Regulation of intestinal homeostasis by dendritic cells

Polysaccharide from Atractylodes macrocephala Koidz Binding with Zinc Oxide Nanoparticles as a Novel Mucosal Immune Adjuvant for H9N2 Inactivated Vaccine

H9N2 avian influenza poses a significant public health risk, necessitating effective vaccines for mass immunization. Oral inactivated vaccines offer advantages like the ease of administration, but their efficacy often requires enhancement through mucosal adjuvants. In a previous study, we established a novel complex of polysaccharide from Atractylodes macrocephala Koidz binding with zinc oxide nanoparticles (AMP-ZnONPs) and preliminarily demonstrated its immune-enhancing function. This work aimed to evaluate the efficacy of AMP-ZnONPs as adjuvants in an oral H9N2-inactivated vaccine and the vaccine's impact on intestinal mucosal immunity. In this study, mice were orally vaccinated on days 0 and 14 after adapting to the environment. AMP-ZnONPs significantly improved HI titers, the levels of specific IgG, IgG1 and IgG2a in serum and sIgA in intestinal lavage fluid; increased the number of B-1 and B-2 cells and dendritic cell populations; and enhanced the mRNA expression of intestinal homing factors and immune-related cytokines. Interestingly, AMP-ZnONPs were more likely to affect B-1 cells than B-2 cells. AMP-ZnONPs showed mucosal immune enhancement that was comparable to positive control (cholera toxin, CT), but not to the side effect of weight loss caused by CT. Compared to the whole-inactivated H9N2 virus (WIV) group, the WIV + AMP-ZnONP and WIV + CT groups exhibited opposite shifts in gut microbial abundance. AMP-ZnONPs serve as an effective and safe mucosal adjuvant for oral WIV, improving cellular, humoral and mucosal immunity and microbiota in the gastrointestinal tract, avoiding the related undesired effects of CT.

The impact of sensory neuropathy and inflammation on epithelial wound healing in diabetic corneas

Diabetic peripheral neuropathy (DPN) is the most common complication of diabetes, with several underlying pathophysiological mechanisms, some of which are still uncertain. The cornea is an avascular tissue and sensitive to hyperglycemia, resulting in several diabetic corneal complications including delayed epithelial wound healing, recurrent erosions, neuropathy, loss of sensitivity, and tear film changes. The manifestation of DPN in the cornea is referred to as diabetic neurotrophic keratopathy (DNK). Recent studies have revealed that disturbed epithelial-neural-immune cell interactions are a major cause of DNK. The epithelium is supplied by a dense network of sensory nerve endings and dendritic cell processes, and it secretes growth/neurotrophic factors and cytokines to nourish these neighboring cells. In turn, sensory nerve endings release neuropeptides to suppress inflammation and promote epithelial wound healing, while resident immune cells provide neurotrophic and growth factors to support neuronal and epithelial cells, respectively. Diabetes greatly perturbs these interdependencies, resulting in suppressed epithelial proliferation, sensory neuropathy, and a decreased density of dendritic cells. Clinically, this results in a markedly delayed wound healing and impaired sensory nerve regeneration in response to insult and injury. Current treatments for DPN and DNK largely focus on managing the severe complications of the disease. Cell-based therapies hold promise for providing more effective treatment for diabetic keratopathy and corneal ulcers.

A Comprehensive Review and Update on the Pathogenesis of Inflammatory Bowel Disease

Inflammatory bowel disease (IBD) is a chronic and life-threating inflammatory disease of gastroenteric tissue characterized by episodes of intestinal inflammation. The pathogenesis of IBD is complex. Recent studies have greatly improved our knowledge of the pathophysiology of IBD, leading to great advances in the treatment as well as diagnosis of IBD. In this review, we have systemically reviewed the pathogenesis of IBD and highlighted recent advances in host genetic factors, gut microbiota, and environmental factors and, especially, in abnormal innate and adaptive immune responses and their interactions, which may hold the keys to identify novel predictive or prognostic biomarkers and develop new therapies.

Polysaccharide A-Dependent Opposing Effects of Mucosal and Systemic Exposures to Human Gut Commensal Bacteroides fragilis in Type 1 Diabetes

Bacteroides fragilis (BF) is an integral component of the human colonic commensal microbiota. BF is also the most commonly isolated organism from clinical cases of intra-abdominal abscesses, suggesting its potential to induce proinflammatory responses upon accessing the systemic compartment. Hence, we examined the impact of mucosal and systemic exposures to BF on type 1 diabetes (T1D) incidence in NOD mice. The impact of intestinal exposure to BF under a chemically induced enhanced gut permeability condition, which permits microbial translocation, in T1D was also examined. While oral administration of heat-killed (HK) BF to prediabetic mice caused enhanced immune regulation and suppression of autoimmunity, resulting in delayed hyperglycemia, mice that received HK BF by intravenous injection showed rapid disease progression. Importantly, polysaccharide A-deficient BF failed to produce these opposing effects upon oral and systemic deliveries. Furthermore, BF-induced modulation of disease progression was observed in wild-type, but not TLR2-deficient, NOD mice. Interestingly, oral administration of BF under enhanced gut permeability conditions resulted in accelerated disease progression and rapid onset of hyperglycemia in NOD mice. Overall, these observations suggest that BF-like gut commensals can cause proinflammatory responses upon gaining access to the systemic compartment and contribute to T1D in at-risk subjects.

