Effect of rSAG-1(P30) immunisation on the circulating and tissue parasites in guinea pigs as determined by quantitative PCR.

The efficacy of immunisation with Toxoplasma gondii recombinant protein (rSAG-1) was evaluated in the guinea pig model. For the infectious challenge, two strains, namely, strain C56 (10,000 tachyzoites) and strain 76K (100 cysts), were used to infect a group of 32 guinea pigs each. The circulating, cerebral and pulmonary parasite loads were determined with the real-time polymerase chain reaction (PCR) after immunisation. With the C56 strain, immunisation showed high activity with a reduction of greater than 1 log of the circulating and tissue parasite loads. Thus, there was a significantly lower circulating parasite load in the rSAG-1 + adjuvant group (0.5+/-1.5 Eq parasites/ml) as compared to that in the control group (67+/-110 Eq parasites/ml; p<0.05). In the same manner, the cerebral parasite load was much lower in the rSAG-1 + adjuvant group (10+/-20 Eq parasites/g) than that in the control group (339+/-291 Eq parasites/g; p<0.01). On the other hand, with the 76K strain, the effect of immunisation was much less and that only on the pulmonary parasite load [p(lung)<0.05]. This could be due to the use of different strains and stages of the parasite and/or the difference in the route of administration for challenge. The quantitative PCR technique used has shown a good correlation with animal inoculation, and when associated with the guinea pig model, it seems to be a useful and reproducible technique for future vaccine studies.