Retinoic acid determines the precise tissue tropism of inflammatory Th17 cells in the intestine

Investigating the In Vivo Biodistribution of Extracellular Vesicles Isolated from Various Human Cell Sources Using Positron Emission Tomography

Positron emission tomography (PET) is a powerful tool for investigating the in vivo behavior of drug delivery systems. We aimed to assess the biodistribution of extracellular vesicles (EVs), nanosized vesicles secreted by cells isolated from various human cell sources using PET. EVs were isolated from mesenchymal stromal cells (MSCs) (MSC EVs), human macrophages (Mϕ EVs), and a melanoma cell line (A375 EVs) by centrifugation and were conjugated with deferoxamine for radiolabeling with Zr-89. PET using conjugated and radiolabeled EVs evaluated their in vivo biodistribution and tissue tropisms. Our study also investigated differences in mouse models, utilizing immunocompetent and immunocompromised mice and an A375 xenograft tumor model. Lastly, we investigated the impact of different labeling techniques on the observed EV biodistribution, including covalent surface modification and membrane incorporation. PET showed that all tested EVs exhibited extended in vivo circulation and generally low uptake in the liver, spleen, and lungs. However, Mϕ EVs showed high liver uptake, potentially attributable to the intrinsic tissue tropism of these EVs from the surface protein composition. MSC EV biodistribution differed between immunocompetent and immunodeficient mice, with increased spleen uptake observed in the latter. PET using A375 xenografts demonstrated efficient tumor uptake of EVs, but no preferential tissue-specific tropism of A375 EVs was found. Biodistribution differences between labeling techniques showed that surface-conjugated EVs had preferential blood circulation and low liver, spleen, and lung uptake compared to membrane integration. This study demonstrates the potential of EVs as effective drug carriers for various diseases, highlights the importance of selecting appropriate cell sources for EV-based drug delivery, and suggests that EV tropism can be harnessed to optimize therapeutic efficacy. Our findings indicate that the cellular source of EVs, labeling technique, and animal model can influence the observed biodistribution.

MAdCAM-1 targeting strategy can prevent colitic cancer carcinogenesis and progression via suppression of immune cell infiltration and inflammatory signals

Chronic inflammation caused by infiltrating immune cells can promote colitis-associated dysplasia/colitic cancer in ulcerative colitis (UC) by activating inflammatory cytokine signalling through the IL-6/p-STAT3 and TNFα/NF-κB pathways. Mucosal addressin cell adhesion molecule-1 (MAdCAM-1) expressed on high endothelial venules promotes the migration of immune cells from the bloodstream to the gut via interaction with α4β7 integrin expressed on the immune cells. MAdCAM-1, has therefore drawn interest as a novel therapeutic target for treating active UC. However, the role of MAdCAM-1-positive endothelial cells in immune cell infiltration in dysplasia/colitic cancers remains unclear. We evaluated the expression of MAdCAM-1, CD31 and immune cell markers (CD8, CD68, CD163 and FOXP3) in samples surgically resected from 11 UC patients with dysplasia/colitic cancer and 17 patients with sporadic colorectal cancer (SCRC), using immunohistochemical staining. We used an azoxymethane/dextran sodium sulphate mouse model (AOM/DSS mouse) to evaluate whether dysplasia/colitic cancer could be suppressed with an anti-MAdCAM-1 blocking antibody by preventing immune cell infiltration. The number of MAdCAM-1-positive vessels and infiltrating CD8+ , CD68+ and CD163+ immune cells was significantly higher in dysplasia/colitic cancer than in normal, SCRC and UC mucosa. In AOM/DSS mice, the anti-MAdCAM-1 antibody reduced the number, mean diameter, depth of tumours, Ki67 positivity, number of CD8+ , CD68+ and CD163+ immune cells and the IL-6/p-STAT3 and TNF-α/NF-κB signalling. Our results indicate that targeting MAdCAM-1 is a promising strategy for controlling not only UC severity but also carcinogenesis and tumour progression by regulating inflammation/immune cell infiltration in patients with UC.

Phytochemical analysis of Origanum majorana L. extract and investigation of its antioxidant, anti-inflammatory and immunomodulatory effects against experimentally induced colitis downregulating Th17 cells

ETHNOPHARMACOLOGICAL RELEVANCE

Origanum majorana L. is a member of the Lamiaceae family and is commonly used in Egyptian cuisine as a seasoning and flavor enhancer. It is also recognized as a well-known traditional medicine in Egypt and is widely used for treating abdominal colic due to its antispasmodic properties. However, the protective effects of Origanum majorana L. against ulcerative colitis and its underlying mechanisms remain unclear.

AIM OF THE STUDY

This study aimed to identify the biologically active components present in methanol extracts of Origanum majorana L. using gas chromatography/mass spectrometry (GC/MS). Additionally, it aimed to investigate the therapeutic effects of these extracts on acetic acid-induced ulcerative colitis and elucidate the potential mechanisms involved.

MATERIALS AND METHODS

We conducted a GC-MS analysis of the methanolic extract obtained from Origanum majorana L. Thirty-two male rats were included in the study and divided into four experimental groups, with eight rats in each group: sham, UC, UC + O. majorana, and UC sulfasalazine. After euthanizing the rats, colon tissue samples were collected for gross and microscopic examinations, assessment of oxidative stress, and molecular evaluation. GC-MS analysis identified 15 components in the extracts. Pretreatment with O. majorana L. extract and sulfasalazine significantly improved the disease activity index (DAI) and resulted in notable improvements in macroscopic and microscopic colon findings. Additionally, both treatments demonstrated preventive effects against colonic oxidative damage by reducing the levels of malondialdehyde (MDA) and increasing the levels of the antioxidant systems superoxide dismutase (SOD), catalase (CAT), and glutathione (GSH), which operate through the Nrf2/HO-1 signaling pathway. Moreover, these treatments downregulated the colonic inflammatory cascade by inhibiting NFκB, TNFα, IL-1β, IL6, IL23, IL17, COX-2, and iNOS, subsequently leading to downregulation of the JAK2/STAT3 signaling pathway and a decrease in the Th17 cell response. Furthermore, a reduction in the number of apoptotic epithelial cells that expressed caspase-3 was observed.

CONCLUSION

pretreatment with O. majorana L. extract significantly ameliorated acetic acid-induced ulcerative colitis. This effect could be attributed to the protective, antioxidant, anti-inflammatory, and anti-apoptotic properties of the extract.

Factors regulating the differences in frequency of infiltration of Th17 and Treg of the blood-brain barrier

Controlling CD4+ immune cell infiltration of the brain is a leading aim in designing therapeutic strategies for a range of neuropathological disorders such as multiple sclerosis, Alzheimer's disease, and depression. CD4+ T cells are a highly heterogeneous and reprogrammable family, which includes various distinctive cell types such as Th17, Th1, and Treg cells. Interestingly Th17 and Treg cells share a related transcriptomic profile, where the TGFβ-SMADS pathway plays a fundamental role in regulating the differentiation of both of these cell types. However, Th17 could be highly pathogenic and was shown to promote inflammation in various neuropathological disorders. Conversely, Treg is anti-inflammatory and is known to inhibit Th17. It could be noticed that Th17 frequencies of infiltration of the blood-brain barrier in various neurological disorders are significantly upregulated. However, Treg infiltration numbers are significantly low. The reasons behind these contradicting observations are still unknown. In this perspective, we propose that the difference in the T-cell receptor repertoire diversity, diapedesis pathways, chemokine expression, and mechanical properties of these two cell types could be contributing to answering this intriguing question.

