MHC II lung cancer vaccines prime and boost tumor-specific CD4+ T cells that cross-react with multiple histologic subtypes of nonsmall cell lung cancer cells

Nonsmall cell lung cancer (NSCLC) is the major cause of lung cancer-related deaths in the United States. We are developing cell-based vaccines as a new approach for the treatment of NSCLC. NSCLC is broadly divided into 3 histologic subtypes: adenocarcinoma, squamous cell carcinoma and large cell carcinoma. Since these subtypes are derived from the same progenitor cells, we hypothesized that they share common tumor antigens, and vaccines that induce immune reactivity against 1 subtype may also induce immunity against other subtypes. Our vaccine strategy has focused on activating tumor-specific CD4(+) T cells, a population of lymphocytes that facilitates the optimal activation of effector and memory cytotoxic CD8(+) T cells. We now report that our NSCLC MHC II vaccines prepared from adeno, squamous or large cell carcinomas each activate CD4(+) T cells that cross-react with the other NSCLC subtypes and do not react with HLA-DR-matched normal lung fibroblasts or other HLA-DR-matched nonlung tumor cells. Using MHC II NSCLC vaccines expressing the DR1, DR4, DR7 or DR15 alleles, we also demonstrate that antigens shared among the different subtypes are presented by multiple HLA-DR alleles. Therefore, MHC II NSCLC vaccines expressing a single HLA-DR allele activate NSCLC-specific CD4(+) T cells that react with the 3 major classes of NSCLC, and the antigens recognized by the activated T cells are presented by several common HLA-DR alleles, suggesting that the MHC II NSCLC vaccines are potential immunotherapeutics for a range of NSCLC patients.

Alternative Ii-independent antigen-processing pathway in leukemic blasts involves TAP-dependent peptide loading of HLA class II complexes

During HLA class II synthesis in antigen-presenting cells, the invariant chain (Ii) not only stabilizes HLA class II complexes in the endoplasmic reticulum, but also mediates their transport to specialized lysosomal antigen-loading compartments termed MIICs. This study explores an alternative HLA class II presentation pathway in leukemic blasts that involves proteasome and transporter associated with antigen processing (TAP)-dependent peptide loading. Although HLA-DR did associate with Ii, Ii silencing in the human class II-associated invariant chain peptide (CLIP)-negative KG-1 myeloid leukemic cell line did not affect total and plasma membrane expression levels of HLA-DR, as determined by western blotting and flow cytometry. Since HLA-DR expression does require peptide binding, we examined the role of endogenous antigen-processing machinery in HLA-DR presentation by CLIP(-) leukemic blasts. The suppression of proteasome and TAP function using various inhibitors resulted in decreased HLA-DR levels in both CLIP(-) KG-1 and ME-1 blasts. Simultaneous inhibition of TAP and Ii completely down-modulated the expression of HLA-DR, demonstrating that together these molecules form the key mediators of HLA class II antigen presentation in leukemic blasts. By the use of a proteasome- and TAP-dependent pathway for HLA class II antigen presentation, CLIP(-) leukemic blasts might be able to present a broad range of endogenous leukemia-associated peptides via HLA class II to activate leukemia-specific CD4(+) T cells.

Myeloid-derived suppressor cells: more mechanisms for inhibiting antitumor immunity

Myeloid-derived suppressor cells (MDSC) accumulate in most cancer patients and experimental animals with cancer. They accumulate in response to pro-inflammatory mediators and they use a variety of mechanisms to block both innate and adaptive antitumor immunity. Because of their critical role in obstructing immune responses, MDSC are a strategic obstacle to immunotherapies that require activation of the host's cell-mediated and innate immune responses. Following a brief description of the factors that induce MDSC accumulation, this article reviews two newly discovered mechanisms that MDSC use to suppress the activation of CD4(+) and CD8(+) T cells. The first mechanism is MDSC sequestration of cysteine, an amino acid that T cells are unable to synthesize de novo and that they require for activation. The second mechanism is MDSC-mediated down-regulation of L: -selectin. T cells must have an L: -selectin(high) phenotype to home to lymph nodes and inflammatory sites where they encounter antigen and are activated. By down-regulating L: -selectin on T cells, MDSC perturb T cell trafficking patterns and thereby inhibit T cell activation. Given the complexity of conditions that regulate MDSC accumulation and the variety of suppressive mechanisms used by MDSC, it is essential to understand which conditions and mechanisms are dominant so MDSC accumulation and/or activity can be targeted in individual patients to minimize MDSC-induced immune suppression.

