CD28 Stimulates tyrosine phosphorylation, cellular redistribution and catalytic activity of the inositol lipid 5-phosphatase SHIP

The D-3 phosphoinositide lipids phosphatidylinositol 3,4,5-trisphophate [Pt dlns(3,4,5)P-3] and phosphatidylinositol 3,4-bisphosphate [Ptdlns(3,4)P-2] represent upstream components of a major signaling pathway that is strongly activated by the T cell costimulatory molecule CD28. A major route for deg radation of PtdIns(3,4,5)P-3, (and hence, regulation of PtdIns(3,4,5)P-3-dr iven effector pathways), involves its conversion to Ptdlns(3,4)P-2 by the 1 45-kDa SH2-containing inositol (poly)phosphate 5-phosphatase (SHIP). In thi s study, we demonstrate using the murine T cell hybridoma DC27.1, that SHIP is strongly tyrosine phosphorylated after ligation of CD28 by either mAb o r the natural ligand B7.1. Ligation of CD3 also stimulates SHIP tyrosine ph osphorylation and an additive effect on tyrosine phosphorylation of SHIP is observed when both CD3 and CD28 are ligated. The tyrosine phosphorylation of SHIP in response to CD28 ligation correlates with a marked redistributio n of SHIP from the cytosol to the plasma membrane, as well as an increase i n the in vitro 5-phosphatase activity associated with SHIP immunoprecipitat es derived from CD28-stimulated cells. However, we have been unable to dete ct a direct association between CD28 and SHIP, so the mechanisms by which C D28 exerts the observed effects on SHIP remain unclear. This is the first d emonstration that SHIP is a biochemical target for CD28 and suggests that S HIP may be involved in the regulation of T cell activation.