The DNA Methylcytosine Dioxygenase Tet2 Sustains Immunosuppressive Function of Tumor-Infiltrating Myeloid Cells to Promote Melanoma Progression.

Ten-Eleven-Translocation-2 (Tet2) is a DNA methylcytosine dioxygenase that functions as a tumor suppressor in hematopoietic malignancies. We examined the role of Tet2 in tumor-tissue myeloid cells and found that Tet2 sustains the immunosuppressive function of these cells. We found that Tet2 expression is increased in intratumoral myeloid cells both in mouse models of melanoma and in melanoma patients and that this increased expression is dependent on an IL-1R-MyD88 pathway. Ablation of Tet2 in myeloid cells suppressed melanoma growth in vivo and shifted the immunosuppressive gene expression program in tumor-associated macrophages to a proinflammatory one, with a concomitant reduction of the immunosuppressive function. This resulted in increased numbers of effector T cells in the tumor, and T cell depletion abolished the reduced tumor growth observed upon myeloid-specific deletion of Tet2. Our findings reveal a non-cell-intrinsic, tumor-promoting function for Tet2 and suggest that Tet2 may present a therapeutic target for the treatment of non-hematologic malignancies.

sFRP2 in the aged microenvironment drives melanoma metastasis and therapy resistance.

Cancer is a disease of ageing. Clinically, aged cancer patients tend to have a poorer prognosis than young. This may be due to accumulated cellular damage, decreases in adaptive immunity, and chronic inflammation. However, the effects of the aged microenvironment on tumour progression have been largely unexplored. Since dermal fibroblasts can have profound impacts on melanoma progression, we examined whether age-related changes in dermal fibroblasts could drive melanoma metastasis and response to targeted therapy. Here we find that aged fibroblasts secrete a Wnt antagonist, sFRP2, which activates a multi-step signalling cascade in melanoma cells that results in a decrease in ß-catenin and microphthalmia-associated transcription factor (MITF), and ultimately the loss of a key redox effector, APE1. Loss of APE1 attenuates the response of melanoma cells to DNA damage induced by reactive oxygen species, rendering the cells more resistant to targeted therapy (vemurafenib). Age-related increases in sFRP2 also augment both angiogenesis and metastasis of melanoma cells. These data provide an integrated view of how fibroblasts in the aged microenvironment contribute to tumour progression, offering new possibilities for the design of therapy for the elderly.

Stem cell-released oncolytic herpes simplex virus has therapeutic efficacy in brain metastatic melanomas.

The recent Food and Drug Administration approval of immunogenic oncolytic virus (OV) has opened a new era in the treatment of advanced melanoma; however, approximately 50% of patients with melanoma develop brain metastasis, and currently there are no beneficial treatment options for such patients. To model the progression of metastases seen in patients and to overcome the hurdles of systemic delivery of OV, we developed melanoma brain metastasis models in immunocompromised and immunocompetent mice, and tested the fate and efficacy of oncolytic herpes simplex virus (oHSV)-armed mesenchymal stem cells (MSCs). Using brain-seeking patient-derived melanoma cells and real-time in vivo imaging, we show a widespread distribution of micrometastases and macrometastases in the brain, recapitulating the progression of multifoci metastases seen in patients. We armed MSCs with different oHSV variants (MSC-oHSV) and found that intracarotid administration of MSC-oHSV, but not of purified oHSV alone, effectively tracks metastatic tumor lesions and significantly prolongs the survival of brain tumor-bearing mice. In a syngeneic model of melanoma brain metastasis, a combination of MSC-oHSV and PD-L1 blockade increases IFNγ-producing CD8+ tumor-infiltrating T lymphocytes and results in a profound extension of the median survival of treated animals. This study thus demonstrates the utility of MSCs as OV carriers to disseminated brain lesions, and provides a clinically applicable therapeutic platform to target melanoma brain metastasis.

Structure of Vps75 and implications for histone chaperone function.

