Opening the crypt: current facts and hypotheses on the function of cryptopatches

Human gut-associated lymphoid tissues (GALT); diversity, structure, and function

Gut-associated lymphoid tissues (GALT) are the key antigen sampling and adaptive immune inductive sites within the intestinal wall. Human GALT includes the multi-follicular Peyer's patches of the ileum, the vermiform appendix, and the numerous isolated lymphoid follicles (ILF) which are distributed along the length of the intestine. Our current understanding of GALT diversity and function derives primarily from studies in mice, and the relevance of many of these findings to human GALT remains unclear. Here we review our current understanding of human GALT diversity, structure, and composition as well as their potential for regulating intestinal immune responses during homeostasis and inflammatory bowel disease (IBD). Finally, we outline some key remaining questions regarding human GALT, the answers to which will advance our understanding of intestinal immune responses and provide potential opportunities to improve the treatment of intestinal diseases.

Sense and immuno-sensibility: innate lymphoid cell niches and circuits

Tissue-resident lymphocytes that lack expression of rearranged antigen receptors and are lineage negative for classical T and B cell markers are collectively known as innate lymphoid cells (ILCs). The ILC family is remarkably heterogeneous and exhibits plasticity; however, mature ILCs can be grouped based on their steady state expression of distinct surface receptors and transcription factors as well as production of signature cytokines following activation. The study of ILC subsets in mouse and human tissues has revealed that the elicitation and magnitude of their effector functions are determined by a combination of extrinsic cues specific to the niches in which they reside. In this short review, we will summarize some recent findings related to tissue-specific signals that govern ILC responses and localization.

ILC3 in Axial Spondyloarthritis: the Gut Angle

PURPOSE OF REVIEW

A growing body of evidence supports the relevance of the interleukin-23/interleukin-17 (IL-23/IL-17) pathway for the pathogenesis of axial spondyloarthritis (axSpA) and its treatment. Recently, innate lymphoid cells (ILC), a heterogeneous family of immune effector cells, have been identified as a relevant contributor in tissue homeostasis, partially via IL-23/IL-17 axis. This review describes the biology and the origins of the group 3 ILCs (ILC3s) in humans, focusing on their role in the pathogenesis of axSpA.

RECENT FINDINGS

Clinical trials showed the effectiveness of IL23/IL-17 axis inhibition in both spondyloarthritis (SpA) and Inflammatory Bowel Disease (IBD). Recent findings confirm the high prevalence of subclinical gut inflammation in patients with SpA. Translational data in humans have demonstrated an increase in the number of ILC3s responsive to IL-23 and producing either IL-22 or IL-17 in the gut of SpA patients. The observation of gut-derived ILC3s in circulation and at inflamed tissues in patients with SpA suggest a recirculation of ILCs from mucosal site to lymphoid tissues and possibly enthesis and joints. Multiple observations demonstrate the expansion of IL-17- and IL-22-producing ILC3 in the subclinically inflamed gut of SpA patients. These innate immune cells, also observed in normal entheses, seem to be able to re-circulate from the gut to inflamed tissues of SpA patients, thus contributing to the disease perpetuation. The development of tools that can provide access to diseased tissue from sacroiliac joint and spinal entheses will provide valuable knowledge on the role of ILC3 in axSpA pathogenesis.

Role of Subclinical Gut Inflammation in the Pathogenesis of Spondyloarthritis

Subclinical gut inflammation occurring in patients affected by spondyloarthritis (SpA) is correlated with the severity of spine inflammation. Several evidences indicate that dysbiosis occurs in SpA, and that may modulate intestinal permeability and intestinal immune responses. The presence of intestinal dysbiosis is accompanied in SpA patients with the presence of zonulin-dependent alterations of gut-epithelial and gut-vascular barriers. The leakage of epithelial and endothelial surface layers is followed by the translocation of bacterial products, such as lipopolysaccharide and intestinal fatty acid binding protein, in the systemic circulation. These bacterial products may downregulate the expression of CD14 on circulating monocytes leading to an “anergic” phenotype. In the gut, IL-23 may induce the expansion of innate immune cells such as mucosal-associated invariant T cells, γδ T cells, and innate lymphoid cells of group 3 that through the interaction with MAdCAM1 may recirculate form the gut to the sites of SpA active inflammation. On the basis of these findings, gut inflammation observed in SpA patient seems to be not only an epiphenomenon of the on going systemic inflammatory process but may also represent the base camp in which inflammatory cells are activated and from whom they shuttle.

