A main feature of MDSCs is immune suppression activity, which are important negative regulators of host innate immunity in tumor microenvironment. MDSCs utilize multiple suppressive mechanism to inhibit CD8${}^{+}$ T, DC, and natural killer cells (NK) through arginase 1 (ARG1), inducible nitric oxide synthase (iNOS), reactive oxygen species (ROS) and so on.

One of the suppressive mechanisms attributed to MDSCs is the inhibition of T cell activation and proliferation. Different subsets of MDSCs might use different mechanisms by which to suppress T-cell. L-arginine (L-Arg) is a conditionally necessary amino acid which serves as a substrate for two enzymes: iNOS and ARG. Lack of L-Arg blocks T-cell proliferation and decreases expression of CD3$\zeta$ chain and interferon $\gamma$ (IFN$\gamma$) production [46, 47, 48]. Recent findings indicate that both MDSCs subsets express ARG1, which converted L-Arg into urea and L-ornithine. In addition, there are substantial differences of depriving L-Arg by MDSCs between mice and human. In murine MDSCs, an increased uptake and intracellular degradation of L-Arg is detected. In human MDSCs, enhanced ARG expression is found in the circulation [49]. Furthermore, multiple studies indicate that the PMN-MDSCs express higher level of ARG than M-MDSCs [50, 51, 52]. In addition, the expression of ARG1 is regulated by inflammatory cytokine and tumor cell-derived factors. In lung cancer, head and neck tumor, colon carcinoma and renal carcinoma, tumor cells derived cyclooxygenase-2 (COX2) induces and maintains ARG1 production, and PGE2 induces ARG1 expression in MDSCs by signaling through the E-prostanoid (EP) 4 receptor [53]. Increased stress-induced activation of $\beta$2-AR signaling in MDSCs modulates the expression of ARG1, which is dependent upon STAT3 phosphorylation [54]. In hepatocellular carcinoma (HCC), wild-type hepatic stellate cells (HSCs)-induced MDSCs expressed increased level of iNOS and ARG1 compared with MDSCs induced by IL-6-deficient HSCs in vitro [55]. Additionally, IL-6/IL-8-ARG1 axis of CD45${}^{+}$CD33${}^{\text{low}}$CD11b${}^{\text{dim}}$ MDSCs in human gastric cancer suppress CD8${}^{+}$ T cell activity [51].

Elevated iNOS expression is a hallmark of MDSCs in tumor-bearing condition. Only M-MDSCs express high level of iNOS, which metabolize L-Arg into NO. Both T-cell receptor and STAT1 are nitrated by MDSCs-produced NO, which results in T-cell activation inhibition and antitumor immune response reduction [56]. HIF-1$\alpha$ is reported to be associated with increased activity of ARG1 and iNOS in MDSCs, leading to strong inhibition of T-cell functions in the tumor microenvironment [57]. In tumor-induced MDSCs, iNOS expression is enhanced by SETD1B, which regulates the trimethylation of histone H3 lysine 4 (H3K4Me3) at the nos2 promoter [56]. Moreover, in ovarian cancer patients, IL-6 and IL-10-driven STAT3 activation upregulates the expression of ARG1 and iNOS in induced M-MDSC [58]. CBP/EP300-BRD pathway maintain the immunosuppressive activity through STAT pathway-related genes and the expression of Arg1 and iNOS [59]. Clearly, the increased STAT3 and NADPH oxidase activity of PMN-MDSCs subset result in high release of ROS but low NO release. Nevertheless, the M-MDSCs subset expresses high levels of STAT1 and iNOS, and enhanced level of NO but low level of ROS are produced [60]. MDSCs-derived ROS and peroxynitrite, which are the product of the reaction of ROS with NO, modify T cell receptor (TCR) and CD8 molecules. The modified TCR and CD8 molecules can not bind phosphorylated MHC, leading to the antigen-specific tolerance of CD8${}^{+}$ T cells [61].

