Enhancement of Reactivation of Murine Gammaherpesvirus 68 from Latency through NF-kB inhibition Karen Bulaklak, ‘12 Principle Investigator: Dr. Laurie Krug, Ph.D. Department of Molecular Genetics and Microbiology Stony Brook University, Stony Brook, NY 11794
Introduction Herpesviruses are double-stranded DNA viruses that establish life-long infections in their hosts. The gammaherpesvirus subfamily of herpesviruses is lymphotropic and is a leading cause of infection-associated cancers in AIDS patients. The human gammaherpesvirus, Epstein-Barr Virus (EBV ), has been linked to the development of multiple lymphomas such as Burkitt’s lymphoma, and Kaposi’s sarcoma-associated herpesvirus (KSHV ) is the causative agent of the neoplasm Kaposi’s sarcoma. Like other herpesviruses, the gammaherpesvirus lifecycle is characterized by intervals of latency and lytic replication. During latency, the viral genome persists within the host cell, but expression of viral genes is limited and no virus particles are made. In this quiescent state, the virus is undetectable by the host and can maintain a chronic infection, thus occluding an effective therapeutic approach. In a process called reactivation, the latent virus can switch to a program of lytic replication under the right circumstances, initiating a cascade of viral gene expression that ends in infectious virus production. The signaling events that trigger this latent to lytic switch are an active area of investigation . Gammaherpesviruses have developed a number of different strategies to persist within their natural hosts. Latency is essential in these techniques and allows the virus to evade the host innate and adaptive immune response. Furthermore, specific viral gene products modulate host signals conducive to latency. Recent studies indicate that the nuclear factor kappa B (NF-kB) signaling pathway is critical for gammaherpesvirus latency in vivo [6,7,8]. The NF-kB transcription factors
are important players in cell survival and inflammation. These proteins include NF-kB1 (p105 and p50), NF-kB2 (p100 and p52), c-Rel, RelB, and RelA (p65), which dimerize within the cytoplasm and are sequestered by inhibitory molecules, called IkBs. In two distinct pathways, different NF-kB-activating signals initiate the phosphorylation and subsequent degradation of the IkBs, either through an IKKk-dependent (canonical pathway) or IKKk-dependent (alternative pathway) manner. The released NF-kB dimers then regulate the expression of multiple genes such as in the inflammatory response. EBV and KSHV lymphomas have constitutively active NF-kB, and studies in vitro show that inhibition of this pathway leads to cell death. The virus may target this pathway to facilitate the survival, proliferation and differentiation of B cells, promoting pathogenesis and tumor formation . Inhibitors of NFkB have also been shown to delay tumor progression of EBV+ and KSHV+ lymphomas in vivo, which suggests that the
pathway may be a good target for treating gammaherpesvirus-related lymphomas . NF-kB inhibitors also lead to reactivation in EBV- and KSHV-positive B cell lymphomas [2,5]. We hypothesize that this pathway promotes latency of the gammaherpesviruses by repressing lytic gene expression. Since we cannot manipulate the virus-host interactions of EBV and KSHV latent B cell infections in vivo, we use murine gammaherpesvirus 68 (MHV68), a naturally occurring mouse pathogen closely related to human gammaherpesviruses. Introduction of MHV68 in mice leads to a robust latent infection in the spleen, which recapitulates many aspects of EBV and KSHV infection. The loss of NF-kB protein p50 also enhances virus replication in MHV68-positive mice, which further suggests that NF-kB plays a role in establishing latency . Here we examined the role of NF-kB signaling in various cell lines infected with MHV68, including the A20-HE mature murine B cell line,
Figure 1: The role of NF-κB inhibitors in reactivation. The NF-κB pathway is important for latency and inhibits lytic replication in vivo (top). Reactivation may result from inhibition of the pathway through treatment with Bay11 or MG132, or upregulation of lytic gene expression through TPA treatment (bottom).
The Stony Brook Young Investigators Review, Fall 2011