Characterization of the Impact of Merkel Cell Polyomavirus-Induced Interferon Signaling on Viral Infection.

Merkel cell polyomavirus (MCPyV) has been associated with approximately 80% of Merkel cell carcinoma (MCC), an aggressive and increasingly incident skin cancer. The link between host innate immunity, viral load control, and carcinogenesis has been established but poorly characterized. We previously established the importance of the STING and NF-κB pathways in the host innate immune response to viral infection. In this study, we further discovered that MCPyV infection of human dermal fibroblasts (HDFs) induces the expression of type I and III interferons (IFNs), which in turn stimulate robust expression of IFN-stimulated genes (ISGs). Blocking type I IFN downstream signaling using an IFN-ß antibody, JAK inhibitors, and CRISPR knockout of the receptor dramatically repressed MCPyV infection-induced ISG expression but did not significantly restore viral replication activities. These findings suggest that IFN-mediated induction of ISGs in response to MCPyV infection is not crucial to viral control. Instead, we found that type I IFN exerts a more direct effect on MCPyV infection postentry by repressing early viral transcription. We further demonstrated that growth factors normally upregulated in wounded or UV-irradiated human skin can significantly stimulate MCPyV gene expression and replication. Together, these data suggest that in healthy individuals, host antiviral responses, such as IFN production induced by viral activity, may restrict viral propagation to reduce MCPyV burden. Meanwhile, growth factors induced by skin abrasion or UV irradiation may stimulate infected dermal fibroblasts to promote MCPyV propagation. A delicate balance of these mutually antagonizing factors provides a mechanism to support persistent MCPyV infection. IMPORTANCE Merkel cell carcinoma is an aggressive skin cancer that is particularly lethal to immunocompromised individuals. Though rare, MCC incidence has increased significantly in recent years. There are no lasting and effective treatments for metastatic disease, highlighting the need for additional treatment and prevention strategies. By investigating how the host innate immune system interfaces with Merkel cell polyomavirus, the etiological agent of most of these cancers, our studies identified key factors necessary for viral control, as well as conditions that support viral propagation. These studies provide new insights for understanding how the virus balances the effects of the host immune defenses and of growth factor stimulation to achieve persistent infection. Since virus-positive MCC requires the expression of viral oncogenes to survive, our observation that type I IFN can repress viral oncogene transcription indicates that these cytokines could be explored as a viable therapeutic option for treating patients with virus-positive MCC.

Decreased IgG Antibody Response to Viral Protein Mimotopes of Oncogenic Merkel Cell Polyomavirus in Sera From Healthy Elderly Subjects.

Merkel cell polyomavirus (MCPyV) is the main causative agent of Merkel cell carcinoma (MCC), a rare but aggressive skin tumor with a typical presentation age >60 years. MCPyV is ubiquitous in humans. After an early-age primary infection, MCPyV establishes a clinically asymptomatic lifelong infection. In immunocompromised patients/individuals, including elders, MCC can arise following an increase in MCPyV replication events. Elders are prone to develop immunesenescence and therefore represent an important group to investigate. In addition, detailed information on MCPyV serology in elders has been debated. These findings cumulatively indicate the need for new research verifying the impact of MCPyV infection in elderly subjects (ES). Herein, sera from 226 ES, aged 66-100 years, were analyzed for anti-MCPyV IgGs with an indirect ELISA using peptides mimicking epitopes from the MCPyV capsid proteins VP1-2. Immunological data from sera belonging to a cohort of healthy subjects (HS) (n = 548) aged 18-65 years, reported in our previous study, were also included for comparisons. Age-/gender-specific seroprevalence and serological profiles were investigated. MCPyV seroprevalence in ES was 63.7% (144/226). Age-specific MCPyV seroprevalence resulted as 62.5% (25/40), 71.7% (33/46), 64.9% (37/57), 63.8% (30/47), and 52.8% (19/36) in ES aged 66-70, 71-75, 76-80, 81-85, and 86-100 years, respectively (p > 0.05). MCPyV seroprevalence was 67% (71/106) and 61% (73/120) in ES males and females, respectively (p > 0.05). Lack of age-/gender-related variations in terms of MCPyV serological profiles was found in ES (p > 0.05). Notably, serological profile analyses indicated lower optical densities (ODs) in ES compared with HS (p < 0.05), while lower ODs were also determined in ES males compared with HS males (p < 0.05). Our data cumulatively suggest that oncogenic MCPyV circulates in elders asymptomatically at a relatively high prevalence, while immunesenescence might be responsible for a decreased IgG antibody response to MCPyV, thereby potentially leading to an increase in MCPyV replication levels. In the worse scenario, alongside other factors, MCPyV might drive MCC carcinogenesis, as described in elders with over 60 years of age.

