Trypanosoma cruzi-infected mice show disturbance in the peripheral immune system such as polyclonal lymphocyte activation, autoantibody production, and immunosuppression of T lymphocytes. Previous observations in our laboratory showed that some stocks ofT. cruzi can be contaminated with mouse hepatitis virus type 3 (MHV-3). Literature has shown that MHV-3 infection induces immunologic disorders characterized by thymic involution with marked cell depletion. However, the effects of interactions between MHV-3 and the parasite on the immune system are not well understood. In the present study specific-pathogen-free CBA mice were inoculated with MHV-3, alone or associated with different stocks of T. cruzi. Concurrent murine virus infection resulted in increased pathogenicity of T. cruzi infection shown by profound thymic atrophy; loss of cortical thymocytes; depletion of Thy1.2+, CD4+, and CD8+ cells; enhancement of in situ labeling of nuclear DNA fragmentation; and eventually, death of the animals. Such lines of evidence show that the mechanism underlying this thymic atrophy is associated with apoptosis. These results also suggest that MHV-3 can account for the increased immunosuppression observed during experimental infection with the parasite.

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The possibility that some virus contaminants could be altering host response to Trypanosoma cruzi experimental infection was investigated. Data obtained showed that CBA/J mice infected with stocks of parasite maintained in mice (YIUEC) presented higher level of parasitemia and shorter survival times than those infected with a stock (YITC) which was also maintained in mice but had been previously passaged in cell culture. Mouse antibody production tests, performed with the filtered plasma of mice infected with YIUEC, indicated the presence of mouse hepatitis virus (MHV) while no virus was detected when testing the plasma of YITC infected mice. Filtered plasma of YIEUC infected mice was shown to contain a factor able to enhance the level of parasitemia and to reduce the mean survival time of mice challenged with 10(5) YITC. This factor, that could be serially passaged to naïve mice was shown to be a coronavirus by neutralization tests.