# Toxoplasma effector GRA15-driven CCL5 secretion enhances brain parasite load through microvascular sequestration of phagocytes

## TL;DR

Toxoplasma gondii uses a parasite protein to make host cells produce a signaling molecule that helps infected immune cells stick to brain blood vessels, aiding parasite spread.

## Contribution

Discovery of GRA15's role in inducing CCL5 to enhance brain colonization by Toxoplasma through microvascular sequestration of infected phagocytes.

## Key findings

- GRA15 triggers CCL5 secretion in brain endothelial cells and dendritic cells via NF-κB signaling.
- CCL5 increases infected dendritic cell motility and adhesion to brain vessels, boosting parasite load.
- Blocking CCL5 with Maraviroc reduces infected cell sequestration and parasite burden in mice.

## Abstract

Toxoplasma gondii exploits host phagocytes to disseminate systemically and establish infection in the central nervous system (CNS). Yet, the mechanisms governing the interaction between parasitized phagocytes and the brain endothelium remain elusive. Here, we show that T. gondii infection, but not parasite lysates, robustly induces transcriptional and secretory upregulation of the chemokine C–C motif ligand 5 (CCL5/RANTES) in primary brain endothelial cells and dendritic cells (DCs). This response was triggered by the parasite effector GRA15 through NF-κB signaling, while the effector TEEGR counteracted CCL5 induction in an MYR-translocon-dependent manner. In response to recombinant CCL5, infected DCs displayed increased hypermotility, chemotaxis toward CCL5 gradients, and enhanced transmigration across polarized endothelial monolayers. Intraperitoneally infected mice rapidly upregulated CCL5 in the blood and Ccl5 expression in the cerebral microvasculature, thereby increasing the adhesion of parasitized DCs and cerebral parasite loads. Pretreatment of mice with recombinant CCL5 dramatically elevated the sequestration of infected DCs, while treatment with the selective chemokine receptor 5 (CCR5) antagonist Maraviroc reverted sequestration. Together, these findings reveal that T. gondii co-opts the host CCL5/CCR5 axis via GRA15-mediated signaling to promote leukocyte-dependent dissemination and early colonization of the CNS.

The intracellular parasite Toxoplasma gondii invades immune cells to spread through the circulatory system, eventually reaching the brains of humans and animals. It is not well understood how parasitized immune cells interact with blood vessel walls, a process that ultimately helps Toxoplasma colonize the brain tissue. We found that when Toxoplasma infects the cells lining the blood vessels (endothelium), these produce C-C motif chemokine ligand 5 (CCL5), a potent signaling and attractant molecule. CCL5 production was triggered by a parasite-derived secreted protein, GRA15. CCL5 activated and attracted infected immune cells. In mice, the levels of CCL5 increased quickly in the brain microvasculature after infection, helping the infected immune cells adhere to brain vessels. When the effect of CCL5 was pharmacologically blocked, fewer infected cells sequestered in the brain vessels, lowering the parasite loads. These findings reveal a mechanism through which Toxoplasma manipulates host cells to produce factors that facilitate its colonization of the brain.