The vacuolar protein sorting 75 (Vps75) histone chaperone participates in chromatin assembly and disassembly at both active and inactive genes through the preferential binding to histone H3-H4. Vps75 is also one of two histone chaperones, along with antisilencing factor 1, that promotes histone H3-Lys-56 acetylation by the regulation of Ty1 transposition protein 109 (Rtt109) histone acetyltransferase. Here, we report the x-ray crystal structure of Vps75 and carry out biochemical studies to characterize its interaction with Rtt109. We find that the Vps75 structure forms a homodimeric "headphone" architecture that includes an extended helical dimerization domain and earmuff domains at opposite ends and sides of the dimerization domain. Despite the similar overall architecture with the yeast nucleosome assembly protein 1 and human SET/TAF-1beta/INHAT histone chaperones, Vps75 shows several unique features including the relative disposition of the earmuff domains to the dimerization domain, characteristics of the earmuff domains, and a pronounced cleft at the center of the Vps75 dimer. These differences appear to correlate with the unique function of Vps75 to interact with Rtt109 for histone acetylation. Our biochemical studies reveal that two surfaces on the earmuff domain of Vps75 participate in Rtt109 interaction with a stoichiometry of 2:1, thus leaving the pronounced central cleft of the Vps75 dimer largely accessible for histone binding. Taken together, our data provide a structural framework for understanding how Vps75 mediates both nucleosome assembly and histone acetylation by Rtt109.

Site-specific introduction of an acetyl-lysine mimic into peptides and proteins by cysteine alkylation.

Protein acetylation on Lys residues is recognized as a significant post-translational modification in cells, but it is often difficult to discern the direct structural and functional effects of individual acetylation events. Here we describe a new tool, methylthiocarbonyl-aziridine, to install acetyl-Lys mimics site-specifically into peptides and proteins by alkylation of Cys residues. We demonstrate that the resultant thiocarbamate modification can be recognized by the Brdt bromodomain and site-specific antiacetyl-Lys antibodies, is resistant to histone deacetylase cleavage, and can confer activation of the histone acetyltransferase Rtt109 by simulating autoacetylation. We also use this approach to obtain functional evidence that acetylation of CK2 protein kinase on Lys102 can stimulate its catalytic activity.

TLR Stimulation Produces IFN-ß as the Primary Driver of IFN Signaling in Nonlymphoid Primary Human Cells.

Several human autoimmune diseases are characterized by increased expression of type 1 IFN-stimulated genes in both the peripheral blood and tissue. The contributions of different type I IFNs to this gene signature are uncertain as the type I IFN family consists of 13 alphas and one each of ß, ε, κ, and ω subtypes. We sought to investigate the contribution of various IFNs to IFN signaling in primary human cell types. We stimulated primary skin, muscle, kidney, and PBMCs from normal healthy human donors with various TLR ligands and measured the expression of type I IFN subtypes and activation of downstream signaling by quantitative PCR. We show that IFNB1 is the dominant type I IFN expressed upon TLR3 and TLR4 stimulation, and its expression profile is associated with subsequent MX1 transcription. Furthermore, using an IFN-ß-specific neutralizing Ab, we show that MX1 expression is inhibited in a dose-dependent manner, suggesting that IFN-ß is the primary driver of IFN-stimulated genes following TLR3 and TLR4 engagement. Stimulation with TLR7/8 and TLR9 ligands induced IFNB1 and IFNA subtypes and MX1 expression only in PBMCs and not in tissue resident cell types. Concordantly, IFN-ß neutralization had no effect on MX1 expression in PBMCs potentially because of the combination of IFNB1 and IFNA expression. Combined, these data highlight the potential role for IFN-ß in driving local inflammatory responses in clinically relevant human tissue types and opportunities to treat local inflammation by targeting IFN-ß.

Novel Protocol for Generating Physiologic Immunogenic Dendritic Cells.

