High Frequency of Mononuclear Myeloid-Derived Suppressor Cells is Associated with Exacerbation of Inflammatory Bowel Disease

Unraveling the Complexities of Myeloid-Derived Suppressor Cells in Inflammatory Bowel Disease

Myeloid-derived suppressor cells (MDSCs) regulate immune responses in many pathological conditions, one of which is inflammatory bowel disease (IBD), an incurable chronic disorder of the digestive tract and beyond. The pathophysiology of IBD remains unclear, likely involving aberrant innate and adaptive immunity. Studies have reported altered population of MDSCs in patients with IBD. However, their distribution varies among patients and different preclinical models of IBD. The expansion and activation of MDSCs are likely driven by various stimuli during intestinal inflammation, but the in-depth mechanisms remain poorly understood. The role of MDSCs in the pathogenesis of IBD appears to be paradoxical. In addition to intestinal inflammation, suppressive MDSCs may promote colitis-to-colon cancer transition. In this Review, we summarize recent progresses on the features, activation, and roles of MDSCs in the development of IBD and IBD-associated colon cancer.

Myeloid-Derived Suppressor Cell Accumulation Drives Intestinal Fibrosis through mCCL6/hCCL15 Chemokine-Mediated Fibroblast Activation

Intestinal fibrosis, a severe complication of Crohn's disease (CD), is linked to chronic inflammation, but the precise mechanism by which immune-driven intestinal inflammation leads to fibrosis development is not fully understood. This study investigates the role of myeloid-derived suppressor cells (MDSCs) in intestinal fibrosis in CD patients and a 2,4,6-trinitrobenzene sulfonic acid (TNBS)-induced mouse model. Elevated MDSCs are observed in inflamed intestinal tissues prior to fibrosis and their sustained presence in fibrotic tissues of both CD patients and murine models. Depletion of MDSCs significantly reduces fibrosis, highlighting their key role in the fibrotic process. Mechanistically, MDSC-derived mCCL6 activates fibroblasts via the CCR1-MAPK signaling, and interventions targeting this axis, including neutralizing antibodies, a CCR1 antagonist, or fibroblast-specific Ccr1 knockout mice reduce fibrosis. In CD patients with stenosis, human CCL15, analogous to mCCL6, is found to be elevated in MDSCs and activated fibroblasts. Additionally, CXCR2 and CCR2 ligands are identified as key mediators of MDSC recruitment in intestinal fibrosis. Blocking MDSC recruitment with CXCR2 and CCR2 antagonists alleviates intestinal fibrosis. These findings suggest that strategies targeting MDSC recruitment and mCCL6/hCCL15 signaling could offer therapeutic benefits for intestinal fibrosis.

17β-estradiol promotes myeloid-derived suppressor cells functions and alleviates inflammatory bowel disease by activation of Stat3 and NF-κB signalings

Inflammatory bowel disease (IBD) describes a group of clinically common autoimmune diseases characterized by chronic intestinal inflammation, with gender differences in prevalence. Estrogen has been previously shown to exert anti-inflammatory action in IBD development, however, the mechanisms remain obscure. Recent research has revealed that myeloid-derived suppressor cells (MDSCs) play a protective role in IBD pathogenesis. To investigate the molecular mechanisms of estrogen steroid 17β-estradiol (E2) in IBD progression, we established IBD mouse models (DNB-induced) with or without prior ovariectomy (OVX) and E2 implantation. We found that OVX led to worse IBD symptoms and reduced MDSCs frequency, whereas E2 significantly alleviated these effects in vivo. Moreover, in vitro experiments showed that E2 promoted the proliferation and immunosuppressive function of MDSCs through phosphorylation of Stat3 and p65. Mechanistically, E2-mediated Stat3/p65 phosphorylation depends on the interaction between HOTAIR, a long non-coding RNA that are well-known in MDSCs proliferation, and Stat3/p65 respectively. In conclusion, our study revealed that E2 promotes the expansion and immunosuppressive function of MDSCs, and thus diminished the occurrence and development of IBD.

