CD4 and infection: In murine and preclinical animals studies cellular immunity is known to play an essential role in protection against rickettsial infections, with both CD4+ and CD8+ T cell-deficient mice demonstrating increased susceptibility to infection with TG, SFG, and O. tsutsugamushi (Walker et al., 2001; Moderzynski et al., 2016; Xu et al., 2017).