Cutaneous Dendritic Cells Research Articles

Resolvin E1 regulates contact hypersensitivity responses by inhibiting dendritic cell functions through leukotriene B4-BLT1 signaling blockade (MUC2P.936)

Abstract Resolvin E1 (RvE1) is a lipid mediator derived from ω3 polyunsaturated fatty acids that exert potent anti-inflammatory roles in several disease models. However, the precise underlying mechanism how RvE1 exerts its action in vivo remains largely unknown. In this study, we investigated the mechanisms of the anti-inflammatory effect of RvE1 using a contact hypersensitivity model. We found that administration of RvE1 significantly suppressed skin inflammation through a blockade of BLT1, a leukotriene B4 (LTB4) receptor, on dendritic cells (DCs). Live imaging of cutaneous DCs by two-photon microscopy revealed that RvE1 significantly impaired DC motility in the skin and migration to draining lymph nodes as well as antigen presentation ability in the skin. LTB4-BLT1 signaling induced actin formation in DCs and increased DC motility by activating Cdc42 and Rac1 through the PI3 kinase pathway, which was canceled by the administration of RvE1. Taken together, our results indicate that RvE1 attenuates cutaneous immune responses by inhibiting cutaneous DC functions through LTB4-BLT1 signaling blockade.

Dermal Vγ4+ γδ T Cells Possess a Migratory Potency to the Draining Lymph Nodes and Modulate CD8+ T-Cell Activity through TNF-α Production

A large number of gamma delta T cells (γδ T cells) are located within epithelial tissues including the skin. In mice, epidermal and dermal γδ T cells consist of distinct subsets and have specific roles in cutaneous immune responses. A recent study demonstrated that γδ T cells and cutaneous dendritic cells migrate from the skin to the draining lymph nodes (LNs). However, it remains unclear whether they regulate the antigen-specific immune response within the LNs. Herein, we investigated their properties and role in the LNs using the Mycobacterium bovis bacille Calmette-Guérin (BCG) infection model. In vivo cell labeling analysis revealed that most of the migratory subset comprised dermal Vγ4(+) cells. This population transmigrated from the skin to the LNs in a Gi-coupled chemokine receptor-independent manner. By depleting Vγ4(+) cells, the intranodal expansion of CD8(+) T cell against BCG was significantly attenuated. In addition, in vitro analysis revealed that Vγ4(+) cells produced TNF-α and enhanced IL-12 production by dendritic cells. Taken together, these findings suggest that dermal Vγ4(+) cells are a unique subset that possesses a migratory potency to the skin-draining LNs and enhances the dendritic cell function therein.

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.

Interleukin-12 preserves the cutaneous physical and immunological barrier after radiation exposure.

The United States continues to be a prime target for attack by terrorist organizations in which nuclear detonation and dispersal of radiological material are legitimate threats. Such attacks could have devastating consequences to large populations, in the form of radiation injury to various human organ systems. One of these at risk organs is the cutaneous system, which forms both a physical and immunological barrier to the surrounding environment and is particularly sensitive to ionizing radiation. Therefore, increased efforts to develop medical countermeasures for treatment of the deleterious effects of cutaneous radiation exposure are essential. Interleukin-12 (IL-12) was shown to elicit protective effects against radiation injury on radiosensitive systems such as the bone marrow and gastrointestinal tract. In this article, we examined if IL-12 could protect the cutaneous system from a combined radiation injury in the form of sublethal total body irradiation and beta-radiation burn (β-burn) directly to the skin. Combined radiation injury resulted in a breakdown in skin integrity as measured by transepidermal water loss, size of β-burn lesion and an exacerbated loss of surveillant cutaneous dendritic cells. Interestingly, intradermal administration of IL-12 48 h postirradiation reduced transepidermal water loss and burn size, as well as retention of cutaneous dendritic cells. Our data identify IL-12 as a potential mitigator of radiation-induced skin injury and argue for the further development of this cytokine as a radiation countermeasure.

