Classic atopic dermatitis is complicated by asthma, allergic rhinitis, and food allergies, cumulatively referred to as atopic diseases. Recent discoveries of mutations in the filaggrin gene as predisposing factors for atopic diseases have refocused investigators' attention on epidermal barrier dysfunction as a causative mechanism. The skin's barrier function has three elements: the stratum corneum (air-liquid barrier), tight junctions (liquid-liquid barrier), and the Langerhans cell network (immunological barrier). Clarification of the molecular events underpinning epidermal barrier function and dysfunction should lead to a better understanding of the pathophysiological mechanisms of atopic diseases. IntroductionAtopic dermatitis (AD) is a chronic relapsing eczematous skin disorder that is frequently associated with elevated serum IgE levels and a family history of AD, allergic rhinitis, and/or asthma. Clinical manifestations of classic AD are dry skin and relapsing eczema, which usually start during early infancy or childhood and become complicated by food allergies, asthma, and/or allergic rhinitis during the first several years of life, in a process called "atopic march" (1). AD is highly prevalent in industrialized countries, where it affects approximately 15%-30% of children and 2%-10% of adults (2). The various observations of the disease indicate that AD has a complex etiology with genetic, immunological, and environmental aspects.Living organisms rely critically on surface barriers to isolate themselves from the external environment and to maintain homeostasis. While unicellular organisms are enclosed by cell membranes and cell walls, epithelial barrier structures, in several forms, cover the surfaces of multicellular organisms (3). In mammals, the airway and gastrointestinal tract are lined by simple epithelia covered with mucus. In contrast, the outer surface of the body is covered by a stratified epithelial cellular sheet called the epidermis, the outermost layer of which is cornified. Recent findings have shown that disruption of epithelial barrier systems are involved in the pathogenesis of immune disorders such as inflammatory bowel disease, asthma, and AD (4-12). In this review, we describe the barrier system of the epidermis, which is far more sophisticated than previously thought (13,14), and attempt to discuss its function with special focus on antigen penetration through these barriers and antigen capture by dendritic cells in the context of AD.

Langerhans cells (LCs) are epidermal dendritic cells with incompletely understood origins that associate with hair follicles for unknown reasons. Here we show that in response to external stress, mouse hair follicles recruited Gr-1hi monocyte-derived precursors of LCs whose epidermal entry was dependent on the chemokine receptors CCR2 and CCR6, whereas the chemokine receptor CCR8 inhibited the recruitment of LCs. Distinct hair-follicle regions had differences in their expression of ligands for CCR2 and CCR6. The isthmus expressed the chemokine CCL2; the infundibulum expressed the chemokine CCL20; and keratinocytes in the bulge produced the chemokine CCL8, which is the ligand for CCR8. Thus, distinct hair-follicle keratinocyte subpopulations promoted or inhibited repopulation with LCs via differences in chemokine production, a feature also noted in humans. Pre-LCs failed to enter hairless skin in mice or humans, which establishes hair follicles as portals for LCs.

Outermost barriers are critical for terrestrial animals to avoid desiccation and to protect their bodies from foreign insults. Mammalian skin consists of two sets of barriers: stratum corneum (SC) and tight junctions (TJs). How acquisition of external antigens (Ags) by epidermal Langerhans cells (LCs) occur despite these barriers has remained unknown. We show that activation-induced LCs elongate their dendrites to penetrate keratinocyte (KC) TJs and survey the extra-TJ environment located outside of the TJ barrier, just beneath the SC. Penetrated dendrites uptake Ags from the tip where Ags colocalize with langerin/Birbeck granules. TJs at KC–KC contacts allow penetration of LC dendrites by dynamically forming new claudin-dependent bicellular- and tricellulin-dependent tricellular TJs at LC–KC contacts, thereby maintaining TJ integrity during Ag uptake. Thus, covertly under keratinized SC barriers, LCs and KCs demonstrate remarkable cooperation that enables LCs to gain access to external Ags that have violated the SC barrier while concomitantly retaining TJ barriers to protect intra-TJ environment.

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The skin harbors a variety of resident leukocyte subsets that must be tightly regulated to maintain immune homeostasis. Hair follicles are unique structures in the skin that contribute to skin dendritic cell homeostasis via chemokine production. We demonstrate that CD4+ and CD8+ skin resident memory T cells (TRM), responsible for long-term skin immunity, resided predominantly within the hair follicle epithelium of unperturbed epidermis. TRM tropism for the epidermis and follicles was herein termed epidermotropism. Hair follicle-derived IL-15 was required for CD8+ TRM, and IL-7 for CD8+ and CD4+ TRM, to exert epidermotropism. The lack of either cytokine impaired hapten-induced contact hypersensitivity responses. In a model of cutaneous T cell lymphoma, epidermotropic CD4+ TRM lymphoma cell localization depended on hair follicle-derived IL-7. These findings implicate hair follicle-derived cytokines as regulators of malignant and non-malignant TRM cell tissue residence and suggest they may be targeted therapeutically in inflammatory skin disease and lymphoma.

