Myeloid-Derived Suppressor Cell

A myeloid-derived suppressor cell (MDSC) is a pathologically activated, developmentally arrested immature myeloid leukocyte that expands in the bone marrow, spleen, peripheral blood, and tissue microenvironments during malignancy, chronic bacterial or viral infection, trauma, sepsis, and pregnancy, where it potently inhibits T-lymphocyte and natural killer (NK) cell effector functions. First described in the late 1970s and 1990s as "natural suppressor cells" or "immature Gr-1⁺CD11b⁺ myeloid cells" and unified under the consensus nomenclature myeloid-derived suppressor cells in 2007 by Dmitry I. Gabrilovich, Vincenzo Bronte, Suzanne Ostrand-Rosenberg, Augusto C. Ochoa, and Hans Schreiber, MDSCs arise through a two-signal model of emergency myelopoiesis. In the tumour microenvironment (TME), MDSCs represent one of the primary barriers to checkpoint blockade and adoptive cell therapy by depleting extracellular L-arginine and L-cysteine, generating peroxynitrite (ONOO⁻) that nitrates T-cell receptors across the immunological synapse, producing adenosine and prostaglandin E₂ (PGE₂), and inducing Foxp3⁺ regulatory T cells (T_reg).

Under healthy steady-state hematopoiesis, bone-marrow common myeloid progenitors (CMPs) and granulocyte–monocyte progenitors (GMPs) rapidly differentiate into mature neutrophils, monocytes, macrophages, and conventional dendritic cells. During chronic inflammation or neoplasia, however, sustained low-grade myelopoietic growth factors combined with pro-inflammatory danger signals reprogram GMPs via the Gabrilovich two-signal model:

• Signal 1 (Myeloid Expansion & Differentiation Block): Chronic tumor- or stroma-derived GM-CSF, G-CSF, M-CSF (CSF-1), VEGF, and IL-6 activate STAT3, C/EBPβ, and IRF8 downregulation, driving rapid proliferation of immature myeloid precursors while blocking their terminal maturation into antigen-presenting dendritic cells or M1 macrophages.
• Signal 2 (Pathological Immunosuppressive Activation): Pro-inflammatory cytokines and damage-associated molecular patterns (IFN-γ, IL-1β, TNF-α, IL-4/IL-13, HMGB1, S100A8/S100A9, and PGE₂) signal through STAT1, STAT6, NF-κB, and endoplasmic reticulum (ER) stress sensor CHOP / XBP1, upregulating the immunosuppressive enzymatic machinery (ARG1, iNOS/NOS2, NOX2, COX-2, and LOX-1).

By international consensus (Bronte, Brandau, Chen, Colombo, Ochoa, Ostrand-Rosenberg, & Gabrilovich, 2016), MDSCs are classified into three major lineage subsets:

Notably, human PMN-MDSCs share canonical surface markers (CD11b⁺CD15⁺CD66b⁺) with mature polymorphonuclear neutrophils, but segregate into the low-density mononuclear cell layer during Ficoll density-gradient centrifugation (1.077 g/mL) and uniquely express the oxidized low-density lipoprotein receptor LOX-1 (OLR1) alongside elevated ER-stress transcription factor CHOP (DDIT3) and FATP2 (SLC27A2).

1. L-Arginine Depletion by Arginase-1 (ARG1) and CD3ζ Chain Downregulation

As demonstrated by Augusto C. Ochoa, Paulo C. Rodriguez, and Vincenzo Bronte, both PMN-MDSCs and M-MDSCs express high levels of cytosolic and exosome-packaged arginase-1 (ARG1) (induced by STAT6/IL-4/IL-13 and STAT3/PGE₂) alongside the cationic amino acid transporter CAT-2B (SLC7A2), which imports extracellular L-arginine and hydrolyzes it into L-ornithine and urea. Depleting L-arginine in the local microenvironment below 5–20 μM arrests T-cell proliferation in the G₀–G₁ phase of the cell cycle via three mechanisms:

1. Loss of the TCR CD3ζ Signaling Subunit: L-arginine starvation selectively decreases the mRNA stability and translation of the CD3ζ (CD247) chain of the T-cell receptor complex, uncoupling surface TCRαβ from intracellular ZAP-70 tyrosine kinase phosphorylation.
2. GCN2 Kinase Activation and Cyclin D3 Suppression: Accumulation of uncharged arginyl-tRNA activates GCN2 (general control nonderepressible 2) kinase, which phosphorylates eukaryotic initiation factor eIF2α, halting de novo synthesis of cyclin D3 and cyclin-dependent kinase 4 (CDK4) and blocking retinoblastoma (Rb) phosphorylation.
3. Polyamine and Proline Synthesis: Ornithine generated by ARG1 is converted by ornithine decarboxylase into polyamines and by ornithine aminotransferase into proline, fueling tumor cell proliferation and extracellular matrix remodeling.

