Toxoplasma gondii is an obligate intracellular parasite that infects virtually all warm-blooded organisms. Systemic dissemination of T. gondii in the organism can cause life-threatening infection that manifests as Toxoplasma encephalitis in immune-compromised patients. In addition, mounting evidence from epidemiological studies indicates a link between chronic Toxoplasma infection and mental disorders. To better understand the pathogenesis of toxoplasmosis, basic knowledge on the host-parasite interactions and the dissemination mechanisms are essential. Previous findings have established that, upon infection with T. gondii, dendritic cells (DCs) and microglia exhibit enhanced migration, which was termed the hypermigratory phenotype. As a result of this enhanced migration, DCs and microglia are used as vehicle cells for dissemination (‘Trojan horse’) which potentiates dissemination of T. gondii in mice. However, the precise mechanisms behind the hypermigratory phenotype remained unknown. In this thesis, we characterized host-parasite interactions upon infection with T. gondii and investigated the basic mechanisms behind the hypermigratory phenotype of T. gondii-infected DCs and microglia.

In paper I, we observed that upon infection with T. gondii, DCs underwent rapid morphological changes such as loss of adhesiveness and podosomes, with integrin redistribution. These rapid morphological changes were linked to hypermotility and were induced by active invasion of T. gondii within minutes. T. gondii-infected DCs exhibited up-regulation of the C-C chemokine receptor CCR7 and chemotaxis towards the CCR7 chemotactic cue, CCL19.

In paper II, we developed a 3-dimensional migration assay in a collagen matrix, which allowed us to characterize the hypermigratory phenotype in a more in vivo-like environment. The migration of T. gondii-infected DCs exhibited features consistent with integrin-independent amoeboid type of migration. T. gondii-induced hypermigration of DCs was further potentiated in the presence of CCL19 in a 3D migration assay.

In paper III, we identified a parasite effector molecule, a Tg14-3-3 protein derived from parasite secretory organelles. Tg14-3-3 was sufficient to induce the hypermigratory phenotype. Transfection with Tg14-3-3-containing fractions or recombinant Tg14-3-3 protein induced the hypermigratory phenotype in primary DCs and in a microglial cell line. In addition, Tg14-3-3 localized in the parasitophorous vacuolar space and host 14-3-3 proteins were rapidly recruited around the parasitophorous vacuole.

In paper IV, we found that mouse DCs dominantly express the L-type voltage-dependent calcium channel, Ca_v1.3. Ca_v1.3 was linked to the GABAergic signaling-induced hypermigratory phenotype. Pharmacological inhibition of Ca_v1.3 and knockdown of Ca_v1.3 abolished the hypermigratory phenotype in T. gondii infected DCs. Blockade of voltage-dependent calcium channels reduced the dissemination of T. gondii in a mouse model.

In paper V, we showed that microglia, resident immune cells in the brain, also exhibited rapid morphological changes and hypermotility upon infection with T. gondii. However, an alternative GABA synthesis pathway was shown to be involved in the hypermigratory phenotype in microglia.

In summary, this thesis describes novel host-parasite interactions, including host cell migratory responses and key molecular mechanisms that mediate the hypermigratory phenotype. The findings define a novel motility-related signaling axis in DCs. Thus, T. gondii employs GABAergic non-canonical pathways to hijack host cell migration and facilitate dissemination. We believe that these findings represent a significant step forward towards a better understanding of the pathogenesis of T. gondii infection.