The data showed consistency with a recent report suggesting the expression of Il10 mRNA in CD19+ B cells of draining LN of susceptible mice at the first day post-inoculation, and it was shown that B cells play as a source of IL-10 which influences the susceptibility of BALB/c
mice to L. major infection . At the late stage of the infection, augmented expression of this cytokine was documented at W3 and then tended to gradually decrease at W5 and W8 post-infection. DE5 strain showed the highest level of expression at W3 post-infection. It seems that IL-10 along with IL-4 cytokine is responsible for the susceptibility of the BALB/c mice to L. major infection, as suggested before . Hence, our results showed that the contribution of DNA Damage inhibitor DA39 strain in eliciting Il10 mRNA expression is lower than most strains at 16 h and during the late stage of infection. Taken together, the results of this study show that different strains of L. major display different virulence and induce different patterns of cytokine expression in BALB/c mice. While DA39 strain induced the lowest parasite load, high-level expression of Th1-related cytokines mRNA RGFP966 and higher Ifng/Il4 mRNA ratio in LN of BALB/c mice, the SH25 strain elicited the highest number
of viable parasite in LN of the infected mice and a lower level of Ifng/Il4 mRNA ratio than DA39 strain at 40 h and 8 weeks post-infection. Interestingly, DA39 strain has failed to induce higher expressions of both Il4 and Il10 mRNA, especially at the late stage of the infection. It is noteworthy that in our previous study, similar results in the parasite burden and the generation of IFN-γ induced by DA39 strain were reported at 4 weeks post-infection, however at that study, we reported
higher levels of IFN- produced by DE5 strain than DA39 at W8 post-infection . The reason for this discrepancy may be attributed to the methods used for the cytokine evaluation. It might be considered that the expression of the cytokines mRNA by real-time PCR seems to be a more precise method than assessment of the cytokine in lymphocyte culture. Moreover, the Thymidylate synthase present study was repeated for three times, and the third experiment results were reported as representative. Therefore, DA39 strain might be considered as an ideal strain for the vaccine studies. In conclusion, our results showed variable parasite loads and different expressions of cytokine mRNA in LN of mice infected with the four strains of L. major. Amongst the four strains isolated from the four endemic areas of Iran and analysed by SSCP, DA39 strain induced lower load of parasites in LN of the inoculated BALB/c mice. Moreover, this strain elicited higher expressions of Ifng and Il12 mRNA and lower expressions of Il4 and Il10 mRNA in draining LN of the infected BALB/c mice at early and late stages post-infection.