SIGNALING THROUGH CD28 T-CELL ACTIVATION PATHWAY INVOLVES AN INOSITOLPHOSPHOLIPID-SPECIFIC PHOSPHOLIPASE-C ACTIVITY

Stimulation of the human T-cell line, Jurkat, by a monoclonal antibody (mAb) directed against the CD28 molecule leads to sustained increases in intracellular levels of Ca2+([Ca 2-]i) ; the initial rise in Ca2comes from internal stores, followed by Ca2+ entry into the cells. The CD28 molecule also appears to activate polyphosphoinositide (InsPL)-s pecific phospholipase C (PLC) activity in Jurkat cells, as demonstrate d by PtdInsP2 breakdown, InsP3 and 1,2-diacylglycerol generation and P tdIns resynthesis. We also observed that interleukin-2 (IL2) productio n induced via CD28 triggering was sensitive to a selective protein kin ase C inhibitor. Of the four other anti-CD28 mAbs (CD28.2, CD28.4, CD2 8.5, CD28.6) tested, only one (CD28.5) was unable to generate any InsP L-specific PLC or IL2 secretion. However, the cross-linking of cell-bo und CD28.5 with anti-mouse Ig antibodies led to an increase in [Ca2+]i . CD28-molecule clustering in itself appears to be a sufficient signal for induction of PLC activity.