Type I interferon restricts type 2 immunopathology through the regulation of group 2 innate lymphoid cells.

Viral respiratory tract infections are the main causative agents of the onset of infection-induced asthma and asthma exacerbations that remain mechanistically unexplained. Here we found that deficiency in signaling via type I interferon receptor led to deregulated activation of group 2 innate lymphoid cells (ILC2 cells) and infection-associated type 2 immunopathology. Type I interferons directly and negatively regulated mouse and human ILC2 cells in a manner dependent on the transcriptional activator ISGF3 that led to altered cytokine production, cell proliferation and increased cell death. In addition, interferon-γ (IFN-γ) and interleukin 27 (IL-27) altered ILC2 function dependent on the transcription factor STAT1. These results demonstrate that type I and type II interferons, together with IL-27, regulate ILC2 cells to restrict type 2 immunopathology.

Non-redundant functions of group 2 innate lymphoid cells.

Protective immunity relies on the interplay of innate and adaptive immune cells with complementary and redundant functions. Innate lymphoid cells (ILCs) have recently emerged as tissue-resident, innate mirror images of the T cell system, with which they share lineage-specifying transcription factors and effector machinery1. Located at barrier surfaces, ILCs are among the first responders against invading pathogens and thus could potentially determine the outcome of the immune response2. However, so far it has not been possible to dissect the unique contributions of ILCs to protective immunity owing to limitations in specific targeting of ILC subsets. Thus, all of the available data have been generated either in mice lacking the adaptive immune system or with tools that also affect other immune cell subsets. In addition, it has been proposed that ILCs might be dispensable for a proper immune response because other immune cells could compensate for their absence3-7. Here we report the generation of a mouse model based on the neuromedin U receptor 1 (Nmur1) promoter as a driver for simultaneous expression of Cre recombinase and green fluorescent protein, which enables gene targeting in group 2 ILCs (ILC2s) without affecting other innate and adaptive immune cells. Using Cre-mediated gene deletion of Id2 and Gata3 in Nmur1-expressing cells, we generated mice with a selective and specific deficiency in ILC2s. ILC2-deficient mice have decreased eosinophil counts at steady state and are unable to recruit eosinophils to the airways in models of allergic asthma. Further, ILC2-deficient mice do not mount an appropriate immune and epithelial type 2 response, resulting in a profound defect in worm expulsion and a non-protective type 3 immune response. In total, our data establish non-redundant functions for ILC2s in the presence of adaptive immune cells at steady state and during disease and argue for a multilayered organization of the immune system on the basis of a spatiotemporal division of labour.

Untimely TGFß responses in COVID-19 limit antiviral functions of NK cells.

SARS-CoV-2 is a single-stranded RNA virus that causes COVID-19. Given its acute and often self-limiting course, it is likely that components of the innate immune system play a central part in controlling virus replication and determining clinical outcome. Natural killer (NK) cells are innate lymphocytes with notable activity against a broad range of viruses, including RNA viruses1,2. NK cell function may be altered during COVID-19 despite increased representation of NK cells with an activated and adaptive phenotype3,4. Here we show that a decline in viral load in COVID-19 correlates with NK cell status and that NK cells can control SARS-CoV-2 replication by recognizing infected target cells. In severe COVID-19, NK cells show defects in virus control, cytokine production and cell-mediated cytotoxicity despite high expression of cytotoxic effector molecules. Single-cell RNA sequencing of NK cells over the time course of the COVID-19 disease spectrum reveals a distinct gene expression signature. Transcriptional networks of interferon-driven NK cell activation are superimposed by a dominant transforming growth factor-ß (TGFß) response signature, with reduced expression of genes related to cell-cell adhesion, granule exocytosis and cell-mediated cytotoxicity. In severe COVID-19, serum levels of TGFß peak during the first two weeks of infection, and serum obtained from these patients severely inhibits NK cell function in a TGFß-dependent manner. Our data reveal that an untimely production of TGFß is a hallmark of severe COVID-19 and may inhibit NK cell function and early control of the virus.

