Toxoplasma gondii impairs CX3CL1/fractalkine shedding from mouse cortical neurons, leading to microglia activation

Toxoplasmosis is caused by infection with Toxoplasma gondii and is one of the most prevalent food-borne parasitic disease worldwide. T. gondii disseminates through the host organism and forms a latency-specific structure called bradyzoite cysts, found primarily in muscle and neuronal cells. In mice, Toxoplasma leads to sustained brain microvascular abnormalities, including capillary rarefaction, microglial activation, and blood-brain barrier (BBB) breakdown, resulting in synaptic and neuronal loss, behavioral and cognitive damages. We hypothesized that cyst-bearing neurons could signal distinct classes of molecules that would orchestrate neurovascular and neuroinflammatory processes. Primary mouse cortical neurons were infected with T. gondii (ME49 strain) tachyzoites, which, 7 days post infection, generated cysts. We determined angiogenic-regulating factors from the neuronal conditioned media (nCM) using a proteome array and found nine molecules, belonging to four main functional clusters: (i) angiogenic signaling (VEGFA); (ii) endothelial-regulating growth factors (IGFBP-2, -3, -9 and PDGF-AA), (iii) chemoattractants (CCL-2, CCL-3, and CXCL12), and (iv) fractalkine signaling (CX3CL1). The main targets were validated in neuronal culture samples and in brain cortices by ELISA, RT-qPCR, or immunoblotting. CX3CL1 secretion was reduced in infected cultures and accumulated on neuronal surface. In vivo, the CX3CL1 receptor (CX3CR1) was upregulated, whereas the CX3CL1 soluble fraction was decreased. Recombinant CX3CL1 decreased arginase-1 and increased iNOS expression in nCM-treated microglial cells, indicating that CX3CL1 polarizes microglia to a pro-resolutive state. Our data suggest that CX3CL1 plays a key role in regulating neuroinflammatory signaling in acquired Toxoplasmosis, highlighting its potential to prevent the neurocognitive damage observed in infected individuals.

Toxoplasma gondii is a widespread parasite that forms latent cysts in neurons during chronic brain infection. How these infected neurons contribute to long-term brain damage is not well understood. In this study, we used a neuron-specific culture system and a mouse model to show that T. gondii infection alters the release of key signaling molecules by neurons. We found that infected neurons reduce secretion of fractalkine, a molecule that normally helps keep brain immune cells (microglia) in a resting state. At the same time, infected neurons showed increased expression of inflammatory and vascular-related genes, but not always matching increases in protein levels, pointing to complex regulation. These changes may contribute to blood-brain barrier dysfunction and persistent inflammation seen in chronic infection. Our findings highlight the role of neuron-derived signals in driving T. gondii-induced brain pathology and identify fractalkine as a potential target to reduce inflammation.