Vitamin C combined with anti-PD-1 antibody alleviates peripheral lymphopenia and enhances CD4+ T-cell antiviral immunity during BVDV infection.

Bovine viral diarrhea virus (BVDV)-induced lymphopenia is associated with immune dysfunction. Vitamin C (VC) improves immune response by increasing peripheral blood lymphocyte (PBL) count, thereby promoting T-cell activation and release of IL-2 and IFN-γ. However, it remains unclear whether VC plays a critical positive regulatory role in the antiviral activities of CD4+ T cells during BVDV infection, or whether the combined treatment with VC and programmed death-1 (PD-1) blockade demonstrates a stronger effect in alleviating BVDV-induced lymphopenia. In this study, we found that both cytopathic (CP) and non-cytopathic (NCP) BVDV infection caused significant reductions in VC in plasma and PBLs of mice. VC supplementation alone or combined with PD-1 blockade significantly increased lymphocyte count and proliferation and upregulated the expression of CD25 and p-ERK in PBLs during CP and NCP BVDV infection. Furthermore, VC supplementation dramatically downregulated PD-1 expression, ameliorated CD4+ T-cell activation and proliferation, and inhibited apoptosis. We further investigated the effect of combined treatment with VC and PD-1 blockade on promoting CD4+ T-cell activation, increasing IFN-γ production, upregulating p-JAK2/p-STAT1 expression, and inhibiting viral replication during NCP BVDV infection. Remarkably, VC supplementation significantly increased IFN-γ production, upregulated p-JAK2/p-STAT1 expression, and reduced viral load in CD4+ T cells after NCP BVDV infection but not after CP BVDV infection. Our findings confirmed an essential regulatory role for VC in alleviating BVDV-induced lymphopenia and enhancing CD4+ T-cell antiviral immunity, as well as the combination's effect on VC and anti-PD-1 antibody during BVDV infection, thereby providing new insights to explore potential therapeutic strategies to control BVDV infection.