Retinoic Acid Signaling Is Required for Dendritic Cell Maturation and the Induction of T Cell Immunity

Vitamin A and its biologically active metabolites, all-trans and 9-cis retinoic acid (RA), are thought to be important in generating and modulating immune function. However, RA modulates the function of many types of immune cells, and its specific role in dendritic cell (DC) activation, Ag presentation, and T cell effector function has not been fully characterized. Because RA works primarily through RA receptor (RAR)α, we examined mice with a myeloid cell-specific defect in RA signaling. These transgenic mice have a CD11c-cre-driven expression of a truncated form of RARα that specifically blocks the signaling of all forms of RARs in myeloid cells. This defect results in abnormal DC function, with impaired DC maturation and activation, and reduced Ag uptake and processing. These DC abnormalities were associated with a reduced ability to mount Ag-specific T cell responses to immunization despite having normally functioning T cells. In contrast, the loss of DC-specific RA signaling did not significantly alter levels of Ag-specific Abs postimmunization and resulted in an increase in bronchial IgA. Our findings indicate that RA signaling in DCs is crucial for immune activation, and its absence impairs the development of Ag-specific effector functions of T cell immunity.

Gut immune cells and intestinal niche imprinting

The intestine comprises the largest proportion of immune cells in the body. It is continuously exposed to new antigens and immune stimuli from the diet, microbiota but also from intestinal pathogens. In this review, we describe the main populations of immune cells present along the intestine, both from the innate and adaptive immune system. We later discuss how intestinal niches significantly impact the phenotype and function of gut immune populations at steady state and upon infection.

Vitamin A Ameliorated Irinotecan-Induced Diarrhea in a Piglet Model Involving Enteric Glia Modulation and Immune Cells Infiltration

Vitamin A (VA) and its metabolite, retinoic acid (RA), play important roles in modulating intestinal mucosal immunity, yet little is known about their regulatory effects on enteric nervous system function. The study aims to explore the protective effects of dietary VA on diarrhea in a piglet model involving enteric glia and immune cell modulation. Twenty-eight weaned piglets were fed either the basal or VA (basal diet supplemented with 18,000 IU/kg VA) diet and with or without irinotecan (CPT-11) injection. CPT-11 induced increased diarrhea incidence, immune infiltration, and reactive enteric gliosis. A diet supplemented with 18,000 IU/kg VA ameliorated the adverse effects of CPT-11 on the gut barrier. VA reduced diarrhea incidence and attenuated enteric glial gliosis, immune cell infiltrations, and inflammatory responses of CPT-induced piglets. An in vitro experiment with 1 nmol/L RA showed direct protective effects on monocultures of enteric glial cells (EGCs) or macrophages in LPS-simulated inflammatory conditions. Furthermore, 1 ng/mL glial-derived neurotropic factors (GDNF) could inhibit M1-macrophage polarization and pro-inflammatory cytokines production. In summary, VA exerted protective effects on the intestinal barrier by modulating enteric glia and immune cells, perhaps enhancing epithelial recovery under CPT-11 challenge. Our study demonstrated that RA signaling might promote the roles of enteric glia in intestinal immunity and tissue repair, which provided a reference for the VA supplementation of patient diets.

A gut-oral microbiome-driven axis controls oropharyngeal candidiasis through retinoic acid

A side effect of antibiotics is outgrowth of the opportunistic fungus Candida albicans in the oropharynx (oropharyngeal candidiasis, OPC). IL-17 signaling is vital for immunity to OPC, but how the microbiome impacts antifungal immunity is not well understood. Mice in standard specific pathogen-free (SPF) conditions are resistant to OPC, whereas we show that germ-free (GF) or antibiotic-treated mice are susceptible. Oral type 17 cells and IL-17-dependent responses were impaired in antibiotic-treated and GF mice. Susceptibility could be rescued in GF mice by mono-colonization with segmented filamentous bacterium (SFB), an intestine-specific constituent of the microbiota. SFB protection was accompanied by restoration of oral IL-17+CD4+ T cells and gene signatures characteristic of IL-17 signaling. Additionally, RNA-Seq revealed induction of genes in the retinoic acid (RA) and RA receptor-α (RARα) pathway. Administration of RA rescued immunity to OPC in microbiome-depleted or GF mice, while RAR inhibition caused susceptibility in immunocompetent animals. Surprisingly, immunity to OPC was independent of serum amyloids. Moreover, RAR inhibition did not alter oral type 17 cytokine levels. Thus, mono-colonization with a component of the intestinal microflora confers protection against OPC by type 17 and RA/RARα, which act in parallel to promote antifungal immunity. In principle, manipulation of the microbiome could be harnessed to maintain antifungal immunity.

Faecalibaculum rodentium remodels retinoic acid signaling to govern eosinophil-dependent intestinal epithelial homeostasis

The intestinal epithelium plays critical roles in sensing and integrating dietary and microbial signals. How microbiota and intestinal epithelial cell (IEC) interactions regulate host physiology in the proximal small intestine, particularly the duodenum, is unclear. Using single-cell RNA sequencing of duodenal IECs under germ-free (GF) and different conventional microbiota compositions, we show that specific microbiota members alter epithelial homeostasis by increasing epithelial turnover rate, crypt proliferation, and major histocompatibility complex class II (MHCII) expression. Microbiome profiling identified Faecalibaculum rodentium as a key species involved in this regulation. F. rodentium decreases enterocyte expression of retinoic-acid-producing enzymes Adh1, Aldh1a1, and Rdh7, reducing retinoic acid signaling required to maintain certain intestinal eosinophil populations. Eosinophils suppress intraepithelial-lymphocyte-mediated production of interferon-γ that regulates epithelial cell function. Thus, we identify a retinoic acid-eosinophil-interferon-γ-dependent circuit by which the microbiota modulates duodenal epithelial homeostasis.

Retinoic acid signaling acts as a rheostat to balance Treg function

Regulatory T cells (Tregs) promote immune homeostasis by maintaining self-tolerance and regulating inflammatory responses. Under certain inflammatory conditions, Tregs can lose their lineage stability and function. Previous studies have reported that ex vivo exposure to retinoic acid (RA) enhances Treg function and stability. However, it is unknown how RA receptor signaling in Tregs influences these processes in vivo. Herein, we employed mouse models in which RA signaling is silenced by the expression of the dominant negative receptor (DN) RARα in all T cells. Despite the fact that DNRARα conventional T cells are hypofunctional, Tregs had increased CD25 expression, STAT5 pathway activation, mTORC1 signaling and supersuppressor function. Furthermore, DNRARα Tregs had increased inhibitory molecule expression, amino acid transporter expression, and metabolic fitness and decreased antiapoptotic proteins. Supersuppressor function was observed when wild-type mice were treated with a pharmacologic pan-RAR antagonist. Unexpectedly, Treg-specific expression of DNRARα resulted in distinct phenotypes, such that a single allele of DNRARα in Tregs heightened their suppressive function, and biallelic expression led to loss of suppression and autoimmunity. The loss of Treg function was not cell intrinsic, as Tregs that developed in a noninflammatory milieu in chimeric mice reconstituted with DNRARα and wild-type bone marrow maintained the enhanced suppressive capacity. Fate mapping suggested that maintaining Treg stability in an inflammatory milieu requires RA signaling. Our findings indicate that RA signaling acts as a rheostat to balance Treg function in inflammatory and noninflammatory conditions in a dose-dependent manner.

Th17 CD4+ T-Cell as a Preferential Target for HIV Reservoirs

Among CD4+ T-cells, T helper 17 (Th17) cells play a sentinel role in the defense against bacterial/fungal pathogens at mucosal barriers. However, Th17 cells are also highly susceptible to HIV-1 infection and are rapidly depleted from gut mucosal sites, causing an imbalance of the Th17/Treg ratio and impairing cytokines production. Consequently, damage to the gut mucosal barrier leads to an enhanced microbial translocation and systemic inflammation, a hallmark of HIV-1 disease progression. Th17 cells’ expression of mucosal homing receptors (CCR6 and α4β7), as well as HIV receptors and co-receptors (CD4, α4β7, CCR5, and CXCR4), contributes to susceptibility to HIV infection. The up-regulation of numerous intracellular factors facilitating HIV production, alongside the downregulation of factors inhibiting HIV, helps to explain the frequency of HIV DNA within Th17 cells. Th17 cells harbor long-lived viral reservoirs in people living with HIV (PLWH) receiving antiretroviral therapy (ART). Moreover, cell longevity and the proliferation of a fraction of Th17 CD4 T cells allow HIV reservoirs to be maintained in ART patients.