ILC3 GM-CSF production and mobilisation orchestrate acute intestinal inflammation

Innate lymphoid cells (ILCs) contribute to host defence and tissue repair but can induce immunopathology. Recent work has revealed tissue-specific roles for ILCs; however, the question of how a small population has large effects on immune homeostasis remains unclear. We identify two mechanisms that ILC3s utilise to exert their effects within intestinal tissue. ILC-driven colitis depends on production of granulocyte macrophage-colony stimulating factor (GM-CSF), which recruits and maintains intestinal inflammatory monocytes. ILCs present in the intestine also enter and exit cryptopatches in a highly dynamic process. During colitis, ILC3s mobilize from cryptopatches, a process that can be inhibited by blocking GM-CSF, and mobilization precedes inflammatory foci elsewhere in the tissue. Together these data identify the IL-23R/GM-CSF axis within ILC3 as a key control point in the accumulation of innate effector cells in the intestine and in the spatio-temporal dynamics of ILCs in the intestinal inflammatory response.

DOI:http://dx.doi.org/10.7554/eLife.10066.001

Crohn’s disease and ulcerative colitis are diseases in which the body’s own immune system causes inflammation of the large intestine. These autoimmune diseases can be severely debilitating and difficult to treat. However an improved understanding of the factors that contribute to the intestinal inflammation may lead to new and effective treatments.

Immune cells called innate lymphoid cells were discovered recently, and shown quickly to play a role in host defense, tissue repair and inflammation regulation. Several groups of innate lymphoid cells are now known; each group is characterized by the genes that control the cell’s development and the small proteins (called cytokines) that the cells release. One group of innate lymphoid cells, the ILC3s, are generally found in the intestinal tract, albeit in small numbers. Given that innate lymphoid cells are known to manage inflammatory responses, it is possible that ILC3s contribute to intestinal inflammation. However, it remains unclear how such a small population of cells could so dramatically inflame the gut.

Pearson et al. now reveal two mechanisms that these innate lymphoid cells use to amplify the inflammatory response and exacerbate intestinal inflammation. First, in both mice and humans, ILC3s were found to be a key source of a cytokine called GM-CSF, which recruits additional immune cells that further promote intestinal inflammation. Secondly, while ILC3s were traditionally regarded as immobile immune cells, Pearson et al. discovered that these cells can move within the intestinal tissue and mobilize from their starting points within this tissue if they are activated. These two mechanisms could explain how ILC3s can trigger inflammation that occurs throughout the gut.

The experiments suggest that blocking production of the GM-CSF cytokine or altering ILC3 movement or activity may help reduce intestinal inflammation. However, the use of GM-CSF blocking drugs to protect against colitis and similar conditions could be problematic, because GM-CSF also plays an important protective role in the intestines. Nevertheless, clinical trials are underway to investigate the use of anti-GM-CSF drugs to treat other inflammatory conditions (such as rheumatoid arthritis). These studies could offer insight into whether these drugs provide relief to trial participants who suffer from intestinal inflammation as well.

DOI:http://dx.doi.org/10.7554/eLife.10066.002

Influence of nutrient-derived metabolites on lymphocyte immunity

Organisms need to protect themselves against potential dangers from their surroundings, yet they require constant and intimate interactions with the same environment for their survival. The immune system is instrumental for protection against invading organisms and their toxins. The immune system consists of many cell types and is highly integrated within other tissues. Immune activity is particularly enriched at surfaces that separate the host from its environment, such as the skin and the gastrointestinal tract. This enables protection at sites directly at risk but also enables environmental factors to influence the maturation and function of immune structures and cells. Recent work has indicated that the diet in particular is able to influence the immune system and thus affect the development of inflammatory disease. This review aims to highlight recent work on how external factors, with a focus on those derived from the diet such as vitamin A, can have a direct or indirect deterministic influence on the activity and function of immunity.

Interleukin-22: immunobiology and pathology

Interleukin-22 (IL-22) is a recently described IL-10 family cytokine that is produced by T helper (Th) 17 cells, γδ T cells, NKT cells, and newly described innate lymphoid cells (ILCs). Knowledge of IL-22 biology has evolved rapidly since its discovery in 2000, and a role for IL-22 has been identified in numerous tissues, including the intestines, lung, liver, kidney, thymus, pancreas, and skin. IL-22 primarily targets nonhematopoietic epithelial and stromal cells, where it can promote proliferation and play a role in tissue regeneration. In addition, IL-22 regulates host defense at barrier surfaces. However, IL-22 has also been linked to several conditions involving inflammatory tissue pathology. In this review, we assess the current understanding of this cytokine, including its physiologic and pathologic effects on epithelial cell function.

