An important component of lytic infection by Kaposi's sarcoma-associated herpesvirus (KSHV) is the ability of the virus to evade the innate immune response, specifically type I interferon (IFN) responses that are triggered by recognition of viral nucleic acids. Inhibition of type I IFN responses by the virus promotes viral replication. Here, we report that KSHV uses a caspase-dependent mechanism to block type I IFN, in particular IFN-β, responses during lytic infection. Inhibition of caspases during KSHV reactivation resulted in increased TBK1/IKKε-dependent phosphorylation of IRF3 as well as elevated levels of IFN-β transcription and secretion. The increased secretion of IFN-β upon caspase inhibition reduced viral gene expression, viral DNA replication, and virus production. Blocking IFN-β production or signaling restored viral replication. Overall, our results show that caspase-mediated regulation of pathogen sensing machinery is an important mechanism exploited by KSHV to evade innate immune responses. KSHV is the causative agent of Kaposi's sarcoma (KS), an AIDS-defining tumor that is one of the most common causes of cancer death in sub-Saharan Africa. In this study, we examined the role of a set of cellular proteases, called caspases, in the regulation of immune responses during KSHV infection. We demonstrate that caspases prevent the induction and secretion of the antiviral factor IFN-β during replicative KSHV infection. The reduced IFN-β production allows for high viral gene expression and viral replication. Therefore, caspases are important for maintaining KSHV replication. Overall, our results suggest that KSHV utilizes caspases to evade innate immune responses, and that inhibiting caspases could boost the innate immune response to this pathogen and potentially be a new antiviral strategy.

Nanoparticles have garnered widespread interest for both the imaging and treatment of cancer due to their unique and tunable pharmacokinetics and their ability to carry a high payload of diverse compounds. However, despite these favorable attributes, the extent of tumor accumulation can be severely restricted due to the dense stroma surrounding the often-permeable blood vessel wall and high intratumoral pressure. In this study, we investigated whether modifying the surface of pegylated gold nanoparticles (AuNPs) with collagenase could improve the accumulation of nanoparticles within a murine tumor xenograft. It was determined that collagenase remains active after surface conjugation and the presence of collagenase has no measureable effect on cell proliferation in vitro. Following intravenous injection, the largest fractions of collagenase-labeled AuNPs were found in the liver and spleen. Histological analysis revealed no signs of toxicity in either of these organs. Blood chemistry revealed normal levels of liver enzymes, but a slightly elevated level of total bilirubin. Within the tumor, AuNPs labeled with collagenase exhibited a 35% increase in accumulation compared with unlabeled AuNPs. Therefore, these studies provide preliminary evidence that the functionalization of nanoparticles with collagenase represent an effective and safe approach to improve tumor accumulation.

Due to their roles in the regulation of programmed cell death and inflammation, the cellular caspase proteases are considered antiviral factors. However, recent studies have revealed examples of proviral functions for caspases. Here, we review a growing body of literature on the role of caspases in promoting the replication of human gammaherpesviruses. We propose that gammaherpesviruses have evolved ways to redirect these enzymes and to use their activation to support viral replication and immune evasion.

Type I interferons are key factors that dictate the outcome of infectious and inflammatory diseases. Thus, intricate cellular regulatory mechanisms are in place to control IFN responses.

During the ongoing virus-host arms race, viruses have evolved numerous immune subversion strategies aimed at the production of type I interferons (IFNs). This focus on IFN evasion highlights the essentiality of these cytokines in controlling viral infections. Apoptotic caspases have recently emerged as important regulators of type I IFN signaling in both non-infectious contexts and during viral infection. Surprisingly, they promote viral immune evasion despite being widely considered antiviral from triggering cell death. Indeed, we previously discovered that the AIDS-associated oncogenic gammaherpesvirus Kaposi's sarcoma-associated herpesvirus (KSHV) exploits caspase-8 activity to suppress the antiviral type I IFN response and promote viral replication. However, the mechanisms of this novel viral immune evasion strategy are poorly understood, particularly how caspase-8 antagonizes IFN signaling. Here we show that caspase activity inhibits the DNA sensor cGAS during KSHV lytic replication to block IFN induction. Furthermore, we use single-cell RNA-sequencing to reveal that the potent antiviral state conferred upon caspase inhibition is mediated by an exceptionally small percentage of infected cells expressing IFN-β, thus uncovering further complexity of IFN regulation during viral infection. Collectively, these results provide insight into multiple levels of type I IFN regulation by the cell and the way in which viruses co-opt them for immune evasion. Unraveling these mechanisms can inform targeted therapeutic strategies for viral infections and reveal cellular mechanisms of regulating interferon signaling in the context of cancer and chronic inflammatory diseases.