Intestinal cryptopatch formation in mice requires lymphotoxin alpha and the lymphotoxin beta receptor

Early-life programming of mesenteric lymph node stromal cell identity by the lymphotoxin pathway regulates adult mucosal immunity

Redundant mechanisms support immunoglobulin A (IgA) responses to intestinal antigens. These include multiple priming sites [mesenteric lymph nodes (MLNs), Peyer's patches, and isolated lymphoid follicles] and various cytokines that promote class switch to IgA, even in the absence of T cells. Despite these backup mechanisms, vaccination against enteric pathogens such as rotavirus has limited success in some populations. Genetic and environmental signals experienced during early life are known to influence mucosal immunity, yet the mechanisms for how these exposures operate remain unclear. Here, we used rotavirus infection to follow antigen-specific IgA responses through time and in different gut compartments. Using genetic and pharmacological approaches, we tested the role of the lymphotoxin (LT) pathway-known to support IgA responses-at different developmental stages. We found that LT-β receptor (LTβR) signaling in early life programs intestinal IgA responses in adulthood by affecting antibody class switch recombination to IgA and subsequent generation of IgA antibody-secreting cells within an intact MLN. In addition, early-life LTβR signaling dictates the phenotype and function of MLN stromal cells to support IgA responses in the adult. Collectively, our studies uncover new mechanistic insights into how early-life LTβR signaling affects mucosal immune responses during adulthood.

Innate lymphoid cell and adaptive immune cell cross-talk: A talk meant not to forget

Innate lymphoid cells (ILCs) are a relatively new class of innate immune cells with phenotypical characters of lymphocytes but genotypically or functionally behave as typical innate immune cells. They have been classically divided into 3 groups (group 1 ILCs or ILC1s, group 2 ILCs or ILC2s, and group 3 ILCs or ILC3s). They serve as the first line of defense against invading pathogens and allergens at mucosal surfaces. The adaptive immune response works effectively in association with innate immunity as innate immune cells serve as APCs to directly stimulate the adaptive immune cells (various sets of T and B cells). Additionally, innate immune cells also secrete various effector molecules, including cytokines or chemokines impacting the function, differentiation, proliferation, and reprogramming among adaptive immune cells to maintain immune homeostasis. Only superantigens do not require their processing by innate immune cells as they are recognized directly by T cells and B cells. Thus, a major emphasis of the current article is to describe the cross-talk between different ILCs and adaptive immune cells during different conditions varying from normal physiological situations to different infectious diseases to allergic asthma.

Immunoregulatory Sensory Circuits in Group 3 Innate Lymphoid Cell (ILC3) Function and Tissue Homeostasis

Recent years have seen a revolution in our understanding of how cells of the immune system are modulated and regulated not only via complex interactions with other immune cells, but also through a range of potent inputs derived from diverse and varied biological systems. Within complex tissue environments, such as the gastrointestinal tract and lung, these systems act to orchestrate and temporally align immune responses, regulate cellular function, and ensure tissue homeostasis and protective immunity. Group 3 Innate Lymphoid Cells (ILC3s) are key sentinels of barrier tissue homeostasis and critical regulators of host-commensal mutualism—and respond rapidly to damage, inflammation and infection to restore tissue health. Recent findings place ILC3s as strategic integrators of environmental signals. As a consequence, ILC3s are ideally positioned to detect perturbations in cues derived from the environment—such as the diet and microbiota—as well as signals produced by the host nervous, endocrine and circadian systems. Together these cues act in concert to induce ILC3 effector function, and form critical sensory circuits that continually function to reinforce tissue homeostasis. In this review we will take a holistic, organismal view of ILC3 biology and explore the tissue sensory circuits that regulate ILC3 function and align ILC3 responses with changes within the intestinal environment.

Fibroblastic reticular cells at the nexus of innate and adaptive immune responses

Lymphoid organs guarantee productive immune cell interactions through the establishment of distinct microenvironmental niches that are built by fibroblastic reticular cells (FRC). These specialized immune‐interacting fibroblasts coordinate the migration and positioning of lymphoid and myeloid cells in lymphoid organs and provide essential survival and differentiation factors during homeostasis and immune activation. In this review, we will outline the current knowledge on FRC functions in secondary lymphoid organs such as lymph nodes, spleen and Peyer's patches and will discuss how FRCs contribute to the regulation of immune processes in fat‐associated lymphoid clusters. Moreover, recent evidence indicates that FRC critically impact immune regulatory processes, for example, through cytokine deprivation during immune activation or through fostering the induction of regulatory T cells. Finally, we highlight how different FRC subsets integrate innate immunological signals and molecular cues from immune cells to fulfill their function as nexus between innate and adaptive immune responses.

Intestinal epithelial cell-specific RARα depletion results in aberrant epithelial cell homeostasis and underdeveloped immune system

Retinoic acid (RA), a dietary vitamin A metabolite, is crucial in maintaining intestinal homeostasis. RA acts on intestinal leukocytes to modulate their lineage commitment and function. Although the role of RA has been characterized in immune cells, whether intestinal epithelial cells (IECs) rely on RA signaling to exert their immune-regulatory function has not been examined. Here we demonstrate that lack of RA receptor α (RARα) signaling in IECs results in deregulated epithelial lineage specification, leading to increased numbers of goblet cells and Paneth cells. Mechanistically, lack of RARα resulted in increased KLF4+ goblet cell precursors in the distal bowel, whereas RA treatment inhibited klf4 expression and goblet cell differentiation in zebrafish. These changes in secretory cells are associated with increased Reg3g, reduced luminal bacterial detection, and an underdeveloped intestinal immune system, as evidenced by an almost complete absence of lymphoid follicles and gut resident mononuclear phagocytes. This underdeveloped intestinal immune system shows a decreased ability to clear infection with Citrobacter rodentium. Collectively, our findings indicate that epithelial cell-intrinsic RARα signaling is critical to the global development of the intestinal immune system.

Regulation of Innate Lymphoid Cells by Aryl Hydrocarbon Receptor

With striking similarity to their adaptive T helper cell counterparts, innate lymphoid cells (ILCs) represent an emerging family of cell types that express signature transcription factors, including T-bet+ Eomes+ natural killer cells, T-bet+ Eomes- group 1 ILCs, GATA3+ group 2 ILCs, RORγt+ group 3 ILCs, and newly identified Id3+ regulatory ILC. ILCs are abundantly present in barrier tissues of the host (e.g., the lung, gut, and skin) at the interface of host-environment interactions. Active research has been conducted to elucidate molecular mechanisms underlying the development and function of ILCs. The aryl hydrocarbon receptor (Ahr) is a ligand-dependent transcription factor, best known to mediate the effects of xenobiotic environmental toxins and endogenous microbial and dietary metabolites. Here, we review recent progresses regarding Ahr function in ILCs. We focus on the Ahr-mediated cross talk between ILCs and other immune/non-immune cells in host tissues especially in the gut. We discuss the molecular mechanisms of the action of Ahr expression and activity in regulation of ILCs in immunity and inflammation, and the interaction between Ahr and other pathways/transcription factors in ILC development and function with their implication in disease.

