HBcAg (residues1-183) has been expressed in E.coli where it assembles in the bacterial cytoplasm into icosahedral capsids. Deletion of the polybasic C-terminal 34 residues (protamine domain) produces assembly competent protein (residues 1- 149) which is suitable for structural analysis. The structure of the capsids has been previously determined by cryo-electron microscopy and by X-ray crystallography. The closely related HBeAg is a soluble secreted protein which is thought to modulate both the innate and adaptive immune responses so as to favor persistent or chronic infection. HBeAg is also an important clinical marker of HBV infection. HBeAg (159 residues) is truncated at position 149 and in addition contains a 10 residue N-terminal extension derived from partial processing of precursor protein. An HBeAg-specific Fab was found to form a stable complex with the recombinant produced HBeAg protein. The immune complex was crystallized and the structure of HBeAg was determined for the first time and published Jan 2013. This structure precludes capsid assembly and forms a distinct antigenic repertoire, explaining why HBcAg and HBeAg are cross-reactive at the T cell level (through sequence identity) but not at the B cell level (through conformation). HBeAg is thought to regulate immune responses by direct interaction with proteins, for example Mal, which regulate the innate immune system. Using the structure of HBeAg we have been modeling its interaction (docking) with target proteins as a guide for new structural studies. The direct targeting of HBeAg is also a potential for the treatment of chronic HBV, and to assist in these studies we have used phage display technology to generate a panel of high-affinity humanized antibodies against HBeAg. Biophysical and structural studies are being carried out to screen these antibodies, with the aim of generating small molecules which bind and inhibit HBeAg function. For clinical use, we also are using the characterized antibodies to develop a highly specific assay for the HBeAg. Previous structural determinations of nucleocapsid-antibody immune complexes by cryo-electron microscopy were performed with a panel of murine antibodies. This work was extended to include human antibodies from clinical samples and a specific antibody (E1) binding to HBcAg capsids which may contribute to acute liver failure (ALF). The monoclonal antibody E1 Fab was generated from previous work (R.H. Purcell, NIAID) based on immunological and molecular biological studies of tissue from patients with ALF. The E1 Fab binds with high affinity to HBcAg core particles. The massive deposit of immune complexes leads to the liver damage associated with ALF. We are currently attempting to co-crystallize the E1 with soluble capsids dimers to determine the structure of the immune complex. This information may be useful for designing drugs which block the E1-capsid interaction, hence preventing or modulating ALF.

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National Institute of Arthritis and Musculoskeletal and Skin Diseases
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