Rapid tyrosine phosphorylation of Lck following ligation of the tumor-associated cell surface molecule A6H

We have recently described the A6H antigen as a novel 120-140 kDa molecule which is coexpressed on human peripheral blood T cells and renal cell carci noma cells. Engagement of the A6H antigen results in cc-stimulation of CD4( +) T cells but it remained unknown how cross-talk between the A6H antigen a nd the TCR-CD3 complex takes place and which signaling pathway might be inv olved. Here we show that ligation of the A6H antigen with mAb induces tyros ine phosphorylation of the Lck protein tyrosine kinase (PTK), Co-ligation o f the A6H antigen with CD3 resulted in augmented Lck phosphorylation and mi togenesis. In addition, A6H ligation induced an up-regulation of CD3-mediat ed phosphorylation of the 23 kDa high mel, wt form of Ton zeta and the zeta -associated protein, ZAP-70, Go-precipitation of Lck and ZAP-70 was only se en in T cells activated by combined A6H and anti-CD3 stimulation. In contra st, another Src family PTK, Fyn, was not affected by A6H ligation, In concl usion, we now demonstrate, for the first time, that A6H ligation triggers L ck phosphorylation, and that cross-talk between A6H and the TCR-CD3 complex involves Lck, ZAP-70 and the slow migrating isoform of TCR 5. These result s further suggests that A6H ligation is sufficient for triggering some of t he early events in T cell activation, whereas full activation of the T cell , characterized by proliferation and cytokine production, requires co-ligat ion of the TCR-CD3 complex.