Extracorporeal photochemotherapy (ECP) is a widely used cancer immunotherapy for cutaneous T cell lymphoma (CTCL), operative in over 350 university centers worldwide. While ECP's clinical efficacy and exemplary safety profile have driven its widespread use, elucidation of the underlying mechanisms has remained a challenge, partly owing to lack of a laboratory ECP model. To overcome this obstacle and create a simple, user-friendly platform for ECP research, we developed a scaled-down version of the clinical ECP leukocyte-processing device, suitable for work with both mouse models, and small human blood samples. This device is termed the Transimmunization (TI) chamber, or plate. In a series of landmark experiments, the miniaturized device was used to produce a cellular vaccine that regularly initiated therapeutic anti-cancer immunity in several syngeneic mouse tumor models. By removing individual factors from the experimental system and ascertaining their contribution to the in vivo anti-tumor response, we then elucidated key mechanistic drivers of ECP immunizing potential. Collectively, our results revealed that anti-tumor effects of ECP are initiated by dendritic cells (DC), physiologically generated through blood monocyte interaction with platelets in the TI plate, and loaded with antigens from tumor cells whose apoptotic cell death is finely titrated by exposure to the photoactivatable DNA cross-linking agent 8-methoxypsoralen and UVA light (8-MOPA). When returned to the mouse, this cellular vaccine leads to specific and transferable anti-tumor T cell immunity. We verified that the TI chamber is also suitable for human blood processing, producing human DCs fully comparable in activation state and profile to those derived from the clinical ECP chamber. The protocols presented here are intended for ECP studies in mouse and man, controlled generation of apoptotic tumor cells with 8-MOPA, and rapid production of physiologic human and mouse monocyte-derived DCs for a variety of applications.

Increased Serine Synthesis Provides an Advantage for Tumors Arising in Tissues Where Serine Levels Are Limiting.

Tumors exhibit altered metabolism compared to normal tissues. Many cancers upregulate expression of serine synthesis pathway enzymes, and some tumors exhibit copy-number gain of the gene encoding the first enzyme in the pathway, phosphoglycerate dehydrogenase (PHGDH). However, whether increased serine synthesis promotes tumor growth and how serine synthesis benefits tumors is controversial. Here, we demonstrate that increased PHGDH expression promotes tumor progression in mouse models of melanoma and breast cancer, human tumor types that exhibit PHGDH copy-number gain. We measure circulating serine levels and find that PHGDH expression is necessary to support cell proliferation at lower physiological serine concentrations. Increased dietary serine or high PHGDH expression is sufficient to increase intracellular serine levels and support faster tumor growth. Together, these data suggest that physiological serine availability restrains tumor growth and argue that tumors arising in serine-limited environments acquire a fitness advantage by upregulating serine synthesis pathway enzymes.

A novel immunogenic mouse model of melanoma for the preclinical assessment of combination targeted and immune-based therapy.

Both targeted therapy and immunotherapy have been used successfully to treat melanoma, but the development of resistance and poor response rates to the individual therapies has limited their success. Designing rational combinations of targeted therapy and immunotherapy may overcome these obstacles, but requires assessment in preclinical models with the capacity to respond to both therapeutic classes. Herein, we describe the development and characterization of a novel, immunogenic variant of the BrafV600ECdkn2a-/-Pten-/- YUMM1.1 tumor model that expresses the immunogen, ovalbumin (YOVAL1.1). We demonstrate that, unlike parental tumors, YOVAL1.1 tumors are immunogenic in vivo and can be controlled by immunotherapy. Importantly, YOVAL1.1 tumors are sensitive to targeted inhibitors of BRAFV600E and MEK, responding in a manner consistent with human BRAFV600E melanoma. The YOVAL1.1 melanoma model is transplantable, immunogenic and sensitive to clinical therapies, making it a valuable platform to guide strategic development of combined targeted therapy and immunotherapy approaches in BRAFV600E melanoma.

Extracorporeal Photochemotherapy Drives Monocyte-to-Dendritic Cell Maturation to Induce Anticancer Immunity.

