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. Here, PDCD1 is linked to tuberculosis.