At the bedside: the emergence of group 2 innate lymphoid cells in human disease

Evolving Perspectives on Innate Immune Mechanisms of IPF

While epithelial-fibroblast interactions are viewed as the primary drivers of Idiopathic Pulmonary Fibrosis (IPF), evidence gleaned from animal modeling and human studies implicates innate immunity as well. To provide perspective on this topic, this review synthesizes the available data regarding the complex role of innate immunity in IPF. The role of substances present in the fibrotic microenvironment including pathogen associated molecular patterns (PAMPs) derived from invading or commensal microbes, and danger associated molecular patterns (DAMPs) derived from injured cells and tissues will be discussed along with the proposed contribution of innate immune populations such as macrophages, neutrophils, fibrocytes, myeloid suppressor cells, and innate lymphoid cells. Each component will be considered in the context of its relationship to environmental and genetic factors, disease outcomes, and potential therapies. We conclude with discussion of unanswered questions and opportunities for future study in this area.

A WAO - ARIA - GA2LEN consensus document on molecular-based allergy diagnosis (PAMD@): Update 2020

Precision allergy molecular diagnostic applications (PAMD@) is increasingly entering routine care. Currently, more than 130 allergenic molecules from more than 50 allergy sources are commercially available for in vitro specific immunoglobulin E (sIgE) testing. Since the last publication of this consensus document, a great deal of new information has become available regarding this topic, with over 100 publications in the last year alone. It thus seems quite reasonable to publish an update. It is imperative that clinicians and immunologists specifically trained in allergology keep abreast of the new and rapidly evolving evidence available for PAMD@.

PAMD@ may initially appear complex to interpret; however, with increasing experience, the information gained provides relevant information for the allergist. This is especially true for food allergy, Hymenoptera allergy, and for the selection of allergen immunotherapy. Nevertheless, all sIgE tests, including PAMD@, should be evaluated within the framework of a patient's clinical history, because allergen sensitization does not necessarily imply clinical relevant allergies.

The skin as an immune organ: Tolerance versus effector responses and applications to food allergy and hypersensitivity reactions

Skin is replete with immunocompetent cells that modulate signaling pathways to maintain a salubrious immunogenic/tolerogenic balance. This fertile immune environment plays a significant role in the development of allergic responses and sensitivities, but the mechanisms underlying these pathways have been underappreciated and underused with respect to developing therapeutics. Among the complex repertoire of cells that promote tolerogenic pathways in the periphery, 2 key classes include dendritic cells and regulatory T (Treg) cells. Immature dendritic cells are the first line of defense, patrolling the periphery, sampling antigens, and secreting cytokines that suppress immune cells and promote the survival of Treg cells. Skin-homing Treg cells also play a critical role in mitigating the reactivity of immune cells, secreting high levels of cytokines that promote tolerance. Therapeutic approaches that capitalize on our knowledge of the rich cellular and molecular environment are emerging and show great promise. We will discuss the advantages and challenges of 5 such strategies and how these therapies might mitigate the atopic march by facilitating tolerance. We conclude that skin is a multifaceted structure that provides a fertile ground for therapeutic discovery. Accordingly, ongoing work in this domain will no doubt continue to deliver exciting progress for improved health outcomes.

ILC2s-Trailblazers in the Host Response Against Intestinal Helminths

Group 2 innate lymphoid cells (ILC2s) were first discovered in experimental studies of intestinal helminth infection—and much of our current knowledge of ILC2 activation and function is based on the use of these models. It is perhaps not surprising therefore that these cells have also been found to play a key role in mediating protection against these large multicellular parasites. ILC2s have been intensively studied over the last decade, and are known to respond quickly and robustly to the presence of helminths—both by increasing in number and producing type 2 cytokines. These mediators function to activate and repair epithelial barriers, to recruit other innate cells such as eosinophils, and to help activate T helper 2 cells. More recent investigations have focused on the mechanisms by which the host senses helminth parasites to activate ILC2s. Such studies have identified novel stromal cell types as being involved in this process—including intestinal tuft cells and enteric neurons, which respond to the presence of helminths and activate ILC2s by producing IL-25 and Neuromedin, respectively. In the current review, we will outline the latest insights into ILC2 activation and discuss the requirement for—or redundancy of—ILC2s in providing protective immunity against intestinal helminth parasites.

