We have found that a slightly modified insulin-like growth factor II (IGF II) consisting of a chimaera of bombyxin and human IGF II (BOMIGF) is properly secreted in insect cells by using the baculovirus expression system. Human interleukin 3 (IL-3) was attached to the C-terminal amino acid residue of BOMIGF with peptide linkers containing five or twelve residues. Only the chimaera with the 12-residue linker had biological activities of both IGF II and IL-3. The chimaera had a significantly higher mitogenic activity than IL-3 in cell cultures of the human haemopoietic cell line TF-1 and its effect could be observed even at femtomolar concentrations. It was also able to stimulate thymidine incorporation in IGF II-dependent bovine fetal erythroid cells. The chimaera significantly increased the number of macroscopic haemopoietic colonies in cultures of human peripheral blood in comparison with IL-3 or mixtures of IL-3 and BOMIGF in vitro. Subcutaneous injection of a BOMIGF–mouse IL-3 chimaera in normal C57BL/6 mice resulted in a significant increase of the number of spleen stem cells producing macroscopic haemopoietic colonies. This new system for the biosynthesis of IGF–cytokine fusion proteins in insect cells might prove advantageous for the low-cost and high-yield production of molecules with complementary or synergistic biological activities.

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