Characterization and modeling of monocyte-derived macrophages after spinal cord injury

Spinal cord injury (SCI) elicits a neuroinflammatory reaction dominated by microglia and monocyte-derived macrophages (MDM). Because MDM do not infiltrate the spinal cord until days after injury, it may be possible to control whether they differentiate into neuroprotective or neurotoxic effector cells. However, doing so will require better understanding of the factors controlling MDM differentiation and activation. Our goal was to develop an in vitro model of MDM that is relevant in the context of SCI. This tool would allow future studies to define mechanisms and intracellular signaling pathways that are associated with MDM-mediated neuroprotection or neurotoxicity. We first characterized SCI-induced cytokine expression in MDM using laser capture microdissection and real-time PCR. Based on this data, we assessed which easily procurable primary macrophage subset would mimic this phenotype in vitro. We established the baseline and inductive potential of resident peritoneal, thioglycollate-elicited peritoneal and bone marrow-derived macrophages (BMDM) at the molecular, cellular and functional level. Of these cells, only BMDM retained the phenotypic, molecular and functional characteristics of MDM that infiltrate the injured spinal cord. Thus, peripheral macrophages should not be used interchangeably in vitro to model the functional consequences of the MDM response elicited by SCI.