CD4 and infection: CD4 T cells are critical for both clearance of S. Tm infection and protection from reinfection.5,6 In particular, Th1 cells are required for S. Tm clearance and protective immunity in systemic infection.5,7–9 In addition to Th1 cells, an early Th17 response has been identified in the intestines, which is important for limiting epithelial damage and systemic dissemination.10–12 Thus, an effective immune response to S. Tm involves both Th17 and Th1 cells.