Diverse Subpopulations of Reactive Astrocytes Following Chronic Toxoplasma Infection

Astrocytes provide physical and metabolic support for neurons, regulate the blood-brain barrier, and react to injury, infection, and disease. When astrocytes become reactive, maintenance of the inflammatory state and its functional implications throughout chronic neuroinflammation are all poorly understood. Several models of acute inflammation have revealed astrocyte subpopulations that go beyond a two-activation state model, instead encompassing distinct functional subsets. However, how reactive astrocyte (RA) subsets evolve over time during chronic inflammatory disease or infection has been difficult to address. Here we use a prolific human pathogen, Toxoplasma gondii, that causes lifelong infection in the brain alongside a Lcn2CreERT2 reporter mouse line to examine reactive astrocyte subsets during chronic neuroinflammation. Single-cell RNA sequencing revealed diverse astrocyte populations including transcriptionally unique Lcn2CreERT2+ RAs which change over the course of infection in a subset-dependent manner. In addition to an immune-regulating Lcn2CreERT2+ astrocyte population enriched with gene transcripts encoding chemokines CCL5, CXCL9, CXCL10, and receptors CCR7 and IL7R, a specific subset of Lcn2CreERT2+ astrocytes highly expressed transthyretin (Ttr), a secreted carrier protein involved in glycolytic enzyme activation and potential vasculature regulation and angiogenesis. These findings provide novel information about the evolution and diversity of reactive astrocyte subtypes and functional signatures at different stages of infection, revealing an undocumented role for transthyretin-expressing astrocytes in immune regulation at the central nervous system (CNS) vasculature.