Protective immunity against Trypanosoma cruzi provided by oral immunization with Phytomonas serpens: role of nitric oxide.

We have previously demonstrated that Phytomonas serpens, a tomato parasite, shares antigens with Trypanosoma cruzi, the protozoa that causes Chagas' disease. These antigens are recognized by human sera and induce protective immunity in Balb/c mice. In the present study, inducible nitric oxide synthase (iNOS) knockout (KO) mice and C57BL/6 mice treated with the nitric oxide inhibitor, aminoguanidine (AG, 50 mg kg(-1)) infected with T. cruzi, were used to demonstrate the role of nitric oxide (NO) to host protection against T. cruzi infection achieved by oral immunization with live P. serpens. A reduction in parasitaemia and an increase in survival were observed in C57BL/6 infected mice and previously immunized with P. serpens, when compared to non-immunized mice. iNOS (KO) mice immunized and C57BL/6 immunized and treated with AG presented parasitaemia and mortality rates comparable to those of infected and non-immunized mice. By itself, immunization with P. serpens did not induce inflammation in the myocardium, but C57BL/6 mice so immunized showed fewer amastigotes nests in the heart following an acute T. cruzi infection than those in non-immunized mice. These results suggest that protective immunity against T. cruzi infection induced by immunization with P. serpens is dependent upon enhanced NO production during the acute phase of T. cruzi infection.