Immunology of Leprosy and Related Chronic Infections of the Skin

Abstract

Mycobacterium leprae is one of a group of microorganisms that can cause disease by its particular property of being able to avoid host resistance mechanisms, especially those dependent on immunological processes. Dissemination of M. leprae, which is a particular feature of the LLp and LLs polar forms of the disease, develops to a considerable extent as a result of a failure of immunological mechanisms to control the proliferation of this microorganism. In common with a number of organisms, both bacterial and parasitic, M. leprae is a facultative intracellular parasite. In this case, the organism resides mainly within cells of the mononuclear phagocytic series, where it would appear to need mainly cell-mediated immune processes for its elimination. It is a feature of leprosy that the disseminated lepromatous form of the disease is due to a failure of cells of the mononuclear phagocyte series to eliminate the organisms that have been phagocytosed. A similar mechanism underlies the pathological features of diffuse cutaneous leishmaniasis and kala-azar, as well as certain forms of deep fungal infections such as South American blastomycosis, coccidiomycosis, and histoplasmosis. The ability of these cells to eliminate the ingested organisms under conditions of normally developing cell-mediated immunity is demonstrated by the pattern found in tuberculoid leprosy or in a healing oriental sore. Inability of macrophages to eliminate intracellular organisms could result from a direct failure of these cells to respond to the specific immunological reaction. Such a failure of macrophage response would be nonspecific and would allow the development of a wide range of diseases in which the cells are parasitized by intracellular parasites.