STAT3 and neoplasm: Several cytokines and growth factors secreted by tumor cells and immune cells have been shown to promote the accumulation and expansion of MDSCs by activating the JAK/STAT1 and JAK/STAT3 signaling pathways (Fleming et al., 2018), including granulocyte-macrophage colony-stimulating factor (GM-CSF), vascular endothelial growth factor (VEGF), transforming growth factor-β (TGF-β), interleukin-6 (IL-6), and interleukin-10 (IL-10).