Natural killer cell activation, trafficking, and contribution to immune responses to viral pathogens

Natural killer (NK) cells are a critical component of the immune response against viral infections. NK cell depletion prior to murine cytomegalovirus (MCMV) infections results in increased susceptibility to infection in several mouse strains. The mechanism of protection in C57Bl/6 mice is dependent on the activation of NK cells by Ly49H recognition of m157. Our previous studies have examined important residues of m157 for Ly49H recognition, as well as the contribution of m157 glycosylation to NK cell activation. However, what role the glycophosphatidyl inositol (GPI) anchor of m157 plays in Ly49H activation was unknown. Here we demonstrate that the GPI anchor of m157 regulates the surface expression of the protein. While the GPI anchor was not required for recognition of m157 by the activating or inhibitory Ly49 receptors, expression of GPI-anchored m157 resulted in greater receptor downregulation on NK cells, as well as increased NK cell cytotoxicity compared to transmembrane m157.

In addition to MCMV infections, NK cells have been shown to participate in the immune response to influenza A virus (IAV). However the exact role of NK cells in IAV infection is less clear, as some studies have found NK cells to be protective, while others have shown that NK cells cause lethal immunopathology. It is likely that the severity of IAV infection may dictate the NK cell response to IAV infection (i.e. protective vs. immunopathogenic). Herein we show that NK cell accumulation in IAV-infected lungs and lung-draining lymph nodes (DLN) is regulated by the severity of IAV infection, where there is increased NK cell accumulation in the lungs during high dose IAV infection, and greater NK cell accumulation in the DLN in low dose IAV infections. Despite significant NK cell recruitment to the lung during IAV infection, as well as previously published studies demonstrating the importance of NK cells to IAV immunity, NK cell depletion prior to IAV infection did not result in a significant change in morbidity or mortality. Interestingly, NK cell depletion resulted in a significantly greater number of CD4 T cells in the IAV infected lung. Further, both CD4 and CD8 T cells in NK-depleted mice showed increased IFN-Γ production. Finally, while not statistically significant, NK cell depletion resulted in a trend toward greater protection from heterosubtypic IAV challenge infections. Taken together these results suggest that NK cells may either regulate the adaptive immune response to IAV infection through suppression of CD4 and CD8 T cells, or that the T cell response to IAV infection is able to compensate for the loss of NK cells. Moreover, while NK cell suppression of T cell function during a primary IAV infection does not result in increased susceptibility to primary IAV infections, NK cell regulation of adaptive immune responses may suppress the memory T cell response, and therefore leave the host more susceptible to secondary infections.

Overall the studies presented herein demonstrate a complex role for NK cells in the immune response against viral infections. Ly49H+ NK cells directly kill MCMV-infected cells and m157-bearing targets, but NK cell activation is regulated by ligand density, as well as the ligand membrane anchor. Additionally, NK cells suppress adaptive immune responses during a primary IAV infection, resulting in changes to the T cell response during both primary and memory responses.