Vaccinia virus (VACV) envelope protein D8 is one of three glycosaminoglycan adhesion molecules and binds to the linear polysaccharide chondroitin sulfate (CS). D8 is also a target for neutralizing antibody responses that are elicited by the smallpox vaccine, which has enabled the first eradication of a human viral pathogen and is a useful model for studying antibody responses. However, to date, VACV epitopes targeted by human antibodies have not been characterized at atomic resolution. Here, we characterized the binding properties of several human anti-D8 antibodies and determined the crystal structures of three VACV-MAb variants, VACV-66, VACV-138, and VACV-304, separately bound to D8. While all these antibodies bound D8 with high affinity and were moderately neutralizing in the presence of complement, VACV-138 and VACV-304 also fully blocked D8 binding to CS-A, the low affinity ligand for D8. VACV-138 also abrogated D8 binding to the high-affinity ligand CS-E, but we observed residual CS-E binding was observed in the presence of VACV-304. Analysis of the VACV-138 and VACV-304 binding sites along the CS binding crevice of D8, combined with different efficiencies of blocking D8 adhesion to CS-A and CS-E allowed us to propose that D8 has a high and low affinity CS binding region within its central crevice. The crevice is amenable to protein engineering to further enhance both specificity and affinity of binding to CS-E. Finally, a wild-type D8 tetramer specifically bound to structures within the developing glomeruli of the kidney, which express CS-E. We propose that through structure-based protein engineering, an improved D8 tetramer
Human antibody responses to vaccinia virus D82 could be used as a potential diagnostic tool to detect expression of CS-E, which is a possible biomarker for ovarian cancer.Smallpox is caused by infection with variola virus and was a major health threat until successful global vaccination efforts led to its complete eradication from the general human population (1). This eradication of an infectious agent was achieved by immunizing with Vaccinia virus (VACV), a related orthopoxvirus with low virulence and the active ingredient in the smallpox vaccine (2,3). Immunization with VACV leads to the robust production of highly neutralizing antibodies targeting several VACV envelope proteins, including A27, A33, B5, D8, H3, L1, and others, which are expressed on different viral envelopes (3,4). A27, D8, H3 and L1 are expressed on the outer membrane of the intracellular mature virion (IMV), while A33 and B5 are found in the more fragile extracellular enveloped virion (EEV), which has an additional host cell derived envelope. Both the IMV and the EEV particles are infectious virions, with IMV being more abundant and mainly responsible for spread between hosts, and EEV being involved in cell-to-cell spread within the host after infection with the IMV (5). As a result, a potent antibody-mediated immune response against VACV targets both infectious virions.Among the targeted envelope prote...