Myeloid-derived Suppressor Cells In Mice Research Articles

Peptostreptococcus anaerobius mediates anti-PD1 therapy resistance and exacerbates colorectal cancer via myeloid-derived suppressor cells in mice

Bacteria such as the oral microbiome member Peptostreptococcus anaerobius can exacerbate colorectal cancer (CRC) development. Little is known regarding whether these immunomodulatory bacteria also affect antitumour immune checkpoint blockade therapy. Here we show that administration of P. anaerobius abolished the efficacy of anti-PD1 therapy in mouse models of CRC. P. anaerobius both induced intratumoral myeloid-derived suppressor cells (MDSCs) and stimulated their immunosuppressive activities to impair effective T cell responses. Mechanistically, P. anaerobius administration activated integrin α2β1–NF-κB signalling in CRC cells to induce secretion of CXCL1 and recruit CXCR2+ MDSCs into tumours. The bacterium also directly activated immunosuppressive activity of intratumoral MDSCs by secreting lytC_22, a protein that bound to the Slamf4 receptor on MDSCs and promoted ARG1 and iNOS expression. Finally, therapeutic targeting of either integrin α2β1 or the Slamf4 receptor were revealed as promising strategies to overcome P. anaerobius-mediated resistance to anti-PD1 therapy in CRC.

ALKBH5 regulates arginase 1 expression in MDSCs and their immunosuppressive activity in tumor-bearing host

Myeloid-derived suppressor cells (MDSCs) are closely related to the occurrence and development of many cancers, but the specific mechanism is not fully understood. It has been found that N6-methyladenosine (m6A) plays a key role in RNA metabolism, but its function in MDSCs has yet to be revealed. In this study, we found that MDSCs in mice with colorectal cancer (CRC) have significantly elevated levels of m6A, while ALKBH5 expression is decreased. Overexpression of ALKBH5 can reduce the immunosuppressive function of MDSCs in vivo and in vitro, and attenuates the protumorigenic ability of MDSCs. Mechanism study found that the overexpression of ALKBH5 in MDSCs reduced the m6A modification level of Arg-1 mRNA, and then weakened the stability of Arg-1 mRNA and protein expression. These data suggest that the decreased expression of ALKBH5 in CRC tumor mice may promote the expression of Arg-1, enhance the immunosuppressor function of MDSCs, and promote tumor growth. These findings highlight that ALKBH5 may regulate the function of MDSCs in tumor-bearing mice and may be a new target for immunotherapy. This research provides a new perspective for our understanding of the role of MDSCs in cancer development, and also brings new hope for cancer treatment.

Myeloid-derived suppressor cells exacerbate poly(I:C)-induced lung inflammation in mice with renal injury and older mice.

Viral pneumonia is a global health burden with a high mortality rate, especially in the elderly and in patients with underlying diseases. Recent studies have found that myeloid-derived suppressor cells (MDSCs) are abundant in these patient groups; however, their roles in the progression of viral pneumonia remain unclear. In this study, we observed a substantial increase in MDSCs in a mouse model of renal ischemia/reperfusion (I/R) injury and in older mice. When intranasal polyinosinic-polycytidylic acid (poly(I:C)) administration was used to mimic viral pneumonia, mice with renal I/R injury exhibited more severe lung inflammation than sham mice challenged with poly(I:C). In addition, MDSC depletion attenuated lung inflammation in mice with I/R injury. Similar results were obtained in older mice compared with those in young mice. Furthermore, adoptive transfer of in vitro-differentiated MDSCs exacerbated poly(I:C)-induced lung inflammation. Taken together, these experimental results suggest that the increased proportion of MDSCs in mice with renal I/R injury and in older mice exacerbates poly(I:C)-induced lung inflammation. These findings have important implications for the treatment and prevention of severe lung inflammation caused by viral pneumonia.

