[Academic Presentation] Recent Advances in Research on Keratinocytes and Cutaneous Immunity

Chinese Journal of Dermatology and Venereology, Issue 01, 2017

Authors: Hu Bin, Xie Yao, Wang Feng, Tao Mengying, Jiang Dan, Liu Mi, Zhu Mingfang

The human body is demarcated from the external environment by the skin; however, the skin’s functions extend far beyond its role as a physical barrier. It also serves as a vast site for immune responses, with diverse immune cells within the skin mediating reactions that help maintain harmonious coexistence between the body and its surroundings. Keratinocytes, the principal cellular component of the epidermis, play a critical role in cutaneous immune responses. Under pathological conditions, dysregulation at various stages of these immune processes can contribute to the onset and progression of disease. This review summarizes the relationships among keratinocytes, skin immunity, and related disorders.

1. The immune surveillance function of keratinocytes

In the skin’s immune response, keratinocytes express Toll‑like receptors and NOD‑like receptors that can sense antigens and modulate immune reactions, distinguishing harmless commensal microbes from harmful pathogens and thereby performing immune surveillance. These receptors detect microbial metabolites and recognize highly conserved microbial structures—such as lipopolysaccharides, peptidoglycans, bacterial flagellin, and nucleic acids—in a process known as pathogen‑associated molecular pattern recognition (PAMPs). Among these receptors, Toll‑like receptors have been studied most extensively; they belong to the family of pattern‑recognition receptors (PRRs) and are found on the cell surface, within cellular membranes, and in other intracellular compartments. Toll‑like receptors play a critical role in maintaining tissue homeostasis. Phosphoinositide 3‑kinase (PI3K) can bidirectionally regulate Toll‑like receptor signaling; upon endotoxin stimulation of monocytes, the PI3K pathway employs negative feedback mechanisms to limit pro‑inflammatory responses and apoptosis, while also helping to balance Th1/Th2 cell subsets. Toll‑like receptors can be activated by a variety of endogenous molecules, triggering inflammatory responses or leading to tissue damage. Epidermal keratinocytes express numerous Toll‑like receptors: some are transmembrane receptors, such as TLR1, TLR2, TLR4, TLR5, and TLR6, whereas others reside intracellularly, including TLR3 and TLR9. The Toll‑like receptors expressed by keratinocytes significantly promote cutaneous immune responses; their activation induces the expression of nuclear factor‑κB (NF‑κB) and interferons, thereby initiating immune and inflammatory cascades.

Nod-like receptors (NLRs) are also a family of pattern-recognition receptors. NLRs comprise three subfamilies: CARD-containing NODs, PYD-containing NALPs, and BIR-containing NAIPs. Among these, the CARD-containing NOD subfamily includes NOD1 and NOD2 proteins, which were among the first intracellular microbial sensors identified; they recognize bacterial peptidoglycan and other pathogen‑associated molecular patterns. The PYD-containing NALP subfamily detects viral, fungal, and self‑derived signals. Activation of these receptors triggers the inflammasome—a multiprotein complex composed of NLRs, adaptor ASC protein, and pro‑caspases—leading to the activation of pro‑inflammatory signaling pathways. Inflammasome assembly results in the cleavage and activation of pro‑caspases, which in turn process pro‑IL‑1β and pro‑IL‑18 into their active forms, yielding pro‑inflammatory cytokines. Moreover, upon exposure to danger signals, NLR activation elicits an inflammatory response via the recognition of danger‑associated molecular patterns. Studies have shown that ultraviolet irradiation can activate the inflammasome in human keratinocytes, thereby mediating inflammatory responses. Consequently, when the skin is exposed to haptens or high doses of UV radiation, receptors within the keratinocyte’s complex inflammasome are activated, triggering caspase activation and promoting the production and release of key pro‑inflammatory cytokines.

In summary, receptors expressed by keratinocytes activate innate immune cells in tissues and initiate and sustain inflammatory responses via pathogen-associated molecular patterns (PAMPs) or danger-associated molecular patterns (DAMPs). Following disruption of the skin’s barrier function, keratinocytes contribute to immune surveillance by secreting IL‑1β and recruiting and activating leukocytes.

