Myeloid cell involvement during the resolution of acute brain inflammation
Davies, Claire Linzi
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Excessive tissue-damaging inflammation can exacerbate acute brain injury, and non-resolving inflammation is implicated in chronic neurodegeneration. Understanding the mechanisms that resolve deleterious inflammation in the brain is imperative to develop new therapeutic strategies. However current knowledge is limited, partly due to a lack of tractable models. Studies in extra-cerebral tissues have shown that myeloid cells are central to the inflammatory response. The aim of this thesis was to develop a model of self-limiting acute brain inflammation that is optimised to address mechanisms controlling resolution. The model was used to define the temporal profile of myeloid cell accumulation in the brain and establish the precise identities, origin and functional contribution of cell subsets in the resolution of the inflammatory response in the brain. Cerebral inflammation was induced by stereotaxic injection of inflammatory stimuli (LPS, HMGB1, MSU); LPS produced a robust inflammatory response and neutrophil influx and loss defined clear phases of initiation and resolution. Cellular changes (e.g. glial activation, endothelial activation and leukocyte influx) in response to LPS were characteristic of acute inflammation. Bone marrow chimaeric (Csf1r-EGFPC57Bl/6J) and monocyte reporter (Ccr2+/RFP) mice were used to distinguish between infiltrating macrophages and resident microglia. Analysis over 28 d showed the temporal profile of myeloid cells during brain inflammation, and monocyte accumulation contributed to expansion of the total mononuclear phagocyte population. Ccr2RFP/RFP knock-in mice showed that monocyte recruitment and resolution were independent of CCR2, and selective depletion of Ly6Clo monocytes with an anti- CSF1R antibody did not affect macrophage recruitment. Monocyte depletion using clodronate failed to deplete the Ly6Cint population and monocytes were still recruited into the brain. Together these results suggest multiple monocyte subsets could be involved in the inflammatory response in the brain. These data show that myeloid cell subsets of distinct origins accumulate in the inflamed brain. This work establishes a model system to identify endogenous mechanisms of resolution in cerebral inflammation and provides a platform to test CNS-targeted pro-resolution agents.