Myeloid-derived suppressor cells in the therapy of autoimmune diseases

Myeloid-derived suppressor cells (MDSCs) are well recognized as critical factors in the pathology of tumors. However, their roles in autoimmune diseases are still unclear, which hampers the development of efficient immunotherapies. The role of different MDSCs subsets in multiple sclerosis, inflammatory bowel diseases, rheumatoid arthritis, type 1 diabetes, and systemic lupus erythematosus displayed different mechanisms of immune suppression, and several studies pointed to MDSCs' capacity to induce T-helper (Th)17 cells and tissue damage. These results also suggested that MDSCs could be present in different functional states and utilize different mechanisms for controlling the activity of T and B cells. Therefore, various therapeutic strategies should be employed to restore homeostasis in autoimmune diseases. The therapies harnessing MDSCs could be designed either as cell therapy or rely on the expansion and activation of MDSCs in vivo, or their depletion. Cumulatively, MDSCs are inevitable players in autoimmunity, and rational approaches in developing therapies are required to avoid the adverse effects of MDSCs and harness their suppressive mechanisms to improve the overall efficacy of autoimmunity therapy.

The paradoxical role of MDSCs in inflammatory bowel diseases: From bench to bedside

Myeloid-derived suppressor cells (MDSCs) are a group of bone marrow derived heterogeneous cells, which is known for their immunosuppressive functions especially in tumors. Recently, MDSCs have receiving increasing attention in pathological conditions like infection, inflammation and autoimmune diseases. Inflammatory bowel diseases (IBD) are a series of immune-dysfunctional autoimmune diseases characterized by relapsing intestinal inflammation. The role of MDSCs in IBD remains controversial. Although most studies in vitro demonstrated its anti-inflammatory effects by inhibiting the proliferation and function of T cells, it was reported that MDSCs failed to relieve inflammation but even promoted inflammatory responses in experimental IBD. Here we summarize recent insights into the role of MDSCs in the development of IBD and the potential of MDSCs-targeted therapy.

Granulocytic myeloid-derived suppressor cells to prevent and treat murine immune-mediated bone marrow failure

Myeloid-derived suppressor cells (MDSCs) are immature myeloid cells that originate in the bone marrow (BM) and have immunoregulatory functions. MDSCs have been implicated in the pathogenesis of several autoimmune diseases but have not been investigated in immune aplastic anemia (AA). We examined the roles of granulocytic-MDSCs (G-MDSCs) in murine models of human AA and BM failure (BMF). As both prophylaxis and therapy, BM-derived G-MDSCs improved pancytopenia and BM cellularity and suppressed BM T-cell infiltration in major histocompatibility complex (MHC)-matched C.B10 BMF mice. These effects were not obtained in the MHC-mismatched CByB6F1 AA model, likely because of MHC disparity between G-MDSCs and donor T cells. Single-cell RNA sequencing demonstrated that G-MDSCs downregulated cell cycle-related genes in BM-infiltrated T cells, consistent with suppression of T-cell proliferation by G-MDSCs through reactive oxygen species pathways. Clearance of G-MDSCs in the MHC-mismatched CByB6F1 model using anti-Ly6G antibody facilitated T cell-mediated BM destruction, suggesting an intrinsic immunosuppressive property of G-MDSCs. However, the same anti-Ly6G antibody in the MHC-matched C.B10 AA model mildly mitigated BMF, associated with expansion of an intermediate Ly6G population. Our results demonstrate that G-MDSC eradication and therapeutic efficacy are immune context-dependent.