Identification of the Merkel Cell Polyomavirus Large Tumor Antigen Ubiquitin Conjugation Residue.

Merkel cell polyomavirus (MCPyV) large tumor (LT) antigen is a DNA binding protein essential for viral gene transcription and genome replication. MCPyV LT interacts with multiple E3 ligases in a phosphorylation-dependent manner, limiting its own viral replication by enhancing LT protein degradation, which is a unique mechanism for MCPyV latency. Thus, identifying LT ubiquitination sites is an important step toward understanding the biological role of these virus-host interactions that can potentially result in viral oncogenesis. The ubiquitin (Ub) attachment sites in LT were predicted by using Rapid UBIquitination (RUBI), a sequence-based ubiquitination web server. Using an immunoprecipitation approach, the lysine (Lys, K) 585 residue in LT is identified as the ubiquitin conjugation site. Lysine 585 is deleted from tumor-derived truncated LTs (tLTs), resulting in stable expression of tLTs present in cancers. Substitution of lysine 585 to arginine (Arg, R) increased LT protein stability, but impaired MCPyV origin replication, due to a loss of ATP hydrolysis activity. These findings uncover a never-before-identified ubiquitination site of LT and its importance not only in the regulation of protein turnover, but also in MCPyV genome replication.

Serum Antibodies Against the Oncogenic Merkel Cell Polyomavirus Detected by an Innovative Immunological Assay With Mimotopes in Healthy Subjects.

Merkel cell polyomavirus (MCPyV), a small DNA tumor virus, has been detected in Merkel cell carcinoma (MCC) and in normal tissues. Since MCPyV infection occurs in both MCC-affected patients and healthy subjects (HS), innovative immunoassays for detecting antibodies (abs) against MCPyV are required. Herein, sera from HS were analyzed with a novel indirect ELISA using two synthetic peptides mimicking MCPyV capsid protein epitopes of VP1 and VP2. Synthetic peptides were designed to recognize IgGs against MCPyV VP mimotopes using a computer-assisted approach. The assay was set up evaluating its performance in detecting IgGs anti-MCPyV on MCPyV-positive (n=65) and -negative (n=67) control sera. Then, the ELISA was extended to sera (n=548) from HS aged 18-65 yrs old. Age-specific MCPyV-seroprevalence was investigated. Performance evaluation indicated that the assay showed 80% sensitivity, 91% specificity and 83.9% accuracy, with positive and negative predictive values of 94.3% and 71%, respectively. The ratio expected/obtained data agreement was 86%, with a Cohen's kappa of 0.72. Receiver-operating characteristic (ROC) curves analysis indicated that the areas under the curves (AUCs) for the two peptides were 0.82 and 0.74, respectively. Intra-/inter-run variations were below 9%. The overall prevalence of serum IgGs anti-MCPyV in HS was 62.9% (345/548). Age-specific MCPyV-seroprevalence was 63.1% (82/130), 56.7% (68/120), 64.5% (91/141), and 66.2% (104/157) in HS aged 18-30, 31-40, 41-50 and 51-65 yrs old, respectively (p>0.05). Performance evaluation suggests that our indirect ELISA is reliable in detecting IgGs anti-MCPyV. Our immunological data indicate that MCPyV infection occurs asymptomatically, at a relatively high prevalence, in humans.

