Field production and functional evaluation of chloroplast-derived interferon-α2b

Type I interferons (IFNs) inhibit viral replication and cell growth and enhance the immune response, and therefore have many clinical applications. IFN-alpha 2b ranks third in world market use for a biopharmaceutical, behind only insulin and erythropoietin. The average annual cost of IFN-alpha 2b for the treatment of hepatitis C infection is $26 000, and is therefore unavailable to the majority of patients in developing countries. Therefore, we expressed IFN-alpha 2b in tobacco chloroplasts, and transgenic lines were grown in the field after obtaining United States Department of Agriculture Animal and Plant Health Inspection Service (USDA-APHIS) approval. Stable, site-specific integration of transgenes into chloroplast genomes and homoplasmy through several generations were confirmed. IFN-alpha 2b levels reached up to 20% of total soluble protein, or 3 mg per gram of leaf (fresh weight). Transgenic IFN-alpha 2b had similar in vitro biological activity to commercially produced PEG-Intron (TM) when tested for its ability to protect cells against cytopathic viral replication in the vesicular stomatitis virus cytopathic effect (VSV CPE) assay and to inhibit early-stage human immunodeficiency virus (HIV) infection. The antitumour and immunomodulating properties of IFN-alpha 2b were also seen in vivo. Chloroplast-derived IFN-alpha 2b increased the expression of major histocompatibility complex class I (MHC I) on splenocytes and the total number of natural killer (NK) cells. Finally, IFN-alpha 2b purified from chloroplast transgenic lines (cpIFN-alpha 2b) protected mice from a highly metastatic tumour line. This demonstration of high levels of expression of IFN-alpha 2b, transgene containment and biological activity akin to that of commercial preparations of IFN-alpha 2b facilitated the first field production of a plant-derived human blood protein, a critical step towards human clinical trials and commercialization.