Restoration of lymphoid organ integrity through the interaction of lymphoid tissue-inducer cells with stroma of the T cell zone

The immune suppressive function of transforming growth factor-β (TGF-β) in human diseases

Transforming growth factor-β (TGF-β) functions as an immune suppressor by influencing immune cells' development, differentiation, tolerance induction and homeostasis. In human diseases, TGF-β has been revealed as an essential regulator of both innate and adaptive functions in autoimmune diseases. Furthermore, it plays a significant role in cancer by inhibiting immunosurveillance in the tumor-bearing host. A variety of TGF-β neutralizing anti-cancer therapies have been investigated based on the role of TGF-β in immunosuppression. New studies are focusing on combining TGF-β blockade with tumor vaccinations and immunogene therapies.

Pivotal roles of ILCs in hepatic diseases

Innate lymphoid cells (ILCs) are involved in the development of mucosal-associated lymphoid tissues and serve as a rapid and early source of the effector cytokines that are typically associated with the T helper cell subsets in response to pathogen-induced changes in the microenvironment. Recent research has implicated ILCs as potential contributing factors to the spectrum of inflammation-related hepatic diseases, particularly hepatitis, fibrosis and carcinoma. In this review, we summarize the current knowledge on the roles of ILCs in these hepatic pathogeneses, providing insights into the underlying cellular and signaling mechanisms to help guide the future research to elucidate the ILCs' characters under normal and diseased conditions and provide interventional targets with therapeutic potential.

The neurotrophic receptor Ntrk2 directs lymphoid tissue neovascularization during Leishmania donovani infection

The neurotrophic tyrosine kinase receptor type 2 (Ntrk2, also known as TrkB) and its ligands brain derived neurotrophic factor (Bdnf), neurotrophin-4 (NT-4/5), and neurotrophin-3 (NT-3) are known primarily for their multiple effects on neuronal differentiation and survival. Here, we provide evidence that Ntrk2 plays a role in the pathologic remodeling of the spleen that accompanies chronic infection. We show that in Leishmania donovani-infected mice, Ntrk2 is aberrantly expressed on splenic endothelial cells and that new maturing blood vessels within the white pulp are intimately associated with F4/80(hi)CD11b(lo)CD11c(+) macrophages that express Bdnf and NT-4/5 and have pro-angiogenic potential in vitro. Furthermore, administration of the small molecule Ntrk2 antagonist ANA-12 to infected mice significantly inhibited white pulp neovascularization but had no effect on red pulp vascular remodeling. We believe this to be the first evidence of the Ntrk2/neurotrophin pathway driving pathogen-induced vascular remodeling in lymphoid tissue. These studies highlight the therapeutic potential of modulating this pathway to inhibit pathological angiogenesis.

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.

Host response. Inflammation-induced disruption of SCS macrophages impairs B cell responses to secondary infection

The layer of macrophages at the subcapsular sinus (SCS) captures pathogens entering the lymph node, preventing their global dissemination and triggering an immune response. However, how infection affects SCS macrophages remains largely unexplored. Here we show that infection and inflammation disrupt the organization of SCS macrophages in a manner that involves the migration of mature dendritic cells to the lymph node. This disrupted organization reduces the capacity of SCS macrophages to retain and present antigen in a subsequent secondary infection, resulting in diminished B cell responses. Thus, the SCS macrophage layer may act as a sensor or valve during infection to temporarily shut down the lymph node to further antigenic challenge. This shutdown may increase an organism's susceptibility to secondary infections.

Role of group 3 innate lymphoid cells in antibody production

Innate lymphoid cells (ILCs) constitute a heterogeneous family of effector lymphocytes of the innate immune system that mediate lymphoid organogenesis, tissue repair, immunity and inflammation. The initial view that ILCs exert their protective functions solely during the innate phase of an immune response has been recently challenged by evidence indicating that ILCs shape adaptive immunity by establishing both contact-dependent and contact-independent interactions with multiple hematopoietic and non-hematopoietic cells, including B cells. Some of these interactions enhance antibody responses both systemically and at mucosal sites of entry.

Deciphering the transcriptional switches of innate lymphoid cell programming: the right factors at the right time

Innate lymphoid cells (ILCs) are increasingly recognised as an innate immune counterpart of adaptive T_H cells. In addition to their similar effector cytokine production, there is a strong parallel between the transcription factors that control the differentiation of T_H1, T_H2 and T_H17 cells and ILC Groups 1, 2 and 3, respectively. Here, we review the transcriptional circuit that specifies the development of a common ILC progenitor and its subsequent programming into distinct ILC groups. Notch, GATA-3, Nfil3 and Id2 are identified as early factors that suppress B and T cell potentials and are turned on in favour of ILC commitment. Natural killer cells, which are the cytotoxic ILCs, develop along a pathway distinct from the rest of the helper-like ILCs that are derived from a common progenitor to all helper-like innate lymphoid cells (CHILPs). PLZF⁻ CHILPs give rise to lymphoid tissue inducer cells while PLZF⁺ CHILPs have multi-lineage potential and could give rise to ILCs 1, 2 and 3. Such lineage specificity is dictated by the controlled expression of T-bet, RORα, RORγt and AHR. In addition to the type of transcription factors, the developmental stages at which these factors are expressed are crucial in specifying the fate of the ILCs.

