Toxoplasma gondii, an obligate intracellular parasite of the phylum Apicomplexa, can cause severe disease in humans with an immature or suppressed immune system. The outcome of Toxoplasma infection is highly dependent on the strain type, as are many of its in vitro growth properties 1 . Here we use genetic crosses between type II and III lines to show that strain-specific differences in the modulation of host cell transcription are mediated by a putative protein kinase, ROP16. Upon invasion by the parasite, this polymorphic protein is released from the apical organelles known as rhoptries and injected into the host cell, where it ultimately affects the activation of signal transducer and activator of transcription (STAT) signalling pathways and consequent downstream effects on a key host cytokine, interleukin (IL)-12. Our findings provide a new mechanism for how an intracellular eukaryotic pathogen can interact with its host and reveal important differences in how different Toxoplasma lineages have evolved to exploit this interaction.Most Toxoplasma gondii isolates that have been identified in Europe and North America belong to three distinct clonal lines 2,3 , referred to as types I, II and III. The three types differ widely in a number of phenotypes in mice such as virulence, persistence, migratory capacity, attraction of different cell types and induction of cytokine expression 1 . Recent results indicate that such differences might also exist in human infection 4-9 . To test the hypothesis that some of these strain-specific differences are a result of how the strains interact with the host cell, we infected human foreskin fibroblasts (HFFs) with each of the three types and used microarray analysis to investigate differences in host gene expression 24 h later. Significance analysis of microarrays 10 (SAM) identified 105 human complementary DNAs, representing at least 88 unique genes that were regulated in a strain-specific manner (false discovery rate 15%) (Fig. 1a).If the strain-specific regulation of a host gene has a genetic basis, it should segregate among F1 progeny derived from a cross between two strains that differ in its regulation. We thereforeCorrespondence and requests for materials should be addressed to J.B. (john.boothroyd@stanford.edu). * These authors contributed equally to this work.Author Contributions J.P.J.S. and S.C. contributed equally to this work. J.P.J.S. performed the microarrays and pathway analyses. S.C. and J.P.J.S. performed the experiments in Fig. 3. S.C. performed the experiments in Fig. 4 and Fig 5. J.P.B., M.E.J. and M.W.W. performed the genetic crosses that produced the progeny D3X1 and JD4. J.P.B. genotyped D3X1 and JD4. J.P.J.S., S.C., J.P.B. and J.C.B. wrote the paper. All authors discussed the results and commented on the manuscript. Author InformationThe microarray data have been deposited in ArrayExpress with the accession number E-MEXP-783. Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financia...

Toxoplasma gondii strains differ dramatically in virulence despite being genetically very similar. Genetic mapping revealed two closely adjacent quantitative trait loci on parasite chromosome VIIa that control the extreme virulence of the type I lineage. Positional cloning identified the candidate virulence gene ROP18, a highly polymorphic serine-threonine kinase that was secreted into the host cell during parasite invasion. Transfection of the virulent ROP18 allele into a nonpathogenic type III strain increased growth and enhanced mortality by 4 to 5 logs. These attributes of ROP18 required kinase activity, which revealed that secretion of effectors is a major component of parasite virulence.