Arginase 1+microglia reduce Aβ plaque deposition during IL-1β-dependent neuroinflammation

Background: Neuroinflammation has long been considered a driver of Alzheimer's disease progression. However, experiments developed to explore the interaction between neuroinflammation and Alzheimer's disease (AD) pathology showed a surprising reduction in amyloid beta (A beta) plaque deposition. We sought to understand this unexpected outcome by examining microglia phenotypes during chronic neuroinflammation. Methods: Using an adeno-associated virus vector carrying hIL-1 beta cDNA, inflammation was induced in one hippocampus of 8-month-old amyloid precursor protein (APP)/PS1 mice for 4 weeks, while the other hemisphere received control injections. Bone marrow chimeras and staining analysis were used to identify the origins and types of immune cells present during sustained inflammation. Arginase 1 (Arg1) and inducible nitric oxide synthase (iNOS) immunoreactivity were used as markers of alternatively activated and classically activated cells, respectively, and changes in cellular uptake of A beta by Arg1+ or iNOS+ microglia was demonstrated by confocal microscopy. To determine if an anti-inflammatory phenotype was present during neuroinflammation, RNA was extracted on flow-sorted microglia and rt-PCR was performed. Interleukin-4 injection was used to induce alternatively activated cells, whereas a minipump and intrahippocampal cannula was used to deliver an interleukin (IL)-4R alpha antibody to block the induction of Arg1+ cells in the setting of sustained IL-1 beta expression. Results: We observed a robust upregulation of centrally derived Arg1+ microglia present only in the inflamed hemisphere. Furthermore, in the inflamed hemisphere, greater numbers of Arg1+ microglia contained A beta when compared to iNOS+ microglia. RNA isolated from flow-sorted microglia from the inflamed hemisphere demonstrated elevation of mRNA species consistent with alternative activation as well as neuroprotective genes such as BDNF and IGF1. To explore if Arg1+ microglia mediated plaque reduction, we induced Arg1+ microglia with IL-4 and observed significant plaque clearance. Moreover, when we reduced Arg1+ microglia induction in the context of neuroinflammation using an anti-IL-4R alpha antibody delivered via intrahippocampal cannula, we observed a clear correlation between numbers of Arg1+ microglia and plaque reduction. Conclusions: Together, these findings suggest that Arg1+ microglia are involved in A beta plaque reduction during sustained, IL-1 beta-dependent neuroinflammation, opening up possible new avenues for immunomodulatory therapy of AD.