Th17 cell
Summary
Th17 cells are CD4+ helper T cells that secrete IL-17, IL-21, and IL-22 and are essential for neutrophil recruitment and defense against extracellular bacteria and fungi (e.g., Candida, S. aureus). They differentiate in response to TGF-β + IL-6 (IL-1β, IL-23 maintain/expand them) via STAT3 and RORγt. Defects cause hyper-IgE syndrome (Job syndrome); excess activity contributes to autoimmune disease (psoriasis, IBD, MS, RA, ankylosing spondylitis).
Detail
**Differentiation** - Naive CD4+ T cell + TGF-β + IL-6 (also IL-1β, IL-21, IL-23) → Th17. - Key signaling: IL-6 → JAK/STAT3 → master transcription factor RORγt (RORC). - IL-23 (from dendritic cells/macrophages) is needed for expansion, survival, and pathogenicity of Th17 cells. - Inhibited by IFN-γ (from Th1) and IL-4 (from Th2). Note TGF-β alone (without IL-6) favors Treg (FoxP3) differentiation, so Th17/Treg balance is a classic comparison.
**Effector functions** - Secrete IL-17A/F, IL-21, IL-22 (and GM-CSF in pathogenic subsets). - IL-17 acts on epithelial cells, keratinocytes, and fibroblasts to induce G-CSF, CXCL8 (IL-8), and other chemokines → neutrophil recruitment and granulopoiesis. - IL-22 promotes epithelial barrier integrity and antimicrobial peptides (defensins) at skin and mucosal surfaces. - Main role: defense against extracellular bacteria and fungi, especially at mucocutaneous barriers (Candida albicans, Staphylococcus aureus, Klebsiella).
**Clinical correlates: deficiency** - **Hyper-IgE syndrome (Job syndrome):** AD-STAT3 loss-of-function mutation → impaired Th17 differentiation → low IL-17/IL-22 → poor neutrophil recruitment. Features: FATCHEL (coarse Facies, cold (non-inflamed) staphylococcal Abscesses, retained primary Teeth, Candidiasis, high IgE, Eczema/dermatitis, Leukocyte-recruitment defect/bone fractures). Also vertebral/skeletal abnormalities and pneumatoceles. - **Chronic mucocutaneous candidiasis:** defects in IL-17 signaling or Th17 axis (IL-17F, IL-17RA, STAT1 gain-of-function, AIRE/APECED with anti-IL-17 and anti-IL-22 autoantibodies). - Patients on IL-17 inhibitors have increased risk of mucocutaneous candidiasis.
**Clinical correlates: excess/autoimmunity** - Psoriasis, psoriatic arthritis, ankylosing spondylitis, IBD (especially Crohn disease), multiple sclerosis (EAE model), rheumatoid arthritis. - Targeted therapies: - IL-17A inhibitors: secukinumab, ixekizumab; IL-17RA: brodalumab (boxed warning for suicidality). - IL-23 (p19) inhibitors: guselkumab, risankizumab, tildrakizumab. - IL-12/23 (p40) inhibitor: ustekinumab. - IL-17 inhibitors can worsen or unmask IBD, so they are generally avoided in Crohn disease.
**Comparison with other CD4+ subsets (high-yield)** - Th1: IL-12, STAT4, T-bet → IFN-γ; intracellular pathogens (macrophage activation). - Th2: IL-4, STAT6, GATA3 → IL-4, IL-5, IL-13; helminths, allergy (eosinophils, IgE). - Th17: IL-6 + TGF-β, STAT3, RORγt → IL-17, IL-22; extracellular bacteria/fungi (neutrophils). - Treg: TGF-β + IL-2, STAT5, FOXP3 → IL-10, TGF-β; tolerance. FOXP3 mutation → IPEX. - Tfh: IL-6/IL-21, BCL6 → IL-21; B-cell help and class switching in germinal centers.
**Pearls** - "Th17 = neutrophils + barrier defense"; Th1 = macrophages; Th2 = eosinophils/IgE. - Hyper-IgE: high IgE but "cold" abscesses, due to low IFN-γ/IL-17-mediated inflammatory signaling and neutrophil recruitment. - Both Th17 and Treg need TGF-β; IL-6 tips the balance toward Th17.
Sources
- First Aid for the USMLE Step 1 (Immunology: Lymphocytes/T cell differentiation; Immunodeficiencies)
- Robbins & Cotran Pathologic Basis of Disease (Diseases of the Immune System)
- Abbas, Lichtman & Pillai: Cellular and Molecular Immunology (Differentiation and Functions of CD4+ Effector T Cells)
- Pathoma (Immunology)
- Sketchy Medical (Immunology: T-cell subsets and Hyper-IgE syndrome)
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