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Brain scar formation in acute injury and chronic neurodegeneration leads to permanent lesions with dramatic consequences for individual patients and healthcare systems. Previous work has described the scar- forming contributions from glial and infiltrating immune cells. Yet, recent findings challenged this notion and showed that the lesion core is formed by a newly discovered cell type - the brain perivascular fibroblast with glial cells surrounding the injury area. Single cell RNA data suggests that fibroblasts enlarge perivascular space during injury and secrete chemokines facilitating immune cell infiltration. However, without functional interventions in-vivo we still will not understand how the fibroblasts contribute to the newly formed lesions and how they interact with the immune system. We propose to examine the role of the fibroblasts in the brain injury response by examining their contribution to vascular remodeling and immune cell infiltration in an acute model of the BBB opening (Dorian McGavern at NIH) and in ALS neurodegeneration (Sebastian Lewandowski at Karolinska Institute). We propose to deplete the brain fibroblast cells using a recombinant diphtheria toxin. We will first use this approach to examine their role in a brain injury response using an acute model of blood-brain barrier (BBB) opening with focal ultrasound developed by the host lab (Mastorakos et al. Nature 1 Neuroscience 2021) . Finally, since our previous studies showed that the brain fibroblasts are activated in early stages of ALS (MaÌŠnberg et al. Nature Medicine 2021)2, we will also deplete these cells in a mouse model of ALS to examine their effect on immune infiltration and disease dynamics (see Fig. 1). Completion of the proposed project will allow us to determine how fibroblasts contribute to brain repair versus neurodegeneration. By comparing the contributions of fibroblasts to these two different outcomes, we aim to identify fibroblast programs that may lead to CNS repair therapies.