Blockade of Myd88 signaling induces antitumor effects by skewing the immunosuppressive function of myeloid-derived suppressor cells

MDSC suppresses T cell antitumor immunity in CAC via GPNMB in a MyD88-dependent manner

BACKGROUND

Myeloid-derived suppressor cells (MDSCs) played an essential role in tumor microenvironment to suppress host antitumor immunity and help cancer cells escape immune surveillance. However, the molecular mechanism behind tumor evasion mediated by MDSCs is not fully understood. Glycoprotein nonmetastatic melanoma protein B (GPNMB) is considered to associate with tumor initiation, metastasis and angiogenesis. Blocking GPNMB function is a potentially valuable therapy for cancer by eliminating GPNMB+ MDSCs. Our previous study has proved that blockage the MyD88 signaling with the MyD88 inhibitor, TJ-M2010-5, may completely prevent the development of CAC in mice, accompanying with downregulation of GPNMB mRNA in the inhibitor-treated mice of CAC.

METHODS

We here focus on the underlying the relationship between GPNMB function and MyD88 signaling pathway activation in MDSCs' antitumor activity in CAC.

RESULTS

CAC development in the mouse model is associated with expanded GPNMB+ MDSCs by a MyD88-dependent pathway. The GPNMB expression on MDSCs is associated with MyD88 signaling activation. The inhibitory effect of MDSCs on T cell proliferation, activation and antitumor cytotoxicity in CAC is mediated by GPNMB in a MyD8-dependent manner.

CONCLUSION

MyD88 signaling pathway plays an essential role in GPNMB+ MDSC-mediated tumor immune escape during CAC development and is a promising focus for revealing the mechanisms of MDSC that facilitate immunosuppression and tumor progression.

Blocking MyD88 signaling with MyD88 inhibitor prevents colitis-associated colorectal cancer development by maintaining colonic microbiota homeostasis

Certain intestinal microbiota alterations appear to positively correlate with tumorigenesis of CAC due to the disruption of the balance between the host and microorganisms. It is proven that blocking MyD88 signaling can prevent colitis-associated colorectal cancer (CAC) development in mice. We are aim to reveal the role of MyD88 signaling of maintaining colonic microbiota homeostasis for preventing CAC development. We here analyzed the landscape of gut microbiome in the mice model of AOM/DSS-induced CAC with MyD88 inhibitor treatment. PCoA revealed significant reduction in Lactobacillus load and increase in Escherichia load in the mucosal microbial composition of mice with CAC, compared with normal controls (NCs). Inhibitor-treatment led to almost undetectable Proteobacteria (Escherichia) and the retention of the dominance of Firmicutes and Bacteroidota (Muribaculaceae) in the mucosa. RNA sequencing analysis identified genes were up-regulated (Hp, SAA3 and IL-1F9) and down-regulated (CYP3A44, SLC30A10, GPNMB and OTC) in Inhibitor-treated mice (vs. CAC). Meanwhile, Inhibitor-treated mice had higher percentage of MUC2-positive area in colon sections (vs. CAC, which was less than NCs) by IF staining and decreased Escherichia in the mucus layer (vs. CAC) by FISH. And intestinal microbiota from mice with MyD88 inhibitor treatment could lessen the outcome of CAC by fecal microbiota transplantation. The development of CAC was involved in the increasing and ectopic Escherichia in the decreasing colonic mucus layer. MyD88 signaling blockade may maintain the host-microbiota homeostasis by up-regulating MUC2 production, increasing probiotics and their protective effects, and inhibiting the reproduction of Escherichia.

MDSCs in sepsis-induced immunosuppression and its potential therapeutic targets

Sepsis is a life-threatening organ dysfunction caused by a dysregulated host response to infection. In sepsis, a complicated immune response is initiated, which varies over time with sustained excessive inflammation and immunosuppression. Identifying a promising way to orchestrate sepsis-induced immunosuppression is a challenge. Myeloid-derived suppressor cells (MDSCs) comprise pathologically activated neutrophils and monocytes with potent immunosuppressive activity. They play an important part in inhibiting innate and adaptive immune responses, and have emerged as part of the immune response in sepsis. MDSCs numbers are persistently high in sepsis patients, and associated with nosocomial infections and other adverse clinical outcomes. However, their characteristics and functional mechanisms during sepsis have not been addressed fully. Our review sheds light on the features and suppressive mechanism of MDSCs. We also review the potential applications of MDSCs as biomarkers and targets for clinical treatment of sepsis.

The role and application of small extracellular vesicles in breast cancer

Breast cancer (BC) is the most common malignancy and the leading cause of cancer-related deaths in women worldwide. Currently, patients’ survival remains a challenge in BC due to the lack of effective targeted therapies and the difficult condition of patients with higher aggressiveness, metastasis and drug resistance. Small extracellular vesicles (sEVs), which are nanoscale vesicles with lipid bilayer envelopes released by various cell types in physiological and pathological conditions, play an important role in biological information transfer between cells. There is growing evidence that BC cell-derived sEVs may contribute to the establishment of a favorable microenvironment that supports cancer cells proliferation, invasion and metastasis. Moreover, sEVs provide a versatile platform not only for the diagnosis but also as a delivery vehicle for drugs. This review provides an overview of current new developments regarding the involvement of sEVs in BC pathogenesis, including tumor proliferation, invasion, metastasis, immune evasion, and drug resistance. In addition, sEVs act as messenger carriers carrying a variety of biomolecules such as proteins, nucleic acids, lipids and metabolites, making them as potential liquid biopsy biomarkers for BC diagnosis and prognosis. We also described the clinical applications of BC derived sEVs associated MiRs in the diagnosis and treatment of BC along with ongoing clinical trials which will assist future scientific endeavors in a more organized direction.

Pharmacological modulation of myeloid-derived suppressor cells to dampen inflammation

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous cell population with potent suppressive and regulative properties. MDSCs’ strong immunosuppressive potential creates new possibilities to treat chronic inflammation and autoimmune diseases or induce tolerance towards transplantation. Here, we summarize and critically discuss different pharmacological approaches which modulate the generation, activation, and recruitment of MDSCs in vitro and in vivo, and their potential role in future immunosuppressive therapy.

