Incidence of Centrocestus formosanus Infection in Snails

Abstract

fluent monolayers of host cells that had been thoroughly rinsed for several hours with serumfree medium. Each monolayer was then covered with a 12-mm-diameter circular coverslip and examined by phase-contrast microscopy. We observed many of the sporozoites undergoing active motility on the surface of both HepG2 and WI38 cells; this was manifested as rapid circular gliding, probing of the cell surface by one end of the sporozoite, attached waving, and an unusual snakelike flexion of the sporozoite body into hairpin turns. We speculate that the reason sporozoites are motile and nominally invasive when in contact with WI38 cells is that these cells, even after thorough rinsings in serum-free medium, probably still have some serum proteins, such as albumin, bound to their surface. Because HepG2 cells are known to synthesize and secrete many serum proteins, including albumin (Knowles et al., 1980, Science 209: 497-499), this would provide an even greater induction of sporozoite motility and invasiveness than the residual serum bound to washed WI38 cells. This would explain the better relative success sporozoites have in invading washed HepG2 cells than WI38 cells. Alternatively, sporozoite motility may have been induced by a nonalbumin factor secreted by, or associated with the surface of, these cell lines. Regardless of the identity of the motility-inducing factor, the important point is that sporozoites settling on the surface of cultured cells incubated in the absence of serum are motile. Thus, these results are still compatible with our motility-invasiveness hypothesis. This study was supported by the U.S. National Institutes of Health through Research Grant AI 09560 and NIAID Training Grant NIH AI T32AI-07180-06AI.