Hepatitis delta virus (HDV) set the precedent for an animal virus harboring a circular RNA genome that displays similarities with the plant viroids and virusoids. As such, HDV represents a new class of viral agent in humans with pathogenic properties. In addition, HDV is a defective virus that cannot replicate autonomously, but requires the helper functions of coinfecting hepatitis B virus (HBV) to initiate an infection. The viral particle is composed of an envelope of HBV surface antigen which encloses the RNA genome and an HDV encoded polypeptide bearing the delta antigen. Upon entry of HDV virions into susceptible cells, HDV RNA replicates autonomously, but the presence of HBV envelope proteins is required for the formation of viral particles that will ensure propagation. Therefore, HDV can be considered a defective agent that needs the HBV envelope, and an interfering agent that suppresses the replication of the helper HBV. This proposal will focus on the interaction between HBV and HDV, including the identification of the HBV and HDV functions required for the formation of infectious HDV particles, and the interference phenomenon of HDV on the replication of HBV. An in vitro tissue culture system will be developed to facilitate the achievement of this investigation. The specific aims of this proposal are: 1) to determine the experimental conditions for the production of HDV particles In vitro; 2) to establish the conditions for infectivity assays in vitro; 3) to identify the domains of the HBV surface proteins required for the morphogenesis and infectivity of HDV particles in vitro; 4) to identify the domains of the delta antigen required for the interaction with HDV RNA and HBV surface protein for formation of particles; 5) to study the requirement for the expression of other HDV ORF products for the production of HDV particles; and 6) to study the effect of HDV replication and particle formation on the replication of HBV. Answers to these questions should provide valuable insight in the designing of antiviral strategies. Identification of the domain(s) on the HBV surface protein that bind to the HDV receptor on the hepatocyte should be facilitated as well. The identification of such a receptor may benefit also from the achievement of this project and will be considered as a future extension of this project.