Fluorescent Detection of Peroxynitrite Produced by Myeloid-Derived Suppressor Cells in Cancer and Inhibition by Dasatinib.

Myeloid-derived suppressor cells (MDSCs) are immature myeloid cells that expand dramatically in many cancer patients. This expansion contributes to immunosuppression in cancer and reduces the efficacy of immune-based cancer therapies. One mechanism of immunosuppression mediated by MDSCs involves production of the reactive nitrogen species peroxynitrite (PNT), where this strong oxidant inactivates immune effector cells through destructive nitration of tyrosine residues in immune signal transduction pathways. As an alternative to analysis of nitrotyrosines indirectly generated by PNT, we used an endoplasmic reticulum (ER)-targeted fluorescent sensor termed PS3 that allows direct detection of PNT produced by MDSCs. When the MDSC-like cell line MSC2 and primary MDSCs from mice and humans were treated with PS3 and antibody-opsonized TentaGel microspheres, phagocytosis of these beads led to production of PNT and generation of a highly fluorescent product. Using this method, we show that splenocytes from a EMT6 mouse model of cancer, but not normal control mice, produce high levels of PNT due to elevated numbers of granulocytic (PMN) MDSCs. Similarly, peripheral blood mononuclear cells (PBMCs) isolated from blood of human melanoma patients produced substantially higher levels of PNT than healthy human volunteers, coincident with higher peripheral MDSC levels. The kinase inhibitor dasatinib was found to potently block the production of PNT both by inhibiting phagocytosis in vitro and by reducing the number of granulocytic MDSCs in mice in vivo, providing a chemical tool to modulate the production of this reactive nitrogen species (RNS) in the tumor microenvironment.

Abstract 1259: Disturbing the Siglec-Sialoglycan axis to target myeloid- derived suppressor cells in the tumor microenvironment

Abstract Background: Overexpression of sialic acids on glycans, called hypersialylation is a common alteration in cancer that drives immune evasion via interaction with Sialic acid-binding immunoglobulin-like lectin (Siglec) immunoregulatory receptors on tumor-infiltrating immune cells. Myeloid derived suppressor cells (MDSCs) generated by cancer-induced aberrant myelopoiesis are highly immunosuppressive cells inhibiting immune cells via direct interaction or secretion of suppressive cytokines. Targeting the complex suppressive tumor microenvironment (TME) remains a challenge. Methods: Siglec expression of murine and human MDSCs in healthy conditions and tumor setting was assessed by flow cytometry. Functional analysis of Siglec-E knockout on MDSCs in mice was evaluated using SigE-LysMCre mice and suppressive capacity was tested in vitro in combination with SiglecE blocking and cleavage of Sialoglycans using Sialidase. Results were confirmed in the human setting using an in vitro assay to generate MDSC-like cells including RNA-Sequencing and a MDSC suppression assay with cancer-derived MDSCs. Results: MDSCs express multiple inhibitory Siglec receptors in humans and mice. Knockout of SiglecE in the myeloid compartment in vivo resulted in prolonged survival and increased T cell infiltration compared to litter mates expressing Siglec-E. Of note, desialylation of MDSCs and Siglec-E blocking in vitro reduced MDSCs suppressive function against T cells. Similarly, reducing Sialoglycans by Sialidase in human MDSC generated in vitro resulted in increased suppressive capacity and downregulation of functional MDSC markers on RNA level. Findings were validated with lung cancer patient derived MDSCs. Conclusion: Our results provide first insights into the importance of the Siglec-Sialoglycans axis to modulate MDSCs in the TME. Targeting glycosylation of MDSCs with Sialidase reduces their suppressive capacity making it a powerful approach to improve cancer immunotherapy. Further studies are needed to reveal the underlying mechanisms. Citation Format: Ronja Wieboldt, Andreas Zingg, Emanuele Carlini, Anastasiya Börsch, Heinz Läubli, Natalia Rodrigues Manutano. Disturbing the Siglec-Sialoglycan axis to target myeloid- derived suppressor cells in the tumor microenvironment [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1259.