The biology of innate lymphoid cells

The innate immune system is composed of a diverse array of evolutionarily ancient haematopoietic cell types, including dendritic cells, monocytes, macrophages and granulocytes. These cell populations collaborate with each other, with the adaptive immune system and with non-haematopoietic cells to promote immunity, inflammation and tissue repair. Innate lymphoid cells are the most recently identified constituents of the innate immune system and have been the focus of intense investigation over the past five years. We summarize the studies that formally identified innate lymphoid cells and highlight their emerging roles in controlling tissue homeostasis in the context of infection, chronic inflammation, metabolic disease and cancer.

The effect of infection with mixed Eimeria species on hematology and immune responses following Newcastle disease and infectious bursal disease booster vaccination in broilers

BACKGROUND

Coccidiosis is an important parasitic disease of chickens, causing high mortality and morbidity. This morbidity is believed to be correlated with altered population dynamics of blood cells and immunocompromisation.

OBJECTIVES

This study investigated the effects of mixed Eimeria species (viz., tenella, maxima, acervulina and necatrix) infection on hematology and immune responses following Newcastle disease (ND) and infectious bursal disease (IBD) booster vaccination in broilers.

ANIMALS AND METHODS

One-day-old broiler chicks (Hubbard; n = 200) were divided into two equal groups A and B. On day 16, group A was infected orally with Eimeria species (7 × 10(4) sporulated oocysts), whereas group B served as control. Both groups were analyzed for hematological parameters on post-infection days 6-8. Sera from both groups were analyzed for antibody titers against ND and IBD vaccines. On day 8 post-infection, lymphoid organs were also examined.

RESULTS

Significantly lower (P < 0.05) levels of plasma proteins, globular volume, hemoglobin concentration, packed cell volume, total erythrocytes, mean corpuscular volume and mean corpuscular hemoglobin concentration were found in infected chickens compared with non-infected control chickens. In addition, the infected group exhibited significantly increased (P < 0.05) numbers of different leukocytes. Infected chickens also showed significantly lower antibody titers against ND and IBD with decreased relative organ weights of all lymphoid organs except spleen.

CONCLUSION AND RECOMMENDATIONS

Mixed species of Eimeria adversely affected the hematology and immune efficiency of broilers. Thus, inexpensive immune potentiators and hemotonics along with appropriate anti-coccidial medications are suggested to avoid the complications and subsequent economic losses.

Diversification and Functional Specialization of Human NK Cell Subsets

Natural killer (NK) cells are lymphocytes that participate in different facets of immunity. They can act as innate sentinels through recognition and eradication of infected or transformed target cells, so-called immunosurveillance. In addition, they can contain immune responses through the killing of other activated immune cells, so-called immunoregulation. Furthermore, they instruct and regulate immune responses by producing pro-inflammatory cytokines such as IFN-γ, either upon direct target cell recognition or by relaying cytokine cues from various cell types. Recent studies in mouse and man have uncovered infection-associated expansions of NK cell subsets with specific receptor repertoires and diverse patterns of intracellular signaling molecule expression. Moreover, distinct attributes of NK cells in tissues, including tissue-resident subsets, are being further elucidated. Findings support an emerging theme of ever-increasing diversification and functional specialization among different NK cell subsets, with a functional dichotomy between subsets involved in immunoregulation or immunosurveillance. The epigenetic landscapes and transcriptional profiles of different NK cell subsets are providing insights into the molecular regulation of effector functions. Here, we review phenotypic, functional, and developmental characteristics of a spectrum of human NK cell subsets. We also discuss the molecular underpinnings of different NK cell subsets and their potential contributions to immunity as well as disease susceptibility.

Stromal infrastructure of the lymph node and coordination of immunity

The initiation of adaptive immune responses depends upon the careful maneuvering of lymphocytes and antigen into and within strategically placed lymph nodes (LNs). Non-hematopoietic stromal cells form the cellular infrastructure that directs this process. Once regarded as merely structural features of lymphoid tissues, these cells are now appreciated as essential regulators of immune cell trafficking, fluid flow, and LN homeostasis. Recent advances in the identification and in vivo targeting of specific stromal populations have resulted in striking new insights to the function of stromal cells and reveal a level of complexity previously unrealized. We discuss here recent discoveries that highlight the pivotal role that stromal cells play in orchestrating immune cell homeostasis and adaptive immunity.

B cell homeostasis and follicle confines are governed by fibroblastic reticular cells

Naive B and T cells exist in discrete zones in lymph nodes. Turley and colleagues demonstrate that a distinct subset of fibroblastic reticular cells reside in B cell zones, where they sustain B cell survival by providing BAFF.

Fibroblastic reticular cells (FRCs) are known to inhabit T cell–rich areas of lymphoid organs, where they function to facilitate interactions between T cells and dendritic cells. However, in vivo manipulation of FRCs has been limited by a dearth of genetic tools that target this lineage. Here, using a mouse model to conditionally ablate FRCs, we demonstrated their indispensable role in antiviral T cell responses. Unexpectedly, loss of FRCs also attenuated humoral immunity due to impaired B cell viability and follicular organization. Follicle-resident FRCs established a favorable niche for B lymphocytes via production of the cytokine BAFF. Thus, our study indicates that adaptive immunity requires an intact FRC network and identifies a subset of FRCs that control B cell homeostasis and follicle identity.

