Interleukin-22

Microbiome-to-barrier signaling#

One route begins with microbial Tryptophan Metabolism. Indole metabolites can activate the aryl hydrocarbon receptor, which regulates ILC3 activity and promotes IL-22 production in mucosal immunity.

This provides a mechanistic bridge from microbial metabolism to a host epithelial response, but the direction and magnitude depend on the specific ligand and cell type.^{[3]Microbiota-derived tryptophan metabolites in vascular inflammation and cardiovascular diseaseNadja Paeslack, Maximilian Mimmler, Stefanie Becker et al. · 2022Open reference 3 ↓}

A second route begins with Short-Chain Fatty Acids (SCFAs). A review of SCFA receptors describes FFAR2 signaling in ILC3s and places IL-22 downstream of this immune-cell program, where it supports tissue integrity and host defense.

The same review emphasizes that SCFA receptors are expressed across epithelial and immune-cell populations, so an observed SCFA association should not automatically be assigned to IL-22 without pathway-level measurements.^{[1]Participation of short-chain fatty acids and their receptors in gut inflammation and colon cancerMaria Daniella Carretta, John Quiroga, Rodrigo Lopez et al. · 2021Open reference 1 ↓}

These pathways explain why loss of indole-producing or SCFA-producing organisms can be discussed as loss of IL-22-supporting ecological function. They do not establish that the abundance of any single organism determines a person's IL-22 level.

Barrier defense and tissue repair#

Within homeostatic mucosal signaling, IL-22 is linked to epithelial repair and antimicrobial defense. It is often discussed alongside tight-junction integrity, mucin, IgA, and antimicrobial-peptide programs.

These outcomes are related but are not interchangeable measurements: detecting IL-22 does not prove that the barrier has recovered, and improved barrier markers do not prove that IL-22 caused the improvement.^{[1]Participation of short-chain fatty acids and their receptors in gut inflammation and colon cancerMaria Daniella Carretta, John Quiroga, Rodrigo Lopez et al. · 2021Open reference 1 ↓}

The producing-versus-responding-cell distinction is important. The evidence summarized here places IL-22 production in immune populations such as ILC3 or Th17-associated cells; epithelial cells are represented primarily as the responding tissue. Claims that colonocytes themselves produce IL-22 require separate direct evidence.

Context-dependent inflammatory effects#

An animal study of DSS colitis illustrates the context problem. A ketogenic diet altered the microbiome, reduced colonic ILC3 abundance and IL-22 expression, improved barrier markers, and reduced disease severity; fecal transfer reproduced part of the protective effect in germ-free mice.^{[4]Ketogenic Diet Alleviates Colitis by Reduction of Colonic Group 3 Innate Lymphoid Cells Through Altering Gut MicrobiomeKong C, Yan X, Liu Y et al. · 2021Open reference 4 ↓}

In that model, lower IL-22 accompanied improvement even though IL-22 can support barrier integrity in other settings. The study therefore argues against labeling every increase as protective or every decrease as harmful.

Outside the gut, a clinical review of nickel allergic contact dermatitis places IL-22 within a Th1/Th17-skewed T-cell response that also includes IFN-gamma and IL-17. This is a skin hypersensitivity context and should not be generalized to intestinal homeostasis.^{[2]Nickel Allergy and Allergic Contact Dermatitis: A Clinical ReviewAhlström MG, Thyssen JP, Wennervaldt M et al. · 2019Open reference 2 ↓}

Fungal and tumor contexts#

Fungal signals add another layer. A cancer-mycobiome review describes Candida- and other fungus-associated IL-22/ILC pathways within immunosuppressive and tumor-promoting microenvironments. It also reports poorer survival in a Candida-dominant gastrointestinal-cancer pattern associated with IL-22 and several tumor pathways.^{[5]The mycobiome in human cancer: analytical challenges, molecular mechanisms, and therapeutic implicationsTing Ding, Chang Liu, Zhengyu Li · 2025Open reference 5 ↓}

These findings support an IL-22 connection in cancer ecology but do not show that IL-22 alone caused the survival difference.

This is why “barrier-protective cytokine” is an incomplete definition. IL-22 can participate in epithelial defense while also appearing in chronic inflammatory or tumor-associated networks. A page or study must specify tissue, cell source, disease state, and measured outcome before assigning direction.