Elimination of murine erythroleukemic stem cells with a novel anti‐erythroid antibody conjugated to ricin a‐chain: A model for studies of bone‐marrow transplantation therapy

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Abstract
We have produced a rat monoclonal antibody (MAb) MAE 15 (IgG), specific for murine erythroid cells, using a murine erythroid cell line as immunogen. This MAb specifically binds to the surface of normal and neoplastic murine erythroid cells. Murine mature erythrocytes and non‐erythroid cells as well as rat and human erythroid and non‐erythroid cells are not recognized by MAb MAE 15. Immunoblotting analysis and mixed precipitation in agar gel showed MAb MAE 15 to be specific for murine epitope of 69 kDa antigen of erythroblasts (Ag‐Eb), an interspecies antigenic marker of nucleated red cells and reticulocytes. A conjugate (immunotoxin) was prepared, comprising ricin A‐chain and MAb MAE15. The immunotoxin inhibited protein synthesis of murine erythroleukemic Ag‐Eb‐positive K‐2 cells and completely inhibited (at the concentration of 2 × 10−7 m) spleen colony formation by erythroleukemic stem cells of the Ag‐Eb‐positive RAL cell line. Approximately 35% of the murine normal stem‐cell (CFU‐S) population was not affected by the immunotoxin at the concentration of 2 × 10−7 m. This experimental system may be a convenient model for studies of bone marrow transplantation therapy of erythroleukemias.