Dynamics of macrophage polarization reveal new mechanism to inhibit IL-1beta release through pyrophosphates

Abstract

In acute inflammation extracellular ATP activates P2X7 ion channel receptors (P2X7R) on M1 polarized macrophages to release pro-inflammatory IL-1bvia activation of the caspase-1/Nucleotide-binding domain and Leucine-rich repeat receptor containing Pyrin domain 3 (NLRP3) inflammasome. In contrast, M2 polarized macrophages are critical to the resolution of inflammation but neither actions of P2X7R on these macrophages, nor mechanisms by which macrophages switch from pro-inflammatory to anti-inflammatory phenotypes, are known. Here we investigated extracellular ATP signaling over a dynamic macrophage polarity gradient from M1 through M2 phenotypes. In macrophages polarized towards, but not at, M2 phenotype, where intracellular IL-1b remains high and the inflammasome is intact, P2X7R activation selectively uncouples to the NLRP3-inflammasome activation but not to upstream ion channel activation. In these intermediate M1/M2 polarized macrophages, extracellular ATP now acts via its pyrophosphate chains, independently of other purine receptors, to inhibit IL-1b release by other stimuli via two independent mechanisms: inhibition of ROS production and trapping of the inflammasome complex via intracellular clustering of actin filaments.