The Development of Steady-State Activation Hubs between Adult LTi ILC3s and Primed Macrophages in Small Intestine

Group 3 innate lymphoid cells (ILC3s) are important for intestinal health, particularly in controlling inflammation in response to epithelial dysregulation, but their role during homeostasis remains less well understood. We generated IL-22 reporter mice to assess production of this key cytokine by ILC3s in the small intestine during development and under basal conditions. Although IL-22 is produced by a variety of lymphocyte populations, constitutively high IL-22 expression was limited to lymphoid-tissue inducer (LTi) cells residing in lymph node-like structures in the gut called solitary intestinal lymphoid tissues (SILT). Constitutive IL-22 expression was dependent on the microbiota and MyD88 signaling, appeared upon weaning, and was present across the spectrum of SILT, including in cryptopatches. Activated SILT LTi cells colocalized with a rare subpopulation of activated macrophages constitutively positive for IL-12/23 p40 and capable of activating neonatal LTi cells in response to TLR stimulus. Thus, weaning leads to the organization of innate immune activation hubs at SILT that mature and are continuously sustained by signals from the microbiota. This functional and anatomic organization constitutes a significant portion of the steady-state IL-23/IL-22 axis.

Innate lymphoid cell function in the context of adaptive immunity

Innate lymphoid cells (ILCs) are a family of innate immune cells that have diverse functions during homeostasis and disease. Subsets of ILCs have phenotypes that mirror those of polarized helper T cell subsets in their expression of core transcription factors and effector cytokines. Given the similarities between these two classes of lymphocytes, it is important to understand which functions of ILCs are specialized and which are redundant with those of T cells. Here we discuss genetic mouse models that have been used to delineate the contributions of ILCs versus those of T cells and review the current understanding of the specialized in vivo functions of ILCs.

Development and Function of Secondary and Tertiary Lymphoid Organs in the Small Intestine and the Colon

The immune system of the gut has evolved a number of specific lymphoid structures that contribute to homeostasis in the face of microbial colonization and food-derived antigenic challenge. These lymphoid organs encompass Peyer’s patches (PP) in the small intestine and their colonic counterparts that develop in a programed fashion before birth. In addition, the gut harbors a network of lymphoid tissues that is commonly designated as solitary intestinal lymphoid tissues (SILT). In contrast to PP, SILT develop strictly after birth and consist of a dynamic continuum of structures ranging from small cryptopatches (CP) to large, mature isolated lymphoid follicles (ILF). Although the development of PP and SILT follow similar principles, such as an early clustering of lymphoid tissue inducer (LTi) cells and the requirement for lymphotoxin beta (LTβ) receptor-mediated signaling, the formation of CP and their further maturation into ILF is associated with additional intrinsic and environmental signals. Moreover, recent data also indicate that specific differences exist in the regulation of ILF formation between the small intestine and the colon. Importantly, intestinal inflammation in both mice and humans is associated with a strong expansion of the lymphoid network in the gut. Recent experiments in mice suggest that these structures, although they resemble large, mature ILF in appearance, may represent de novo-induced tertiary lymphoid organs (TLO). While, so far, it is not clear whether intestinal TLO contribute to the exacerbation of inflammatory pathology, it has been shown that ILF provide the critical microenvironment necessary for the induction of an effective host response upon infection with enteric bacterial pathogens. Regarding the importance of ILF for intestinal immunity, interfering with the development and maturation of these lymphoid tissues may offer novel means for manipulating the immune response during intestinal infection or inflammation.

Ectopic Tertiary Lymphoid Tissue in Inflammatory Bowel Disease: Protective or Provocateur?

Organized lymphoid tissues like the thymus first appeared in jawed vertebrates around 500 million years ago and have evolved to equip the host with a network of specialized sites, strategically located to orchestrate strict immune-surveillance and efficient immune responses autonomously. The gut-associated lymphoid tissues maintain a mostly tolerant environment to dampen our responses to daily dietary and microbial products in the intestine. However, when this homeostasis is perturbed by chronic inflammation, the intestine is able to develop florid organized tertiary lymphoid tissues (TLT), which heralds the onset of regional immune dysregulation. While TLT are a pathologic hallmark of Crohn's disease (CD), their role in the overall process remains largely enigmatic. A critical question remains; are intestinal TLT generated by the immune infiltrated intestine to modulate immune responses and rebuild tolerance to the microbiota or are they playing a more sinister role by generating dysregulated responses that perpetuate disease? Herein, we discuss the main theories of intestinal TLT neogenesis and focus on the most recent findings that open new perspectives to their role in inflammatory bowel disease.

The Gut-Associated Lymphoid Tissues in the Small Intestine, Not the Large Intestine, Play a Major Role in Oral Prion Disease Pathogenesis

Prion diseases are infectious neurodegenerative disorders characterized by accumulations of abnormally folded cellular prion protein in affected tissues. Many natural prion diseases are acquired orally, and following exposure, the early replication of some prion isolates upon follicular dendritic cells (FDC) within gut-associated lymphoid tissues (GALT) is important for the efficient spread of disease to the brain (neuroinvasion). Prion detection within large intestinal GALT biopsy specimens has been used to estimate human and animal disease prevalence. However, the relative contributions of the small and large intestinal GALT to oral prion pathogenesis were unknown. To address this issue, we created mice that specifically lacked FDC-containing GALT only in the small intestine. Our data show that oral prion disease susceptibility was dramatically reduced in mice lacking small intestinal GALT. Although these mice had FDC-containing GALT throughout their large intestines, these tissues were not early sites of prion accumulation or neuroinvasion. We also determined whether pathology specifically within the large intestine might influence prion pathogenesis. Congruent infection with the nematode parasite Trichuris muris in the large intestine around the time of oral prion exposure did not affect disease pathogenesis. Together, these data demonstrate that the small intestinal GALT are the major early sites of prion accumulation and neuroinvasion after oral exposure. This has important implications for our understanding of the factors that influence the risk of infection and the preclinical diagnosis of disease.

IMPORTANCE

Many natural prion diseases are acquired orally. After exposure, the accumulation of some prion diseases in the gut-associated lymphoid tissues (GALT) is important for efficient spread of disease to the brain. However, the relative contributions of GALT in the small and large intestines to oral prion pathogenesis were unknown. We show that the small intestinal GALT are the essential early sites of prion accumulation. Furthermore, congruent infection with a large intestinal helminth (worm) around the time of oral prion exposure did not affect disease pathogenesis. This is important for our understanding of the factors that influence the risk of prion infection and the preclinical diagnosis of disease. The detection of prions within large intestinal GALT biopsy specimens has been used to estimate human and animal disease prevalence. However, our data suggest that using these biopsy specimens may miss individuals in the early stages of oral prion infection and significantly underestimate the disease prevalence.

