Beyond NK cells: the expanding universe of innate lymphoid cells

For a long time, natural killer (NK) cells were thought to be the only innate immune lymphoid population capable of responding to invading pathogens under the influence of changing environmental cues. In the last few years, an increasing amount of evidence has shown that a number of different innate lymphoid cell (ILC) populations found at mucosal sites rapidly respond to locally produced cytokines in order to establish or maintain homeostasis. These ILC populations closely mirror the phenotype of adaptive T helper subsets in their repertoire of secreted soluble factors. Early in the immune response, ILCs are responsible for setting the stage to mount an adaptive T cell response that is appropriate for the incoming insult. Here, we review the diversity of ILC subsets and discuss similarities and differences between ILCs and NK cells in function and key transcriptional factors required for their development.

The interplay between the gut microbiota and the immune system

The impact of the gut microbiota on immune homeostasis within the gut and, importantly, also at systemic sites has gained tremendous research interest over the last few years. The intestinal microbiota is an integral component of a fascinating ecosystem that interacts with and benefits its host on several complex levels to achieve a mutualistic relationship. Host-microbial homeostasis involves appropriate immune regulation within the gut mucosa to maintain a healthy gut while preventing uncontrolled immune responses against the beneficial commensal microbiota potentially leading to chronic inflammatory bowel diseases (IBD). Furthermore, recent studies suggest that the microbiota composition might impact on the susceptibility to immune-mediated disorders such as autoimmunity and allergy. Understanding how the microbiota modulates susceptibility to these diseases is an important step toward better prevention or treatment options for such diseases.

Recognition strategies of group 3 innate lymphoid cells

During the early phase of an inflammatory response, innate cells can use different strategies to sense environmental danger. These include the direct interaction of specific activating receptors with pathogen-encoded/danger molecules or the engagement of cytokine receptors by pro-inflammatory mediators produced by antigen presenting cells in the course of the infection. These general recognition strategies, which have been extensively described for innate myeloid cells, are shared by innate lymphoid cells (ILC), such as Natural Killer (NK) cells. The family of ILC has recently expanded with the discovery of group 2 (ILC2) and group 3 ILC (ILC3), which play an important role in the defense against extracellular pathogens. Although ILC3 and NK cells share some phenotypic characteristics, the recognition strategies employed by the various ILC3 subsets have been only partially characterized. In this review, we will describe and comparatively discuss how ILC3 sense environmental cues and how the triggering of different receptors may regulate their functional behavior during an immune response.

Evaluation of the T helper 17 cell specific genes and the innate lymphoid cells counts in the peripheral blood of patients with the common variable immunodeficiency

BACKGROUND

Common variable immunodeficiency (CVID) is characterized by a deficiency in the immune system with a heterogeneous collection of disorders resulting in antibody deficiency and recurrent infections. T helper 17 (Th17) cells promote B-cell survival and synergize with the B-cell activating factor to induce their differentiation into the plasma cells. A sub-population of innate lymphoid cells (ILCs) also produces interleukin 17 (IL-17). This study aimed to measure the Th17 specific genes and ILCs counts in the CVID patients in comparison with control subjects.

MATERIALS AND METHODS

Total messenger ribonucleic acid (mRNA) was extracted from the whole blood samples of 10 CVID patients and 10 healthy individuals. IL-17, retinoic acid receptor-related orphan receptor C2 (RORC2), IL-23R, and IL-9 gene expression were measured using the quantitative reverse transcriptase-polymerase chain reaction. Count of lineage negative/CD127(+)/CD90(+) ILCs in the blood samples was performed by the flow cytometry method.

RESULTS

The transcript levels of IL-17 and RORC2 in CVID patients was strongly lower than control subjects (P = 0.049 and P = 0.046, respectively), but slight reduction in the IL-23R expression (P = 0.252) have seen in the CVID patients. Accordingly, the number of ILCs decreased significantly (P = 0.04). Interestingly, IL-9 mRNA level was more significantly in the CVID patients (P = 0.001).

CONCLUSION

The results presented in this study show that the Th17 cell specific genes expression (as the determiner Th17 cells) and ILCs (another lymphoid source of IL-17) are decreased in patients with CVID and this could be an explanation for the defect of their humoral immune response. In addition, elevation of the IL-9 gene expression may shed a new light into the way toward the understanding of the mechanism of autoimmunity in the CVID patients.

