A Highly Productive CHO Cell Line Secreting Human Blood Clotting Factor IX

Hemophilia B patients suffer from an inherited blood-clotting defect and require regular administration of blood-clotting factor IX replacement therapy. Recombinant human factor IX produced in cultured CHO cells is nearly identical to natural, plasma-derived factor IX and is widely used in clinical practice. Development of a biosimilar recombinant human factor IX for medical applications requires the generation of a clonal cell line with the highest specific productivity possible and a high level of specific procoagulant activity of the secreted factor IX. We previously developed plasmid vectors, p1.1 and p1.2, based on the untranslated regions of the translation elongation factor 1 alpha gene from Chinese hamster. These vectors allow one to perform the methotrexate- driven amplification of the genome-integrated target genes and co-transfect auxiliary genes linked to various resistance markers. The natural open reading frame region of the factor IX gene was cloned in the p1.1 vector plasmid and transfected to CHO DG44 cells. Three consecutive amplification rounds and subsequent cell cloning yielded a producer cell line with a specific productivity of 10.7 0.4 pg/cell/day. The procoagulant activity of the secreted factor IX was restored nearly completely by co-transfection of the producer cells by p1.2 plasmids bearing genes of the soluble truncated variant of human PACE/furin signal protease and vitamin K oxidoreductase from Chinese hamster. The resulting clonal cell line 3B12-86 was able to secrete factor IX in a protein-free medium up to a 6 IU/ml titer under plain batch culturing conditions. The copy number of the genome- integrated factor IX gene for the 3B12-86 cell line was only 20 copies/genome; the copy numbers of the genome-integrated genes of PACE/furin and vitamin K oxidoreductase were 3 and 2 copies/genome, respectively. Factor IX protein secreted by the 3B12-86 cell line was purified by three consecutive chromatography rounds to a specific activity of up to 230 IU/mg, with the overall yield 30%. The developed clonal producer cell line and the purification process employed in this work allow for economically sound industrial-scale production of biosimilar factor IX for hemophilia B therapy.