We have generated six high affinity monoclonal antibodies (MoAbs) to the human erythropoietin receptor (hEPO-R) polypeptide. All six MoAbs bind to the extracytoplasmic domain of the hEPO-R, and all immunoprecipitate 35S-labeled hEPO-R from metabolically labeled Ba/F3- hEPO-R cells. Four of the MoAbs neutralize the EPO-dependent growth of Ba/F3-hEPO-R cells, whereas two MoAbs are non-neutralizing. None of the MoAbs inhibit the EPO-dependent growth of Ba/F3 cells expressing the murine EPO-R (mEPO-R), even though the hEPO-R and mEPO-R share 82% amino acid identity. All six of the anti-EPO-R MoAbs bind to the cell surface human EPO-R but none bind to the cell surface murine EPO-R. Of the four neutralizing MoAbs, the one-half maximal inhibition occurs at MoAb concentrations ranging from 1 nmol/L to 50 nmol/L. These MoAbs also compete with radiolabeled EPO for hEPO-R binding. The two non- neutralizing MoAbs fail to inhibit EPO-dependent growth or compete with EPO-binding, even at antibody concentrations as high as 500 nmol/L. The four neutralizing MoAbs, designated group I, compete with each other for an epitope of the hEPO-R polypeptide required for EPO-binding. The two non-neutralizing MoAbs recognize discrete epitopes, and are designated group II and group III MoAbs. In conclusion, this is the first description of MoAbs specific for the hEPO-R. The MoAbs, which recognize three discrete epitopes, may be useful in characterizing the spectrum of cells that display the hEPO-R and in further defining the role of EPO in hematopoiesis.

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