UC Irvine

Immune defense against the ubiquitous brain-resident parasite Toxoplasma gondii requires a balanced immune response that ultimately results in chronic infection of the host. Monocytes play a key role in host defense and are critical for controlling T. gondii in the periphery. Monocytes also enter the brain and protect against Toxoplasmic encephalitis. However, the dynamic and regional relationship between brain infiltration of the parasite and host monocytes is unknown. In addition, the ability of T. gondii to disseminate through the brain is poorly understood. In this dissertation, we report the mobilization of inflammatory and patrolling monocytes into the blood and brain during T. gondii infection of mice and define their regional localization. We also describe previously unappreciated effects of T. gondii infection on the blood-brain barrier (BBB) and the novel observation that infected cells can shuttle T. gondii at the BBB and in the brain parenchyma.

Many of these questions were addressed using longitudinal analysis of mice using two-photon, intravital imaging of the brain microvasculature through cranial windows. This imaging approach revealed that CCR2-RFP monocytes were recruited to the blood-brain barrier (BBB) within two weeks of T. gondii infection, exhibited distinct rolling and crawling behavior at the BBB, and accumulated within the vessel lumen before entering the parenchyma. Optical clearing of intact T. gondii-infected brains and light sheet microscopy coupled with computational alignment of brains to the Allen annotated reference atlas indicated consistent patterns of monocyte infiltration during T. gondii infection, specifically in the olfactory tubercle and pontine areas. These data provide novel insights into the localization of monocytes in the brain during CNS infection and raise the provocative hypothesis that regional neuroinflammation may contribute to the behavioral phenotypes observed during T. gondii infection of mice.

At the BBB, we observed profound vascular remodeling in T. gondii-infected mice expressing the tight junction protein claudin-5 fused to eGFP in endothelial cells. We detected increased tortuosity of individual endothelial cells and reorganization of the vascular network. In addition, two-photon imaging revealed T. gondii within highly motile cells travelling along cerebral blood vessels. These parasite-infected cells were highly dynamic and traveled at speeds greater than 5 µm/min. As a result, infected cell motility resulted in significantly greater displacement of T. gondii in the brain than individual parasites migrating using their own motility. These data revealed that the infection of motile cells enhanced the dissemination of T. gondii in the brain.

Collectively, this work highlights inherent trade-offs in T. gondii immunity, revealing the delicate balance between mounting a protective monocyte immune response while suggesting it may also contribute to parasite-induced pathology and spread.