Toxoplasma gondii is an obligate intracellular Apicomplexan parasite that could possibly infect all warm-blooded animals. Acute infections with T. gondii produce generally mild symptoms in healthy individuals, but infections of the fetus during pregnancy and infections in those that are immunocompromised can cause severe and life-threatening pathology. The parasite gains entry to the host by crossing the biological barriers of the intestine or placenta. Mononuclear phagocytes (MPs) and other leukocytes at these barriers become infected by T. gondii. Previous work has established that upon T. gondii infection, dendritic cells (DCs) undergo morphological and phenotypical changes and display enhanced migration. Components of this ‘hypermigratory phenotype’ have been confirmed in other MPs, such as monocytes, macrophages and microglia. The mechanisms underlying the hypermigratory phenotype are the subject of this thesis.

In paper I, we describe a non-canonical extended upregulation of the transcription factor Egr1 in T. gondii-infected DCs. While the rapid and transient canonical induction of Egr1 depends on the ERK1/2-pathway, the extended upregulation was dependent on p38 MAPK and p38-activating parasite-derived effector GRA24. The hypermotility component of the hypermigratory phenotype did not depend on GRA24/p38, but on ERK1/2. We determined that EGR1 acts as an inhibitor of phenotypic maturation and that GRA24 stimulates Il2 and Il12p40 expression in T. gondii-infected DCs.

In paper II, we characterize actors upstream of ERK1/2 in hypermotility of T. gondii-infected DCs. Two axes that output on the ERK1/2 pathway were found to be required for hypermotility. The first involves Ca²⁺ influx through voltage-gated calcium channel Ca_v1.3, resulting in activation of calcium/calmodulin-dependent protein kinase II (CaMKII) via Ca²⁺ sensor calmodulin (CaM). The other axis relies on hepatocyte growth factor (HGF), which is secreted by DCs, and its receptor Met. Both axes converge on the ERK1/2 pathway via the GTPase Ras.

In paper III, we study the migratory behavior of T. gondii-infected DCs on and across endothelial cell monolayers. Particularly infected DCs transmigrated across endothelial cell monolayers, but were, unlike on 2D surfaces, not hypermotile on endothelial cells. We characterize the differential involvement of β1 and β2 integrins, cell adhesion molecules ICAM-1 and PECAM-1 and pan-integrin-cytoskeleton linker talin in transmigration across endothelial cells and in migration on endothelial cells and 2D surfaces.

Finally, we report in paper IV that T. gondii imparts a DC-like transcriptional signature on infected macrophages. Infected macrophages upregulate chemokine receptor CCR7 and display chemotaxis to CCR7-ligand CCL19, like DCs. Concomitantly, these macrophages upregulate the expression of transcription factors associated with DCs and of immune activation-related genes and markers. T. gondii-infected macrophages thus display a remarkable transcriptional and functional plasticity. We identify GRA28 as the primary T. gondii-derived effector protein responsible for these phenotypes, with parasite-derived ROP16 having partially opposing effects.

Altogether, my thesis identifies novel aspects of the hypermigratory phenotype in T. gondii-infected MPs and provides insights into the molecular components and signaling that underlie them.