Immunogenicity of Trypanosoma cruzi strains determined by neutralization test

Trypomastigotes of the CA-I strain of Trypanosoma cruzi, which is practically non-lethal for mice, exhibited adhered antibodies when harvested 25-28 days pi and handled at 4 degrees C, whereas at 37 degrees C they became negative within 10-15 min. Mice injected with these tryp pre-incubated with anti-RA rabbit serum (shown to contained neutralizing antibodies against the homologous strain) developed a significantly lower parasitemia than controls while those pre-incubated with anti-CA-I serum showed only a delay in the parasitemia pattern. Neutralizing activity was undetectable in anti-CA-I rabbit and mouse sera when the test was performed under standard conditions against RA-tryp. On the other hand, neutralizing antibodies were demonstrated in few anti-CA-I mouse serum samples against RA strain when the test was performed increasing the sensitivity in order to reach no more than 75% lethality for controls. In these cases, a close relationship between "infective dose size-immune response achieved" was found, being only a narrow range of inocula able to trigger detectable neutralizing activity (10(4) CA-I tryp inoculum). Variations in the degree of neutralizing activity as a function of the infective doze size was also demonstrated for anti-RA mouse sera and, despite every antiserum prepared with this T. cruzi strain was reactive, the best values were proved for those prepared with inocula up to 10(2) tryp. Above optimal parasite dose size, both strains induced a drop of neutralizing activity.