2. Inducible Nitric Oxide Synthase (iNOS), NOX2, and Peroxynitrite Nitration of TCR–CD8

In M-MDSCs, STAT1 and NF-κB drive high-level expression of inducible nitric oxide synthase (iNOS / NOS2), which oxidizes L-arginine to generate sustained micromolar fluxes of nitric oxide (NO^•) that inhibit JAK3/STAT5 signaling downstream of the IL-2 receptor, block MHC class II transcription, and induce T-cell apoptosis.

In PMN-MDSCs, the phagocyte NADPH oxidase complex NOX2 (gp91^phox / CYBB and p47^phox)—activated by STAT3—generates high concentrations of superoxide anion (O₂^•−) and hydrogen peroxide (H₂O₂). When superoxide (O₂^•−) reacts at diffusion-controlled rates with nitric oxide (NO^•)—especially when L-arginine is simultaneously depleted, causing iNOS uncoupling—it forms the highly reactive oxidant peroxynitrite (ONOO⁻). As discovered by Srinivas Nagaraj and Dmitry Gabrilovich (2007) in Nature Medicine, during brief antigen-specific contact between a PMN-MDSC and a CD8⁺ T cell, peroxynitrite nitrates tyrosine residues within the T-cell receptor (TCR) and the CD8 co-receptor (forming 3-nitrotyrosine). Tyrosine nitration conformationally rigidifies the TCR–CD8 complex so that the CD8⁺ T cell can no longer bind its cognate peptide–MHC class I complex on tumor cells, while leaving the T cell responsive to non-specific stimuli such as phorbol esters or anti-CD3 antibodies. Furthermore, peroxynitrite nitrates the chemokine CCL2 (N-CCL2) in the tumor stroma, which remains able to recruit CCR2^high myeloid cells while losing the ability to attract CCR2^low cytotoxic CD8⁺ T cells, trapping CTLs in the peritumoral stroma.

3. Cysteine Sequestration, FATP2/PGE₂ Lipid Reprogramming, and T_reg Induction

MDSCs deploy three additional metabolic and immunoregulatory weapons:

• Extracellular Cysteine Deprivation (Ostrand-Rosenberg et al., 2010): Naive and activated T cells require L-cysteine for protein synthesis and intracellular glutathione (GSH) antioxidant production, but T cells lack both the cystathionase enzyme (which converts methionine to cysteine) and the x_c⁻ cystine/glutamate antiporter (SLC7A11/SLC3A2) during early activation. Consequently, T cells depend entirely on neighboring dendritic cells and macrophages to import disulfide L-cystine via x_c⁻, reduce it intracellularly to L-cysteine, and export L-cysteine via the ASC neutral amino acid transporter. MDSCs express high levels of x_c⁻ (importing all extracellular cystine) but lack the ASC exporter, hoarding cysteine and starving adjacent T cells.
• FATP2 Arachidonic Acid Uptake and Prostaglandin E₂ (Veglia et al., 2019): Tumor-infiltrating PMN-MDSCs upregulate fatty acid transport protein 2 (FATP2 / SLC27A2), which imports arachidonic acid and converts it via cyclooxygenase-2 (COX-2) and mPGES-1 into prostaglandin E₂ (PGE₂).
• T_reg Expansion and Ectonucleotidase Adenosine Generation: Activated by HIF-1α in hypoxic tumors, MDSCs upregulate CD39 and CD73 (converting extracellular ATP into immunosuppressive adenosine), express PD-L1 and VISTA, and secrete IL-10 and TGF-β to expand Foxp3⁺ regulatory T cells and type 1 regulatory T cells while impairing NK-cell NKG2D expression and perforin cytotoxicity.

Because elevated circulating PMN-MDSC and M-MDSC counts correlate with resistance to anti-PD-1/PD-L1 and anti-CTLA-4 immune checkpoint blockade across melanoma, non-small-cell lung cancer, renal cell carcinoma, and colorectal cancer, four clinical classes of MDSC-targeted therapies have been developed:

1. Promoting Myeloid Differentiation (ATRA & Vitamin D₃): All-trans retinoic acid (ATRA) upregulates glutathione synthase (GSS) in MDSCs, neutralizing intracellular reactive oxygen species and driving immature MDSCs to differentiate into mature, immunogenic dendritic cells and macrophages.
2. Inhibiting Suppressive Machinery (PDE5, COX-2, HDAC, PI3Kγ, and FATP2 Inhibitors): Phosphodiesterase-5 (PDE5) inhibitors (sildenafil, tadalafil), class I histone deacetylase inhibitor entinostat, STAT3 antisense oligonucleotide danvatirsen, PI3Kγ inhibitor eganelisib (IPI-549), and FATP2 inhibitor lipofermata downregulate ARG1, iNOS, and PGE₂ in intratumoral MDSCs.
3. Blocking MDSC Chemotactic Trafficking Into Tumors: Antagonists of CXCR2 (SX-682, blocking CXCL1/CXCL2/CXCL5/IL-8-driven PMN-MDSC recruitment), CCR2 (blocking CCL2-driven M-MDSC recruitment), and CSF-1R prevent MDSC infiltration into the tumor bed.
4. Selective MDSC Depletion: Low-dose 5-fluorouracil, gemcitabine, paclitaxel, and TRAIL-R2 (DR5) agonistic antibodies (targeting ER-stress-induced DR5 on MDSCs) selectively induce apoptosis of MDSCs while sparing effector T cells.

