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

Effect of Probiotics on Respiratory Tract Allergic Disease and Gut Microbiota

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

AVIAN INNATE IMMUNITY WITH AN EMPHASIS ON CHICKEN MELANOMA DIFFERENTIATION-ASSOCIATED GENE 5 (MDA5)

Avian species have immune system to fight invading pathogens. The immune system comprises innate and adaptive immunity. Innate immunity relies on pattern recognition receptors to sense particular molecules present in pathogens, i.e. pathogen-associated molecular patterns (PAMPs), or danger signals in the environment, i.e. danger-associated molecular patterns (DAMPs). Cytoplasmic retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs) and nucleotide-binding oligomerization domain-like receptors (NLRs) are the sensors recognizing cytoplasmic PAMP and/or DAMP. Among common avian species, chickens do not have RIG-I whereas ducks and finches do. Therefore, the other RLR member, melanoma differentiation-associated gene 5 (MDA5), is believed to play an important role to recognize intracellular pathogens in chickens. Chicken MDA5 has been identified and its function determined. Chicken MDA5 maintains the same domain architecture compared with MDA5 analogs in other animal species. The expression of chicken MDA5 was upregulated when a synthetic double-stranded RNA (dsRNA), polyriboinosinic:polyribocytidylic acids (poly(I:C)), was transfected into chicken cells, whereas that did not change when cells were incubated with poly(I:C). The enhanced expression of chicken MDA5 in chicken cells upregulated the expression of chicken interferon-[Formula: see text] (IFN-[Formula: see text]). The infection of dsRNA infectious bursal disease virus (IBDV) in non-immune cells triggered the activation of chicken MDA5 signaling pathway, leading to the production of IFN-[Formula: see text] and subsequent response of IFN-stimulated genes. Furthermore, in immune cells like macrophages, chicken MDA5 participated in sensing the infection of IBDV by activating downstream antiviral genes and molecules and modulating adaptive immunity.

On the contrary, one of cytoplasmic NLR member, NLR family pyrin domain containing 3 (NLRP3), was cloned and functionally characterized in chicken cells. Chicken NLRP3 conserved the same domain architecture compared with NLRP3 analogs in other animal species. Chicken NLRP3 was highly expressed in kidney, bursa of Fabricius and spleen. The production of mature chicken interleukin 1 [Formula: see text] (IL-1[Formula: see text] in chicken macrophages was stimulated by lipopolysaccharide (LPS) treatment followed by short ATP exposure.

In summary, chicken MDA5 was a cytoplasmic dsRNA sensor that mediated the production of type I IFN upon ligand engagement, whereas NLRP3 sensed danger signals, such as ATP, in the cytoplasm and cleaved pro-IL-1[Formula: see text] to produce mature IL-1[Formula: see text]. Chicken MDA5 was not only involved in the activation of innate immune responses in non-immune and immune cells, but it also participated in modulating adaptive immunity in immune cells. Chicken NLRP3 participated in the production of mature chicken IL-1[Formula: see text] upon ligand engagement.

Pathogenic Fungal Infection in the Lung

Respiratory fungal infection is a severe clinical problem, especially in patients with compromised immune functions. Aspergillus, Cryptococcus, Pneumocystis, and endemic fungi are major pulmonary fungal pathogens that are able to result in life-threatening invasive diseases. Growing data being reported have indicated that multiple cells and molecules orchestrate the host's response to a fungal infection in the lung. Upon fungal challenge, innate myeloid cells including macrophages, dendritic cells (DC), and recruited neutrophils establish the first line of defense through the phagocytosis and secretion of cytokines. Natural killer cells control the fungal expansion in the lung via the direct and indirect killing of invading organisms. Adaptive immune cells including Th1 and Th17 cells confer anti-fungal activity by producing their signature cytokines, interferon-γ, and IL-17. In addition, lung epithelial cells (LEC) also participate in the resistance against fungal infection by internalization, inflammatory cytokine production, or antimicrobial peptide secretion. In the host cells mentioned above, various molecules with distinct functions modulate the immune defense signaling: Pattern recognition receptors (PRRs) such as dectin-1 expressed on the cell surface are involved in fungal recognition; adaptor proteins such as MyD88 and TRAF6 are required for transduction of signals to the nucleus for transcriptional regulation; inflammasomes also play crucial roles in the host's defense against a fungal infection in the lung. Furthermore, transcriptional factors modulate the transcriptions of a series of genes, especially those encoding cytokines and chemokines, which are predominant regulators in the infectious microenvironment, mediating the cellular and molecular immune responses against a fungal infection in the lung.