Activated group 3 innate lymphoid cells promote T-cell-mediated immune responses

Group 3 innate lymphoid cells (ILC3s) have emerged as important cellular players in tissue repair and innate immunity. Whether these cells meaningfully regulate adaptive immune responses upon activation has yet to be explored. Here we show that upon IL-1β stimulation, peripheral ILC3s become activated, secrete cytokines, up-regulate surface MHC class II molecules, and express costimulatory molecules. ILC3s can take up latex beads, process protein antigen, and consequently prime CD4(+) T-cell responses in vitro. The cognate interaction of ILC3s and CD4(+) T cells leads to T-cell proliferation both in vitro and in vivo, whereas its disruption impairs specific T-cell and T-dependent B-cell responses in vivo. In addition, the ILC3-CD4(+) T-cell interaction is bidirectional and leads to the activation of ILC3s. Taken together, our data reveal a novel activation-dependent function of peripheral ILC3s in eliciting cognate CD4(+) T-cell immune responses.

Enteric mucosa integrity in the presence of a preserved innate interleukin 22 compartment in HIV type 1-treated individuals

BACKGROUND

Interleukin 22 (IL-22) is emerging as a key cytokine for gut epithelial homeostasis and mucosal repair. Gut disruption is a hallmark of human immunodeficiency virus (HIV) infection. Here, we investigated IL-22 production and gut mucosal integrity in HIV type 1 (HIV-1)-infected individuals receiving long-term antiretroviral therapy (ART).

METHODS

Biopsy specimens from 37 individuals who underwent colonoscopy primarily for cancer screening and from 17 HIV-1-infected and 20 healthy age-matched controls were assessed.

RESULTS

We found significant depletion of sigmoid IL-22-producing CD4(+) T cells (T-helper type 22 [Th22] cells) even after prolonged ART, contrasting with the apparently normal compartments of regulatory and interleukin 17 (IL-17)-producing CD4(+) T cells, as well as total mucosal CD4(+) T cells. Despite the preferential Th22 cell depletion, IL-22 production by innate lymphoid cells (ILCs) was similar to that observed in HIV-1-seronegative subjects, and transcription of genes encoding molecules relevant for IL-22 production (ie, AHR, IL23, IL23R, IL1B, IL6, and TGFB1) was preserved. Remarkably, levels of transcripts of IL-22-target genes (ie, REG3G, DEFB4A, S100A9, MUC1, and MUC13) were unaltered, suggesting an adequate production of antimicrobial peptides and mucins. In agreement, enteric epithelial architecture was fully preserved.

CONCLUSIONS

Despite the reduced Th22 cell subset, innate IL-22-mediated mechanisms, essential for sigmoid mucosa integrity, were fully operational in long-term-treated HIV-1-infected individuals. Our data highlight IL-22 production by ILCs as an important target for therapies aimed at facilitating human mucosal reconstitution.

Plasticity and complexity of B cell responses against persisting pathogens

Vaccines against acute infections execute their protective effects almost exclusively via the induction of antibodies. Development of protective vaccines against persisting pathogens lags behind probably because standard immunogens and application regimen do not sufficiently stimulate those circuits in B cell activation that mediate protection. In general, B cell responses against pathogen derived-antigens are generated through complex cellular interactions requiring the coordination of innate and adaptive immune mechanisms. In this review, we summarize recent findings from prototypic infection models to exemplify how generation of protective antibodies against persisting pathogens is imprinted by particular pathogen-derived factors and how distinct CD4(+) T cell populations determine the quality of these antibodies. Clearly, it is the high plasticity of these processes that is instrumental to drive tailored B cell responses that protect the host. In sum, application of novel knowledge on B cell plasticity and complexity can guide the development of rationally designed vaccines that elicit protective antibodies against persisting pathogens.

Lymph node stromal cells negatively regulate antigen-specific CD4+ T cell responses

Lymph node (LN) stromal cells (LNSCs) form the functional structure of LNs and play an important role in lymphocyte survival and the maintenance of immune tolerance. Despite their broad spectrum of function, little is known about LNSC responses during microbial infection. In this study, we demonstrate that LNSC subsets display distinct kinetics following vaccinia virus infection. In particular, compared with the expansion of other LNSC subsets and the total LN cell population, the expansion of fibroblastic reticular cells (FRCs) was delayed and sustained by noncirculating progenitor cells. Notably, newly generated FRCs were preferentially located in perivascular areas. Viral clearance in reactive LNs preceded the onset of FRC expansion, raising the possibility that viral infection in LNs may have a negative impact on the differentiation of FRCs. We also found that MHC class II expression was upregulated in all LNSC subsets until day 10 postinfection. Genetic ablation of radioresistant stromal cell-mediated Ag presentation resulted in slower contraction of Ag-specific CD4(+) T cells. We propose that activated LNSCs acquire enhanced Ag-presentation capacity, serving as an extrinsic brake system for CD4(+) T cell responses. Disrupted function and homeostasis of LNSCs may contribute to immune deregulation in the context of chronic viral infection, autoimmunity, and graft-versus-host disease.

Murine spleen tissue regeneration from neonatal spleen capsule requires lymphotoxin priming of stromal cells

Spleen is a tissue with regenerative capacity, which allows autotransplantation of human spleen fragments to counteract the effects of splenectomy. We now reveal in a murine model that transplant of neonatal spleen capsule alone leads to the regeneration of full spleen tissue. This finding indicates that graft-derived spleen stromal cells, but not lymphocytes, are essential components of tissue neogenesis, a finding verified by transplant and regeneration of Rag1KO spleen capsules. We further demonstrate that lymphotoxin and lymphoid tissue inducer cells participate in two key elements of spleen neogenesis, bulk tissue regeneration and white pulp organization, identifying a lymphotoxin-dependent pathway for neonatal spleen regeneration that contrasts with previously defined lymphotoxin-independent embryonic spleen organogenesis.

