Meningeal macrophages protect against viral neuroinfection.

The surface of the central nervous system (CNS) is protected by the meninges, which contain a dense network of meningeal macrophages (MMs). Here, we examined the role of tissue-resident MM in viral infection. MHC-II- MM were abundant neonatally, whereas MHC-II+ MM appeared over time. These barrier macrophages differentially responded to in vivo peripheral challenges such as LPS, SARS-CoV-2, and lymphocytic choriomeningitis virus (LCMV). Peripheral LCMV infection, which was asymptomatic, led to a transient infection and activation of the meninges. Mice lacking macrophages but conserving brain microglia, or mice bearing macrophage-specific deletion of Stat1 or Ifnar, exhibited extensive viral spread into the CNS. Transcranial pharmacological depletion strategies targeting MM locally resulted in several areas of the meninges becoming infected and fatal meningitis. Low numbers of MHC-II+ MM, which is seen upon LPS challenge or in neonates, corelated with higher viral load upon infection. Thus, MMs protect against viral infection and may present targets for therapeutic manipulation.

Role of meningeal immunity in brain function and protection against pathogens.

The brain and spinal cord collectively referred to as the Central Nervous System (CNS) are protected by the blood-brain barrier that limits molecular, microbial and immunological trafficking. However, in the last decade, many studies have emphasized the protective role of 'border regions' at the surface of the CNS which are highly immunologically active, in contrast with the CNS parenchyma. In the steady-state, lymphoid and myeloid cells residing in the cranial meninges can affect brain function and behavior. Upon infection, they provide a first layer of protection against microbial neuroinvasion. The maturation of border sites over time enables more effective brain protection in adults as compared to neonates. Here, we provide a comprehensive update on the meningeal immune system and its role in physiological brain function and protection against infectious agents.

Flow cytometry and immunohistochemistry of the mouse dural meninges for immunological and virological assessments.

The highly vascularized meninges protect the surface of the central nervous system and contain a dense network of immune cells controlling neuroinfection and neuroinflammation. Here, we present techniques for the immunological and virological assessment of mouse dural meninges. We describe steps for immunophenotyping including meninges extraction and digestion, immunostaining, and flow cytometry. We then describe viral assessment upon lymphocytic choriomeningitis virus infection including steps for fixation of the meninges in the skull, whole-mount immunohistochemistry, and confocal imaging. For complete details on the use and execution of this protocol, please refer to Rebejac et al. (2022).1.