Polymorphisms in Vitamin A-Related Genes and Their Functions in Autoimmune Thyroid Disease

Background: Vitamin A is a factor that suppresses immune responses, including T helper (Th)1 and Th17 responses. However, there has been no report showing the association between vitamin A-related genes (CYP26B1, RARB, and RARG) and the prognosis of autoimmune thyroid disease (AITD). The objective of this study was to clarify the association between vitamin A-related genes and the susceptibility and prognosis of AITD. Methods: We genotyped polymorphisms in genes encoding vitamin A-related molecules using the polymerase chain reaction-restriction fragment length polymorphism method. The proportion of T helper cells was analyzed by flow cytometry. Serum interleukin (IL)-17 and interferon (IFN)-γ were examined by enzyme-linked immunosorbent assay. Results:CYP26B1 rs3768641 GG genotype and G allele were significantly more frequent in patients with mild Hashimoto's thyroiditis (HT) than in those with severe HT (p = 0.0013 and 0.0024, respectively). The RARB rs1997352 CC genotype was significantly more frequent in HT patients than in controls (p = 0.0207). The proportion of Th17 cells was significantly higher in CYP26B1 rs2241057 TT genotype than C carrier (CC+CT genotypes) (p = 0.0385), in RARB rs1997352 A carrier (AA+AC genotypes) than those with CC genotype (p = 0.0246), and in RARG rs7398676 G carrier (GG+GT genotypes) than in TT genotype (p = 0.0249). In the RARB rs1997352 polymorphism, HT patients with a high concentration of IFN-γ (≥150 ng/mL) were more frequent in the CC genotype than in A carriers (AA+AC genotypes) (p = 0.0226). Serum levels of IL-17 were significantly elevated in subjects with the TT genotype of the CYP26B1 rs2241057 single nucleotide polymorphism (SNP) (p = 0.0026) and in subjects with the GG genotype of the CYP26B1 rs3798641 SNP (p = 0.030). Subjects with a high concentration of IL-17 (≥0.71 pg/mL) were more frequent in RARG 7398676 G carriers (GG+GT genotypes) than in TT genotype (p = 0.0218). Conclusions: Polymorphisms in the CYP26B1 gene were related to the proportion of Th17 cells, the level of IL-17 and the severity of HT. Polymorphisms in RAR were related to the proportion of Th17 cells, concentrations of IFN-γ and IL-17, and susceptibility to HT.

Commensal segmented filamentous bacteria-derived retinoic acid primes host defense to intestinal infection

Interactions between the microbiota and mammalian host are essential for defense against infection, but the microbial-derived cues that mediate this relationship remain unclear. Here, we find that intestinal epithelial cell (IEC)-associated commensal bacteria, segmented filamentous bacteria (SFB), promote early protection against the pathogen Citrobacter rodentium, independent of CD4⁺ T cells. SFB induced histone modifications in IECs at sites enriched for retinoic acid receptor motifs, suggesting that SFB may enhance defense through retinoic acid (RA). Consistent with this, inhibiting RA signaling suppressed SFB-induced protection. Intestinal RA levels were elevated in SFB mice, despite the inhibition of mammalian RA production, indicating that SFB directly modulate RA. Interestingly, RA was produced by intestinal bacteria, and the loss of bacterial-intrinsic aldehyde dehydrogenase activity decreased the RA levels and increased infection. These data reveal RA as an unexpected microbiota-derived metabolite that primes innate defense and suggests that pre- and probiotic approaches to elevate RA could prevent or combat infections.

IL-17 and CCR9+α4β7- Th17 Cells Promote Salivary Gland Inflammation, Dysfunction, and Cell Death in Sjögren's Syndrome

Previous studies have evaluated the roles of T and B cells in the pathogenesis of Sjögren's syndrome (SS); however, their relationships with age-dependent and metabolic abnormalities remain unclear. We examined the impacts of changes associated with aging or metabolic abnormalities on populations of T and B cells and SS disease severity. We detected increased populations of IL-17-producing T and B cells, which regulate inflammation, in the salivary glands of NOD/ShiLtJ mice. Inflammation-induced human submandibular gland cell death, determined based on p-MLKL and RIPK3 expression levels, was significantly increased by IL-17 treatment. Among IL-17-expressing cells in the salivary gland, peripheral blood, and spleen, the α4β7 (gut-homing integrin)-negative population was significantly increased in aged NOD/ShiLtJ mice. The α4β7-positive population markedly increased in the intestines of aged NOD/ShiLtJ mice following retinoic acid (RA) treatment. A significant increase in α4β7-negative IL-17-expressing cells in salivary glands may be involved in the onset and progression of SS. These results suggest the potential therapeutic utility of RA in SS treatment.

Illuminating the Role of Vitamin A in Skin Innate Immunity and the Skin Microbiome: A Narrative Review

Vitamin A is a fat-soluble vitamin that plays an important role in skin immunity. Deficiencies in Vitamin A have been linked to impaired immune response and increased susceptibility to skin infections and inflammatory skin disease. This narrative review summarizes recent primary evidence that elucidates the role of vitamin A and its derivatives on innate immune regulators through mechanisms that promote skin immunity and sustain the skin microbiome.

In Ovo Inoculation of Vitamin A Modulates Chicken Embryo Immune Functions

Vitamin A mediates many important biological functions in humans and animals. Presence of vitamin A receptors on immune system cells emphasizes their role in immune functions. To assess the effects of in ovo inoculation of vitamin A on the immune system of chicken embryos, 18 days old embryonated eggs were inoculated with 3 different concentrations of retinoic acid (the active form of vitamin A) at 30, 90, and 270 μmol/egg via the amniotic sac. After 6, 18, and 24 h, the spleen and bursa of the embryos were collected for RNA extraction and real-time polymerase chain reaction. The results were dose dependant. After 24 h, inoculation with 270 μmol/egg downregulated relative expression of interferon IFN-α, IFN-β, IFN-γ, interleukin (IL)-1β, IL-2, CXCLi2, IL-12, and IL-13 compared to control in the spleen, indicating an anti-inflammatory effect at this concentration. In comparison, 90 μmol/egg induced greater expressions of the above genes at the same timepoint compared to the 270 μmol. The results of this study indicate that in ovo inoculation of vitamin A can modulate immune functions of the chicken embryo, which might be beneficial for induction of immune responses by in ovo vaccines.

The Need to Consider Context in the Evaluation of Anti-infectious and Immunomodulatory Effects of Vitamin A and its Derivatives

Vitamin A and its derivatives (retinoids) act as potent regulators in many aspects of mammalian reproduction, development, repair, and maintenance of differentiated tissue functioning. Unlike other vitamins, Vitamin A and retinoids, which have hormonal actions, present significant toxicity, which plays roles in clinically relevant situations, such as hypervitaminosis A and retinoic acid ("differentiation") syndrome. Although clinical presentation is conspicuous in states of insufficient or excessive Vitamin A and retinoid concentration, equally relevant effects on host resistance to specific infectious agents, and in the general maintenance of immune homeostasis, may go unnoticed, because their expression requires either pathogen exposure or the presence of inflammatory co-morbidities. There is a vast literature on the roles played by retinoids in the maintenance of a tolerogenic, noninflammatory environment in the gut mucosa, which is considered by many investigators representative of a general role played by retinoids as anti-inflammatory hormones elsewhere. However, in the gut mucosa itself, as well as in the bone marrow and inflammatory sites, context determines whether one observes an anti-inflammatory or proinflammatory action of retinoids. Both interactions between specialized cell populations, and interactions between retinoids and other classes of mediators/regulators, such as cytokines and glucocorticoid hormones, must be considered as important factors contributing to this overall context. We review evidence from recent studies on mucosal immunity, granulocyte biology and respiratory allergy models, highlighting the relevance of these variables as well as their possible contributions to the observed outcomes.

