The Toxoplasma Surface Protein SAG1 Triggers Efficient in Vitro Secretion of Chemokine Ligand 2 (CCL2) from Human Fibroblasts

Chemokines play an important role in the physiopathology of toxoplasmosis in murine models. Infection of different human cell types by Toxoplasma gondii induces the secretion of these immune mediators. The aim of our study was to identify parasite molecules that could be involved in the triggering of chemokine ligand 2 (CCL2) secretion during T. gondii host cell invasion: surface, micronemal, rhoptry and dense granule proteins. The secretion of CCL2 was studied 1) after infection of human fibroblasts with mutants of Toxoplasma RH strain deficient either for GRA5, GRA2-GRA6, ROP1 or SAG1; 2) after stimulation by micronemal proteins or by the immunodominant surface antigen 1 of T. gondii. CCL2 secretion was quantified by ELISA at 3 h and/or 24 h after infection or stimulation. Infection by Deltagra2-Deltagra6, Deltagra5 or Deltarop1 mutants did not modify the level of CCL2, as compared with the level measured after infection with the wild-type strain. Moreover, stimulation with micronemal proteins did not increase the secretion of this chemokine. By contrast, the level of CCL2 was increased 3 h post-stimulation by purified or recombinant SAG1. Specificity of this effect was confirmed by the decrease in CCL2 secretion when human fibroblasts were infected with the Deltasag1 mutant (48%) as compared with the wild-type strain (100%). In conclusion, this major Toxoplasma surface protein SAG1, specific to the tachyzoite stage, is directly or indirectly involved in the cellular mechanisms triggering CCL2 secretion after T. gondii infection. These results could explain the parasitic mechanisms leading to cell infiltrates detected only in the presence of tachyzoites, a phenomenon observed in toxoplasmic reactivation.