Reciprocal regulation of lymphoid tissue development in the large intestine by IL-25 and IL-23

Isolated lymphoid follicles (ILFs) develop after birth in the small and large intestines (SI and LI) and represent a dynamic response of the gut immune system to the microbiota. Despite their similarities, ILF development in the SI and LI differs on a number of levels. We show that unlike ILF in the SI, the microbiota inhibits ILF development in the colon as conventionalization of germ-free mice reduced colonic ILFs. From this, we identified a novel mechanism regulating colonic ILF development through the action of interleukin (IL)-25 on IL-23 and its ability to modulate T regulatory cell (Treg) differentiation. Colonic ILF develop in the absence of a number of factors required for the development of their SI counterparts and can be specifically suppressed by factors other than IL-25. However, IL-23 is the only factor identified that specifically promotes colonic ILFs without affecting SI-ILF development. Both IL-23 and ILFs are associated with inflammatory bowel disease, suggesting that disruption to this pathway may have an important role in the breakdown of microbiota-immune homeostasis.

Colonic patch and colonic SILT development are independent and differentially regulated events

Intestinal lymphoid tissues have to simultaneously ensure protection against pathogens and tolerance toward commensals. Despite such vital functions, their development in the colon is poorly understood. Here, we show that the two distinct lymphoid tissues of the colon-colonic patches and colonic solitary intestinal lymphoid tissues (SILTs)-can easily be distinguished based on anatomical location, developmental timeframe, and cellular organization. Furthermore, whereas colonic patch development depended on CXCL13-mediated lymphoid tissue inducer (LTi) cell clustering followed by LTα-mediated consolidation, early LTi clustering at SILT anlagen did not require CXCL13, CCR6, or CXCR3. Subsequent dendritic cell recruitment to and gp38(+)VCAM-1(+) lymphoid stromal cell differentiation within SILTs required LTα; B-cell recruitment and follicular dendritic cell differentiation depended on MyD88-mediated signaling, but not the microflora. In conclusion, our data demonstrate that different mechanisms, mediated mainly by programmed stimuli, induce the formation of distinct colonic lymphoid tissues, therefore suggesting that these tissues may have different functions.

A role for Th17 cells in the regulation of tertiary lymphoid follicles

Immune responses propagate in secondary lymphoid organs (SLOs), such as the spleen and lymph nodes. These highly organized structures are typified by distinct B-cell follicles and T-cell zones, and are orchestrated by interactions between the TNF superfamily molecules expressed on hematopoietic cells and their receptors on mesenchymal cells and the subsequent cytokines and chemokines that are elicited. During chronic immune responses, cellular effectors of the immune response can infiltrate target tissue and organize anatomically into de novo B-cell follicles and T-cell areas, a phenomenon called lymphoid neogenesis or the formation of tertiary lymphoid organs (TLOs). Critical to the development of SLOs are lymphoid-tissue inducer (LTi) cells, that is innate lymphoid cells that arise from common precursor cells within the fetal liver. Of interest, Th17 cells, a subset of CD4(+) T cells most associated with autoimmune pathogenesis, share many developmental and effector markers with LTi cells. Here, we compare and contrast LTi and Th17 cells, and review recent evidence that Th17 cells and Th17 cytokines, such as IL-17 and IL-22, contribute to the development of ectopic lymphoid structures in chronic-ally inflamed tissue.

Role of the Aryl Hydrocarbon Receptor in Controlling Maintenance and Functional Programs of RORγt(+) Innate Lymphoid Cells and Intraepithelial Lymphocytes

Mucosal retinoic receptor-related orphan receptor (ROR)γt-expressing innate lymphoid cells (ILC) play an important role in the defense against intestinal pathogens and in promoting epithelial homeostasis and adaptation, thereby effectively protecting the vertebrate host against intestinal inflammatory disorders. The functional activity of RORγt(+) ILC is under the control of environmental cues. However, the molecular sensors for such environmental signals are largely unknown. Recently, the aryl hydrocarbon receptor (AhR) has emerged as a master regulator for the postnatal maintenance of intestinal RORγt(+) ILC and intraepithelial lymphocytes. AhR is a highly conserved transcription factor whose activity is regulated by environmental and dietary small molecule ligands. Here, we review the role of AhR signaling for the maintenance of intestinal immune cells and its impact on the immunological protection against intestinal infections and debilitating chronic inflammatory disorders.

Isolated Lymphoid Follicles are Dynamic Reservoirs for the Induction of Intestinal IgA

IgA is one of the most important molecules in the regulation of intestinal homeostasis. Peyer's patches have been traditionally recognized as sites for the induction of intestinal IgA responses, however more recent studies demonstrate that isolated lymphoid follicles (ILFs) can perform this function as well. ILF development is dynamic, changing in response to the luminal microbial burden, suggesting that ILFs play an important role providing an expandable reservoir of compensatory IgA inductive sites. However, in situations of immune dysfunction, ILFs can over-develop in response to uncontrollable enteric flora, resulting in ILF hyperplasia. The ability of ILFs to expand and respond to help control the enteric flora makes this dynamic reservoir an important arm of IgA inductive sites in intestinal immunity.

Epithelial expression of the cytosolic retinoid chaperone cellular retinol binding protein II is essential for in vivo imprinting of local gut dendritic cells by lumenal retinoids

Dendritic cells (DCs) use all-trans retinoic acid (ATRA) to promote characteristic intestinal responses, including Foxp3(+) Treg conversion, lymphocyte gut homing molecule expression, and IgA production. How this ability to generate ATRA is conferred to DCs in vivo remains largely unstudied. Here, we observed that among DCs, retinaldehyde dehydrogenase (ALDH1), which catalyzes the conversion of retinal to ATRA, was preferentially expressed by small intestine CD103(+) lamina propria (LP) DCs. Retinoids induced LP CD103(+) DCs to generate ATRA via ALDH1 activity. Either biliary or dietary retinoids were required to confer ALDH activity to LP DCs in vivo. Cellular retinol-binding protein II (CRBPII), a cytosolic retinoid chaperone that directs enterocyte retinol and retinal metabolism but is redundant to maintain serum retinol, was required to confer ALDH activity to CD103(+) LP DCs. CRBPII expression was restricted to small intestine epithelial cells, and ALDH activity in CRBPII(-/-) DCs was restored by transfer to a wild-type recipient. CD103(+) LP DCs from CRBPII(-/-) mice had a decreased capacity to promote IgA production. Moreover, CD103(+) DCs preferentially associated with the small intestine epithelium and LP CD103(+) DC ALDH activity, and the ability to promote IgA production was reduced in mice with impaired DC-epithelia associations. These findings demonstrate in vivo roles for the expression of epithelial CRBPII and lumenal retinoids to imprint local gut DCs with an intestinal phenotype.