Inhibition of myeloid-derived suppressor cell arginase-1 production enhances T-cell-based immunotherapy against Cryptococcus neoformans infection

Cryptococcosis is a potentially lethal disease that is primarily caused by the fungus Cryptococcus neoformans, treatment options for cryptococcosis are limited. Here, we show glucuronoxylomannan, the major polysaccharide component of C. neoformans, induces the recruitment of neutrophilic myeloid-derived suppressor cells in mice and patients with cryptococcosis. Depletion of neutrophilic myeloid-derived suppressor cells enhances host defense against C. neoformans infection. We identify C-type lectin receptor-2d recognizes glucuronoxylomannan to potentiate the immunosuppressive activity of neutrophilic myeloid-derived suppressor cells by initiating p38-mediated production of the enzyme arginase-1, which inhibits T-cell mediated antifungal responses. Notably, pharmacological inhibition of arginase-1 expression by a specific inhibitor of p38, SB202190, or an orally available receptor tyrosine kinase inhibitor, vandetanib, significantly enhances T-cell mediated antifungal responses against cryptococcosis. These data reveal a crucial suppressive role of neutrophilic myeloid-derived suppressor cells during cryptococcosis and highlight a promising immunotherapeutic application by inhibiting arginase-1 production to combat infectious diseases.

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

SummaryBackground. 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.

Development of immunosuppressive myeloid cells to induce tolerance in solid organ and hematopoietic cell transplant recipients

AbstractReplacement of failed organs followed by safe withdrawal of immunosuppressive drugs has long been the goal of organ transplantation. We studied changes in the balance of T cells and myeloid cells in the blood of HLA-matched and -mismatched patients given living donor kidney transplants followed by total lymphoid irradiation, anti-thymocyte globulin conditioning, and donor hematopoietic cell transplant to induce mixed chimerism and immune tolerance. The clinical trials were based on a conditioning regimen used to establish mixed chimerism and tolerance in mice. In preclinical murine studies, there was a profound depletion of T cells and an increase in immunosuppressive polymorphonuclear (pmn) myeloid-derived suppressor cells (MDSCs) in the spleen and blood following transplant. Selective depletion of pmn MDSCs in mice abrogated mixed chimerism and tolerance. In our clinical trials, patients given an analogous tolerance conditioning regimen developed similar changes, including profound depletion of T cells and a marked increase in MDSCs in blood posttransplant. Posttransplant pmn MDSCs transiently increased expression of lectin-type oxidized LDL receptor-1, a marker of immunosuppression, and production of the T-cell inhibitor arginase-1. These posttransplant pmn MDSCs suppressed the activation, proliferation, and inflammatory cytokine secretion of autologous T-cell receptor microbead-stimulated pretransplant T cells when cocultured in vitro. In conclusion, we elucidated changes in receptors and function of immunosuppressive myeloid cells in patients enrolled in the tolerance protocol that were nearly identical to those of MDSCs required for tolerance in mice. These trials were registered at www.clinicaltrials.gov as #NCT00319657 and #NCT01165762.

Distinct mechanisms govern populations of myeloid-derived suppressor cells in chronic viral infection and cancer.

