Epitope mapping and key amino acid identification of anti-CD22 immunotoxin CAT-8015 using hybrid β-lactamase display

Monoclonal antibodies are a commercially successful class of drug molecules and there are now a growing number of antibodies coupled to toxic payloads, which demonstrate clinical efficacy. Determining the precise epitope of therapeutic antibodies is beneficial in understanding the structure-activity relationship of the drug, but in many cases is not done due to the structural complexity of, in particular, conformational protein epitopes. Using the immunotoxin CAT-8015 as a test case, this study demonstrates that a new methodology, hybrid β-lactamase display, can be employed to elucidate a complex epitope on CD22. Following insertion of random CD22 gene fragments into a permissive site within β-lactamase, proteins expressed in Escherichia coli were first screened for correct folding by resistance to ampicillin and then selected by phage display for affinity to CAT-8015. The optimal protein region recognised by CAT-8015 could then be used as a tool for fine epitope mapping, using alanine-scanning analysis, demonstrating that this technology is well suited to the rapid characterisation of antibody epitopes.