyclospori ne A, Both pathways are blocked by genistein, a general protein tyrosi ne kinase inhibitor, The magnitude of induction of FasL mRNA is not pr oportional to the manifested FasL-dependent cytotoxicity, We also foun d a calcium requirement for cytotoxicity that is unrelated to FasL mRN A induction, In addition to the positive effects of constitutive and i nduced calcium levels on FasL-mediated cytotoxicity, calcium may play a role in the rapid down-regulation of the response, We also present d ata suggesting that CD2 and LFA-1 contribute to FasL-mediated cytotoxi city, Together these results suggest pathways by which partial TCR act ivation could, among the many activation-induced functions of a T cell , selectively lead to the induction of FasL-mediated cytotoxicity that can be regulated by lineage/activation-dependent accessory molecules on the target.