Complementary diversification of dendritic cells and innate lymphoid cells

Dendritic cells (DCs) are professional antigen presenting cells conventionally thought to mediate cellular adaptive immune responses. Recent studies have led to the recognition of a non-redundant role for DCs in orchestrating innate immune responses, and in particular, for DC subset-specific interactions with innate lymphoid cells (ILCs). Recently recognized as important effectors of early immune responses, ILCs develop into subsets which mirror the transcriptional and cytokine profile of their T cell subset counterparts. DC diversification into functional subsets provides for modules of pathogen sensing and cytokine production that direct pathogen-appropriate ILC and T cell responses. This review focuses on the recent advances in the understanding of DC development, and their function in orchestrating the innate immune modules.

Activated innate lymphoid cells are associated with a reduced susceptibility to graft-versus-host disease

Allogeneic hematopoietic stem cell transplantation (HSCT) is widely used to treat hematopoietic cell disorders but is often complicated by graft-versus-host disease (GVHD), which causes severe epithelial damage. Here we have investigated longitudinally the effects of induction chemotherapy, conditioning radiochemotherapy, and allogeneic HSCT on composition, phenotype, and recovery of circulating innate lymphoid cells (ILCs) in 51 acute leukemia patients. We found that reconstitution of ILC1, ILC2, and NCR(-)ILC3 was slow compared with that of neutrophils and monocytes. NCR(+) ILC3 cells, which are not present in the circulation of healthy persons, appeared both after induction chemotherapy and after allogeneic HSCT. Circulating patient ILCs before transplantation, as well as donor ILCs after transplantation, expressed activation (CD69), proliferation (Ki-67), and tissue homing markers for gut (α4β7, CCR6) and skin (CCR10 and CLA). The proportion of ILCs expressing these markers was associated with a decreased susceptibility to therapy-induced mucositis and acute GVHD. Taken together, these data suggest that ILC recovery and treatment-related tissue damage are interrelated and affect the development of GVHD.

Human innate lymphoid cells

Innate lymphoid cells (ILCs) are lymphoid cells that do not express rearranged receptors and have important effector and regulatory functions in innate immunity and tissue remodeling. ILCs are categorized into 3 groups based on their distinct patterns of cytokine production and the requirement of particular transcription factors for their development and function. Group 1 ILCs (ILC1s) produce interferon γ and depend on Tbet, group 2 ILCs (ILC2s) produce type 2 cytokines like interleukin-5 (IL-5) and IL-13 and require GATA3, and group 3 ILCs (ILC3s) include lymphoid tissue inducer cells, produce IL-17 and/or IL-22, and are dependent on RORγt. Whereas ILCs play essential roles in the innate immune system, uncontrolled activation and proliferation of ILCs can contribute to inflammatory autoimmune diseases. In this review, we provide an overview of the characteristics of ILCs in the context of health and disease. We will focus on human ILCs but refer to mouse studies if needed to clarify aspects of ILC biology.

Laminins affect T cell trafficking and allograft fate

Lymph nodes (LNs) are integral sites for the generation of immune tolerance, migration of CD4⁺ T cells, and induction of Tregs. Despite the importance of LNs in regulation of inflammatory responses, the LN-specific factors that regulate T cell migration and the precise LN structural domains in which differentiation occurs remain undefined. Using intravital and fluorescent microscopy, we found that alloreactive T cells traffic distinctly into the tolerant LN and colocalize in exclusive regions with alloantigen-presenting cells, a process required for Treg induction. Extracellular matrix proteins, including those of the laminin family, formed regions within the LN that were permissive for colocalization of alloantigen-presenting cells, alloreactive T cells, and Tregs. We identified unique expression patterns of laminin proteins in high endothelial venule basement membranes and the cortical ridge that correlated with alloantigen-specific immunity or immune tolerance. The ratio of laminin α4 to laminin α5 was greater in domains within tolerant LNs, compared with immune LNs, and blocking laminin α4 function or inducing laminin α5 overexpression disrupted T cell and DC localization and transmigration through tolerant LNs. Furthermore, reducing α4 laminin circumvented tolerance induction and induced cardiac allograft inflammation and rejection in murine models. This work identifies laminins as potential targets for immune modulation.

Regulation of the adaptive immune system by innate lymphoid cells

Innate lymphoid cells (ILCs) are a group of lymphocytes that promote rapid cytokine-dependent innate immunity, inflammation and tissue repair. In addition, a growing body of evidence suggests ILCs can influence adaptive immune cell responses. During fetal development a subset of ILCs orchestrate the generation and maturation of secondary lymphoid tissues. Following birth, ILCs continue to modulate adaptive immune cell responses indirectly through interactions with stromal cells in lymphoid tissues and epithelial cells at barrier surfaces. In this review we summarize the current understanding of how ILCs modulate the magnitude and quality of adaptive immune cell responses, and in particular focus on recent evidence suggesting that ILCs can also directly regulate CD4(+) T cells. Further, we discuss the implications that these pathways may have on human health and disease.