Merkel Cell Polyomavirus Infection Induces an Antiviral Innate Immune Response in Human Dermal Fibroblasts.

Merkel cell polyomavirus (MCPyV) infects most of the human population asymptomatically, but in rare cases it leads to a highly aggressive skin cancer called Merkel cell carcinoma (MCC). MCC incidence is much higher in aging and immunocompromised populations. The epidemiology of MCC suggests that dysbiosis between the host immune response and the MCPyV infectious cycle could contribute to the development of MCPyV-associated MCC. Insufficient restriction of MCPyV by normal cellular processes, for example, could promote the incidental oncogenic MCPyV integration events and/or entry into the original cell of MCC. Progress toward understanding MCPyV biology has been hindered by its narrow cellular tropism. Our discovery that primary human dermal fibroblasts (HDFs) support MCPyV infection has made it possible to closely model cellular responses to different stages of the infectious cycle. The present study reveals that the onset of MCPyV replication and early gene expression induces an inflammatory cytokine and interferon-stimulated gene (ISG) response. The cGAS-STING pathway, in coordination with NF-κB, mediates induction of this innate immune gene expression program. Further, silencing of cGAS or NF-κB pathway factors led to elevated MCPyV replication. We also discovered that the PYHIN protein IFI16 localizes to MCPyV replication centers but does not contribute to the induction of ISGs. Instead, IFI16 upregulates inflammatory cytokines in response to MCPyV infection by an alternative mechanism. The work described herein establishes a foundation for exploring how changes to the skin microenvironment induced by aging or immunodeficiency might alter the fate of MCPyV and its host cell to encourage carcinogenesis. IMPORTANCE MCC has a high rate of mortality and an increasing incidence. Immune-checkpoint therapies have improved the prognosis of patients with metastatic MCC. Still, a significant proportion of the patients fail to respond to immune-checkpoint therapies or have a medical need for iatrogenic immune-suppression. A greater understanding of MCPyV biology could inform targeted therapies for MCPyV-associated MCC. Moreover, cellular events preceding MCC oncogenesis remain largely unknown. The present study aims to explore how MCPyV interfaces with innate immunity during its infectious cycle. We describe how MCPyV replication and/or transcription elicit an innate immune response via cGAS-STING, NF-κB, and IFI16. We also explore the effects of this response on MCPyV replication. Our findings illustrate how healthy cellular conditions may allow low-level infection that evades immune destruction until highly active replication is restricted by host responses. Conversely, pathological conditions could result in unbridled MCPyV replication that licenses MCC tumorigenesis.

Quality Is King: Fundamental Insights into Tumor Antigenicity from Virus-Associated Merkel Cell Carcinoma.

Merkel cell carcinoma (MCC) is a rare skin malignancy that is a paradigm cancer for solid tumor immunotherapy. MCCs associated with Merkel cell polyomavirus (virus-positive MCC [VP-MCC]) or chronic UV exposure (virus-negative MCC [VN-MCC]) are anti-PD(L)1 responsive, despite VP-MCC's low mutational burden. This suggests that antigen quality, not merely mutation quantity, dictates immunotherapy responsiveness, and cell-based therapies targeting optimal antigens may be effective. Despite VP-MCC's antigenic homogeneity, diverse T-cell infiltration patterns are observed, implying microenvironment plasticity and multifactorial contributions to immune recognition. Moreover, VP-MCC exemplifies how antitumor adaptive immunity can provide tumor burden biomarkers for early detection and disease monitoring.

Robust Production of Merkel Cell Polyomavirus Oncogene Specific T Cells From Healthy Donors for Adoptive Transfer.

