Single-cell transcriptomics reveal PD-1-loss-driven immune dysregulation of pulmonary lymphocytes during early Mycobacterium tuberculosis infection.

Although PD-1 blockade shows strong antitumor efficacy and is being considered for chronic infections including Mycobacterium tuberculosis (M.tb), its influence on early lymphocyte responses to M.tb remains poorly understood. In this study, we characterized the transcriptional and developmental states of pulmonary B and T cells in mice lacking PD-1 after 7 days post-M.tb infection. We found that loss of PD-1 disturbed pulmonary lymphocyte homeostasis, characterized by abnormal regulatory T cell (Tregs) expansion, early exhaustion-like differentiation of cytotoxic T cells, and impaired maturation of memory B cells. These alterations are associated with changes in key transcriptional regulators, including reduced activity of factors promoting effector differentiation (e.g., Runx2, Runx3 in T cells; Tcf4, Pou2f2 in B cells) and upregulation of regulators driving regulatory or suppressive phenotypes (e.g., Ikzf2 in Tregs). Additionally, PD-1 deficiency rewired B-T cell communication with diminished antigen presentation and co-stimulation while amplifying proinflammatory signals. These insights provide a transcriptional blueprint for PD-1-mediated immune balance, with implications for host-directed therapies in tuberculosis.IMPORTANCEWhile PD-1 inhibition can boost protective T cell responses, it has also been linked to increased TB susceptibility. Our study reveals that PD-1 plays a previously unrecognized role in shaping the earliest immune responses to M.tb. In mice lacking PD-1, lung lymphocytes failed to develop in a balanced manner: regulatory T cells expanded excessively, cytotoxic T cells showed signs of early exhaustion, and memory B cell maturation was delayed. These changes disrupted the normal communication between B and T cells, weakening adaptive immunity while amplifying inflammation. By defining PD-1 as a critical organizer of early lymphocyte fate, our findings highlight the risks of indiscriminate PD-1 blockade in infectious contexts and point toward more precise strategies for host-directed TB therapies and vaccine design.