PD-1 signaling in neonates restrains CD8⁺ T cell function and protects against respiratory viral immunopathology

Respiratory viral infections, including human metapneumovirus (HMPV), remain a leading cause of morbidity and mortality in neonates and infants. However, the mechanisms behind the increased sensitivity to those respiratory viral infections in neonates are poorly understood. Neonates, unlike adults, have several anti-inflammatory mechanisms in the lung, including elevated baseline expression of programmed death ligand 1 (PD-L1), a ligand for the inhibitory receptor programmed cell death protein 1 (PD-1). We thus hypothesized that neonates would rely on PD-1:PD-L1 signaling to restrain antiviral CD8 responses. To test this, we developed a neonatal primary HMPV infection model using wild-type C57BL/6 (B6) and Pdcd1^-/- (lacking PD-1) mice. HMPV-infected neonatal mice had increased PD-L1/PD-L2 co-expression on innate immune cells but a similar number of antigen-specific CD8⁺ T cells and upregulation of PD-1 to that of adult B6 mice. Neonatal CD8⁺ T cells had reduced interferon-gamma (IFN-γ), granzyme B, and interleukin-2 production compared with B6 adults. Pdcd1^-/- neonatal CD8⁺ T cells had markedly increased production of IFN-γ and granzyme B compared with B6 neonates. Pdcd1^-/- neonates had increased acute pathology with HMPV or influenza. Pdcd1^-/- neonates infected with HMPV had long-term changes in pulmonary physiology with evidence of immunopathology and a persistent CD8⁺ T-cell response with increased granzyme B production. Using single-cell ribonucleic acid sequencing from a child lacking PD-1 signaling, a similar activated CD8⁺ T-cell signature with increased granzyme B expression was observed. These data indicate that PD-1 signaling critically limits CD8⁺ T-cell effector functions and prevents immunopathology in response to neonatal respiratory viral infections.