T Cells in CTCL Have an Exhausted Phenotype While Cutaneous Dendritic Cells Display a Normally Activated Mature Phenotype

Background: Mycosis fungoides (MF) and Sézary syndrome (SS) represent the most common types of cutaneous T-cell lymphoma (CTCL). CTCL develops from clonally-expanded, effector/central memory CD4+ T cells in a background of chronic inflammation. The eliciting Ag(s) and how this inflammation contributes to the development and progression of CTCL remain important unknowns. Despite tumor-infiltrating CD8+ T cells in skin, the malignant CD4+ T-cell population persists, indicating that these CD8+ T cells cannot exert an effective anti-tumor response, potentially due to T-cell exhaustion, which has been observed in chronic viral infections and is associated with disease progression in hematologic malignancies like ATLL and CLL. Exhausted T-cells are characterized by immunoinhibitory molecules, like programmed death-1 (PD-1). Engagement of PD-1 by PD-Ligand (L)-1/L-2 transduces a signal that leads to inhibition of T-cell functions, but the cells are not deleted. PD-L1 is expressed on T cells, dendritic cells (DCs), and many tumor cells, whereas PD-L2 expression is limited to antigen-presenting cells. Although malignant CD4+ T cells in Sézary syndrome lesions express PD-1, the relevance of the PD-1/PD-L1/2 pathway in CTCL and the immune cells in CTCL that express PD-L1/L2 are not known.Methods: Epidermis and dermis of skin biopsies from consented CTCL pts (12) and healthy donors (3) were separated using dispase-II and incubated in RPMI-10% pooled human serum for 24 hrs. Migrated cutaneous T cells and DCs were phenotyped by flow cytometry for a variety of maturation markers and immune checkpoint molecules. In 3 pts skin findings were compared with matched PBMCs.Results: Freshly isolated cutaneous CD8+ T cells displayed distinct effector and memory subsets with a higher frequency of effector-memory (CD28-CD45RA-CCR7-) and terminally differentiated effector cells (CD28-CD45RA+CCR7-). In addition, CD8+ T-cells exhibited enhanced expression of PD-1, Lag-3, Tim-3, and ICOS. PD-1 was the receptor most overexpressed in CD8+ T cells compared with healthy controls. PD-1 expression on circulating CD8+ T cells was lower compared with skin-infiltrating CD8+ T-cells. In contrast, the malignant CD4+ T-cells expressed predominantly a central memory or transient memory subtype (CD28+CD45RA-CCR7+/-), with overexpression of PD-1, CTLA-4, Lag-3, and ICOS. Immune-suppressive CD3+CD4+FoxP3+CD127-Tregs were not increased in PBMCs of CTCL pts. Migrated DCs displayed an activated, mature phenotype expressing HLA-DR+, CD1a+/-, CD207 (Langerin)+/-, CD14+/-, CD11c+, CD83low/med, CD86low/med and PD-L1+/- phenotype.Conclusion: Both the malignant CD4+ T cells and tumor-infiltrating CD8+ T cells display an exhausted phenotype, while DC subsets display a normal activated phenotype. Our preliminary data imply a role for PD-1 and other checkpoint molecules like ICOS, Tim-3, Lag-3 in attenuating the anti-tumor response. Studies are underway in vitro to establish the function of PD-1, Lag-3, Tim-3, and ICOS on T cells in response to DC/LC stimulation to develop a rationale for therapeutic checkpoint blockade inhibitors to reverse the exhausted T cell phenotype and attendant lack of immune reactivity. DisclosuresNo relevant conflicts of interest to declare.

CCR8 regulates contact hypersensitivity by restricting cutaneous dendritic cell migration to the draining lymph nodes.

Allergic contact dermatitis (ACD) is a typical occupational disease in industrialized countries. Although various cytokines and chemokines are suggested to be involved in the pathogenesis of ACD, the roles of these molecules remain to be elucidated. CC chemokine receptor 8 (CCR8) is one such molecule, of which expression is up-regulated in inflammatory sites of ACD patients. In this study, we found that Ccr8(-/-) mice developed severer contact hypersensitivity (CHS) responses to 2,4-dinitrofluorobenzene, a murine model of ACD, compared with wild-type mice. T cells from Ccr8(-/-) mice showed enhanced proliferative recall responses and Th1 and Th17 cell populations were expanded in these mice. However, CHS responses were similar between SCID mice adoptively transferred with Ccr8(-/-) and wild-type T cells, suggesting that CCR8 in T cells is not responsible for the exacerbation of CHS. Notably, skin-resident dendritic cells (DCs), such as Langerhans cells and dermal DCs, and inflammatory DCs were highly accumulated in lymph nodes (LNs) of Ccr8(-/-) mice after sensitization. Consistent with this, Ccr8(-/-) antigen-presenting cells readily migrated from the skin to the draining LNs after sensitization. These observations suggest that CCR8 negatively regulates migration of cutaneous DCs from the skin to the draining LNs in CHS by keeping these cells in the skin.

