Furthermore, macrophages may shift from one phenotype to another . In considering the role of macrophages in brain injury, it may be important to distinguish between macrophage subsets. Thus, in vitro studies have demonstrated that M1
macrophages are neurotoxic, while M2 macrophages promote regenerative neuronal growth . CCL2, which is expressed post-TBI in the brain and cerebrospinal fluid, has been thought Opaganib price to elicit primarily M1 macrophages, and the presence of macrophages/microglia early after TBI by histology is often associated with the expression of TNF, IL-6, and IL-1 [1, 13, 25-27]. These findings previously suggested that there is a prominent M1 phenotype in early macrophage recruitment following TBI. Characterization of macrophages in TBI by histology has been complicated by difficulty in distinguishing them from microglia; there is no known marker that is expressed by macrophages but not microglia or vice versa. By flow cytometry, however, the two cell populations can be distinguished by the
level of CD45 expression. Using this approach, we have examined the nature of macrophages responding to TBI in mice. To facilitate macrophage subset identification, we examined TBI in YARG mice, in which yellow fluorescent protein (YFP) is expressed under the promoter for the M2 marker, Arg1 Epigenetics Compound Library cell line [28, 29], and Yet40 mice, in which YFP is expressed under the promoter for the M1 marker, IL-12p40. We here demonstrate that a subset of brain wound macrophages upregulate
Arg1 and home to the site of injury. At day 1 after injury, 21 ± 1.5% of the ipsilateral hemisphere macrophages express high levels of Arg1, but the number of Arg1+ cells falls thereafter and cannot be detected after 1 week. Whole genome expression analysis of Arg1+ and Arg1− macrophages following TBI Thymidylate synthase revealed that these macrophage subsets differ in their expression of over 1300 genes, with notable differences in genes encoding chemokines. The pattern of gene expression in neither population is characteristic of in vitro derived M2 or M1 cells. Our results indicate that the macrophage response to TBI is heterogeneous, and the early response includes at least two distinct subsets. As assessed by expression of Arg1, the ratio of these subsets changes with time. To assess the immune response following TBI, we used an adult murine controlled cortical impact model. Histological analysis of brain sections following TBI confirmed cortical injury, which extended into the hippocampus (Fig. 1A). Hematoxylin and eosin (H&E) staining revealed increased cellular recruitment to cortical tissues adjacent to the lesion (Fig. 1A). Immunohistochemical staining for F4/80 showed that macrophages/microglia are widely present at the pericontusional site (e.g. in areas of the cortex adjacent to the lesion) (Fig. 1B).