The tumorigenic potential of mouse polyomavirus (MPyV) has been studied for decades in cell culture models and has been mainly attributed to nonstructural middle T antigen (MT), which acts as a scaffold signal adaptor, activates Src tyrosine kinases, and possesses transforming ability. We hypothesized that MPyV could also transform mouse cells independent of MT via a Toll-like receptor 4 (TLR4)-mediated inflammatory mechanism. To this end, we investigated the interaction of MPyV with TLR4 in mouse embryonic fibroblasts (MEFs) and 3T6 cells, resulting in secretion of interleukin 6 (IL-6), independent of active viral replication. TLR4 colocalized with MPyV capsid protein VP1 in MEFs. Neither TLR4 activation nor recombinant IL-6 inhibited MPyV replication in MEFs and 3T6 cells. MPyV induced STAT3 phosphorylation through both direct and MT-dependent and indirect and TLR4/IL-6-dependent mechanisms. We demonstrate that uninfected mouse fibroblasts exposed to the cytokine environment from MPyV-infected fibroblasts upregulated the expressions of MCP-1, CCL-5, and α-SMA. Moreover, the cytokine microenvironment increased the invasiveness of MEFs and CT26 carcinoma cells. Collectively, TLR4 recognition of MPyV induces a cytokine environment that promotes the cancer-associated fibroblast (CAF)-like phenotype in noninfected fibroblasts and increases cell invasiveness.

Hepatitis B virus (HBV) is a hepatotropic DNA virus that infects over 250 million people worldwide and causes serious liver diseases. HBV infection can modulate host cellular processes, potentially inducing proteomic changes in hepatocytes. In this study, we investigated how acute HBV infection alters the proteome and secretome of primary human hepatocytes, a physiologically relevant in vitro model that retains essential liver-specific functions. Protein-level changes in cell lysates and culture supernatants were quantified 8 days post-infection using data-independent acquisition MS. We used HBV infection in the presence of the entry inhibitor bulevirtide as a control to separate the effects of productive infection from those caused by inoculum-associated components. Despite robust infection, active HBV replication induced only subtle changes in host protein levels. Orthogonal validation of MS-identified candidates confirmed reticulocalbin-2 as a novel host factor downregulated during productive HBV infection. The functional role of candidate proteins identified by MS was assessed in vitro by siRNA-mediated knockdown and measurement of viral replication markers. Knockdown had no impact on viral RNA or antigen levels, suggesting that the observed proteomic changes may reflect stress responses or broader modulation of the hepatic microenvironment. Our findings support the concept of HBV as a stealth virus and underscore the importance of carefully controlled experimental systems for studying host responses to infection in vitro .