Myeloid-derived suppressor cells (MDSCs) are major negative regulators of immune responses in cancer and chronic infections. It remains unclear if regulation of MDSC activity in different conditions is controlled by similar mechanisms. We compared MDSCs in mice with cancer and lymphocytic choriomeningitis virus (LCMV) infection. Chronic LCMV infection caused the development of monocytic MDSCs (M-MDSCs) but did not induce polymorphonuclear MDSCs (PMN-MDSCs). In contrast, both MDSC populations were present in cancer models. An acquisition of immune-suppressive activity by PMN-MDSCs in cancer was controlled by IRE1α and ATF6 pathways of the endoplasmic reticulum (ER) stress response. Abrogation of PMN-MDSC activity by blockade of the ER stress response resulted in an increase in tumor-specific immune response and reduced tumor progression. In contrast, the ER stress response was dispensable for suppressive activity of M-MDSCs in cancer and LCMV infection. Acquisition of immune-suppressive activity by M-MDSCs in spleens was mediated by IFN-γ signaling. However, it was dispensable for suppressive activity of M-MDSCs in tumor tissues. Suppressive activity of M-MDSCs in tumors was retained due to the effect of IL-6 present at high concentrations in the tumor site. These results demonstrate disease- and population-specific mechanisms of MDSC accumulation and the need for targeting different pathways to achieve inactivation of these cells.

TIGIT/CD155 blockade enhances anti-PD-L1 therapy in head and neck squamous cell carcinoma by targeting myeloid-derived suppressor cells

ObjectivesAnti-PD-1/PD-L1 therapy has recently been approved for head and neck squamous cell carcinoma (HNSCC). However, given that large numbers of patients with HNSCC do not respond to PD-1/PD-L1 antibodies, combination strategies for elevating the response rate need to be further investigated. The goal of this study was to explore the possibility of dual-targeting CD155/TIGIT and PD-1/PD-L1 signalling in HNSCC. Materials and methodsMultiplex flow cytometry was performed to determine the co-expression of CD155 and PD-L1 in human HNSCC and transgenic HNSCC mouse models. The combined application of TIGIT mAb and PD-L1 mAb in a mouse model was used to explore the therapeutic effect. ResultsCD155 and PD-L1 were highly co-expressed on myeloid-derived suppressor cells (MDSCs) derived from patients with HNSCC and were inversely associated with the percentage of tumour CD3+ T and effector memory T cells. CD155+PD-L1+ MDSCs in the mouse model were gradually enriched in the tumour microenvironment in the middle and late stages of tumour progression. Anti-PD-L1 treatment alone upregulated the expression of CD155 on MDSCs and while anti-TIGIT treatment upregulated the expression of PD-L1 on MDSCs in mice. The combined blockade of TIGIT/CD155 and PD-1/PD-L1 signalling in mice significantly inhibited tumour growth, enhanced the percentages of effector T cells and cytokine secretion and elicited immune memory effects. ConclusionOur study indicated that CD155+PD-L1+ MDSCs are enriched in the tumour microenvironment and blocking TIGIT/CD155 can effectively enhance the response rate of HNSCC to PD-L1 mAb therapy, which provides the clinical potential of co-targeting TIGIT/CD155 and PD-1/PD-L1 signalling.

MDSC in Mice and Men: Mechanisms of Immunosuppression in Cancer.

Myeloid-derived suppressor cells (MDSCs) expand during pathological conditions in both humans and mice and their presence is linked to poor clinical outcomes for cancer patients. Studying MDSC immunosuppression is restricted by MDSCs’ rarity, short lifespan, heterogeneity, poor viability after freezing and the lack of MDSC-specific markers. In this review, we will compare identification and isolation strategies for human and murine MDSCs. We will also assess what direct and indirect immunosuppressive mechanisms have been attributed to MDSCs. While some immunosuppressive mechanisms are well-documented in mice, e.g., generation of ROS, direct evidence is still lacking in humans. In future, bulk or single-cell genomics could elucidate which phenotypic and functional phenotypes MDSCs adopt in particular microenvironments and help to identify potential targets for therapy.

Myeloid-derived suppressor cells improve corneal graft survival through suppressing angiogenesis and lymphangiogenesis.