IL-12 from endogenous cDC1, and not vaccine DC, is required for Th1 induction

Success of DC vaccines relies on the quality of antigen presentation, costimulation, lymph node migration, and the release of IL-12, in case of Th1 priming. Here, we provide evidence for interaction between the injected vaccine DCs with endogenous lymph node–resident DCs for Th1 induction. While migration of the injected DCs was essential for antigen delivery to the lymph node, the injected DCs contributed only partially to Th0 priming and were unable to instruct Th1 generation. Instead, we provide evidence that the lymph node–resident XCR1⁺ DCs are activated by the injected DCs to present the cognate antigen and release IL-12 for Th1 polarization. The timing of interactions in the draining lymph nodes appeared step-wise as (a) injected DCs with cognate T cells, (b) injected DCs with bystander DCs, and (c) bystander DCs with T cells. The transcriptome of the bystander DCs showed a downregulation of Treg- and Th2/Th9-inducing genes and self-antigen presentation, as well as upregulation of MHC class II and genes required for Th1 instruction. Together, these data show that injected mature lymph node migratory DCs direct T cell priming and bystander DC activation, but not Th1 polarization, which is mediated by endogenous IL-12p70⁺XCR1⁺ resident bystander DCs. Our results are of importance for clinical DC-based vaccinations against tumors where endogenous DCs may be functionally impaired by chemotherapy.

Successful Th1 priming by DC vaccines in mice depends on IL-12 from endogenous and XCR1⁺ cDC1 population.

Retinoids in health and disease: A role for hepatic stellate cells in affecting retinoid levels

Vitamin A (retinol) is important for normal growth, vision and reproduction. It has a role in the immune response and the development of metabolic syndrome. Most of the retinol present in the body is stored as retinyl esters within lipid droplets in hepatic stellate cells (HSCs). In case of liver damage, HSCs release large amounts of stored retinol, which is partially converted to retinoic acid (RA). This surge of RA can mediate the immune response and enhance the regeneration of the liver. If the damage persists activated HSCs change into myofibroblast-like cells producing extracellular matrix, which increases the chance of tumorigenesis to occur. RA has been shown to decrease proliferation and metastasis of hepatocellular carcinoma. The levels of RA and RA signaling are influenced by the possibility to esterify retinol towards retinyl esters. This suggests a complex regulation between different retinoids, with an important regulatory role for HSCs.

Association of serum retinoic acid with depression in patients with acute ischemic stroke

Retinoic acid (RA), produced by the metabolism of vitamin A, makes effects on depression and stroke. This study was aimed to evaluate the relationship between RA levels in serum and post-stroke depression (PSD). A single-center (Chengdu, China) prospective cohort study was conducted on patients with acute ischemic stroke. The RA serum level was measured at admission. The PSD was assessed in the 3-month follow-up. The RA-PSD relationship was evaluated with conditional logistic regression. In total, 239 ischemic stroke cases and 100 healthy controls were included. The median RA serum level in patients with ischemic stroke was 2.45 ng/ml (interquartile range [IQR], 0.72-4.33), lower(P<0.001) than 3.89 ng/ml of those in control cases ([IQR]: 2.62-5.39). The crude and adjusted odds ratios [OR] (and 95% confidence intervals [CI]) of PSD associated with an IQR increase for RA were 0.54 (0.44, 0.67) and 0.66 (0.52, 0.79), respectively. Higher ORs of PSD associated with reduced RA levels ( Collapse

Key Words

• depression
• ischemic stroke
• retinoic acid
• stroke

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MESH Headings

• Aged
• Biomarkers/blood
• Cohort Studies
• Depression/blood
• Depression/etiology
• Female
• Humans
• Ischemic Stroke/blood
• Ischemic Stroke/complications
• Ischemic Stroke/psychology
• Male
• Middle Aged
• Prospective Studies
• Tretinoin/blood

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Grants Collapse

Affiliation(s)

Cai-Di Yang Department of Neurology, Eastern Hospital, Sichuan Academy of Medical Sciences and Sichuan Provincial People's Hospital, Chengdu 610101, China
Ming-Li Cheng Department of Neurology, People's Hospital of Jianyang, Jianyang 641400, China
Wen Liu The Clinical Laboratory Department, Affiliated Hospital of North Sichuan Medical College, Nanchong 637000, China
Ding-Hua Zeng Department of Neurology, Eastern Hospital, Sichuan Academy of Medical Sciences and Sichuan Provincial People's Hospital, Chengdu 610101, China

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Impact of gut microbiota on gut-distal autoimmunity: a focus on T cells

The immune system is essential for maintaining a delicate balance between eliminating pathogens and maintaining tolerance to self-tissues to avoid autoimmunity. An enormous and complex community of gut microbiota provides essential health benefits to the host, particularly by regulating immune homeostasis. Many of the metabolites derived from commensals can impact host health by directly regulating the immune system. Many autoimmune diseases arise from an imbalance between pathogenic effector T cells and regulatory T (Treg) cells. Recent interest has emerged in understanding how cross-talk between gut microbiota and the host immune system promotes autoimmune development by controlling the differentiation and plasticity of T helper and Treg cells. At the molecular level, our recent study, along with others, demonstrates that asymptomatic colonization by commensal bacteria in the gut is capable of triggering autoimmune disease by molecular mimicking self-antigen and skewing the expression of dual T-cell receptors on T cells. Dysbiosis, an imbalance of the gut microbiota, is involved in autoimmune development in both mice and humans. Although it is well known that dysbiosis can impact diseases occurring within the gut, growing literature suggests that dysbiosis also causes the development of gut-distal/non-gut autoimmunity. In this review, we discuss recent advances in understanding the potential molecular mechanisms whereby gut microbiota induces autoimmunity, and the evidence that the gut microbiota triggers gut-distal autoimmune diseases.

β7 integrins contribute to intestinal tumor growth in mice

The gut homing receptor integrin α4β7 is essential for the migration of pro-inflammatory T cells into the gut mucosa. Since intestinal neoplasia has been associated with chronic inflammation, we investigated whether interfering with gut-homing affects intestinal tumorigenesis. Using chemically induced and spontaneous intestinal tumor models we showed that lack of β7 integrin significantly impairs tumor growth without affecting tumor frequencies, with a mild translatable effect on overall survival. This correlates with human data showing lower MAdCAM-1 expression and disease-free survival in colorectal cancer patients. Thus, paradoxically in contrast to extra-intestinal tumors, blocking migration of immune cells into the gut might have a positive therapeutic effect on intestinal neoplasia.

Role of Vitamin A in the Immune System

Vitamin A (VitA) is a micronutrient that is crucial for maintaining vision, promoting growth and development, and protecting epithelium and mucus integrity in the body. VitA is known as an anti-inflammation vitamin because of its critical role in enhancing immune function. VitA is involved in the development of the immune system and plays regulatory roles in cellular immune responses and humoral immune processes. VitA has demonstrated a therapeutic effect in the treatment of various infectious diseases. To better understand the relationship between nutrition and the immune system, the authors review recent literature about VitA in immunity research and briefly introduce the clinical application of VitA in the treatment of several infectious diseases.

Impact of Retinoic Acid on Immune Cells and Inflammatory Diseases

Vitamin A metabolite retinoic acid (RA) plays important roles in cell growth, differentiation, organogenesis, and reproduction and a key role in mucosal immune responses. RA promotes dendritic cells to express CD103 and to produce RA, enhances the differentiation of Foxp3⁺ inducible regulatory T cells, and induces gut-homing specificity in T cells. Although vitamin A is crucial for maintaining homeostasis at the intestinal barrier and equilibrating immunity and tolerance, including gut dysbiosis, retinoids perform a wide variety of functions in many settings, such as the central nervous system, skin aging, allergic airway diseases, cancer prevention and therapy, and metabolic diseases. The mechanism of RA is interesting to explore as both a mucosal adjuvant and a combination therapy with other effective agents. Here, we review the effect of RA on innate and adaptive immunity with a special emphasis on inflammatory status.

