To determine the influence of interferon (IFN)-γ on the organ infectivity and on the genetic susceptibility of susceptible (C57BL/6) and resistant (BALB/c) strains after peroral infection with cysts of Toxoplasma gondii, IFN-γ knockout (KO) mice in C57BL/6 and BALB/c backgrounds were utilized. The kinetics of the changes in T. gondii abundance were evaluated with a quantitative competitive polymerase chain reaction assay in various organs at different times after peroral infection. In IFN-γ KO mice, a T. gondii-specific gene, SAG1, was detected in all organs examined, and the protozoan proliferated much more actively than in wild-type mice. The abundance of T. gondii was much higher in mesenteric lymph nodes and the heart than in other organs. In contrast, in the nervous system organs and kidneys, only a weakly detectable reaction was observed. Toxoplasma gondii grew at a more rapid rate in the organs of IFN-γ KO C57BL/6 mice than in the organs of IFN-γ KO BALB/c mice during the course of infection. Destruction of the IFN-γ gene showed remarkable effects on the infectivity in both susceptible and resistant mice.

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