The epidermis is the outermost layer of the skin and constantly protects the body against pathogens and environmental threats. It provides a physical barrier with stratified epithelial cells, as well as an immunological barrier with tissue-resident immune cells. Communication between these two systems is essential for the maintenance of skin homeostasis and effective host defense. Upon injury, the epidermis has the critical task of closing the wounded area through a process called re-epithelialization. During wound repair, both immune and epithelial cells in the epidermis work together to prevent further damage to the organism and restore the skin protective barriers. This review will highlight our current knowledge of immune-epithelial cell interactions in the skin epidermis and their role in maintaining tissue structure, homeostasis, and contribution to wound repair. We will also discuss the pathological consequences of dysregulated communication between immune and epithelial cells.

Epithelial cells in the skin, also called keratinocytes, are the most abundant cell type in the epidermis. Keratinocytes are named after their high expression of cytoskeletal protein keratin. Keratin monomers assemble to create intermediate filaments that control keratinocyte properties such as structure, mobility, and metabolic signaling. Keratinocytes build the stratified structure of the epidermis by creating its four distinct layers (basal, spinous, granular, and cornified). The basal layer is the bottom-most layer of the epidermis and is anchored to the basement membrane, which separates the epidermis from the dermis. Proliferative keratinocytes in the basal layer continuously renew and give rise to differentiated keratinocytes that move towards the upper layers. Keratinocytes in the basal layer express keratin 5 (K5) and K14. Keratinocytes that commit to departure of the basal layer begin the expression of K1 and K10 (1). As keratinocytes transition into upper layers, their cellular properties undergo differentiation and terminally differentiated keratinocytes are eventually shed off as dead cells from the cornified layer (2). This continuous keratinocyte replenishment maintains a resilient epidermal stratified barrier, which provides protection from physical damage, water loss, and invasion of foreign materials.

The epidermal landscape created by keratinocytes is composed of vast areas of interfollicular epidermis with appendage structures such as hair follicles, sebaceous glands, and sweat glands. In addition to the keratinocytes, there are four co-existing tissue-resident immune cells in the interfollicular epidermis: Langerhans cells (LCs), dendritic epidermal T cells (DETCs), CD8^＋ tissue-resident memory T (T_RM) cells, and innate lymphoid cells (ILCs) (3, 4). These immune cells represent the innate and adaptive arms of the epidermal immune response.

LCs are well-known antigen presenting cells. LCs are characterized by their dendritic morphology as well as the expression of Langerin (CD207), which is a membrane-bound C-type lectin receptor that is involved in antigen capture and the formation of Birbeck granules (5). LCs first appear in the epidermis from a first wave of yolk sac-derived myeloid progenitor cells during early embryogenesis. Later in embryogenesis, a second wave of fetal liver monocytes seed into the epidermis and burst in proliferation to become the LC network observed in adult skin (6, 7). LCs are able to self-renew in the adult skin and maintain their epidermal residency through anchoring to neighboring keratinocytes via cell adhesion molecules, such as EpCAM and E-cadherin (8, 9).

DETCs are epidermis-resident T cells bearing γδ T-cell receptors (TCRs). It is noteworthy that DETCs are exclusive to the murine skin, whereas DETCs do not exist in humans (10). During development, yolk-sac derived lymphoid progenitors first populate the thymus, where they express the invariant Vγ5Vδ1 TCRs to become DETC progenitor cells (11). Later in embryogenesis, DETC progenitors downregulate CCR6 and upregulate S1PR1 to depart the thymus and migrate to the fetal skin (12, 13). Entry into the epidermis remains unclear, but loss of the chemokine receptors CCR4 or CCR10 in DETC progenitors reduces their ability to populate the epidermis (14). DETCs are capable of self-renewal through clonal expansion, albeit at a much slower turnover rate compared to LCs (15).