Control of Innate and Adaptive Lymphocytes by the RAR-Retinoic Acid Axis

Lymphocytes, such as T cells, B cells, and innate lymphoid cells (ILCs), play central roles in regulating immune responses. Retinoic acids (RAs) are vitamin A metabolites, produced and metabolized by certain tissue cells and myeloid cells in a tissue-specific manner. It has been established that RAs induce gut-homing receptors on T cells, B cells, and ILCs. A mounting body of evidence indicates that RAs exert far-reaching effects on functional differentiation and fate of these lymphocytes. For example, RAs promote effector T cell maintenance, generation of induced gut-homing regulatory and effector T cell subsets, antibody production by B cells, and functional maturation of ILCs. Key functions of RAs in regulating major groups of innate and adaptive lymphocytes are highlighted in this article.

T Helper 17 Promotes Induction of Antigen-Specific Gut-Mucosal Cytotoxic T Lymphocytes following Adenovirus Vector Vaccination

Few current vaccines can establish antigen (Ag)-specific immune responses in both mucosal and systemic compartments. Therefore, development of vaccines providing defense against diverse infectious agents in both compartments is of high priority in global health. Intramuscular vaccination of an adenovirus vector (Adv) has been shown to induce Ag-specific cytotoxic T lymphocytes (CTLs) in both systemic and gut-mucosal compartments. We previously found that type I interferon (IFN) signaling is required for induction of gut-mucosal, but not systemic, CTLs following vaccination; however, the molecular mechanism involving type I IFN signaling remains unknown. Here, we found that T helper 17 (Th17)-polarizing cytokine expression was down-regulated in the inguinal lymph nodes (iLNs) of Ifnar2^−/− mice, resulting in the reduction of Ag-specific Th17 cells in the iLNs and gut mucosa of the mice. We also found that prior transfer of Th17 cells reversed the decrease in the number of Ag-specific gut-mucosal CTLs in Ifnar2^−/− mice following Adv vaccination. Additionally, prior transfer of Th17 cells into wild-type mice enhanced the induction of Ag-specific CTLs in the gut mucosa, but not in systemic compartments, suggesting a gut mucosa-specific mechanism where Th17 cells regulate the magnitude of vaccine-elicited Ag-specific CTL responses. These data suggest that Th17 cells translate systemic type I IFN signaling into a gut-mucosal CTL response following vaccination, which could promote the development of promising Adv vaccines capable of establishing both systemic and gut-mucosal protective immunity.

Retinoic Acid and Retinoic Acid Receptors as Pleiotropic Modulators of the Immune System

Vitamin A is a multifunctional vitamin implicated in a wide range of biological processes. Its control over the immune system and functions are perhaps the most pleiotropic not only for development but also for the functional fate of almost every cell involved in protective or regulatory adaptive or innate immunity. This is especially key at the intestinal border, where dietary vitamin A is first absorbed. Most effects of vitamin A are exerted by its metabolite, retinoic acid (RA), which through ligation of nuclear receptors controls transcriptional expression of RA target genes. In addition to this canonical function, RA and RA receptors (RARs), either as ligand-receptor or separately, play extranuclear, nongenomic roles that greatly expand the multiple mechanisms employed for their numerous and paradoxical functions that ultimately link environmental sensing with immune cell fate. This review discusses RA and RARs and their complex roles in innate and adaptive immunity.

Differential T cell homing to colon vs. small intestine is imprinted by local CD11c+ APCs that determine homing receptors

Mechanisms that imprint T cell homing to the small intestine have been well studied; however, those for homing to the colon are poorly understood. Recently, we found that these are distinct subcompartments of the gut mucosal immune system, which implies differential homing. Here, we show that colonic CD11c⁺ APCs imprint CD8⁺ T cell preferential homing to the colon, in contrast to those from the small intestine that imprint CD8⁺ T cell homing to the small intestine, and that the differences are related to the variable ability of APCs to induce α4β7-integrin and CCR9 expression on T cells. Colon APCs also expressed lower levels of retinoic acid-producing enzymes that are known to control the mucosal homing of T cells. These findings are the first to our knowledge to directly demonstrate that colon APCs imprint T cells to selectively home to the large bowel, which is critical for the design of successful T cell-based therapies and vaccines, such as colon cancer immunotherapy and HIV vaccines.

Host mechanisms involved in cattle Escherichia coli O157 shedding: a fundamental understanding for reducing foodborne pathogen in food animal production

The host mechanisms involved in Escherichia coli O157 super-shedding in cattle is largely unknown. In this study, the comparison of transcriptomes of intestinal tissues between super-shedders (SS) and cattle negative for E. coli O157 (NS) was performed, aiming to identify genes that are potentially associated with super-shedding. In total, 16,846 ± 639 (cecum) to 18,137 ± 696 (distal jejunum) were expressed throughout the intestine, with the expressed genes associated with immune functions more pronounced in the small intestine. In total, 351 differentially expressed (DE) genes were identified throughout the intestine between SS and NS, with 101 being up-regulated and 250 down-regulated in SS. Functional analysis revealed DE genes were involved in increased T-cell responses and cholesterol absorption in the distal jejunum and descending colon, and decreased B-cell maturation in the distal jejunum of SS. RNA-Seq based SNP discovery revealed that the mutations in seven DE genes involved in leukocyte activation and cholesterol transportation were associated with E. coli O157 shedding. Our findings suggest that T-cell responses and cholesterol metabolism in the intestinal tract may be associated with super-shedding phenomenon, and the SNPs in the DE genes are possibly associated with the observed gene expression difference between SS and NS.

HIV-1 selectively targets gut-homing CCR6+CD4+ T cells via mTOR-dependent mechanisms

Gut-associated lymphoid tissues are enriched in CCR6+ Th17-polarized CD4+ T cells that contribute to HIV-1 persistence during antiretroviral therapy (ART). This raises the need for Th17-targeted immunotherapies. In an effort to identify mechanisms governing HIV-1 permissiveness/persistence in gut-homing Th17 cells, we analyzed the transcriptome of CCR6+ versus CCR6- T cells exposed to the gut-homing inducer retinoic acid (RA) and performed functional validations in colon biopsies of HIV-infected individuals receiving ART (HIV+ART). Although both CCR6+ and CCR6- T cells acquired gut-homing markers upon RA exposure, the modulation of unique sets of genes coincided with preferential HIV-1 replication in RA-treated CCR6+ T cells. This molecular signature included the upregulation of HIV-dependency factors acting at entry/postentry levels, such as the CCR5 and PI3K/Akt/mTORC1 signaling pathways. Of note, mTOR expression/phosphorylation was distinctively induced by RA in CCR6+ T cells. Consistently, mTOR inhibitors counteracted the effect of RA on HIV replication in vitro and viral reactivation in CD4+ T cells from HIV+ART individuals via postentry mechanisms independent of CCR5. Finally, CCR6+ versus CCR6- T cells infiltrating the colons of HIV+ART individuals expressed unique molecular signatures, including higher levels of CCR5, integrin β7, and mTOR phosphorylation. Together, our results identify mTOR as a druggable key regulator of HIV permissiveness in gut-homing CCR6+ T cells.

Contraction of intestinal effector T cells by retinoic acid-induced purinergic receptor P2X7

The intestinal environment harbors a large number of activated T cells, which are potentially inflammatory. To prevent inflammatory responses, intestinal T cells are controlled by various tolerogenic mechanisms, including T-cell apoptosis. We investigated the expression mechanism and function of the purinergic receptor P2X7 in contraction of intestinal CD4⁺ effector T cells. We found that P2X7 upregulation on CD4⁺ effector T cells is induced by retinoic acid through retinoic acid receptor α binding to an intragenic enhancer region of the P2rx7 gene. P2X7 is highly expressed by most intestinal αβ and γδ T cells, including T-helper type 1 (Th1) and Th17 cells. The intestinal effector T cells are effectively deleted by P2X7 activation-dependent apoptosis. Moreover, P2X7 activation suppressed T-cell-induced colitis in Rag1^-/- mice. The data from vitamin A-deficient and P2rx7^-/- mice indicate that the retinoic acid-P2X7 pathway is important in preventing aberrant buildup of activated T cells. We conclude that retinoic acid controls intestinal effector T-cell populations by inducing P2X7 expression. These findings have important ramifications in preventing inflammatory diseases in the intestine.

