Activated group 3 innate lymphoid cells promote T-cell-mediated immune responses

PD1+ innate lymphoid cells 3 predict JAK-dependent inflammation in rheumatoid arthritis

Innate lymphoid cells (ILCs) play a key role in maintaining immune homeostasis and are linked to inflammation and autoimmunity. This study investigates the role of ILCs in the pathogenesis of rheumatoid arthritis (RA) and their response to two targeted therapies - JAK inhibitors (JAKi), which block critical signaling pathways required for their activation, and TNF inhibitors (TNFi), which target a key inflammatory mediator - offering insights into how these interventions shape ILC-driven inflammation. ILC distribution correlated with RA activity, as indicated by the DAS28 score, and this imbalance was improved significantly within four weeks of JAKi, underscoring its early therapeutic impact on ILC-mediated inflammation. While levels of ILC3-activating cytokines such as IL-1β and IL-23 declined under JAKi therapy, they remained unchanged with TNFi. Although JAKi and TNFi showed similar treatment efficacy, multivariate regression analysis showed that improvement in DAS28 score was strongly associated with increase in CTLA-4+ILC3 and reduction in both PD1+ILC3 frequency and systemic IL12p40/IL-23 levels only with JAKi. Notably, this two ILC3 subtypes determined the DAS28 score after 50 d of JAKi. In contrast, patients showing limited response to JAKi (ΔDAS28 < 1.2) maintained high systemic IL-18 levels, a cytokine that induces signaling independent of the JAK pathway, suggesting a potential resistance mechanism. These findings highlight that monitoring PD1+ILC3s or IL-12p40/IL-23 may serve as an indicator of JAKi responsiveness, while elevated IL-18 may identify patients benefiting from alternative therapies. These results also emphasize the clinical relevance of targeting innate immunity for more personalized, pathway-focused RA therapies.

RORγt-expressing dendritic cells are functionally versatile and evolutionarily conserved antigen-presenting cells

Conventional dendritic cells (cDCs) are potent antigen-presenting cells (APCs) that integrate signals from their environment allowing them to direct situation-adapted immunity. Thereby they harbor great potential for being targeted in vaccination, autoimmunity, and cancer. Here, we use fate mapping, functional analyses, and comparative cross-species transcriptomics to show that RORγt+ DCs are a conserved, functionally versatile, and transcriptionally distinct type of DCs. RORγt+ DCs entail various populations described in different contexts including Janus cells/RORγt-expressing extrathymic Aire-expressing cells (eTACs), subtypes of Thetis cells, RORγt+-DC (R-DC) like cells, cDC2C and ACY3+ DCs. We show that in response to inflammatory triggers, RORγt+ DCs can migrate to lymph nodes and in the spleen can activate naïve CD4+ T cells. These findings expand the functional repertoire of RORγt+ DCs beyond the known role of eTACs and Thetis cells in inducing T cell tolerance to self-antigens and intestinal microbes in mice. We further show that RORγt+ DCs with proinflammatory features accumulate in autoimmune neuroinflammation in mice and men. Thus, our work establishes RORγt+ DCs as immune sentinel cells that exhibit a broad functional spectrum ranging from inducing peripheral T cell tolerance to T cell activation depending on signals they integrate from their environment.

Role of innate immune cells in multiple sclerosis

Multiple sclerosis (MS) is a chronic autoimmune, inflammatory and neurodegenerative disease affecting the central nervous system (CNS). MS is associated with a complex interplay between neurodegenerative and inflammatory processes, mostly attributed to pathogenic T and B cells. However, a growing body of preclinical and clinical evidence indicates that innate immunity plays a crucial role in MS promotion and progression. Accordingly, preclinical and clinical studies targeting different innate immune cells to control MS are currently under study, highlighting the importance of innate immunity in this pathology. Here, we reviewed recent findings regarding the role played by innate immune cells in the pathogenesis of MS. Additionally, we discuss potential new treatments for MS based on targets against innate immune components.

Circulating subpopulations of non-cytotoxic ILCs in diffuse large B-cell lymphoma

Non-cytotoxic innate lymphoid cells (ILCs) have been added to the list of immune cells that may contribute to the tumor microenvironment. Elevated levels of total ILCs and their subgroups have been reported in peripheral blood and tissue samples from patients with solid tumors, but their frequency in non-Hodgkin lymphomas, particularly diffuse large B-cell lymphoma (DLBCL), has not been clearly established. This study examined frequency and subset distribution in newly diagnosed DLBCL patients (nodal and extra-nodal) and compared it with blood specimens from healthy donors. The percentage of total ILCs (Lin - CD127+) was assessed by flow cytometry, as well as the four ILC subsets, defined as ILC1 (Lin - CD127 + cKit - CRTH2-), ILC2 (Lin - CD127 + cKit+/- CRTH2+), ILCp NCR- (Lin - CD127 + cKit + CRTH2- NKp46-) and NCR + ILC3 (Lin - CD127 + cKit + NKp46+). In the studied group of patients (n = 54), significantly lower levels of circulating total ILCs, ILC1, and ILCp NCR- were observed compared to the control group (n = 43). Similarly, there was a statistically significant decrease in the median frequency of NKp46 + ILC3 cells in lymphoma patients. Analysis of the ILC2 subpopulation showed no significant differences. The correlation of the distribution of individual subpopulations of ILCs with the stage and location of the tumor was also demonstrated. Our results suggest that circulating ILCs are activated and differentiated and/or differentially recruited to the lymph nodes or tumor microenvironment where they may be involved in antitumor defense. However, our observations require confirmation in functional studies.

ANDROGENS PROTECT ILC2S FROM FUNCTIONAL SUPPRESSION DURING INFLUENZA VIRUS INFECTION

Biological sex differences in morbidity upon influenza A virus (IAV) infection are linked to stronger IFN-centered immune responses in females, yet the regulatory role of sex hormone receptors in immune cell subsets is incompletely understood. Lung-resident group 2 innate lymphoid cells (ILC2s) express notably high levels of androgen receptors (AR). In IAV infection, ILC2s produce type 2 cytokines and facilitate tissue repair, but they also may be functionally suppressed by type 1 cytokines. Here we report sex differences in the magnitude of lung ILC2 functional suppression at the peak of sublethal IAV infection. Relative to males, ILC2s in females show attenuated proliferation, decreased propensity for IL-5 and amphiregulin production and reduced expression of GATA3 and IL-33R, features supported by divergent transcriptomes. Equivalent inflammatory cytokine levels and viral load suggested sex differences in ILC2-intrinsic factors. Indeed, naïve female ILC2s showed elevated IFNGR expression and higher phospho-STAT1 levels following IFNγ stimulation, and lymphocyte-restricted STAT1 deficiency reversed IAV-induced suppression of female ILC2s. Lymphocyte-restricted AR deficiency or loss of androgens via orchiectomy led to increased IFNGR expression and suppression of male ILC2s. These data support the hypothesis that intrinsic AR activity regulates IFNGR-STAT1 signaling pathways to preserve canonical ILC2 function in males during IAV infection.

