Opsonization of Toxoplasma gondii tachyzoites with nonspecific immunoglobulins promotes their phagocytosis by macrophages and inhibits their proliferation in nonphagocytic cells in tissue culture

We have recently shown that Toxoplasma gondii tachyzoites grown in in vitro culture can bind unspecific immunoglobulin (Ig) through their Fc moiety. W e show now that Fc receptors are also present on T. gondii within the host animal, and that intraperitoneal parasites in immunocompetent mice are satu rated with unspecific Ig. We have also investigated the effect of the paras ite's Fc receptor on the interaction of tachyzoites with mammalian cells, u sing the Vero cell line as a model for nonphagocytic host cells and murine peritoneal macrophages in primary culture as a model for phagocytic cells. Coating of tachyzoites with parasite-unrelated Ig did not enhance their inv asive capacity in either target cell type, but slightly decreased the paras ite proliferation. Moreover, phagocytosis by macrophages was increased by a pproximately 50% when parasites were coated with unspecific Ig. These resul ts indicate that the Fe receptor on T. gondii affects the balance between i nvasion and phagocytosis in a way that is detrimental to the parasites.