LAUSR

ABSTRACT We studied the expression of pro‐ and anti‐inflammatory molecules in microglia and infiltrating monocyte‐derived macrophages after permanent Middle Cerebral Artery Occlusion (pMCAO). LysM‐EGFP knock‐in mice were used to distinguish between these two cell types, as peripheral myeloid cells are LysM‐EGFP+, while microglia are not. This was confirmed with P2ry12 (a microglial specific marker), Iba‐1 and EGFP immunostaining. The peak of LysM‐EGFP+ myeloid cell infiltration was 72 h post‐ischemia, and were distributed evenly in the lesion core, surrounded by a dense region of microglia. Flow cytometry showed that a higher percentage of microglia expressed TNF‐&agr; at 3 (24.3% vs 1.4%) and 7 (18.8% vs 3.4%) days post‐pMCAO as compared to infiltrating macrophages. Microglia and macrophages were purified by fluorescence activated cell sorting 72 h post‐ischemia to assess the mRNA expression of inflammatory markers. Macrophages upregulated expression of mRNA for arginase‐1 (Arg‐1) by 1000‐fold, and IL‐1&bgr; by 90‐fold as compared to microglia. At the protein level, a significantly number of macrophages expressed Arg‐1, while few if any microglia expressed Arg‐1. However, IL‐1&bgr; protein was not detected in macrophages by flow cytometry or immunofluorescence labeling of tissue sections. It was, however, detected in astrocytes along the lesion border. A PCR‐array screen of 84 inflammatory genes revealed that pro‐inflammatory chemokines and cytokines were predominantly upregulated in macrophages but down‐regulated in microglia in the ischemic brain. Our results show clear differences in the inflammatory expression profiles between microglia and macrophages 72 h post‐ischemia which may shape repair and pro‐regenerative mechanisms after stroke. HIGHLIGHTSLysM‐EGFP knock‐in mouse is a good tool to differentiate between microglia and infiltrating macrophages in the CNS.Microglia and macrophages after ischemic stroke differ in their inflammatory profile.Macrophages upregulate the expression of pro‐inflammatory and neutrophil recruitment genes.Microglia down‐regulate pro‐inflammatory and immune cell recruitment genes.Microglia in the peri‐infarct region, which express TNF‐&agr;, may help limit the expansion of the lesion.