Soluble CD80 restores T cell activation and overcomes tumor cell Programmed Death Ligand-1-mediated suppression (P2041)

Many tumor cells escape anti-tumor immunity through their expression of Programmed Death Ligand 1 (PDL1 or B7-H1), which interacts with T cell-expressed PD1 and results in T cell apoptosis. We previously reported that transfection of human tumor cells with a membrane-bound form of the costimulatory molecule CD80 prevented PD1 binding and restored T cell activation. We now report that membrane-bound CD80 similarly reduces PDL1-PD1-mediated suppression by mouse tumor cells, and that a fusion protein consisting of the extracellular domains of CD80 fused to an Fc domain of IgG1 overcomes PDL1-mediated suppression by human tumor cells. T cell activation experiments assessing costimulation indicate that the soluble CD80 fusion protein mediates its effects by binding to PDL1 and inhibiting PDL1-PD1 interactions. Comparison of the CD80 fusion protein to antibodies specific for PD1 or PDL1 demonstrate that soluble CD80 treatment is more effective in restoring T cell activation than treatment with mAb to either PD1 or PDL1. These studies identify soluble CD80 as an alternative and potentially more efficacious therapeutic agent for overcoming PDL1-induced immune suppression and facilitating tumor-specific immunity.

Abstract 2876: Inflammation of the tumor microenvironment is regulated by myeloid derived suppressor cell and macrophage crosstalk

Myeloid-Derived Suppressor Cells (MDSC) polarize macrophages to a Type II tumor-promoting phenotype via MDSC-macrophage crosstalk. We previously demonstrated that MDSC produce IL-10 which inhibits macrophage production of IL-12, and MDSC production of IL-10 is dependent on TLR4 and enhanced by cell-cell contact with macrophages. However, the role of inflammation in MDSC-macrophage crosstalk is not well defined. To determine the role of inflammation, we utilized wild type, IL-6-/-, and IL-10-/- mice bearing syngeneic 4T1 mammary carcinoma. IL-10 is classically an anti-inflammatory cytokine. In contrast, IL-6 is a pro-inflammatory cytokine that is secreted by activated Type 1 macrophages, is found at higher levels in tumor-bearing patients, and is an inducer of MDSC. 4T1-induced MDSC and macrophages express both IL-6 and IL-10 receptors, suggesting that they have the potential to respond to both cytokines. To determine if IL-6 and IL-10 contribute to tumor progression by modulating MDSC-macrophage crosstalk, MDSC from wild type or IL-10-/- mice bearing 4T1 tumors were cultured with macrophages from wild type or IL-6-/- BALB/c mice. Although MDSC are typically pro-inflammatory cells, they significantly decreased macrophage production of IL-6, suggesting that MDSC can also function as anti-inflammatory cells. IL-6 levels are important in vivo since primary 4T1 tumors grow more slowly in IL-6-deficient mice, and these mice have significantly extended survival compared to wild type BALB/c mice. Anti-inflammatory effects are further exacerbated by macrophages themselves, since their production of IL-6 increases MDSC production of IL-10. However, macrophage-produced IL-6 affects MDSC indirectly, as incubation of MDSC with exogenous IL-6 in the absence of macrophages does not increase IL-10 production. Although IL-10 is classically considered as an anti-inflammatory cytokine, it contributes to tumor progression because 4T1-bearing IL-10-/- mice have delayed primary tumor progression and extended survival vs. wild type mice. These data demonstrate that MDSC have both pro-inflammatory and anti-inflammatory effects and that MDSC-macrophage cross-talk contributes to the overall milieu of IL-10 and IL-6 within the tumor microenvironment. Since IL-6 is a signature cytokine of anti-tumor M1-like macrophages, these data also suggest that although MDSC may decrease inflammation through their production of IL-10, this effect neither promotes the development of tumoricidal macrophages nor decreases tumor progression.

Supported by NIH R01 CA115880, RO1CA84232, and DOD BCRP W81XWH-11-1-0115

Citation Format: Daniel W. Beury, Katherine H. Parker, Suzanne Ostrand-Rosenberg. Inflammation of the tumor microenvironment is regulated by myeloid derived suppressor cell and macrophage crosstalk. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2876. doi:10.1158/1538-7445.AM2013-2876

Abstract 1264: Mutated CD80 may facilitate T-cell activation by inhibiting PDL1-PD1 suppression and by costimulating

