Novel transcriptome signatures and markers defining murine macrophages at the extremes of the canonical M1 and M2 polarization spectrum

Classically (M1) and alternatively activated (M2) macrophages play distinct roles in various physiological and disease processes. Understanding the gene transcription programs that contribute to macrophage polarization along the M1/M2 spectrum may lead to new tools to detect and therapeutically manipulate macrophage phenotype. Here, we define the M1 and M2 macrophage signature through mRNA microarray. The M1 macrophage signature was defined by 629 up-regulated and 732 down-regulated genes while the M2 macrophage signature was formed by 388 up-regulated and 425 down-regulated genes. While a subset of probes was common to both M1 and M2 cells, others were exclusive to each macrophage subset. The common M1/M2 pathways were characterized by changes in various transcriptional regulators and signaling partners, including increases in Kruppel-like Factor (Klf) 4, but decreases in Klf2. To identify M1 and M2 biomarkers that help discriminate these populations, we selected genes that were increased during M1 or M2 differentiation but decreased in the opposite population. Among top novel M1-distinct genes, we identified CD38, G-protein coupled receptor 18 (Gpr18) and Formyl peptide receptor 2 (Fpr2). Among top M2 genes, we found early growth response protein 2 (Egr2) and Myc. We validated these genes by Real-Time PCR and developed a CD38/Egr2-based flow cytometry assay that discriminates between M1 and M2 macrophages. Overall, this work defines the M1 and M2 signature and identifies several novel M1 and M2 genes that may be used to distinguish and manipulate M1 and M2 macrophages.