Abstract 5312: Myeloid-derived suppressor cells express the death receptor Fas and apoptose in response to T cell-expressed FasL

Myeloid-derived suppressor cells (MDSC) accumulate to high levels in individuals with a variety of conditions including cancer, chronic inflammation, and stress, where they are potent inhibitors of adaptive and innate immunity. Some of the factors that induce their accumulation are known; however, physiological conditions that regulate their turn-over have not been identified. Mass spectrometry analysis revealed prominent expression of apoptosis pathway proteins by MDSC and led us to hypothesize that Fas/FasL-mediated apoptosis may be a key mechanism that regulates MDSC turn-over. This hypothesis was confirmed by the findings that MDSC levels significantly regress after removal of primary mammary tumors from STAT6-deficient or CD-1-deficient mice, but do not regress in mice doubly deficient for FasL and STAT6 or CD-1. Flow cytometry and confocal microscopy studies examining activated caspase 3 expression and cytochrome C localization revealed that MDSC apoptosis is triggered via the extrinsic, and not the intrinsic, pathway. Surprisingly, activated FasL+ T cells mediated MDSC apoptosis in vivo, demonstrating a retaliatory relationship between these two cell populations, and suggesting that Fas-FasL interactions could be exploited as a strategy to reduce in vivo levels of MDSC.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5312.

Abstract 5316: Inflammation-induced myeloid-derived suppressor cells have enhanced resistance to apoptosis

The purpose of this study is to deduce the influence of inflammation on apoptosis of Myeloid-Derived Suppressor cells. Myeloid-derived suppressor cells (MDSC) accumulate in patients and animals with cancer where they mediate systemic immune suppression. MDSC induced in heightened inflammatory environments with high levels of IL-1β and/or IL-6 accumulate more rapidly, are more potent suppressors of T cell activation, produce more IL-10, and have a greater ability to down-regulate macrophage production of IL-12, as compared to MDSC that develop in environments with lower levels of these cytokines. To determine how inflammation enhances MDSC levels and activity we used mass spectrometry to identify proteins produced by MDSC induced in highly inflammatory vs. less inflammatory settings. Proteomic pathway analysis identified Fas pathway and caspase network proteins, leading us to hypothesize that inflammation enhances MDSC accumulation by down-regulating Fas-mediated MDSC apoptosis. Using cytochrome C and activated caspase-3 and caspase-8 as indicators of apoptosis, confocal microscopy and flow cytometry demonstrated that inflammation-induced MDSC have enhanced resistance to Fas-mediated apoptosis. These results suggest that an inflammatory environment 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. (supported by NIH R01CA84232 and RO1CA115880).

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5316.

Abstract 5309: Programmed Death Ligand-1 expression on the cell surface is regulated by Costimulatory Molecule CD80 expressed by cancer vaccines

With the lack of effective treatments for metastatic cancers, the employment of immunotherapies, cell-based vaccines particularly, to activate a patient's own immune system is a promising approach for the treatment of disseminated metastatic cancer. Many tumor cells constitutively express or are induced by Interferonγ (IFNγ) to express the inhibitory molecule Programmed Death Ligand 1 (PDL1). Since many cell-based vaccines are derived from tumor cells, the expression of PDL1 could reduce the therapeutic efficacy of the vaccines. PDL1 binds to its receptor Programmed Death 1 (PD1) on T cells and can signal for either apoptosis of T cells or can prevent their activation. As determined by flow cytometry using three anti-PDL1 monoclonal antibodies (mAb) that detect distinct PDL1 epitopes, and human lung, uveal melanoma, cutaneous melanoma, and breast cancer cells, the over-expression of CD80 blocks constitutive and IFNγ-induced cell surface expression of PDL1. Quantitative analysis of CD80 and PDL1 expression indicates an inverse relationship between PDL1 and CD80. CD80+ tumor cells contain intracellular PDL1, as assessed by flow cytometry, suggesting that CD80 regulates PDL1 at the protein level. Tumor cells transfected with CD80 constructs lacking a cytoplasmic domain or with an irrelevant non-signaling cytoplasmic domain, retain the ability to block PDL1 expression at the cell surface indicating that CD80 does not modulate PDL1 transcription via intracellular signaling. These results suggest that in addition to serving as a costimulatory molecule, CD80 facilitates T cell activation by blocking coinhibitory signals and preventing T cell apoptosis.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5309.

