SHIP Represses the Generation of IL-3-Induced M2 Macrophages by Inhibiting IL-4 Production from Basophils

There is a great deal of interest in determining what regulates the generation of classically activated (M1) vs alternatively activated (M2) macrophages (M phi s) because of the opposing effects that these two M phi subsets have on tumor progression. We show herein that IL-3 and, to a lesser extent, GM-CSF skew murine M phi progenitors toward an M2 phenotype, especially in the absence of SHIP. Specifically, the addition of these cytokines, with or without M-CSF, to adherence- or lineage-depleted (Lin(-)) SHIP-/- bone marrow (BM) cells induces high levels of the M2 markers, arginase 1, and Ym1 in the resulting mature M phi s. These in vitro-derived mature M phi s also display other M2 characteristics, including an inability to enhance anti-CD3-stimulated splenic T cell secretion of IFN-gamma and low IL-12 and high IL-10 production in response to LPS. Not surprisingly, given that IL-3 and GM-CSF utilize STAT5 to trigger many downstream signaling pathways, this M2 phenotype is suppressed when STAT5(-/-) BM cells are used. Unexpectedly, however, this M2 phenotype is also suppressed when STAT6(-/-) BM cells are used, suggesting that IL-4- or IL-13-induced signaling might be involved. Consistent with this, we found that IL-3 and GM-CSF stimulate the production of IL-4, especially from SHIP-/- Lin(-) BM cells, and that neutralizing anti-IL-4 Abs block IL-3-induced M2 skewing. Moreover, we found that basophil progenitors within the Lin(-) BM are responsible for this IL-3- and GM-CSF-induced IL-4 production, and that SHIP represses M2 skewing not by preventing skewing within M phi s themselves but by inhibiting IL-4 production from basophils. The Journal of Immunology, 2009, 183: 3652-3660.