Blockade of Myd88 signaling by a novel MyD88 inhibitor prevents colitis-associated colorectal cancer development by impairing myeloid-derived suppressor cells

Background. In cancer, myeloid-derived suppressor cells (MDSCs) are known to escape the host immune system by developing a highly suppressive environment. However, little is known about the molecular mechanism behind MDSC-mediated tumor cell evasion of the immune system. Toll-like receptor (TLR) signaling elicited in the tumor microenvironment has the potential to induce MDSC differentiations in different organs. Therefore, MDSC elimination by blocking the action of myeloid differentiation factor 88 (MyD88), which is a key adaptor-signaling molecule that affects TLR activity, seems to be an ideal tumor immunotherapy. Previous studies have proven that blocking MyD88 signaling with a novel MyD88 inhibitor (TJ-M2010-5, synthesized by Zhou’s group) completely prevented colitis-associated colorectal cancer (CAC) development in mice. Methods. In the present study, we investigated the impact of the novel MyD88 inhibitor on the number, phenotype, and function of MDSC in the mice model of CAC. Results. We showed that CAC growth inhibition was involved in diminished MDSC generation, expansion, and suppressive function and that MDSC-mediated immune escape was dependent on MyD88 signaling pathway activation. MyD88 inhibitor treatment decreased the accumulation of CD11b⁺Gr1⁺ MDSCs in mice with CAC, thereby reducing cytokine (GM-CSF, G-CSF, IL-1β, IL-6 and TGF-β) secretion associated with MDSC accumulation, and reducing the expression of molecules (iNOS, Arg-1 and IDO) associated with the suppressive capacity of MDSCs. In addition, MyD88 inhibitor treatment reduced the differentiation of MDSCs from myeloid cells and the suppressive capacity of MDSCs on the proliferation of activated CD4⁺ T cells in vitro. Conclusion. MDSCs are primary cellular targets of a novel MyD88 inhibitor during CAC development. Our findings prove that MyD88 signaling is involved in the regulation of the immunosuppressive functions of MDSCs. The novel MyD88 inhibitor TJ-M2010-5 is a new and effective agent that modulates MyD88 signaling to overcome MDSC suppressive functions, enabling the development of successful antitumor immunotherapy.

Novel Characterization of Myeloid-Derived Suppressor Cells in Tumor Microenvironment

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous group of cells generated in various pathologic conditions, which have been known to be key components of the tumor microenvironment (TME) involving in tumor immune tolerance. So MDSCs have been extensively researched recently. As its name suggests, immunosuppression is the widely accepted function of MDSCs. Aside from suppressing antitumor immune responses, MDSCs in the TME also stimulate tumor angiogenesis and metastasis, thereby promoting tumor growth and development. Therefore, altering the recruitment, expansion, activation, and immunosuppression of MDSCs could partially restore antitumor immunity. So, this view focused on the favorable TME conditions that promote the immunosuppressive effects of MDSCs and contribute to targeted therapies with increased precision for MDSCs.

NF-κB in Cancer Immunity: Friend or Foe?

The emergence of immunotherapies has definitely proven the tight relationship between malignant and immune cells, its impact on cancer outcome and its therapeutic potential. In this context, it is undoubtedly critical to decipher the transcriptional regulation of these complex interactions. Following early observations demonstrating the roles of NF-κB in cancer initiation and progression, a series of studies converge to establish NF-κB as a master regulator of immune responses to cancer. Importantly, NF-κB is a family of transcriptional activators and repressors that can act at different stages of cancer immunity. In this review, we provide an overview of the selective cell-intrinsic contributions of NF-κB to the distinct cell types that compose the tumor immune environment. We also propose a new view of NF-κB targeting drugs as a new class of immunotherapies for cancer.

Immunotherapy Targeting Myeloid-Derived Suppressor Cells (MDSCs) in Tumor Microenvironment

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of immature myeloid cells that accumulate in tumor-bearing hosts to reduce T cells activity and promote tumor immune escape in the tumor microenvironment (TME). The immune system in the TME can be stimulated to elicit an anti-tumor immune response through immunotherapy. The main theory of immunotherapy resides on the plasticity of the immune system and its capacity to be re-educated into a potent anti-tumor response. Thus, MDSCs within the TME became one of the major targets to improve the efficacy of tumor immunotherapy, and therapeutic strategies for tumor MDSCs were developed in the last few years. In the article, we analyzed the function of tumor MDSCs and the regulatory mechanisms of agents targeting MDSCs in tumor immunotherapy, and reviewed their therapeutic effects in MDSCs within the TME. Those data focused on discussing how to promote the differentiation and maturation of MDSCs, reduce the accumulation and expansion of MDSCs, and inhibit the function, migration and recruitment of MDSCs, further preventing the growth, invasion and metastasis of tumor. Those investigations may provide new directions for cancer therapy.

Dual roles of myeloid-derived suppressor cells induced by Toll-like receptor signaling in cancer

Myeloid-derived suppressor cells (MDSCs) are one of the major components of the tumor microenvironment (TME), and are the main mediators of tumor-induced immunosuppression. Recent studies have reported that the survival, differentiation and immunosuppressive activity of MDSCs are affected by the Toll-like receptor (TLR) signaling pathway. However, the regulatory effect of TLR signaling on MDSCs remains controversial. TLR-induced MDSC can acquire different immunosuppressive activities to influence the immune response that can be either beneficial or detrimental to cancer immunotherapy. The present review summarizes the effects of TLR signals on the number, phenotype and inhibitory activity of MDSCs, and their role in cancer immunotherapy, which cannot be ignored if effective cancer immunotherapies are to be developed for the immunosuppression of the TME.

The role of non-coding genome in the behavior of infiltrated myeloid-derived suppressor cells in tumor microenvironment; a perspective and state-of-the-art in cancer targeted therapy

Cancer is one of the healthcare problems that affect many communities around the world. Many factors contribute to cancer development. Besides, these factors are counted as the main impediment in cancer immunotherapy. Myeloid-derived suppressor cells (MDSCs) are one of these impediments. MDSCs inhibit the immune responses through various mechanisms such as inhibitory cytokine release and nitric oxide metabolite production. Several factors are involved in forming these cells, including tumor secreted cytokine and chemokines, transcription factors, and non-coding RNA. In the meantime, micro-RNAs (miRNAs) and long non-coding RNAs (lncRNAs) are the vital gene regulatory elements that affect gene expression. In this study, we are going to discuss the role of miRNAs and lncRNAs in MDSCs development in a cancer situation. It is hoped that miRNA and lncRNAs targeting may prevent the growth and development of these inhibitory cells in the cancer environment.