CD80 functions as a costimulatory signal to activate T cells when it binds to CD28 or as an apoptotic signal when it binds to CTLA4. We recently identified another function for CD80 and showed that co-expression of CD80 by Program Death Ligand-1+ (PDL1+) human tumor cells prevents PDL1 binding to its receptor PD1 on activated T cells, and enhances activation of tumor-reactive T cells. To distinguish if CD80 is facilitating T cell activation by preventing PDL1-PD1 interactions and/or by functioning as a costimulatory molecule, we have generated a recombinant CD80 molecule that does not bind to CD28. Since we hypothesize that CD80 may be a therapeutic for inhibiting tumor-mediated immune suppression via PDL1-PD1 interactions, the recombinant CD80 is also designed to not interact with CTLA4. Using site directed mutagenesis we generated CD80 with the following mutations: H96A, L97A, and E99A (CD8096,97,99). CD28 and CTLA4 bind to 35.3% and 37.4%, respectively, of tumor cells expressing wild type CD80. In contrast, only 1.2% and 1.4% of tumor cells expressing CD8096,97,99 bind CD28 and CTLA4, respectively, demonstrating that the mutant CD80 does not bind to CD28 or CTLA4. Previous studies demonstrated that CD80 prevents PDL1-PD1 interactions by binding to PDL1 and obscuring its detection. Only 7.05% of CD8096,97,99 mutants have detectable PDL1 (vs. 98% of untransfected parental human tumor cells), demonstrating that the CD8096,97,99 mutant does not bind CD28 or CTLA4, but retains the ability to inhibit PDL1-PD1 interactions. We are also generating a soluble form of CD8096,97,99 (sCD8096,97,99) that could be administered to cancer patients to prevent tumor cell-mediated PDL1 immune suppression. The construct for the soluble form was generated by ligating the two extracellular domains of CD80 containing the 96,97,99 mutations to the Fc region of human IgG1, followed by incorporation into the Pet21a+ plasmid containing the T7 promoter. Recombinant protein will be isolated from transformed E. coli using a protein G affinity column, and tested in functional experiments with PDL1+ tumor cells to determine if CD80 increases T-cell activation by inhibiting PDL1-PD-1 immune suppression and/or by facilitating costimulation.

This work was funded, in part, by grants from the HHMI Precollege and Undergraduate Science Education Program (52006949), CBI (T32 GM066706), and NIH (R01CA84232 and RO1CA115880).

Citation Format: Sonia P. Dalal, Samuel Haile, Preethi Somasundaram, Suzanne Ostrand-Rosenberg. Mutated CD80 may facilitate T-cell activation by inhibiting PDL1-PD1 suppression and by costimulating. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1264. doi:10.1158/1538-7445.AM2013-1264

Abstract 4718: Withaferin A, a potent and abundant component of Withania somnifera root extract, reduces myeloid-derived suppressor cell function

Myeloid cells play a crucial role in growth and metastasis of malignant tumors. Tumor infiltrating myeloid cells includes myeloid-derived suppressor cells (MDSC), tumor-associated macrophages (TAMS) and dendritic cells (DC). These cells infiltrate into tumor and suppress tumor immunity by their inherent immune suppressive activity which is enhanced by interactions with each other (cross-talk). Therapeutic strategies targeting manipulation of these cells are being developed. The root extract of the plant Withania somnifera (Ashwagandha) (WRE) has been reported to reduce tumor cell proliferation and angiogenesis. Whether the anti-tumor effects of WRE are due to direct effects on malignant cells, or whether WRE also impacts immune suppressive myeloid cells is unknown. We hypothesize that WRE or its constituents impact tumor infiltrating myeloid cells and thereby boost anti-tumor immunity. HPLC and mass spectrometry analysis revealed that Withaferin A (WA) is the most abundant constituent of Withania somnifera root powder (ChromaDex, Inc. Irvine, CA). In comparison with the other constituents, WA has the most potent effect on reducing suppressive functions of myeloid cells. A prominent effect of MDSC is their production of IL-10 which increases upon cross-talk with macrophages, thus polarizing immunity to a tumor promoting type-2 phenotype. WA reduces inherent and cross-talk induced IL-10 secretion from MDSC in a dose dependent manner. Macrophage secretion of IL-6 and TNFα cytokines that are characteristic of M1-type macrophages and that also increase MDSC accumulation and function, are also reduced by WA. Much of the T cell suppressive activity of MDSC is due to MDSC production of reactive oxygen species (ROS), and WA significantly reduces MDSC production of ROS. Thus adjunctive treatment with WA has the potential to concomitantly reduce myeloid cell mediated immune suppression, to polarize immunity towards a tumor-rejecting type I phenotype, and to facilitate the development of anti-tumor immunity.

