TGF-beta1 dampens the susceptibility of dendritic cells to environmental stimulation, leading to the requirement for danger signals for activation

Alternative Methods for Skin-Sensitization Assessment

Skin sensitization is a term used to refer to the regulatory hazard known as allergic contact dermatitis (ACD) in humans or contact hypersensitivity in rodents, an important health endpoint considered in chemical hazard and risk assessments. Information on skin sensitization potential is required in various regulatory frameworks, such as the Directive of the European Parliament and the Council on Registration, Evaluation and Authorization of Chemicals (REACH). The identification of skin-sensitizing chemicals previously required the use of animal testing, which is now being replaced by alternative methods. Alternative methods in the field of skin sensitization are based on the measurement or prediction of key events (KE), i.e., (i) the molecular triggering event, i.e., the covalent binding of electrophilic substances to nucleophilic centers in skin proteins; (ii) the activation of keratinocytes; (iii) the activation of dendritic cells; (iv) the proliferation of T cells. This review article focuses on the current state of knowledge regarding the methods corresponding to each of the key events in skin sensitization and considers the latest trends in the development and modification of these methods.

Blockade of TGF-β signaling reactivates HIV-1/SIV reservoirs and immune responses in vivo

TGF-β plays a critical role in maintaining immune cells in a resting state by inhibiting cell activation and proliferation. Resting HIV-1 target cells represent the main cellular reservoir after long-term antiretroviral therapy (ART). We hypothesized that releasing cells from TGF-β-driven signaling would promote latency reversal. To test our hypothesis, we compared HIV-1 latency models with and without TGF-β and a TGF-β type 1 receptor inhibitor, galunisertib. We tested the effect of galunisertib in SIV-infected, ART-treated macaques by monitoring SIV-env expression via PET/CT using the 64Cu-DOTA-F(ab')2 p7D3 probe, along with plasma and tissue viral loads (VLs). Exogenous TGF-β reduced HIV-1 reactivation in U1 and ACH-2 models. Galunisertib increased HIV-1 latency reversal ex vivo and in PBMCs from HIV-1-infected, ART-treated, aviremic donors. In vivo, oral galunisertib promoted increased total standardized uptake values in PET/CT images in gut and lymph nodes of 5 out of 7 aviremic, long-term ART-treated, SIV-infected macaques. This increase correlated with an increase in SIV RNA in the gut. Two of the 7 animals also exhibited increases in plasma VLs. Higher anti-SIV T cell responses and antibody titers were detected after galunisertib treatment. In summary, our data suggest that blocking TGF-β signaling simultaneously increases retroviral reactivation events and enhances anti-SIV immune responses.

Novel Discoveries in Immune Dysregulation in Inborn Errors of Immunity

With the expansion of our knowledge on inborn errors of immunity (IEI), it gradually becomes clear that immune dysregulation plays an important part. In some cases, autoimmunity, hyperinflammation and lymphoproliferation are far more serious than infections. Thus, immune dysregulation has become significant in disease monitoring and treatment. In recent years, the wide application of whole-exome sequencing/whole-genome sequencing has tremendously promoted the discovery and further studies of new IEI. The number of discovered IEI is growing rapidly, followed by numerous studies of their pathogenesis and therapy. In this review, we focus on novel discovered primary immune dysregulation diseases, including deficiency of SLC7A7, CD122, DEF6, FERMT1, TGFB1, RIPK1, CD137, TET2 and SOCS1. We discuss their genetic mutation, symptoms and current therapeutic methods, and point out the gaps in this field.

Therapeutic targeting of TGF-β in cancer: hacking a master switch of immune suppression

Cancers may escape elimination by the host immune system by rewiring the tumour microenvironment towards an immune suppressive state. Transforming growth factor-β (TGF-β) is a secreted multifunctional cytokine that strongly regulates the activity of immune cells while, in parallel, can promote malignant features such as cancer cell invasion and migration, angiogenesis, and the emergence of cancer-associated fibroblasts. TGF-β is abundantly expressed in cancers and, most often, its abundance associated with poor clinical outcomes. Immunotherapeutic strategies, particularly T cell checkpoint blockade therapies, so far, only produce clinical benefit in a minority of cancer patients. The inhibition of TGF-β activity is a promising approach to increase the efficacy of T cell checkpoint blockade therapies. In this review, we briefly outline the immunoregulatory functions of TGF-β in physiological and malignant contexts. We then deliberate on how the therapeutic targeting of TGF-β may lead to a broadened applicability and success of state-of-the-art immunotherapies.