MHC class II of different non-professional antigen-presenting cells mediate multiple effects of crosstalk with CD4+T cells in lung diseases

The respiratory system is continuously exposed to the outside world, making it vulnerable to airborne particles and harmful pathogens like bacteria and viruses that can enter through breathing. Antigen presenting cells (APCs) have a vital function in the innate immune response as they present antigens to T cells and initiate the response of adaptive immune cells. Professional APCs engulf foreign microorganisms and display their peptides to T lymphocytes using MHC molecules. MHC II on their cell surface and potentially present antigen to CD4+T cells. Furthermore, various other types of cells have similar function that can also serve as APCs by expressing MHC II, thus impacting the progression of lung diseases, such as alveolar epithelial cells (AECs), endothelial cells (ECs), fibroblasts, innate lymphoid cells (ILCs), eosinophils, interstitial cells, mast cells, etc. express MHC II and present antigen. The non-professional APCs type and the extra signals it provides have a direct impact on CD4+T cell programming and downstream effector mechanisms. Here, we summarize the existing research on the expression of MHC II on non-professional APCs in different lung diseases and its influence on CD4+T differentiation types and disease outcomes, in order to further clarify the role of MHC II of different non-professional APCs in lung diseases, such as asthma, chronic obstructive pulmonary disease (COPD), etc.

Emerging roles of astrocytes as immune effectors in the central nervous system

The astrocyte, a major glial cell type in the central nervous system (CNS), is widely regarded as a functionally diverse mediator of homeostasis. During development and throughout adulthood, astrocytes have essential roles, such as providing neuron metabolic support, modulating synaptic function, and maintaining the blood-brain barrier (BBB). Recent evidence continues to underscore their functional heterogeneity and importance for CNS maintenance, as well as how these cells ensure optimal CNS and immune responses to disease, acute trauma, and infection. Advances in our understanding of neuroimmune interactions complement our knowledge of astrocyte functional heterogeneity, where astrocytes are now regarded as key effectors and propagators of immune signaling. This shift in perspective highlights the role of astrocytes not merely as support cells, but as active participants in CNS immune responses.

How underappreciated autoinflammatory (innate immunity) mechanisms dominate disparate autoimmune disorders

Historically inflammation against self was considered autoimmune which stems back to the seminal observations by Ehrlich who described serum factors, now known to be autoantibodies produced by B lineage cells that mediate "horror autotoxicus". The 20th century elucidation of B- and T-cell adaptive immune responses cemented the understanding of the key role of adaptive immune responses in mediating pathology against self. However, Mechnikov shared the Nobel Prize for the discovery of phagocytosis, the most rudimentary aspect of innate immunity. Fast forward some 100 years and an immunogenetic understanding of innate immunity led to the categorising of innate immunopathology under the umbrella term 'auto inflammation' and terminology such as "horror autoinflammaticus" to highlight the schism from the classical adaptive immune understanding of autoimmunity. These concepts lead to calls for a two-tiered classification of inflammation against self, but just as innate and adaptive immunity are functionally integrated, so is immunopathology in many settings and the concept of an autoimmune to autoinflammation continuum emerged with overlaps between both. Herein we describe several historically designated disorders of adaptive immunity where innate immunity is key, including rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), systemic juvenile idiopathic arthritis (sJIA) and adult-onset Still's disease (AOSD) where the immunopathology phenotype is strongly linked to major histocompatibility complex (MHC) class II associations and responds to drugs that target T-cells. We also consider MHC-I-opathies including psoriasis and Behcet's disease(BD) that are increasingly viewed as archetype CD8 T-cell related disorders. We also briefly review the key role of barrier dysfunction in eczema and ulcerative colitis (UC) where innate tissue permeability barrier dysfunction and microbial dysbiosis contributes to prominent adaptive immune pathological mechanisms. We also highlight the emerging roles of intermediate populations of lymphocytes including gamma delta (γδ) and mucosal-associated invariant T (MAIT) cells that represent a blend of adaptive immune plasticity and innate immune rapid responders that may also determine site specific patterns of inflammation.

The crosstalk between ILC3s and adaptive immunity in diseases

RORγt+ group 3 innate lymphoid cells (ILC3s), the innate counterpart of Th17 cells, are enriched in the mucosal area and lymphoid tissues. ILC3s interact with a variety of cells through their effector molecules and play an important role in the host defense against a spectrum of infections. Recent studies suggest that the extensive crosstalk between ILC3s and adaptive immune cells, especially T cells, is essential for maintaining tissue homeostasis. Here we discuss recent advances in the crosstalk between ILC3s and adaptive immune responses in multiple tissues and diseases. Understanding how ILC3s engage with adaptive immune cells will enhance our comprehension of diseases and facilitate the identification of novel therapeutic targets.

Integrative transcriptome analysis reveals the molecular events underlying impaired T-cell responses in EGFR-mutant lung cancer

EGFR mutations are critical oncogenic drivers in lung adenocarcinoma (LUAD). However, the mechanisms by which they impact the tumor microenvironment (TME) and tumor immunity are unclear. Furthermore, the reasons underlying the poor response of EGFR-mutant (EGFR-MU) LUADs to immunotherapy with PD-1/PD-L1 inhibitors are unknown. Utilizing single-cell RNA (sc-RNA) and bulk RNA sequencing datasets, we conducted high-dimensional weighted gene coexpression network analysis to identify key genes and immune-related pathways contributing to the immunosuppressive TME. EGFR-MU cancer cells downregulated MHC class I genes to evade CD8+ cytotoxic T cells, expressed substantial levels of MHC class II molecules, and engaged with CD4+ regulatory T cells (Tregs). EGFR-MU tumors may recruit Tregs primarily through the CCL17/CCL22/CCR4 axis, leading to a Treg-enriched TME. High levels of MHC class II-positive cancer-associated fibroblasts and tumor endothelial cells were found within EGFR-MU tumors. Owing to the absence of costimulatory factors, they may inhibit rather than activate the tumor antigen-specific CD4+ T-cell response, contributing further to immune suppression. Multiplex immunohistochemistry analyses in a LUAD cohort confirmed increased expression of MHC class II molecules in cancer cells and fibroblasts in EGFR-MU tumors. Our research elucidates the highly immunosuppressive TME in EGFR-MU LUAD and suggests potential targets for effective immunotherapy.

IL-21 conditions antigen-presenting human γδ T-cells to promote IL-10 expression in naïve and memory CD4+ T-cells

Direct interaction between T-cells exerts a major influence on tissue immunity and inflammation across multiple body sites including the human gut, which is highly enriched in 'unconventional' lymphocytes such as γδ T-cells. We previously reported that microbial activation of human Vγ9/Vδ2+ γδ T-cells in the presence of the mucosal damage-associated cytokine IL-15 confers the ability to promote epithelial barrier defence, specifically via induction of IL-22 expression in conventional CD4+ T-cells. In the current report, we assessed whether other cytokines enriched in the gut milieu also functionally influence microbe-responsive Vγ9/Vδ2 T-cells. When cultured in the presence of IL-21, Vγ9/Vδ2 T-cells acquired the ability to induce expression of the immunoregulatory cytokine IL-10 in both naïve and memory CD4+ T-cells, at levels surpassing those induced by monocytes or monocyte-derived DCs. These findings identify an unexpected influence of IL-21 on Vγ9/Vδ2 T-cell modulation of CD4+ T-cell responses. Further analyses suggested a possible role for CD30L and/or CD40L reverse signalling in mediating IL-10 induction by IL-21 conditioned Vγ9/Vδ2 T-cells. Our findings indicate that the local microenvironment exerts a profound influence on Vγ9/Vδ2 T-cell responses to microbial challenge, leading to induction of distinct functional profiles among CD4+ T-cells that may influence inflammatory events at mucosal surfaces. Targeting these novel pathways may offer therapeutic benefit in disorders such as inflammatory bowel disease.

