Unique functions of CD11b+, CD8 alpha+, and double-negative Peyer's patch dendritic cells

Geography and plumbing control the T cell response to infection

The orchestrated movement of cells of the immune system is essential to generation of productive responses leading to protective memory development. Recent advances have allowed the direct microscopic visualization of lymphocyte and antigen-presenting cell migration and interaction during immune response initiation and progression. These studies have defined important characteristics of the microanatomy of lymphocyte movement, particularly in the lymph node. Moreover, the ability to track endogenous antigen-specific T cells has revealed a coordinated pathway of CD8 T cell movement in the spleen following primary and secondary infection. As a consequence, the local anatomy of secondary lymphoid tissues during infection has emerged as a critical regulator of immunity. While some of the factors responsible for the migratory cues instructing immune cell movement have been identified, much remains to be learned. Here, we provide a brief overview of studies examining CD8 T cell localization during the immune response to infection in the context of our current understanding of immune system structure.

Microbe sampling by mucosal dendritic cells is a discrete, MyD88-independent step in DeltainvG S. Typhimurium colitis

Intestinal dendritic cells (DCs) are believed to sample and present commensal bacteria to the gut-associated immune system to maintain immune homeostasis. How antigen sampling pathways handle intestinal pathogens remains elusive. We present a murine colitogenic Salmonella infection model that is highly dependent on DCs. Conditional DC depletion experiments revealed that intestinal virulence of S. Typhimurium SL1344 ΔinvG mutant lacking a functional type 3 secretion system-1 (ΔinvG)critically required DCs for invasion across the epithelium. The DC-dependency was limited to the early phase of infection when bacteria colocalized with CD11c⁺CX3CR1⁺ mucosal DCs. At later stages, the bacteria became associated with other (CD11c⁻CX3CR1⁻) lamina propria cells, DC depletion no longer attenuated the pathology, and a MyD88-dependent mucosal inflammation was initiated. Using bone marrow chimeric mice, we showed that the MyD88 signaling within hematopoietic cells, which are distinct from DCs, was required and sufficient for induction of the colitis. Moreover, MyD88-deficient DCs supported transepithelial uptake of the bacteria and the induction of MyD88-dependent colitis. These results establish that pathogen sampling by DCs is a discrete, and MyD88-independent, step during the initiation of a mucosal innate immune response to bacterial infection in vivo.

In vivo depletion of CD11c+ cells impairs scrapie agent neuroinvasion from the intestine

Following oral exposure, some transmissible spongiform encephalopathy (TSE) agents accumulate first upon follicular dendritic cells (DCs) in the GALT. Studies in mice have shown that TSE agent accumulation in the GALT, in particular the Peyer's patches, is obligatory for the efficient transmission of disease to the brain. However, the mechanism through which TSE agents are initially conveyed from the gut lumen to the GALT is not known. Studies have implicated migratory hemopoietic DCs in this process, but direct demonstration of their involvement in vivo is lacking. In this study, we have investigated the contribution of CD11c(+) DCs in scrapie agent neuroinvasion through use of CD11c-diptheria toxin receptor-transgenic mice in which CD11c(+) DCs can be specifically and transiently depleted. Using two distinct scrapie agent strains (ME7 and 139A scrapie agents), we show that when CD11c(+) DCs were transiently depleted in the GALT and spleen before oral exposure, early agent accumulation in these tissues was blocked. In addition, CD11c(+) cell depletion reduced susceptibility to oral scrapie challenge indicating that TSE agent neuroinvasion from the GALT was impaired. In conclusion, these data demonstrate that migratory CD11c(+) DCs play a key role in the translocation of the scrapie agent from the gut lumen to the GALT from which neuroinvasion subsequently occurs.

Secretory IgA mediates bacterial translocation to dendritic cells in mouse Peyer's patches with restriction to mucosal compartment

In addition to fulfilling its function of immune exclusion at mucosal surfaces, secretory IgA (SIgA) Ab exhibits the striking feature to adhere selectively to M cells in the mouse and human intestinal Peyer's patches (PPs). Subsequent uptake drives the SIgA Ab to dendritic cells (DCs), which become partially activated. Using freshly isolated mouse DCs, we found that the interaction with SIgA was tissue and DC subtype dependent. Only DCs isolated from PPs and mesenteric lymph nodes interacted with the Ab. CD11c(+)CD11b(+) DCs internalized SIgA, while CD11c(+)CD19(+) DCs only bound SIgA on their surface, and no interaction occurred with CD11c(+)CD8alpha(+) DCs. We next examined whether SIgA could deliver a sizeable cargo to PP DCs in vivo by administering SIgA-Shigella flexneri immune complexes into a mouse ligated intestinal loop containing a PP. We found that such immune complexes entered the PPs and were internalized by subepithelial dome PP DCs, in contrast to S. flexneri alone that did not penetrate the intestinal epithelium in mice. Dissemination of intraepithelial S. flexneri delivered as immune complexes was limited to PPs and mesenteric lymph nodes. We propose that preexisting SIgA Abs associated with microbes contribute to mucosal defense by eliciting responses that prevent overreaction while maintaining productive immunity.

Cholera toxin, E. coli heat-labile toxin, and non-toxic derivatives induce dendritic cell migration into the follicle-associated epithelium of Peyer's patches

The follicle-associated epithelium (FAE) of Peyer's patches (PPs) transports antigens and microorganisms into mucosal lymphoid tissues where they are captured by subepithelial dendritic cells (DCs). Feeding of cholera toxin (CT) induced migration of subepithelial DCs to interfollicular T-cell areas within 24 h. This study investigated short-term effects of CT, Escherichia coli heat-labile toxin, and non-toxic derivatives on DC migration. CT or CTB injected into ligated intestinal loops induced significant increase in CD11c+ DCs within the FAE within 90 min. In mice fed CT intragastrically, DC numbers in the FAE increased by 1 h, were maximal by 2 h, declined between 8 and 12 h, and were reversed by 24 h. Feeding of native LT, recombinant CTB, dibutyryl cyclic AMP, and to a lesser extent mutated CT(E29H) or mutated LT(R192G) had the same effect. Thus, both A and B subunits of enterotoxins, presumably acting through distinct signaling pathways, may promote capture of incoming antigens and pathogens by PP DCs.

