Abstract

Targeted intracellular delivery of therapeutic peptides and proteins remains an important but unresolved goal in biotechnology. A promising approach is to engineer bacterial exotoxins that deliver their cytotoxic enzymes into cells and can be engineered to target cancer cells as is the case with immunotoxins. The well-studied diphtheria toxin translocation domain is ideally suited as a delivery platform as it has been shown to be capable of delivering a wide range of macromolecular cargo. Widespread deployment of DT-based therapeutics in humans, however, is complicated by the prevalence of pre-existing anti-DT antibodies from childhood vaccinations that reduce the exposure, efficacy and safety of this important class of protein drugs. Thus, there is a great need for delivery platforms with no pre-existing immunity in humans. Here, we describe the discovery and characterization of a distant diphtheria toxin homolog from the ancient reptile pathogen Austwickia chelonae that we have named Chelona Toxin (CT). We show that CT is comparable to DT structure and function in all respects except that it is not recognized by pre-existing anti-DT antibodies present in human sera. Moreover, we demonstrate that the CT translocase is superior to the DT translocase at delivering therapeutic protein cargo into target cells. These findings highlight CT as a potentially class-enabling new chassis for developing safer and more efficacious immunotoxins and intracellular protein delivery platforms for cancer therapy.