Interleukin-17C (IL-17C) is a distinctive member of the IL-17 cytokine family that plays important roles in mucosal immunity, epithelial defense, and neuroinflammatory processes. Unlike other IL-17 family members primarily produced by T cells, IL-17C is predominantly expressed by epithelial cells, providing front-line defense at barrier surfaces. Within the central nervous system (CNS), IL-17C and its receptor are expressed on various cell types including neurons, astrocytes, and microglia, where they participate in neuroinflammatory processes relevant to neurodegenerative diseases.
The gene encoding IL-17C is located on chromosome 5q33.1 in humans, a region that has been implicated in various inflammatory and autoimmune conditions through genetic association studies. The IL-17 family includes six members (IL-17A through IL-17F) that share structural features but have distinct biological functions and expression patterns. IL-17C is unique among family members in its primarily epithelial source and its utilization of a distinct receptor complex.
Interleukin-17C (IL-17C) is a distinctive member of the IL-17 cytokine family that plays important roles in mucosal immunity, epithelial defense, and neuroinflammatory processes. Unlike other IL-17 family members primarily produced by T cells, IL-17C is predominantly expressed by epithelial cells, providing front-line defense at barrier surfaces. Within the central nervous system (CNS), IL-17C and its receptor are expressed on various cell types including neurons, astrocytes, and microglia, where they participate in neuroinflammatory processes relevant to neurodegenerative diseases.
The gene encoding IL-17C is located on chromosome 5q33.1 in humans, a region that has been implicated in various inflammatory and autoimmune conditions through genetic association studies. The IL-17 family includes six members (IL-17A through IL-17F) that share structural features but have distinct biological functions and expression patterns. IL-17C is unique among family members in its primarily epithelial source and its utilization of a distinct receptor complex.
The role of IL-17C in the CNS has emerged as an important area of investigation given its dual nature: it can be protective in some contexts by promoting antimicrobial defense and tissue repair, while contributing to pathology in others through promotion of inflammatory responses. This context-dependent behavior makes IL-17C a complex but potentially important therapeutic target in neurodegenerative diseases characterized by neuroinflammation, including Alzheimer's disease (AD), Parkinson's disease (PD), and multiple sclerosis (MS).
<div class="infobox infobox-gene">
| Property | Value |
|----------|-------|
| Gene Symbol | IL17C |
| Full Name | Interleukin 17C |
| Chromosomal Location | 5q33.1 |
| NCBI Gene ID | 151306 |
| OMIM ID | 607246 |
| Ensembl ID | ENSG00000109339 |
| UniProt ID | Q9P0M4 |
| Encoded Protein | Interleukin-17C |
| Protein Length | 219 amino acids |
| Molecular Weight | ~25 kDa |
| Associated Diseases | Inflammatory bowel disease, psoriasis, rheumatoid arthritis, asthma |
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The IL17C gene spans approximately 4.8 kilobases and consists of three exons encoding a 219-amino-acid polypeptide. The protein adopts a characteristic IL-17 family cysteine-rich protein fold, with structural features shared with other family members including a cysteine-knot motif formed by conserved disulfide bonds. Like other IL-17 cytokines, IL-17C forms homodimers that interact with its receptor.
The promoter region of IL17C contains binding sites for various transcription factors including NF-κB, AP-1, and STAT3, reflecting its regulation by inflammatory signals. Epigenetic modifications including DNA methylation and histone modifications dynamically control IL17C expression in response to environmental stimuli and cellular differentiation signals. This regulatory complexity allows IL-17C production to be tightly controlled and context-dependent.
IL-17C is produced primarily by epithelial cells including intestinal epithelial cells, bronchial epithelial cells, and skin keratinocytes. Innate lymphoid cells (ILCs) particularly ILC3s also produce IL-17C in response to cytokine cues including IL-23, IL-1β, and TGF-β. Some T cell subsets including Th17 cells can also produce IL-17C, though at lower levels than epithelial cells.
IL-17C signals through a heterodimeric receptor complex consisting of IL-17 receptor A (IL-17RA) and IL-17 receptor E (IL-17RE). IL-17RA is widely expressed on various cell types including epithelial cells, fibroblasts, immune cells, and CNS cells, explaining the broad tissue distribution of IL-17C effects. IL-17RE expression is more restricted, with highest expression on epithelial cells and certain immune cell subsets.