Division of labor, plasticity, and crosstalk between dendritic cell subsets

For years, dendritic cell (DC) biologists have oscillated between two seemingly antagonistic ideas: functional specialization (division of labor) of DC subsets and plasticity (multitasking). More recently, a third hypothesis is gathering support: crosstalk between functionally distinct DC subsets. This reveals a previously unappreciated hierarchy of organization within the DC system, and provides a conceptual framework to understand how cooperation between functionally distinct, yet plastic, DC subsets can shape adaptive immunity and immunological memory. Here we review the recent advances in this area.

Regulatory role of lymphoid chemokine CCL19 and CCL21 in the control of allergic rhinitis

The lymphoid chemokines CCL19 and CCL21 are known to be crucial both for lymphoid cell trafficking and for the structural organization of lymphoid tissues such as nasopharynx-associated lymphoid tissue (NALT). However, their role in allergic responses remains unclear, and so our current study aims to shed light on the role of CCL19/CCL21 in the development of allergic rhinitis. After nasal challenge with OVA, OVA-sensitized plt (paucity of lymph node T cells) mice, which are deficient in CCL19/CCL21, showed more severe allergic symptoms than did identically treated wild-type mice. OVA-specific IgE production, eosinophil infiltration, and Th2 responses were enhanced in the upper airway of plt mice. Moreover, in plt mice, the number of CD4(+)CD25(+) regulatory T cells declined in the secondary lymphoid tissues, whereas the number of Th2-inducer-type CD8alpha(-)CD11b(+) myeloid dendritic cells (m-DCs) increased in cervical lymph nodes and NALT. Nasal administration of the plasmid-encoding DNA of CCL19 resulted in the reduction of m-DCs in the secondary lymphoid tissues and the suppression of allergic responses in plt mice. These results suggest that CCL19/CCL21 act as regulatory chemokines for the control of airway allergic disease and so may offer a new strategy for the control of allergic disease.

The IL-4Ralpha pathway in macrophages and its potential role in silica-induced pulmonary fibrosis

Crystalline silica exposure can result in pulmonary fibrosis, where the pulmonary macrophage is key as a result of its ability to react to silica particles. In the mouse silicosis model, there is initial Th1-type inflammation, characterized by TNF-alpha and IFN-gamma. Previous studies determined that Th2 mediators (i.e., IL-13) are vital to development of pulmonary fibrosis. The present study, using in vivo and in vitro techniques, compares silica exposures between Balb/c and Th2-deficient mice in an effort to determine the link between Th2 immunity and silicosis. In long-term experiments, a significant increase in fibrosis and activated interstitial macrophages was observed in Balb/c but not IL-4Ralpha(-/-) mice. Additionally, a significant increase in Ym1 mRNA levels, a promoter of Th2 immunity, was determined in the interstitial leukocyte population of silica-exposed Balb/c mice. To elucidate the effects of silica on macrophage function, bone marrow-derived macrophages (BMdM) were exposed to particles and assayed for T cell (TC) stimulation activity. As a control, Ym1 mRNA expression in Balb/c BMdM was determined using IL-4 stimulation. In the in vitro assay, a significant increase in TC activation, as defined by surface markers and cytokines, was observed in the cultures containing the silica-exposed macrophages in wild-type and IL-4Ralpha(-/-) mice, with one exception: IL-4Ralpha(-/-) BMdM were unable to induce an increase in IL-13. These results suggest that crystalline silica alters cellular functions of macrophages, including activation of TC, and that the increase in Th2 immunity associated with silicosis is via the IL-4Ralpha-Ym1 pathway.

Temporal regulation of interleukin-12p70 (IL-12p70) and IL-12-related cytokines in splenic dendritic cell subsets during Leishmania donovani infection

Dendritic cells (DC) play an essential role in initiating and directing T-cell responses, in part by production of interleukin-12p70 (IL-12p70), IL-23, and IL-27. However, comparative studies on the capacity for cytokine production of DC subsets are rare. Here, we compare splenic CD8alpha+, CD4+, and double-negative (DN) DC, isolated 5 h to 28 days after Leishmania donovani infection, for (i) production of IL-12p70, (ii) accumulation of IL-12/23p40, IL-12p35, IL-23p19, and IL-27p28 mRNAs, and (iii) their capacity to direct CD4+ T-cell differentiation. At 5 h, conventional DC (cDC) accumulated mRNA for IL-12/23p40 (CD8alpha>CD4>DN), IL-23p19 (CD4>CD8alpha>DN), and IL-27p28 (CD8alpha>CD4>DN), in an infection dose-dependent manner. IL-12p70 was restricted to CD8alpha+ cDC, reflecting the subset-specific accumulation of IL-12p35 mRNA. In contrast, cDC from mice infected for 14 to 28 days accumulated little mRNA for IL-12p40 and IL-12p19, though IL-27p28 mRNA remained detectable (CD8alpha>DN>CD4). IL-12p70 secretion by CD8alpha+ cDC was also absent, reflecting deficient IL-12/23p40, rather than IL-12p35, mRNA accumulation. The capacity of CD8alpha+ cDC isolated early after infection to direct Th1 cell differentiation was mediated through IL-12/23p40, whereas this ability in CD4+ and DN cDC was independent of IL-12/23p40 and did not result from overexpression of Delta 4 Notch-like ligand. However, DN cDC produced gamma interferon (IFN-gamma) and also contained a rare population of CD11c(hi) DX5+ IFN-gamma-producing cells. Our data illustrate the extensive diversity in, and temporal regulation of, splenic cDC subsets during infection and suggest caution in interpreting data obtained with unfractionated or minimally purified DC.

Psg18 is specifically expressed in follicle-associated epithelium

Pregnancy-specific glycoproteins (Psgs) secreted by the placenta regulate the immune system to ensure the survival of the fetal allograft by inducing IL-10, an anti-inflammatory cytokine. However, it is unknown whether Psgs are involved in more general aspects of immune response other than maternal immunity. Here, we report that Psg18 is highly expressed in the follicle-associated epithelium (FAE) overlaying Peyer's patches (PPs). Bioinformatics analysis with Reference Database for Immune Cells (RefDIC) as well as RT-PCR data demonstrated that Psg18 is exclusively expressed in FAE in adult mice, in contrast to other Psg family members that are either not expressed or only slightly expressed in FAE. Psg18 expression was observed in FAE of germ-free-conditioned mice, and was slightly upregulated after bacterial inoculation. In situ hybridization analysis revealed that Psg18 is widely expressed throughout FAE. Furthermore, Psg18 protein is deposited on the extracellular matrix in the subepithelial dome beneath FAE, where antigen-presenting cells accumulate. These results suggest that Psg18 is an FAE-specific marker protein that could promote interplay between FAE and immune cells in mucosa-associated lymphoid tissues.