Expansion of monocytic myeloid-derived suppressor cells ameliorated intestinal inflammatory response by radiation through SOCS3 expression

Radiation-induced colitis is a common clinical problem after radiation therapy and accidental radiation exposure. Myeloid-derived suppressor cells (MDSCs) have immunosuppressive functions that use a variety of mechanisms to alter both the innate and the adaptive immune systems. Here, we demonstrated that radiation exposure in mice promoted the expansion of splenic and intestinal MDSCs and caused intestinal inflammation due to the increased secretion of cytokines. Depletion of monocytic MDSCs using anti-Ly6C exacerbated radiation-induced colitis and altered the expression of inflammatory cytokine IL10. Adoptive transfers of 0.5 Gy-derived MDSCs ameliorated this radiation-induced colitis through the production IL10 and activation of both STAT3 and SOCS3 signaling. Intestinal-inflammation recovery using 0.5 Gy-induced MDSCs was assessed using histological grading of colitis, colon length, body weight, and survival rate. Using in vitro co-cultures, we found that 0.5 Gy-induced MDSCs had higher expression levels of IL10 and SOCS3 compared with 5 Gy-induced MDSCs. In addition, IL10 expression was not enhanced in SOCS3-depleted cells, even in the presence of 0.5 Gy-induced monocytic MDSCs. Collectively, the results indicate that 0.5 Gy-induced MDSCs play an important immunoregulatory role in this radiation-induced colitis mouse model by releasing anti-inflammatory cytokines and suggest that IL10-overexpressing mMDSCs may be potential immune-therapy targets for treating colitis.

The diverse roles of myeloid derived suppressor cells in mucosal immunity

The mucosal immune system plays a vital role in protecting the host from the external environment. Its major challenge is to balance immune responses against harmful and harmless agents and serve as a 'homeostatic gate keeper'. Myeloid derived suppressor cells (MDSCs) are a heterogeneous population of undifferentiated cells that are characterized by an immunoregulatory and immunosuppressive phenotype. Herein we postulate that MDSCs may be involved in shaping immune responses related to mucosal immunity, due to their immunomodulatory and tissue remodeling functions. Until recently, MDSCs were investigated mainly in cancerous diseases, where they induce and contribute to an immunosuppressive and inflammatory environment that favors tumor development. However, it is now becoming clear that MDSCs participate in non-cancerous conditions such as chronic infections, autoimmune diseases, pregnancy, aging processes and immune tolerance to commensal microbiota at mucosal sites. Since MDSCs are found in the periphery only in small numbers under normal conditions, their role is highlighted during pathologies characterized by acute or chronic inflammation, when they accumulate and become activated. In this review, we describe several aspects of the current knowledge characterizing MDSCs and their involvement in the regulation of the mucosal epithelial barrier, their crosstalk with commensal microbiota and pathogenic microorganisms, and their complex interactions with a variety of surrounding regulatory and effector immune cells. Finally, we discuss the beneficial and harmful outcomes of the MDSC regulatory functions in diseases affecting mucosal tissues. We wish to illuminate the pivotal role of MDSCs in mucosal immunity, the limitations in our understanding of all the players and the intricate challenges stemming from the complex interactions of MDSCs with their environment.

Tim-4 expressing monocytes as a novel indicator to assess disease activity and severity of ulcerative colitis

AIMS

The dysregulation of the immune response has been shown to be involved in ulcerative colitis (UC) pathogenesis. Tim-4 is a potential regulator of the immune system which plays key roles in multiple autoimmune diseases. However, whether it is involved in UC remains unclear. The aim of this research was to determine the expression of Tim-4 on circulating monocytes and its clinical significance in UC patients.

MAIN METHODS

In total, 36 UC patients and 34 healthy controls (HCs) were enrolled in this study. The frequencies of CD14⁺Tim-4⁺ cells, regulatory T cells (Treg) and CD14⁺HLA-DR^-/low myeloid-derived suppressor cells (MDSCs) in the peripheral blood were determined by flow cytometry. Serum IL-6 levels were determined by chemiluminescence immunoassay.

