The transmembrane isoform of mucin 1 (MUC1/TM) is a well recognized tumor antigen, contributing to tumorigenesis and immune evasion. While MUC1/TM has been correlated with malignancy, it appears that a secreted splice variant of MUC1 (MUC1/sec) has antitumor properties and prevents tumor development. It was discovered that MUC1/sec expressing tumor cells (DA-3/sec) have a significant reduction in expression of urokinase plasminogen activator (uPA) relative to the parental tumor line, and tumor cells expressing MUC1/TM (DA-3/TM). The serine protease uPA, has been found to be involved in growth promoting signaling, angiogenesis, and induction of matrix remodeling leading to metastasis. Furthermore, the tumor suppressive and interferon responsive Stat1 transcription factor is dramatically upregulated in DA-3/sec cells. In addition, treatment of various murine and human cell lines with conditioned media containing MUC1/sec results in up-regulation of Stat1. DA-3/sec tumor cells are also sensitized to the anti-proliferative effects of IFN-g. Furthermore, transfection of the Stat1 gene into DA-3 tumor cells leads to a downregulation of uPA, and delays tumor progression. Since myeloid-derived suppressor cells (MDSC) play a critical role in tumor-induced immunosuppression, we investigated their recruitment by DA-3/sec and DA-3/TM cells. DA-3/sec tumor cells recruit dramatically lower levels of MDSC, relative to DA-3/TM cells. Since MUC1/sec down-regulates tumor expression of uPA, its potential role in MDSC recruitment was investigated. Tumor-derived uPA is capable of recruiting MDSC, and correlates with tumor development. In addition to diminishing recruitment of MDSC, the effect of MUC1/sec on MDSC suppressive mechanisms was investigated. MUC1/sec, or its unique immunoenhancing peptide (IEP), is capable of blocking expression of arginase 1 and production of reactive oxygen species (ROS) in MDSC, implicated in the suppression of T cells. These findings demonstrate a new mechanism of MDSC recruitment, and provide evidence that MUC1/sec has antitumor properties affecting both tumor cells and MDSC. Furthermore, it was discovered that MDSC home to the liver in addition to the tumor, bone marrow, blood, and spleen of tumor bearers, as previously described. The liver is thus an organ where MDSC accumulate and can contribute to immunosuppression directly and indirectly, via interactions with a variety of immune cells.</p