The interaction of intestinal microbiota and innate lymphoid cells in health and disease throughout life.

Immunity is shaped by commensal microbiota. From early life onwards, microbes colonize mucosal surfaces of the body and thereby trigger the establishment of immune homeostasis and defense mechanisms. Recent evidence reveals that the family of innate lymphoid cells (ILCs), which are mainly located in mucosal tissues, are essential in the maintenance of barrier functions as well as in the initiation of an appropriate immune response upon pathogenic infection. In this review, we summarize recent insights on the functional interaction of microbiota and ILCs at steady-state and throughout life. Furthermore, we will discuss the interplay of ILCs and the microbiota in mucosal infections focusing on intestinal immunity.

Regulating the development of pulmonary Group 2 innate lymphoid cells.

Group 2 innate lymphoid cells (ILC2s) are members of the family of innate lymphoid cells and are innately committed to type 2 immune responses. In the lungs, ILC2s are the predominant population of innate lymphoid cells (ILCs) and their development is orchestrated by several different transcription factors ensuring lineage commitment by intrinsic regulation. ILC2s are present in the lungs from the foetal period onwards and are thus exposed to extrinsic regulation due to the airways' continuous morphological changes upon birth. In this review, we will briefly summarise the dependence of ILC2s on transcription factors and discuss recently described characteristics and function of early life ILC2s in the lungs.

Adenosine receptors differentially regulate type 2 cytokine production by IL-33-activated bone marrow cells, ILC2s, and macrophages.

Group 2 innate lymphoid cells (ILC2s) represent a rapid source of type 2 cytokines, such as IL-5 and IL-13, and play an important role in orchestrating type 2 immune response. Adenosine is an endogenous purine nucleoside, a catabolite of ATP that binds and activates ≥1 of 4 transmembrane G protein-coupled cell-surface adenosine receptors (ARs)-A1, A2A, A2B, and A3. Here, we studied the role of ARs in the regulation of cytokine production by ILC2s. We found that A2BARs suppress the production of both IL-5 and IL-13 by ILC2s, whereas A2AARs augment IL-5 production and fail to affect IL-13 release. Combined stimulation of all ARs led to the suppression of both IL-5 and IL-13 production, which indicated that A2BARs dominate A2AARs. Both pre- and post-transcriptional processes may be involved in the AR modulation of ILC2 IL-5 and IL-13 production. Thus, we identify adenosine as a novel negative regulator of ILC2 activation.-Csóka, B., Németh, Z. H., Duerr, C. U., Fritz, J. H., Pacher, P., Haskó, G. Adenosine receptors differentially regulate type 2 cytokine production by IL-33-activated bone marrow cells, ILC2s, and macrophages.

Regulation of group 2 innate lymphoid cells.

Group 2 innate lymphoid cells (ILC2) exert critical roles in type 2 immune responses, epithelial repair at mucosal tissues and metabolic homeostasis. ILC2 rapidly provide large amounts of type 2 signature cytokines, thereby driving type 2 immune responses such as the defense against helminths. However, if deregulated, ILC2 facilitate tissue fibrosis and trigger unwanted type 2 immunopathologies such as allergies, asthma and atopic dermatitis. Therefore, ILC2 need to be tightly regulated and we are just beginning to understand which mediators activate or inhibit this rare but important cell population. In this review, we summarize current knowledge about positive and negative regulation of ILC2 and discuss its immunological consequences.

TRIF signaling drives homeostatic intestinal epithelial antimicrobial peptide expression.