Supported by NIH RO1CA115880, RO1CA84232, and American Cancer Society IRG-97-153-07

Citation Format: Pratima Sinha, Suzanne Ostrand-Rosenberg. Withaferin A, a potent and abundant component of Withania somnifera root extract, reduces myeloid-derived suppressor cell function. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4718. doi:10.1158/1538-7445.AM2013-4718

Major histocompatibility complex class II+ invariant chain negative breast cancer cells present unique peptides that activate tumor-specific T cells from breast cancer patients

The major histocompatibility complex (MHC) class II-associated Invariant chain (Ii) is present in professional antigen presenting cells where it regulates peptide loading onto MHC class II molecules and the peptidome presented to CD4+ T lymphocytes. Because Ii prevents peptide loading in neutral subcellular compartments, we reasoned that Ii- cells may present peptides not presented by Ii+ cells. Based on the hypothesis that patients are tolerant to MHC II-restricted tumor peptides presented by Ii+ cells, but will not be tolerant to novel peptides presented by Ii- cells, we generated MHC II vaccines to activate cancer patients' T cells. The vaccines are Ii- tumor cells expressing syngeneic HLA-DR and the costimulatory molecule CD80. We used liquid chromatography coupled with mass spectrometry to sequence MHC II-restricted peptides from Ii+ and Ii- MCF10 human breast cancer cells transfected with HLA-DR7 or the MHC Class II transactivator CIITA to determine if Ii- cells present novel peptides. Ii expression was induced in the HLA-DR7 transfectants by transfection of Ii, and inhibited in the CIITA transfectants by RNA interference. Peptides were analyzed and binding affinity predicted by artificial neural net analysis. HLA-DR7-restricted peptides from Ii- and Ii+ cells do not differ in size or in subcellular location of their source proteins; however, a subset of HLA-DR7-restricted peptides of Ii- cells are not presented by Ii+ cells, and are derived from source proteins not used by Ii+ cells. Peptides from Ii- cells with the highest predicted HLA-DR7 binding affinity were synthesized, and activated tumor-specific HLA-DR7+ human T cells from healthy donors and breast cancer patients, demonstrating that the MS-identified peptides are bonafide tumor antigens. These results demonstrate that Ii regulates the repertoire of tumor peptides presented by MHC class II+ breast cancer cells and identify novel immunogenic MHC II-restricted peptides that are potential therapeutic reagents for cancer patients.

Regulating the suppressors: apoptosis and inflammation govern the survival of tumor-induced myeloid-derived suppressor cells (MDSC)

Immune suppressive myeloid-derived suppressor cells (MDSC) are present in most cancer patients where they inhibit innate anti-tumor immunity and are a significant obstacle to cancer immunotherapy. Inflammation is a known inducer of Gr1(+)CD11b(+) MDSC; however, the factors/conditions that regulate MDSC survival and half-life have not been identified. We have used mass spectrometry (MS) and proteomic analysis to identify proteins and pathways that regulate MDSC survival. This analysis revealed high expression of caspase family proteins and the Fas-FasL, p38 MAPK, and TGFβ pathways, suggesting that Fas-FasL apoptosis regulates MDSC survival. Flow cytometry, confocal microscopy, and western blot analyses confirmed the MS findings and demonstrated that Fas(+) MDSC are susceptible to Fas-mediated killing in vitro. In vivo studies with FasL-deficient and Fas-deficient mice demonstrated that Fas-FasL interactions are essential for MDSC apoptosis and for rejection of established metastatic disease and survival and that FasL(+) T cells are the effector population mediating MDSC apoptosis. MS findings validated by biological experiments demonstrated that inflammation increases MDSC levels by protecting MDSC from Fas-mediated apoptosis, possibly by activating p38 MAPK. These results demonstrate that MDSC half-life in vivo is regulated by FasL(+) T cells and that inflammation increases MDSC levels by conferring resistance to Fas-mediated apoptosis and identifies T cells as the relevant effector cells causing MDSC apoptosis in vivo. This newly recognized mechanism for regulating MDSC levels identifies potential new targets for decreasing MDSC in cancer patients.

