Date Published: April 17, 2014
Publisher: Public Library of Science
Author(s): Melissa A. Edeling, S. Kyle Austin, Bimmi Shrestha, Kimberly A. Dowd, Swati Mukherjee, Christopher A. Nelson, Syd Johnson, Manu N. Mabila, Elizabeth A. Christian, Joseph Rucker, Theodore C. Pierson, Michael S. Diamond, Daved H. Fremont, Félix A. Rey.
We recently described our most potently neutralizing monoclonal antibody, E106, which protected against lethal Dengue virus type 1 (DENV-1) infection in mice. To further understand its functional properties, we determined the crystal structure of E106 Fab in complex with domain III (DIII) of DENV-1 envelope (E) protein to 2.45 Å resolution. Analysis of the complex revealed a small antibody-antigen interface with the epitope on DIII composed of nine residues along the lateral ridge and A-strand regions. Despite strong virus neutralizing activity of E106 IgG at picomolar concentrations, E106 Fab exhibited a ∼20,000-fold decrease in virus neutralization and bound isolated DIII, E, or viral particles with only a micromolar monovalent affinity. In comparison, E106 IgG bound DENV-1 virions with nanomolar avidity. The E106 epitope appears readily accessible on virions, as neutralization was largely temperature-independent. Collectively, our data suggest that E106 neutralizes DENV-1 infection through bivalent engagement of adjacent DIII subunits on a single virion. The isolation of anti-flavivirus antibodies that require bivalent binding to inhibit infection efficiently may be a rare event due to the unique icosahedral arrangement of envelope proteins on the virion surface.
Dengue virus (DENV) infection in humans causes symptoms ranging from a mild febrile illness to a severe and sometimes fatal disease. Over 3.6 billion people globally are at risk for DENV infection, with an estimated 390 million infections annually and no currently approved vaccine or antiviral therapy . DENV belongs to the Flaviviridae family of medically important positive-stranded RNA viruses. Within the DENV serocomplex, there is significant diversity, including four serotypes (DENV-1, -2, -3, and 4) that differ at the amino acid level of the envelope (E) protein by ∼25 to 40 percent and multiple genotypes within a serotype that vary by up to ∼3 percent , .
Epitope mapping studies have enhanced our understanding of the mechanisms of virus neutralization and identified sites on the E protein of flaviviruses that are targeted by neutralizing antibodies . These include the lateral ridge of DIII of WNV and JEV , , the A-strand of DIII of DENV , , the CC′ loop of DIII of DENV-1 , the fusion loop of DII of WNV and DENV , a DI epitope of DENV-4 , and a complex epitope centered at the hinge of DI and DII on WNV  and DENV , . Here, we describe a composite epitope, comprised of regions of the lateral ridge and A-strand of DIII that is targeted by the therapeutic MAb E106. DIII residues contacted by E106 were highly conserved among DENV-1 genotypes but variable in other DENV serotypes. Consistent with this, E106 potently neutralized all five DENV-1 genotypes, but not other DENV serotypes nor WNV .