L10 Pathology Tuberculosis

Overview

This lecture covers tuberculosis (TB) caused by Mycobacterium tuberculosis: its transmission, the type IV hypersensitivity immune response that produces the granuloma, the pathology of primary infection (the Ghon complex and its outcomes), reactivation/secondary TB, extrapulmonary (isolated-organ) TB, and the principles of diagnosis and treatment.

Epidemiology

• TB is the top infectious disease killer worldwide. In 2018, 1.5 million people died from TB, including 251,000 deaths among people with HIV; TB is the leading killer of people with HIV and a major cause of death due to antimicrobial resistance.
• 10 million people fell ill with TB in 2018: 5.7 million men, 3.2 million women, 1.1 million children.
• In New Zealand, TB notification rates fell from ~15/100,000 in 1980 to ~6.3/100,000 by 2016; TB screening was introduced for international students staying over 6 months at the end of 2004, coinciding with a further decline from the mid-2000s.

Causative organism and transmission

• Causative organism: Mycobacterium tuberculosis, an aerobic bacillus with a lipid-containing capsule.
• Not visible on Gram stain; the organisms resist decolorisation by acid during staining, hence “acid-fast” bacilli. Visualised with Ziehl-Neelsen stain (stain pink) or, better, by PCR.
• Transmitted by airborne droplet infection: an infectious person with pulmonary TB releases droplets by coughing, sneezing, laughing or singing. Prolonged contact is needed for transmission.
• People with extrapulmonary TB alone cannot transmit infection; people with latent infection are not infectious.
• On exposure, ~30% of people become infected, but only 5-10% of those go on to develop active disease (30% in people with HIV). Infection is distinct from disease.
• Intestinal and oropharyngeal TB is caused by Mycobacterium bovis, contracted by drinking unpasteurised milk from infected cows; rarely seen in developed countries.
• The BCG (Bacillus Calmette-Guerin) vaccine is an attenuated strain of M. bovis and is the vaccine currently used against TB.

Immune response and granuloma formation

• Inhaled M. tuberculosis travels deep into lung tissue and is phagocytosed by alveolar macrophages. Within the macrophage, the organism resists phagolysosome formation and its microbicidal effect, and proliferates unchecked.
• This proliferation within alveolar macrophages and air spaces causes bacteraemia and seeding of multiple organs during primary infection; most people are asymptomatic at this stage.
• Before the T-cell response begins, bacteria travel within macrophages to other organs, where they can remain dormant and later reactivate as extrapulmonary/isolated-organ TB.
• About 3 weeks after infection, a T-cell-mediated response begins: mycobacterial antigens reach the draining hilar lymph nodes, where antigen-presenting cells present them (via class II MHC) to T-cells, which differentiate into TH1 cells (with IL-12 signalling).
• TH1 cells produce IFN-γ, which activates macrophages, driving phagolysosome maturation, nitric oxide and reactive oxygen species production, autophagy, and mycobacterial killing (this activation is also what underlies a positive tuberculin skin reaction).
• Activated macrophages secrete TNF and chemokines, recruiting further monocytes.
• Activated macrophages are now called epithelioid cells (indistinct cell boundaries, resembling epithelial cells). Epithelioid macrophages, recruited monocytes and sensitised T-lymphocytes aggregate to form the TB granuloma, with caseous necrosis at its centre - collateral tissue damage from the immune response and cytokine release.
• This is the prototype of granulomatous inflammation, a specific type of chronic inflammation; granulomas can also occur in other diseases or as a foreign-body reaction (e.g. to sutures). Immune granulomas result from persistent T-cell activation by an organism that is difficult to eradicate.
• Granuloma formation in TB is CD4+ T cell-mediated inflammation, an example of type IV hypersensitivity. Wherever there is TB, there are granulomas.
• Structure of the granuloma, outside to inside: an outer fibrous collar (a healing response to chronic inflammation); a collar of activated T-lymphocytes; epithelioid macrophages forming most of the granuloma, with scattered Langhans giant cells (formed by fusion of multiple macrophages); and central caseous necrosis, which grossly resembles cheese.
• On histology (H&E), the granuloma shows a peripheral rim of lymphocytes, a “sea” of epithelioid macrophage cytoplasm with indistinct borders, scattered Langhans giant cells, and central structureless pink necrosis devoid of nuclei, surrounded by normal lung tissue.
• Key point: whichever clinical type of TB (primary, secondary, miliary, extrapulmonary), the underlying unit lesion is always the granuloma, seen microscopically; what is visible to the naked eye is a collection of granulomas plus a variable degree of caseation. Naked-eye lesions differ because granulomas differ in size and in the proportions of their components - e.g. secondary TB shows large confluent granulomas with extensive caseous necrosis and fibrosis, while miliary TB shows multiple scattered tiny granulomas with minimal caseation.