T_RM cells are CD8^＋ memory T cells that can be found in the adult epidermis, specifically at sites affected by previous infections. T_RM cells excel at remembering encountered antigens and providing strong immune responses against pathogen reinfections. In mice, only CD8^＋ T_RM cells are found in the epidermis, whereas CD4^＋ T_RM cells can be found in the dermis. Although CD4^＋ T_RM cells have been observed in human epidermis, these are poorly understood (16). T_RM cells in the epithelial tissues are imperfectly identified by their expression of glycoprotein CD69 and integrin CD103; imperfectly, because expression of such markers does not always infer epithelial residency (17). Unlike LCs and DETCs, T_RM cells do not automatically seed the epidermis during development. Instead, T_RM cells are recruited to the epidermis by skin dendritic cells in response to infection or inflammation (18). T_RM cells in mice can be derived from priming naïve T cells showing low expression of the effector-cell marker KLRG1 in the lymph nodes (19). Notably, T_RM cell recruitment can also be driven by inflammation, independent of antigens in the skin (20). Interestingly, T_RM cells do not overlap territories with DETCs and outcompete them for epidermal space (21). T_RM cells persist for at least 6 months in the affected area and have the ability to expand through proliferation and recruitment upon allergen re-exposure (22, 23).

ILCs have recently been found to reside in the epidermis (24). ILCs are a heterogenous group of immune cells that act as a component of the innate immunity system. ILCs are distinguished by their lymphoid morphology and the absence of antigen receptors. Therefore, contrary to T cells, ILCs become activated in an antigen-independent manner (25). ILCs arise from common lymphoid progenitors and can be classified into three major groups according to their signature cytokines and transcription factors: ILC1, ILC2, and ILC3. Briefly, ILC1s produce IFNγ and depend on T-bet, ILC2s produce type 2 cytokines (including interleukin [IL]-5 and IL-13) and depend on GATA-3, and ILC3s produce cytokines IL-17A and/or IL-22 and depend on RORγt(25). ILCs have demonstrated a high degree of plasticity where ILC2s and ILC3s show bi-directional regulation of T-bet and RORγt gradients to adopt an ILC1 effector program. The flexibility of the ILC transcriptional program has been suggested to provide an adaptable state in response to the variability of assaults at border tissues (26).

Interactions between LCs and keratinocytes are important for the development, survival, and function of LCs. Keratinocytes secrete IL-34, an alternative ligand for the Csf-receptor, to control the development of LCs during embryogenesis and their survival in adult skin (Fig. 1) (27, 28). Keratinocytes can also recruit LCs in the absence of inflammation by expression of chemokine (C-C motif) ligand 2 (CCL2) (MCP-1) (29). LCs established in the epidermis create an evenly distributed network that is regulated by keratinocyte density. In vivo mice experiments demonstrated that the density of LCs is directly correlated and adaptable to changes in basal layer keratinocyte density (30). Keratinocyte-LC interactions are also important for LCs to perform their immunological functions. As antigen-presenting cells, LCs constantly survey the surface of the skin in search of foreign material. LCs accomplish this by extending and retracting their dendrites to penetrate tight junctions created by keratinocytes of the granular layer. This remarkable cooperation between LCs and keratinocytes allows LCs to sample the skin surface without compromising the integrity of epidermal barrier function (31). Upon antigen uptake, LCs mature into an active form and migrate to the lymph nodes to prime naïve T cells. Activated LCs show a higher expression of MHC-II and costimulatory proteins, CD80 and CD86 (32). Furthermore, activated LCs become highly mobile by downregulating expression of adhesion molecules (E-cadherin and EpCAM) (9, 33, 34), therefore, LCs lose their anchorage and physical communication with surrounding keratinocytes, which functions to keep LCs static and present in the epidermis. Controversially, CD11c-specific (dendritic cell marker including LCs) deletion of E-cadherin demonstrated that E-cadherin is dispensable for the maturation and migration of LCs, but LCs become more rounded and less dendritic (35). In addition, keratinocytes can also affect immune tolerance via crosstalk with LCs. Keratinocyte-derived glucocorticoids inhibit the migration of epidermal antigen-presenting cells towards the lymph nodes and consequently dampen the activation of pro-inflammatory responses (36). Moreover, transforming growth factor-β1 (TGF-β1) produced by keratinocytes and LCs, prevents the premature maturation of LCs, and contributes to their function as antigen-presenting cells in the steady state skin (37, 38). This was further supported by a later study where β-catenin was identified as a positive regulator of LC differentiation in response to TGF-β1 (39). The inverted expression pattern of bone morphogenetic protein 7 (BMP7) by basal keratinocytes relative to TGF-β1 by suprabasal keratinocytes is suggested to govern LC proliferation and differentiation (40).