The skin as an immune organ: Tolerance versus effector responses and applications to food allergy and hypersensitivity reactions

Skin is replete with immunocompetent cells that modulate signaling pathways to maintain a salubrious immunogenic/tolerogenic balance. This fertile immune environment plays a significant role in the development of allergic responses and sensitivities, but the mechanisms underlying these pathways have been underappreciated and underused with respect to developing therapeutics. Among the complex repertoire of cells that promote tolerogenic pathways in the periphery, 2 key classes include dendritic cells and regulatory T (Treg) cells. Immature dendritic cells are the first line of defense, patrolling the periphery, sampling antigens, and secreting cytokines that suppress immune cells and promote the survival of Treg cells. Skin-homing Treg cells also play a critical role in mitigating the reactivity of immune cells, secreting high levels of cytokines that promote tolerance. Therapeutic approaches that capitalize on our knowledge of the rich cellular and molecular environment are emerging and show great promise. We will discuss the advantages and challenges of 5 such strategies and how these therapies might mitigate the atopic march by facilitating tolerance. We conclude that skin is a multifaceted structure that provides a fertile ground for therapeutic discovery. Accordingly, ongoing work in this domain will no doubt continue to deliver exciting progress for improved health outcomes.

Prostaglandin E2 Antagonizes TGF-β Actions During the Differentiation of Monocytes Into Dendritic Cells

Inflammatory dendritic cells (DCs) are a distinct subset of DCs that derive from circulating monocytes infiltrating injured tissues. Monocytes can differentiate into DCs with different functional signatures, depending on the presence of environment stimuli. Among these stimuli, transforming growth factor-beta (TGF-β) and prostaglandin E2 (PGE2) have been shown to modulate the differentiation of monocytes into DCs with different phenotypes and functional profiles. In fact, both mediators lead to contrasting outcomes regarding the production of inflammatory and anti-inflammatory cytokines. Previously, we have shown that human semen, which contains high concentrations of PGE2, promoted the differentiation of DCs into a tolerogenic profile through a mechanism dependent on signaling by E-prostanoid receptors 2 and 4. Notably, this effect was induced despite the huge concentration of TGF-β present in semen, suggesting that PGE2 overrides the influence exerted by TGF-β. No previous studies have analyzed the joint actions induced by PGE2 and TGF-β on the function of monocytes or DCs. Here, we analyzed the phenotype and functional profile of monocyte-derived DCs differentiated in the presence of TGF-β and PGE2. DC differentiation guided by TGF-β alone enhanced the expression of CD1a and abrogated LPS-induced expression of IL-10, while differentiation in the presence of PGE2 impaired CD1a expression, preserved CD14 expression, abrogated IL-12 and IL-23 production, stimulated IL-10 production, and promoted the expansion of FoxP3+ regulatory T cells in a mixed lymphocyte reaction. Interestingly, DCs differentiated in the presence of TGF-β and PGE2 showed a phenotype and functional profile closely resembling those induced by PGE2 alone. Finally, we found that PGE2 inhibited TGF-β signaling through an action exerted by EP2 and EP4 receptors coupled to cyclic AMP increase and protein kinase A activity. These results indicate that PGE2 suppresses the influence exerted by TGF-β during DC differentiation, imprinting a tolerogenic signature. High concentrations of TGF-β and PGE2 are usually found in infectious, autoimmune, and neoplastic diseases. Our observations suggest that in these scenarios PGE2 might play a mandatory role in the acquisition of a regulatory profile by DCs.

Collagen scaffold microenvironments modulate cell lineage commitment for differentiation of bone marrow cells into regulatory dendritic cells

The microenvironment plays a pivotal role for cell survival and functional regulation, and directs the cell fate determination. The biological functions of DCs have been extensively investigated to date. However, the influences of the microenvironment on the differentiation of bone marrow cells (BMCs) into dendritic cells (DCs) are not well defined. Here, we established a 3D collagen scaffold microenvironment to investigate whether such 3D collagen scaffolds could provide a favourable niche for BMCs to differentiate into specialised DCs. We found that BMCs embedded in the 3D collagen scaffold differentiated into a distinct subset of DC, exhibiting high expression of CD11b and low expression of CD11c, co-stimulator (CD40, CD80, CD83, and CD86) and MHC-II molecules compared to those grown in 2D culture. DCs cultured in the 3D collagen scaffold possessed weak antigen uptake ability and inhibited T-cell proliferation in vitro; in addition, they exhibited potent immunoregulatory function to alleviate allo-delay type hypersensitivity when transferred in vivo. Thus, DCs differentiated in the 3D collagen scaffold were defined as regulatory DCs, indicating that collagen scaffold microenvironments probably play an important role in modulating the lineage commitment of DCs and therefore might be applied as a promising tool for generation of specialised DCs.

