Leishmania immune adherence reaction in vertebrates

In normal human blood C3-opsonized Leishmania promastigotes immune adhere t o erythrocytes, a mechanism believed to enhance their clearance from blood and phagocytosis. Given the potential importance of this reaction in host d efence against infection, the promastigote-erythrocyte interaction was stud ied in blood of individuals from one avian and 12 mammalian genera; [In-111 ]-labelled promastigotes were found to bind only to primate erythrocytes. N evertheless, previous experiments coincubating platelets isolated from nonp rimate mammals with C3-opsonized promastigotes led to promastigote-platelet adherence. To ascertain whether this is a natural mechanism in nonprimate Leishmania infection, normal blood from members of Leishmania animal models of interest, dog, guinea-pig, hamster mouse and rabbit, was infected ex vi vo with promastigotes. Within 1 min of blood contact, the promastigote surf ace was loaded with platelets, rapidly evolving into large aggregates. Thes e results confirm the physiological nature of the reaction and demonstrate that promastigote-erythrocyte and promastigote-platelet binding are the fir st parasite-host cell encounters after Leishmania invasion of primates and nonprimate mammals, respectively. Leishmania immune adherence shares the ch aracteristics of the nonanticipatory immune systems, and we consider it sho uld be viewed as an innate vertebrate host effector mechanism.