Th1 differentiation and function are inhibited in neonates following human metapneumovirus infection

Human metapneumovirus (HMPV) is a leading cause of lower respiratory tract infection in children accounting for 7% of acute care visits and hospitalizations. In particular, neonates and infants have worse outcomes with HMPV infection. The neonatal immune system is regulated to favor anti-inflammatory and tolerogenic responses compared to adults, including prior work demonstrating epigenetic factors in neonatal CD4+ T cells promoting Th2 formation rather than antiviral Th1 differentiation. To interrogate the neonatal immune response to HMPV, 4-to-6 day-old mice or adult 6-to-8 week-old mice were infected with HMPV. Neonates had a decreased Th1 population and increased Th2 and regulatory T-cell (Treg) populations compared to adults. Neonatal Th1 function, but not cell number, was restrained by surface PD-1 expression. To assess if neonatal Th1 formation was intrinsically inhibited after HMPV, neonatal and adult CD4s were transferred into immunocompetent or immunodeficient neonates. Both adult and neonatal CD4s demonstrated reduced Th1 differentiation in the immunocompetent neonates, but robust Th1 differentiation in immunodeficient neonates and immunocompetent adults, suggesting an extrinsic mechanism. Loss of neonatal Tregs led to increased Th1 differentiation after HMPV infection. Neonatal Tregs had increased TGF-β production compared to adult Tregs, and disruption of TGF-β signaling increased Th1 induction. These data demonstrate Tregs provide extrinsic regulation of Th1 formation in the context of respiratory viral infections, rather than an intrinsic limitation of neonatal CD4s. Collectively, these findings identify a nuanced neonatal response to respiratory viruses limiting Th1 formation and function.