Combining metabolic and checkpoint therapy: 1,25-dihydroxyvitamin D₃ enhances the antiviral efficacy of anti-PD-1 in BVDV infection.

Bovine viral diarrhea virus (BVDV) infection induces severe immunosuppression and peripheral blood lymphopenia, driven by PD-1/PD-L1-mediated T cell exhaustion. Emerging evidence highlights 1,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃, VD), the active metabolite of vitamin D, as a critical modulator of antiviral immunity. However, its interplay with PD-1 signaling in BVDV pathogenesis remains unexplored. This study investigated the therapeutic potential of VD alone or combined with PD-1 blockade in a murine BVDV infection model. We found that BVDV infection significantly reduced serum and intracellular concentrations of VD. Supplementation with VD effectively improved these levels, downregulated PD-1/PD-L1 expression, reduced lymphopenia, and reduced CD8+ T cell apoptosis. While both monotherapies showed benefits, their combination yielded superior outcomes. The combined therapy significantly enhanced viral clearance, improved pathological changes in spleen and duodenum, and enhanced CD8⁺ T cell proliferation and IFN-γ production. Mechanistically, the combination uniquely upregulated key anti-apoptotic signaling molecules (p-Akt, p-mTOR, p-ERK) in CD8⁺ T cells. Notably, therapeutic efficacy differed between cytopathic (CP) and non-cytopathic (NCP) biotypes, with PD-1 blockade showing greater dependency in CP infection. These findings highlight the therapeutic potential of targeting both metabolic dysregulation (via VD) and immune checkpoint inhibition (via PD-1 blockade) to combat BVDV-induced immunosuppression. This dual strategy may offer a novel approach for treating viral infections characterized by metabolic-viral-immune interplay.