Recent results indicate a significant contribution of innate immune signaling to maintain mucosal homeostasis, but the precise underlying signal transduction pathways are ill-defined. By comparative analysis of intestinal epithelial cells isolated from conventionally raised and germ-free mice, as well as animals deficient in the adaptor molecules MyD88 and TRIF, the TLR3 and TLR4, as well as the type I and III IFN receptors, we demonstrate significant TLR-mediated signaling under homeostatic conditions. Surprisingly, homeostatic expression of Reg3γ and Paneth cell enteric antimicrobial peptides critically relied on TRIF and, in part, TLR3 but was independent of IFN receptor signaling. Reduced antimicrobial peptide expression was associated with significantly lower numbers of Paneth cells and a reduced Paneth cell maturation and differentiation factor expression in TRIF mutant compared with wild-type epithelium. This phenotype was not transferred to TRIF-sufficient germ-free animals during cohousing. Low antimicrobial peptide expression in TRIF-deficient mice caused reduced immediate killing of orally administered bacteria but was not associated with significant alterations in the overall composition of the enteric microbiota. The phenotype was rapidly restored in a TRIF-independent fashion after transient epithelial damage. Our results identify TRIF signaling as a truly homeostatic pathway to maintain intestinal epithelial barrier function revealing fundamental differences in the innate immune signaling between mucosal homeostasis and tissue repair.

A TNF-α-CCL20-CCR6 axis regulates Nod1-induced B cell responses.

Innate immune responses provoke the accumulation of leukocytes at sites of inflammation. In addition to monocytes and granulocytes, B cells also participate in antimicrobial innate immune responses; however, the mechanisms for accumulation of B cells to sites of inflammation are not well understood. To study B cell accumulation following systemic inflammation, we used a model synthetic ligand that stimulates a specific pattern recognition molecule, nucleotide-binding oligomerization domain-containing protein 1 (Nod1). Upon exposure to Nod1 agonists, both B cells and neutrophils rapidly accumulate within the spleen, and dendritic cells migrate into the periarterial lymphoid sheath. Nod1 stimulation led to a marked increase in several chemokines within the spleen, including CXCL13, CCL2, and CCL20. Whereas the lymphotoxin pathway was critical for the induction of the B cell chemoattractant CXCL13 in response to Nod1 agonists, B cell accumulation within the spleen following Nod1-induced systemic inflammation was independent of the lymphotoxin pathway. In contrast, a CCR6/CCL20 chemokine loop instructed rapid increase of B cells in the spleen in response to systemic administration of Nod1 agonists in a TNF-α-dependent manner. Moreover, CCR6 was required to regulate Nod1-mediated B cell responses. These results reveal a novel mechanism of B cells during inflammation and shed light on how B cells participate in innate immune responses to microbial stimulation.

The mammalian intestinal epithelium as integral player in the establishment and maintenance of host-microbial homeostasis.

Only one single layer of epithelial cells separates the densely colonized and environmentally exposed intestinal lumen from the largely sterile subepithelial tissue. Together with the overlaying mucus and the subepithelial mucosal immune system the epithelium has evolved to maintain homeostasis in the presence of the enteric microbiota. It also contributes to rapid and efficient antimicrobial host defence in the event of infection with pathogenic microorganisms. Both, epithelial antimicrobial host defence and homeostasis rely on signalling pathways induced by innate immune receptors demonstrating the active role of epithelial cells in the host-microbial interplay. The interaction of epithelial cells with professional immune cells illustrates the integrated function within the mucosal tissue. In the present review we focus on structural and functional changes of the intestinal epithelium during the fetal-neonatal transition and infancy and try to delineate its role in the induction and maintenance of host-microbial homeostasis. We also address factors that impair epithelial functions and may lead to disruption of the mucosal barrier, tissue damage and the development of symptomatic disease.

IL-33 signaling regulates innate and adaptive immunity to Cryptococcus neoformans.

