Innate lymphoid cells: parallel checkpoints and coordinate interactions with T cells

Insights into the tumor microenvironment of B cell lymphoma

The standard therapies in lymphoma have predominantly focused on targeting tumor cells with less of a focus on the tumor microenvironment (TME), which plays a critical role in favoring tumor growth and survival. Such an approach may result in increasingly refractory disease with progressively reduced responses to subsequent treatments. To overcome this hurdle, targeting the TME has emerged as a new therapeutic strategy. The TME consists of T and B lymphocytes, tumor-associated macrophages (TAMs), myeloid-derived suppressor cells (MDSCs), cancer-associated fibroblasts (CAFs), and other components. Understanding the TME can lead to a comprehensive approach to managing lymphoma, resulting in therapeutic strategies that target not only cancer cells, but also the supportive environment and thereby ultimately improve survival of lymphoma patients. Here, we review the normal function of different components of the TME, the impact of their aberrant behavior in B cell lymphoma and the current TME-direct therapeutic avenues.

Type 1 Innate Lymphoid Cells Are Proinflammatory Effector Cells in Ischemia-Reperfusion Injury of Steatotic Livers

Innate lymphoid cells (ILCs), the most recently described family of lymphoid cells, play fundamental roles in tissue homeostasis through the production of key cytokine. Group 1 ILCs, comprised of conventional natural killer cells (cNKs) and type 1 ILCs (ILC1s), have been implicated in regulating immune-mediated inflammatory diseases. However, the role of ILC1s in nonalcoholic fatty liver disease (NAFLD) and ischemia-reperfusion injury (IRI) is unclear. Here, we investigated the role of ILC1 and cNK cells in a high-fat diet (HFD) murine model of partial warm IRI. We demonstrated that hepatic steatosis results in more severe IRI compared to non-steatotic livers. We further elicited that HFD-IRI mice show a significant increase in the ILC1 population, whereas the cNK population was unchanged. Since ILC1 and cNK are major sources of IFN-γ and TNF-α, we measured the level of ex vivo cytokine expression in normal diet (ND)-IRI and HFD-IRI conditions. We found that ILC1s in HFD-IRI mice produce significantly more IFN-γ and TNF-α when compared to ND-IRI. To further assess whether ILC1s are key proinflammatory effector cells in hepatic IRI of fatty livers, we studied both Rag1−/− mice, which possess cNK cells, and a substantial population of ILC1s versus the newly generated Rag1−/−Tbx21−/− double knockout (Rag1-Tbet DKO) mice, which lack type 1 ILCs, under HFD IRI conditions. Importantly, HFD Rag1-Tbet DKO mice showed significant protection from hepatic injury upon IRI when compared to Rag1−/− mice, suggesting that T-bet-expressing ILC1s play a role, at least in part, as proinflammatory effector cells in hepatic IRI under steatotic conditions.

A nonredundant role for T cell-derived interleukin 22 in antibacterial defense of colonic crypts

Interleukin (IL)-22 is central to immune defense at barrier sites. We examined the contributions of innate lymphoid cell (ILC) and T cell-derived IL-22 during Citrobacter rodentium (C.r) infection using mice that both report Il22 expression and allow lineage-specific deletion. ILC-derived IL-22 activated STAT3 in C.r-colonized surface intestinal epithelial cells (IECs) but only temporally restrained bacterial growth. T cell-derived IL-22 induced a more robust and extensive activation of STAT3 in IECs, including IECs lining colonic crypts, and T cell-specific deficiency of IL-22 led to pathogen invasion of the crypts and increased mortality. This reflected a requirement for T cell-derived IL-22 for the expression of a host-protective transcriptomic program that included AMPs, neutrophil-recruiting chemokines, and mucin-related molecules, and it restricted IFNγ-induced proinflammatory genes. Our findings demonstrate spatiotemporal differences in the production and action of IL-22 by ILCs and T cells during infection and reveal an indispensable role for IL-22-producing T cells in the protection of the intestinal crypts.