Long noncoding RNA: a dazzling dancer in tumor immune microenvironment

Long noncoding RNAs (lncRNAs) are a class of endogenous, non-protein coding RNAs that are highly linked to various cellular functions and pathological process. Emerging evidence indicates that lncRNAs participate in crosstalk between tumor and stroma, and reprogramming of tumor immune microenvironment (TIME). TIME possesses distinct populations of myeloid cells and lymphocytes to influence the immune escape of cancer, the response to immunotherapy, and the survival of patients. However, hitherto, a comprehensive review aiming at relationship between lncRNAs and TIME is missing. In this review, we focus on the functional roles and molecular mechanisms of lncRNAs within the TIME. Furthermore, we discussed the potential immunotherapeutic strategies based on lncRNAs and their limitations.

Contribution of long-chain fatty acid to induction of myeloid-derived suppressor cell (MDSC)-like cells - induction of MDSC by lipid vesicles (liposome)

CONTEXT

Effects of liposomal particles on immune function have not been adequately investigated. Earlier reports indicate that intravenous injection of rats with pegylated liposomes comprising chemically defined specific lipids produces myeloid derived suppressor-cell (MDSC)-like cells in the spleen.

OBJECTIVES

After liposome injection, we sought a cell surface marker expressed specifically on splenic macrophages. Then we assessed the immunosuppressive activity of macrophages positive for the marker. Furthermore, we investigated whether immunosuppression induction is an immunopharmacological action specific to this pegylated liposome, or not.

MATERIALS AND METHODS

After using a microarray system to screen genes enhanced by this liposome, we evaluated cell surface expression of gene products using flow cytometry. Liposomes of several kinds, each comprising one type of phospholipid, were prepared and evaluated for their ability to induce T-cell suppression.

RESULTS

Microarray analysis indicated enhanced B7-H3 expression. Flow cytometry revealed that the B7-H3 molecule was expressed on splenic macrophages after liposome injection. B7-H3⁺ macrophages were positive for iNOS. Removing B7-H3⁺ cells restored T-cell proliferation. Similarly to this liposome, various liposomes with different long chain fatty acids induced T-cell suppression when accumulated in the spleen.

CONCLUSIONS

Immunosuppressive cells induced by this pegylated liposome closely resemble MDSCs, especially B7-H3⁺ MDSCs. Immunosuppression induction is not a phenomenon specific to this liposome. Accumulation of long chain fatty acid in macrophages by internalization of liposomal nanoparticles might be related to macrophage acquisition of immunosuppressive activity in vivo.

Dendritic Cells and Myeloid Derived Suppressor Cells Fully Responsive to Stimulation via Toll-Like Receptor 4 Are Rapidly Induced from Bone-Marrow Cells by Granulocyte-Macrophage Colony-Stimulating Factor

Dendritic cells (DCs) are commonly generated from bone marrow (BM) progenitor cells with granulocyte-macrophage colony-stimulating factor (GM-CSF) alone or in combination with interleukin 4 (IL-4). These cells are often harvested post day 5, when they acquire maturation markers and can stimulate T cells. Apart from DCs, myeloid derived suppressor cells (MDSCs) are also found within these cultures. However, little is known about the functional characteristics of DCs and MDSCs before day 5. Herein, using a murine model, it is shown that early DCs and MDSCs, even in cultures with GM-CSF alone, upregulate fully maturation and activation surface molecules in response to the toll-like receptor 4 (TLR4) ligand lipopolysaccharide (LPS) stimulation. Despite initially displaying lower marker expression levels, these cells efficiently induced T cell stimulation and cytokine production. Interestingly, Gr-1^int MDSCs increased their T cell co-stimulatory activity upon TLR4 stimulation. Additionally, early DCs and MDSCs exhibited differential endocytic capacity for viral sized nanoparticles and bacterial sized microparticles. DCs internalized both particle sizes, whilst MDSCs only internalized the larger microparticles, with reduced endocytic activity over time in the culture. These findings have unveiled an important role for the rapid initiation of productive immunity by GM-CSF, with promising implications for future vaccine and DC immunotherapy developments.

Plasmacytoid dendritic cells regulate colitis-associated tumorigenesis by controlling myeloid-derived suppressor cell infiltration

Toll-like receptor (TLR)3 and TLR7 are important for stimulating plasmacytoid dendritic cells (pDCs), which secrete type I interferon. Mice deficient for TLR3 and TLR7 (TLR3^-/-TLR7^-/-) reportedly exhibit deteriorated colitis because of impaired pDCs. However, the role of pDCs in tumorigenesis-associated inflammation progression has not been studied. We treated wild-type or TLR3^-/-TLR7^-/- mice with dextran sulfate sodium (DSS) and/or azoxymethane (AOM) and examined colon mucosa, measured body weight and colon length of mice, and examined pDC and myeloid-derived suppressor cell (MDSC) accumulation. Further, we depleted pDCs in AOM/DSS-treated wild-type mice by treating them with anti-PDCA-1 antibodies. We found that MDSCs significantly increased, while pDCs decreased in TLR3^-/-TLR7^-/- mice. Moreover, TLR3^-/-TLR7^-/- mice developed colitis-associated colon cancer following AOM/DSS treatment. Additionally, we showed that a defect in TLR7 of pDCs is responsible for the aggravation of colitis-associated colon cancer. Further, we showed that TLR7 ligand mitigates colitis-associated colon cancer. Collectively, our results demonstrate that gut pDCs play a crucial role in reducing colorectal cancer development via the regulation of infiltrating MDSCs.