Abstract 4363: Identification of ABCB5 multidrug transporter in retinoblastoma

Purpose: Retinoblastoma (Rb) is the most common intraocular tumor in children. Chemotherapy has improved the outcome for both unilateral and bilateral early stage disease, yet late stage bilateral Rb remains difficult to treat, and metastasis are often fatal. Treatment with carboplatin, vincristine and etoposide is only effective in 40% of bilateral Rb patients and results in a high incidence of secondary malignancies. ATP-binding cassette (ABC) transporters act as efflux pumps of different substrates, including drugs. ABCB5, a member of the ABC-B subfamily, is expressed on both normal tissues (CNS, mammary gland, testis, retina) and in skin melanoma, where it identifies a subpopulation of cancer stem cells with enhanced tumorigenicity. Unexpectedly, the retina displays the highest level of ABCG5 in normal tissues, suggesting it might be expressed in Rb tumors. Here we identify for the first time in Rb, an ABCB5+ subpopulation of cells that we predict will possess drug resistance and/or enhanced tumorigenicity.

Methods: Rb143, Rb116, Rb125 and Rb107 cell lines were developed in our laboratory from primary explants recovered from patients with large tumors. Sequencing of the Rb gene (27 exons) was used to validate all Rb cell lines. Flow cytometry was used to identify ABCB5+ cells using a monoclonal anti-ABCB5 antibody or isotype control. Calcein AM and/or Aqua fluorescent dye was used to determine viability. ABCB5 pump function was determined by using Calcein AM, a pump substrate that becomes fluorescent within viable cells. Optical coherence tomography (OCT) was used to quantitate the size of intraocular tumors, following orthotopic injection into the sub-retinal space of NOD-scid IL2rg−/− mice.

Results: All Rb cell lines possessed a subpopulation of ABCB5+ cells with a frequency ranging from 3-12%. To determine if the ABCB5 pump was functional, Rb cells were cultured for 30 mins with Calcein AM (ABCB5 pump substrate). ABCB5 positive, but not negative, Rb cells excluded the Calcein AM. The ABCB5 pump mediated Calcein exclusion, which was terminated by the addition of an ABCB5 specific blocking antibody. Rb143 cells (originally 3% ABCB5+ cells) were separated by cell sorting into (i) ABCB5+ cells (90% pure), and (ii) ABCB5- cells (100% pure). To test the growth potential and drug resistance of these two subpopulations in vivo, we developed a murine transient retinal detachment model. Injection of 50μl of PBS into the sub-retinal space induces a retinal detachment, but as the PBS is subsequently absorbed over the next 24 hrs the retina reattaches. Up to 1×106 cells were orthotopically injected beneath the detached retina, resulting in tumor growth in < 1 wk. This model will allow us to determine the drug resistance and growth potential of ABCB5+ and ABCB5- Rb cells.

Conclusion: Retinoblastoma possesses a small sub-population of ABCB5+ tumor cells, which displays an active pump with the potential of drug resistance and preferential tumor growth.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4363.

Abstract 1922: Do stress response genes play a role in maintaining MDSC survival and suppression

