Viral acute lower respiratory infections impair CD8+ T cells through PD-1

Viruses are leading causes of severe acute lower respiratory infections (LRIs). These infections evoke incomplete immunity, as individuals can be repeatedly reinfected throughout life. We report that acute viral LRI causes rapid pulmonary CD8(+) cytotoxic T lymphocyte (T-CD8) functional impairment via programmed death-1/ programmed death ligand-1 (PD-1/PD-L1) signaling, a pathway previously associated with prolonged antigenic stimulation during chronic infections and cancer. PD-1-mediated T-CD8 impairment occurred acutely in mice following infection with human metapneumovirus or influenza virus. Viral antigen was sufficient for PD-1 upregulation, but induction of PD-L1 was required for impairment. During secondary viral infection or epitope-only challenge, memory T-CD8 rapidly reexpressed PD-1 and exhibited severe functional impairment. Inhibition of PD-1 signaling using monoclonal antibody blockade prevented T-CD8 impairment, reduced viral titers during primary infection, and enhanced protection of immunized mice against challenge infection. Additionally, PD-1 and PD-L1 were upregulated in the lungs of patients with 2009 H1N1 influenza virus, respiratory syncytial virus, or parainfluenza virus infection. These results indicate that PD-1 mediates T-CD8 functional impairment during acute viral infection and may contribute to recurrent viral LRIs. Therefore, the PD-1/PD-L1 pathway may represent a therapeutic target in the treatment of respiratory viruses.