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

Soon after MERS-CoV emergence, a diagnostic assay was designed (21) . Similarly, antivirals inhibiting virus replication, such as cyclosporine A, IFN-␣, or ribavirin, have been described (14, 22, 23) . In contrast, reliable vaccines have not yet been developed, although the S protein and the receptor-binding site within this protein induce neutralizing antibodies and, in principle, could serve as a subunit vaccine (17) . CoVs infect respiratory and enteric mucosal areas, and thus, induction of mucosal immunity is necessary to protect these tissues from infection. Live attenuated viruses are expected to elicit mucosal immunity more efficiently than nonreplicating antigens, which elicit reduced secretory immune responses. Live attenuated viruses can be generated by the deletion of genes conferring virulence, a procedure that requires the availability of a reverse genetics system for MERS-CoV. In this article, we describe the construction of an infectious cDNA clone of MERS-CoV using a bacterial artificial chromosome (BAC). Using this clone, recombinant MERS-CoV (rMERS-CoV) deletion mutants were constructed lacking genes nonessential for virus replication. In addition, we deleted the structural envelope (E) protein gene, because in previous work from our laboratory, deletion of the E gene in two other CoVs led to mutants that were either replication-competent, propagation-defective viruses or attenuated viruses (24) (25) (26) . All deletion mutants efficiently replicated and spread in cell cultures except the one in which the E gene was deleted, which was replication competent but propagation defective. This virus was propagated in cells by providing E protein in trans. Therefore, this deletion mutant missing the E gene can serve as the basis for a safe vaccine candidate.