CCL25 mediates the localization of recently activated CD8alphabeta(+) lymphocytes to the small-intestinal mucosa

Single-cell atlas of the small intestine throughout the human lifespan demonstrates unique features of fetal immune cells

Proper development of mucosal immunity is critical for human health. Over the past decade, it has become evident that in humans, this process begins in utero. However, there are limited data on the unique features and functions of fetal mucosal immune cells. To address this gap, we integrated several single-cell ribonucleic acid sequencing datasets of the human small intestine (SI) to create an SI transcriptional atlas throughout the human life span, ranging from the first trimester to adulthood, with a focus on immune cells. Fetal SI displayed a complex immune landscape comprising innate and adaptive immune cells that exhibited distinct transcriptional programs from postnatal samples, especially compared with pediatric and adult samples. We identified shifts in myeloid populations across gestation and progression of memory T-cell states throughout the human lifespan. In particular, there was a marked shift of memory T cells from those with stem-like properties in the fetal samples to fully differentiated cells with a high expression of activation and effector function genes in adult samples, with neonatal samples containing both features. Finally, we demonstrate that the SI developmental atlas can be used to elucidate improper trajectories linked to mucosal diseases by implicating developmental abnormalities underlying necrotizing enterocolitis, a severe intestinal complication of prematurity. Collectively, our data provide valuable resources and important insights into intestinal immunity that will facilitate regenerative medicine and disease understanding.

Characterization of Bovine Intraepithelial T Lymphocytes in the Gut

Intraepithelial T lymphocytes (T-IELs), which constitute over 50% of the total T lymphocytes in the animal, patrol the mucosal epithelial lining to defend against pathogen invasion while maintaining gut homeostasis. In addition to expressing T cell markers such as CD4 and CD8, T-IELs display T cell receptors (TCR), including either TCRαβ or TCRγδ. Both humans and mice share similar T-IEL subsets: TCRγδ+, TCRαβ+CD8αα+, TCRαβ+CD4+, and TCRαβ+CD8αβ+. Among these subsets, human T-IELs are predominantly TCRαβ+ (over 80%), whereas those in mice are mostly TCRγδ+ (~60%). Of note, the majority of the TCRγδ+ subset expresses CD8αα in both species. Although T-IELs have been extensively studied in humans and mice, their profiles in cattle have not been well examined. Our study is the first to characterize bovine T-IELs using flow cytometry, where we identified several distinct features. The percentage of TCRγδ+ was comparable to that of TCRαβ+ T-IELs (both ~50% of CD3+), and the majority of bovine TCRγδ+ T-IELs did not express CD8 (CD8-) (above 60%). Furthermore, about 20% of TCRαβ+ T-IELs were CD4+CD8αβ+, and the remaining TCRαβ+ T-IELs were evenly distributed between CD4+ and CD8αβ+ (~40% of TCRαβ+ T-IELs each) with no TCRαβ+CD8αα+ identified. Despite these unique properties, bovine T-IELs, similar to those in humans and mice, expressed a high level of CD69, an activation and tissue-retention marker, and a low level of CD62L, a lymphoid adhesion marker. Moreover, bovine T-IELs produced low levels of inflammatory cytokines such as IFNγ and IL17A, and secreted small amounts of the immune regulatory cytokine TGFβ1. Hence, bovine T-IELs' composition largely differs from that of human and mouse, with the dominance of the CD8- population among TCRγδ+ T-IELs, the substantial presence of TCRαβ+CD4+CD8αβ+ cells, and the absence of TCRαβ+CD8αα+ T-IELs. These results provide the groundwork for conducting future studies to examine how bovine T-IELs respond to intestinal pathogens and maintain the integrity of the gut epithelial barrier in animals.

Memory T Cells in the Immunoprevention of Cancer: A Switch from Therapeutic to Prophylactic Approaches

Cancer immunoprevention, the engagement of the immune system to prevent cancer, is largely overshadowed by therapeutic approaches to treating cancer after detection. Vaccines or, alternatively, the utilization of genetically engineered memory T cells could be methods of engaging and creating cancer-specific T cells with superb memory, lenient activation requirements, potent antitumor cytotoxicity, tumor surveillance, and resilience against immunosuppressive factors in the tumor microenvironment. In this review we analyze memory T cell subtypes based on their potential utility in cancer immunoprevention with regard to longevity, localization, activation requirements, and efficacy in fighting cancers. A particular focus is on how both tissue-resident memory T cells and stem memory T cells could be promising subtypes for engaging in immunoprevention.

Downregulation of chemokine receptor 9 facilitates CD4+CD8αα+ intraepithelial lymphocyte development

Intestinal intraepithelial lymphocytes (IELs) reside in the gut epithelial layer, where they help in maintaining intestinal homeostasis. Peripheral CD4+ T cells can develop into CD4+CD8αα+ IELs upon arrival at the gut epithelium via the lamina propria (LP). Although this specific differentiation of T cells is well established, the mechanisms preventing it from occurring in the LP remain unclear. Here, we show that chemokine receptor 9 (CCR9) expression is low in epithelial CD4+CD8αα+ IELs, but CCR9 deficiency results in CD4+CD8αα+ over-differentiation in both the epithelium and the LP. Single-cell RNA sequencing shows an enriched precursor cell cluster for CD4+CD8αα+ IELs in Ccr9-/- mice. CD4+ T cells isolated from the epithelium of Ccr9-/- mice also display increased expression of Cbfβ2, and the genomic occupancy modification of Cbfβ2 expression reveals its important function in CD4+CD8αα+ differentiation. These results implicate a link between CCR9 downregulation and Cbfb2 splicing upregulation to enhance CD4+CD8αα+ IEL differentiation.

Conserved and cell type-specific transcriptional responses to IFN-γ in the ventral midbrain

Dysregulated inflammation within the central nervous system (CNS) contributes to neuropathology in infectious, autoimmune, and neurodegenerative disease. With the exception of microglia, major histocompatibility complex (MHC) proteins are virtually undetectable in the mature, healthy central nervous system (CNS). Neurons have generally been considered incapable of antigen presentation, and although interferon gamma (IFN-γ) can elicit neuronal MHC class I (MHC-I) expression and antigen presentation in vitro, it has been unclear whether similar responses occur in vivo. Here we directly injected IFN-γ into the ventral midbrain of mature mice and analyzed gene expression profiles of specific CNS cell types. We found that IFN-γ upregulated MHC-I and associated mRNAs in ventral midbrain microglia, astrocytes, oligodendrocytes, and GABAergic, glutamatergic, and dopaminergic neurons. The core set of IFN-γ-induced genes and their response kinetics were similar in neurons and glia, but with a lower amplitude of expression in neurons. A diverse repertoire of genes was upregulated in glia, particularly microglia, which were the only cells to undergo cellular proliferation and express MHC classII (MHC-II) and associated genes. To determine if neurons respond directly via cell-autonomous IFN-γ receptor (IFNGR) signaling, we produced mutant mice with a deletion of the IFN-γ-binding domain of IFNGR1 in dopaminergic neurons, which resulted in a complete loss of dopaminergic neuronal responses to IFN-γ. Our results demonstrate that IFN-γ induces neuronal IFNGR signaling and upregulation of MHC-I and related genes in vivo, although the expression level is low compared to oligodendrocytes, astrocytes, and microglia.

Tissue adaptation and clonal segregation of human memory T cells in barrier sites

T lymphocytes migrate to barrier sites after exposure to pathogens, providing localized immunity and long-term protection. Here, we obtained blood and tissues from human organ donors to examine T cells across major barrier sites (skin, lung, jejunum), associated lymph nodes, lymphoid organs (spleen, bone marrow), and in circulation. By integrating single-cell protein and transcriptome profiling, we demonstrate that human barrier sites contain tissue-resident memory T (T_RM) cells that exhibit site-adapted profiles for residency, homing and function distinct from circulating memory T cells. Incorporating T cell receptor and transcriptome analysis, we show that circulating memory T cells are highly expanded, display extensive overlap between sites and exhibit effector and cytolytic functional profiles, while T_RM clones exhibit site-specific expansions and distinct functional capacities. Together, our findings indicate that circulating T cells are more disseminated and differentiated, while T_RM cells exhibit tissue-specific adaptation and clonal segregation, suggesting that strategies to promote barrier immunity require tissue targeting.