IL-22 bridges the lymphotoxin pathway with the maintenance of colonic lymphoid structures during infection with Citrobacter rodentium

Colonic patches (CLPs) and isolated lymphoid follicles (ILFs) are two main lymphoid structures in the colon. Lymphoid tissue-inducer cells (LTi cells) are indispensable for the development of ILFs. LTi cells also produce interleukin 17 (IL-17) and IL-22, signature cytokines secreted by IL-17-producing helper T cells. Here we report that IL-22 acted downstream of the lymphotoxin pathway and regulated the organization and maintenance of mature CLPs and ILFs in the colon during infection with Citrobacter rodentium. Lymphotoxin (LTα(1)β(2)) regulated the production of IL-22 during infection with C. rodentium, but the lymphotoxin-like protein LIGHT did not. IL-22 signaling was sufficient to restore the organization of CLPs and ILFs and host defense against infection with C. rodentium in mice lacking lymphotoxin signals, which suggests that IL-22 connects the lymphotoxin pathway to mucosal epithelial defense mechanisms.

Lymphotoxin-β receptor-independent development of intestinal IL-22-producing NKp46+ innate lymphoid cells

The natural cytotoxicity receptor NKp46 is an activating receptor expressed by several distinct innate lymphoid cell (ILC) subsets, including NK cells, some γδ T cells and intestinal RORγt(+) IL-22(+) cells (NCR22 cells, IL-22-producing NKp46(+) cell). NCR22 cells may play a role in mucosal barrier function through IL-22-mediated production of anti-bacterial peptides from intestinal epithelial cells. Previous studies identified a predominant proportion of NCR22 cells in gut cryptopatches (CP), lymphoid structures that are strategically positioned to collect and integrate signals from luminal microbes; however, whether CP or other lymphoid structures condition NCR22 cell differentiation is not known. Programmed and inducible lymphoid tissue development requires cell-surface-expressed lymphotoxin (LT)α(1) β(2) heterotrimers (provided by lymphoid tissue inducer (LTi) cells) to signal lymphotoxin-β receptor (LTR)(+) stromal cells. Here, we analyzed NCR22 cells in LTβR-deficient Ncr1(GFP/+) mice that lack organized secondary lymphoid tissues. We found that NCR22 cells develop in the absence of LTβR, become functionally competent and localize to the lamina propria under steady-state conditions. Following infection of LTβR(-/-) mice with the Gram-negative pathogen Citrobacter rodentium, IL-22 production from NCR22 cells was not affected. These results indicate that organized lymphoid tissue structures are not critical for the generation of an intact and fully functional intestinal NCR22 cell compartment.

Adaptive immune regulation in the gut: T cell-dependent and T cell-independent IgA synthesis

In mammals, the gastrointestinal tract harbors an extraordinarily dense and complex community of microorganisms. The gut microbiota provide strong selective pressure to the host to evolve adaptive immune responses required for the maintenance of local and systemic homeostasis. The continuous antigenic presence in the gut imposes a dynamic remodeling of gut-associated lymphoid tissues (GALT) and the selection of multiple layered strategies for immunoglobulin (Ig) A production. The composite and dynamic gut environment also necessitates heterogeneous, versatile, and convertible T cells, capable of inhibiting (Foxp3(+) T cells) or helping (T(FH) cells) local immune responses. In this review, we describe recent advances in our understanding of dynamic pathways that lead to IgA synthesis, in gut follicular structures and in extrafollicular sites, by T cell-dependent and T cell-independent mechanisms. We discuss the finely tuned regulatory mechanisms for IgA production and emphasize the role of mucosal IgA in the selection and maintenance of the appropriate microbial composition that is necessary for immune homeostasis.

Dendritic cells produce CXCL13 and participate in the development of murine small intestine lymphoid tissues

In the adult intestine, luminal microbiota induce cryptopatches to transform into isolated lymphoid follicles (ILFs), which subsequently act as sites for the generation of IgA responses. The events leading to this conversion are incompletely understood. Dendritic cells (DCs) are components of cryptopatches (CPs) and ILFs and were therefore evaluated in this process. We observed that the adult murine intestine contains clusters of DCs restricted to the CP/ILF continuum. A numerical and cell associative hierarchy in the adult intestine and a chronologic hierarchy in the neonatal intestine demonstrated that these clusters form after the coalescence of CD90+ cells to form CPs and before the influx of B220+ B lymphocytes to form ILFs. Cluster formation was dependent on lymphotoxin and the lymphotoxin beta receptor and independent of lymphocytes. The ILF DC population was distinguished from that of the lamina propria by the absence of CD4+CD11c+ cells and an increased proportion of CD11c+B220+ cells. The formation of clusters was not limited by DC numbers but was induced by luminal microbiota. Moreover, in the absence of the chemokine CXCL13, CP transformation into ILF was arrested. Furthermore, ILF DCs express CXCL13, and depletion of DCs resulted in regression of ILFs and disorganization of CPs. These results reveal DC participation in ILF transformation and maintenance and suggest that in part this may be due to CXCL13 production by these cells.

CCR6 identifies lymphoid tissue inducer cells within cryptopatches

The chemokine receptor CCR6 is expressed by dendritic cells, B and T cells predominantly within the organized structures of the gut-associated lymphatic tissue. Its ligand CCL20 is synthesized by the follicle-associated epithelium and is crucial for the development of M cells within Peyer's patches. In addition, lineage-negative c-kit positive lymphocytes within cryptopatches (CP) express CCR6. CCR6-deficient mice exhibit an altered intestinal immune system containing increased amounts of intraepithelial lymphocytes and show smaller Peyer's patches, while progression of cryptopatches to mature isolated lymphoid follicles (ILF) is inhibited. In this report, we show that lin(-) c-kit(+) lymphocytes express a variety of different chemokine receptors and that CCR6 identifies those cells located within CP. In contrast, cells found outside CP are positive for CXCR3 and exhibit a different surface marker profile, suggesting that at least two different populations of lin(-) c-kit(+) cells are present. The presence of CCR6 does not influence the expression of Notch molecules on lin(-) c-kit(+) cells, nor does it influence Notch ligand expression on bone marrow-derived dendritic cells. In the human gut, CCR6 identifies clusters of lymphocytes resembling murine CP. CCR6 seems to have an important role for lin(-) c-kit(+) cells inside CP, is expressed in a regulated manner and identifies potential human CP.