Regulation of IgA Production by Intestinal Dendritic Cells and Related Cells

The intestinal mucosa is a physiological barrier for most microbes, including both commensal bacteria and invading pathogens. Under homeostatic conditions, immunoglobulin A (IgA) is the major immunoglobulin isotype in the intestinal mucosa. Microbes stimulate the production of IgA, which controls bacterial translocation and neutralizes bacterial toxins at the intestinal mucosal surface. In the intestinal mucosa, dendritic cells (DCs), specialized antigen-presenting cells, regulate both T-cell-dependent (TD) and -independent (TI) immune responses. The intestinal DCs are a heterogeneous population that includes unique subsets that induce IgA synthesis in B cells. The characteristics of intestinal DCs are strongly influenced by the microenvironment, including the presence of commensal bacterial metabolites and epithelial cell-derived soluble factors. In this review, we summarize the ontogeny, classification, and function of intestinal DCs and how the intestinal microenvironment conditions DCs and their precursors to become the mucosal phenotype, in particular to regulate IgA production, after they arrive at the intestine. Understanding the mechanism of IgA synthesis could provide insights for designing effective mucosal vaccines.

CCR7-deficient mice exhibit a delayed antigen-specific mucosal IgA antibody response to an oral recombinant Salmonella strain

The migration of antigen (Ag)-loading dendritic cells (DCs) from Peyer's patches (PPs) to the draining mesenteric lymph nodes (MLNs) via chemokine receptor 7 (CCR7) is thought to be an important step in the initiation of acquired immunity. Our previous study showed that PPs were indispensable for Ag-specific secretory (S)IgA antibody (Ab) responses against oral recombinant Salmonella (rSalmonella). In this study, we attempted to show direct PP DC migration to MLNs by employing photoconvertible protein transgenic mice and investigated the role of the CCR7 signaling pathway in mucosal IgA induction. Our results demonstrated an actual flux of DCs from PPs to MLNs. The frequency of CCR7+ CD11c+ DCs in MLNs of PP-deficient mice was reduced, suggesting that some PP DCs migrated via CCR7. Immunization of CCR7−/− mice elicited significantly lower levels of Ag-specific SIgA Ab responses, which was associated with diminished formation of the germinal center in PPs. However, increased SIgA Ab production and dissemination of rSalmonella were observed at later time points. These results suggest that, although CCR7 was required for SIgA induction at normal velocity, the CCR7-mediated pathway is not essential for the induction of Ag-specific SIgA Ab responses to rSalmonella.

Protective effect of TSLP and IL-33 cytokines in ulcerative colitis

PURPOSE

Inflammatory bowel disease (IBD) primarily includes ulcerative colitis (UC) and Crohn's disease (CD). Thymic stromal lymphopoietin (TSLP) is a cytokine produced by intestinal epithelial cells (IECs) with immunomodulatory properties that plays an important role in the development of regulatory T cell (Treg) responses and tolerance in the gut. On the other hand, IL-33 has been considered as a cytokine with two different properties, inflammatory and anti-inflammatory functions, the latter may play a protective role against chronic intestinal inflammation. In the present study, we investigated the relative gene expression levels of TSLP and IL-33 molecules in ulcerative colitis.

METHODS

Patients with clinical symptoms of colitis undergoing a routine diagnostic colonoscopy were included in this study. Biopsy specimens were collected and divided into two parts. One part was fixed and processed for routine histopathological examinations and the other part was stored for RNA extraction. TSLP and IL-33 gene expression were determined using the SYBR Green qRT-PCR.

RESULTS

The expression level of TSLP and IL-33 were significantly lower in UC patients compared with the control group. Moreover, the expressions of these cytokines were more down-regulated in severe UC patients compared with mild and moderate ones and the control group. We also showed a positive correlation between low expression of TSLP and IL-33 and the severity of UC disease.

CONCLUSIONS

In this study, we showed decreased mRNA expression levels of TSLP and IL-33 in UC patients and also a negative correlation between expression of TSLP and IL-33 and severity of UC disease.

Candida albicans infection and intestinal immunity

Fungal infections cause high rates of morbidity and mortality in intensive care and immunocompromised patients, and can represent a life-threatening disease. As a microorganism commonly found in the intestine, Candida albicans (C. albicans) can invade the gut epithelium barrier via microfold cells and enter the bloodstream. The defensive potential of the intestinal barrier against invasive C. albicans is dependent on innate and adaptive immune responses which enable the host to eliminate pathogenic fungi. The lamina propria layer of the intestine contains numerous immune cells capable of inducing an innate cellular immune response against invasive fungi. This review focuses on the immune response triggered by a C. albicans infection in the intestine.

In situ proximity of CX3CR1-positive mononuclear phagocytes and VIP-ergic nerve fibers suggests VIP-ergic immunomodulation in the mouse ileum

Being continuously exposed to a plethora of antigens ranging from food antigens to potential pathogenic organisms, the gastrointestinal (GI) tract harbors the largest collection of immune cells in the mammalian body. This immune system has to maintain a delicate balance between mounting an active immune response and maintaining tolerance. The GI tract is also home to an elaborate intrinsic nervous system, the enteric nervous system (ENS). Various in vitro studies of neuro-immune communication have suggested that vasoactive intestinal peptide (VIP), an important GI neurotransmitter, modulates mononuclear phagocytes (MNPs), i.e., dendritic cells and macrophages. Using a combined approach of reverse transcription plus the polymerase chain reaction, immunofluorescence, three-dimensional maximum intensity projections and immunoelectron microscopy, we investigate the interaction between the enteric innervation and MNPs in the ileal lamina propria (LP). We demonstrate that VIP-ergic fibers of the ENS lie adjacent to CX3CR1⁺ MNPs and that VPAC1 is constitutively expressed on ileal CX3CR1+ cells in the LP of the mouse. We also identify, for the first time, CX3CR1⁺ immune cells in the LP at the ultrastructural level. Our data thus reveal the in situ presence of the molecular components that are necessary for a VIP-mediated neuro-immune interaction between the ENS and CX3CR1-expressing immune cells in the LP of the ileum.