Extracorporeal photochemotherapy (ECP) is a cancer immunotherapy for cutaneous T-cell lymphoma (CTCL) operative in more than 350 centers worldwide. Although its efficacy and favorable safety profile have driven its widespread use, elucidation of its underlying mechanism has been difficult. In this study, we identify the principal contributors to the anticancer immunotherapeutic effects of ECP, with the goal of enhancing potency and broadening applicability to additional malignancies. First, we scaled down the clinical ECP leukocyte-processing device to mouse size. Second, we used that miniaturized device to produce a cellular vaccine that regularly initiated therapeutic antimelanoma immunity. Third, we individually subtracted key factors from either the immunizing inoculum or the treated animal to ascertain their contribution to the in vivo antimelanoma response. Platelet-signaled monocyte-to-dendritic cell (DC) differentiation followed by sorting/processing/presentation of tumor antigens derived from internalized apoptotic tumor cells were absolute requirements. As in clinical ECP, immunogenic cell death of tumor cells was finely titrated by DNA cross-linkage mediated by photoactivated 8-methoxypsoralen (8-MOPA). ECP-induced tumor-loaded DC were effective immunotherapeutic agents only if they were spared exposure to 8-MOPA, indicating that healthy DC are required for ECP. Infusion of responder T cells into naïve tumor-challenged mice established the protective role of stimulated T-cell antitumor immunity. Collectively, these results reveal that selective antitumor effects of ECP are initiated by tumor antigen-loaded, ECP-induced DC, which promote potent collaboration between CD4 and CD8 tumor-specific T cells. These mechanistic insights suggest potential therapeutic applicability of ECP to solid tumors in addition to CTCL.Significance: These findings identify principal cellular contributors to the anticancer immunotherapeutic impact of ECP and suggest this treatment may be applicable to a broad spectrum of immunogenic malignancies. Cancer Res; 78(14); 4045-58. ©2018 AACR.

DKK2 imparts tumor immunity evasion through ß-catenin-independent suppression of cytotoxic immune-cell activation.

Immunotherapy offers new options for cancer treatment, but efficacy varies across cancer types. Colorectal cancers (CRCs) are largely refractory to immune-checkpoint blockade, which suggests the presence of yet uncharacterized immune-suppressive mechanisms. Here we report that the loss of adenomatosis polyposis coli (APC) in intestinal tumor cells or of the tumor suppressor PTEN in melanoma cells upregulates the expression of Dickkopf-related protein 2 (DKK2), which, together with its receptor LRP5, provides an unconventional mechanism for tumor immune evasion. DKK2 secreted by tumor cells acts on cytotoxic lymphocytes, inhibiting STAT5 signaling by impeding STAT5 nuclear localization via LRP5, but independently of LRP6 and the Wnt-ß-catenin pathway. Genetic or antibody-mediated ablation of DKK2 activates natural killer (NK) cells and CD8+ T cells in tumors, impedes tumor progression, and enhances the effects of PD-1 blockade. Thus, we have identified a previously unknown tumor immune-suppressive mechanism and immunotherapeutic targets particularly relevant for CRCs and a subset of melanomas.

Myeloid-targeted immunotherapies act in synergy to induce inflammation and antitumor immunity.

Eliciting effective antitumor immune responses in patients who fail checkpoint inhibitor therapy is a critical challenge in cancer immunotherapy, and in such patients, tumor-associated myeloid cells and macrophages (TAMs) are promising therapeutic targets. We demonstrate in an autochthonous, poorly immunogenic mouse model of melanoma that combination therapy with an agonistic anti-CD40 mAb and CSF-1R inhibitor potently suppressed tumor growth. Microwell assays to measure multiplex protein secretion by single cells identified that untreated tumors have distinct TAM subpopulations secreting MMP9 or cosecreting CCL17/22, characteristic of an M2-like state. Combination therapy reduced the frequency of these subsets, while simultaneously inducing a separate polyfunctional inflammatory TAM subset cosecreting TNF-α, IL-6, and IL-12. Tumor suppression by this combined therapy was partially dependent on T cells, and on TNF-α and IFN-γ. Together, this study demonstrates the potential for targeting TAMs to convert a "cold" into an "inflamed" tumor microenvironment capable of eliciting protective T cell responses.

UV-induced somatic mutations elicit a functional T cell response in the YUMMER1.7 mouse melanoma model.

