Recombinant expression of porcine lactoferrin peptide LF-6 with intein technology and its immunomodulatory function in ETEC K88-infected mice

• Designed LF-6 had distinctive structure compared with parental peptide LFP-20.
• Expression LF-6 by intein system is no enzymatic and self-cleaving process.
• LF-6 is not only confined to antibacterial but immune regulation potential.

LF-6 is a modified antibacterial peptide derived from LFP-20, a major active ingredient of porcine lactoferrin, whose antibacterial activity is 200 times higher than its native protein counterpart. Moreover, LF-6 displays even higher antibacterial activity than LFP-20 and negligible toxic adverse effects, make it a potential therapeutic agent for antibacterial purposes. Escherichia coli expression system has been a preferred choice and workhorse for most recombinant proteins. However, LF-6 must be coexpressed with a fusion partner to avoid its potentially fatal toxicity which would threat E. coli expression system. In this study, we successfully introduced intein system to solve this problem, which LF-6 was N-terminally fused to dithiothreitol (DTT)-induced self-cleavable intein, and it conduct cleavage when the intein-fusion peptide passing through a chromatography column filled with chitin, then the spliced peptide was purified with RP-HPLC and identified with mass spectroscopy. A bacteriostatic test showed that the recombinant LF-6 displayed nearly the same antibacterial activity as the chemically synthetized LF-6, and an in vivo immunoprotection analysis showed that the recombinant LF-6 exerted protective effects on Escherichia coli (ETEC)-K88-infected mice, which significantly reduced the pro-inflammatory cytokines level in plasma and intestine, and resistant to intestinal mucosal injury compared to the infective alone groups. Our study indicates that the intein system allows a safe and efficient method to produce recombinant LF-6, which not only has antibacterial activity, but more importantly, has an immunomodulatory function.