Generation and characterization of a neutralizing monoclonal antibody against erythroid cell stimulating factor

Erythroid cell stimulating factor (ESF) is present in mouse serum and has been reported to function in concert with erythropoietin (EPO) in the formation of erythroid cells in in vitro culture systems. We report here the generation and characterization of a monoclonal antibody (MAb) directed against ESF, with potent anti-ESF-neutralizing activity. A hybridoma-producing MAb to ESF was selected following enzyme-linked immunosorbent assay (ELISA)-based screening of 270 colonies obtained from a fusion of immunized mouse splenocytes with NS1 myeloma cells. Western blot analyses of mouse serum using this antibody specifically detected a single protein (approximate molecular weight of 60 kDa and 120 kDa, under reducing and nonreducing conditions, respectively) corresponding to ESF, with no reactivity to EPO. Furthermore, this MAb demonstrated reactivity to a protein similar in molecular mass, across species, showing reactivity in sera obtained from human, horse, goat, guinea pig, rabbit, and rat. Immuno-chemical characterization demonstrated this antibody to be of IgG3 isotype, bearing kappa light chains. Injection of this monoclonal anti-ESF antibody to exhypoxic polycythemic mice at 6 and 24 h after EPO injection significantly reduced 59Fe incorporation into red blood cells, demonstrating its ability to neutralize in vivo erythropoiesis in our mouse model system. Thus, this novel erythroid cell-specific MAb will be an invaluable tool for further delineating the physiological role of ESF in in vivo erythropoiesis.