Virus positive Merkel cell carcinoma (VP-MCC) is an aggressive but immunogenic skin malignancy driven by Merkel cell polyomavirus (MCPyV) T antigen (TAg). Since adoptive T cell transfer (ACT) can be effective against virus-driven malignancies, we set out to develop a methodology for generating MCPyV TAg specific T cells. MCPyV is a common, asymptomatic infection and virus-exposed healthy donors represent a potential source of MCPyV TAg specific T cells for ACT. Virus specific T cells were generated using monocyte-derived dendritic cells (moDCs) pulsed with MCPyV TAg peptide libraries and co-cultured with autologous T cells in supplemented with pro-inflammatory and homeostatic cytokines for 14 days. Specific reactivity was observed predominantly within the CD4+ T cell compartment in the cultures generated from 21/46 random healthy donors. Notably, responses were more often seen in donors aged 50 years and older. TAg specific CD4+ T cells specifically secreted Th1 cytokines and upregulated CD137 upon challenge with MCPyV TAg peptide libraries and autologous transduced antigen presenting cells. Expanded T cells from healthy donors recognized epitopes of both TAg splice variants found in VP-MCC tumors, and minimally expressed exhaustion markers. Our data show that MCPyV specific T cells can be expanded from healthy donors using methods appropriate for the manufacture of clinical grade ACT products.

Merkel cell polyomavirus small tumour antigen activates the p38 MAPK pathway to enhance cellular motility.

Merkel cell carcinoma (MCC) is an aggressive skin cancer with high rates of recurrence and metastasis. Merkel cell polyomavirus (MCPyV) is associated with the majority of MCC cases. MCPyV-induced tumourigenesis is largely dependent on the expression of the small tumour antigen (ST). Recent findings implicate MCPyV ST expression in the highly metastatic nature of MCC by promoting cell motility and migration, through differential expression of cellular proteins that lead to microtubule destabilisation, filopodium formation and breakdown of cell-cell junctions. However, the molecular mechanisms which dysregulate these cellular processes are yet to be fully elucidated. Here, we demonstrate that MCPyV ST expression activates p38 MAPK signalling to drive cell migration and motility. Notably, MCPyV ST-mediated p38 MAPK signalling occurs through MKK4, as opposed to the canonical MKK3/6 signalling pathway. In addition, our results indicate that an interaction between MCPyV ST and the cellular phospatase subunit PP4C is essential for its effect on p38 MAPK signalling. These results provide novel opportunities for the treatment of metastatic MCC given the intense interest in p38 MAPK inhibitors as therapeutic agents.

Polyomavirus-driven Merkel cell carcinoma: Prospects for therapeutic vaccine development.

Great strides have been made in cancer immunotherapy including the breakthrough successes of anti-PD-(L)1 checkpoint inhibitors. In Merkel cell carcinoma (MCC), a rare and aggressive skin cancer, PD-(L)1 blockade is highly effective. Yet, ~50% of patients either do not respond to therapy or develop PD-(L)1 refractory disease and, thus, do not experience long-term benefit. For these patients, additional or combination therapies are needed to augment immune responses that target and eliminate cancer cells. Therapeutic vaccines targeting tumor-associated antigens, mutated self-antigens, or immunogenic viral oncoproteins are currently being developed to augment T-cell responses. Approximately 80% of MCC cases in the United States are driven by the ongoing expression of viral T-antigen (T-Ag) oncoproteins from genomically integrated Merkel cell polyomavirus (MCPyV). Since T-Ag elicits specific B- and T-cell immune responses in most persons with virus-positive MCC (VP-MCC), and ongoing T-Ag expression is required to drive VP-MCC cell proliferation, therapeutic vaccination with T-Ag is a rational potential component of immunotherapy. Failure of the endogenous T-cell response to clear VP-MCC (allowing clinically evident tumors to arise) implies that therapeutic vaccination will need to be potent ansd synergize with other mechanisms to enhance T-cell activity against tumor cells. Here, we review the relevant underlying biology of VP-MCC, potentially applicable therapeutic vaccine platforms, and antigen delivery formats. We also describe early successes in the field of therapeutic cancer vaccines and address several clinical scenarios in which VP-MCC patients could potentially benefit from a therapeutic vaccine.

