Engineering a Replication-Competent, Propagation-Defective Middle East Respiratory Syndrome Coronavirus as a Vaccine Candidate

Almazán, Fernando; DeDiego, Marta L.; Sola, Isabel; Zuñiga, Sonia; Nieto-Torres, Jose L.; Marquez-Jurado, Silvia; Andrés, German; Enjuanes, Luis

rMERS-CoV-⌬E was a replication-competent, propagationdeficient virus and was only efficiently disseminated in cells expressing the E protein in trans. In the presence of transiently ex-pressed E protein, rMERS-CoV-⌬E yielded maximum progeny viral titers of around 10 3 TCID 50 /ml. This modest yield could be improved by the generation of cell lines stably expressing the E protein. In fact, a direct relation between viral titers and the amount of E protein expressed was previously observed for TGEV (45) . However, high expression levels of E protein could induce apoptosis, as described for MHV E protein expression (46) . To overcome this potential adverse effect in the case of MERS-CoV, an inducible system for E protein expression would have to be established.