Flt3L Dependence Helps Define an Uncharacterized Subset of Murine Cutaneous Dendritic Cells

Skin-derived dendritic cells (DC) are potent antigen presenting cells with critical roles in both adaptive immunity and tolerance to self. Skin DC carry antigens and constitutively migrate to the skin draining lymph nodes (LN). In mice, Langerin-CD11b− dermal DC are a low-frequency, heterogeneous, migratory DC subset that traffic to LN (Langerin-CD11b-migDC). Here, we build on the observation that Langerin-CD11b− migDC are Fms-like tyrosine kinase 3 ligand (Flt3L) dependent and strongly Flt3L responsive, which may relate them to classical DCs. Examination of DC capture of FITC from painted skin, DC isolation from skin explant culture, and from the skin of CCR7 knockout mice which accumulate migDC, demonstrate these cells are cutaneous residents. Langerin-CD11b-Flt3L responsive DC are largely CD24(+) and CX3CR1low and can be depleted from Zbtb46-DTR mice, suggesting classical DC lineage. Langerin-CD11bmigDC present antigen with equal efficiency to other DC subsets ex vivo including classical CD8α cDC and Langerin+CD103+ dermal DC. Finally, transcriptome analysis suggests a close relationship to other skin DC, and a lineage relationship to other classical DC. This work demonstrates that Langerin- CD11b− dermal DC, a previously overlooked cell subset, may be an important player in the cutaneous immune environment.

Depletion of Cutaneous Macrophages and Dendritic Cells Promotes Growth of Basal Cell Carcinoma in Mice

Basal cell carcinoma (BCC) belongs to the group of non-melanoma skin tumors and is the most common tumor in the western world. BCC arises due to mutations in the tumor suppressor gene Patched1 (Ptch). Analysis of the conditional Ptch knockout mouse model for BCC reveals that macrophages and dendritic cells (DC) of the skin play an important role in BCC growth restraining processes. This is based on the observation that a clodronate-liposome mediated depletion of these cells in the tumor-bearing skin results in significant BCC enlargement. The depletion of these cells does not modulate Ki67 or K10 expression, but is accompanied by a decrease in collagen-producing cells in the tumor stroma. Together, the data suggest that cutaneous macrophages and DC in the tumor microenvironment exert an antitumor effect on BCC.

Distinct Molecular Signature of Human Skin Langerhans Cells Denotes Critical Differences in Cutaneous Dendritic Cell Immune Regulation

Langerhans cells (LCs) are professional antigen-presenting cells (APCs) residing in the epidermis. Despite their high potential to activate T lymphocytes, current understanding of human LC biology is limited. Genome-wide comparison of the transcriptional profiles of human skin migratory CD1a+ LCs and CD11c+ dermal dendritic cells (DDCs) demonstrated significant differences between these "dendritic cell (DC)" types, including preferential expression of 625 genes (P<0.05) in LC and 914 genes (P<0.05) in DDC. Analysis of the temporal regulation of molecular networks activated after stimulation with tumor necrosis factor-α (TNF-α) confirmed the unique molecular signature of LCs. Although LCs conformed to the phenotype of professional APC, inflammatory signaling activated primarily genes associated with cellular metabolism and mitochondrial activation (e.g., CYB561 and MRPS35), cell membrane re-organization, and antigen acquisition and degradation (CAV1 and PSMD14; P<0.05-P<0.0001). Conversely, TNF-α induced classical activation in DDCs with early downregulation of surface receptors (mannose receptor-1 (MRC1) and C-type lectins), and subsequent upregulation of cytokines, chemokines (IL1a, IL1b, and CCL18), and matrix metalloproteinases (MMP1, MMP3, and MMP9; P<0.05-P<0.0001). Functional interference of caveolin abrogated LCs superior ability to cross-present antigens to CD8+ T lymphocytes, highlighting the importance of these networks to biological function. Taken together, these observations support the idea of distinct biological roles of cutaneous DC types.

