Question

The causative microorganism for leprosy is Mycobacterium leprae, an intracellular bacterium which can grow inside of macrophages. The disease caused by this bacterium exists as a continuum with two extremes. One extreme form of the disease is called tuberculoid leprosy, which is typified by an interferon-g and TNF-a dominated delayed type hypersensitivity (DTH) cellular immune response. The second extreme is called lepromatous leprosy dominated by an IL-4, IL-5, IL-10 helper T cell immune response and high levels of serum IgG1 antibody. Between these two diseases: tuberculoid leprosy and lepromatous leprosy, which immune response is more likely to result in long term control of a Mycobacterium leprae infection and which response will result in continual disease progression. Why did you choose a particular infection for long term control; why did you choose a particular infection for disease progression?

Answer 1

Delayed type hypersensitivity (DTH) cellular immune response will lead to long time control where as the other response will lead to disease progression.

Mycobacterium leprae is an intracellular organism. So there is very less or no role of antibodies while the maximum role is played by cell mediated immunity (CMI) to control the infection.

In case of Lepromatous leprosy (LL):-

• People with low CMI usually develops LL lesions. Delayed hypersensitivity (DTH) to the lepra antigens (which is usually positive in sensitized individual) is absent.
• Delayed hypersensitivity against antigens of other organism is not suppressed. So there is increased chances of opportunistic infection also such as cancer, AIDS etc.
• It has predominant T​​​​​​H​​​​​2 release of TH2 specific cytokines such as IL4, IL5, IL6, IL 10 and also serum IgG1 which leads to an exaggerated antibody response.
• CD4:CD8 ratio is inversed (1:2).

In case of tuberculoid leprosy (TT) :-

• People with intact CMI develops this kind of lesions. Delayed type hypersensitivity reaction is present in TT patients.
• Since CMI is intact chances of opportunistic infection is less.
• It has predominant T​​​​​​H​​​​​1 response. So there is releaseof TH1 specific such as IL2 , INF γ which activate Macrophages. In turn phagocytose or kill the leprae bacilli.
• CD4: CD8 ratio is normal (2:1).

So, it is the intact or suppressed CMI in TT type and LL type that decides whether the response will be long time control of disease or infection leads to disease progression.

• LL leads to disease progression.
• TT leads to a long time control of the disease.