Natural Killer Cells and Type 1 Innate Lymphoid Cells in Hepatocellular Carcinoma: Current Knowledge and Future Perspectives

Natural killer (NK) cells and type 1 innate lymphoid cells (ILC1) are specific innate lymphoid cell subsets that are key for the detection and elimination of pathogens and cancer cells. In liver, while they share a number of characteristics, they differ in many features. These include their developmental pathways, tissue distribution, phenotype and functions. NK cells and ILC1 contribute to organ homeostasis through the production of key cytokines and chemokines and the elimination of potential harmful bacteria and viruses. In addition, they are equipped with a wide range of receptors, allowing them to detect “stressed cells’ such as cancer cells. Our understanding of the role of innate lymphoid cells in hepatocellular carcinoma (HCC) is growing owing to the development of mouse models, the progress in immunotherapeutic treatment and the recent use of scRNA sequencing analyses. In this review, we summarize the current understanding of NK cells and ILC1 in hepatocellular carcinoma and discuss future strategies to take advantage of these innate immune cells in anti-tumor immunity. Immunotherapies hold great promise in HCC, and a better understanding of the role and function of NK cells and ILC1 in liver cancer could pave the way for new NK cell and/or ILC1-targeted treatment.

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.

The cancer-natural killer cell immunity cycle

Immunotherapy with checkpoint blockade induces rapid and durable immune control of cancer in some patients and has driven a monumental shift in cancer treatment. Neoantigen-specific CD8⁺ T cells are at the forefront of current immunotherapy strategies, and the majority of drug discovery and clinical trials revolve around further harnessing these immune effectors. Yet the immune system contains a diverse range of antitumour effector cells, and these must function in a coordinated and synergistic manner to overcome the immune-evasion mechanisms used by tumours and achieve complete control with tumour eradication. A key antitumour effector is the natural killer (NK) cells, cytotoxic innate lymphocytes present at high frequency in the circulatory system and identified by their exquisite ability to spontaneously detect and lyse transformed or stressed cells. Emerging data show a role for intratumoural NK cells in driving immunotherapy response and, accordingly, there have been renewed efforts to further elucidate and target the pathways controlling NK cell antitumour function. In this Review, we discuss recent clinical evidence that NK cells are a key immune constituent in the protective antitumour immune response and highlight the major stages of the cancer-NK cell immunity cycle. We also perform a new analysis of publicly available transcriptomic data to provide an overview of the prognostic value of NK cell gene expression in 25 tumour types. Furthermore, we discuss how the role of NK cells evolves with tumour progression, presenting new opportunities to target NK cell function to enhance cancer immunotherapy response rates across a more diverse range of cancers.

Pulmonary paracoccidioidomycosis in AhR deficient hosts is severe and associated with defective Treg and Th22 responses

AhR is a ligand-activated transcription factor that plays an important role in the innate and adaptive immune responses. In infection models, it has been associated with host responses that promote or inhibit disease progression. In pulmonary paracoccidioidomycosis, a primary fungal infection endemic in Latin America, immune protection is mediated by Th1/Th17 cells and disease severity with predominant Th2/Th9/Treg responses. Because of its important role at epithelial barriers, we evaluate the role of AhR in the outcome of a pulmonary model of paracoccidioidomycosis. AhR^−/− mice show increased fungal burdens, enhanced tissue pathology and mortality. During the infection, AhR^−/− mice have more pulmonary myeloid cells with activated phenotype and reduced numbers expressing indoleamine 2,3 dioxygenase 1. AhR-deficient lungs have altered production of cytokines and reduced numbers of innate lymphoid cells (NK, ILC3 and NCR IL-22). The lungs of AhR^−/− mice showed increased presence Th17 cells concomitant with reduced numbers of Th1, Th22 and Foxp3⁺ Treg cells. Furthermore, treatment of infected WT mice with an AhR-specific antagonist (CH223191) reproduced the main findings obtained in AhR^−/− mice. Collectively our data demonstrate that in pulmonary paracoccidioidomycosis AhR controls fungal burden and excessive tissue inflammation and is a possible target for antifungal therapy.

