The Ability of SOCS1 to Cross-Regulate GM-CSF Signaling is Dose Dependent

Suppressor of cytokine signaling (SOCS) 1 is a key negative regulator of interferon (IFN), interleukin (IL)12, and IL-2 family cytokine signaling through inhibition of the Janus kinase-signal transducer and activator of transcription (JAK-STAT) pathway. To investigate the temporal induction of SOCS1 in response to cytokine in live cells and its selective regulation of signaling pathways, we generated a mouse expressing a Halo-tag-SOCS1 fusion protein (Halo-SOCS1) under control of the endogenous <i>Socs1</i> promoter. Homozygous Halo-SOCS1 mice (<i>Halo-Socs1^KI/KI</i>) were viable with minor T cell abnormalities, most likely due to enhanced Halo-SOCS1 expression in thymocytes compared with the untagged protein. IFNγ and IL-4 induced Halo-SOCS1 expression in macrophages derived from <i>Halo-Socs1^KI/KI</i> mice, and a critical level of SOCS1 expression was required for inhibition of both IFNγ and granulocyte macrophage-colony stimulating factor (GM-CSF)-driven JAK-STAT signaling. In contrast, IFNγ priming to induce SOCS1 did not cross-regulate IL-4 signaling. This study indicates that while SOCS1 expression needs to exceed a critical threshold to inhibit IFNγ signaling, its selective regulation of cytokine signaling results from an as yet undetermined, level of regulatory control.