Terminally exhausted CD8⁺ T cells contribute to age-dependent severity of human metapneumovirus

Abstract Objective Human metapneumovirus (HMPV) is a leading cause of acute respiratory infection with severe morbidity and mortality among older adults. HMPV seropositivity is universal due to childhood exposure, yet antibodies do not fully protect. We hypothesized that severe HMPV disease in the elderly was due to deficient CD8 +T cell responses. Methods B6 aged 70–71wk mice and young 6–7wk mice were infected with 2×10 6PFU HMPV. Seven days post infection, mice were euthanized and lung, bronchoalveolar lavage (BAL), spleen, and draining lymph nodes were collected for flow cytometry. Lung and BAL supernatant was collected for viral titer, histology, and cytokine measurement. Results Aged 70–71wk C57BL/6 mice exhibited worsened weight loss, disease, and lung pathology with delayed viral clearance compared to young adult mice. Aged mice developed fewer HMPV tetramer +CD8 +T cells, which demonstrated higher expression of PD-1 and other inhibitory receptors and were less polyfunctional with decreased granzyme B. Transplant of aged T cells into young mice and vice versa, as well as adoptive transfer of young or aged CD8 +T cells into Rag1−/−mice, recapitulated the HMPV aged phenotype, suggesting a cell-intrinsic age-associated defect. HMPV-specific aged CD8 +T cells exhibited a terminally exhausted TCF1 −TOX +EOMES +phenotype. PD-1 blockade or 4-1BB agonist failed to improve the aged CD8 +T cell response. Conclusion Aged mice exhibit more severe respiratory disease when infected with HMPV. There is a cell-intrinsic age-associated defect in CD8 +T cells as shown through the transplant models. This study also identifies terminal CD8 +T cell exhaustion as a mechanism of severe HMPV disease in the elderly. NIH AI085062 (JVW) 1F30HL159915-01A1 (OBP) T32 GM-008208 (OBP, JS) Henry L. Hillman Foundation (JVW) T32 AI089443 (YZ) K12 HD000850 (TE)