IDO is a nodal pathogenic driver of lung cancer and metastasis development

Indoleamine 2,3-dioxygenase (IDO) enzyme inhibitors have entered clinical trials for cancer treatment based on preclinical studies, indicating that they can defeat immune escape and broadly enhance other therapeutic modalities. However, clear genetic evidence of the impact of IDO on tumorigenesis in physiologic models of primary or metastatic disease is lacking. Investigating the impact of Ido1 gene disruption in mouse models of oncogenic KRAS-induced lung carcinoma and breast carcinoma-derived pulmonary metastasis, we have found that IDO deficiency resulted in reduced lung tumor burden and improved survival in both models. Micro-computed tomographic (CT) imaging further revealed that the density of the underlying pulmonary blood vessels was significantly reduced in Ido1-nullizygous mice. During lung tumor and metastasis outgrowth, interleukin (IL)-6 induction was greatly attenuated in conjunction with the loss of IDO. Biologically, this resulted in a consequential impairment of protumorigenic myeloid-derived suppressor cells (MDSC), as restoration of IL-6 recovered both MDSC suppressor function and metastasis susceptibility in Ido1-nullizygous mice. Together, our findings define IDO as a prototypical integrative modifier that bridges inflammation, vascularization, and immune escape to license primary and metastatic tumor outgrowth.

SIGNIFICANCE

This study provides preclinical, genetic proof-of-concept that the immunoregulatory enzyme IDO contributes to autochthonous carcinoma progression and to the creation of a metastatic niche. IDO deficiency in vivo negatively impacted both vascularization and IL-6–dependent, MDSC-driven immune escape, establishing IDO as an overarching factor directing the establishment of a protumorigenic environment.

Tumor-induced myeloid-derived suppressor cell function is independent of IFN-γ and IL-4Rα

Myeloid-derived suppressor cells (MDSCs) are present in most cancer patients and experimental animals where they exert a profound immune suppression and are a significant obstacle to immunotherapy. IFN-γ and IL-4 receptor alpha (IL-4Rα) have been implicated as essential molecules for MDSC development and immunosuppressive function. If IFN-γ and IL-4Rα are critical regulators of MDSCs, then they are potential targets for preventing MDSC accumulation or inhibiting MDSC function. Because data supporting a role for IFN-γ and IL-4Rα are not definitive, we have examined MDSCs induced in IFN-γ-deficient, IFN-γR-deficient, and IL-4Rα-deficient mice carrying three C57BL/6-derived (B16 melanoma, MC38 colon carcinoma, and 3LL lung adenocarcinoma), and three BALB/c-derived (4T1 and TS/A mammary carcinomas, and CT26 colon carcinoma) tumors. We report that although MDSCs express functional IFN-γR and IL-4Rα, and have the potential to signal through the STAT1 and STAT6 pathways, respectively, neither IFN-γ nor IL-4Rα impacts the phenotype, accumulation, or T-cell suppressive potency of MDSCs, although IFN-γ and IL-4Rα modestly alter MDSC-macrophage IL-10 crosstalk. Therefore, neither IFN-γ nor IL-4Rα is a key regulator of MDSCs and targeting these molecules is unlikely to significantly alter MDSC accumulation or function.

Transcription factor Nrf2 (NF-E2 Related Factor 2) enhances myeloid-derived suppressor cell (MDSC) accumulation and tumor progression (46.13)

Previous studies have shown that MDSC block adaptive anti-tumor immunity by producing high levels of oxidizing agents such as reactive oxygen species (ROS), nitric oxide (NO), and peroxynitrite. Despite high levels of the toxic oxidizing agents, MDSC survive and suppress anti-tumor immunity. We hypothesize that MDSC survival is mediated by the antioxidant-regulating transcription factor Nrf2. To test this hypothesis, BALB/c wild type and Nrf2-/- mice were injected with 4T1 mammary carcinoma cells and assayed weekly for percentage of MDSC in the blood and for MDSC levels of ROS and glutathione. Mice were also followed for survival. Nrf2-/- MDSC had more ROS and less glutathione than wild type MDSC, indicating that Nrf2-/- MDSC were more oxidatively stressed. Increased oxidative stress is likely to lead to increased apoptosis of MDSC and reduced immune suppression. Tumor-bearing Nrf2-/- and wild type mice accumulated MDSC in the blood with increasing tumor-burden. However, tumor-bearing Nrf2-/- mice had less MDSC than wild type mice for a given sized primary tumor. Therefore, MDSC either accumulate more slowly or they apoptose more quickly in Nrf2-/- vs. wild type mice. Tumor-bearing Nrf2-/- mice lived longer than wild type mice demonstrating that Nrf2 contributes to tumor progression; consistent with our hypothesis that Nrf2 regulates MDSC survival, resulting in fewer MDSC in Nrf2-/- mice, thereby increasing anti-tumor immunity against metastatic disease.