Myeloid- derived suppressor cells (MDSC) contribute to immune suppression in tumor bearing individuals. Their suppression and survival are increased by inflammation. MDSC production of toxic radicals is the primary mechanism of suppression but the toxic mediators do not affect the MDSC themselves. We are investigating stress response genes that may maintain MDSC survival and facilitate suppression. BALB/c mice were injected with BALB/c derived mammary carcinoma 4T1 or 4T1 constitutively expressing the pro-inflammatory cytokine IL-1β (4T1/IL-1β). Primary tumors were surgically removed after 25 days. At 35 days, spleens were harvested and assayed for the percentage of Gr1+ CD11b+ MDSC. Surgical removal of 4T1 results in a regression of the MDSC population, however MDSC do not regress in mice bearing 4T1/IL-1β tumors, confirming that inflammation increases MDSC survival. Western blots and confocal microscopy of MDSC induced by 4T1 and 4T1/IL-1β tumors localize Nrf2 in the nucleus, demonstrating that Nrf2 is activated in MDSC. In flow cytometry assays, MDSC induced by 4T1/IL-1β have higher expression of one chain of the cystine transporter, xCT, compared to 4T1 induced MDSC. 4T1/IL-1β induced MDSC also expressed more reactive oxygen species (ROS) compared to 4T1 MDSC. The increased expression of ROS and xCT in 4T1/IL-1β induced MDSC represent mechanisms for the increased suppression of CD4+ and CD8+ T cells compared to 4T1 induced MDSC. Catalase, a protein transcriptionally regulated by Nrf2, when added to a T cell activation assay eliminates MDSC suppressive activity. Addition of catalase to T cells co-cultured with MDSC and peptide causes a synergistic effect, activating T cells to a greater level than peptide alone. Collectively these data suggest that stress genes may play a role in maintaining MDSC survival and suppressive activity.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1922.

Do stress response genes play a role in maintaining MDSC survival and suppression? (100.17)

Myeloid- derived suppressor cells (MDSC) contribute to immune suppression in tumor bearing individuals. Their suppression and survival are increased by inflammation. MDSC production of toxic radicals is the primary mechanism of suppression but the toxic mediators do not affect the MDSC themselves. We are investigating stress response genes that may maintain MDSC survival and facilitate suppression. BALB/c mice were injected with BALB/c derived mammary carcinoma 4T1 or 4T1 constitutively expressing the pro-inflammatory cytokine IL-1β (4T1/IL-1β). Primary tumors were surgically removed after 25 days. At 35 days, spleens were harvested and assayed for the percentage of Gr1+ CD11b+ MDSC. Surgical removal of 4T1 results in a regression of the MDSC population, however MDSC do not regress in mice bearing 4T1/IL-1β tumors, confirming that inflammation increases MDSC survival. Western blots and confocal microscopy of MDSC induced by 4T1 and 4T1/IL-1β tumors localize Nrf2 in the nucleus, demonstrating that Nrf2 is activated in MDSC. Catalase, a protein transcriptionally regulated by Nrf2, when added to a T cell activation assay eliminates MDSC suppressive activity. Addition of catalase to T cells co-cultured with MDSC and peptide causes a synergistic effect, activating T cells to a greater level than peptide alone. Collectively these data suggest that stress genes may play a role in maintaining MDSC survival and suppressive activity.

Programmed Death Ligand-1 expression on the cell surface is regulated by costimulatory molecule CD80 expressed by cancer vaccines (101.4)

Programmed Death Ligand 1 (PDL1) is a major obstacle in the development of cancer immunotherapies. PDL1 is constitutively expressed or induced by IFNγ on many tumor cells, resulting in the prevention of T cell activation or the apoptotic cell death of activated T cells. Cell-based vaccines previously developed in our lab were made by modifying tumor cells to express Major Histocompatibility Complex Class II (MHCII) and costimulatory B7-1 (CD80) molecules. These cell-based vaccines do not express PDL1, even after IFNγ stimulation. As determined by flow cytometry using three anti-PDL1 monoclonal antibodies that detect distinct PDL1 epitopes, and human lung, uveal and cutaneous melanoma, and breast cancer cells, the over-expression of CD80 blocks constitutive and IFNγ-induced cell surface expression of PDL1. Quantitative analysis of CD80 and PDL1 expression indicates an inverse relationship between PDL1 and CD80. CD80+ tumor cells contain intracellular PDL1, as assessed by flow cytometry, suggesting that CD80 regulates PDL1 at the protein level. Tumor cells transfected with CD80 constructs lacking a cytoplasmic domain or with an irrelevant non-signaling cytoplasmic domain, retain the ability to block PDL1 expression at the cell surface indicating that CD80 does not modulate PDL1 transcription. These results suggest that in addition to serving as a costimulatory molecule, CD80 facilitates T cell activation by blocking coinhibitory signals and preventing T cell apoptosis.