KEY FINDINGS

The percentage of CD14⁺Tim-4⁺ cells was higher in UC patients than in HCs. The frequency of Treg cells was significantly decreased, while that of MDSCs was significantly increased in UC patients. The frequency of CD14⁺Tim-4⁺ cells was significantly elevated in subjects with high severity, high number of defecations per day, high UC disease activity index Mayo score, high IgG, and high levels of inflammatory markers. And the percentages of Tim-4-expressing monocytes were significantly decreased in UC patients that received a 3-week treatment with mesalazine. Furthermore, the frequency of CD14⁺Tim-4⁺ cells was also positively correlated with MDSCs and negatively correlated with Treg cells.

SIGNIFICANCE

CD14⁺Tim-4⁺ cells was elevated in UC patients and could be a novel indicator to assess disease severity and activity of UC.

Myeloid - derived suppressor cells in Type 1 diabetes are an expanded population exhibiting diverse T-cell suppressor mechanisms

Myeloid-derived suppressor cells (MDSC) represent a heterogeneous group of immature myeloid cells with immunoregulatory function in cancer and autoimmune diseases. In humans, two subsets of MDSC were determined based on the characteristic surface markers, monocytic MDSC (M-MDSC) and granulocytic MDSC (G-MDSC). Expansion of MDSC has been reported in some murine models and patients with autoimmune diseases and their immune-suppressive properties were characterized. However, the exact role of MDSC in the pathogenesis of autoimmune diseases is more complex and/or controversial. In type 1 diabetes mellitus (T1D), the increased frequency of MDSC was found in the blood of T1D patients but their suppressor capacity was diminished. In our study, we assessed the role of M-MDSC in the pathogenesis of T1D and showed for the first time the increased frequency of M-MDSC not only in the blood of T1D patients but also in their at-risk relatives compared to healthy donors. T1D patients with inadequate long term metabolic control showed an elevation of M-MDSC compared to patients with better disease control. Furthermore, we described the positive correlation between the percentage of M-MDSC and Th17 cells and IFN-γ producing T cells in T1D patients and their at-risk relatives. Finally, we found that the ability of M-MDSC to suppress autologous T cells is efficient only at the high MDSC: T cells ratio and dependent on cell-cell-contact and TGF-β production. Our data show that the engagement of MDSC in the pathogenesis of T1D is evident, yet not entirely explored and more experiments are required to clarify whether MDSC are beneficial or harmful in T1D.

Myeloid-Derived Suppressor Cell-Derived Arginase-1 Oppositely Modulates IL-17A and IL-17F Through the ESR/STAT3 Pathway During Colitis in Mice

Myeloid-derived suppressor cells (MDSC) play a crucial role in regulating the intestinal immune response during colitis. We previously revealed an essential role of MDSC in promoting TH17 cell polarization, which was found to be arginase-1 (Arg-1)-dependent; however, the underlying mechanism remains obscure. Here we report that percentage of MDSC decreased in Arg^myeKO mice during DSS-induced colitis. IL-17A levels reduced but IL-17F levels increased significantly in the colorectum of Arg^myeKO mice, leading to severe tissue damage and high risk of mortality rate. Activation of estrogen receptor (ESR) increased pSTAT3 level in MDSC and consequently led to elevated percentage of MDSC and more Arg-1 and inducible nitric oxide synthase expression in MDSC. Increased level of IL-17A and reduced level of IL-17F alleviated colitis in mice consequently. Together, these findings demonstrate a protective role of MDSC-derived Arg-1 during colitis after activates ESR/STAT3 signaling in MDSC. High level of Arg-1 favors accumulation of IL-17A, but reduced IL-17F expression in the colorectum of mice and ultimately leading to relief of colitis, indicating a potential clinical impact of MDSC-derived Arg-1 for controlling inflammatory bowel disease.

