Liver type 1 innate lymphoid cells lacking IL-7 receptor are a native killer cell subset fostered by parenchymal niches

Role of liver-resident NK cells in liver immunity

The tolerogenic immune microenvironment of the liver (the immune system avoids attacking harmless antigens, such as antigens derived from food and gut microbiota) has garnered significant attention in recent years. Inherent immune cells in the liver play a unique role in regulating this microenvironment. Liver-resident natural killer (LrNK) cells, also known as liver type 1 innate lymphoid cells (ILC1s), are a recently discovered subset of immune cells that possess properties distinct from those of conventional NK (cNK) cells. Accumulating evidence suggests that there are significant differences between LrNK and cNK cells, with LrNK cells potentially exhibiting immunosuppressive functions in the liver. This review summarizes the latest findings on LrNK cells, focusing on their phenotype, heterogeneity, plasticity, origin, development, and the required transcription factors. In addition, immune functions of LrNK cells in various liver diseases, including liver cancer, viral infections, liver injury, and cirrhosis, were analyzed. By elucidating the role of LrNK cells in liver immunity, this review aims to enhance our understanding of the mechanisms underlying liver immunity and contribute to the improvement of liver disease treatment.

The transcription factor RORα is required for the development of type 1 innate lymphoid cells in adult bone marrow

Type 1 innate lymphoid cells (ILC1s) respond to infections and tumors by producing IFN-γ. Although RAR-related orphan receptor α (RORα) is required for ILC2s and some ILC3s, its role in ILC1 development remains controversial. To investigate the role of RORα in ILC1s, we analyzed bone marrow (BM) chimeras of RORα-deficient mice. ILC1s derived from RORα-deficient BM cells were significantly reduced in various tissues, including the intestine, indicating a hematopoietic cell-intrinsic need for RORα in ILC1 development. Developmental stage-specific RORα-deficient mice showed a decrease in adult liver and BM IL-7R+ ILC1s and an increase in BM NK cells, whereas fetal liver ILC1s and adult liver IL-7R- ILC1s remained unchanged. Furthermore, RORα is primarily required for IL-7R+ precursor stages and partially affects small intestine ILC1 after differentiation. This study suggests that RORα promotes ILC1 differentiation while suppressing NK cell differentiation at the ILC precursor stage in the adult BM.

GPR34 is a metabolic immune checkpoint for ILC1-mediated antitumor immunity

Type 1 innate lymphoid cells (ILC1s) are a class of tissue-resident cells with antitumor activity, suggesting its possible role in solid tumor immune surveillance, but it is not clear whether manipulating ILC1s can induce potent antitumor immune responses. Here, we found that G-protein-coupled receptor 34 (GPR34), a receptor for lysophosphatidylserine (LysoPS), was highly expressed on ILC1s but not on conventional natural killer cells in the tumor microenvironment. LysoPS was enriched in the tumor microenvironment and could inhibit ILC1 activation via GPR34. Genetic deletion of LysoPS synthase Abhd16a expression in tumors or Gpr34 expression in ILC1s or antagonizing GPR34 enhanced ILC1 antitumor activity. In individuals with cancer, ABHD16A expression in tumors or GPR34 expression in ILC1s was inversely correlated with the antitumor activity of ILC1s or ILC1-like cells. Thus, our results demonstrate that manipulating ILC1s can induce potent antitumor immunity, and GPR34 is a metabolic immune checkpoint that can be targeted to develop ILC1-based immunotherapy.

IFNγ initiates TLR9-dependent autoimmune hepatitis in DNase II deficient mice

Patients with biallelic hypomorphic mutation in DNASE2 develop systemic autoinflammation and early-onset liver fibrosis. Prior studies showed that Dnase2 -/- Ifnar -/- double knockout (DKO) mice develop Type I IFN-independent liver inflammation, but immune mechanisms were unclear. We now show that DKO mice recapitulate many features of human autoimmune hepatitis (AIH), including periportal and interstitial inflammation and fibrosis and elevated ALT. Infiltrating cells include CD8+ tissue resident memory T cells, type I innate lymphoid cells, and inflammatory monocyte/macrophage cells that replace the Kupffer cell pool. Importantly, TLR9 expression by bone marrow-derived cells is required for the the development of AIH. TLR9 is highly expressed by inflammatory myeloid cells but not long-lived Kupffer cells. Furthermore, the initial recruitment of TLR9 expressing monocytes and subsequent activation of lymphocytes requires IFNγ signaling. These findings highlight a critical role of feed forward loop between TLR9 expressing monocyte-lineage cells and IFNg producing lymphocytes in autoimmune hepatitis.

