The role of CD8+ T cells was further addressed by the infection of CD8+ T cell-deficient C57BL/6 MHCI-/- mice and C57BL/6 Perforin-/- mice that lack the cytotoxic activity of CD8+ T cells and NK cells with R. australis. Both animals showed enhanced lethality in this infection [15], demonstrating a critical role of CD8+ T cells and their cytotoxic activity in protection against SFG rickettsiae. The gene discussed is PRF1; the disease is infection.