CD4 and tuberculosis: Our in vitro findings of Mtb antigen-induced PD-1 expression further corroborate our ex vivo findings that PD-1 is significantly upregulated on Mtb-specific CD4 T cells in the setting of high bacterial load in smear+ TB, indicating recent Mtb antigen exposure in vivo, compared with LTBI individuals who have either cleared Mtb or maintain bacterial loads below the level of detection by sputum culture.