Dissemination of Toxoplasma gondii to the central nervous system : with special reference to in vivo bioluminescence imaging

Abstract

Toxoplasma gondii causes an asymtomatic chronic infection in immune competent individuals that can be life-threatening in individuals that become immuno-compromised or to the developing fetus. This thesis aimed to (1) establish a novel murine model to study acute and reactivated toxoplasmosis in real-time using in vivo bioluminescence imaging (BLI), (2) investigate the migratory pathways utilized by Toxoplasma for systemic dissemination, especially to the central nervous system (CNS) and (3) address the role of resident brain glia cells in the setting of recrudescent infection in mice.

Firstly, dissemination of T. gondii to distant organs was monitored in vivo by BLI. Dramatic differences in the kinetics of dissemination of virulent and non-virulent T. gondii strains were observed in vivo. Protective host responses in vivo were partly explored for the Toll/Interleukin-1 receptor (TIR) pathway showing that signaling mediated by Toll-like receptors (TLRs) to the adaptor protein MyD88 is crucial for the outcome of the disease. Secondly, Toxoplasma could take advantage of cells of the immune system such as dendritic cells (DC) to assure systemic dissemination. Infected DC exhibited a dramatic hypermotility phenotype in vitro. Adoptive transfer of infected DC potentiated dissemination of parasites to distant organs in syngeneic mice. Thirdly, we established a model to study the onset of toxoplasmic encephalitis using BLI and investigated the pathophysiology associated with recrudescence in mice. Interestingly, an uneven distribution of foci of reactivation was found in the CNS. In our model, recrudescence preferentially occurred in the parietal and frontal cortex, similar to localizations described in human disease. Also, parasitic foci co-localized with microvasculature along with massive leukocyte infiltration. Activated astrocytes and microglia co-localized with foci of parasite reactivation. Similar to DC, infected microglia exhibited hypermotility whereas astrocytes did not. This suggests a role for infected microglia in the local dissemination of Toxoplasma in the CNS.

This thesis has addressed some of the mechanisms underlying Toxoplasma s success in establishing infections in its host. The application of BLI to the Toxoplasma infection model in mice provides a non-invasive versatile tool to study the behavior of this parasite in vivo. The results presented here reveal that the dynamics of parasite dissemination is strain specific and that Toxoplasma may use infected cells as Trojan horses to assure systemic dissemination to distant organs and within the CNS.