2 Keratinocytes and Innate Immune Mediators

The production of antimicrobial peptides represents a defense mechanism in eukaryotes against evolutionarily conserved antigens. These peptides are synthesized on the surface of damaged epidermal cells and can neutralize microbial invasion by directly eliminating pathogens. Some antimicrobial peptides exert their protective effects by disrupting bacterial cell membranes, thereby rapidly inhibiting bacterial growth prior to the onset of systemic nonspecific immune responses. However, during active infections, this direct bactericidal activity is not the primary basis for their protective role; rather, their actions are also linked to mechanisms such as activating chemokines, recruiting host immune cells (e.g., dendritic cells or T cells), modulating cytokine release, and regulating Toll‑like receptor signaling pathways. Keratinocytes serve as a major source of antimicrobial peptides, including β‑defensins and cathelicidins. During skin infections, cytokines secreted by T cells can upregulate local keratinocyte expression of antimicrobial peptides, with IL‑17A and IL‑22 produced by Th17 cells exerting particularly robust stimulatory effects. Antimicrobial peptides play a critical role in immune regulation of both skin and mucosal surfaces, thereby establishing an important link between keratinocytes and adaptive immune cells in the skin. Clinical studies have demonstrated elevated expression of antimicrobial peptides in the skin of patients with psoriasis, which helps explain why cutaneous infections are relatively uncommon in this population. Conversely, other research suggests that in psoriasis, keratinocytes may breach self‑tolerance by producing the antimicrobial peptide LL37, thereby promoting disease pathogenesis.

In human keratinocytes, the expression of antimicrobial peptides is influenced by multiple factors, including antigenic stimulation, vitamin D3, and skin wounds. Studies on keratinocytes surrounding wounds have revealed a close interplay among antimicrobial peptides, Toll‑like receptors, and vitamin D. Specifically, keratinocytes in the wound microenvironment exhibit high levels of Toll‑like receptor 2, the antimicrobial peptide LL37, and CYP27B1, which facilitates the conversion of 25‑(OH)D3 to 1,25‑(OH)2D3. The active form of 1,25‑(OH)2D3, in concert with IL‑17A secreted by T cells, exerts positive and negative feedback regulation on LL37 expression in keratinocytes. Moreover, LL37 enhances Toll‑like receptor signaling in immune cells, thereby initiating a cytokine cascade during bacterial recognition. Consequently, the epidermis serves as a critical site for the precise regulation of antimicrobial peptide expression.

In addition, keratinocytes constitutively secrete or induce the release of numerous cytokines, including IL‑1, IL‑6, IL‑10, IL‑18, IL‑33, and tumor necrosis factor. Among these, IL‑1 produced by keratinocytes plays a critical role in cutaneous immune responses. IL‑1 is a pleiotropic cytokine with broad biological effects, such as activating helper T cells and dendritic cells, as well as promoting B‑cell maturation and clonal expansion. In the skin, keratinocytes constitutively synthesize Pro‑IL‑1α and Pro‑IL‑1β; upon rapid exposure to bacterial threats, activation of the inflammasome triggers the release of pro‑inflammatory cytokines. This process, in turn, activates caspase‑1 to cleave Pro‑IL‑1β into the mature inflammatory cytokine IL‑1β, while further processing of IL‑1α amplifies the signal, eliciting responses in neighboring epidermal cells. However, the regulatory mechanisms governing IL‑1α secretion remain incompletely understood. A recent study suggests that, in rat epidermal keratinocytes, reduced levels of caspase‑8 lead to the release of active IL‑1α from its precursor form, implying that caspase‑8 may exert a negative regulatory effect in keratinocytes. Transgenic mouse models overexpressing IL‑1α in keratinocytes exhibit a skin‑inflammatory phenotype, underscoring the pivotal role of IL‑1α in cutaneous inflammation. Moreover, IL‑1α has several family members, such as IL‑1F6, which also play important roles in cutaneous immunopathology; transgenic mice with keratinocyte‑specific overexpression of IL‑1F6 similarly display a skin‑inflammatory phenotype. Furthermore, increased expression of IL‑1F6 in the epidermis of psoriasis patients suggests that IL‑1F6 may be involved in the immunopathogenic mechanisms underlying psoriasis.

Keratinocytes are also a major source of chemokines and express chemokine receptors, thereby modulating immune responses by recruiting diverse cell types to the skin. In pathological conditions such as psoriasis and cutaneous T-cell lymphoma, keratinocytes selectively attract effector T cells to the skin by expressing CCL20, CXCL9, CXCL10, and CXCL11, resulting in characteristic T-cell infiltration. In patients with psoriasis, activated keratinocytes further recruit neutrophils to inflamed skin tissue by producing CXCL1 and IL‑8. Moreover, the expression of CCL20 can regulate the migration of Langerhans cell precursors into the epidermis.

Therefore, keratinocytes, as pro-inflammatory effector cells, can synergistically produce antimicrobial peptides, pro-inflammatory cytokines, and chemokines, enabling a rapid response to harmful stimuli at the body’s outermost barrier.

