Conserved and cell type-specific transcriptional responses to IFN-γ in the ventral midbrain

Dysregulated inflammation within the central nervous system (CNS) contributes to neuropathology in infectious, autoimmune, and neurodegenerative disease. With the exception of microglia, major histocompatibility complex (MHC) proteins are virtually undetectable in the mature, healthy central nervous system (CNS). Neurons have generally been considered incapable of antigen presentation, and although interferon gamma (IFN-γ) can elicit neuronal MHC class I (MHC-I) expression and antigen presentation in vitro, it remains unclear whether similar responses occur in vivo. Here we directly injected IFN-γ into the ventral midbrain of mature mice and analyzed gene expression profiles of specific CNS cell types. We find that IFN-γ induces cellular proliferation and expression of MHC-II and associated genes only in microglia. However, IFN-γ upregulated MHC-I and associated mRNAs in ventral midbrain microglia, astrocytes, oligodendrocytes, and GABAergic, glutamatergic, and dopaminergic neurons. The core set of IFN-γ-induced genes and their response kinetics were conserved across neurons and glia, with a lower amplitude of expression in neurons. A diverse repertoire of genes was upregulated in glia, particularly microglia, while no neuron-specific responses to IFN-γ were observed. Using mutant mice to selectively delete the IFN-γ-binding domain of IFNGR1 in dopaminergic neurons, we demonstrate that dopaminergic neurons respond directly to IFN-γ. Our results suggest that most neurons are capable of responding directly to IFN-γ and upregulating MHC-I and related genes in vivo, but their expression amplitude and repertoire is limited compared to oligodendrocytes, astrocytes, and microglia. One-sentence summary We find that IFN-γ induces transcription of MHC class I antigen processing and presentation machinery in all major parenchymal cell types in the ventral midbrain; however, neuronal responses are low amplitude and limited to a small set of genes, MHC class II expression and cellular proliferation are restricted to microglia, and dopamine neuronal responses require cell autonomous expression of IFNGR1.