Identification and characterization of murine glycoprotein 2-expressing intestinal dendritic cells

The intestinal lamina propria (LP) contains distinct subsets of classical dendritic cells (cDC), each playing key non-redundant roles in intestinal immune homeostasis. Here, we show that glycoprotein 2 (GP2), a GPI-anchored protein and receptor for bacterial type-I fimbriae, is selectively expressed by CD103+CD11b+ cDC in the murine small intestine (SI). GP2 expression was induced on CD103+CD11b+ cDC within the SI-LP and was regulated by IRF4, TGFβR1- and retinoic acid signalling. Mice selectively lacking Gp2 on CD103+CD11b+ cDC (huLang-Cre.gp2fl/fl mice) had normal numbers and proportions of innate and adaptive immune cells in the SI-LP suggesting that GP2 expression by CD103+CD11b+ cDC is not required for intestinal immune homoeostasis.

SARS-CoV-2-specific T cells in unexposed adults display broad trafficking potential and cross-react with commensal antigens

The baseline composition of T cells directly affects later response to pathogens, but the complexity of precursor states remains poorly defined. Here, we examined the baseline state of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-specific T cells in unexposed individuals. SARS-CoV-2-specific CD4+ T cells were identified in prepandemic blood samples by major histocompatibility complex (MHC) class II tetramer staining and enrichment. Our data revealed a substantial number of SARS-CoV-2-specific T cells that expressed memory phenotype markers. Integrated phenotypic analyses demonstrated diverse preexisting memory states that included cells with distinct polarization features and trafficking potential to barrier tissues. T cell clones generated from tetramer-labeled cells cross-reacted with antigens from commensal bacteria in the skin and gastrointestinal tract. Direct ex vivo tetramer staining for one spike-specific population showed a similar level of cross-reactivity to sequences from endemic coronavirus and commensal bacteria. These data highlight the complexity of precursor T cell repertoire and implicate noninfectious exposures to common microbes as a key factor that shapes human preexisting immunity to SARS-CoV-2.

Intestinal single-cell atlas reveals novel lymphocytes in pigs with similarities to human cells

Single-cell RNA sequencing of porcine ileal lymphocytes reveals similarities to human cells and discovery of porcine intestinal innate lymphoid cells.

Lymphocytes can heavily influence intestinal health, but resolving intestinal lymphocyte function is challenging as the intestine contains a vastly heterogeneous mixture of cells. Pigs are an advantageous biomedical model, but deeper understanding of intestinal lymphocytes is warranted to improve model utility. Twenty-six cell types were identified in the porcine ileum by single-cell RNA sequencing and further compared with cells in human and murine ileum. Though general consensus of cell subsets across species was revealed, some porcine-specific lymphocyte subsets were identified. Differential tissue dissection and in situ analyses conferred spatial context, revealing similar locations of lymphocyte subsets in Peyer’s patches and epithelium in pig-to-human comparisons. Like humans, activated and effector lymphocytes were abundant in the ileum but not periphery of pigs, suggesting tissue-specific and/or activation-associated gene expression. Gene signatures for peripheral and ileal innate lymphoid cells newly discovered in pigs were defined and highlighted similarities to human innate lymphoid cells. Overall, we reveal novel lymphocyte subsets in pigs and highlight utility of pigs for intestinal research applications.

CCL25/CCR9 interaction promotes the malignant behavior of salivary adenoid cystic carcinoma via the PI3K/AKT signaling pathway

Background

CC chemokine receptor 9 (CCR9), an organ-specific chemokine receptor, interacts with its exclusive ligand CCL25 to promote tumor proliferation and metastasis. However, the effect of CCR9 on salivary adenoid cystic carcinoma (SACC) malignant behavior remains unknown. This study aimed to investigate the specific molecular mechanism by which CCR9/CCL25 modulates malignant progression in SACC.

Methods

Immunohistochemistry staining and RT-qPCR analyses were performed to detect the correlation of CCR9 expression and tumor progression-associated markers in SACC. In vitro, SACC cell proliferation and apoptosis were evaluated using Cell Counting Kit-8 and colon formation, and cell migration and invasion were detected by wound healing and transwell assays. Vercirnon was used as an inhibitor of CCR9, and LY294002 was used as an inhibitor of the PI3K/AKT pathway in this study. Western blot and RT-qPCR assays were carried out to measure the downstream factors of the interaction of CCL25 and CCR9. The effect of CCL25 on the development of SACC in vivo was examined by a xenograft tumor model in nude mice following CCL25, Vercirnon and LY294002 treatment.

Results

CCR9 was highly expressed in SACC compared with adjacent salivary gland tissues, and its level was associated with tumor proliferation and metastases. CCL25 enhanced cell proliferation, migration, and invasion through its interaction with CCR9 and exerted an antiapoptotic effect on SACC cells. Targeting CCR9 via Vercirnon significantly reduced the phosphorylation level of AKT induced by CCL25. CCL25/CCR9 could activate its downstream factors through the PI3K/AKT signaling pathway, such as cyclin D1, BCL2 and SLUG, thus promoting SACC cell proliferation, antiapoptosis, invasion and metastasis. The in vivo data from the xenograft mouse models further proved that CCL25 administration promoted malignant tumor progression by activating the PI3K/AKT pathway.

Conclusion

The interaction of CCL25 and CCR9 promotes tumor growth and metastasis in SACC by activating the PI3K/AKT signaling pathway, offering a promising strategy for SACC treatment.

Emerging translational strategies and challenges for enhancing regulatory T cell therapy for graft-versus-host disease

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is a curative therapy for many types of cancer. Genetic disparities between donor and host can result in immune-mediated attack of host tissues, known as graft versus host disease (GVHD), a major cause of morbidity and mortality following HSCT. Regulatory CD4+ T cells (Tregs) are a rare cell type crucial for immune system homeostasis, limiting the activation and differentiation of effector T cells (Teff) that are self-reactive or stimulated by foreign antigen exposure. Adoptive cell therapy (ACT) with Treg has demonstrated, first in murine models and now in patients, that prophylactic Treg infusion can also suppress GVHD. While clinical trials have demonstrated Treg reduce severe GVHD occurrence, several impediments remain, including Treg variability and practical need for individualized Treg production for each patient. Additionally, there are challenges in the use of in vitro expansion techniques and in achieving in vivo Treg persistence in context of both immune suppressive drugs and in lymphoreplete patients being treated for GVHD. This review will focus on 3 main translational approaches taken to improve the efficacy of tTreg ACT in GVHD prophylaxis and development of treatment options, following HSCT: genetic modification, manipulating TCR and cytokine signaling, and Treg production protocols. In vitro expansion for Treg ACT presents a multitude of approaches for gene modification to improve efficacy, including: antigen specificity, tissue targeting, deletion of negative regulators/exhaustion markers, resistance to immunosuppressive drugs common in GVHD treatment. Such expansion is particularly important in patients without significant lymphopenia that can drive Treg expansion, enabling a favorable Treg:Teff ratio in vivo. Several potential therapeutics have also been identified that enhance tTreg stability or persistence/expansion following ACT that target specific pathways, including: DNA/histone methylation status, TCR/co-stimulation signaling, and IL-2/STAT5 signaling. Finally, this review will discuss improvements in Treg production related to tissue source, Treg subsets, therapeutic approaches to increase Treg suppression and stability during tTreg expansion, and potential for storing large numbers of Treg from a single production run to be used as an off-the-shelf infusion product capable of treating multiple recipients.

Anti-CCR9 chimeric antigen receptor T cells for T-cell acute lymphoblastic leukemia

T cell acute lymphoblastic leukemia (T-ALL) is an aggressive malignancy of immature T lymphocytes, associated with higher rates of induction failure compared with those in B cell acute lymphoblastic leukemia. The potent immunotherapeutic approaches applied in B cell acute lymphoblastic leukemia, which have revolutionized the treatment paradigm, have proven more challenging in T-ALL, largely due to a lack of target antigens expressed on malignant but not healthy T cells. Unlike B cell depletion, T-cell aplasia is highly toxic. Here, we show that the chemokine receptor CCR9 is expressed in >70% of cases of T-ALL, including >85% of relapsed/refractory disease, and only on a small fraction (<5%) of normal T cells. Using cell line models and patient-derived xenografts, we found that chimeric antigen receptor (CAR) T-cells targeting CCR9 are resistant to fratricide and have potent antileukemic activity both in vitro and in vivo, even at low target antigen density. We propose that anti-CCR9 CAR-T cells could be a highly effective treatment strategy for T-ALL, avoiding T cell aplasia and the need for genome engineering that complicate other approaches.