Lung type 3 innate lymphoid cells respond early following Mycobacterium tuberculosis infection

Tuberculosis is the leading cause of death due to an infectious disease worldwide. Innate lymphoid type 3 cells (ILC3s) mediate early protection during Mycobacterium tuberculosis (Mtb) infection. However, the early signaling mechanisms that govern ILC3 activation or recruitment within the lung during Mtb infection are unclear. scRNA-seq analysis of Mtb-infected mouse lung innate lymphoid cells (ILCs) has revealed the presence of different clusters of ILC populations, suggesting heterogeneity. Using mouse models, we show that during Mtb infection, interleukin-1 receptor (IL-1R) signaling on epithelial cells drives ILC3 expansion and regulates ILC3 accumulation in the lung. Furthermore, our data revealed that C-X-C motif chemokine receptor 5 (CXCR5) signaling plays a crucial role in ILC3 recruitment from periphery during Mtb infection. Our study thus establishes the early responses that drive ILC3 accumulation during Mtb infection and points to ILC3s as a potential vaccine target.

IMPORTANCE

Tuberculosis is a leading cause of death due to a single infectious agent accounting for 1.6 million deaths each year. In our study, we determined the role of type 3 innate lymphoid cells in early immune events necessary for achieving protection during Mtb infection. Our study reveals distinct clusters of ILC2, ILC3, and ILC3/ILC1-like cells in Mtb infection. Moreover, our study reveal that IL-1R signaling on lung type 2 epithelial cells plays a key role in lung ILC3 accumulation during Mtb infection. CXCR5 on ILC3s is involved in ILC3 homing from periphery during Mtb infection. Thus, our study provides novel insights into the early immune mechanisms governed by innate lymphoid cells that can be targeted for potential vaccine-induced protection.

Are innate lymphoid cells friend or foe in human pregnancy?

Innate lymphoid cells (ILCs) are involved in the innate immune system because they lack specific antigen receptors and lineage markers. ILCs also display phenotypic and characteristic features of adaptive immune cells. Therefore, ILCs are functional in essential interactions between adaptive and innate immunity. ILCs are found in both lymphoid and nonlymphoid tissues and migrate to the area of inflammation during the inflammatory process. ILCs respond to pathogens by producing a variety of cytokines and are involved in the barrier defense of antigens and in many immunological processes such as allergic events. Recent research has shown that ILCs are functional during human pregnancy and have been suggested to be essential for the healthy progression of pregnancy. In this review, we focus on the role of ILCs in human pregnancy by discussing the relationship between ILCs and the pregnancy microenvironment, specifically summarizing the role of ILCs in physiological and pathological pregnancies.

Understanding Type 3 Innate Lymphoid Cells and Crosstalk with the Microbiota: A Skin Connection

Innate lymphoid cells (ILCs) are a diverse population of lymphocytes classified into natural killer (NK) cells, ILC1s, ILC2s, ILC3s, and ILCregs, broadly following the cytokine secretion and transcription factor profiles of classical T cell subsets. Nonetheless, the ILC lineage does not have rearranged antigen-specific receptors and possesses distinct characteristics. ILCs are found in barrier tissues such as the skin, lungs, and intestines, where they play a role between acquired immune cells and myeloid cells. Within the skin, ILCs are activated by the microbiota and, in turn, may influence the microbiome composition and modulate immune function through cytokine secretion or direct cellular interactions. In particular, ILC3s provide epithelial protection against extracellular bacteria. However, the mechanism by which these cells modulate skin health and homeostasis in response to microbiome changes is unclear. To better understand how ILC3s function against microbiota perturbations in the skin, we propose a role for these cells in response to Cutibacterium acnes, a predominant commensal bacterium linked to the inflammatory skin condition, acne vulgaris. In this article, we review current evidence describing the role of ILC3s in the skin and suggest functional roles by drawing parallels with ILC3s from other organs. We emphasize the limited understanding and knowledge gaps of ILC3s in the skin and discuss the potential impact of ILC3-microbiota crosstalk in select skin diseases. Exploring the dialogue between the microbiota and ILC3s may lead to novel strategies to ameliorate skin immunity.

Microbiota and plasticity of antigen-presenting ILC3s: impact on antitumor immune response

Growing evidence highlights the pivotal role of RORγt-innate lymphoid cells (ILCs) in the establishment of antitumor immune response and in enhancing tumor sensitivity to immunotherapy. Noteworthy, type 3 ILCs (ILC3s) have been recently acknowledged as an important class of antigen-presenting cells (APCs) in the context of host-microorganism interactions shaping the adaptive immune response in the intestinal mucosa. Although a broad range of mouse models has led to significant progress in untangling the role of ILC3s as APCs, the outcome of major histocompatibility complex (MHC)-dependent ILC-T cell crosstalk in colorectal cancer (CRC) remains underexplored in human. Moreover, expression of MHCII is confined to ILC3 subset, endowed with lymphoid tissue-inducing properties, that adopts tissue-specific fates and functions. Intestinal microbiota could dictate the plasticity of antigen-presenting ILC3s and we here summarize our current understanding of the functions of these cells in both mouse and human CRC discussing the role of microbiota as a key modulator of their tumor-suppressive activity.

The emerging family of RORγt+ antigen-presenting cells

Antigen-presenting cells (APCs) are master regulators of the immune response by directly interacting with T cells to orchestrate distinct functional outcomes. Several types of professional APC exist, including conventional dendritic cells, B cells and macrophages, and numerous other cell types have non-classical roles in antigen presentation, such as thymic epithelial cells, endothelial cells and granulocytes. Accumulating evidence indicates the presence of a new family of APCs marked by the lineage-specifying transcription factor retinoic acid receptor-related orphan receptor-γt (RORγt) and demonstrates that these APCs have key roles in shaping immunity, inflammation and tolerance, particularly in the context of host-microorganism interactions. These RORγt+ APCs include subsets of group 3 innate lymphoid cells, extrathymic autoimmune regulator-expressing cells and, potentially, other emerging populations. Here, we summarize the major findings that led to the discovery of these RORγt+ APCs and their associated functions. We discuss discordance in recent reports and identify gaps in our knowledge in this burgeoning field, which has tremendous potential to advance our understanding of fundamental immune concepts.

Disease pathogenesis and barrier functions regulated by group 3 innate lymphoid cells

The mucosal surface is in constant contact with foreign antigens and is regulated by unique mechanisms that are different from immune responses in the peripheral organs. For the last several decades, only adaptive immune cells such as helper T (Th) cells, Th1, Th2, or Th17 were targeted to study a wide variety of immune responses in the mucosal tissues. However, since their discovery, innate lymphoid cells (ILCs) have been attracting attention as a unique subset of immune cells that provide border defense with various functions and tissue specificity. ILCs are classified into different groups based on cell differentiation and functions. Group 3 innate lymphoid cells (ILC3s) are particularly in close proximity to mucosal surfaces and therefore have the opportunity to be exposed to a variety of bacteria including pathogenic bacteria. In recent years, studies have also provided much evidence that ILC3s contribute to disease pathogenesis as well as the defense of mucosal surfaces by rapidly responding to pathogens and coordinating other immune cells. As the counterpart of helper T cells, ILC3s together with other ILC subsets establish the immune balance between adaptive and innate immunity in protecting us from invasion or encounter with non-self-antigens for maintaining a complex homeostasis. In this review, we summarize recent advances in our understanding of ILCs, with a particular focus on the function of ILC3s in their involvement in bacterial infection and disease pathogenesis.