Prevalent and Diverse Intratumoral Oncoprotein-Specific CD8+ T Cells within Polyomavirus-Driven Merkel Cell Carcinomas.

Merkel cell carcinoma (MCC) is often caused by persistent expression of Merkel cell polyomavirus (MCPyV) T-antigen (T-Ag). These non-self proteins comprise about 400 amino acids (AA). Clinical responses to immune checkpoint inhibitors, seen in about half of patients, may relate to T-Ag-specific T cells. Strategies to increase CD8+ T-cell number, breadth, or function could augment checkpoint inhibition, but vaccines to augment immunity must avoid delivery of oncogenic T-antigen domains. We probed MCC tumor-infiltrating lymphocytes (TIL) with an artificial antigen-presenting cell (aAPC) system and confirmed T-Ag recognition with synthetic peptides, HLA-peptide tetramers, and dendritic cells (DC). TILs from 9 of 12 (75%) subjects contained CD8+ T cells recognizing 1-8 MCPyV epitopes per person. Analysis of 16 MCPyV CD8+ TIL epitopes and prior TIL data indicated that 97% of patients with MCPyV+ MCC had HLA alleles with the genetic potential that restrict CD8+ T-cell responses to MCPyV T-Ag. The LT AA 70-110 region was epitope rich, whereas the oncogenic domains of T-Ag were not commonly recognized. Specific recognition of T-Ag-expressing DCs was documented. Recovery of MCPyV oncoprotein-specific CD8+ TILs from most tumors indicated that antigen indifference was unlikely to be a major cause of checkpoint inhibition failure. The myriad of epitopes restricted by diverse HLA alleles indicates that vaccination can be a rational component of immunotherapy if tumor immune suppression can be overcome, and the oncogenic regions of T-Ag can be modified without impacting immunogenicity.

Conversion of Sox2-dependent Merkel cell carcinoma to a differentiated neuron-like phenotype by T antigen inhibition.

Viral cancers show oncogene addiction to viral oncoproteins, which are required for survival and proliferation of the dedifferentiated cancer cell. Human Merkel cell carcinomas (MCCs) that harbor a clonally integrated Merkel cell polyomavirus (MCV) genome have low mutation burden and require viral T antigen expression for tumor growth. Here, we showed that MCV+ MCC cells cocultured with keratinocytes undergo neuron-like differentiation with neurite outgrowth, secretory vesicle accumulation, and the generation of sodium-dependent action potentials, hallmarks of a neuronal cell lineage. Cocultured keratinocytes are essential for induction of the neuronal phenotype. Keratinocyte-conditioned medium was insufficient to induce this phenotype. Single-cell RNA sequencing revealed that T antigen knockdown inhibited cell cycle gene expression and reduced expression of key Merkel cell lineage/MCC marker genes, including HES6, SOX2, ATOH1, and KRT20 Of these, T antigen knockdown directly inhibited Sox2 and Atoh1 expression. MCV large T up-regulated Sox2 through its retinoblastoma protein-inhibition domain, which in turn activated Atoh1 expression. The knockdown of Sox2 in MCV+ MCCs mimicked T antigen knockdown by inducing MCC cell growth arrest and neuron-like differentiation. These results show Sox2-dependent conversion of an undifferentiated, aggressive cancer cell to a differentiated neuron-like phenotype and suggest that the ontology of MCC arises from a neuronal cell precursor.

Human CD4+ T Cells Specific for Merkel Cell Polyomavirus Localize to Merkel Cell Carcinomas and Target a Required Oncogenic Domain.