Gut-Specific Delivery of T-Helper 17 Cells Reduces Obesity and Insulin Resistance in Mice

BACKGROUND & AIMS

Obesity and metabolic syndrome have been associated with alterations to the intestinal microbiota. However, few studies examined the effects of obesity on the intestinal immune system. We investigated changes in subsets of intestinal CD4⁺ T-helper (T_H) cells with obesity and the effects of gut-tropic T_H17 cells in mice on a high-fat diet (HFD).

METHODS

We isolated immune cells from small intestine and adipose tissue of C57BL/6 mice fed a normal chow diet or a HFD for 10 weeks and analyzed the cells by flow cytometry. Mice fed a vitamin A-deficient HFD were compared with mice fed a vitamin A-sufficient HFD. Obese RAG1-deficient mice were given injections of only regulatory T cells or a combination of regulatory T cells and T_H17 cells (wild type or deficient in integrin β7 subunit or interleukin 17 [IL17]). Mice were examined for weight gain, fat mass, fatty liver, glucose tolerance, and insulin resistance. Fecal samples were collected before and after T cell transfer and analyzed for microbiota composition by metagenomic DNA sequencing and quantitative polymerase chain reaction.

RESULTS

Mice placed on a HFD became obese, which affected the distribution of small intestinal CD4⁺ T_H cells. Intestinal tissues from obese mice had significant reductions in the proportion of T_H17 cells but increased proportion of T_H1 cells, compared with intestinal tissues from nonobese mice. Depletion of vitamin A in obese mice further reduced the proportion of T_H17 cells in small intestine; this reduction correlated with more weight gain and worsening of glucose intolerance and insulin resistance. Adoptive transfer of in vitro-differentiated gut-tropic T_H17 cells to obese mice reduced these metabolic defects, which required the integrin β7 subunit and IL17. Delivery of T_H17 cells to intestines of mice led to expansion of commensal microbes associated with leanness.

CONCLUSIONS

In mice, intestinal T_H17 cells contribute to development of a microbiota that maintains metabolic homeostasis, via IL17. Gut-homing T_H17 cells might be used to reduce metabolic disorders in obese individuals.

Gut Microbiota Species Can Provoke both Inflammatory and Tolerogenic Immune Responses in Human Dendritic Cells Mediated by Retinoic Acid Receptor Alpha Ligation

Dendritic cells are considered as the main coordinators of both mucosal and systemic immune responses, thus playing a determining role in shaping the outcome of effector cell responses. However, it is still uncovered how primary human monocyte-derived DC (moDC) populations drive the polarization of helper T (Th) cells in the presence of commensal bacteria harboring unique immunomodulatory properties. Furthermore, the individual members of the gut microbiota have the potential to modulate the outcome of immune responses and shape the immunogenicity of differentiating moDCs via the activation of retinoic acid receptor alpha (RARα). Here, we report that moDCs are able to mediate robust Th1 and Th17 responses upon stimulation by Escherichia coli Schaedler or Morganella morganii, while the probiotic Bacillus subtilis strain limits this effect. Moreover, physiological concentrations of all-trans retinoic acid (ATRA) are able to re-program the differentiation of moDCs resulting in altered gene expression profiles of the master transcription factors RARα and interferon regulatory factor 4, and concomitantly regulate the cell surface expression levels of CD1 proteins and also the mucosa-associated CD103 integrin to different directions. It was also demonstrated that the ATRA-conditioned moDCs exhibited enhanced pro-inflammatory cytokine secretion while reduced their co-stimulatory and antigen-presenting capacity thus reducing Th1 and presenting undetectable Th17 type responses against the tested microbiota strains. Importantly, these regulatory circuits could be prevented by the selective inhibition of RARα functionality. These results altogether demonstrate that selected commensal bacterial strains are able to drive strong effector immune responses by moDCs, while in the presence of ATRA, they support the development of both tolerogenic and inflammatory moDC in a RARα-dependent manner.

Synthetic Retinoid AM80 Ameliorates Lung and Arthritic Autoimmune Responses by Inhibiting T Follicular Helper and Th17 Cell Responses

Rheumatoid arthritis is an autoimmune disorder that affects the joints and other organs. Pulmonary complications contribute significantly to rheumatoid arthritis mortality. Retinoic acid and its synthetic compound AM80 play roles in immunoregulation but their effect on mucosal autoimmunity remains largely unknown. T follicular helper (Tfh) and Th17 cells are known to promote inflammation and autoantibody production. Using the K/BxN autoimmune arthritis model, we elucidate a novel mechanism whereby oral AM80 administration suppressed lung mucosa-associated Tfh and autoantibody responses by increasing the gut-homing α4β7 integrin expression on Tfh cells. This diverted Tfh cells from systemic (non-gut) inflamed sites such as the lung into the gut-associated lymphoid tissues, Peyer's patches, and thus reduced the systemic autoantibodies. AM80 also inhibited the lung Th17 response. AM80's effect in the lungs was readily applied to the joints as AM80 also inhibited Tfh and Th17 responses in the spleen, the major autoantibody producing site known to correlate with K/BxN arthritis severity. Finally, we used anti-β7 treatment as an alternative approach, demonstrating that manipulating T cell migration between the gut and systemic sites alters the systemic disease outcome. The β7 blockade prevented both Tfh and Th17 cells from entering the non-immunopathogenic site, the gut, and retained these T effector cells in the systemic sites, leading to augmented arthritis. These data suggest a dual beneficial effect of AM80, targeting both Tfh and Th17 cells, and warrant strict safety monitoring of gut-homing perturbing agents used in treating intestinal inflammation.

Metabolites: deciphering the molecular language between DCs and their environment

Dendritic cells (DCs) determine the outcome of the immune response based on signals they receive from the environment. Presentation of antigen under various contexts can lead to activation and differentiation of T cells for immunity or dampening of immune responses by establishing tolerance, primarily through the priming of regulatory T cells. Infections, inflammation and normal cellular interactions shape DC responses through direct contact or via cytokine signaling. Although it is widely accepted that DCs sense microbial components through pattern recognition receptors (PRRs), increasing evidence advocates for the existence of a set of signals that can profoundly shape DC function via PRR-independent pathways. This diverse group of host- or commensal-derived metabolites represents a newly appreciated code from which DCs can interpret environmental cues. In this review, we discuss the existing information on the effect of some of the most studied metabolites on DC function, together with the implications this may have in immune-mediated diseases.

Beta 1-integrin ligation and TLR ligation enhance GM-CSF-induced ALDH1A2 expression in dendritic cells, but differentially regulate their anti-inflammatory properties

Retinoic acid (RA)–producing CD103⁺ mature dendritic cells (DCs) in mesenteric lymph nodes (MLNs) play crucial roles in gut immunity. GM-CSF and RA contribute to the expression of the RA-producing enzyme ALDH1A2. However, additional signals appeared to be required for inducing ALDH1A2^high mature DCs from immature DCs. We found here that TLR ligands (Ls) and immobilized E-cadherin could provide such signals in FLT3-L–generated bone marrow (BM)–derived DCs after treatment with GM-CSF and the RA receptor agonist Am80. The TLR-L-treated DCs produced proinflammatory cytokines unlike normal ALDH1A2^high MLN-DCs, whereas the E-cadherin-treated DCs did not. Immobilized VCAM-1 and semaphorin 7 A exerted effects similar to those of E-cadherin. Soluble anti-integrin β1 antibodies or inhibitors of integrin signaling molecules suppressed the effects of these immobilized proteins, whereas immobilized anti-integrin β1 antibodies enhanced the GM-CSF/Am80-induced ALDH1A2 expression without inducing proinflammatory cytokines. Sequential stimulation of splenic pre-DCs with GM-CSF/Am80 and immobilized E-cadherin or anti-integrin β1 antibody also induced differentiation to mature DCs with high ALDH activity. The E-cadherin-treated BM-DCs induced gut-tropic Foxp3⁺ T cells and alleviated DSS–induced colitis, whereas the TLR-L-treated DCs aggravated DSS–induced colitis. The results suggest that integrin β1-mediated signals contribute to the differentiation and maturation of RA-producing anti-inflammatory DCs.