Enhanced dual function of osteoclast precursors following calvarial Porphyromonas gingivalis infection

BACKGROUND AND OBJECTIVE

Excessive osteoclast activity is a major characteristic of pathogenic bone loss in inflammatory bone diseases including periodontitis. However, beyond the knowledge that osteoclasts are differentiated from the monocyte/macrophage lineage and share common ancestry with macrophages and DC, the nature and function of osteoclast precursors are not completely understood. Furthermore, little is known about how osteoclast precursors respond to bacterial infection in vivo. We have previously demonstrated in vitro that the periodontal pathogen Porphyromonas gingivalis (Pg) plays a biphasic role on the receptor activator of nuclear factor kappa B ligand (RANKL)-induced osteoclast differentiation. In this study, we investigated the in vivo effect of Pg infection on the regulation of osteoclast precursors, using a mouse calvarial infection model.

METHODS AND RESULTS

C57BL/6 wild-type and the myeloid differentiation factor 88 knockout (MyD88-/- ) mice were infected with Pg by calvarial injection. Local and systemic bone loss, and the number and function of CD11b+ c-fms+ cells from bone marrow and spleen were analyzed. Our results show that Pg infection induces localized inflammatory infiltration and osteoclastogenesis, as well as increased number and osteoclastogenic potential of CD11b+ c-fms+ osteoclast precursors in the bone marrow and periphery. We also show that CD11b+ c-fms+ RANK+ and CD11b+ c-fms+ RANK- are precursors with similar osteoclastogenic and pro-inflammatory potentials. In addition, CD11b+ c-fms+ cells exhibit an antigen-specific T-cell immune-suppressive activity, which are increased with Pg infection. Moreover, we demonstrate that MyD88 is involved in the regulation of osteoclast precursors upon Pg infection.

CONCLUSIONS

In this study, we demonstrate an enhanced dual function of osteoclast precursors following calvarial Pg infection. Based on our findings, we propose the following model: Pg infection increases a pool of precursor cells that can be shunted toward osteoclast formation at the infection/inflammation sites, while at the same time dampening host immune responses, which is beneficial for the persistence of infection and maintenance of the characteristic chronic nature of periodontitis. Understanding the nature, function, and regulation of osteoclast precursors will be helpful for identifying therapeutic interventions to aid in the control and prevention of inflammatory bone loss diseases including periodontitis.

Myeloid Derived Suppressor Cells: Key Drivers of Immunosuppression in Ovarian Cancer

The presence of tumor infiltrating lymphocytes (TILs) is associated with a longer overall survival in advanced stage epithelial ovarian cancer. Despite the prognostic impact of TILs, response to checkpoint-inhibitors and antigen-specific active immunotherapy is limited in ovarian cancer. The goal of our study was to investigate the interaction between ovarian cancer and the innate and adaptive immune system in the ID8-fLuc syngeneic ovarian cancer mouse model. For the in vivo experiments C57BL/6, B6.129S7-Rag1^tm1Mom/J, and B6.129P2(SJL)-Myd88^tm1.1Defr/J mice were inoculated with ID8-fLuc. In vivo depletion experiments were performed using clodronate liposomes (CL), anti-CD8a, anti-GR1, anti-colony stimulating factor 1 (anti-CSF1), and TMβ1 (anti-CD122). Immune read out was performed by fluorescent activated cell sorting analysis for effector T cells, regulatory T cells, natural killer cells, B cells, macrophages, and myeloid derived suppressor cells (MDSC), immunohistochemistry for MDSC and tumor-associated macrophages (TAM) and immunofluorescence for M1 and M2 TAM in the vascular context. The effect of MDSC on T cell proliferation and phenotype were studied in vitro. We discovered that the absence of T and B cells did not influence tumor growth or survival of B6.129S7-Rag1^tm1Mom/J mice compared to immunocompetent C57BL/6 mice. CL-induced macrophage depletion promoted tumor proliferation and shortened survival in C57BL/6 mice (p = 0.004) and in B6.129S7-Rag1^tm1Mom/J mice (p = 0.0005). During CL treatment, we observed a clear increase of pro-inflammatory cytokines (p ≤ 0.02) and monocytic MDSC (p ≤ 0.01). Selective depletion of MDSC by anti-GR1 improved survival, certainly in comparison to mice treated with anti-CSF1 (p = 0.01-median survival 91 vs. 67.5 days). B6.129P2(SJL)-Myd88^tm1.1Defr/J mice displayed to a longer median survival compared to C57BL/6 mice (90 vs. 76 days). MDSC activated by ID8-fLuc conditioned medium or ascites of tumor-bearing mice showed T cell suppressive functions in vitro. Based on these findings, we conclude that the adaptive immune system does not efficiently control tumor growth in the ID8-fLuc model. In addition, we discovered a prominent role for MDSC as the driver of immunosuppression in the ID8-fLuc ovarian cancer mouse model.

AMPK activation inhibits the functions of myeloid-derived suppressor cells (MDSC): impact on cancer and aging

AMP-activated protein kinase (AMPK) has a crucial role not only in the regulation of tissue energy metabolism but it can also control immune responses through its cooperation with immune signaling pathways, thus affecting immunometabolism and the functions of immune cells. It is known that AMPK signaling inhibits the activity of the NF-κB system and thus suppresses pro-inflammatory responses. Interestingly, AMPK activation can inhibit several major immune signaling pathways, e.g., the JAK-STAT, NF-κB, C/EBPβ, CHOP, and HIF-1α pathways, which induce the expansion and activation of myeloid-derived suppressor cells (MDSC). MDSCs induce an immunosuppressive microenvironment in tumors and thus allow the escape of tumor cells from immune surveillance. Chronic inflammation has a key role in the expansion and activation of MDSCs in both tumors and inflammatory disorders. The numbers of MDSCs also significantly increase during the aging process concurrently with the immunosenescence associated with chronic low-grade inflammation. Increased fatty acid oxidation and lactate produced by aerobic glycolysis are important immunometabolic enhancers of MDSC functions. However, it seems that AMPK signaling regulates the functions of MDSCs in a context-dependent manner. Currently, the activators of AMPK signaling are promising drug candidates for cancer therapy and possibly for the extension of healthspan and lifespan. We will describe in detail the AMPK-mediated regulation of the signaling pathways controlling the expansion and activation of immunosuppressive MDSCs. We will propose that the beneficial effects mediated by AMPK activation, e.g., in cancers and the aging process, could be induced by the inhibition of MDSC functions.

