Natural killer cells: NK cells stroll down the memory lane

HIF1α is required for NK cell metabolic adaptation during virus infection

Natural killer (NK) cells are essential for early protection against virus infection and must metabolically adapt to the energy demands of activation. Here, we found upregulation of the metabolic adaptor hypoxia-inducible factor-1α (HIF1α) is a feature of mouse NK cells during murine cytomegalovirus (MCMV) infection in vivo. HIF1α-deficient NK cells failed to control viral load, causing increased morbidity. No defects were found in effector functions of HIF1αKO NK cells; however, their numbers were significantly reduced. Loss of HIF1α did not affect NK cell proliferation during in vivo infection and in vitro cytokine stimulation. Instead, we found that HIF1α-deficient NK cells showed increased expression of the pro-apoptotic protein Bim and glucose metabolism was impaired during cytokine stimulation in vitro. Similarly, during MCMV infection HIF1α-deficient NK cells upregulated Bim and had increased caspase activity. Thus, NK cells require HIF1α-dependent metabolic functions to repress Bim expression and sustain cell numbers for an optimal virus response.

Natural killer cells: multifaceted players with key roles in hepatitis C immunity

Natural killer cells (NKs) are involved in every stage of hepatitis C viral (HCV) infection, from protection against HCV acquisition and resolution in the acute phase to treatment-induced clearance. In addition to their direct antiviral actions, NKs are involved in the induction and priming of appropriate downstream T-cell responses. In the setting of chronic HCV, overall NK cell levels are decreased, subset distribution is altered, and changes in NK receptor (NKR) expression have been demonstrated, although the contribution of individual NKRs to viral clearance or persistence remains to be clarified. Enhanced NK cell cytotoxicity accompanied by insufficient interferon-γ production may promote liver damage in the setting of chronic infection. Treatment-induced clearance is associated with activation of NK cells, and it will be of interest to monitor NK cell responses to triple therapy. Activated NK cells also have anti-fibrotic properties, and the same hepatic NK cell populations that are actively involved in control of HCV may also be involved in control of HCV-associated liver damage. We still have much to learn, in particular: how do liver-derived NKs influence the outcome of HCV infection? Do NK receptors recognize HCV-specific components? And, are HCV-specific memory NK populations generated?

The interaction between CD300a and phosphatidylserine inhibits tumor cell killing by NK cells

The activity of NK cells is controlled by inhibitory and activating receptors. The inhibitory receptors interact mostly with MHC class I proteins, however, inhibitory receptors such as CD300a, which bind to non-MHC class I ligands, also exist. Recently, it was discovered that phosphatidylserine (PS) is a ligand for CD300a and that the interaction between PS expressed on apoptotic cells and CD300a inhibits the uptake of apoptotic cells by phagocytic cells. Whether PS can inhibit NK-cell activity through CD300a is unknown. Here, we have generated specific antibodies directed against CD300a and we used these mAbs to demonstrate that various NK-cell clones express different levels of CD300a. We further demonstrated that both CD300a and its highly homologous molecule CD300c bind to the tumor cells equally well and that they recognize PS and additional unknown ligand(s) expressed by tumor cells. Finally, we showed that blocking the PS-CD300a interaction resulted in increased NK-cell killing of tumor cells. Collectively, we demonstrate a new tumor immune evasion mechanism that is mediated through the interaction between PS and CD300a and we suggest that CD300c, similarly to CD300a, also interacts with PS.