IL26 Gene

IL26 (Interleukin 26)

Overview

Interleukin 26 (IL-26) is a member of the IL-10 cytokine family that plays important roles in immune modulation and inflammatory responses. Originally identified as AK155, IL-26 is encoded by the IL26 gene located on chromosome 12q13.12[@donnelly2010]. This cytokine signals through a heterodimeric receptor complex composed of IL-20RA (IL-20 Receptor Alpha) and IL-20RB (IL-20 Receptor Beta), activating downstream JAK-STAT signaling pathways that regulate gene expression in target cells[@kotenko2001]. IL-26 is primarily expressed by memory CD4+ T cells, Th17 cells, and natural killer cells, where it contributes to both pro-inflammatory and immunomodulatory functions[@wilson2007]. Recent research has begun to explore the role of IL-26 in neuroinflammatory processes associated with neurodegenerative diseases, making it a subject of interest for understanding immune-mediated mechanisms in Alzheimer's Disease (AD) and Parkinson's Disease (PD)[@lyman2020].

<div class="infobox infobox-gene">

| Property | Value |
|----------|-------|
| Gene Symbol | IL26 |
| Full Name | Interleukin 26 |
| Chromosomal Location | 12q13.12 |
| NCBI Gene ID | 55801 |
| OMIM ID | 605450 |
| Ensembl ID | ENSG00000136646 |
| UniProt ID | Q9NPC3 |
| Encoded Protein | Interleukin-26 |
| Protein Family | IL-10 cytokine family |
| Associated Diseases | Autoimmune disorders, Rheumatoid arthritis, Inflammatory bowel disease |

</div>

Molecular Mechanism

Receptor Binding and Signaling Cascade

IL26 (Interleukin 26)

Overview

Interleukin 26 (IL-26) is a member of the IL-10 cytokine family that plays important roles in immune modulation and inflammatory responses. Originally identified as AK155, IL-26 is encoded by the IL26 gene located on chromosome 12q13.12[@donnelly2010]. This cytokine signals through a heterodimeric receptor complex composed of IL-20RA (IL-20 Receptor Alpha) and IL-20RB (IL-20 Receptor Beta), activating downstream JAK-STAT signaling pathways that regulate gene expression in target cells[@kotenko2001]. IL-26 is primarily expressed by memory CD4+ T cells, Th17 cells, and natural killer cells, where it contributes to both pro-inflammatory and immunomodulatory functions[@wilson2007]. Recent research has begun to explore the role of IL-26 in neuroinflammatory processes associated with neurodegenerative diseases, making it a subject of interest for understanding immune-mediated mechanisms in Alzheimer's Disease (AD) and Parkinson's Disease (PD)[@lyman2020].

<div class="infobox infobox-gene">

| Property | Value |
|----------|-------|
| Gene Symbol | IL26 |
| Full Name | Interleukin 26 |
| Chromosomal Location | 12q13.12 |
| NCBI Gene ID | 55801 |
| OMIM ID | 605450 |
| Ensembl ID | ENSG00000136646 |
| UniProt ID | Q9NPC3 |
| Encoded Protein | Interleukin-26 |
| Protein Family | IL-10 cytokine family |
| Associated Diseases | Autoimmune disorders, Rheumatoid arthritis, Inflammatory bowel disease |

</div>

Molecular Mechanism

Receptor Binding and Signaling Cascade

IL-26 signals through a heterodimeric receptor complex consisting of IL-20RA (also known as IL-20R1) and IL-20RB (also known as IL-20R2)[@kotenko2001]. Unlike other IL-10 family cytokines that can signal through multiple receptor combinations, IL-26 specifically requires this particular receptor heterodimer for signal transduction[@logsdon2012]. The IL-20RB subunit is shared with other IL-20 family cytokines (IL-20 and IL-24), providing a molecular basis for functional overlap and cross-talk between these signaling pathways[@rich2011].

Upon ligand binding, the IL-26 receptor complex undergoes conformational changes that activate intracellular signaling through the JAK (Janus Kinase) family of tyrosine kinases, primarily JAK1 and TYK2[@dumoutier2001]. These kinases phosphorylate tyrosine residues on the intracellular domains of the receptor, creating docking sites for STAT (Signal Transducer and Activator of Transcription) proteins. IL-26 signaling predominantly activates STAT3, although STAT1 can also be phosphorylated in certain cellular contexts[@wei2013]. The phosphorylated STAT proteins dimerize and translocate to the nucleus, where they bind to specific DNA response elements and regulate transcription of target genes involved in immune modulation, cell survival, and inflammatory responses.

Downstream Gene Expression

The JAK-STAT3 signaling cascade induced by IL-26 regulates expression of various genes involved in inflammation and immune regulation. Key target genes include:

• IL6: Pro-inflammatory cytokine amplification
• CXCL8 (IL-8): Chemokine for neutrophil recruitment
• CCL20: Chemokine for lymphocyte recruitment
• MMP1: Matrix metalloproteinase involved in tissue remodeling
• BCL2L1: Anti-apoptotic protein promoting cell survival[@hsu2015]

Cellular Functions

Immune Cell Modulation

IL-26 exerts multiple effects on various immune cell populations. In monocytes and macrophages, IL-26 promotes the production of pro-inflammatory cytokines including IL-1β, IL-6, and TNF-α, creating a positive feedback loop that amplifies inflammatory responses[@che2018]. This cytokine also enhances the expression of antimicrobial peptides such as LL-37 (CAP18), which has been shown to have direct antimicrobial activity against bacterial pathogens[@guerralaso2019].

