Targeting inflamed and non-inflamed melanomas: biological background and clinical challenges

Immune checkpoint inhibitors (ICIs) have demonstrated impressive antitumor activity in patients with advanced and early stage melanoma, thus improving long-term survival outcomes. However, most patients derive limited benefit from immunotherapy, due to the development of primary, adaptive, or acquired resistance mechanisms. Immunotherapy resistance is a complex phenomenon that depends on genetic and epigenetic mechanisms which, in turn, drive the interplay between cancer cells and the tumor microenvironment (TME). Immunologically "cold" (i.e. non-inflamed) tumors lack or have few tumor infiltrating lymphocytes (TILs) as a result of low tumor mutational burden (TMB), defective antigen presentation, or physical barriers to lymphocyte migration, resulting in a minimal benefit from immunotherapy. In contrast, in most cases immunologically "hot" (i.e. inflamed) tumors display high TMB, implying a higher load of neoantigens and increased programmed cell death ligand 1 (PD-L1) expression, with a consequently higher rate of TILs. However, the presence of TILs does not necessarily denote the tumor as immunologically "hot", since the presence of tumor-specific CD8+ T cells persistently exposed to antigenic stimulation induces a dysfunctional state called "exhaustion", which leads to a reduced response to immunotherapy. In recent years, efforts have been made to characterize mechanisms of resistance to immunotherapy, and to investigate strategies to overcome treatment resistance. Indeed, predictors of response and toxicity to immunotherapy are still lacking and, to date, there are no reliable predictive biomarkers to select patients according to baseline clinical, histological, or genomic characteristics. In this review, we will focus on the morphologic and immunohistochemical characteristics of the TME, and on the molecular determinants of resistance to immunotherapy, differentiating between inflamed and non-inflamed melanomas. Then, we will provide a thorough overview of preclinical data on genetic and epigenetic mechanisms with a potential impact on the immune response and patient outcome. Finally, we will focus our attention on the role of potential biomarkers in determining disease response to immunotherapy, in the adjuvant and metastatic setting, providing an insight into current and future research in this field

A Preclinical Model of Malignant Peripheral Nerve Sheath Tumor-like Melanoma Is Characterized by Infiltrating Mast Cells

Human melanomas exhibit considerable genetic, pathologic, and microenvironmental heterogeneity. Genetically engineered mice have successfully been used to model the genomic aberrations contributing to melanoma pathogenesis, but their ability to recapitulate the phenotypic variability of human disease and the complex interactions with the immune system have not been addressed. Here, we report the unexpected finding that immune cell-poor pigmented and immune cell-rich amelanotic melanomas developed simultaneously in Cdk4R24C-mutant mice upon melanocyte-specific conditional activation of oncogenic BrafV600E and a single application of the carcinogen 7,12-dimethylbenz(a) anthracene. Interestingly, amelanotic melanomas showed morphologic and molecular features of malignant peripheral nerve sheath tumors (MPNST). A bioinformatic cross-species comparison using a gene expression signature of MPNST-like mouse melanomas identified a subset of human melanomas with a similar histomorphology. Furthermore, this subset of human melanomas was found to be highly associated with a mast cell gene signature, and accordingly, mouse MPNST-like melanomas were also extensively infiltrated by mast cells and expressed mast cell chemoattractants similar to human counterparts. A transplantable mouse MPNST-like melanoma cell line recapitulated mast cell recruitment in syngeneic mice, demonstrating that this cell state can directly reconstitute the histomorphologic and microenvironmental features of primary MPNST-like melanomas. Our study emphasizes the importance of reciprocal, phenotype-dependent melanoma-immune cell interactions and highlights a critical role for mast cells in a subset of melanomas. Moreover, our BrafV600E-Cdk4R24C model represents an attractive system for the development of therapeutic approaches that can target the heterogeneous tumor microenvironment characteristic of human melanomas. (C) 2015 AACR

Immune Cell-Poor Melanomas Benefit from PD-1 Blockade after Targeted Type I IFN Activation

