SHIP represses the generation of alternatively activated macrophages

We recently reported that SHIP restrains LPS-induced classical (M1) activation of in vitro differentiated, bone marrow-derived macrophages (BMM Phi s) and that SHIP upregulation is essential for endotoxin tolerance. Herein, we show that in vivo differentiated SHIP-/- peritoneal (PM Phi s) and alveolar (AM Phi s) macrophages, unlike their wild-type counterparts, are profoundly M2 skewed (alternatively activated), possessing constitutively high arginase I (Argl) and Ym1 levels and impaired LPS-induced NO production. Consistent with this, SHIP-/- mice display M2-mediated lung pathology and enhanced tumor implant growth. Interestingly, BMM Phi s from SHIP-/- mice do not display this M2 phenotype unless exposed to TGF beta within normal mouse plasma (MP) during in vitro differentiation. Our results suggest that SHIP functions in vivo to repress M2 skewing and that macrophage polarization can occur during differentiation in response to TGF beta if progenitors have elevated PIP3.