Abstract The tumor microenvironment is a complex and heterogeneous
milieu in which multiple interactions occur between
tumor and host cells. Immunosuppressive cells which
are present in this microenvironment, such as regulatory T
(Treg) cells and myeloid-derived suppressor cells (MDSCs),
play an important role in tumor progression, via downregulation
of antitumor responses. MDSCs represent a heterogeneous
group of cells originated from the myeloid lineage
that are in the immature state. These cells markedly accumulate
under pathologic conditions, such as cancer, infection,
and inflammation, and use various mechanisms to inhibit both
adaptive and innate immune responses. These immunosuppressive
mechanisms include deprivation of T cells from essential
amino acids, induction of oxidative stress, interference
with viability and trafficking of T cells, induction of immunosuppressive
cells, and finally polarizing immunity toward a
tumor-promoting type 2 phenotype. In addition to suppression
of antitumor immune responses,MDSCs can also enhance the
tumor metastasis and angiogenesis. Previous studies have
shown that increased frequency of MDSCs is related to the
improve antitumor immune responses as well as increase
the efficacy of immunotherapeutic intervention. In this review,
we will first discuss on the immunobiology of
MDSCs in an attempt to find the role of these cells in
tumor progression and then discuss about therapeutic approaches
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