Towards a novel polio VLP vaccine: Stabilising the PV-1 capsid by thermal selection.

Poliomyelitis (polio) is a highly infectious and debilitating viral disease caused by poliovirus (PV). The use of an oral (OPV) and an inactivated (IPV) polio vaccine over the years has led to remarkable progress towards the eradication of polio. In order to safeguard against reintroduction of polio, post-eradication, vaccination will continue. However, current OPV and IPV require the propagation of live virus and therefore constitute biological hazards post-eradication. Genome-free empty capsids (ECs) are produced during the PV lifecycle but are conformationally unstable at physiological temperatures, rapidly losing native antigenicity. If stabilised in the native conformation, recombinantly expressed PV ECs could have applications as alternative virus-free vaccines for use post-eradication. In this study, thermally-stable variants were selected through cycles of increasing thermal pressures from 51oC through 53oC to 57oC. Selected viruses were shown to have evolved thermally-stable ECs that retained native antigenicity at elevated temperatures. The capsid-stabilising mutations were identified and stabilising combinations were further investigated. The structural precursor protein (P1) of two mutant virus candidates were co-expressed with the viral protease (3CD) in a plant system which resulted in the production of thermally-stable PV-1 ECs, some of which retained the native antigenicity at temperatures higher than current IPV. The investigated potential for expression of thermally-stable VLPs in yeast is also discussed.