Role of the Lymphotoxin/LIGHT System in the Development and Maintenance of Reticular Networks and Vasculature in Lymphoid Tissues

Lymphoid organs are meeting zones where lymphocytes come together and encounter antigens present in the blood and lymph or as delivered by cells migrating from the draining tissue bed. The exquisite efficiency of this process relies heavily on highly specialized anatomy to direct and position the various players. Gated entry and exit control access to these theaters and reticular networks and associated chemokines guide cells into the proper sections. Lymphoid tissues are remarkably plastic, being able to expand dramatically and then involute upon resolution of the danger. All of the reticular scaffolds and vascular and lymphatic components adapt accordingly. As such, the lymph node (LN) is a wonderful example of a physiologic remodeling process and is potentially a guide to study such elements in pathological settings such as fibrosis, chronic infection, and tumor metastasis. The lymphotoxin/LIGHT axis delivers critical differentiation signals that direct and hone differentiation of both reticular networks and the vasculature. Considerable progress has been made recently in understanding the mesenchymal differentiation pathways leading to these specialized networks and in the remodeling that occurs in reactive LNs. In this article, we will review some new advances in the area in terms of developmental, differentiation, and maintenance events mediated by this axis.

Trapping of naive lymphocytes triggers rapid growth and remodeling of the fibroblast network in reactive murine lymph nodes

Adaptive immunity is initiated in T-cell zones of secondary lymphoid organs. These zones are organized in a rigid 3D network of fibroblastic reticular cells (FRCs) that are a rich cytokine source. In response to lymph-borne antigens, draining lymph nodes (LNs) expand several folds in size, but the fate and role of the FRC network during immune response is not fully understood. Here we show that T-cell responses are accompanied by the rapid activation and growth of FRCs, leading to an expanded but similarly organized network of T-zone FRCs that maintains its vital function for lymphocyte trafficking and survival. In addition, new FRC-rich environments were observed in the expanded medullary cords. FRCs are activated within hours after the onset of inflammation in the periphery. Surprisingly, FRC expansion depends mainly on trapping of naïve lymphocytes that is induced by both migratory and resident dendritic cells. Inflammatory signals are not required as homeostatic T-cell proliferation was sufficient to trigger FRC expansion. Activated lymphocytes are also dispensable for this process, but can enhance the later growth phase. Thus, this study documents the surprising plasticity as well as the complex regulation of FRC networks allowing the rapid LN hyperplasia that is critical for mounting efficient adaptive immunity.

Stromal cell regulation of homeostatic and inflammatory lymphoid organogenesis

Secondary lymphoid organs function to increase the efficiency of interactions between rare, antigen-specific lymphocytes and antigen presenting cells, concentrating antigen and lymphocytes in a supportive environment that facilitates the initiation of an adaptive immune response. Homeostatic lymphoid tissue organogenesis proceeds via exquisitely controlled spatiotemporal interactions between haematopoietic lymphoid tissue inducer populations and multiple subsets of non-haematopoietic stromal cells. However, it is becoming clear that in a range of inflammatory contexts, ectopic or tertiary lymphoid tissues can develop inappropriately under pathological stress. Here we summarize the role of stromal cells in the development of homeostatic lymphoid tissue, and assess emerging evidence that suggests a critical role for stromal involvement in the tertiary lymphoid tissue development associated with chronic infections and inflammation.

Recent advances in inflammatory bowel disease: mucosal immune cells in intestinal inflammation

The intestine and its immune system have evolved to meet the extraordinary task of maintaining tolerance to the largest, most complex and diverse microbial commensal habitat, while meticulously attacking and containing even minute numbers of occasionally incoming pathogens. While our understanding is still far from complete, recent studies have provided exciting novel insights into the complex interplay of the many distinct intestinal immune cell types as well as the discovery of entirely new cell subsets. These studies have also revealed how proper development and function of the intestinal immune system is dependent on its specific microbiota, which appears to have evolutionarily co-evolved. Here we review key immune cells that maintain intestinal homeostasis and, conversely, describe how altered function and imbalances may lead to inflammatory bowel disease (IBD). We highlight the latest developments within this field, covering the major players in IBD including intestinal epithelial cells, macrophages, dendritic cells, adaptive immune cells, and the newly discovered innate lymphoid cells, which appear of characteristic importance for immune function at mucosal surfaces. We set these mucosal immune pathways in the functional context of IBD risk genes where such insight is available. Moreover, we frame our discussion of fundamental biological pathways that have been elucidated in model systems in the context of results from clinical trials in IBD that targeted key mediators secreted by these cells, as an attempt of 'functional' appraisal of these pathways in human disease.

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.

Aryl hydrocarbon receptor promotes RORγt⁺ group 3 ILCs and controls intestinal immunity and inflammation

Unlike adaptive immune cells that require antigen recognition and functional maturation during infection, innate lymphoid cells (ILCs) usually respond to pathogens promptly and serve as the first line of defense in infectious diseases. RAR-related orphan receptor (RORγt)⁺ group 3 ILCs are one of the innate cell populations that have recently been intensively studied. During the fetal stage of development, RORγt⁺ group 3 ILCs (e.g., lymphoid tissue inducer cells) are required for lymphoid organogenesis. In adult mice, RORγt⁺ group 3 ILCs are abundantly present in the gut to exert immune defensive functions. Under certain circumstances, however, RORγt⁺ group 3 ILCs can be pathogenic and contribute to intestinal inflammation. Aryl hydrocarbon receptor (Ahr), a ligand-dependent transcriptional factor, is widely expressed by various immune and non-immune cells. In the gut, the ligand for Ahr can be derived/generated from diet, microflora, and/or host cells. Ahr has been shown to regulate different cell populations in the immune system including RORγt⁺ group 3 ILCs, T helper (Th)17/22 cells, γδT cells, regulatory T cells (Tregs), Tr1 cells, and antigen presenting cells. In this review, we will focus on the development and function of RORγt⁺ group 3 ILCs, and discuss the role of Ahr in intestinal immunity and inflammation in mice and in humans. A better understanding of the function of Ahr in the gut is important for developing new therapeutic means to target Ahr in future treatment of infectious and autoimmune diseases.

