Changes in morphology and infectivity of cell culture-derived trypomastigotes of Trypanosoma cruzi.

Trypomastigotes of Trypanosoma cruzi, obtained from the first burst of infected Vero cells, have only a limited invasive capability for fibroblastic cells. Intracellular amastigotes and epimastigotes do not infect these cells at all. Preincubation of the isolated trypomastigotes with Eagle's minimal essential medium/10% fetal calf serum increases 5- to 15-fold their in vitro infectivity. This increased invasive capability is accompanied, in the case of the EP strain of T. cruzi, by a morphological transformation into an amastigote-like or spheromastigote form, which is similar, but not identical to replicating intracellular amastigotes. Trypomastigotes from another isolate (BEC) also increase their infectivity several fold upon preincubation, but before any morphological differentiation occurs, suggesting that these two events are independent. The phenomenon of increased infective capability of the parasite is expressed similarly in different host cells. Parasite adhesion is stimulated 4- to 12-fold upon preincubation of the trypomastigotes. The type of serum used (fetal calf, calf, human) affects the development of infectivity, as well as the process of cell infection in itself, but not the morphological differentiation. These processes are also temperature-dependent. The highly infective parasitic forms do not synthetize DNA, but are active in RNA and protein synthesis. The results obtained indicate the existence in T. cruzi trypomastigotes of an active system for infecting fibroblastic cells, which is only partially expressed in trypomastigotes recently released from host fibroblasts but which can undergo a further extracellular maturation, thus allowing studies on the mechanism of infection in cell-free media.