Aryl hydrocarbon receptor plays protective roles in ConA-induced hepatic injury by both suppressing IFN-γ expression and inducing IL-22

The aryl hydrocarbon receptor (AhR), a ligand-activated nuclear transcription factor, is known to mediate the toxic and carcinogenic effects of various environmental pollutants, while AhR has been shown to protect animals from various types of tissue injury. ConA-induced hepatitis is known as a mouse model of acute liver injury. Here, we found a protective role of AhR in ConA-induced hepatitis. AhR is induced in the liver during ConA-induced hepatitis, and Ahr (-/-) mice were highly sensitive to this model. Bone marrow chimera experiments indicate that Ahr (-/-) hematopoietic cells are responsible for hypersensitivity to ConA-induced hepatitis. We found that IFN-γ from invariant NKT cells was up-regulated and IL-22 from innate lymphoid cells (ILCs) was abolished in Ahr (-/-) mice. In addition, IL-22 production was still observed in Rag2 (-/-) mice but it was severely reduced in Ahr (-/-) Rag2 (-/-) mice. ConA-induced IL-22 production was also dependent on retinoic acid-related orphan receptor γt. These results show that AhR has crucial protective roles in ConA-induced liver injury via promoting IL-22 production from ILCs and suppressing IFN-γ expression from NKT cells.

Parasites: what are they good for?

Parasitic diseases caused by helminth and protozoan infections remain one of the largest global public health problems for mankind. While natural immunity in man is rare or slow to develop for many parasites, the immune response is capable of recognizing and responding to infection by utilizing a number of different immunological mechanisms. This special topics journal issue examines many of the key findings in the recent literature regarding the immune response against helminth and protozoan infections, as well as highlighting areas in which our current knowledge falls short. The question of how we can tailor immune responses to prevent or reduce disease burden is a burning question within the field of immunoparasitology.

Interleukin-17-producing innate lymphoid cells and the NLRP3 inflammasome facilitate obesity-associated airway hyperreactivity

Obesity is associated with the development of asthma, which is often difficult to control. To understand the immunological pathways that lead to obesity-associated asthma, we fed mice a high-fat diet for 12 weeks, which resulted in obesity and the development of airway hyperreactivity (AHR), a cardinal feature of asthma. This AHR was independent of adaptive immunity, as it occurred in obese Rag1(-/-) mice, which lack B and T cells, and was dependent on interleukin-17A (IL-17A) and the NLRP3 inflammasome, as it did not develop in obese Il17a(-/-) or Nlrp3(-/-) mice. AHR was also associated with the expansion of CCR6(+) type 3 innate lymphoid cells (ILCs) producing IL-17A (ILC3 cells) in the lung, which could by themselves mediate AHR when adoptively transferred into Rag2(-/-); Il2rg(-/-) mice treated with recombinant IL-1β. Macrophage-derived IL-1β production was induced by HFD and expanded the number of lung ILC3 cells. Blockade of IL-1β with an IL-1 receptor antagonist abolished obesity-induced AHR and reduced the number of ILC3 cells. As we found ILC3-like cells in the bronchoalveolar lavage fluid of individuals with asthma, we suggest that obesity-associated asthma is facilitated by inflammation mediated by NLRP3, IL-1β and ILC3 cells.

Roles for helper T cell lineage-specifying transcription factors in cellular specialization

The development of specialized helper T cells has garnered much attention because of their critical role in coordinating the immune response to invading pathogens. Recent research emphasizing novel functions for specialized helper T cells in a variety of infectious disease settings, as well as autoimmune states, has reshaped our view on the capabilities of helper T cells. Notably, one previously underappreciated aspect of the lifespan of helper T cells is that they often retain the capacity to respond to changes in the environment by altering the composition of helper T cell lineage-specifying transcription factors they express, which, in turn, changes their phenotype. This emerging realization is changing our views on the stability versus flexibility of specialized helper T cell subtypes. Now, there is a new concerted effort to define the mechanistic events that contribute to the potential for flexibility in specialized helper T cell gene expression programs in the different environmental circumstances that allow for the re-expression of helper T cell lineage-specifying transcription factors. In addition, we are also now beginning to appreciate that "helper T cell" lineage-specifying transcription factors are expressed in diverse types of innate and adaptive immune cells and this may allow them to play roles in coordinating aspects of the immune response. Our current challenges include defining the conserved mechanisms that are utilized by these lineage-specifying transcription factors to coordinate gene expression programs in different settings as well as the mechanistic events that contribute to the differential downstream consequences that these factors mediate in unique cellular environments. In this review, we will explore our evolving views on these topics, often times using the Th1-lineage-specifying transcription factor T-bet as an example.