IFN-gamma-producing dendritic cells are important for priming of gut intraepithelial lymphocyte response against intracellular parasitic infection

The importance of intraepithelial lymphocytes (IEL) in immunoprotection against orally acquired pathogens is being increasingly recognized. Recent studies have demonstrated that Ag-specific IEL can be generated and can provide an important first line of defense against pathogens acquired via oral route. However, the mechanism involved in priming of IEL remains elusive. Our current study, using a microsporidial model of infection, demonstrates that priming of IEL is dependent on IFN-gamma-producing dendritic cells (DC) from mucosal sites. DC from mice lacking the IFN-gamma gene are unable to prime IEL, resulting in failure of these cells to proliferate and lyse pathogen-infected targets. Also, treatment of wild-type DC from Peyer's patches with Ab to IFN-gamma abrogates their ability to prime an IEL response against Encephalitozoon cuniculi in vitro. Moreover, when incubated with activated DC from IFN-gamma knockout mice, splenic CD8(+) T cells are not primed efficiently and exhibit reduced ability to home to the gut compartment. These data strongly suggest that IFN-gamma-producing DC from mucosal sites play an important role in the generation of an Ag-specific IEL response in the small intestine. To our knowledge, this report is the first demonstrating a role for IFN-gamma-producing DC from Peyer's patches in the development of Ag-specific IEL population and their trafficking to the gut epithelium.

Dynamic changes in conjunctival dendritic cell numbers, anatomical position and phenotype during experimental allergic conjunctivitis

Dendritic cells (DCs) are a subset of antigen-presenting cells (APCs) that are involved in the initiation and control of the immune response to antigens present at the interface with the environment. A limited number of groups have studied DCs in human and animal conjunctiva but no data is available concerning the different DC subsets present in the conjunctival tissue. The aims of this study are to characterize the phenotypes and numbers of DCs present in the murine model of allergic conjunctivitis using the technique of immunohistochemistry so as to aid the understanding of the mechanisms involved in allergic eye disease. A double immunofluorescence method was used to analyze the phenotypic distribution and density of DC subsets in the mouse conjunctival tissues of the allergic model using a panel of antibodies: CD11c, as a general marker of DCs, coupled with another DC subset marker such as Langerin for Langerhans cells (LCs), CD11b for myeloid DCs (mDCs) and mPDCA-1 for plasmacytoid DCs (pDCs). In the naïve conjunctiva, mDCs were consistently detected in the subepithelial layer and substantia propria. In the epithelium and the subepithelial layer, very few LCs and virtually no pDCs were observed. Following allergen challenge, there was a marked influx of mDCs and pDCs, but no LCs, into the subepithelial layer and throughout the substantia propria. These results indicate that conjunctival DC subsets may play an important role in the immune-regulatory processes involved in the inflammatory component of allergic conjunctivitis.

The yin and yang of intestinal epithelial cells in controlling dendritic cell function

Recent work suggests that dendritic cells (DCs) in mucosal tissues are "educated" by intestinal epithelial cells (IECs) to suppress inflammation and promote immunological tolerance. After attack by pathogenic microorganisms, however, "non-educated" DCs are recruited from nearby areas, such as the dome of Peyer's patches (PPs) and the blood, to initiate inflammation and the ensuing immune response to the invader. Differential epithelial cell (EC) responses to commensals and pathogens may control these two tolorogenic and immunogenic functions of DCs.

Dendritic cells support angiogenesis and promote lesion growth in a murine model of endometriosis

Endometriosis affects 10-15% of women and is associated with pelvic pain and infertility. Angiogenesis plays an essential role in its pathogenesis. Dendritic cells (DCs) were recently implicated in supporting tumor angiogenesis. As both tumors and endometriosis lesions depend on angiogenesis, we investigated the possibility that DCs may also play a role in endometriosis. We induced endometriosis in 8-wk-old female C57BL/6 mice by implantation of autologous endometrium into the peritoneal cavity. We observed an abundance of CD11c(+) DCs infiltrating sites of angiogenesis in endometriosis lesions. We noticed a similar pattern of infiltrating DCs at sites of angiogenesis in the peritoneal Lewis lung carcinoma tumor model. These DCs were immature (major histocompatability complex class II(low)) and expressed vascular endothelial growth factor receptor 2. Peritoneal implanted bone marrow-derived DCs (BMDCs) incorporated into both endometriosis lesions and into B16 melanoma tumors and enhanced their growth at 8 days compared with controls (5.1+/-2.5 vs. 1.5+/-0.5 mm(2), n=4 and 4, P<0.0001 for endometriosis; 67.6+/-15.1 vs. 22.7+/-14.6 mm(2), n=5 and 7, P=0.0004 for mouse melanoma). Finally, immature BMDCs but not mature BMDCs enhanced microvascular endothelial cell migration in vitro (219+/-51 vs. 93+/-32 cells, P=0.02). Based on these findings, we suggest a novel role for DCs in supporting angiogenesis and promoting lesion growth both in endometriosis and in tumors.

Anatomic location defines antigen presentation by dendritic cells to T cells in response to intravenous soluble antigens

In the spleen, exogenous antigen is preferentially presented by CD8alpha+CD11b- DC to CD8 T cells and by CD8alpha-CD11b+ DC to CD4 T cells. However, it is not yet clear whether the same rule applies to other secondary lymphoid organs. To address this issue, we first classified secondary lymphoid tissues into three categories based on the expression pattern of CD8alpha and CD11b in C57BL/6 and BALB/c mice: (a) spleen, (b) mesenteric lymph node (MLN) and (c) other peripheral lymph nodes (PLN). We then analyzed the OVA-specific T cell-stimulating capacity of each DC subset after intravenous injection with soluble OVA. Our results show that, regardless of tissue origin, CD8alpha-CD11b+ DC generally present OVA to CD4 T cells, a finding that held true as well for CD8alpha+CD11b+ DC in PLN. In striking contrast, CD8alpha+CD11b- DC in spleen, CD8alpha-CD11b+ DC in MLN and CD8alpha+CD11b+ DC in PLN mainly cross-present OVA to CD8 T cells in their respective tissues. Of note, CD8alpha-CD11b+ DC in MLN and CD8alpha+CD11b+ DC in PLN present OVA to both CD4 T and CD8 T cells. Therefore, the antigen-presenting capacity of each distinct DC subset is determined by its anatomic environment in combination with its surface phenotype.

