Abstract
Macrophage polarization plays an important role in tissue damage and repair. In this study, we show that Substance-P (SP) can directly induce M2 polarization of inflammatory macrophages. SP induced the differentiation of GM-CSF-differentiated pro-inflammatory macrophages into alternatively activated phagocytic M2 like macrophages (M2SP) through direct activation of the PI3K/Akt/mTOR/S6kinase pathway and induction of Arginase-1, CD163, and CD206, all of which were nullified by pretreatment with the neurokinin-1 receptor (NK-1R) antagonist RP67580 and specific signaling pathway inhibitors. M2SP were distinct from IL-4/IL-13-induced M2a and IL-10-induced M2c subtypes; they did not show STAT activation and exhibited high phagocytic and endothelial adhesive activity. Furthermore, SP had a dominant effect on M2 polarization over Interferon gamma (IFNγ), a potent M1-skewing cytokine, and effectively induced the M2 phenotype in monocytes and the human THP-1 cell line. Finally, adoptively transferred M2SP migrated to a spinal cord injury (SCI) lesion site and improved functional recovery. Collectively, our findings show that SP, a neuropeptide, plays a role as a novel cytokine by inducing tissue-repairing M2SP macrophages and thus may be developed for pharmacological intervention in diseases involving chronic inflammation and acute injury.