A Murine Cytokine Fusion Toxin Specifically Targeting the Murine Granulocyte-Macrophage Colony-Stimulating Factor (GM-CSF) Receptor on Normal Committed Bone Marrow Progenitor Cells and GM-CSF-Dependent Tumor Cells

A fusion protein was synthesized consisting of the murine granulocyte-macrophage colony-stimulating factor (mGM-CSF) gene spliced to a truncated form of the diphtheria toxin (DT₃₉₀) gene coding for a molecule that retained full enzymatic activity, but excluded the native binding domain. The DT₃₉₀-mGM-CSF hybrid gene was cloned into a vector under the control of an inducible promoter and the protein expressed in Escherichia coli. After induction, a protein was purified from inclusion bodies in accord with the deduced molecular weight of DT₃₉₀ mGM-CSF. Cell-free studies of the adenosine diphosphate-ribosylating activity of DT₃₉₀ mGM-CSF showed results that were similar to those of native DT. The DT₃₉₀ mGM-CSF immunotoxin inhibited FDCP2.1d, a murine myelomonocytic tumor line expressing the GM-CSF receptor with an IC₅₀ (concentration inhibiting 50% activity) of 5 × 10^-1 mol/L. The fusion toxin was specifically cytotoxic and directed by the GM-CSF portion of the molecule because addition of a monoclonal antibody directed against GM-CSF inhibited its ability to kill the cell line. Cell lines that do not express GM-CSF receptor were not inhibited by the fusion toxin. DT₃₉₀ mGM-CSF was also able to specifically inhibit normal committed bone marrow (BM) progenitor cells as measured in clonal colony-forming unit granulocyte-macrophage assays. Together, these findings indicate that DT₃₉₀ mGM-CSF may be useful as a novel tool for purging BM of contaminating leukemia cells or in vivo for eliminating residual leukemia cells. Also, it can be used to determine whether committed and/or noncommitted BM progenitor cells express the GM-CSF receptor.