Inflammation-Induced Myeloid-Derived Suppressor Cells have enhanced resistance to apoptosis. (100.31)

The purpose of this study is to deduce the influence of inflammation on apoptosis of Myeloid-Derived Suppressor cells. Myeloid-derived suppressor cells (MDSC) accumulate in patients and animals with cancer where they mediate systemic immune suppression. MDSC induced in heightened inflammatory environments with high levels of IL-1β and/or IL-6 accumulate more rapidly, are more potent suppressors of T cell activation, produce more IL-10, and have a greater ability to down-regulate macrophage production of IL-12, as compared to MDSC that develop in environments with lower levels of these cytokines. To determine how inflammation enhances MDSC levels and activity we used mass spectrometry to identify proteins produced by MDSC induced in highly inflammatory vs. less inflammatory settings. Proteomic pathway analysis identified Fas pathway and caspase network proteins, leading us to hypothesize that inflammation enhances MDSC accumulation by down-regulating Fas-mediated MDSC apoptosis. Using cytochrome C and activated caspase 3 and caspase 8 as indicators of apoptosis, confocal microscopy and flow cytometry demonstrated that inflammation-induced MDSC have enhanced resistance to Fas-mediated apoptosis. These results suggest that an inflammatory environment 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.

Myeloid-derived suppressor cells express the death receptor Fas and apoptose in response to T cell-expressed FasL (100.18)

Myeloid-derived suppressor cells (MDSC) accumulate to high levels in individuals with a variety of conditions including cancer, chronic inflammation, and stress, where they are potent inhibitors of adaptive and innate immunity. Some of the factors that induce their accumulation are known; however, physiological conditions that regulate their turn-over have not been identified. Mass spectrometry analysis revealed prominent expression of apoptosis pathway proteins by MDSC and led us to hypothesize that Fas/FasL-mediated apoptosis may be a key mechanism that regulates MDSC turn-over. This hypothesis was confirmed by the findings that MDSC levels significantly regress after removal of primary mammary tumors from STAT6-deficient or CD-1-deficient mice, but do not regress in mice doubly deficient for FasL and STAT6 or CD-1. Flow cytometry and confocal microscopy studies examining activated caspase 3 expression and cytochrome C localization revealed that MDSC apoptosis is triggered via the extrinsic, and not the intrinsic, pathway. Surprisingly, activated FasL+ T cells mediated MDSC apoptosis in vivo, demonstrating a retaliatory relationship between these two cell populations, and suggesting that Fas-FasL interactions could be exploited as a strategy to reduce in vivo levels of MDSC.

Myeloid-derived suppressor cells inhibit T-cell activation by depleting cystine and cysteine

Myeloid-derived suppressor cells (MDSC) are present in most cancer patients and are potent inhibitors of T-cell-mediated antitumor immunity. Their inhibitory activity is attributed to production of arginase, reactive oxygen species, inducible nitric oxide synthase, and interleukin-10. Here we show that MDSCs also block T-cell activation by sequestering cystine and limiting the availability of cysteine. Cysteine is an essential amino acid for T-cell activation because T cells lack cystathionase, which converts methionine to cysteine, and because they do not have an intact xc- transporter and therefore cannot import cystine and reduce it intracellularly to cysteine. T cells depend on antigen-presenting cells (APC), such as macrophages and dendritic cells, to export cysteine, which is imported by T cells via their ASC neutral amino acid transporter. MDSCs express the xc- transporter and import cystine; however, they do not express the ASC transporter and do not export cysteine. MDSCs compete with APC for extracellular cystine, and in the presence of MDSCs, APC release of cysteine is reduced, thereby limiting the extracellular pool of cysteine. In summary, MDSCs consume cystine and do not return cysteine to their microenvironment, thereby depriving T cells of the cysteine they require for activation and function.

Class II-associated invariant chain peptide down-modulation enhances the immunogenicity of myeloid leukemic blasts resulting in increased CD4+ T-cell responses

BACKGROUND

Disease recurrence in patients with acute myeloid leukemia may be partially explained by the escape of leukemic blasts from CD4(+) T-cell recognition. The current study investigates the role of aberrant HLA class II antigen presentation on leukemic blasts by determining both the clinical and functional impact of the class II-associated invariant chain peptide (CLIP).

