Investigating the role of innate immune memory in microglial-mediated synaptic remodeling

Innate immune memory is the process by which the immune response led the innate immune system is enhanced or dampened according to previous inflammatory challenges. Recently, microglia, the tissue-resident macrophages of the brain, were found to display immune training and tolerance, two key elements of innate immune memory, characterized by epigenetic molecular mechanisms which lead to sustained transcriptional changes. Considering microglia’s active role in regulating neural circuits in adulthood and disease, by engulfing and removing synaptic connections, this thesis aims to assess the hypothesis that immune memory shapes microglia-mediated synapse remodeling. I took advantage of the barrel cortex system, where microglia have been shown to engulf excitatory synapses following unilateral whisker cauterization in neonatal mice, leading to synapse loss. In adult mice, I further show that sensory lesioning elicited microglia to engulf thalamocortical inputs in the deprived barrels. However, this assay required me to establish a new immunostaining protocol that eliminates microglial autofluorescence from the synaptic engulfment analysis, allowing proper quantification. Strikingly, after optimizing this protocol, I found that microglia no longer engulfed synapses in adult mice that had received a single injection of lipopolysaccharide one month before lesioning. On the other hand, microglia from mice that received four consecutive injections displayed a higher baseline of engulfment in both the control and deprived hemispheres. These results demonstrate that immune memory alters microglia-mediated synapse remodeling prior to and in response to lesioning