Immunoregulatory natural killer cells

This review discusses a broader scope of functional roles for NK cells. Despite the well-known cytolytic and inflammatory roles of NK cells against tumors and pathogenic diseases, extensive evidence demonstrates certain subsets of NK cells have defacto immunoregulatory effects and have a role in inducing anergy or lysis of antigen-activated T cells and regulating several autoimmune diseases. Furthermore, recent evidence suggests certain subsets of immunoregulatory NK cells can cause anergy or lysis of antigen-activated T cells to regulate hyperinflammatory diseases, including multisystem inflammatory syndrome. Several pathogens induce T cell and NK cell exhaustion and/or suppression, which impair the immune system's control of the replication speed of virulent pathogens and tumors and result in extensive antigens and antigen-antibody immune complexes, potentially inducing to some extent a Type III hypersensitivity immune reaction. The Type III hypersensitivity immune reaction induces immune cell secretion of proteinases, which can cleave specific proteins to create autoantigens which activate T cells to initiate autoimmune and/or hyperinflammatory diseases. Furthermore, pathogen induced NK cell exhaustion and/or suppression will inhibit NK cells which would have induced the anergy or lysis of activated T cells to regulate autoimmune and hyperinflammatory diseases. Autoimmune and hyperinflammatory diseases can be consequences of the dual lymphocyte exhaustion and/or suppression effects during infections, by creating autoimmune and/or hyperinflammatory diseases, while also impairing immunoregulatory lymphocytes which otherwise would have regulated these diseases.

Group 1 ILCs: Heterogeneity, plasticity, and transcriptional regulation

Group 1 innate lymphoid cells (ILCs), comprising ILC1s and natural killer cells (NK cells), belong to a large family of developmentally related innate lymphoid cells that lack rearranged antigen-specific receptors. NK cells and ILC1s both require the transcription factor T-bet for lineage commitment but additionally rely on Eomes and Hobit, respectively, for their development and effector maturation programs. Both ILC1s and NK cells are essential for rapid responses against infections and mediate cancer immunity through production of effector cytokines and cytotoxicity mediators. ILC1s are enriched in tissues and hence generally considered tissue resident cells whereas NK cells are often considered circulatory. Despite being deemed different cell types, ILC1s and NK cells share many common features both phenotypically and functionally. Recent studies employing single cell RNA sequencing (scRNA-seq) technology have exposed previously unappreciated heterogeneity in group 1 ILCs and further broaden our understanding of these cells. Findings from these studies imply that ILC1s in different tissues and organs share a common signature but exhibit some unique characteristics, possibly stemming from tissue imprinting. Also, data from recent fate mapping studies employing Hobit, RORγt, and polychromic reporter mice have greatly advanced our understanding of the developmental and effector maturation programs of these cells. In this review, we aim to outline the fundamental traits of mouse group 1 ILCs and explore recent discoveries related to their developmental programs, phenotypic heterogeneity, plasticity, and transcriptional regulation.

Emerging roles of type 1 innate lymphoid cells in tumour pathogenesis and cancer immunotherapy

Innate lymphoid cells (ILCs) are the most recently discovered class of innate immune cells found to have prominent roles in various human immune-related pathologies such as infection and autoimmune diseases. However, their role in cancer was largely unclear until recently, where several emerging studies over the past few years unanimously demonstrate ILCs to be critical players in tumour immunity. Being the innate counterpart of T cells, ILCs are potent cytokine producers through which they orchestrate the overall immune response upstream of adaptive immunity thereby modulating T cell function. Out of the major ILC subsets, ILC1s have gained significant traction as potential immunotherapeutic candidates due to their central involvement with the anti-tumour type 1 immune response. ILC1s are potent producers of the well-established anti-tumour cytokine interferon γ (IFNγ), and exert direct cytotoxicity against cancer cells in response to the cytokine interleukin-15 (IL-15). However, in advanced diseases, ILC1s are found to demonstrate an exhausted phenotype in the tumour microenvironment (TME) with impaired effector functions, characterised by decreased responsiveness to cytokines and reduced IFNγ production. Tumour cells produce immunomodulatory cytokines such as transforming growth factor β (TGFβ) and IL-23, and through these suppress ILC1 anti-tumour actfivities and converts ILC1s to pro-tumoural ILC3s respectively, resulting in disease progression. This review provides a comprehensive overview of ILC1s in tumour immunity, and discusses the exciting prospects of harnessing ILC1s for cancer immunotherapy, either alone or in combination with cytokine-based treatment. The exciting prospects of targeting the upstream innate immune system through ILC1s may surmount the limitations associated with adaptive immune T cell-based strategies used in the clinic currently, and overcome cancer immunotherapeutic resistance.

Control of the Development, Distribution, and Function of Innate-Like Lymphocytes and Innate Lymphoid Cells by the Tissue Microenvironment

Recently, considerable attention has been directed toward innate-like T cells (ITCs) and innate lymphoid cells (ILCs) owing to their indispensable contributions to immune responses, tissue homeostasis, and inflammation. Innate-like T cells include NKT cells, MAIT cells, and γδ T cells, whereas ILCs include NK cells, type 1 ILCs (ILC1s), type 2 ILCs (ILC2s), and type 3 ILCs (ILC3s). Many of these ITCs and ILCs are distributed to specific tissues and remain tissue-resident, while others, such as NK cells and some γδ T cells, circulate through the bloodstream. Nevertheless, recent research has shed light on novel subsets of innate immune cells that exhibit characteristics intermediate between tissue-resident and circulating states under normal and pathological conditions. The local microenvironment frequently influences the development, distribution, and function of these innate immune cells. This review aims to consolidate the current knowledge on the functional heterogeneity of ITCs and ILCs, shaped by local environmental cues, with particular emphasis on IL-15, which governs the activities of the innate immune cells involved in type 1 immune responses.