Dendritic cells as gatekeepers of tolerance

Dendritic cells (DC) are unique hematopoietic cells, linking innate and adaptive immune responses. In particular, they are considered as the most potent antigen presenting cells, governing both T cell immunity and tolerance. In view of their exceptional ability to present antigen and to interact with T cells, DC play distinct roles in shaping T cell development, differentiation and function. The outcome of the DC-T cell interaction is determined by the state of DC maturation, the type of DC subset, the cytokine microenvironment and the tissue location. Both regulatory T cells (Tregs) and DC are indispensable for maintaining central and peripheral tolerance. Over the past decade, accumulating data indicate that DC critically contribute to Treg differentiation and homeostasis.

Regulation of Dendritic Cell Function in Inflammation

Dendritic cells (DC) are professional antigen presenting cells and link the innate and adaptive immune system. During steady state immune surveillance in skin, DC act as sentinels against commensals and invading pathogens. Under pathological skin conditions, inflammatory cytokines, secreted by surrounding keratinocytes, dermal fibroblasts, and immune cells, influence the activation and maturation of different DC populations including Langerhans cells (LC) and dermal DC. In this review we address critical differences in human DC subtypes during inflammatory settings compared to steady state. We also highlight the functional characteristics of human DC subsets in inflammatory skin environments and skin diseases including psoriasis and atopic dermatitis. Understanding the complex immunoregulatory role of distinct DC subsets in inflamed human skin will be a key element in developing novel strategies in anti-inflammatory therapy.

The TGF-β superfamily in dendritic cell biology

The TGF-β superfamily consists of a large group of pleiotropic cytokines that are involved in the regulation of many developmental, physiological and pathological processes. Dendritic cells are antigen-presenting cells that play a key role in innate and adaptive immune responses. Dendritic cells have a complex relationship with the TGF-β cytokine superfamily being both source and targets for many of these cytokines. Some TGF-β family members are expressed by dendritic cells and modulate immune responses, for instance through the induction of T cell polarization. Others play a crucial role in the development and function of the different dendritic cell subsets. This review summarizes the current knowledge on the role of TGF-β family cytokines in dendritic cell biology, focusing on TGF-β as well as on other, less characterized, members of these important immune mediators.

Inflammatory conditions distinctively alter immunological functions of Langerhans-like cells and dendritic cells in vitro

The specific function of human skin-resident dendritic cell (DC) subsets in the regulation of immunity or tolerance is still a matter of debate. Langerhans cells (LC) induce anti-viral immune responses but, conversely to dermal DC, maintain tolerance to bacteria. However, the definite function of epidermal LC and cutaneous DC appears even more complex under inflammatory conditions. Here we investigated the immune responses of human immature monocyte-derived DC (MoDC) and LC-like cells (MoLC) upon stimulation with different Toll-like receptor ligands in the presence or absence of pro-inflammatory cytokines tumour necrosis factor-α (TNF-α) and interleukin-1β (IL-1β). In MoDC, bacterial antigens selectively up-regulated CD83 and CD86 expression and induced the release of T helper type 1 (Th1) and Th17 cytokines and led to a higher CCR7-dependent migratory capacity compared with a low responsiveness of MoLC. Importantly, MoLC activation with lipopolysaccharide under inflammatory conditions strongly enhanced a phenotypically mature state, increased IL-12p70, IL-23 and IL-6 production and Th1 cytokine secretion by CD4(+) T cells. Treatment with poly(I:C) specifically up-regulated surface expression of co-stimulatory molecules and increased release of IL-12p70 in MoLC and co-stimulation with TNF-α and IL-1β further elevated Th1 and Th17 cytokine production. Poly(I:C)-induced up-regulation of type I interferon mRNA levels in MoLC and MoDC was Toll-like receptor 3-dependent but not, or only weakly, modulated by pro-inflammatory cytokines. Our results indicate that inflammatory conditions greatly facilitate recognition of bacteria by MoLC. Furthermore, we suggest a critical involvement of both subsets in innate defence against viruses, whereas inflammatory skin environments additionally favour MoLC as potent inducers of Th1 and Th17 cytokines.

