World-wide, some 350 million people are chronically infected with Hepatitis B Virus (HBV), of whom 620,000 die each year from HBV-related liver disease. Like any other pathogen, HBV expresses protein antigens that trigger the body’s immune system to defend itself. A relatively small and simple virus, HBV has three major clinical antigens that elicit an immune response: the surface antigen (which is also used safely and effectively to vaccinate individuals against HBV), the core antigen (HBcAg), and the e-antigen (HBeAg).
The HBV core antigen and the e-antigen are basically two versions of the same protein, but the core antigen is important for virus production, while the e-antigen is not. The e-antigen plays a role in establishing immune tolerance and chronic HBV infection. In addition, the core antigen assembles into the shell (capsid) that houses the genetic material of the virus, while the e-antigen is secreted into the bloodstream in an unassembled form. The relationship between the e-antigen and the core antigen has been a mystery for the past three decades.
In the new study, the NIH scientists developed a unique antibody that binds to and forms a stable complex with e-antigen. This complex was found to form well-diffracting crystals whose analysis allowed the structure of the complex to be determined. They discovered that the e-antigen subunit has essentially the same fold as the core antigen subunit, but that it pairs into dimers (two associated subunits) in an entirely different way, with a relative rotation of 140 degrees between the subunits. The rotation obviates the protein's ability to assemble and transforms its antigenic character. This switch represents a novel mechanism for regulating a protein's structure and function.
Understanding the e-antigen structure provides a framework upon which future studies can build to fully elucidate its role in HBV persistence and possibly a way to prevent the establishment of chronic liver infections. For more information, visit NIAMS