uPA and uPA-Receptor Are Involved in Cancer-associated Myeloid-derived Suppressor Cell Accumulation

Background: 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. Materials and Methods: 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. Results: 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. Conclusion: MDSC accumulation is tumor burden-dependent, and tumor-derived factors such as uPA and its receptor uPAR play a role in their recruitment. The immune system plays an important role in preventing tumor development, and it is now widely accepted that tumor cells induce and are shaped by host anti-tumor immune responses. The theory that the immune system is involved in tumor surveillance was proposed by Paul Ehrlich in the early 1900's and has since evolved into a very complex process. In fact the specific assessment of immune cell infiltration for a given tumor has been shown to serve as a better predictor of patient survival than histopathological staging (1). However, tumors are believed to exploit a chronic inflammatory environment, leading to the recruitment of immune cells with a regulatory phenotype, such as T-regulatory cells and myeloid-derived suppressor cells (MDSC). These cells have been shown to prevent tumor immunity (2), and their depletion has proven to be effective for immune cell therapies (3). Over twenty years ago, our laboratory described accumulation of myeloid cells in the spleen due to tumor- derived factors (4). Furthermore, the laboratory reported that these CD11b + (MAC-1) cells, now recognized as MDSC, down-regulate T- and B-cell responses (5). MDSC in the mouse have been phenotypically identified as GR-1 + CD11b + (6), while in humans the markers are less definitive and described as LIN - HLA - DR - CD33 + or CD11b + CD14 - CD33 + (7). MDSC accumulation has been associated with inflammation in the tumor microenvironment, and several tumor-derived cytokines have now been shown to influence their phenotypes and suppressive pathways (8-10). These cells have been shown to suppress T-cells (11), repress innate immune cells such as NK cells (12), and skew macrophage differentiation from M1 type to M2, normally associated with immunosuppression (13). We have shown that MDSC accumulate in tumor-bearers in various compartments such as the bone marrow, blood, spleen, and liver, and can interact with various immune cells in those organs (14). Tumor cells have several means of evading the immune system, one of which is by the production of immune- modulating factors which can, in part, recruit immune regulatory cells such as MDSC. One such factor is urokinase plasminogen activator (uPA), a serine protease produced by many tumors at high levels. It is also increased in the serum of cancer patients, and correlates with the extent of disease. High uPA levels have been found in patients with various