Respiratory Influenza Virus Infection Induces Memory-like Liver NK Cells in Mice.

Although NK cells are classified as innate immune cells, recent studies have demonstrated the transformation of NK cells into long-lived memory cells that contribute to secondary immune responses in certain mouse models. However, whether NK cells mount an Ag-specific memory response to acute influenza virus infection has not yet been examined. Here, we show that, consistent with previous studies, lung NK cells play an important role in controlling viral proliferation after primary influenza virus infection. However, although lung NK cells display a memory phenotype at the late stage of infection, these cells do not protect mice against secondary influenza virus infection. Interestingly, liver NK cells from influenza virus-infected mice possess a memory phenotype and protect mice against secondary influenza virus infection. Memory-like liver NK cells display a CD49aDX5 phenotype, and the adoptive transfer of purified liver CD49aDX5 NK cells into naive mice followed by viral infection results in protective immunity and decreased viral titer. Moreover, we demonstrate that primary inactivated influenza virus induces memory NK cells residing in the liver of Rag1 mice. Collectively, these data suggest that liver CD49aDX5 NK cells remember encountered Ag from influenza virus after primary infection and are more protective upon subsequent infection.