The IRE1 endoplasmic reticulum stress sensor activates natural killer cell immunity and promotes natural killer cell memory by regulating c-Myc and mitochondrial respiration

Natural killer (NK) cells are critical mediators of host immunity to pathogens. Though historically being considered as a part of the innate immune system, NK cells exhibit adaptive features and can ‘remember’ stimuli like inflammation or viral infection and hence are capable of responding more robustly upon re-encountering the stimulation. Successful design of an NK cell-based therapy would ideally rely on harnessing NK cell memory, the key drivers and regulators of which remain poorly understood. Here, we demonstrate that the endoplasmic reticulum stress sensor IRE1α and its substrate transcription factor XBP1 drive NK cell responses against viral infection and tumors in vivo. IRE1α-XBP1 were essential for expansion of activated mouse and human NK cells and are situated downstream of the mTOR pathway. Transcriptome and chromatin immunoprecipitation analysis revealed c-Myc as a new and direct downstream target of XBP1 for regulation of NK cell proliferation. NK cells with c-Myc haploinsufficency phenocopied IRE1α deficiency, while c-Myc overexpression largely rescued the proliferation defect in IRE1α−/− NK cells. Like c-Myc, IRE1α also promotes oxidative phosphorylation in NK cells. Combined, our study identifies an IRE1α-XBP1-cMyc axis in NK cell immunity; and based on this work, we are currently investigating the role of IRE1 in NK cell memory using novel transgenic mouse models and small molecule drug treatment in primary human cells. We are also evaluating the therapeutic benefit of manipulating IRE1 function in CAR-NK cells to treat cancer. Our ongoing study should advance the current knowledge of NK cell memory and also open new opportunities in cancer therapy for patients with otherwise extremely poor prognoses.

MicroRNA-21 is a critical regulator of the antiviral NK cell response

Natural killer (NK) cells are innate lymphocytes that possess adaptive features, including antigen-specific clonal expansion and memory responses. Global loss of microRNAs (miRNAs) leads to defects in NK cell homeostasis and antiviral responses; however, the role of individual miRNAs in these processes is poorly defined. Here, we investigated miR-21, a highly expressed miRNA in both human and mouse NK cells. While miR-21 was largely dispensable for NK cell development, miR-21-deficient NK cells failed to mediate significant host protection against mouse cytomegalovirus (MCMV) due to defective proliferation. Although activated NK cells upregulated miR-21 early after MCMV exposure, loss of miR-21 did not impact NK cell activation or effector molecule expression during viral infection. Transcriptome analysis of miR-21-deficient NK cells demonstrated dysregulation of cell cycle regulators, including several predicted targets of miR-21. Altogether, our findings demonstrate a novel role for miR-21 as a crucial regulator of adaptive NK cell responses during MCMV infection.

Adipose-resident group 1 innate lymphoid cells promote obesity-associated insulin resistance

Innate lymphoid cells (ILCs) are the most recently identified arm of the innate immune system that function to protect epithelial barriers against pathogens and maintain tissue homeostasis. While ILCs can also promote pathology at these mucosal sites, it remains unknown whether aberrant activation of tissue-resident ILCs can contribute to disease in non-barrier tissues. Here, we identify a subset of long-term adipose-resident ILC1 that are dependent on the transcription factors Nfil3 and T-bet, but phenotypically and functionally distinct from circulating mature natural killer (NK) cells. Diet-induced obesity drove early proinflammatory cytokine production in adipose tissue depots, leading to selective proliferation and accumulation of adipose-resident ILC1s. ILC1-derived IFN-γ was both necessary and sufficient to drive proinflammatory macrophage polarization to promote obesity-associated insulin resistance. Therefore, these findings suggest that adipose-resident ILC1s contribute to obesity-associated pathology in response to dysregulated local proinflammatory cytokine production.

The transcription factor IRF8 regulates natural killer cell development and response to cytomegalovirus infection

Interferon regulatory factor 8 (IRF8) is a transcription factor critical for specifying monocyte and dendritic cell lineage fate and for directing B cell development, yet its function in natural killer (NK) cells is not understood. NK cells are innate lymphocytes that develop in the bone marrow and are capable of antigen-specific clonal proliferation. We observe in mixed bone marrow chimeric mice that despite a prevalent NK progenitor (NKP) population compared to wild-type, peripheral Irf8−/− NK cells exhibit proper maturation and full functionality. However, using an adoptive transfer system, we demonstrate that IRF8 is indispensable for robust expansion of NK cells in response to mouse cytomegalovirus (MCMV). Consistent with this finding, Ly49H+ NK cells upregulate Irf8 transcript during the early stages of MCMV infection. Furthermore, the Irf8 locus is dynamically regulated upon activation, showing extensive STAT4 binding at promoter and enhancer regions, and upregulation of the activating epigenetic marks H3K4me3 and H3K4Ac27. Thus, our work elucidates the functional contribution of IRF8 during NK cell development and antiviral responses.