IL‐3 specifically inhibits GM‐CSF binding to the higher affinity receptor

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Abstract
The inhibition of binding between human granulocyte‐macrophage colony‐stimulating factor (GM‐CSF) and its receptor by human interleukin‐3 (IL‐3) was observed in myelogenous leukemia cell line KG‐1 which bore the receptors both for GM‐CSF and IL‐3. In contrast, this phenomenon was not observed in histiocytic lymphoma cell line U‐937 or in gastric carcinoma cell line KATO III, both of which have apparent GM‐CSF receptor but an undetectable IL‐3 receptor. In KG‐1 cells, the cross‐inhibition was preferentially observed when the binding of GM‐CSF was performed under the high‐affinity binding condition; i.e., a low concentration of 125I‐GM‐CSF was incubated. Scatchard analysis of 125I‐GM‐CSF binding to KG‐1 cells in the absence and in the presence of unlabeled IL‐3 demonstrated that IL‐3 inhibited GM‐CSF binding to the higher‐affinity component of GM‐CSF receptor on KG‐1 cells. Moreover, a chemical cross‐linking study has revealed that the cross‐inhibition of the GM‐CSF binding observed in KG‐1 cells is specific for the β‐chain, Mr 135,000 binding protein which has been identified as a component forming the high‐affinity GM‐CSF receptor existng specifically on hemopoietic cells.