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.

Th1- and Th2-like subsets of innate lymphoid cells

Innate lymphoid cells (ILCs) constitute a family of effectors in innate immunity and regulators of tissue remodeling that have a cytokine and transcription factor expression pattern that parallels that of the T-helper (Th) cell family. Here, we discuss how ILCs can be categorized and summarize the current knowledge of Th1- and Th2-like ILCs with regard to the molecular mechanisms of development, effector functions, and their interplay with other cell types.

Nkx2-5(+)islet1(+) mesenchymal precursors generate distinct spleen stromal cell subsets and participate in restoring stromal network integrity

Secondary lymphoid organ stromal cells comprise different subsets whose origins remain unknown. Herein, we exploit a genetic lineage-tracing approach to show that splenic fibroblastic reticular cells (FRCs), follicular dendritic cells (FDCs), marginal reticular cells (MRCs), and mural cells, but not endothelial cells, originate from embryonic mesenchymal progenitors of the Nkx2-5(+)Islet1(+) lineage. This lineage include embryonic mesenchymal cells with lymphoid tissue organizer (LTo) activity capable also of supporting ectopic lymphoid-like structures and a subset of resident spleen stromal cells that proliferate and regenerate the splenic stromal microenvironment following resolution of a viral infection. These findings identify progenitor cells that generate stromal diversity in spleen development and repair and suggest the existence of multipotent stromal progenitors in the adult spleen with regenerative capacity.

Naive B-cell trafficking is shaped by local chemokine availability and LFA-1–independent stromal interactions

CXCR5, but not CXCR4 or CCR7, acts with LFA-1 to mediate random B-cell migration in the T-cell area and B-cell follicles. In contrast, stromal guidance during B-cell migration is LFA-1 independent and CXCR5 independent.

Lineage relationships of human interleukin-22-producing CD56+ RORγt+ innate lymphoid cells and conventional natural killer cells

Human interleukin (IL)-22-producing RORγt(+) innate lymphoid cells (ILC22) and conventional natural killer (cNK) cells are present in secondary lymphoid tissues. Both have an immunophenotype corresponding to stage III NK progenitors (CD56(+/-)CD117(high)CD94(-)). Using an in vitro differentiation and primary human tissues, we investigated their developmental relationships. cNK cells showed a CD56(+)CD117(+)CD7(+/-)LFA-1(high) phenotype and expressed surface receptors, cytokines, and transcription factors found on mature cNK cells. In contrast, ILC22 cells were contained within the CD56(+)CD117(high)CD94(-)CD7(-)LFA-1(-) fraction and produced IL-22, IL-8, and granulocyte macrophage colony stimulating factor. Although ILC22 cells expressed NKp44 and CD161, they lacked most other NK receptors and NK-associated transcription factors (T-bet and Eomes) and were incapable of interferon-γ production or cytotoxic responses. Most purified CD56(+)CD117(+)CD7(+/-)LFA-1(-) remained as ILC22 cells and never became cNK cells. In the absence of IL-15, CD34(+) cells showed a complete block in cNK differentiation and instead gave rise to a CD56(+) population of ILC22 cells. Conversely, in the absence of IL-7 and stem cell factor, cNK cells were generated but ILC22 cells showed minimal differentiation. Although human ILC22 cells and cNK progenitors have a phenotype that overlaps with stage III NK progenitors, they have unique cytokine requirements and can be distinguished by LFA-1 expression.

Innate lymphoid cells: balancing immunity, inflammation, and tissue repair in the intestine

Innate lymphoid cells (ILCs) are a recently described group of innate immune cells that can regulate immunity, inflammation, and tissue repair in multiple anatomical compartments, particularly the barrier surfaces of the skin, airways, and intestine. Broad categories of ILCs have been defined based on transcription factor expression and the ability to produce distinct patterns of effector molecules. Recent studies have revealed that ILC populations can regulate commensal bacterial communities, contribute to resistance to helminth and bacterial pathogens, promote inflammation, and orchestrate tissue repair and wound healing. This review will examine the phenotype and function of murine and human ILCs and discuss the critical roles these innate immune cells play in health and disease.

Endothelial cell-specific lymphotoxin-β receptor signaling is critical for lymph node and high endothelial venule formation

The development of lymph nodes (LNs) and formation of LN stromal cell microenvironments is dependent on lymphotoxin-β receptor (LTβR) signaling. In particular, the LTβR-dependent crosstalk between mesenchymal lymphoid tissue organizer and hematopoietic lymphoid tissue inducer cells has been regarded as critical for these processes. Here, we assessed whether endothelial cell (EC)-restricted LTβR signaling impacts on LN development and the vascular LN microenvironment. Using EC-specific ablation of LTβR in mice, we found that conditionally LTβR-deficient animals failed to develop a significant proportion of their peripheral LNs. However, remnant LNs showed impaired formation of high endothelial venules (HEVs). Venules had lost their cuboidal shape, showed reduced segment length and branching points, and reduced adhesion molecule and constitutive chemokine expression. Due to the altered EC-lymphocyte interaction, homing of lymphocytes to peripheral LNs was significantly impaired. Thus, this study identifies ECs as an important LTβR-dependent lymphoid tissue organizer cell population and indicates that continuous triggering of the LTβR on LN ECs is critical for lymphocyte homeostasis.