Tissue-resident memory CD8+ T cells possess unique transcriptional, epigenetic and functional adaptations to different tissue environments

Tissue-resident memory T cells (T_RM cells) provide protective immunity, but the contributions of specific tissue environments to T_RM cell differentiation and homeostasis are not well understood. In the present study, the diversity of gene expression and genome accessibility by mouse CD8⁺ T_RM cells from distinct organs that responded to viral infection revealed both shared and tissue-specific transcriptional and epigenetic signatures. T_RM cells in the intestine and salivary glands expressed transforming growth factor (TGF)-β-induced genes and were maintained by ongoing TGF-β signaling, whereas those in the fat, kidney and liver were not. Constructing transcriptional-regulatory networks identified the transcriptional repressor Hic1 as a critical regulator of T_RM cell differentiation in the small intestine and showed that Hic1 overexpression enhanced T_RM cell differentiation and protection from infection. Provision of a framework for understanding how CD8⁺ T_RM cells adapt to distinct tissue environments, and identification of tissue-specific transcriptional regulators mediating these adaptations, inform strategies to boost protective memory responses at sites most vulnerable to infection.

Natural killer cell homing and trafficking in tissues and tumors: from biology to application

Natural killer (NK) cells, a subgroup of innate lymphoid cells, act as the first line of defense against cancer. Although some evidence shows that NK cells can develop in secondary lymphoid tissues, NK cells develop mainly in the bone marrow (BM) and egress into the blood circulation when they mature. They then migrate to and settle down in peripheral tissues, though some special subsets home back into the BM or secondary lymphoid organs. Owing to its success in allogeneic adoptive transfer for cancer treatment and its "off-the-shelf" potential, NK cell-based immunotherapy is attracting increasing attention in the treatment of various cancers. However, insufficient infiltration of adoptively transferred NK cells limits clinical utility, especially for solid tumors. Expansion of NK cells or engineered chimeric antigen receptor (CAR) NK cells ex vivo prior to adoptive transfer by using various cytokines alters the profiles of chemokine receptors, which affects the infiltration of transferred NK cells into tumor tissue. Several factors control NK cell trafficking and homing, including cell-intrinsic factors (e.g., transcriptional factors), cell-extrinsic factors (e.g., integrins, selectins, chemokines and their corresponding receptors, signals induced by cytokines, sphingosine-1-phosphate (S1P), etc.), and the cellular microenvironment. Here, we summarize the profiles and mechanisms of NK cell homing and trafficking at steady state and during tumor development, aiming to improve NK cell-based cancer immunotherapy.

Sphingosine 1-phosphate modulation and immune cell trafficking in inflammatory bowel disease

Immune cell trafficking is a critical element of the intestinal immune response, both in homeostasis and in pathological conditions associated with inflammatory bowel disease (IBD). This process involves adhesion molecules, chemoattractants and receptors expressed on immune cell surfaces, blood vessels and stromal intestinal tissue as well as signalling pathways, including those modulated by sphingosine 1-phosphate (S1P). The complex biological processes of leukocyte recruitment, activation, adhesion and migration have been targeted by various monoclonal antibodies (vedolizumab, etrolizumab, ontamalimab). Promising preclinical and clinical data with several oral S1P modulators suggest that inhibition of lymphocyte egress from the lymph nodes to the bloodstream might be a safe and efficacious alternative mechanism for reducing inflammation in immune-mediated disorders, including Crohn's disease and ulcerative colitis. Although various questions remain, including the potential positioning of S1P modulators in treatment algorithms and their long-term safety, this novel class of compounds holds great promise. This Review summarizes the critical mediators and mechanisms involved in immune cell trafficking in IBD and the available evidence for efficacy, safety and pharmacokinetics of S1P receptor modulators in IBD and other immune-mediated disorders. Further, it discusses potential future approaches to incorporate S1P modulators into the treatment of IBD.

Chemokine receptor CCR9 suppresses the differentiation of CD4+CD8αα+ intraepithelial T cells in the gut

The chemokine receptor CCR9 equips T cells with the ability to respond to CCL25, a chemokine that is highly expressed in the thymus and the small intestine (SI). Notably, CCR9 is mostly expressed on CD8 but not on CD4 lineage T cells, thus imposing distinct tissue tropism on CD4 and CD8 T cells. The molecular basis and the consequences for such a dichotomy, however, have not been fully examined and explained. Here, we demonstrate that the forced expression of CCR9 interferes with the tissue trafficking and differentiation of CD4 T cells in SI intraepithelial tissues. While CCR9 overexpression did not alter CD4 T cell generation in the thymus, the forced expression of CCR9 was detrimental for the proper tissue distribution of CD4 T cells in the periphery, and strikingly also for their terminal differentiation in the gut epithelium. Specifically, the differentiation of SI epithelial CD4 T cells into immunoregulatory CD4+CD8αα+ T cells was impaired by overexpression of CCR9 and conversely increased by the genetic deletion of CCR9. Collectively, our results reveal a previously unappreciated role for CCR9 in the tissue homeostasis and effector function of CD4 T cells in the gut.

Marginal Zone B-Cell Populations and Their Regulatory Potential in the Context of HIV and Other Chronic Inflammatory Conditions

Inflammation in the context of Human Immunodeficiency Virus (HIV) establishes early and persists beyond antiretroviral therapy (ART). As such, we have shown excess B-cell activating factor (BAFF) in the blood of HIV-infected progressors, as soon as in the acute phase, and despite successful ART. Excess BAFF was associated with deregulation of the B-cell compartment; notably, with increased frequencies of a population sharing features of both transitional immature (TI) and marginal zone (MZ) B-cells, we termed Marginal Zone precursor-like (MZp). We have reported similar observations with HIV-transgenic mice, Simian Immunodeficiency Virus (SIV)-infected macaques, and more recently, with HIV-infected Beninese commercial sex workers, which suggests that excess BAFF and increased frequencies of MZp B-cells are reliable markers of inflammation in the context of HIV. Importantly, we have recently shown that in healthy individuals, MZps present an important regulatory B-cell (Breg) profile and function. Herein, we wish to review our current knowledge on MZ B-cell populations, especially their Breg status, and that of other B-cell populations sharing similar features. BAFF and its analog A Proliferation-Inducing Ligand (APRIL) are important in shaping the MZ B-cell pool; moreover, the impact that excess BAFF—encountered in the context of HIV and several chronic inflammatory conditions—may exert on MZ B-cell populations, Breg and antibody producing capacities is a threat to the self-integrity of their antibody responses and immune surveillance functions. As such, deregulations of MZ B-cell populations contribute to autoimmune manifestations and the development of MZ lymphomas (MZLs) in the context of HIV and other inflammatory diseases. Therefore, further comprehending the mechanisms regulating MZ B-cell populations and their functions could be beneficial to innovative therapeutic avenues that could be deployed to restore MZ B-cell immune competence in the context of chronic inflammation involving excess BAFF.

Vasoactive intestinal peptide promotes host defense against enteric pathogens by modulating the recruitment of group 3 innate lymphoid cells

Group 3 innate lymphoid cells (ILC3s) control the formation of intestinal lymphoid tissues and play key roles in intestinal defense. They express neuropeptide vasoactive intestinal peptide (VIP) receptor 2 (VPAC2), through which VIP modulates their function, but whether VIP exerts other effects on ILC3 remains unclear. We show that VIP promotes ILC3 recruitment to the intestine through VPAC1 independent of the microbiota or adaptive immunity. VIP is also required for postnatal formation of lymphoid tissues as well as the maintenance of local populations of retinoic acid (RA)-producing dendritic cells, with RA up-regulating gut-homing receptor CCR9 expression by ILC3s. Correspondingly, mice deficient in VIP or VPAC1 suffer a paucity of intestinal ILC3s along with impaired production of the cytokine IL-22, rendering them highly susceptible to the enteric pathogen Citrobacter rodentium This heightened susceptibility to C. rodentium infection was ameliorated by RA supplementation, adoptive transfer of ILC3s, or by recombinant IL-22. Thus, VIP regulates the recruitment of intestinal ILC3s and formation of postnatal intestinal lymphoid tissues, offering protection against enteric pathogens.