Modulation of Signal Strength Switches Notch from an Inducer of T Cells to an Inducer of ILC2

Innate lymphoid cells (ILCs) are emerging key players of the immune system with close lineage relationship to T cells. ILC2 play an important role in protective immunity against multicellular parasites, but are also involved in the pathogenesis of type 2 immune diseases. Here, we have studied the developmental requirements for human ILC2. We report that ILC2 are present in the thymus of young human donors, possibly reflecting local differentiation. Furthermore, we show that uncommitted lineage⁻CD34⁺CD1a⁻human thymic progenitors have the capacity to develop into ILC2 in vitro under the influence of Notch signaling, either by stimulation with the Notch ligand Delta like 1 (Dll1) or by expression of the active intracellular domain of NOTCH1 (NICD1). The capacity of NICD1 to mobilize the ILC2 differentiation program was sufficiently potent to override commitment to the T cell lineage in CD34⁺CD1a⁺ progenitors and force them into the ILC2 lineage. As Notch is an important factor also for T cell development, these results raise the question how one and the same signaling pathway can elicit such distinct developmental outcomes from the same precursors. We provide evidence that Notch signal strength is a critical determinant in this decision: by tuning signal amplitude, Notch can be converted from a T cell inducer (low signal strength) to an ILC2 inducer (high signal strength). Thus, this study enhances our understanding of human ILC2 development and identifies a mechanism determining specificity of Notch signal output during T cell and ILC2 differentiation.

Interleukin-33/ST2 axis promotes breast cancer growth and metastases by facilitating intratumoral accumulation of immunosuppressive and innate lymphoid cells

The role of IL-33/ST2 pathway in antitumor immunity is unclear. Using 4T1 breast cancer model we demonstrate time-dependent increase of endogenous IL-33 at both the mRNA and protein levels in primary tumors and metastatic lungs during cancer progression. Administration of IL-33 accelerated tumor growth and development of lung and liver metastases, which was associated with increased intratumoral accumulation of CD11b(+) Gr-1(+) TGF-β1(+) myeloid-derived suppressor cells (MDSCs) that expressed IL-13α1R, IL-13-producing Lin(-) Sca-1(+) ST2(+) innate lymphoid cells (ILCs) and CD4(+) Foxp3(+) ST2(+) IL-10(+) Tregs compared to untreated mice. Higher incidence of monocytic vs. granulocytic MDSCs and plasmocytoid vs. conventional dendritic cells (DCs) was present in mammary tumors of IL-33-treated mice. Intratumoral NKp46(+) NKG2D(+) and NKp46(+) FasL(+) cells were markedly reduced after IL-33 treatment, while phosphate-buffered saline-treated ST2-deficient mice had increased frequencies of these tumoricidal natural killer (NK) cells compared to untreated wild-type mice. IL-33 promoted intratumoral cell proliferation and neovascularization, which was attenuated in the absence of ST2. Tumor-bearing mice given IL-33 had increased percentages of splenic MDSCs, Lin(-) Sca-1(+) ILCs, IL-10-expressing CD11c(+) DCs and alternatively activated M2 macrophages and higher circulating levels of IL-10 and IL-13. A significantly reduced NK cell, but not CD8(+) T-cell cytotoxicity in IL-33-treated mice was observed and the mammary tumor progression was not affected when CD8(+) T cells were in vivo depleted. We show a previously unrecognized role for IL-33 in promoting breast cancer progression through increased intratumoral accumulation of immunosuppressive cells and by diminishing innate antitumor immunity. Therefore, IL-33 may be considered as an important mediator in the regulation of breast cancer progression.

