Paradoxically, tumor development and progression can be inhibited and promoted by the immune system. After three stages of immune editing, namely, elimination, homeostasis and escape, tumor cells are no longer restricted by immune surveillance and thus develop into clinical tumors. The mechanisms of immune escape include abnormalities in antitumor-associated immune cells, selection for immune resistance to tumor cells, impaired transport of T cells, and the formation of an immunosuppressive tumor microenvironment. A population of distinct immature myeloid cells, myeloid-derived suppressor cells (MDSCs), mediate immune escape primarily by exerting immunosuppressive effects and participating in the constitution of an immunosuppressive microtumor environment. Clinical trials have found that the levels of MDSCs in the peripheral blood of cancer patients are strongly correlated with tumor stage, metastasis and prognosis. Moreover, animal experiments have confirmed that elimination of MDSCs inhibits tumor growth and metastasis to some extent. Therefore, MDSCs may become the target of immunotherapy for many cancers, and eliminating MDSCs can help improve the response rate to cancer treatment and patient survival. However, a clear definition of MDSCs and the specific mechanism involved in immune escape are lacking. In this paper, we review the role of the MDSCs population in tumor development and the mechanisms involved in immune escape in different tumor contexts. In addition, we discuss the use of these cells as targets for tumor immunotherapy. This review not only contributes to a systematic and comprehensive understanding of the essential role of MDSCs in immune system reactions against tumors but also provides information to guide the development of cancer therapies targeting MDSCs.

中文翻译：

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癌症中的骨髓源性抑制细胞：克服肿瘤免疫逃避的治疗靶点

矛盾的是，免疫系统可以抑制和促进肿瘤的发生和进展。经过消除、稳态和逃逸三个阶段的免疫编辑，肿瘤细胞不再受到免疫监视的限制，从而发展成为临床肿瘤。免疫逃逸的机制包括抗肿瘤相关免疫细胞的异常、对肿瘤细胞的免疫抵抗的选择、T细胞运输受损以及免疫抑制性肿瘤微环境的形成。一群不同的未成熟骨髓细胞，即骨髓源性抑制细胞（MDSC），主要通过发挥免疫抑制作用并参与免疫抑制微肿瘤环境的构建来介导免疫逃逸。临床试验发现，癌症患者外周血中MDSCs的水平与肿瘤分期、转移和预后密切相关。而且，动物实验证实，消除MDSCs可以在一定程度上抑制肿瘤的生长和转移。因此，MDSCs可能成为许多癌症免疫治疗的靶点，消除MDSCs有助于提高癌症治疗的反应率和患者的生存率。然而，MDSCs的明确定义以及参与免疫逃逸的具体机制尚缺乏。在本文中，我们回顾了 MDSC 群体在肿瘤发展中的作用以及不同肿瘤背景下参与免疫逃逸的机制。此外，我们还讨论了使用这些细胞作为肿瘤免疫治疗的靶标。该综述不仅有助于系统、全面地了解 MDSC 在免疫系统抗肿瘤反应中的重要作用，而且还为指导针对 MDSC 的癌症疗法的开发提供信息。