Differential induction of isolated lymphoid follicles in the gut by 18β-glycyrrhetinic acid

18β-glycyrrhetinic acid (GRA) is a pharmacologically active component of licorice root with documented immunomodulatory properties. We reported that GRA administered orally to mice induces B cell recruitment to isolated lymphoid follicles (ILF) in the small intestine and shortens the duration of rotavirus antigen shedding. ILF are dynamic lymphoid tissues in the gut acquired post-natally upon colonization with commensal bacteria and mature through B cell recruitment to the follicles, resulting in up-regulation of IgA synthesis in response to changes in the composition of microbiota. In this study, we investigated potential mechanisms by which GRA induces ILF maturation in the ileum and the colon using mice depleted of enteric bacteria and a select group of mice genetically deficient in pattern recognition receptors. The data show GRA was unable to induce ILF maturation in ileums of mice devoid of commensal bacteria, MyD88-/- or NOD2-/- mice, but differentially induced ILF in colons. Increased expression of chemokine and chemokine receptor genes that modulate B and T cell recruitment to the mucosa were in part dependent on NOD2, TLR, and signaling adaptor protein MyD88. Together the results suggest GRA induces ILF through cooperative signals provided by bacterial ligands under normal conditions to induce B cell recruitment to ILF to the gut, but that the relative contribution of these signals differ between ileum and colon.

Mucosal immunology of HIV infection

Recent advances in the immunology, pathogenesis, and prevention of human immunodeficiency virus (HIV) infection continue to reveal clues to the mechanisms involved in the progressive immunodeficiency attributed to infection, but more importantly have shed light on the correlates of immunity to infection and disease progression. HIV selectively infects, eliminates, and/or dysregulates several key cells of the human immune system, thwarting multiple arms of the host immune response, and inflicting severe damage to mucosal barriers, resulting in tissue infiltration of 'symbiotic' intestinal bacteria and viruses that essentially become opportunistic infections promoting systemic immune activation. This leads to activation and recruitment or more target cells for perpetuating HIV infection, resulting in persistent, high-level viral replication in lymphoid tissues, rapid evolution of resistant strains, and continued evasion of immune responses. However, vaccine studies and studies of spontaneous controllers are finally providing correlates of immunity from protection and disease progression, including virus-specific CD4(+) T-cell responses, binding anti-bodies, innate immune responses, and generation of antibodies with potent antibody-dependent cell-mediated cytotoxicity activity. Emerging correlates of immunity indicate that prevention of HIV infection may be possible through effective vaccine strategies that protect and stimulate key regulatory cells and immune responses in susceptible hosts. Furthermore, immune therapies specifically directed toward boosting specific aspects of the immune system may eventually lead to a cure for HIV-infected patients.

Insight into lymphoid tissue morphogenesis

Secondary lymphoid organs (SLO) are crucial structures for immune-surveillance and rapid immune responses allowing resident lymphocytes to encounter antigen-presenting cells that carry antigens from peripheral tissues. These structures develop during embryonic life through a tightly regulated process that involves interactions between haematopoietic and mesenchymal cells. Importantly, this morphogenesis potential is maintained throughout life since in chronic inflammatory conditions novel "tertiary lymphoid organs" can be generated by processes that are reminiscent of embryonic SLO development. In this review we will discuss early events in SLO morphogenesis, focusing on haematopoietic and mesenchymal cell subsets implicated on the development of lymphoid organs.

New friendships and old feuds: relationships between innate lymphoid cells and microbial communities

Mammalian barrier surfaces, including the skin, lung and intestine, are constantly exposed to environmental stimuli, including beneficial and pathogenic microbes, dietary substances and non-organic materials. At these anatomical sites it is essential to maintain barrier integrity to promote tissue homeostasis and prevent local and systemic inflammation. As such, changes in the composition and localization of commensal bacteria are closely associated with inflammatory, metabolic and infectious disease in mammals. Cells of the innate and adaptive immune systems have a crucial role in the tight regulation of host-commensal relationships. A recently described family of immune cells, termed innate lymphoid cells (ILCs), contributes to inflammation, modulates adaptive immunity and regulates wound healing and tissue regeneration. ILCs are present at barrier surfaces, and thus are in close proximity to environmental antigens, including commensal bacteria. The composition and localization of microbial communities have a profound impact on immunity at barrier surfaces as well as at distant sites. This review will summarize the phenotypic characteristics of ILC family members and discuss recent findings about the interactions between ILCs and the microbiota in the contexts of homeostasis, immunity, inflammation and tissue organization and repair.

