Diminished tyrosine protein kinase activity in T cells unresponsive to TCR stimulation

Abstract

Tyrosine phosphorylation is thought to be one of the earliest steps in antigenic activation of T cells. Three nonreceptor tyrosine kinases, p56lck, p60fyn, and ZAP-70, are known to be involved in T cell receptor (TCR) signaling, albeit their functional roles appear to be different. Whereas p60fyn and ZAP-70 are functionally associated with the T cell antigen receptor, p56lck is essential for TCR signaling without being directly coupled to the TCR. We have studied a mutant variant of the Jurkat T cell line (J32-3.2), in which basal activities of p56lck and p60fyn are 2- to 2.5-fold reduced relative to those in its parental line (J32) while basal activity of ZAP-70 remains unchanged, and compared responses of J32-3.2 and J32 to TCR stimulation. We have demonstrated that tyrosine phosphorylation following CD3 cross-linking in J32-3.2 cells was extremely short-lived and thus insufficient for the induction of subsequent physiological responses. This was at least partially due to the diminished tyrosine kinase activity in these cells. A decrease in the activity of src-related kinases was caused primarily by their lower expression, whereas expression of ZAP-70 was unchanged but its response to CD3 cross-linking was diminished, correlating 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 recruits ZAP-70 required to sustain the signal.