Expression of immunogenic glycoprotein S polypeptides from transmissible gastroenteritis coronavirus in transgenic plants

Exigency of Plant-Based Vaccine against COVID-19 Emergence as Pandemic Preparedness

After two years since the declaration of COVID-19 as a pandemic by the World Health Organization (WHO), more than six million deaths have occurred due to SARS-CoV-2, leading to an unprecedented disruption of the global economy. Fortunately, within a year, a wide range of vaccines, including pathogen-based inactivated and live-attenuated vaccines, replicating and non-replicating vector-based vaccines, nucleic acid (DNA and mRNA)-based vaccines, and protein-based subunit and virus-like particle (VLP)-based vaccines, have been developed to mitigate the severe impacts of the COVID-19 pandemic. These vaccines have proven highly effective in reducing the severity of illness and preventing deaths. However, the availability and supply of COVID-19 vaccines have become an issue due to the prioritization of vaccine distribution in most countries. Additionally, as the virus continues to mutate and spread, questions have arisen regarding the effectiveness of vaccines against new strains of SARS-CoV-2 that can evade host immunity. The urgent need for booster doses to enhance immunity has been recognized. The scarcity of "safe and effective" vaccines has exacerbated global inequalities in terms of vaccine coverage. The development of COVID-19 vaccines has fallen short of the expectations set forth in 2020 and 2021. Furthermore, the equitable distribution of vaccines at the global and national levels remains a challenge, particularly in developing countries. In such circumstances, the exigency of plant virus-based vaccines has become apparent as a means to overcome supply shortages through fast manufacturing processes and to enable quick and convenient distribution to millions of people without the reliance on a cold chain system. Moreover, plant virus-based vaccines have demonstrated both safety and efficacy in eliciting robust cellular immunogenicity against COVID-19 pathogens. This review aims to shed light on the advantages and disadvantages of different types of vaccines developed against SARS-CoV-2 and provide an update on the current status of plant-based vaccines in the fight against the COVID-19 pandemic.

Review: A systematic review of virus-like particles of coronavirus: Assembly, generation, chimerism and their application in basic research and in the clinic

Virus-like particles (VLPs) are nano-scale particles that are morphologically similar to a live virus but which lack a genetic component. Since the pandemic spread of COVID-19, much focus has been placed on coronavirus (CoV)-related VLPs. CoVs contain four structural proteins, though the minimum requirement for VLP formation differs among virus species. CoV VLPs are commonly produced in mammalian and insect cell systems, sometimes in the form of chimeric VLPs that enable surface display of CoV epitopes. VLPs are an ideal model for virological research and have been applied as vaccines and diagnostic reagents to aid in clinical disease control. This review summarizes and updates the research progress on the characteristics of VLPs from different known CoVs, mainly focusing on assembly, in vitro expression systems for VLP generation, VLP chimerism, protein-based nanoparticles and their applications in basic research and clinical settings, which may aid in development of novel VLP vaccines against emerging coronavirus diseases such as SARS-CoV-2.

Producing Vaccines against Enveloped Viruses in Plants: Making the Impossible, Difficult

The past 30 years have seen the growth of plant molecular farming as an approach to the production of recombinant proteins for pharmaceutical and biotechnological uses. Much of this effort has focused on producing vaccine candidates against viral diseases, including those caused by enveloped viruses. These represent a particular challenge given the difficulties associated with expressing and purifying membrane-bound proteins and achieving correct assembly. Despite this, there have been notable successes both from a biochemical and a clinical perspective, with a number of clinical trials showing great promise. This review will explore the history and current status of plant-produced vaccine candidates against enveloped viruses to date, with a particular focus on virus-like particles (VLPs), which mimic authentic virus structures but do not contain infectious genetic material.

