Differential impact of inflammatory microglia signals and chronic HIV-1 Tat exposure on A1-like polarization of astrocytes

In the context of neurodegeneration, activated microglia facilitate inflammation via secretion of TNF-α, IL-1α, and C1q. Astrocytes exposed to this signaling array polarize to a reactive inflammatory phenotype, termed A1 or A1-like. Astrocytes are essential for neuronal survival, synaptic support, and blood-brain barrier (BBB) function, but A1-like astrocytes upregulate inflammatory gene expression, downregulate neurotrophic factors, and secrete neurotoxic signals. The consequences of A1-like polarization on BBB function are unknown but may have etiological implications for some diseases. Frequently identified by upregulation of complement component 3 (C3), A1-like astrocytes have been characterized in neurodegenerative disorders like Alzheimer’s disease, with polarization correlated with disease progression and severity. However, the role of A1-like astrocytes in neurodegeneration associated with chronic viral infections, like HIV-1-associated neurocognitive disorder (HAND), remains unclear. An in vitro system using primary human astrocytes, as well as a BBB model featuring primary human brain microvascular endothelial cells (BMECs) co-cultured with astrocytes, was used to elucidate cellular and molecular consequences of chronic astrocyte activation. As measured by whole transcriptome analysis and protein expression assays, repeated treatment with TNF-α, IL-1α, and C1q induced A1-like polarization of astrocytes both in monoculture and in a BBB model, resulting in increased secretion of pro-inflammatory signals. No substantial change to BBB permeability was observed. In contrast, exposure to HIV-1 viral protein Tat did not independently induce A1-like polarization. Ongoing investigations into the effect of astrocyte polarization on BBB integrity and treatment with pathogenic proteins may provide insights into the role of neurotoxic astrocytes in neuroviral pathologies.