The dynamics of dendritic cell: mediated innate immune regulation

After taking up pathogen-derived antigens, dendritic cells (DCs) leave peripheral organs and migrate into sentinel lymph nodes via afferent lymphatic vessels. During this process, they undergo maturation and produce proinflammatory cytokines, which leads to efficient antigen (Ag) presentation and activation of the innate and acquired immune systems. Recent evidence indicates that DC subsets cooperate to activate the innate immune system. It is becoming clear that the total DC population is composed of a network of DC subsets with distinct functions that are critical for sensing pathogens and orchestrating immune responses.

Lung dendritic cells have a potent capability to induce production of immunoglobulin A

The mucosal immune system provides the first line of defense against inhaled pathogens in the lung. This system is largely mediated by immunoglobulin A (IgA) locally produced by plasma cells, which originate from homing IgA-committed B cells. It has not been determined what types of antigen-presenting cells (APCs) primarily induce B cell differentiation for IgA production in the lung. In addition, although mucosal dendritic cells (DCs) are functionally distinct from DCs in other tissues, it is unclear whether IgA-inducing capability differs between mucosal lung DCs (LDCs) and nonmucosal DCs. The present study was conducted to identify APCs principally responsible for IgA induction in the lung, and to determine potential differences in IgA-inducing capacity between LDCs and nonmucosal DCs. We measured immunoglobulin and cytokine production in a coculture system containing naive IgD(+) B cells, naive T cells from ovalbumin-specific T cell-receptor transgenic mice, and APCs including LDCs, alveolar macrophages (AMs), or spleen DCs (SDCs). LDCs induced significantly greater levels of IgA, IgG1, IL-6, and TGF-beta than AMs and SDCs, whereas no differences were found in the production of IgM or IgG2a. In addition, the IgA percentage of total class-switched immunoglobulin was highest in cocultures with LDCs (38.4%) when compared with those with AMs (15.1%) and SDCs (22.7%). Neutralizing TGF-beta, but not IL-6, significantly decreased IgA induction by LDCs and SDCs, but not by AMs. This study suggests that LDCs are the primary APCs introducing IgA to the lung, and have a more potent IgA-inducing capacity than nonmucosal DCs.

Oral vaccines: new needs, new possibilities

Vaccination is an important tool for handling healthcare programs both in developed and developing countries. The current global scenario calls for a more-efficacious, acceptable, cost-effective and reliable method of immunization for many fatal diseases. It is hoped that the adoption of oral vaccines will help to provide an effective vaccination strategy, especially in developing countries. Mucosal immunity generated by oral vaccines can serve as a strong first line of defense against most of the pathogens infecting through the mucosal lining. Advances in elucidating the mechanism of action of oral vaccines will facilitate the design of more effective, new generation vaccines. There are promising developments in the use of different agents to effectively deliver the vaccine candidate. It is hoped that ongoing research may be able to set another cardinal point, after polio vaccine, in eradicating infectious diseases.

Epithelial cells prime the immune response to an array of gut-derived commensals towards a tolerogenic phenotype through distinct actions of thymic stromal lymphopoietin and transforming growth factor-beta

Humans and other mammals coexist with a diverse array of microbes colonizing the intestine, termed the microflora. The relationship is symbiotic, with the microbes benefiting from a stable environment and nutrient supply, and the host gaining competitive exclusion of pathogens and continuously maintenance of the gut immune homeostasis. Here we report novel crosstalk mechanisms between the human enterocyte cell line, Caco2, and underlying human monocyte-derived DC in a transwell model where Gram-positive (G+) commensals prevent Toll-like receptor-4 (TLR4)-dependent Escherichia coli-induced semimaturation in a TLR2-dependent fashion. These findings add to our understanding of the hypo-responsiveness of the gut epithelium towards the microflora. Gut DC posses a more tolerogenic phenotype than conventional DC. Here we show that Caco2 spent medium (SM) induces tolerogenic DC with lower expression of maturation markers, interleukin (IL)-12p70, and tumour necrosis factor-alpha when matured with G+ and Gram-negative (G-) commensals, while IL-10 production is enhanced in DC upon encountering G+ commensals and reduced upon encountering G- bacteria. The Caco2 SM-induced tolerogenic phenotype is also seen in DC priming of naive T cells with elevated levels of transforming growth factor-beta (TGF-beta) and markedly reduced levels of bacteria-induced interferon-gamma production. Caco2 cell production of IL-8, thymic stromal lymphopoietin (TSLP) and TGF-beta increases upon microbial stimulation in a strain dependent manner. TSLP and TGF-beta co-operate in inducing the tolerogenic DC phenotype but other mediators might be involved.

Interactions between immune cells and their microenvironment

In the past, studies of the immune system have identified molecules and cell types with immunological function and focussed on interactions between these components. However, it is increasingly apparent that the ability of immune cells to interact with elements from their microenvironment such as extracellular matrix components and stromal cells, that are often considered unlinked to the adaptive immune system, is central to the successful and correct functioning of the immune system. As a consequence, a number of sophisticated techniques have been used to analyse these interactions in vitro and in vivo. Here we describe the interactions involved in immune function and some of the methods used to examine them, focussing particularly on those that use imaging techniques.

Small intestine lamina propria dendritic cells promote de novo generation of Foxp3 T reg cells via retinoic acid

To maintain immune homeostasis, the intestinal immune system has evolved redundant regulatory strategies. In this regard, the gut is home to a large number of regulatory T (T reg) cells, including the Foxp3⁺ T reg cell. Therefore, we hypothesized that the gut environment preferentially supports extrathymic T reg cell development. We show that peripheral conversion of CD4⁺ T cells to T reg cells occurs primarily in gut-associated lymphoid tissue (GALT) after oral exposure to antigen and in a lymphopenic environment. Dendritic cells (DCs) purified from the lamina propria (Lp; LpDCs) of the small intestine were found to promote a high level of T reg cell conversion relative to lymphoid organ–derived DCs. This enhanced conversion by LpDCs was dependent on TGF-β and retinoic acid (RA), which is a vitamin A metabolite highly expressed in GALT. Together, these data demonstrate that the intestinal immune system has evolved a self-contained strategy to promote T reg cell neoconversion.