Emerging role of immune cells as drivers of pulmonary fibrosis

The pathogenesis of pulmonary fibrosis, including idiopathic pulmonary fibrosis (IPF) and other forms of interstitial lung disease, involves a complex interplay of various factors including host genetics, environmental pollutants, infection, aberrant repair and dysregulated immune responses. Highly variable clinical outcomes of some ILDs, in particular IPF, have made it difficult to identify the precise mechanisms involved in disease pathogenesis and thus the development of a specific cure or treatment to halt and reverse the decline in patient health. With the advent of in-depth molecular diagnostics, it is becoming evident that the pathogenesis of IPF is unlikely to be the same for all patients and therefore will likely require different treatment approaches. Chronic inflammation is a cardinal feature of IPF and is driven by both innate and adaptive immune responses. Inflammatory cells and activated fibroblasts secrete various pro-inflammatory cytokines and chemokines that perpetuate the inflammatory response and contribute to the recruitment and activation of more immune cells and fibroblasts. The balance between pro-inflammatory and regulatory immune cell subsets, as well as the interactions between immune cell types and resident cells within the lung microenvironment, ultimately determines the extent of fibrosis and the potential for resolution. This review examines the role of the innate and adaptive immune responses in pulmonary fibrosis, with an emphasis on IPF. The role of different immune cell types is discussed as well as novel anti-inflammatory and immunotherapy approaches currently in clinical trial or in preclinical development.

IL-23 receptor signaling licenses group 3-like innate lymphoid cells to restrict a live-attenuated oral Chlamydia vaccine in the gut

An IFNγ-susceptible mutant of Chlamydia muridarum is attenuated in pathogenicity in the genital tract and was recently licensed as an intracellular Oral vaccine vector or intrOv. Oral delivery of intrOv induces transmucosal protection in the genital tract, but intrOv itself is cleared from the gut (without shedding any infectious particles externally) by IFNγ from group 3-like innate lymphoid cells (ILC3s). We further characterized the intrOv interactions with ILC3s in the current study, since the interactions may impact both the safety and efficacy of intrOv as an oral Chlamydia vaccine. Intracolonic inoculation with intrOv induced IFNγ that in return inhibited intrOv. The intrOv-IFNγ interactions were dependent on RORγt, a signature transcriptional factor of ILC3s. Consistently, the transfer of oral intrOv-induced ILC3s from RORγt-GFP reporter mice to IFNγ-deficient mice rescued the inhibition of intrOv. Thus, IFNγ produced by intrOv-induced ILC3s is likely responsible for inhibiting intrOv, which is further supported by the observation that oral intrOv did induce significant levels of IFNγ-producing LC3s (IFNγ+ILC3s). Interestingly, IL-23 receptor knockout (IL-23R-/-) mice no longer inhibited intrOv, which was accompanied by reduced colonic IFNγ. Transfer of oral intrOv-induced ILC3s rescued the IL-23R-/- mice to inhibit intrOv, validating the dependence of ILC3s on IL-23R signaling for inhibiting intrOv. Clearly, intrOv induces intestinal IFNγ+ILC3s for its own inhibition in the gut, which is facilitated by IL-23R signaling. These findings have provided a mechanism for ensuring the safety of intrOv as an oral Chlamydia vaccine and a platform for investigating how oral intrOv induces transmucosal protection in the genital tract.

mTORC2 acts as a gatekeeper for mTORC1 deficiency-mediated impairments in ILC3 development

Group 3 innate lymphoid cells (ILC3s) are mediators of intestinal immunity and barrier function. Recent studies have investigated the role of the mammalian target of rapamycin complex (mTOR) in ILC3s, whereas the mTORC1-related mechanisms and crosstalk between mTORC1 and mTORC2 involved in regulating ILC3 homeostasis remain unknown. In this study, we found that mTORC1 but not mTORC2 was critical in ILC3 development, IL-22 production, and ILC3-mediated intestinal homeostasis. Single-cell RNA sequencing revealed that mTORC1 deficiency led to disruption of ILC3 heterogeneity, showing an increase in differentiation into ILC1-like phenotypes. Mechanistically, mTORC1 deficiency decreased the expression of NFIL3, which is a critical transcription factor responsible for ILC3 development. The activities of both mTORC1 and mTORC2 were increased in wild-type ILC3s after activation by IL-23, whereas inhibition of mTORC1 by Raptor deletion or rapamycin treatment resulted in increased mTORC2 activity. Previous studies have demonstrated that S6K, the main downstream target of mTORC1, can directly phosphorylate Rictor to dampen mTORC2 activity. Our data found that inhibition of mTORC1 activity by rapamycin reduced Rictor phosphorylation in ILC3s. Reversing the increased mTORC2 activity via heterozygous or homozygous knockout of Rictor in Raptor-deleted ILC3s resulted in severe ILC3 loss and complete susceptibility to intestinal infection in mice with mTORC1 deficiency (100% mortality). Thus, mTORC1 acts as a rheostat of ILC3 heterogeneity, and mTORC2 protects ILC3s from severe loss of cells and immune activity against intestinal infection when mTORC1 activity is diminished.

ILC3: a case of conflicted identity

Innate lymphoid cells type 3 (ILC3s) are the first line sentinels at the mucous tissues, where they contribute to the homeostatic immune response in a major way. Also, they have been increasingly appreciated as important modulators of chronic inflammatory and autoimmune responses, both locally and systemically. The proper identification of ILC3 is of utmost importance for meaningful studies on their role in immunity. Flow cytometry is the method of choice for the detection and characterization of ILC3. However, the analysis of ILC3-related papers shows inconsistency in ILC3 phenotypic definition, as different inclusion and exclusion markers are used for their identification. Here, we present these discrepancies in the phenotypic characterization of human and mouse ILC3s. We discuss the pros and cons of using various markers for ILC3 identification. Furthermore, we consider the possibilities for the efficient isolation and propagation of ILC3 from different organs and tissues for in-vitro and in-vivo studies. This paper calls upon uniformity in ILC3 definition, isolation, and propagation for the increased possibility of confluent interpretation of ILC3's role in immunity.

Microbiota-dependent regulation of costimulatory and coinhibitory pathways via innate immune sensors and implications for immunotherapy

Our bodies are inhabited by trillions of microorganisms. The host immune system constantly interacts with the microbiota in barrier organs, including the intestines. Over decades, numerous studies have shown that our mucosal immune system is dynamically shaped by a variety of microbiota-derived signals. Elucidating the mediators of these interactions is an important step for understanding how the microbiota is linked to mucosal immune homeostasis and gut-associated diseases. Interestingly, the efficacy of cancer immunotherapies that manipulate costimulatory and coinhibitory pathways has been correlated with the gut microbiota. Moreover, adverse effects of these therapies in the gut are linked to dysregulation of the intestinal immune system. These findings suggest that costimulatory pathways in the immune system might serve as a bridge between the host immune system and the gut microbiota. Here, we review mechanisms by which commensal microorganisms signal immune cells and their potential impact on costimulation. We highlight how costimulatory pathways modulate the mucosal immune system through not only classical antigen-presenting cells but also innate lymphocytes, which are highly enriched in barrier organs. Finally, we discuss the adverse effects of immune checkpoint inhibitors in the gut and the possible relationship with the gut microbiota.