Interactions with keratinocytes regulate the development, survival, and activity of DETCs in the steady-state skin. During development, DETCs entry into the epidermis depends on TCR signaling, which has been strongly correlated with expression of Skint1 by keratinocytes (41). Direct TCR signaling is essential to promote the expression of the skin homing genes of DETCs: S1PR, CCR4, CCR10, E and P selectins. Keratinocytes are in part responsible for directing DETCs towards the epidermis through the release of the CCR10 ligand CCL27 (12, 42, 43). Keratinocytes also promote the survival and proliferation of DETCs through the secretion of IL-15 and IL-7 (44, 45). Indeed, mice deficient for IL-15 downstream targets, JAK3 and STAT5, show severe reduction of DETCs (46, 47). DETCs show similar interactions with keratinocytes, like LCs during homeostasis. DETCs in the adult epidermis also maintain regular distribution via dendritic activity, and keratinocyte density is directly correlated with DETC density (30). DETC dendrites at tight junctions with keratinocytes show enriched expression of δTCR and integrin CD103 (48, 49). Fully activated DETCs are characterized by the rounding of their cell body, downregulation of cell adhesion molecules CD103 and E-cadherin, and upregulation of occludin (50-52). Occludin has been implicated in promoting the migration of activated DETCs from the epidermis to the lymph nodes (51). However, in vivo evidence for the gained mobility and migration of activated DETCs is yet to be seen. Remarkably, DETCs promote keratinocyte survival. Activated DETCs act as negative regulators of keratinocyte apoptotic rates through the production of insulin-like growth factor 1 (IGF-1) (53). Keratinocytes, in turn, secrete IL-15 to promote the production of IGF-1 in DETCs, creating a positive feedback loop of mutual proliferation and survival (54).

T_RM cells depend on their interactions with keratinocytes to establish tissue residency and survival. As mentioned earlier, T_RM cells arise from primed naïve T cells in respoto infection (55). The epidermal presence of T_RM cells depends on their expression of glycoprotein CD69 and integrin CD103. CD69 in T_RM cells promotes tissue residency by forming a complex with S1PR1 which, when bound to S1P, promotes tissue egression instead (56). The alpha integrin CD103 allows T_RM cells to bind E-cadherin expressed by local keratinocytes. In return, keratinocyte production of TGF-β induces CD103 expression in T_RM cells (19). A later study further described that keratinocyte integrins α_vβ₆ and α_vβ₈ are responsible for the activation of TGF-β in the skin (57). Recently, it was reported that the transcription factor Runx3 enforces tissue residency in T_RM cells through a TGF-β-dependent transcriptional mechanism (58, 59). Keratinocyte-derived fatty acids may promote the survival and longevity of T_RM cells. Indeed, genetic deletion of peroxisome proliferator-activated receptor γ (PPARγ), a regulator of lipid uptake, in T_RM cells results in reduced cell survival and compromised tissue residency (60). T_RM cells are maintained in homeostasis through IL-7 and IL-15 cytokines derived from keratinocytes in the hair follicle. Depletion of IL-7 and IL-15 or their receptors results in reduced numbers of T_RM cells and impaired immune responses in the skin (61). Under steady state conditions, the main function of T_RM cells is to patrol the epidermis in search of local antigen (62). In contrast to LCs and DETCs, T_RM cells extend their dendrites laterally to probe for cognate antigen among keratinocytes of the basal layer (63).

