Niacin stimulates M2 macrophage polarization in vitro. (A) Cultured primary peritoneal mouse macrophages were treated with LPS/IFN γ or IL-4 for 24 hours after washing, then the cells were exposed to niacin (3 mM) for 30 minutes, the supernatants were collected, and PGD 2 was analyzed by mass spectrometry. * P