Spatial and Temporal Mapping of Human Innate Lymphoid Cells Reveals Elements of Tissue Specificity

Innate lymphoid cells (ILC) play critical roles in regulating immunity, inflammation, and tissue homeostasis in mice. However, limited access to non-diseased human tissues has hindered efforts to profile anatomically-distinct ILCs in humans. Through flow cytometric and transcriptional analyses of lymphoid, mucosal, and metabolic tissues from previously healthy human organ donors, here we have provided a map of human ILC heterogeneity across multiple anatomical sites. In contrast to mice, human ILCs are less strictly compartmentalized and tissue localization selectively impacts ILC distribution in a subset-dependent manner. Tissue-specific distinctions are particularly apparent for ILC1 populations, whose distribution was markedly altered in obesity or aging. Furthermore, the degree of ILC1 population heterogeneity differed substantially in lymphoid versus mucosal sites. Together, these analyses comprise a comprehensive characterization of the spatial and temporal dynamics regulating the anatomical distribution, subset heterogeneity, and functional potential of ILCs in non-diseased human tissues.

Immunomodulatory effects of probiotics: Can they be used to treat allergies and autoimmune diseases?

As a person ages, physiological, immunological and gut microbiome changes collectively result in an array of chronic conditions. According to the 'hygiene hypothesis' the increasing prevalence of immune-mediated disorders may be related to intestinal dysbiosis, leading to immune dysfunction and associated conditions such as eczema, asthma, allergies and autoimmune diseases. Beneficial probiotic bacteria can be utilized by increasing their abundance within the gastrointestinal lumen, which in turn will modulate immune cells, such as, T helper (Th)-1, Th2, Th17, regulatory T (Treg) cells and B cells, which have direct relevance to human health and the pathogenesis of immune disorders. Here, we describe the cross-talk between probiotics and the gastrointestinal immune system, and their effects in relation to inflammatory bowel disease, multiple sclerosis, allergies and atopic dermatitis.

Cytokine Networks between Innate Lymphoid Cells and Myeloid Cells

Innate lymphoid cells (ILCs) are an essential component of the innate immune system in vertebrates. They are developmentally rooted in the lymphoid lineage and can diverge into at least three transcriptionally distinct lineages. ILCs seed both lymphoid and non-lymphoid tissues and are locally self-maintained in tissue-resident pools. Tissue-resident ILCs execute important effector functions making them key regulator in tissue homeostasis, repair, remodeling, microbial defense, and anti-tumor immunity. Similar to T lymphocytes, ILCs possess only few sensory elements for the recognition of non-self and thus depend on extrinsic cellular sensory elements residing within the tissue. Myeloid cells, including mononuclear phagocytes (MNPs), are key sentinels of the tissue and are able to translate environmental cues into an effector profile that instructs lymphocyte responses. The adaptation of myeloid cells to the tissue state thus influences the effector program of ILCs and serves as an example of how environmental signals are integrated into the function of ILCs via a tissue-resident immune cell cross talks. This review summarizes our current knowledge on the role of myeloid cells in regulating ILC functions and discusses how feedback communication between ILCs and myeloid cells contribute to stabilize immune homeostasis in order to maintain the healthy state of an organ.