Intraepithelial dendritic cells and sensory nerves are structurally associated and functional interdependent in the cornea

The corneal epithelium consists of stratified epithelial cells, sparsely interspersed with dendritic cells (DCs) and a dense layer of sensory axons. We sought to assess the structural and functional correlation of DCs and sensory nerves. Two morphologically different DCs, dendriform and round-shaped, were detected in the corneal epithelium. The dendriform DCs were located at the sub-basal space where the nerve plexus resides, with DC dendrites crossing several nerve endings. The round-shaped DCs were closely associated with nerve fiber branching points, penetrating the basement membrane and reaching into the stroma. Phenotypically, the round-shaped DCs were CD86 positive. Trigeminal denervation resulted in epithelial defects with or without total tarsorrhaphy, decreased tear secretion, and the loss of dendriform DCs at the ocular surface. Local DC depletion resulted in a significant decrease in corneal sensitivity, an increase in epithelial defects, and a reduced density of nerve endings at the center of the cornea. Post-wound nerve regeneration was also delayed in the DC-depleted corneas. Taken together, our data show that DCs and sensory nerves are located in close proximity. DCs may play a role in epithelium innervation by accompanying the sensory nerve fibers in crossing the basement membrane and branching into nerve endings.

Immunoregulatory Role of Myeloid-derived Cells in Inflammatory Bowel Disease

As the frontiers of immunological research expand, new insights into the pathogenesis of long poorly understood diseases, such as inflammatory bowel disease (IBD), are opening up new possible avenues for treatment. Myeloid-derived cells (i.e., monocytes, macrophages, neutrophils, and dendritic cells), long believed to be effector cells driving the initiation of inflammation, have been increasingly shown to have immunoregulatory effects previously underappreciated. Dysfunction in the immunoregulatory roles of these cells may play a part in the pathogenesis of a subset of patients with IBD. The role of myeloid-derived suppressor cells, initially described in cancer, have been shown to play an important role in the balancing of effector and regulatory T cells in inflammation as well, and their role in IBD is also explored. The potential for future cell-based therapies for IBD is enhanced by the advances being made in the understanding of the innate immune system in the intestine.

Effect of schistosomiasis on CX3CR1-expressing mononuclear phagocytes in the ileum and mesenteric lymph nodes of the mouse

BACKGROUND

Intestinal dendritic cells (DCs) maintain immune homeostasis, only initiating an active immune response against invading pathogens. However, little information is available on the reaction of mononuclear phagocytes (MNP) to intestinal trematode infection, a reaction equally important in helminth-based therapies. The CD11c(+)  CX3CR1(+)  F4/80(-) DCs in the ileal lamina propria (LP) of the mouse were proven to migrate to the mesenteric lymph nodes (MLNs). We analyzed all MNP subsets present in the mouse LP and MLNs, under steady-state conditions and during acute Schistosoma mansoni-induced inflammation. Furthermore, we studied the uptake of schistosomal antigens by MNP in vivo in the LP and MLNs.

METHODS

Using a combination of immunohistochemistry and multiparametric flow cytometry, we investigated distributional changes of the MNP during acute intestinal schistosomiasis. Next, S. mansoni-derived products, i.e., S. mansoni soluble worm proteins (SmSWP) and S. mansoni soluble egg antigens (SmSEA) were intraperitoneally injected into CX3CR1(+/) (GFP) C57BL/6 mice and antigen uptake was analyzed using confocal microscopy.

KEY RESULTS

The CD11c(+)  CX3CR1(+)  F4/80(-) DCs significantly increased during intestinal schistosomiasis in the LP and MLNs. Only CX3CR1-expressing DC and MФ subsets, but not other LP DCs, are involved in both SmSWP and SmSEA antigen uptake and processing.

CONCLUSIONS & INFERENCES

The significant upregulation of CD11c(+)  CX3CR1(+)  F4/80(-) DCs during intestinal schistosomiasis and the restriction of phagocytosis of parasitic antigens to CX3CR1-expresssing MNP indicate a crucial role for this immune cell niche in response to trematodiasis. These findings add insight into the functional specialization of LP immune cells and add to the understanding of cellular mechanisms behind helminth-based therapies.

Oral antigen exposure in newborn piglets circumvents induction of oral tolerance in response to intraperitoneal vaccination in later life

Background

We previously determined that newborn piglets orally gavaged with Ovalbumin (OVA) responded to systemic OVA re-exposure with tolerance; if adjuvants were included in oral vaccine, piglets responded with antibody-mediated immunity (Vet Immunol Immunopathol 161(3–4):211–21, 2014). Here, we will investigate whether newborn piglets gavaged with a vaccine comprised of OVA plus unmethylated CpG oligodeoxynucleotides (CpG; soluble component; OVA/CpG) combined with OVA plus CpG encapsulated within polyphosphazene microparticles (MP; particulate component) responded with systemic and mucosal immunity. To monitor the response to systemic antigen re-exposure, piglets were i.p.-immunized with OVA plus Incomplete Freund’s Adjuvant (IFA) one month later.

