STAT3 and neoplasm: MDSC development is supported by several growth factors (GM-CSF and G-CSF) and other factors, including IL-1β, IL-6, IL-11, IFN-γ, and tumor-derived exosomes, through the action of TLR2 and TGF-β, which have been implicated in the activation of signal transducer and activator of transcription (STAT)1, STAT3, and nuclear factor kappa-B (NF-κB) and the down-regulation of IFN regulatory factor 8 (IRF8) expression in MDSCs; these factors are critical for MDSC expansion [34].