Review: Skin and the Immune System

The skin immune system comprises a complex network of cells, functioning both in immunity against invading pathogens but also tolerogenic mechanisms to ensure maintenance of immune homeostasis. The nature of antigens present and interplay between the cutaneous innate and adaptive immune systems determine the type of immune response generated. Dendritic cells are the key players in bridging innate and adaptive immune responses due to their inherent plasticity, direct roles in both type of immune responses, and cross-talk with other immune cells. This review dissects the functional roles of components of both innate and adaptive immune systems in the skin, with a special focus on cutaneous dendritic cells as the only cells capable of inducing primary immune responses, their ability to generate either immunogenic or tolerogenic immune responses, and ability to direct effector cells back to the skin via imprinting of skin-homing properties on T-cells. Finally the reciprocal interactions between the skin microbiota and immune system and their role in host defence and disease have been discussed.

Aldara-induced psoriasis-like skin inflammation: isolation and characterization of cutaneous dendritic cells and innate lymphocytes.

Psoriasis is a chronic auto-inflammatory skin disease of unknown etiology affecting millions of people worldwide. Dissecting the cellular networks and molecular signals promoting the development of psoriasis critically depends on appropriate animal models. Topical application of Aldara cream containing the Toll-like receptor (TLR)7-ligand Imiquimod induces skin inflammation and pathology in mice closely resembling plaque-type psoriasis in humans. The particular power of the Aldara model lies in examining the early events during psoriatic plaque formation, which is difficult to achieve in patients. Hence, recent reports using this model have challenged currently prevailing concepts concerning the pathophysiology of psoriasis. Here, we describe the induction and phenotype of Aldara-mediated dermatitis in mice and, in particular, analysis of the inflammatory cell infiltrate using flow cytometry.

CCR7 Plays No Appreciable Role in Trafficking of Central Memory CD4 T Cells to Lymph Nodes

CCR7⁻/⁻ mice exhibit profound anomalies in lymph node and spleen architecture, which complicates the study of CCR7-mediated T cell trafficking in vivo. To circumvent this problem, we established in vivo models in which wild-type and CCR7⁻/⁻ populations coexist within mice possessing normal lymphoid organs and must compete for developmental niches within the tissues of these mice. Under the conditions we have created in vivo, we find the entry of memory CD4 T cells into lymph nodes from the blood to be independent of CCR7. Thus, the central memory CD4 T cells that traffic though lymph nodes, which are often defined by their expression of CCR7, do not appear to gain any competitive homing advantage by expressing this receptor. Furthermore, in contrast to cutaneous dendritic cell populations, we found that CCR7 deficiency had no appreciable effect on the exit of CD4 T cells from inflamed skin. Finally, we found that wild-type and CCR7⁻/⁻ precursors were equally represented within the major thymic subpopulations, despite previous findings that CCR7 plays a role in seeding the thymus from bone marrow-derived T cell precursors.

Mechanisms by Which Chronic Ethanol Feeding Impairs the Migratory Capacity of Cutaneous Dendritic Cells

Chronic alcoholism is associated with increased incidence and severity of skin infection. Cutaneous dendritic cells (CDCs) play a pivotal role in skin immunity, and chronic ethanol (EtOH) feeding in mice has been shown to inhibit CDC migration to skin-draining lymph nodes (dLNs) following epicutaneous sensitization. Because CDC subsets differentially initiate T-cell responses, it is important to determine how EtOH feeding affects migration of each subset and identify mechanisms responsible for observed defects. Mice received EtOH in the drinking water for ≥ 16weeks. Baseline numbers of CDC subsets and their migration to the dLNs following fluorescein 5-isothiocyanate (FITC) sensitization were assessed by flow cytometry. Epidermal cell suspension and skin explant cultures were used to measure the impact of EtOH upon molecules that influence CDC migration. Cytokine arrays performed on explant culture supernatants assessed local production of inflammatory cytokines. Chronic EtOH feeding reduced migration of all CDC subsets to the dLNs following FITC sensitization. Reduced migration of dermal-resident CDCs did not correspond with reduced baseline numbers of these cells. For Langerhans cells (LCs), EtOH-induced migratory dysfunction corresponded with delayed down-regulation of E-cadherin, chemokine receptor 1 (CCR1), and CCR6 and impaired up-regulation of matrix metalloproteinases (MMPs) 2 and 9. In skin explant assays, EtOH blunted CDC mobilization following stimulation with CCL21/CPG 1826. No alteration in CD54 or CCR7 expression was observed, but production of skin-derived tumor necrosis factor alpha (TNF-α) was reduced. Poor migratory responses in vitro could be improved by supplementing explant cultures from EtOH-fed mice with TNF-α. Chronic EtOH consumption does not alter baseline dermal-resident CDC numbers. However, like LCs, migratory responsiveness of dermal CDCs was decreased following FITC sensitization. Inefficient down-regulation of both CCRs and adhesion molecules and the inability to up-regulate MMPs indicate that EtOH impedes LC acquisition of a promigratory phenotype. These defects, combined with improvement of the migratory defect with in vitro TNF-α replacement, demonstrate intrinsic as well as environmental contributions to defective CDC migration. These findings provide novel mechanisms to explain the observed increased incidence and severity of skin infections in chronic alcoholics.

