Preparation of myeloid derived suppressor cells (MDSC) from naive and pancreatic tumor-bearing mice using flow cytometry and automated magnetic activated cell sorting (AutoMACS).

MDSC are a heterogeneous population of immature macrophages, dendritic cells and granulocytes that accumulate in lymphoid organs in pathological conditions including parasitic infection, inflammation, traumatic stress, graft-versus-host disease, diabetes and cancer(1-7). In mice, MDSC express Mac-1 (CD11b) and Gr-1 (Ly6G and Ly6C) surface antigens(7). It is important to note that MDSC are well studied in various tumor-bearing hosts where they are significantly expanded and suppress anti-tumor immune responses compared to naive counterparts(7-10). However, depending on the pathological condition, there are different subpopulations of MDSC with distinct mechanisms and targets of suppression(11,12). Therefore, effective methods to isolate viable MDSC populations are important in elucidating their different molecular mechanisms of suppression in vitro and in vivo. Recently, the Ghansah group has reported the expansion of MDSC in a murine pancreatic cancer model. Our tumor-bearing MDSC display a loss of homeostasis and increased suppressive function compared to naive MDSC (13). MDSC percentages are significantly less in lymphoid compartments of naive vs. tumor-bearing mice. This is a major caveat, which often hinders accurate comparative analyses of these MDSC. Therefore, enriching Gr-1(+) leukocytes from naive mice prior to Fluorescence Activated Cell Sorting (FACS) enhances purity, viability and significantly reduces sort time. However, enrichment of Gr-1(+) leukocytes from tumor-bearing mice is optional as these are in abundance for quick FACS sorting. Therefore, in this protocol, we describe a highly efficient method of immunophenotyping MDSC and enriching Gr-1(+) leukocytes from spleens of naive mice for sorting MDSC in a timely manner. Immunocompetent C57BL/6 mice are inoculated with murine Panc02 cells subcutaneously whereas naive mice receive 1XPBS. Approximately 30 days post inoculation; spleens are harvested and processed into single-cell suspensions using a cell dissociation sieve. Splenocytes are then Red Blood Cell (RBC) lysed and an aliquot of these leukocytes are stained using fluorochrome-conjugated antibodies against Mac-1 and Gr-1 to immunophenotype MDSC percentages using Flow Cytometry. In a parallel experiment, whole leukocytes from naive mice are stained with fluorescent-conjugated Gr-1 antibodies, incubated with PE-MicroBeads and positively selected using an automated Magnetic Activated Cell Sorting (autoMACS) Pro Separator. Next, an aliquot of Gr-1(+) leukocytes are stained with Mac-1 antibodies to identify the increase in MDSC percentages using Flow Cytometry. Now, these Gr1(+) enriched leukocytes are ready for FACS sorting of MDSC to be used in comparative analyses (naive vs. tumor-bearing) in in vivo and in vitro assays.