Virus-specific CD4+ T cells contribute to clearance of human metapneumovirus despite exhibiting an impaired phenotype

Human metapneumovirus (HMPV) is a leading cause of respiratory tract infection in pediatric, elderly, and immunocompromised populations. Clearance of HMPV relies primarily on the destruction of infected cells by cytotoxic CD8+ T cells. However, signals provided by CD4+ helper T cells significantly impact the magnitude and effectiveness of CD8+ T cells. The role of CD4+ helper T cells in the immune response to HMPV is largely unknown. Antibody-mediated depletion of CD4+ T cells in a mouse model of acute infection led to delayed viral clearance and enhanced PD-1 expression on virus-specific CD8+ T cells. In accord with CD4 depletion experiments, blocking of CD40L reduced viral control and CD8+ T cell responses while stimulation of this pathway with an agonist antibody decreased the duration of infection. We identified a dominant CD4+ T cell epitope in the viral nucleoprotein and constructed the first MHC-II tetramer for HMPV. Analysis of pulmonary T cells revealed that virus-specific cells peak on day 10 post-infection and were TH1-skewed. Additionally, virus-specific CD4+ T cells displayed phenotypic and functional markers of impairment, including inhibitory receptor co-expression and prolonged PD-1 upregulation. However, genetic ablation of PD-1 signaling did not improve virus-specific CD4+ T cell functionality. Further characterization of virus-specific CD4+ helper T cells, their regulation by PD-1, and their role in CD8+ T cell impairment will provide new insights that aid in the design of effective vaccines for HMPV.