ILCs localization and residency in the skin requires keratinocyte-derived cytokines and chemokines. To untangle the ILC heterogeneity in the skin, a recent study aimed to identify ILCs in isolated epidermis. Antibody staining showed epidermal ILCs residing in the hair follicle in close proximity to sebaceous glands (24). Adult murine keratinocytes in the upper hair follicle secrete IL-7, thymic stromal lymphopoietin (TSLP) and CCL20 to regulate the development and localization of ILCs. ILCs, in turn, expressed tumor necrosis factor (TNF) receptor ligands to repress Notch signaling and limit sebaceous growth (24).

Wound repair is a complex process carefully orchestrated by the crosstalk of diverse cell types to restore the structure and integrity of damaged skin. The repair process is generally divided into three phases: inflammation, proliferation, and remodeling (64). Among these, keratinocytes rebuild the damaged epidermis, a process also known as re-epithelialization, during the proliferation phase. Dynamic cellular behaviors of keratinocytes, such as migration, proliferation, and differentiation, are spatiotemporally organized during the re-epithelization (65). Abnormal cellular behaviors can give rise to defects in the re-epithelization. This can eventually lead to skin barrier malfunction as well as development of severe diseases, such as non-healing chronic wounds and cancers.

Keratinocytes regulate epidermal departure and repopulation of LCs after injury. During wound repair, keratinocytes produce proinflammatory cytokines, such as IL-1α and TNF-α, and these enhance LC maturation for migration towards the lymph nodes (Fig. 2) (66). The recruitment of new LCs is dependent on their expression of chemokine receptors CCR2 and CCR6. Indeed, LC precursors enter the epidermis via attractive chemokines, such as CCL2 and CCL20, from keratinocytes located in the upper hair follicles. Keratinocytes from the hair bulge expressed inhibitory chemokines, such as CCL8, to repel LC precursors from the lower hair follicles (67). Moreover, external factors can also influence LC-keratinocyte crosstalk. The microbial metabolite of Trp, IAId, promotes the expression of RANK and RANKL by LCs and keratinocytes, respectively. This interaction activates NF-κB signaling and production of IL-10 in LCs to induce a tolerogenic response (68). The role of LCs in keratinocyte re-epithelialization and overall wound repair is controversial. A Langerin-diphtheria toxin receptor (DTR) ablation study targeting all Langerin^＋ cells concluded that loss of LCs results in accelerated wound healing (69). On the contrary, a study utilizing the dendritic cell marker CD11c to eliminate LCs determined that wound healing was delayed in the absence of LCs (70). The divergent results are likely the consequence of using different mouse models and the unspecific targeting of epidermal LCs. However, a recent study utilized the huLangerin-diptheria toxin A mouse model to specifically eliminate epidermal LCs during wound repair. LC-deficient mice showed abnormal angiogenesis activity, and this eventually delayed wound repair (71). Additional evidence for the contribution of LCs to wound repair comes from non-healing wound studies. Diabetic mice were observed to have less LCs compared to healthy mice during wound healing (72). In addition, a higher number of LCs correlated with better healing in diabetic foot ulcer patients (73). Altogether, these data suggest that LCs have beneficial functions for wound repair.