A short review on key role of plants and their extracts in boosting up immune response to combat COVID-19

In the wake of the recent global pandemic of COVID-19, there has been an increasing concern among the general public to improve their immune system. The causative agent of COVID-19 is SARS CoV-2, similar to its relative viruses SARS Cov-1 and MERS. Up till now, no vaccine has been developed against this disease and the only way one can stay safe is via prevention and developing one's immune system. Plants have been used since ancient times in herbal medicines and many chemicals and extracts in them are found to boost the immune system. Therefore, a consciously maintained diet consisting of plant-based immunity boosters is the need of the hour. Plants are known to provide many chemicals, natural therapeutics and vitamins which naturally enhance our immune system. Recently, vaccination research is also being carried out in transgenic plants. In this review, we have focused on highlighting some of the main players within the Kingdom Plantae which, when taken up in regular diet, can significantly boost our immune system capacity and thus provide the best possible measure to combat this pandemic.

Bioinformatic prediction of immunodominant regions in spike protein for early diagnosis of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)

Background

To contain the pandemics caused by SARS-CoV-2, early detection approaches with high accuracy and accessibility are critical. Generating an antigen-capture based detection system would be an ideal strategy complementing the current methods based on nucleic acids and antibody detection. The spike protein is found on the outside of virus particles and appropriate for antigen detection.

Methods

In this study, we utilized bioinformatics approaches to explore the immunodominant fragments on spike protein of SARS-CoV-2.

Results

The S1 subunit of spike protein was identified with higher sequence specificity. Three immunodominant fragments, Spike_56-94, Spike_199-264, and Spike_577-612, located at the S1 subunit were finally selected via bioinformatics analysis. The glycosylation sites and high-frequency mutation sites on spike protein were circumvented in the antigen design. All the identified fragments present qualified antigenicity, hydrophilicity, and surface accessibility. A recombinant antigen with a length of 194 amino acids (aa) consisting of the selected immunodominant fragments as well as a universal Th epitope was finally constructed.

Conclusion

The recombinant peptide encoded by the construct contains multiple immunodominant epitopes, which is expected to stimulate a strong immune response in mice and generate qualified antibodies for SARS-CoV-2 detection.

COVID-19 Crisis: How Can Plant Biotechnology Help?

The emergence of the COVID-19 pandemic has led to significant public health crisis all over the world. The rapid spreading nature and high mortality rate of COVID-19 places a huge pressure on scientists to develop effective diagnostics and therapeutics to control the pandemic. Some scientists working on plant biotechnology together with commercial enterprises for the emergency manufacturing of diagnostics and therapeutics have aimed to fulfill the rapid demand for SARS-CoV-2 protein antigen and antibody through a rapid, scalable technology known as transient/stable expression in plants. Plant biotechnology using transient/stable expression offers a rapid solution to address this crisis through the production of low-cost diagnostics, antiviral drugs, immunotherapy, and vaccines. Transient/stable expression technology for manufacturing plant-based biopharmaceuticals is already established at commercial scale. Here, current opinions regarding how plant biotechnology can help fight against COVID-19 through the production of low-cost diagnostics and therapeutics are discussed.

Synergistic antiviral effects against SARS-CoV-2 by plant-based molecules

The exponential spread of Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) emphasizes the immediate need for effective antiviral drugs and vaccines that could control and prevent the spread of this pandemic. Several new and repurposed drugs are being tested for their effectiveness in the treatment regime, and the development of vaccines is underway. The availability of genome sequence information of the virus and the identification of potential targets to neutralize and eradicate the infection have enabled the search for novel as well as existing molecules to perform the desired function. However, the application of plants in the development of potential biomolecules, such as antibiotics and vaccines, is limited. Traditional medicines involving plant-based formulations have proven successful in boosting immunity and providing tolerance to virus infections. Still, in-depth studies are not available to explore the bioactive compounds of plant origin and their mechanism of action. Given this, the current opinion article conveys our thoughts and perspectives on the promising usage of plant-based biomolecules in circumventing SARS-CoV-2, and how these molecules can work synergistically with other potential drugs for treating SARS-CoV-2.