Tissue-specific differentiation of CD8+ resident memory T cells

CD8⁺ tissue-resident memory T (T_RM) cells play crucial roles in defense against infections and cancer and have been implicated in autoimmune diseases such as psoriasis. In mice and humans, they exist in all nonlymphoid organs and share key characteristics across all tissues, including downregulation of tissue egress and lymph node homing pathways. However, recent studies demonstrate considerable heterogeneity across T_RM cells lodged in different tissues - linked to the activity of tissue-specific molecules, including chemokines, cytokines, and transcription factors. Current work indicates that transforming growth factor (TGF)-β plays a major role in generating T_RM heterogeneity at phenotypic and functional levels. Here, we review common and unique features of T_RM cells in different tissues and discuss putative strategies aimed at harnessing T_RM cells for site-specific protection against infectious and malignant diseases.

The Roles of CCR9/CCL25 in Inflammation and Inflammation-Associated Diseases

Chemokine is a structure-related protein with a relatively small molecular weight, which can target cells to chemotaxis and promote inflammatory response. Inflammation plays an important role in aging. C-C chemokine receptor 9 (CCR9) and its ligand C-C chemokine ligand 25 (CCL25) are involved in the regulating the occurrence and development of various diseases, which has become a research hotspot. Early research analysis of CCR9-deficient mouse models also confirmed various physiological functions of this chemokine in inflammatory responses. Moreover, CCR9/CCL25 has been shown to play an important role in a variety of inflammation-related diseases, such as cardiovascular disease (CVD), rheumatoid arthritis, hepatitis, inflammatory bowel disease, asthma, etc. Therefore, the purpose of this review gives an overview of the recent advances in understanding the roles of CCR9/CCL25 in inflammation and inflammation-associated diseases, which will contribute to the design of future experimental studies on the potential of CCR9/CCL25 and advance the research of CCR9/CCL25 as pharmacological inflammatory targets.

Location is important: differentiation between ileal and colonic Crohn's disease

Crohn's disease can affect any part of the gastrointestinal tract; however, current European and national guidelines worldwide do not differentiate between small-intestinal and colonic Crohn's disease for medical treatment. Data from the past decade provide evidence that ileal Crohn's disease is distinct from colonic Crohn's disease in several intestinal layers. Remarkably, colonic Crohn's disease shows an overlap with regard to disease behaviour with ulcerative colitis, underlining the fact that there is more to inflammatory bowel disease than just Crohn's disease and ulcerative colitis, and that subtypes, possibly defined by location and shared pathophysiology, are also important. This Review provides a structured overview of the differentiation between ileal and colonic Crohn's disease using data in the context of epidemiology, genetics, macroscopic differences such as creeping fat and histological findings, as well as differences in regard to the intestinal barrier including gut microbiota, mucus layer, epithelial cells and infiltrating immune cell populations. We also discuss the translation of these basic findings to the clinic, emphasizing the important role of treatment decisions. Thus, this Review provides a conceptual outlook on a new mechanism-driven classification of Crohn's disease.

Disparate Recruitment and Retention of Plasmacytoid Dendritic Cells to The Small Intestinal Mucosa between Young and Aged Mice

Plasmacytoid dendritic cells (pDC), a highly specialized class of innate immune cells that serve as rapid sensors of danger signals in circulation or in lymphoid tissue are well studied. However, there remains knowledge gaps about age-dependent changes of pDC function in the intestinal mucosa. Here, we report that under homeostatic conditions, the proportion of pDC expressing C-C chemokine receptor 9 (CCR9) in the intestinal intraepithelial cell (iIEC) population is comparable between young (2-4 months) and aged (18-24 months) mice, but the absolute numbers of iIEC and pDC are significantly lower in aged mice. Employing the classic model of acute endotoxemia induced by lipopolysaccharide (LPS), we found a decrease in the proportion and absolute number of intraepithelial pDC in both young and aged mice despite the LPS-induced increased expression of the chemokine C-C ligand 25 (CCL25), the ligand of CCR9, in the intestinal mucosa of young mice. In adoptive transfer experiments, a significantly lower number of pDC was retained into the intestinal layer of aged host mice after LPS administration. This was associated with recoverable pDC numbers in the intestinal lumen. Furthermore, co-adoptive transfer of young and aged pDC into young hosts also showed significantly lower retention of aged pDC in the epithelial layer compared to the co-transferred young pDC. Collectively, these data show age-associated changes in mucosal CCL25 gene expression and in pDC number. These may underlie the reported inadequate responses to gastrointestinal pathogens during chronologic aging.

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

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

Intestinal cDC1 drive cross-tolerance to epithelial-derived antigen via induction of FoxP3+CD8+ Tregs

Although CD8⁺ T cell tolerance to tissue-specific antigen (TSA) is essential for host homeostasis, the mechanisms underlying peripheral cross-tolerance and whether they may differ between tissue sites remain to be fully elucidated. Here, we demonstrate that peripheral cross-tolerance to intestinal epithelial cell (IEC)-derived antigen involves the generation and suppressive function of FoxP3⁺CD8⁺ T cells. FoxP3⁺CD8⁺ T_reg generation was dependent on intestinal cDC1, whose absence led to a break of tolerance and epithelial destruction. Mechanistically, intestinal cDC1-derived PD-L1, TGFβ, and retinoic acid contributed to the generation of gut-tropic CCR9⁺CD103⁺FoxP3⁺CD8⁺ T_regs Last, CD103-deficient CD8⁺ T cells lacked tolerogenic activity in vivo, indicating a role for CD103 in FoxP3⁺CD8⁺ T_reg function. Our results describe a role for FoxP3⁺CD8⁺ T_regs in cross-tolerance in the intestine for which development requires intestinal cDC1.

CCL25 and CCR9 is a unique pathway that potentiates pannus formation by remodeling RA macrophages into mature osteoclasts

This study elucidates the mechanism of CCL25 and CCR9 in rheumatoid arthritis (RA). RA synovial fluid (SF) expresses elevated levels of CCL25 compared to OA SF and plasma from RA and normal. CCL25 was released into RA SF by fibroblasts (FLS) and macrophages (MΦs) stimulated with IL-1β and IL-6. CCR9 is also presented on IL-1β and IL-6 activated RA FLS and differentiated MΦs. Conversely, in RA PBMCs neither CCL25 nor CCR9 are impacted by 3-month longitudinal TNF inhibitor therapy. CCL25 amplifies RA FLS and monocyte infiltration via p38 and ERK phosphorylation. CCL25-stimulated RA FLS secrete potentiated levels of IL-8 which is disrupted by p38 and ERK inhibitors. CCL25 polarizes RA monocytes into nontraditional M1 MΦs that produce IL-8 and CCL2. Activation of p38 and ERK cascades are also responsible for the CCL25-induced M1 MΦ development. Unexpectedly, CCL25 was unable to polarize RA PBMCs into effector Th1/Th17 cells. Consistently, lymphokine like RANKL was uninvolved in CCL25-induced osteoclastogenesis; however, this manifestation was regulated by osteoclastic factors such as RANK, cathepsin K (CTSK), and TNF-α. In short, we reveal that CCL25/CCR9 manipulates RA FLS and MΦ migration and inflammatory phenotype in addition to osteoclast formation via p38 and ERK activation.

Natural Killer Cell Integrins and Their Functions in Tissue Residency

Integrins are transmembrane receptors associated with adhesion and migration and are often highly differentially expressed receptors amongst natural killer cell subsets in microenvironments. Tissue resident natural killer cells are frequently defined by their differential integrin expression compared to other NK cell subsets, and integrins can further localize tissue resident NK cells to tissue microenvironments. As such, integrins play important roles in both the phenotypic and functional identity of NK cell subsets. Here we review the expression of integrin subtypes on NK cells and NK cell subsets with the goal of better understanding how integrin selection can dictate tissue residency and mediate function from the nanoscale to the tissue environment.