Myeloid-derived suppressor cells (MDSC) are one of the major negative regulators of immune responses during many pathological conditions such as cancer and transplantation. Emerging evidence indicates that MDSC also contribute to tumor progression through their pro-angiogenic activity in addition to immunosuppressive function. However, virtually nothing is known about the role of MDSC in the regulation of neovascularization after transplantation. Here we showed that antibody-mediated depletion of MDSC in mice led to robust growth of blood and lymphatic neovessels and rapid allograft rejection after corneal penetrating keratoplasty. In contrast, adoptive transfer of ex vivo generated MDSC from cytokine-treated bone marrow cells (evMDSC) suppressed neovascularization and prolonged corneal allograft survival in an inducible nitric oxide synthase (iNOS)-dependent manner. Mechanistically, compared to naïve MDSC control, evMDSC have increased expression of an anti-angiogenic factor thrombospondin 1 (Tsp-1) and decreased expression of two critical pro-angiogenic factors, vascular endothelial growth factor A (VEGF-A), and VEGF-C. These findings demonstrate MDSC as a critical anti-angiogenic regulator during transplantation. Our study also indicates that evMDSC are a valuable candidate agent for development of novel cell therapy to improve allograft survival after transplantation.

Stem cell factor produced by tumor cells expands myeloid-derived suppressor cells in mice

Immunotherapy is a novel treatment approach for cancers; however, its therapeutic effects are impeded by myeloid-derived suppressor cells (MDSCs). This study aimed to determine how MDSCs are expanded in cancer hosts. MDSCs were positive for Gr-1 and CD11b. Hepa1-6 hepatoma cells, EL4 lymphoma cells, and mice bearing Hepa1-6 hepatoma or lymphoma were examined. Following the inoculation of Hepa1-6 cells into the flanks of mice, a linear correlation was evident between the frequency of MDSCs in the spleen and tumor sizes. MDSC numbers diminished gradually and returned to the normal level within 3 weeks if the tumors were excised. To identify the cytokines produced by tumor cells that allowed expansion of MDSCs, cytokines in Hepa1-6 cell culture medium and murine serum were examined using a cytokine array. Stem cell factor (SCF) was implicated as the relevant cytokine. When recombinant SCF was added to the spleen cell culture medium, MDSC expansion could occur. In the presence of c-kit blockade, this effect of SCF was partially reversed. In conclusion, MDSCs can be expanded in tumor cells in a process that involves SCF released by tumor cells.

Sildenafil might impair maternal-fetal immunotolerance by suppressing myeloid-derived suppressor cells in mice

Myeloid-derived suppressor cells (MDSCs) as an important population of immune cells were found to restrain T cell function, polarize T-helper cells (Th) 1/Th2 toward Th2 response and induce regulatory T cells (Tregs), therefore enhancing the immunotolerance during pregnancy. Sildenafil has been applied for poor endometrial quality in implantation failure patients. Nevertheless, investigations have shown that sildenafil could reduce MDSCs-dependent immunosuppression. Whether sildenafil affects embryo implantation by suppressing MDSCs? To address this question, using the mice model, we investigated the amounts of immune cells in peripheral blood and endometrial cells from control group (CG), sildenafil low-dose group (LDG) and high-dose group (HDG). We found that both treatment groups displayed a marked deficiency in polymorphonuclear (PMN)-MDSCs and Th2 from mice blood and endometrium as compared to these from CG. The frequency of Tregs in endometrium from HDG was lower than those from CG. Th1/Th2 ratio in both periphery and uterus from study groups showed a significant increase as compared to those from CG. By relevance analysis, we found that the level of Tregs positively correlated with the level of PMN-MDSCs, whereas the Th1/Th2 ratio negatively correlated with the frequency of PMN-MDSCs in uterus. Moreover, there was a positive relationship between the amount of blood PMN-MDSCs and endometrial PMN-MDSCs. These results suggest that we should carefully weigh the pros and cons of using sildenafil when applied to patients with poor endometrial receptivity.

