rs763780 — IL17F His161Arg

IL-17F His161Arg — The Natural Th17 Dampener

Interleukin-17F (IL-17F)¹¹ Interleukin-17F (IL-17F)
a Th17 effector cytokine closely related to IL-17A that shares receptor components IL-17RA and IL-17RC and drives overlapping but distinct inflammatory programs is one of the signature outputs of Th17 cells — the immune subset central to defense against extracellular bacteria and fungi and, when dysregulated, to the tissue damage seen in autoimmune conditions like psoriasis, ankylosing spondylitis, and inflammatory bowel disease. While IL-17A (encoded nearby on chromosome 6p12) has received more attention, IL-17F is produced at higher levels by Th17 cells and contributes substantially to neutrophil recruitment, epithelial antimicrobial peptide production, and mucosal barrier defense.

The rs763780 variant introduces a single amino acid change — histidine to arginine at position 161 (His161Arg) — in a protein only 163 residues long after signal peptide cleavage. That position sits at the very C-terminus of the mature protein within the cystine-knot fold that both IL-17A and IL-17F share. The result is a natural variant that functions as a dominant-negative antagonist of its own wild-type counterpart.

The Mechanism

IL-17F signals through a heteromeric receptor complex of IL-17RA and IL-17RC²² IL-17RA and IL-17RC
IL-17RA is ubiquitously expressed; IL-17RC provides ligand-binding specificity. Both are required for downstream signaling, which proceeds through the adaptor Act1 to NF-κB and MAPK pathways. In bronchial epithelial cells, wild-type IL-17F activates MAPK signaling cascades and induces production of IL-8, IL-6, and other pro-inflammatory mediators that recruit neutrophils and amplify mucosal inflammation.

The His161Arg variant (H161R) fails to activate the MAPK pathway and cannot induce cytokine or chemokine production in bronchial epithelial cells³³ His161Arg variant (H161R) fails to activate the MAPK pathway and cannot induce cytokine or chemokine production in bronchial epithelial cells
Functional assays in Kawaguchi et al. 2006 showed H161R protein had no measurable MAPK activation or IL-8 induction, unlike wild-type IL-17F at equivalent concentrations. More strikingly, the variant protein actively blocked wild-type IL-17F signaling, demonstrating dominant-negative behavior: heterozygous carriers produce both wild-type and H161R protein, and the H161R form competes with wild-type at the receptor level, reducing net IL-17F signaling below even that of non-carriers.

The histidine at position 161 lies in the mature protein's C-terminal region within the cystine-knot fold critical for homodimerization and receptor engagement. The arginine substitution likely disrupts dimer interface geometry or receptor contact residues, producing a protein that can occupy receptor-proximal space without transducing signal — the structural basis for its antagonist activity.

The Evidence

The landmark study by Kawaguchi et al.⁴⁴ landmark study by Kawaguchi et al.
Journal of Allergy and Clinical Immunology, 2006; n=867 Japanese subjects demonstrated that CC homozygosity was inversely associated with asthma risk with an odds ratio of 0.06 (95% CI 0.01–0.43; p=0.0039) — a striking 94% reduction in asthma odds for carriers of two His161Arg alleles, remaining significant after adjustment for atopy (p=0.0079). This was one of the first demonstrations of a naturally occurring dominant-negative cytokine variant with direct impact on inflammatory disease.

The protective signal extends to other Th17-driven conditions. In Takayasu arteritis — a large-vessel vasculitis with prominent Th17 involvement⁵⁵ Takayasu arteritis — a large-vessel vasculitis with prominent Th17 involvement
Danda et al. 2017; combined analysis across two replication phases, n=218 cases, 220 controls, the C allele was significantly protective (OR 0.44, 95% CI 0.25–0.77; p=0.0029). In ulcerative colitis, the wild-type T allele (His161) confers increased risk⁶⁶ ulcerative colitis, the wild-type T allele (His161) confers increased risk
Arisawa et al. 2008, with the T allele identified as an independent risk factor, particularly for the chronic continuous phenotype and pancolitis.

A meta-analysis of IL-17F rs763780 across 14 studies and 8,124 gastric cancer cases⁷⁷ meta-analysis of IL-17F rs763780 across 14 studies and 8,124 gastric cancer cases
Elshazli et al. 2018 found no significant association with gastric cancer risk, suggesting the variant's protective effects are not universal across all inflammatory disease contexts.

The trade-off becomes apparent in infectious disease data. In a Chinese Han tuberculosis study⁸⁸ Chinese Han tuberculosis study
Peng et al. 2013; 596 PTB + 176 EPTB cases, 622 controls, the C allele was enriched in tuberculosis cases, suggesting that reduced IL-17F activity — while dampening autoimmune inflammation — impairs protective Th17 responses against intracellular pathogens. Similarly, bladder cancer studies found the T allele (functional IL-17F) associated with greater antitumor immune surveillance effects.

The ClinVar classification as "benign" for familial candidiasis 6 reflects that the His161Arg variant alone does not cause the severe mucocutaneous candidiasis phenotype linked to complete IL-17F loss-of-function; the Ser95Leu variant in the same gene is responsible for that rare disease. However, reduced IL-17F activity from His161Arg may contribute to increased susceptibility to mucosal Candida colonization at a subclinical level.

Practical Actions

For TC heterozygotes, the dominant-negative activity of the Arg161 protein partially dampens IL-17F signaling from the wild-type allele. This is broadly beneficial for reducing autoimmune inflammatory burden, but carriers should maintain vigilance for mucosal infections — oral thrush, recurrent vaginal yeast infections, nail fungal infections — as these may signal reduced IL-17F-dependent mucosal defense.

For CC homozygotes, the strong protective effect against autoimmune disease means this variant is more likely to be clinically relevant as a protective marker than a risk factor. However, the near-complete loss of IL-17F activity warrants proactive monitoring for signs of impaired mucosal immunity: persistent Candida infections, recurrent sinopulmonary bacterial infections, and delayed wound healing at mucosal surfaces all warrant prompt evaluation.

If anti-IL-17 biologics (secukinumab, ixekizumab, bimekizumab) are considered for any Th17-driven condition, TC or CC genotype status provides biological context: these individuals already have naturally reduced IL-17F activity, so IL-17 inhibition adds to that baseline reduction; monitoring for fungal infections during biologic therapy is especially warranted.

Interactions

The rs763780 His161Arg variant operates in direct functional opposition to the IL-17A rs2275913 promoter variant⁹⁹ IL-17A rs2275913 promoter variant
the -197G>A variant increases NFAT binding affinity and amplifies IL-17A transcription, elevating Th17-driven inflammation. Individuals carrying the IL-17A risk allele (rs2275913 A) together with functional IL-17F (rs763780 TT) have the highest net Th17 cytokine output; those carrying both the IL-17A promoter risk allele and the IL-17F His161Arg variant exist in a partially offset state, where elevated IL-17A is partially counterbalanced by reduced IL-17F.

Upstream, IL-23R rs2201841¹⁰¹⁰ IL-23R rs2201841
intronic variant that increases IL-23 pathway activity and expands Th17 cells influences how many Th17 cells produce both IL-17A and IL-17F. Carrying the IL-23R risk allele alongside rs763780 CC produces an unusual combination: expanded Th17 pools making high IL-17A but with most IL-17F output neutralized by the dominant-negative variant.