Several unique features of anthrax toxin are being exploited to make novel, cell-specific cytotoxins for killing HIV-1 infected cells. The strategies for killing cells utilize highly-toxic fusion proteins in which the amino-terminal portion of anthrax toxin lethal factor (LF) is genetically fused to the ADP- ribosylation domain of Pseudomonas exotoxin A (PE). Delivery of these LF-PE fusion proteins to the cytosol of cells requires the prior binding and proteolytic activation of the protective antigen (PA) component of the toxin. Three separate approaches to targeting cells are being used: 1. The site in PA which must be proteolytically cleaved was replaced by consensus sequences recognized by HIV-1 protease, so that PA will be activated only in HIV-1-infected cells. Four of five mutant PA proteins were cleaved by HIV-1 protease. It is anticipated that these PA mutants will sensitize infected cells to exogenously-added LF-PE fusions. 2. The receptor-binding portion of PA is to be replaced by CD4, IL-2, or single-chain antibodies directed to HIV gp120 or gp41, so that the high efficiency anthrax toxin translocation mechanism will internalize the LF-PE fusions into infected lymphocytes. A model system is being used in which a peptide, EQKLISEEDLN, is fused on the carboxyl-terminus of PA, replacing the normal receptor-binding region of PA. A hybridoma cell line having surface-displayed antibodies to this peptide is expected to bind and internalize the PA-peptide fusion. 3. The gene encoding PA will be transfected into cells under control of transcriptional regulators encoded by HIV-1, so that only infected cells will produce PA and become sensitized to exogenous addition of the LF-PE fusions. To test whether PA produced within cells will be able to internalize LF-PE fusions, well-characterized vectors able to express PA are being transfected into appropriate cultured cell lines.