The role and mechanism of myeloid-derived suppressor cells in mice with nonalcoholic steatohepatitis

Objective To investigate the effects of myeloid-derived suppressor cells (MDSCs) on nonalcoholic steatohepatitis mice. Methods ⑴ Male C57BL/6 mice aged 6-8 weeks were randomly divided into normal diet group, CDAA group(choline deficient amino acid diet) and CSAA group (choline sufficient amino acid diet). ⑵ After the success of the non-alcoholic steatohepatitis model, serum was collected from some mice to detect biochemical indexes; Liver tissue was retained for microscopic observation by hematoxylin-eosin (HE) staining; The changes of T cell subsets in peripheral blood of mice in each group were detected by flow cytometry. In addition, The proportion and subtype of CD11b+ Gr-1+ MDSCs cells in liver, spleen, blood and bone marrow were also detected by flow cytometry. ⑶ The bone marrow-derived Gr-1highLy-6G+ was purified by magnetic bead sorting technique, and the purified Gr-1highLy-6G+ was transferred into non-alcoholic steatohepatitis (NASH) mice by tail vein injection. The role of MDSCs in NASH was analyzed by detection of serological indexes of liver function and pathological dyeing. Results ⑴ There was no significant difference in body weight and liver index between the groups (P>0.05). Serological indicators: alanine aminotransferase (ALT), aspartate aminotransferase (AST), blood glucose, interleukin (IL)-6 and interferon-γ (INF-γ) in the model group were significantly higher than those in the normal group (P 0.05); the MDSCs of liver in CDAA group is lower than that of normal group (P normal group (P<0.01), CD11b+ Gr-1dimLy-6G-(M-MDSC) showed a downward trend, CDAA group < normal group; ⑶ Serum AST and ALT levels of NASH mice who receiving Gr-1highLy-6G+ MDSCs from normal bone marrow were significantly decreased (P<0.001), and histopathological changes were alleviated. Conclusions ⑴ The NASA mouse model can be successfully established on the CDAA diet. ⑵ The CDAA-induced NASH mice may have immune dysfunction, mainly manifesting in the change of bone marrow MDSCs subpopulations and the increase of CD11b+ Gr-1highLy-6G+ (G-MDSC). ⑶ MDSCs derived from normal mouse bone marrow can alleviate the pathological changes of NASH. Key words: Myeloid-derived suppressor cells; Non-alcoholic steatohepatitis; T-lymphocytes; Interleukin-6; Mice

Lactoferrin deficiency induces a pro-metastatic tumor microenvironment through recruiting myeloid-derived suppressor cells in mice.

Lactoferrin, an innate immunity molecule, is involved in anti-inflammatory, anti-microbial, and anti-tumor activities. We previously reported that lactoferrin is downregulated in specimens of nasopharyngeal carcinoma and negatively associated with tumor progression and metastasis of patients with nasopharyngeal carcinoma. However, the relationship between lactoferrin and the pro-metastatic microenvironment has not been reported yet. Here, by using the lactoferrin knockout mouse, we found that lactoferrin deficiency facilitated melanoma cells metastasizing to lungs, through recruiting myeloid-derived suppressor cells (MDSCs) in the lungs. Mechanistic studies showed that in the lung microenvironment of the lactoferrin knockout mice, the TLR9 signaling was the most repressed signaling. Lactoferrin can induce MDSCs differentiation and apoptosis, as well as upregulate TLR9 expression. TLR9 agonist or lactoferrin treatment can rescue this phenotype in the tumor metastasis mouse model. Our results suggest a protective role of lactoferrin in cancer metastasis, along with a deficiency in certain components of the innate immune system, may lead to a pro-metastatic tumor microenvironment.

Lipid Metabolic Pathways Confer the Immunosuppressive Function of Myeloid-Derived Suppressor Cells in Tumor.

