The morphology and phenotype of granulocytic MDSCs (G-MDSCs or polymorphonuclear (PMN)-MDSCs) and monocytic MDSCs (M-MDSCs) are similar to those of neutrophils and monocytes, respectively. Murine MDSCs are defined as Gr-1⁺CD11b⁺ cells. G-MDSCs and M-MDSCs are defined as CD11b⁺Ly6G⁺Ly6C^lo cells and CD11b⁺Ly6G⁻Ly6C^hi cells, respectively.5 Human MDSCs are extensively described as HLA-DR⁻CD11b⁺CD33⁺ cells. G-MDSCs and M-MDSCs are described as HLA-DR⁻CD11b⁺CD33⁺CD15⁺ cells and HLA-DR⁻CD11b⁺CD33⁺CD14⁺ cells, respectively.6 Intestine-infiltrating MDSCs were first described as CD11b⁺Gr-1⁺ cells with immunosuppressive functions in CD8⁺ T cell-mediated colitis mice.4 Later, colonic G-MDSCs were described as Mac-1⁺Ly6C^intGr-1⁺ cells.7 Colonic M-MDSCs are described as CD11b⁺Ly6C^hiCD103⁻CX3CR1^int or CD11b⁺CD14⁺CX3CR1⁺ cells that produce high levels of IL-12, IL-23, inducible nitric oxide synthase (iNOS), and tumor necrosis factor-α (TNF-α).8 9 MDSCs in the peripheral blood of patients with CAC have been described as Lin^−/loHLA-DR⁻CD11b⁺CD33⁺ cells.10 Human colorectal tumor-infiltrating MDSCs express high levels of CD13 and CD39, and low levels of CD115, CD117, CD124 and programmed death ligand-1 (PD-L1).11 We previously sorted human colorectal tumor-infiltrating CD45⁺HLA-DR⁻CD11b⁺CD33⁺ MDSCs with immunosuppressive functions by flow cytometry.12 Figure 1A illustrates the current understanding of the surface markers on myeloid cells in patients with CRC and the gating strategy for the identification of colorectal-infiltrating myeloid cells.12–15 These markers provide an initial framework and can be complemented by other markers. Human M-MDSCs expressed IL-4Rα.16 Human G-MDSCs express CD62L, CD54, CD63, PD-L1, and lectin-like oxidized low-density lipoprotein receptor-1 (Lox-1).17 In colitis mice and CT-26 tumor-bearing mice, CD49d⁺ MDSCs are phenotypically and morphologically similar to M-MDSCs and can suppress antigen-specific T cell responses. CD49d⁻ MDSCs are similar to G-MDSCs.18 These results suggest that CD49d is a promising marker to distinguish G-MDSCs and M-MDSCs. Therefore, human MDSC subsets should be identified based on multiple phenotypic features.

G-MDSCs are pathologically activated and relatively immature neutrophils that have been implicated in the pathological regulation of CAC. The transition from G-MDSCs to neutrophils is halted in CAC, which results in the continuous accumulation and activation of G-MDSCs. It is difficult to distinguish G-MDSCs from neutrophils only by phenotypic differences, although the classification of G-MDSCs as CD11b⁺Ly6G⁺Ly6C^lo cells in mice and HLA-DR⁻CD33⁺CD11b⁺CD15⁺ cells in humans has been extensively reported. New knowledge of both G-MDSCs and neutrophils provides an opportunity to distinguish these cell types (table 1).

Compared with neutrophils, G-MDSCs exhibit reduced expression of CD16 and CD62L, and increased expression of CD11b, CD66b, CD115, and CD244, and have increased reactive oxygen species (ROS) and arginase 1 (Arg-1) activity.19 Neutrophils have higher levels of lysosomal and proteasomal enzymes and stronger cytotoxicity, lytic activity and phagocytosis than G-MDSCs.20 Furthermore, neutrophils are on top of the erythrocyte fraction, and G-MDSCs are in the peripheral blood mononuclear cell fraction after density centrifugation.6 Finally, the nuclear morphology of neutrophils is hypersegmented, with more than four nuclear lobes (human) or a cloverleaf shape (mouse). G-MDSCs have a horseshoe-shaped or banded-shaped nucleus (human) or a ring-shaped nucleus (mice) (figure 1C).20