ILC3s integrate glycolysis and mitochondrial production of reactive oxygen species to fulfill activation demands

Group 3 innate lymphoid cells (ILC3s) are the innate counterparts of Th17 that require the transcription factor RORγt for development and contribute to the defense against pathogens through IL-22 and IL-17 secretion. Proliferation and effector functions of Th17 require a specific mTOR-dependent metabolic program that utilizes high-rate glycolysis, while mitochondrial lipid oxidation and production of reactive oxygen species (mROS) support alternative T reg cell differentiation. Whether ILC3s employ a specific metabolic program is not known. Here, we find that ILC3s rely on mTOR complex 1 (mTORC1) for proliferation and production of IL-22 and IL-17A after in vitro activation and Citrobacter rodentium infection. mTORC1 induces activation of HIF1α, which reprograms ILC3 metabolism toward glycolysis and sustained expression of RORγt. However, in contrast to Th17, ILC3 activation requires mROS production; rather than inducing an alternative regulatory fate as it does in CD4 T cells, mROS stabilizes HIF1α and RORγt in ILC3s and thereby promotes their activation. We conclude that ILC3 activation relies on a metabolic program that integrates glycolysis with mROS production.

A Gene Signature Predicting Natural Killer Cell Infiltration and Improved Survival in Melanoma Patients

Natural killer (NK) cell activity is essential for initiating antitumor responses and may be linked to immunotherapy success. NK cells and other innate immune components could be exploitable for cancer treatment, which drives the need for tools and methods that identify therapeutic avenues. Here, we extend our gene-set scoring method singscore to investigate NK cell infiltration by applying RNA-seq analysis to samples from bulk tumors. Computational methods have been developed for the deconvolution of immune cell types within solid tumors. We have taken the NK cell gene signatures from several such tools, then curated the gene list using a comparative analysis of tumors and immune cell types. Using a gene-set scoring method to investigate RNA-seq data from The Cancer Genome Atlas (TCGA), we show that patients with metastatic cutaneous melanoma have an improved survival rate if their tumor shows evidence of NK cell infiltration. Furthermore, these survival effects are enhanced in tumors that show higher expression of genes that encode NK cell stimuli such as the cytokine IL15 Using this signature, we then examine transcriptomic data to identify tumor and stromal components that may influence the penetrance of NK cells into solid tumors. Our results provide evidence that NK cells play a role in the regulation of human tumors and highlight potential survival effects associated with increased NK cell activity. Our computational analysis identifies putative gene targets that may be of therapeutic value for boosting NK cell antitumor immunity.

Role of innate lymphoid cells and dendritic cells in intradermal immunization of the enterovirus antigen

Enterovirus type 71 (EV71) and coxsackievirus A 16 (CA16) are the major pathogens of human hand, foot, and mouth disease (HFMD). In our previous study, intramuscular immunization with the inactivated EV71 vaccine elicited effective immunity, while immunization with the inactivated CA16 vaccine did not. In this report, we focused on innate immune responses elicited by inactivated EV71 and CA16 antigens administered intradermally or intramuscularly. The distributions of the EV71 and CA16 antigens administered intradermally or intramuscularly were not obviously different, but the antigens were detected for a shorter period of time when administered intradermally. The expression levels of NF-κB pathway signaling molecules, which were identified as being capable of activating DCs, ILCs, and T cells, were higher in the intradermal group than in the intramuscular group. Antibodies for the EV71 and CA16 antigens colocalized with ILCs and DCs in skin and muscle tissues under fluorescence microscopy. Interestingly, ILC colocalization decreased over time, while DC colocalization increased over time. ELISpot analysis showed that coordination between DCs and ILCs contributed to successful adaptive immunity against vaccine antigens in the skin. EV71 and/or CA16 antigen immunization via the intradermal route was more capable of significantly increasing neutralizing antibody titers and activating specific T cell responses than immunization via the intramuscular route. Furthermore, neonatal mice born to mothers immunized with the EV71 and CA16 antigens were 100% protected against wild-type EV71 or CA16 viral challenge. Together, our results provide new insights into the development of vaccines for HFMD.