MDSCs can also exert their immunosuppression by inducing regulatory T cells through cytokine [62]. For example, in tumor-bearing host, Gr-1${}^{+}$CD115${}^{+}$ MDSCs mediate the development of Treg cell through a combination of multiple pathways dependent on TGF-$\beta$, IL-10, and cell-cell contact [63]. miR-130a and miR-145 directly target TGF$\beta$ receptor II (T$\beta$RII) and are down-regulated in the Gr-1${}^{+}$CD11b${}^{+}$ immature myeloid cells, leading to increased T$\beta$RII, and increased response to TGF$\beta$ [64]. In CD33${}^{+}$ MDSCs, the indoleamine 2,3-dioxygenase (IDO) expression is positively related with Foxp3${}^{+}$Tregs, which were all correlated with advanced clinical stage prior to neoadjuvant chemotherapy in tumor tissues [65].

In addition to inhibiting T cells, MDSCs can also regulate the functions of other immune cells, including NK cells, macrophages, and DCs. NK cells play a crucial role in anti-tumor immunity because of their innate ability to distinguish malignant cells from normal cells. In head and neck cancer model, PMN-MDSCs suppress NK-cell function through TGF$\beta$ and production of H${}_2$O${}_2$ [66]. In vitro, binding of PGE2 to EP2 and EP4 receptors on M-MDSCs activates the p38MAPK/ERK pathway and elevates the secretion of TGF$\beta$ as a result. Therefore, PGE2-treated M-MDSCs potently suppress NK-cell activity through production of TGF$\beta$ [67]. In syngeneic orthotopic mammary cancer model, the cytotoxicity of NK cells is significantly decreased in the presence of MDSCs in pre-metastatic niche, resulting in a reduced anti-tumor response and an increased successful metastasis in the secondary organs [29]. Furthermore, in addition to decrease cytotoxicity, MDSCs are reported to inhibit NKG2D expression and IFN-$\gamma$ production of NK cells both in vivo and in vitro [68]. However, a study reports that intraperitoneal polyinosinic:polycytidylic acid (poly I:C) treatment in B16-bearing mice induces MDSC activation, driving CD69 expression and IFN-$\gamma$ production in NK cells [69], implying that the effect of MDSCs to NK cells is dependent on stimulating factors. In addition, DCs are a critical component of immune responses in cancer which cross-present tumor-associated antigens, and PMN-MDSCs are reported to block the cross-presentation by DCs, which is dependent on myeloperoxidase (MPO) [70].

Last but not least, HIF-1$\alpha$ can also enhance the suppressive activity of MDSCs by inducing expression of programmed death-ligand 1 (PD-L1), leading to reduced production of IL-2 and decreased proliferation of cytotoxic T cells [71, 72].

Recent findings revealed that type I interferons (IFN1) receptor signaling can also restrict acquisition of suppressive activity through an universal mechanism. In G-MDSCs, the downregulation of IFN1 signaling activates the PI3K-Akt/mTOR pathway through SOCS1 downregulation [73], and the downregulation of IFNAR1 dependent on tumor-derived factors-driven p38 kinase leads to an overwhelmed IFN1 pathway in myeloid cells during tumorigenesis, leading to the acquirement of immune-suppressive activities [74]. In addition, through type I IFN signaling, DNMTi 5-azacytidine (AZA) can reduce the percentage of MDSCs in the tumor microenvironment [75].

