MDSC are a mixed population of monocyte-lineage (CD14/CD15/CD16) PBMC and granulocytes. If you wan the human monoycte MDSC, I advise you to check out a Miltneyi positive-selection magnetic column separation loaded with CD33 beads if you are doing Ficoll first for your PBMC. https://www.miltenyibiotec.com/US-en/products/macs-cell-separation/cell-separation-reagents/microbeads-and-isolation-kits/granulocytes-and-myeloid-cells/cd33-microbeads-human.html
I've used the Miltenyi systems and had good luck. They make a whole blood kit which skips Ficoll by lots of RBC get through. Best to do Ficoll first if you want a clean sample.