Deficiency in astrocyte CCL2 production reduces neuroimmune control of Toxoplasma gondii infection

by Stephanie B. Orchanian, Katherine Still, Tajie H. Harris, Melissa B. Lodoen

Toxoplasma gondii is an obligate intracellular parasite that infects one-third of the world’s human population and establishes infection in the brain. Cerebral immune cell infiltration is critical for controlling the parasite, but little is known about the molecular cues guiding immune cells to the brain during infection. Activated astrocytes produce CCL2, a chemokine that mediates inflammatory monocyte recruitment to tissues by binding to the CCR2 receptor. We detected elevated CCL2 production in the brains of C57BL/6J mice by 15 days after T. gondii infection. Utilizing confocal microscopy and intracellular flow cytometry, we identified microglia and brain-infiltrating myeloid cells as the main producers of CCL2 during acute infection, and CCL2 was specifically produced in regions of parasite infection in the brain. In contrast, astrocytes became the dominant CCL2 producer during chronic T. gondii infection. To determine the role of astrocyte-derived CCL2 in mobilizing immune cells to the brain and controlling T. gondii infection, we generated GFAP-Cre x CCL2^fl/fl mice, in which astrocytes are deficient in CCL2 production. We observed significantly decreased immune cell recruitment and increased parasite burden in the brain during chronic, but not acute, infection of mice deficient in astrocyte CCL2 production, without an effect on peripheral immune responses. To investigate potential mechanisms explaining the reduced control of T. gondii infection, we analyzed key antimicrobial and immune players in host defense against T. gondii and detected a reduction in iNOS⁺ myeloid cells, and T. gondii-specific CD4⁺ T cells in the knockout mice. These data uncover a critical role for astrocyte-derived CCL2 in immune cell recruitment and parasite control in the brain during chronic, but not acute, T. gondii infection.