Kras Is Critical for CD8 T Cell Antiviral Function

Small GTPases regulate multiple signaling pathways and individual Ras member can have distinct biological function. To overcome embryonic lethality of Kras-deficient mice, we generated and examined mice with hematopoietic- and T cell-specific deletion of Kras. In VavCreKras^fl/fl mice with hematopoietic deletion of Kras, thymic T-cell development was normal based on the presence of normal populations of total, CD4^- CD8^-, CD4⁺ CD8⁺, CD4⁺ and CD8⁺ thymocytes. The populations of splenic CD4⁺ and CD8⁺ T cells were also comparable between VavCreKras^fl/fl relative to control mice. In addition, no consistent defects in the ³ H-thymidine incorporation rate of Kras-deficient splenic CD4⁺ or CD8⁺ T cells in response to anti-CD3 or anti-CD3 plus IL-2 was detected. Nonetheless, we studied the effect of Kras deficiency on CD8 T-cell immune response to acute infection of the Armstrong strain of lymphocytic choriomeningitis virus (LCMV). Sub-lethally irradiated Rag1-deficient mice transplanted with bone marrow (BM) cells from VavCreKras^fl/fl or control mice were subjected to LCMV infection. Infection-induced expansion of CD8 T cells and generation of LCMV epitope gp33-specific CD8 T cells were markedly reduced in the recipients that received the BM from VavCreKras^fl/fl relative to control mice. Following in vitro stimulation with the LCMV epitope gp33, the induction of IFNg-expressing CD8 T cells from LCMV-infected recipients that received the BM from VavCreKras^fl/fl mice was dramatically reduced. Further, BM chimeric mice with CD8 T cell-specific deficiency generated by transplantation of lethally irradiated CD8 T cell-depleted CD45.1 congenic mice with a mixture of BM cells from VavCreKras^fl/fl mice and BM cells from CD8 T cell-deficient mice exhibited an impaired CD8 T-cell immune response to LCMV infection. Lastly, we examined the role of Kras in TCR signaling. The level of total TCR-activated Ras (Ras-GTP) was markedly reduced in Kras-deficient relative to control CD8 T cells. Importantly, TCR-induced ERK1/2 activation was impaired in Kras-deficient relative to control CD8 T cells. Consistently, TCR-induced activation of Raf-1 and MEK1/2 was markedly reduced in mutant CD8 T cells. However, TCR-induced JNK and p38 activation as well as Ca²⁺ flux were normal in Kras-deficient CD8 T cells. Of note, TCR-induced activation of Ca²⁺ flux, JNK and p38 as well as ERK1/2, MEK1/2 and Raf1 was normal in Kras-deficient relative to control CD4 cells. Taken together, these data demonstrate that Kras is dispensable for T cell development or TCR-induced proliferation of CD4 or CD8 T cells in vitro, but regulates TCR-induced activation of the Raf-1/MEK/ERK pathway in CD8 but not CD4 T cells and intrinsically controls CD8 T-cell immune response to viral infection.