Dynamic CD4 T cell heterogeneity defines subset-specific suppression and PD-L1-blockade-driven functional restoration in chronic infection.

Inhibiting PD-1:PD-L1 signaling has transformed therapeutic immune restoration. CD4 T cells sustain immunity in chronic infections and cancer, yet little is known about how PD-1 signaling modulates CD4 helper T (T) cell responses or the ability to restore CD4 T-mediated immunity by checkpoint blockade. We demonstrate that PD-1:PD-L1 specifically suppressed CD4 T1 cell amplification, prevents CD4 T1 cytokine production and abolishes CD4 cytotoxic killing capacity during chronic infection in mice. Inhibiting PD-L1 rapidly restored these functions, while simultaneously amplifying and activating T1-like T regulatory cells, demonstrating a system-wide CD4-T1 recalibration. This effect coincided with decreased T cell antigen receptor signaling, and re-directed type I interferon (IFN) signaling networks towards dominant IFN-γ-mediated responses. Mechanistically, PD-L1 blockade specifically targeted defined populations with pre-established, but actively suppressed proliferative potential, with limited impact on minimally cycling TCF-1 follicular helper T cells, despite high PD-1 expression. Thus, CD4 T cells require unique differentiation and functional states to be targets of PD-L1-directed suppression and therapeutic restoration.