Results

Newborn piglets (n = 5/group) were gavaged with a combined soluble and particulate vaccine consisting of OVA (0.5-0.05 mg) plus 50 μg CpG and 0.5 mg OVA plus 50 μg CpG encapsulated within a polyphosphazene MP (0.5 mg) referred to as OVA/CpG + MP. Control piglets were gavaged with saline alone. Piglets were i.p. immunized with 10 mg OVA (or saline) in IFA at four weeks of age and then euthanized at eight weeks of age. We observed significantly higher titres of serum anti-OVA immunoglobulin (Ig) IgM, IgA, IgG, IgG1, IgG2 and IgG in piglets immunized with 0.05 mg OVA/CpG + MP relative to saline control animals. Thus, a single oral exposure at birth to a combined soluble and particulate OVA vaccine including adjuvants can circumvent induction of oral tolerance which impacts response to i.p. vaccination in later life. Further, piglets gavaged with 0.05 mg OVA/CpG + MP generated significant anti-OVA IgG and IgG1 titres in lung compared to saline control piglets but results were comparable to titres measured in parenteral control piglets. Peripheral blood mononuclear cells (PBMCs) ex vivo-stimulated with OVA showed markedly decreased production of IL-10 cytokine after 72 hours relative to animal-matched cells incubated with media alone. No production of IFN-γ was observed from any groups.

Conclusion

Newborn piglets gavaged with low dose soluble and particulate OVA plus CpG ODN and polyphosphazene adjuvants produced antigen-specific antibodies in serum and lung after systemic re-exposure in later life. These data indicate circumvention of oral tolerance but not induction of oral immunity.

Natural killer T cells play a necessary role in modulating of immune-mediated liver injury by gut microbiota

Gut microbiota are implicated in many liver diseases. Concanavalin A (ConA)-induced hepatitis is a well-characterized murine model of fulminant immunological hepatic injury. Oral administration of pathogenic bacteria or gentamycin to the mice before ConA injection, liver injury and lymphocyte distribution in liver and intestine were assessed. Our data show that administration of pathogenic bacteria exacerbated the liver damage. There was more downregulation of activation-induced natural killer T (NKT) cells in the liver of pathogenic bacteria-treated ConA groups. Also, there was a negative correlation between the numbers of hepatic NKT cells and liver injury in our experiments. Moreover, intestinal dendritic cells (DCs) were increased in pathogenic bacteria-treated ConA groups. The activation of DCs in Peyer's patches and the liver was similar to the intestine. However, depletion of gut gram-negative bacteria alleviated ConA-induced liver injury, through suppressed hepatic NKT cells activation and DCs homing in liver and intestine. In vitro experiments revealed that DCs promoted NKT cell cytotoxicity against hepatocyte following stimulation with pathogenic bacteria. Our study suggests that increased intestinal pathogenic bacteria facilitate immune-mediated liver injury, which may be due to the activation of NKT cells that mediated by intestinal bacterial antigens activated DCs.

Linking immunity, epigenetics, and cancer in inflammatory bowel disease

Most of what is known about the pathogenesis of inflammatory bowel disease (IBD) pertains to complex interplay between host genetics, immunity, and environmental factors. Epigenetic modifications play pivotal roles in intestinal immunity and mucosal homeostasis as well as mediating gene-environment interactions. In this article, we provide a historical account of epigenetic research either directly related or pertinent to the pathogenesis and management of IBD. We further collate emerging evidence supporting roles for epigenetic mechanisms in relevant aspects of IBD biology, including deregulated immunity, host-pathogen recognition and mucosal integrity. Finally, we highlight key epigenetic mechanisms that link chronic inflammation to specific IBD comorbidities, including colitis-associated cancer and discuss their potential utility as novel biomarkers or pharmacologic targets in IBD therapy.

Eosinophils promote generation and maintenance of immunoglobulin-A-expressing plasma cells and contribute to gut immune homeostasis

Although in normal lamina propria (LP) large numbers of eosinophils are present, little is known about their role in mucosal immunity at steady state. Here we show that eosinophils are needed to maintain immune homeostasis in gut-associated tissues. By using eosinophil-deficient ΔdblGATA-1 and PHIL mice or an eosinophil-specific depletion model, we found a reduction in immunoglobulin A(+) (IgA(+)) plasma cell numbers and in secreted IgA. Eosinophil-deficient mice also showed defects in the intestinal mucous shield and alterations in microbiota composition in the gut lumen. In addition, TGF-β-dependent events including class switching to IgA in Peyer's patches (PP), the formation of CD103(+) T cells including Foxp3(+) regulatory (Treg), and also CD103(+) dendritic cells were disturbed. In vitro cultures showed that eosinophils produce factors that promote T-independent IgA class switching. Our findings show that eosinophils are important players for immune homeostasis in gut-associated tissues and add to data suggesting that eosinophils can promote tissue integrity.

Intestinal dendritic cell and macrophage subsets: Tipping the balance to Crohn's disease?