The interplay between innate lymphoid cells and microbiota

Innate lymphoid cells (ILCs) are mainly resident in mucosal tissues such as gastrointestinal tract and respiratory tract, so they are closely linked to the microbiota. ILCs can protect commensals to maintain homeostasis and increase resistance to pathogens. Moreover, ILCs also play an early role in defense against a variety of pathogenic microorganisms including pathogenic bacteria, viruses, fungi and parasites, before the intervention of adaptive immune system. Due to the lack of adaptive antigen receptors expressed on T cells and B cells, ILCs need to use other means to sense the signals of microbiota and play a role in corresponding regulation. In this review, we focus on and summarize three major mechanisms used in the interaction between ILCs and microbiota: the mediation of accessory cells represented by dendritic cells; the metabolic pathways of microbiota or diet; the participation of adaptive immune cells.

Reciprocal costimulatory molecules control the activation of mucosal type 3 innate lymphoid cells during engagement with B cells

Innate lymphoid cells (ILCs) are the counterpart of T helper cells in the innate immune system and share multiple phenotypes with T helper cells. Inducible T-cell costimulator (ICOS) is recognized on T cells and participates in T-cell activation and T and B-cell engagement in lymphoid tissues. However, the role of ICOS in ILC3s and ILC3-involved interactions with the immune microenvironment remains unclear. Here, we found that ICOS expression on human ILC3s was correlated with the activated state of ILC3s. ICOS costimulation enhanced the survival, proliferation, and capacity of ILC3s to produce cytokines (IL-22, IL-17A, IFN-γ, TNF, and GM-CSF). Via synergistic effects of ICOS and CD40 signaling, B cells promoted ILC3 functions, and ILC3-induced T-cell-independent B-cell IgA and IgM secretion primarily required CD40 signaling. Hence, ICOS is essential for the nonredundant role of ILC3s and their interaction with adjacent B cells.

Host Immunity and Immunization Strategies for Clostridioides difficile Infection

Clostridioides difficile infection (CDI) represents a significant challenge to public health. C. difficile-associated mortality and morbidity have led the U.S. CDC to designate it as an urgent threat. Moreover, recurrence or relapses can occur in up to a third of CDI patients, due in part to antibiotics being the primary treatment for CDI and the major cause of the disease. In this review, we summarize the current knowledge of innate immune responses, adaptive immune responses, and the link between innate and adaptive immune responses of the host against CDI. The other major determinants of CDI, such as C. difficile toxins, the host microbiota, and related treatments, are also described. Finally, we discuss the known therapeutic approaches and the current status of immunization strategies for CDI, which might help to bridge the knowledge gap in the generation of therapy against CDI.

Regulation of systemic metabolism by tissue-resident immune cell circuits

Recent studies have demonstrated that tissue homeostasis and metabolic function are dependent on distinct tissue-resident immune cells that form functional cell circuits with structural cells. Within these cell circuits, immune cells integrate cues from dietary contents and commensal microbes in addition to endocrine and neuronal signals present in the tissue microenvironment to regulate structural cell metabolism. These tissue-resident immune circuits can become dysregulated during inflammation and dietary overnutrition, contributing to metabolic diseases. Here, we review the evidence describing key cellular networks within and between the liver, gastrointestinal tract, and adipose tissue that control systemic metabolism and how these cell circuits become dysregulated during certain metabolic diseases. We also identify open questions in the field that have the potential to enhance our understanding of metabolic health and disease.

Helios+ and RORγt+ Treg populations are differentially regulated by MHCII, CD28, and ICOS to shape the intestinal Treg pool

Foxp3+ regulatory T cells (Tregs) are essential for intestinal homeostasis. Tregs in the small intestine include Helios+ thymus-derived Tregs (tTregs) and RORγt+ Tregs that differentiate in the periphery after antigenic stimulation (pTregs). TCR and costimulatory signals sustain Tregs with effector phenotypes, including those in the intestine, but it is unknown if tTregs and pTregs have similar requirements for these pathways. We previously used mice lacking peripheral expression of MHCII to demonstrate that the small intestine sustains tTregs independently of peripheral antigen. Here, we show that the effector phenotype and tissue-resident signature of tTregs are also MHCII-independent. Using this model, we define the distinct costimulatory requirements of intestinal tTregs and pTregs. Helios+ effector tTregs proliferate through CD28 and require neither ICOS nor MHCII for maintenance. In contrast, RORγt+ pTregs use CD28 and ICOS. Notably, the differential costimulatory utilization allows tTregs and pTregs to dynamically respond to perturbations to support a fixed number of intestinal Tregs. This suggests that the environmental regulation of costimulatory ligands might shape the subpopulations of intestinal Tregs and promote effective homeostasis and defense. Our data reveal new complexity in effector Treg biology and costimulatory signaling of tTregs and pTregs and highlight the importance of analyzing both subpopulations.

Crosstalk between ILC3s and Microbiota: Implications for Colon Cancer Development and Treatment with Immune Check Point Inhibitors

Type 3 innate lymphoid cells (ILC3s) are primarily tissue-resident cells strategically localized at the intestinal barrier that exhibit the fast-acting responsiveness of classic innate immune cells. Populations of these lymphocytes depend on the transcription factor RAR-related orphan receptor and play a key role in maintaining intestinal homeostasis, keeping host-microbial mutualism in check. Current evidence has indicated a bidirectional relationship between microbiota and ILC3s. While ILC3 function and maintenance in the gut are influenced by commensal microbiota, ILC3s themselves can control immune responses to intestinal microbiota by providing host defense against extracellular bacteria, helping to maintain a diverse microbiota and inducing immune tolerance for commensal bacteria. Thus, ILC3s have been linked to host-microbiota interactions and the loss of their normal activity promotes dysbiosis, chronic inflammation and colon cancer. Furthermore, recent evidence has suggested that a healthy dialog between ILC3s and gut microbes is necessary to support antitumor immunity and response to immune checkpoint inhibitor (ICI) therapy. In this review, we summarize the functional interactions occurring between microbiota and ILC3s in homeostasis, providing an overview of the molecular mechanisms orchestrating these interactions. We focus on how alterations in this interplay promote gut inflammation, colorectal cancer and resistance to therapies with immune check point inhibitors.

The single-cell transcriptional landscape of innate and adaptive lymphocytes in pediatric-onset colitis

Innate lymphoid cells (ILCs) are considered innate counterparts of adaptive T cells; however, their common and unique transcriptional signatures in pediatric inflammatory bowel disease (pIBD) are largely unknown. Here, we report a dysregulated colonic ILC composition in pIBD colitis that correlates with inflammatory activity, including accumulation of naive-like CD45RA⁺CD62L^- ILCs. Weighted gene co-expression network analysis (WGCNA) reveals modules of genes that are shared or unique across innate and adaptive lymphocytes. Shared modules include genes associated with activation/tissue residency, naivety/quiescence, and antigen presentation. Lastly, nearest-neighbor-based analysis facilitates the identification of "most inflamed" and "least inflamed" lymphocytes in pIBD colon with unique transcriptional signatures. Our study reveals shared and unique transcriptional signatures of colonic ILCs and T cells in pIBD. We also provide insight into the transcriptional regulation of colonic inflammation, deepening our understanding of the potential mechanisms involved in pIBD.