Coxsackievirus A 16 (CA16) and enterovirus 71 (EV71) infections are the most common cause of hand-foot-and-mouth diseases. Inactivated virus has been evaluated as potential vaccine for both viruses in animal models, but protection was only achieved for EV71. In this study, led by Qihan Li from the Chinese Academy of Medical Sciences, researchers show that intradermal, as compared to intramuscular immunization, results in an elevated immune response and improved protection from EV71 and CA16 infection in mice. Intradermal vaccination increases interaction of vaccine antigen with dendritic cells and innate lymphoid cells at the site of inoculation, as compared to intramuscular vaccination. Intradermal vaccination furthermore improves the antibody and T cell response and protects mice from infection. However, complete protection of mice from CA16 infection was only achieved after intradermal immunization with a combination of inactivated EV71 and CA16 vaccine, suggesting that further improvements of this vaccine candidate will be necessary.

Isolation and Identification of Innate Lymphoid Cells (ILCs) for Immunotoxicity Testing

Innate lymphoid cells (ILCs) comprise a family of innate immune cells that orchestrate mucosal immune responses: initiating, sustaining, and even curbing immune responses. ILCs are relatively rare (≤1% of lymphocytes in mucosal tissues), lack classical cell-surface markers, and can be divided into three subsets (type 1-3 ILCs) based on differences in cytokine production, phenotype, and developmental pathway. Because ILCs can only be identified by combinations of cell-surface markers and cytokine production, multicolor flow cytometry is the most reliable method to purify, characterize, and assess the functionality of ILCs. Here, we describe the methods for cell preparation, flow cytometric analysis, and purification of murine ILCs from the lung.

Innate Lymphoid Cells: Diversity, Plasticity, and Unique Functions in Immunity

Type 1, 2, and 3 innate lymphoid cells (ILCs) have emerged as tissue-resident innate correlates of T helper 1 (Th1), Th2, and Th17 cells. Recent studies suggest that ILCs are more diverse than originally proposed; this might reflect truly distinct lineages or adaptation of ILCs to disparate tissue microenvironments, known as plasticity. Given that ILCs strikingly resemble T cells, are they redundant? While the regulation, timing, and magnitude of ILC and primary T cell responses differ, tissue-resident memory T cells may render ILCs redundant during secondary responses. The unique impact of ILCs in immunity is probably embodied in the extensive array of surface and intracellular receptors that endow these cells with the ability to distinguish between normal and pathogenic components, interact with other cells, and calibrate their cytokine secretion accordingly. Here I review recent advances in elucidating the diversity of ILCs and discuss their unique and redundant functions.

Interplay of innate lymphoid cells and the microbiota

Innate lymphoid cells (ILC) are a recently identified group of innate lymphocytes that are preferentially located at barrier surfaces. Barrier surfaces are in direct contact with complex microbial ecosystems, collectively referred to as the microbiota. It is now believed that the interplay of the microbiota with host components (i.e. epithelial cells and immune cells) promotes host fitness by regulating organ homeostasis, metabolism, and host defense against pathogens. In this review, we will give an overview of this multifaceted interplay between ILC and components of the microbiota.

Regulation of neutrophils in type 2 immune responses

Type 2 immune responses contribute to the resistance to helminths and toxins as well as several physiological processes. Although they usually do not participate in type 2 immune responses, neutrophils have been shown in mice to enhance the anti-helminth response, but they also contribute to increased target tissue damage. Increased pathology and morbidity is also observed in type 2 immune-mediated disorders, such as allergic asthma, when neutrophils become a predominant subset of the infiltrate. How neutrophil recruitment is regulated during type 2 immune responses is now starting to become clear, with recent data showing that signaling via the prototypic type 2 cytokine interleukin-4 receptor mediates direct and indirect inhibitory actions on neutrophils in mice and humans.

