Abstract Hepatitis delta virus (HDV) is a sub-viral agent of hepatitis B virus (HBV). It uses HBV envelope proteins (or surface antigen, HBsAg) to form its virions and infect hepatocytes via HBV receptor. About 257 million people worldwide are chronically infected with HBV, of which ~20 million are also chronically infected with HDV. HDV infection causes additional liver damage, and may accelerate liver disease, cause more rapid and frequent cirrhosis, and increase risk of hepatocellular carcinoma. Anti-HBV drugs do not block HDV infection, and no drugs exist in clinic that directly target HDV. Mechanism of HDV infection is not fully understood, which affects the treatment options. HDV-HBV and HDV-host interactions are complex and under-studied. The regulation of HDV life cycle by the immune response to HBV infection is practically unexplored, and it is the subject for this study. Thus, HBV-host interactions lead to emergence of the immune escape mutations (IEMs) in the major antigenic loop (MAL) of HBsAg. IEMs can make HBsAg poorly recognized by natural anti-HBsAg antibodies and undetectable by the immunoassays, cause HBV vaccine failure, facilitate re-infection of grafted liver with HBV, and promote maintenance of chronic HBV infection. IEMs can profoundly affect secretion of HBsAg and HBV virions. However, the understanding how IEMs affect HDV assembly and infectivity, and recognition of HDV by anti-HBsAg antibodies (i.e., HDV antigenicity) is very limited, and the vast majority of IEMs were not analyzed in this respect. We found that HBsAg regulates HDV infectivity by facilitating two distinct functions that could be functionally separated. It determines the number of the infected hepatocytes by regulating the attachment/entry. It also determines the number of HDV genomes that post-entry reach the nucleus and start the replication by regulating the trafficking and uncoating (loss of the envelope in infected cell). Our work suggested that the virus infectivity, spread and super-infection are likely critical factors for the establishment and maintenance of chronic HDV infection. In addition, the rate of HDV assembly can greatly affect HDV spread/super-infection. The amino acids in MAL can affect HBsAg yield/secretion, and thus can affect HDV assembly. MAL is expected to regulate the (i) attachment/entry and (ii) possibly trafficking/uncoating, and thus to regulate HDV infectivity. MAL also regulates the recognition of the virions by anti-HBsAg antibodies. Thus, our central hypothesis is that IEMs could considerably affect the assembly, infectivity and antigenicity of HDV. Aim 1 using HBsAg of HBV genotypes A-D (for which most IEMs were reported) will find how HBsAg bearing IEMs can regulate the yield of secreted HDV virions and affect the recognition of HDV virions by hepatitis B immune globulin (HBIg). Aim 2 using the above mentioned HBsAg bearing natural IEMs will find how IEMs can regulate the infectivity of HDV virions. The proposed study w...