In T cells, IL-26 can influence Th17 cell differentiation and function. Th17 cells are a subset of CD4+ T helper cells that produce IL-17 (IL-17A) and other pro-inflammatory cytokines. IL-26 can act in an autocrine or paracrine manner to enhance IL-17 production, thereby contributing to the pathogenic functions of Th17 cells in autoimmune diseases[@pne2018]. Additionally, IL-26 has been shown to promote T cell migration and tissue infiltration through upregulation of chemokine receptors.

Epithelial and Stromal Cell Effects

IL-26 acts on non-hematopoietic cells including epithelial cells and fibroblasts. In these cell types, IL-26 induces the production of chemokines (CXCL1, CXCL8, CCL20) and pro-inflammatory mediators, contributing to the recruitment of immune cells to sites of inflammation[@kanda2016]. This epithelial-stromal-immune cell cross-talk is particularly relevant in barrier tissues such as the intestinal epithelium and the blood-brain barrier.

Role in Neuroinflammation

Central Nervous System Expression

While IL-26 is not constitutively expressed in the healthy central nervous system (CNS), emerging evidence suggests that it may be induced under inflammatory conditions. Astrocytes and microglia, the resident immune cells of the brain, can potentially produce and respond to IL-26, suggesting a role in CNS immune surveillance[@lively2019]. The expression of IL-26 in the CNS may be induced by pro-inflammatory cytokines such as IL-1β and TNF-α, which are elevated in neurodegenerative disease contexts.

Alzheimer's Disease

In Alzheimer's Disease, neuroinflammation is a hallmark feature characterized by activation of microglia and astrocytes surrounding amyloid-beta plaques and neurofibrillary tangles. IL-26 may contribute to AD-related neuroinflammation through several mechanisms:

Parkinson's Disease

In Parkinson's Disease, neuroinflammation contributes to dopaminergic neuron loss in the substantia nigra pars compacta. IL-26 may play a role in PD pathogenesis through:

Disease Associations

Autoimmune Disorders

IL-26 is strongly implicated in the pathogenesis of several autoimmune diseases:

• Rheumatoid Arthritis (RA): Elevated IL-26 levels have been detected in synovial fluid and serum of RA patients. IL-26 promotes synovial fibroblast activation and inflammatory cytokine production, contributing to joint destruction[@choi2012].
• Systemic Lupus Erythematosus (SLE): IL-26 levels correlate with disease activity in SLE patients, particularly in those with renal involvement[@cheng2019].
• Inflammatory Bowel Disease (IBD): IL-26 is overexpressed in the intestinal mucosa of patients with Crohn's disease and ulcerative colitis, where it contributes to epithelial inflammation[@dambacher2014].
• Psoriasis: IL-26 is produced by Th17 cells in psoriatic skin lesions and promotes keratinocyte proliferation and inflammatory responses[@harper2009].

Therapeutic Implications

Given its pro-inflammatory role, IL-26 represents a potential therapeutic target for inflammatory and autoimmune diseases. Strategies under investigation include:

Expression Patterns

Tissue Distribution

IL-26 expression is restricted primarily to immune tissues and cells:

• Peripheral Blood: Low basal expression in circulating immune cells
• Lymphoid Tissues: Higher expression in tonsils, lymph nodes, and spleen
• Skin: Expressed in psoriatic skin lesions
• Intestine: Expressed in inflamed intestinal mucosa
• Synovium: Elevated expression in rheumatoid arthritis synovium
• CNS: Inducible expression in brain under inflammatory conditions

Cellular Sources

The primary cellular sources of IL-26 include:

• Memory CD4+ T cells
• Th17 cells and Th22 cells
• Natural killer (NK) cells
• NKT cells
• Potentially astrocytes and microglia (under inflammatory conditions)

Genetic Associations

Single Nucleotide Polymorphisms (SNPs)

Genetic variants in the IL26 gene have been associated with susceptibility to several autoimmune diseases:

• rs1559906: Associated with increased IL-26 expression and rheumatoid arthritis risk[@tian2020]
• rs2872840: Linked to inflammatory bowel disease susceptibility
• rs4082314: Associated with systemic lupus erythematosus

Gene Regulation

IL26 expression is regulated by multiple factors:

• Transcription Factors: STAT3, RORC (Th17 differentiation)
• Epigenetic Modifications: DNA methylation at promoter regions
• Environmental Factors: Microbial exposure, cytokine milieu

See Also

• [Alzheimer's Disease](/diseases/alzheimers-disease)
• [Parkinson's Disease](/diseases/parkinsons-disease)
• [Neuroinflammation](/mechanisms/neuroinflammation-pathway)
• [JAK-STAT Signaling Pathway](/mechanisms/jak-stat-signaling)
• [Th17 Cells](/cell-types/th17-cells)
• [Microglia](/cell-types/microglia)
• [IL6 Gene](/genes/il6)
• [IL17 Gene](/genes/il17)
• [TNF Gene](/genes/tnf)

External Links

• [Ensembl: ENSG00000136646](https://www.ensembl.org/Homo_sapiens/Gummary?g=ENSG00000136646)
• [NCBI Gene: IL26](https://www.ncbi.nlm.nih.gov/gene/55801)
• [GeneCards: IL26](https://www.genecards.org/cgi-bin/carddisp.pl?gene=IL26)
• [OMIM: IL26](https://omim.org/entry/605450)
• [UniProt: Q9NPC3](https://www.uniprot.org/uniprot/Q9NPC3)
• [Allen Brain Atlas: IL26](https://human.brain-map.org/microarray/search/show?search_term=IL26)

References