The cytoplasmic domain of IL-17RA associates with the adaptor protein Act1 (encoded by TRAF3IP2), which recruits TRAF6 and activates downstream signaling pathways including NF-κB, MAPK, and C/EBP. This signaling cascade leads to production of pro-inflammatory cytokines, chemokines, and antimicrobial peptides that contribute to host defense and inflammatory responses.
Within the CNS, IL-17RA expression has been detected on neurons, astrocytes, and microglia, with expression levels varying across brain regions and cell types. IL-17RE expression in the CNS remains an area of investigation, with some studies detecting expression on specific cell types while others find minimal production.
IL-17C plays crucial roles in mucosal immunity and barrier defense at various epithelial surfaces. In the gut, IL-17C is produced by intestinal epithelial cells and ILC3s in response to microbial signals and inflammatory cytokines, where it acts in an autocrine and paracrine manner to enhance barrier function and antimicrobial defense.
The IL-17C-IL-17RE axis in the intestine promotes production of antimicrobial peptides including REGIIIγ and β-defensins, stimulates mucin production, and enhances tight junction integrity. These effects collectively contribute to protection against enteric pathogens and maintenance of intestinal homeostasis. Dysregulation of IL-17C signaling has been implicated in inflammatory bowel disease (IBD), with both protective and pathogenic roles reported depending on disease context.
In the skin, IL-17C contributes to psoriasis pathogenesis through promotion of keratinocyte proliferation and inflammatory cytokine production. The IL-17C-IL-17RA signaling axis in keratinocytes induces production of IL-6, IL-8, and other pro-inflammatory mediators that drive epidermal hyperplasia and inflammatory cell infiltration.
The CNS has its own immune system largely mediated by microglia, the resident macrophages that populate the brain and spinal cord. These cells respond to pathogens, injury, and metabolic disturbances through activation and production of inflammatory mediators. Chronic microglial activation drives progressive neuroinflammation that contributes to neuronal dysfunction and death in various neurodegenerative conditions.
IL-17C receptor expression on CNS cells creates opportunities for IL-17C to modulate neuroinflammatory processes. Studies have demonstrated that IL-17C can act on microglia to influence their activation state, with effects that vary depending on experimental conditions. The context-dependence of IL-17C effects reflects the complexity of microglial responses and the multiple signaling pathways activated by IL-17R.
In astrocytes, IL-17C signaling can induce production of chemokines and cytokines that influence neuroinflammation and recruit peripheral immune cells into the CNS. The blood-brain barrier (BBB), composed of endothelial cells and associated pericytes, represents another target of IL-17C effects, as cytokine signaling can modulate BBB permeability and immune cell trafficking.
Neurons also express IL-17RA and respond to IL-17C exposure, though the functional consequences of neuronal IL-17C signaling remain incompletely characterized. Some studies suggest protective effects against pathogens, while others indicate potential contributions to pathology, highlighting the need for further investigation.
Alzheimer's disease, the most common neurodegenerative disorder and cause of dementia worldwide, is characterized by accumulation of amyloid-β (Aβ) plaques and neurofibrillary tangles composed of hyperphosphorylated tau protein. Neuroinflammation, particularly chronic microglial activation, is increasingly recognized as a major contributor to disease progression rather than merely a secondary response to pathological protein accumulation.
Evidence for IL-17C involvement in Alzheimer's disease includes detection of IL-17C in the cerebrospinal fluid (CSF) and brain tissue of AD patients, with some studies suggesting correlations with disease severity. Genetic polymorphisms in the IL17C gene have been investigated for associations with AD risk, with some studies suggesting potential links though findings have been inconsistent.
In mouse models of AD, IL-17C has been shown to modulate microglial activation and amyloid clearance. Some studies suggest that IL-17C can enhance microglial phagocytosis of Aβ, potentially facilitating clearance of pathological aggregates. However, chronic IL-17C exposure may also promote pro-inflammatory microglial phenotypes that exacerbate neuroinflammation. The net effect likely depends on disease stage, local cytokine environment, and other factors.
