A new checkpoint for Natural Killer cell activation

Natural Killer (NK) cells are innate lymphocytes with spontaneous antitumor and anti-viral effector functions. As such, there is intense interest in understanding how these functions are regulated in order to devise novel therapeutic strategies to harness NK cell function in disease. In a recent issue of Nature, a team led by Alberto Mantovani and Cecilia Garlanda have identified a new “checkpoint” or negative regulator of NK cell function known as IL-1R8. Their data demonstrate that IL-1R8 expression by NK cells dampens their response to the inflammatory cytokine IL-18, and that genetic deletion of Il1r8 unleashes NK cell dependent resistance to hepatic carcinoma, tumor metastasis and cytomegalovirus infection. If translated to humans, therapeutic inhibition of IL-1R8 could act as a novel NK cell checkpoint inhibitor in cancer immunotherapy. IL-1R8, also known as TIR8 or SIGIRR, acts as a negative regulator of interleukin-1 receptor family members (IL1Rs) and toll-like receptors (TLRs) to suppress the inflammatory responses mediated via ligation of these receptors. In Molgora et al., the authors focus on the expression of IL1R family members by the immune system and noted high IL-1R8 expression by NK cells compared to other immune cells. In both human and murine NK cells, IL-1R8 expression increased during maturation and was particularly elevated at the stage where NK cells gain potent cytolytic function (CD11b stage in mice and CD56CD16 stage in humans). To investigate the consequences of IL-1R8 signaling in vivo, NK cells from Il1r8 / mice were analyzed extensively. A major alteration in NK cell maturation was noted with a skewing toward more mature (KLRG1) and less immature (CD27) Il1r8 / NK cells in most organs interrogated. Irrespective of this skewing in maturation, all NK cell subsets in Il1r8 / mice displayed functional hyperresponsiveness in vitro with higher levels of intracellular IFN-c and Granzyme B following stimulation with IL-18 and IL-12 or when cocultured with bone marrow derived dendritic cells primed with CpG or LPS (which induces IL-12 and IL-18 production). NK cells lacking IL-1R8 responded to IL-18 with elevated phospho-IRAK4 and phospho-S6 compared to control NK cells, strengthening the evidence for IL-18 receptor hyper-activity. These data suggest that IL-18 signaling in vivo might contribute to NK cell differentiation and potentially reduce the threshold for NK cell activation by other stimuli. Of note, the authors did not present data on the levels of soluble IL-18 or other signaling components of the IL-18 receptor in Il1r8 / mice as this could impact on the interpretation, although the NK cell phenotype was cell intrinsic as demonstrated using bone marrow chimeras. Similarly, NK cell activation by IL-18 alone is typically weak, however, IL-18 synergizes strongly with IL-12 to drive NK cell effector functions including IFN-c production. Alterations in IL-18 signaling may also impact on IL-12 receptor expression on Il1r8 / NK cells and their responsiveness to IL-12 in the steady-state. There are confounding reports around the role of IL-18 in NK cell homeostasis. IL-18 bioavailability is also negatively regulated by IL-18 binding protein (IL-18BP), which sequesters free IL18. IL-18BP-null mice thus could be expected to present with a NK cell phenotype similar to Il1r8 / mice, however, Il18 bp / mice were recently shown to have the opposite NK cell maturation phenotype to Il1r8 / mice with an accumulation of immature (CD11b ) and reduction in mature KLRG1 NK cells. In the same study, NK cells from Il18 / mice displayed a normal NK cell maturation, consistent with our earlier study questioning the direct role of IL-18 in NK cell maturation. Consistently, both Il18 bp / and Il1r8 / mice presented with significantly elevated IFN-c levels when challenged with pathogens/tumor supporting the notion that IL-18 reduces the activation threshold and primes NK cells in vivo. Indeed, administration of IL-18BP has been shown to reduce IFN-c production from NK Correspondence Nicholas D Huntington, Department of Medical Biology, University of Melbourne, Vic. 3050 Australia. E-mail: huntington@wehi.edu. au