TLR2 activation by proteosomes promotes uptake of particulate vaccines at mucosal surfaces

Proteosome-based vaccines have TLR2-based adjuvant activity and show promise for mucosal immunization. We examined the effects of proteosomes on mucosal uptake in Peyer's patches in vivo. Proteosomes accelerated transepithelial transport of microparticles by M cells and induced migration of dendritic cells (DCs) into the follicle-associated epithelium (FAE); both effects were dependent on TLR2. Proteosomes induced the release of the DC-attracting chemokine MIP3alpha from Caco-2 epithelial cells in vitro. In HEK cells, proteosome-mediated MIP3alpha release was dependent on TLR2 expression and matrix metalloproteinase activation. Thus, TLR2 activation by proteosomes may promote mucosal uptake of particulate vaccines, and this may contribute to their adjuvanticity.

CD8- DCs induce IL-12-independent Th1 differentiation through Delta 4 Notch-like ligand in response to bacterial LPS

Toll-like receptor (TLR) ligation is believed to skew T cell responses toward T helper (Th)1 differentiation by inducing interleukin (IL)-12 secretion by CD8⁺ dendritic cells (DCs). However, TLR-dependent Th1 responses occur in the absence of IL-12. To determine how DCs induce Th1 differentiation in the absence of IL-12, we examined the response of IL-12–deficient DCs to bacterial lipopolysaccharide (LPS). We find that LPS activates MyD88-dependent Delta 4 Notch-like ligand expression by CD8⁻ DCs, and that these cells direct Th1 differentiation by an IL-12–independent and Notch-dependent mechanism in vitro and in vivo. Thus, activation of the two DC subsets by TLR4 leads to Th1 responses by two distinct MyD88-dependent pathways.

Mucosal dendritic cells

The internal surfaces of the human body are covered by distinct types of epithelial cells and mucus-secreting cells. The mucosal surfaces serve many vital functions, such as respiration (nasal passage and lung), absorption (gastrointestinal tract), excretion (lung, urinary tract, large intestine), and reproduction (reproductive tract). In performing these functions, the host is inevitably exposed to environmental antigens, food particles, commensal flora, and pathogens. Mucosal surfaces contain specialized dendritic cells (DCs) capable of sensing these external stimuli and mounting appropriate local responses depending on the nature of the elements they encounter. In the absence of pathogens, mucosal DCs either ignore the antigen or induce regulatory responses. Upon recognition of microorganisms that invade the mucosal barrier, mucosal DCs mount robust protective immunity. This review highlights progress in our understanding of how mucosal DCs process external information and direct appropriate responses by mobilizing various cells of the innate and adaptive immune systems to achieve homeostasis and protection.

Phenotype and function of intestinal dendritic cells

It is now appreciated that dendritic cells (DCs) play a primary role in oral tolerance and defense against mucosal pathogens. Specific DC subpopulations are localized to discrete regions within primary inductive tissues, like the Peyer's patch and mesenteric lymph node, and effector sites, like the lamina propria, and may have unique roles in driving regulatory, effector and memory T cell responses. Certain DC subpopulations may also help maintain T cell responses at sites of abnormal intestinal inflammation. While early in our understanding, knowledge about the involvement of DC subpopulations in the regulation of mucosal immunity may well provide a basis for the development of novel vaccines and therapeutics.

Induction of an anti-inflammatory cytokine, IL-10, in dendritic cells after toll-like receptor signaling

Interleukin-10 (IL-10) is an anti-inflammatory cytokine that modulates innate and adaptive immunity. IL-10 transcripts and the protein were induced in murine bone marrow-derived dendritic cells (BMDCs) after toll-like receptor (TLR) stimulation. IL-10 induction was TLR ligand selective, in that CpG DNA, imidazoquinolin, peptidoglycan, and zymosan but not lipopolysaccharide (LPS) and poly I:C led to IL-10 production. IL-10 induction was, however, completely absent in MyD88(/) DCs that lacked a TLR adaptor showing that IL-10 induction depends on TLR signaling. Kinetic analysis of IL-10 induction by CpG and imidazoquinolin revealed a prolonged lag phase prior to a measurable rise in transcript levels, which peaked at 12-24 h after stimulation. Stat3, implicated in IL-10 gene transcription, was also induced after TLR stimulation with the kinetics similar to those of IL-10 induction. Further, Stat3 was phosphorylated and bound to the IL-10 promoter in TLR-stimulated DCs. Supporting a link with IL-10 induction, STAT3 induction was absent in MyD88(/) DCs. These data suggest a two-step model where the initial TLR signaling induced proinflammatory cytokines, which then activated Stat3, leading to the induction of IL-10. TLR-stimulated IL-10 production may regulate DC maturation steps, thereby influencing the ensuing immune responses.

Epithelial-cell-intrinsic IKK-beta expression regulates intestinal immune homeostasis

Intestinal epithelial cells (IECs) provide a primary physical barrier against commensal and pathogenic microorganisms in the gastrointestinal (GI) tract, but the influence of IECs on the development and regulation of immunity to infection is unknown. Here we show that IEC-intrinsic IkappaB kinase (IKK)-beta-dependent gene expression is a critical regulator of responses of dendritic cells and CD4+ T cells in the GI tract. Mice with an IEC-specific deletion of IKK-beta show a reduced expression of the epithelial-cell-restricted cytokine thymic stromal lymphopoietin in the intestine and, after infection with the gut-dwelling parasite Trichuris, fail to develop a pathogen-specific CD4+ T helper type 2 (T(H)2) response and are unable to eradicate infection. Further, these animals show exacerbated production of dendritic-cell-derived interleukin-12/23p40 and tumour necrosis factor-alpha, increased levels of CD4+ T-cell-derived interferon-gamma and interleukin-17, and develop severe intestinal inflammation. Blockade of proinflammatory cytokines during Trichuris infection ablates the requirement for IKK-beta in IECs to promote CD4+ T(H)2 cell-dependent immunity, identifying an essential function for IECs in tissue-specific conditioning of dendritic cells and limiting type 1 cytokine production in the GI tract. These results indicate that the balance of IKK-beta-dependent gene expression in the intestinal epithelium is crucial in intestinal immune homeostasis by promoting mucosal immunity and limiting chronic inflammation.