Human Immunodeficiency Virus Infection and Host Defense in the Lungs

Immunosuppression associated with human immunodeficiency virus (HIV) infection impacts all components of host defense against pulmonary infection. Cells within the lung have altered immune function and are important reservoirs for HIV infection. The host immune response to infected lung cells further compromises responses to a secondary pathogenic insult. In the upper respiratory tract, mucociliary function is impaired and there are decreased levels of salivary immunoglobulin A. Host defenses in the lower respiratory tract are controlled by alveolar macrophages, lymphocytes, and polymorphonuclear leukocytes. As HIV infection progresses, lung CD4 T cells are reduced in number causing a lack of activation signals from CD4 T cells and impaired defense by macrophages. CD8 T cells, on the other hand, are increased in number and cause lymphocytic alveolitis. Specific antibody responses by B-lymphocytes are decreased and opsonization of microorganisms is impaired. These observed defects in host defense of the respiratory tract explain the susceptibility of HIV-infected persons for oropharyngeal candidiasis, bacterial pneumonia, Pneumocystis pneumonia, and other opportunistic infections.

Transcription factors controlling innate lymphoid cell fate decisions

The mucosal epithelium is in direct contact with symbiotic and pathogenic microorganisms. Therefore, the mucosal surface is the principal portal of entry for invading pathogens and immune cells accumulated in the intestine to prevent infections. In addition to these conventional immune system functions, it has become clear that immune cells during steady-state continuously integrate microbial and nutrient-derived signals from the environment to support organ homeostasis. A major role in both processes is played by a recently discovered group of lymphocytes referred to as innate lymphoid cells (ILCs) Innate lymphoid cells (ILCs) that are specifically enriched at mucosal surfaces but are rather rare in secondary lymphoid organs. In analogy to the dichotomy between CD8 and CD4 T cells, we propose to classify ILCs into interleukin-7 receptor α-negative cytotoxic ILCs and IL-7Rα(+) helper-like ILCs. Dysregulated immune responses triggered by the various ILC subsets have been linked to inflammatory diseases such as inflammatory bowel disease, atopic dermatitis and airway hyperresponsiveness. Here, we will review recent progress in determining the transcriptional and developmental programs that control ILC fate decisions.

Human natural killer cell development in secondary lymphoid tissues

For nearly a decade it has been appreciated that critical steps in human natural killer (NK) cell development likely occur outside of the bone marrow and potentially necessitate distinct microenvironments within extramedullary tissues. The latter include the liver and gravid uterus as well as secondary lymphoid tissues such as tonsils and lymph nodes. For as yet unknown reasons these tissues are naturally enriched with NK cell developmental intermediates (NKDI) that span a maturation continuum starting from an oligopotent CD34(+)CD45RA(+) hematopoietic precursor cell to a cytolytic mature NK cell. Indeed despite the detection of NKDI within the aforementioned tissues, relatively little is known about how, why, and when these tissues may be most suited to support NK cell maturation and how this process fits in with other components of the human immune system. With the discovery of other innate lymphoid subsets whose immunophenotypes overlap with those of NKDI, there is also need to revisit and potentially re-characterize the basic immunophenotypes of the stages of the human NK cell developmental pathway in vivo. In this review, we provide an overview of human NK cell development in secondary lymphoid tissues and discuss the many questions that remain to be answered in this exciting field.

Of worms, mice and man: an overview of experimental and clinical helminth-based therapy for inflammatory bowel disease

The incidence of inflammatory and autoimmune disorders is highest in well-developed countries which is directly related to their higher hygienic standards: it is suggested that the lack of exposure to helminths contributes to the susceptibility for immune-related diseases. Epidemiological, experimental and clinical data support the idea that helminths provide protection against immune-mediated diseases such as inflammatory bowel disease (IBD). The most likely mechanism for the suppression of immune responses by helminths is the release of helminth-derived immunomodulatory molecules. This article reviews the experimental and clinical studies investigating the therapeutic potential of helminth-based therapy in IBD and also focuses on the current knowledge of its immunomodulatory mechanisms of action highlighting innate as well as adaptive immune mechanisms. Identifying the mechanisms by which these helminths and helminth-derived molecules modulate the immune system will help in creating novel drugs for the treatment of IBD and other disorders that result from an overactive immune response.