MDSCs were once regarded as endogenous antagonists of immune system functionality in mucosal inflammation due to their immunosuppressive effects on effector T cells. MDSCs are recruited and activated in intestinal tissue when challenged with inflammation. However, colonic MDSCs fail to inhibit the inflammatory response and instead promote effector T cell expansion.7–9 IEC-derived and activated immune cell-derived cytokines and gut microbiota-derived factors lead to the recruitment and activation of MDSCs in the intestine (figure 2).22 23 In chronic colitis mice, colonic G-MDSCs acquire stimulatory antigen-presenting functions and induce T cell activation and IL-17 production.7 Adoptively transferred CD11b⁺Ly6C^hi cells are converted into proinflammatory cells and promote intestinal inflammation.8 9 Human HLA-DR^−/loCD33⁺CD15⁺CD14⁻ MDSCs from the peripheral blood of patients with IBD not only fail to suppress the autologous T cell response but also enhance T cell proliferation.24 Furthermore, activated MDSCs inhibit the antigen uptake and processing by DCs and subsequent CD4⁺ T cell proliferation and activation, which leads to inadequate clearance of pathogenic bacteria at sites of bacterial penetration, resulting in a sustained inflammatory stimulus (figure 2).25 26 MDSC-derived ROS are the major inducers of the IEC damage, but MDSC-derived transforming growth factor-β (TGF-β) promotes IEC repair.27 These double effects drive chronic inflammation. Under the colonic inflammatory milieu, MDSCs downregulate the expression of CCAAT/enhancer-binding protein beta (CEBPβ), a critical transcription factor associated with the suppressive function of MDSCs, and secreted increased levels of proinflammatory molecules.24 In addition, MDSC-derived proinflammatory molecules, such as IL-6, TNF-α, granulocyte-macrophage colony stimulating factor (GM-CSF) and C-X-C motif chemokine ligand 1 (CXCL1), promote Th17 cell differentiation and the influx of macrophages and neutrophils, resulting in a strong inflammatory response (figure 2).28 Therefore, MDSCs are the key factors in the continual reinitialization of chronic intestinal inflammation over long periods of time and the targetable link between acute inflammation and chronic inflammation in IBD.

Susceptibility loci for IBD are closely linked to ILC function and the depletion of ILCs reduces the severity of colonic inflammation.29 30 ILC1s and ILC3s are enriched in the intestinal tissue of patients with CD and promote intestinal inflammation by producing interferon-γ (IFN-γ) and IL-17.30 Fontolizumab and secukinumab, two humanized neutralizing monoclonal antibodies (mAbs) against IFN-γ and IL-17, respectively, can effectively inhibit CD activity.31 32 ILCs isolated from patients with active CD showed increased gene expression of IL-23R, which is a key ILC3 cytokine receptor.33 IL-23 regulates the selective accumulation of the ILC3 population, which is characterized by inflammatory cytokine expression and is associated with intestinal inflammation in CD.33 IL-23 produced by MDSCs is a driver of inflammation and tumors.34 Thus, IL-23 from MDSCs may promote the expansion of ILC3s through IL-23R and is expected to be a target for the treatment of IBD and CAC.

Intestinal ILC2s induced strong type 2 innate inflammation by secreting cytokines, such as IL-4, IL-5, and IL-13.35 Previous studies showed that ILC2-derived IL-4, IL-5, and IL-13 were linked to impaired epithelial barrier function in the gut and drove UC.36 37 G-MDSCs effectively suppressed the cytokine production of ILC2s in allergy-induced airway inflammation models, thereby alleviating airway inflammation,38 although the potential relationship between ILC2s and MDSCs in IBD remains largely unknown. Interestingly, ILC2s could prevent acute gastrointestinal graft-versus-host disease following hematopoietic stem cell transplantation, which was associated with ILC2-derived IL-13 promotion of MDSCs and reductions in proinflammatory Th1 and Th17 cells.39 In the context of acute promyelocytic leukemia, activated ILC2s promoted M-MDSC activation through IL-13 secretion and were associated with poor tumor control.40 Thus, ILC2s may limit antitumor immune responses indirectly by modulating MDSC functions. However, ILC-derived IL-22 and amphiregulin play protective roles in IBD by promoting mucus secretion and epithelial cell repair.41 42 Thus, the crosstalk between MDSCs and ILCs plays an important and complex role in the pathogenesis of IBD. There are more regulatory mechanisms of MDSCs in the pathogenesis of CAC than those of ILCs, although additional regulatory effects of ILCs in cancer are being discovered. Investigating the relationship between ILCs and MDSCs will help clarify the regulatory mechanism of ILCs and MDSCs in IBD, which could provide therapeutic benefit in the treatment of IBD and CAC.