Potential Applications of Plant Biotechnology against SARS-CoV-2

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a novel coronavirus responsible for an ongoing human pandemic (COVID-19). There is a massive international effort underway to develop diagnostic reagents, vaccines, and antiviral drugs in a bid to slow down the spread of the disease and save lives. One part of that international effort involves the research community working with plants, bringing researchers from all over the world together with commercial enterprises to achieve the rapid supply of protein antigens and antibodies for diagnostic kits, and scalable production systems for the emergency manufacturing of vaccines and antiviral drugs. Here, we look at some of the ways in which plants can and are being used in the fight against COVID-19.

Protein-Based Bioproducts

Plant proteins can be used for the production of a variety of bioproducts, including films and coatings, adhesives, fibres and pharmaceuticals. Proteins derived from plant production systems have many advantages: they are safe, low-cost and rapidly deployable, allow for simple product storage and result in proteins that are properly folded, assembled and post-translationally modified. While plant-derived protein-based products are natural, renewable, biodegradable and environmentally friendly, they tend to be lower in strength and elasticity than their corresponding synthetic products. Current research in this area is focused on overcoming challenges in plant production platforms related to yield, purification, regulatory approval and customer acceptance.

Mucosal Vaccines from Plant Biotechnology

The use of plants for production of recombinant proteins has evolved over the past 25 years. The first plant-based vaccines were expressed in stably transgenic plants, with the idea to conveniently deliver “edible vaccines” by ingestion of the antigen-containing plant material. These systems provided a proof of concept that oral delivery of vaccines in crude plant material could stimulate antigen-specific serum and mucosal antibodies. Transgenic grains like rice in particular provide a stable and robust vehicle for antigen delivery. However, some issues exist with stably transgenic plants, including relatively low expression levels and regulatory issues. Thus, many recent studies use transient expression with plant viral vectors to achieve rapid high expression in Nicotiana benthamiana, followed by purification of antigen and intranasal delivery for effective stimulation of mucosal immune responses.

An Oral Vaccine for TGEV Immunization of Pigs

Transmissible gastroenteritis virus (TGEV) is a commercially important pathogen of hog farms and causes contagious, lethal diarrhea in piglets. While orally and parenterally administered vaccines made from inactivated or attenuated TGEV are commercially available, they require individual administration to piglets, which is time and labor intensive, and run the risk of reversion to pathogenicity. Also, parenteral vaccines produce neutralizing serum antibodies which may be less effective against an orally transmitted pathogen, compared to an oral vaccine that would induce the production of mucosal antibodies. There has been an effort to produce subunit vaccines in an edible form in plants for convenient administration through feed. These efforts towards the expression of the S-antigen of TGEV in maize seed, its effectiveness at inducing neutralizing antibody production in the colostrum of gilts, and its efficacy in protecting piglets against challenge by virulent TGEV are summarized here.

Oral immunization of mice with recombinant Lactococcus lactis expressing porcine transmissible gastroenteritis virus spike glycoprotein

Lactococcus lactis NZ9000 was selected as an antigen delivery vehicle for mucosal immunization against porcine transmissible gastroenteritis virus (TGEV) infection. An approximately 70 kDa fragment of the N-terminal globular domain of the spike (S) protein (S_N protein) from the coronavirus TGEV was used as the transmissible gastroenteritis virus antigen model. Recombinant L. lactis, expressing the S_N protein, was constructed with the pNZ8112 plasmid. Expression and localization of the transcribed S_N protein from the recombinant L_NZ9000-rTGEV-S_N were detected via SDS-PAGE, Western blot, and immunofluorescence. BALB/c mice, orally immunized with L_NZ9000-rTGEV-S_N, produced local mucosal immune responses against TGEV. The induced antibodies demonstrated neutralizing effects on TGEV infection. These data indicated that the recombinant L. lactis could be a valuable tool in the development of future vaccines against TGEV.