How are human PMN-MDSCs distinguished experimentally from normal mature neutrophils?

Although human PMN-MDSCs and mature polymorphonuclear neutrophils share the standard surface markers CD11b⁺, CD14⁻, CD15⁺, and CD66b⁺, they differ in three ways: (1) on standard density-gradient centrifugation (1.077 g/mL), mature high-density neutrophils pellet with erythrocytes at the bottom of the tube, whereas hypodense PMN-MDSCs float in the low-density peripheral blood mononuclear cell (PBMC) ring; (2) human PMN-MDSCs specifically express the lectin-type oxidized LDL receptor LOX-1 (OLR1) and ER-stress factor CHOP, which are absent on healthy neutrophils; and (3) functionally, PMN-MDSCs suppress anti-CD3/CD28-stimulated T-cell proliferation and IFN-γ production in co-culture assays.

Why does peroxynitrite produced by PMN-MDSCs cause antigen-specific rather than global T-cell unresponsiveness?

Peroxynitrite (ONOO⁻) has a biological half-life of less than 10 milliseconds at physiological pH and can diffuse only across nanometer distances during direct cell-to-cell contact between a PMN-MDSC (presenting a peptide on MHC class I) and a cognate CD8⁺ T cell. Within this synaptic cleft, peroxynitrite chemically nitrates tyrosine residues on the TCR and CD8 molecules engaged at the contact zone. This conformational modification prevents the T cell's TCR–CD8 complex from binding its specific peptide–MHC ligand on tumor cells, while leaving intracellular signaling pathways intact if the T cell is stimulated with TCR-bypassing mitogens.

How does hypoxia inside solid tumors alter MDSC fate and function?

When circulating M-MDSCs extravasate into hypoxic regions of a solid tumor, stabilization of hypoxia-inducible factor 1-alpha (HIF-1α) accomplishes two things: first, HIF-1α binds directly to hypoxia-response elements in the promoters of CD274 (PD-L1), ARG1, NOS2 (iNOS), ENTPD1 (CD39), and NT5E (CD73), amplifying MDSC suppressive potency across all T-cell specificities; second, HIF-1α downregulates STAT3 activity via CD45 protein tyrosine phosphatase activation, triggering rapid differentiation of M-MDSCs into immunosuppressive M2-like tumor-associated macrophages (TAMs).

1. Gabrilovich, D. I., & Nagaraj, S. (2009). "Myeloid-derived suppressor cells as regulators of the immune system." Nature Reviews Immunology, 9(3), 162–174. https://doi.org/10.1038/nri2506
2. Nagaraj, S., Gupta, K., Pisarev, V., Kinarsky, L., Sherman, S., Kang, L., Herber, D. L., Schneck, J., & Gabrilovich, D. I. (2007). "Altered recognition of antigen is a mechanism of CD8⁺ T cell tolerance in cancer." Nature Medicine, 13(7), 828–835. https://doi.org/10.1038/nm1609
3. Bronte, V., Brandau, S., Chen, S.-H., Colombo, M. P., Frey, A. B., Greten, T. F., Mandruzzato, S., Murray, P. J., Ochoa, A., Ostrand-Rosenberg, S., Rodriguez, P. C., Sica, A., Umansky, V., Vonderheide, R. H., & Gabrilovich, D. I. (2016). "Recommendations for myeloid-derived suppressor cell nomenclature and characterization standards." Nature Communications, 7, 12150. https://doi.org/10.1038/ncomms12150
4. Veglia, F., Tyurin, V. A., Blasi, M., De Leo, A., Kossenkov, A. V., Donthireddy, L., To, T. K. J., Schug, Z., Basu, S., Wang, F., Ricciotti, E., DiRusso, C., Murphy, M. E., Vonderheide, R. H., Lieberman, P. M., Mulligan, C., Nam, B., Hockstein, N., Masters, G., Guarino, M., Lin, C., Nefedova, Y., Black, P., Kagan, V. E., & Gabrilovich, D. I. (2019). "Fatty acid transport protein 2 reprograms neutrophils in cancer." Nature, 569(7754), 73–78. https://doi.org/10.1038/s41586-019-1118-2