Induction of interleukin 2-responsiveness in thymocytes of the transgenic mice carrying lck-transgene.

The role of lck gene in T cell proliferation and differentiation was investigated with transgenic mice carrying human lck cDNA whose expression was regulated by the promoter of mouse H-2Kb and the enhancer element of mouse IgH. RNase protection assay revealed that the lck transgene was expressed in the thymus and spleen, whereas immunoblot analysis demonstrated that amounts of p56lck in freshly isolated lymphoid organs were almost equal between transgenic mice and negative littermates. Cell-surface marker analyses of the thymocytes and peripheral lymphocytes revealed no remarkable difference between both groups. Notable finding is that the thymocytes from transgenic mice showed a significant proliferative response to the stimulation with IL-2, but not the thymocytes from negative littermates. Further analysis revealed that CD4+8- single positive thymocytes proliferated in response to IL-2. While surface expression levels of IL-2R alpha and IL-2R beta of these CD4+8- thymocytes from transgenic and control mice were almost equal before stimulation with IL-2, the expression of IL-2R beta was induced only in transgenic thymocytes after stimulation with IL-2. Immunoblot analysis demonstrated that the expression of p56lck of transgenic thymocytes was not down-regulated at 4 hr after stimulation with IL-2, whereas p56lck of control ones were not detectable any more at 4 hr after stimulation with IL-2. Moreover, in vitro kinase assay substantiated such unchanged expression of p56lck in the thymocytes from transgenic mice: the kinase activities of p56lck did not decrease in thymocytes from transgenic mice after stimulation with IL-2, while kinase activities of control ones were significantly down-regulated by stimulation of IL-2. These results suggested that a significant proliferative response found in the thymocytes from lck-transgenic mice after the stimulation with IL-2 was caused by a constitutive expression of p56lck in these thymocytes even after the stimulation. Our findings, therefore, support a possibility that p56lck may play a role in the IL-2R-mediated signaling system in CD4+8- thymocytes.