Although CD4+ T cells likely play key roles in antitumor immune responses, most immuno-oncology studies have been limited to CD8+ T-cell responses due to multiple technical barriers and a lack of shared antigens across patients. Merkel cell carcinoma (MCC) is an aggressive skin cancer caused by Merkel cell polyomavirus (MCPyV) oncoproteins in 80% of cases. Because MCPyV oncoproteins are shared across most patients with MCC, it is unusually feasible to identify, characterize, and potentially augment tumor-specific CD4+ T cells. Here, we report the identification of CD4+ T-cell responses against six MCPyV epitopes, one of which included a conserved, essential viral oncogenic domain that binds/disables the cellular retinoblastoma (Rb) tumor suppressor. We found that this epitope (WEDLT209-228) could be presented by three population-prevalent HLA class II alleles, making it a relevant target in 64% of virus-positive MCC patients. Cellular staining with a WEDLT209-228-HLA-DRB1*0401 tetramer indicated that specific CD4+ T cells were detectable in 78% (14 of 18) of evaluable MCC patients, were 250-fold enriched within MCC tumors relative to peripheral blood, and had diverse T-cell receptor sequences. We also identified a modification of this domain that still allowed recognition by these CD4+ T cells but disabled binding to the Rb tumor suppressor, a key step in the detoxification of a possible therapeutic vaccine. The use of these new tools for deeper study of MCPyV-specific CD4+ T cells may provide broader insight into cancer-specific CD4+ T-cell responses.

Merkel cell polyomavirus Tumor antigens expressed in Merkel cell carcinoma function independently of the ubiquitin ligases Fbw7 and ß-TrCP.

Merkel cell polyomavirus (MCPyV) accounts for 80% of all Merkel cell carcinoma (MCC) cases through expression of two viral oncoproteins: the truncated large T antigen (LT-t) and small T antigen (ST). MCPyV ST is thought to be the main driver of cellular transformation and has also been shown to increase LT protein levels through the activity of its Large-T Stabilization Domain (LSD). The ST LSD was reported to bind and sequester several ubiquitin ligases, including Fbw7 and ß-TrCP, and thereby stabilize LT-t and several other Fbw7 targets including c-Myc and cyclin E. Therefore, the ST LSD is thought to contribute to transformation by promoting the accumulation of these oncoproteins. Targets of Fbw7 and ß-TrCP contain well-defined, conserved, phospho-degrons. However, as neither MCPyV LT, LT-t nor ST contain the canonical Fbw7 phospho-degron, we sought to further investigate the proposed model of ST stabilization of LT-t and transformation. In this study, we provide several lines of evidence that fail to support a specific interaction between MCPyV T antigens and Fbw7 or ß-TrCP by co-immunoprecipitation or functional consequence. Although MCPyV ST does indeed increase LT protein levels through its Large-T Stabilization domain (LSD), this is accomplished independently of Fbw7. Therefore, our study indicates a need for further investigation into the role and mechanism(s) of MCPyV T antigens in viral replication, latency, transformation, and tumorigenesis.

Expression of the IDO1/TDO2-AhR pathway in tumor cells or the tumor microenvironment is associated with Merkel cell polyomavirus status and prognosis in Merkel cell carcinoma.

Merkel cell carcinoma (MCC) is a rare, aggressive neuroendocrine skin cancer, with approximately 80% of cases related to Merkel cell polyomavirus (MCPyV). Indoleamine 2,3-dioxygenase 1 (IDO1) and tryptophan 2,3-dioxygenase 2 (TDO2) are the key rate-limiting enzymes of the tryptophan-to-kynurenine metabolic pathway. With aryl hydrocarbon receptor (AhR), an intracellular transcription factor, they play a role in escaping the immunosurveillance process in several cancers. IDO1/TDO2/AhR expression associated with the MCPyV status and prognosis in MCC was investigated. Samples included 24 MCPyV-positive MCCs, 12 MCPyV-negative MCCs with squamous cell carcinoma, and 7 MCPyV-negative pure MCCs. They were stained immunohistochemically with IDO1, TDO2, and AhR antibodies and analyzed. Higher IDO1 expression in MCC tumor cells was found in MCPyV-negative than in MCPyV-positive MCC (P < .001). The tumor microenvironment (TME) in MCPyV-negative MCC expressed higher TDO2 than in MCPyV-positive MCC (P < .001). Kaplan-Meier and log-rank tests showed that MCC with lower IDO1 expression in tumor cells and with lower TDO2 and AhR expressions in TME had better overall survival than otherwise (P = .043, .008, and .035, respectively); lower TDO2 expression in TME was also associated with longer disease-specific survival (P = .016). This suggests that IDO1, TDO2, and AhR express differentially in tumor cells or TME and play different roles in tumorigenesis between MCPyV-positive and MCPyV-negative MCC that may affect the MCC biology. Evaluating IDO1/TDO2/AhR expression is important for selecting the most likely patients with MCC for immunotherapies targeting the IDO1/TDO2-AhR pathway.

