SARS Vaccine Development

A novel coronavirus (SCoV) is the etiological agent of severe acute respiratory syndrome (SARS). SCoV-like viruses were isolated from Himalayan palm civets found in a live-animal market in Guangdong, China. Evidence of virus infection was also detected in other animals (including a raccoon dog, Nyctereutes procyonoides) and in humans working at the same market. All the animal isolates retain a 29-nucleotide sequence that is not found in most human isolates. The detection of SCoV-like viruses in small, live wild mammals in a retail market indicates a route of interspecies transmission, although the natural reservoir is not known.

Angiotensin-converting enzyme 2 (ACE2) is a receptor for SARS-CoV, the novel coronavirus that causes severe acute respiratory syndrome [Li, W. Moore, M. J., Vasilieva, N., Sui, J., Wong, S. K., Berne, M. A., Somasundaran, M., Sullivan, J. L., Luzuriaga, K., Greenough, T. C., et al. (2003) Nature 426, 450-454]. We have identified a different human cellular glycoprotein that can serve as an alternative receptor for SARS-CoV. A human lung cDNA library in vesicular stomatitis virus G pseudotyped retrovirus was transduced into Chinese hamster ovary cells, and the cells were sorted for binding of soluble SARS-CoV spike (S) glycoproteins, S(590) and S(1180). Clones of transduced cells that bound SARS-CoV S glycoprotein were inoculated with SARS-CoV, and increases in subgenomic viral RNA from 1-16 h or more were detected by multiplex RT-PCR in four cloned cell lines. Sequencing of the human lung cDNA inserts showed that each of the cloned cell lines contained cDNA that encoded human CD209L, a C-type lectin (also called L-SIGN). When the cDNA encoding CD209L from clone 2.27 was cloned and transfected into Chinese hamster ovary cells, the cells expressed human CD209L glycoprotein and became susceptible to infection with SARS-CoV. Immunohistochemistry showed that CD209L is expressed in human lung in type II alveolar cells and endothelial cells, both potential targets for SARS-CoV. Several other enveloped viruses including Ebola and Sindbis also use CD209L as a portal of entry, and HIV and hepatitis C virus can bind to CD209L on cell membranes but do not use it to mediate virus entry. Our data suggest that the large S glycoprotein of SARS-CoV may use both ACE2 and CD209L in virus infection and pathogenesis.

Immunization with a killed or inactivated viral vaccine provides significant protection in animals against challenge with certain corresponding pathogenic coronaviruses (CoVs). However, the promise of this approach in humans is hampered by serious concerns over the risk of leaking live severe acute respiratory syndrome (SARS) viruses. In this study, we generated a SARS vaccine candidate by using the live-attenuated modified vaccinia virus Ankara (MVA) as a vector. The full-length SARS-CoV envelope Spike (S) glycoprotein gene was introduced into the deletion III region of the MVA genome. The newly generated recombinant MVA, ADS-MVA, is replication incompetent in mammalian cells and highly immunogenic in terms of inducing potent neutralizing antibodies in mice, rabbits, and monkeys. After two intramuscular vaccinations with ADS-MVA alone, the 50% inhibitory concentration in serum was achieved with reciprocal sera dilutions of more than 1,000- to 10,000-fold in these animals. Using fragmented S genes as immunogens, we also mapped a neutralizing epitope in the region of N-terminal 400 to 600 amino acids of the S glycoprotein (S400-600), which overlaps with the angiotensin-converting enzyme 2 (ACE2) receptor-binding region (RBR; S318-510). Moreover, using a recombinant soluble RBR-Fc protein, we were able to absorb and remove the majority of the neutralizing antibodies despite observing that the full S protein tends to induce a broader spectrum of neutralizing activities in comparison with fragmented S proteins. Our data suggest that a major mechanism for neutralizing SARS-CoV likely occurs through blocking the interaction between virus and the cellular receptor ACE2. In addition, ADS-MVA induced potent immune responses which very likely protected Chinese rhesus monkeys from pathogenic SARS-CoV challenge.