Isolation and immunosuppressive functions of myeloid-derived suppressor cell-derived exosomes

Myeloid-derived suppressor cells (MDSCs) are an integral part of the tumor microenvironment (TME). MDSC's involvement in the TME starts as soon as the primary tumor starts to get its blood supply causing an immunosuppressive environment and tumor cell invasion, and then at the formation of premetastatic niche through full-blown metastasis in distal organs. All of these functions don't require physical interaction of MDSC as some of the MDSC's functions can be replicated by secreted exosomes (MDSC-derived exosomes), which can alter the microenvironment through cellular interaction by fusion with the plasma membrane and subsequent release of their cargo, consisting of proteins, soluble factors, lipids, DNAs, microRNAs (miRNAs), and RNAs. In this method paper, we explained how to isolate MDSC exosomes and how to use the exosome to observe immunosuppressive function. We also discussed how to measure the number of exosomes by nanoparticle tracking analysis. Additionally, we outlined how to measure the protein of exosomes as well as the types of protein by Bradford assay and membrane cytokine array respectively. We also provided instructions on how to utilize MDSC-derived exosomes to get knowledge about in vitro immune cell migration, scratch assay with the tumor cells, and in vivo effect of MDSC exosome along with T cell function and proliferation.

Exploring the dynamic interplay between cancer stem cells and the tumor microenvironment: implications for novel therapeutic strategies

Cancer stem cells (CSCs) have emerged as key contributors to tumor initiation, growth, and metastasis. In addition, CSCs play a significant role in inducing immune evasion, thereby compromising the effectiveness of cancer treatments. The reciprocal communication between CSCs and the tumor microenvironment (TME) is observed, with the TME providing a supportive niche for CSC survival and self-renewal, while CSCs, in turn, influence the polarization and persistence of the TME, promoting an immunosuppressive state. Consequently, these interactions hinder the efficacy of current cancer therapies, necessitating the exploration of novel therapeutic approaches to modulate the TME and target CSCs. In this review, we highlight the intricate strategies employed by CSCs to evade immune surveillance and develop resistance to therapies. Furthermore, we examine the dynamic interplay between CSCs and the TME, shedding light on how this interaction impacts cancer progression. Moreover, we provide an overview of advanced therapeutic strategies that specifically target CSCs and the TME, which hold promise for future clinical and translational studies in cancer treatment.

Myeloid-derived suppressor cells impair CD4+ T cell responses during chronic Staphylococcus aureus infection via lactate metabolism

Staphylococcus aureus is an important cause of chronic infections resulting from the failure of the host to eliminate the pathogen. Effective S. aureus clearance requires CD4+ T cell-mediated immunity. We previously showed that myeloid-derived suppressor cells (MDSC) expand during staphylococcal infections and support infection chronicity by inhibiting CD4+ T cell responses. The aim of this study was to elucidate the mechanisms underlying the suppressive effect exerted by MDSC on CD4+ T cells during chronic S. aureus infection. It is well known that activated CD4+ T cells undergo metabolic reprogramming from oxidative metabolism to aerobic glycolysis to meet their increased bioenergetic requirements. In this process, pyruvate is largely transformed into lactate by lactate dehydrogenase with the concomitant regeneration of NAD+, which is necessary for continued glycolysis. The by-product lactate needs to be excreted to maintain the glycolytic flux. Using SCENITH (single-cell energetic metabolism by profiling translation inhibition), we demonstrated here that MDSC inhibit CD4+ T cell responses by interfering with their metabolic activity. MDSC are highly glycolytic and excrete large amount of lactate in the local environment that alters the transmembrane concentration gradient and prevent removal of lactate by activated CD4+ T. Accumulation of endogenous lactate impedes the regeneration of NAD+, inhibit NAD-dependent glycolytic enzymes and stop glycolysis. Together, the results of this study have uncovered a role for metabolism on MDSC suppression of CD4+ T cell responses. Thus, reestablishment of their metabolic activity may represent a mean to improve the functionality of CD4+ T cells during chronic S. aureus infection.

