FIBRONECTIN INDUCES PHOSPHORYLATION OF A 120-KDA PROTEIN AND SYNERGIZES WITH THE T-CELL RECEPTOR TO ACTIVATE CYTOTOXIC T-CELL CLONES

Fibronectin (FN) has been shown to act as a costimulator in both CD4() and CD8(+) T cell activation through the T cell receptor (TcR). Cons istent with previous studies, we found that FN is able to both enhance the maximal amount of TcR-triggered degranulation and lower the thres hold for activation. The density of immobilized anti-CD3 or anti-TcR r equired to induce degranulation and tyrosine phosphorylation of cellul ar proteins by several cytotoxic T lymphocyte clones is quantitatively about tenfold lower in the presence of FN. We further demonstrate tha t FN alone stimulates transient tyrosine phosphorylation of a 120-kDa protein (pp120) in CD8(+) T cells and when FN is coimmobilized with su bstimulatory amounts of anti-CD3 or anti-TcR there is a synergistic re sponse, resulting in prolonged and enhanced phosphorylation of pp120. To determine if FN acts as a costimulator in CD8(+) cells solely throu gh mediating adhesion events or if it also transduces signals in T cel ls we conducted remote stimulation experiments. Degranulation was indu ced when FN and sub-stimulatory anti-CD3 were presented on separate su rfaces, indicating that FN induces independent transmembrane signals c apable of augmenting TcR-induced signals resulting in a functional res ponse. Both FN plus TcR-induced tyrosine phosphorylation of pp120 and degranulation are inhibited by RGD-containing peptides, implying that an RGD-dependent FN receptor is mediating phosphorylation of pp120 and enhancing TcR-mediated degranulation.