Id2-, RORgammat-, and LTbetaR-independent initiation of lymphoid organogenesis in ocular immunity

The eye is protected by the ocular immunosurveillance system. We show that tear duct–associated lymphoid tissue (TALT) is located in the mouse lacrimal sac and shares immunological characteristics with mucosa-associated lymphoid tissues (MALTs), including the presence of M cells and immunocompetent cells for antigen uptake and subsequent generation of mucosal immune responses against ocularly encountered antigens and bacteria such as Pseudomonas aeruginosa. Initiation of TALT genesis began postnatally; it occurred even in germ-free conditions and was independent of signaling through organogenesis regulators, including inhibitor of DNA binding/differentiation 2, retinoic acid–related orphan receptor γt, lymphotoxin (LT) α1β2–LTβR, and lymphoid chemokines (CCL19, CCL21, and CXCL13). Thus, TALT shares immunological features with MALT but has a distinct tissue genesis mechanism and plays a key role in ocular immunity.

Secondary lymphoid organs: responding to genetic and environmental cues in ontogeny and the immune response

Secondary lymphoid organs (SLOs) include lymph nodes, spleen, Peyer's patches, and mucosal tissues such as the nasal-associated lymphoid tissue, adenoids, and tonsils. Less discretely anatomically defined cellular accumulations include the bronchus-associated lymphoid tissue, cryptopatches, and isolated lymphoid follicles. All SLOs serve to generate immune responses and tolerance. SLO development depends on the precisely regulated expression of cooperating lymphoid chemokines and cytokines such as LTalpha, LTbeta, RANKL, TNF, IL-7, and perhaps IL-17. The relative importance of these factors varies between the individual lymphoid organs. Participating in the process are lymphoid tissue initiator, lymphoid tissue inducer, and lymphoid tissue organizer cells. These cells and others that produce crucial cytokines maintain SLOs in the adult. Similar signals regulate the transition from inflammation to ectopic or tertiary lymphoid tissues.

Targeting lymphocyte activation through the lymphotoxin and LIGHT pathways

SUMMARY

Cytokines mediate key communication pathways essential for regulation of immune responses. Full activation of antigen-responding lymphocytes requires cooperating signals from the tumor necrosis factor (TNF)-related cytokines and their specific receptors. LIGHT, a lymphotoxin-beta (LTbeta)-related TNF family member, modulates T-cell activation through two receptors, the herpesvirus entry mediator (HVEM) and indirectly through the LT-beta receptor. An unexpected finding revealed a non-canonical binding site on HVEM for the immunoglobulin superfamily member, B and T lymphocyte attenuator (BTLA), and an inhibitory signaling protein suppressing T-cell activation. Thus, HVEM can act as a molecular switch between proinflammatory and inhibitory signaling. The non-canonical HVEM-BTLA pathway also acts to counter LTbetaR signaling that promotes the proliferation of antigen-presenting dendritic cells (DCs) within lymphoid tissue microenvironments. These results indicate LTbeta receptor and HVEM-BTLA pathways form an integrated signaling circuit. Targeting these cytokine pathways with specific antagonists (antibody or decoy receptor) can alter lymphocyte differentiation and activation. Alternately, agonists directed at their cell surface receptors can restore homeostasis and potentially reset immune and inflammatory processes, which may be useful in treating autoimmune and infectious diseases and cancer.

Induction of intestinal lymphoid tissue formation by intrinsic and extrinsic signals

Since the discovery of inducer cells as a separate lineage for organogenesis of Peyer's patches in the small intestine of fetal mice, a lot of progress has been made in understanding the molecular pathways involved in the generation of lymphoid tissue and the maintenance of the lymphoid architecture. The findings that inducer cells also exist in adult mice and in humans, have a lineage relationship to natural killer cells, and can be stimulated during infections highlight their possible role in establishing innate and adaptive immune responses. Novel concepts in the development of intestinal lymphoid tissues have been made in the past few years suggesting that lymphoid organs are more plastic as previously thought and depend on antigenic stimulation. In addition, the generation of novel lymphoid organs in the gut under inflammatory conditions indicates a function in chronic diseases. The present review summarizes current knowledge on the basic framework of signals required for developing lymphoid tissue under normal and inflammatory conditions.

Chemokine receptor CXCR5 supports solitary intestinal lymphoid tissue formation, B cell homing, and induction of intestinal IgA responses

Solitary intestinal lymphoid tissue (SILT) comprises a spectrum of phenotypically diverse lymphoid aggregates interspersed throughout the small intestinal mucosa. Manifestations of SILT range from tiny lymphoid aggregates almost void of mature lymphocytes to large structures dominated by B cells. Large SILT phenotypically resemble a single Peyer's patch follicle, suggesting that SILT might contribute to intestinal humoral immune responses. In this study, we track the fate of individual SILT in vivo over time and analyze SILT formation and function in chemokine receptor CXCR5-deficient mice. We show that, in analogy to Peyer's patches, formation of SILT is invariantly determined during ontogeny and depends on CXCR5. Young CXCR5-deficient mice completely lack SILT, suggesting that CXCR5 is essential for SILT formation during regular postnatal development. However, microbiota and other external stimuli can induce the formation of aberrant SILT distinguished by impaired development of B cell follicles in CXCR5-deficient mice. Small intestinal transplantation and bone marrow transplantation reveal that defect follicle formation is due to impaired B cell homing. Moreover, oral immunization with cholera toxin or infection with noninvasive Salmonella fail to induce efficient humoral immune responses in CXCR5-deficient mice. Bone marrow transplantation of CXCR5-deficient recipients with wild-type bone marrow rescued B cell follicle formation in SILT but failed to restore full humoral immune responses. These results reveal an essential role of CXCR5 in Peyer's patch and SILT development and function and indicate that SILT do not fully compensate for the lack of Peyer's patches in T cell-dependent humoral immune responses.

Alpha4beta7/MAdCAM-1 interactions play an essential role in transitioning cryptopatches into isolated lymphoid follicles and a nonessential role in cryptopatch formation

The alpha(4) integrins alpha(4)beta(7) and alpha(4)beta(1), and their ligands mucosal vascular addressin cell adhesion molecule 1 (MAdCAM-1) and VCAM-1, have diverse functions, including roles in the formation of secondary lymphoid tissues at early time points during the colonization and clustering of the fetal lymphoid tissue inducer (LTi) cells and at later time points during the recruitment of lymphocytes. In this study, we evaluated the role of alpha(4) integrins in the development of a recently appreciated class of intestinal lymphoid tissues, isolated lymphoid follicles (ILFs). We observed that diverse ILF cellular populations express alpha(4)beta(7) and alpha(4)beta(1), including the LTi-like cells and lymphocytes, while ILF stromal cells and vessels within ILFs express VCAM-1 and MAdCAM-1, respectively. Evaluation of adult and neonatal beta(7)(-/-) mice and adult and neonatal mice given blocking Abs to alpha(4)beta(7), MAdCAM-1, or VCAM-1 did not identify a role for alpha(4) integrins in cryptopatch (CP) development; however, these studies demonstrated that alpha(4)beta(7) and MAdCAM-1 are required for the transitioning of CP into lymphoid tissues containing lymphocytes or ILFs. Competitive bone marrow transfers demonstrated that beta(7)(-/-) LTi-like cells had a reduced but not significantly impaired ability to localize to CP. Bone marrow transfers and adoptive transfers of B lymphocytes revealed that beta(7) expression by B lymphocytes was essential for their entry into the developing ILFs. These findings demonstrate an essential role for alpha(4)beta(7)/MAdCAM-1 in ILF development corresponding to the influx of beta(7)-expressing lymphocytes and a nonessential role for beta(7)-localizing LTi-like cells to the small intestine.