Control of intestinal homeostasis by regulatory T cells and dendritic cells

Many different pathways contribute to the maintenance of tolerance to harmless antigens in the intestine. When these important pathways are compromised, chronic intestinal inflammation can develop. In particular, naturally occurring CD4+CD25+ regulatory T cells have been shown to play an important role in the prevention and cure of colitis in animal models of intestinal inflammation. These regulatory T cell responses may be influenced by the local environment in the intestine. For example, functionally specialised populations of dendritic cells exist in the intestine which may favour regulatory type responses. Understanding how these pathways intersect may lead to the development of more specific therapies for the treatment of inflammatory bowel disease.

Generation of gut-homing T cells and their localization to the small intestinal mucosa

The intestinal mucosa represents the largest body surface toward the external environment and harbors numerous T lymphocytes that take up resident within the intestinal epithelium or in the underlying lamina propria (LP). The intraepithelial lymphocytes include subsets of 'unconventional' T cells with unclear ontogeny and reactivity that localize to this site independently of antigen-specific activation in secondary lymphoid organs. In contrast, the majority of the 'conventional' gut T cells are recruited into the intestinal mucosa subsequent to their activation in intestinal inductive sites, including Peyer's patches (PPs) and mesenteric lymph nodes (MLNs). T cells homing to the small intestine express a distinct pattern of homing molecules, allowing them to interact with and transmigrate across intestinal postcapillary endothelium. At least some of these homing molecules, including the integrin alpha(4)beta(7) and the chemokine receptor CCR9, are induced on T cells during their activation in PPs or MLNs. Mucosal dendritic cells (DCs) play a key role in this process, but not all intestinal DCs possess the ability to confer a gut-homing capacity to T cells. Instead, functionally specialized CD103(+) DCs derived from the small intestinal LP appear to selectively regulate T-cell homing to the small intestine.

Ingestion of heat-treated Lactobacillus rhamnosus GG prevents development of atopic dermatitis in NC/Nga mice

BACKGROUND

Continuous oral administration of live Lactobacillus rhamnosus GG (L. GG) to pregnant subjects with atopic dermatitis and their children, suppressed the frequency of atopic dermatitis. The details of mechanisms and immune systems involved in this suppressive effect, however, remain speculative.

OBJECTIVE

We sought to clarify suppressive mechanisms of L. GG on atopic dermatitis by using NC/Nga mice, a model of human atopic dermatitis.

METHODS

Maternal mice and infant mice were fed with food containing or not containing heat-treated L. GG during pregnancy and breastfeeding, and after weaning.

RESULTS

Control NC/Nga mice raised under an air-uncontrolled condition spontaneously manifested typical skin lesions very similar to those in patients with atopic dermatitis. On the other hand, administration of food containing heat-treated L. GG inhibited the onset and development of atopic skin lesions, accompanied by smaller numbers of mast cells and eosinophils in the affected skin sites. Mice fed with L. GG showed a significant increase in plasma IL-10 levels compared with control mice, while there was no significant difference in the proportion of splenic CD4(+)CD25(+) regulatory T cells between mice fed with L. GG and control mice. The IL-10 mRNA expression was enhanced in both Peyer's patches and mesenteric lymph nodes in mice fed with L. GG.

CONCLUSION

These findings suggest that some components of heat-treated L. GG may have an ability to delay the onset and suppress the development of atopic dermatitis, probably through a strong induction of IL-10 in intestinal lymphoid organs and systemic levels.

Mouse respiratory tract dendritic cell subsets and the immunological fate of inhaled antigens

It is widely accepted that tissue dendritic cells (DC) function as immune sentinels by alerting T cells to foreign antigen after delivering and presenting it in the draining lymph nodes. Over the last two decades, studies in animal models, particularly rodents, have demonstrated that respiratory tract DC are crucial for the adaptive immune response to inhaled antigen. Indeed, the fate of inhaled antigen is inextricably linked to the function of respiratory tract DC. In this review, we will discuss the characteristics of respiratory tract DC from mice and recent data that may help to explain their role in the fate of inhaled antigen.

Immune privilege in the gut: the establishment and maintenance of non-responsiveness to dietary antigens and commensal flora

Immune privilege in the gut is the result of a complex interplay between the gut microbiome, gut luminal antigens, and the intestinal epithelial barrier. Composed of both physical and immunochemical components, the intestinal barrier secretes immunoregulatory mediators that promote the generation of tolerogenic antigen-presenting cells, phagocytic innate immune cells characterized by 'inflammatory anergy', and regulatory cells of the adaptive immune system. Innate immune cells mediate controlled transepithelial transport of luminal antigens as far as the mesenteric lymph nodes, where the intestinal and peripheral immune systems intersect. This promotes the generation of adaptive regulatory lymphocytes that actively suppress effector cell responses against gut luminal antigens and flora. The net result is the generation of tolerance to dietary antigens and the maintenance of gut homeostasis. Dysregulation of this complex immunoregulatory network leads to diseases such as food allergy and inflammatory bowel disease. Future therapies for these diseases will likely involve the functional restoration of the barrier and regulatory cell functions at the epithelial/luminal interface.

Initiating mechanisms of food allergy: Oral tolerance versus allergic sensitization

Immediately after birth the mucosa of the gastrointestinal tract, which represents the greatest body surface area exposed to the outside environment, is confronted with a large variety of foreign antigens. The immune system of the intestine now has to meet the task of discriminating between pathogens and harmless antigens, such as food proteins and commensal bacteria, and to respond accordingly. This important job is fulfilled by cells of the gut-associated lymphoid tissue, the largest immunologic organ in the body. Despite the large extent of food antigen exposure, only a small percentage of individuals experience adverse immunologic reactions to food. This is due to the fact that the normal immune response to dietary proteins is associated with the induction of oral tolerance, which refers to a state of active inhibition of immune responses to an antigen by means of prior exposure to that antigen via the oral route. Abrogation of oral tolerance or failure to induce oral tolerance may result in the development of food hypersensitivity. In the present review, factors that may play a role in the outcome of oral tolerance versus sensitization to food proteins are discussed.