Primary pulmonary TB: the Ghon complex and its outcomes

• The area of primary lung infection that develops granulomas and caseous necrosis (from the TH1 response) is the Ghon focus, located deep in the midzone of the lung - the lower part of the upper lobe or upper part of the lower lobe, just under the pleura.
• The hilar lymph nodes involved in the initial infection also develop granulomas and caseation.
• The Ghon focus plus the involved hilar lymph nodes together are called the Ghon complex.
• Possible outcomes after primary infection:
  1. In most people (~90%), the TH1 response halts and kills the organism; the Ghon complex heals by fibrosis, often followed by calcification seen on X-ray. The patient acquires immunity against TB.
  2. Organisms may remain dormant within the initial lung focus or extrapulmonary sites, awaiting reactivation if host immunity drops - this is latent disease.
  3. In others, infection progresses (possibly due to advanced age or immunosuppression), with ongoing immune response causing extensive caseous necrosis and lung damage - progressive primary TB (active disease).
• Progressive primary TB can spread haematogenously, producing “millet seed” lesions (granulomas) across multiple organs including the lung - miliary TB.

Secondary (reactivation/reinfection) pulmonary TB

• Reactivation of infection, or re-exposure to bacilli in a previously sensitised host, produces a rapid, vigorous immune response.
• This forms multiple, confluent (fused), large granulomas with extensive tissue damage - massive central caseous necrosis surrounded by fibrosis (healing in the setting of chronic inflammation). Hence secondary TB is also called chronic fibro-caseous TB.
• Secondary lesions typically occur in the apical parts of the lungs, where oxygen tension is highest (M. tuberculosis is aerobic).
• Extensive tissue destruction can erode adjacent bronchial walls; the patient coughs up bacteria-laden caseous material, leaving empty cavities in the lung - hence secondary TB is also called cavitary TB.
• Erosion of neighbouring blood vessels by caseous necrosis can cause haemoptysis (blood in sputum).
• Patients with these “open” TB lesions are infectious to others living with them.
• Patients may also re-inhale their own organisms and re-infect themselves, so the whole lung can show TB lesions (TB pneumonia), which histologically is entirely granulomas.
• Secondary TB can also disseminate to cause miliary TB, though less commonly than in primary TB.
• On the natural-history model: latent TB (TST/IGRA positive) can progress bidirectionally with reactivation TB.

Extrapulmonary (isolated-organ) TB

• Organ seeding during primary infection (bacteria travelling within macrophages before the TH1 response begins) can later reactivate as extrapulmonary/isolated-organ TB, sometimes without evidence of pulmonary disease.
• Can occur in essentially any organ: bone (tuberculous osteomyelitis), kidney, nervous system (e.g. tuberculous meningitis), uterus, adrenal glands (TB was historically the most common cause of Addison disease).
• Lymph nodes are the most common site for extrapulmonary TB - tuberculous lymphadenitis, historically known as “scrofula”.
• Pott’s disease: tuberculous osteomyelitis of the spinal vertebrae, causing caseating destruction that can lead to pathological fracture and vertebral collapse.
• Renal TB (tuberculous pyelonephritis): extensive caseous necrosis destroys renal parenchyma and distends the renal pelvis (hydronephrosis) due to obstruction of urine flow from ureteric narrowing by TB infection.
• Tuberculous meningitis: small nodules on the meningeal surface of the base of the brain, each a collection of adjacent granulomas on microscopy.

