Infectivity and pathogenicity of toxoplasma gondii in vitro and in vivo

Abstract

While T. gondii (RH strain) infection in mice is always lethal, rats resist acute infection with a comparable dose and are generally considered resistant to T. gondii infection. IFN-y is found to play an important role in pathogenesis and infectivity in
mice, though its role in the control of infection in rat is still not very clear. Therefore, DA and AO rats that differ in the level of IFN-y production were employed to get an insight into the pathogenicity and infectivity ofT. gondii. T. gondii, being an obligate intracellular parasite, can only be maintained in tissue culture or by passage through mice. Maintenance of parasite in laboratory animals may not be the method of choice due to cost and ethical issues involved. Permissiveness of two cell lines (Fen, bladder epithelium and L929, fibroblast) for the propagation of RH 7'. gondii was compared with the standard cell line (Vero, monkey kidney) traditionally employed for in vitro. L929 did not support the growth very well. Comparison of both infectivity and intensity of infection in Fen and Vero showed that there was no difference in percent infectivity, though the intensity of infection was significantly higher in Vero, thus accounting for higher yield in spite of the larger cell size. Since continuous maintenance of 7' gondii in vitro results in loss of infectivity for mice and rats, cryopreserved RH tachyzoites from Vero cell line were passaged twice through C57BL6 mice (intraperitoneally) before using for rat inoculation.
Three week old DA and AO rats, when injected intraperitoneally with RH tachyzoites, were found to differ significantly in their susceptibility. DA rats infected intraperitoneally with 50 x 106 RH tachyzoites died of neurological symptoms within 96 hours (100% mortality), while AO rats were resistant to this dose. A dose of 100 x 106 Ri-I tachyzoites (intraperitoneal injection) in AO rats, however, resulted in 100% mortality. Although tachyzoites were demonstrable in tissue homogenates by
microscopy and by bioassay in C57BL6 mice, there was no evidence of cyst formation either in the brain or in other organs and also no lesion could be seen by H&E staining of tissue sections in either strain. 11 DA rats infected with LD50 (25 x 106) either died within 5 days or cleared the parasites and did not develop any symptoms of the infection though the presence of
tachyzoite antigens in the brain of infected DA rats could be demonstrated by immunoblotting technique.
It was interesting to see that both AO and DA rats infected with 25 x 10 6 RH tachyzoites and sacrificed one year post intraperitoneal infection had evidence of a tachyzoite antigen. This could possibly be attributed either to the presence of very small cysts that escaped detection and/or attenuation of the virulence of the parasite for mice due to a passage through rats, a finding also reported previously.
Thus, despite being three times more efficient producers of IFN-y, DA rats were found to be significantly more susceptible to RH tachyzoite infection than AO. There was, however, no difference in the distribution of parasites in various organs of
infected DA and AO rats and none of the organs investigated showed a histological lesion on H&E staining. Immunoblotting appears to be a sensitive technique for the detection of parasites in organ homogenates of sub-lethally infected rats. Use of this technique for the diagnosis ofT. gondii infection remains to be elucidated.