Tumor-induced myeloid-derived suppressor cell function is independent of IFNγ and IL-4Rα (127.37)

Myeloid-derived suppressor cells (MDSC) are present in most cancer patients and experimental animals where they exert a profound immune suppression and are a significant obstacle to immunotherapy. IFNγ and IL-4Rα have been implicated as essential molecules for MDSC development and immunosuppressive function. If IFNγ and IL-4Rα are critical regulators of MDSC, then they are potential targets for preventing MDSC accumulation or inhibiting MDSC function. Because data supporting a role for IFNγ and IL-4Rα are not definitive, we have examined tumor-induced MDSC from IFNγ-deficient, IFNγR-deficient, and IL-4Rα-deficient mice. Although MDSC express functional IFNγR and IL-4Rα and signal via the STAT1 and STAT6 pathways, respectively, neither IFNγ nor IL-4Rα impacts the phenotype, accumulation, or T cell suppressive potency of MDSC, although IFNγ and IL-4Rα modestly alter macrophage -induced MDSC production of IL-10. Therefore, neither IFNγ nor IL-4Rα is a key regulator of MDSC and targeting these molecules is unlikely to significantly alter MDSC function.

Soluble CD80 restores T cell activation and overcomes tumor cell Programmed Death Ligand-1-mediated suppression (46.8)

One of the mechanisms tumor cells employ to escape anti-tumor immunity is expression of Programmed Death Ligand 1 (PD-L1or B7-H1). PD-L1 protects tumor cells by binding to its receptor Programmed Death 1 (PD-1) on activated T cells, thereby causing T cell apoptosis. We previously reported that transfection of tumor cells to express the costimulatory molecule CD80 prevents PD-1 binding and restores T cell activation, consistent with the concept that CD80 binds to PD-L1 and inhibits PD-L1/PD-1 interaction. Since it is not therapeutically feasible to transfect tumor cells with CD80, we have tested a soluble form of CD80, consisting of the extracellular domains of the molecule, for its ability to overcome PD-L1-mediated suppression. Treatment of human lung adenocarcinoma, mammary carcinoma, and cutaneous melanoma cells with soluble CD80 prevents PD-L1-mediated suppression and restores T cell activation. Because on-going clinical trials have shown that mAbs to PD-1 facilitate tumor rejection, we have compared the efficacy of soluble CD80 vs. mAbs to PD-1 or PD-L1. Soluble CD80 treatment is more effective in restoring T cell activation than treatment with mAbs to either PD-1or PD-L1. Therefore, soluble CD80 may be a therapeutic agent for facilitating tumor-specific immunity by overcoming PD-L1-induced immune suppression, and is potentially more effective than mAbs to either PD-1 or PD-L1.

Ii- tumor cells present novel MHC II peptides that are tumor specific and immunogenic (52.18)

We have generated tumor cell-based vaccines to activate patients’ CD4+ T-cells to facilitate tumor immunity. The vaccines are based on the hypothesis that invariant chain negative (Ii-), MHC II+ , CD80+ human breast cancer cells (MCF10) present novel MHC II peptides and circumvent patients’ tolerance to their cancer. We sequenced MHC II peptides from Ii- (MCF10/DR7/CD80 and MCF10/CIITA/CD80/Ii siRNA) and Ii+ (MCF10/DR7/CD80/Ii and MCF10/CIITA/CD80) cells using LC-MS/MS. 64/116 peptides identified were unique to MCF10/DR7/CD80 and 176/228 peptides were unique to MCF10/DR7/CD80/Ii cells. 102/108 and 22/28 peptides were uniquely identified for MCF10/CIITA/CD80 and MCF10/CIITA/CD80/Ii siRNA cells, respectively. Therefore, the absence of Ii enables the presentation of unique peptides that are not presented by Ii+ or professional antigen presenting cells. Five peptides unique to Ii- and 2 peptides shared by Ii- and Ii+ cells and with the highest MHC II binding affinity, as predicted by artificial neural net analysis, were tested for their ability to activate T-cells from the blood of healthy donors or breast cancer patients. All peptides expanded tumor-reactive T-cells and induced IFNγ production. This is the first study to compare the human MHC II peptide repertoire in the absence or presence of Ii and to identify novel immunogenic MHC II-restricted peptides that are potential therapeutic reagents for cancer patients.

