Vaccines to prevent severe acute respiratory syndrome coronavirus-induced disease

Abstract

An important effort has been performed after the emergence of severe acute respiratory syndrome (SARS) epidemic in 2003 to diagnose and prevent virus spreading. Several types of vaccines have been developed including inactivated viruses, subunit vaccines, virus-like particles (VLPs), DNA vaccines, heterologous expression systems, and vaccines derived from SARS-CoV genome by reverse genetics. This review describes several aspects essential to develop SARS-CoV vaccines, such as the correlates of protection, virus serotypes, vaccination side effects, and bio-safeguards that can be engineered into recombinant vaccine approaches based on the SARS-CoV genome. The production of effective and safe vaccines to prevent SARS has led to the development of promising vaccine candidates, in contrast to the design of vaccines for other coronaviruses, that in general has been less successful. After preclinical trials in animal models, efficacy and safety evaluation of the most promising vaccine candidates described has to be performed in humans.

Fig. 1

Phylogenetic analysis of human, bat, and civet cat-racoon dog virus spike sequences. An unrooted Bayesian phylogenetic gene tree of 24 SARS viruses divided into four groups. Group 1 includes viruses isolated from animals in southern China in 2003. Group 2 is a cluster of viruses isolated from animals and humans (*) in 2003. Group 3 includes viruses from all three phases of the human SARS epidemic of 2002–2003. Group 4 represents a cluster of viruses isolated from bats in 2005–06. A multiple sequence alignment of the spike gene of each virus was created using ClustalX 1.83 with default settings. Bayesian inference was conducted with Mr. Bayes, with Markov chain Monte Carlo sampling of four chains for 500,000 generations, and a consensus tree was generated using the 50% majority rule with a burn in of 1000. Branch confidence values are shown as posterior probabilities. The three human isolates that fall within the animal cluster (GZ0402, GD03, and GZ0401) may represent infections where a human acquired the virus from animals. The dashed line between Group 3 and Group 4 is used to represent a much longer line in the tree (∼10 times longer), thus the distance of the line is not representative of the distance between bat and human SARS.

Fig. 2

Structure and genome organization of SARS-CoV. (A). Schematic diagram of SARS-CoV structure. S, spike protein; M, membrane protein; E, envelope protein; N, nucleoprotein; 3a, 7a, and 7b, structural proteins of SARS-CoV. (B). Representation of a prototype SARS-CoV genome. Poly(A) tail is indicated by AAA. Numbers and letters indicate viral genes.

Fig. 3

Neutralization of SARS-CoV pseudotypes and heterologous challenge studies. (A) Cross neutralization responses between wt Urbani (●) and icGD03 (■). About 100 PFU of each virus was incubated for 30 min with varying concentrations of human antisera from a convalescent SARS patient or control serum and titered by plaque assay. (B) Compilation of vaccination results in mice inoculated with VRP-vectored vaccines and challenged with icSARS or icGDO3-S (Deming et al., 2006). The percent of mice without detectable replicating virus are shown as bars while the average titers of detectable virus are shown as red circles. Error bars represent the standard deviation of the measured samples. Mice were vaccinated with VRP-S, VRP-N, a cocktail of VRP-S and VRP-N (VRP-S+N), or mock vaccinated with either VRP-HA (VRP expressing influenza A HA protein) or PBS. Mice were intranasally challenged with either Urbani derived from the infectious clone (icSARS) or a chimeric virus expressing the GD03 spike glycoprotein (GD03-S). Mice challenged with icSARS were vaccinated when young (4–5 weeks), boosted 4 weeks later, and challenged either 8 weeks post boost (young) or 54 weeks post boost when old (Senescent). VRP-S and VRP-S+N provided protection in both groups against icSARS. Mice challenged with GD03-S were either vaccinated when 7 weeks old (young) or older than 26 weeks old (senescent), boosted approximately 4 weeks later, and challenged either 7 weeks post boost (young) or 32 weeks post boost (senescent).

Fig. 4

Propagation of SARS-CoV with a deleted E gene in different cell lines. Vero E6 (A), Huh-7 (B), and CaCo-2 (C) cells were infected at a moi of 0.5 with either the rSARS-CoV-ΔE or the recombinant wild-type virus. At different times post-infection, virus titers were determined by plaque assay on Vero E6 cells. Error bars represent standard deviations from the mean from three experiments.

Fig. 5

Assembly of SARS-CoV-ΔE deletion mutant in the ERGIG compartment. Electron micrographs of Vero E6 cells infected (moi 1.0) with SARS-CoV (A) and SARS-CoV-ΔE (B) at 24 h post-infection.

Fig. 6

Growth of rSARS-CoV in the respiratory tract of hamsters. Hamsters were inoculated with 10³TCID₅₀ of rSARS-CoV or rSARS-CoV-ΔE. Animals were sacrified and tissues were harvested at different times post-infection. Viral titers in lung (A) and nasal turbinates (B) were determined in Vero E6 cells monolayers. The non-parametric Mann–Whitney U-statistical method was used for ascertaining the significance of observed differences. Statistical significance was indicated by (^*p-value < 0.05). The dotted line indicates the lower limit of detection.

Fig. 7

(A) Genome organization of SARS-CoV recombinant viruses to generate safe attenuated viruses. The wild-type SARS-CoV TRS, ACGAAC (blue circles), were changed to CCGGAT (red circles). Since the wild-type and mutant TRS signals are not compatible in regulating subgenomic transcription (Yount et al., 2006), a recombination event resulting in a viral genome with mixed TRS signals is not viable. (B) The icSARS-CoV TRS sequence is unique from that of other described coronaviruses. TRS sequences for select group 1, 2 and 3 coronaviruses are summarized. The TRS selected for the remodeled virus is shown at the bottom.