Novel Markers to Delineate Murine M1 and M2 Macrophages

TLDR

Classically (M1) and alternatively activated (M2) macrophages have distinct phenotypes and functions, yet dissecting these phenotypes in vivo and in vitro has been challenging due to a spectrum of states and low marker expression. The study aims to map transcriptional signatures that distinguish murine M0, M1, and M2 macrophages. The authors performed a comprehensive transcriptional analysis of M0, M1, and M2 macrophages to identify genes common or exclusive to each subset. They validated that CD38, Gpr18, and Fpr2 are M1‑exclusive markers, while Egr2 and c‑Myc are M2‑exclusive, and showed that CD38 and Egr2 reliably distinguish M1 and M2 macrophages in vitro and in vivo, with Egr2 labeling ~70 % of M2 cells versus 24 % for Arginase‑1 and CD38 labeling 71 % of M1 cells, thereby establishing novel, robust markers for murine macrophage subsets.

Abstract

Classically (M1) and alternatively activated (M2) macrophages exhibit distinct phenotypes and functions. It has been difficult to dissect macrophage phenotypes in vivo, where a spectrum of macrophage phenotypes exists, and also in vitro, where low or non-selective M2 marker protein expression is observed. To provide a foundation for the complexity of in vivo macrophage phenotypes, we performed a comprehensive analysis of the transcriptional signature of murine M0, M1 and M2 macrophages and identified genes common or exclusive to either subset. We validated by real-time PCR an M1-exclusive pattern of expression for CD38, G-protein coupled receptor 18 (Gpr18) and Formyl peptide receptor 2 (Fpr2) whereas Early growth response protein 2 (Egr2) and c-Myc were M2-exclusive. We further confirmed these data by flow cytometry and show that M1 and M2 macrophages can be distinguished by their relative expression of CD38 and Egr2. Egr2 labeled more M2 macrophages (~70%) than the canonical M2 macrophage marker Arginase-1, which labels 24% of M2 macrophages. Conversely, CD38 labeled most (71%) in vitro M1 macrophages. In vivo, a similar CD38+ population greatly increased after LPS exposure. Overall, this work defines exclusive and common M1 and M2 signatures and provides novel and improved tools to distinguish M1 and M2 murine macrophages.