Targeting tumour-reprogrammed myeloid cells: the new battleground in cancer immunotherapy

Tumour microenvironment is a complex ecosystem in which myeloid cells are the most abundant immune elements. This cell compartment is composed by different cell types, including neutrophils, macrophages, dendritic cells, and monocytes but also unexpected cell populations with immunosuppressive and pro-tumour roles. Indeed, the release of tumour-derived factors influences physiological haematopoiesis producing unconventional cells with immunosuppressive and tolerogenic functions such as myeloid-derived suppressor cells. These pro-tumour myeloid cell populations not only support immune escape directly but also assist tumour invasion trough non-immunological activities. It is therefore not surprising that these cell subsets considerably impact in tumour progression and cancer therapy resistance, including immunotherapy, and are being investigated as potential targets for developing a new era of cancer therapy. In this review, we discuss emerging strategies able to modulate the functional activity of these tumour-supporting myeloid cells subverting their accumulation, recruitment, survival, and functions. These innovative approaches will help develop innovative, or improve existing, cancer treatments.

Prolonged survival of anaplastic thyroid carcinoma is associated with resectability, low tumor-infiltrating neutrophils/myeloid-derived suppressor cells, and low peripheral neutrophil-to-lymphocyte ratio

PURPOSE

Anaplastic thyroid carcinoma (ATC) is the most lethal form of thyroid cancer with most patients dying of their disease within a few months. Only a very small percentage of long-term survivors (LTS) are alive for 2 years or longer. In this retrospective case-control study, we provided a comprehensive comparison between 46 ATC LTSs and 75 ATC control patients who suffered disease-specific mortality within 2 years, aiming to identify factors that may be associated with prolonged survival in ATC.

METHODS

A comprehensive clinicopathologic and molecular comparison was performed between 46 ATC LTSs and 75 ATC control patients. Peripheral neutrophil count and neutrophil-to-lymphocyte ratio (NLR) were recorded. The composition of the tumor microenvironment was compared using immunohistochemistry.

RESULTS

Compared with ATC control patients, ATC LTSs were characterized by 1) higher frequency of (primary) resection as well as clinicopathologic parameters attributed to resectability; 2) lower rate of concurrent RAS/BRAF and TERT promoter mutations; 3) lower peripheral neutrophil count and NLR; and 4) lower number of tumor-infiltrating neutrophils/myeloid-derived suppressor cells (MDSC). The survival benefits of low peripheral neutrophil counts and low NLR persisted even when controlling for distant metastasis status at presentation.

CONCLUSIONS

In addition to traditional beneficial prognostic factors, e.g., surgical resection, factors attributed to resectability, and absence of co-existing RAS/BRAF and TERT promoter mutations, we herein show that tumor-infiltrating and circulating neutrophils/MDSC are adverse prognostic factors in ATC.

Systemic but not MDSC-specific IRF4 deficiency promotes an immunosuppressed tumor microenvironment in a murine pancreatic cancer model

Pancreatic ductal adenocarcinoma is characterized by a strong immunosuppressive network with a dense infiltration of myeloid cells including myeloid-derived suppressor cells (MDSC). Two distinct populations of MDSC have been defined: polymorphonuclear MDSC (PMN-MDSC) and monocytic MDSC (M-MDSC). Several factors influence the development and function of MDSC including the transcription factor interferon regulatory factor 4 (IRF4). Here, we show that IRF4 deficiency accelerates tumor growth and reduces survival, accompanied with a dense tumor infiltration with PMN-MDSC and reduced numbers of CD8⁺ T cells. As IRF4 has been described to modulate myeloid cell development and function, particularly of PMN-MDSC, we analyzed its role using MDSC-specific IRF4 knockout mice with the Ly6G or LysM knock-in allele expressing Cre recombinase and Irf4^flox. In GM-CSF-driven bone marrow cultures, IRF4 deficiency increased the frequency of MDSC-like cells with a strong T cell suppressive capacity. Myeloid (LysM)-specific depletion of IRF4 led to increased tumor weight and a moderate splenic M-MDSC expansion in tumor-bearing mice. PMN cell (Ly6G)-specific depletion of IRF4, however, did not influence tumor progression or MDSC accumulation in vivo in accordance with our finding that IRF4 is not expressed in PMN-MDSC. This study demonstrates a critical role of IRF4 in the generation of an immunosuppressive tumor microenvironment in pancreatic cancer, which is independent of IRF4 expression in PMN-MDSC.

