C1q and immunoglobulins mediate activity-dependent synapse loss in the adult brain

C1q, the initiating protein of the classical complement cascade, mediates synapse loss in development and disease. In various mouse models of neurologic diseases, including Alzheimer's disease, C1q, which is secreted by microglia, the brain's resident macrophages, is found deposited on synapses in vulnerable brain regions. However, what underlies C1q deposition on synapses in the adult brain is unclear. Using in vivo chemogenetics, we demonstrate that neuronal hyperactivity acts as a trigger for region-specific deposition of C1q, which is required for activity-dependent synapse loss. Further, using spatial transcriptomics, live cell tracking, super-resolution microscopy and other molecular and cellular tools, we report a role for B lymphocyte lineage cells and immunoglobulins in the activity-dependent C1q deposition and synapse loss. Overall, our work suggests a link between neuronal hyperactivity and C1q-mediated synapse loss in the adult brain and introduces immunoglobulins as players in this process.