Non-canonical Binding of an Antibody Resembling a Naïve B Cell Receptor Immunoglobulin to Hepatitis B Virus Capsids

The hepatitis B virus capsid (core antigen) is able to bind to and activate naïve B cells and these become efficient primary antigen-presenting cells for the priming of T cells. We have investigated this interaction by using cryo-electron microscopy, three-dimensional image reconstruction, and molecular modeling to visualize capsids decorated with Fab fragments of a receptor immunoglobulin, and surface plasmon resonance to measure the binding affinity. By both criteria, the mode of binding differs from those of the six monoclonal anti-core antigen antibodies previously characterized. The Fab interacts with two sites ∼30 Å apart. One interaction is canonical, whereby the CDR loops engage the tip of one of the 25 Å spikes that protrude from the capsid surface. The second interaction is non-canonical; in it, the Fab framework contacts the tip of an adjacent spike. The binding affinity of this Fab for capsids, KD ∼4×10−7 M, is relatively low for an antibody-antigen interaction, but is ∼150-fold lower still (∼2.5×10−5 M) for unassembled capsid protein dimers. The latter observation indicates that both of the observed interactions are required to achieve stable binding of capsids by this receptor immunoglobulin. Considerations of conserved sequence motifs in other such molecules suggest that other naïve B cells may interact with HBV capsids in much the same way.