eased in LPSnorNOHA co-treated cells. At 18 hrs post-infection, the number of T. gondii per 100 cells was also significantly lower in LPSnorNOHA co-treated macrophages, Aglafoline compared to LPS-treated only or control cells. These results showed that the inhibition of arginase activity reduced the infection and proliferation of T. gondii in mouse macrophages. 4 Mechanism of Rat Resistance to T. gondii Discussion Previous research has shown that rat peritoneal macrophages do not support the multiplication of Toxoplasma gondii in vitro, but those of mice do. Some explanations have been suggested regarding the mechanism that accounts for this difference, but it is far from understood. A large number of reports have demonstrated that NO is a major effector molecule for macrophage-mediated cytotoxicity in mouse macrophages and is a key anti-pathogen factor used by the infected host to control progression of intracellular pathogens including Toxoplasma. We speculated whether there would be any difference in NO between mouse and rat resident macrophages. Our results show that rat peritoneal macrophages express a high level of iNOS and produce much more NO although difference was found within the strains of rats, whereas NO is undetectable in mouse macrophages, which indicates that NO could be an important factor accounting for the resistance of rat peritoneal macrophages against T. gondii infection. We have shown that the number of tachyzoites is significantly higher in rat macrophages treated with L-NAME than in control cells, while the proliferation of T. gondii is obviously inhibited in “8813645 the rat or mouse macrophages treated with LPSIFN-c. These data demonstrate that a high concentration of NO in rat peritoneal macrophages is closely associated with their resistance to T. gondii infection, supporting our hypothesis that NO in rat macrophages is linked to the resistance to T. gondii infection, as implied in published results regarding mouse activated macrophages. Macrophages have been considered one of the key cells for distribution of T. gondii to other organs after infection, and therefore are suggested to play a part in the natural resistance of rats against the parasite. We have confirmed the fact that rats, even newborns, are naturally resistant to the RH strain of T. gondii, while mice are highly susceptible to its fatal infection. Results from the analysis of genetic recombination between BN and Lewis rats, and their F1 progeny, have revealed that a major locus on chromosome 10, called Toxo1, mediates resistance to T. gondii infection. It was suggested that Toxo1 is associated with the ability of the macrophage to impede the proliferation of the parasite in the parasitophorous vacuole. We found that the number of tachyzoites of T. gondii RH strain in the peritoneal ” macrophages of the F1 progeny of BN6Lewis was significantly Mechanism of Rat Resistance to T. gondii higher than those from Lewis rats but much lower than those from BN rats. Our results also showed that the iNOS expression level and NO concentration in the peritoneal macrophages from the F1 progeny of BN6Lewis was significantly lower than in Lewis rats, but higher than in BN rats. When considering the studies on the Toxo1 locus, we note that the iNOS gene is also located on chromosome 10. From our studies, we suggest that the Toxo1 locus is likely to be associated with the iNOS gene although additional research will be needed in order to ascertain this matter. Why is NO so much highe