Th1 cell
Summary
Th1 cells are CD4+ T helper cells that differentiate in response to IL-12 (from macrophages/dendritic cells) and IFN-γ, driven by the transcription factor T-bet. They secrete IFN-γ (and IL-2, TNF-β), activating macrophages and promoting cell-mediated immunity against intracellular pathogens (e.g., mycobacteria, Listeria, Leishmania) and mediating type IV hypersensitivity.
Detail
**Differentiation** - Naive CD4+ T cell (Th0) is activated by an APC presenting antigen on MHC II (signal 1) plus B7–CD28 costimulation (signal 2). - IL-12 (from dendritic cells/macrophages) and IFN-γ (from NK cells) drive Th1 differentiation. IL-12 acts through STAT4. IFN-γ acts through STAT1. - Master transcription factor: **T-bet (TBX21)**. - Surface markers: CD4+, with IL-12 receptor and CXCR3.
**Effector cytokines and function** - **IFN-γ**: the signature cytokine. - Activates macrophages, which increases phagolysosome fusion and NO/ROS production ("classically activated," M1 macrophages). - Upregulates MHC I and II expression. - Promotes isotype switching to opsonizing IgG (IgG1/IgG3 in humans, IgG2a in mice). - Promotes granuloma formation. - **IL-2**: T-cell proliferation. - **TNF-α/β**: inflammation and granuloma maintenance. - Role: defense against **intracellular pathogens**, including *Mycobacterium tuberculosis*, *M. leprae* (tuberculoid form), *Listeria*, *Salmonella*, *Toxoplasma*, *Leishmania*, and *Histoplasma*.
**Cross-regulation** - IFN-γ inhibits Th2 differentiation. - IL-4 and IL-10 (from Th2) inhibit Th1 differentiation.
**Clinical correlations** - **Type IV (delayed-type) hypersensitivity**: PPD/tuberculin skin test, contact dermatitis (via CD4+ Th1 and CD8+ cells), and granulomatous inflammation. - **Granulomatous disease**: Th1-driven in TB, sarcoidosis, and Crohn disease. - **Autoimmunity (Th1-mediated)**: type 1 diabetes, multiple sclerosis, Hashimoto thyroiditis, rheumatoid arthritis, and Crohn disease. Th17 cells also contribute to many of these. - **Leprosy**: Th1 response gives **tuberculoid leprosy** (strong cell-mediated response, few bacilli, good prognosis). Th2 response gives **lepromatous leprosy** (weak cell-mediated response, many bacilli, poor prognosis). - **Mendelian susceptibility to mycobacterial disease (MSMD)**: defects in the IL-12/IFN-γ axis (IL-12 receptor β1, IL-12p40, IFN-γ receptor, STAT1) cause severe disseminated infections with atypical mycobacteria and BCG. - **Anti-TNF therapy** (infliximab, adalimumab) can reactivate latent TB because it impairs granuloma maintenance. Screen with PPD or IGRA before starting. - **HIV**: loss of CD4+ Th1 cells leads to opportunistic intracellular infections. - **IFN-γ release assay (QuantiFERON-TB)** measures the Th1 response to TB antigens. - **Therapeutic use**: recombinant IFN-γ is used in chronic granulomatous disease (CGD) and osteopetrosis. Ustekinumab (anti-IL-12/23 p40) is used in Crohn disease and psoriasis.
**High-yield comparison**
| | Th1 | Th2 | Th17 | |---|---|---|---| | Inducers | IL-12, IFN-γ | IL-4 | TGF-β, IL-6, IL-23 | | Transcription factor | T-bet | GATA-3 | RORγt | | Cytokines | IFN-γ, IL-2 | IL-4, IL-5, IL-10, IL-13 | IL-17, IL-22 | | Targets | Macrophages, intracellular microbes | Eosinophils, IgE, helminths, allergy | Neutrophils, extracellular bacteria and fungi |
**Memory aid**: "Th1 = IFN-γ = macrophages = intracellular bugs = type IV hypersensitivity."
Sources
- First Aid for the USMLE Step 1 (Immunology: Cellular Components, Lymphoid Structures, Hypersensitivity)
- Robbins & Cotran Pathologic Basis of Disease (Diseases of the Immune System)
- Abbas, Lichtman & Pillai: Cellular and Molecular Immunology (CD4+ T cell subsets)
- Pathoma (Immunology/Hypersensitivity)
- Janeway's Immunobiology (Effector T cell responses)
Reviewed by Medusa editorial — medical student review. Information here is for study reference only and is not medical advice. Spotted an error? Let us know.