Expression and characterization of recombinant human alpha-antitrypsin in transgenic rice seed

Human alpha-antitrypsin (AAT) is the most abundant circulating protease inhibitor in the human plasma. It is produced in the liver and exerts a primary physiological role as inhibitor for the neutrophil elastase in the lung. Individuals with one or several gene mutations in AAT causing reduction of the protein are related to lung, liver and pancreatic emphysema diseases and are treated lifelong with infusions of human plasma-derived AAT. Due to shortage of plasma and low expression levels of recombinant AAT in conventional gene expression systems, we explored the possibility to produce recombinant AAT in rice grains (Oryza sativa AAT, OsrAAT). An expression level of up to 2.24g/kg brown rice and a final recovery of purified 0.366g/kg OsrAAT has been obtained. OsrAAT has the same secondary structure and protease inhibitory activity as plasma-derived AAT (pAAT), but was highly heterogeneous with regard to glycan modifications. Thus 32.8% of OsrAAT were glycosylated and 67.2% were free of glycans as determined by MALDI-MS. Of the N-glycan structures 64.8% were vacuole-specific paucimannosidic molecules. Immune electron microscopy located OsrAAT in the endoplasmic reticulum lumen as precursor-accumulating (PAC)-like vesicle structures. The pharmacokinetic study indicated that the half-life of OsrAAT was prolonged, while the clearance rate was faster than that of pAAT in vivo. The results demonstrate that rice endosperm is a promising system to express this biopharmaceutical protein.