Functional consequences of a close encounter between microglia and brain-infiltrating monocytes during CNS pathology and repair

Neuroinflammation is recognized as an important factor contributing to the development and progression of several central nervous system (CNS) disorders. Upon CNS trauma or disease, parenchymal microglia highly proliferate and accumulate in and around the lesion site. In addition, blood-derived monocytes can infiltrate the inflamed CNS in response to cellular damage and/or a compromised blood-brain barrier. Both microglia and infiltrating monocytes are characterized by multiple functional states and can either display highly proinflammatory properties or promote resolution of inflammation and tissue regeneration. Despite sharing some basic immunologic functions, microglia and monocytes display many distinctive features, which ultimately define their contribution to neuropathology. Understanding how the innate immune system participates to brain disease is imperative to identify novel treatment options for CNS inflammatory disorders. In this context, existing and newly developed in vitro platforms for disease modeling are fundamental tools to investigate and modulate microglia and monocyte immune functions within a specific neuropathologic context. In this review, we first briefly summarize the current knowledge on microglia and monocyte ontogenesis, as well as their complex and interconnected contributions to the development of various CNS pathologies. Following the well-recognized concept that both microglia and monocytes can either exert neuroprotective functions or exacerbate tissue damage, we provide a comprehensive overview of cellular models currently available for in vitro study of neuroinflammatory responses. In this context, we highlight how simplified single-cell models may not always correctly recapitulate in vivo biology, hence future research should move toward novel models with higher and multicellular complexity.