Revisiting the Page & Schroeder model: the good, the bad and the unknowns in the periodontal host response 40 years later

In their classic 1976 paper, Page & Schroeder described the histopathologic events and the types of myeloid cells and lymphocytes involved in the initiation and progression of inflammatory periodontal disease. The staging of periodontal disease pathogenesis as 'initial', 'early', 'established' and 'advanced' lesions productively guided subsequent research in the field and remains fundamentally valid. However, major advances regarding the cellular and molecular mechanisms underlying the induction, regulation and effector functions of immune and inflammatory responses necessitate a reassessment of their work and its integration with emerging new concepts. We now know that each type of leukocyte is actually represented by functionally distinct subsets with different, or even conflicting, roles in immunity and inflammation. Unexpectedly, neutrophils, traditionally regarded as merely antimicrobial effectors in acute conditions and protagonists of the 'initial' lesion, are currently appreciated for their functional versatility and critical roles in chronic inflammation. Moreover, an entirely new field of study, osteoimmunology, has emerged and sheds light on the impact of immunoinflammatory events on the skeletal system. These developments and the molecular dissection of crosstalk interactions between innate and adaptive leukocytes, as well as between the immune system and local homeostatic mechanisms, offer a more nuanced understanding of the host response in periodontitis, with profound implications for treatment. At the same time, deeper insights have generated new questions, many of which remain unanswered. In this review, 40 years after Page & Schroeder proposed their model, we summarize enduring and emerging advances in periodontal disease pathogenesis.

Innate immune mediators in cancer: between defense and resistance

Chronic inflammation in the tumor microenvironment and evasion of the antitumor effector immune response are two of the emerging hallmarks required for oncogenesis and cancer progression. The innate immune system not only plays a critical role in perpetuating these tumor-promoting hallmarks but also in developing antitumor adaptive immune responses. Thus, understanding the dual role of the innate system in cancer immunology is required for the design of combined immunotherapy strategies able to tackle established tumors. Here, we review recent advances in the understanding of the role of cell populations and soluble components of the innate immune system in cancer, with a focus on complement, the adapter molecule Stimulator of Interferon Genes, natural killer cells, myeloid cells, and B cells.

The IDO-AhR Axis Controls Th17/Treg Immunity in a Pulmonary Model of Fungal Infection

In infectious diseases, the enzyme indoleamine 2,3 dioxygenase-1 (IDO1) that catalyzes the tryptophan (Trp) degradation along the kynurenines (Kyn) pathway has two main functions, the control of pathogen growth by reducing available Trp and immune regulation mediated by the Kyn-mediated expansion of regulatory T (Treg) cells via aryl hydrocarbon receptor (AhR). In pulmonary paracoccidioidomycosis (PCM) caused by the dimorphic fungus Paracoccidioides brasiliensis, IDO1 was shown to control the disease severity of both resistant and susceptible mice to the infection; however, only in resistant mice, IDO1 is induced by TGF-β signaling that confers a stable tolerogenic phenotype to dendritic cells (DCs). In addition, in pulmonary PCM, the tolerogenic function of plasmacytoid dendritic cells was linked to the IDO1 activity. To further evaluate the function of IDO1 in pulmonary PCM, IDO1-deficient (IDO1^-/-) C57BL/6 mice were intratracheally infected with P. brasiliensis yeasts and the infection analyzed at three postinfection periods regarding several parameters of disease severity and immune response. The fungal loads and tissue pathology of IDO1^-/- mice were higher than their wild-type controls resulting in increased mortality rates. The evaluation of innate lymphoid cells showed an upregulated differentiation of the innate lymphoid cell 3 phenotype accompanied by a decreased expansion of ILC1 and NK cells in the lungs of infected IDO1^-/- mice. DCs from these mice expressed elevated levels of costimulatory molecules and cytokine IL-6 associated with reduced production of IL-12, TNF-α, IL-1β, TGF-β, and IL-10. This response was concomitant with a marked reduction in AhR production. The absence of IDO1 expression caused an increased influx of activated Th17 cells to the lungs with a simultaneous reduction in Th1 and Treg cells. Accordingly, the suppressive cytokines IL-10, TGF-β, IL-27, and IL-35 appeared in reduced levels in the lungs of IDO1^-/- mice. In conclusion, the immunological balance mediated by the axis IDO/AhR is fundamental to determine the balance between Th17/Treg cells and control the severity of pulmonary PCM.