Dendritic cells and macrophages play an essential role in immune homeostasis in the intestine. They have the critical task of maintaining the balance between tolerance to the intestinal microflora and potential food antigens while retaining the ability to initiate immunity against pathogens. For patients with Crohn's Disease, the tolerance/immunity balance is disturbed and these individuals suffer from chronic intestinal inflammation driven by aberrant T cell reactivity to intestinal bacteria. As antigen presenting cells are required for T cell activation, intestinal phagocytes with the capacity to present antigens from intestinal bacteria to T cells are likely involved in initiating and propagating Crohn's Disease. Recent data describe unique subsets of human intestinal phagocytes that may be involved in the aberrant reactivity to commensal flora that drives Crohn's Disease pathogenesis. This review summarizes the current knowledge of phagocyte subsets in the intestine and mesenteric lymph nodes in healthy individuals and Crohn's Disease patients. Deciphering the function of intestinal phagocytes in health and disease is crucial to advance our understanding of the cellular mechanisms underlying this debilitating disease, provides a potential way to improve treatment for patients with inflammatory bowel disease.

Mucosal co-immunization with AIM2 enhances protective SIgA response and increases prophylactic efficacy of chitosan-DNA vaccine against coxsackievirus B3-induced myocarditis

Coxsackievirus B3 (CVB3) infection is considered as the most common cause of viral myocarditis with no available vaccine. Considering that CVB3 mainly invades through the gastrointestinal mucosa, the development of CVB3-specific mucosal vaccine, which is the most efficient way to induce mucosal immune responses, gains more and more attention. In this study, we used absent in melanoma 2 (AIM2) as a mucosal adjuvant to enhance the immunogenicity and immunoprotection of CVB3-specific chitosan-pVP1 vaccine. Mice were intranasally co-immunized with 50 μg chitosan-pAIM2 and equal amount of chitosan-pVP1 vaccine 4 times at 2 week-intervals, and then challenged with CVB3 2 weeks after the last immunization. Compared with chitosan-pVP1 vaccine immunization alone, chitosan-pAIM2 co-immunization enhanced resistance to CVB3-induced myocarditis evidenced by significantly enhanced ejection fractions from 55.40 ± 9.35 to 80.31 ± 11.35, improved myocarditis scores from 1.50 ± 0.45 to 0.30 ± 0.15, reduced viral load from 3.33 ± 0.50 to 0.50 ± 0.65, and increased survival rate from 40.0% to 75.5%. This increased immunoprotection might be attributed to the augmented level of CVB3-specific fecal SIgA with high affinity and neutralizing ability. In addition, co-immunization with chitosan-pAIM2 remarkably facilitated dendritic cells (DCs) recruitment to mesenteric lymph nodes (MLN), and promoted the expression of IgA-inducing factors (BAFF, APRIL, iNOS, RALDH1, IL-6, TGF-β), which might account for its mucosal adjuvant effect. This strategy may represent a promising prophylactic vaccine against CVB3-induced myocarditis.

Granulocyte macrophage colony-stimulating factor and the intestinal innate immune cell homeostasis in Crohn's disease

Current literature consolidates the view of Crohn's disease (CD) as a form of immunodeficiency highlighting dysregulation of intestinal innate immunity in the pathogenesis of CD. Intestinal macrophages derived from blood monocytes play a key role in sustaining the innate immune homeostasis in the intestine, suggesting that the monocyte/macrophage compartment might be an attractive therapeutic target for the management of CD. Granulocyte macrophage colony-stimulating factor (GM-CSF) is a hematopoietic growth factor that also promotes myeloid cell activation, proliferation, and differentiation. GM-CSF has a protective effect in human CD and mouse models of colitis. However, the role of GM-CSF in immune and inflammatory reactions in the intestine is not well defined. Beneficial effects exerted by GM-CSF during intestinal inflammation could relate to modulation of the mucosal barrier function in the intestine, including epithelial cell proliferation, survival, restitution, and immunomodulatory actions. The aim of this review is to summarize potential mechanistic roles of GM-CSF in intestinal innate immune cell homeostasis and to highlight its central role in maintenance of the intestinal immune barrier in the context of immunodeficiency in CD.

Dendritic cell-based approaches for therapeutic immune regulation in solid-organ transplantation

To avoid immune rejection, allograft recipients require drug-based immunosuppression, which has significant toxicity. An emerging approach is adoptive transfer of immunoregulatory cells. While mature dendritic cells (DCs) present donor antigen to the immune system, triggering rejection, regulatory DCs interact with regulatory T cells to promote immune tolerance. Intravenous injection of immature DCs of either donor or host origin at the time of transplantation have prolonged allograft survival in solid-organ transplant models. DCs can be treated with pharmacological agents before injection, which may attenuate their maturation in vivo. Recent data suggest that injected immunosuppressive DCs may inhibit allograft rejection, not by themselves, but through conventional DCs of the host. Genetically engineered DCs have also been tested. Two clinical trials in type-1 diabetes and rheumatoid arthritis have been carried out, and other trials, including one trial in kidney transplantation, are in progress or are imminent.

Tolerogenic dendritic cells: key regulators of peripheral tolerance in health and disease

Dendritic cells (DCs) as professional antigen-presenting cells are able to induce immunity and tolerance in different conditions, which leads to strong immune responses against hostile agents or tolerance to self-antigens. Although for subsets of DCs, a regulatory function has been shown, the role of the microenvironment is momentous to generate tolerogenic DCs (tDCs). Different microorganisms and tumor cells escape from immune responses by producing mediators and components that influence DCs to show tolerogenic characteristics. In this review, tDC induction was explained in steady state and disease conditions. Moreover, an overview was presented on the efforts to generate in vitro tDCs, their clinical applications and the problems which remain to be solved in this field.

