The epidermis provides a physical barrier to microbial invasion. The epidermis consists of multiple layers of stratified squamous epithelium, made up almost entirely of specialized epithelial cells called keratinocytes. The basal layer of keratinocytes, anchored onto the basement membrane, continuously proliferate, and their maturing progeny cells are displaced upward and differentiate to form several different layers. In the top layer, called the stratum corneum, the cells undergo programmed death, thereby forming a keratin- and lipid-rich permeability barrier that is important for protection against microbes and harmful physical and chemical agents. Filaggrin, a protein that is expressed and proteolytically processed in maturing keratinocytes, is critical for the barrier function of the epidermis. The importance of the skin’s innate immune barrier function is reflected by the high frequency of serious systemic infections when the barrier is damaged by burns, trauma, pressure ulcers, and other forms of injury.
In addition to forming a physical barrier, keratinocytes actively respond to pathogens and injury by producing antimicrobial peptides, which kill microbes, and various cytokines, which promote and regulate immune responses. The antimicrobial peptides that keratinocytes produce include defensins, S100, and cathelicidins. The cytokines made by keratinocytes include: TNF, TSLP, IL-1, IL-6, IL-18, IL-25, and IL-33, which promote inflammation; granulocyte-macrophage colony-stimulating factor (GM-CSF), which induces differentiation and activation of DCs in the epidermis; and IL-10, which controls immune responses. Keratinocytes produce the chemokine CCL27, which participates in recruitment of lymphocytes expressing CCR10. The expression of defensins, cytokines, and chemokines by keratinocytes is stimulated by innate immune receptors, including TLRs and NLRs (see Chapter 4). Keratinocytes in normal skin constitutively synthesize pro-IL-1β and pro-IL-18, which are processed following inflammasome activation to generate the biologically active cytokines. Stimuli such as ultraviolet irradiation activate the inflammasome to process these procytokines to the active forms, which explains the inflammatory response to sunburn. When signal transduction pathways linked to inflammatory responses, such as the NF-κB (nuclear factor κB) and STAT3 (signal transducer and activator of transcription 3) pathways, are activated only in keratinocytes in mice, the animals develop inflammatory skin diseases, showing the potential of keratinocytes to initiate cutaneous immune responses.
Innate immune responses to pathogens that breach the epi dermal barrier are induced by macrophages, mast cells, and ILCs in the dermis. As we have described for other tissues, tissue resident sentinel cells such as macrophages and mast cells express TLRs and other innate pattern recognition receptors and respond to PAMPs and DAMPs (damage-associated molecular patterns) by secreting inflammatory cytokines and lipid mediators. ILCs are activated by cytokines secreted by keratinocytes and sentinel cells and in turn secrete inflammatory cytokines, which influence the type of inflammatory responses that follow. For example, ILC2s are activated by keratinocyte-derived TSLP, IL-25, and IL-33, and the ILC2s then secrete IL-5, which promotes eosinophilic inflammation. IL-18 production by keratinocyte and sentinel cells activates ILC1s to secrete IFN-γ, which promotes macrophage-mediated defense. DCs also play an important sentinel role in the skin.
Several DC populations are normally present in the skin and contribute to innate immune responses and initiation of T-cell responses to microbial and environmental antigens that enter through the skin. In the epidermis, the most abundant DCs are Langerhans cells (see Fig. 1). These cells express a C-type lectin receptor called langerin (CD207), which contributes to the capture of microbes in the skin. The microbes are ingested into vacuolar cytoplasmic structures known as Birbeck granules, which are a characteristic feature of these cells. Langerhans cells populate the skin during embryonic development and are developmentally related to other tissue-resident macrophages rather than conventional DCs. The dendrites of Langerhans cells form a dense meshwork between the keratinocytes of the epidermis. In the dermis, there are relatively sparse langerin-expressing DCs, which express CD103 in mice and CD141 in humans and are a distinct lineage from Langerhans cells. Each of these DC populations expresses innate pattern recognition receptors for PAMPs as well as for DAMPs derived from injured cells. The DCs respond to these ligands by secreting inflammatory cytokines.
Fig1. Dendritic cells. (A) Light micrograph of cultured dendritic cells (DCs) derived from bone marrow precursors. (B) A scanning electron micrograph of a DC showing extensive membrane projections. (C) A section of the skin stained with antibodies specific for Langerhans cells, a special type of skin DC found in the epithelial layer (which appear dark brown in this immunoenzyme stain). (D and E) DCs in a lymph node, illustrated schematically (D) and in a section of a mouse lymph node (E) stained with fluorescently labeled antibodies against B cells in follicles (green) and DCs in the T-cell zone (red). A and B, Courtesy of Dr. Y-J Liu, MD, Anderson Cancer Center, Houston, Texas. C, Courtesy of Dr. Jarish Cohen, University of California San Francisco School of Medicine, San Francisco, CA. E, Courtesy of Drs. Kathryn Pape and Jennifer Walter, University of Minnesota School of Medicine, Minneapolis, Minnesota.