There are already abundant data confirming the importance of the gut microbiota in the pathogenesis of IBD and CAC by inducing host immune response disturbance and chronic inflammation. Some pathogenic microbiota, such as Mycobacterium avium subspecies paratuberculosis,43 Cytomegalovirus,44 and adherent-invasive Escherichia coli,45 participate directly in the pathogenesis of IBD. Furthermore, reduced microbial diversity is an important reason for IBD. The loss of Bacteroides suppresses the conversion of non-digestible dietary fiber into short-chain fatty acids.46 A reduction in Clostridium enhances mucosal permeability, which results in the exposure of antigens and bacterial Toll-like receptor ligands, thereby activating pathogenic immune responses.47 Fecal microbiota transplantation can increase fecal microbial diversity in patients with active UC and is a promising treatment to induce remission.48 Interestingly, IBD-associated microbiota can induce the accumulation and proinflammatory functions of MDSCs. The combined action of enterotoxigenic Bacteroides fragilis and IL-17 on IECs promoted the differentiation of proinflammatory M-MDSCs.49 This may explain why bone marrow-MDSCs potently suppress CD4⁺ T cell responses but fail to control colitis-associated immune responses in vivo. Bone marrow cells incubated with Candida tropicalis exhibited MDSCs features.50 Mice deficient in fungal killing exhibit dysbiosis and have increased susceptibility to colitis and CAC.50 The role of the gut microbiota in shaping MDSC phenotype and function remains to be further studied. The important task is to identify the unique microbiota that drives IBD and the relationship of the gut microbiota with myeloid cells, which could help in the development of personalized intervention strategies that correct abnormalities and induce sustainable treatment responses in patients with IBD.

Under CAC conditions, stromal cell-derived and tumor-derived factors mobilize myeloid progenitors to develop the phenotypic features of MDSCs and acquire suppressive activity toward immune cells through multiple mechanisms. CD8⁺ cytotoxic T lymphocytes (CTLs) are central players in controlling cancer.63 MDSCs predominantly suppress CTLs via effector molecules such as Arg-1, ROS, and nitric oxide (NO). Peroxynitrite formed by the cooperative activity of ROS and NO leads to the nitration of tyrosines in the T cell receptor (TCR)-CD8 complex, which disrupts the conformational flexibility of the TCR-CD8 complex. The damaged TCR-CD8 complex cannot interact with peptide-loaded major histocompatibility complex I (MHC-I), which results in the unresponsiveness of CD8⁺ T cells to antigen-specific stimulation.64 65 MDSCs directly cleave CD62L on the surface of naïve T cells by expressing A disintegrin and metalloproteinase domain 17 (ADAM17), which suppresses naïve T cell differentiation into CTLs (figure 4).66 MDSCs also induce regulatory T cell (Treg) expansion by producing TGF-β.67 Tumor-infiltrating MDSCs are exposed to the hypoxic microenvironment, which results in an increase in hypoxia-inducible factor 1α (HIF-1α).12 HIF-1α promoted the production of Arg-1 and iNOS and the upregulation of inhibitory V-domain Ig suppressor of T cell activation and PD-L1, which promoted MDSC-mediated T cell suppression.68 69 The hypoxic microenvironment causes the activation of CD45 protein tyrosine phosphatases, which results in the downregulation of STAT3 activity and promotes M-MDSC differentiation to tumor-associated macrophages.70 These alterations result in the potent non-specific immunosuppressive activity of MDSCs within the tumor.

MDSCs have been shown to play pivotal and intricate roles in promoting CAC metastasis. Great efforts focusing on the regulatory network of MDSCs in CAC have been made. We will discuss some new advances in the field, documenting an increase in the sphere of influence of MDSCs (figure 4).