Pharmacological Inhibition of Serine Palmitoyl Transferase and Sphingosine Kinase-1/-2 Inhibits Merkel Cell Carcinoma Cell Proliferation.

The majority of Merkel cell carcinoma, a highly aggressive neuroendocrine cancer of the skin, is associated with Merkel cell polyomavirus infection. Polyomavirus binding, internalization, and infection are mediated by glycosphingolipids. Besides receptor function, bioactive sphingolipids are increasingly recognized as potent regulators of several hallmarks of cancer. Merkel cell polyomavirus+ and Merkel cell polyomavirus- cells express serine palmitoyl transferase subunits and sphingosine kinase (SK) 1/2 mRNA. Induced expression of Merkel cell polyomavirus-large tumor antigen in human lung fibroblasts resulted in upregulation of SPTLC1-3 and SK 1/2 expression. Therefore, we exploited pharmacological inhibition of sphingolipid metabolism as an option to interfere with proliferation of Merkel cell polyomavirus+ Merkel cell carcinoma cell lines. We used myriocin (a serine palmitoyl transferase antagonist) and two SK inhibitors (SKI-II and ABC294640). In MKL-1 and WaGa cells myriocin decreased cellular ceramide, sphingomyelin, and sphingosine-1-phosphate content. SKI-II increased ceramide species but decreased sphingomyelin and sphingosine-1-phosphate concentrations. Aberrant sphingolipid homeostasis was associated with reduced cell viability, increased necrosis, procaspase-3 and PARP processing, caspase-3 activity, and decreased AKTS473 phosphorylation. Myriocin and SKI-II decreased tumor size and Ki-67 staining of xenografted MKL-1 and WaGa tumors on the chorioallantoic membrane. Our data suggest that pharmacological inhibition of sphingolipid synthesis could represent a potential therapeutic approach in Merkel cell carcinoma.

Merkel cell polyomavirus-specific immune responses in patients with Merkel cell carcinoma receiving anti-PD-1 therapy.

BACKGROUND: Merkel cell carcinoma (MCC) is an aggressive skin cancer that frequently responds to anti-PD-1 therapy. MCC is associated with sun exposure and, in 80% of cases, Merkel cell polyomavirus (MCPyV). MCPyV-specific T and B cell responses provide a unique opportunity to study cancer-specific immunity throughout PD-1 blockade therapy. METHODS: Immune responses were assessed in patients (n = 26) with advanced MCC receiving pembrolizumab. Peripheral blood mononuclear cells (PBMC) were collected at baseline and throughout treatment. MCPyV-oncoprotein antibodies were quantified and T cells were assessed for MCPyV-specificity via tetramer staining and/or cytokine secretion. Pre-treatment tumor biopsies were analyzed for T cell receptor clonality. RESULTS: MCPyV oncoprotein antibodies were detectable in 15 of 17 (88%) of virus-positive MCC (VP-MCC) patients. Antibodies decreased in 10 of 11 (91%) patients with responding tumors. Virus-specific T cells decreased over time in patients who had a complete response, and increased in patients who had persistent disease. Tumors that were MCPyV(+) had a strikingly more clonal (less diverse) intratumoral TCR repertoire than virus-negative tumors (p = 0.0001). CONCLUSIONS: Cancer-specific T and B cell responses generally track with disease burden during PD-1 blockade, in proportion to presence of antigen. Intratumoral TCR clonality was significantly greater in VP-MCC than VN-MCC tumors, suggesting expansion of a limited number of dominant clones in response to fewer immunogenic MCPyV antigens. In contrast, VN-MCC tumors had lower clonality, suggesting a diverse T cell response to numerous neoantigens. These findings reveal differences in tumor-specific immunity for VP-MCC and VN-MCC, both of which often respond to anti-PD-1 therapy.

