Identification of a functional domain of human granulocyte colony-stimulating factor using neutralizing monoclonal antibodies

Human granulocyte colony-stimulating factor (G-CSF) is a hemopoietic growth factor that is being used successfully to treat various forms of neutropenia. To define functionally important regions of G-CSF, we have prepared 37 monoclonal anti-G-CSF antibodies and mapped the regions of G-CSF recognized by different antibody groups. Antibodies recognizing similar epitopes were identified by competition assays, neutralization assays, conformation dependence and cross-reactivity with canine G-CSF. Seven of eight neutralizing antibodies fell into two related epitope groups and were conformation-dependent. The eighth was unrelated and conformation-independent. Peptides of G-CSF were generated by chemical or enzymatic digestion and tested for antibody reactivity. One of the neutralizing antibodies (LMM351) recognized a small, disulfide-bonded peptide from the V8 protease digest (residues 34-46). A synthetic peptide (residues 20-58) was recognized by all the neutralizing antibodies, implicating this disulfide-bonded loop in receptor binding. The epitopes recognized by nonneutralizing antibodies were found throughout G-CSF. Thus, regions of G-CSF that are not involved in receptor binding have also been defined. A CNBr peptide (residues 1-121) had greatly reduced biological activity, indicating that the COOH terminus is required for receptor binding. We predict that residues 20-46 and the COOH terminus bind to the G-CSF receptor.