Growing evidence suggests a role of the gut microbiota in controlling the activity of CNS innate immune cells, especially microglia . In the first funding period we observed that host microbiota controls aging of microglia and phagocytic clearance of β-amyloid (Aβ) deposits in the 5xFAD mouse model of Alzheimer’s disease (AD). In young germ-free (GF) mice, microglia displayed several defects in metabolism, including changes in mitochondrial mass, mitochondrial membrane potential and disruption of oxidative phosphorylation. This was connected to a decrease in AD pathology in GF mice. However, we also found diminished signs of microglial aging in GF mice. In the second funding period, we propose to investigate the role of the microbiota in controlling the function of CNS-associated macrophages (CAMs), including meningeal (mMɸ), perivascular (pMɸ) and choroid plexus macrophages (cpMɸ) . Building on our previous work on the role of microglia in the context of aging and Alzheimer’s disease (AD) , as well as on the regional specification of microglia, we will now (i) investigate the role of the microbiota in controlling CAMs under homeostatic conditions, during aging and AD, (ii) characterize metabolic parameters in CAMs of specific pathogen-free (SPF) and GF mice during steady-state, aging and AD, and (iii) determine how localization influences the phenotype and function of CAMs.