Diagnosis

• Definitive diagnosis requires isolation of the organism from sputum or from tissue at an infected extrapulmonary site, using PCR or Ziehl-Neelsen stain. Finding granulomas histologically is highly consistent with TB but must be confirmed by demonstrating the organism.
• Supporting clinical findings: cough, sputum, haemoptysis, night fever and sweats (active pulmonary disease); or, e.g., enlarged cervical lymph nodes in extrapulmonary disease. Also supported by chest X-ray.
• Mantoux test (tuberculin skin test, TST): intracutaneous injection of purified protein derivative (PPD) induces a visible, palpable induration peaking at 48-72 hours. A positive reaction signifies T-cell-mediated immunity against mycobacterial antigens (cutaneous granuloma formation via type IV hypersensitivity), but does not distinguish latent infection from active disease (active disease requires active symptoms plus positive PCR/Z-N stain). False negatives occur in certain viral infections, malnutrition, or immunosuppression; false positives can result from prior BCG vaccination.
• Interferon-Gamma Release Assay (IGRA): now the preferred blood test for latent or active disease. TH1 cells from infected individuals produce interferon-gamma when mixed with M. tuberculosis antigens, which can be quantified (in New Zealand: QuantiFERON-TB Gold). IGRA should remain positive even after successful treatment. Preferred over Mantoux in people previously vaccinated with BCG (less likely to give false positives) and in immunosuppressed people (less likely to give false negatives).

Treatment

• Multiple-drug therapy is used to prevent drug resistance, and because different drugs act on different bacterial populations: some attack dormant organisms within macrophages (pyrazinamide), others attack extracellular organisms (isoniazid).
• Standard 6-month regimen for active disease, in two phases:
  • Intensive bactericidal phase: 4 drugs for 2 months - isoniazid, rifampicin, ethambutol, pyrazinamide.
  • Sterilisation phase: 2 drugs for 4 months - isoniazid, rifampicin.
• More prolonged treatment is required for extrapulmonary TB.
• Several treatment regimens exist for latent TB infection (LTBI).
• Second-line antibiotics are available for multi-drug-resistant TB.

Self-test

1. Describe the mode of transmission of pulmonary TB, and explain why people with extrapulmonary TB alone or latent infection are not infectious.
2. Describe the steps by which alveolar macrophages first handle inhaled M. tuberculosis, from phagocytosis through to bacteraemia and organ seeding, before the T-cell response begins.
3. Describe the steps of the TH1-mediated immune response to M. tuberculosis, from antigen presentation in the hilar lymph nodes to granuloma formation.
4. Describe the structure of a TB granuloma from outside to inside.
5. What is the Ghon complex, and where in the lung is the Ghon focus typically located?
6. List the three possible outcomes after primary TB infection, with the approximate proportion and key feature of each.
7. Explain how miliary TB arises, and describe its characteristic appearance.
8. Distinguish secondary (reactivation) TB from primary TB in terms of granuloma pattern, tissue damage, and typical lung location, and explain why secondary TB is called both “chronic fibro-caseous” and “cavitary” TB.
9. A patient with secondary TB develops haemoptysis and coughs up caseous material, leaving an empty cavity in the lung apex. Explain the pathological process behind each of these two findings.
10. List examples of extrapulmonary/isolated-organ TB and their affected sites, and state which site is the most common.
11. Distinguish the Mantoux test (TST) from the Interferon-Gamma Release Assay (IGRA) in terms of what each measures, and give one situation in which IGRA is preferred over TST.
12. Describe the standard drug regimen for active pulmonary TB, including the drugs used in each phase and the rationale for multi-drug therapy.
13. Trace the full pathogenesis pathway of TB from initial inhalation of the organism through to either healing, latency, or progression to secondary and miliary disease, integrating the roles of the granuloma at each stage.