3 Non-classical antigen presentation by keratinocytes

Keratinocytes also possess the capacity to express MHC class II molecules, and studies have demonstrated that MHC class II expression by keratinocytes is observed in numerous skin diseases characterized by T-cell infiltration. This suggests that keratinocytes may participate in cutaneous immune responses in multiple capacities, potentially serving as non‑classical antigen‑presenting cells that contribute to the initiation of skin immunity.

Both in vivo and in vitro studies have demonstrated that keratinocytes induce anergy and immune tolerance in T cells more prominently than T‑cell activation. However, keratinocytes can also promote the proliferation of quiescent T cells activated by superantigens, eliciting a robust immune response, which further indicates that keratinocytes provide signals essential for T‑cell expansion. Recent research has shown that keratinocytes can elicit functional responses from both CD4+ and CD8+ memory T cells. They are capable of processing peptide antigens and presenting them to CD4+ T cells, thereby driving the secretion of Th1‑ and Th2‑type cytokines; additionally, they can process viral‑encoded and exogenous peptides and present them to CD8+ T cells, ultimately triggering cytokine production and target cell lysis. Thus, although keratinocytes cannot initiate naive T cells, they can potentially activate memory T cells—those previously exposed to antigen—to mount an immune response. These findings suggest that keratinocytes serve as antigen‑presenting cells in the context of both antigen‑specific tolerance and activation.

4 Keratinocytes and the Inflammatory Response

The discovery of the immune functions of keratinocytes originated from the skin’s response to contact hypersensitivity induced by topical agents, suggesting that keratinocyte activation may precede T‑cell recruitment. This finding further clarifies that keratinocytes and the cytokines they secrete may play a role in the propagation of cutaneous inflammatory responses. Animal studies have demonstrated that dysregulation of keratinocyte function can trigger systemic autoimmune lymphocyte responses. Overexpression of CD40 on keratinocytes leads to a reduction of more than 90% of epidermal Langerhans cells and an increase in dendritic cells, indicating that CD40 promotes the migration of Langerhans cells from the epidermis into the dermis—processes that may contribute to the development of enlarged lymph nodes and autoantibodies, while also disrupting immune tolerance to skin antigens. Moreover, targeted blockade of the NF‑κB pathway via knockout of IκB kinase β (IKKβ) likewise amplifies cutaneous inflammation; and in vitro experiments stimulating keratinocytes with Cutibacterium acnes show that pro‑inflammatory cytokines or chemokines produced by these cells can activate TLR2, TLR4, NF‑κB, and MAPK signaling pathways, ultimately eliciting localized inflammatory responses through the recruitment of macrophages and other immune cells. Collectively, these findings underscore the pivotal role of NF‑κB in inflammation and its bidirectional regulatory influence on inflammatory processes. Additionally, constitutive activation of signal transducer and activator of transcription 3 (STAT3) in mice can, under psoriasis‑like conditions, downregulate receptors for inflammatory cytokines such as IL‑23. Thus, cytokine secretion by keratinocytes and the resulting activation of key signaling pathways constitute critical immunopathological mechanisms mediating cutaneous inflammation.

Changes in cytokine expression by keratinocytes can both activate immune cells and modulate the development of skin tumors. For example, constitutive expression of the NKG2D ligand retinoic acid early transcription factor 1 (RAE1) leads to downregulation of NKG2D receptor expression on NK cells, γδ T cells, and CD8+ αβ T cells, which in turn results in impaired cytotoxicity of NK cells, thereby enhancing tumor cell susceptibility. The biological effects of NKG2D binding to its ligand RAE1 also play a critical regulatory role in cutaneous viral infections. Furthermore, transient upregulation of RAE1 expression in keratinocytes can induce an inflammatory phenotype characterized by the redistribution of γδ T cells and Langerhans cells within the epidermis, followed by abnormal recruitment of αβ T cells, suggesting that acute alterations in the expression of the NKG2D ligand RAE1 can trigger a rapid, multifaceted immune response.

In summary, keratinocytes participate in multiple steps of the skin’s immune response. They can transduce signals by recognizing pathogen‑associated molecular patterns via specific receptors, thereby mediating inflammatory and immune reactions. Upon antigen stimulation, they also produce antimicrobial peptides, cytokines, and chemokines, which recruit other immune cells to the epidermis and confer protective immune functions. Furthermore, keratinocytes can directly present antigens; as non‑classical antigen‑presenting cells, they deliver antigens to T cells, thus completing the immune response. The roles of keratinocytes in cutaneous immunity are closely linked to skin‑related immune disorders, and in-depth investigation of these functions may offer new therapeutic strategies for such conditions.