A Guide to Monocytic MDSC Markers

Monocytic MDSC (myeloid-derived suppressor cells) are a specialized subset of monocytes with potent immunosuppressive activity. These cells regulate immune responses in diverse contexts, ranging from chronic infection and obesity to cancer and autoimmunity. Mo-MDSCs also help maintain immune homeostasis and maternal-fetal tolerance during pregnancy.

Under normal circumstances, the myeloid cell compartment generates monocytes and granulocytes in response to pathogens or tissue damage. Under chronic inflammatory conditions, sustained production of cytokines such as M-CSF, IL-6, and IL-1β drives the conversion of monocytes and granulocytes into Mo-MDSCs and granulocytic MDSCs, respectively. The chief function of these cell types is to inhibit immune responses mediated by lymphocytes (T cells, B cells, and natural killer cells).

Mouse Mo-MDSC Markers

In mice, Mo-MDSCs bear many of the same cell surface markers as classical monocytes and other myeloid populations. For example, both cell populations are CD11b⁺Ly6G⁻Ly6C^hi. Other markers shared by both populations include ​​CD115 (CSF1R), CCR2 and CD49d (VLA4). The pan-macrophage marker F4/80 also labels mouse Mo-MDSCs, although its expression is lower on these cells than tumor-associated macrophages, enabling these populations to be distinguished in tumors. Expression of CD115 and IL-4R (CD124) in Mo-MDSCs correlates with immunosuppressive activity; both markers are common on Mo-MDSCs found in tumors. CD49d (VLA4) is a useful marker for discriminating between Mo-MDSCs and granulocytic MDSCs.

Image: This figure highlights some key differences between mouse and human MDSC markers (only membrane protein markers are shown).

Apart from this core set of markers, Mo-MSDCs are rather heterogeneous. Indeed, cellular markers such as CD11c (ITGAX), MHCII, Sca-1 (Ly6a), and Integrin α4β1 (ITGA4) are expressed by some but not all mouse Mo-MDSCs. All of these markers are shared by other populations within the myeloid lineage. Dendritic cells, for example, express all four of these markers.

Markers that discriminate between Mo-MDSCs and classical monocyte populations have been identified recently, namely, CD84. In mice, this marker is present on mouse Mo-MDSCs (and granulocytic MDSCs, for that matter) but not classical monocytes (nor granulocytes).

In mice, immature Mo-MDSCs express PECAM-1 (CD31) as well as low levels of MHCII.

Human Mo-MDSC Markers

Like their mouse counterparts, human Mo-MDSCs also share many cell surface markers with human classical monocytes, including CD14 and CD15. However, human Mo-MDSCs can be distinguished from human monocytes by HLA-DR expression: the former expresses the marker at lower levels than the latter. This difference in HLA-DR expression is consistent with the poor antigen presentation capabilities of Mo-MDSCs. Additional markers that label human Mo-MDSCs but not monocytes include CD66b⁻, CXCR1⁺, and CD84⁺.

Human Mo-MDSCs are a heterogeneous population, too. The list of markers expressed nonuniformly across human Mo-MDSCs is long and includes CD11c (ITGAX), CXCR2, CD13, CD16^lo, CD33, CD34, CD38, and Integrin α4β1 (ITGA4).

In humans, early-stage Mo-MDSCs are negative for the lineage markers CD14, CD15, CD3, CD19, and CD56. These five markers are sometimes referred to as lineage markers or, collectively, as LIN⁻.

Markers of Mo-MDSC Function

With so many Mo-MDSCs markers being shared by other cell populations within the myeloid lineage in mice and humans, Mo-MDSCs are primarily distinguished based on functional markers. Whereas classical monocytes and other myeloid populations are primarily immunostimulatory, Mo-MDSCs are immunosuppressive.

The immunosuppressive phenotype of Mo-MDSCs is characterized by upregulation of the transcription factor STAT3, immune modulating S100 proteins (S100A8/9), arginase-1 (ARG1), nitric oxide synthase 2 (NOS2), the immune checkpoint molecule PD-L1, and the anti-inflammatory cytokines IL-10 and TGF-β. TLR2/Myd88-dependent induction of STAT3 is essential for the immunosuppressive activity of Mo-MDSCs, as inhibition or genetic ablation of STAT3 in hematopoietic cells almost entirely abolishes this activity and results in markedly improved antitumor immune responses in cancer models. STAT3 is induced by a number of stimuli, including estradiol (during pregnancy), tumor-derived exosomes-associated Hsp72 (in cancer), and pathogen-associated molecular patterns (such as those derived from hepatitis C virus).

S100A8 and S100A9 primarily serve to promote MDSC expansion. Arginase-1 and inducible nitric oxide synthase (iNOS; encoded by NOS2) inhibit T cell immunity by metabolizing a common substrate, L-arginine. Because L-arginine promotes T cell proliferation, its catabolism essentially halts this process. The byproduct of iNOS-catabolized L-arginine—nitric oxide—inhibits T cell MHCII expression and promotes T cell apoptosis. PD-L1 also acts to inhibit T cell proliferation. Finally, membrane-bound TGF-β on MDSCs has been shown to suppress natural killer cells in cancer, and IL-10 induces M2, or anti-inflammatory, polarization in macrophages.

In humans, a population of CD14⁺HLA-DR^−/low MDSCs has been shown to inhibit dendritic cell maturation, which supports immune responses through antigen presentation, and induces T regulatory cells, an immunosuppressive subset of T cells. Thus, in addition to inhibiting the proliferation and activation of immunostimulatory immune cell populations (including T cells, natural killer cells, and dendritic cells), Mo-MDSCs also suppress immune responses by recruiting help from other immunosuppressive cell types.

Table of Monocytic MDSC Markers

The table below lists human and mouse proteins characterizing Mo-MDSC as described by recent review literature. The majority of proteins listed are membrane markers expressed on the cell surface, but also included are other defining proteins, like transcription factors and signaling proteins such as cytokines. Accompanying each marker are links to relevant antibodies and ELISA kits, as these immunodetection tools are routinely used in cell characterization studies via flow cytometry and immunostaining. The associated products are offered by a variety of manufacturers and can serve as a useful reference for MDSC immunophenotyping.

Note: *Some markers are protein isoforms, multi-subunit protein complexes, or protein families composed of several distinct genes. Information on Protein Type, Localization, and Size (kDa) obtained from UniProt.org (for human genes only). 

References

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2. Damuzzo V, Pinton L, Desantis G, et al. Complexity and challenges in defining myeloid-derived suppressor cells. Cytometry B Clin Cytom. 2015;88(2):77-91. doi:10.1002/cyto.b.21206

3. Zhao Y, Wu T, Shao S, Shi B, Zhao Y. Phenotype, development, and biological function of myeloid-derived suppressor cells. Oncoimmunology. 2015;5(2):e1004983. Published 2015 Oct 14. doi:10.1080/2162402X.2015.1004983

4. Veglia F, Perego M, Gabrilovich D. Myeloid-derived suppressor cells coming of age. Nat Immunol. 2018;19(2):108-119. doi:10.1038/s41590-017-0022-x

5. Hegde S, Leader AM, Merad M. MDSC: Markers, development, states, and unaddressed complexity. Immunity. 2021;54(5):875-884. doi:10.1016/j.immuni.2021.04.004

6. Veglia F, Sanseviero E, Gabrilovich DI. Myeloid-derived suppressor cells in the era of increasing myeloid cell diversity. Nat Rev Immunol. 2021;21(8):485-498. doi:10.1038/s41577-020-00490-y