IL17A — Interleukin 17A

Overview

IL17A (Interleukin-17A) is the founding member of the IL-17 cytokine family, encoded by the IL17A gene on chromosome 6p12.2. It is a 155-amino acid secreted glycoprotein that forms disulfide-linked homodimers. IL-17A is produced primarily by Th17 cells (a distinct CD4+ T helper subset), as well as by innate immune cells including gamma-delta (γδ) T cells, innate lymphoid cells type 3 (ILC3s), natural killer T (NKT) cells, and neutrophils. IL-17A is a potent pro-inflammatory cytokine that plays critical roles in host defense against extracellular bacteria and fungi, but is also central to the pathogenesis of autoimmune diseases and has been increasingly recognized as a contributor to neuroinflammation in neurodegenerative diseases ^{1Interleukin-17 family members and inflammation2012 · Immunity · PMID 23077250Open reference2Interleukin-17A in neuroinflammation: from pathogenesis to therapy2023 · Int J Mol Sci · PMID 37686209Open reference}.

Molecular Function

Receptor and Signaling

IL-17A signals through a heterodimeric receptor complex:

• IL17RA (IL-17 Receptor A): ubiquitously expressed, forms the signaling subunit

• IL17RC: co-receptor, especially highly expressed in non-immune cells including neurons and glia

The IL-17A/RA complex recruits the adaptor protein ACT1 (encoded by TNF receptor-associated factor 3 interacting protein 2, TRAF3IP2), which activates downstream signaling pathways:

• NF-κB pathway: Classical pro-inflammatory signaling via IKK complex activation

• MAPK pathways: C/EBPβ, C/EBPδ, and AP-1 family transcription factors

• C/EBP activation: Gene expression programs for chemokines and inflammatory mediators

The downstream effects include:

Cellular Sources

• Th17 cells: CD4+ T helper cells differentiated under TGF-β, IL-6, IL-21, and IL-23

• γδ T cells: Innate-like T cells producing IL-17A rapidly in response to stress

• ILC3s: Innate lymphoid cells providing early IL-17A response

• Neutrophils: Can produce IL-17A in chronic inflammatory settings

Role in Neurodegeneration

Alzheimer’s Disease

IL-17A levels are elevated in the cerebrospinal fluid and brain tissue of Alzheimer’s Disease patients. Key contributions include:

• Neuroinflammation amplification: IL-17A promotes chronic inflammation in the brain microenvironment, activating microglia and astrocytes

• Blood-brain barrier disruption: IL-17A increases BBB permeability, allowing peripheral immune cell infiltration

• Synapse toxicity: IL-17A signaling in neurons can lead to synaptic dysfunction and loss

• Aβ interaction: IL-17A may synergize with amyloid-β to drive microglial activation and neurotoxicity ^{3Elevated IL-17A levels in cerebrospinal fluid of patients with neurodegenerative diseases2023 · J Neurol Sci · PMID 37245492Open reference4IL-17A as a potential biomarker and therapeutic target for Alzheimer\'s disease2022 · J Transl Med}

Parkinson’s Disease

In Parkinson’s Disease, IL-17A contributes to dopaminergic neuron death through multiple mechanisms:

• Direct toxicity to dopaminergic neurons: IL-17A activates apoptotic pathways in substantia nigra neurons

• Microglial activation: IL-17A is a potent activator of pro-inflammatory microglia (M1 phenotype)

• Th17 infiltration: Peripheral Th17 cells may cross the BBB and exacerbate nigral inflammation

• MPTP model evidence: In the MPTP mouse model of PD, IL-17A neutralization protects dopaminergic neurons and improves motor function ^{5IL-17A promotes neuroinflammation and contributes to dopaminergic neuronal death in MPTP mouse model2018 · Inflamm Res · PMID 29487988Open reference6Th17 cells and IL-17A in Parkinson's disease: from pathogenesis to therapeutics2023 · Front Cell Neurosci · PMID 37475764Open reference}

Multiple Sclerosis

IL-17A is central to Multiple Sclerosis pathogenesis:

• EAE induction: IL-17A is critical for experimental autoimmune encephalomyelitis (EAE), the mouse model of MS

• Demyelination: IL-17A drives oligodendrocyte death and myelin damage

• Clinical trials: Secukinumab (anti-IL-17A) showed efficacy in MS Phase II trials, with further studies ongoing ^{7IL-17A in multiple sclerosis and experimental autoimmune encephalomyelitis2019 · Cold Spring Harb Perspect Med · PMID 30918116Open reference}

Stroke and Ischemia

• Exacerbates ischemic injury: IL-17A levels rise rapidly after stroke and contribute to secondary neuronal damage

