IL-23 (Interleukin-23): The "Stabilizer" of Autoinflammation and the "Regulator" of Barrier Immunity

I. Overview: Origin, Structure, and Unique Receptor System

IL-23 is primarily produced by activated myeloid antigen-presenting cells, including dendritic cells, macrophages, and monocytes. It is a unique heterodimeric cytokine.

Heterodimeric structure: IL-23 consists of the p19 subunit (IL-23A) and the p40 subunit (IL-12B) linked by disulfide bonds. The p19 subunit is unique to IL-23, while the p40 subunit is shared with IL-12. This shared and unique characteristic determines the overlapping and divergent functions of IL-23 and IL-12.

IL-23 receptor complex: IL-23 exerts its effects through its specific receptor (IL-23R), the expression pattern of which is key to its functional specificity:

IL-23R complex: Composed of a unique IL-23R chain (ligand-binding chain) and the IL-12Rβ1 chain (signal transduction chain, shared with the IL-12 receptor).

Inducible and restricted expression: IL-23R is not expressed on naïve T cells. Its expression requires induction by cytokines such as TGF-β and IL-6 (in mice) or IL-1β and IL-6 (in humans) and is primarily found on T cells polarized toward the Th17 lineage, certain γδ T cells, type 3 innate lymphoid cells (ILC3s), and some myeloid cells. This restricted expression ensures that IL-23's action is precisely targeted to activated "type III immune" effector cell populations.

II. Core Mechanism: Amplification and Stabilization of Type III Immune Effectors

The core function of IL-23 is not to initiate differentiation programs but to act as an "amplifier" and "maintenance signal" during the effector phase, primarily manifested at three levels:

1. Amplification and stabilization of Th17 cell responses

Maintenance of Th17 cell phenotype and survival: Naïve T cells differentiate into Th17 precursor cells under the induction of cytokines such as TGF-β and IL-6, and begin expressing IL-23R. Subsequently, IL-23 signaling activates STAT3, upregulates RORγt (the master transcription factor of Th17 cells), enhances its activity, and promotes the production of IL-17A, IL-17F, IL-22, and GM-CSF, thereby stabilizing and amplifying the pathogenic phenotype of Th17 cells.

Driving pathogenic Th17 cells: Studies show that IL-23 is a key factor in inducing "pathogenic" Th17 cells (distinct from non-pathogenic "homeostatic" Th17 cells), which highly express pro-inflammatory cytokines and are the core effector cells driving tissue inflammation and autoimmune diseases.

2. Activation of innate-like effector cells

Activation of γδ T cells and ILC3s: These tissue-resident innate-like lymphocytes also express IL-23R. IL-23 can directly activate them, enabling rapid production of large amounts of IL-17 and IL-22, which play a role in early infection responses and tissue homeostasis.

3. Induction of characteristic inflammatory mediators

Promotion of IL-17 family cytokines: IL-17A/F are core mediators for neutrophil recruitment, tissue barrier disruption, and antimicrobial peptide induction.

Promotion of IL-22 production: IL-22 acts on epithelial cells, promoting proliferation, regeneration, and antimicrobial peptide production, playing a dual role in barrier repair and pathological hyperplasia.

4. Signaling pathway: The central role of the JAK-STAT3 pathway

Primary activation of the JAK2/TYK2-STAT3 axis: Upon binding to its receptor, IL-23 primarily activates JAK2 (coupled with IL-12Rβ1) and TYK2 (coupled with IL-23R), leading to strong phosphorylation of STAT3.

The critical role of STAT3: Phosphorylated STAT3 dimers translocate to the nucleus and directly drive the expression of genes related to Th17 cell function, survival, and expansion. STAT3 activation is essential for nearly all biological functions of IL-23.

Secondary activation of pathways such as MAPK.

III. Downstream Applications: From Barrier Defense to Chronic Inflammatory Diseases

Abnormal activation of the IL-23 signaling pathway is a common pathological basis for many chronic inflammatory diseases, while its physiological functions are vital for barrier defense.

1. Autoimmune and inflammatory diseases (core pathological domains)

Psoriasis: The IL-23/Th17 axis is central to the pathogenesis of this disease. Elevated IL-23 levels in lesions drive abnormal keratinocyte proliferation and inflammation. Monoclonal antibodies targeting the IL-23p19 subunit (e.g., guselkumab, tildrakizumab, risankizumab) have demonstrated high efficacy and durable remission in treating moderate-to-severe plaque psoriasis, underscoring the pathway's central role.