Myeloid-Derived Suppressor Cells: Ductile Targets in Disease

Myeloid-derived suppressor cells (MDSCs) represent a heterogeneous population of immature myeloid cells with major regulatory functions and rise during pathological conditions, including cancer, infections and autoimmune conditions. MDSC expansion is generally linked to inflammatory processes that emerge in response to stable immunological stress, which alter both magnitude and quality of the myelopoietic output. Inability to reinstate physiological myelopoiesis would fall in an “emergency state” that perpetually reprograms myeloid cells toward suppressive functions. While differentiation and reprogramming of myeloid cells toward an immunosuppressive phenotype can be considered the result of a multistep process that originates in the bone marrow and culminates in the tumor microenvironment, the identification of its driving events may offer potential therapeutic approaches in different pathologies. Indeed, whereas expansion of MDSCs, in both murine and human tumor bearers, results in reduced immune surveillance and antitumor cytotoxicity, placing an obstacle to the effectiveness of anticancer therapies, adoptive transfer of MDSCs has shown therapeutic benefits in autoimmune disorders. Here, we describe relevant mechanisms of myeloid cell reprogramming leading to generation of suppressive MDSCs and discuss their therapeutic ductility in disease.

Immune biomarkers for chronic inflammation related complications in non-cancerous and cancerous diseases

Chronic inflammation arising in a diverse range of non-cancerous and cancerous diseases, dysregulates immunity and exposes patients to a variety of complications. These include immunosuppression, tissue damage, cardiovascular diseases and more. In cancer, chronic inflammation and related immunosuppression can directly support tumor growth and dramatically reduce the efficacies of traditional treatments, as well as novel immune-based therapies, which require a functional immune system. Nowadays, none of the immune biomarkers, regularly used by clinicians can sense a developing chronic inflammation, thus complications can only be detected upon their appearance. This review focuses on the necessity for such immune status biomarkers, which could predict complications prior to their appearance. Herein we bring examples for the use of cellular and molecular biomarkers in diagnosis, prognosis and follow-up of patients suffering from various cancers, for prediction of response to immune-based anti-cancer therapy and for prediction of cardiovascular disease in type 2 diabetes patients. Monitoring such biomarkers is expected to have a major clinical impact in addition to unraveling of the entangled complexity underlying dysregulated immunity in chronic inflammation. Thus, newly discovered biomarkers and those that are under investigation are projected to open a new era towards combating the silent damage induced by chronic inflammation.

Aberrant function of myeloid-derived suppressor cells (MDSCs) in experimental colitis and in inflammatory bowel disease (IBD) immune responses

BACKGROUND AND AIMS

Myeloid-derived suppressor cells (MDSCs) encompass a novel population of suppressor cells and a potential candidate for cell-based therapies in inflammatory diseases. Herein, we investigated their immunomodulatory properties in experimental inflammatory colitis and T cell-mediated immune responses in inflammatory bowel disease (IBD) patients.

METHODS

MDSCs (defined as CD14^-HLA^-DR^-/lowCD33⁺CD15⁺) numbers were determined in peripheral blood (PB) from IBD patients. PB MDSC function was assessed in vitro. Experimental colitis was induced upon 2,4,6-trinitrobenzene sulfonic acid (TNBS) treatment and MDSCs were characterized by flow cytometry. The in vivo suppressive potential of bone marrow (BM)-derived MDSCs (BM-MDSCs) was tested by using both depleting and adoptive transfer strategies.

RESULTS

MDSCs were enriched in the periphery of IBD patients during active disease. TNBS colitis induced amplification of MDSCs, particularly of the granulocytic (Ly6G⁺) subset during the effector phase of disease. Of interest, BM-MDSCs potently suppressed CD4⁺T cell responses under steady state but failed to control colitis-associated immune responses in vivo. Mechanistically, under the colonic inflammatory milieu MDSCs switched phenotype (decreased proportion of Gr1^high and increased numbers of Gr1^low) and downregulated CCAAT/enhancer-binding protein beta (CEBPβ) expression, a critical transcription factor for the suppressive function of MDSCs. In accordance with the murine data, human CD33 ⁺ CD15⁺MDSCs from peripheral blood of IBD patients not only failed to suppress autologous T cell responses but instead enhanced T cell proliferation in vitro.