Potential of immunoglobulin A to prevent allergic asthma

Allergic asthma is characterized by bronchial hyperresponsiveness, a defective barrier function, and eosinophilic lower airway inflammation in response to allergens. The inflammation is dominated by Th2 cells and IgE molecules and supplemented with Th17 cells in severe asthma. In contrast, in healthy individuals, allergen-specific IgA and IgG4 molecules are found but no IgE, and their T cells fail to proliferate in response to allergens, probably because of the development of regulatory processes that actively suppress responses to allergens. The presence of allergen-specific secretory IgA has drawn little attention so far, although a few epidemiological studies point at a reverse association between IgA levels and the incidence of allergic airway disease. This review highlights the latest literature on the role of mucosal IgA in protection against allergic airway disease, the mechanisms described to induce secretory IgA, and the role of (mucosal) dendritic cells in this process. Finally, we discuss how this information can be used to translate into the development of new therapies for allergic diseases based on, or supplemented with, IgA boosting strategies.

The role of the vascular dendritic cell network in atherosclerosis

A complex role has been described for dendritic cells (DCs) in the potentiation and control of vascular inflammation and atherosclerosis. Resident vascular DCs are found in the intima of atherosclerosis-prone vascular regions exposed to disturbed blood flow patterns. Several phenotypically and functionally distinct vascular DC subsets have been described. The functional heterogeneity of these cells and their contributions to vascular homeostasis, inflammation, and atherosclerosis are only recently beginning to emerge. Here, we review the available literature, characterizing the origin and function of known vascular DC subsets and their important role contributing to the balance of immune activation and immune tolerance governing vascular homeostasis under healthy conditions. We then discuss how homeostatic DC functions are disrupted during atherogenesis, leading to atherosclerosis. The effectiveness of DC-based "atherosclerosis vaccine" therapies in the treatment of atherosclerosis is also reviewed. We further provide suggestions for distinguishing DCs from macrophages and discuss important future directions for the field.

The mucosal adjuvant cholera toxin B instructs non-mucosal dendritic cells to promote IgA production via retinoic acid and TGF-β

It is currently unknown how mucosal adjuvants cause induction of secretory immunoglobulin A (IgA), and how T cell-dependent (TD) or -independent (TI) pathways might be involved. Mucosal dendritic cells (DCs) are the primary antigen presenting cells driving TI IgA synthesis, by producing a proliferation-inducing ligand (APRIL), B cell activating factor (BAFF), Retinoic Acid (RA), TGF-β or nitric oxide (NO). We hypothesized that the mucosal adjuvant Cholera Toxin subunit B (CTB) could imprint non-mucosal DCs to induce IgA synthesis, and studied the mechanism of its induction. In vitro, CTB-treated bone marrow derived DCs primed for IgA production by B cells without the help of T cells, yet required co-signaling by different Toll-like receptor (TLR) ligands acting via the MyD88 pathway. CTB-DC induced IgA production was blocked in vitro or in vivo when RA receptor antagonist, TGF-β signaling inhibitor or neutralizing anti-TGF-β was added, demonstrating the involvement of RA and TGF-β in promoting IgA responses. There was no major involvement for BAFF, APRIL or NO. This study highlights that synergism between CTB and MyD88-dependent TLR signals selectively imprints a TI IgA-inducing capacity in non-mucosal DCs, explaining how CTB acts as an IgA promoting adjuvant.

Low dose antigen exposure for a finite period in newborn rats prevents induction of mucosal tolerance

Background

In adult rats, initial exposure to antigens by a mucosal route triggers tolerance such that any subsequent re-exposure, even by a systemic route, results in suppression of immunity. The newborn’s gut is semi-permeable for a finite period to allow maternal antibodies to enter the newborn’s circulation. We propose that antigens introduced in extreme early life can readily traverse the gut wall and therefore circumvent induction of mucosal tolerance.

Methodology/Principle Findings

Rat pups were gavaged with low-doses of ovalbumin (OVA; oral exposure group) or saline (parenteral control group) every second day for several weeks followed by an intraperitoneal (i.p.) injection at 1 month of age. When gavage was initiated the day after birth, newborn oral exposure pups responded with significantly higher anti-OVA IgA, IgM, IgG2a, and IgG1 titres in their serum and anti-OVA IgA, IgG2a and IgG1 titres in their lungs compared to negative control pups. Oral exposure alone failed to induce immunity. Pups exposed to the same treatment regimen starting at 14 days of age showed induction of mucosal tolerance after i.p. immunization. Newborn oral exposure groups subjected to secondary i.p. immunization responded with significantly increased humoral immunity in lung and sera suggesting that once antigen-specific mucosal tolerance if circumvented, it persists. Lymphocytes derived from mesenteric lymph node cells re-simulated with OVA ex vivo, from newborn oral exposure pups exposed to secondary immunization produced significantly higher IFN-γ expression and lymphocyte proliferation relative to control pups indicating prevention of tolerance in the cell-mediated immune system.

Conclusions/Significance

This work demonstrates that newborns may be uniquely qualified to prevent induction of mucosal tolerance to oral antigens. These results should be further explored to establish whether prevention of tolerance by early life oral vaccination can be exploited to prime for mucosal as well as systemic immunity and thus protect this susceptible population against infectious diseases.

