Effect of the silk protein sericin on the production of adenovirus‐based gene‐therapy vectors

Adenoviral vectors are extensively used as gene-delivery vehicles in gene therapy. They are usually produced by HEK-293 cell (human embryonic kidney-293 cell) culture, which requires specially formulated serum-free medium, the cost of which is considerable or by supplementation with FBS (fetal bovine serum). The risk of infectious diseases such as BSE (bovine spongiform encephalopathy) and endogenous retrovirus derived from cattle is a serious concern. The present study reports the use of sericin protein derived from silkworm (Bombyx mori) as an effective supplement instead of FBS. Without FBS, HEK-293 cells significantly proliferated in the presence of 0.025-0.4% sericin, especially at 0.1%, but the effect was inferior to that of FBS. When a lower titre [MOI (multiplicity of infection) 0.03] of adenoviral vector pAxCAiLacZ was used as the inoculum, HEK-293 cells in the presence of 0.1% sericin produced a nearly 3-fold higher vector titre than culture in the presence of 5% (v/v) FBS. However, when a higher vector titre (MOI 3.7) was used as the inoculum, HEK-293 cells in the presence of sericin produced a slightly higher vector titre than in the presence of FBS, which might suggest that HEK-293 cells produce a maximum amount when a higher vector titre is used as the inoculum. These increases in vector production with sericin were confirmed by LacZ (beta-galactosidase reporter gene) activity assay. Supplementation with sericin decreased lactate dehydrogenase activity, an indicator of cell death, suggesting that sericin improved cell survival; hence, prolonging the culture period might be one of the reasons for increased vector production. On the basis of these results, sericin peptide seems to be a potent and effective alternative supplement for production of adenoviral vectors without such risks as BSE and retrovirus.