1a.Objectives (from AD-416)
Risk-based management systems for the control of zoonotic parasites in swine.
1b.Approach (from AD-416)
Will determine genetic and antigenic variation among toxoplasma isolates from food animals.
Toxoplasma gondii is a widespread parasitic infection that can cause severe disease in immunocompromised patients. Our studies seek to define the factors that contribute to the severity of disease caused by this parasite. By understanding the virulence determinants of the parasite that enable it to cause harm, it may be possible to develop new therapeutic approaches to combat human infection. Towards this goal we have successfully performed a genetic cross between the virulent type I GT-1 strain and avirulent type II ME49 strain. Here, we explored the genetic basis of differences in virulence between the highly virulent type I lineage and moderately virulent type II based on a new genetic cross and linkage mapping. Genome-wide association revealed a single quantitative trait locus controls the > 4 log difference in lethality between these strains. Neither ROP16 nor ROP18, previously implicated in virulence differences in T. gondii, were found to contribute to differences between types I and II. Instead, the major virulence locus contained a cluster of pseudokinases denoted as rhoptry protein 5 (ROP5); this locus contains a tandem cluster of polymorphic alleles that differed in expression levels between strains. ROP5 alleles contained only part of the catalytic triad of canonical S/T kinases, and consistent with this they lack demonstrable kinase activity in vitro. Genetic disruption of the rop5 locus in the type I lineage lead to a > 4 log increase in the lethal dose, and surviving mice developed lasting immunity and were protected from an otherwise lethal challenge. These findings reveal that amplification of a polymorphic cluster of pseudokinases plays an important role in pathogenesis of toxoplasmosis in the mouse model. Project plans, goals, and accomplishments were discussed via conference calls and e-mail; technical advice was provided to the Cooperator in writing and by teleconference.