The late endosomal adaptor molecule p14 (LAMTOR2) regulates TGFβ1-mediated homeostasis of Langerhans cells

Langerhans cells (LCs), a sub-population of dendritic cells (DCs) in the skin, participate in the regulation of immunity and peripheral tolerance. The adaptor molecule p14 is part of the late endosomal/lysosomal adaptor and mitogen-activated protein kinase and mammalian target of rapamycin (mTOR) activator/regulator (LAMTOR) complex, which mediates the activation of lysosome-associated extracellular signaling-regulated kinase (ERK) and the mTOR cascade. In previous work, we demonstrated that CD11c-specific deficiency of p14 disrupts LC homeostasis by affecting the LAMTOR-mediated ERK and mTOR signaling. In this study, we extended our analysis on p14 deficiency specifically in LCs. Langerin-specific ablation of p14 caused a complete loss of LCs, accompanied by an increased maturational phenotype of LCs. The absence of LCs in p14-deficient mice reduced contact hypersensitivity (CHS) responses to the contact sensitizer trinitrochlorobenzene. Analysis using bone marrow-derived DCs (BMDCs) revealed that p14 deficiency in DCs/LCs interfered with the LC-relevant transforming growth factor β1 (TGFβ1) pathway, by lowering TGFβ receptor II expression on BMDCs and LCs, as well as surface binding of TGFβ1 on BMDCs. We conclude that p14 deficiency affects TGFβ1 sensitivity of LCs, which is mandatory for their homeostasis and subsequently for their immunological function during CHS.

Tumor-promoting role of TGFβ1 signaling in ultraviolet B-induced skin carcinogenesis is associated with cutaneous inflammation and lymph node migration of dermal dendritic cells

Transforming growth factor beta 1 (TGFβ1) is a pleiotropic cytokine in the skin that can function both as a tumor promoter and suppressor in chemically induced skin carcinogenesis, but the function in ultraviolet B (UVB) carcinogenesis is not well understood. Treatment of SKH1 hairless mice with the activin-like kinase 5 (ALK5) inhibitor SB431542 to block UVB-induced activation of cutaneous TGFβ1 signaling suppressed skin tumor formation but did not alter tumor size or tumor cell proliferation. Tumors that arose in SB-treated mice after 30 weeks had significantly reduced percentage of IFNγ(+) tumor-infiltrating lymphocytes compared with control mice. SB431542 blocked acute and chronic UVB-induced skin inflammation and T-cell activation in the skin-draining lymph node (SDLN) and skin but did not alter UVB-induced epidermal proliferation. We tested the effect of SB431542 on migration of skin dendritic cell (DC) populations because DCs are critical mediators of T-cell activation and cutaneous inflammation. SB431542 blocked (i) UVB-induced Smad2 phosphorylation in dermal DC (dDC) and (ii) SDLN and ear explant migration of CD103(+) CD207(+) and CD207(-) skin DC subsets but did not affect basal or UV-induced migration of Langerhans cells. Mice expressing a dominant-negative TGFβ type II receptor in CD11c(+) cells had reduced basal and UVB-induced SDLN migration of CD103(+) CD207(+) and CD207(-) DC subsets and a reduced percentage of CD86(high) dDC following UVB irradiation. Together, these suggest that TGFβ1 signaling has a tumor-promoting role in UVB-induced skin carcinogenesis and this is mediated in part through its role in UVB-induced migration of dDC and cutaneous inflammation.

Secondary lymphoid organ homing phenotype of human myeloid dendritic cells disrupted by an intracellular oral pathogen

Several intracellular pathogens, including a key etiological agent of chronic periodontitis, Porphyromonas gingivalis, infect blood myeloid dendritic cells (mDCs). This infection results in pathogen dissemination to distant inflammatory sites (i.e., pathogen trafficking). The alteration in chemokine-chemokine receptor expression that contributes to this pathogen trafficking function, particularly toward sites of neovascularization in humans, is unclear. To investigate this, we utilized human monocyte-derived DCs (MoDCs) and primary endothelial cells in vitro, combined with ex vivo-isolated blood mDCs and serum from chronic periodontitis subjects and healthy controls. Our results, using conditional fimbria mutants of P. gingivalis, show that P. gingivalis infection of MoDCs induces an angiogenic migratory profile. This profile is enhanced by expression of DC-SIGN on MoDCs and minor mfa-1 fimbriae on P. gingivalis and is evidenced by robust upregulation of CXCR4, but not secondary lymphoid organ (SLO)-homing CCR7. This disruption of SLO-homing capacity in response to respective chemokines closely matches surface expression of CXCR4 and CCR7 and is consistent with directed MoDC migration through an endothelial monolayer. Ex vivo-isolated mDCs from the blood of chronic periodontitis subjects, but not healthy controls, expressed a similar migratory profile; moreover, sera from chronic periodontitis subjects expressed elevated levels of CXCL12. Overall, we conclude that P. gingivalis actively "commandeers" DCs by reprogramming the chemokine receptor profile, thus disrupting SLO homing, while driving migration toward inflammatory vascular sites.