Candida/Staphylococcal Polymicrobial Intra-Abdominal Infection: Pathogenesis and Perspectives for a Novel Form of Trained Innate Immunity

Polymicrobial sepsis is difficult to diagnose and treat and causes significant morbidity and mortality, especially when fungi are involved. In vitro, synergism between Candida albicans and various bacterial species has been described for many years. Our laboratory has developed a murine model of polymicrobial intra-abdominal infection with Candida albicans and Staphylococcus aureus, demonstrating that polymicrobial infections cause high levels of mortality, while monoinfections do not. By contrast, closely related Candida dubliniensis does not cause synergistic lethality and rather provides protection against lethal polymicrobial infection. This protection is thought to be driven by a novel form of trained innate immunity mediated by myeloid-derived suppressor cells (MDSCs), which we are proposing to call "trained tolerogenic immunity". MDSC accumulation has been described in patients with sepsis, as well as in in vivo sepsis models. However, clinically, MDSCs are considered detrimental in sepsis, while their role in in vivo models differs depending on the sepsis model and timing. In this review, we will discuss the role of MDSCs in sepsis and infection and summarize our perspectives on their development and function in the spectrum of trained innate immune protection against fungal-bacterial sepsis.

The Pseudogene Olfr29-ps1 Promotes the Suppressive Function and Differentiation of Monocytic MDSCs

Long noncoding RNA (lncRNA) plays a critical role in many biological processes, such as cell differentiation and development. However, few studies about lncRNAs regulating the differentiation and development of myeloid-derived suppressor cells (MDSCs) exist. In this study, we identified a lncRNA pseudogene, Olfr29-ps1, which was expressed in MDSCs and upregulated by the proinflammatory cytokine IL6. The Olfr29-ps1 in vertebrates is conserved, and the similarity between the Olfr29-ps1 and human OR1F2P sequence is 43%. This lncRNA promoted the immunosuppressive function and differentiation of monocytic (Mo-)MDSCs in vitro and in vivo It directly sponged miR-214-3p to downregulate miR-214-3p, which may target MyD88 to modulate the differentiation and development of MDSCs. The functions of Olfr29-ps1 were dependent on IL6-mediated N ⁶-methyladenosine (m6A) modification, which not only enhanced Olfr29-ps1, but also promoted the interaction of Olfr29-ps1 with miR-214-3p Thus, our results demonstrated that the pseudogene Olfr29-ps1 may regulate the differentiation and function of MDSCs through a m6A-modified Olfr29-ps1/miR-214-3p/MyD88 regulatory network, revealing a mechanism for the regulation of myeloid cells and also providing potential targets for antitumor immunotherapy.

Impaired antigen-specific lymphocyte priming in mice after Toll-like receptor 4 activation via induction of monocytic myeloid-derived suppressor cells

In sepsis, the pathology involves a shift from a proinflammatory state toward an immunosuppressive phase. We previously showed that an agonistic anti-TLR4 antibody induced long-term endotoxin tolerance and suppressed antigen-specific secondary IgG production when primed prior to immunization with antigen. These findings led us to speculate that TLR4-induced innate tolerance due to primary infection causes an immunosuppressive pathology in sepsis. Therefore, the mechanism underlying impaired antigen-specific humoral immunity by the TLR4 antibody was investigated. We showed, in a mouse model, that primary antigen-specific IgG responses were impaired in TLR4 antibody-induced tolerized mice, which was the result of reduced numbers of antigen-specific GC B cells and plasma cells. Ovalbumin-specific CD4 and CD8 T-cell responses were impaired in TLR4 antibody-injected OT-I and -II transgenic mice ex vivo. Adoptive transfer studies demonstrated suppression of OVA-specific CD4 and CD8 T-cell responses by the TLR4 antibody in vivo. The TLR4 antibody induced Gr1⁺ CD11b⁺ myeloid-derived suppressor cell (MDSC) expansion with suppression of T-cell activation. Monocytic MDSCs were more suppressive and exhibited higher expression of PD-L1 and inducible nitric oxidase compared with granulocytic MDSCs. In conclusion, immune tolerance conferred by TLR4 activation induces the expansion of monocytic MDSCs, which impairs antigen-specific T-cell priming and IgG production.

Myeloid-Derived Suppressor Cells: Immune-Suppressive Cells That Impair Antitumor Immunity and Are Sculpted by Their Environment

Myeloid-derived suppressor cells (MDSC) are a diverse population of immature myeloid cells that have potent immune-suppressive activity. Studies in both mice and humans have demonstrated that MDSC accumulate in most individuals with cancer, where they promote tumor progression, inhibit antitumor immunity, and are an obstacle to many cancer immunotherapies. As a result, there has been intense interest in understanding the mechanisms and in situ conditions that regulate and sustain MDSC, and the mechanisms MDSC use to promote tumor progression. This article reviews the characterization of MDSC and how they are distinguished from neutrophils, describes the suppressive mechanisms used by MDSC to mediate their effects, and explains the role of proinflammatory mediators and the tumor microenvironment in driving MDSC accumulation, suppressive potency, and survival.

Dual functional roles of the MyD88 signaling in colorectal cancer development

The myeloid differentiation factor 88 (MyD88), an adaptor protein in regulation of the innate immunity, functions to regulate immune responses against viral and bacterial infections in the human body. Toll-like receptors (TLRs) and interleukin 1 receptors (IL-1R) can recognize microbes or endogenous ligands and then recruit MyD88 to activate the MyD88-dependent pathway, while MyD88 mutation associated with lymphoma development and altered MyD88 signaling also involved in cancer-associated cell intrinsic and extrinsic inflammation progression and carcinogenesis. Detection of MyD88 expression was to predict prognosis of various human cancers, e.g., lymphoid, liver, and colorectal cancers. In human cancers, MyD88 protein acts as a bridge between the inflammatory signaling from the TLR/IL-1R and Ras oncogenic signaling pathway. However, the MyD88 signaling played dual functional roles in colorectal cancer, i.e., the tumor-promoting role that enhances cancer inflammation and intestinal flora imbalance to induce tumor invasion and tumor cell self-renewal, and the anti-tumor role that helps to maintain the host-microbiota homeostasis to induce tumor cell cycle arrest and immune responses against cancer cells. This review precisely discusses the up to date literature for these contrasting effects of MyD88 signaling on colorectal cancer development and progression.