Peripheral Lymphoid Volume Expansion and Maintenance Are Controlled by Gut Microbiota via RALDH+ Dendritic Cells

Lymphocyte homing to draining lymph nodes is critical for the initiation of immune responses. Secondary lymphoid organs of germ-free mice are underdeveloped. How gut commensal microbes remotely regulate cellularity and volume of secondary lymphoid organs remains unknown. We report here that, driven by commensal fungi, a wave of CD45(+)CD103(+)RALDH(+) cells migrates to the peripheral lymph nodes after birth. The arrival of these cells introduces high amounts of retinoic acid, mediates the neonatal to adult addressin switch on endothelial cells, and directs the homing of lymphocytes to both gut-associated lymphoid tissues and peripheral lymph nodes. In adult mice, a small number of these RALDH(+) cells might serve to maintain the volume of secondary lymphoid organs. Homing deficiency of these cells was associated with lymph node attrition in vitamin-A-deficient mice, suggesting a perpetual dependence on retinoic acid signaling for structural and functional maintenance of peripheral immune organs.

Retinoic Acid as a Modulator of T Cell Immunity

Vitamin A, a generic designation for an array of organic molecules that includes retinal, retinol and retinoic acid, is an essential nutrient needed in a wide array of aspects including the proper functioning of the visual system, maintenance of cell function and differentiation, epithelial surface integrity, erythrocyte production, reproduction, and normal immune function. Vitamin A deficiency is one of the most common micronutrient deficiencies worldwide and is associated with defects in adaptive immunity. Reports from epidemiological studies, clinical trials and experimental studies have clearly demonstrated that vitamin A plays a central role in immunity and that its deficiency is the cause of broad immune alterations including decreased humoral and cellular responses, inadequate immune regulation, weak response to vaccines and poor lymphoid organ development. In this review, we will examine the role of vitamin A in immunity and focus on several aspects of T cell biology such as T helper cell differentiation, function and homing, as well as lymphoid organ development. Further, we will provide an overview of the effects of vitamin A deficiency in the adaptive immune responses and how retinoic acid, through its effect on T cells can fine-tune the balance between tolerance and immunity.

Retinoic Acid Differentially Regulates the Migration of Innate Lymphoid Cell Subsets to the Gut

Distinct groups of innate lymphoid cells (ILCs) such as ILC1, ILC2, and ILC3 populate the intestine, but how these ILCs develop tissue tropism for this organ is unclear. We report that prior to migration to the intestine ILCs first undergo a "switch" in their expression of homing receptors from lymphoid to gut homing receptors. This process is regulated by mucosal dendritic cells and the gut-specific tissue factor retinoic acid (RA). This change in homing receptors is required for long-term population and effector function of ILCs in the intestine. Only ILC1 and ILC3, but not ILC2, undergo the RA-dependent homing receptor switch in gut-associated lymphoid tissues. In contrast, ILC2 acquire gut homing receptors in a largely RA-independent manner during their development in the bone marrow and can migrate directly to the intestine. Thus, distinct programs regulate the migration of ILC subsets to the intestine for regulation of innate immunity.

Agonists of Co-stimulation in Cancer Immunotherapy Directed Against CD137, OX40, GITR, CD27, CD28, and ICOS

T and natural killer (NK) lymphocytes are considered the main effector players in the immune response against tumors. Full activation of T and NK lymphocytes requires the coordinated participation of several surface receptors that meet their cognate ligands through structured transient cell-to-cell interactions known as immune synapses. In the case of T cells, the main route of stimulation is driven by antigens as recognized in the form of short polypeptides associated with major histocompatibility complex (MHC) antigen-presenting molecules. However, the functional outcome of T-cell stimulation towards clonal expansion and effector function acquisition is contingent on the contact of additional surface receptor-ligand pairs and on the actions of cytokines in the milieu. While some of those interactions are inhibitory, others are activating and are collectively termed co-stimulatory receptors. The best studied belong to either the immunoglobulin superfamily or the tumor necrosis factor-receptor (TNFR) family. Co-stimulatory receptors include surface moieties that are constitutively expressed on resting lymphocytes such as CD28 or CD27 and others whose expression is induced upon recent previous antigen priming, ie, CD137, GITR, OX40, and ICOS. Ligation of these glycoproteins with agonist antibodies actively conveys activating signals to the lymphocyte. Those signals, acting through a potentiation of the cellular immune response, give rise to anti-tumor effects in mouse models. Anti-CD137 antibodies are undergoing clinical trials with evidence of clinical activity and anti-OX40 monoclonal antibodies (mAbs) induce interesting immunomodulation effects in humans. Antibodies anti-CD27 and GITR have recently entered clinical trials. The inherent dangers of these immunomodulation strategies are the precipitation of excessive systemic inflammation or/and invigorating silent autoimmunity. Agonist antibodies, recombinant forms of the natural ligands, and polynucleotide-based aptamers constitute the pharmacologic tools to manipulate such receptors. Preclinical data suggest that the greatest potential of these agents is achieved in combined treatment strategies.

All-Trans Retinoic Acid Improves the Effects of Bone Marrow-Derived Mesenchymal Stem Cells on the Treatment of Ankylosing Spondylitis: An In Vitro Study

Previous studies have demonstrated the immunosuppressive effects of both all-trans retinoic acid (ATRA) and mesenchymal stem cells (MSCs). The present study aimed to assess the immunoregulatory effects of ATRA on MSCs in the treatment of ankylosing spondylitis (AS). Bone marrow-derived MSCs from healthy donors were pretreated with ATRA and cocultured with CD3/28-activated peripheral blood mononuclear cells (PBMCs) derived from AS patients. Frequencies of Th17 and regulatory T (Treg) cells were analyzed using flow cytometry. The secretion and the mRNA level of key cytokines were measured with cytometric bead array and quantitative real-time PCR, respectively. ATRA pretreatment increased interleukin-6 (IL-6) secretion of MSCs. Th17 and Treg subset populations were increased and reduced by ATRA-pretreated MSCs, respectively. ATRA-pretreated MSCs significantly decreased not only the vital pathogenic cytokine in AS, tumor necrosis factor-α (TNF-α), but also AS-boosting factors interleukin-17 (IL-17A) and interferon-γ (IFN-γ). These results indicated that IL-6 may be a potential protective factor in AS and highlighted the promising role of ATRA in improving the efficacy of MSC-based treatment of AS.

Seeing through the dark: New insights into the immune regulatory functions of vitamin A

The importance of vitamin A for host defense is undeniable and the study of its mechanisms is paramount. Of the estimated 250 million preschool children who are vitamin A-deficient (VAD), 10% will die from their increased susceptibility to infectious disease. Vitamin A supplementation was established in the 1980s as one of the most successful interventions in the developing world. Understanding how vitamin A controls immunity will help curb the mortality and morbidity associated with vitamin A deficiency and exploit the immune-enhancing capacity of vitamin A to heighten host resistance to infectious disease. The discoveries that retinoic acid (RA) imprints the homing of leukocytes to the gut and enhances the induction of regulatory T cells, highlighted a potential role for RA in mucosal tolerance. However, more recently emerging data tell of a more profound systemic impact of RA on leukocyte function and commitment. In animal models using genetic manipulation of RA signaling, we learned when and how RA controls T cell fate. Here, we review the role for RA as a critical checkpoint regulator in the differentiation of CD4(+) T cells within the immune system.