Requirement for lymphoid tissue-inducer cells in isolated follicle formation and T cell-independent immunoglobulin A generation in the gut

Immunoglobulin A (IgA) is generated in the gut by both T cell-dependent and T cell-independent processes. The sites and the mechanisms for T cell-independent IgA synthesis remain elusive. Here we show that isolated lymphoid follicles (ILFs) were sites where induction of activation-induced cytidine deaminase (AID) and IgA class switching of B cells took place in the absence of T cells. We also show that formation of ILFs was regulated by interactions between lymphoid tissue-inducer cells expressing the nuclear receptor ROR gamma t (ROR gamma t(+)LTi cells) and stromal cells (SCs). Activation of SCs by ROR gamma t(+)LTi cells through lymphotoxin (LT)-beta receptor (LT beta R) and simultaneously by bacteria through TLRs induced recruitment of dendritic cells (DCs) and B cells and formation of ILFs. These findings provide insight into the crosstalk between bacteria, ROR gamma t(+)LTi cells, SCs, DCs, and B cells required for ILF formation and establish a critical role of ILFs in T cell-independent IgA synthesis in gut.

Villous B cells of the small intestine are specialized for invariant NK T cell dependence

B cells are important in mucosal microbial homeostasis through their well-known role in secretory IgA production and their emerging role in mucosal immunoregulation. Several specialized intraintestinal B cell compartments have been characterized, but the nature of conventional B cells in the lamina propria is poorly understood. In this study, we identify a B cell population predominantly composed of surface IgM(+) IgD(+) cells residing in villi of the small intestine and superficial lamina propria of the large intestine, but distinct from the intraepithelial compartment or organized intestinal lymphoid structures. Small intestinal (villous) B cells are diminished in genotypes that alter the strength of BCR signaling (Bruton tyrosine kinase(xid), Galphai2(-/-)), and in mice lacking cognate BCR specificity. They are not dependent on enteric microbial sensing, because they are abundant in mice that are germfree or genetically deficient in TLR signaling. However, villous B cells are reduced in the absence of invariant NK T cells (Jalpha18(-/-) or CD1d(-/-) mice). These findings define a distinct population of conventional B cells in small intestinal villi, and suggest an immunologic link between CD1-restricted invariant NK T cells and this B cell population.

Effects of donor T-cell trafficking and priming site on graft-versus-host disease induction by naive and memory phenotype CD4 T cells

Graft-versus-host disease (GVHD) remains a major cause of morbidity and mortality in allogeneic stem cell transplantation. Effector memory T cells (T(EM)) do not cause GVHD but engraft and mount immune responses, including graft-versus-tumor effects. One potential explanation for the inability of T(EM) to cause GVHD is that T(EM) lack CD62L and CCR7, which are instrumental in directing naive T cells (T(N)) to lymph nodes (LN) and Peyer patches (PP), putative sites of GVHD initiation. Thus T(EM) should be relatively excluded from LN and PP, possibly explaining their inability to cause GVHD. We tested this hypothesis using T cells deficient in CD62L or CCR7, transplant recipients lacking PNAd ligands for CD62L, and recipients without LN and PP or LN, PP, and spleen. Surprisingly, CD62L and CCR7 were not required for T(N)-mediated GVHD. Moreover, in multiple strain pairings, GVHD developed in recipients that lacked LN and PP. Mild GVHD could even be induced in mice lacking all major secondary lymphoid tissues (SLT). Conversely, enforced constitutive expression of CD62L on T(EM) did not endow them with the ability to cause GVHD. Taken together, these data argue against the hypothesis that T(EM) fail to induce GVHD because of inefficient trafficking to LN and PP.

TLR signaling mediated by MyD88 is required for a protective innate immune response by neutrophils to Citrobacter rodentium

Enteropathogenic Escherichia coli, enterohemorrhagic E. coli, and Citrobacter rodentium are classified as attaching and effacing pathogens based on their ability to adhere to intestinal epithelium via actin-filled membranous protrusions (pedestals). Infection of mice with C. rodentium causes breach of the colonic epithelial barrier, a vigorous Th1 inflammatory response, and colitis. Ultimately, an adaptive immune response leads to clearance of the bacteria. Whereas much is known about the adaptive response to C. rodentium, the role of the innate immune response remains unclear. In this study, we demonstrate for the first time that the TLR adaptor MyD88 is essential for survival and optimal immunity following infection. MyD88(-/-) mice suffer from bacteremia, gangrenous mucosal necrosis, severe colitis, and death following infection. Although an adaptive response occurs, MyD88-dependent signaling is necessary for efficient clearance of the pathogen. Based on reciprocal bone marrow transplants in conjunction with assessment of intestinal mucosal pathology, repair, and cytokine production, our findings suggest a model in which TLR signaling in hemopoietic and nonhemopoietic cells mediate three distinct processes: 1) induction of an epithelial repair response that maintains the protective barrier and limits access of bacteria to the lamina propria; 2) production of KC or other chemokines that attract neutrophils and thus facilitate killing of bacteria; and 3) efficient activation of an adaptive response that facilitates Ab-mediated clearance of the infection. Taken together, these experiments provide evidence for a protective role of innate immune signaling in infections caused by attaching and effacing pathogens.