Myeloid-derived suppressor cells (MDSCs) play crucial roles in tumorigenesis and their inhibition is critical for successful cancer immunotherapy. MDSCs undergo metabolic reprogramming from glycolysis to fatty acid oxidation (FAO) and oxidative phosphorylation led by lipid accumulation in tumor. Increased exogenous fatty acid uptake by tumor MDSCs enhance their immunosuppressive activity on T-cells thus promoting tumor progression. Tumor-infiltrating MDSCs in mice may prefer FAO over glycolysis as a primary source of energy while treatment with FAO inhibitors improved anti-tumor immunity. This review highlights the immunosuppressive functions of lipid metabolism and its signaling pathways on MDSCs in the tumor microenvironment. The manipulation of these pathways in MDSCs is relevant to understand the tumor microenvironment therefore, could provide novel therapeutic approaches to enhance cancer immunotherapy.

TRAF3 is a critical regulator of the expansion of myeloid-derived suppressor cells in mice

Abstract TRAF3 is an adaptor protein and E3 ligase that regulates the signal transduction pathways of the TNFR superfamily, pattern recognition receptors, and cytokine receptors. We previously showed that specific deletion of TRAF3 in myeloid cells leads to spontaneous inflammation and tumor development in aging M-TRAF3−/− mice, which is consistently associated with a striking expansion of CD11b+Gr1+ myeloid cells. Here we report that these CD11b+Gr1+ cells were myeloid-derived suppressor cells (MDSCs), as they exhibited a strong suppressive activity on CD8 T cell proliferation and also displayed a nuclear morphology of immature myeloid cells. Using a chronic inflammation model induced by repeated injections of heat-killed Bacillus Calmette-Guérin (BCG), we showed that young adult M-TRAF3−/− mice also exhibited hyperexpansion of MDSCs after BCG injections. Our in vivo BrdU labeling experiments revealed that MDSC hyperexpansion in M-TRAF3−/− mice was mainly due to the increased proliferation of MDSCs or progenitor cells in the spleen but not in the bone marrow (BM). Interestingly, TRAF3 does not appear to be an intrinsic regulator of the development, survival or proliferation of MDSCs, as suggested by the results of cultured BM progenitor cells and purified splenic MDSCs, in the absence or presence of CSFs or agonists of TLRs or CD40. Furthermore, we identified environmental factors that induced the aberrant expansion of MDSCs in M-TRAF3−/− mice using cytokine protein array analyses, which were further verified by macrophage cultures and in vivo neutralization studies. Taken together, our findings identify TRAF3 as a new regulator of MDSC expansion, and therefore also a critical regulator of chronic inflammation and immunosuppression.

Abstract 4756: Secretory IgM exacerbates tumor progression by inducing accumulations of myeloid-derived suppressor cells in mice

