Clinical human metapneumovirus isolates show distinct pathogenesis and inflammatory profiles but similar CD8⁺ T cell impairment

Human metapneumovirus (HMPV) is a negative-sense single-stranded RNA virus in the Pneumoviridae family and a leading cause of acute upper and lower respiratory infections, particularly in children, immunocompromised patients, and the elderly. Although nearly every person is infected with HMPV during early childhood, re-infections occur often, highlighting difficulty in building long-term immunity. Inflammatory responses, including PD-1-mediated impairment of virus-specific CD8(+) T cells (T-CD8), contribute to HMPV disease severity. HMPV strains are divided into four lineages: A1, A2, B1, and B2. However, little is known about immune responses to different viral subtypes. Here, we characterize responses to four HMPV clinical isolates-TN/94-344 (A1), TN/94-49 (A2), C2-202 (B1), and TN/96-35 (B2)-in vivo in C57BL/6 (B6) mice. TN/94-49 was avirulent, while TN/94-344, C2-202, and TN/96-35 showed varying degrees of weight loss and clinical disease. Differences in disease did not correlate to virus burden in upper or lower tracts. TN/94-49 HMPV exhibited highest nose titers and delayed lung clearance. Cytokine profiles differed between HMPV isolates, with TN/96-35 inducing the broadest lung inflammatory cytokines. TN/96-35 also showed lower HMPV burden and less weight loss than other virulent isolates, suggesting a more efficient antiviral response. Interestingly, disease correlated with higher expression of T-cell chemoattractant CXCL9. All isolates elicited PD-1 upregulation and decreased IFN gamma and CD107a expression in virus-specific T-CD8, with little difference between HMPV subtypes. This work uncovers previously uncharacterized variations in immune responses to clinical HMPV isolates of different lineages.IMPORTANCE This study extensively explored differences in T-cell-mediated immunity between human metapneumovirus (HMPV) clinical isolates. Much existing HMPV research has been done with strains passaged extensively in cell lines, likely acquiring mutations advantageous to in vitro replication. Clinical isolates are collected directly from human patients and have undergone <10 passages, serving as more physiologically relevant models of HMPV infection. Additionally, existing animal studies of HMPV disease mainly focus on lung pathogenesis, while HMPV infects both upper and lower airways of humans. This work highlights distinct differences in HMPV burden in upper and lower tracts between clinical isolates. Lastly, this study uniquely explores differences in host immunity between all four HMPV genetic lineages. The predominant HMPV subtype in circulation varies seasonally; thus, understanding host responses to all subgroups is critical for developing effective HMPV vaccines.