A role for the carboxyl terminus of human granulocyte-macrophage colony-stimulating factor in the binding of ligand to the alpha-subunit of the high affinity receptor.

A synthetic segment (110-127) of the carboxyl terminus of recombinant human granulocyte-macrophage colony-stimulating factor (rh-GM-CSF) was used to generate a rabbit polyclonal antibody (345-6), which recognized both peptide and full-length Escherichia coli-derived rh-GM-CSF in a direct enzyme-linked immunosorbent assay. Antibody 345-6 was shown to antagonize the binding of 125I-labeled rh-GM-CSF to its receptor on the KG-1 cell line and to inhibit human GM-CSF-dependent proliferation of the AML-193 cell line. The purified IgG fraction of neutralizing antibody 345-6 was used as immunogen to obtain sheep anti-serum 1418. Antibody 1418 recognized antibody 345-6 on direct enzyme-linked immunosorbent assay but did not recognize rh-GM-CSF or the peptide 110-127 to which antibody 345-6 was raised. Antiserum 1418, as well as a purified IgG fraction of this serum, inhibited both rh-GM-CSF-stimulated cell proliferation and 125I-labeled rh-GM-CSF receptor binding but not 125I-labeled recombinant human interleukin-4 receptor binding. The anti-idiotypic antibody response derived from the anti-(110-127) antibody strongly suggests that the carboxyl-terminal region of rh-GM-CSF may be directly involved in the receptor-ligand interaction of this protein. The high affinity receptor consists of two different components (GM-R alpha beta) a cytokine-specific alpha-subunit and a beta-subunit that is shared by human GM-CSF, interleukin-3, and interleukin-5. In an effort to localize the epitope of antibody 1418 to either GMR alpha or GMR beta, several cell lines containing high, low, or both high and low affinity receptors were examined. Each was specifically and completely inhibited by antibody 1418. Interleukin-3-dependent cell proliferation of the AML-193 cell line was found to be unaffected by the antibody 1418. Thus, the carboxyl-terminal region of rh-GM-CSF is likely to be involved in the interaction of the ligand with the alpha-subunit of the high affinity receptor.