Role of CC chemokine receptor 9 in the progression of murine and human non-alcoholic steatohepatitis

BACKGROUND & AIMS

The number of patients with non-alcoholic steatohepatitis (NASH) is increasing globally. Recently, specific chemokine receptors have garnered interest as therapeutic targets in NASH. This is the first report to examine the role of the C-C chemokine receptor 9 (CCR9)/C-C chemokine receptor ligand 25 (CCL25) axis, and to reveal its therapeutic potential in NASH.

METHODS

Patients with biopsy-proven non-alcoholic liver disease (NAFLD) were recruited and their serum and hepatic chemokine expression was examined. Furthermore, wild-type (WT) and Ccr9^-/- mice were fed a high-fat high-cholesterol (HFHC) diet for 24 weeks to establish NASH.

RESULTS

Serum CCL25, and hepatic CCR9 and CCL25 expression levels were increased in patients with NASH compared to healthy volunteers. Furthermore, Ccr9^-/- mice were protected from HFHC diet-induced NASH progression both serologically and histologically. Flow cytometry and immunohistochemistry analysis showed that CCR9⁺CD11b⁺ inflammatory macrophages accumulated in the inflamed livers of HFHC diet-fed mice, while the number was reduced in Ccr9^-/- mice. Consistent with human NASH livers, CCR9 was also expressed on hepatic stellate cells (HSCs) in mice with NASH, while CCR9-deficient HSCs showed less fibrogenic potential in vitro. Administration of a CCR9 antagonist hampered further fibrosis progression in mice with NASH, supporting its potential clinical application. Finally, we showed that CCR9 blockade attenuated the development of NAFLD-related hepatocellular carcinoma in HF diet-fed mice injected with diethylnitrosamine.

CONCLUSIONS

These results highlight the role of the CCR9/CCL25 axis on macrophage recruitment and fibrosis formation in a murine NASH model, providing new insights into therapeutic strategies for NASH.

LAY SUMMARY

Herein, we show that a specific chemokine axis involving a receptor (CCR9) and its ligand (CCL25) contributes to the progression of non-alcoholic steatohepatitis and carcinogenesis in humans and mice. Furthermore, treatment with a CCR9 antagonist ameliorates the development of steatohepatitis and holds promise for the treatment of patients with non-alcoholic steatohepatitis.

Oral immunization induces a novel CXCR6+ β7+ intraepithelial lymphocyte subset predominating in the small intestine

Intestinal T cells form a central part of the front-line defence against foreign organisms and need to be situated in the mucosa where infection occurs. It is well accepted that immunization by a mucosal route favours localization of antigen-specific effector T cells in the mucosal epithelium, while systemic immunization does not. The aim of the study is to determine how homing receptors are specifically involved in retaining effector T cells in the small intestine after oral immunization. We here demonstrate that the chemokine receptor CXCR6, integrins β7 and CD29 contribute differentially to the epithelial retention phenotype of CD8⁺ T cells in the small intestine of mice. CD8⁺ intraepithelial lymphocytes (IELs) of unvaccinated mice are predominantly β7 single positives, and subcutaneous immunization-induced antigen-specific CD8⁺ effector IELs are mainly composed of CXCR6⁺ , CD29⁺ and CXCR6⁺ CD29⁺ cells. Strikingly, the majority of oral immunization-induced antigen-specific CD8⁺ effector IELs exhibit a distinct, tissue-specific CXCR6⁺ β7⁺ double-positive phenotype, cytotoxic potential and enhanced intraepithelial localization. Transfer of antigen-specific CD8⁺ T cells preactivated with certain immuno-stimuli (such as monophosphoryl lipid A) results in increased accumulation of donor IELs with the CXCR6⁺ β7⁺ phenotype. As β7 exclusively paired with αE on IELs, our results strongly suggest that CXCR6 may cooperate with the heterodimer αEβ7 to preferentially retain intestinally induced effector IELs in the epithelium. The identification of this novel IEL phenotype has significant implications for the development of vaccines and therapeutic strategies to enhance gut immunity.

The role of the CCL25-CCR9 axis in beta-cell function: potential for therapeutic intervention in type 2 diabetes

BACKGROUND AND PURPOSE

Chemokines are known to play essential roles mediating immunity and inflammation in many physiological and pathophysiological processes, with reports linking their action to the development of obesity, insulin resistance and type 2 diabetes (T2D). Given our findings of highly upregulated mRNA expression of the chemokine receptor CCR9 in islets from obese human donors, we have determined the effects of CCR9 activation by CCL25 on islet function and viability.

BASIC PROCEDURES

RT-qPCR was used to measure expression of 384 GPCR mRNAs in human islets from organ donors with normal and elevated BMI. mRNA encoding CCR9, a receptor that was highly upregulated in islets from obese donors, was also quantified in islets from lean and high-fat diet (HFD) mice. The effects of CCR9 activation by exogenous CCL25 in human and mouse islets and its inhibition by the CCR9 antagonist vercirnon on insulin secretion, apoptosis and cAMP accumulation were examined using standard techniques.

MAIN FINDINGS

The qPCR analysis showed altered expression of several GPCRs in islets isolated from lean and obese donors. CCR9 displayed over 90-fold upregulation in islets from obese individuals, and it was also significantly upregulated in islets from obese mice. In isolated human and mouse islets exogenous CCL25 inhibited glucose-induced insulin secretion in a concentration-dependent manner, enhanced cytokine-induced apoptosis and significantly reduced forskolin-induced elevation in cAMP levels. These detrimental effects of CCL25 in islets were blocked by vercirnon, which had no effect on its own.

PRINCIPAL CONCLUSIONS

We have shown that CCL25 acts via the Gαi-coupled receptor CCR9 to impair beta-cell function by inhibiting insulin secretion and promoting cytokine-induced apoptosis. Upregulation of CCR9 in islets in obesity, possibly secondary to accumulation of passenger immune cells, may predispose to metabolic dysfunction and our data suggest that CCL25 downregulation or CCR9 inhibition could be explored to treat T2D.

Pathogenesis and clinical perspectives of extraintestinal manifestations in inflammatory bowel diseases

A considerable number of patients with inflammatory bowel disease (IBD) experience extraintestinal manifestations (EIMs), which can present either before or after IBD diagnosis. Unraveling the pathogenic pathways of EIMs in IBD is challenging because of the lack of reliable criteria for diagnosis and difficulty in distinguishing EIMs from external pathologies caused by drugs or other etiologies. Optimizing treatment can also be difficult. Early diagnosis and management of EIM revolve around multidisciplinary teams, and they should have the resources necessary to make and implement appropriate decisions. In addition, specialists of the affected organs should be trained in IBD treatment. Furthermore, patient awareness regarding the extraintestinal symptoms of IBD is of paramount importance for improving patient understanding of disease and health outcomes. Herein, we review the pathogenesis and clinical perspectives of EIMs in IBD.

The effect of dietary cecropin AD on intestinal health, immune response and disease resistance of juvenile turbot (Scophthalmus maximus L.)

Cecropin AD (CAD) is a commercial cationic antimicrobial peptide that has been seldom studied in marine fish. This study investigated the effects of dietary CAD on intestinal health, immune response, disease resistance, and growth performance of turbot. A diet using fishmeal and plant protein as the main protein resources was used as the control (crude protein 53%, crude lipid 12%). CAD was supplemented into the control diet at the level of 250, 500, 750, and 1000 mg kg^-1 to formulate four experimental diets, C1, C2, C3, and C4, respectively. No significant difference was observed in fish growth performance, feed utilization efficiency and whole-body composition among all groups. Dietary CAD significantly increased the activity of lysozyme and complement component 3 level in both serum and distal intestine (DI), as well as the immunoglobulin M content in DI. The gene expression of immune cytokines such as IFN-γ, IL-1β, and chemokine SmCCL19, and the goblet cell number in DI were also significantly increased by dietary CAD supplementation. Compared with the control group, the microbiota analysis indicated group C4 showed significantly decreased α-diversity, obvious alternation in dominant bacteria composition at phylum level, different clustering, and significantly decreased relative abundance of Lactobacillus. Besides, the relative abundance of Bacteroides was significantly decreased in groups C1, C3, and C4. In addition, the lowest mortality of turbot challenged with Edwardsiella tarda was observed in fish fed diets C2 and C3. In conclusion, moderate levels of CAD in diet of turbot improved the intestinal immune response without disrupting the intestinal bacterial community, and enhanced the disease resistance. However, dietary CAD at 1000 mg kg^-1 greatly affected the intestinal bacterial composition and showed potentially inhibitory effects towards Lactobacillus.