Runx/Cbfβ complexes protect group 2 innate lymphoid cells from exhausted-like hyporesponsiveness during allergic airway inflammation

Group 2 innate lymphoid cells (ILC2s) have tissue-resident competence and contribute to the pathogenesis of allergic diseases. However, the mechanisms regulating prolonged ILC2-mediated TH2 cytokine production under chronic inflammatory conditions are unclear. Here we show that, at homeostasis, Runx deficiency induces excessive ILC2 activation due to overly active GATA-3 functions. By contrast, during allergic inflammation, the absence of Runx impairs the ability of ILC2s to proliferate and produce effector TH2 cytokines and chemokines. Instead, functional deletion of Runx induces the expression of exhaustion markers, such as IL-10 and TIGIT, on ILC2s. Finally, these 'exhausted-like' ILC2s are unable to induce type 2 immune responses to repeated allergen exposures. Thus, Runx confers competence for sustained ILC2 activity at the mucosa, and contributes to allergic pathogenesis.

The Role of Immune and Inflammatory Cells in Idiopathic Pulmonary Fibrosis

The contribution of the immune system to idiopathic pulmonary fibrosis (IPF) remains poorly understood. While most sources agree that IPF does not result from a primary immunopathogenic mechanism, evidence gleaned from animal modeling and human studies suggests that innate and adaptive immune processes can orchestrate existing fibrotic responses. This review will synthesize the available data regarding the complex role of professional immune cells in IPF. The role of innate immune populations such as monocytes, macrophages, myeloid suppressor cells, and innate lymphoid cells will be discussed, as will the activation of these cells via pathogen-associated molecular patterns derived from invading or commensural microbes, and danger-associated molecular patterns derived from injured cells and tissues. The contribution of adaptive immune responses driven by T-helper cells and B cells will be reviewed as well. Each form of immune activation will be discussed in the context of its relationship to environmental and genetic factors, disease outcomes, and potential therapies. We conclude with discussion of unanswered questions and opportunities for future study in this area.

A Major Population of Functional KLRG1- ILC2s in Female Lungs Contributes to a Sex Bias in ILC2 Numbers

Humans show significant sex differences in the incidence and severity of respiratory diseases, including asthma and virus infection. Sex hormones contribute to the female sex bias in type 2 inflammation associated with respiratory diseases, consistent with recent reports that female lungs harbor greater numbers of GATA-3-dependent group 2 innate lymphoid cells (ILC2s). In this study, we determined whether sex hormone levels govern sex differences in the numbers, phenotype, and function of ILC2s in the murine lung and bone marrow (BM). Our data show that lungs of female mice harbor significantly greater ILC2 numbers in homeostasis, in part due to a major subset of ILC2s lacking killer-cell lectin like receptor G1 (KLRG1), a population largely absent in male lungs. The KLRG1^- ILC2s were capable of type 2 cytokine production and increased with age after sexual maturity, suggesting that a unique functional subset exists in females. Experiments with gonadectomized mice or mice bearing either global or lymphocyte restricted estrogen receptor α (Esr1) deficiency showed that androgens rather than estrogens regulated numbers of the KLRG1^- ILC2 subset and ILC2 functional capacity in the lung and BM, as well as levels of GATA-3 expression in BM ILC2s. Furthermore, the frequency of BM PLZF⁺ ILC precursors was higher in males and increased by excess androgens, suggesting that androgens act to inhibit the transition of ILC precursors to ILC2s. Taken together, these data show that a functional subset of KLRG1^- ILC2s in females contributes to the sex bias in lung ILC2s that is observed after reproductive age.

Group 2 ILCs: A way of enhancing immune protection against human helminths?

