Monoclonal antibodies are promising for neutralizing ricin
For more than 30 years, the Département Médicaments & Technologies pour la Santé (DMTS) of Joliot institute has been developing immunotherapeutic and vaccine-based approaches through its contribution to the French interministerial CBRN-E R&D program coordinated by the CEA. In the context of bioterrorism preparedness, the Service de Pharmacologie and Immunoanalyses (SPI/DMTS) has designed monoclonal antibodies (mAbs) for the detection and neutralization of several toxic or pathogenic agents (bacteria, viruses, toxins). Among these agents is ricin, a protein easily extracted from castor bean seeds and highly toxic to humans.
For such applications, mAbs represent the most promising therapeutic option. By targeting a single, precisely characterized epitope of the toxin, mAbs can provide more effective and reproducible neutralization than polyclonal antibodies, while limiting response heterogeneity and nonspecific immunological effects. However, the development of effective neutralizing mAbs against ricin is challenging and requires targeting epitopes that ensure neutralization of both natural isoforms of the protein (D and E). Moreover, high binding affinity does not necessarily correlate with toxin neutralization, highlighting the critical importance of epitope specificity for protection.
PROTEIN ENGINEERING TO IDENTIFY THE BEST CANDIDATES
Building on the expertise of Hervé Nozach's team (SIMoS/DMTS), which develops innovative antibody engineering technologies, and that of the Robert Koch Institute, the authors applied two state-of-the-art biophysical methods to characterize a panel of 17 candidate mAbs directed against ricin D and E. Binding affinities were evaluated using surface plasmon resonance (SPR) and biolayer interferometry (BLI) (see Box). Epitope determination was achieved by :
i) clustering antibodies according to their compatibility and competition profiles for toxin binding, enabling simplified visualization of epitope groups ;
ii) deep mutational scanning using yeast surface display, allowing the identification of key epitope residues.
Thanks to these precise measurements, the authors were able to classify the 17 candidates according to their binding epitopes and identify, at the amino-acid level, the ricin residues involved in functional recognition. The consistency of the epitope mapping results revealed six unique, partially overlapping epitope groups.
The two epitope-mapping strategies described in this study yielded highly consistent results and demonstrated that several epitopes are involved in the mechanism of ricin toxicity. By deepening our understanding of ricin neutralization by monoclonal antibodies, these approaches contribute to the rational design of increasingly effective immunotherapies for ricin intoxication.
Joliot contacts : Hervé Nozach (herve.nozach@cea.fr); Stéphanie Simon (stephanie.simon@cea.fr)
- An epitope, also known as an antigenic determinant, is the part of a molecule that can be recognised by an antibody.
- Surface Plasmon Resonance (SPR) is an optical technique used to detect a ligand (organic molecule, biological molecule, microorganism, etc.) binding to a receptor immobilised on a surface. It does not require prior labelling of the target molecules and allows real-time detection, which can be quantitative.
- Bio-Layer Interferometry (BLI) is based on light interference measurement applied to bio-layers formed by protein interactions. It involves measuring a spectral shift that allows the stages of association and dissociation of bimolecular interactions, particularly antigen/monoclonal antibody interactions, to be monitored using ready-to-use biosensors.
- Yeast Surface Display is a protein engineering technique that uses the expression of recombinant proteins integrated into the cell wall of yeast.
See also :
Promising anti-ricin antibodies for effective passive immunotherapy