Immunological roles of intestinal mesenchymal cells

The intestine is continuously exposed to an enormous variety and quantity of antigens and innate immune stimuli derived from both pathogens and harmless materials, such as food and commensal bacteria. Accordingly, the intestinal immune system is uniquely adapted to ensure appropriate responses to the different kinds of challenge; maintaining tolerance to harmless antigens in the steady-state, whilst remaining poised to deal with potential pathogens. To accomplish this, leucocytes of the intestinal immune system have to adapt to a constantly changing environment and interact with many different non-leucocytic intestinal cell types, including epithelial and endothelial cells, neurons, and a heterogenous network of intestinal mesenchymal cells (iMC). These interactions are intricately involved in the generation of protective immunity, the elaboration of inflammatory responses, and the development of inflammatory conditions, such as inflammatory bowel diseases. Here we discuss recent insights into the immunological functions of iMC under homeostatic and inflammatory conditions, focusing particularly on iMC in the mucosa and submucosa, and highlighting how an appreciation of the immunology of iMC may help understand the pathogenesis and treatment of disease.

Immune Profiling of Human Gut-Associated Lymphoid Tissue Identifies a Role for Isolated Lymphoid Follicles in Priming of Region-Specific Immunity

The intestine contains some of the most diverse and complex immune compartments in the body. Here we describe a method for isolating human gut-associated lymphoid tissues (GALTs) that allows unprecedented profiling of the adaptive immune system in submucosal and mucosal isolated lymphoid follicles (SM-ILFs and M-ILFs, respectively) as well as in GALT-free intestinal lamina propria (LP). SM-ILF and M-ILF showed distinct patterns of distribution along the length of the intestine, were linked to the systemic circulation through MAdCAM-1⁺ high endothelial venules and efferent lymphatics, and had immune profiles consistent with immune-inductive sites. IgA sequencing analysis indicated that human ILFs are sites where intestinal adaptive immune responses are initiated in an anatomically restricted manner. Our findings position ILFs as key inductive hubs for regional immunity in the human intestine, and the methods presented will allow future assessment of these compartments in health and disease.

Dissecting the Heterogeneity in T-Cell Mediated Inflammation in IBD

Infiltration of the lamina propria by inflammatory CD4⁺ T-cell populations is a key characteristic of chronic intestinal inflammation. Memory-phenotype CD4⁺ T-cell frequencies are increased in inflamed intestinal tissue of IBD patients compared to tissue of healthy controls and are associated with disease flares and a more complicated disease course. Therefore, a tightly controlled balance between regulatory and inflammatory CD4⁺ T-cell populations is crucial to prevent uncontrolled CD4⁺ T-cell responses and subsequent intestinal tissue damage. While at steady state, T-cells display mainly a regulatory phenotype, increased in Th1, Th2, Th9, Th17, and Th17.1 responses, and reduced Treg and Tr1 responses have all been suggested to play a role in IBD pathophysiology. However, it is highly unlikely that all these responses are altered in each individual patient. With the rapidly expanding plethora of therapeutic options to inhibit inflammatory T-cell responses and stimulate regulatory T-cell responses, a crucial need is emerging for a robust set of immunological assays to predict and monitor therapeutic success at an individual level. Consequently, it is crucial to differentiate dominant inflammatory and regulatory CD4⁺ T helper responses in patients and relate these to disease course and therapy response. In this review, we provide an overview of how intestinal CD4⁺ T-cell responses arise, discuss the main phenotypes of CD4⁺ T helper responses, and review how they are implicated in IBD.

Gut microbiota-dependent CCR9+CD4+ T cells are altered in secondary progressive multiple sclerosis

The mechanism underlying the progression of relapsing-remitting multiple sclerosis to secondary progressive multiple sclerosis (SPMS), characterized by accumulating fixed disability, is yet to be fully understood. Although alterations in the gut microbiota have recently been highlighted in multiple sclerosis pathogenesis, the mechanism linking the altered gut environment with the remote CNS pathology remains unclear. Here, we analyse human CD4⁺ memory T cells expressing the gut-homing chemokine receptor CCR9 and found a reduced frequency of CCR9⁺ memory T cells in the peripheral blood of patients with SPMS relative to healthy controls. The reduction in the proportion of CCR9⁺ cells among CD4⁺ memory T cells (%CCR9) in SPMS did not correlate with age, disease duration or expanded disability status scale score, although %CCR9 decreased linearly with age in healthy controls. During the clinical relapse of both, relapsing-remitting multiple sclerosis and neuromyelitis optica, a high proportion of cells expressing the lymphocyte activating 3 gene (LAG3) was detected among CCR9⁺ memory T cells isolated from the CSF, similar to that observed for mouse regulatory intraepithelial lymphocytes. In healthy individuals, CCR9⁺ memory T cells expressed higher levels of CCR6, a CNS-homing chemokine receptor, and exhibited a regulatory profile characterized by both the expression of C-MAF and the production of IL-4 and IL-10. However, in CCR9⁺ memory T cells, the expression of RORγt was specifically upregulated, and the production of IL-17A and IFNγ was high in patients with SPMS, indicating a loss of regulatory function. The evaluation of other cytokines supported the finding that CCR9⁺ memory T cells acquire a more inflammatory profile in SPMS, reporting similar aspects to CCR9⁺ memory T cells of the elderly healthy controls. CCR9⁺ memory T cell frequency decreased in germ-free mice, whereas antibiotic treatment increased their number in specific pathogen-free conditions. Here, we also demonstrate that CCR9⁺ memory T cells preferentially infiltrate into the inflamed CNS resulting from the initial phase and that they express LAG3 in the late phase in the experimental autoimmune encephalomyelitis mouse model of multiple sclerosis. Antibiotic treatment reduced experimental autoimmune encephalomyelitis symptoms and was accompanied by an increase in CCR9⁺ memory T cells in the peripheral blood. Antibodies against mucosal vascular addressin cell adhesion molecule 1 (MADCAM1), which is capable of blocking cell migration to the gut, also ameliorated experimental autoimmune encephalomyelitis. Overall, we postulate that the alterations in CCR9⁺ memory T cells observed, caused by either the gut microbiota changes or ageing, may lead to the development of SPMS.

T Cell Recruitment to the Intestinal Stem Cell Compartment Drives Immune-Mediated Intestinal Damage after Allogeneic Transplantation

The key sites within the gastrointestinal (GI) tract where T cells mediate effector responses and the impact of these responses on intestinal stem cells (ISCs) remain unclear. Using experimental bone marrow transplantation to model immune-mediated GI damage and 3D imaging to analyze T cell localization, we found that the ISC compartment is the primary intestinal site targeted by T cells after transplantation. Recruitment to the crypt base region resulted in direct T cell engagement with the stem cell compartment and loss of crypt base columnar ISCs, which expressed both MHC classes I and II. Vasculature expressing the adhesion molecule MAdCAM-1 clustered near the crypt base, preferentially regulating crypt compartment invasion and ISC reduction without affecting T cell migration to villi. These findings indicate that allogeneic T cells rapidly access the stem cell niche after transplantation, and this targeted recruitment to the stem cell compartment results in ISC loss during immune-mediated GI damage.

CCL28 Is Involved in Mucosal IgA Responses, Olfaction, and Resistance to Enteric Infections

CCL28 is a mucosal chemokine that has been involved in various responses, including IgA production. We have analyzed its production in human tissues using a comprehensive microarray database. Its highest expression is in the salivary gland, indicating that it is an important component of saliva. It is also expressed in the trachea, bronchus, and in the mammary gland upon onset of lactation. We have also characterized a Ccl28-/- mouse that exhibits very low IgA levels in milk, and the IgA levels in feces are also reduced. These observations confirm a role for the CCL28/CCR10 chemokine axis in the recruitment of IgA plasmablasts to the lactating mammary gland. CCL28 is also expressed in the vomeronasal organ. We also detected olfactory defects (anosmia) in a Ccl28-/- mouse suggesting that CCL28 is involved in the function/development of olfaction. Importantly, Ccl28-/- mice are highly susceptible to Salmonella enterica serovar Typhimurium in an acute model of infection, indicating that CCL28 plays a major role in innate immunity against Salmonella in the gut. Finally, microbiome studies revealed modest differences in the gut microbiota between Ccl28-/- mice and their cohoused wild-type littermates. The latter observation suggests that under homeostatic conditions, CCL28 plays a limited role in shaping the gut microbiome.

