Natural killer cells and IFN-γ protect against liver injury during HAV infection in mice.

A limited number of studies on human subjects with type-A hepatitis have found a positive correlation between activated natural killer (NK) cell frequencies in blood and the severity of liver injury. However, without knowledge of differences in viral burdens, it remains unclear whether NK cells are elevated due to hepatitis A virus (HAV) replication and ongoing associated injury, or if NK cell responses directly cause pathogenesis. To gain insight into how NK cells respond during the early stages of hepatitis A virus infection, we generated mice that are permissive for HAV infection in the liver but are otherwise immunocompetent. HAV readily infected the liver of these hepatocyte-specific type-I interferon receptor knockout mice (<em>Ifnar1^&Delta;Hep</em>), resulting in fecal virus shedding, elevated serum alanine aminotransferase levels, and immune cell infiltrates in the liver. Single-cell RNA sequencing analyses of liver leukocytes from infected <em>Ifnar1^&Delta;Hep</em> mice revealed a rapid accumulation of activated NK cells in the liver within 2 days. HAV infection and liver inflammation were brief and substantially reduced in magnitude in these mice compared to global <em>Ifnar1^-/-</em> knockout mice that failed to induce substantial antiviral NK cell responses. NK cells were the principal source of interferon (IFN)-&gamma; in <em>Ifnar1^&Delta;Hep</em> mice. <em>Ifnar1^&Delta;Hep</em> mice that were experimentally depleted of NK cells or IFN-&gamma; showed greatly increased viral replication and liver damage. Moreover, IFN-&gamma; alone was sufficient to suppress HAV replication in primary mouse hepatocytes. Collectively, these findings establish a critical role for type-I IFN induction of NK cells, their expression of IFN-&gamma;, and protection against hepatitis A.IMPORTANCEHepatitis A virus remains a leading cause of foodborne illness among unvaccinated individuals. Infection can result in severe liver injury that can progress to fatal fulminant viral hepatitis. Despite its clinical significance, the molecular, genetic, and cellular factors influencing infection severity remain poorly understood. Previous studies of patient blood have suggested that natural killer (NK) cells cause liver injury during infection. In this study, we used mouse models of infection to characterize early cellular defenses to infection and identified a critical role for NK cells and interferon-&gamma; in conferring rapid immune protection rather than pathogenesis.