Abstract 295: IDO drives tumor-promoting, pathogenic inflammation in lung

First identified as a mediator of acquired immune tolerance of the ‘foreign’ fetus from maternal immunity, the tryptophan-catabolizing enzyme IDO (indoleamine 2,3-dioxygenase) has since been implicated in tumor escape from the host immune system. Insight into the intricate role of IDO in the classical DMBA/TPA skin carcinogenesis model suggested that inflammatory tumor environments can induce IDO production resulting in de novo tumor development. In the genetically deficient model of IDO, mice show resistance to tumor formation. This provided a basis for our current studies exploring the importance of IDO in the microenvironment of the lung. To this end, we have investigated both primary tumor formation and metastatic disease in the lungs of IDO-deficient mice using the KRAS-induced lung adenocarcinoma and the metastatic 4T1 breast cancer models. Elevation of the inflammatory cytokine IL6 was associated with tumor outgrowth in the lungs in both models but was greatly attenuated with the loss of IDO, consistent with the in vitro demonstration that IDO activity markedly potentiates IL6 production. MDSCs (myeloid derived suppressor cells) exhibited reduced T cell suppressive activity when isolated from tumor-bearing, IDO-deficient animals that could be rescued by ectopic production of IL6 in the tumor. IL6 production could likewise reverse the pulmonary metastasis resistance exhibited by IDO-deficient mice. Interestingly, while there is a clear role of the immune system in lung tumor and metastatic outgrowth, IDO-deficient mice appear to have reduced vascularization in the lung which may partly contribute to reduced tumor formation. Together, these findings genetically validate IDO as a therapeutic target in the settings of pulmonary cancer and metastasis and establish the importance of IDO as a driver of IL6 production and MDSC function. Furthermore, the correlation of IDO to angiogenesis may be a new insight into the role of this enzyme in cancer.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 295. doi:1538-7445.AM2012-295

Cross-talk between myeloid-derived suppressor cells (MDSC), macrophages, and dendritic cells enhances tumor-induced immune suppression

The tumor microenvironment is a complex milieu of tumor and host cells. Host cells can include tumor-reactive T cells capable of killing tumor cells. However, more frequently the tumor and host components interact to generate a highly immune suppressive environment that frustrates T cell cytotoxicity and promotes tumor progression through a variety of immune and non-immune mechanisms. Myeloid-derived suppressor cells (MDSC) are a major host component contributing to the immune suppressive environment. In addition to their inherent immune suppressive function, MDSC amplify the immune suppressive activity of macrophages and dendritic cells via cross-talk. This article will review the cell-cell interactions used by MDSC to inhibit anti-tumor immunity and promote progression, and the role of inflammation in promoting cross-talk between MDSC and other cells in the tumor microenvironment.

Gr-1+ CD11b+ myeloid-derived suppressor cells suppress inflammation and promote insulin sensitivity in obesity

Activation of immune cells, including macrophages and CD8(+) T cells, contributes significantly to the advancement of obesity and its associated medical complications, such as atherosclerosis, insulin resistance, and type 2 diabetes. However, how the activation of these immune cells is regulated in vivo remains largely unexplored. Here we show that a group of immature myeloid cells with cell surface markers of Gr-1(+) CD11b(+) are highly enriched in peripheral tissues (i.e. liver and adipose tissues) during obesity. Down-regulation of these cells in obese animals significantly increases inflammation and impairs insulin sensitivity and glucose tolerance, whereas elevation of these cells via adoptive transfer has the opposite effects. Mechanistically, we show that under obese conditions, the Gr-1(+) cells suppress proliferation and induce apoptosis of CD8(+) T cells and are capable of skewing differentiation of macrophages into insulin-sensitizing, alternatively activated M2 macrophages. Taken together, our study demonstrates that immature myeloid cells provide a checks-and-balances platform to counter proinflammatory immune cells in the liver and adipose tissue during obesity to prevent overt immune responses.

Tumor cell programmed death ligand 1-mediated T cell suppression is overcome by coexpression of CD80

Programmed death ligand 1 (PDL1, or B7-H1) is expressed constitutively or is induced by IFN-γ on the cell surface of most human cancer cells and acts as a "molecular shield" by protecting tumor cells from T cell-mediated destruction. Using seven cell lines representing four histologically distinct solid tumors (lung adenocarcinoma, mammary carcinoma, cutaneous melanoma, and uveal melanoma), we demonstrate that transfection of human tumor cells with the gene encoding the costimulatory molecule CD80 prevents PDL1-mediated immune suppression by tumor cells and restores T cell activation. Mechanistically, CD80 mediates its effects through its extracellular domain, which blocks the cell surface expression of PDL1 but does not prevent intracellular expression of PDL1 protein. These studies demonstrate a new role for CD80 in facilitating antitumor immunity and suggest new therapeutic avenues for preventing tumor cell PDL1-induced immune suppression.