Functional aspects of fish lymphocytes

After almost 40 years of studies in comparative immunology, some light has been shed on the evolutive immunobiology of vertebrates, and experimental evidences have shown that acquired immunity, defined by somatic recombination of antigen-binding molecules and memory, is an achievement as ancient as jawless vertebrates. However, the molecular processes generating antigen receptors evolved independently between jawless and jawed fishes, and produced lymphocytic cells with similar functions but employing different sets of genes. In recent years, data have been provided describing some in vitro and in vivo functional responses of fish lymphocytes. After a long gap, the number of specific markers for fish lymphocytes is increasing, thus allowing a first characterisation of lymphocyte subsets. Overall, in the near future it will be possible to open a new chapter in fish immunology and investigate functional immunity of lymphocyte responses by combining the extensive knowledge on immune gene products with markers for molecules and cells. The present review summarizes current knowledge on functional features of fish lymphocytes.

Th2-type innate immune responses mediated by natural helper cells

Natural helper (NH) cells are a newly identified innate lymphocyte population that responds to a combination of interleukin (IL)-2 and either IL-25 or IL-33 to produce large amounts of T helper cell type 2 (Th2) cytokines. NH cells have been identified in fat-associated lymphoid clusters (FALCs), produce Th2 cytokines constitutively without any stimulation, and support the self-renewal of B1 cells and IgA production by B cells. Large amounts of IL-5 and IL-13 produced upon helminth infection or in response to IL-33 can induce eosinophilia and goblet cell hyperplasia in the lung and intestine; these cytokines, which activate NH cells, play important roles in antihelminth immunity and allergic diseases such as asthma.

Innate lymphoid cells: from border protection to the initiation of inflammatory diseases

Innate lymphoid cells (ILC) are a recently discovered group of innate lymphocytes found at mucosal surfaces. The transcriptional and effector programs of ILC strikingly resemble those of the various T-helper (Th) cell fates (that is, Th1, Th2, Th9, Th17, Th22). ILC are involved in protecting the mucosal borders by producing tissue protective factors. More recently, evidence has been provided that inappropriately activated ILC can be drivers of various inflammatory disorders. Here, we will highlight recent developments in our understanding of the transcriptional and developmental programs controlling ILC specification and fate decisions. We will also review the roles assigned to ILC in protecting barriers and in promoting inflammatory diseases. Finally, we will outline how the power of ILC may be harnessed for clinical application, and how interference with ILC function may be used as a new strategy to treat inflammatory diseases.

Natural helper cells and TH2-type innate immunity

Natural helper (NH) cells are a newly identified innate lymphocyte population, which respond to a combination of interleukin (IL)-2 and IL-25 or IL-33 to produce large amounts of TH2 cytokines, most notably, IL-5 and IL-13. We identified NH cells in lymphoid clusters in adipose tissues, which we termed fat-associated lymphoid clusters. IL-5 and IL-13 produced by activated NH cells induce eosinophilia and goblet cell hyperplasia, respectively, and play an important role in anti-helminth immunity and allergic diseases, such as asthma. In addition, NH cells produce TH2 cytokines constitutively in the absence of stimulation and support the self-renewal of B1 cells and IgA production by B cells. After our identification of NH cells, other groups have also reported novel TH2 cytokine-producing cells, such as nuocytes, and innate helper type 2 cells. There are similarities and differences between these newly identified cell populations and NH cells. Multipotent progenitor cell type 2 (MPP) cells can differentiate to other myeloid cells, making this a distinct cell type from the others. MPP cells, nuocytes, and innate helper type 2 cells respond to IL-25 alone in vivo, whereas NH cells do not respond to IL-25 without IL-2, although they respond strongly to IL-33. Localization of NH cells in fat-associated lymphoid clusters is an intriguing difference compared with the other cell types, which are found in the lymph node and/or spleen. This article summarizes current information regarding the emerging field of TH2-type innate lymphocytes.

