A 52-year-old man with a 10-year history of rheumatoid arthritis (RA) on methotrexate and infliximab (TNF-α inhibitor) presents with fever, cough, and bilateral pulmonary infiltrates on chest X-ray. Blood cultures grow Mycobacterium tuberculosis. A tuberculin skin test (Mantoux) performed 1 week earlier was negative (0 mm induration). Which immunological mechanism best explains the failure of cell-mediated immunity to contain TB in this patient?

Explanation

## TNF-α Inhibitors and Impaired Cell-Mediated Immunity **Key Point:** TNF-α is essential for **granuloma formation and maintenance** — the hallmark of protective immunity against *Mycobacterium tuberculosis*. Blockade of TNF-α signalling abrogates Type IV hypersensitivity and allows TB reactivation or primary progression. ### Role of TNF-α in TB Immunity ```mermaid flowchart TD A[M. tuberculosis infection]:::outcome --> B[Antigen presentation to CD4+ T cells]:::action B --> C[IFN-γ production by Th1 cells]:::action C --> D[Macrophage activation]:::action D --> E[TNF-α secretion]:::action E --> F[Granuloma formation & maintenance]:::outcome E --> G[Apoptosis of infected macrophages]:::action G --> H[Mycobacterial containment]:::outcome I[TNF-α inhibitor]:::urgent --> J[Blocked TNF signalling]:::urgent J --> K[Granuloma breakdown]:::urgent K --> L[TB reactivation/progression]:::urgent ``` ### Mechanism of TNF-α Blockade **High-Yield:** TNF-α has dual roles in TB immunity: 1. **Granuloma formation:** TNF-α drives recruitment and activation of macrophages and fibroblasts to wall off infected cells. 2. **Macrophage apoptosis:** TNF-α induces programmed cell death of infected macrophages, preventing intracellular mycobacterial replication. 3. **Th1 differentiation:** TNF-α supports IL-12 signalling and IFN-γ production by CD4+ T cells. When TNF-α is blocked (by infliximab, adalimumab, etanercept): - Granulomas regress or fail to form. - Infected macrophages survive longer, allowing mycobacterial multiplication. - CD4+ T cell help is reduced (less IL-2, IFN-γ). - **Result:** Reactivation TB or rapid primary TB progression. ### Clinical Correlation **Clinical Pearl:** This patient had a **negative Mantoux test despite active TB** — a classic sign of severe immunosuppression. In immunocompetent individuals, TB infection triggers a robust Type IV hypersensitivity response (induration ≥5 mm in high-risk patients). The negative test here reflects: - Failure of CD4+ T cell recruitment to the skin (no IFN-γ, no macrophage activation). - Impaired IL-2 production and T cell proliferation. **Mnemonic:** **TNF-TB** — TNF-α is essential for TB control; its blockade leads to TB reactivation. Screen all patients for TB (TST, IGRA) **before** starting TNF-α inhibitors; give isoniazid prophylaxis if latent TB is detected. ### Comparison: Types of Hypersensitivity and TB | Type | Mediator | Cells | TB Context | Example | |------|----------|-------|------------|----------| | **I** | IgE, histamine | Mast cells, basophils | Not involved in TB immunity | Anaphylaxis | | **II** | IgG, IgM (cytotoxic) | Complement, NK cells | Not primary in TB | Graves' disease | | **III** | Immune complexes | Neutrophils, complement | Occurs in TB (erythema nodosum) but not the primary failure here | Serum sickness | | **IV** | IFN-γ, IL-2 (T cells) | CD4+ Th1, CD8+ cells | **PRIMARY DEFENSE AGAINST TB** — blocked by TNF-α inhibition | TB granuloma, Mantoux test | **Warning:** Do NOT confuse TB-associated immune complex disease (erythema nodosum, Type III hypersensitivity) with the primary mechanism of TB control. Type IV hypersensitivity is the protective response; Type III is a complication of immune response.