Binding of iodinated multipotential colony‐stimulating factor (interleukin‐3) to murine bone marrow cells

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Abstract
Multipotential colony‐stimulating factor (Multi‐CSF or interleukin‐3) was radioiodinated to high specific radioactivity (1–4 × 105 cpm/ng) with no detectable loss of biological activity and its binding to murine bone marrow cells and factor‐dependent cell lines studied. Both the native glycosylated molecule purified from a cloned T‐cell line (LB3) and the purified non‐glycosylated recombinant molecule produced by E. coli could be radioiodinated. Comparative binding studies with these derivatives demonstrated equal binding affinities and equal numbers of binding sites on various cell types indicating that carbohydrate moieties are not involved in the binding interactions. Binding of 125I‐Multi‐CSF to several factor‐dependent continuous hemopoietic cell lines showed the presence of specific receptors on all cell lines, the receptor number per cell varying from 700 to 13,000 and the apparent dissociation constant from 400 pM to 1 nM. Specific binding of 125I‐Multi‐CSF was also observed to normal murine hemopoietic cells and the binding to murine bone marrow cells was studied in detail. Bone marrow cells showed 117–130 receptors per cell on average and an apparent dissociation constant of 126–233 pM. However, quantitative autoradiographic analysis indicated that receptors for 125I‐Multi‐CSF were not distributed randomly on bone marrow cells–‐nucleated erythroid and lymphoid cells were not labeled while essentially all neutrophilic granulocyte, eosinophilic granulocyte and monocytic cells were labeled. Moreover, in each of the labeled cell lineages grain counts (reflecting receptor number) decreased with increasing maturation and a small subpopulation of marrow cells (0.4–1.5% and including blast cells, monocytes, promyelocytes, and myelocytes) exhibited very high grain counts. The existence of such a subset of marrow cells raises the possibility of functional heterogeneity among marrow cells in their response to Multi‐CSF.