Proteomic pathway analysis reveals inflammation increases myeloid-derived suppressor cell resistance to apoptosis

Granulocytic myeloid-derived suppressor cells are cryosensitive and their frequency does not correlate with serum concentrations of colony-stimulating factors in head and neck cancer

Granulocytic myeloid-derived suppressor cells (MDSC) are a MDSC subset expanded in various cancer types. As many clinical studies rely on the use of stored collections of frozen blood samples, we first tested the influence of freezing/thawing procedures on immunophenotyping and enumeration of granulocytic MDSC (G-MDSC). To identify factors involved in expansion of human G-MDSC, we then analyzed correlations between G-MDSC frequencies, clinical parameters and granulocyte-related factors in the peripheral blood of head and neck cancer patients. HLA-DR, CD14, CD33 and CD66b allowed a clear discrimination of G-MDSC from monocytic MDSC and immature myeloid cells. MDSC subsets were sensitive to cryopreservation with immature G-MDSC showing the highest sensitivity. G-MDSC frequencies were increased in advanced disease stage and associated with the level of CCL4 and CXCL8, but not with colony-stimulating factors, IL-6, S100A8/9, CXCL1 and other cytokines. Our results indicate that the frequency of MDSC, in particular G-MDSC, may be underestimated in retrospective clinical analyses using frozen blood samples. Increased G-MDSC frequencies correlate with advanced disease and increased concentrations of CXCL8, but, unexpectedly, not with growth factors (such as granulocyte colony-stimulating factor), IL-6 and CXCL1. Our data suggest that CXCL8 promotes accumulation of G-MDSC in cancer patients independent of classical colony-stimulating factors.

Regulating the suppressors: apoptosis and inflammation govern the survival of tumor-induced myeloid-derived suppressor cells (MDSC)

Immune suppressive myeloid-derived suppressor cells (MDSC) are present in most cancer patients where they inhibit innate anti-tumor immunity and are a significant obstacle to cancer immunotherapy. Inflammation is a known inducer of Gr1(+)CD11b(+) MDSC; however, the factors/conditions that regulate MDSC survival and half-life have not been identified. We have used mass spectrometry (MS) and proteomic analysis to identify proteins and pathways that regulate MDSC survival. This analysis revealed high expression of caspase family proteins and the Fas-FasL, p38 MAPK, and TGFβ pathways, suggesting that Fas-FasL apoptosis regulates MDSC survival. Flow cytometry, confocal microscopy, and western blot analyses confirmed the MS findings and demonstrated that Fas(+) MDSC are susceptible to Fas-mediated killing in vitro. In vivo studies with FasL-deficient and Fas-deficient mice demonstrated that Fas-FasL interactions are essential for MDSC apoptosis and for rejection of established metastatic disease and survival and that FasL(+) T cells are the effector population mediating MDSC apoptosis. MS findings validated by biological experiments demonstrated that inflammation increases MDSC levels by protecting MDSC from Fas-mediated apoptosis, possibly by activating p38 MAPK. These results demonstrate that MDSC half-life in vivo is regulated by FasL(+) T cells and that inflammation increases MDSC levels by conferring resistance to Fas-mediated apoptosis and identifies T cells as the relevant effector cells causing MDSC apoptosis in vivo. This newly recognized mechanism for regulating MDSC levels identifies potential new targets for decreasing MDSC in cancer patients.

Myeloid-derived suppressor cells: general characteristics and relevance to clinical management of pancreatic cancer

Recent studies describe a heterogeneous population of cells of the myeloid lineage, termed myeloid derived suppressor cells (MDSC), which are observed with increased prevalence in the peripheral blood and tumor microenvironment of cancer patients, including pancreatic cancer. Accumulation of MDSC in the peripheral circulation has been related to extent of disease, and correlates with stage. MDSC have primarily been implicated in promoting tumor growth by suppressing antitumor immunity. There is also compelling evidence MDSC are also involved in angiogenesis and metastatic spread. Two main subsets of MDSC have been identified in cancer patients: a monocytic subset, characterized by expression of CD14, and a granulocytic subset characterized by expression of CD15. Both subsets of MDSC actively suppress host immunity through a variety of mechanisms including production of reactive oxygen species and arginase. Just as in humans, accumulation of monocytic and granulocytic MDSC has been noted in the bone marrow, spleen, peripheral circulation, and tumors of tumor bearing mice. Successful targeting of MDSC in mice is associated with improved immune responses, delayed tumor growth, improved survival, and increased efficacy of vaccine therapy. By further elucidating mechanisms of MDSC recruitment and maintenance in the tumor environment, strategies could be developed to reverse immune tolerance to tumor. We discuss here what is currently known about MDSC as well as some potential strategies targeting MDSC in the context of our work on pancreatic cancer and recent literature. Due to the number of new reports on MDSC, the most pertinent ones have been selected.

Myeloid-derived suppressor cells--their role in haemato-oncological malignancies and other cancers and possible implications for therapy

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of immature myeloid cells at different stages of maturation that play a role in cancer tolerance and function as an immune-suppressive cell subpopulation. They utilize different mechanisms to block both innate and adaptive arms of anti-tumour immunity, mostly through inhibition of T cell activation and expansion. Further advances in our understanding of this cell population in both murine models and humans has enabled more accurate characterization of their phenotype and the recognition of two major classes of MDSCs: granulocytic and monocytic. Recently, the mechanism of action and clinical importance of MDSCs has been more clearly defined and their interactions with cancer cells have been shown to be among the factors influencing tumour development and induction of tolerance. Most of the earlier studies were performed using murine models, but recent clinical investigations have shown their potential role in human cancers. Here, we review the origin of MDSCs, their mechanisms of action, the factors influencing their production and related signalling pathways. We focus on their role in human solid tumours and haemato-oncological malignancies, and relate to possible novel therapeutic approaches targeting MDSCs which could be considered together with other anticancer strategies in the not too distant future.