Immunosuppression by monocytic myeloid-derived suppressor cells in patients with pancreatic ductal carcinoma is orchestrated by STAT3

Background

Pancreatic ductal adenocarcinoma (PDAC) is a highly devastating disease with an overall 5-year survival rate of less than 8%. New evidence indicates that PDAC cells release pro-inflammatory metabolites that induce a marked alteration of normal hematopoiesis, favoring the expansion and accumulation of myeloid-derived suppressor cells (MDSCs). We report here that PDAC patients show increased levels of both circulating and tumor-infiltrating MDSC-like cells.

Methods

The frequency of MDSC subsets in the peripheral blood was determined by flow cytometry in three independent cohorts of PDAC patients (total analyzed patients, n = 117). Frequency of circulating MDSCs was correlated with overall survival of PDAC patients. We also analyzed the frequency of tumor-infiltrating MDSC and the immune landscape in fresh biopsies. Purified myeloid cell subsets were tested in vitro for their T-cell suppressive capacity.

Results

Correlation with clinical data revealed that MDSC frequency was significantly associated with a shorter patients’ overall survival and metastatic disease. However, the immunosuppressive activity of purified MDSCs was detectable only in some patients and mainly limited to the monocytic subset. A transcriptome analysis of the immunosuppressive M-MDSCs highlighted a distinct gene signature in which STAT3 was crucial for monocyte re-programming. Suppressive M-MDSCs can be characterized as circulating STAT3/arginase1-expressing CD14⁺ cells.

Conclusion

MDSC analysis aids in defining the immune landscape of PDAC patients for a more appropriate diagnosis, stratification and treatment.

Differences of tumor-recruiting myeloid cells in murine squamous cell carcinoma influence the efficacy of immunotherapy combined with a TLR7 agonist and PD-L1 blockade

OBJECTIVES

The immune status of the tumor microenvironment has a marked impact on clinical outcomes. Here we examined the immune environments of tumor-infiltrating leukocytes (TILes) in two murine models of squamous cell carcinoma and compared the effects of immunotherapeutic agents, including a TLR7 agonist and an immune checkpoint inhibitor, and a chemotherapeutic agent, gemcitabine, in these models.

MATERIALS AND METHODS

TILes from NR-S1- and SCCVII-grafted mice were analyzed by flow cytometry. NR-S1-inoculated mice received resiquimod (a synthetic TLR7 agonist), an anti-PD-L1 antibody, or both, and tumor growth and TILs were examined. Gemcitabine was administered to deplete CD11b⁺ cells.

RESULTS

More than 50% of TILes from NR-S1- and SCCVII-inoculated mice were CD11b⁺Gr-1⁺ cells. A major fraction of NR-S1 CD11b⁺ cells was Ly6G^highLy6C^low-negaF4/80^- tumor-associated neutrophils (TANs) and the majority of SCCVII CD11b⁺ cells were Ly6G^lowLy6C^-F4/80⁺ tumor-associated macrophages. NR-S1 TANs did not express MHC class II and CD86, but did express reactive oxygen species and PD-L1. Resiquimod, alone and in combination with an anti-PD-L1 antibody, did not regress NR-S1 tumors, but the combination increased the CD8/regulatory T cell-ratio, and IFN-γ and PD-1 expression in CD8⁺ TILes. Pre-administration of low-dose gemcitabine prior to the combination treatment suppressed the progression of NR-S1 tumors.

