Type III Interferon-Mediated Responses in Upper and Lower Respiratory Tracts During Infection with Human Metapneumovirus Clinical Isolates

Human metapneumovirus (HMPV) is a leading cause of acute upper and lower airway infections. While all people are infected with HMPV before age 5, re-infections occur often and highlight difficulty in building long-term immunity. There are no vaccines or antiviral therapies for HMPV. Here, we uncover several factors that impact host immune responses to HMPV, with a focus on interferons. First, we established a more physiologically relevant mouse model of HMPV disease through 1) use of clinical isolates, viral strains isolated directly from patients and undergoing <10 passages in cells, and 2) HMPV dosing that results in mild, self-limiting disease that resolves in 10-14 days, similar to human infection. We showed in this model that HMPV clinical isolates of 4 genetic lineages efficiently replicate in upper and lower airways, but exhibited varying degrees of disease severity, HMPV burden, and lung inflammatory cytokine induction. Disease did not correlate to upper or lower HMPV burden but did correlate with higher expression of T-cell chemoattractant CXCL9. Second, we employed this model of HMPV disease to characterize early immune responses to HMPV infection, focusing on the potent host antiviral cytokine IFN-λ. We showed IFN-λ is highly upregulated in in vitro and in vivo models of infection. We found that type-II alveolar epithelial cells are major producers of IFN-λ, and that IFN-λ signaling is regulated through CD45- non-immune mediators. IFN-λ was critical in limiting lung HMPV replication and restricting viral spread to lower airways but did not contribute to HMPV pathology. Mice lacking IFN-λ signaling showed differential recruitment of innate immune cells and higher inflammatory cytokine induction, suggesting an immunomodulatory role. IFN-λ prophylaxis or post-infection treatment reduced viral load without inflammation-driven clinical disease, showing promise for HMPV therapy. Third, we characterized responses in the upper airway to HMPV, finding HMPV burden was higher in nasal airways and exhibited delayed clearance. Despite high burden, there was limited interferon production and minimal immune response to infection. HMPV-infected nasal airways suppressed nasal IFN induction by downregulating interferon regulatory factor-3. IFN-λ treatment rescued this quiescent nasal landscape, and IFN-λ adjuvant of nasal HMPV immunization was essential for reducing clinical disease and clearing virus.