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.

The Ubiquitin-Specific Protease Usp7, a Novel Merkel Cell Polyomavirus Large T-Antigen Interaction Partner, Modulates Viral DNA Replication.

Merkel cell polyomavirus (MCPyV) is the major cause for Merkel cell carcinoma (MCC), a rare but highly aggressive skin cancer predominantly found in elderly and immunosuppressed patients. The early viral gene products large T-antigen (LT) and small T-antigen (sT) are important for efficient viral DNA replication, and both contribute to transformation processes. These functions are executed mainly through interactions with host factors. Here, we identify the cellular ubiquitin-specific processing protease 7 (Usp7) as a new interaction partner of the MCPyV LT. Using glutathione S-transferase pulldown experiments, we show that MCPyV LT directly binds to Usp7 and that N- as well as C-terminal regions of LT bind to the TRAF (tumor necrosis factor receptor-associated) domain of Usp7. We demonstrate that endogenous Usp7 coprecipitates with MCPyV T-antigens and relocalizes to viral DNA replication centers in cells actively replicating MCPyV genomes. We show that Usp7 does not alter ubiquitination levels of the T-antigens; however, Usp7 binding increases the binding affinity of LT to the origin of replication, thereby negatively regulating viral DNA replication. Together, these data identify Usp7 as a restriction factor of MCPyV replication. In contrast to other DNA viruses, Usp7 does not affect MCPyV gene expression via its ubiquitination activity but influences MCPyV DNA replication solely via a novel mechanism that modulates binding of LT to viral DNA.IMPORTANCE MCPyV is the only human polyomavirus that is associated with cancer; the majority of Merkel cell cancers have a viral etiology. While much emphasis was placed on investigations to understand the transformation process by MCPyV oncoproteins and cellular factors, we have only limited knowledge of cellular factors participating in the MCPyV life cycle. Here, we describe Usp7, a cellular deubiquitination enzyme, as a new factor involved in MCPyV replication. Usp7 is known in the context of large DNA tumor viruses, Epstein-Barr virus (EBV) and Kaposi's sarcoma herpesvirus, to restrict viral replication. Similar to EBV, where Usp7 binding to EBNA1 increases EBNA1 binding affinity to viral DNA, we find MCPyV LT binding to the origin of replication to be increased in the presence of Usp7, resulting in restriction of viral DNA replication. However, Usp7-induced restriction of MCPyV replication is independent of its enzymatic activity, thereby constituting a novel mechanism of Usp7-induced restriction of viral replication.

Investigation of simian virus 40 (SV40) and human JC, BK, MC, KI, and WU polyomaviruses in glioma.

The gliomagenesis remains not fully established and their etiological factors still remain obscure. Polyomaviruses were detected and involved in several human tumors. Their potential implication in gliomas has been not yet surveyed in Africa and Arab World. Herein, we investigated the prevalence of six polyomaviruses (SV40, JCPyV, BKPyV, MCPyV, KIPyV, and WUPyV) in 112 gliomas from Tunisian patients. The DNA sequences of polyomaviruses were examined by PCR assays. Viral infection was confirmed by DNA in situ hybridization (ISH) and/or immunohistochemistry (IHC). The relationships between polyomavirus infection and tumor features were evaluated. Specific SV40 Tag, viral regulatory, and VP1 regions were identified in 12 GBM (10.7%). DNA ISH targeting the whole SV40 genome and SV40 Tag IHC confirmed the PCR findings. Five gliomas yielded JCPyV positivity by PCR and DNA ISH (2.7%). However, no BKPyV, KIPyV, and WUPyV DNA sequences were identified in all samples. MCPyV DNA was identified in 30 gliomas (26.8%). For GBM samples, MCPyV was significantly related to patient age (p = 0.037), tumor recurrence (p = 0.024), and SV40 (p = 0.045) infection. No further significant association was identified with the remaining tumor features (p > 0.05) and patient survival (Log Rank, p > 0.05). Our study indicates the presence of SV40, JCPyV, and MCPyV DNA in Tunisian gliomas. Further investigations are required to more elucidate the potential involvement of polyomaviruses in these destructive malignancies.

Characterization of an early passage Merkel cell polyomavirus-positive Merkel cell carcinoma cell line, MS-1, and its growth in NOD scid gamma mice.

Merkel cell carcinoma (MCC) is an aggressive skin cancer with a high mortality rate. The majority of MCC (70-80%) harbor clonally integrated Merkel cell polyomavirus (MCV) in the tumor genome and express viral T antigen oncoproteins. The characterization of an early passage MCV-positive MCC cell line MS-1 is described, and its cellular, immunohistochemical, and virological features to MCV-negative (UISO, MCC13, and MCC26) and MCV-positive cell lines (MKL-1 and MKL-2) were compared. The MS-1 cellular genome harbors integrated MCV, which preserves an identical viral sequence from its parental tumor. Neither VP2 gene transcripts nor VP1 protein are detectable in MS-1 or other MCV-positive MCC cell lines tested. Mapping of viral and cellular integration sites in MS-1 and MCC tumor samples demonstrates no consistent viral or cellular gene integration locus. All MCV-positive cell lines show cytokeratin 20 positivity and grow in suspension. When injected subcutaneously into NOD scid gamma (NSG) mice, MS-1 forms a discrete macroscopic tumor. Immunophenotypic analysis of the MS-1 cell line and xenografts in mice show identical profiles to the parental tumor biopsy. Hence, MS-1 is an early passage cell line that provides a useful in vitro model to characterize MCV-positive MCC.

Merkel cell polyomavirus-positive Merkel cell carcinoma cells do not require expression of the viral small T antigen.

Increasing evidence suggests that Merkel cell carcinoma (MCC) is caused by the Merkel cell polyomavirus (MCV). The viral sequence encodes for two potential oncoproteins, i.e., the small T antigen (sT) and the large T antigen (LT). Indeed, sT has recently been shown to bear transforming activity. Here, we confirm this observation by demonstrating focus formation upon expression of MCV sT in NIH3T3 fibroblasts. On the other hand, however, we provide evidence that established MCC cells do not require sT for growth and survival. Silencing of sT protein expression by two different sT-specific short hairpin RNAs (shRNAs) leads to variable degrees of growth retardation in MCV-positive MCC cell lines. However, these effects are not sT specific, as proliferation of MCV-negative cell lines is similarly affected by these sT shRNAs. Furthermore, ectopic expression of shRNA-insensitive sT does not revert the growth inhibition implicated by sT silencing. Finally, the unambiguous and specific growth inhibition induced by means of an shRNA targeting both T antigens, can be completely rescued by ectopic expression of LT alone, thus demonstrating a dispensable role of sT. Altogether, our results indicate that MCV LT is more relevant in maintaining the proliferation and survival of established MCC cell lines.