Epithelial-mesenchymal transition (EMT) plays pivotal roles in CAC metastasis. The TGF-β/Smad signaling pathway is an essential driver of EMT. In a model of uveal melanoma, G-MDSCs are recruited to primary tumors by CXCL5 and then induce EMT by the TGF-β, epidermal growth factor, and hepatocyte growth factor signaling pathways, which promote cancer cell dissemination.71 MDSC-derived matrix metallopeptidase 9 (MMP9) increases the bioavailability of vascular endothelial growth factor and promotes angiogenesis, which was crucial in CAC metastasis. The promotion of tumor metastasis by forming a premetastatic niche (PMN) is widely accepted. In CRC liver metastasis mice, MDSCs promote liver-specific metastasis by promoting PMN formation.10 In other tumor models, MDSC-derived prometastatic proteins such as Bv8, MMP9, S100A8/9, TGF-β, and inhibitory molecules participate in multiple stages of PMN formation and evolution by increasing vascular permeability and the degradation of tight junction proteins, suppressing the immune response, and promoting the formation of an inflammatory microenvironment.72 Our previous research showed that G-MDSCs promoted CAC cell stemness through exosomal S100A9.12

CAC and sporadic colorectal cancer (sCRC) are the two major forms of CRC. Chronic inflammation and immune dysregulation predispose IECs to dysplasia and eventually lead to the development of CAC in patients with IBD, whereas the accumulation of mutations in oncogenes and tumor suppressor genes drives the initiation of sCRC. The unique initiation mechanisms lead to the unique and complex microenvironment of CAC. First, sustained chronic inflammation leads to DNA damage that exceeds the capacity of DNA repair, which is the major mechanism responsible for the microsatellite instability (MSI) phenotype.73 MSI is more likely to occur in patients with CAC than in patients with sCRC (approximately 50% compared with 15%).73–77 The microenvironment of MSI⁺ CRC contains strong Th1 and CTL components. The increased number of tumor ‘neoantigens’ created by the chronic inflammation-driven high mutational load may be an important factor.78 Multiple immune checkpoints, such as cytotoxic T-lymphocyte associated protein 4 (CTLA-4), programmed cell death protein 1 (PD-1), and PD-L1, are highly elevated in MSI⁺ CRC relative to microsatellite stability (MSS) CRC.79 Additionally, chronic inflammation damages epithelial integrity and increases intestinal permeability in patients with CAC, which allows luminal microbiota to freely enter the lamina propria (LP) and promote tumor progression.80

Persistent chronic inflammation plays an important role in the initiation of CAC. Anti-inflammatory treatment is an important means to prevent the development of CAC. In experimental mouse models, a variety of anti-inflammatory agents, such as zileuton,81 infliximab,82 and omeprazole,83 have been suggested to prevent the occurrence of CAC. Earlier and intensive anti-inflammatory therapy should be considered for either mitigating clinical courses or preventing the development of CAC in high-risk patients with IBD.

Relapse and intolerance to existing drugs, such as antibiotics, corticosteroids, and aminosalicylate, always occur during the treatment of IBD. Enormous efforts have been made to develop new treatment strategies based on immunosuppressive interventions. MDSCs are one cell type with a well-recognized role in limiting immune reactions and play an important role in the response to IBD treatment. Dexamethasone suppresses HIF-1α-dependent glycolysis in MDSCs through glucocorticoid receptor signaling and thus promotes the immunosuppressive activity of MDSCs.84 INK128, an mTOR kinase inhibitor, is in clinical development. In dextran sodium sulfate (DSS)-induced murine colitis, INK128 can maintain the immature state of MDSCs by elevating S100A8/9 expression and reducing the production of inflammatory cytokines.85 Glatiramer acetate enhances IL-10 and TGF-β secretion while reducing IL-23 and IL-6 secretion from MDSCs via the recognition of paired Ig-like receptor B, thus favoring conditions that suppress Th17 maturation but enhance Treg induction.86 Atorvastatin promotes the expansion of G-MDSCs, which suppress T cell responses via NO production. Transfer of these G-MDSCs attenuates chronic colitis.87 In an IBD model induced by the bacterium Helicobacter hepaticus, oral administration of diallyl trisulfide reduces colon inflammation by limiting the recruitment of G-MDSCs in the colon.88 GSK343, an inhibitor of enhancer of zeste homolog 2 (EZH2), shows beneficial effects on DSS-induced colitis by increasing the number of MDSCs in the LP.89 Therefore, the immunosuppressive effect of MDSCs plays an important role in limiting IBD-related inflammation and may be used as an indicator to assess the response to IBD therapeutic strategies.