Mechanisms of persistence by small DNA tumor viruses.

Virus infection contributes to nearly 15% of human cancers worldwide. Many of the oncogenic viruses tend to cause cancer in immunosuppressed individuals, but maintain asymptomatic, persistent infection for decades in the general population. In this review, we discuss the tactics employed by two small DNA tumor viruses, Human papillomavirus (HPV) and Merkel cell polyomavirus (MCPyV), to establish persistent infection. We will also highlight recent key findings as well as outstanding questions regarding the mechanisms by which HPV and MCPyV evade host immune control to promote their survival. Since persistent infection enables virus-induced tumorigenesis, identifying the mechanisms by which small DNA tumor viruses achieve latent infection may inform new approaches for preventing and treating their respective human cancers.

Acquired cancer resistance to combination immunotherapy from transcriptional loss of class I HLA.

Understanding mechanisms of late/acquired cancer immunotherapy resistance is critical to improve outcomes; cellular immunotherapy trials offer a means to probe complex tumor-immune interfaces through defined T cell/antigen interactions. We treated two patients with metastatic Merkel cell carcinoma with autologous Merkel cell polyomavirus specific CD8+ T cells and immune-checkpoint inhibitors. In both cases, dramatic remissions were associated with dense infiltration of activated CD8+s into the regressing tumors. However, late relapses developed at 22 and 18 months, respectively. Here we report single cell RNA sequencing identified dynamic transcriptional suppression of the specific HLA genes presenting the targeted viral epitope in the resistant tumor as a consequence of intense CD8-mediated immunologic pressure; this is distinguished from genetic HLA-loss by its reversibility with drugs. Transcriptional suppression of Class I loci may underlie resistance to other immunotherapies, including checkpoint inhibitors, and have implications for the design of improved immunotherapy treatments.

Cellular sheddases are induced by Merkel cell polyomavirus small tumour antigen to mediate cell dissociation and invasiveness.

Merkel cell carcinoma (MCC) is an aggressive skin cancer with a high propensity for recurrence and metastasis. Merkel cell polyomavirus (MCPyV) is recognised as the causative factor in the majority of MCC cases. The MCPyV small tumour antigen (ST) is considered to be the main viral transforming factor, however potential mechanisms linking ST expression to the highly metastatic nature of MCC are yet to be fully elucidated. Metastasis is a complex process, with several discrete steps required for the formation of secondary tumour sites. One essential trait that underpins the ability of cancer cells to metastasise is how they interact with adjoining tumour cells and the surrounding extracellular matrix. Here we demonstrate that MCPyV ST expression disrupts the integrity of cell-cell junctions, thereby enhancing cell dissociation and implicate the cellular sheddases, A disintegrin and metalloproteinase (ADAM) 10 and 17 proteins in this process. Inhibition of ADAM 10 and 17 activity reduced MCPyV ST-induced cell dissociation and motility, attributing their function as critical to the MCPyV-induced metastatic processes. Consistent with these data, we confirm that ADAM 10 and 17 are upregulated in MCPyV-positive primary MCC tumours. These novel findings implicate cellular sheddases as key host cell factors contributing to virus-mediated cellular transformation and metastasis. Notably, ADAM protein expression may be a novel biomarker of MCC prognosis and given the current interest in cellular sheddase inhibitors for cancer therapeutics, it highlights ADAM 10 and 17 activity as a novel opportunity for targeted interventions for disseminated MCC.