E-cadherin interactions are required for Langerhans cell differentiation

Human skin contains the following two distinct DC subsets: (i) Langerhans cells (LCs), expressing Langerin but not DC-specific intercellular adhesion molecule-3-grabbing nonintegrin (DC-SIGN), are predominantly localized in the epidermis; and (ii) dermal DCs, expressing DC-SIGN but not Langerin, are observed mainly in the dermis. It is not known whether localization in the epidermis provides cues for LC differentiation. Here, we show that E-cadherin expressed by epidermal keratinocytes (KCs) is crucial for differentiation of LCs. Monocytes differentiated into LC-like cells in presence of IL-4, GM-CSF, and TGF-β1. However, these LC-like cells expressed not only Langerin but also DC-SIGN. Notably, co-culturing of these LC-like cells with KCs expressing E-cadherin or recombinant E-cadherin strongly decreased expression of DC-SIGN and further induced a phenotype similar to purified epidermal LCs. Moreover, pretreatment of LC-like cells with anti-E-cadherin-specific antibody completely abolished their Langerin expression, indicating the requirement of E-cadherin–E-cadherin interactions for the differentiation into Langerin⁺ cells. These findings suggest that E-cadherin expressed by KCs provide environmental cues that induce differentiation of LCs in the epidermis.

TGFβ1 overexpression by keratinocytes alters skin dendritic cell homeostasis and enhances contact hypersensitivity

Overexpression of Transforming Growth Factor Beta1 (TGFβ1) in mouse epidermis causes cutaneous inflammation and keratinocyte hyperproliferation. Here, we examined acute effects of TGFβ1 overproduction by keratinocytes on skin dendritic cells (DCs). TGFβ1 induction for 2 and 4 days increased numbers and CD86 expression of B220+ plasmacytoid DCs (pDCs) and CD207+CD103+, CD207−CD103−CD11b+ and CD207−CD103−CD11b− dermal DCs (dDCs) in skin draining lymph nodes (SDLN). The dermis of TGFβ1-overexpressing mice had significantly more pDCs, CD207+CD103+ dDCs and CD207-CD11b+ dDCs in the absence of increased dermal proliferation. Application of dye, TRITC, in dibutylpthalate (DBP) solution after TGFβ1 induction increased the numbers of TRITC+CD207− dDCs in SDLN, and augmented TRITC/DBP-induced Langerhans cell (LC) migration 72 hrs post-TRITC treatment. Consistent with this, LC migration was increased in vitro by TGFβ1 overexpression in skin explants and by exogenous TGFβ1 in culture media. Transient TGFβ1 induction during DNFB sensitization increased contact hypersensitivity responses by 1.5-fold. Thus, elevated epidermal TGFβ1 alone is sufficient to alter homeostasis of multiple cutaneous DC subsets and enhance DC migration and immune responses to contact sensitizers. These results highlight a role for keratinocyte-derived TGFβ1 in DC trafficking and the initiation of skin inflammation.

A regulatory dendritic cell signature correlates with the clinical efficacy of allergen-specific sublingual immunotherapy

BACKGROUND

Given their pivotal role in the polarization of T-cell responses, molecular changes at the level of dendritic cells (DCs) could represent an early signature indicative of the subsequent orientation of adaptive immune responses during immunotherapy.

OBJECTIVE

We sought to investigate whether markers of effector and regulatory DCs are affected during allergen immunotherapy in relationship with clinical benefit.

METHODS

Differential gel electrophoresis and label-free mass spectrometry approaches were used to compare whole proteomes from human monocyte-derived DCs differentiated toward either regulatory or effector functions. The expression of those markers was assessed by using quantitative PCR in PBMCs from 79 patients with grass pollen allergy enrolled in a double-blind, placebo-controlled clinical study evaluating the efficacy of sublingual tablets in an allergen exposure chamber over a 4-month period.

RESULTS

We identified several markers associated with DC1 and/or DC17 effector DCs, including CD71, FSCN1, IRF4, NMES1, MX1, TRAF1. A substantial phenotypic heterogeneity was observed among various types of tolerogenic DCs, with ANXA1, Complement component 1 (C1Q), CATC, GILZ, F13A, FKBP5, Stabilin-1 (STAB1), and TPP1 molecules established as shared or restricted regulatory DC markers. The expression of 2 of those DCs markers, C1Q and STAB1, was increased in PBMCs from clinical responders in contrast to that seen in nonresponders or placebo-treated patients.

CONCLUSION

C1Q and STAB1 represent candidate biomarkers of early efficacy of allergen immunotherapy as the hallmark of a regulatory innate immune response predictive of clinical tolerance.