Tissue-specific effector functions of innate lymphoid cells

Innate lymphoid cells (ILCs) is the collective term for a group of related innate lymphocytes, including natural killer (NK) cells and the more recently discovered non-NK ILCs, which all lack rearranged antigen receptors such as those expressed by T and B cells. Similar to NK cells, the newly discovered ILCs depend on the transcription factor Id2 and the common γ-chain of the interleukin-2 receptor for development. However, in contrast to NK cells, non-NK ILCs also require interleukin-7. In addition to the cytotoxic functions of NK cells, assuring protection against tumour development and viruses, new data indicate that ILCs contribute to a wide range of homeostatic and pathophysiological conditions in various organs via specialized cytokine production capabilities. Here we summarize current knowledge on ILCs with a particular emphasis on their tissue-specific effector functions, in the gut, liver, lungs and uterus. When possible, we try to highlight the role that these cells play in humans.

T-bet in the spot light: roles in distinct T-cell fate determination

The transcription factor T-bet was originally described to be important for the differentiation of the CD4(+) Th1 subset. More recent investigations implicate T-bet in the lineage commitment of a variety of innate immune cells also. The T-bet appears to have a dual role in the immune system. Under certain conditions T-bet provides a beneficial role, whereas the exaggerated expression of T-bet in certain innate lymphoid cells can be detrimental to the host. Therefore, this transcription factor needs to be carefully regulated. The feedback control and the epigenetic mechanisms involved in the expression of T-bet remain to be fully elucidated.

Functional perturbation of classical natural killer and innate lymphoid cells in the oral mucosa during SIV infection

Despite the fact that the majority of human pathogens are transmitted across mucosal surfaces, including the oral mucosae, oral immunity is poorly understood. Furthermore, because the normal flora of the oral cavity is vast and significantly diverse, host immunity must balance a complex system of tolerance and pathogen recognition. Due to the rapid recognition and response to pathogens, the innate immune system, including natural killer (NK) cells, likely plays a critical role in mediating this balance. Because logistical and ethical restraints limit access to significant quantities of human mucosal tissues, non-human primate models offer one of the best opportunities to study mucosal NK cells. In this study we have identified both classical NK cells, as well as innate lymphoid cells (ILCs) in tonsillar and buccal tissues and oral-draining lymph nodes. Identified by mutually exclusive expression of NKG2A and NKp44, NK cells, and ILCs in the oral mucosa are generally phenotypically and functionally analogous to their gut counterparts. NKG2A⁺ NK cells were more cytotoxic while NKp44⁺ ILCs produced copious amounts of IL-17 and TNF-α. However, in contrast to gut, oral NK cells and ILCs both produced large quantities of IFN-γ and the beta-chemokine, MIP-1β. Also in contrast to what we have previously found in gut tissues of SIV-infected macaques, we found no reduction in NK cells during chronic SIV infection, but rather an expansion of ILCs in oral-draining lymph nodes and tonsils. These data suggest that the lentivirus-induced depletion of the NK cell/ILC compartment in the gut may be absent in the oral mucosa, but the inherent differences and SIV-induced alterations are likely to have significant impact on preventing oral opportunistic infections in lentiviral disease. Furthermore, these data extend our understanding of the oral innate immune system in general and could aid future studies evaluating the regulation of both normal oral flora and limiting transmission of oral mucosal pathogens.

Immunoregulation by the gut microbiota

The human intestinal mucosa is constantly exposed to commensal microbiota. Since the gut microbiota is beneficial to the host, hosts have evolved intestine-specific immune systems to co-exist with the microbiota. On the other hand, the intestinal microbiota actively regulates the host's immune system, and recent studies have revealed that specific commensal bacterial species induce the accumulation of specific immune cell populations. For instance, segmented filamentous bacteria and Clostridium species belonging to clusters XIVa and IV induce the accumulation of Th17 cells in the small intestine and Foxp3(+) regulatory T cells in the large intestine, respectively. The immune cells induced by the gut microbiota likely contribute to intestinal homeostasis and influence systemic immunity in the host.

