Regulation of pp56lck during T-cell activation: functional implications for the src-like protein tyrosine kinases

The lymphocyte-specific protein tyrosine kinase pp56lck, encoded by a member of the src gene family, is implicated in the control of T-cell growth and differentiation. Purified resting human T lymphocytes contain appreciable levels of lck mRNA and of pp56lck. Upon activation of these T cells, levels of lck mRNA and of pp56lck promptly decline. These reductions in lck mRNA and protein expression are closely correlated with the induction of lymphokine production. Both require identical stimuli and follow a similar time course of response. Down-regulation of lck expression, however, is not correlated with proliferation. Our results provide an example of regulation of a src-like protein tyrosine kinase in a normal fully differentiated cell population and suggest that modulation of lck RNA and protein expression is an important feature of the lymphocyte activation sequence leading to lymphokine production.