LAUSR

356Background: M-MDSCs promote tumor progression through complex mechanisms, including immunosuppression and the production of mediators of angiogenesis and invasion. M-MDSCs are associated with poor outcomes in a number of malignancies. We conducted an exploratory analysis enumerating M-MDSCs (Lin-CD14+/HLA-DRlow/-) in the blood of pts with mUC and assessed for assay variability and correlation with clinical outcomes. Methods: Whole blood was collected at a single time point for each pt and stabilized in Cyto-Chex tubes. M-MDSC% was calculated by flow cytometric analysis of HLA-DR expression on CD14+ monocytes using an MSKCC developed computational algorithm-based approach (Kitano et al. 2014). Individual samples were run in quadruplicate at MSKCC. The mean MDSC% was used in subsequent analyses and the replicate standard deviation (SD) was considered a reflection of intra-assay variability. Clinical variables were collected and pts followed for OS, which was calculated from time of sample collection. Res...