Susceptibility to progressive infection with the fungus Cryptococcus neoformans is associated with an allergic pattern of lung inflammation, yet the factors that govern this host response are not clearly understood. Using a clinically relevant mouse model of inhalational infection with virulent C. neoformans H99, we demonstrate a role for IL-33-dependent signaling in host immune defense. Infection of BALB/c mice with 10(4) CFU of C. neoformans H99 caused a time-dependent induction of IL-33 with accumulation of type 2 pulmonary innate lymphoid cells and alternatively activated macrophages in the lungs as well as Th2-polarized CD4(+) T cells in draining lymph nodes. IL-33R subunit T1/ST2-deficient (T1/ST2(-/-)) mice infected with C. neoformans H99 had improved survival with a decreased fungal burden in the lungs, spleen, and brain, compared with wild-type mice. Signaling through T1/ST2 was required for the accumulation and early production of IL-5 and IL-13 by lung type 2 pulmonary innate lymphoid cells. Further analysis of T1/ST2(-/-) mice revealed increased fungicidal exudate macrophages in the lungs and decreased C. neoformans-specific Th2 cells in the mediastinal lymph nodes. T1/ST2 deficiency also diminished goblet cell hyperplasia, mucus hypersecretion, bronchoalveolar lavage eosinophilia, alternative activation of macrophages, and serum IgE. These observations demonstrate that IL-33-dependent signaling contributes to the expansion of innate type 2 immunity and subsequent Th2-biased lung immunopathology that facilitates C. neoformans growth and dissemination.

IFN-lambda determines the intestinal epithelial antiviral host defense.

Type I and type III IFNs bind to different cell-surface receptors but induce identical signal transduction pathways, leading to the expression of antiviral host effector molecules. Despite the fact that type III IFN (IFN-λ) has been shown to predominantly act on mucosal organs, in vivo infection studies have failed to attribute a specific, nonredundant function. Instead, a predominant role of type I IFN was observed, which was explained by the ubiquitous expression of the type I IFN receptor. Here we comparatively analyzed the role of functional IFN-λ and type I IFN receptor signaling in the innate immune response to intestinal rotavirus infection in vivo, and determined viral replication and antiviral gene expression on the cellular level. We observed that both suckling and adult mice lacking functional receptors for IFN-λ were impaired in the control of oral rotavirus infection, whereas animals lacking functional receptors for type I IFN were similar to wild-type mice. Using Mx1 protein accumulation as marker for IFN responsiveness of individual cells, we demonstrate that intestinal epithelial cells, which are the prime target cells of rotavirus, strongly responded to IFN-λ but only marginally to type I IFN in vivo. Systemic treatment of suckling mice with IFN-λ repressed rotavirus replication in the gut, whereas treatment with type I IFN was not effective. These results are unique in identifying a critical role of IFN-λ in the epithelial antiviral host defense.

ILC2 require cell-intrinsic ST2 signals to promote type 2 immune responses.

The initiation of type 2 immune responses at mucosal barriers is regulated by rapidly secreted cytokines called alarmins. The alarmins IL-33, IL-25 and TSLP are mainly secreted by stromal and epithelial cells in tissues and were linked to chronic inflammatory diseases, such as allergic lung inflammation, or to resistance against worm infections. Receptors for alarmins are expressed by a variety of immune cells, including group 2 innate lymphoid cells (ILC2s), an early source of the type 2 cytokines, such as IL-5 and IL-13, which have been linked to atopic diseases and anti-worm immunity as well. However, the precise contribution of the IL-33 receptor signals for ILC2 activation still needs to be completed due to limitations in targeting genes in ILC2. Using the newly established Nmur1 iCre-eGFP mouse model, we obtained specific conditional genetic ablation of the IL-33 receptor subunit ST2 in ILC2s. ST2-deficient ILC2s were unresponsive to IL-33 but not to stimulation with the alarmin IL-25. As a result of defective ST2 signals, ILC2s produced limited amounts of IL-5 and IL-13 and failed to support eosinophil homeostasis. Further, ST2-deficient ILC2s were unable to expand and promote the recruitment of eosinophils during allergic lung inflammation provoked by papain administration. During infection with Nippostrongylus brasiliensis, ILC2-intrinsic ST2 signals were required to mount an effective type 2 immune response against the parasite leading to higher susceptibility against worm infection in conditional knockout mice. Therefore, this study argues for a non-redundant role of cell-intrinsic ST2 signals triggering proper activation of ILC2 for initiation of type 2 immunity.

O-antigen delays lipopolysaccharide recognition and impairs antibacterial host defense in murine intestinal epithelial cells.