DESIGN AND METHODS

The levels of expression of CLIP and HLA-DR on blood and bone marrow samples from 207 patients with acute myeloid leukemia were correlated with clinical outcome. Irradiated CLIP(-) and CLIP(+) leukemic blasts were compared for their ability to induce CD4(+) T cells during mixed leukocyte reactions. To discriminate between these blasts, we down-modulated CLIP expression on myeloid leukemic cell lines by RNA interference of the invariant chain, a chaperone protein critically involved in HLA-DR processing, and performed flow cytometric sorting for their isolation from primary acute myeloid leukemia samples.

RESULTS

We found that patients with leukemic blasts characterized by a high amount of HLA-DR occupied by CLIP (relative amount of CLIP) had a significantly shortened disease-free survival. The clear reductions in amount of HLA-DR occupied by CLIP on blasts of the THP-1 and Kasumi-1 myeloid leukemic cell lines after treatment with invariant chain short interfering RNA resulted in enhanced rates of allogeneic CD4(+) T-cell proliferation. Similar findings were obtained in an autologous setting, in which there were strong increases in proliferation of remission CD4(+) T cells stimulated with CLIP(-)-sorted leukemic blasts from HLA-DR(+) acute myeloid leukemia patients, in contrast to CLIP(+)-sorted leukemic blasts from the same patients.

CONCLUSIONS

These data highlight the relevance of CLIP expression on leukemic blasts and the potential of CLIP as a target for immunomodulatory strategies to enhance HLA class II antigen presentation and CD4(+) T-cell reactivity in acute myeloid leukemia.

Myeloid-derived suppressor cells down-regulate L-selectin expression on CD4+ and CD8+ T cells

Effective cell-mediated antitumor immunity requires the activation of tumor-reactive T cells and the trafficking of activated T cells to tumor sites. These processes involve the extravasation of lymphocytes from the blood and lymphatics, and their homing to lymph nodes and tumors. L-selectin (CD62L) is an important molecule in these processes. It directs naive lymphocytes to peripheral lymph nodes where they become activated and it traffics naive lymphocytes to inflammatory environments, such as tumors. Individuals with advanced cancer are immune suppressed due to myeloid-derived suppressor cells (MDSC), a population of immature myeloid cells that accumulate to high levels in response to tumor-secreted and proinflammatory factors. We now demonstrate that the reduction in T cell levels of L-selectin that is commonly seen in individuals with cancer inversely correlates with MDSC levels. Three lines of evidence demonstrate that MDSC directly down-regulate L-selectin on naive T cells: 1) naive T cells cocultured with tumor-induced MDSC have reduced L-selectin; 2) T cells in tumor-free aged mice with elevated levels of MDSC have reduced L-selectin, and 3) peritoneal exudate T cells of tumor-free mice treated with plasminogen activator urokinase to elevate MDSC have reduced levels of L-selectin. MDSC are likely to down-regulate L-selectin through their plasma membrane expression of ADAM17 (a disintegrin and metalloproteinase domain 17), an enzyme that cleaves the ectodomain of L-selectin. Therefore, MDSC down-regulate L-selectin levels on naive T cells, decreasing their ability to home to sites where they would be activated. This is another mechanism by which MDSC inhibit antitumor immunity.

Gene dosage and antigenic expression on the cell surface of bovine erythrocytes

Electron microscopic and serological techniques have been used to study the relationship between cell surface expression of bovine erythrocyte antigens and the genes coding for these antigens. Using cells which are genetically and serologically defined for their zygosity with respect to the Z allele, it was found that homozygous (Z/Z) cells have approximately twice as much surface Z antigen as heterozygous (Z/-) cells. Cells labeled for the J antigen, a soluble serum substance which secondarily adsorbs to the erythrocyte surface, display a quantity of antigen which is directly related to the J titer of the cells. A new antigen is described which is independent of the J antigen, and which is detectable by EM labeling and by indirect agglutination, but not by hemolysis.