Human melanomas exhibit relatively high somatic mutation burden compared to other malignancies. These somatic mutations may produce neoantigens that are recognized by the immune system, leading to an antitumor response. By irradiating a parental mouse melanoma cell line carrying three driver mutations with UVB and expanding a single-cell clone, we generated a mutagenized model that exhibits high somatic mutation burden. When inoculated at low cell numbers in immunocompetent C57BL/6J mice, YUMMER1.7 (Yale University Mouse Melanoma Exposed to Radiation) regresses after a brief period of growth. This regression phenotype is dependent on T cells as YUMMER1.7 tumors grow significantly faster in immunodeficient Rag1-/- mice and C57BL/6J mice depleted of CD4 and CD8 T cells. Interestingly, regression can be overcome by injecting higher cell numbers of YUMMER1.7, which results in tumors that grow without effective rejection. Mice that have previously rejected YUMMER1.7 tumors develop immunity against higher doses of YUMMER1.7 tumor challenge. In addition, escaping YUMMER1.7 tumors are sensitive to anti-CTLA-4 and anti-PD-1 therapy, establishing a new model for the evaluation of immune checkpoint inhibition and antitumor immune responses.

The YUMM lines: a series of congenic mouse melanoma cell lines with defined genetic alterations.

The remarkable success of immune therapies emphasizes the need for immune-competent cancer models. Elegant genetically engineered mouse models of a variety of cancers have been established, but their effective use is limited by cost and difficulties in rapidly generating experimental data. Some mouse cancer cell lines are transplantable to immunocompetent host mice and have been utilized extensively to study cancer immunology. Here, we describe the Yale University Mouse Melanoma (YUMM) lines, a comprehensive system of mouse melanoma cell lines that are syngeneic to C57BL/6, have well-defined human-relevant driver mutations, and are genomically stable. This will be a useful tool for the study of tumor immunology and genotype-specific cancer biology.

DNMT3b Modulates Melanoma Growth by Controlling Levels of mTORC2 Component RICTOR.

DNA methyltransferase DNMT3B is frequently overexpressed in tumor cells and plays important roles during the formation and progression of several cancer types. However, the specific signaling pathways controlled by DNMT3B in cancers, including melanoma, are poorly understood. Here, we report that DNMT3B plays a pro-tumorigenic role in human melanoma and that DNMT3B loss dramatically suppresses melanoma formation in the Braf/Pten mouse melanoma model. Loss of DNMT3B results in hypomethylation of the miR-196b promoter and increased miR-196b expression, which directly targets the mTORC2 component Rictor. Loss of RICTOR in turn prevents mTORC2 activation, which is critical for melanoma formation and growth. These findings establish Dnmt3b as a regulator of melanoma formation through its effect on mTORC2 signaling. Based on these results, DNMT3B is a potential therapeutic target in melanoma.

Co-inhibition of colony stimulating factor-1 receptor and BRAF oncogene in mouse models of BRAFV600E melanoma.

The presence of colony stimulating factor-1 (CSF1)/CSF1 receptor (CSF1R)-driven tumor-infiltrating macrophages and myeloid-derived suppressor cells (MDSCs) is shown to promote targeted therapy resistance. In this study, we demonstrate the superior effect of a combination of CSF1R inhibitor, PLX3397 and BRAF inhibitor, PLX4720, in suppressing primary and metastatic mouse BRAFV600E melanoma. Using flow cytometry to assess SM1WT1 melanoma-infiltrating leukocytes immediately post therapy, we found that PLX3397 reduced the recruitment of CD11b+ Gr1lo and CD11b+ Gr1int M2-like macrophages, but this was accompanied by an accumulation of CD11b+ Gr1hi cells. PDL1 expression on remaining myeloid cells potentially dampened the antitumor efficacy of PLX3397 and PLX4720 in combination, since PD1/PDL1 axis blockade improved outcome. We also reveal a role for PLX3397 in reducing tumor-infiltrating lymphocytes, and interestingly, this feature was rescued by the co-administration of PLX4720. Our findings, from three different mouse models of BRAF-mutated melanoma, support clinical approaches that co-target BRAF oncogene and CSF1R.