Simultaneous detection of antibodies to mouse hepatitis virus recombinant structural proteins by a microsphere-based multiplex fluorescence immunoassay

We describe a new microsphere-based multiplex fluorescent immunoassay (MFI) using recombinant mouse hepatitis virus (MHV) proteins to detect antibodies to coronaviruses in mouse and rat sera. All the recombinant proteins, including nucleocapsid (N) and 3 subunits of spike protein, S1, S2, and Smid, showed positive reactivity in MFI with mouse antisera to 4 MHV strains (MHV-S, -A59, -JHM, and -Nu67) and rat antiserum to a strain of sialodacryoadenitis virus (SDAV-681). The MFI was evaluated for its diagnostic power, with panels of mouse sera classified as positive or negative for anti-MHV antibodies by enzyme-linked immunosorbent assay (ELISA) using MHV virion antigen and indirect fluorescent antibody assay. The reactivities of 236 naturally infected mouse sera were examined; 227 samples were positive by MFI using S2 antigen (96% sensitivity), and 208 samples were positive using N antigen (88% sensitivity). Based on the assessment by MFI using the S2 and N antigens, only 3 serum samples showed double-negative results, indicating a false-negative rate of 1.3%. In 126 uninfected mouse sera, including 34 ELISA false-positive sera, only 7 samples showed false-positive results by MFI using either the S2 or N antigen (94% specificity). Similarly, the S2 and N antigen-based MFI was 98% sensitive and 100% specific in detecting anticoronavirus antibodies in rat sera. Thus, this MFI-based serologic assay using the S2 and N antigens promises to be a reliable diagnostic method, representing a highly sensitive and specific alternative to traditional ELISA for detection of coronavirus infections in laboratory mouse and rat colonies.

Immunogenicity of recombinant GP5 protein of porcine reproductive and respiratory syndrome virus expressed in tobacco plant

The aim of the study was to evaluate the immunogenicity of the ORF5-encoded major envelop glycoprotein 5 (GP5) of porcine reproductive and respiratory syndrome virus (PRRSV) expressed in tobacco plant as a potential pig oral vaccine in protection against PRRSV infection. Six-week-old PRRSV-free pigs were fed four times orally with 50g of chopped fresh GP5 transgenic tobacco leaves (GP5-T) (GP5 reaching 0.011% of total soluble protein) or wild-type tobacco leaves (W-T) each on days 0, 14, 28, and 42. Samples of serum, saliva, and peripheral blood mononuclear cells (PBMCs) were collected on days -1, 6, 13, 20, 27, 34, 41, and 48 after the initial oral vaccination. A similar vaccination-dependent gradual increase in the responses of serum and saliva anti-PRRSV total IgG and IgA, respectively, and in the levels of PRRSV-specific blastogenic response of PBMCs was seen in GP5-T-treated pigs; all statistically significant elevations occurred after the 2nd vaccination and were revealed after 20 days post-initial oral vaccination (DPIOV). Pigs fed on GP5-T also developed serum neutralizing antibodies to PRRSV at a titer of 1:4-1:8 after the 4th vaccination by 48 DPIOV. No detectable anti-PRRSV antibody responses and PRRSV-specific blastogenic response were seen in W-T-treated pigs. The present study has demonstrated that pigs fed on GP5-T could develop specific mucosal as well as systemic humoral and cellular immune responses against PRRSV. The results also support that transgenic plant as GP5-T can be an effective system for oral delivery of recombinant subunit vaccines in pigs.

Plant-made vaccines for humans and animals

The concept of using plants to produce high-value pharmaceuticals such as vaccines is 20 years old this year and is only now on the brink of realisation as an established technology. The original reliance on transgenic plants has largely given way to transient expression; proofs of concept for human and animal vaccines and of efficacy for animal vaccines have been established; several plant-produced vaccines have been through Phase I clinical trials in humans and more are scheduled; regulatory requirements are more clear than ever, and more facilities exist for manufacture of clinic-grade materials. The original concept of cheap edible vaccines has given way to a realisation that formulated products are required, which may well be injectable. The technology has proven its worth as a means of cheap, easily scalable production of materials: it now needs to find its niche in competition with established technologies. The realised achievements in the field as well as promising new developments will be reviewed, such as rapid-response vaccines for emerging viruses with pandemic potential and bioterror agents.

