Internalization of a Bacillus anthracis protective antigen c-Myc fusion protein mediated by cell surface anti-c-Myc antibodies

Background: Anthrax toxin, secreted by Bacillus anthracis, consists of protective antigen (PA) and either lethal factor (LF) or edema factor (EF). PA, the receptor-binding component of the toxin, translocates LF or EF into the cytosol, where the latter proteins exert their toxic effects. We hypothesized that anthrax toxin fusion proteins could be used to kill virus-infected cells and tumor cells, if PA could be redirected to unique receptors found only on these cells. Materials And Methods: To test this hypothesis in a model system, amino acids 410-419 of the human p62(c-myc) epitope were fused to the C-terminus of PA to redirect PA to the c-Myc-specific hybridoma cell line 9E10. Results: The PA-c-Myc fusion protein killed both mouse macrophages and 9E10 hybridoma cells when administered with LF or an LF fusion protein (FP59), respectively. Similar results were obtained with PA, which suggests that PA-c-Myc used the endogenous PA receptor to enter the cells. By blocking the endogenous PA receptors on 9E10 cells with the competitive inhibitor PA SNKE Delta FF, the PA-c-Myc was directed to an alternate receptor, i.e., the anti-c-Myc antibodies presented on the cell surface. The c-Myc IgG were proven to act as receptors because the addition of a synthetic peptide containing the c-Myc epitope along with PA SNFK Delta FF further reduced the toxicity of PA-c-Myc + FP59. Conclusion: This study shows that PA can be redirected to alternate receptors by adding novel epitopes to the C-terminus of PA, enabling the creation of cell-directed toxins for therapeutic purposes.