Lymphotoxin-independent expression of TNF-related activation-induced cytokine by stromal cells in cryptopatches, isolated lymphoid follicles, and Peyer's patches

Stromal cells play a crucial role in the organogenesis of lymphoid tissues. We previously identified VCAM-1(+) stromal cells in cryptopatches (CP) and isolated lymphoid follicles (ILF) in the small intestine of C57BL/6 mice. Nonhemopoietic stromal cell networks in CP and ILF of adult mice also expressed FDC-M1, CD157 (BP-3), and TNF-related activation-induced cytokine (TRANCE). Individual stromal cells were heterogeneous in their expression of these markers, with not all stromal cells expressing the entire set of stromal cell markers. Expression of VCAM-1, FDC-M1, and CD157 on CP stromal cells was absent in alymphoplasia mice deficient in NF-kappaB-inducing kinase (NIK) and NIK knockout mice. Administration of lymphotoxin beta receptor (LTbetaR)-Ig to wild-type mice on day 13 resulted in the absence of CP on day 20; delaying administration of LTbetaR-Ig until day 18 resulted in an 80% decrease in the number of CP on day 22 and diminished expression of VCAM-1, FDC-M1, and CD157 on the remaining CP. In sharp contrast, TRANCE expression by stromal cells was completely independent of NIK and LTbetaR. In addition, expression of TRANCE in ILF was concentrated just beneath the follicle-associated epithelium, a pattern of polarization that was also observed in Peyer's patches. These findings suggest that TRANCE on stromal cells contributes to the differentiation and maintenance of organized lymphoid aggregates in the small intestine.

Curriculum vitae of intestinal intraepithelial T cells: their developmental and behavioral characteristics

The alimentary tract has an epithelial layer, consisting mainly of intestinal epithelial cells (IECs), that is exposed to the exterior world through the intestinal lumen. The IEC layer contains many intestinal intraepithelial T cells (IELs), and the total number of IELs constitutes the largest population in the peripheral T-cell pool. Virtually all gammadelta-IELs and many alphabeta-IELs in the mouse small intestine are known to express CD8 alpha alpha homodimers. A wide range of evidence that supports extrathymic development of these CD8 alpha alpha(+) IELs has been collected. In addition, while several studies identified cells with precursor T-cell phenotypes within the gut epithelium, how these precursors, which are dispersed along the length of the intestine, develop into gammadelta-IELs and/or alphabeta-IELs has not been clarified. The identification of lymphoid cell aggregations named 'cryptopatches' (CPs) in the intestinal crypt lamina propria of mice as sites rich in T-cell precursors in 1996 by our research group, however, provided evidence for a central site, whereby precursor IELs could give rise to T-cell receptor-bearing IELs. In this review, we discuss the development of IELs in the intestinal mucosa and examine the possibility that CPs serve as a production site of extrathymic IELs.

Adaptation of solitary intestinal lymphoid tissue in response to microbiota and chemokine receptor CCR7 signaling

Besides Peyer's patches, solitary intestinal lymphoid tissue (SILT) provides a structural platform to efficiently initiate immune responses in the murine small intestine. SILT consists of dynamic lymphoid aggregates that are heterogeneous in size and composition, ranging from small clusters of mostly lineage-negative cells known as cryptopatches to larger isolated lymphoid follicles rich in B cells. In this study, we report that in chemokine receptor CCR7-deficient mice SILT is enlarged, although unchanged in frequency and cellular composition compared with wild-type mice. This phenotype is conferred by bone marrow-derived cells and is independent of the presence of intestinal bacteria. Remarkably, particularly small-sized SILT predominates in germfree wild-type mice. Colonization of wild-type mice with commensal bacteria provokes an adjustment of the spectrum of SILT to that observed under specific pathogen-free conditions by the conversion of pre-existing lymphoid structures into larger-sized SILT. In conclusion, our findings establish that intestinal microbes influence the manifestation of gut-associated lymphoid tissues and identify CCR7 signaling as an endogeneous factor that controls this process.

Recent understanding of IBD pathogenesis: implications for future therapies

The inflammatory bowel diseases (IBD) are comprised of two major phenotypes, Crohn's disease (CD) and ulcerative colitis (UC). Research over the last couple of years has led to great advances in understanding the inflammatory bowel diseases and their underlying pathophysiologic mechanisms. From the current understanding, it is likely that chronic inflammation in IBD is due to aggressive cellular immune responses to a subset of luminal bacteria. Susceptibility to disease is thereby determined by genes encoding immune responses which are triggered by environmental stimuli. Based on extensive research over the last decade, there are several new and novel pathways and specific targets on which to focus new therapeutics. The following review summarizes the current view on the four basic tenets of the pathophysiological basis of IBD and its implications for therapies of IBD: genetics, immune dysregulation, barrier dysfunction and the role of the microbial flora.

Molecular Networks Orchestrating GALT Development

During evolution, the development of secondary lymphoid organs has evolved as a strategy to promote adaptive immune responses at sites of antigen sequestration. Mesenteric lymph nodes (LNs) and Peyer's patches (PPs) are localized in proximity to mucosal surfaces, and their development is coordinated by a series of temporally and spatially regulated molecular events involving the collaboration between hematopoietic, mesenchymal, and, for PPs, epithelial cells. Transcriptional control of cellular differentiation, production of cytokines as well as adhesion molecules are mandatory for organogenesis, recruitment of mature leukocytes, and lymphoid tissue organization. Similar to fetal and neonatal organogenesis, lymphoid tissue neoformation can occur in adult individuals at sites of chronic stimulation via cytokines and TNF-family member molecules. These molecules represent new therapeutic targets to manipulate the microenvironment during autoimmune diseases.

Lymphoid Tissue Inducer Cells in Intestinal Immunity

During fetal development, lymphoid tissue inducer cells (LTis) seed the developing lymph node and Peyer's patch anlagen and initiate the formation of both types of lymphoid organs. In the adult, a similar population of cells, termed lymphoid tissue inducer-like cells (LTi-like cells), supports the formation of organized gut-associated lymphoid tissue (GALT) in the intestine, including both isolated lymphoid follicles (ILFs) and cryptopatches (CPs). Both LTi and LTi-like cells require expression of the transcription factor RORgammat for their differentiation and function, and mice lacking RORgammat lack lymph nodes, Peyer's patches, and other organized GALT. In ILFs and cryptopatches, LTi-like cells are in close contact with different populations of intestinal dendritic cells (DCs), including a subpopulation recently shown to extend dendrites and sample luminal microflora. This interaction may allow for communication between the intestinal lumen and the immune cells in the lamina propria, which is necessary for maintaining homeostasis between the commensal microflora and the intestinal immune system. The potential functional implications of the organization of LTi-like cells, DCs, and lymphocytes in the lamina propria are discussed in the context of maintenance of homeostasis and of infectious diseases, particularly HIV infection.

Induction of Intestinal Lymphoid Tissue: The Role of Cryptopatches

The intestinal immune system includes several organized structures, such as Peyer's patches, isolated lymphoid follicles (ILFs), cryptopatches (CPs) as well as mesenteric lymph nodes (MLNs) that constitute an extensive network with other nonorganized parts, such as intraepithelial and lamina propria lymphocytes. CPs are small clusters of lymphoid cells with an immature lymphocyte phenotype and dendritic cells. Initial observations in transfer experiments suggested that the immature lymphocytes are T cell precursors and CPs are a potential site of extrathymic intraepithelial lymphocyte (IEL) differentiation. This feature has recently been challenged particularly by the observation that CP cells express the orphan receptor RORgammat and are phenotypically indistinguishable from lymphoid tissue-inducer (LTi) cells, suggesting that CP cells are the adult counterpart of fetal LTi cells. In addition, the chemokine receptor CCR6 is specifically expressed by precursor cells within CPs and its deletion inhibits the development of ILFs. Therefore, it is likely that ILFs derive from CPs under the control of CCR6 under inflammatory conditions and might constitute a valuable target for anti-inflammatory therapies.

