Ultrastructure of Trypanosoma cruzi in mouse liver

The induction of Trypanosoma cruzi trypomastigote to amastigote transformation by low pH

Following cell invasion, Trypanosoma cruzi trypomastigotes transform into amastigotes, which are the mammalian replicative forms of the parasite. Although amastigotes represent a critical stage in the life-cycle of T. cruzi, little is known of the factors controlling trypomastigote to amastigote transformation. Kanbera et al. (1990) observed that exposure of trypomastigotes to acidic pH induced their transformation into rounded forms resembling amastigotes. We confirm their observation and, using two strains of T. cruzi, establish that these transformants are ultrastructurally and biochemically indistinguishable from natural amastigotes. Incubation of trypomastigotes in medium at pH 5.0 for 2 h was sufficient to trigger their transformation into forms resembling amastigotes. Electron microscopical analysis confirmed that the kinetoplast structure, and general morphological features of the acid-induced, extracellular amastigotes were indistinguishable from those of intracellular-derived amastigotes. The extracellular transformation was accompanied by the acquisition of the stage-specific surface antigen of the naturally transformed amastigotes (Ssp-4), and loss of a stage-specific trypomastigote antigen (Ssp-3). Trypomastigotes incubated at neutral pH did not transform into amastigotes, and did not acquire the Ssp-4 epitope or lose the Ssp-3 epitope. Finally, acid-induced amastigotes subsequently incorporated [3H]thymidine into their DNA, indicating that the important replicative property of intracellular amastigotes is also exhibited by these in vitro transformants. This effect of low pH appears to be of physiological relevance, and acid-induced extracellular transformation appears to represent a valid experimental technique for studies of the molecular mechanisms involved in the differentiation process.

Comparative study of kinetoplast DNA in culture, blood and intracellular forms of Trypanosoma cruzi

A comparative study has been made on the kinetoplast DNA (kDNA) from I in culture epimastigote, blood trypomastigote and intracellular amastigote stages of Trypanosoma cruzi. The basic properties of the kDNA in all 3 forms were identical. Thus the DNA was in the form of networks of density 1.698-9 g/cm3 and with sedimentation coefficients (S20w) of approximately 5500, the networks being composed of large complexes of minicircular and apparently linear molecules, the former having contour lengths of 0.45 MICROMETer. Several differences were noted. The ultrastructural arrangement of the kDNA in the kinetoplast of the blood stage consisted of three to four double rows of DNA as compared to one double layered row in the other two stages. There was proportionately more kDNA in the blood stages, suggesting that, since the networks have apparently the same size (see above), more than one is present. DNA loops situated at the periphery of the kDNA networks were observed in higher proportion in blood and intracellular forms. Dimeric and oligomeric circles were present in the kDNA of the blood and intracellular stages in much greater proportion than in culture epimastigote stages. Few large circular molecules, heterogeneous in size, were also observed in intracellular blood stages. There were some differences, mainly quantitative, in the gel electrophoresis patterns after endonuclease digestion.

Trypanosoma cruzi: mechanism of entry and intracellular fate in mammalian cells

The mode of entry and intracellular fate of epimastigotes and trypomastigotes of Trypanosoma cruzi in cultured cells was studied. Electron microscopic observations indicated the uptake by phagocytosis of both forms into mouse peritoneal macrophages and of trypomastigotes and transition forms into other cultured cell types. In each instance the organisms were initially surrounded by a plasma membrane-derived phagosome. Trypsin and chymotrypsin treatment of the macrophages completely abolished attachment and ingestion of both forms, indicating that protease-sensitive structures on the macrophage plasma membrane mediate ingestion. The macrophage Fc or C3b receptors were not essential for uptake of T. cruzi in the conditions used. Cytochalasin B inhibited ingestion but not the attachment of both forms by macrophages. Epimastigotes were not taken up by HeLa, L cells, and calf embryo fibroblasts. In macrophages, epimastigotes were killed and digested within phagolysosomes. In contrast, trypomastigotes and transition forms escaped from the phagocytic vacuole and then multiplied in the cytoplasmic matrix. Amastigotes released from infected cells exhibited properties similar to those of trypomastigotes and were able to enter all cell types studied and multiply intracellularly.