What is the pathogenesis of tuberculosis as described in Harrison's Principles of Internal Medicine?

Pathogenesis of Tuberculosis

The pathogenesis of tuberculosis follows a sequential process beginning with inhalation of Mycobacterium tuberculosis in aerosol droplets, followed by alveolar macrophage phagocytosis, immune cell recruitment, granuloma formation with Th1-mediated responses, and potential progression to either latent infection or active disease depending on the balance between bacterial replication and host immunity 1.

Initial Infection and Immune Response

M. tuberculosis is transmitted through airborne droplet nuclei (1-5 microns) that reach the distal airways and alveoli, predominantly in the lower lung lobes 1, 2. Upon arrival:

• Alveolar macrophages phagocytose the bacteria but in their quiescent state provide minimal antimicrobial resistance
• Neutrophils accumulate early but cannot directly kill M. tuberculosis
• Dendritic cells capture mycobacterial antigens from dying neutrophils and macrophages, then migrate to regional lymph nodes 1

Adaptive Immune Response Development

In the lymph nodes (2-10 weeks post-infection), a critical adaptive response develops 1, 2:

• CD4 T-cells differentiate into IFN-γ secreting Th1 cells
• CD8 T-cells become cytotoxic Tc1 cells producing granulysin and granzymes
• Th17 cells secrete IL-17, IL-21, and IL-22
• Regulatory T-cells produce IL-10 and TGF-β to balance the response 1

These effector cells enter circulation and home to infection sites, where tuberculin skin testing becomes positive as delayed-type hypersensitivity develops 1.

Granuloma Formation and the Ghon Complex

The primary granulomatous lesion in the lung plus the enlarged hilar lymph node forms the "Ghon complex" 1. During initial growth:

• M. tuberculosis spreads bronchogenically within the lung
• Hematogenous dissemination occurs after reaching regional lymph nodes
• Bacteria enter epithelial cells, fibroblasts, adipocytes, or tissue macrophages 1

Granuloma Structure and Function

Well-organized granulomas are entirely dependent on an effector Th1 response 1. The architecture consists of:

• Central zone: Mycobacteria-containing macrophages that differentiate into epithelioid cells
• Peripheral zone: Surrounding lymphocytes that activate macrophages via IFN-γ 1

This organization enables:

• Delivery of reactive oxygen and nitrogen species to phagolysosomes
• Perforin-dependent delivery of granulysin and mycobactericidal molecules by Tc1 cells
• Reduction (though not complete elimination) of mycobacterial growth 1

Caseous Necrosis and Latency

Depending on bacterial burden and immune response intensity 1:

• Macrophage destruction leads to central granuloma necrosis
• An overwhelming Th2 response (IL-4/IL-13) counteracts mycobactericidal mechanisms, contributing to caseation 1
• This solid necrotic mass becomes oxygen-depleted, potentially sclerotic and calcified
• M. tuberculosis enters nonreplicating persistence within the necrotic caseum 1

Granulomas are dynamic structures with continuous cell turnover 1. Disruption of this structure (e.g., by TNF-α inhibitors blocking adhesion molecules) causes granuloma disintegration and bacterial dissemination 1.

Latent Infection Characteristics

Within granulomas, an equilibrium exists between 1:

• Actively dividing M. tuberculosis
• Stress-adapted bacteria in foamy macrophages that prevent growth without complete killing
• Genes for fatty acid hydrolysis become highly upregulated 1

The viability of M. tuberculosis during latency remains debated: earlier studies found viable bacteria in most fibrocaseous lesions, while other studies showed 50% of necrotic and 85% of calcified lesions were sterile 1.

Post-Primary (Reactivation) Tuberculosis

Tuberculosis scars are most commonly found in lung apices on autopsy, and post-primary reactivation typically begins its apicocaudal propagation from these apical sites 1. Reactivation occurs when:

• T-cell immunity wanes (5% per year tuberculin skin test reversion rate) 1
• Immunosuppression develops (HIV, immunosuppressive drugs)
• Mycobacterial growth recrudesces, triggering hyperinflammatory responses
• Necrotic and cavitating lesions form 1

Clinical Outcomes

Following initial infection 2:

• <1% progress rapidly to clinical illness
• 5-10% develop disease after months to decades
• Risk markedly increases with HIV infection 2

Important Caveats

• Primary infection provides limited protection: reinfection with new strains can occur even in immunocompetent hosts 1
• BCG vaccination accelerates containment and reduces dissemination risk (e.g., to meninges) but does not fully protect against pulmonary disease 1
• No animal model fully replicates the human sequence of primary and post-primary tuberculosis 1
• Inadequate T-cell immunity results in uncontained bacterial growth but paradoxically causes less tissue damage 1