Trypanosoma cruzi: attenuation of virulence by culture in tissues

Trypomastigotes of the 'Brazil' strain of Trypanosoma cruzi were cultured in Vero and fish (Pimephales promelas) cells at 30 and 37 C. Those harvested from 37 C Vero cell cultures killed all inoculated C3H mice no matter how long the cultures had been maintained. Those harvested from 120-day 30 C Vero and fish cultures killed only 1 out of 10 mice each, indicating attenuation of virulence. When the surviving mice were later challenged with Vero-cultured trypomastigotes known to be lethal to controls, only 2 of the 18 animals developed even light parasitaemias, indicating previous immunization by the attenuated parasites. Varying the number of attenuated trypomastigotes cultured at 30 C for 30 days at most delayed the rise of parasitaemia. Possible mechanisms of these phenomenon are discussed.