Investigation of host responses upon infection of distinct <i>Toxoplasma</i> strains

Toxoplasma gondii is the causative agent of Toxoplasmosis in human and animals. T. gondii isolates are highly diverse. Hundreds of genotypes have been identified, but only three clonal lineages, namely Type I, II and III are prevalent worldwide. In mouse model, T. gondii strains can be divided into three groups based on their virulence, including the virulent (LD₁₀₀=1), the intermediately virulent (LD₅₀ = 10³-10⁴) and the non virulent (LD₅₀ > 10⁵). The clonal Type I, II and III T. gondii strains belong to these three groups, respectively. Epidemiologic studies suggest the difference of virulence in mice may relate to the severity of toxoplasmosis in human infection. Therefore, it is necessary to understand biological differences in genetically different T. gondii strains and their effect on the host responses. To date, the majority of data published on this aspect has been limited to in vitro assays. Here, we used in vivo assays to investigate host responses upon infection of distinct Toxoplasma strains.

Our studies examined host response to infection of the three widespread clonal lineages of T. gondii using a mouse model. The following results were revealed: (i) increased tissue burden in mice is the indicator of virulence of T. gondii. Quantification of parasite burden in the spleen of mice showed significantly more parasites for Type I strain than that of Type II and III strains, with the latter two having comparable parasite burdens. Given that the Type II strains are more virulent than the Type III strains in mice; this result suggests that difference in host response is the result of specific parasite-host interaction, which is not simply due to the difference of parasite tissue load. (ii) gene expression in the host is strongly influenced by parasite genetic background. Transcriptional profiles of mice infected with the above three types of T. gondii strains showed that the overall gene expression patterns are similar between Type I and Type II infected mice and both stimulated stronger and more polarized change comparing to Type III strain. These results emphasize the importance of studying T. gondii pathogenesis in the host with the consideration of parasite genetic diversity. Such research could possibly aid in select appropriate regimes to treat toxoplasmosis caused by diverse T. gondii strains.