CONCLUSIONS

NR-S1 tumors with abundant recruitment of TANs were resistant to treatments with a TLR7 agonist, alone and in combination with PD-1 blockade, and required an additional gemcitabine treatment. The phenotype and status of tumor-infiltrating CD11b⁺ myeloid cells may influence the efficacy of immunotherapeutic agents.

Transmembrane tumor necrosis factor-α promotes the recruitment of MDSCs to tumor tissue by upregulating CXCR4 expression via TNFR2

Myeloid-derived suppressor cells (MDSCs) accumulated in tumor sites promote immune evasion. We found that TNFR deficiency-induced rejection of transplanted tumor was accompanied with markedly decreased accumulation of MDSCs. However, the mechanism(s) behind this phenomenon is not completely understood. Here, we demonstrated that TNFR deficiency did not affect the amount of MDSCs in bone marrow (BM), but decreased accumulation of Gr-1⁺CD11b⁺ MDSCs in the spleen and tumor tissues. The chemotaxis of Tnfr^-/- MDSCs was prominently decreased in response to both tumor cell culture supernatants and tumor tissue homogenates from Tnfr^-/- and wild-type mice, indicating an effect of TNFR signaling on chemokine receptor expression in MDSCs. We used real-time PCR to detect gene expression for several chemokine receptors in MDSCs from BM and found that CXCR4 was the most affected molecule at the transcriptional level in Tnfr^-/- MDSCs. Neutralizing CXCR4 in wild-type MDSCs by a specific antibody blocked their chemotactic migration. Interestingly, it was tmTNF-α, but not sTNF-α, that induced CXCR4 expression in MDSCs. This effect of tmTNF-α was totally blocked in TNFR2^-/- but not in TNFR1^-/- MDSCs, and partially inhibited by PDTC or SB203580, an inhibitor of NF-κB or p38 MAPK pathway, respectively. Adoptive transfer of wild-type MDSCs restored MDSCs accumulation in tumors of Tnfr^-/- mice, but this could be partially blocked by treatment with a CXCR4 inhibitor AMD3100. Our data suggest that tmTNF-α upregulates CXCR4 expression that promotes chemotaxis of MDSCs to tumor, and give a new insight into a novel mechanism by which tmTNF-α facilitates tumor immune evasion.

Attenuation of Hepatic Graft-versus-host Disease in Allogeneic Recipients of MyD88-deficient Donor Bone Marrow

Acute graft-versus-host-disease (GVHD) is characterized by selective damage to the liver, the skin, and the gastrointestinal tract. Following allogeneic hematopoietic stem cell transplantation, donor bone marrow (BM) cells repopulate the immune system of the recipient. We previously demonstrated that the acute intestinal GVHD (iGVHD) mortality rate was higher in MyD88-deficient BM recipients than that in the control BM recipients. In the present study, the role of MyD88 (expressed by donor BM) in the pathophysiology of hepatic GVHD (hGVHD) was examined. Unlike iGVHD, transplantation with MyD88-deficient T-cell depleted (TCD) BM attenuated hGVHD severity and was associated with low infiltration of T cells into the liver of the recipients. Moreover, GVHD hosts, transplanted with MyD88-deficient TCD BM, exhibited markedly reduced expansion of CD11b⁺Gr-1⁺ myeloid-derived suppressor cells (MDSC) in the liver. Adoptive injection of the MDSC from wild type mice, but not MyD88-deficient mice, enhanced hepatic T cell infiltration in the MyD88-deficient TCD BM recipients. Pre-treatment of BM donors with LPS increased MDSC levels in the liver of allogeneic wild type BM recipients. In conclusion, hGVHD and iGVHD may occur through various mechanisms based on the presence of MyD88 in the non-T cell compartment of the allograft.