COMPLETE IMMUNIZATION AGAINST TRYPANOSOMA-CRUZI VERIFIED IN INDIVIDUAL MICE BY COMPLEMENT-MEDIATED LYSIS

Experimental systems to assay immunity against Trypanosoma cruzi usual ly demonstrate partial resistance without excluding the establishment of sub-patent infections in protected animals. To test whether Swiss m ice immunized with attenuated parasites might develop complete resista nce against virulent T cruzi, experiments were performed involving cha llenge with low numbers of parasites, enhancement of local inflammatio n and the combination of natural and acquired resistance. Absence of i nfection was established after repeated negative parasitological tests (including xenodiagnosis and hemoculture), and lack of lytic antibody was tested by complement mediated lysis. Immunization with 10(7) atte nuated epimastigotes conferred protection against the development of h igh levels of parasitemia after challenge with Tulahuen strain, but wa r unable to reduce the number of infected animals. However when a stro ng, delayed-type hypersensitivity reaction was triggered at the site o f infection by injecting a mixture of virulent and attenuated T cruzi, a significant proportion of immunized animals remained totally free o f virulent infection. The same result was obtained when the immunizati on experiment was performed in four month old Swiss mice, displaying a relatively high natural resistance and challenged with wild, vector-b orne parasites. These experiments demonstrate that complete resistance against T. cruzi can be obtained in a significant proportion of anima ls, under conditions which replicate natural, vector delivered infecti on by the parasite.