Late Expression of Granulysin by Microbicidal CD4+ T Cells Requires PI3K- and STAT5-Dependent Expression of IL-2Rβ That Is Defective in HIV-Infected Patients1

Granulysin is a cytolytic effector molecule used by lymphocytes to kill tumor and microbial cells. Regulation of granulysin production is complex. A significant delay (5 days) following stimulation of CD4 + T cells with IL-2 occurs before granulysin is produced. Unfortunately, the mechanisms responsible for this delay are unknown. We have recently demonstrated that granulysin-mediated killing of Cryptococcus neoformans by CD4 + T cells is defective during HIV infection. This is because CD4 + T cells from HIV-infected patients fail to produce granulysin in response to IL-2 activation. The present studies examined the mechanism of delayed production of granulysin and the mechanism of the defect in HIV patients. We demonstrate that IL-2 initially requires both STAT5 and PI3K activation to increase expression of IL-2R β , produce granulysin, and kill C. neoformans . The increased expression of IL-2R β precedes granulysin, and preventing the increased expression of IL-2R β using small interfering RNA knockdown abrogates granulysin expression. Moreover, following the increased expression of IL-2R β , blocking subsequent signaling by IL-2 using IL-2R β -specific blocking Abs abrogates expression of granulysin. Finally, CD4 + T cells from HIV-infected patients, who are defective in both STAT5 and PI3K signaling, fail to express IL-2R β and fail to produce granulysin. These results suggest that IL-2 signals via PI3K and STAT5 to increase expression of IL-2R β , which in turn is required for production of granulysin. These results provide a mechanism to explain the “late” production of granulysin during normal T cell responses, as well as for defective granulysin production by CD4 + T cells in HIV-infected patients.