CD4 and tuberculosis: Conventional T cells are essential for maintaining a low bacterial load, as shown using antibody depletion or adoptive transfer of CD4+ or CD8+ T cells in mice (Flynn and Chan, 2001; Behar, 2013), increased risk of active tuberculosis with reduced CD4+ T cell frequency in HIV co-infection of humans (Cooper, 2009; Prezzemolo et al., 2014), and loss of protection against tuberculosis in primates with CD8+ T cell depletion (Chen et al., 2009).