['Myeloid-derived suppressor cells (MDSC) have been shown to play a critical role in tumor-induced immunosuppression, in many mouse and human cancers. The aim of this study was to show that MDSC accumulation is tumor burden-dependent, and to investigate the role of the tumor-derived urokinase plasminogen activator (uPA) and its receptor (uPAR) on MDSC recruitment.', 'Levels of MDSC were assessed in tumor-bearers, and the ability to recruit MDSC by uPA was investigated in normal, tumor-bearers, uPAR(-/-), and CD11b(-/-) mice. uPAR expression in MDSC was also explored.', 'MDSC accumulate to dramatic levels in tumor-bearers, and tumor-derived factors such as uPA also increase to great levels in circulation. MDSC can be recruited by uPA, and uPAR but not CD11b are required for such recruitment.'] Are uPA and uPA-receptor involved in cancer-associated myeloid-derived suppressor cell accumulation?