Moving beyond metagenomics to find the next pandemic virus.

Movements of viruses from animals to humans underlie outbreaks of diseases, such as Ebola hemorrhagic fever, influenza, and Middle East respiratory syndrome. The severe acute respiratory syndrome (SARS) virus pandemic of 2003 was caused by a novel coronavirus (CoV) that originated in Chinese horseshoe bats (1). Results of sequence analyses have shown that viruses related to SARS-CoV continue to circulate in bats, but their potential for infecting humans is not known. Gazing at viral sequences has its limits; experiments need to be done. In PNAS, Menachery et al. (2) develop a framework for deriving viruses from these genome sequences and examining their potential to cause the next SARS pandemic (Fig. 1). Entry of SARS-CoV into human cells begins with binding of the viral spike glycoprotein to the cell surface receptor human angiotensin converting enzyme 2 (ACE2) (3). Most of the presumed ancestors of SARS-CoV found in bats are unable to bind this receptor (4). Several years ago in China, a bat CoV, WIV1-CoV, was found to bind to human ACE2 and replicate in human cells (1). To determine if WIV1-CoV has the potential to infect humans,Menachery et al. (2) synthesized aDNA copy of the genome and introduced it into cells to recover infectious virus. The virus replicated as well as SARS-CoV in differentiated primary human airway epithelial cell cultures, the closest model to the human lung. These findings demonstrate that WIV1-CoV does not require adaptation for efficient replication in human cells. An important question is whether WIV1-CoV causes disease in an animal model of infection. SARS-CoV does not cause disease in mice, but multiple passages of the virus in this host produced a mouse-adapted virus called SARS-CoVMA15. This virus induces rapid weight loss and lethality by 4 d after intranasal infection (5). When Menachery et al. (2) substituted the spike glycoprotein gene from SARS-CoV MA15 with the corresponding gene from WIV1-CoV, the resulting virus replicated poorly in mice, and did not cause weight loss or lethality. When wild-type viruses (e.g., not mouseadapted) were inoculated into mice, neither SARSCoV nor WIV1-CoV caused weight loss or lethality. However, the two viruses differedmarkedly in their ability to replicate in mice: SARS-CoV replicated to higher titers in the lung and brain compared with WIV1-CoV. These observations show that although the spike glycoprotein of WIV1-CoV can mediate entry of the virus into human cells, the virus does not cause disease in mice. A very different outcome was observed when transgenic mice that produce the human ACE2 receptor were used by Menachery et al. (2). In contrast to wildtypemice, intranasal infection of these mice with SARSCoV did result in weight loss and death. However, WIV1-CoV replicated to lower titers in ACE2 transgenic Fig. 1. Experimental platform for moving beyond metagenomics to identify viruses with pandemic potential. Samples from animals are subjected to deep, high-throughput sequencing to identify viral genomes. Sequence data are subjected to phylogenetic analyses to identify evolutionary relationships. Selected viral genomes are synthesized as DNAs and transfected into cells to recover virus. Viruses are evaluated for the ability to replicate in relevant human cell culture models and to cause disease in animal models. The latter can also be used to evaluate therapeutics (antivirals and monoclonal antibodies) and vaccines. Images courtesy of (clockwise, Left to Right) Flickr/Michael Pennay, Flickr/Pablo Gonzalez, Andrew Rambaut, Flickr/J. N. Stuart, and the National Cancer Institute.