Innate lymphoid cells and innate-like T cells in cancer - at the crossroads of innate and adaptive immunity

Immunotherapies targeting conventional T cells have revolutionized systemic treatment for many cancers, yet only a subset of patients benefit from these approaches. A better understanding of the complex immune microenvironment of tumours is needed to design the next generation of immunotherapeutics. Innate lymphoid cells (ILCs) and innate-like T cells (ILTCs) are abundant, tissue-resident lymphocytes that have recently been shown to have critical roles in many types of cancers. ILCs and ILTCs rapidly respond to changes in their surrounding environment and act as the first responders to bridge innate and adaptive immunity. This places ILCs and ILTCs as pivotal orchestrators of the final antitumour immune response. In this Review, we outline hallmarks of ILCs and ILTCs and discuss their emerging role in antitumour immunity, as well as the pathophysiological adaptations leading to their pro-tumorigenic function. We explore the pleiotropic, in parts redundant and sometimes opposing, mechanisms that underlie the delicate interplay between the different subsets of ILCs and ILTCs. Finally, we highlight their role in amplifying and complementing conventional T cell functions and summarize immunotherapeutic strategies for targeting ILCs and ILTCs in cancer.

Controversial role of ILC3s in intestinal diseases: A novelty perspective on immunotherapy

ILC3s have been identified as crucial immune regulators that play a role in maintaining host homeostasis and modulating the antitumor response. Emerging evidence supports the idea that LTi cells play an important role in initiating lymphoid tissue development, while other ILC3s can promote host defense and orchestrate adaptive immunity, mainly through the secretion of specific cytokines and crosstalk with other immune cells or tissues. Additionally, dysregulation of ILC3-mediated overexpression of cytokines, changes in subset abundance, and conversion toward other ILC subsets are closely linked with the occurrence of tumors and inflammatory diseases. Regulation of ILC3 cytokines, ILC conversion and LTi-induced TLSs may be a novel strategy for treating tumors and intestinal or extraintestinal inflammatory diseases. Herein, we discuss the development of ILCs, the biology of ILC3s, ILC plasticity, the correlation of ILC3s and adaptive immunity, crosstalk with the intestinal microenvironment, controversial roles of ILC3s in intestinal diseases and potential applications for treatment.

FXR mediates ILC-intrinsic responses to intestinal inflammation

The pleiotropic actions of the Farnesoid X Receptor (FXR) are required for gut health, and reciprocally, reduced intestinal FXR signaling is seen in inflammatory bowel diseases (IBDs). Here, we show that activation of FXR selectively in the intestine is protective in inflammation-driven models of IBD. Prophylactic activation of FXR restored homeostatic levels of pro-inflammatory cytokines, most notably IL17. Importantly, these changes were attributed to FXR regulation of innate lymphoid cells (ILCs), with both the inflammation-driven increases in ILCs, and ILC3s in particular, and the induction of Il17a and Il17f  in ILC3s blocked by FXR activation. Moreover, a population of ILC precursor-like cells increased with treatment, implicating FXR in the maturation/differentiation of ILC precursors. These findings identify FXR as an intrinsic regulator of intestinal ILCs and a potential therapeutic target in inflammatory intestinal diseases.

Group 3 innate lymphoid cells require BATF to regulate gut homeostasis in mice

Group 3 innate lymphoid cells (ILC3s) are crucial for the maintenance of host-microbiota homeostasis in gastrointestinal mucosal tissues. The mechanisms that maintain lineage identity of intestinal ILC3s and ILC3-mediated orchestration of microbiota and mucosal T cell immunity are elusive. Here, we identified BATF as a gatekeeper of ILC3 homeostasis in the gut. Depletion of BATF in ILC3s resulted in excessive interferon-γ production, dysbiosis, aberrant T cell immune responses, and spontaneous inflammatory bowel disease (IBD), which was considerably ameliorated by the removal of adaptive immunity, interferon-γ blockade, or antibiotic treatment. Mechanistically, BATF directly binds to the cis-regulatory elements of type 1 effector genes, restrains their chromatin accessibility, and inhibits their expression. Conversely, BATF promotes chromatin accessibility of genes involved in MHCII antigen processing and presentation pathways, which in turn directly promotes the transition of precursor ILC3s to MHCII+ ILC3s. Collectively, our findings reveal that BATF is a key transcription factor for maintaining ILC3 stability and coordinating ILC3-mediated control of intestinal homeostasis.

The role of ILC subsets in cancer

The family of innate lymphoid cells (ILCs) are composed of five canonical subsets, NK cells, ILC1, ILC2, ILC3 and Lymphoid tissue inducer cells. ILCs have important functions in early stages of immune response towards infectious agents. ILCs are highly plastic enabling rapid modification of their functions dependent on the type of microbe and tissue environment to optimally counter these microbes. Data that still accumulate in a rapid pace indicate that these cells are also involved in immunity against tumor cells. Paradoxically ILC subsets have been shown to have tumor suppressing and tumor promoting activities. In this brief review we provide a snapshot of our current knowledge of characteristics and functions of tumor infiltrating ILC subsets and speculate on how these cells can be harnessed to mediate anti-tumor immunity.

Intestinal single-cell atlas reveals novel lymphocytes in pigs with similarities to human cells

Lymphocytes can heavily influence intestinal health, but resolving intestinal lymphocyte function is challenging as the intestine contains a vastly heterogeneous mixture of cells. Pigs are an advantageous biomedical model, but deeper understanding of intestinal lymphocytes is warranted to improve model utility. Twenty-six cell types were identified in the porcine ileum by single-cell RNA sequencing and further compared with cells in human and murine ileum. Though general consensus of cell subsets across species was revealed, some porcine-specific lymphocyte subsets were identified. Differential tissue dissection and in situ analyses conferred spatial context, revealing similar locations of lymphocyte subsets in Peyer's patches and epithelium in pig-to-human comparisons. Like humans, activated and effector lymphocytes were abundant in the ileum but not periphery of pigs, suggesting tissue-specific and/or activation-associated gene expression. Gene signatures for peripheral and ileal innate lymphoid cells newly discovered in pigs were defined and highlighted similarities to human innate lymphoid cells. Overall, we reveal novel lymphocyte subsets in pigs and highlight utility of pigs for intestinal research applications.

