Differential Neutralizing Activities of a Single Domain Camelid Antibody (VHH) Specific for Ricin Toxin’s Binding Subunit (RTB)

摘要: Ricin, a member of the A-B family ribosome-inactivating proteins, is classified as Select Toxin by Centers for Disease Control and Prevention because its potential use biothreat agent. In an effort to engineer therapeutics ricin, we recently produced collection alpaca-derived, heavy-chain only antibody VH domains (VHH or "nanobody") specific ricin's enzymatic (RTA) binding (RTB) subunits. We reported that one particular RTB-specific VHH, RTB-B7, when covalently linked via peptide spacer different RTA-specific VHHs, resulted in heterodimers like VHH D10/B7 were capable passively protecting mice against lethal dose challenge with ricin. However, RTB-B7 itself, mixed ricin at 1 ∶ 10 toxin:antibody ratio did not afford any protection vivo, even though it had demonstrable toxin-neutralizing activity vitro. To better define attributes antibodies associated neutralization vitro undertook more thorough characterization RTB-B7. report 100-fold molar excess (toxin:antibody) was unable alter toxicity mouse model. On other hand, two well-established cytotoxicity assays, neutralized 50% inhibitory concentration (IC50) equivalent 24B11, well-characterized potent murine monoclonal antibody. fact, 24B11 virtually identical compared across series including adherence after toxin pre-bound cell surface receptors. differed from both relatively less effective blocking attachment receptors on host cells able form high molecular weight complexes solution. Whether either these activities important neutralizing vivo remains be determined.