KITLG and neoplasm: The factors triggering MDSC expansion and activation are well-studied in tumor models, including cytokines such as IL-1β, IL-6, IL-10, and IL-13, growth factor such as SCF, VEGF, GM-CSF, G-CSF, and M-CSF, as well as calcium binding pro-inflammatory proteins such as S100A8, S100A9, cyclooxygenase-2, and prostaglandin E2 (25, 26).