Leprosy, or Hansen disease, is a slowly progressive infection caused by Mycobacterium leprae, affecting the skin and peripheral nerves and resulting in disabling deformities. M.leprae is for the most part contained within the skin, but leprosy is likely to be transmitted from person to person through aerosols from lesions in the upper respiratory tract. Inhaled M.leprae, like M.tuberculosis, is taken up by alveolar macrophages, disseminates through the blood, but grows only in relatively cool tissues of the skin and extremities. Despite its low communicability , leprosy remains endemic among an estimated 10 to 15 million people living in poor tropical countries.



Pathogenesis

M. leprae is an acid-fast obligate intracellular organism that does not grow in culture but can be grown in the armadillo. It grows more slowly than other mycobacteria and grows best at 32 to 34 degree C, the temperature of the human skin and the core temperature of armadillos. Like M.tuberculosis, M.leprae secrets no toxins, but its virulence is based on properties of its cell wall. The cell wall is similar enough to that of M.tuberculosis that immunization to bacille Camette-Guerin alone confers 50% protection against M.leprae infection. Further immunization may work as an adjuvant to treatment with antimycobacterial drugs. Cell mediacted immunity is reflected by delayed type hypersensitivity reactions to dermal injections of a bacterial extract called lepromin. A 10 kD heat shock protein of M.leprae is an important antigen recognized by the cellular immune system.

Leprosy is a bipolar disease, determined by the host cellular immune response. Two forms of the disease occur depending on whether the host mounts a T cell mediated immune response or is anergic. Patients with tuberculoid leprosy from granulomas similar to those seen in tuberculosis containing epithelioid macrophages, giant cells and few surviving mycobacteria. The 48 hour lepromin skin test reaction is strongly positive. At the circumference of the granulomas are CD 4+ type 1 helper T cells that like mouse TH1 cells secrete IL-2 and interferon ?ŷ. There are also a few CD8+ lymphocytes in the center of the tuberculoid leprosy lesions. Damage to the nervous system occurs and granulomas form in the nerve sheaths.

In contrast, patients with lepromatous leprosy lack T cell-mediated immunity, are anergic to lepromin and have diffuse lesions containing foamy macrophages stuffed with large numbers of mycobacteria (multibacillary disease). Lepromatous leprosy lesions lack CD4+ type 1 T cells at their margins but instead contain many CD8+ suppressor T cells in a diffuse pattern. The CD8+ suppressor T cells secrete IL-10, which inhibits helper T cells and may mediate the anergy seen in lepromatous leprosy. These CD8+ suppressor T cells also secrete IL-4 which induces antibody production by B cells. The antibody is not protective, and indeed the formation of antigen-antibody complexes in lepromatous leprosy leads to erythema nodosum leprosum, a life threatening vasculitis and glomerulonephritis. In lepromatous leprosy, damage to the nervous system comes from widespread invasion of the mycobacteria into Schwann cells and into endoneural and perineural macrophages. Because of the diffuse, parasite-filled lesions, patients with lepromatous leprosy are more infectious than are those with tuberculoid leprosy.

Priliminary studies of the genetic basis of human responsiveness to M.leprae shed some light on why leprosy runs varying courses in different persons. Individuals who recognized certain M.leprae antigens and had no disease showed different alleles at the human Bcg locus than did those who were anergic and had multibacillary disease. The mouse bcg gene controls responses to intracellular bacteria and parasites. Particular polymorphisms in T-cell receptor genes also correlated with presence of multibacillary or paucibacillary disease.