Tumor cell Programmed Death Ligand-1-mediated T cell suppression is overcome by co-expression of CD80 (66.29)

Programmed death ligand 1 (PDL1 or B7-H1) is expressed constitutively or is induced by IFNγ on the cell surface of most human cancer cells and acts as a “molecular shield” by protecting tumor cells from T cell-mediated destruction. Using seven cell lines representing four histologically distinct solid tumors (lung adenocarcinoma, mammary carcinoma, cutaneous melanoma, and uveal melanoma) we demonstrate that transfection of human tumor cells with the gene encoding the costimulatory molecule CD80 prevents PDL1- mediated immune suppression by tumor cells and restores the ability of tumor cells to activate tumor-specific T lymphocytes. Mechanistically, CD80 mediates its effects through its extracellular domain which blocks the cell surface expression of PDL1 but does not prevent intracellular expression of PDL1 protein. These studies demonstrate a new role for CD80 in facilitating anti-tumor immunity and suggest new therapeutic avenues for preventing tumor cell PDL1-induced immune suppression.

Blockade of cystine transport inhibits glutathione synthesis and survival of inflammation-induced myeloid-derived suppressor cells (66.6)

Tumor-induced myeloid derived suppressor cells (MDSC) are responsible for immune suppression in tumor bearing individuals and are a major obstacle to effective immunotherapy. MDSC suppress T cell activation through several mechanisms including their production of reactive oxygen species (ROS). MDSC accumulation, suppressive activity, and ROS production are enhanced by inflammation. However, MDSC production of ROS does not negatively impact MDSC themselves. Since MDSC sequester cystine/cysteine from their local environment, and cysteine is the rate limiting reagent for glutathione (GSH), which is the predominant antioxidant, we hypothesized that inflammation-induced MDSC utilize high GSH production to attenuate ROS-mediated damage. This hypothesis is supported by our data showing that inflammation-induced MDSC upregulate the subunits of the xc- (cystine/glutamate) transporter, resulting in greater uptake of cystine and higher GSH synthesis. Since MDSC only generate cysteine via the uptake of cystine, the xc- inhibitor sulfasalazine (SASP) may decrease MDSC survival and suppressive activity. This concept is supported by our findings that MDSC treated with SASP are depleted of GSH and are less viable and that inflammation further enhances these effects. Because of their ability to reduce MDSC survival and block MDSC-mediated suppression, xc- inhibitors are potential supplements to cancer immunotherapies.

HMGB1: a regulator of myeloid-derived suppressor cell potency? (66.37)

Myeloid-derived suppressor cells (MDSC) are a heterogeneous population of cells that accumulate in individuals with cancer and inflammation and play a pivotal role in tumor immunity by suppressing T-cell activation and secreting proinflammatory molecules. The suppressive capacity of MDSC is mediated by immune suppressive factors such as arginase and reactive oxygen species (ROS). Nuclear protein, High Mobility Group Box1 (HMGB1), is present in nearly all cells and is released from myeloid cells as a danger response to sepsis, infection, or arthritis. Its release promotes inflammatory responses. HMGB1 signals through a multitude of receptors including TLR4, which is expressed by MDSC. In contrast to other inflammatory mediators which increase MDSC potency, HMGB1 reduced the suppressive capacity of TLR4(-/-) and wildtype MDSC, and reduced ROS levels in TLR4(-/-) MDSC. These findings suggest that HMGB1 may diminish MDSC function and may lead to new immunotherapeutic uses of HMGB1.

Glutathione S-transferases as regulators of tumor-induced myeloid-derived suppressor cell survival (66.38)

Tumor-induced myeloid-derived suppressor cells (MDSC) are a major barrier to tumor immunotherapy because they inhibit T-cell anti-tumor immunity through various mechanisms, including cystine sequestration and production of reactive oxygen species (ROS). MDSC accumulation, suppressive potency, and survival are driven by inflammation, which also increases MDSC production of ROS. Surprisingly, ROS do not adversely affect MDSC, suggesting that MDSC neutralize endogenous ROS. Because MDSC survival is likely to be controlled by the mechanisms that protect them against endogenous ROS, we are identifying molecules that regulate ROS levels with the goal of identifying potential targets for inducing MDSC apoptosis. Glutathione (GSH) is the major antioxidant that detoxifies ROS in conjunction with various isoforms of GSH S-transferase (GST) that catalyze the detoxification process. Because of their role in detoxifying intracellular ROS, we are analyzing intracellular levels of GSH and multiple GST isoenzymes in conventional and inflammatory MDSC to understand how inflammation protects MDSC and thus promotes tumorigenesis. GST isoforms that facilitate MDSC survival will be novel drug targets for reducing tumor-induced immune suppression and facilitating tumor immunotherapy.

