Cell Mediated Immunity Notes

Pathogenesis (Hypersensitivity And Immunity)

Tubercle bacilli as such do not produce any toxins. Tissue changes seen in tuberculosis, therefore, are not the result of any exotoxin or endotoxin.

Instead, the pathogenesis of tissue changes in the infected host is explained by development of delayed-type hypersensitivity (or type IV hypersensitivity) and cell-mediated immunity (CMI), both of which are closely related. While CMI provides some protection against M. tuberculosis, there is very little role of humoral immunity in protection against tuberculosis, except for the existence of antibodies against mycobacterial lipoarabinomannan (LAM) which may prevent the dissemination of tuberculosis in children.

Read And Learn More Infammation And Repair Pathology 

Two types of cells play an essential role in CMI by the host against the organism:

1. Macrophages (alveolar macrophages, blood monocytes, dendritic cells), and
2. T lymphocytes (primarily CD4+ and CD8+).

After 2-4 weeks of mycobacterial infection, following two host responses occur:

1. Macrophage-activating CMI response: TB-infected macrophages stimulate CD4+ T lymphocytes to release various lymphokines and activate macrophages. Activated macrophages are capable of killing and phagocytosing tubercle bacilli.
   These macrophages aggregate in the centre of lesions that undergoes necrosis which resembles cheese i.e. caseous necrosis. Tubercle bacilli remain dormant within macrophages or in the necrotic foci for several years even in healed lesions.
2. Tissue damaging response: This response is due to delayed-type hypersensitivity to various mycobacterial antigens. Unactivated macrophages that contain multiplying mycobacteria cause caseous necrosis, occurring due to lipids and lipoproteins present in the cell wall or cytoplasm of tubercle bacilli.

TB Vaccine Name

Primary And Secondary Infection Concept:

It has been known since the time of Robert Koch that the tissue reaction to tubercle bacilli is different in healthy animal not previously infected (primary infection) from an animal who is previously infected (secondary infection), the best experiment being on guinea pig because this animal does not possess any natural resistance to tubercle bacilli.

• Primary infection: An intradermal injection of tubercle bacilli into the skin of a healthy guinea pig evokes no visible reaction for 10-14 days.
  • After this period, a nodule develops at the inoculation site which subsequently ulcerates and heals poorly because the guinea pig, unlike human beings, does not possess any natural resistance.
  • The regional lymph nodes also develop tubercles.
  • This process is a manifestation of delayed-type hypersensitivity (type IV reaction) and is comparable to primary tuberculosis in children although healing invariably occurs in children.
• Secondary infection: Here, the sequence of changes is different. When the tubercle bacilli are injected into the skin of the guinea pig which has been previously infected with tuberculosis 4-6 weeks earlier, the sequence and duration of development of lesions is different.
  • In 1-2 days, the site of inoculation is indurated and dark, attaining a diameter of about 1 cm.
  • The skin lesion ulcerates which heals quickly and the regional lymph nodes are not affected.
  • This is called Koch’s phenomenon and is indicative of hypersensitivity and immunity in the host i.e. guinea pig in this case.

TB Vaccine Name

Similar type of changes can be produced if injection of live tubercle bacilli is replaced with old tuberculin (OT) which is used in tuberculin skin tests (TST) in human beings.

Immunisation Against Tuberculosis

Protective immunisation against tuberculosis is induced by injection of attenuated strains of bovine type of tubercle bacilli,

BCG Vaccine Full Form –  Bacille Calmette-Guérin (BCG). Cell-mediated immunity with consequent delayed hypersensitivity reaction develops with the healing of the lesion, but the cell-mediated immunity persists, rendering the host tuberculin-positive and hence immune.

While BCG vaccination is routinely done at birth in countries with a high prevalence of tuberculosis, it has never been recommended in US for general use due to lower prevalence of tuberculosis and the impact of the test on an interpretation of skin test.

Tuberculin Skin Test (TST)

TST or Mantoux test is done by intradermal injection of 0.1 ml of tuberculoprotein, purified protein derivative (PPD).

Delayed type of hypersensitivity develops in individuals who are having or have been previously infected with tuberculous infection which is identified as an indurated area of more than 15 mm in 72 hours; reaction larger than 15 mm is unlikely to be due to previous BCG vaccination.

Patients having disseminated tuberculosis may show negative tests due to the release of large amount of tuberculoproteins from the endogenous lesions masking the hypersensitivity test.