Fibroblastic reticular cells regulate intestinal inflammation via IL-15-mediated control of group 1 ILCs

Fibroblastic reticular cells influence the function of lymphocytes in secondary lymphoid organs. Ludewig and colleagues demonstrate that they also specifically restrain the activation of group 1 innate lymphoid cells in the presence of microbial stimulation to prevent immunopathology.

Fibroblastic reticular cells (FRCs) of secondary lymphoid organs form distinct niches for interaction with hematopoietic cells. We found here that production of the cytokine IL-15 by FRCs was essential for the maintenance of group 1 innate lymphoid cells (ILCs) in Peyer's patches and mesenteric lymph nodes. Moreover, FRC-specific ablation of the innate immunological sensing adaptor MyD88 unleashed IL-15 production by FRCs during infection with an enteropathogenic virus, which led to hyperactivation of group 1 ILCs and substantially altered the differentiation of helper T cells. Accelerated clearance of virus by group 1 ILCs precipitated severe intestinal inflammatory disease with commensal dysbiosis, loss of intestinal barrier function and diminished resistance to colonization. In sum, FRCs act as an 'on-demand' immunological 'rheostat' by restraining activation of group 1 ILCs and thereby preventing immunopathological damage in the intestine.

Evidence of innate lymphoid cell redundancy in humans

Innate lymphoid cells (ILCs) have potent immune functions in experimental conditions in mice, but their contribution to immunity in natural conditions in humans remains unclear. We investigated the presence of ILCs in a cohort of patients with severe combined immunodeficiency (SCID). All ILC subsets were absent in SCID patients carrying mutations of IL2RG or JAK3. T cell reconstitution was observed in SCID patients upon hematopoietic stem cell transplantation (HSCT), but the patients still exhibited drastic reduction of ILCs in the absence of myeloablation, at the exception of rare cases of ILC1 reconstitution. Remarkably, the observed ILC deficiencies were not associated with any particular susceptibility to disease, with a follow-up extending from 7 to 39 years after HSCT. We thus report here the first cases of selective ILC deficiency in humans, and show that ILCs may be dispensable in natural conditions, if T cells are present and B cell function is preserved.

Graft-Infiltrating Macrophages Adopt an M2 Phenotype and Are Inhibited by Purinergic Receptor P2X7 Antagonist in Chronic Rejection

Macrophages exhibit diverse phenotypes and functions; they are also a major cell type infiltrating chronically rejected allografts. The exact phenotypes and roles of macrophages in chronic graft loss remain poorly defined. In the present study, we used a mouse heart transplant model to examine macrophages in chronic allograft rejection. We found that treatment of C57BL/6 mice with CTLA4 immunoglobulin fusion protein (CTLA4-Ig) prevented acute rejection of a Balb/c heart allograft but allowed chronic rejection to develop over time, characterized by prominent neointima formation in the graft. There was extensive macrophage infiltration in the chronically rejected allografts, and the graft-infiltrating macrophages expressed markers associated with M2 cells but not M1 cells. In an in vitro system in which macrophages were polarized into either M1 or M2 cells, we screened phenotypic differences between M1 and M2 cells and identified purinergic receptor P2X7 (P2x7r), an adenosine triphosphate (ATP)-gated ion channel protein that was preferentially expressed by M2 cells. We further showed that blocking the P2x7r using oxidized ATP (oATP) inhibited M2 induction in a dose-dependent fashion in vitro. Moreover, treatment of C57BL/6 recipients with the P2x7r antagonist oATP, in addition to CTLA4-Ig treatment, inhibited graft-infiltrating M2 cells, prevented transplant vasculopathy, and induced long-term heart allografts survival. These findings highlight the importance of the P2x7r-M2 axis in chronic rejection and establish P2x7r as a potential therapeutic target in suppression of chronic rejection.