MHC II-restricted peptides derived from invariant chain negative breast cancer cell-based vaccines activate healthy donor T-cells (156.17)

We have generated cell-based MHC II vaccines to activate patients’ CD4+ T-cell to facilitate tumor immunity. This was based on the hypothesis that invariant chain (Ii) negative, MHC II and CD80 positive tumor cells present novel MHC II peptides and circumvent patients’ tolerance to their cancer. To confirm that absence of Ii facilitates presentation of novel immunogenic tumor peptides we sequenced MHC II peptides from MCF10/DR7/CD80, MCF10/CIITA/CD80/Ii siRNA (Ii-) and MCF10/DR7/CD80/Ii, MCF10/CIITA/CD80 (Ii+) vaccines using LC-MS/MS. 116 Peptides were identified for MCF10/DR7/CD80, 228 peptides for MCF10/DR7/CD80/Ii. 52 Peptides were in both cell lines. 108 Peptides were identified for MCF10/CIITA/CD80, 28 peptides for MCF10/CIITA/CD80/Ii siRNA, 6 peptides in both cell lines. This confirms that Ii+ and Ii- MHC II vaccines present distinct peptide repertoires and agrees with in vitro studies showing that human MHC II Ii- and Ii+ vaccines activate distinct T-cell subsets. 7 peptides with the highest MHC II binding affinity were tested for their ability to activate tumor-specific CD4+ T-cells. 5 of the peptides activated CD4+ T-cells to the same extent as breast cancer peptide Her2 p776, (measured by IFNγ production). This is the first study to compare the human MHC II peptide repertoire in the absence or presence of Ii and to identify novel immunogenic MHC II-restricted peptides that are potential therapeutic reagents for cancer patients.

IL-1β regulates a novel myeloid-derived suppressor cell subset that impairs NK cell development and function

Chronic inflammation is associated with promotion of malignancy and tumor progression. Many tumors enhance the accumulation of myeloid-derived suppressor cells (MDSC), which contribute to tumor progression and growth by suppressing anti-tumor immune responses. Tumor-derived IL-1β secreted into the tumor microenvironment has been shown to induce the accumulation of MDSC possessing an enhanced capacity to suppress T cells. In this study, we found that the enhanced suppressive potential of IL-1β-induced MDSC was due to the activity of a novel subset of MDSC lacking Ly6C expression. This subset was present at low frequency in tumor-bearing mice in the absence of IL-1β-induced inflammation; however, under inflammatory conditions, Ly6C(neg) MDSC were predominant. Ly6C(neg) MDSC impaired NK cell development and functions in vitro and in vivo. These results identify a novel IL-1β-induced subset of MDSC with unique functional properties. Ly6C(neg) MDSC mediating NK cell suppression may thus represent useful targets for therapeutic interventions.

Proteomic pathway analysis reveals inflammation increases myeloid-derived suppressor cell resistance to apoptosis

Myeloid-derived suppressor cells (MDSC) accumulate in patients and animals with cancer where they mediate systemic immune suppression and obstruct immune-based cancer therapies. We have previously demonstrated that inflammation, which frequently accompanies tumor onset and progression, increases the rate of accumulation and the suppressive potency of MDSC. To determine how inflammation enhances MDSC levels and activity we used mass spectrometry to identify proteins produced by MDSC induced in highly inflammatory settings. Proteomic pathway analysis identified the Fas pathway and caspase network proteins, leading us to hypothesize that inflammation enhances MDSC accumulation by increasing MDSC resistance to Fas-mediated apoptosis. The MS findings were validated and extended by biological studies. Using activated caspase 3 and caspase 8 as indicators of apoptosis, flow cytometry, confocal microscopy, and Western blot analyses demonstrated that inflammation-induced MDSC treated with a Fas agonist contain lower levels of activated caspases, suggesting that inflammation enhances resistance to Fas-mediated apoptosis. Resistance to Fas-mediated apoptosis was confirmed by viability studies of MDSC treated with a Fas agonist. These results suggest that an inflammatory environment, which is frequently present in tumor-bearing individuals, protects MDSC against extrinsic-induced apoptosis resulting in MDSC with a longer in vivo half-life, and may explain why MDSC accumulate more rapidly and to higher levels in inflammatory settings.