The newly emerging Middle East respiratory syndrome coronavirus (MERS-CoV) causes a severe respiratory infection with a high mortality rate (~35%). MERS-CoV has been a global threat due to continuous outbreaks in the Arabian peninsula and international spread by infected travelers since 2012. From May to July 2015, a large outbreak initiated by an infected traveler from the Arabian peninsula swept South Korea and resulted in 186 confirmed cases with 38 deaths (case fatality rate, 20.4%). Here, we show the rapid emergence and spread of a mutant MERS-CoV with reduced affinity to the human CD26 receptor during the South Korean outbreak. We isolated 13 new viral genomes from 14 infected patients treated at a hospital and found that 12 of these genomes possess a point mutation in the receptor-binding domain (RBD) of viral spike (S) protein. Specifically, 11 of these genomes have an I529T mutation in RBD, and 1 has a D510G mutation. Strikingly, both mutations result in reduced affinity of RBD to human CD26 compared to wild-type RBD, as measured by surface plasmon resonance analysis and cellular binding assay. Additionally, pseudotyped virus bearing an I529T mutation in S protein showed reduced entry into host cells compared to virus with wild-type S protein. These unexpected findings suggest that MERS-CoV adaptation during human-to-human spread may be driven by host immunological pressure such as neutralizing antibodies, resulting in reduced affinity to host receptor, and thereby impairs viral fitness and virulence, rather than positive selection for a better affinity to CD26.

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Hantaan (HTN) virus, the etiologic agent of clinically severe hemorrhagic fever with renal syndrome (HFRS), was first isolated in 1976 from lung tissue of a striped-field mouse (Apodemus agrarius) captured in Songnae-ri, Gyeonggi Province, Republic of Korea. Found primarily in mountainous areas, the Korean field mouse (A. peninsulae) is the second-most dominant field rodent species found throughout Korea. A new hantavirus, designated Soochong (SOO), was isolated in Vero E6 cells from four A. peninsulae captured in August 1997 at Mt. Gyebang in Hongcheon-gun, Mt. Gachil, Inje-gun, Gangwon Province, and in September 1998 at Mt. Deogyu, Muju-gun, Jeollabuk Province. The entire S, M, and L genomic segments of SOO virus, amplified by RT-PCR from lung tissues of seropositive A. peninsulae and from virus-infected Vero E6 cells, diverged from HTN virus (strain 76-118) by 15.6%, 22.8%, and 21.7% at the nucleotide level and 3.5%, 9.5%, and 4.6% at the amino acid level, respectively. Phylogenetic analyses of the nucleotide and deduced amino acid sequences, using the maximum parsimony and neighbor-joining methods, indicated that SOO virus was distinct from A. agrarius-borne HTN virus. SOO virus shared a common ancestry with Amur virus from Far East Russia, as well as with H5 and B78 hantaviruses, previously isolated from HFRS patients in China. Cross-focus-reduction neutralizating antibody tests showed that SOO virus, which is the first hantavirus isolated in cell culture from A. peninsulae, could be classified as a new hantavirus serotype.

Hantaviruses replicate in primary and cultured animal cells with little or no cytopathic effect. We report here that the cultured Vero E6 cells infected by the Hantaan or by the Prospect Hill viruses exhibited characteristic features of apoptosis, including condensation and segmentation of nuclei and internucleosomal cleavage of nuclear DNA. Apoptosis was not seen in the cells adsorbed by UV-inactivated virus, indicating that the viral replication is required for the induction of apoptosis. Furthermore, level of the proto-oncogenic Bcl-2 protein was significantly reduced, whereas its mRNA level remained unchanged in Hantaan virus-infected cells, suggesting possible involvement and posttranscriptional regulation of this antiapoptotic protein in the process.

a b s t r a c tThe latent ribonuclease RNase L and the interferon-inducible 2 0 ,5 0 -oligoadenylate synthetase (OAS) have been implicated in the antiviral response against hepatitis C virus (HCV). However, the specific roles of these enzymes against HCV have not been fully elucidated. In this study, a scarce endogenous expression and RNA degrading activity of RNase L in human hepatoma Huh7 cells enabled us to demonstrate the antiviral activity of RNase L against HCV replication through the transient expression of the enzyme. The antiviral potential of specific members of the OAS family was further examined through overexpression and RNA interference approaches. Our data suggested that among the members of the OAS family, OAS1 p46 and OAS3 p100 mediate the RNase L-dependent antiviral activity against HCV.