Updated project metadata. GM-CSF paradoxically possesses ability to differentiate both classically activated macrophages (MØs) with dominant proinflammatory function (M1-like MØs) and alternative activated MØs with strong immunosuppressive function (M2-like MØs). The intrinsic regulatory mechanism responsible for functional polarization of MØs under GM-CSF signalling remains elusive. Here we revealed that cytokine-inducible SH2-containing protein (CIS), induced by GM-CSF, is a key determinant in controlling MØ polarization. Compared to WT MØs, Cish-/- MØs gained characteristics of alternative activated MØs (M2 MØs), showing high expression of prototypic M2 markers Arginase 1, Tgm2 and YM-1; strong suppression of T cell proliferation; and low production of IL-12 and other proinflammatory cytokines¬¬. Differing from canonical IL-4/STAT6/IRF4 signaling axis of M2 induction, development of M2 MØ characteristics in Cish-/- MØs was associated with intensified STAT5 activation and consequent IRF8 downregulation. Attenuation of GM-CSF signalling via JAK inhibition and IRF8 rescue corrected certain functional defects in Cish-/- MØs. As CIS inhibition in NK and T cells promotes anti-tumour immunity, we showed that CIS deficiency enhanced the development of intra-tumoural M2 MØs and reduced CTL induction within tumour microenvironment with elevated GM-CSF. Overall, we conclude that CIS acts as an intrinsic rheostat to control intense GM-CSF signalling in order to maintain proinflammatory functions of MØs. Targeting CIS as a checkpoint in cancer immunotherapy should consider its role in regulating myeloid cell function.