Mesenchymal Stromal Cell Preconditioning: The Next Step Toward a Customized Treatment For Severe Burn

Over the last century, the clinical management of severe skin burns significantly progressed with the development of burn care units, topical antimicrobials, resuscitation methods, early eschar excision surgeries, and skin grafts. Despite these considerable advances, the present treatment of severe burns remains burdensome, and patients are highly susceptible to skin engraftment failure, infections, organ dysfunction, and hypertrophic scarring. Recent researches have focused on mesenchymal stromal cell (MSC) therapy and hold great promises for tissue repair, as reported in several animal studies and clinical cases. In the present review, we will provide an up-to-date outlook of the pathophysiology of severe skin burns, clinical treatment modalities and current limitations. We will then focus on MSCs and their potential in the burn wound healing both in in vitro and in vivo studies. A specific attention will be paid to the cell preconditioning approach, as a means of improving the MSC efficacy in the treatment of major skin burns. In particular, we will debate how several preconditioning cues would modulate the MSC properties to better match up with the burn pathophysiology in the course of the cell therapy. Finally, we will discuss the clinical interest and feasibility of a MSC-based therapy in comparison to their paracrine derivatives, including microvesicles and conditioned media for the treatment of major skin burn injuries.

Exosome: emerging biomarker in breast cancer

Exosomes are nano-sized membrane vesicles released by a variety of cell types, and are thought to play important roles in intercellular communications. In breast cancer, through horizontal transfer of various bioactive molecules, such as proteins and mRNAs, exosomes are emerging as local and systemic cell-to-cell mediators of oncogenic information and play an important role on cancer progression. This review outlines the current knowledge and concepts concerning the exosomes involvement in breast cancer pathogenesis (including tumor initiation, invasion and metastasis, angiogenesis, immune system modulation and tumor microenvironment) and cancer therapy resistance. Moreover, the potential use of exosomes as promising diagnostic and therapeutic biomarkers in breast cancer are also discussed.

Inhibition of Myeloid Differentiation Factor 88 Reduces Human and Mouse T-Cell Interleukin-17 and IFNγ Production and Ameliorates Experimental Autoimmune Encephalomyelitis Induced in Mice

Myeloid differentiation factor 88 (MyD88) recruits signaling proteins to the intracellular domain of receptors belonging to the toll-like/interleukin-1 (IL-1) receptor superfamily. Mice lacking MyD88 are highly susceptible to infectious diseases, but tend to resist experimentally induced autoimmune diseases such as experimental autoimmune encephalomyelitis (EAE) and manifest diminished allograft rejection. We reasoned that inhibition of MyD88 should influence the cytokine profile of responding T cells by blocking costimulatory molecule expression by antigen-presenting cells (APCs) and by inhibiting T-cell responses to IL-18. We now report that inhibition of MyD88 in human APCs led to decreased IFNγ and IL-17 production and a shift to IL-4 production by responding T cells in a mixed lymphocyte reaction. Direct inhibition of Myd88 in mouse and human T cells also reduced their production of IFNγ in response to IL-12/IL-18 stimulation. Finally, systemic MyD88 antagonism significantly reduced the clinical manifestations of EAE in mice. Thus, MyD88 appears to be a key factor in determining T cell phenotype and represents a potential target for therapeutic intervention.

Myeloid-derived suppressor cells-a new therapeutic target to overcome resistance to cancer immunotherapy

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of immature myeloid cells that accumulate during pathologic conditions, such as cancer. Patients diagnosed with advanced metastatic cancers have an average survival of 12-24 mo, a survival time that hasn't changed significantly in the past 30 yr. Despite some encouraging improvements in response rates and overall survival in patients receiving immunotherapies, such as immune checkpoint inhibitors, most patients will ultimately progress. MDSCs contribute to immunotherapeutic resistance by actively inhibiting antitumor T cell proliferation and cytotoxic activity as well as by promoting expansion of protumorigenic T regulatory cells, thereby, dampening the host immune responses against the tumor. In addition, MDSCs promote angiogenesis, tumor invasion, and metastasis. Thus, MDSCs are potential therapeutic targets in cases of multiple cancers. This review focuses on the phenotypic and functional characteristics of MDSCs and provides an overview of the mono- and combinatorial-therapeutic strategies that target MDSCs with an objective of enhancing the efficacy of cancer immunotherapies.

CXCL2/MIF-CXCR2 signaling promotes the recruitment of myeloid-derived suppressor cells and is correlated with prognosis in bladder cancer

The accumulation of myeloid-derived suppressor cells (MDSCs) has been observed in solid tumors and is correlated with tumor progression; however, the underlying mechanism is still poorly understood. In this study, we identified a mechanism by which tumor cells induce MDSC accumulation and expansion in the bladder cancer (BC) microenvironment via CXCL2/MIF-CXCR2 signaling. Elevated expression of CXCL2 and MIF and an increased number of CD33⁺ MDSCs were detected in BC tissues, and these increases were significantly associated with advanced disease stage and poor patient prognosis (P<0.01). A positive association was observed between CXCL2 or MIF expression and the number of tumor-infiltrating CD33⁺ MDSCs (P<0.01). Subsequently, we demonstrated that CD45⁺CD33⁺CD11b⁺HLA-DR^- MDSCs from fresh BC tissues displayed high levels of suppressive molecules, including Arg1, iNOS, ROS, PDL-1 and P-STAT3, and stronger suppression of T-cell proliferation. Interestingly, these CD45⁺CD33⁺CD11b⁺HLA-DR^- MDSCs exhibited increased CXCR2 expression compared with that in peripheral blood from BC patients or healthy controls (P<0.05). Chemotaxis assay revealed that bladder cancer cell line J82 induced MDSC migration via CXCL2/MIF-CXCR2 signaling in vitro. Mechanistic studies demonstrated that J82-induced MDSC trafficking and CXCR2 expression were associated with increased phosphorylation of p38, ERK and p65. Conversely, inhibition of the phosphorylation of p38, ERK or p65 decreased J82-induced MDSC trafficking and CXCR2 expression. CXCL2/MIF-stimulated activation of the mitogen-activated protein kinase and nuclear factor kappa B pathways in MDSCs was MyD88 dependent. Overall, our results identify the CXCL2/MIF-CXCR2 axis as an important mediator in MDSC recruitment and as predictors and potential therapeutic targets in BC patients.

