Recommendations for myeloid-derived suppressor cell nomenclature and characterization standards

Abstract

Myeloid-derived suppressor cells (MDSCs) have emerged as major regulators of immune responses in cancer and other pathological conditions. In recent years, ample evidence supports key contributions of MDSC to tumour progression through both immune-mediated mechanisms and those not directly associated with immune suppression. MDSC are the subject of intensive research with >500 papers published in 2015 alone. However, the phenotypic, morphological and functional heterogeneity of these cells generates confusion in investigation and analysis of their roles in inflammatory responses. The purpose of this communication is to suggest characterization standards in the burgeoning field of MDSC research.

Figure 1. Gating strategy for the identification of mouse MDSC subsets.

Gating strategy used to define MDSC subpopulations in BM, blood and spleen of C57Bl/6 tumour-free or MCA203 tumour-bearing mice. After exclusion of doublets (not shown), live CD11b⁺ cells were gated and the proportion of Ly6C and Ly6G cells was evaluated.

Figure 2. Gating strategy for the identification of MDSC subsets in the peripheral blood of healthy donors and melanoma patients.

Doublets were excluded and live PBMC were gated (not shown). (a) CD14⁺HLA-DR^−/lo M-MDSC. Monocytes were gated on the basis of FSC and SSC parameters and HLA-DR downregulation was defined by FMO control. (b) Lin⁻HLA-DR⁻CD33⁺ eMDSC. (c) CD14⁻CD15⁺CD11b⁺ PMN-MDSC.

Figure 3. Overview of MDSC involvement in myeloid cell differentiation in cancer.

In cancer and chronic inflammation, the bone marrow and spleen increase the output of mature and immature myeloid cells that comprise a spectrum between monocytes and neutrophils. In mice, MDSC toward the monocytic end of the spectrum (M-MDSC) are CD11b⁺Ly6C⁺Ly6G⁻, while towards the neutrophil end of the spectrum (PMN-MDSC) are CD11b⁺LyG⁺Ly6C⁻. Within solid tumours M-MDSC develop through intermediate steps towards macrophages where Ly6C is progressively downregulated and MHCII, F4/80 and CX3CR1 are upregulated. Under chronic inflammation, monocytic lineages show an increasing requirement for anti-apoptotic survival pathways (mediated primarily by GM-CSF signalling) to block the intrinsic mitochondrial death pathway. A similar scheme is likely to occur in humans; however, the cell markers are different.

Figure 4. Algorithm for identification of cells as MDSC.

Step-by-step approach to identify cells as MDSC for reporting. It is important, wherever possible, to use cells with the same phenotype from control mice or healthy donors as controls.