Malakoplakia
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Malakoplakia is a rare chronic inflammatory process which most often affects the urinary tract in association with E. coli infection. It may, however, to a lesser extent, involve a variety of other organ systems and may be associated with infection with microorganisms other than E. coli including other bacteria, mycobacteria, and fungi. Malakoplakia is the result of an acquired defect in macrophage function causing impairment of bactericidal activity. The large macrophages with foamy, eosoinphilic cytoplasm that are present at sites of infection (Von Hansemann cells) exhibit numerous secondary lysosomes containing partially digested organisms. Fusion and calcification of these lysosomes results in the formation of intracytoplasmic crystalline bodies with a central hydroxy- apatite core called Michaelis-Gutmann (M-G) bodies. By light microscopy the M-G bodies often exhibit a targetoid appearance with a dense central core. The typical targetoid appearance may not be apparent if the plane of section does not pass through the dense central core. M-G bodies are PAS positive and stain for iron and calcium. Malakoplakia is associated with immunodeficiency states including HIV infection, hematopoietic malignancies, and corticosteroid and chemotherapy-induced immunodeficiency. Pulmonary malakoplakia occurring in association with HIV infection is usually the result of infection with the gram positive coccobacillus Rhodococcus equi, transmitted by contact with the excreta of infected horses, cattle, pigs, and sheep.
