IMMUNOLOGY OF TUBERCULOSIS

(15 Marks — MD Examination Answer)

INTRODUCTION

I. THE ORGANISM: IMMUNOLOGICALLY RELEVANT FEATURES

• Acid-fast cell wall: Composed of mycolic acids with acyl chains up to 90 carbons; resists drying, acid-alcohol decolorization, and certain antimicrobials
• Lipid-rich envelope: Contains lipoarabinomannan (LAM), phosphatidylinositol mannosides (PIMs), and cord factor (trehalose-6,6′-dimycolate) — key virulence and immunomodulatory molecules
• Slow growth: 14–24 hour doubling time; evades rapid immune clearance
• Type VII secretion system (ESX systems): Secretes virulence proteins (e.g., ESAT-6, CFP-10) that perforate phagosomal membranes and modulate immunity
• Only human reservoir: No animal reservoir for M. tuberculosis proper — transmission is exclusively person-to-person

II. INNATE IMMUNITY

A. Initial Encounter and Phagocytosis

B. Macrophage Subcellular Niches — Phagosome Arrest

• Normally, after microbial ingestion, the phagosome acidifies, acquires lysosomal hydrolases, antimicrobial peptides, and restricts iron
• The NADPH oxidase generates reactive oxygen species (ROS) that kill microbes
• Mtb arrests phagosomal maturation: The mycobacterial vacuole retains early endosomal markers (Rab5, Rab14, EEA1) and prevents fusion with lysosomes
• Mtb prevents phagolysosome formation by:
  • Excluding vacuolar ATPase (prevents acidification)
  • Inhibiting PI3P (phosphatidylinositol 3-phosphate) signaling
  • Secreting ESAT-6 via the ESX-1 secretion system, which perforates the phagosomal membrane, allowing Mtb to access the cytosol

C. Cytokine Responses in Innate Immunity

• TNF-α: Critical for granuloma formation and containment; blockade (e.g., anti-TNF biologics) reactivates latent TB
• IL-12: Drives Th1 differentiation; links innate and adaptive immunity
• IL-18: Induces IFN-γ from NK cells
• IL-1β: Recruits inflammatory cells; activates local antimicrobial responses
• IL-10: Immunomodulatory; induced by ManLAM on Mtb — dampens protective responses
• Type I IFNs (IFN-α/β): Paradoxically harmful in TB — associated with disease progression; suppress IL-1β signaling

D. Vitamin D and Innate Killing

• Macrophages activated by IFN-γ upregulate 25-hydroxyvitamin D 1α-hydroxylase → converts 25-OH-D3 to active 1,25-(OH)₂-D3 (calcitriol)
• Calcitriol induces expression of cathelicidin (LL-37) and β-defensin-2 → antimycobacterial peptides
• Vitamin D deficiency is a significant risk factor for active TB — explains TB epidemiology in populations with limited sun exposure

E. Cell Death Pathways

• Apoptosis of infected macrophages: Protective — packages bacteria into apoptotic vesicles that can be phagocytosed and degraded by neighboring macrophages (efferocytosis)
• Necrosis/Necroptosis: Favored by Mtb — releases bacteria extracellularly, promotes tissue damage and dissemination
• Mtb blocks apoptosis via lipoarabinomannan and promotes necrosis via ESAT-6 — this is a key virulence mechanism

III. GRANULOMA FORMATION

Structure of the Granuloma

Central zone: Caseous necrosis (cheese-like; from Latin caesum)
           ↓
Epithelioid macrophages (activated, abundant cytoplasm)
           ↓
Langhans giant cells (fused macrophages, nuclei at periphery)
           ↓
Lymphocytic collar (predominantly CD4+ T cells, some CD8+ T cells)
           ↓
Outer fibrotic rim (collagen, fibroblasts)

Mechanism of Granuloma Formation

1. Infected macrophages release TNF-α and IL-12
2. IL-12 drives Th1 differentiation of CD4+ T cells
3. Th1 cells produce IFN-γ → activates macrophages → enhanced microbicidal capacity
4. TNF-α recruits additional monocytes/macrophages
5. Aggregation and fusion of macrophages → epithelioid cells → Langhans giant cells
6. Lymphocytes (CD4+ T, CD8+ T, B cells) surround the macrophage core
7. Fibroblast activation → outer fibrous capsule

Dual Role of Granuloma

IV. ADAPTIVE IMMUNITY

A. Antigen Presentation and T Cell Priming

• Dendritic cells (DCs) phagocytose Mtb and migrate to regional lymph nodes (hilum) to prime T cells
• This process takes 2–6 weeks — explaining the delay in tuberculin skin test (TST) positivity
• MHC Class II presents peptide antigens to CD4+ T cells
• MHC Class I (and cross-presentation) activates CD8+ T cells
• Mtb delays DC migration to lymph nodes — a key evasion mechanism that slows adaptive priming

B. CD4+ T Cells (Th1 Response) — The Central Pillar

• Produce IFN-γ: Activates macrophages → upregulates NADPH oxidase, ROS production, nitric oxide synthase (iNOS) → NO production → antimycobacterial activity
• Produce TNF-α: Granuloma formation and maintenance
• Produce IL-2: T cell proliferation and survival