Plant production of veterinary vaccines and therapeutics

Plant-derived biologicals for use in animal health are becoming an increasingly important target for research into alternative, improved methods for disease control. Although there are no commercial products on the market yet, the development and testing of oral, plant-based vaccines is now beyond the proof-of-principle stage. Vaccines, such as those developed for porcine transmissible gastroenteritis virus, have the potential to stimulate both mucosal and systemic, as well as, lactogenic immunity as has already been seen in target animal trials. Plants are a promising production system, but they must compete with existing vaccines and protein production platforms. In addition, regulatory hurdles will need to be overcome, and industry and public acceptance of the technology are important in establishing successful products.

Intragastric administration of attenuated Salmonella typhimurium harbouring transmissible gastroenteritis virus (TGEV) DNA vaccine induced specific antibody production

Attenuated Salmonella typhimurium was selected as a transgenic vehicle for the development of live mucosal vaccines against transmissible gastroenteritis virus (TGEV). A 2.2kb DNA fragment, encoding for N-terminal domain glycoprotein S of TGEV, was amplified by RT-PCR and cloned into eukaryotic expression vector pVAX1. The recombinant plasmid pVAX-S was transformed by electroporation into attenuated S. typhimurium SL7207, the expression and translation of the pVAX-S delivered by recombinant S. typhimurium SL7207 (pVAX-S) was detected in vitro and in vivo respectively. BALB/c mice were inoculated orally with SL7207 (pVAX-S) at different dosages, the bacterium was safe to mice at dosage of 2x10(9)CFU and eventually eliminated from the spleen and liver at week 4 post-immunization. Mice immunized with different dosages of SL7207 (pVAX-S) elicited specific anti-TGEV local mucosal and humoral responses as measured by indirect ELISA assay. Moreover, the immunogenicity of the DNA vaccine was highly dependent on the dosage of the attenuated bacteria used for oral administration, 10(9)CFU dosage group showed higher antibody response than 10(8)CFU and 10(7)CFU dosages groups during week 4-8 post-immunization. The results indicated that attenuated S. typhimurium could be used as a delivery vector for oral immunization of TGEV DNA vaccine.

Plants as bioreactors for the production of vaccine antigens

Plants have been identified as promising expression systems for commercial production of vaccine antigens. In phase I clinical trials several plant-derived vaccine antigens have been found to be safe and induce sufficiently high immune response. Thus, transgenic plants, including edible plant parts are suggested as excellent alternatives for the production of vaccines and economic scale-up through cultivation. Improved understanding of plant molecular biology and consequent refinement in the genetic engineering techniques have led to designing approaches for high level expression of vaccine antigens in plants. During the last decade, several efficient plant-based expression systems have been examined and more than 100 recombinant proteins including plant-derived vaccine antigens have been expressed in different plant tissues. Estimates suggest that it may become possible to obtain antigen sufficient for vaccinating millions of individuals from one acre crop by expressing the antigen in seeds of an edible legume, like peanut or soybean. In the near future, a plethora of protein products, developed through ‘naturalized bioreactors’ may reach market. Efforts for further improvements in these technologies need to be directed mainly towards validation and applicability of plant-based standardized mucosal and edible vaccines, regulatory pharmacology, formulations and the development of commercially viable GLP protocols. This article reviews the current status of developments in the area of use of plants for the development of vaccine antigens.

