Protective immunity to Toxoplasma gondii depends on innate immune activation and the establishment of a robust Th1 response. Notably, however, T. gondii can subvert host immunity by altering the transcriptional program and the proteome of infected cells. The mechanisms by which the parasite alters the expression and function of cell surface receptors, which play a key role in immune activation, are not well understood. As critical regulators of T cell activation, the expression of the costimulatory receptors B7-1 and B7-2 are tightly controlled. We examined the molecular mechanisms governing their induction in macrophages. We found that all three strains of T. gondii (types I, II, and III) up-regulated the expression of B7-2. Transcriptional profiling by microarray analysis revealed nodes of regulation in infected cells, in particular the induction of MAPK signaling. Using specific inhibitors against MAPKs, we determined that parasite-induced B7-2 is dependent on JNK. We demonstrate that B7-2 induced by T. gondii is capable of activating T cells and promoting their proliferation.

We also examined the regulation of the CD40 receptor on the surface of T. gondii-infected macrophages. CD40 expression was dramatically increased only in type II T. gondii-infected cells. Forward genetic analysis suggested that GRA15 from type II strains (GRA15_II) was the parasite factor responsible for the CD40-inducing phenotype. Using type I parasites and THP-1 cells stably expressing GRA15_II, we confirmed that GRA15_II was sufficient to induce CD40. We also found that engagement of CD40 in type II-infected cells amplified the IL-12 response. These data indicate that GRA15_II may promote parasite immunity through CD40-mediated production of IL-12.

Using a proteomic approach, we sought to determine how T. gondii infection globally alters the expression of cell surface proteins. Using this approach we found that the expression of Fc&gammaR is altered on infected cells, which may represent a previously undescribed immune evasion strategy by T. gondii. The dataset generated using this protocol may lead to the identification of novel surface markers specific to infected cells that can serve as therapeutic targets.

Collectively, our data suggest that changes in the expression of surface proteins on infected cells may profoundly shape T. gondii immunity.