SARS-CoV-2 XBB.1.5 infects wild-type C57BL/6 mice and induces a protective CD4 T cell response required for viral clearance.

Mouse models are critical for studying SARS-CoV-2 pathogenesis and evaluating therapeutic and preventive strategies. Standard C57BL/6 mice are generally resistant to infection with the ancestral SARS-CoV-2 strain due to inefficient binding of the viral spike protein to the murine angiotensin-converting enzyme 2 (ACE2) receptor. Although human ACE2 transgenic mice can support robust pulmonary infection, these models often develop fatal encephalitis, a pathology not commonly observed in humans. We and others have previously shown that certain SARS-CoV-2 variants can infect wild-type C57BL/6 mice and cause discernible disease. However, the susceptibility of C57BL/6 mice to recently emerged Omicron subvariants, and the role of T cell-mediated immunity in controlling these infections, remain incompletely understood. Herein, we evaluated the susceptibility of wild-type C57BL/6 mice to infection with the SARS-CoV-2 Omicron subvariant XBB.1.5. We assessed viral burden, innate and adaptive immune responses, and virus-induced lung pathology. Our findings demonstrate that XBB.1.5 efficiently replicates in both the upper and lower respiratory tracts of C57BL/6 mice, inducing significant lung inflammation and pathology. Infection elicited a robust pulmonary CD4 and CD8 T cell response. Through antibody-mediated depletion studies, we further show that CD4 T cells are critical for viral clearance, particularly in the nasal airways, as their depletion resulted in persistent viral RNA in the upper respiratory tract. These findings underscore the importance of CD4 T cell responses in controlling XBB.1.5 infection and provide a valuable model for studying variant-specific immune responses and pathogenesis.