Project Summary Merkel cell polyomavirus (MCPyV), the most recently discovered tumor virus, can cause a highly aggressive form of skin cancer called Merkel cell carcinoma (MCC). While the incidence of MCC has tripled over the past twenty years, there is no effective therapy for metastatic MCCs, highlighting the need to better understand MCPyV oncogenic mechanism in order to develop more successful therapies. MCPyV asymptomatically infects most of the human population, but tends to cause MCC in the elderly and immunocompromised individuals. These observations suggest that host immunity plays a critical role in controlling MCPyV-induced tumorigenesis. However, very little is known about the innate immune response elicited by MCPyV. Neither is it clear how a dysregulated immune system contributes to MCC tumorigenesis. This is largely because MCPyV tropism was previously unknown and there was a lack of biologically relevant culture system for MCPyV. Recently, we discovered that human dermal fibroblasts (HDFs) support productive MCPyV infection and established the first in vitro as well as ex vivo infection models for MCPyV. Using these systems, we demonstrated that MCPyV infection activates STING-mediated innate immune responses, which in turn restrict viral amplification and spread. In addition, we discovered that STING is silenced in MCPyV(+) MCC tumors, revealing that loss of STING function is needed to drive MCC tumorigenesis. Our studies suggest that disruption of STING function may cause pathologic rampant replication of MCPyV to promote viral genome integration into the host genome, which is a key event in MCPyV- driven tumorigenesis. In addition, loss of STING function may allow MCPyV-induced pre-cancerous cells to circumvent its tumor suppressive effects, thus stimulating cell proliferation and tumorigenesis. Building on these observations, we hypothesize that STING functions not only as a key antiviral immune mediator for controlling MCPyV infection but also a prime tumor suppressor that blocks MCPyV-driven tumorigenesis. To test this hypothesis, we will combine the in vitro and ex vivo MCPyV infection models with 3D “artificial human skin” reconstructed in mice to examine the impact of STING innate immune sensing pathways on MCPyV infection (Aim 1) and to determine how disruption of STING signaling impacts MCPyV-driven MCC tumorigenesis (Aim 2). Through revealing the largely unknown interplay between MCPyV and the innate immune system, our ultimate goal is to understand how poorly controlled MCPyV infection leads to MCC development. Identification of immune effectors that normally restrict MCPyV propagation could also unveil novel strategies for preventing and treating the devastating MCC cancers.