TIMP1 secretion induced by <i>Toxoplasma</i> effector GRA24 via p38 MAPK signaling promotes non-disruptive parasite translocation across polarized brain endothelial monolayers

The protozoan <i>Toxoplasma gondii</i> first crosses the intestinal wall and then the blood-brain barrier (BBB) to establish chronic, latent infections in humans and other warm-blooded vertebrates. However, the molecular mechanisms underlying this stealthy colonization remain poorly understood. In this study, we investigated the passage of <i>T. gondii</i> tachyzoites across polarized monolayers of murine brain endothelial cells (bEnd.3) and human intestinal cells (Caco-2). We found that exposure to live <i>T. gondii</i> tachyzoites, but not to tachyzoite lysate or lipopolysaccharide, induced elevated transcription and secretion of tissue inhibitor of metalloproteinases 1 (TIMP1), a pleiotropic protein linked to BBB maintenance. Recombinant TIMP1 consistently increased <i>T. gondii</i> transmigration across monolayers, while pharmacological inhibition of matrix metalloproteinases (MMPs) non-significantly impacted transmigration. Through a combined approach of pharmacological inhibition and mutant <i>T. gondii</i> lines, we identified the MYR translocon-associated effector GRA24 and host cell p38 mitogen-activated protein kinase (MAPK) signaling as key mediators of <i>Timp1</i> induction. Moreover, despite <i>T. gondii</i> transmigration, cell polarization and barrier integrity were preserved, suggesting a non-disruptive passage of tachyzoites with minimal or transient barrier dysregulation. These findings reveal a role for GRA24-p38 MAPK signalling and TIMP1's MMP-independent effects in facilitating the translocation of <i>T. gondii</i> across restrictive biological barriers.IMPORTANCEThe parasite <i>Toxoplasma gondii</i>, which is globally widespread, colonizes the brains of humans and other warm-blooded animals. To do so, it first crosses the gut wall before entering the brain via the bloodstream. However, the mechanisms by which <i>Toxoplasma</i> overcomes the body's restrictive biological barriers remain largely unknown. In this study, we used cellular models of the gut and brain barriers to investigate how the parasite passes through. We found that <i>Toxoplasma</i> induces cells to secrete TIMP1, a multifunctional protein that reduces inflammation and is linked to blood-brain barrier protection. Surprisingly, TIMP1 also facilitated <i>Toxoplasma</i>'s passage across cellular barriers. This elevated TIMP1 production and secretion by host cells was triggered by a secreted <i>Toxoplasma</i> effector protein (GRA24) and mediated through host cell signaling pathways (p38 MAPK). These findings suggest that <i>Toxoplasma</i> manipulates host cells to produce factors that aid its colonization while suppressing inflammation.