Prostaglandin E2-EP3 Axis in Fine-Tuning Excessive Skin Inflammation by Restricting Dendritic Cell Functions

Prostaglandin E2 (PGE2) is produced in the skin and is suggested to play a role in the regulation of cutaneous immune homeostasis and responses. However, the multifaceted functions of PGE2 continue to elude our understanding, especially because of the multiplicity of PGE2 receptors—EP1, EP2, EP3, and EP4. While cAMP-elevating EP4 is known to activate the functions of cutaneous dendritic cells (DCs), including Langerhans cells (LCs) and dermal DCs, the role of cAMP-suppressing EP3 in this process remains unknown. Here we demonstrated that an EP3 receptor selective agonist, ONO-AE-248, inhibited chemotaxis and co-stimulatory molecule expressions of DCs in vitro. A suboptimal dose of antigen was sufficient to induce contact hypersensitivity in EP3-deficient mice. Intriguingly, EP3 deficiency did not impair skin inflammation at all when the antigen dose was sufficiently high. EP3 limited the functions of cutaneous DCs only when the antigen dose was low. In contrast to EP4, the observed unappreciated function of EP3 may stabilize the cutaneous DCs to halt the impetuous response to a suboptimal dose of antigen. Taken together, PGE2-EP3 signaling is essential for fine-tuning excessive skin inflammation by restricting DC functions.

Radiation-induced depletion of cutaneous dendritic cells: Functional consequences of a breakdown in the cutaneous immunological barrier (P4259)

Abstract The United States continues to be a prime target for attack by terrorist organizations in which nuclear detonation or dispersal of radiological material are legitimate threats. These types of attacks could result in devastating consequences to a multitude of individuals in the form of radiation injury to particular organ systems. One of these organs, the cutaneous system, which provides the first line of defense against invading pathogens, is particularly sensitive to ionizing radiation (IR) leaving the host vulnerable to cutaneous pathogen invasion. Our laboratory demonstrated that exposure to IR depleted murine cutaneous dendritic cells (cDCs); a network of cells responsible for forming a cutaneous immunological barrier that is critical in protecting the host against microorganism infection. This IR-induced depletion was caused by a migration of cDCs out of the skin responding to abnormal upregulation of cDC migratory factors, CCR7 and CCL21. As a result, this breakdown in the immunological barrier left the host susceptible to a cutaneous Candida albicans challenge where the pathogen was not contained locally but rather disseminated systemically to the kidneys. Interestingly, a subcutaneous treatment with IL-12 prevented the IR-induced depletion of cDCs offering a potential radiation countermeasure to preserve the cutaneous immunological barrier and perhaps prevent the lethal dissemination of opportunistic pathogens following radiation exposure.

Signaling through Purinergic Receptors for ATP Induces Human Cutaneous Innate and Adaptive Th17 Responses: Implications in the Pathogenesis of Psoriasis

Human cutaneous dendritic cells (DCs) have the ability to prime and bias Th17 lymphocytes. However, the factors that stimulate cutaneous DCs to induce Th17 responses are not well known. Alarmins, such as ATP, likely play a pivotal role in the induction and maintenance of cutaneous immune responses by stimulating DC maturation, chemotaxis, and secretion of IL-1β and IL-6, Th17-biasing cytokines. In this study, using a well-established human skin model, we have demonstrated that signaling purinergic receptors, predominantly the P2X7 receptor (P2X7R), via an ATP analog initiate innate proinflammatory inflammation, DC17 differentiation, and the subsequent induction of Th17-biased immunity. Moreover, our results suggest a potential role for P2X7R signaling in the initiation of psoriasis pathogenesis, a Th17-dependent autoimmune disease. In support of this, we observed the increased presence of P2X7R in nonlesional and lesional psoriatic skin compared with normal healthy tissues. Interestingly, there was also a P2X7R variant that was highly expressed in lesional psoriatic skin compared with nonlesional psoriatic and normal healthy skin. Furthermore, we demonstrated that psoriatic responses could be initiated via P2X7R signaling in nonlesional skin following treatment with a P2X7R agonist. Mechanistic studies revealed a P2X7R-dependent mir-21 angiogenesis pathway that leads to the expression of vascular endothelial growth factor and IL-6 and that may be involved in the development of psoriatic lesions. In conclusion, we have established that purinergic signaling in the skin induces innate inflammation, leading to the differentiation of human Th17 responses, which have implications in the pathogenesis and potential treatment of psoriasis.