The role of IL-17C in tau pathology, the other major hallmark of AD, has received less attention but represents an important area for future investigation. Given the connections between neuroinflammation and tau spreading, IL-17C modulation of inflammatory processes may influence tau-mediated neurodegeneration.
Parkinson's disease is characterized by progressive loss of dopaminergic neurons in the substantia nigra pars compacta and the presence of Lewy bodies, cytoplasmic inclusions primarily composed of misfolded α-synuclein. Neuroinflammation, particularly microglial activation in the substantia nigra, is a prominent pathological feature that contributes to disease progression through release of pro-inflammatory mediators.
Studies have detected IL-17C in the CSF and serum of PD patients compared to controls, and IL-17C expression is increased in post-mortem brain tissue from PD patients. The source of IL-17C in the CNS may include infiltrating T cells, resident glial cells, and epithelial cells of the choroid plexus, with the relative contributions remaining to be determined.
In mouse models of PD, including 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated mice and transgenic α-synuclein models, IL-17C has been shown to influence dopaminergic neuron survival. Effects appear to be context-dependent, with both protective and pathogenic functions reported depending on experimental conditions and disease stage.
The role of IL-17C in α-synuclein pathology has particular relevance for PD pathogenesis. IL-17C may influence α-synuclein aggregation, propagation, and clearance through effects on cellular protein homeostasis pathways and inflammatory processes. Interactions between IL-17C signaling and pathways involved in α-synuclein processing warrant further investigation.
Multiple sclerosis is an autoimmune demyelinating disease of the CNS driven by T cell responses against myelin antigens. IL-17C, produced by various cell types including Th17 cells, ILC3s, and epithelial cells, contributes to MS pathogenesis through promotion of inflammatory responses and potentially through effects on BBB permeability and immune cell trafficking into the CNS.
Genetic association studies have identified polymorphisms in the IL17C region that may influence MS susceptibility, suggesting that genetic variation in IL-17C signaling contributes to disease risk. Elevated IL-17C levels have been detected in the serum and CSF of MS patients, with some studies indicating correlations with disease activity and progression.
In experimental autoimmune encephalomyelitis (EAE), the mouse model of MS, IL-17C has complex and context-dependent effects. Some studies suggest pathogenic roles, with IL-17C deficiency resulting in reduced disease severity. However, protective effects have also been reported in specific contexts, highlighting the complexity of IL-17C biology.
Given the evidence for IL-17C involvement in neurodegenerative and neuroinflammatory diseases, therapeutic modulation of IL-17C signaling represents an attractive strategy for intervention. Approaches under investigation include neutralizing antibodies against IL-17C, small molecule inhibitors targeting downstream signaling pathways, and decoy receptors that sequester IL-17C.
In autoimmune conditions including psoriasis and inflammatory bowel disease, IL-17C and its receptor have been targeted with some success in preclinical models. However, clinical translation has been more challenging than for IL-17A blockade, partly due to the broader expression pattern and complex biology of IL-17C.
Translation of IL-17C-targeted approaches to neurodegenerative diseases remains at earlier stages but represents a promising direction. Key considerations include the context-dependent nature of IL-17C effects, potential for beneficial versus detrimental outcomes depending on disease stage and individual patient characteristics, and need for biomarkers to guide patient selection and treatment timing.
Several animal models have been employed to study IL-17C biology and its relevance to neuroinflammation and neurodegeneration. IL-17C knockout (Il17c-/-) mice are viable and fertile, allowing assessment of IL-17C deficiency in various disease models. Transgenic mice expressing IL-17C under various promoters enable investigation of IL-17C overexpression effects.
In EAE models, IL-17C deficiency generally results in altered disease severity, with some studies showing protection and others showing exacerbation depending on model details and timing of assessment. These findings underscore the complexity of IL-17C biology and the importance of context in determining effects.
AAV-mediated gene delivery to the CNS provides a means to manipulate IL-17C signaling in the brain. These approaches have been used to investigate IL-17C effects in models of AD, PD, and other neurodegenerative conditions, providing insights into potential therapeutic targeting strategies.