Dynamic imaging of dendritic cell extension into the small bowel lumen in response to epithelial cell TLR engagement

Cells lining the gastrointestinal tract serve as both a barrier to and a pathway for infectious agent entry. Dendritic cells (DCs) present in the lamina propria under the columnar villus epithelium of the small bowel extend processes across this epithelium and capture bacteria, but previous studies provided limited information on the nature of the stimuli, receptors, and signaling events involved in promoting this phenomenon. Here, we use immunohistochemical as well as dynamic explant and intravital two-photon imaging to investigate this issue. Analysis of CD11c–enhanced green fluorescent protein (EGFP) or major histocompatibility complex CII-EGFP mice revealed that the number of trans-epithelial DC extensions, many with an unusual “balloon” shape, varies along the length of the small bowel. High numbers of such extensions were found in the proximal jejunum, but only a few were present in the terminal ileum. The extensions in the terminal ileum markedly increased upon the introduction of invasive or noninvasive Salmonella organisms, and chimeric mouse studies revealed the key role of MyD88-dependent Toll-like receptor (TLR) signaling by nonhematopoietic (epithelial) elements in the DC extension response. Collectively, these findings support a model in which epithelial cell TLR signaling upon exposure to microbial stimuli induces active DC sampling of the gut lumen at sites distant from organized lymphoid tissues.

[Early bacterial colonisation of the intestine: why it matters?]

The birth process allows the progressive formation of complex intestinal microflora composed of myriad bacteria, leading to this recently identified host-bacterial mutualism in the human intestine. This kind of cross-talk originating from birth is opportunistically used by the young host to initiate its own immune system. Recent epidemiogical data support the hypothesis that some increasing immune deviances observed in the last 2 decades could have originated from a modification of the bacterial environment in young populations. Our modern approach to perinatal care may, to some extent, have modified inadequately the overall quality of this bacterial-host interface. The international medical community has to be made aware of the increasing importance that initial colonising intestinal microflora could have on the health and well-being of the host later in life. It is of great concern to decrease these possible negative influences and to discover in the near future the possible means of helping to manipulate positively the gut microbiotia of infants.

Lymphoid Tissue Inducer Cells in Intestinal Immunity

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

The role of adjuvants in the development of mucosal vaccines

It is well-established that most pathogens that cause infectious diseases enter the host via mucosal membranes of the respiratory, digestive and genital tracts. Some parenterally administered vaccines induce protection against mucosal pathogens. However, there is increasing evidence that mucosal protection is better afforded by mucosal vaccination, particularly for the induction of memory responses. Mucosal vaccines must pass several difficult hurdles before entering the host and inducing an effective and protective immune response. This review deals with present and past efforts in devising effective mucosal vaccines using delivery systems and immunopotentiating adjuvants for protein-based vaccines. The paper will conclude with the authors' opinion on how the field will or should progress in the future and what will be the required components of ideal future mucosal vaccines that can induce immunological memory.

CCR6(+) dendritic cells: the gut tactical-response unit

How an immune response to pathogens is initiated in the gut is still an open question. In this issue of Immunity, it has been found that a distinct subset of CCR6-expressing dendritic cells are absolutely required to activate anti-Salmonella-specific T cells.

Foxp3-expressing CD103+ regulatory T cells accumulate in dendritic cell aggregates of the colonic mucosa in murine transfer colitis

Little is known of the anatomical compartmentalization of colitogenic or regulatory T-cell responses in the murine transfer colitis model. Therefore, we analyzed the putative function of large intestinal dendritic cell (DC) aggregates, to which donor CD4+ T cells selectively home before colitis becomes manifest. The co-stimulatory molecules MHC-II, CD40, CD80, and CD86 were expressed in DC aggregates. IL-23 was primarily absent from DC aggregates at all stages of disease but was expressed at high levels in the severely inflamed lamina propria. Interferon-gamma was up-regulated in the lamina propria during early and advanced disease, whereas in DC aggregates it was detectable to a significant degree only in fully developed colitis. In contrast, Foxp3, a marker of regulatory T cells, was expressed in DC aggregates on T-cell transfer, coinciding with the appearance of CD103+ CD25- T cells in these clusters. Foxp3 was enriched in the CD103+ T-cell fraction isolated from the lamina propria of diseased mice. T-cell grafts depleted of CD103+ T cells generated similar numbers of colonic CD103+ T cells as unfractionated T cells. We conclude that DC aggregates are structures involved in the expansion and/or differentiation of CD103+ CD25- CD4+ Foxp3-expressing regulatory T cells.

Dendritic cells: the commanders-in-chief of mucosal immune defenses

PURPOSE OF REVIEW

Intestinal dendritic cells have emerged as key regulators of immunity to pathogens, oral tolerance and intestinal inflammation. Studies have begun to elucidate the regulatory mechanisms responsible for defining region- and compartment-specific phenotypes and functions of dendritic cells in mucosal tissues.

RECENT FINDINGS

Specific subsets of dendritic cells appear to be associated with the various routes for antigen acquisition in the intestine. The constant sampling of intestinal antigenic content ensures establishment of tolerance to commensal bacteria and food antigens. Tolerance development to oral antigens is restricted to the mucosal immune system. Other advances have provided insight into the molecular basis of microbial recognition and innate immune responses by intestinal dendritic cells. Differences in the involvement of dendritic cells have begun to emerge in Crohn's disease and ulcerative colitis and link gene regulation in dendritic cells to therapeutic responses.

SUMMARY

A major focus of mucosal immunology will be to understand how diverse dendritic cell subsets cooperate in regulating homeostasis and host defense in the different intestinal immune compartments. This will be pivotal to understanding how the mucosal immune system makes the distinction between commensal microbiota, pathogens and self antigens.

Dendritic cells in the recognition of intestinal microbiota

Mucosal dendritic cells (DCs) constantly survey the luminal microenvironment which contains commensal microbiota and potentially harmful organisms regulating pathogen recognition and adaptive as well as innate defense activation. Distinct mechanisms are beginning to emerge by which intestinal antigen sampling and handling is achieved ensuring specificity and contributing to redundancy in pathogen detection. Distinct DC subsets are associated with these mechanisms and regulate specific innate or adaptive immune responses to help distinguish between commensal microbiota, pathogens and self antigens. Understanding DC biology in the mucosal immune system may contribute to the unraveling of infection routes of intestinal pathogens and may aid in developing novel vaccines and therapeutic strategies for the treatment of infectious and inflammatory diseases.

