CD49d is a new marker for distinct myeloid-derived suppressor cell subpopulations in mice

Myeloid-derived suppressor cells (MDSCs) are a heterogenous population of cells that negatively regulate the immune response during tumor progression, inflammation, and infection. In this study, through gene-expression analysis, we have identified a new marker, CD49d, which is expressed exclusively on CD11b(+)Gr-1(dull/int.) MDSCs. We have characterized two subpopulations of MDSCs based on CD49d expression in two different settings, a mouse model of inflammatory bowel disease and tumor-bearing mice. The CD49d(+) subset of MDSCs was mainly monocytic and strongly suppressed Ag-specific T cell proliferation in an NO-dependent mechanism similar to Gr-1(dull/int.) MDSCs. Alternatively, CD49d(-) cells were granulocytic and poorly inhibited T cell proliferation compared with CD11b(+)Gr-1(high) cells. Both mouse models showed preferential expansion of the granulocytic CD49d(-) subset. We suggest that CD49d can be used as an alternative marker for Gr-1 to differentiate between the subpopulations of MDSCs together with CD11b, which will ultimately help in understanding the mechanisms of immune suppression by MDSCs.