538 Major Neutral Prote 1 Nase of Entamoeba Histolytica

Although an infection with Entamoeba histolytica is most commonly asympto-matic, the potential for invasive, metastatic disease, coupled with its high prevalence , makes amoebiasis a major health problem throughout much of the world. Most amoebiasis-related mortality stems from extra-intestinal infection. In these cases, trophozoites penetrate the epithelial layer and lamina propria of the bowel mucosa, enter the bloodstream, and then disseminate to almost any organ or tissue, most commonly the liver. The mechanisms that allow tissue penetration are not well understood. Ultra-structural and histopathological studies (reviewed in reference 1) have shown that, during invasion of the bowel wall, trophozoites are seen at the margin of ulcerative lesions adjacent to healthy tissue. Degeneration of epithelial cells adjacent to invading trophozoites and dissolution of the basement membrane of the mucosa have been observed (2-4). These findings suggest that histolytic and proteolytic factors may be elaborated to facilitate mucosal damage and invasion. Two general types of histolytic factors have been proposed: cytotoxic factors which may directly damage cells, such as a secreted "amoebapore" ion channel (5, 6) and proteolytic enzymes, which may attack both cells and extracellular matrix. The virulence ~ of axenically cultured E. histolytica strains has been correlated with the presence of proteolytic enzymes found on the amoeba surface, in secretions, or in extracts of whole trophozoites (9-14). Unfortunately, as none of these proteinases has been purified to homogeneity, important questions remain as to how many are present and how they may contribute to the pathogenesis of amoebiasis. To determine what role secreted proteinases may play in tissue invasion, we assayed the degradation of extracellular matrix by live trophozoites and their secretory products, using an in vitro model of extracellular matrix successfully used to study other invasive parasites (15-18). These studies allowed us to For axenic amoebae in culture, "virulence" is defined in terms of ability to induce lesions in animals (7) or cytopathic effect on cell monolayers (8).