Cross-linking of the Fas/APO-1 antigen suppresses the CD3-mediated signal transduction events in human T lymphocytes.

The Fas/APO-1 Ag, a member of the TNF-R family, mediates apoptosis (programmed cell death, PCD). PCD has been recognized to be important in the regulation of normal and autoimmune responses. However, the underlying molecular mechanisms are not fully known. To elucidate the role of the Fas Ag in lymphocyte activation, we investigated the influence of Fas Ag cross-linking on CD3/TCR-mediated signal transduction in IL-2-dependent human T cell lines. Early signal transduction events were evaluated in 10-day-old T cell lines using anti-Fas and anti-CD3 mAbs. Incubation of Indo-1-loaded cells with anti-Fas mAb (10 micrograms/ml), but not with an isotypic control mAb almost completely inhibited the anti-CD3-mediated Ca2+ influx. This inhibition was mAb concentration dependent and was detected after incubation for at least 15 min. Cross-linking of the Fas Ag did not affect the anti-CD3-mediated release of Ca2+ from the internal stores or the thapsigargin-mediated Ca2+ influx. Anti-Fas mAb-incubated cells also showed decreased CD3-mediated inositol 1,4,5-trisphosphate production. Preincubation of cells with anti-Fas mAb inhibited the anti-CD3-mediated tyrosine phosphorylation of multiple cellular proteins. Cross-linking of the Fas Ag for up to 6 h did not alter the expression of CD3 on the surface of the T cells. Preincubation of Indo-1-loaded EBV-transformed B cells with anti-Fas mAb did not affect the anti-mu Ab-induced increase in intracellular calcium ([Ca2+]i). Our results show that cross-linking of the Fas Ag can suppress the receptor-mediated signaling process in human T but not B cells.