Meningeal inflammation drives long-term engraftment by monocytes that impair CNS immunity

Abstract

Abstract Tissue-resident macrophages have an embryonic origin and can be replenished by blood monocytes in some tissues during adulthood. Under steady-state conditions, infiltrating monocytes can share phenotypic and genotypic features with their embryonically-derived counterparts. However, little is known about the properties of monocytes/macrophages that establish tissue residency after an inflammatory challenge. The meninges of the central nervous system (CNS) are populated by a dense network of specialized macrophages that serve as immune sentinels. Following infection by lymphocytic choriomeningitis virus (LCMV), these resident macrophages become activated by innate inflammatory cytokines, acquire viral antigens, and are targeted by infiltrating cytotoxic T lymphocytes (CTL), which leads to their depletion. Innate cytokines and chemokines released by CTL also promote a massive recruitment of inflammatory monocytes from the periphery. Surprisingly, these infiltrating monocytes engraft the meningeal niche and remain in situ several months after viral clearance. This leads to significant phenotypic and functional changes in meningeal immunity – a defect that can be partially restored by blocking IFNγ signaling. Importantly, macrophages that establish residency in the meninges after an inflammatory event are deficient in bacterial and immunoregulatory sensors, which impedes their ability to detect pathogens and dampen subsequent meningeal immune responses. Collectively, these data indicate that monocytes can engraft the meninges after an inflammatory challenge and contribute to long-term alterations in CNS immunity.