THE INDUCTION OF TRYPANOSOMA-CRUZI TRYPOMASTIGOTE TO AMASTIGOTE TRANSFORMATION BY LOW PH

Following cell invasion, Trypanosoma cruzi trypomastigotes transform i nto amastigotes, which are the mammalian replicative forms of the para site. Although amastigotes represent a critical stage in the life-cycl e of T. cruzi, little is known of the factors controlling trypomastigo te to amastigote transformation. Kanbera et al. (1990) observed that e xposure of trypomastigotes to acidic pH induced their transformation i nto rounded forms resembling amastigotes. We confirm their observation and, using two strains of T. cruzi, establish that these transformant s are ultrastructurally and biochemically indistinguishable from natur al amastigotes. Incubation of trypomastigotes in medium at pH 5.0 for 2 h was sufficient to trigger their transformation into forms resembli ng amastigotes. Electron microscopical analysis confirmed that the kin etoplast structure, and general morphological features of the acid-ind uced, extracellular amastigotes were indistinguishable from those of i ntracellular-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 amasti gotes subsequently incorporated [H-3]thymidine into their DNA, indicat ing that the important replicative property of intracellular amastigot es is also exhibited by these in vitro transformants. This effect of l ow pH appears to be of physiological relevance, and acid-induced extra cellular transformation appears to represent a valid experimental tech nique for studies of the molecular mechanisms involved in the differen tiation process.