Interactions between Entamoeba histolytica and complement

Abstract

THE pathogenic mechanisms in amoebiasis are not fully understood. Histopathological studies of infected tissues have shown lysis of host cells near the parasites, particularly in the liver where abscesses are formed. In the intestine, inflammatory reactions are often slight, unless there is secondary bacterial involvement. The infiltrating cells belong mainly to the lymphocyte and macrophage cell series, and neutrophil leukocytes are sparse1, suggesting that there is a low-grade chronic inflammation. Tacheuchi and Phillips2 have made systematic studies of experimentally infected germ-free guinea pigs, which have revealed the early events in invasive amoebiasis. When the amoebae had not yet made contact with the colonic epithelial cells, the latter showed degenerative changes such as loss of brush borders, swelling of mitochondria and dilatation of the endoplasmic reticulum. At the same time polymorphonuclear leukocytes (PMN) as well as some macrophages were seen to infiltrate the lamina propria, and extravasated erythrocytes were observed in the same areas. A high proportion of the PMN showed signs of degeneration, including disruption of the cell membrane. As changes were observed some distance from the parasites, Tacheuchi and Phillips2 concluded that the parasites exert their effects by means of a soluble mediator(s), possibly a toxin elaborated by the parasites. Another possibility is that enzymes released from amoebae damage both epithelium and PMN. A third explanation would be that the amoebae can interact with host material to generate factors which are chemotactic for leukocytes and can also exert toxic effects. A likely candidate would be the complement system. For this reason, the experiments described here were undertaken to analyse interactions between amoebae and the complement system. IgG antibody fractions were prepared from sera of individuals suffering from amoebiasis, and unheated sera from normal humans or experimental animals were used as a source of heat-labile complement components. We found that Entamoeba histolytica activates complement by the alternative pathway, and can be lysed by the reaction products.