Interleukin-10 attenuates tumour growth by inhibiting interleukin-6/signal transducer and activator of transcription 3 signalling in myeloid-derived suppressor cells

Interleukin-10 (IL-10) is a well-characterized anti-inflammatory cytokine, but its role in anti-cancer immunity is controversial. After injection with TC-1 cancer cells, we observed more rapid tumour growth and significantly higher interleukin-6 (IL-6) production in IL-10 knockout (IL-10(-/-)) mice than wild-type (WT) mice. Blocking IL-6 with an anti-IL-6 receptor (IL-6R) monoclonal antibody (mAb) inhibited tumour growth and myeloid-derived suppressor cell (MDSC) generation, which were significantly increased in IL-10-deficient mice. MDSCs and tumour cells from IL-10(-/-) mice had increased phosphorylated signal transducer and activator of transcription 3 (p-STAT3) levels. Treatment with a STAT3 inhibitor, S3I, reduced tumour growth, inhibited MDSC expansion, reduced IL-6 in tumours, and relieved T cell suppression. The combination of anti-IL-6R mAb and S3I further inhibited tumour growth compared to S3I treatment alone. These results suggested that the inhibition of the IL-6/STAT3 signalling axis is a candidate anti-cancer strategy, especially under systemic inflammatory conditions with high IL-6.

Myeloid-Derived Suppressor Cells in Bacterial Infections

Myeloid-derived suppressor cells (MDSCs) comprise monocytic and granulocytic innate immune cells with the capability of suppressing T- and NK-cell responses. While the role of MDSCs has been studied in depth in malignant diseases, the understanding of their regulation and function in infectious disease conditions has just begun to evolve. Here we summarize and discuss the current view how MDSCs participate in bacterial infections and how this knowledge could be exploited for potential future therapeutics.

Breast Cancer-Derived Extracellular Vesicles: Characterization and Contribution to the Metastatic Phenotype

The study of extracellular vesicles (EVs) in cancer progression is a complex and rapidly evolving field. Whole categories of cellular interactions in cancer which were originally presumed to be due solely to soluble secreted molecules have now evolved to include membrane-enclosed extracellular vesicles (EVs), which include both exosomes and shed microvesicles (MVs), and can contain many of the same molecules as those secreted in soluble form but many different molecules as well. EVs released by cancer cells can transfer mRNA, miRNA, and proteins to different recipient cells within the tumor microenvironment, in both an autocrine and paracrine manner, causing a significant impact on signaling pathways, mRNA transcription, and protein expression. The transfer of EVs to target cells, in turn, supports cancer growth, immunosuppression, and metastasis formation. This review focuses exclusively on breast cancer EVs with an emphasis on breast cancer-derived exosomes, keeping in mind that breast cancer-derived EVs share some common physical properties with EVs of other cancers.

Phenotype, development, and biological function of myeloid-derived suppressor cells

CD11b⁺Gr-1⁺ myeloid-derived suppressor cells (MDSCs) are an important population of innate regulatory cells mainly comprising monocytic MDSCs (M-MDSCs) with a phenotype of CD11b⁺Ly6G^-Ly6C^high and granulocytic MDSCs (G-MDSCs) with a phenotype of CD11b⁺Ly6G⁺Ly6C^low in mice. They play crucial roles in the pathogenesis of cancers, chronic infections, autoimmune diseases, and transplantation. Various extracellular factors such as lipopolysaccharide (LPS), macrophage colony-stimulating factor (M-CSF), granulocyte macrophage colony-stimulating factor (GM-CSF), stem cell factor (SCF), interleukin (IL)-6, interferon gamma (IFNγ), IL-1β, vascular endothelial growth factor (VEGF), Hsp72, IL-13, C5a, and prostaglandin E2 (PGE2) can induce MDSC differentiation, whereas IL-4 and all-trans-retinoic acid can inhibit this process. For the intracellular signals, signal transducer and activator of transcription (STAT) family members, C/EBPβ and cyclooxigenase-2 (COX-2) promote MDSC function, whereas interferon regulatory factor-8 (IRF-8) and Smad3 downregulate MDSC activity. The immunosuppressive function of MDSCs is mediated through various effector molecules, primarily cellular metabolism-related molecules such as nitric oxide (NO), arginase, reactive oxygen species (ROS), transforming growth factor β (TGFβ), IL-10, indoleamine 2,3-dioxygenase (IDO), heme oxygenase-1 (HO-1), carbon monoxide (CO), and PGE2. In this article, we will summarize the molecules involved in the induction and function of MDSCs as well as the regulatory pathways of MDSCs.

Elevated endoplasmic reticulum stress reinforced immunosuppression in the tumor microenvironment via myeloid-derived suppressor cells

The role of endoplasmic reticulum (ER) stress in cancer has been studied in detail, and ER stress is known to increase tumor cell apoptosis, and thus, reduce tumor growth. However, in our study, persistent ER stress induced by multiple administrations of low-dose thapsigargin (Tg) accelerated tumor growth in mice. Tg-mediated ER stress increased the generation of Ly6G⁺CD11b⁺ myeloid cells, but did not alter anti-tumor effector T cells. 4-Phenylbutyric acid (4-PBA), a chemical chaperone widely used as an ER stress reducer, attenuated Tg-induced myeloid-derived suppressor cell (MDSC) expansion and tumor growth. Tg-mediated ER stress enhanced the immunosuppressive capacity of tumor-infiltrating MDSCs by increasing expression of ARG1, iNOS, and NOX2, although splenic MDSCs were not affected. Consistent with these results, 4-PBA restored the anti-tumor immune response by regulating inflammatory cytokines such as TNF-α and CXCL1/KC, and activated tumor-infiltrating CD8⁺ T cells that were inhibited by Tg-mediated ER stress. These results suggest that significant ER stress in a tumor-bearing host might induce tumor growth mediated by enhancement of MDSC-mediated suppression. Therefore, ER stress reducers such as 4-PBA could restore anti-tumor immunity by inhibiting suppressive MDSCs that are exacerbated by ER stress.