Innate immunity and immunotherapy for hemorrhagic shock

Hemorrhagic shock (HS) is a shock result of hypovolemic injury, in which the innate immune response plays a central role in the pathophysiology ofthe severe complications and organ injury in surviving patients. During the development of HS, innate immunity acts as the first line of defense, mediating a rapid response to pathogens or danger signals through pattern recognition receptors. The early and exaggerated activation of innate immunity, which is widespread in patients with HS, results in systemic inflammation, cytokine storm, and excessive activation of complement factors and innate immune cells, comprised of type II innate lymphoid cells, CD4⁺ T cells, natural killer cells, eosinophils, basophils, macrophages, neutrophils, and dendritic cells. Recently, compelling evidence focusing on the innate immune regulation in preclinical and clinical studies promises new treatment avenues to reverse or minimize HS-induced tissue injury, organ dysfunction, and ultimately mortality. In this review, we first discuss the innate immune response involved in HS injury, and then systematically detail the cutting-edge therapeutic strategies in the past decade regarding the innate immune regulation in this field; these strategies include the use of mesenchymal stem cells, exosomes, genetic approaches, antibody therapy, small molecule inhibitors, natural medicine, mesenteric lymph drainage, vagus nerve stimulation, hormones, glycoproteins, and others. We also reviewed the available clinical studies on immune regulation for treating HS and assessed the potential of immune regulation concerning a translation from basic research to clinical practice. Combining therapeutic strategies with an improved understanding of how the innate immune system responds to HS could help to identify and develop targeted therapeutic modalities that mitigate severe organ dysfunction, improve patient outcomes, and reduce mortality due to HS injury.

Oxygen regulates ILC3 antigen presentation potential and pregnancy-related hormone actions

Early pregnancy is marked by placentation and embryogenesis, which take place under physiological low oxygen concentrations. This oxygen condition is crucial for many aspects of placentation, trophoblast function, vascularization and immune function. Recently, a new family of innate lymphoid cells has been found to be expressed at the fetomaternal interface. Among these, type 3 innate lymphoid cells (ILC3) are important antigen presenting cells in the context of MHC-II. The expression of MHC-II on ILC3s during pregnancy is reduced. We tested the hypothesis that low oxygen concentrations reduce the potential of ILC3s to present antigens promoting fetal tolerance.Using an in vitro approach, NCR⁺ ILC3s generated from cord blood stem cell precursors were incubated under different O₂ concentrations in the presence or absence of the pregnancy-related hormones hCG and TGF-β1. The expression of MHC-II, accessory molecules and an activation marker were assessed by flow cytometry. We observed that 1% O₂ reduced the expression of the MHC-II molecule HLA-DR as compared to 21% O₂ and modulated the relative effects of hCG and TGF-β1.Our data indicate that low oxygen concentrations reduce the antigen presentation potential of NCR⁺ ILC3s and suggest that it may promote fetal tolerance during the first trimester of pregnancy.

Extrathymic expression of Aire controls the induction of effective TH17 cell-mediated immune response to Candida albicans

Patients with loss of function in the gene encoding the master regulator of central tolerance AIRE suffer from a devastating disorder called autoimmune polyendocrine syndrome type 1 (APS-1), characterized by a spectrum of autoimmune diseases and severe mucocutaneous candidiasis. Although the key mechanisms underlying the development of autoimmunity in patients with APS-1 are well established, the underlying cause of the increased susceptibility to Candida albicans infection remains less understood. Here, we show that Aire⁺MHCII⁺ type 3 innate lymphoid cells (ILC3s) could sense, internalize and present C. albicans and had a critical role in the induction of Candida-specific T helper 17 (T_H17) cell clones. Extrathymic Rorc-Cre-mediated deletion of Aire resulted in impaired generation of Candida-specific T_H17 cells and subsequent overgrowth of C. albicans in the mucosal tissues. Collectively, our observations identify a previously unrecognized regulatory mechanism for effective defense responses against fungal infections.

Gliadin-reactive vitamin D-sensitive proinflammatory ILCPs are enriched in celiac patients

Celiac disease (CD) is a multisystem disease in which different organs may be affected. We investigate whether circulating innate lymphoid cells (ILCs) contribute to the CD peripheral inflammatory status. We find that the CD cytokine profile is characterized by high concentrations of IL-12p40, IL-18, and IFN-γ, paralleled by an expansion of ILC precursors (ILCPs). In the presence of the gliadin peptides p31-43 and pα-9, ILCPs from CD patients increase transglutaminase 2 (TG2) expression, produce IL-18 and IFN-γ, and stimulate CD4⁺ T lymphocytes. IFN-γ is also produced upon stimulation with IL-12p40 and IL-18 and is inhibited by the addition of vitamin D. Low levels of blood vitamin D correlate with high IFN-γ and ILCP presence and mark the CD population mostly affected by extraintestinal symptoms. Dietary vitamin D supplementation appears to be an interesting therapeutic approach to dampen ILCP-mediated IFN-γ production.

Transcription factor-driven regulation of ILC1 and ILC3

Mammalian innate lymphoid cells (ILCs) have functional relevance under both homeostatic and disease settings, such as inflammatory bowel disease (IBD), particularly in the context of maintaining the integrity of mucosal surfaces. Early reports highlighted group 1 and 3 ILC regulatory transcription factors (TFs), T-box expressed in T cells (T-bet; Tbx21) and RAR-related orphan nuclear receptor γt (RORγt; Rorc), as key regulators of ILC biology. Since then, other canonical TFs have been shown to have a role in the development and function of ILC subsets. In this review, we focus on recent insights into the balance between mature ILC1 and ILC3 based on these TFs and how they interact with other key cell-intrinsic molecular pathways. We outline how this TF interplay might be explored to identify novel candidate therapeutic avenues for human diseases.

Effect of Probiotics on Respiratory Tract Allergic Disease and Gut Microbiota

Allergy is a hypersensitivity reaction triggered by specific cell or antibody-mediated immune mechanisms. Allergies have increased in industrialized countries in recent decades. The rise in allergic respiratory diseases such as allergic rhinitis (AR) and allergic asthma (AA) is a potential threat to public health. Searches were conducted using PubMed, Google Scholar and Medline using the following key terms: allergic rhinitis OR asthma AND probiotics, allergic airway inflammation AND immune disorders, probiotics OR gut microbiota AND allergic disease, probiotics AND inflammatory. Studies from all years were included, specifically those published within the last 10 years. Some review articles and their reference lists were searched to identify related articles. The role of microbiota in respiratory allergic diseases has attracted more and more attention. Pieces of evidence suggested that the development of allergic diseases causes a possible imbalance in the composition of the gut microbiota. Compared to colonized mice, germ-free mice exhibit exaggerated allergic airway responses, suggesting that microbial host interactions play an important role in the development of allergic diseases. Probiotics modulate both the innate and adaptive inflammatory immune responses, often used as dietary supplements to provide health benefits in gastrointestinal disorders. Probiotics may serve as immunomodulators and activators of host defense pathways. Besides, oral probiotics can modulate the immune response in the respiratory system. Recently, studies in humans and animals have demonstrated the role of probiotic in RA and AA. To understand the characterization, microbiota, and the potential role of probiotics intervention of AA/AR, this review provides an overview of clinical features of AA and AR, probiotics for the prevention and treatment of AR, AA, changes in gut microbiota, and their mechanisms of action.

