Stimulation of phosphatidylinositol turnover by concanavalin A is not sufficient to activate mouse thymocytes

Mouse thymocytes treated with the lectin Concanavalin A do not proliferate nor do they develop responsiveness to interleukin 2. Co-treatment with Concanavalin A and either lectin-activated splenocyte conditioned medium or phorbol ester caused increased interleukin 2 receptor expression and proliferation. Under these conditions, lectin alone stimulated a 3.4 fold increase in phosphatidylinositol turnover which was unaffected by the presence of conditioned medium. Phosphorylation of a 55 kD protein was stimulated in response to conditioned medium or phorbol ester, but not lectin. These results indicate that stimulation of phosphoinositide turnover is not sufficient to activate thymocytes, and suggest that costimulating factors activate a kinase which is distinct from protein kinase C, or alternatively, activate protein kinase C through a process which is not coupled to phosphoinositide turnover.