Activated DETCs are critical players of keratinocyte re-epithelialization and wound inflammation. Damaged keratinocytes express molecules which can activate DETCs (74). Downregulation of the Skint family of genes, expressed by keratinocytes and previously implicated in DETC development, also affects DETC activation and impairs wound healing (75). Stressed keratinocytes at the wound edge upregulate Plexin-B2, which interacts with its receptor CD100 (also known as Semaphorin 4D) to promote the activation of DETCs (76). Inversely, Plexin-B2 stimulation in keratinocytes promotes the NF-κB signaling pathway activation of the NLRP3 inflammasome resulting in secretion of pro-inflammatory molecules (77). Interestingly, subcutaneous injection of CD100 in a chronic wound diabetic mouse model showed enhanced wound healing (78). Another ligand upregulated in stressed keratinocytes is retinoic acid early-inducible 1 (Rae-1). Rae-1 induces DETC activation via NKG2D receptors and promotes the secretion of IL-2 and IL-13 (79). IL-13 was later implicated in enhancing keratinocyte proliferation and survival (80). These activated DETCs at the wound site secrete keratinocyte growth factor 1 (KGF-1) and KGF-2, also known as fibroblast growth factors 7 and 10, to promote keratinocyte proliferation and migration (52). DETCs isolated from chronic wounds fail to produce IGF-1 (10). Indeed, mice lacking DETCs display delayed wound healing (52).

ILCs crosstalk with keratinocytes via stress signals and cytokines during wound repair. Keratinocyte alarmin cytokines IL-33, IL-25 and TSLP are critical activators of ILC2s in homeostasis and injury response. Activated ILC2s express IL-5 and IL-13 to induce type 2 immune activation (81). ILCs interact with keratinocytes to promote wound healing. ILC2s accumulate at the site of injury a few days after induction in response to a rapid increase of epithelial-derived IL-33. Depletion of either ILCs or IL-33 results in inefficient wound closure and impaired re-epithelialization (82). Type 3 ILCs crosstalk with keratinocytes has also been reported to promote wound repair. Damaged keratinocytes express Notch signaling to recruit ILC3s to the wound site in a TNFα-mediated process. Upon arrival, ILC3s produce IL-17F and CCL3 to regulate keratinocyte proliferation and promote re-epithelialization during wound repair (83).

Disruption in the balanced ecosystem inside the epidermis can lead to serious complications. Chronic inflammatory skin diseases, such as atopic dermatitis (AD) and psoriasis can arise from dysregulated crosstalk between keratinocytes and neighboring immune cells. Despite both diseases inducing aberrant prolonged inflammation, they accomplish it differently. AD relies on the IL-4/IL-13 signaling pathway (T helper 2 cell inflammation, Th2), whereas psoriasis relies on the IL-23/IL-17 axis (T helper 17 cell inflammation, Th17) (84). Therefore, details of immune-epithelial interactions are different between AD and psoriasis.

In AD, keratinocytes are the main effector cells responsible for activating abnormal immune responses. Erroneously activated keratinocytes express pro-inflammatory cytokines which stimulate activated antigen-presenting cells to recruit Th2 cells. Ultimately, secretion of Th2 cytokines causes the hyperproliferation of activated keratinocytes and the recruitment of leukocytes to create the hallmark continuous state of inflammation in AD. Another characteristic in AD is a defective permeability barrier that has been linked to the loss of the filament aggregation protein, filaggrin (85). LC-keratinocyte interactions are dysregulated in epidermis affected by AD. In a human AD explant study, LCs showed enhanced interaction with keratinocyte tight junctions to survey the surface for antigens compared to non-lesional atopic skin of the same patients. It was later found that filaggrin deficiency promoted the expression of activation markers in LCs (86, 87). Another example of LC-keratinocyte dysregulation can be found in the MC903 mouse model of AD. In MC903 treated mice, TSLP overexpression by keratinocytes lead to the activation of LCs and the consequential Th2 phenotype (88). Other MC903 studies also revealed that keratinocyte-derived signaling triggers the proliferation of LCs via their vitamin D receptor (VDR), thus increasing the density of LCs in AD lesions (7). In contrast to LCs, DETC-keratinocyte crosstalk has not been found to affect the parameters of AD progression. However, it was noted that DETCs quickly accumulate in the epidermis on the onset of histological abnormalities (89). T_RM and keratinocyte interactions in AD have yet to be elucidated. Nonetheless, it has been described that TSLP increases the presence of CD69^＋ T_RM cells, which contributes to AD by secreting Th2 cytokines (90). Importantly, TCR repertoire analysis in AD skin has suggested that T_RM cells persist in resolved AD lesions, ready for a recurrence (91). ILC-keratinocyte communication is also implicated in AD pathogenesis. ILC2s are recruited to lesional atopic skin via IL-25 and IL-33. The low E-cadherin in AD keratinocytes, which has been linked to filaggrin insufficiency, results in the activation of ILC2s and the subsequent expression of IL-5 and IL-13 (92).