Functional Differences between Human NKp44(-) and NKp44(+) RORC(+) Innate Lymphoid Cells

Human RORC⁺ lymphoid tissue inducer cells are part of a rapidly expanding family of innate lymphoid cells (ILC) that participate in innate and adaptive immune responses as well as in lymphoid tissue (re) modeling. The assessment of a potential role for innate lymphocyte-derived cytokines in human homeostasis and disease is hampered by a poor characterization of RORC⁺ innate cell subsets and a lack of knowledge on the distribution of these cells in adults. Here we show that functionally distinct subsets of human RORC⁺ innate lymphoid cells are enriched for secretion of IL-17a or IL-22. Both subsets have an activated phenotype and can be distinguished based on the presence or absence of the natural cytotoxicity receptor NKp44. NKp44⁺ IL-22 producing cells are present in tonsils while NKp44⁻ IL-17a producing cells are present in fetal developing lymph nodes. Development of human intestinal NKp44⁺ ILC is a programmed event that is independent of bacterial colonization and these cells colonize the fetal intestine during the first trimester. In the adult intestine, NKp44⁺ ILC are the main ILC subset producing IL-22. NKp44⁻ ILC remain present throughout adulthood in peripheral non-inflamed lymph nodes as resting, non-cytokine producing cells. However, upon stimulation lymph node ILC can swiftly initiate cytokine transcription suggesting that secondary human lymphoid organs may function as a reservoir for innate lymphoid cells capable of participating in inflammatory responses.

Natural killer cell receptor-expressing innate lymphocytes: more than just NK cells

Recently, additional subsets that extend the family of innate lymphocytes have been discovered. Among these newly identified innate lymphoid cells is a subset sharing phenotypic characteristics of natural killer cells and lymphoid tissue inducer cells. These cells co-express the transcription factor RORγt and activating NK cell receptors (NKR), but their lineage and functional qualities remain poorly defined. Here, we discuss recent proposals to place these NKR(+)RORγt(+) innate lymphocytes on hematopoietic lineage maps. An overview of the transcriptional circuitry determining fate decisions of innate lymphocytes and a summary of current concepts concerning plasticity and stability of innate lymphocyte effector fates are provided. We will conclude by discussing the function of RORγt-expressing innate lymphocytes during inflammatory bowel diseases and in the immune response to tumors.

Identity, regulation and in vivo function of gut NKp46+RORγt+ and NKp46+RORγt- lymphoid cells

The gut is a major barrier against microbes and encloses various innate lymphoid cells (ILCs), including two subsets expressing the natural cytotoxicity receptor NKp46. A subset of NKp46(+) cells expresses retinoic acid receptor-related orphan receptor γt (RORγt) and produces IL-22, like lymphoid tissue inducer (LTi) cells. Other NKp46(+) cells lack RORγt and produce IFN-γ, like conventional Natural Killer (cNK) cells. The identity, the regulation and the in vivo functions of gut NKp46(+) ILCs largely remain to be unravelled. Using pan-genomic profiling, we showed here that small intestine (SI) NKp46(+)RORγt(-) ILCs correspond to SI NK cells. Conversely, we identified a transcriptional programme conserved in fetal LTi cells and adult SI NKp46(+)RORγt(+) and NKp46(-)RORγt(+) ILCs. We also demonstrated that the IL-1β/IL-1R1/MyD88 pathway, but not the commensal flora, drove IL-22 production by NKp46(+)RORγt(+) ILCs. Finally, oral Listeria monocytogenes infection induced IFN-γ production in SI NK and IL-22 production in NKp46(+)RORγt(+) ILCs, but only IFN-γ contributed to control bacteria dissemination. NKp46(+) ILC heterogeneity is thus associated with subset-specific transcriptional programmes and effector functions that govern their implication in gut innate immunity.

Control of epithelial cell function by interleukin-22-producing RORγt+ innate lymphoid cells

It is rapidly emerging that the defence system of innate lymphocytes is more diverse than previously recognized. In addition to natural killer (NK) cells, lymphoid tissue inducer (LTi) cells, and natural helper cells have now been identified. LTi cells are developmentally dependent on the orphan transcription factor RORγt and instruct lymph node development during embryogenesis. More recently, it has become evident, that in addition to their role for lymph organ development, LTi cells are also potent producers of cytokines such as interleukin-22 (IL-22) and IL-17 in adult mice. In addition to LTi cells, another RORγt-dependent innate lymphocyte subset co-expressing RORγt and NK cell receptors (NKRs) has been identified. These NKR(+) RORγt(+) cells are also potent producers of IL-22 but it is unclear whether they are part of the NK cell or LTi cell lineage. This review will highlight recent progress in understanding development and function of innate IL-22-producing lymphocyte subsets.