Host molecular determinants of toxoplasma gondii dissemination

Abstract

Humans and animals acquire the parasite Toxoplasma gondii through contaminated food or water, upon which the parasite rapidly disseminates in the host. In immunocompetent individuals, this acute phase is transient and leads to chronic colonization of preferentially the central nervous system (CNS). The latent chronic infection may reactivate in immunocompromised individuals, causing potentially lethal encephalitis. It has been established that dendritic cells (DCs) are exploited by T. gondii as “Trojan horses” to facilitate dissemination in the host. T. gondii infected DCs exhibit a hypermigratory phenotype that is a major determinant for enhanced dissemination of the parasite. The host cell pathways involved in the onset and maintenance of this phenotype are largely unknown. This thesis has investigated potential mechanisms of host cell subversion, addressing dissemination during the acute phase of infection and reactivated infection in the brain upon immunosuppression. The main findings of the thesis are listed below.

Significant CD8+ T cell infiltration and activation of CNS resident microglia and astrocytes was observed in close vicinity of reactivated T. gondii foci in mouse brain. Cortical microglia exhibited hypermigration upon T. gondii infection in vitro, as previously shown for DCs. Infected microglia and astrocytes showed increased sensitivity to T cell mediated killing, allowing parasite transmission to surrounding cells. Thus, resident and infiltrating cells may act as Trojan horses, potentiating local T. gondii dissemination in the encephalitic brain.

We developed a motility assay in which cells are tracked while migrating in a collagen matrix. In this assay, T. gondii infected DCs displayed increased velocities as early as 10 min post invasion. This “hypermotility” coincided with the redistribution of actin and integrins. Infected DCs also chemotaxed along a gradient of CCR7 ligand CCL19. Collectively, this series of cellular events contributes to a shift in the migratory capacity of T. gondii infected DCs, expanding the concept of the hypermigratory phenotype.

We showed that DCs express functional γ-aminobutyric acid (GABA) receptors and are capable of producing and secreting GABA. T. gondii infection enhanced GABA secretion, while inhibition of GABA production, transport or GABA receptors abolished the hypermigratory phenotype. This autocrine GABAergic activation was crucial for DCs to act as Trojan horses in T. gondii dissemination in vivo.

This thesis describes the expression of the voltage dependent Ca2+ channel CaV1.3 in murine DCs. We found that GABAergic activation was linked to CaV1.3 and that selective blockade or knockdown of the latter abolished T. gondii induced hypermotility in DCs. This fast- acting signaling axis may allow rapid parasite manipulation of host cell migration.

In summary, we have shown that T. gondii exploits non-canonical migration-related host signaling pathways to enhance its dissemination. Several cell types are likely involved as Trojan horses at different stages of the infection process, e.g. DCs, T cells and microglia. This work also illustrates that the study of parasite-host interactions may further our understanding of basic cellular mechanisms, such as motility, and uncover novel signaling pathways in the host.