Strategies for improving vaccine antigens expression in transgenic plants: fusion to carrier sequences

Transgenic plants are gaining increasing attention from the industry as a natural bioreactor for the production of industrial and chemical products. Optimization of transgene expression in plant cells holds the key to maximizing the potential of plants for producing proteins of commercial interest. This chapter is devoted to the description of the methods utilized for the generation of transgenic plants expressing a canine parvovirus vaccine peptide or virus-like particles from a rabbit calicivirus.

Chloroplast-derived vaccine antigens and biopharmaceuticals: expression, folding, assembly and functionality

Chloroplast genetic engineering offers several advantages, including high levels of transgene expression, transgene containment via maternal inheritance, and multi-gene expression in a single transformation event. Oral delivery is facilitated by hyperexpression of vaccine antigens against cholera, tetanus, anthrax, plague, or canine parvovirus (4%-31% of total soluble protein, TSP) in transgenic chloroplasts (leaves) or non-green plastids (carrots, tomato) as well as the availability of antibiotic free selectable markers or the ability to excise selectable marker genes. Hyperexpression of several therapeutic proteins, including human serum albumin (11.1% TSP), somatotropin (7% TSP), interferon-alpha (19% TSP), interferon-gamma (6% TSP), and antimicrobial peptide (21.5% TSP), facilitates efficient and economic purification. Also, the presence of chaperones and enzymes in chloroplasts facilitates assembly of complex multisubunit proteins and correct folding of human blood proteins with proper disulfide bonds. Functionality of chloroplast-derived vaccine antigens and therapeutic proteins has been demonstrated by several assays, including the macrophage lysis assay, GM1-ganglioside binding assay, protection of HeLA cells or human lung carcinoma cells against encephalomyocarditis virus, systemic immune response, protection against pathogen challenge, and growth or inhibition of cell cultures. Purification of human proinsulin has been achieved using novel purification strategies (inverse temperature transition property) that do not require expensive column chromatography techniques. Thus, transgenic chloroplasts are ideal bio-reactors for production of functional human and animal therapeutic proteins in an environmentally friendly manner.

Development of an enzyme-linked immunosorbent assay-based test with a cocktail of nucleocapsid and spike proteins for detection of severe acute respiratory syndrome-associated coronavirus-specific antibody

A new enzyme-linked immunosorbent assay (ELISA)-based immunoglobulin G (IgG)-plus-IgM antibody detection test for severe acute respiratory syndrome (SARS) has been developed by using a cocktail of four recombinant polypeptides as the antigen. These recombinant fragments were designed as parts of two different structural proteins from SARS-associated coronavirus (SARS-CoV). One recombinant polypeptide, S251-683, was designed as part of the spike glycoprotein, and the other three polypeptides comprised almost the whole nucleocapsid protein, avoiding the last 25 C-terminal amino acids. Immunization with a cocktail of these four polypeptides yielded a specific polyclonal antibody that is able to recognize SARS-CoV-infected cells by an immunofluorescence assay. This polypeptide cocktail was also used to set up an ELISA-based IgG-plus-IgM antibody detection test, which showed 99% specificity and 90% sensitivity upon evaluation using sera from 100 healthy negative controls and 20 SARS patients. Separate immunoreactivity assays with each recombinant polypeptide demonstrated that a combination of N and S protein fragments was more suitable than the individual peptides for developing a serological assay for SARS-CoV.

Foliar extracts from transgenic tomato plants expressing the structural polyprotein, P1-2A, and protease, 3C, from foot-and-mouth disease virus elicit a protective response in guinea pigs

The expression of recombinant antigens in transgenic plants is increasingly used as an alternative method of producing experimental immunogens. In this report, we describe the production of transgenic tomato plants that express the structural polyprotein, P1-2A, and protease, 3C, from foot-and-mouth disease (FMDV). P1-2A3C was inserted into the plant binary vector, pBin438, and transformed into tomato plants using Agrobacterium tumefaciens strain, GV3101. The presence of P1-2A3C was confirmed by PCR, transcription was verified by RT-PCR, and recombinant protein expression was confirmed by sandwich-ELISA and Western blot analyses. Guinea pigs immunized intramuscularly with foliar extracts from P1-2A3C-transgenic tomato plants were found to develop a virus-specific antibody response against FMDV. Vaccinated guinea pigs were fully protected against a challenge infection, while guinea pigs injected with untransformed plant extracts failed to elicit an antibody response and were not protected against challenge. These results demonstrate that transgenic tomato plants expressing the FMDV structural polyprotein, P1-2A, and the protease, 3C, can be used as a source of recombinant antigen for vaccine production.

