The Rat T-Cell Surface Protein RT6 Is Associated With src Family Tyrosine Kinases and Generates an Activation Signal

RT6 is a glycosyl-phosphatidylinositol-linked surface molecule present on most mature rat T-cells. RT6⁺ T-cells can prevent the expression of autoimmune diabetes in the BB rat, but the mechanism is unknown. Because cross-linking of other glycosyl-phosphatidylinositol-linked T-cell proteins is known to activate T-cells, we investigated the signaling properties of RT6. Antibody cross-linking of RT6 enhanced expression of the α subunit of the interleukin-2 (IL-2) receptor and potentiated the proliferation of rat T-cells cultured in the presence of phorbol ester plus recombinant IL-2 (rIL-2) and/or rIL-4. RT6 was found to coimmunoprecipitate with five tyrosine phosphorylated proteins including p60^fyn and p56^lck, members of the src tyrosine kinase family. Pretreatment of T-cells with phorbol ester increased the phosphorylation of proteins that coimmunoprecipitated with RT6, altered the electrophoretic mobility of several of these coimmunoprecipitated phosphoproteins, and increased the amount of p60^fyn and p56^lck coimmunoprecipitated with RT6. These data indicate that RT6-mediated signaling events may prime T-cells to respond to exogenous cytokines, suggesting a possible mechanism by which surface RT6 may influence T-cell function.