Group 2 innate lymphoid cells (ILC2s) play crucial roles in type 2 immune responses associated with allergic and autoimmune diseases, viral and helminth infections and tissue homoeostasis. Experimental models show that in helminth infections ILC2s provide an early source of type 2 cytokines and therefore are essential for the induction of potentially protective type 2 responses. Much of our knowledge of ILC2s in helminth infections has come from experimental mouse models with very few studies analysing ILC2s in natural human infections. In attempts to harness knowledge from paradigms of the development of protective immunity in human helminth infections for vaccine development, the role of ILC2 cells could be pivotal. So far, potential vaccines against human helminth infections have failed to provide effective protection when evaluated in human studies. In addition to appropriate antigen selection, it is apparent that more detailed knowledge on mechanisms of induction and maintenance of protective immune responses is required. Therefore, there is need to understand how ILC2 cells induce type 2 responses and subsequently support the development of a protective immune response in the context of immunizations. Within this review, we summarize the current knowledge of the biology of ILC2s, discuss the importance of ILC2s in human helminth infections and explore how ILC2 responses could be boosted to efficiently induce protective immunity.

Emerging concepts and future challenges in innate lymphoid cell biology

Innate lymphoid cells (ILCs) are innate immune cells that are ubiquitously distributed in lymphoid and nonlymphoid tissues and enriched at mucosal and barrier surfaces. Three major ILC subsets are recognized in mice and humans. Each of these subsets interacts with innate and adaptive immune cells and integrates cues from the epithelium, the microbiota, and pathogens to regulate inflammation, immunity, tissue repair, and metabolic homeostasis. Although intense study has elucidated many aspects of ILC development, phenotype, and function, numerous challenges remain in the field of ILC biology. In particular, recent work has highlighted key new questions regarding how these cells communicate with their environment and other cell types during health and disease. This review summarizes new findings in this rapidly developing field that showcase the critical role ILCs play in directing immune responses through their ability to interact with a variety of hematopoietic and nonhematopoietic cells. In addition, we define remaining challenges and emerging questions facing the field. Finally, this review discusses the potential application of basic studies of ILC biology to the development of new treatments for human patients with inflammatory and infectious diseases in which ILCs play a role.

Human circulating group 2 innate lymphoid cells can express CD154 and promote IgE production

BACKGROUND

Protection against helminths consists of adaptive responses by T_H2 cells and innate responses by group 2 innate lymphoid cells (ILC2s), with these latter being well characterized in mice but less so in human subjects.

OBJECTIVE

We sought to characterize human circulating ILC2s and compare their functional profile with that of autologous T_H2 cells.

METHODS

Circulating ILC2s and T_H2 cells were isolated by means of fluorescence-activated cell sorting and magnetic cell sorting and expanded in vitro. ILC2s were then stimulated with phorbol 12-myristate 13-acetate plus ionomycin, IL-25 plus IL-33 (IL-25/IL-33), or a mixture of Toll-like receptor ligands to evaluate their ability to produce cytokines, express CD154, and induce IgE production by autologous B cells. Cytokines and transcription factor gene methylation were assessed.

RESULTS

ILC2s expressed GATA-3, retinoic acid orphan receptor (RORC) 2, and RORα; were able to produce IL-5, IL-13, and IL-4; and, accordingly, were characterized by demethylation of IL4, IL13, IL5, GATA3, and RORC2, whereas the IFNG, IFNG promoter, and TBX21 regions of interest were methylated. ILC2s expressed TLR1, TLR4, and TLR6, and TLR stimulation induced IL-5 and IL-13 production. Moreover, ILC2s expressed CD154 in response to phorbol 12-myristate 13-acetate plus ionomycin, IL-25/IL-33, or a mixture of TLR ligands. Stimulated ILC2s also induced IgM, IgG, IgA, and IgE production by B cells. Finally, circulating ILC2s from atopic patients were not different in numbers and frequency but expressed higher IL-4 levels than those from nonatopic subjects.