Immunohistochemical localization of T-lymphocyte subsets in the developing lymphoid tissues of the tammar wallaby (Macropus eugenii)

Research into marsupial adaptive immunity during ontogeny has been hampered by the lack of antibodies that react to marsupial immunological cell populations. In this study, newly synthesised polyclonal antibodies to the T cell marker, CD8, have been developed and used to investigate the ontogeny and distribution of this T cell population in the tammar wallaby. Immunohistochemical analysis indicated that the distribution of the CD8 lymphocytes in the lymphoid tissues of tammar neonates during the first 144 days of pouch life was similar to that of the eutherian mammals. However, CD8α(+) lymphocytes were observed in the intestines of tammar neonates prior to their first appearance in the cervical thymus, an observation that has not been found in eutherians. A dual labelling immunohistochemical approach was used for the indirect demonstration of CD4 and enabled the simultaneous detection in the tammar wallaby tissues of the two major T-lymphocyte populations, CD4 and CD8 that are associated with adaptive immunity. As in eutherian mammals, CD4(+) cells were the predominant T cell lymphocyte subset observed in the spleen while in the nodal tissues, an age-related decrease in the CD4(+)/CD8(+) ratio was noted. These antibodies provide a new immunological tool to study the role of T cell subsets in marsupial immunity and disease pathogenesis studies.

Stroma cell priming in enteric lymphoid organ morphogenesis

The lymphoid system is equipped with a network of specialized platforms located at strategic sites, which grant strict immune-surveillance and efficient immune responses. The development of these peripheral secondary lymphoid organs (SLO) occurs mainly in utero, while tertiary lymphoid structures can form in adulthood generally in response to persistent infection and inflammation. Regardless of the lymphoid tissue and intrinsic cellular and molecular differences, it is now well established that the recruitment of fully functional lymphoid tissue inducer (LTi) cells to presumptive lymphoid organ sites, and their consequent close and reciprocal interaction with resident stroma cells, are central to SLO formation. In contrast, the nature of events that initially prime resident sessile stroma cells to recruit and retain LTi cells remains poorly understood. Recently, new findings revealed early phases of SLO development putting emphasis on mesenchymal and lymphoid tissue initiator cells. Herein we discuss the main tenets of enteric lymphoid organs genesis and focus in the most recent findings that open new perspectives to the understanding of the early phases of lymphoid morphogenesis.

Interleukin-22 and interleukin-22-producing NKp44+ natural killer cells in subclinical gut inflammation in ankylosing spondylitis

OBJECTIVE

The intestinal inflammation observed in patients with ankylosing spondylitis (AS) is characterized by an overexpression of interleukin-23 (IL-23). IL-23 is known to regulate IL-22 production through lamina propria NKp44+ natural killer (NK) cells, which are thought to be involved in protective mucosal mechanisms. This study was undertaken to evaluate the frequency of NKp44+ NK cells and the expression of IL-22 in the ileum of AS patients.

METHODS

Tissue NKp44+ NK cells, NKp46+ NK cells, and IL-22-producing cells were analyzed by flow cytometry. Quantitative gene expression analysis of IL-22, IL-23, IL-17, STAT-3, and mucin 1 (MUC-1) was performed by reverse transcriptase-polymerase chain reaction on ileal samples from 15 patients with AS, 15 patients with Crohn's disease (CD), and 15 healthy controls. NKp44, pSTAT-3, and IL-22 expression was analyzed by immunohistochemistry.

RESULTS

The frequency of NKp44+ but not NKp46+ NK cells was increased in the inflamed ileum of AS patients compared to CD patients and controls. The frequency of NKp46+ NK cells was significantly increased only in CD patients. Among CD4+ lymphocytes and NKp44+ NK cell subsets, the latter were the major source of IL-22 on lamina propria mononuclear cells from AS patients. Significant up-regulation of IL-22, IL-23p19, MUC-1, and STAT-3 transcripts in the terminal ileum of patients with AS was observed. Immunohistochemical analysis confirmed the increased IL-22 and pSTAT-3 expression in inflamed mucosa from AS and CD patients.

CONCLUSION

Our findings indicate that overexpression of IL-22, together with an increased number of IL-22-producing NKp44+ NK cells, occurs in the gut of AS patients, where it appears to play a tissue-protective role.

Lymphoid microenvironments and innate lymphoid cells in the gut

Gut-associated lymphoid tissue (GALT) is a sensor region for luminal content and plays an important role in lymphoid maturation, activation and differentiation. It comprises isolated and aggregated lymphoid follicles, cryptopatches (CPs) and tertiary lymphoid tissue. Innate lymphoid cells (ILCs) play a central role within GALT. Prenatal GALT development is dependent on ILC lymphoid-inducer function. Postnatally, these cells rapidly respond to commensal and pathogenic intestinal bacteria, parasites and food components by polarized cytokine production [such as interleukin (IL)-22, IL-17 or IL-13] and further contribute to GALT formation and function. Here, we discuss how ILCs shape lymphoid intestinal microenvironments and act as amplifier cells for innate and adaptive immune responses.