Host Cell Vimentin Restrains <i>Toxoplasma gondii</i> Invasion and Phosphorylation of Vimentin is Partially Regulated by Interaction with <i>Tg</i>ROP18

Abstract

The obligate intracellular parasite, <i>Toxoplasma gondii</i>, manipulates the cytoskeleton of its host cells to facilitate infection. A significant rearrangement of host cell vimentin around <i>Toxoplasma</i> parasitophorous vacuoles is observed during the course of infection. ROP18 (<i>Tg</i>ROP18) is a serine-threonine kinase secreted by <i>T. gondii</i> rhoptry and a major virulence factor; however, the mechanisms by which this kinase modulates host factors remain poorly understood. Different and dynamic patterns of vimentin solubility, phosphorylation, and expression levels were observed in host cells infected with <i>T. gondii</i> strain RH and RH Δ<i>rop18</i> strains, suggesting that <i>Tg</i>ROP18 contributes to the regulation of these dynamic patterns. Additionally, host cell vimentin was demonstrated to interact with and be phosphorylated by <i>Tg</i>ROP18. A significant increase in <i>T. gondii</i> infection rate was observed in vimentin knockout human brain microvessel endothelial cells (HBMEC), while vimentin knockout or knock down in host cells had no impact on parasite proliferation and egress. These results indicate that host cell vimentin can inhibit <i>T. gondii</i> invasion. Interestingly, western blotting of different mouse tissues indicated that the lowest vimentin expression level was present in the brain, which may explain the mechanism underlying the nervous system tropism of <i>T. gondii</i>, and the phenomenon of huge cyst burdens developing in the mouse brain during chronic infection.