Astrocytes promote a protective immune response to brain Toxoplasma gondii infection via IL-33-ST2 signaling

It is of great interest to understand how invading pathogens are sensed within the brain, a tissue with unique challenges to mounting an immune response. The eukaryotic parasite Toxoplasma gondii colonizes the brain of its hosts, and initiates robust immune cell recruitment, but little is known about pattern recognition of T. gondii within brain tissue. The host damage signal IL-33 is one protein that has been implicated in control of chronic T. gondii infection, but, like many other pattern recognition pathways, IL-33 can signal peripherally, and the specific impact of IL-33 signaling within the brain is unclear. Here, we show that IL-33 is expressed by oligodendrocytes and astrocytes during T. gondii infection, is released locally into the cerebrospinal fluid of T. gondii-infected animals, and is required for control of infection. IL-33 signaling promotes chemokine expression within brain tissue and is required for the recruitment and/or maintenance of blood-derived anti-parasitic immune cells, including proliferating, IFN-γ-expressing T cells and iNOS-expressing monocytes. Importantly, we find that the beneficial effects of IL-33 during chronic infection are not a result of signaling on infiltrating immune cells, but rather on radio-resistant responders, and specifically, astrocytes. Mice with IL-33 receptor-deficient astrocytes fail to mount an adequate adaptive immune response in the CNS to control parasite burden–demonstrating, genetically, that astrocytes can directly respond to IL-33 in vivo. Together, these results indicate a brain-specific mechanism by which IL-33 is released locally, and sensed locally, to engage the peripheral immune system in controlling a pathogen.
Author summary Toxoplasma gondii is a highly successful parasite, estimated to infect one-third of the world’s human population and many warm-blooded vertebrates. T. gondii traffics to the brain of its hosts where it persists for their lifetime. Immune pressure is required to control T. gondii in brain tissue, as evidenced by destruction of brain tissue in T. gondii-infected immunosuppressed patients. But how T. gondii presence is sensed by brain cells to orchestrate immune responses is not well understood. Here, we show that a host protein, IL-33, typically sequestered within brain cells in the healthy state, is released as a damage signal during brain T. gondii infection and can induce local changes to the brain environment to recruit immune cells. We show that astrocytes, specifically, are capable of directly responding to IL-33, thus illustrating a local mechanism by which brain-resident cells are alerted to pathogen entry.