T lymphocytes in the intestinal mucosa: defense and tolerance

Although lymphocytes are known to circulate throughout lymphoid tissues and blood, they also establish residency in nonlymphoid organs, most prominently in barrier tissues, such as the intestines. The adaptation of T lymphocytes to intestinal environments requires constant discrimination between natural stimulation from commensal flora and food and pathogens that need to be cleared. Genetic variations that cause a defective defense or a break in tolerance along with environmental cues, such as infection or imbalances in the gut microbiota known as dysbiosis, can trigger several immune disorders via the activation of T lymphocytes in the intestines. Elucidation of the immune mechanisms that distinguish between commensal flora and pathogenic organisms may reveal therapeutic targets for the prevention or modulation of inflammatory diseases and boost the efficacy of cancer immunotherapy. In this review, we discuss the development and adaptation of T lymphocytes in the intestine, how these cells protect the host against pathogenic infections while tolerating food antigens and commensal microbiota, and the potential implications of targeting these cells for disease management and therapeutics.

Dispensable role of chemokine receptors in migration of mycobacterial antigen-specific CD4+ T cells into Mycobacterium-infected lung

Mycobacterial antigen-specific CD4⁺ Th1 cells have pivotal role in protective immunity against mycobacterial infections including pulmonary tuberculosis. In the course of the infection, Th1 cells differentiate in the lung-draining lymph nodes and migrate into the infected lung. Chemokine receptors on T cells are involved in T cell migration into the intestine and skin. However, role of chemokine receptors in the migration of CD4⁺ T cells into the lung is not yet established. To address the issue, the role of chemokine receptors in T cell migration into the mycobacteria-infected lung was analyzed using mycobacterial Ag85B peptide 25-specific T cell receptor-transgenic (P25) CD4⁺ T cells. The P25 T cells in the Mycobacterium bovis BCG-infected lung and lung-draining mediastinal lymph node expressed chemokine receptors CCR5, CCR6, CXCR3 and CXCR5 which bind chemokines expressed by the BCG-infected lung. To further analyze the role of the chemokine receptors in the migration of the BCG-primed P25 T cells into the lung or mediastinal lymph node, the P25 T cells were adoptively transferred into the BCG-infected wild type mice, and their migration into the lung was monitored. Unexpectedly, blocking of chemokine receptor function with pertussis toxin, a G-protein inhibitor, failed to suppress migration of the T cells into the infected lung although the treatment completely blocked migration of the mediastinal lymph node P25 T cells into the recipient lymph node. The results suggest that interaction of chemokine receptors on mycobacterial antigen-specific Th1 cells with chemokines is dispensable in their migration into the mycobacteria-infected lung.

Regulatory T Cells for More Targeted Immunosuppressive Therapies

There has been a prolific amount of research dedicated to the T-regulatory cells (Tregs) and their role in achieving immune homeostasis. Here, the authors briefly discuss the known biology, utilization, and potential of Tregs, for current trials and future immunotherapy. Most current trials of Treg therapies include either ex vivo expanded Tregs transferred into the peripheral blood of patients with diseases of immunologic origin or interleukin 2 injected to stimulate Tregs directly. Ongoing trials designed to measure the clinical efficacy and safety profile of these novel therapeutic approaches have resulted in largely favorable outcomes in a variety of autoimmune and alloimmune diseases.

Defensive Driving: Directing HIV-1 Vaccine-Induced Humoral Immunity to the Mucosa with Chemokine Adjuvants

A myriad of pathogens gain access to the host via the mucosal route; thus, vaccinations that protect against mucosal pathogens are critical. Pathogens such as HIV, HSV, and influenza enter the host at mucosal sites such as the intestinal, urogenital, and respiratory tracts. All currently licensed vaccines mediate protection by inducing the production of antibodies which can limit pathogen replication at the site of infection. Unfortunately, parenteral vaccination rarely induces the production of an antigen-specific antibody at mucosal surfaces and thus relies on transudation of systemically generated antibody to mucosal surfaces to mediate protection. Mucosa-associated lymphoid tissues (MALTs) consist of a complex network of immune organs and tissues that orchestrate the interaction between the host, commensal microbes, and pathogens at these surfaces. This complexity necessitates strict control of the entry and exit of lymphocytes in the MALT. This control is mediated by chemoattractant chemokines or cytokines which recruit immune cells expressing the cognate receptors and adhesion molecules. Exploiting mucosal chemokine trafficking pathways to mobilize specific subsets of lymphocytes to mucosal tissues in the context of vaccination has improved immunogenicity and efficacy in preclinical models. This review describes the novel use of MALT chemokines as vaccine adjuvants. Specific attention will be placed upon the use of such adjuvants to enhance HIV-specific mucosal humoral immunity in the context of prophylactic vaccination.

CCL25 chemokine-guided stem cell attraction: an assessment of possible benefits and risks

Due to its chemoattraction potential on mesenchymal stromal cells of the CCL25/CCR9 axis, local application of CCL25 to severely damaged tissues may be a promising approach for regenerative therapies. Analysis of the given data revealed that CCL25/CCR9 signaling has a crucial role in regulation of an adult immune homeostasis. CCR9 expression variations resulted in dysfunctional immune response in colitis, rheumatoid arthritis and endometriosis. Regarding oncology, different neoplastic tissues exploit CCL25-dependent CCR9 signaling for either local proliferation or migration processes. The CCR9 pathway likely can trigger crosstalk between the Akt and NOTCH pathway and thus participate in the regulation of the neoplastic behavior. In conclusion, the designated application-tissue requires precise molecular analysis of possible CCR9 expression due to its proto-oncogenic characteristics.

Chemokines and Chemokine Receptors as Therapeutic Targets in Inflammatory Bowel Disease; Pitfalls and Promise

The principal targets for anti-chemokine therapy in inflammatory bowel disease (IBD) have been the receptors CCR9 and CXCR3 and their respective ligands CCL25 and CXCL10. More recently CCR6 and its ligand CCL20 have also received attention, the expression of the latter in enterocytes being manipulated through Smad7 signalling. These pathways, selected based on their fundamental role in regulating mucosal immunity, have led to the development of several therapeutic candidates that have been tested in early phase clinical trials with variable clinical efficacy. In this article, we appraise the status of chemokine-directed therapy in IBD, review recent developments, and nominate future areas for therapeutic focus.

Niches for the Long-Term Maintenance of Tissue-Resident Memory T Cells

Tissue-resident memory T cells (T_RM cells) are a population of immune cells that reside in the lymphoid and non-lymphoid organs without recirculation through the blood. These important cells occupy and utilize unique anatomical and physiological niches that are distinct from those for other memory T cell populations, such as central memory T cells in the secondary lymphoid organs and effector memory T cells that circulate through the tissues. CD8⁺ T_RM cells typically localize in the epithelial layers of barrier tissues where they are optimally positioned to act as sentinels to trigger antigen-specific protection against reinfection. CD4⁺ T_RM cells typically localize below the epithelial layers, such as below the basement membrane, and cluster in lymphoid structures designed to optimize interactions with antigen-presenting cells upon reinfection. A key feature of T_RM populations is their ability to be maintained in barrier tissues for prolonged periods of time. For example, skin CD8⁺ T_RM cells displace epidermal niches originally occupied by γδ T cells, thereby enabling their stable persistence for years. It is also clear that the long-term maintenance of T_RM cells in different microenvironments is dependent on multiple tissue-specific survival cues, although the specific details are poorly understood. However, not all T_RM persist over the long term. Recently, we identified a new spatial niche for the maintenance of CD8⁺ T_RM cells in the lung, which is created at the site of tissue regeneration after injury [termed repair-associated memory depots (RAMD)]. The short-lived nature of RAMD potentially explains the short lifespans of CD8⁺ T_RM cells in this particular tissue. Clearly, a better understanding of the niche-dependent maintenance of T_RM cells will be important for the development of vaccines designed to promote barrier immunity. In this review, we discuss recent advances in our understanding of the properties and nature of tissue-specific niches that maintain T_RM cells in different tissues.