Autocrine/paracrine TGF-β1 inhibits Langerhans cell migration

Langerhans cells (LCs) are skin-resident dendritic cells (DC) located in the epidermis that migrate to skin-draining lymph nodes during the steady state and in response to inflammatory stimuli. TGF-β1 is a critical immune regulator that is highly expressed by LCs. The ability to test the functional importance of LC-derived TGF-β1 is complicated by the requirement of TGF-β1 for LC development and by the absence of LCs in mice with an LC-specific ablation of TGF-β1 or its receptor. To overcome these problems, we have engineered transgenic huLangerin-CreER(T2) mice that allow for inducible LC-specific excision. Highly efficient and LC-specific expression was confirmed in mice bred onto a YFP Cre reporter strain. We next generated huLangerin-CreER(T2) × TGF-βRII(fl) and huLangerin-CreER(T2) × TGF-β1(fl) mice. Excision of the TGFβRII or TGFβ1 genes induced mass migration of LCs to the regional lymph node. Expression of costimulatory markers and inflammatory cytokines was unaffected, consistent with homeostatic migration. In addition, levels of p-SMAD2/3 were decreased in LCs from wild-type mice before inflammation-induced migration. We conclude that TGF-β1 acts directly on LCs in an autocrine/paracrine manner to inhibit steady-state and inflammation-induced migration. This is a readily targetable pathway with potential therapeutic implications for skin disease.

Severe dermatitis with loss of epidermal Langerhans cells in human and mouse zinc deficiency

Zinc deficiency can be an inherited disorder, in which case it is known as acrodermatitis enteropathica (AE), or an acquired disorder caused by low dietary intake of zinc. Even though zinc deficiency diminishes cellular and humoral immunity, patients develop immunostimulating skin inflammation. Here, we have demonstrated that despite diminished allergic contact dermatitis in mice fed a zinc-deficient (ZD) diet, irritant contact dermatitis (ICD) in these mice was more severe and prolonged than that in controls. Further, histological examination of ICD lesions in ZD mice revealed subcorneal vacuolization and epidermal pallor, histological features of AE. Consistent with the fact that ATP release from chemically injured keratinocytes serves as a causative mediator of ICD, we found that the severe ICD response in ZD mice was attenuated by local injection of soluble nucleoside triphosphate diphosphohydrolase. In addition, skin tissue from ZD mice with ICD showed increased levels of ATP, as did cultured wild-type keratinocytes treated with chemical irritants and the zinc-chelating reagent TPEN. Interestingly, numbers of epidermal Langerhans cells (LCs), which play a protective role against ATP-mediated inflammatory signals, were decreased in ZD mice as well as samples from ZD patients. These findings suggest that upon exposure to irritants, aberrant ATP release from keratinocytes and impaired LC-dependent hydrolysis of nucleotides may be important in the pathogenesis of AE.

Human myeloid dendritic cells treated with supernatants of rotavirus infected Caco-2 cells induce a poor Th1 response

We have previously shown that human myeloid dendritic cells treated with purified rotavirus induce an allogenic Th1 response. To determine if rotavirus in the context of an intestinal microenvironment modulates the function of dendritic cells, we treated these cells with supernatants from non-infected or infected Caco-2 cells and studied their capacity to promote Th1 or Th2 responses. Dendritic cells treated with supernatants from rotavirus-infected Caco-2 cells promoted a significantly lower Th1 response, in comparison with those treated with purified rotavirus. We wanted to establish if TGF-β1, induced, or TSLP, not induced, during rotavirus infection, could mediate this effect. Neutralization of TGF-β but not TSLP in the supernatant prior to treatment of dendritic cells increased their capacity to promote a Th1 response. The results suggest that the TGF-β1 induced by rotavirus could be an immune evasion mechanism, and may partially explain the poor rotavirus-specific T cell response we have previously evidenced.

Dendritic cell control of tolerogenic responses

One of the most fundamental problems in immunology is the seemingly schizophrenic ability of the immune system to launch robust immunity against pathogens, while acquiring and maintaining a state of tolerance to the body's own tissues and the trillions of commensal microorganisms and food antigens that confront it every day. A fundamental role for the innate immune system, particularly dendritic cells (DCs), in orchestrating immunological tolerance has been appreciated, but emerging studies have highlighted the nature of the innate receptors and the signaling pathways that program DCs to a tolerogenic state. Furthermore, several studies have emphasized the major role played by cellular interactions and the microenvironment in programming tolerogenic DCs. Here, we review these studies and suggest that the innate control of tolerogenic responses can be viewed as different hierarchies of organization, in which DCs, their innate receptors and signaling networks, and their interactions with other cells and local microenvironments represent different levels of the hierarchy.