Answers

Reveal answers

1. TB spreads by airborne droplet infection when an infectious person with pulmonary TB coughs, sneezes, laughs or sings; prolonged contact is needed. Extrapulmonary TB alone cannot transmit infection, and latent infection is not infectious, because in both cases there is no active pulmonary disease releasing infectious droplets.
2. Inhaled organisms travel deep into lung tissue and are phagocytosed by alveolar macrophages; within the macrophage they resist phagolysosome formation and its microbicidal effect and proliferate unchecked; this proliferation in alveolar macrophages and air spaces causes bacteraemia and seeding of multiple organs; most people are asymptomatic at this stage.
3. About 3 weeks after infection, mycobacterial antigens reach the draining hilar lymph nodes and are presented by antigen-presenting cells (via class II MHC) to T-cells, which differentiate into TH1 cells; TH1 cells produce IFN-γ, which activates macrophages, enhancing their bactericidal power; activated macrophages secrete TNF, recruiting more monocytes; activated macrophages (epithelioid cells), recruited monocytes and lymphocytes aggregate to form the granuloma, which develops central caseous necrosis.
4. From outside to inside: an outer collar of fibrosis (healing response); a collar of activated T-lymphocytes; epithelioid macrophages forming most of the granuloma, with scattered Langhans giant cells among them; central caseous necrosis.
5. The Ghon complex is the Ghon focus (the area of primary lung infection with granulomas and caseous necrosis) plus the involved, caseating hilar lymph nodes. The Ghon focus is deep in the midzone of the lung - the lower part of the upper lobe or upper part of the lower lobe, just under the pleura.
6. (1) In ~90%, the TH1 response kills the organism and the Ghon complex heals by fibrosis (often later calcifying), with acquired immunity. (2) Organisms remain dormant in the lung or extrapulmonary sites - latent disease, which can reactivate later. (3) Infection progresses with extensive caseation and lung damage - progressive primary TB (active disease).
7. Miliary TB arises from haematogenous (blood) spread of organisms, usually as part of progressive primary TB (or, less commonly, secondary TB). It produces numerous small “millet seed” granulomas scattered across multiple organs, including the lung.
8. Secondary TB shows multiple, confluent, large granulomas with extensive central caseous necrosis surrounded by fibrosis, typically in the apical lung (highest oxygen tension), versus the single, more localised Ghon focus of primary TB. It is called “chronic fibro-caseous” because of the extensive caseation and surrounding fibrosis, and “cavitary” because tissue destruction erodes bronchial walls, allowing the patient to cough up caseous material and leave empty cavities.
9. Haemoptysis occurs because caseous necrosis erodes neighbouring blood vessels. The cavity forms because extensive tissue destruction erodes adjacent bronchial walls, allowing the patient to cough up the bacteria-laden caseous material and leave an empty space in the lung.
10. Bone (tuberculous osteomyelitis, e.g. Pott’s disease of the spine), kidney (tuberculous pyelonephritis), nervous system (tuberculous meningitis), uterus, adrenal glands (historically the leading cause of Addison disease), and lymph nodes (tuberculous lymphadenitis/“scrofula”). Lymph nodes are the most common extrapulmonary site.
11. The Mantoux test (TST) is an intracutaneous PPD injection that measures a cutaneous type IV hypersensitivity reaction (induration at 48-72 hours), and does not distinguish latent from active disease. IGRA is a blood test measuring interferon-gamma release by TH1 cells exposed to M. tuberculosis antigens. IGRA is preferred over TST in people previously given the BCG vaccine, since it is less likely to give a false positive (or, equivalently, in immunosuppressed people, being less likely to give a false negative).
12. Active TB is treated for 6 months in two phases: an intensive bactericidal phase (2 months) with 4 drugs - isoniazid, rifampicin, ethambutol, pyrazinamide - followed by a sterilisation phase (4 months) with 2 drugs - isoniazid, rifampicin. Multi-drug therapy prevents drug resistance and covers both dormant intracellular organisms (targeted by pyrazinamide) and extracellular organisms (targeted by isoniazid).
13. Inhaled M. tuberculosis is phagocytosed by alveolar macrophages and proliferates unchecked, causing bacteraemia and organ seeding before the T-cell response begins. Around 3 weeks later, a TH1 response activates macrophages and forms a granuloma with caseous necrosis at the primary site (Ghon complex) and, from earlier seeding, dormant foci in other organs. This primary infection then resolves by healing (fibrosis/calcification, ~90%), becomes latent (dormant organisms awaiting reactivation), or progresses to progressive primary TB, which can disseminate haematogenously as miliary TB. Latent disease can later reactivate (or the host can be reinfected) as secondary TB, with vigorous confluent granulomas, cavitation and possible further miliary spread; latent organs seeded during primary infection can likewise reactivate as extrapulmonary TB. At every stage, the granuloma is the underlying unit lesion.