THE CD45 TYROSINE PHOSPHATASE REGULATES SPECIFIC POOLS OF ANTIGEN RECEPTOR-ASSOCIATED P59(FYN) AND CD4-ASSOCIATED P56(LCK) TYROSINE KINASESIN HUMAN T-CELLS

A newly isolated T-cell line (CB1) derived from a T-acute lymphoblasti c leukaemia (T-ALL) patient contained cells (40% of total) which did n ot express the CD45 phosphotyrosine phosphatase. The cells were sorted into CD45(-) and CD45(+) populations and shown to be clonal in origin . T-cell receptor (TCR) cross-linking or coligation of the TCR with it s CD4/CD8 co-receptors induced tyrosine phosphorylation and calcium si gnals in CD45(+) but not in CD45(-) cells. Unexpectedly, whole cell p5 6(lck) and p59(fyn) tyrosine kinase activities were not reduced in CD4 5(-) compared to CD35(+) cells. A novel technique was therefore develo ped to isolate specific pools of aggregated receptors expressed at the cell surface, together with their associated tyrosine kinases. Using this technique it was shown that cell surface CD4-p56(lck) kinase acti vity was 78% lower in CD45(-) than in CD45(+) cells. Phosphorylation o f TCR zeta- and gamma-chains occurred in TCR immunocomplexes from CD45 (+) but not CD45(-) cells, despite comparable levels of p59(fyn) and T CR proteins. Furthermore, TCR-associated tyrosine kinase activity towa rds an exogenous substrate was 84% lower in CD45(-) than in CD45(+) ce lls. Addition of recombinant p5(fyn) to TCR immunocomplexes isolated f rom CD45(-) cells restored the phosphorylation of the TCR zeta- and ga mma-chains. Our results demonstrate that CD45 selectively regulates th e pools of p59(fyn) and p56(lck) kinases which are associated with the TCR and CD4 at the cell surface. Activation by CD45 of these receptor -associated kinase pools correlates with the ability of the TCR and it s co-receptors to couple to intracellular signalling pathways.