PD-1 blockade synergizes with ascorbic acid to restore the activation and anti-viral immune functions of CD8+ T cells in a mouse model of BVDV infection

Bovine viral diarrhea virus (BVDV) can cause typical peripheral lymphopenia and inhibit CD8⁺ T-cell activation and proliferation. Programmed death-1 (PD-1) blockade has been shown to increase CD8⁺ T-cell activation during cytopathic (CP) BVDV infection but not non-cytopathic (NCP) BVDV. Notably, ascorbic acid (AA) restores lymphocyte count and activation during SARS-CoV-2 and influenza virus infections and has a synergistic effect with PD-1 blockade to improve antitumor CD8⁺ T-cell activity. Nevertheless, it remains unclear whether AA exerts an immunomodulatory effect on the activation and proliferation of CD8⁺ T cells during BVDV infection, especially NCP BVDV infection, or whether PD-1 blockade and AA exert a synergistic effect in regulating CD8⁺ T cell antiviral activities. In this study, we found that BVDV infection significantly decreased AA levels in serum and CD8⁺ T cells in a BALB/c mouse model. Interestingly, AA supplementation dramatically downregulated PD-1 expression, restored the activation and proliferation of CD8⁺ T cells, inhibited viral replication, ameliorated BVDV-induced histological lesions, and upregulated the expression of CD25 and p-ERK. More importantly, we also found a synergistic effect of PD-1 blockade with AA in restoring the activation and proliferation of CD8⁺ T cells during CP BVDV infection. However, during NCP BVDV infection, a synergistic effect of PD-1 blockade and AA led to the inhibition of viral replication and the promotion of IFN-γ production. Our findings provided new insights into the immunopathological mechanisms of BVDV and the potential value of anti-viral strategies based on AA treatment alone or in combination with PD-1 blockade.