Study on Herpesviral Suppressor of Interferon Production

Seungmin Hwang, UC Los Angeles

In acquired immunodeficiency by HIV, HIV itself does not directly cause fatal diseases; rather it provides the immunological environment in which other pathogens can escape immune control and cause diseases. Because of their prevalence and association with a wide variety of diseases, herpesviruses are especially notorious pathogens for immune-compromised patients. Herpesvirus can persist in the host through reactivation to the productive lytic cycle thus replenishing the latent pool. Latency allows herpesvirus to avoid elimination by the host immune system. Therefore, elucidating the mechanisms of herpesviral persistence through latency is essential to control herpesviral diseases.

In immune-compromised patients, the two human gamma-herpesviruses Epstein-Barr virus and Kaposi's sarcoma-associated herpesvirus cause a variety of tumors, such as Burkitt's lymphoma and Kaposi's sarcoma, the most frequently found tumors in AIDS patients. The studies of human gamma- herpesvirus infection have been limited mostly to in vitro experiments because of their restricted host range. Based on its genomic and biological similarity to human gamma-herpesviruses, murine herpesvirus 68 (MHV-68) is used as a mouse model to study the host-interaction of human gamma-herpesviruses and to develop therapeutic strategies against herpesviral diseases. Therefore, we used MHV-68 to investigate the function of viral genes in the establishment and maintenance of persistent infection. For the systematic analysis of viral genes, we generated a mutant library across the entire MHV-68 genome and identified viral genes required for in vitro and in vivo replication. To understand the immune evasion mechanism of MHV-68, we screened the library for viral gene(s) which antagonize type I interferons (IFN-I), the first line of host immune defense. By comparing the replication of MHV-68 mutants in normal and IFN-I receptor knock-out (IFNAR-/-) mice, a viral gene was identified and named SIP (suppressor of interferon production). The inhibitory mechanism of SIP will be investigated in this study.

The long-term goal of this research is to understand the mechanism of herpesviral persistence and to apply this knowledge toward the therapy of herpesviral diseases. The objective of this proposal is to elucidate the mechanism of IFN-I inhibition by SIP and investigate the role of SIP in the replication of MHV-68. The hypothesis is that MHV-68 encodes a protein that suppresses the production of interferon beta (IFNb) by inhibiting the transcriptional activator interferon regulatory factor 3 (IRF-3) and this inhibition is important for the replication of MHV-68 both in vitro and in vivo. To achieve this goal, three aims will be pursued. 1) To investigate the inhibitory mechanism of SIP on IRF-3 in the context of the well-defined activation steps of IRF-3. 2) To investigate the role of SIP in the in vitro replication of MHV-68 without functional IRF-3 or IFNb. 3) To monitor the in vivo replication of MHV-68/SIPnull using bioluminescent imaging in de novo infection and reactivation from latency.

The mutant library of MHV-68 generated during this study and the identification of viral genes responsible for different immune evasion strategies of herpesvirus will provide new ground for the basic mechanistic research and more opportunities to develop novel therapeutic approaches against persistent herpesviral infection and associated diseases.