Cooperative regulation of NOTCH1 protein-phosphatidylinositol 3-kinase (PI3K) signaling by NOD1, NOD2, and TLR2 receptors renders enhanced refractoriness to transforming growth factor-beta (TGF-beta)- or cytotoxic T-lymphocyte antigen 4 (CTLA-4)-mediated impairment of human dendritic cell maturation

Dendritic cells (DCs) as sentinels of the immune system are important for eliciting both primary and secondary immune responses to a plethora of microbial pathogens. Cooperative stimulation of a complex set of pattern-recognition receptors, including TLR2 and nucleotide-binding oligomerization domain (NOD)-like receptors on DCs, acts as a rate-limiting factor in determining the initiation and mounting of the robust immune response. It underscores the need for "decoding" these multiple receptor interactions. In this study, we demonstrate that TLR2 and NOD receptors cooperatively regulate functional maturation of human DCs. Intriguingly, synergistic stimulation of TLR2 and NOD receptors renders enhanced refractoriness to TGF-β- or CTLA-4-mediated impairment of human DC maturation. Signaling perturbation data suggest that NOTCH1-PI3K signaling dynamics assume critical importance in TLR2- and NOD receptor-mediated surmounting of CTLA-4- and TGF-β-suppressed maturation of human DCs. Interestingly, the NOTCH1-PI3K signaling axis holds the capacity to regulate DC functions by virtue of PKCδ-MAPK-dependent activation of NF-κB. This study provides mechanistic and functional insights into TLR2- and NOD receptor-mediated regulation of DC functions and unravels NOTCH1-PI3K as a signaling cohort for TLR2 and NOD receptors. These findings serve in building a conceptual foundation for the design of improved strategies for adjuvants and immunotherapies against infectious diseases.

Cross talk between keratinocytes and dendritic cells: impact on the prediction of sensitization

Understanding the mechanistic aspects involved in sensitization by chemicals will help to develop relevant preventive strategies. Many potential sensitizers are not directly immunogenic but require activation outside or inside the skin by nonenzymatic oxidation (prehaptens) or metabolic transformation (prohaptens) prior to being able to induce an immune response. This necessary activation step has not yet been actively integrated into a cell line-based prediction approach. We cocultured HaCaT keratinocytes with THP-1 as dendritic cell-like cells allowing intercellular interactions. The sensitizing potential was determined by analyzing differences in the expression of CD86, CD40, and CD54 on cocultured THP-1 cells. This new assay setup allowed (1) to distinguish irritants from allergens without influencing cell viability and (2) to discriminate pre/prohaptens from haptens. Under coculture conditions, the prohaptens eugenol, 2-methoxy-4-methylphenol, and benzo[a]pyrene induced a significantly higher upregulation of CD86 expression on THP-1. In agreement with the hapten concept, responses to 2,4-dinitrochlorobenzene, Bandrowski's base, and the prehapten isoeugenol were not significantly modified. Inhibition of cytochrome P450 or NAD(P)H:quinone oxidoreductase (NQO1) activity reduced the prohapten-mediated upregulation of CD86 on cocultured THP-1 cells. This coculture assay allowing cross talk between HaCaT and THP-1 cells appears to be suitable for the detection of prohaptens, is reproducible, easy to perform, and avoids donor variations. In addition, this assay is a promising approach to understand the impact of cross talk on the prediction of sensitization and once established may be integrated in a future in vitro toolbox to detect potential skin sensitizers and may thus contribute to reduce animal testing.

Identification of a new phenotype of tolerogenic human dendritic cells induced by fungal proteases from Aspergillus oryzae

We characterized a new pathway to induce tolerogenic dendritic cells (DCs) following treatment of human monocyte-derived DCs with proteases from the fungus Aspergillus oryzae (ASP). ASP-treated DCs (ASP-DCs) exhibit a CD80(-)CD83(-)CD86(-)Ig-like transcript (ILT)2(-)ILT3(-)ILT4(+) phenotype, do not secrete cytokines or chemokines, and express tolerogenic markers such as glucocorticoid-induced leucine zipper, NO synthetase-2, retinaldehyde dehydrogenase-1 or retinaldehyde dehydrogenase-2. When cocultured with naive CD4(+) T cells, ASP-DCs induce an anergic state that can be reversed by IL-2. Generated T cells mediate a suppressive activity in third-party experiments that is not mediated by soluble factors. A comparison between dexamethasone-treated DCs used as a reference for regulatory T cell-inducing DCs and ASP-DCs reveals two distinct phenotypes. In contrast to dexamethasone, ASP treatment induces glucocorticoid-induced leucine zipper independently of glucocorticoid receptor engagement and leads to NF-κB p65 degradation. Abrogation of protease activities in ASP using specific inhibitors reveals that aspartic acid-containing proteases are key inducers of regulatory genes, whereas serine, cysteine, and metalloproteases contribute to NF-κB p65 degradation. Collectively, those features correspond to a previously unreported anergizing phenotype for human DCs. Such regulatory mechanisms may allow fungi to downregulate host immune responses and provide clues for new approaches to treat proinflammatory disorders.