CCR9 antagonism: potential in the treatment of Inflammatory Bowel Disease

Inflammatory Bowel Disease (IBD), mainly comprising Crohn’s disease (CD) and ulcerative colitis (UC), is a chronic condition that primarily affects the intestine and is characterized by leukocytic infiltration. Blocking the migration of leukocytes from the circulation is therefore a reasonable therapeutic goal. Recent clinical trials using this approach have shown promise, with the monoclonal antibody to α₄β₇ integrin, vedolizumab, and previously with the monoclonal antibody to the α₄ subunit, natalizumab. Directly targeting the subset of α₄β₇ expressing cells that co-express CC chemokine receptor 9 (CCR9), using the orally administered antagonist, CCX282-B, also known as vercirnon, has also been evaluated in Phase II and III trials that have produced mixed results. Although CCX282-B showed efficacy in inducing response in active CD in early studies, this was not confirmed in a Phase III study. CCX282-B was also more effective than placebo in maintaining remission, and this result has yet to be confirmed in Phase III. The efficacy of blocking CCR9 in UC, where vedolizumab was effective, has not been tested. The prospect of targeting CCR9 in IBD remains attractive. Much of the local accumulation of inflammatory cells in the intestine arises from migration rather than local proliferation and genetic and pharmacological targeting of CCR9 or its ligand in preclinical models that mimic UC and CD ameliorate inflammation in some cases. Furthermore, binding of chemokine ligands to receptor is a critical step in activating integrin binding, so there is a potential for synergistic action between integrin and chemokine antagonists. CCR9 is expressed on a smaller proportion of circulating cells than α₄β₇ integrin, which may offer greater specificity of effect, particularly in long term use. Furthermore, while α₄β₇ is widely expressed on T and B cell subsets, CCR9 is mainly expressed on effector memory Th1 cells. Indications for the use of intestine-specific integrin and chemokine receptor targeting may also extend beyond IBD, to include, for example, postoperative ileus, and primary sclerosing cholangitis.

All-trans retinoid acid promotes allogeneic corneal graft survival in mice by regulating Treg-Th17 balance in the presence of TGF-β

BACKGROUND

All-trans retinoid acid (ATRA) has been proven to skew Regulatory T cell-T helper 17 cell (Treg-Th17) balance toward Treg in vitro, favoring graft acceptance. However, its in vivo effect after solid organ transplantation is under investigation.

RESULTS

BALB/c mice were given orthotopic corneal grafts from C57BL/6 donors, and recipient mice were administered with ATRA, TGF-β, and the combination of both agents for 8 weeks after surgery. We found that a mixed treatment of ATRA and TGF-β significantly promoted graft survival. Moreover, with the presence of TGF-β, ATRA upregulated CD4(+)CD25(+)Foxp3(+)Treg cells and suppressed Th17 cells in the blood, spleen and draining lymph nodes of recipient mice, as well as enhanced the Foxp3 expression and inhibited the RORγt expression in grafts and peripheral blood mononuclear cells (PBMCs). Simultaneously, increased number of Foxp3+ cells and decreased number of IL-17+ cells in conjunctiva were found in recipients with mixed treatment, along with reduced IL-17 level in serum and aqueous humor and increased IL-10 level in aqueous humor. Tregs isolated from recipient mice treated with ATRA + TGF-β presented the strongest suppressive activity in vitro.

CONCLUSIONS

Combined application of ATRA and TGF-β may shift the Th17-Treg balance toward Tregs, hence facilitating the induction of immunological tolerance after allogenic corneal transplantation and representing a potential therapeutic approach in the treatment of posttransplant rejection.

Leukocyte homing, fate, and function are controlled by retinoic acid

Although vitamin A was recognized as an "anti-infective vitamin" over 90 years ago, the mechanism of how vitamin A regulates immunity is only beginning to be understood. Early studies which focused on the immune responses in vitamin A-deficient (VAD) animals clearly demonstrated compromised immunity and consequently increased susceptibility to infectious disease. The active form of vitamin A, retinoic acid (RA), has been shown to have a profound impact on the homing and differentiation of leukocytes. Both pharmacological and genetic approaches have been applied to the understanding of how RA regulates the development and differentiation of various immune cell subsets, and how RA influences the development of immunity versus tolerance. These studies clearly show that RA profoundly impacts on cell- and humoral-mediated immunity. In this review, the early findings on the complex relationship between VAD and immunity are discussed as well as vitamin A metabolism and signaling within hematopoietic cells. Particular attention is focused on how RA impacts on T-cell lineage commitment and plasticity in various diseases.

Diet and host-microbial crosstalk in postnatal intestinal immune homeostasis

Neonates face unique challenges in the period following birth. The postnatal immune system is in the early stages of development and has a range of functional capabilities that are distinct from the mature adult immune system. Bidirectional immune-microbial interactions regulate the development of mucosal immunity and alter the composition of the microbiota, which contributes to overall host well-being. In the past few years, nutrition has been highlighted as a third element in this interaction that governs host health by modulating microbial composition and the function of the immune system. Dietary changes and imbalances can disturb the immune-microbiota homeostasis, which might alter susceptibility to several autoimmune and metabolic diseases. Major changes in cultural traditions, socioeconomic status and agriculture are affecting the nutritional status of humans worldwide, which is altering core intestinal microbial communities. This phenomenon is especially relevant to the neonatal and paediatric populations, in which the microbiota and immune system are extremely sensitive to dietary influences. In this Review, we discuss the current state of knowledge regarding early-life nutrition, its effects on the microbiota and the consequences of diet-induced perturbation of the structure of the microbial community on mucosal immunity and disease susceptibility.

T cell-associated α4β7 but not α4β1 integrin is required for the induction and perpetuation of chronic colitis

Anti-adhesion therapies that target α(4) integrins (e.g., natalizumab) are thought to work by blocking T-cell recruitment to the intestinal tissues in patients with Crohn's disease (CD); however, little direct evidence is available to confirm this contention. We wished to evaluate the importance of T cell-associated α(4) integrins in a chronic colitis model in mice and to determine the effect of natalizumab treatment on intestinal tissue T-cell accumulation in human CD. Adoptive transfer of T cells lacking α(4) (α(4)(-/-)) but not β(1) integrin into immunodeficient mice produced significantly attenuated disease. This was correlated with reduced numbers of colon CD4 T cells compared with the control mice; however, tissue distribution of T helper type 1 (Th1) and T helper type 17 (Th17) cells and regulatory T cells (Tregs) was not affected by the lack of α(4). Furthermore, α(4)(-/-) T cells demonstrated defective homing to the chronically inflamed small intestines and colons. Finally, patients treated with natalizumab showed significant reduction in mucosal CD4 T cells and no skewing in the foxp3(+) Treg or T-bet(+)Th1 fractions thereof. These results demonstrate a direct role for T cell-associated α(4)β(7) but not α(4)β(1) integrins during initiation and perpetuation of chronic colitis. Moreover, our data demonstrated that natalizumab treatment reduced mucosal CD4 T-cell accumulation in CD patients.

An evolving perspective about the origins of childhood undernutrition and nutritional interventions that includes the gut microbiome

The Sackler Institute for Nutrition Science and the World Health Organization (WHO) have worked together to formulate a research agenda for nutrition science. Undernutrition of children has profound effects on health, development, and achievement of full human capacity. Undernutrition is not simply caused by a lack of food, but results from a complex interplay of intra- and intergenerational factors. Representative preclinical models and comprehensive well-controlled longitudinal clinical studies are needed to further understand the contributions and the interrelationships among these factors and to develop interventions that are effective and durable. This paper summarizes work on mechanisms underlying the varied manifestations of childhood undernutrition and discusses current gaps in knowledge and challenges to our understanding of undernutrition and infection/immunity throughout the human life cycle, focusing on early childhood growth. It proposes a series of basic and clinical studies to address this global health challenge.