A new langerin ؉ DC subset has recently been identified in murine dermis (langerin ؉ dDC), but the lineage and functional relationships between these cells and langerin ؉ epidermal Langerhans cells (LC) are incompletely characterized. Selective expression of the cell adhesion molecule EpCAM by LC allowed viable LC to be easily distinguished from langerin ؉ dDC in skin and lymphoid tissue and ex vivo as well. Differential expression of EpCAM and langerin revealed the presence of at least 3 distinct skin DC subsets. We determined that LC and langerin ؉ dDC exhibit different migratory capabilities in vitro and repopulate distinct anatomic compartments in skin at different rates after conditional depletion in vivo. Langerin ؉ dDC, in contrast to LC, did not require TGF␤1 for development. Carefully timed gene gun immunization studies designed to take advantage of the distinct repopulation kinetics of langerin ؉ dDC and LC revealed that langerin ؉ dDC were required for optimal production of ␤-galactosidase-specific IgG2a/c and IgG2b in the acute phase. In contrast, immunization via LC-deficient skin resulted in persistent and strikingly reduced IgG1 and enhanced IgG2a Ab production. Our data support the concepts that LC and langerin ؉ dDC represent distinct DC subsets that have specialized functions and that LC are important immunoregulatory cells. The presence of at least 3 functionally distinct skin DC subsets may have particular relevance for vaccines that are administered epicutaneously.EpCAM ͉ gene gun ͉ langerin ͉ TGF-beta T he remarkable phenotypic heterogeneity of DC, both between and within certain tissues, has been long recognized. To date, however, it has been possible to clearly relate DC phenotype to DC function in only a few instances, even in mice. For example, plasmacytoid DC are recognized as the primary source of virus-induced type I IFN (1), CD8␣ ϩ lymph node DC are largely responsible for cross-presentation of cell-associated antigen to CD8 T cells (2-4), and 33D1-reactive (DCIR2 ϩ ) splenic DC (as compared with CD205 ϩ DC) preferentially stimulate CD4 T cells (5). Epidermal Langerhans cells (LC) represent perhaps the most striking example of an extensively studied tissue DC subpopulation whose function is incompletely understood.LC have long been thought to play pivotal roles in initiating immunity by acquiring antigens that are encountered in skin, migrating to draining LN after activation, and stimulating antigen-specific T cells (6). However, recent studies suggested that LC do not function as essential antigen-presenting cells for anti-viral immune responses (2, 7) or for contact hypersensitivity reactions (8-12) in established murine models. Studies of LC have been challenging, in part, because readily detectable cell surface proteins that are constitutively expressed by epidermal LC and LC that have emigrated from epidermis have not been well recognized. The C-type lectin langerin has been regarded as a pathognomonic LC marker, but recent studies suggest that this protein is present in at le...

Purpose: The organic anion transporter OATP1B3, encoded by SLCO1B3, is involved in the transport of steroid hormones. However, its role in testosterone uptake and clinical outcome of prostatic cancer is unknown. This study examined (a) the SLCO1B3 genotype in cancer cells as well as the uptake of testosterone by cells transfected with genetic variants of SLCO1B3; (b) the expression of OATP1B3 in normal prostate, benign prostatic hyperplasia, and prostatic cancer; and (c) the role of SLCO1B3 haplotype on clinical outcome of Caucasian patients with androgen-independent prostatic cancer. Experimental Design: SLCO1B3 genotype was assessed in the NCI-60 panel of tumor cells by sequencing, whereas testosterone transport was analyzed in Cos-7 cells transfected with WT, 334G, and 699A SLCO1B3 variants. OATP1B3 expression in prostatic tissues was examined by fluorescence microscopy, and the relationship between SLCO1B3 haplotypes and survival was examined in patients. Results: Cells transfected with wild-type (334T/699G) SLCO1B3, or with a vector containing either the 334G or 699A variants, actively transported testosterone, whereas its uptake was impaired in cells transfected with a gene carrying both 334G and 699A single nucleotide polymorphisms. Prostatic cancer overexpresses OATP1B3 compared with normal or benign hyperplastic tissue; patients with SLCO1B3 334GG/699AA haplotype showed longer median survival (8.5 versus 6.4 years; P = 0.020) and improved survival probability at 10 years (42% versus 23%; P < 0.023) than patients carryingTT/AA and TG/GA haplotypes. Conclusions: The common SLCO1B3 GG/AA haplotype is associated with impaired testosterone transport and improved survival in patients with prostatic cancer.

SUMMARY Immune cells and epithelium form sophisticated barrier systems in symbiotic relationships with microbiota. Evidence suggests that immune cells can sense microbes through intact barriers, but regulation of microbial commensalism remain largely unexplored. Here, we uncovered spatial compartmentalization of skin-resident innate lymphoid cells (ILCs) and modulation of sebaceous glands by a subset of RORγt+ ILCs residing within hair follicles in close proximity to sebaceous glands. Their persistence in skin required IL-7 and thymic stromal lymphopoietin, and localization was dependent on the chemokine receptor CCR6. ILC subsets expressed TNF receptor ligands, which limited sebocyte growth by repressing Notch signaling pathway. Consequently, loss of ILCs resulted in sebaceous hyperplasia with increased production of antimicrobial lipids and restricted commensalism of Gram-positive bacterial communities. Thus, epithelia-derived signals maintain skin-resident ILCs that regulate microbial commensalism through sebaceous gland-mediated tuning of the barrier surface, highlighting an immune-epithelia circuitry that facilitates host-microbe symbiosis.