PD-L1 is an intrinsic switch for natural killer cell-mediated, TRAIL-dependent antiviral function.

Each year, influenza A virus (IAV) infection of the lung causes half a million deaths worldwide. Patients with compromised immunity experience distinct influenza pathogenesis; however, most IAV-related research is done with wild-type mice or people who are otherwise healthy. We utilize a model of immunocompromised recombination-activating gene 1 (Rag1)-knockout (KO) mice to discover natural killer (NK) cell activation and regulation mechanisms during IAV infection. The treatment of IAV-challenged Rag1-KO mice with a monoclonal antibody (mAb) targeting programmed death ligand 1 (PD-L1) triggers NK cell-intrinsic signaling of PD-L1 and significantly delays lethality. This treatment upregulates tumor necrosis factor-related apoptosis-inducing ligand on NK cells downstream of PD-L1 signaling and is required for the benefits of PD-L1 mAb treatment in IAV-challenged Rag1-KO mice. These results present a paradigm shift for understanding the innate immune response to respiratory virus infections, offering an alternative approach for therapeutic treatment of IAV infections in patients with compromised immunity.