Psoriasis is characterized by significant thickening of the epidermis due to keratinocyte hyperproliferation and abnormal differentiation. Previously considered to only function as executors in an immune-driven disease, genetically predisposed keratinocytes have started to emerge as potential triggers in psoriasis. However, the classical inductors in psoriasis are activated dendritic cells. IL-23 produced by activated dendritic cells promotes the secretion of Th17 cytokines such as IL-17 and IL-22. In turn, IL-17 activates keratinocytes and induces their expression of pro-inflammatory cytokines to fuel a self-sustaining cycle of inflammation (93). Dysregulated crosstalk between keratinocytes and LCs contribute to psoriasis pathology. Psoriatic Stat3 activation in keratinocytes promotes LC activation in situ via IL-1α stimulation. In response, LCs contribute to the production of IL-23 (94). Further studies on LC participation in psoriasis comes from the imiquimod (IMQ) mouse model. In mice treated with IMQ, psoriasis-like lesions developed as a consequence of IL-17 secretion by dermal γδ T cells, which was promoted by IL-23 expressed by LCs (95). Another consequence of aberrant keratinocyte-LC interactions in psoriasis is LC impaired function. Psoriatic keratinocyte secretome inhibits LC migration (96). Indeed, LC mobility and function is affected in psoriatic skin, even when treated with LC motility incentives, IL-1α and TNF-α (97). UVB has been suggested to function as an immunosuppressant in psoriasis. A recent study demonstrated that UVB radiation phototherapy promotes the production of cis-uronic acid (cis-UCA) by keratinocytes. This led to the inhibition of IL-23 and the production of PD-L1 in LCs, which resulted in the attenuated proliferation and migration of dermal γδ T cells (98). Despite dermal γδ T cells being considered major contributors in psoriasis pathogenesis, DETCs have not been found to play a role in the disease. This conclusion comes from the fact that DETCs do not secrete IL-17 and IL-22 in IMQ mouse models (99). On the contrary, T_RM cells have been implicated in the recurrence of psoriatic lesions in previously resolved sites. Activated T_RM cells in psoriatic lesions with high expression of CD69 have been highlighted as promoters of psoriatic lesion recurrence (100). CD69 had been previously described to positively regulate the secretion of IL-22 in T_RM cells and contribute to the development of psoriasis (101). ILC-keratinocyte interactions have yet to be explored in the context of psoriasis, however, ILCs are major promoters of the disease. In human studies, ILC3s expressing IL-17 and IL-22 were frequently found in patients with psoriasis (102). In a later study, ILCs demonstrated their on-site plasticity by converting ILC2s into IL-17-producing ILC3s (103).

The skin epidermis is a highly dynamic barrier composed of diverse cell populations that work together to protect the organism. Keratinocytes orchestrate epidermal homeostasis by maintaining the stratified barrier and by dictating the fate and function of tissue-resident immune cells. When faced with barrier disruption, immune cells and keratinocytes cooperate to restore the epidermis. Failure to regulate the intricate communication between keratinocytes and epidermal immune cell populations can lead to chronic inflammatory skin diseases and defects in the stratified barrier. Building upon past investigations, we have already gained the ability to intervene and ameliorate dysregulated immune-epithelial communication in skin diseases. However, there is still much to understand from the cellular crosstalk between keratinocytes and tissue-resident immune cells for the development of better treatment and prevention strategies in skin inflammatory disorders and non-healing wounds.