Indeed, in addition to chronic inflammation, the initiation of CAC is also related to cumulative genetic changes, abnormal immune regulation, and intestinal microfloral imbalance. Anti-inflammatory preparations-mediated MDSC expansion is also likely to shape CAC. MDSC-derived ROS participate in CAC initiation by mediating proliferation signals and DNA damage in IECs, which promotes the transition from inflammation to dysplasia.60 MDSC-derived S100A8/9 activated β-catenin via RAGE, which initiates dysplasia in IECs.54 Furthermore, MDSCs suppress the body’s immune response, which is beneficial for abnormally hyperplastic IECs to escape immune surveillance. MDSC-mediated chronic inflammation and an immunosuppressive microenvironment are suitable for the settlement and survival of tumor cells.7 Persistent application of antibiotics and glucocorticoid drugs leads to imbalance in the gut microbiota, which is a key event leading to chronic tissue injury and CAC initiation. Therefore, anti-inflammatory treatment is expected to become a specific preventive measure for CAC. However, developing true anti-inflammatory treatments to prevent CAC remains inherently challenging due to anti-inflammatory-related side effects and potential risks.

The majority of chemotherapy drugs have been shown to reduce the tumor infiltration of MDSC (table 2). Sunitinib potently decreased MDSC accumulation and rescued T cell inhibition in colon cancer cell-bearing mice.90 MDSCs are the major components of the tumor microenvironment in both murine models of patients with colon cancer and CRC, although the microenvironments are different. Sunitinib-mediated MDSC elimination is beneficial in restoring antitumor immunity in patients with CAC. However, sunitinib malate combined with 5-fluorouracil/leucovorin/irinotecan (FOLFIRI) chemotherapy did not significantly improve progression-free survival in patients with metastatic colorectal cancer (mCRC) compared with that of FOLFIRI alone.91 The sensitivity of CRC cells themselves may be the main reason for the failure of sunitinib in clinical trials. Additionally, in patients with CAC, the microenvironment in which MDSCs exist is more complex due to the microbiota, dietary components and metabolites. In colon cancer cell-bearing mice, both gemcitabine and oxaliplatin induced protective antitumor immunity by eliminating MDSCs.92 93 5-Fluorouracil (5-FU) has a good effect on MDSC depletion. Bevacizumab in combination with 5-fluorouracil/leucovorin calcium/oxaliplatin (FOLFOX) or leucovorin calcium/5-fluorouracil (LV5FU2) and anakinra could decrease the number of G-MDSCs or M-MDSCs in patients with mCRC.94 95 In CT-26 metastatic mice, 5-fluorocytosine in combination with Toca 511 significantly decreased the number of MDSC in both the liver and brain in mCRC mice.96 A phase I study of Toca 511 and Toca FC in solid tumors, including mCRC, has been completed and showed a promising novel treatment strategy.97 Gemcitabine and 5-FU activate the pyrin domain containing-3 protein (NLRP3)-dependent caspase-1 activation complex in MDSCs, resulting in the production of IL-1β, which limits the antitumor efficacy. Gemcitabine and 5-FU exert an improved antitumor effect when combined with an IL-1 receptor antagonist.98 In murine mammary cancer models, anthracyclines not only eliminated peripheral and intratumoral MDSCs but also caused a shift toward a more differentiated phenotype featuring upregulated Gr-1 and downregulated CD11b in MDSCs.99 100 Thus, anthracycline may have synergistic effects with other anti-CAC agents by effectively depleting MDSCs, which provides a promising strategy for potential CAC therapy.

