Invasive phenotype of Trypanosoma cruzi restricted to a population expressing trans-sialidase

Trypanosoma cruzi expresses a developmentally regulated trans-sialidase implicated in the pathogenesis of Chagas' disease. On inhabitation of the extracellular milieu of cultured cells by infective trypomastigotes, the enzyme is restricted to a small (20 to 30%) population of parasites. The biological significance of trans-sialidase expression on this subset, termed TS+, and not on the majority (70 to 80%) of morphologically similar trypanosomes, named TS-, is unknown. To determine the roles of the TS+ and TS- subsets in T. cruzi invasion, we prepared pure populations of TS- and TS+ trypanosomes using magnetic beads coated with a monoclonal antibody specific for the tandem repeat unit of the trans-sialidase C terminus. After removal of nonadherent TS- trypomastigotes, the TS+ trypomastigotes were isolated from the beads by specific elusion with a synthetic peptide epitope of the trans-sialidase monoclonal antibody. Confirmation of TS+ and TS- phenotypes was obtained by immunofluorescence, immunoblotting, and sialidase or sialyl transferase activity measurements. The TS+ trypanosomes were highly invasive, as they attached to, penetrated, and thrived in cultured mammalian cells much more efficiently than did unfractionated parasites. The critical role of the trans-sialidase in invasion was underscored by the observation that infection was neutralized by human antibodies to transsialidase. What's more, the TS- parasites, in sharp contrast to their TS+ counterparts, were extremely inefficient in invading epithelial cells and fibroblasts. Further, introduction of small amounts of exogenous trans-sialidase into suspensions of nonpenetrating TS- parasites converted them to a highly invasive phenotype indistinguishable from that of the TS+ population. Rescue of the invasive phenotype was specific for the T. cruzi enzyme, for it didn't happen with bacterial and viral sialidases. The in vitro results were confirmed in the murine model of Chagas' disease, as TS- trypomastigotes were relatively avirulent while TS+ trypomastigotes were more virulent than unfractionated parasites.