The gut is the epicentre of antibiotic resistance

The gut contains very large numbers of bacteria. Changes in the composition of the gut flora, due in particular to antibiotics, can happen silently, leading to the selection of highly resistant bacteria and Candida species. These resistant organisms may remain for months in the gut of the carrier without causing any symptoms or translocate through the gut epithelium, induce healthcare-associated infections, undergo cross-transmission to other individuals, and cause limited outbreaks. Techniques are available to prevent, detect, and treat the carriage of resistant organisms in the gut. However, evidence on these techniques is scant, the only exception being selective digestive decontamination (SDD), which has been extensively studied in neutropenic and ICU patients. After the destruction of resistant colonizing bacteria, which has been successfully obtained in several studies, the gut could be re-colonized with normal faecal flora or probiotics. Studies are warranted to evaluate this concept.

Functional RNA delivery targeted to dendritic cells by synthetic nanoparticles

Dendritic cells (DCs) are essential to many aspects of immune defense development and regulation. They provide important targets for prophylactic and therapeutic delivery. While protein delivery has had considerable success, RNA delivery is still expanding. Delivering RNA molecules for RNAi has shown particular success and there are reports on successful delivery of mRNA. Central, therein, is the application of cationic entities. Following endocytosis of the delivery vehicle for the RNA, cationic entities should promote vesicular membrane perturbation, facilitating cytosolic release. The present review explains the diversity of DC function in immune response development and control. Promotion of delivered RNA cytosolic release is discussed, relating to immunoprophylactic and therapeutic potential, and DC endocytic machinery is reviewed, showing how DC endocytic pathways influence the handling of internalized material. The potential advantages for application of replicating RNA are presented and discussed, in consideration of their value and development in the near future.

Erythromycin treatment hinders the induction of oral tolerance to fed ovalbumin

The mucosal immune system is constantly exposed to antigen, whether it be food antigen, commensal bacteria, or harmful antigen. It is essential that the mucosal immune system can distinguish between harmful and non-harmful antigens, and initiate an active immune response to clear the harmful antigens, while initiating a suppressive immune response (tolerance) to non-harmful antigens. Oral tolerance is an immunologic hyporesponsiveness to an orally administered antigen and is important in preventing unnecessary gastrointestinal tract inflammation, which can result in a number of autoimmune and hypersensitivity diseases. Probiotics (beneficial intestinal bacteria), T regulatory cells, and dendritic cells (DCs) are all essential for generating tolerance. Antibiotics are commonly prescribed to fight infections and often necessary for maintaining health, but they can disrupt the normal intestinal probiotic populations. There is increasing epidemiologic evidence that suggests that antibiotic usage correlates with the development of atopic or irritable bowel disorders, which often result due to a breakdown in immune tolerance. This study investigated the effect of the antibiotic erythromycin on oral tolerance induction to ovalbumin. The results demonstrated that antibiotic treatment prior to exposure to fed antigen prevents tolerance to that antigen, which may be associated with a reduction in intestinal Lactobacillus populations. Furthermore, antibiotic treatment resulted in a significant decrease in the tolerogenic CD11c⁺/CD11b⁺/CD8α^- mesenteric lymph node DCs independent of tolerizing treatment. These results provide evidence that antibiotic treatment, potentially through its effects on tolerogenic DCs and intestinal microflora, may contribute to autoimmune and atopic disorders via a breakdown in tolerance and support prior epidemiologic studies correlating increased antibiotic usage with the development of these disorders.

Innate immune signaling in defense against intestinal microbes

The gastrointestinal system is a common entry point for pathogenic microbes to access the inner environment of the body. Anti-microbial factors produced by the intestinal mucosa limit the translocation of both commensal and pathogenic microbes across the intestinal epithelial cell barrier. The regulation of these host defense mechanisms largely depends on the activation of innate immune receptors by microbial molecules. Under steady-state conditions, the microbiota provides constitutive signals to the innate immune system, which helps to maintain a healthy inflammatory tone within the intestinal mucosa and, thus, enhances resistance to infection with enteric pathogens. During an acute infection, the intestinal epithelial cell barrier is breached, and the detection of microbial molecules in the intestinal lamina propria rapidly stimulates innate immune signaling pathways that coordinate early defense mechanisms. Herein, we review how microbial molecules shed by both commensal and pathogenic microbes direct host defenses at the intestinal mucosa. We highlight the signaling pathways, effector molecules, and cell populations that are activated by microbial molecule recognition and, thereby, are involved in the maintenance of homeostatic levels of host defense and in the early response to acute enteric infection. Finally, we discuss how manipulation of these host defense pathways by stimulating innate immune receptors is a potential therapeutic strategy to prevent or alleviate intestinal disease.

Innate-adaptive crosstalk: how dendritic cells shape immune responses in the CNS

Dendritic cells (DCs) are a heterogeneous group of professional antigen presenting cells that lie in a nexus between innate and adaptive immunity because they recognize and respond to danger signals and subsequently initiate and regulate effector T-cell responses. Initially thought to be absent from the CNS, both plasmacytoid and conventional DCs as well as DC precursors have recently been detected in several CNS compartments where they are seemingly poised for responding to injury and pathogens. Additionally, monocyte-derived DCs rapidly accumulate in the inflamed CNS where they, along with other DC subsets, may function to locally regulate effector T-cells and/or carry antigens to CNS-draining cervical lymph nodes. In this review we highlight recent research showing that (a) distinct inflammatory stimuli differentially recruit DC subsets to the CNS; (b) DC recruitment across the blood-brain barrier (BBB) is regulated by adhesion molecules, growth factors, and chemokines; and (c) DCs positively or negatively regulate immune responses in the CNS.

