THE GLYCOSYLPHOSPHATIDYLINOSITOL-ANCHORED CD59 PROTEIN STIMULATES BOTH T-CELL RECEPTOR ZETA ZAP-70-DEPENDENT AND ZETA/ZAP-70-INDEPENDENT SIGNALING PATHWAYS IN T-CELLS/

The glycosylphosphatidylinositol (GPI)-anchored CD59 protein (human pr otectin) protects cells against complement-induced lysis, binds to CD2 and also transduces activation signals within T cells. We have furthe r examined the biochemical signals transduced by CD59 and addressed it s role in regard to the CD3-mediated signaling cascade. We show here t hat CD59 cross-linking induces a time-dependent activation of p56(lck) and of p70(zap) (ZAP-70) in CD3-positive Jurkat cells, leading to the stimulation of the T cell receptor zeta/ZAP-70 signaling cascade and interleukin-2 (IL-2) synthesis. Cross-linking of CD59 on peripheral T cells and thymocytes induces tyrosine phosphorylations identical to th ose seen in Jurkat cells and this is followed by lymphokine production and proliferation. In contrast, only activation of CD59-associated p5 6(lck) occurs in CD3-negative Jurkat cells, while IL-2 production is i mpaired, consistent with the lack of ZAP-70 tyrosine phosphorylation o bserved in these cells. CD59 triggers activation events even in the ab sence of CD3/T cell receptor expression in Jurkat cells. CD59 cross-li nking synergizes with sub-optimal doses of phorbol ester for activatio n of the protein kinase C and of the p42(mapk), as shown by in vitro p hosphorylation of histone HIIIS and myelin basic protein, respectively , and leads to CD25 but not CD69 expression. In conclusion, at least t wo signaling pathways are triggered through CD59, the first one involv ing ZAP-70 activation and leading to IL-2 secretion and a second pathw ay observed in the absence of ZAP-70 activation leading to CD25 expres sion. These two pathways are likely to be involved in the modulation o f T cell activation by CD59 protein.