A positive test is indicative of cellmediated hypersensitivity to tubercular antigens but does not distinguish between infection and disease.

The test may be false positive in atypical mycobacterial infection and previous BCG vaccination, false negative in cutaneous energy (due to weakened immune system), sarcoidosis, some viral infections, Hodgkin’s disease, recent tuberculous (8-10 weeks of exposure) infection and fulminant tuberculosis.

Morphologic Evolution Of Tubercle

After infection of tissue with tubercle bacilli, the following sequence of events takes place that culminates in the development of a tubercle in the tissue.

1. When the tubercle bacilli are injected intravenously into the guinea pig, the bacilli are lodged in pulmonary capillaries where an initial response of neutrophils is evoked which are rapidly destroyed by the organisms.
2. After about 12 hours, there is progressive infiltration by macrophages.
3. This is due to the coating of tubercle bacilli with serum complement factors C2a and C3b which act as opsonins and attract the macrophages.
4. Macrophages dominate the picture throughout the remaining life of the lesions.
5. If the tubercle bacilli are, however, inhaled into the lung alveoli, macrophages predominate the picture from the beginning.
6. The macrophages phagocytose the tubercle bacilli and either try to kill the bacteria or die away themselves.
7. In the latter case, there is the production of nitric oxide radicals having antimycobacterial properties and also cause the increased synthesis of cytokines (TNF-α and IL-1) resulting in the proliferation of macrophages locally as well as increased recruitment from blood monocytes.
8. As a part of body’s immune response, T and B cells are activated. Activated CD4+ T cells elaborate cytokines, mainly IFN-γ and IL-2.
9. These cytokines and their regulators determine the host’s response by infiltrating macrophage monocytes and develop the cell-mediated delayed type hypersensitivity reaction.
10. Qualitative and quantitative defects of CD4+ cells in HIV explain their poor ability to deal with tubercle bacilli and hence their proneness to disseminated tuberculosis.
11. In 2-3 days, the macrophages undergo structural changes as a result of immune mechanisms the cytoplasm becomes pale and eosinophilic and their nuclei become elongated and vesicular.
12. These modified macrophages resemble epithelial cells and are called epithelioid cells (i.e. epithelial-like).
13. These epithelioid cells aggregate into tight clusters in the centre of the lesion and form granulomas. Release of cytokines in response to sensitised CD4+ T cells and constituents of mycobacteria play a role in the formation of granuloma.
14. Some macrophages, unable to destroy tubercle bacilli, fuse together and form multinucleated giant cells.
15. These giant cells may be Langhans’ type having peripherally arranged nuclei in the form of horseshoe or ring, or clustered at the two poles of the giant cell; or they may be foreign body type having centrally-placed nuclei.
16. Around the central cluster of epithelioid cells and a few giant cells, a zone of lymphocytes and plasma cells is formed which is further surrounded by fibroblasts. The lesion at this stage is called hard tubercle due to the absence of central necrosis.
17. Within 10-14 days, the centre of the cellular mass begins to undergo caseation necrosis, characterised by cheesy appearance and high lipid content. This stage is called soft tubercle which is the hallmark of tuberculous lesions.
18. The development of caseation necrosis is possibly due to the interaction of mycobacteria with activated T cells (CD4+ helper T cells via IFN-γ and CD8+ suppressor T cells directly) as well as by direct cytotoxicity of mycobacteria on macrophages.

Microscopically, caseation necrosis is structureless, eosinophilic and granular material with nuclear debris.

19. The soft tubercle which is a fully-developed granuloma with caseous centre does not favour the rapid proliferation of tubercle bacilli. Acid-fast bacilli are difficult to find in these lesions and may be demonstrated at the margins of recent necrotic foci and in the walls of the cavities.

• • The fate of a granuloma is variable:
    1. The caseous material may undergo liquefaction and extend into surrounding soft tissues, discharging the contents on the surface. This is called cold abscess although there are no pus cells in it.
    2. In tuberculosis of tissues like bones, joints, lymph nodes and epididymis, sinuses are formed; the sinus tracts are lined by tuberculous granulation tissue.
    3. The adjacent granulomas may coalesce together enlarging the lesion which is surrounded by progressive fibrosis i.e. fibrocaseous tuberculosis.
    4. In the granuloma enclosed by fibrous tissue, calcium salts may get deposited in the caseous material (dystrophic calcification) (page 54); sometimes the lesion may even get ossified over the years.