An inflammatory microenvironment is implicated as a contributory factor to tumor development and metastasis in many cases. MDSCs promote the inflammatory microenvironment formation in pre-metastatic niche. During inflammation, S100A8/A9 is actively released and plays an important role in regulating the inflammatory response [76]. In cancer, S100A8/A9 is reported as a “soil signal” to attract cancer cells that with TLR4, and a study showed that abundant production and release of S100A8/A9 in the M-MDSCs is detected in the pre-metastatic region [77]. Our work showed that the recruited M-MDSCs in pre-metastatic niche produce IL-1$\beta$, which is a hallmark of inflammation, thereby increasing the expression of E-selectin and contributing to the arrest of tumor cell on endothelial cells [23]. Another study showed that accompanied with the recruitment of CD11b${}^{+}$Gr1${}^{+}$ MDSCs, the proinflammatory cytokines, such as IL-1$\beta$, monocyte chemotactic protein-1 (MCP1), stromal cell derived factor 1 (SDF-1), and macrophage-derived chemokine are also significantly elevated. Ex vivo coculture of lung single-cell suspension with CD11b${}^{+}$Gr-1${}^{+}$ MDSCs significantly upregulated the production of basic fibroblast growth factor (bFGF), insulin-like growth factor-I (IGFI), IL-5, macrophage-derived chemokine, SDF-1, MMP9 and VEGFR1. This work suggested that the CD11b${}^{+}$Gr-1${}^{+}$ MDSCs is likely induce an inflammatory microenvironment in the pre-metastatic lung [78].

Except for the recruited MDSCs induce the inflammatory responses in pre-metastatic niche, MDSCs development and activation are also influenced by inflammation. In murine melanoma model, upregulated inflammatory factors such as IL-1$\beta$, GM-CSF, and IFN-$\gamma$ are accompanied with MDSC recruitment and increased immunosuppressive activity, which correlated with tumor progression. Upon manipulation with the phosphodiesterase-5 inhibitor sildenafil, reduced levels of numerous inflammatory mediators (e.g., IL-1$\beta$, IL-6, VEGF, S100A9) are detected, which are in association with decreased MDSC amounts and immunosuppressive function [79]. Besides tooth loss, periodontitis can also increase the patient’s risk for multiple disease [80]. It is reported that periodontal inflammation promotes metastasis of breast cancer through MDSCs recruitment, which is partially dependent on pyroptosis-induced multiple inflammatory factors and chemokines generation, such as IL-1$\beta$, CCL2, CXCL5 and CCL5 in the early steps of metastasis [81]. The pro-inflammatory cytokines S100A8 and S100A9 as well as these two factors-induced SAA protein and recruited CD11b${}^{+}$ myeloid cells by SAA together contribute to the development of pre-metastatic niches in the lung [82].

Neoangiogenesis and high vascular permeability are effective methods for tumor metastasis. MDSCs are defined as one of major contributors to stimulate angiogenesis and induce high vascular permeability in pre-metastatic niche. Angiogenesis is considered as a hallmark of cancer and an imperative process for tumor growth and metastatic dissemination [83]. Multiple investigations have been reported to illuminate the contribution of MDSCs to angiogenesis. It is well known that proteolysis mediated by matrix metalloproteinases (MMPs) promotes angiogenesis and inflammation in the tumor microenvironment. MDSCs can produce high levels of MMPs, such as MMP14, MMP13, MMP2 and MMP9, to promote angiogenesis and accelerate tumor neovasculature [83, 84]. In addition, CD11b${}^{+}$Gr1${}^{+}$ MDSCs can also acquire endothelial cell properties or being incorporated into the vessel wall directly to contribute to tumor angiogenesis in the tumor microenvironment [85]. The activated MDSCs induce the production of VEGF and FGF2 so as to promote tumor angiogenesis [86]. Bv8 protein, which is produced by G-CSF-mobilized Ly6G${}^{+}$Ly6C${}^{+}$ cells, is implicated in angiogenesis and mobilization of myeloid cells. Anti-Bv8 antibody significantly decreases lung metastasis [39]. In response to the melanoma-derived exosomes, the MDSCs are reprogrammed into a pro-vasculogenic phenotype that is positive for c-kit, the receptor tyrosine kinase Tie2 and Met [87]. In the models of breast carcinoma, the MDSCs recruited to the pre-metastatic lungs appear to be a angiogenic switching through secreting several proangiogenic factors in tumors, including IL-1$\beta$ and MMP-9 [88]. Most cancers will appear organ-specific metastasis, a process also considered as “organotropism”. This process is related with multiple factors, including the tumor-intrinsic properties and their interaction with unique features of host organs. Studies show that the organotropic metastasis of tumor cells is mediated by tumor-derived extracellular vesicles. In addition to the angiogenesis, the vascular hyperpermeability in pre-metastatic niche contributes to subsequent tumor cell homing in lung vessels. The CCR2-CCL2 system has been reported to induce the abundant secretion of SAA3 and S100A8 to increase the permeability of vessels [89], which are also secreted by recruited MDSCs in pre-metastatic niche [77]. The interaction of MDSCs with epithelial cells increase the permeability in blood vessel [90], although the exact mechanism remains to be elucidated.