Infiltration of human melanomas with cytotoxic immune cells correlates with spontaneous type I IFN activation and a favorable prognosis. Therapeutic blockade of immune-inhibitory receptors in patients with preexisting lymphocytic infiltrates prolongs survival, but new complementary strategies are needed to activate cellular antitumor immunity in immune cell-poor melanomas. Here, we show that primary melanomas in Hgf-Cdk4(R24C) mice, which imitate human immune cell-poor melanomas with a poor outcome, escape IFN-induced immune surveillance and editing. Peritumoral injections of immunostimulatory RNA initiated a cytotoxic inflammatory response in the tumor microenvironment and significantly impaired tumor growth. This critically required the coordinated induction of type I IFN responses by dendritic, myeloid, natural killer, and T cells. Importantly, antibody-mediated blockade of the IFN-induced immune-inhibitory interaction between PD-L1 and PD-1 receptors further prolonged the survival. These results highlight important interconnections between type I IFNs and immune-inhibitory receptors in melanoma pathogenesis, which serve as targets for combination immunotherapies. SIGNIFICANCE: Using a genetically engineered mouse melanoma model, we demonstrate that targeted activation of the type I IFN system with immunostimulatory RNA in combination with blockade of immune-inhibitory receptors is a rational strategy to expose immune cell-poor tumors to cellular immune surveillance. (C) 2014 AACR

ICOS DNA methylation regulates melanoma cell-intrinsic ICOS expression, is associated with melanoma differentiation, prognosis, and predicts response to immune checkpoint blockade

Abstract Background Inducible T cell costimulator ICOS is an emerging target in immuno-oncology. The aim of this study was to investigate the epigenetic regulation of ICOS in melanoma by DNA methylation. Methods We comprehensively investigate ICOS DNA methylation of specific CpG sites and expression pattern within the melanoma microenvironment with regard to immune correlates, differentiation, clinical outcomes, and immune checkpoint blockade (ICB) response. Results Our study revealed a sequence-contextual CpG methylation pattern consistent with an epigenetically regulated gene. We found a cell type-specific methylation pattern and locus-specific correlations and associations of CpG methylation with ICOS mRNA expression, immune infiltration, melanoma differentiation, prognosis, and response to ICB. High ICOS mRNA expression was identified as a surrogate for enriched immune cell infiltration and was associated with favorable overall survival (OS) in non-ICB-treated patients and predicted response and a prolonged progression-free survival (PFS) following ICB therapy initiation. ICOS hypomethylation, however, significantly correlated with poor OS in non-ICB patients but predicted higher response and prolonged PFS and OS in ICB-treated patients. Moreover, we observed cytoplasmic and sporadically nuclear tumor cell-intrinsic ICOS protein expression. Tumor cell-intrinsic ICOS protein and mRNA expression was inducible by pharmacological demethylation with decitabine. Conclusion Our study identified ICOS DNA methylation and mRNA expression as promising prognostic and predictive biomarkers for immunotherapy in melanoma and points towards a hitherto undescribed role of ICOS in tumor cells

Adoptive T Cell Therapy Targeting Different Gene Products Reveals Diverse and Context-Dependent Immune Evasion in Melanoma

Tumor immune escape limits durable responses to T cell therapy. Here, we examined how regulation and function of gene products that provide the target epitopes for CD8(+) T cell anti-tumor immunity influence therapeutic efficacy and resistance. We used a CRISPR-Cas9-based method (CRISPitope) in syngeneic melanoma models to fuse the same model CD8(+) T cell epitope to the C-termini of different endogenous gene products. Targeting melanosomal proteins or oncogenic CDK4(R24C )(Cyclin-dependent kinase 4) by adoptive cell transfer (ACT) of the same epitope-specific CD8(+) T cells revealed diverse genetic and non-genetic immune escape mechanisms. ACT directed against melanosomal proteins, but not CDK4(R24C), promoted melanoma dedifferentiation, and increased myeloid cell infiltration. CDK4(R24C) antigen persistence was associated with an interferon-high and T-cell-rich tumor microenvironment, allowing for immune checkpoint inhibition as salvage therapy. Thus, the choice of target antigen determines the phenotype and immune contexture of recurrent melanomas, with implications to the design of cancer immunotherapies

MAPK signaling and inflammation link melanoma phenotype switching to induction of CD73 during immunotherapy

Evolution of tumor cell phenotypes promotes heterogeneity and therapy resistance. Here we found that induction of CD73, the enzyme that generates immunosuppressive adenosine, is linked to melanoma phenotype switching. Activating MAPK mutations and growth factors drove CD73 expression, which marked both nascent and full activation of a mesenchymal-like melanoma cell state program. Proinflammatory cytokines like TNF alpha cooperated with MAPK signaling through the c-Jun/AP-1 transcription factor complex to activate CD73 transcription by binding to an intronic enhancer. In a mouse model of T-cell immunotherapy, CD73 was induced in relapse melanomas, which acquired a mesenchymal-like phenotype. We also detected CD73 upregulation in melanoma patients progressing under adoptive T-cell transfer or immune checkpoint blockade, arguing for an adaptive resistance mechanism. Our work substantiates CD73 as a target to combine with current immunotherapies, but its dynamic regulation suggests limited value of CD73 pretreatment expression as a biomarker to stratify melanoma patients. (C) 2017 AACR