Lysates of blood MDSCs (> 90% Gr1⁺CD11b⁺ cells) were digested with trypsin, and the tryptic peptides were analyzed on an LTQ-FT mass spectrometer interfaced with an Agilent 1100 nanoLC system. Mass spectra were searched against the NCBI mouse protein database²⁸  with the use of the MASCOT (Matrix Sciences) data analysis tool. Proteins that were identified by ≥ 2 peptides and which had a MASCOT MOWSE score > 30 were considered reliable identifications. MOWSE score is a probability-based score and is equal to −10*Log(P), where P is the probability of an observed event being random. A MOWSE score of ≥ 31 indicates that protein identification is probably correct. Proteins were identified in each of 2 independent mass spectroscopic runs with the use of independent cell preparations.