Transgenic plants for animal health: plant-made vaccine antigens for animal infectious disease control

A variety of plant species have been genetically modified to accumulate vaccine antigens for human and animal health and the first vaccine candidates are approaching the market. The regulatory burden for animal vaccines is less than that for human use and this has attracted the attention of researchers and companies, and investment in plant-made vaccines for animal infectious disease control is increasing. The dosage cost of vaccines for animal infectious diseases must be kept to a minimum, especially for non-lethal diseases that diminish animal welfare and growth, so efficient and economic production, storage and delivery are critical for commercialization. It has become clear that transgenic plants are an economic and efficient alternative to fermentation for large-scale production of vaccine antigens. The oral delivery of plant-made vaccines is particularly attractive since the expensive purification step can be avoided further reducing the cost per dose. This review covers the current status of plant-produced vaccines for the prevention of disease in animals and focuses on barriers to the development of such products and methods to overcome them.

Immune response induced by spike protein from transmissible gastroenteritis coronavirus expressed in mouse mammary cells

The present study is undertaken to investigate the immune response that was induced by the recombinant spike (S) protein from swine-transmissible gastroenteritis virus (TGEV) expressed in mouse mammary cells. A mammary-specific expression vector pEBS containing the full-length cDNA of S gene was constructed and expressed in the mouse mammary cells (EMT6). The recombinant S protein from culture supernatant of transgenic EMT6 was harvested and immunized BALB/c mice. The results demonstrated recombinant S protein was expressed at high levels in mammary cells by Western blotting and enzyme-linked immunosorbent assay (ELISA) detection. The antibody titer in BALB/c mice following immunization with recombinant S protein was detectable after the first immunization. Maximum titers of antibody (8.86 ± 0.19 ng/ml of serum) were attained after the second immunization. In conclusion, the recombinant S protein expressed in mammary cells was able to elicit substantial immunological response against TGEV. This lays the basis for using mammary gland bioreactor generating edible vaccine.

Molecular farming for antigen (vaccine) production in plants

Genomic and proteomic approaches to the study of fundamental cell mechanisms are rapidly contributing to broaden our knowledge on metabolic pathways for the optimal exploitation of the cell as a factory. In the last few years this knowledge has led to important advances in the large scale production of diagnostic and therapeutic proteins in heterologous hosts (bacteria, yeasts, mammalian and insect cells or transgenic animals and plants), allowing the comparison of the most efficient methods in terms of costs, product quality and safety.

Tissue specific expression of hepatitis B virus surface antigen in transgenic plant cells and tissue culture

The tobacco plants (Nicotiana tabacum L.) carrying the HBsAg gene controlled by (Aocs)₃AmasPmas, the hybrid promoter that includes regulatory elements of the agrobacterial octopine and mannopine synthase genes, as well as plants controlled by the same promoter and adh1, maize alcohol dehydrogenase gene intron were obtained. The presence of the adh1 gene intron did not significantly change the level of expression of the HBsAg gene in plants. The analysis of expression of hepatitis B virus surface antigen (HBs-antigen) in transformed plants expressing the HBsAg under the control of different promoters was made. The level of HBs-antigen in plants carrying the HBsAg gene controlled by (Aocs)₃AmasPmas, the hybrid agrobacterium-derived promoter, was the highest in roots and made up to 0.01% of total amount of soluble protein. The level of HBs-antigen in plants carrying the HBsAg gene controlled by the dual 35S RNA cauliflower mosaic virus promoter was the same in all organs of the plant and made up to 0.06% of the total amount of soluble protein. Hairy root and callus cultures of plants carrying the HBsAg gene and expressing the HBs-antigen were obtained.