Myeloid-derived suppressor cells: linking inflammation and cancer

Many cancer immunotherapies developed in experimental animals have been tested in clinical trials. Although some have shown modest clinical effects, most have not been effective. Recent studies have identified myeloid-origin cells that are potent suppressors of tumor immunity and therefore a significant impediment to cancer immunotherapy. "Myeloid-derived suppressor cells" (MDSC) accumulate in the blood, lymph nodes, and bone marrow and at tumor sites in most patients and experimental animals with cancer and inhibit both adaptive and innate immunity. MDSC are induced by tumor-secreted and host-secreted factors, many of which are proinflammatory molecules. The induction of MDSC by proinflammatory mediators led to the hypothesis that inflammation promotes the accumulation of MDSC that down-regulate immune surveillance and antitumor immunity, thereby facilitating tumor growth. This article reviews the characterization and suppressive mechanisms used by MDSC to block tumor immunity and describes the mechanisms by which inflammation promotes tumor progression through the induction of MDSC.

Inflammation enhances myeloid-derived suppressor cell cross-talk by signaling through Toll-like receptor 4

Myeloid-derived suppressor cells (MDSC) are potent inhibitors of anti-tumor immunity that facilitate tumor progression by blocking the activation of CD4(+) and CD8(+) T cells and by promoting a type 2 immune response through their production of IL-10 and down-regulation of macrophage production of IL-12. MDSC accumulate in many cancer patients and are a significant impediment to active cancer immunotherapies. Chronic inflammation has been shown recently to enhance the accumulation of MDSC and to increase their suppression of T cells. These findings led us to hypothesize that inflammation contributes to tumor progression through the induction of MDSC, which create a favorable environment for tumor growth. As chronic inflammation also drives type 2 immune responses, which favor tumor growth, we asked if inflammation mediates this effect through MDSC. We find that IL-1beta-induced inflammation increased IL-10 production by MDSC and induces MDSC, which are more effective at down-regulating macrophage production of IL-12 as compared with MDSC isolated from less-inflammatory tumor microenvironments, thereby skewing tumor immunity toward a type 2 response. Inflammation heightens MDSC phenotype by signaling through the TLR4 pathway and involves up-regulation of CD14. Although this pathway is well-recognized in other myeloid cells, it has not been implicated previously in MDSC function. These studies demonstrate that MDSC are an intermediary through which inflammation promotes type 2 immune responses, and they identify the TLR4 pathway in MDSC as a potential target for down-regulating immune suppression and promoting anti-tumor immunity.

Proinflammatory S100 proteins regulate the accumulation of myeloid-derived suppressor cells

Chronic inflammation is a complex process that promotes carcinogenesis and tumor progression; however, the mechanisms by which specific inflammatory mediators contribute to tumor growth remain unclear. We and others recently demonstrated that the inflammatory mediators IL-1beta, IL-6, and PGE(2) induce accumulation of myeloid-derived suppressor cells (MDSC) in tumor-bearing individuals. MDSC impair tumor immunity and thereby facilitate carcinogenesis and tumor progression by inhibiting T and NK cell activation, and by polarizing immunity toward a tumor-promoting type 2 phenotype. We now show that this population of immature myeloid cells induced by a given tumor share a common phenotype regardless of their in vivo location (bone marrow, spleen, blood, or tumor site), and that Gr1(high)CD11b(high)F4/80(-)CD80(+)IL4Ralpha(+/-)Arginase(+) MDSC are induced by the proinflammatory proteins S100A8/A9. S100A8/A9 proteins bind to carboxylated N-glycans expressed on the receptor for advanced glycation end-products and other cell surface glycoprotein receptors on MDSC, signal through the NF-kappaB pathway, and promote MDSC migration. MDSC also synthesize and secrete S100A8/A9 proteins that accumulate in the serum of tumor-bearing mice, and in vivo blocking of S100A8/A9 binding to MDSC using an anti-carboxylated glycan Ab reduces MDSC levels in blood and secondary lymphoid organs in mice with metastatic disease. Therefore, the S100 family of inflammatory mediators serves as an autocrine feedback loop that sustains accumulation of MDSC. Since S100A8/A9 activation of MDSC is through the NF-kappaB signaling pathway, drugs that target this pathway may reduce MDSC levels and be useful therapeutic agents in conjunction with active immunotherapy in cancer patients.