CCR6 and CCL20: Partners in Intestinal Immunity and Lymphorganogenesis

The CCR6 chemokine receptor is expressed by most B cells and subsets of T cells and dendritic cells (DCs) found in the gut mucosal immune system. CCL20, the single chemokine ligand for CCR6, is selectively made by the follicle-associated epithelium (FAE) overlying Peyer's patches (PPs) and isolated lymphoid follicles (ILFs). CCL20 contributes to the recruitment of CCR6-expressing B cells to these structures. CCL20 expression by the intestinal epithelium is also highly inducible in response to inflammatory stimuli. Thus, CCL20 functions as both an inflammatory and homeostatic chemokine. Interactions between CCR6 and CCL20 play a role at several stages in the development of intestinal lymphoid structures. A subset of the c-kit(+) lymphoid precursors found in cryptopatches (CPs) expresses CCR6. Recruitment of B cells to CPs and the subsequent expansion and organization of these B cells allows differentiation of some of these structures into ILFs. In CCR6 knockout mice, PPs are smaller with fewer follicles and the development of ILFs is compromised. These defects in the development of mucosal inductive sites in CCR6-deficient mice are responsible for decreased IgA production to oral antigens. CCR6 can be included with CXCR5 and CCR7 in a list of chemokine receptors that participate in shaping the organized lymphoid structures that are part of the intestinal immune system.

T cells in cryptopatch aggregates share TCR gamma variable region junctional sequences with gamma delta T cells in the small intestinal epithelium of mice

The role of cryptopatch aggregates in the development of intestinal intraepithelial lymphocytes (IEL) is a matter of controversy. Therefore, an important question is whether T cells in cryptopatch aggregates are lineally related to IEL. We hypothesized that if gammadelta+ IEL derive from T cells in cryptopatch aggregates, then a clonal relationship would exist between the two populations. To test this hypothesis, we compared the sequence of rearranged TCR gamma variable region 5 genes in gammadelta+ IEL and cryptopatch cells. We purified IEL by FACS and cryptopatch cells were isolated from frozen sections of the intestine by laser-assisted microdissection. PCR showed that TCR gamma variable region 5 was rearranged in gammadelta+ IEL and in CD3+ cryptopatch cells, but not in CD3- cryptopatch cells. DNA sequence analysis showed that the frequency of in-frame junctions in cryptopatch aggregates was at a level consistent with positive selection in both wild-type and athymic nude mice. In addition, the predicted amino acid sequences of V-J junctions present in gammadelta+ IEL and cryptopatch cells were encoded by identical nucleotide sequences. By contrast, the frequency of in-frame joints was significantly reduced in cryptopatch cells isolated from TCR delta-deficient mice, indicating that the enrichment of in-frame joints in cryptopatch cells must normally depend on expression of surface gammadelta TCR. Our results are consistent with the hypothesis that a subset of gammadelta+ IEL are related to T cells in cryptopatch aggregates. The precise role of cryptopatch aggregates in intestinal gammadelta+ T cell homeostasis still needs to be determined.

A review of trafficking and activation of uterine natural killer cells

PROBLEM

Enrichment of uterine natural killer (uNK) cells occurs during pregnancy in many species. However, functions of uNK cells and regulation of their uterine homing are not fully defined. In mice and women, uNK cells contribute to angiogenesis, a role reviewed here and now addressed in a mammal with an alternative placental type.

METHODS OF STUDY

To address lymphocyte functions, RNA from murine or porcine endometrium and lymphocytes purified from endometrium were analyzed using real-time or reverse transcription PCR. To address homing potential, human blood CD56(+) lymphocytes were evaluated using both RNA and functional adhesion to endothelium presented under shear force in frozen sections of gestation day 7 C57Bl/6J implantation sites. Women were serially sampled over a menstrual cycle or a clinical preparatory cycle for embryo transfer.

RESULTS

Activation of murine uNK cells is associated with much greater increases in transcription for Eomes than for T-bet (Tbx21). Lymphocytes from normal porcine implantation sites transcribe vascular endothelial growth factor, placental growth factor, interferon-gamma and hypoxia-inducible factor (HIF)-1alpha. In fertile women, increases in L-selectin- and alpha4-integrin-mediated interactions between CD56(+) cells and endothelium occur at luteinizing hormone (LH) surge (cycling women) to oocyte pick up or embryo transfer, then return to pre-LH levels.

CONCLUSIONS

Uterine lymphocytes may universally promote pregnancy-associated endometrial angiogenesis. Recruitment of uNK precursor cells from blood appears to occur in a window promoted by rising plasma estrogen and LH and limited by rising progesterone.

Organizing a mucosal defense

Gastrointestinal associated lymphoid tissue can be divided into loosely organized effector sites, which include the lamina propria and intraepithelial lymphocytes, and more organized structures, such as mesenteric lymph nodes (LNs), Peyer's patches (PPs), isolated lymphoid follicles, and cryptopatches (CPs). These organized structures in the gastrointestinal tract have been hypothesized to play the role of primary lymphoid organ, supporting the extrathymic development of T lymphocytes (CPs), secondary lymphoid organs involved in the induction of the mucosal immune response (PPs), and tertiary lymphoid structures whose function is still under debate (isolated lymphoid follicles). The most widely studied lymphoid structure found in the small intestine is the PP. PPs are secondary lymphoid structures, and their development and function have been extensively investigated. However, single lymphoid aggregates resembling PPs have been also described in humans and in the murine small intestines. These isolated lymphoid follicles have both germinal centers and an overlying follicle-associated epithelium, suggesting that they also can function as inductive sites for the mucosal immune response. This review compares and contrasts the development and function of the four main organized gastrointestinal lymphoid tissues: CPs, isolated lymphoid follicles, PPs, and mesenteric LNs.

Inducible lymphoid tissues in the adult gut: recapitulation of a fetal developmental pathway?

The intestinal immune system faces an extraordinary challenge from the large numbers of commensal bacteria and potential pathogens that are restrained by only a single layer of epithelial cells. Here, I discuss evidence that the intestinal immune system develops an extensive network of inducible, reversible lymphoid tissues that contributes to the vital equilibrium between the gut and the bacterial flora. I propose that this network is induced by cryptopatches, which are small clusters of dendritic cells and lymphoid cells that are identical to fetal inducers of lymph-node and Peyer's-patch development.