Temporal regulation of human reactive astrocytes reveals their capacity for antigen presentation

Astrocytes adapt to injury and disease by entering reactive states with altered gene expression, morphology, and function, but we still do not know how these states evolve over time in human cells or whether they acquire immune effector properties. Using human cortical organoids (hCOs) and primary fetal cortical tissue, we mapped the temporal dynamics and plasticity of inflammatory human astrocytes. Brief and prolonged exposure to inflammatory cytokines elicited robust, time-dependent changes in astrocyte transcriptomes and chromatin accessibility, and these exposures produced distinct acute and chronic reactive signatures. Despite these widespread genomic alterations, astrocytes that experienced either acute or chronic cytokine exposure reverted to a quiescent gene expression and chromatin state within days of cytokine withdrawal, which demonstrates a high degree of plasticity. Chronic inflammatory settings, but not acute ones, revealed a second layer of reactivity in which astrocytes induced major histocompatibility complex class II (MHCII) genes and surface MHCII protein. We validated MHCII expression and surface localization in primary human fetal astrocytes, in organotypic fetal cortical slices that include microglia, and in human brain tissue from patients with chronic inflammatory lesions. We propose that delayed MHCII emergence reflects progressive activation of interferon linked signaling in astrocytes, including low level interferon gamma production. Through co-immunoprecipitation and mass spectrometry of MHCII complexes from TIC-treated fetal astrocytes we identified candidate MHCII-presented peptides and confirmed that astrocytes can process and display exogenous peptides derived from human fetal neurons.