CONCLUSION

This study provides the first evidence that human ILC2s can express CD154 and stimulate the production of IgE by B lymphocytes through IL-25/IL-33 stimulation or TLR triggering.

Targeting key proximal drivers of type 2 inflammation in disease

Systemic type 2 inflammation encompassing T helper 2 (TH2)-type responses is emerging as a unifying feature of both classically defined allergic diseases, such as asthma, and a range of other inflammatory diseases. Rather than reducing inflammation with broad-acting immunosuppressants or narrowly targeting downstream products of the TH2 pathway, such as immunoglobulin E (IgE), efforts to target the key proximal type 2 cytokines - interleukin-4 (IL-4), IL-5 and IL-13 - represent a promising strategy to achieve therapeutic benefit across multiple diseases. After several initial disappointing clinical results with therapies targeting IL-4, IL-5 or IL-13 in asthma, applying a personalized approach achieved therapeutic benefit in an asthma subtype exhibiting an 'allergic' phenotype. More recently, efficacy was extended into a broad population of people with asthma. This argues that the Type 2 inflammation is broadly relevant across the severe asthma population if the key upstream drivers are properly blocked. Moreover, the simultaneous inhibition of IL-4 and IL-13 has shown significant clinical activity in diseases that are often co-morbid with asthma - atopic dermatitis and chronic sinusitis with nasal polyps - supporting the hypothesis that targeting a central 'driver pathway' could benefit multiple allergic diseases.

Interleukin-1 Family Cytokines in Liver Diseases

The gene encoding IL-1 was sequenced more than 30 years ago, and many related cytokines, such as IL-18, IL-33, IL-36, IL-37, IL-38, IL-1 receptor antagonist (IL-1Ra), and IL-36Ra, have since been identified. IL-1 is a potent proinflammatory cytokine and is involved in various inflammatory diseases. Other IL-1 family ligands are critical for the development of diverse diseases, including inflammatory and allergic diseases. Only IL-1Ra possesses the leader peptide required for secretion from cells, and many ligands require posttranslational processing for activation. Some require inflammasome-mediated processing for activation and release, whereas others serve as alarmins and are released following cell membrane rupture, for example, by pyroptosis or necroptosis. Thus, each ligand has the proper molecular process to exert its own biological functions. In this review, we will give a brief introduction to the IL-1 family cytokines and discuss their pivotal roles in the development of various liver diseases in association with immune responses. For example, an excess of IL-33 causes liver fibrosis in mice via activation and expansion of group 2 innate lymphoid cells to produce type 2 cytokines, resulting in cell conversion into pro-fibrotic M2 macrophages. Finally, we will discuss the importance of IL-1 family cytokine-mediated molecular and cellular networks in the development of acute and chronic liver diseases.

At the bench: understanding group 2 innate lymphoid cells in disease

The conventional paradigm of type 2 inflammatory responses is characterized by activation of CD4(+) Th2 cells that produce IL-4, IL-5, and IL-13, resulting in tissue eosinophil infiltration, mucus metaplasia, AHR, and IgE production. However, the recent discovery of ILC2s in mice and humans has brought forth a novel pathway in type 2 immunity that may work independent of, or in concert with, adaptive Th2 responses. ILC2s were described initially as lineage-negative lymphocytes that produce high levels of Th2 cytokines IL-5 and IL-13 in response to IL-25 and IL-33 and promote protection against helminth infections. More recent investigations have identified novel upstream regulators, as well as novel ILC2 products. ILC2s are found in mucosal surfaces, including respiratory tract and skin, and studies from experimental asthma and atopic dermatitis models support a role for ILC2s in promoting type 2 inflammatory responses. There are many unanswered questions about the role of ILC2s in chronic allergic diseases, including how ILC2s or upstream pathways can be targeted for therapy. As ILC2s are not antigen specific and may be activated after exposures to a variety of infectious agents and irritants thought to contribute to respiratory and skin diseases, future strategies to target ILC2 function in human disease may be promising. Our intent is to identify priority areas for ILC2 translational research based on basic research insights.