Response to Programmed Cell Death-1 Blockade in a Murine Melanoma Syngeneic Model Requires Costimulation, CD4, and CD8 T Cells.

The programmed cell death protein 1 (PD-1) limits effector T-cell functions in peripheral tissues, and its inhibition leads to clinical benefit in different cancers. To better understand how PD-1 blockade therapy modulates the tumor-host interactions, we evaluated three syngeneic murine tumor models, the BRAFV600E-driven YUMM1.1 and YUMM2.1 melanomas, and the carcinogen-induced murine colon adenocarcinoma MC38. The YUMM cell lines were established from mice with melanocyte-specific BRAFV600E mutation and PTEN loss (BRAFV600E/PTEN-/-). Anti-PD-1 or anti-PD-L1 therapy engendered strong antitumor activity against MC38 and YUMM2.1, but not YUMM1.1. PD-L1 expression did not differ between the three models at baseline or upon interferon stimulation. Whereas mutational load was high in MC38, it was lower in both YUMM models. In YUMM2.1, the antitumor activity of PD-1 blockade had a critical requirement for both CD4 and CD8 T cells, as well as CD28 and CD80/86 costimulation, with an increase in CD11c+CD11b+MHC-IIhigh dendritic cells and tumor-associated macrophages in the tumors after PD-1 blockade. Compared with YUMM1.1, YUMM2.1 exhibited a more inflammatory profile by RNA sequencing analysis, with an increase in expression of chemokine-trafficking genes that are related to immune cell recruitment and T-cell priming. In conclusion, response to PD-1 blockade therapy in tumor models requires CD4 and CD8 T cells and costimulation that is mediated by dendritic cells and macrophages. Cancer Immunol Res; 4(10); 845-57. ©2016 AACR.

BRAF Inhibition Decreases Cellular Glucose Uptake in Melanoma in Association with Reduction in Cell Volume.

BRAF kinase inhibitors have dramatically affected treatment of BRAF(V600E) (/) (K)-driven metastatic melanoma. Early responses assessed using [(18)F]fluorodeoxyglucose uptake-positron emission tomography (FDG-PET) have shown dramatic reduction of radiotracer signal within 2 weeks of treatment. Despite high response rates, relapse occurs in nearly all cases, frequently at sites of treated metastatic disease. It remains unclear whether initial loss of (18)FDG uptake is due to tumor cell death or other reasons. Here, we provide evidence of melanoma cell volume reduction in a patient cohort treated with BRAF inhibitors. We present data demonstrating that BRAF inhibition reduces melanoma glucose uptake per cell, but that this change is no longer significant following normalization for cell volume changes. We also demonstrate that volume normalization greatly reduces differences in transmembrane glucose transport and hexokinase-mediated phosphorylation. Mechanistic studies suggest that this loss of cell volume is due in large part to decreases in new protein translation as a consequence of vemurafenib treatment. Ultimately, our findings suggest that cell volume regulation constitutes an important physiologic parameter that may significantly contribute to radiographic changes observed in clinic.

The broad-spectrum receptor tyrosine kinase inhibitor dovitinib suppresses growth of BRAF-mutant melanoma cells in combination with other signaling pathway inhibitors.

BRAF inhibitors have revolutionized treatment of mutant BRAF metastatic melanomas. However, resistance develops rapidly following BRAF inhibitor treatment. We have found that BRAF-mutant melanoma cell lines are more sensitive than wild-type BRAF cells to the small molecule tyrosine kinase inhibitor dovitinib. Sensitivity is associated with inhibition of a series of known dovitinib targets. Dovitinib in combination with several agents inhibits growth more effectively than either agent alone. These combinations inhibit BRAF-mutant melanoma and colorectal carcinoma cell lines, including cell lines with intrinsic or selected BRAF inhibitor resistance. Hence, combinations of dovitinib with second agents are potentially effective therapies for BRAF-mutant melanomas, regardless of their sensitivity to BRAF inhibitors.