DIMINISHED TYROSINE PROTEIN-KINASE ACTIVITY IN T-CELLS UNRESPONSIVE TO TCR STIMULATION

Tyrosine phosphorylation is thought to be one of the earliest steps in antigenic activation of T cells. Three nonreceptor tyrosine kinases, p56(lck), p60(fyn), and ZAP-70, are known to be involved in T cell rec eptor (TCR) signaling, albeit their functional roles appear to be diff erent. Whereas p60(fyn) and ZAP-70 are functionally associated with th e T cell antigen receptor, p56(lck) is essential for TCR signaling wit hout being directly coupled to the TCR. We have studied a mutant varia nt of the Jurkat T cell line (J32-3.2), in which basal activities of p 56(lck) and p60(fyn) are 2- to 2.5-fold reduced relative to those in i ts parental line (J32) while basal activity of ZAP-70 remains unchange d, and compared responses of J32-3.2 and J32 to TCR stimulation. We ha ve demonstrated that tyrosine phosphorylation following CD3 cross-link ing in J32-3.2 cells was extremely short-lived and thus insufficient f or the induction of subsequent physiological responses. This was at le ast partially due to the diminished tyrosine kinase activity in these cells. A decrease in the activity of src-related kinases was caused pr imarily by their lower expression, whereas expression of ZAP-70 was un changed but its response to CD3 crosslinking was diminished, correlati ng with the deficient tyrosine phosphorylation of the CD3 zeta-chain, recently observed in J32-3.2. These data are consistent with the idea that src-related kinases phosphorylate the zeta-chain, which in turn r ecruits ZAP-70 required to sustain the signal.