Transcriptional regulation of myeloid-derived suppressor cells

Myeloid-derived suppressor cells are a heterogeneous group of pathologically activated immature cells that play a major role in the negative regulation of the immune response in cancer, autoimmunity, many chronic infections, and inflammatory conditions, as well as in the regulation of tumor angiogenesis, tumor cell invasion, and metastases. Accumulation of myeloid-derived suppressor cells is governed by a network of transcriptional regulators that could be combined into 2 partially overlapping groups: factors promoting myelopoiesis and preventing differentiation of mature myeloid cells and factors promoting pathologic activation of myeloid-derived suppressor cells. In this review, we discuss the specific nature of these factors and their impact on myeloid-derived suppressor cell development.

MDSCs in cancer: Conceiving new prognostic and therapeutic targets

The incomplete clinical efficacy of anti-tumor immunotherapy can depend on the presence of an immunosuppressive environment in the host that supports tumor progression. Tumor-derived cytokines and growth factors induce an altered hematopoiesis that modifies the myeloid cell differentiation process, promoting proliferation and expansion of cells with immunosuppressive skills, namely myeloid derived suppressor cells (MDSCs). MDSCs promote tumor growth not only by shaping immune responses towards tumor tolerance, but also by supporting several processes necessary for the neoplastic progression such as tumor angiogenesis, cancer stemness, and metastasis dissemination. Thus, MDSC targeting represents a promising tool to eliminate host immune dysfunctions and increase the efficacy of immune-based cancer therapies.

Waldenström macroglobulinemia: What a hematologist needs to know

Waldenström macroglobulinemia (WM) is a distinct hematologic malignancy characterized by a lymphoplasmacytic bone marrow infiltration and the presence of immunoglobulin (Ig)M monoclonal protein. Patients typically present at an advanced age, and a substantial proportion are asymptomatic at diagnosis. A unifying diagnosis of WM may be missed by an unsuspecting hematologist, as symptomatic patients present with a multitude of non-specific manifestations. Although constitutional and neuropathy-related symptoms predominate, concomitant IgM-induced hyperviscosity-associated features can provide useful diagnostic clues. There are specific indications for initiation of therapy. This review focuses on the most up-to-date management strategies of WM, in addition to highlighting the recent discoveries of MYD88 and CXCR4 mutations that have shed unprecedented light on the complex signaling pathways, and opened avenues for novel therapeutic targeting. Although WM remains incurable, with the rapid emergence and integration of effective novel therapies, its clinical course appears poised to improve in the foreseeable future.

Unique Roles of TLR9- and MyD88-Dependent and -Independent Pathways in Adaptive Immune Responses to AAV-Mediated Gene Transfer

The immune system represents a significant barrier to successful gene therapy with adeno-associated viral (AAV) vectors. In particular, adaptive immune responses to the viral capsid or the transgene product are of concern. The sensing of AAV by toll-like receptors (TLRs) TLR2 and TLR9 has been suggested to play a role in innate immunity to the virus and may also shape subsequent adaptive immune responses. Here, we investigated the functions of TLR2, TLR9 and the downstream signaling adaptor MyD88 in antibody and CD8+ T-cell responses. Antibody formation against the transgene product occurred largely independently of TLR signaling following gene transfer with AAV1 or AAV2 vectors, whereas loss of signaling through the TLR9-MyD88 pathway substantially reduced CD8+ T-cell responses. In contrast, MyD88 (but neither of the TLRs) regulated antibody responses to capsid. B cell-intrinsic MyD88 was required for the formation of anti-capsid IgG2c independently of vector serotype or route of administration. However, MyD88(-/-) mice instead produced anti-capsid IgG1 that emerged with delayed kinetics but nonetheless completely prevented in vivo readministration. We conclude that there are distinct roles for TLR9 and MyD88 in promoting adaptive immune responses to AAV-mediated gene transfer and that there are redundant MyD88-dependent and MyD88-independent mechanisms that stimulate neutralizing antibody formation against AAV.

Formation and role of exosomes in cancer

Exosomes offer new insight into cancer biology with both diagnostic and therapeutic implications. Because of their cell-to-cell communication, exosomes influence tumor progression, metastasis, and therapeutic efficacy. They can be isolated from blood and other bodily fluids to reveal disease processes occurring within the body, including cancerous growth. In addition to being a reservoir of cancer biomarkers, they can be re-engineered to reinstate tumor immunity. Tumor exosomes interact with various cells of the microenvironment to confer tumor-advantageous changes that are responsible for stromal activation, induction of the angiogenic switch, increased vascular permeability, and immune escape. Exosomes also contribute to metastasis by aiding in the epithelial-to-mesenchymal transition and formation of the pre-metastatic niche. Furthermore, exosomes protect tumor cells from the cytotoxic effects of chemotherapy drugs and transfer chemoresistance properties to nearby cells. Thus, exosomes are essential to many lethal elements of cancer and it is important to understand their biogenesis and role in cancer.

Myeloid-derived suppressor cells: the dark knight or the joker in viral infections?

Myeloid derived suppressor cells (MDSCs) are immature cells of myeloid origin, frequently found in tumor microenvironments and in the blood of cancer patients. In recent years, MDSCs have also been found in non-cancer settings, including a number of viral infections. The evasion of host immunity employed by viruses to establish viral persistence strikingly parallels mechanisms of tumor escape, prompting investigations into the generation and function of MDSCs in chronic viral infections. Importantly, analogous to the tumor microenvironment, MDSCs effectively suppress antiviral host immunity by limiting the function of several immune cells including T cells, natural killer cells, and antigen-presenting cells. In this article, we review studies on the mechanisms of MDSC generation, accumulation, and survival in an effort to understand their emergent importance in viral infections. We include a growing list of viral infections in which MDSCs have been reported. Finally, we discuss how MDSCs might play a role in establishing chronic viral infections and identify potential therapeutics that target MDSCs.