In this project, we have been studying developmental adaptations of Trypanosoma cruzi to the vertebrate host and, in particular, surface membrane changes occurring during the morphogenesis of epimastigotes (vector stage) to trypomastigotes (vertebrate stage). During the past year we focused most of our effort in studying interactions of the complement system with these parasite stages and, in particular, we investigated the epimastigote acceptor site for C3 during activation of the alternative pathway (AP). Epimastigotes were surface labeled with 125I by the iodogen method and reacted with fresh human serum. The parasites were then lysed in detergent and the C3-parasite membrane complex purified by affinity chromatography on anti-C3 Sepharose. Electrophoretic and immunochemical analysis revealed that the C3 had bound selectively to one T. cruzi surface component - a 72,000 MWR glycoprotein. Minimal C3 deposition was observed when metacyclic trypomastigotes (derived from in vitro culture) were tested in the same reaction. These results provide the first demonstration of a C3 acceptor molecule on a parasite. In related projects, an epimastigote cDNA library was constructed in the expression vector lambda gt-11 in preparation for identification of the gene encoding the 72 kd glycoprotein. A T. cruzi insect vector, Dipetalogastor maximums, was propogated in preparation for studies on bug derived metacyclic trypomastigotes.