Melanoma Extracellular Vesicles Generate Immunosuppressive Myeloid Cells by Upregulating PD-L1 via TLR4 Signaling.

Tumor cell-derived extracellular vesicles (EV) convert normal myeloid cells into myeloid-derived suppressor cells (MDSC), inhibiting antitumor immune responses. Here, we show that EV from Ret mouse melanoma cells upregulate the expression of programmed cell death ligand 1 (PD-L1) on mouse immature myeloid cells (IMC), leading to suppression of T-cell activation. PD-L1 expression and the immunosuppressive potential of EV-generated MDSC were dependent on the expression of Toll-like receptors (TLR).

Immunosuppression mediated by myeloid-derived suppressor cells (MDSCs) during tumour progression.

Under steady-state conditions, bone marrow-derived immature myeloid cells (IMC) differentiate into granulocytes, macrophages and dendritic cells (DCs). This differentiation is impaired under chronic inflammatory conditions, which are typical for tumour progression, leading to the accumulation of IMCs. These cells are capable of inducing strong immunosuppressive effects through the expression of various cytokines and immune regulatory molecules, inhibition of lymphocyte homing, stimulation of other immunosuppressive cells, depletion of metabolites critical for T cell functions, expression of ectoenzymes regulating adenosine metabolism, and the production of reactive species.

Tumor-derived microRNAs induce myeloid suppressor cells and predict immunotherapy resistance in melanoma.

The accrual of myeloid-derived suppressor cells (MDSCs) represents a major obstacle to effective immunotherapy in cancer patients, but the mechanisms underlying this process in the human setting remain elusive. Here, we describe a set of microRNAs (miR-146a, miR-155, miR-125b, miR-100, let-7e, miR-125a, miR-146b, miR-99b) that are associated with MDSCs and resistance to treatment with immune checkpoint inhibitors in melanoma patients. The miRs were identified by transcriptional analyses as being responsible for the conversion of monocytes into MDSCs (CD14+HLA-DRneg cells) mediated by melanoma extracellular vesicles (EVs) and were shown to recreate MDSC features upon transfection.

Myeloid-Derived Suppressor Cells Hinder the Anti-Cancer Activity of Immune Checkpoint Inhibitors.

Immune checkpoint inhibitors (ICI) used for cancer immunotherapy were shown to boost the existing anti-tumor immune response by preventing the inhibition of T cells by tumor cells. Antibodies targeting two negative immune checkpoint pathways, namely cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), programmed cell death protein 1 (PD-1), and programmed cell death-ligand 1 (PD-L1), have been approved first for patients with melanoma, squamous non-small cell lung cancer (NSCLC), and renal cell carcinoma. Clinical trials are ongoing to verify the efficiency of these antibodies for other cancer types and to evaluate strategies to block other checkpoint molecules.

Targeting Myeloid-Derived Suppressor Cells to Bypass Tumor-Induced Immunosuppression.

The immune system has many sophisticated mechanisms to balance an extensive immune response. Distinct immunosuppressive cells could protect from excessive tissue damage and autoimmune disorders. Tumor cells take an advantage of those immunosuppressive mechanisms and establish a strongly immunosuppressive tumor microenvironment (TME), which inhibits antitumor immune responses, supporting the disease progression.

CCR5 Myeloid-Derived Suppressor Cells Are Enriched and Activated in Melanoma Lesions.

Accumulation of myeloid-derived suppressor cells (MDSC) in melanoma microenvironment is supported by chemokine receptor/chemokine signaling. Although different chemokines were suggested to be involved in this process, the role of CCR5 and its ligands is not established. Using a transgenic mouse melanoma model, we found an accumulation of CCR5 MDSCs in melanoma lesions associated with both increased concentrations of CCR5 ligands and tumor progression.

Expression of CD24 and Siglec-10 in first trimester placenta: implications for immune tolerance at the fetal-maternal interface.

During pregnancy, the fetal-maternal interface establishes immune tolerance between the fetus and the mother. CD24, a mucin-like glycoprotein expressed at the surface of hematopoietic cells and diverse tumor cells, is known to interact with the sialic acid-binding immunoglobulin-type lectins (Siglecs). This interaction was assessed as a candidate complex for the immune suppression response in the placenta.

Myeloid-derived suppressor cells and tumor escape from immune surveillance.

Tumor progression is known to be supported by chronic inflammatory conditions developed in the tumor microenvironment. It is characterized by the long-term secretion of various inflammatory soluble factors (including cytokines, chemokines, growth factors, reactive oxygen and nitrogen species, prostaglandins, etc.) and strong leukocyte infiltration.

Impaired gp100-Specific CD8(+) T-Cell Responses in the Presence of Myeloid-Derived Suppressor Cells in a Spontaneous Mouse Melanoma Model.

Murine tumor models that closely reflect human diseases are important tools to investigate carcinogenesis and tumor immunity. The transgenic (tg) mouse strain tg(Grm1)EPv develops spontaneous melanoma due to ectopic overexpression of the metabotropic glutamate receptor 1 (Grm1) in melanocytes. In the present study, we characterized the immune status and functional properties of immune cells in tumor-bearing mice.