Antibodies have been found to block the expansion of MDSCs in CRC (table 2). Preclinical studies demonstrated that blocking semaphorin 4D (SEMA4D) inhibited MDSC expansion in patients with CRC.101 Phase I clinical studies of anti-SEMA4D mAb (VX15/2503) in mCRC are underway (NCT03373188). In MCA-26-bearing mice, blocking the stem cell factor (SCF) receptor–SCF interaction by anti-cKit (anti-cellular kit proto-oncogene) mAbs significantly reduced MDSC expansion and restored the proliferative responses of tumor-infiltrating T cells.102 An anti-chemokine (C-C motif) receptor 2 (CCR2) antibody blocked radiation-induced M-MDSC infiltration.103 In CT-26-bearing mice, the oxford 40 (OX40) agonist antibody significantly decreased the expression of TGF-β in MDSCs.104

MSI is a main type of genomic instability factor. Carcinogenesis and malignant potential in CAC are closely associated with MSI caused by TGFβRII mutations and hMLH1 hypermethylation. Chronic inflammation and folate deficiency lead to high frequencies of MSI in patients with IBD and CAC. Brentnall et al 73 detected MSI in 46% of high-grade dysplasias, in 40% of carcinomas, and in 50% of non-neoplastic mucosa. Ishitsuka et al 76 found MSI in 8% of dysplasias, in 50% of carcinomas, and in 9% of non-neoplastic mucosa from patients with UC who had long durations of neoplasms. Schulmann et al 77 detected MSI in 67% of high-grade dysplasias and in 67% of carcinomas. Therefore, approximately half of the CAC cases are MSI-H. MSI-H⁺ is detected in only approximately 15% of sCRC cases.73 75 MSI-H tumors selectively upregulate the expression of various immune checkpoint factors, such as PD-1, PD-L1, and CTLA-4.79 Humanized anti-PD-1/PD-L1 mAbs have shown promising results in patients with CRC, and the clinical efficacy is especially significant in MSI⁺ CRC.105 106 Therefore, CAC is more likely to respond effectively to immunotherapy than sCRC, although this has not been analyzed separately. PD-1/PD-L1 mAbs are another available drug for CAC immunotherapy, although the exact clinical efficacy needs to be further defined. It is worth noting that non-neoplastic mucosa and high grade-dysplasia in CAC have also been found to have MSI. Early application of anti-PD-1/PD-L1 mAbs may be beneficial for either mitigating clinical courses or preventing the ultimate development of CAC in high-risk patients with IBD.

The alternative strategy to suppress MDSCs is to inhibit their effector molecules and catabolic enzymes (table 2). In CAC mice, Arg-1 and iNOS can be blocked by nitroaspirin and phosphodiesterase-5 inhibitors.107 In MC-38-bearing mice, both triterpenoids and etomoxir dampened the immunosuppressive effects of MDSCs by reducing ROS secretion.108 109 In APC^min/+ mice and MC-38-bearing mice, GSK872 inhibited MDSC to produce IL-23, IL-1β, and COX-2, which inhibited tumor growth.110 Metformin downregulated the inhibitory functions of G-MDSCs by reducing STAT3 phosphorylation levels in MC-38-bearing mice.111 In addition, fluconazole, trefoil factor 2, and an EZH2 inhibitor have been found to suppress CAC susceptibility by decreasing MDSC accumulation.22 50 89 In CT-26-bearing mice, Juglone reduced the accumulation of MDSCs and impaired the immunosuppressive functions of MDSCs.112 In CAC mice, hyperoxia decreased G-MDSC exosome production by inhibiting HIF-1α-dependent Rab27a expression, which suppressed CAC susceptibility.12

Overall, MDSC infiltration is a common feature in CAC models or patients. Various strategies targeting MDSCs may be beneficial in restoring antitumor immunity in CAC and are emerging opportunities for enhancing the effectiveness of anti-CAC therapy. It is noteworthy that the experimental conditions are different in colon cancer cell-bearing mice, chemically induced CAC mice, and patients with CAC and sCRC. The characteristics of MDSCs in each organ could be different. In the bone marrow (BM) and spleen, the major role of MDSCs is immune suppression. These cells may play a protective role in IBD. MDSCs existing underneath the intestinal mucosal surface have multiple functions due to the complex intestinal microenvironment. There are close and complex relationships between MDSC characteristics and the microbiota and dietary components. Research in this area is essential for the precise treatment of CAC, and the exact effect of these factors on CAC treatments based on MDSCs remains to be further studied.