Developing an effective and economical bacterial expression system for the production of animal vaccines in Thailand and South East Asia

The lack of capacity to produce animal vaccines within low and middle income countries in South East Asia poses a significant risk to their food security and economic stability; Thailand's economy relies heavily on their production of pork, however viral diseases have spread within livestock populations to endemic levels. In the last few decades, porcine circoviruses have become ubiquitous almost worldwide, with developing countries like Thailand needing to rely on expensive, imported vaccine technology to combat the risk of economic losses caused by livestock diseases. Current imported vaccines against PCV2 are mismatched to the local strains in circulation, and due to the high rate of genetic shift occurring among viruses, the effectiveness of the imported vaccines is limited. This research set out to develop a low cost recombinant expression system in E. coli to generate subunit vaccines to protect against porcine circovirus 2 (PCV2) and porcine circovirus 3 (PCV3) strains circulating locally in Thailand, with the wider goal to establish production capacity within Thailand to allow for local production of vaccines for appropriate circulating strains. Through optimising expression of PCV2d and PCV3 capsid (Cap) genes in E. coli to generate high protein yields at 1.5 L fed-batch fermentation and purify using a single-step cation exchange purification, we were able to achieve high purity (>95%) PCV2d proteins capable of inducing neutralising antibody responses in rabbits and mice as demonstrated by PK-15 cell-based virus neutralisation assays. Purified PCV2d protein was also developed into an adjuvanted vaccine candidate, showing an immunological response in a pig viral challenge trial. In addition, promising steps were made towards developing a chimeric PCV2d-PCV3 dual vaccine candidate for protection against both viruses in a single vaccine. We have established a minimal step, low-cost, laboratory-scale vaccine production process that is able to generate a virus-neutralising subunit vaccine against PCV2d, with the potential for scale-up, to contribute towards facilitating vaccine production capabilities locally within Thailand.