Psoriatic arthritis: IL-23 also contributes to synovial inflammation and bone destruction in joints, and anti-IL-23p19 therapy is effective for PsA.

Inflammatory bowel disease:

Crohn's disease: IL-23 drives Th17 responses and innate immune cell activation in the intestinal mucosa, leading to chronic transmural inflammation. Ustekinumab (targeting p40, blocking both IL-12 and IL-23) and guselkumab (anti-IL-23p19) have become important treatments for moderate-to-severe CD.

Ulcerative colitis: The IL-23 pathway is similarly activated, and therapies targeting this pathway have shown efficacy.

Ankylosing spondylitis: Although traditionally associated with IL-17, upstream IL-23 may also play a role.

2. Anti-infective immunity (physiological protective functions)

Defense against extracellular bacteria and fungi: At mucosal and skin sites, IL-23 activates Th17, γδ T cells, and ILC3s to promote IL-17 and IL-22 production, which is critical for clearing pathogens such as Candida albicans, Staphylococcus aureus, and Klebsiella pneumoniae.

Barrier immune homeostasis: In the gut and other sites, appropriate IL-23 signaling helps maintain immune tolerance to commensal bacteria and barrier integrity.

3. Tumor immunity (a complex double-edged sword)

Pro-tumor effects: In the tumor microenvironment, IL-23 may promote inflammation, angiogenesis, and suppression of anti-tumor immunity.

Anti-tumor potential: In some contexts, IL-23 may enhance anti-tumor immunity, such as by promoting memory T cell responses. Its role depends heavily on tumor type and microenvironmental context.

IV. Future Perspectives: From Targeted Blockade to Precision Immune Remodeling

The success of IL-23-targeted therapies has ushered in a new era in autoimmune disease treatment, with future research advancing toward greater precision and broader applications.

Expanding therapeutic indications:

Exploring IL-23 inhibitors in other Th17/IL-17-mediated diseases, such as hidradenitis suppurativa, Behçet's disease, and specific types of uveitis.

Optimizing combination therapy strategies:

Investigating combinations of anti-IL-23 drugs with other biologics (e.g., anti-IL-17, anti-TNF) or small-molecule drugs (e.g., JAK inhibitors) to achieve deeper remission or treat refractory cases.

Exploring combinations with mucosal addressin-targeted therapies (e.g., anti-integrin drugs) for IBD to achieve multi-mechanistic synergy.

Precision medicine and biomarker development:

Identifying biomarkers (e.g., genetic signatures, microbiome profiles, cytokine patterns) to predict patient responses to anti-IL-23 therapy, enabling personalized treatment selection.

Distinguishing IL-23-dependent vs. independent disease subtypes, such as defining IBD patient subgroups most suitable for IL-23-targeted therapy.

Tissue-specific delivery and localized therapy:

Developing localized formulations (e.g., modified topical agents for psoriasis or colonic delivery systems for IBD) to maximize local efficacy while minimizing systemic exposure and risks.

Beyond blockade: Agonist and vaccine adjuvant potential:

Exploring IL-23 as an adjuvant in specific immunodeficiencies or vaccine scenarios to enhance mucosal immunity and Th17 responses, such as for antifungal or anti-tuberculosis vaccines.

Deep mechanistic insights:

Further investigating IL-23's roles in non-Th17 cells (e.g., regulatory T cells, B cells, myeloid cells) to fully understand its immunoregulatory network.

Using single-cell sequencing and spatial omics to precisely map IL-23-producing and responding cells in tissues.

Summary

IL-23 is a functionally precise and potent "effector amplifier" in the immune system. It does not initiate new immune responses but precisely acts on pre-"marked" type III effector cells, endowing and maintaining their robust inflammatory and tissue-remodeling capabilities. This characteristic makes it a loyal guardian of barrier defense under physiological conditions but a stubborn driver of chronic inflammation when dysregulated. From the groundbreaking success of anti-IL-23p19 antibodies in treating psoriasis and IBD to its use as a core molecular model for exploring immune-epithelial crosstalk, research on IL-23 has profoundly transformed our understanding of chronic inflammatory diseases and therapeutic paradigms. Moving forward, by refining its cellular action maps, developing more precise patient stratification strategies, and exploring smarter drug delivery methods, we can elevate the art of IL-23 regulation to new heights, offering lasting relief and fundamentally improved quality of life for patients suffering from chronic inflammation.