CONCLUSIONS

Our findings demonstrate an aberrant function of MDSCs in experimental inflammatory colitis and in IBD-associated immune responses in vitro. Delineation of the mechanisms that underlie the loss of MDSCs function in IBD may provide novel therapeutic targets.

Associations between NRAMP1 Polymorphisms and Susceptibility to Ulcerative Colitis/Crohn's Disease: A Meta-Analysis

OBJECTIVE

Multiple environmental and genetic factors contribute to the risks of ulcerative colitis (UC) and Crohn's disease (CD). Several allelic variants have been identified in natural resistance associated macrophage protein 1 (NRAMP1) gene; however, their association with UC/CD remains conflicting. The purpose of this study was to evaluate whether NRAMP1 polymorphisms are associated with the susceptibility to UC/CD.

METHODS

A meta-analysis on the association between the NRAMP1 polymorphisms and susceptibility to UC/CD was performed. Relevant studies were retrieved from the databases. After eligible data were extracted, Mantel-Haenszel statistics and random/fixed effects model were applied to calculate the pooled odds radio (OR) and 95% confidence interval (95% CI).

RESULTS

Seven articles containing 536 UC cases, 997 CD cases, and 1361 controls were collected. No significant association between allele 2 frequency of NRAMP1 and susceptibility to UC/CD was detected in overall population (all p > 0.05). However, increased UC/CD risk for allele 3 was observed in Caucasian population (OR = 1.27, 95% CI = 1.08~1.50, p = 0.04), whereas decreased UC/CD risk was detected in non-Caucasian population (OR = 0.72, 95% CI = 0.60~0.87, p < 0.001), under "allele 3 vs. other alleles" model. Moreover, a significant increase in CD risk for T carrier frequency of -237 C/T (OR = 0.44, 95% CI, 0.26~0.75, p = 0.003) was detected, but not 274 C/T and 1729+55del4 (TGTG) +/del.

CONCLUSIONS

The polymorphism of -237 C/T is related to the risk of CD; and the association of allele 3 with UC/CD risk differs in Caucasian and non-Caucasian population, which might be the potential biomarkers for clinical diagnosis of UC/CD.

Current knowledge on psoriasis and autoimmune diseases

Psoriasis is a prevalent, chronic inflammatory disease of the skin, mediated by crosstalk between epidermal keratinocytes, dermal vascular cells, and immunocytes such as antigen presenting cells (APCs) and T cells. Exclusive cellular “responsibility” for the induction and maintenance of psoriatic plaques has not been clearly defined. Increased proliferation of keratinocytes and endothelial cells in conjunction with APC/T cell/monocyte/macrophage inflammation leads to the distinct epidermal and vascular hyperplasia that is characteristic of lesional psoriatic skin. Despite the identification of numerous susceptibility loci, no single genetic determinant has been identified as responsible for the induction of psoriasis. Thus, numerous other triggers of disease, such as environmental, microbial and complex cellular interactions must also be considered as participants in the development of this multifactorial disease. Recent advances in therapeutics, especially systemic so-called “biologics” have provided new hope for identifying the critical cellular targets that drive psoriasis pathogenesis. Recent recognition of the numerous co-morbidities and other autoimmune disorders associated with psoriasis, including inflammatory bowel disease, multiple sclerosis, rheumatoid arthritis, and systemic lupus erythematosus suggest common signaling elements and cellular mediators may direct disease pathogenesis. In this review, we discuss common cellular pathways and participants that mediate psoriasis and other autoimmune disorders that share these cellular signaling pathways.