Production of Zika virus-like particles (VLPs) by perfusion processes

Zika virus (ZIKV) emerged as a major international public health concern in 2015 and rapidly spread to more than 80 countries in Africa, Asia and the Americas. ZIKV infection has been shown to cause Guillain-Barré syndrome in adults, as well as severe congenital malformations in fetuses from as much as 42% of infected mothers (Brasil et al., 2016, doi:10.1056/NEJMoa1602412). While no ZIKV vaccine becomes approved for human use, periodic outbreaks will continue to occur in endemic regions and the risk of spreading to non-endemic regions will continue to exist, especially because ZIKV persists in body fluids for very long time after infection and can be transmitted via the sexual route. Among many different vaccine platforms currently under study, virus-like particles (VLPs) are a promising alternative for the development of vaccines, since three-dimensional structures, constituted by recombinant structural proteins of the virus but lacking the viral genome, are able to display the antigen in a repetitive pattern, triggering a robust immune response. In this work, we investigated the production of Zika virus-like particles by both intermittent and continuous perfusion processes, using a recombinant HEK293 cell pool previously generated in our laboratory, which constitutively expresses the VLPs. In order to improve production levels, we first enriched the recombinant cell pool for high producers by means of fluorescence-activated cell sorting (FACS). Using this FACS-enriched cell pool, small-scale shake flask studies showed that intermittent perfusion (also known as pseudoperfusion) with daily medium exchange enhanced viable cell density by 3.5 fold and VLP titer by 4 fold when compared to batch cultures. Continuous perfusion in a controlled stirred-tank bioreactor was carried out using an ATF-2 unit as cell retention device. A steady-state viable cell concentration of 25-30 × 106 cells/mL was maintained at a cell-specific perfusion rate (CSPR) of 50-60 pL/cell/day. VLP titers inside the bioreactor were higher than in the harvest, evidencing product retention by the ATF hollow fiber, especially from day 14 of cultivation on. Our results show that the use of cell lines constitutively expressing zika VLPs, cultured in stirred-tank perfusion bioreactors, represents a promising system for the production of a VLP-based Zika vaccine candidate. This process could potentially be more cost-effective than traditional viral vaccine platforms based on batch production of whole viruses, especially considering that VLPs can be produced in lower biosafety level plants, and that perfusion systems are characterized by higher volumetric productivities, reduced bioreactor sizes, smaller plant footprint and lower investment costs when compared to batch processes