Adaptation of candidate vaccine strains of South African Territories (SAT) type foot-and-mouth disease virus to suspension cell culture

Abstract

African buffalo (Syncerus caffer) are efficient maintenance hosts of three serotypes of foot-and-mouth disease (FMD) virus in South Africa, namely the South African Territories (SAT) types 1-3 viruses. Of these, the SAT-1 and SAT-2 serotypes historically caused the most FMD outbreaks in the country, however, SAT-3 outbreaks have also occurred in recent years. Because buffaloes represent a permanent source of infection for cattle, control of the disease hinges on the separation of buffalo and cattle through fencing and the establishment of a protection zone and a surveillance zone between the infected buffalo population in the Kruger National Park and the FMD-free zone. As fences can, and have been breached by buffalo in the past, cattle in the buffer zone are vaccinated biannually with an inactivated trivalent vaccine. Historically, the FMD vaccine contained five virus strains (two SAT-1, two SAT-2 and one SAT-3) to ensure adequate antigenic coverage and protection against the highly variable field strains in circulation in buffalo. As effective control through vaccination requires the continuous development of vaccines that better match field and outbreak strains, it is important to identify suitable vaccine strain candidates for rapid formulation of novel vaccines in response to outbreaks and/or the detection of new virus variants. Vaccine strain selection is, however, time-intensive as only a small number of viruses meet the stringent requirements of rapid adaptation to monolayer and suspension cell cultures, sustained high virus yield and genome stability over successive passages. In this study, four SAT-type viruses that are not part of the current FMD vaccine formulation for South Africa were assessed for their vaccine strain potential. PCR of the P1 region using NCR1 and WDA primers and Sanger sequencing was done as an initial verification step. Each low-passage virus was then adapted to baby hamster kidney (BHK) 21 monolayer cells, after which the virus was passaged three times on BHK-21 suspension cells. Samples from the infected suspension cell cultures were collected at regular intervals from 16 to 40 hours post-infection to monitor changes in the tissue culture infectious dose (TCID)50. Lastly, next-generation sequencing was employed to determine the genome stability of each virus. Integration of the resulting data on changes in variant composition associated with adaptation to suspension cell culture, and virus yield allowed for ranking of the candidate vaccine strains assessed in this study. The approach and results will ultimately guide the selection of the virus isolates that are best-suited to vaccine production in BHK-21 suspension cell culture.