Improving Anti-Ricin Antibodies: Chimerization and Selection of Ricin-Resistant Hybridoma Cell Lines

Antibodies provide the best defense against the effects of biological toxins. We have previously made a panel of 45 different murine monoclonal antibodies (mAbs) to ricin A chain, B chain, and determinants found on both chains. Comparative in vitro and in vivo studies have identified one anti-A chain mAb, designated RAC18, as the most effective at both in vitro neutralization and in vivo protection from ricin toxin. Here we describe experiments to improve the utility of this mAb. We have made a chimeric mouse/human Ab by grafting the murine V regions onto human IgG1/ constant regions, and show that it retained full in vitro and in vivo activity. We also used a novel approach to generate higher affinity Abs, based on our observation that hybridoma cell lines were resistant to ricin in proportion to the affinity of the Ab they produce. We induced AID-dependent V region mutagenesis while selecting cells in increasing concentrations of ricin. We were able to isolate cells that were 100X more resistant to ricin than the parental hybridomas, but the quality of the Ab was no different. Rather, cells had down-regulated the expression of cell surface structures that are bound by ricin. These results demonstrate a unique mechanism whereby cells become resistant to ricin’s lethal effects.

Keywords: Ricin, anti-ricin, antibodies, chimeric, ricin-resistant, hybridoma, protection.