IMiDs regulate SOCS1-dependent cytokine-negative feedback mechanism in effector cells in MM. Flow cytometric analysis of intracellular expression of SOCS1 protein in effector cells from cocultures are shown as representative dot plots. To provide direct tumor cell-effector cell contact and -BM contact, healthy donor's PBMCs were cocultured for 16 hours with either MM cell lines (MM1.S and U266) or with MM cell lines and MM-BMSCs, in the absence or presence of lenalidomide and pomalidomide. Intracytoplasmic expression of SOCS1 (PE) protein was then evaluated by flow cytometry in the gated CD4 PECy5 (CD4T cells; A), CD8 PEcy7 (CD8T cells; B), CD56 PE⁺CD8 PECy7⁺ (NKT cells; C), and CD56PE⁺CD8PECy7⁻ (NK cells). (D) Top panel demonstrates intracellular SOCS1 protein expression in effector cells of healthy PBMC cocultured with allogeneic healthy PBMCs. Middle panel demonstrates intracellular SOCS1 protein expression in effector cells of healthy PBMC cocultured with MM cell line MM1.S, and bottom panel demonstrates intracellular SOCS1 protein expression in effector cells of healthy PBMCs cocultured with MM cell line MM1.S and MM-BMSC. Numbers indicate percentage of positive cells. One representative of 3 independent experiments is shown. Statistical significance indicated (t test, 1-tailed distribution, P < .05).