Although Toll-like receptor (TLR) 4 signals from the cell surface of myeloid cells, it is restricted to an intracellular compartment and requires ligand internalization in intestinal epithelial cells (IECs). Yet, the functional consequence of cell-type specific receptor localization and uptake-dependent lipopolysaccharide (LPS) recognition is unknown. Here, we demonstrate a strikingly delayed activation of IECs but not macrophages by wildtype Salmonella enterica subsp. enterica sv. (S.) Typhimurium as compared to isogenic O-antigen deficient mutants. Delayed epithelial activation is associated with impaired LPS internalization and retarded TLR4-mediated immune recognition. The O-antigen-mediated evasion from early epithelial innate immune activation significantly enhances intraepithelial bacterial survival in vitro and in vivo following oral challenge. These data identify O-antigen expression as an innate immune evasion mechanism during apical intestinal epithelial invasion and illustrate the importance of early innate immune recognition for efficient host defense against invading Salmonella.

The NF-κB Transcription Factor c-Rel Modulates Group 2 Innate Lymphoid Cell Effector Functions and Drives Allergic Airway Inflammation.

Group 2 innate lymphoid cells (ILC2s) play a key role in the initiation and orchestration of early type 2 immune responses. Upon tissue damage, ILC2s are activated by alarmins such as IL-33 and rapidly secrete large amounts of type 2 signature cytokines. ILC2 activation is governed by a network of transcriptional regulators including nuclear factor (NF)-κB family transcription factors. While it is known that activating IL-33 receptor signaling results in downstream NF-κB activation, the underlying molecular mechanisms remain elusive. Here, we found that the NF-κB subunit c-Rel is required to mount effective innate pulmonary type 2 immune responses. IL-33-mediated activation of ILC2s in vitro as well as in vivo was found to induce c-Rel mRNA and protein expression. In addition, we demonstrate that IL-33-mediated activation of ILC2s leads to nuclear translocation of c-Rel in pulmonary ILC2s. Although c-Rel was found to be a critical mediator of innate pulmonary type 2 immune responses, ILC2-intrinsic deficiency of c-Rel did not have an impact on the developmental capacity of ILC2s nor affected homeostatic numbers of lung-resident ILC2s at steady state. Moreover, we demonstrate that ILC2-intrinsic deficiency of c-Rel alters the capacity of ILC2s to upregulate the expression of ICOSL and OX40L, key stimulatory receptors, and the expression of type 2 signature cytokines IL-5, IL-9, IL-13, and granulocyte-macrophage colony-stimulating factor (GM-CSF). Collectively, our data using Rel-/- mice suggest that c-Rel promotes acute ILC2-driven allergic airway inflammation and suggest that c-Rel may contribute to the pathophysiology of ILC2-mediated allergic airway disease. It thereby represents a promising target for the treatment of allergic asthma, and evaluating the effect of established c-Rel inhibitors in this context would be of great clinical interest.

The MNK1/2-eIF4E Axis Supports Immune Suppression and Metastasis in Postpartum Breast Cancer.

Breast cancer diagnosed within 10 years following childbirth is defined as postpartum breast cancer (PPBC) and is highly metastatic. Interactions between immune cells and other stromal cells within the involuting mammary gland are fundamental in facilitating an aggressive tumor phenotype. The MNK1/2-eIF4E axis promotes translation of prometastatic mRNAs in tumor cells, but its role in modulating the function of nontumor cells in the PPBC microenvironment has not been explored. Here, we used a combination of in vivo PPBC models and in vitro assays to study the effects of inactivation of the MNK1/2-eIF4E axis on the protumor function of select cells of the tumor microenvironment. PPBC mice deficient for phospho-eIF4E (eIF4ES209A) were protected against lung metastasis and exhibited differences in the tumor and lung immune microenvironment compared with wild-type mice. Moreover, the expression of fibroblast-derived IL33, an alarmin known to induce invasion, was repressed upon MNK1/2-eIF4E axis inhibition. Imaging mass cytometry on PPBC and non-PPBC patient samples indicated that human PPBC contains phospho-eIF4E high-expressing tumor cells and CD8+ T cells displaying markers of an activated dysfunctional phenotype. Finally, inhibition of MNK1/2 combined with anti-PD-1 therapy blocked lung metastasis of PPBC. These findings implicate the involvement of the MNK1/2-eIF4E axis during PPBC metastasis and suggest a promising immunomodulatory route to enhance the efficacy of immunotherapy by blocking phospho-eIF4E. SIGNIFICANCE: This study investigates the MNK1/2-eIF4E signaling axis in tumor and stromal cells in metastatic breast cancer and reveals that MNK1/2 inhibition suppresses metastasis and sensitizes tumors to anti-PD-1 immunotherapy.