Analysis of IL17C expression across tissues reveals highest expression in epithelial tissues including intestine, skin, lung, and urinary tract. In peripheral blood mononuclear cells (PBMCs), IL17C expression is detectable in various immune cell subsets including ILC3s, Th17 cells, and γδ T cells, though at lower levels than in epithelial cells.
Within the CNS, IL17C expression has been detected in infiltrating immune cells and, to lesser extent, in resident glial cells under inflammatory conditions. The relative contributions of peripheral versus CNS-derived IL-17C to neuroinflammatory processes remain an area of active investigation.
Single-cell RNA sequencing studies have provided detailed insights into IL17C expression patterns in various tissues and disease states. These approaches reveal cellular heterogeneity in IL-17C production and identify novel cellular sources that may be relevant to specific disease contexts.
The structure of IL-17C has been solved, revealing a characteristic IL-17 family cysteine-rich protein fold. The protein forms a homodimer, with each monomer consisting of β-strands organized in a cysteine-knot configuration. Receptor binding occurs through specific regions of the protein that interact with the extracellular domains of IL-17RA and IL-17RE.
The IL-17RA extracellular domain contains multiple cytokine receptor modules (CRMs) that mediate ligand binding. The receptor undergoes conformational changes upon ligand binding that bring intracellular signaling domains into proximity for activation. The Act1 adaptor protein (encoded by TRAF3IP2) is essential for IL-17C signaling, recruiting TRAF6 and activating downstream pathways.
Structure-function studies have identified residues important for receptor binding and signal activation. Mutations that disrupt IL-17C binding to its receptor abrogate biological activity, while certain amino acid substitutions alter signaling outcomes. These studies inform design of IL-17C variants with modified properties for research or therapeutic applications.
IL-17C signaling intersects with numerous other cytokine pathways and cellular processes relevant to neurodegeneration. The IL-17 family of cytokines shares structural features and signaling mechanisms but has distinct biological functions and expression patterns. Cross-talk between IL-17C and other family members including IL-17A and IL-17F creates complexity in understanding net effects.
NF-κB activation by IL-17C overlaps with signaling by other pro-inflammatory cytokines including IL-1β, TNF-α, and IL-17A, creating both redundancy and competition in downstream gene expression. The MAPK and C/EBP pathways activated by IL-17C also intersect with signaling by other cytokines and growth factors.
In the context of neurodegenerative diseases, IL-17C interactions with pathways including NLRP3 inflammasome, TGF-β signaling, and type I interferon responses are particularly relevant. These pathways integrate inflammatory signals and regulate cellular responses that determine survival or death outcomes.
Despite significant progress in understanding IL-17C biology, important knowledge gaps remain regarding its role in neurodegeneration. The precise mechanisms through which IL-17C influences neuronal survival in different disease contexts are incompletely characterized, and the relative contributions of direct versus indirect effects require further investigation.
The temporal dynamics of IL-17C effects in neurodegenerative diseases are poorly understood. It remains unclear whether IL-17C plays different roles at different disease stages, with potential beneficial effects early in disease transitioning to detrimental effects later. This complexity has implications for therapeutic targeting and highlights the need for biomarkers that can guide intervention timing.
Better understanding of IL-17C signaling in specific CNS cell types is needed. How IL-17C modulates microglial activation states, astrocyte responses, and neuronal function under different conditions will inform therapeutic strategies. Development of cell-type-specific targeting approaches may enable more precise modulation of IL-17C effects.
Interleukin-17C is a cytokine with important roles in mucosal immunity, epithelial defense, and neuroinflammation. Through its receptor expressed on neurons, astrocytes, and microglia, IL-17C modulates inflammatory processes that contribute to neuronal dysfunction and death in Alzheimer's disease, Parkinson's disease, and related disorders. The context-dependent nature of IL-17C effects, varying with disease stage, brain region, and cellular environment, presents both challenges and opportunities for therapeutic targeting. While IL-17C can have protective effects through promotion of barrier integrity and antimicrobial defense, chronic or dysregulated signaling may contribute to neuroinflammation and neurodegeneration. Continued research into IL-17C biology and its contributions to neurodegeneration will inform development of novel interventions for these devastating conditions.