Intestinal and pulmonary mucosal T cells: local heroes fight to maintain the status quo

Mucosal immunity in the lung and intestine is controlled by complex multifaceted systems. While mucosal T cells are essential for protection against invading pathogens owing to their proximity to the outside world, powerful systems must also be in place to harness ongoing inflammatory processes. In each site, distinct anatomical structures play key roles in mounting and executing both protective and deleterious mucosal T cell responses. Although analogies can be drawn regarding the immune systems of these two organs, there are substantial dissimilarities necessitated by unique physiologic constraints. Here, we discuss how T cell activation and effector function are generated in the mucosae.

Tight mucosal compartmentation of the murine immune response to antigens of the enteric microbiota

BACKGROUND & AIMS

The normal host immune response to antigens of the enteric microbiota is poorly defined. In this study, we isolated recombinant microbial antigens from commensal bacteria and used them to probe the normal murine immune response.

METHODS

A plasmid DNA expression library was generated from cecal bacteria of C3H/HeJ mice and used to express 20 recombinant intestinal bacterial proteins (rIBs). Antibody responses in serum and secretions were measured by an enzyme-linked immunosorbent assay, and CD4+ T-cell responses were measured by [3H]-thymidine incorporation. Two immunodominant commensal flagellins were also tested.

RESULTS

No baseline serum immunoglobulin (Ig)G antibody or splenic CD4+ T-cell systemic response to any rIB or to either flagellin was detected in normal C3H/HeJ mice. However, there were strong systemic responses to all 20 rIBs after parenteral immunization, which were equivalent to the responses to ovalbumin. Substantial levels of intestinal IgA were detected to half the rIBs and to both commensal flagellins. Mucosal immunization with flagellin plus ovalbumin stimulated an intestinal IgA but not a serum IgG response. Antigen-pulsed dendritic cells (DCs) stimulated production of specific IgA in the absence of T-cell help via costimulation by BAFF and/or APRIL, members of the TNF family.

CONCLUSIONS

The host immune response to enteric bacteria is tightly compartmentalized to the mucosa in normal mice, with systemic B cells and CD4+ T cells remaining naive rather than tolerant. We postulate that mucosal DCs play a crucial role in this compartmentation.

Modulation of dendritic cell phenotype and function in an in vitro model of the intestinal epithelium

A network of dendritic cells (DC) can be detected in close proximity to the epithelial cells overlying Peyer's patches in the gut. Intestinal DC show distinct phenotypes as compared to DC from the systemic lymph nodes (relatively low MHC and costimulatory molecules and high IL-10 and TGFbeta) and may play a role in maintaining tolerance to enteric antigens. We show that a similar phenotype is induced in the presence of a polarised epithelial cell monolayer in vitro. Monocyte-derived DC were co-cultured with Caco-2 intestinal epithelial monolayers for 24 h. Co-culture resulted in DC with reduced expression of MHC class II, CD86, and CD80, and poor T cell stimulatory capacity. Cytokine profiles showed reduced levels of inflammatory cytokine production, and co-cultured DC were less sensitive to stimulation via Toll-like receptors (TLR2, 4, and 6) as a result of increased levels of autocrine TGFbeta production. However, phenotypic changes in co-cultured DC could not be blocked by removal of apoptotic cells or addition of anti-TGFbeta antibodies, suggesting that other soluble factors are involved in DC modulation. Thus, polarised epithelial cell monolayers create a 'tolerogenic' environment which modulates the activity of DC. These results highlight the regulatory importance of the epithelial microenvironment at mucosal surfaces.

TLRs regulate the gatekeeping functions of the intestinal follicle-associated epithelium

Initiation of adaptive mucosal immunity occurs in organized mucosal lymphoid tissues such as Peyer's patches of the small intestine. Mucosal lymphoid follicles are covered by a specialized follicle-associated epithelium (FAE) that contains M cells, which mediate uptake and transepithelial transport of luminal Ags. FAE cells also produce chemokines that attract Ag-presenting dendritic cells (DCs). TLRs link innate and adaptive immunity, but their possible role in regulating FAE functions is unknown. We show that TLR2 is expressed in both FAE and villus epithelium, but TLR2 activation by peptidoglycan or Pam(3)Cys injected into the intestinal lumen of mice resulted in receptor redistribution in the FAE only. TLR2 activation enhanced transepithelial transport of microparticles by M cells in a dose-dependent manner. Furthermore, TLR2 activation induced the matrix metalloproteinase-dependent migration of subepithelial DCs into the FAE, but not into villus epithelium of wild-type and TLR4-deficient mice. These responses were not observed in TLR2-deficient mice. Thus, the FAE of Peyer's patches responds to TLR2 ligands in a manner that is distinct from the villus epithelium. Intraluminal LPS, a TLR4 ligand, also enhanced microparticle uptake by the FAE and induced DC migration into the FAE, suggesting that other TLRs may modulate FAE functions. We conclude that TLR-mediated signals regulate the gatekeeping functions of the FAE to promote Ag capture by DCs in organized mucosal lymphoid tissues.

Chlamydia Infection Induces ICOS Ligand-Expressing and IL-10-Producing Dendritic Cells That Can Inhibit Airway Inflammation and Mucus Overproduction Elicited by Allergen Challenge in BALB/c Mice

Our previous study has shown that the adoptive transfer of dendritic cells (DCs) freshly isolated from Chlamydia-infected mice (iIDCs), unlike those from control naive mice (iNDCs), can inhibit systemic and cutaneous eosinophilia induced by OVA exposure. In the present study, we examined the mechanism by which iIDC inhibits allergen-specific Th2 cell differentiation in vitro and in vivo. The study revealed that iIDCs exhibited higher surface expression of CD8alpha and the ICOS ligand (ICOS-L), as well as higher IL-10 and IL-12 production than iNDCs. In vitro DC:CD4(+) T cell coculture experiments showed that iIDCs could inhibit allergen-specific Th2 cell differentiation and that the inhibitory effect could be abolished by the blockage of IL-10 or IL-12 activity. More interestingly, the coblockade of IL-10 and the ICOS-L showed synergistic effect in enhancing allergen-driven Th2 cytokine production. Furthermore, adoptive transfer of iIDCs, but not iNDCs, to OVA sensitized mice significantly inhibited airway eosinophilia and mucus overproduction following intranasal challenge with OVA. Overall, the data demonstrate a critical role played by ICOS-L-expressing and IL-10-producing DCs from Chlamydia-infected mice in the infection-mediated inhibition of allergic responses.