Abstract PL03-02: Inflammation-driven myeloid-derived suppressor cells: co-opting anti-tumor immunity to promote tumor progression

PL03-02

Inflammation facilitates tumor onset and promotes tumor progression through a variety of mechanisms. One of these mechanisms is by activating myeloid-derived suppressor cells (MDSC), a population of cells that block anti-tumor immunity. MDSC are present in most patients and experimental animals with primary and/or metastatic cancer. They inhibit adaptive anti-tumor immunity by preventing the activation of CD4+ and CD8+ T cells, and by blocking innate immunity by inhibiting natural killer cell cytotoxicity and polarizing immunity towards a tumor-promoting type 2 phenotype. Because of their extreme immune suppressive activity, MDSC will be a significant impediment to active immunotherapy in patients with established malignancies. The accumulation and retention of MDSC are driven by multiple pro-inflammatory factors that arise from chronic inflammation and that are produced by malignant cells and by the tumor microenvironment. Multiple pro-inflammatory mediators, including IL-1β, IL-6, prostaglandin E2 (PGE-2), and the S100A8/A9 proteins, accentuate the accumulation and retention of MDSC in tumor-bearing individuals. IL-1β mediates its effects indirectly, while IL-6, PGE-2, and S100A8/A9 proteins appear to directly interact with MDSC through their respective receptors. In addition to increasing the T cell suppressive activity of MDSC, IL-1β increases the cross-talk between MDSC and macrophages. This cross-talk enhances a type 2 immune response that favors tumor progression by increasing MDSC production of IL-10 and decreasing macrophage production of IL-12. MDSC not only respond to these pro-inflammatory molecules, but they also exacerbate an inflammatory tumor microenvironment by synthesizing and secreting proinflammatory mediators, including the S100A8/A9 proteins. As a result, once induced and activated, MDSC maintain an autocrine feedback loop that sustains their levels in the blood, lymph nodes, and at tumor sites. Multiple approaches are being developed to block the induction/activation of MDSC. Since both IL-1β and S100A8/A9 activate MDSC through the NF-κB signaling pathway, drugs that target this pathway may reduce MDSC levels and be useful therapeutic agents in conjunction with active immunotherapy in cancer patients. (Supported by NIH R01CA84232, RO1CA115880)

Citation Information: Cancer Prev Res 2008;1(7 Suppl):PL03-02.

Antagonism of the prostaglandin E receptor EP4 inhibits metastasis and enhances NK function

Cyclooxygenase-2 (COX-2) is associated with aggressive breast cancers. The COX-2 product prostaglandin E(2) (PGE(2)) acts through four G-protein-coupled receptors designated EP1-4. Malignant and immortalized normal mammary epithelial cell lines express all four EP. The EP4 antagonist AH23848 reduced the ability of tumor cells to colonize the lungs or to spontaneously metastasize from the mammary gland. EP4 gene silencing by shRNA also reduced the ability of mammary tumor cells to metastasize. Metastasis inhibition was lost in mice lacking either functional Natural Killer (NK) cells or interferon-gamma. EP4 antagonism inhibited MHC class I expression resulting in enhanced ability of NK cells to lyse mammary tumor target cells. These studies support the hypothesis that EP4 receptor antagonists reduce metastatic potential by facilitating NK-mediated tumor cell killing and that therapeutic targeting of EP4 may be an alternative approach to the use of COX inhibitors to limit metastatic disease.

Mouse 4T1 breast tumor model

The 4T1 mammary carcinoma is a transplantable tumor cell line that is highly tumorigenic and invasive and, unlike most tumor models, can spontaneously metastasize from the primary tumor in the mammary gland to multiple distant sites including lymph nodes, blood, liver, lung, brain, and bone The 4T1 tumor has several characteristics that make it a suitable experimental animal model for human mammary cancer. First, tumor cells are easily transplanted into the mammary gland so that the primary tumor grows in the anatomically correct site, as described in this unit. Second, as in human breast cancer, 4T1 metastatic disease develops spontaneously from the primary tumor. Also, the progressive spread of 4T1 metastases to the draining lymph nodes and other organs is very similar to that of human mammary cancer. In this unit, a protocol describes surgical removal of the primary tumor, so that metastatic disease can be studied in an animal setting comparable to the clinical situation where the primary tumor is surgically removed, and metastatic foci remain intact. Another advantage of 4T1 is its resistance to 6-thioguanine. This property enables precise quantitation of metastatic cells, even when they are disseminated and at sub-microscopic levels in distant organs, as described here.