Memory CD4+ T cells are required for optimal NK cell effector functions against the opportunistic fungal pathogen Pneumocystis murina

Little is known about the role of NK cells or their interplay with other immune cells during opportunistic infections. Using our murine model of Pneumocystis pneumonia, we found that loss of NK cells during immunosuppression results in substantial Pneumocystis lung burden. During early infection of C57B/6 CD4(+) T cell-depleted mice, there were significantly fewer NK cells in the lung tissue compared with CD4(+) T cell-intact animals, and the NK cells present demonstrated decreased upregulation of the activation marker NKp46 and production of the effector cytokine, IFN-γ. Furthermore, coincubation studies revealed a significant increase in fungal killing when NK cells were combined with CD4(+) T cells compared with either cell alone, which was coincident with a significant increase in perforin production by NK cells. Finally, however, we found through adoptive transfer that memory CD4(+) T cells are required for significant NK cell upregulation of the activation marker NK group 2D and production of IFN-γ, granzyme B, and perforin during Pneumocystis infection. To the best of our knowledge, this study is the first to demonstrate a role for NK cells in immunity to Pneumocystis pneumonia, as well as to establish a functional relationship between CD4(+) T cells and NK cells in the host response to an opportunistic fungal pathogen.

Dysregulated balance of retinoid-related orphan receptor γt-dependent innate lymphoid cells is involved in the pathogenesis of chronic DSS-induced colitis

Retinoid-related orphan receptor (ROR) γt-expressing and IL-22-producing NKp46(+) innate lymphoid (ILC22) cells reside in the lamina propria of the intestine in mice, suggesting that ILC22 cells contribute to host defense during intestinal damage in models of colitis in mice. Nevertheless, another set of pathological interferon (IFN)-γ and/or IL-17A-producing innate lymphoid cells (ILC1 and ICL17) may participate in the pathogenesis in different models of colitis. We here showed that RORγt(+)IL-22(+) ILC22 cells were localized in Thy-1(high)SCA-1(high) and/or Thy-1(high)SCA-1(low) subpopulations of the intestine in normal and dextran sodium sulfate (DSS)-induced colitic RORγt-sufficient Rag-2(-/-) mice. RORγt-deficient Rag-2(-/-) mice developed more severe DSS-induced colitis accompanied with lower expression of REG3β and REG3γ in the colon, but with a lower ratio and absolute number of IFN-γ-producing ILC1 cells as compared to control RORγt-sufficient Rag-2(-/-) mice. Collectively, not only the presence of ILC22 cells but also the balance of protective and pathogenic ILCs may be involved in the prevention of colitis.

Multifaceted roles of interleukin-7 signaling for the development and function of innate lymphoid cells

Recently, additional innate lymphocyte subsets have been identified that express germline encoded immunoreceptors and respond to cytokine cues. Among these, innate lymphoid cells (ILC) at mucosal surfaces are of significant interest because they were found to play important roles for lymphoid organogenesis, tissue homeostasis and repair, for immunity to various infections but also have been involved as disease-promoting cells in models of chronic inflammatory diseases and of autoimmunity. Their functional and transcriptional programs strikingly resemble that of the various T helper cell subsets suggesting that these programs are already pre-formed in the innate immune system and that these may be more conserved than previously appreciated. Interestingly, all ILC subsets express the interleukin 7 receptor α chain and IL-7 signaling has been involved in various aspects of their developmental and functional programs. Here, we will review the role of IL-7 signaling for the differentiation, maintenance and function of two important ILC subsets, lymphoid tissue inducer cells (i.e., RORγt(+) ILC) and natural helper cells (i.e., type 2 ILC). We will also put emphasis on the recently discovered role of IL-7 in controlling plasticity of RORγt(+) ILC.

Innate lymphoid cells: critical regulators of allergic inflammation and tissue repair in the lung

Maintenance of epithelial barrier function in the skin, respiratory tract and intestine is critical to limit exposure to commensal and pathogenic microbes and to maintain tissue homeostasis. Innate lymphoid cells (ILCs) are a recently recognized innate immune cell population that plays critical roles in host defense, regulation of inflammation and promotion of wound healing and tissue repair at barrier surfaces. In this review we discuss recent advances in the understanding of how ILC populations in the respiratory tract impact allergic airway inflammation and lung epithelial repair.