CD4 and infection: To determine whether increased IFN-γ production in the KO mice is due to higher MAPK activity in NK and CD4 T cells, we isolated these cells from the lungs of WT and KO mice and treated them with MAPK inhibitor prior to infection with S. aureus. However, blocking MAPK did not reduce IFN-γ secretion by NK and CD4 T cells (S3E Fig and S3F Fig).