Toxoplasma gondii is an obligate intracellular parasite that infects all warm-blooded vertebrates including one third of the global human population. While infection is typically asymptomatic in healthy human hosts, reactivated and acute infection in immunosuppressed or immunecompromised individuals can lead to lethal toxoplasmic encephalitis After ingestion, the parasite crosses the intestinal epithelium and rapidly achieves systemic dissemination, ultimately establishing chronic infection in the brain. Shortly after crossing the intestinal epithelium T. gondii encounters dendritic cells (DCs). Paradoxically, T. gondii tachyzoites exploit the inherent migratory ability of DCs for dissemination via a “Trojan horse” mechanism. Within minutes of active invasion by T. gondii tachyzoites, DCs adopt a hypermigratory phenotype that mediates rapid systemic dissemination of T. gondii in mice. Previous studies have demonstrated that the hypermigratory phenotype involves cytoskeletal rearrangement, redistribution of integrins and high-velocity in vitro cell migration (termed hypermotility), which is initiated by GABAergic signaling. However, the downstream effectors of GABAergic signaling in parasitized DCs remain enigmatic. Leukocyte migration often relies on adhesion and proteolysis of extracellular matrix (ECM). However, the role of ECM proteolysis in hypermigration has not been addressed. In this thesis, the migratory activation of T. gondii-infected DCs is characterized in terms of cell signaling and ECM proteolysis.

In paper I we demonstrate that MMP-mediated proteolytic activity of DCs is abolished upon T. gondii infection. To investigate DC pericellular proteolysis at the single cell level, we developed a high-content imaging and automated image analysis method. With pharmacological inhibitors and gene silencing, we show that T. gondii-infected DCs lose the ability to degrade ECM through the upregulation of TIMP1 and the loss of podosome structures.

In paper II we show that the hypermigratory phenotype induced by GABAergic signaling in T. gondii-infected DCs is dependent on L-type voltage dependent Ca²⁺ channel (L-VDCC) activation, predominantly Ca_V1.3. Pharmacological antagonism of Ca_V1.3 and gene silencing of cav1.3 blocked hypermotility. Further, inhibition of L-VDCCs with benidipine significantly reduced T. gondii dissemination in a mouse model.

In paper III we address the impact of TIMP1 on the migratory activation of T. gondii-infected DCs. Using pharmacological antagonism and shRNA-mediated gene silencing, we demonstrate that secreted TIMP1 induces motility and migration in T. gondii-infected DCs by activating ITGB1-FAK signaling through interactions with CD63.

In paper IV we report that the GTPase Ras functions as a molecular switch in the migratory activation of T. gondii-infected DCs. We identify that VDCC-CaM-CaMkII and Met signaling converge on Ras-mediated Erk phosphorylation leading to migratory activation of T. gondii-infected DCs.

In summary, my thesis details novel host signaling pathways hijacked by the protozoan parasite T. gondii in Trojan horse DCs for dissemination. Through the investigation of host-parasite interactions, we shed new light on mechanisms that govern leukocyte migration and strategies employed by T. gondii to achieve pervasive dissemination. Gaining further insights into the biology that underlies T. gondii pathogenesis and persistence will help ameliorate toxoplasmosis in at-risk groups.

Toxoplasma gondii is an obligate intracellular parasite that can likely infect all warm-blooded vertebrates, with estimates of up to 30% of the global human population being infected. Although infection with T. gondii is usually asymptomatic or mild, in immunocompromised individuals infection can lead to lethal toxoplasmic encephalitis. Infection acquired during pregnancy can also lead to serious ocular or neurological damage and even death of the foetus. Following ingestion, the parasite is able to cross the first biological barrier it encounters, the gut epithelium and convert to the rapidly replicating tachyzoite stage. It can then disseminate throughout the body of the host, eventually reaching sites such as the brain, after crossing the blood-brain barrier (BBB). Previous findings have shown that T. gondii can use leukocytes, such as dendritic cells (DCs), for dissemination via a “Trojan horse”-type mechanism, but how T. gondii then crosses restrictive barriers such as the BBB is still not fully understood. The overall objective of this work has been to investigate how T. gondii crosses biological barriers and how infection impacts host cell signalling.

In paper I we demonstrate that T. gondii can cross polarised cell monolayers without significantly perturbing barrier integrity. Reduced phosphorylation of focal adhesion kinase (FAK) was observed in cell monolayers upon T. gondii challenge, and inhibition or gene silencing of FAK (Ptk2) facilitated transmigration of T. gondii across polarised cell monolayers. In paper II we found that upon T. gondii infection of DCs, secreted TIMP-1 induces hypermotility by activating β1 integrin-FAK signalling through interactions with CD63. In paper III we show that T. gondii can cross polarised endothelial cell monolayers inside DCs. We also report that parasitised DCs on endothelium do not display a hypermotile phenotype, switching to integrin-dependent motility. Blockade of β1 and β2 integrins or ICAM-1, and gene silencing of β1 (Itgb1) or talin (Tln1) restored infected-DC motility, and reduced the frequency of transmigration of T. gondii-challenged DCs across endothelium. In paper IV we demonstrate that, shortly after T. gondii inoculation in mice, parasites mainly localised to cortical capillaries of the brain. Early invasion to the brain parenchyma occurred in absence of a significant increase in BBB permeability, perivascular leukocyte cuffs or haemorrhage. Further, pharmacological inhibition or endothelial cell-specific knockout of FAK facilitated parasite transmigration to the brain parenchyma.

In paper V we report that DCs challenged with type II T. gondii transmigrate across polarised endothelial cell monolayers at a higher frequency than type I T. gondii, while type I infected DCs exhibited increased migratory velocities on endothelium. We also show that T. gondii-induced upregulation of ICAM-1 in DCs is genotype-dependent, and requires the T. gondii secreted effector GRA15. Finally, gene silencing of leukocyte ICAM-1 (Icam-1) or deletion of T. gondii GRA15 reduced transmigration across endothelial cell monolayers.

In summary, the work in this thesis provides novel insights into how T. gondii can potentially cross biological barriers on its journey to the brain. We find that T. gondii can cross polarised monolayers both as free parasites and using DCs as a “Trojan horse”, and identify new ways in which T. gondii can alter host cell dynamics to benefit its own dissemination.