Fig. 6: Sequestration of T. gondii-infected leukocytes at the BBB facilitates CNS colonization.

The cartoon model illustrates the sequential processes involved in the CNS invasion by T. gondii, based on experimental data. 1 Initial adhesion: Circulating phagocytes infected with T. gondii adhere to the endothelium of cortical capillaries. This adhesion is mediated by ICAM-1/CD18 interactions, which facilitate the attachment of infected DCs. Microvascular inflammation, induced either by intraperitoneal infection with T. gondii or by LPS treatment, rapidly exacerbates the sequestration of infected leukocytes. In contrast, heparin treatment or treatments with blocking antibodies to ICAM-1 or CD18 reverse this sequestration. The T. gondii effectors TgWIP and GRA15, secreted into the cytosol of infected leukocytes, promote adhesion to the endothelium. 2 Intracellular replication and egress: After intracellular replication, tachyzoites egress from the sequestered leukocytes. In absence of parasite replication and egress, infected leukocytes remain sequestered for >48 h. 3 and 3’ Infection of endothelial cells or transmigration: Egressed tachyzoites can either infect endothelial cells or directly transmigrate across the endothelium. 4 Infection of neurons: Upon transmigration, T. gondii rapidly infects cortical neurons. Vacuoles with intra-neurally replicating parasites were retrieved 16 h after inoculation in the cerebral circulation. Additionally, experimental data indicate two alternative routes of CNS invasion, which are not depicted in the cartoon. First, T. gondii-infected DCs can directly transmigrate into the parenchyma. Second, circulating extracellular tachyzoites can either infect endothelium or directly transmigrate to the parenchyma following intracarotid or intravenous inoculation, without replication in the endothelium. However, the frequency of these alternative routes appears significantly lower than the primary pathway early during infection, which involves sequestration of parasitized leukocytes and parasite egress, followed by either endothelial cell invasion or transmigration across the endothelium. Created in BioRender. Pairoto, M. (2025) https://BioRender.com/i14p970.