Both epidermal Langerhans cells and dermal DCs take up protein antigens, process them into peptides, and migrate to draining lymph nodes, where they present peptide-MHC complexes to naive T cells. The contributions of the different skin DC subsets to the initiation of different types of T-cell responses are not fully defined. Mouse models have been developed in which particular DC subsets are eliminated, and these models show that mouse Langerhans cells are not required for the activation of CD4+ and CD8+ T-cell responses to many types of antigens in the skin, but they do appear to play a role in Th17 responses to extracellular pathogens, and in tolerance to some skin antigens. Langerin-expressing cDC1s in mice and humans are required for cross-presentation of antigens to naive CD8+ T cells.
Normal human skin contains many T cells, 95% of which have a memory phenotype. There are about 2 × 1010 total T cells in the skin, about 98% of which are present in the dermis, and 2% are intraepidermal lymphocytes. Dermal T lymphocytes (both CD4+ and CD8+ cells) are located predominantly adjacent to blood vessels and hair follicles. Most of these dermal T cells are tissue-resident memory cells generated within lymph nodes during prior skin infections, which then home to and remain in the skin for long periods without recirculating. Smaller numbers of both CD4+ and CD8+ resident memory T cells are present in the epidermis and express the integrin CD103, which binds to E-cadherin on epithelial cells and retains the T cells in the skin. All of these resident memory T cells display potent effector or regulatory functions when activated by antigens and include CD4+ cells of each major helper subset, Th1, Th2, Th17, and Treg. Th1 and Th17 cells are important for microbial defense against intracellular and extracellular microbes, respectively, as in other tissues. The two signature Th17 cytokines, IL-17 and IL-22, induce expression of defensins and cathelicidins by keratinocytes and IL-22 induces epidermal cell proliferation. In contrast, the Th2 cytokines IL-4 and IL-13 suppress production of defensins and cathelicidin, which can result in infections in Th2-driven skin diseases. Dermal γδ T cells may be a source of IL-17 in some chronic inflammatory skin diseases.
T cells in the skin express homing molecules that direct their migration out of dermal microvessels (Fig. 2). Migration of effector or memory T cells into the skin depends on T-cell expression of cutaneous lymphocyte antigen (CLA), which is an E-selectin-binding carbohydrate moiety displayed on various glycoproteins on the endothelial cell plasma membrane. In addition, T-cell expression of CCR4, CCR8, and CCR10, which bind the chemokines CCL17, CCL1, and CCL27, respectively, is also required for T-cell trafficking to skin. The skin-homing properties of T cells are imprinted during activation in skin draining lymph nodes, by a process analogous to imprinting of gut-homing properties of T cells in mesenteric lymph nodes, dis cussed earlier in the chapter. When naive T cells recognize anti gens presented by DCs in skin-draining lymph nodes, the T cells receive signals from the DCs that not only induce proliferation and differentiation into effector cells but also induce expression of the skin-homing molecules CLA, CCR4, CCR8, and CCR10. Interestingly, sunlight and vitamin D appear to play an important role in T-cell migration to the skin, analogous to the role of vitamin A and its metabolite retinoic acid in lymphocyte migration to the gut. UVB rays in sunlight act on 7-dehydrocholesterol made in the basal layer of the epidermis, converting it to pre vitamin D3 . Dermal DCs express vitamin D3 hydroxylases that convert previtamin D3 to the active form, 1,25(OH)2 D3 , which may be transported in free form or within migrating DCs to skin draining lymph nodes. Within the node, 1,25(OH)2 D3 enters T cells that have been activated by antigen-presenting DCs and binds to its cytosolic receptor, which then translocates into the nucleus and induces transcription of CCR10. IL-12 made by the DCs participates in the induction of CLA. CCR4 and CCR8 are also upregulated, and the gut-homing integrin α4 β7 is downregulated, by unknown signals, during T-cell activation in skin-draining lymph nodes. Thus, naive T cells activated in skin-draining lymph nodes differentiate into effector T cells that preferentially home back into the skin. 1,25(OH)2 D3 may also act locally within the dermis on effector and memory T cells to upregulate CCR10 and promote migration of the T cells into the epidermis in response to the CCR10 ligand CCL27 made by keratinocytes.
Fig2. Homing properties of skin lymphocytes. The skin-homing properties of effector lymphocytes are imprinted in skin-draining lymph nodes, where they have undergone differentiation from naive precursors. Ultraviolet rays (UVB) in sunlight stimulate production of vitamin D, which induces expression of CCR10; interleukin-12 (IL-12) induces expression of the E-selectin ligand cutaneous lymphocyte antigen (CLA); and other signals induce CCR4, CCR8, and CCR10 expression. These homing molecules direct migration of the effector T cells into the skin.