TGF-beta is required to maintain the pool of immature Langerhans cells in the epidermis

The pivotal role of TGF-beta in Langerhans cell (LC) development has been previously established in TGF-beta-deficient mice, which lack epidermal LCs. As to whether TGF-beta also governs LC homeostasis and function remains elusive. To assess the role of TGF-beta-mediated control of cutaneous dendritic cells (DCs) in vivo, we generated mice with a conditional knockout of the TGF-beta receptor 1 (TbetaR1) under a DC-specific promoter (DC-TbetaR1(del) mice). While initial LC seeding occurred in DC-TbetaR1(del) mice, the cells disappeared from the epidermis during the first week of life. TbetaR1-deficient LCs demonstrated spontaneous maturation and gained migratory potential based on increased surface expression of MHC class II, costimulatory molecules, and CCR7 and downregulation of E-cadherin. In parallel to their early loss from the epidermis, migrating LCs were reduced in the dermis and skin-draining lymph nodes of adult DC-TbetaR1(del) mice, whereas the number of Langerin(+) dermal DCs was similar to wild-type. In the absence of LCs, low-dose contact hypersensitivity in DC-TbetaR1(del) mice was significantly diminished. In contrast, ear swelling was restored to wild-type levels when a higher hapten dose was applied to efficiently target TbetaR1-deficient dermal DCs. In conclusion, TGF-beta inhibits in vivo LC maturation and migratory phenotype, identifying TGF-beta as a critical factor controlling LC homeostasis in the steady state.

miR-146a is differentially expressed by myeloid dendritic cell subsets and desensitizes cells to TLR2-dependent activation

Langerhans cells (LCs) in epithelia and interstitial dendritic cells (intDCs) in adjacent connective tissues represent two closely related myeloid-derived DC subsets that exert specialized functions in the immune system and are of clinical relevance for cell therapy. Both subsets arise from monocyte-committed intermediates in response to tissue-associated microenvironmental signals; however, molecular mechanisms underlying myeloid DC subset specification and function remain poorly defined. Using microarray profiling, we identified microRNA (miRNA) miR-146a to be constitutively expressed at higher levels in human LCs compared with intDCs. Moreover, miR-146a levels were low in monocytes and nondetectable in neutrophil granulocytes. Interestingly, constitutive high miR-146a expression in LCs is induced by the transcription factor PU.1 in response to TGF-beta1, a key microenvironmental signal for epidermal LC differentiation. We identified miR-146a as a regulator of monocyte and DC activation but not myeloid/DC subset differentiation. Ectopic miR-146a in monocytes and intDCs interfered with TLR2 downstream signaling and cytokine production, without affecting phenotypic DC maturation. Inversely, silencing of miR-146a in LCs enhanced TLR2-dependent NF-kappaB signaling. We therefore conclude that high constitutive miR-146a levels are induced by microenvironmental signals in the epidermis and might render LCs less susceptible to inappropriate activation by commensal bacterial TLR2 triggers at body surfaces.

Langerhans cells: critical regulators of skin immunity?

Langerhans cells (LC) are members of the heterogenous family of professional antigen presenting dendritic cells (DC). They are identified by the C-type lectin receptor Langerin and form a contiguous network in the epidermis. Consequently, LC are an integral part of the skin barrier to the environment and were considered to be critical inducers of skin immunity, whereas dermal DC were largely overlooked. However, with the identification of a distinct subset of Langerin expressing dermal DC, the situation in the skin has become more complex and the relative contribution of the different cutaneous DC populations in balancing immunity and tolerance has become a matter of active debate. Here, we briefly review the classical paradigm and recent challenges of LC function, before focusing on advances concerning their role in contact hypersensitivity and ultraviolet radiation-induced immunosuppression obtained with in vivo LC ablation models. We then discuss novel LC/DC-specific gene targeting approaches currently used to dissect the role of the regulatory cytokines transforming growth factor-beta and interleukin-10 to govern LC and DC function in vivo. This second generation of LC-specific genetically engineered mice will considerably extend our understanding of the molecular control of LC function in regulating skin immunity and tolerance in the near future.