• Interfering with antigen presentation (reducing MHC II expression)
• Inducing FoxP3+ regulatory T cells (Tregs) that suppress effector responses
• Producing IL-10 that downregulates Th1 activity
• Inducing T cell exhaustion via PD-1/PD-L1 pathway in chronic infection

C. CD8+ T Cells

• Kill Mtb-infected cells via perforin/granzyme pathway and Fas-FasL interactions
• Produce IFN-γ and TNF-α
• Important for controlling intracellular bacilli that evade CD4-dependent macrophage activation
• MHC-Ib restricted CD8+ cells can recognize non-peptide antigens (lipids, metabolites)

D. Non-Classical T Cells

E. B Cells and Humoral Immunity

• Humoral immunity plays a lesser role in Mtb than in extracellular pathogens
• B cells form lymphoid follicle-like structures at the periphery of granulomas
• Antibodies may have protective functions: opsonization, enhanced phagocytosis, complement activation
• IgG antibodies to surface antigens facilitate macrophage recognition via FcγR
• Recent evidence suggests B cell-rich granulomas correlate with better containment of infection

V. IMMUNOLOGICAL BASIS OF LATENCY AND REACTIVATION

Latent TB Infection (LTBI)

• ~1.7 billion people globally have LTBI
• Immune system contains but does not eradicate Mtb
• Bacteria persist in granulomas, adipose tissue, lymphatic endothelial cells, and hematopoietic stem cells in a non-replicating/dormant state
• LTBI is detected by TST or IGRA (which measure T cell memory responses to ESAT-6, CFP-10)
• 5–10% of LTBI individuals progress to active TB during their lifetime

Immunological Balance in LTBI

Effective Immunity (CD4+ Th1, IFN-γ, TNF-α, granuloma)
        ↕
Bacterial dormancy (hypoxia adaptation, DosS/DosT system)
        ↕
Host immune surveillance

Reactivation Factors and Their Mechanisms

• DosS/DosT two-component system: Adapts to hypoxia; promotes recovery from dormancy when oxygen levels change
• Resuscitation-promoting factors (Rpf): Act on bacterial peptidoglycan to promote growth from dormancy
• Sigma factors: Enable responses to nitric oxide and carbon monoxide gradients

IFN-γ Counter-Evasion by Mtb

VI. EVASION OF ADAPTIVE IMMUNITY — SUMMARY

VII. IMMUNOLOGICAL TESTS IN CLINICAL PRACTICE

A. Tuberculin Skin Test (TST / Mantoux)

• Intradermal injection of purified protein derivative (PPD) — mixture of Mtb antigens
• Positive at ≥10 mm (≥5 mm in HIV/immunosuppressed)
• Measures delayed-type hypersensitivity (DTH) — CD4+ Th1 mediated
• False positives: BCG vaccination, NTM infection
• False negatives: Severe immunosuppression, miliary TB, sarcoidosis

B. Interferon-Gamma Release Assay (IGRA)

• Measures IFN-γ produced by T cells in response to ESAT-6 and CFP-10
• More specific than TST (not affected by BCG)
• QuantiFERON-TB Gold Plus, T-SPOT.TB
• Same limitations as TST in severe immunosuppression

VIII. BCG VACCINE AND IMMUNOLOGY

• BCG (Bacille Calmette-Guérin): Live attenuated M. bovis
• Given at birth in endemic countries
• Stimulates Th1 response and long-lived memory T cells
• Highly effective against severe childhood forms (miliary TB, TB meningitis): ~80% protection
• Poor protection against adult pulmonary TB (~0–80%; variable by region)
• Mechanisms of protective immunity targeted by BCG: CD4+ Th1 cells, IFN-γ, memory T cells
• Next-generation vaccines: M72:AS01E (adjuvanted subunit) — 54% efficacy in IGRA+ adults; BCG revaccination — ~45% efficacy against sustained LTBI

IX. IMMUNOPATHOLOGY: WHY IMMUNITY CAUSES DISEASE

• Caseation: Central necrosis in granulomas from excessive Th1/macrophage activation
• Cavity formation: Liquefaction of caseous material → aerobic milieu → explosive bacterial replication (10⁸–10⁹ organisms/mL in cavity)
• Post-primary TB: An exuberant DTH response in a sensitized host → rapid tissue necrosis
• Paradoxical reactions: Immune reconstitution inflammatory syndrome (IRIS) in HIV patients starting ART — reactivation/worsening of TB as immunity recovers

SUMMARY TABLE: INNATE vs. ADAPTIVE IMMUNITY IN TB

CONCLUSION

Sources: Murray & Nadel's Textbook of Respiratory Medicine (Chapter 52 — Immunology of TB, Phillips & Ernst); Robbins & Cotran Pathologic Basis of Disease; Cellular and Molecular Immunology (Abbas, Lichtman & Pillai); Sharma Mohan (attached reference text)