A guide to chemokines and their receptors

The chemokines (or chemotactic cytokines) are a large family of small, secreted proteins that signal through cell surface G protein-coupled heptahelical chemokine receptors. They are best known for their ability to stimulate the migration of cells, most notably white blood cells (leukocytes). Consequently, chemokines play a central role in the development and homeostasis of the immune system, and are involved in all protective or destructive immune and inflammatory responses. Classically viewed as inducers of directed chemotactic migration, it is now clear that chemokines can stimulate a variety of other types of directed and undirected migratory behavior, such as haptotaxis, chemokinesis, and haptokinesis, in addition to inducing cell arrest or adhesion. However, chemokine receptors on leukocytes can do more than just direct migration, and these molecules can also be expressed on, and regulate the biology of, many nonleukocytic cell types. Chemokines are profoundly affected by post-translational modification, by interaction with the extracellular matrix (ECM), and by binding to heptahelical 'atypical' chemokine receptors that regulate chemokine localization and abundance. This guide gives a broad overview of the chemokine and chemokine receptor families; summarizes the complex physical interactions that occur in the chemokine network; and, using specific examples, discusses general principles of chemokine function, focusing particularly on their ability to direct leukocyte migration.

S1P-dependent interorgan trafficking of group 2 innate lymphoid cells supports host defense

Innate lymphoid cells (ILCs) are innate counterparts of adaptive T lymphocytes, contributing to host defense, tissue repair, metabolic homeostasis, and inflammatory diseases. ILCs have been considered to be tissue-resident cells, but whether ILCs move between tissue sites during infection has been unclear. We show here that interleukin-25- or helminth-induced inflammatory ILC2s are circulating cells that arise from resting ILC2s residing in intestinal lamina propria. They migrate to diverse tissues based on sphingosine 1-phosphate (S1P)-mediated chemotaxis that promotes lymphatic entry, blood circulation, and accumulation in peripheral sites, including the lung, where they contribute to anti-helminth defense and tissue repair. This ILC2 expansion and migration is a behavioral parallel to the antigen-driven proliferation and migration of adaptive lymphocytes to effector sites and indicates that ILCs complement adaptive immunity by providing both local and distant tissue protection during infection.

Chemokine regulation of innate lymphoid cell tissue distribution and function

Three groups of innate lymphoid cells (ILCs) can be defined based on transcription factor requirements, cytokine production profiles, and roles in immunity. Given their strategic anatomical location into barrier tissues and the ability to rapidly produce cytokines and to cross-talk with other immune and non-immune cells, ILCs play fundamental functions in tissue homeostasis and regulation of immune responses. Several members of the chemokine family influence ILC tissue localization in the correct microenvironment by regulating their release from the bone marrow as well as their homing and retention in the tissues. In this review, we discuss the recent advances on how chemokine regulation of ILC tissue-positioning and functional interaction with other cells play essential roles in tissue-specific regulation of innate and adaptive immune responses.

[Retinoic Acid Prevents Dendritic Cells from Inducing Novel Inflammatory T Cells That Produce Abundant Interleukin-13]

 Vitamin A (VA) plays critical roles in gut homeostasis. Dendritic cells in mesenteric lymph nodes (MLN-DCs) can metabolize VA to retinoic acid (RA), thereby inducing gut-tropic lymphocytes and enhancing peripheral differentiation of regulatory T cells expressing forkhead box P3. We found that MLN-DCs from VA-deficient mice induced a distinct inflammatory T helper type 2 (Th2)-cell subset that produced abundant interleukin-13 (IL-13) and expressed receptors for homing to skin and inflammatory sites but not to the intestine. IL-6-neutralizing antibodies or RA abrogated the induction of this subset. On the other hand, RA receptor antagonists allowed MLN-DCs from VA-sufficient mice to induce a similar T-cell subset. IL-6 induced the differentiation of this subset from naive CD4⁺ T cells upon activation with antibodies against CD3 and CD28, and RA receptor antagonists enhanced this induction. It has been considered that VA deficiency reduces Th2-dependent antibody responses. However, oral administration of an antigen to VA-deficient mice failed to induce immune tolerance but primed strong IL-13-dependent immunoglobulin G1 (IgG1) responses and IgE responses that caused skin allergy. These results suggest that MLN-DCs possess the latent ability to induce IL-13-producing inflammatory Th2 cells and that RA prevents them from inducing IL-13-dependent allergic or inflammatory responses to orally administered antigens.

Differential Expression of Homing Receptor Ligands on Tumor-Associated Vasculature that Control CD8 Effector T-cell Entry

Although CD8⁺ T cells are critical for controlling tumors, how they are recruited and home to primary and metastatic lesions is incompletely understood. We characterized the homing receptor (HR) ligands on tumor vasculature to determine what drives their expression and their role in T-cell entry. The anatomic location of B16-OVA tumors affected the expression of E-selectin, MadCAM-1, and VCAM-1, whereas the HR ligands CXCL9 and ICAM-1 were expressed on the vasculature regardless of location. VCAM-1 and CXCL9 expression was induced by IFNγ-secreting adaptive immune cells. VCAM-1 and CXCL9/10 enabled CD8⁺ T-cell effectors expressing α₄β₁ integrin and CXCR3 to enter both subcutaneous and peritoneal tumors, whereas E-selectin enabled E-selectin ligand⁺ effectors to enter subcutaneous tumors. However, MadCAM-1 did not mediate α₄β₇⁺ effector entry into peritoneal tumors due to an unexpected lack of luminal expression. These data establish the relative importance of certain HRs expressed on activated effectors and certain HR ligands expressed on tumor vasculature in the effective immune control of tumors. Cancer Immunol Res; 5(12); 1062-73. ©2017 AACR.

Colorectal cancer-infiltrating T lymphocytes display a distinct chemokine receptor expression profile

Background

T lymphocytes exert important homeostatic functions in the healthy intestinal mucosa, whereas in case of colorectal cancer (CRC), infiltration of T lymphocytes into the tumor is crucial for an effective anti-tumor immune response. In both situations, the recruitment mechanisms of T lymphocytes into the tissues are essential for the immunological functions deciding the outcome. The recruitment of T lymphocytes is largely dependent on their expression of various chemokine receptors. The aim of this study was to identify potential chemokine receptors involved in the recruitment of T lymphocytes to normal human colonic mucosa and to CRC tissue, respectively, by examining the expression of 16 different chemokine receptors on T lymphocytes isolated from these tissues.

Methods

Tissues were collected from patients undergoing bowel resection for CRC. Lymphocytes were isolated through enzymatic tissue degradation of CRC tissue and nearby located unaffected mucosa, respectively. The expression of a broad panel of chemokine receptors on the freshly isolated T lymphocytes was examined by flow cytometry.

Results

In the normal colonic mucosa, the frequencies of cells expressing CCR2, CCR4, CXCR3, and CXCR6 differed significantly between CD4⁺ and CD8⁺ T lymphocytes, suggesting that the molecular mechanisms mediating T lymphocyte recruitment to the gut differ between CD4⁺ and CD8⁺ T lymphocytes. In CRC, the frequencies of cells expressing CCR2 and CXCR5 were significantly lower in both the CD4⁺ and CD8⁺ T lymphocyte populations compared to unaffected colonic mucosa, and the frequency of CCR9⁺ cytotoxic T lymphocytes was significantly decreased in CRC tissue.

Conclusions

With regard to the normal gut mucosa, the results suggest that the molecular mechanisms mediating T lymphocyte recruitment differ between CD4⁺ and CD8⁺ T lymphocytes, which are important for understanding gut homeostasis. Importantly, T lymphocytes from CRC compared to normal colonic tissue displayed a distinct chemokine receptor expression profile, suggesting that mechanisms for recruitment of T lymphocytes to CRC tissue are skewed compared to normal colonic mucosa. Understanding these mechanisms could help in developing new strategies in cancer immunotherapy and to optimize already available alternatives such as immune checkpoint inhibitors.