EVIDENCE FOR CD45HIGH REACTIVE MICROGLIA, T-CELLS, AND NEUTROPHILS—BUT NOT INFILTRATING MACROPHAGES—IN ALZHEIMER’S MOUSE MODEL BRAINS

Abstract

The level of CD45 staining is currently being used to distinguish brain resident microglia (CD11b+CD45int) from infiltrating peripheral macrophages (PIM)(CD11b+CD45high). An increase in CD11b+CD45high cell fraction is observed in Alzheimer's disease (AD) models, interpreted as evidence of PIM in plaque-ridden brain parenchyma. Because PIM have been implicated in amyloid clearance in AD models, we used Cx3cr1-GFP along with CD11b and CD45 to distinguish the brain's resident myeloid cell types (microglia and PVM, perivascular macrophages) from PIM, which are GFP–. In normal brain (NTG), essentially all CD11b+ cells were GFP+, with a low frequency of CD45high cells among them. As expected, the CD11b+CD45intGFP+ cells expressed microglial markers, and the CD11b+CD45highGFP+ cells expressed macrophage markers, interpreted to be PVM since they expressed Cx3cr1. In brains from our PS2APP TG mouse model, like others, we saw increased frequency of CD11b+CD45high cells. In order to identity them, we looked at expression of various myeloid subtype markers such as Ccr2 (PVM and PIM), Cx3cr1 (PVM and microglia but not PIM), Ly6G (Neutrophils but not PVM or PIM) in the CD45high population. This revealed that a fraction of the CD45 high cells are PVM and neutrophils but not PIM. Indeed, a majority of CD45 high myeloid cells in TG brains expressed several microglial markers (by qPCR) such as Tmem119, Fcrls and Cx3cr1. Taken together the data suggest that a subset of microglia (“Reactive Microglia”) upregulate CD45 expression in neurodegenerative conditions. Consistent with this, CD45 high microglia express higher levels of several neurodegenerative markers (Ccl3, Igf1, Bhlhe40) in aged PS2APP TG brains. We will present this and RNA sequencing data from these various immune subtypes (microglia, reactive microglia, PVM, neutrophils and T-cells) that were isolated from PS2APP TG and NTG animals. Collectively, our data suggest that 1) in plaque-ridden brains, a subset of microglia increase CD45; 2) this population accounts for most of the pathology-driven changes in myeloid gene expression; and 3) presence of infiltrating macrophages is negligible.