The present invention provides methods for inhibiting interleukin-3 receptor-expressing cells, and, in particular, inhibiting the growth of such cells byusing a diphtheria toxin-human interleukin-3 conjugate (DT-IL3) that is toxicto cells expressing the interleukin-3 receptor. In preferred embodiments,the DT-IL3 conjugate is a fusion protein comprising amino acids 1-388 of diphtheriatoxin fused via a peptide linker to full-length, human interleukin-3. In certainembodiments, the methods of the present invention relate to the administrationof a DT-IL3 conjugate to inhibit the growth of cancer cells and/or cancer stemcells in humans, which cells express one or more subunits of the interleukin-3receptor. Exemplary cells include myeloid leukemia cancer stem cells. In otherembodiments, the methods of the present invention relate to ex vivo purging ofbone marrow or peripheral blood to remove cells that express one or more subunitsof the interleukin-3 receptor such that the purged bone marrow or peripheralblood is suitable, e.g., for autologous stem cell transplantation to restorehematopoietic function.