IL-17-producing γδ T cells and innate lymphoid cells

The inflammatory cytokine IL-17 plays a critical role in immunity to infection and is involved in the inflammatory pathology associated with certain autoimmune diseases, such as psoriasis and rheumatoid arthritis. While CD4(+) and CD8(+) T cells are important sources of this cytokine, recent evidence has suggested that γδ T cells and a number of families of innate lymphoid cells (ILCs) can secrete IL-17 and related cytokines. The production of IL-17 by γδ T cells appears to be largely independent of T-cell receptor activation and is promoted through cytokine signalling, in particular by IL-23 in combination with IL-1β or IL-18. Therefore IL-17-secreting γδ T cells can be categorised as a family of cells similar to innate-like lymphoid cells. IL-17-secreting γδ T cells function as a part of mucosal defence against infection, with most studies to date focusing on their response to bacterial pathogens. γδ T cells also play a pathological role in certain autoimmune diseases, where they provide an early source of IL-17 and IL-21, which initiate responses mediated by conventional IL-17-secreting CD4(+) T cells (Th17 cells). ILCs lack an antigen receptor or other lineage markers, and ILC subsets that express the transcriptional factor RORγt have been found to secrete IL-17. Evidence is emerging that these newly recognised sources of IL-17 play both pathological and protective roles in inflammatory diseases as discussed in this article.

Human type 1 innate lymphoid cells accumulate in inflamed mucosal tissues

Innate lymphoid cells (ILCs) are effectors of innate immunity and regulators of tissue modeling. Recently identified ILC populations have a cytokine expression pattern that resembles that of the helper T cell subsets T(H)2, T(H)17 and T(H)22. Here we describe a distinct ILC subset similar to T(H)1 cells, which we call 'ILC1'. ILC1 cells expressed the transcription factor T-bet and responded to interleukin 12 (IL-12) by producing interferon-γ (IFN-γ). ILC1 cells were distinct from natural killer (NK) cells as they lacked perforin, granzyme B and the NK cell markers CD56, CD16 and CD94, and could develop from RORγt(+) ILC3 under the influence of IL-12. The frequency of the ILC1 subset was much higher in inflamed intestine of people with Crohn's disease, which indicated a role for these IFN-γ-producing ILC1 cells in the pathogenesis of gut mucosal inflammation.

Innate lymphoid cells--how did we miss them?

Innate lymphoid cells (ILCs) are newly identified members of the lymphoid lineage that have emerging roles in mediating immune responses and in regulating tissue homeostasis and inflammation. Here, we review the developmental relationships between the various ILC lineages that have been identified to date and summarize their functions in protective immunity to infection and their pathological roles in allergic and autoimmune diseases.

Blood flukes exploit Peyer's Patch lymphoid tissue to facilitate transmission from the mammalian host

Schistosomes are blood-dwelling parasitic helminths which produce eggs in order to facilitate transmission. Intestinal schistosomes lay eggs in the mesenteries, however, it is unclear how their eggs escape the vasculature to exit the host. Using a murine model of infection, we reveal that Schistosoma mansoni exploits Peyer's Patches (PP) gut lymphoid tissue as a preferential route of egress for their eggs. Egg deposition is favoured within PP as a result of their more abundant vasculature. Moreover, the presence of eggs causes significant vascular remodeling leading to an expanded venule network. Egg deposition results in a decrease in stromal integrity and lymphoid cellularity, including secretory IgA producing lymphocytes, and the focal recruitment of macrophages. In mice lacking PP, egg excretion is significantly impaired, leading to greater numbers of ova being entrapped in tissues and consequently, exacerbated morbidity. Thus, we demonstrate how schistosomes directly facilitate transmission from the host by targeting lymphoid tissue. For the host, PP-dependency of egg egress represents a trade-off, as limiting potentially life-threatening morbidity is balanced by loss of PP structure and perturbed PP IgA production.

Schistosomes are parasitic helminths that parasitise >200 million people worldwide. Adult worm pairs of intestinal schistosomes lay their eggs in the mesenteric veins from which the eggs need to pass into the lumen prior to excretion and completion of their life cycle. However, it is not known how eggs transfer from the intestinal vasculature to reach the gut lumen. Here, we reveal using a mouse model of infection, that Schistosoma mansoni exploits Peyer's Patches (PP) lymphoid tissues in the wall of the small intestine as a preferential route of egg egress. The eggs cause vascular remodelling in the PP leading to an expanded venule network, reduced stromal integrity, and decreased lymphoid cellularity. Most significantly, in mice rendered deficient in PP, egg excretion is impaired (despite intact immune responses), leading to greater numbers of eggs entrapped in tissues, and consequently exacerbated host morbidity. Thus, we demonstrate how schistosomes directly facilitate transmission from the host by targeting lymphoid tissue. For the host, this represents a trade-off as limiting life-threatening morbidity is balanced by loss of PP structure and function. The requirement of PP for efficient schistosome egress may be a significant risk factor of developing severe disease within heavily infected human populations.