Whole Body Irradiation Induces Cutaneous Dendritic Cells Depletion via NF-κB Activation

Background: Effect of ionizing radiation on cutaneous dendritic cells (cDC) is critical to its influence on immune status of the skin, which plays an important role in the progression and recovery of radiation skin sickness. This study was to study the influence of whole body irradiation (WBI) on the cDC. Methods: Density of epidermal and dermal DC was determined with a fluorescent microscopy and the DC numbers in lymph node were measured by flow cytometry. A FITC induced migration assay was also used to study the migration of DC. The expressions of cytokines and chemokines were evaluated by Realtime PCR, and the protein level of was measured by Western blot. Results: WBI caused depletion of cDC in epidermal as well as dermal and augmented FITC-induced migration of DC to the draining lymph node (LN). The number of DC migrated from ear explants to the CCL19-containing medium also increased after exposure to WBI. It was also found that WBI increased mRNA level of CCL19/CCL21 as well as CCR7 in LN and skin tissue. The expressions of TNFa, IL-1a, IL-1ß, and IL-6 in skin tissues were also greatly induced by WBI in a dose dependent manner. Finally, we found that WBI induced translocation of nuclear factor κB (NF-κB) and that the radiation-induced migration of DC was blocked by NF-κB inhibitor or TLR4 knockout. Conclusion: WBI caused cDC depletion through induction of DC migration to the draining LN, which might result from the activation of NF-κB and the induction of inflammatory microenvironment within the skin.

TGFβ1, an Epidermal Controller of Skin Dendritic Cell Homeostasis

Transforming growth factor-β1 (TGFβ1) is generally regarded as an anti-inflammatory cytokine. However, previous studies suggest that TGFβ1 can promote immune responses under certain conditions. In this issue, Mohammed et al. report that epidermal keratinocyte-derived TGFβ1 alone can alter homeostasis of multiple cutaneous dendritic cell (DC) subsets, which may enhance skin inflammation. These findings may provide a better understanding of the pathogenesis of inflammatory skin disorders such as psoriasis, although how keratinocyte-derived TGFβ1 regulates cutaneous DCs under physiological and inflammatory conditions should be further addressed.

The chemokine CX3CL1 promotes trafficking of dendritic cells through inflamed lymphatics

SummaryTissue inflammation is characterised by increased trafficking of antigen-loaded dendritic cells (DCs) from the periphery via afferent lymphatics to draining lymph nodes, with a resulting stimulation of ongoing immune responses. Transmigration across lymphatic endothelium constitutes the first step in this process and is known to involve the chemokine CCL21 and its receptor CCR7. However, the precise details of DC transit remain obscure and it is likely that additional chemokine-receptor pairs have roles in lymphatic vessel entry. Here, we report that the transmembrane chemokine CX3CL1 (fractalkine) is induced in inflamed lymphatic endothelium, both in vitro in TNF-α-treated human dermal lymphatic endothelial cells (HDLECs) and in vivo in a mouse model of skin hypersensitivity. However, unlike blood endothelial cells, which express predominantly transmembrane CX3CL1 as a leukocyte adhesion molecule, HDLECs shed virtually all CX3CL1 at their basolateral surface through matrix metalloproteinases. We show for the first time that both recombinant soluble CX3CL1 and endogenous secreted CX3CL1 promote basolateral-to-luminal migration of DCs across HDLEC monolayers in vitro. Furthermore, we show in vivo that neutralising antibodies against CX3CL1 dramatically reduce allergen-induced trafficking of cutaneous DCs to draining lymph nodes as assessed by FITC skin painting in mice. Finally, we show that deletion of the CX3CL1 receptor in Cx3cr1−/− DCs results in markedly delayed lymphatic trafficking in vivo and impaired translymphatic migration in vitro, thus establishing a previously unrecognised role for this atypical chemokine in regulating DC trafficking through the lymphatics.

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.