Innate immunity in allergic disease

The innate immune system consists of multiple cell types that express germline-encoded pattern recognition receptors that recognize pathogen-associated molecular patterns (PAMPs) or danger-associated molecular patterns (DAMPs). Allergens are frequently found in forms and mixtures that contain PAMPs and DAMPs. The innate immune system is interposed between the external environment and the internal acquired immune system. It is also an integral part of the airways, gut, and skin. These tissues face continuous exposure to allergens, PAMPs, and DAMPs. Interaction of allergens with the innate immune system normally results in immune tolerance but, in the case of allergic disease, this interaction induces recurring and/or chronic inflammation as well as the loss of immunologic tolerance. Upon activation by allergens, the innate immune response commits the acquired immune response to a variety of outcomes mediated by distinct T-cell subsets, such as T-helper 2, regulatory T, or T-helper 17 cells. New studies highlighted in this review underscore the close relationship between allergens, the innate immune system, and the acquired immune system that promotes homeostasis versus allergic disease.

Dendritic cell-epithelium interplay is a determinant factor for corneal epithelial wound repair

The functions of intraepithelial dendritic cells (DCs) are critical for mucosal innate and adaptive immunity, but little is known about the role of tissue-specific DCs in epithelial homeostasis and tissue repair. By using the epithelial debridement wound model and CD11c-diphtheria toxin receptor mice that express a CD11c promoter-driven diphtheria toxin receptor, we showed that DCs migrate along with the epithelial sheet to cover the wound and that local depletion of DCs resulted in a significant delay in epithelial wound closure. In response to wounding, migratory epithelia produce CXCL10, thymic stromal lymphopoietin, and IL-1β and its antagonist soluble IL-1 receptor antagonist (sIL-1Ra); depletion of corneal DCs reversed their elevated expressions to a different extent, suggesting a DC-mediated positive feedback loop in epithelial gene expression. Furthermore, both CXCL10 and thymic stromal lymphopoietin were localized in migratory epithelia, suggesting that epithelial cells play a key role in DC infiltration and activation in injured corneas. On the other hand, DC depletion resulted in suppressed epithelial AKT activation, increased cell apoptosis, and decreased polymorphonuclear leukocyte infiltration in the healing cornea. These results indicate that DCs and epithelium form a functional entity at mucosal surfaces for maintaining corneal homeostasis and for tissue repair.

GM-CSF: a role in immune and inflammatory reactions in the intestine

Granulocyte macrophage colony-stimulating factor (GM-CSF) is a cytokine that promotes myeloid cell development and maturation, and dendritic cell differentiation and survival in vitro. Growing evidence supports the notion that GM-CSF has a major role in some inflammatory and autoimmune reactions and in the host's response to pulmonary infection, but few studies have addressed its functions and importance in the GI tract. Recent studies demonstrated that administration of GM-CSF can result in clinical improvement in patients with Crohn's disease. Mice deficient in GM-CSF (GM-CSF(-/-) ) developed more severe intestinal and systemic infection after an enteric infection, and more severe colitis in response to enteric exposure to dextran sodium sulfate. Both the severity of infection and colitis were largely prevented by GM-CSF administration. Such studies indicate that GM-CSF has an important role in the regulation of intestinal immune and inflammatory responses.

Mucosal dendritic cell diversity in the gastrointestinal tract

The discovery of dendritic cells (DCs) in skin by Paul Langerhans in 1868 identified a cell type which has since been recognized as a key link between innate and adaptive immunity. DCs originate from bone marrow and disseminate through blood to all tissues in the body, and distinct DC subpopulations have been identified in many different tissues. DC diversity is apparent throughout all mucosal surfaces of the body, but the focus of this review article is DC diversity throughout the gastro-intestinal tract (GIT). DC subpopulations have been well characterized in the oral cavity and small intestine, but DC characterization in other regions, such as the esophagus and stomach, is limited. Substantial research has focused on DC function during disease, but understanding the regulation of inflammation and the induction of acquired immune responses requires combined phenotypic and functional characterization of individual DC subpopulations. Furthermore, little is known regarding mucosal DC subpopulations in the GIT of the neonate and how these DC populations change following colonization by commensal microflora. The current review will highlight mucosal DC diversity and discuss factors that may influence mucosal DC differentiation.

Central nervous system demyelinating disease protection by the human commensal Bacteroides fragilis depends on polysaccharide A expression

The importance of gut commensal bacteria in maintaining immune homeostasis is increasingly understood. We recently described that alteration of the gut microflora can affect a population of Foxp3(+)T(reg) cells that regulate demyelination in experimental autoimmune encephalomyelitis (EAE), the experimental model of human multiple sclerosis. We now extend our previous observations on the role of commensal bacteria in CNS demyelination, and we demonstrate that Bacteroides fragilis producing a bacterial capsular polysaccharide Ag can protect against EAE. Recolonization with wild type B. fragilis maintained resistance to EAE, whereas reconstitution with polysaccharide A-deficient B. fragilis restored EAE susceptibility. Enhanced numbers of Foxp3(+)T(reg) cells in the cervical lymph nodes were observed after intestinal recolonization with either strain of B. fragilis. Ex vivo, CD4(+)T cells obtained from mice reconstituted with wild type B. fragilis had significantly enhanced rates of conversion into IL-10-producing Foxp3(+)T(reg) cells and offered greater protection against disease. Our results suggest an important role for commensal bacterial Ags, in particular B. fragilis expressing polysaccharide A, in protecting against CNS demyelination in EAE and perhaps human multiple sclerosis.