Innate (and Innate-like) Lymphoid Cells: Emerging Immune Subsets With Multiple Roles Along Transplant Life

Transplant immunology is currently largely focused on conventional adaptive immunity, particularly T and B lymphocytes, which have long been considered as the only cells capable of allorecognition. In this vision, except for the initial phase of ischemia/reperfusion, during which the role of innate immune effectors is well established, the latter are largely considered as "passive" players, recruited secondarily to amplify graft destruction processes during rejection. Challenging this prevalent dogma, the recent progresses in basic immunology have unraveled the complexity of the innate immune system and identified different subsets of innate (and innate-like) lymphoid cells. As most of these cells are tissue-resident, they are overrepresented among passenger leukocytes. Beyond their role in ischemia/reperfusion, some of these subsets have been shown to be capable of allorecognition and/or of regulating alloreactive adaptive responses, suggesting that these emerging immune players are actively involved in most of the life phases of the grafts and their recipients. Drawing upon the inventory of the literature, this review synthesizes the current state of knowledge of the role of the different innate (and innate-like) lymphoid cell subsets during ischemia/reperfusion, allorecognition, and graft rejection. How these subsets also contribute to graft tolerance and the protection of chronically immunosuppressed patients against infectious and cancerous complications is also examined.

Antigen-presenting innate lymphoid cells orchestrate neuroinflammation

Pro-inflammatory T cells in the central nervous system (CNS) are causally associated with multiple demyelinating and neurodegenerative diseases1-6, but the pathways that control these responses remain unclear. Here we define a population of inflammatory group 3 innate lymphoid cells (ILC3s) that infiltrate the CNS in a mouse model of multiple sclerosis. These ILC3s are derived from the circulation, localize in proximity to infiltrating T cells in the CNS, function as antigen-presenting cells that restimulate myelin-specific T cells, and are increased in individuals with multiple sclerosis. Notably, antigen presentation by inflammatory ILC3s is required to promote T cell responses in the CNS and the development of multiple-sclerosis-like disease in mouse models. By contrast, conventional and tissue-resident ILC3s in the periphery do not appear to contribute to disease induction, but instead limit autoimmune T cell responses and prevent multiple-sclerosis-like disease when experimentally targeted to present myelin antigen. Collectively, our data define a population of inflammatory ILC3s that is essential for directly promoting T-cell-dependent neuroinflammation in the CNS and reveal the potential of harnessing peripheral tissue-resident ILC3s for the prevention of autoimmune disease.

ILC3s control airway inflammation by limiting T cell responses to allergens and microbes

Group 3 innate lymphoid cells (ILC3s) critically regulate host-microbe interactions in the gastrointestinal tract, but their role in the airway remains poorly understood. Here, we demonstrate that lymphoid-tissue-inducer (LTi)-like ILC3s are enriched in the lung-draining lymph nodes of healthy mice and humans. These ILC3s abundantly express major histocompatibility complex class II (MHC class II) and functionally restrict the expansion of allergen-specific CD4⁺ T cells upon experimental airway challenge. In a mouse model of house-dust-mite-induced allergic airway inflammation, MHC class II⁺ ILC3s limit T helper type 2 (Th2) cell responses, eosinophilia, and airway hyperresponsiveness. Furthermore, MHC class II⁺ ILC3s limit a concomitant Th17 cell response and airway neutrophilia. This exacerbated Th17 cell response requires exposure of the lung to microbial stimuli, which can be found associated with house dust mites. These findings demonstrate a critical role for antigen-presenting ILC3s in orchestrating immune tolerance in the airway by restricting pro-inflammatory T cell responses to both allergens and microbes.

In this study, Teng et al. demonstrate that an innate immune cell type, ILC3, is enriched in the lung draining lymph node of healthy humans and mice and functions to limit airway inflammation through antigen presentation and control of T cell responses directed against allergens or microbes.

T-bet and RORα control lymph node formation by regulating embryonic innate lymphoid cell differentiation

The generation of lymphoid tissues during embryogenesis relies on group 3 innate lymphoid cells (ILC3) displaying lymphoid tissue inducer (LTi) activity and expressing the master transcription factor RORγt. Accordingly, RORγt-deficient mice lack ILC3 and lymphoid structures, including lymph nodes (LN). Whereas T-bet affects differentiation and functions of ILC3 postnatally, the role of T-bet in regulating fetal ILC3 and LN formation remains completely unknown. Using multiple mouse models and single-cell analyses of fetal ILCs and ILC progenitors (ILCP), here we identify a key role for T-bet during embryogenesis and show that its deficiency rescues LN formation in RORγt-deficient mice. Mechanistically, T-bet deletion skews the differentiation fate of fetal ILCs and promotes the accumulation of PLZFhi ILCP expressing central LTi molecules in a RORα-dependent fashion. Our data unveil an unexpected role for T-bet and RORα during embryonic ILC function and highlight that RORγt is crucial in counteracting the suppressive effects of T-bet.

Innate lymphoid cells and gastrointestinal disease

Innate lymphoid cells (ILCs) are a group of innate immune cells, which constitute the first line of defense in the immune system, together with skin and mucous membrane. ILCs also play an important role in maintaining the homeostasis of the body, particularly in the complex and diverse environment of the intestine. ILCs respond to different microenvironments, maintaining homeostasis directly or indirectly through cytokines. As a result, ILCs, with complex and pleiotropic characteristics, are associated with many gastrointestinal diseases. Their ability of transition among those subgroups makes them function as both promoting and inhibiting cells, thus affecting homeostasis and disease progressing to either alleviation or deterioration. With these special characteristics, ILCs theoretically can be used in the new generation of immunotherapy as an alternative and supplement to current tumor therapy. Our review summarizes the characteristics of ILCs with respect to category, function, and the relationship with intestinal homeostasis and gastrointestinal diseases. In addition, potential tumor immunotherapies involving ILCs are also discussed to shed light on the perspectives of immunotherapy.

Dysregulation of ILC3s unleashes progression and immunotherapy resistance in colon cancer

Group 3 innate lymphoid cells (ILC3s) regulate immunity and inflammation, yet their role in cancer remains elusive. Here, we identify that colorectal cancer (CRC) manifests with altered ILC3s that are characterized by reduced frequencies, increased plasticity, and an imbalance with T cells. We evaluated the consequences of these changes in mice and determined that a dialog between ILC3s and T cells via major histocompatibility complex class II (MHCII) is necessary to support colonization with microbiota that subsequently induce type-1 immunity in the intestine and tumor microenvironment. As a result, mice lacking ILC3-specific MHCII develop invasive CRC and resistance to anti-PD-1 immunotherapy. Finally, humans with dysregulated intestinal ILC3s harbor microbiota that fail to induce type-1 immunity and immunotherapy responsiveness when transferred to mice. Collectively, these data define a protective role for ILC3s in cancer and indicate that their inherent disruption in CRC drives dysfunctional adaptive immunity, tumor progression, and immunotherapy resistance.

Innate lymphoid cells are double-edged swords under the mucosal barrier

As the direct contacting site for pathogens and allergens, the mucosal barrier plays a vital role in the lungs and intestines. Innate lymphoid cells (ILCs) are particularly resident in the mucosal barrier and participate in several pathophysiological processes, such as maintaining or disrupting barrier integrity, preventing various pathogenic invasions. In the pulmonary mucosae, ILCs sometimes aggravate inflammation and mucus hypersecretion but restore airway epithelial integrity and maintain lung tissue homeostasis at other times. In the intestinal mucosae, ILCs can increase epithelial permeability, leading to severe intestinal inflammation on the one hand, and assist mucosal barrier in resisting bacterial invasion on the other hand. In this review, we will illustrate the positive and negative roles of ILCs in mucosal barrier immunity.