Cold-adapted live attenuated MERS-CoV vaccine strain remains attenuated in mice after multiple passages in Vero cells at 37 °C

Middle East respiratory syndrome coronavirus (MERS-CoV) is a zoonotic disease affecting camels and humans. The live attenuated vaccine represents a candidate human vaccine because it can induce strong immune responses in immunized hosts. The attenuated vaccine strain of the highly pathogenic virus can also be used to produce a cell-based vaccine in the BSL2 GMP facility. In this study, we evaluated the reversion potential of pathogenicity to pathogenic wild-type virus to ensure the safety of the live attenuated vaccine strain. We passaged our previously developed cold-adapted live attenuated MERS-CoV vaccine strain at 22 °C (EMC2012-CA22°C) in Vero cells at 37 °C as often as 15 times to determine the potential of pathogenicity reversion in hDPP4 (human dipeptidyl peptidase 4)-transgenic mice, K18-hDPP4. The serial passage of EMC2012-CA22°C in Vero cells at 37 °C up to 15 times did not result in pathogenicity reversion to wild-type MERS-CoV. In K18-hDPP4 mice infected with this virus, no weight loss or mortality was observed, and no virus was detected in tissues such as the lung, kidney, brain, and nasal turbinate. In addition, mice immunized with this virus produced a robust neutralizing antibody response and were fully protected from lethal challenge with wild-type MERS-CoV. The cold-adapted attenuated MERS-CoV vaccine strain (EMC2012-CA22°C) was not reverted to wild-type pathogenic virus after 15 passages in Vero cells at 37 °C.