Abstract To explore the role of B cell receptor (BCR) in promoting malignant progression of chronic lymphocytic leukemia (CLL) in mice, we generate MD4/Eμ-TCL1 mice, whose B cells harbor a monoclonal BCR against hen egg lysozyme (HEL) and secrete IgM against HEL. MD4/Eμ-TCL1 mice survive significantly shorter than Eμ-TCL1 mice. While precancerous B cells in young MD4/Eμ-TCL1 mice recognize HEL, CLL cells developed in older MD4/Eμ-TCL1 mice fail to recognize HEL. Nevertheless, MD4/Eμ-TCL1 CLL cells can be activated by goat F(ab')2 anti-mouse IgM and respond by phosphorylation of Igα, Syk and ERK1/2, indicating reactivation of a parental Ig gene allele. MD4/Eμ-TCL1 CLL cells also produce large quantities of secretory IgM (sIgM), which does not react with HEL. Compared with age-matched Eμ-TCL1 mice, MD4/Eμ-TCL1 mice also generate a significantly increased population of CD11b+/Ly6G+ granulocytic cells in the peripheral blood, spleens and bone marrow. CD11b+/Ly6G+ granulocytic cells purified from spleens of MD4/Eμ-TCL1 mice can suppress CD3/CD28-mediated proliferation of CD8+ T cells and gp100-loaded class I MHC-mediated proliferation of CD8+ T cells from PMEL-1 mice, qualifying these cells as myeloid-derived suppressor cells (MDSCs). Increased MDSCs may account for significantly decreased T cells and poor prognosis in CLL-bearing MD4/Eμ-TCL1 mice. Because MD4/Eμ-TCL1 mouse CLL cells and human CLL cells can produce sIgM, we hypothesize that sIgM may account for the accumulation of MDSCs. To test this hypothesis, we cross μS-/- mice, which cannot produce sIgM, with Eμ-TCL1 mice. The μS-/-/Eμ-TCL1 mice indeed develop significantly lower numbers of MDSCs, and survive significantly longer than Eμ-TCL1 mice. We decide to target the synthesis of sIgM by deleting the function of XBP-1, because the synthesis of sIgM is tightly regulated by a mechanism called regulated IRE-1-dependent decay (RIDD), which is hyperactivated in B cells as a response to XBP-1 deficiency. We cross B cell-specific XBP-1KO mice with MD4/Eμ-TCL1 mice, and the resultant XBP-1KO/MD4/Eμ-TCL1 mice indeed produce significantly reduced amounts of sIgM and decreased numbers of MDSCs. In addition, μS-/- mice grafted with Lewis lung carcinoma exhibit reduced functions of MDSCs in suppressing T cells, resulting in significantly slower tumor growth. These results clearly demonstrate that sIgM produced by B cells can upregulate the immunosuppressive functions of MDSCs in tumor-bearing mice to aggravate cancer progression. Citation Format: Chih-Hang Tang, Shiun Chang, Ayumi Hashimoto, Yi-Ju Chen, Chang Won Kang, Anthony Mato, Juan Del Valle, Dmitry Gabrilovich, Chih-Chi Andrew Hu. Secretory IgM exacerbates tumor progression by inducing accumulations of myeloid-derived suppressor cells in mice [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4756.

Secretory IgM Exacerbates Tumor Progression by Inducing Accumulations of MDSCs in Mice.

Chronic lymphocytic leukemia (CLL) cells can secrete immunoglobulin M. However, it is not clear whether secretory IgM (sIgM) plays a role in disease progression. We crossed the Eμ-TCL1 mouse model of CLL, in which the expression of human TCL1 oncogene was driven by the V(H) promoter-Ig(H)-Eμ enhancer, with MD4 mice whose B cells produced B-cell receptor (membrane-bound IgM) and sIgM with specificity for hen egg lysozyme (HEL). CLL cells that developed in these MD4/Eμ-TCL1 mice reactivated a parental Ig gene allele and secreted IgM, and did not recognize HEL. The MD4/Eμ-TCL1 mice had reduced survival, increased myeloid-derived suppressor cells (MDSC), and decreased numbers of T cells. We tested whether sIgM could contribute to the accumulation of MDSCs by crossing μS-/- mice, which could not produce sIgM, with Eμ-TCL1 mice. The μS-/-/Eμ-TCL1 mice survived longer than Eμ-TCL1 mice and developed decreased numbers of MDSCs which were less able to suppress proliferation of T cells. We targeted the synthesis of sIgM by deleting the function of XBP-1s and showed that targeting XBP-1s genetically or pharmacologically could lead to decreased sIgM, accompanied by decreased numbers and reduced functions of MDSCs in MD4/Eμ-TCL1 mice. Additionally, MDSCs from μS-/- mice grafted with Lewis lung carcinoma were inefficient suppressors of T cells, resulting in slower tumor growth. These results demonstrate that sIgM produced by B cells can upregulate the functions of MDSCs in tumor-bearing mice to aggravate cancer progression. In a mouse model of CLL, production of secretory IgM led to more MDSCs, fewer T cells, and shorter survival times for the mice. Thus, secretory IgM may aggravate the progression of this cancer. Cancer Immunol Res; 6(6); 696-710. ©2018 AACR.