Internalization-dependent recognition of Mycobacterium avium ssp. paratuberculosis by intestinal epithelial cells.

Mycobacterium avium ssp. paratuberculosis (MAP) is the causative agent of Johne's disease, a highly prevalent chronic intestinal infection in domestic and wildlife ruminants. The microbial pathogenesis of MAP infection has attracted additional attention due to an association with the human enteric inflammatory Crohn's disease. MAP is acquired by the faecal-oral route prompting us to study the interaction with differentiated intestinal epithelial cells. MAP was rapidly internalized and accumulated in a late endosomal compartment. In contrast to other opportunistic mycobacteria or M. bovis, MAP induced significant epithelial activation as indicated by a NF-kappaB-independent but Erk-dependent chemokine secretion. Surprisingly, MAP-induced chemokine production was completely internalization-dependent as inhibition of Rac-dependent bacterial uptake abolished epithelial activation. In accordance, innate immune recognition of MAP by differentiated intestinal epithelial cells occurred through the intracellularly localized pattern recognition receptors toll-like receptor 9 and NOD1 with signal transduction via the adaptor molecules MyD88 and RIP2. The internalization-dependent innate immune activation of intestinal epithelial cells is in contrast to the stimulation of professional phagocytes by extracellular bacterial constituents and might significantly contribute to the histopathological changes observed during enteric MAP infection.

Group 2 Innate Lymphoid Cells in Respiratory Allergic Inflammation.

Millions of people worldwide are suffering from allergic inflammatory airway disorders. These conditions are regarded as a consequence of multiple imbalanced immune events resulting in an inadequate response with the exact underlying mechanisms still being a subject of ongoing research. Several cell populations have been proposed to be involved but it is becoming increasingly evident that group 2 innate lymphoid cells (ILC2s) play a key role in the initiation and orchestration of respiratory allergic inflammation. ILC2s are important mediators of inflammation but also tissue remodeling by secreting large amounts of signature cytokines within a short time period. Thereby, ILC2s instruct innate but also adaptive immune responses. Here, we will discuss the recent literature on allergic inflammation of the respiratory tract with a focus on ILC2 biology. Furthermore, we will highlight different therapeutic strategies to treat pulmonary allergic inflammation and their potential influence on ILC2 function as well as discuss the perspective of using human ILC2s for diagnostic purposes.

Group 2 Innate Lymphoid Cells in Pulmonary Immunity and Tissue Homeostasis.

Group 2 innate lymphoid cells (ILC2) represent an evolutionary rather old but only recently identified member of the family of innate lymphoid cells and have received much attention since their detailed description in 2010. They can orchestrate innate as well as adaptive immune responses as they interact with and influence several immune and non-immune cell populations. Moreover, ILC2 are able to rapidly secrete large amounts of type 2 cytokines that can contribute to protective but also detrimental host immune responses depending on timing, location, and physiological context. Interestingly, ILC2, despite their scarcity, are the dominant innate lymphoid cell population in the lung, indicating a key role as first responders and amplifiers upon immune challenge at this site. In addition, the recently described tissue residency of ILC2 further underlines the importance of their respective microenvironment. In this review, we provide an overview of lung physiology including a description of the most prominent pulmonary resident cells together with a review of known and potential ILC2 interactions within this unique environment. We will further outline recent observations regarding pulmonary ILC2 during immune challenge including respiratory infections and discuss different models and approaches to study ILC2 biology in the lung.