A CD40-OX40 co-stimulatory circuit orchestrates protective CD4+ T cell immunity in tuberculosis.

Effective CD4+ T cell responses are essential for controlling Mycobacterium tuberculosis (Mtb), but they fail to achieve sterilizing immunity in infected lungs. In mouse models of Mtb infection, single-cell transcriptomic profiling identified a dominant population of OX40+ CD4+ T cells that emerged during Mtb infection yet exhibited impaired effector differentiation. This dysfunction was linked to defective co-stimulatory signaling, corresponding to persistently low OX40 ligand (OX40L) expression on antigen-presenting cells. Therapeutic activation of OX40 restored CD4+ T cell functionality, expanded protective clonotypes, and reduced pulmonary bacterial burden in vivo. These effects required intact CD40-CD40L interactions to sustain OX40 expression and promote T cell differentiation, and were critically dependent on interferon (IFN)-γ signaling for antimicrobial activity. CD40L blockade abolished the immunotherapeutic benefits of OX40 stimulation, revealing a cooperative CD40-OX40 axis that orchestrates protective immunity in tuberculosis. These findings identify impaired co-stimulatory signaling as a central mechanism of CD4+ T cell dysfunction during chronic infection and highlight this axis as a target for host-directed immunotherapy in tuberculosis.