• Post-stroke inflammation: IL-17A from γδ T cells drives early inflammatory response that worsens outcomes

Therapeutic Targeting

Approved Biologicals

Clinical Trials in Neurodegeneration

• Secukinumab in MS: Phase II trials showed reduced MRI lesions and clinical progression

• Rinvoq (JAK inhibitor): Indirectly reduces IL-17A signaling via JAK-STAT pathway; approved for inflammatory diseases

Challenges

• Infection risk: IL-17A blockade increases susceptibility to extracellular bacterial and fungal infections

• Mucosal immunity: Disruption of IL-17A-dependent host defense at barrier surfaces

• CNS delivery: Ensuring adequate CNS penetration for brain-resident inflammation

See Also

• Neuroinflammation — Central role of IL-17A

• Alzheimer’s Disease — AD neuroinflammation

• Parkinson’s Disease — PD neuroinflammation

• Multiple Sclerosis — MS autoimmune mechanisms

• Th17 Cells — IL-17A producing cells

• Microglia — Target cells in neuroinflammation

• NF-κB Signaling — Downstream pathway of IL-17A

Pathway Diagram

The following diagram shows the key molecular relationships involving il17a discovered through SciDEX knowledge graph analysis:

flowchart TD
    IL17A["IL17A"] -.->|"inhibits"| PARKIN["PARKIN"]
    IL17A["IL17A"] -->|"promotes"| mitochondrial_dysfunction["mitochondrial dysfunction"]
    IL17A["IL17A"] -->|"mediates"| myeloid_cell_immune_response["myeloid cell immune response"]
    IL17A["IL17A"] -->|"involved in"| Staphylococcus_Aureus_Infectio["Staphylococcus Aureus Infection"]
    IL17A["IL17A"] -->|"mediates"| Staphylococcus_Aureus_Infectio["Staphylococcus Aureus Infection"]
    IL17A["IL17A"] -.->|"inhibits"| BMAL1["BMAL1"]
    IL17A["IL17A"] -->|"activates"| DNMT1["DNMT1"]
    IL17A["IL17A"] -->|"associated with"| Senescence["Senescence"]
    hyperforin["hyperforin"] -.->|"inhibits"| IL17A["IL17A"]
    IL10["IL10"] -->|"associated with"| IL17A["IL17A"]
    TP53["TP53"] -->|"associated with"| IL17A["IL17A"]
    APOE["APOE"] -->|"associated with"| IL17A["IL17A"]
    IL6["IL6"] -->|"associated with"| IL17A["IL17A"]
    style IL17A fill:#4a1a6b,stroke:#333,color:#e0e0e0
    style PARKIN fill:#4a1a6b,stroke:#333,color:#e0e0e0
    style mitochondrial_dysfunction fill:#5d2900,stroke:#333,color:#e0e0e0
    style myeloid_cell_immune_response fill:#006494,stroke:#333,color:#e0e0e0
    style Staphylococcus_Aureus_Infectio fill:#ef5350,stroke:#333,color:#e0e0e0
    style BMAL1 fill:#006494,stroke:#333,color:#e0e0e0
    style DNMT1 fill:#006494,stroke:#333,color:#e0e0e0
    style Senescence fill:#ef5350,stroke:#333,color:#e0e0e0
    style hyperforin fill:#6d3000,stroke:#333,color:#e0e0e0
    style IL10 fill:#4a1a6b,stroke:#333,color:#e0e0e0
    style TP53 fill:#4a1a6b,stroke:#333,color:#e0e0e0
    style APOE fill:#4a1a6b,stroke:#333,color:#e0e0e0
    style IL6 fill:#4a1a6b,stroke:#333,color:#e0e0e0

Pathway Diagram

The following diagram shows the key molecular relationships involving IL17A — Interleukin 17A discovered through SciDEX knowledge graph analysis:

graph TD
    hyperforin["hyperforin"] -.->|"inhibits"| IL17A["IL17A"]
    IL10["IL10"] -->|"associated with"| IL17A["IL17A"]
    TP53["TP53"] -->|"associated with"| IL17A["IL17A"]
    APOE["APOE"] -->|"associated with"| IL17A["IL17A"]
    IL6["IL6"] -->|"associated with"| IL17A["IL17A"]
    style hyperforin fill:#ff8a65,stroke:#333,color:#000
    style IL17A fill:#ce93d8,stroke:#333,color:#000
    style IL10 fill:#ce93d8,stroke:#333,color:#000
    style TP53 fill:#ce93d8,stroke:#333,color:#000
    style APOE fill:#ce93d8,stroke:#333,color:#000
    style IL6 fill:#ce93d8,stroke:#333,color:#000