What is the pathophysiology of tuberculosis?

Pathophysiology of Tuberculosis

Tuberculosis pathophysiology is fundamentally a dynamic interplay between M. tuberculosis immune evasion and host cell-mediated immunity, centered on granuloma formation—a Th1-dependent structure that either contains or fails to contain bacterial replication, determining whether infection remains latent or progresses to active disease.

Initial Infection and Bacterial Entry

M. tuberculosis is inhaled as aerosol droplet nuclei, reaching the distal bronchoalveolar tree, predominantly in the lower lung lobes 1. The infectious dose is remarkably low—typically only 1-5 bacilli 2. Upon reaching the alveoli, the bacteria are phagocytosed by alveolar macrophages, which in their quiescent state mount minimal antimicrobial resistance 1. Neutrophils accumulate early but cannot directly kill M. tuberculosis 1.

Critical early mechanism: M. tuberculosis blocks phagolysosome fusion within macrophages, allowing intracellular survival and replication 1. This represents the pathogen's primary immune evasion strategy.

Adaptive Immune Response Initiation

Dying neutrophils and macrophages release M. tuberculosis antigens, which are captured by alveolar dendritic cells 1. These dendritic cells migrate to regional lymph nodes where they present antigens to:

• MHC class-II restricted CD4 T-cells → differentiate into IFN-γ secreting Th1 cells
• MHC class-I restricted CD8 T-cells → differentiate into cytotoxic Tc1 cells that accumulate granulysin and granzymes 1

Additional T-cell populations coordinate the response 1:

• Th17 cells (secreting IL-17, IL-21, IL-22)
• Regulatory T-cells (secreting IL-10, TGF-β)

At this stage, delayed-type hypersensitivity becomes positive on tuberculin skin testing 1.

Granuloma Formation: The Central Pathophysiologic Structure

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

• Central core: Mycobacteria-containing macrophages that differentiate into epithelioid cells
• Surrounding layer: Lymphocytes in organized juxtaposition 1

This structure enables effective macrophage activation via IFN-γ, delivering antimycobacterial effector molecules including reactive oxygen and nitrogen species to the phagolysosome 1. Tc1 cells deliver granulysin and mycobactericidal molecules via perforin-dependent mechanisms 1.

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 to other lung areas
• Hematogenously (the rule) after reaching regional lymph nodes, potentially seeding any body site 1

Granuloma Evolution: Latency vs. Active Disease

Path to Latency (Containment)

When cellular immunity is robust, the granuloma restricts M. tuberculosis growth through 1:

1. IFN-γ-activated macrophages containing bacterial replication
2. Central caseation necrosis: A solid necrotic mass devoid of oxygen that becomes sclerotic and calcified, resulting in bacterial containment or death 1

Granulomas are dynamic lesions with continuous cell turnover 1. Within the granuloma exists an equilibrium between actively dividing M. tuberculosis and bacteria adapted to stress conditions in a state of non-replicating persistence 1. These dormant bacteria upregulate genes for fatty acid hydrolysis for energy generation 1.

Path to Active Disease (Breakdown)

Granuloma failure occurs when 1:

• Overwhelming Th2 response (IL-4/IL-13 secreting T-cells) counteracts mycobactericidal mechanisms, contributing to central granuloma caseation 1
• TNF-α blockade (e.g., from biologics) prevents cell entry, causing granuloma disintegration and bacterial dissemination 1
• T-cell deficiencies result in uncontained bacterial growth, though paradoxically with less tissue damage 1

Post-Primary Reactivation Disease

Tuberculosis scars are most commonly found in lung apices on autopsy 1. Post-primary reactivation disease characteristically starts its apicocaudal propagation from the apices 1. The mechanism for apical predilection involves the "vulnerable region" hypothesis—tubercle bacilli must reach apical regions for disease progression, either via:

• Bacillemia during first infection (in low-risk populations)
• Repeated airborne exposure (in high-risk populations) 2

Clinical Pitfalls

1. B-cell/antibody contribution is controversial and probably minor 1—do not rely on serological tests for diagnosis
2. Immunosuppression explains only 10% of active TB cases; the remaining 90% involve comorbidities, proinflammatory environments, and genetic factors 3
3. The immune response is 90% protective against active disease but cannot prevent reinfection because it relies on Th1 proliferation rather than sterilizing immunity 3

This pathophysiologic framework explains why TB remains a leading cause of death despite available chemotherapy—the bacteria's sophisticated immune evasion mechanisms and the host's inability to achieve sterilizing immunity create a persistent global reservoir of latent infection 4, 5.