The recruitment and survival of disseminated circulating tumor cells in target organs are essential for successful metastasis. E-selectin mediates adhesion of circulating cells to the vessel surface via interaction with its ligand present on the circulating cells [91]. In the pre-metastatic lung, the recruited M-MDSCs ahead of tumor cells increase the expression of E-selectin through IL-1$\beta$, and thereby promote the arrest of tumor cells on endothelial cells. Depletion of M-MDSCs in the pre-metastatic lungs results in reduced E-selectin expression [23].

The survival of tumor cells in the hostile distant organs rate-limited the successful metastasis in cancer. CCL9 was highly induced in Gr1${}^{+}$CD11b${}^{+}$ immature myeloid cells and in pre-metastatic lung in tumor-bearing mice. It promotes tumor cell survival by increasing phospho-AKT (P-AKT) and BLC-2, in addition to decreasing the expression of poly (ADP-ribose) polymerase (PARP) [92]. PMN-MDSCs isolated from pre-metastatic livers of NSG mice bearing HCT-116 cecal tumors or LS-174T cecal tumors inhibits HCT-116 cell apoptosis, and the PMN-MDSCs enhancement of tumor cell survival is ratio dependent [10]. Tissue factor (TF) recruited CD11b${}^{+}$ macrophages enhances the survival of tumor cells after arrest in the lung, and impairment of macrophage function decreases tumor cell survival [93].

Based on the crucial function of MDSCs in the formation of pre-metastatic microenvironment, targeting MDSCs could be an effective means to hamper the establishment of pre-metastatic microenvironment. Multiple strategies have been proposed to target MDSCs which is including but not limited in the pre-metastatic niche. Studies have been conducted to explore drugs that focused on inhibiting the immunosuppressive activity of MDSCs. Sunitinib is a small molecule synthetic dihydroindole receptor tyrosine kinase (RTK) inhibitor. In various types of cancer patients, Sunitinib treatment can weaken the immunosuppressive activity of M-MDSC by reducing Arg-1 and pSTAT3 [94]. Another drug targeting MDSC is 5 phosphodiesterase (PDE5) inhibitor, which has been proved to reduce IL-4R$\alpha$, pSTAT6 and Arg-1, thereby effectively blocking the function of MDSC [95, 96]. Drugs targeting corresponding chemokines and receptors have also been developed. SX-682, a small molecule inhibitor targeting CXCR1 and CXCR2, significantly reduces the invasion of CXCR2+PMN MDSCs to tumors [97]. Blocking the interaction of CCL2-CCR2 by using carlumab or CCR2 antagonist PF-04136309 has been proved to have potential anti-tumor effect in several preclinical cancer models [98, 99]. Maraviroc, a CCR5 antagonist, effectively block the CCL5/CCR5 axis and exhibit a significant anti-tumor effect in a phase I clinical trial [100]. CSF-1R inhibitors, such as IMC-CS4, GW2580, PLX3397, AMG820, imatizumab, and pecidatinib, also show an anti-tumor effect by inhibiting the recruitment of M-MDSC [101, 102]. Another way targeting MDSCs is the elimination of MDSCs. The traditional chemotherapy drugs such as gemcitabine and 5-fluorouracil can induce the apoptosis of MDSCs and reduce the number of MDSCs in the tumor microenvironment [103, 104]. In addition, an anti-Gr1 antibody (RB6-8C5) is used to clear MDSCs in mice [105].