What's the next best cytokine target in IBD?

In the gut of patients with inflammatory bowel disease (IBD), immune and nonimmune cells produce large amounts of cytokines that drive the inflammatory process leading to the tissue damage. Cytokine blockers, such as anti-tumor necrosis factor alpha (TNF-α), have been used with some success in IBD. However, not all patients respond, and the therapeutic effects wane with time, demonstrating the need for more effective and long-lasting antiinflammatory strategies. A key question is whether neutralizing other proinflammatory cytokines such as interleukin (IL)-12, IL-21, IL-27, or IL-33 will lead to a better clinical response than with anti-TNF-α antibodies. Equally, we now know that IBD-related inflammation is marked by defective production/activity of antiinflammatory cytokines, and there are strategies to correct these defects. An alternative approach is to try to target individual therapies to individual patients, to improve clinical efficacy in subsets of patients, but this has proven difficult. Here we try to evaluate the potential of each of these choices.

Immune response and immunopathology during toxoplasmosis

Toxoplasma gondii is a protozoan parasite of medical and veterinary significance that is able to infect any warm-blooded vertebrate host. In addition to its importance to public health, several inherent features of the biology of T. gondii have made it an important model organism to study host-pathogen interactions. One factor is the genetic tractability of the parasite, which allows studies on the microbial factors that affect virulence and allows the development of tools that facilitate immune studies. Additionally, mice are natural hosts for T. gondii, and the availability of numerous reagents to study the murine immune system makes this an ideal experimental system to understand the functions of cytokines and effector mechanisms involved in immunity to intracellular microorganisms. In this article, we will review current knowledge of the innate and adaptive immune responses required for resistance to toxoplasmosis, the events that lead to the development of immunopathology, and the natural regulatory mechanisms that limit excessive inflammation during this infection.

IL-7-producing stromal cells are critical for lymph node remodeling

Nonhematopoietic stromal cells of secondary lymphoid organs form important scaffold and fluid transport structures, such as lymph node (LN) trabeculae, lymph vessels, and conduits. Furthermore, through the production of chemokines and cytokines, these cells generate a particular microenvironment that determines lymphocyte positioning and supports lymphocyte homeostasis. IL-7 is an important stromal cell-derived cytokine that has been considered to be derived mainly from T-cell zone fibroblastic reticular cells. We show here that lymphatic endothelial cells (LECs) are a prominent source of IL-7 both in human and murine LNs. Using bacterial artificial chromosome transgenic IL-7-Cre mice, we found that fibroblastic reticular cells and LECs strongly up-regulated IL-7 expression during LN remodeling after viral infection and LN reconstruction after avascular transplantation. Furthermore, IL-7-producing stromal cells contributed to de novo formation of LyveI-positive lymphatic structures connecting reconstructed LNs with the surrounding tissue. Importantly, diphtheria toxin-mediated depletion of IL-7-producing stromal cells completely abolished LN reconstruction. Taken together, this study identifies LN LECs as a major source of IL-7 and shows that IL-7-producing stromal cells are critical for reconstruction and remodeling of the distinct LN microenvironment.

Cutting edge: lymphoid tissue inducer cells maintain memory CD4 T cells within secondary lymphoid tissue

Phylogeny shows that CD4 T cell memory and lymph nodes coevolved in placental mammals. In ontogeny, retinoic acid orphan receptor (ROR)γ-dependent lymphoid tissue inducer (LTi) cells program the development of mammalian lymph nodes. In this study, we show that although primary CD4 T cell expansion is normal in RORγ-deficient mice, the persistence of memory CD4 T cells is RORγ-dependent. Furthermore, using bone marrow chimeric mice we demonstrate that LTi cells are the key RORγ-expressing cell type sufficient for memory CD4 T cell survival in the absence of persistent Ag. This effect was specific for CD4 T cells, as memory CD8 T cells survived equally well in the presence or absence of LTi cells. These data demonstrate a novel role for LTi cells, archetypal members of the innate lymphoid cell family, in supporting memory CD4 T cell survival in vivo.

Tight control - decision-making during T cell-vascular endothelial cell interaction

Vascular endothelial cells (ECs) form the inner layer of blood vessels and exert crucial functions during immune reactions including coagulation, inflammation, and regulation of innate immunity. Importantly, ECs can interact with T cells in an antigen-specific, i.e., T cell receptor-dependent manner. In this review, we will discuss EC actions and reactions during acute inflammation and focus on the interaction of T cells with ECs at two vascular sites: the high endothelial venule (HEV) of lymph nodes, and the vascular lesion during transplant vasculopathy (TV). HEVs are characterized by a highly active endothelium that produces chemoattracting factors and expresses adhesion molecules to facilitate transit of lymphocytes into the lymph node (LN) parenchyma. Yet, T cell-EC interaction at this anatomical location results neither in T cell activation nor tolerization. In contrast, the endothelium at sites of chronic inflammation, such as solid organ transplants, can promote T cell activation by upregulation of major histocompatibility complex (MHC) and costimulatory molecules. Importantly, a major function of ECs in inflamed tissues must be the maintenance of vascular integrity including the efficient attenuation of effector T cells that may damage the vascular bed. Thus, antigen-specific T cell-EC interaction is characterized by a tightly controlled balance between immunological ignorance, immune activation, and tolerization.