Ca2+-binding allergens from olive pollen exhibit biochemical and immunological activity when expressed in stable transgenic Arabidopsis

Employing transgenic plants as alternative systems to the conventional Escherichia coli, Pichia pastoris or baculovirus hosts to produce recombinant allergens may offer the possibility of having available edible vaccines in the near future. In this study, two EF-hand-type Ca2+-binding allergens from olive pollen, Ole e 3 and Ole e 8, were produced in transgenic Arabidopsis thaliana plants. The corresponding cDNAs, under the control of the constitutive CaMV 35S promoter, were stably incorporated into the Arabidopsis genome and encoded recombinant proteins, AtOle e 3 and AtOle e 8, which exhibited the molecular properties (i.e. MS analyses and CD spectra) of their olive and/or E. coli counterparts. Calcium-binding assays, which were carried out to assess the biochemical activity of AtOle e 3 and AtOle e 8, gave positive results. In addition, their mobilities on SDS/PAGE were according to the conformational changes derived from their Ca2+-binding capability. The immunological behaviour of Arabidopsis-expressed proteins was equivalent to that of the natural- and/or E. coli-derived allergens, as shown by their ability to bind allergen-specific rabbit IgG antiserum and IgE from sensitized patients. These results indicate that transgenic plants constitute a valid alternative to obtain allergens with structural and immunological integrity not only for scaling up production, but also to develop new kind of vaccines for human utilization.

Antibodies against trimeric S glycoprotein protect hamsters against SARS-CoV challenge despite their capacity to mediate FcgammaRII-dependent entry into B cells in vitro

Vaccine-induced antibodies can prevent or, in the case of feline infectious peritonitis virus, aggravate infections by coronaviruses. We investigated whether a recombinant native full-length S-protein trimer (triSpike) of severe acute respiratory syndrome coronavirus (SARS-CoV) was able to elicit a neutralizing and protective immune response in animals and analyzed the capacity of anti-S antibodies to mediate antibody-dependent enhancement (ADE) of virus entry in vitro and enhancement of replication in vivo. SARS-CoV-specific serum and mucosal immunoglobulins were readily detected in immunized animals. Serum IgG blocked binding of the S-protein to the ACE2 receptor and neutralized SARS-CoV infection in vitro. Entry into human B cell lines occurred in a FcγRII-dependent and ACE2-independent fashion indicating that ADE of virus entry is a novel cell entry mechanism of SARS-CoV. Vaccinated animals showed no signs of enhanced lung pathology or hepatitis and viral load was undetectable or greatly reduced in lungs following challenge with SARS-CoV. Altogether our results indicate that a recombinant trimeric S protein was able to elicit an efficacious protective immune response in vivo and warrant concern in the safety evaluation of a human vaccine against SARS-CoV.

Select what you need: a comparative evaluation of the advantages and limitations of frequently used expression systems for foreign genes

The expression of heterologous proteins in microorganisms using genetic recombination is still the high point in the development and exploitation of modern biotechnology. People can produce bioactive proteins from relatively cheap culture medium instead of expensive extraction. Host cell systems for the expression of heterologous genes are generally prokaryotic or eukaryotic systems, both of which have inherent advantages and drawbacks. An optimal expression system can be selected only if the productivity, bioactivity, purpose, and physicochemical characteristics of the interest protein are taken into consideration, together with the cost, convenience and safety of the system itself. Here, we concisely review the most frequently used prokaryotic, yeast, insect and mammalian expression systems, as well as expression in eukaryote individuals. The merits and demerits of these systems are discussed.

Assembly of pseudorabies virus genome-based transfer vehicle carrying major antigen sites of S gene of transmissible gastroenteritis virus: potential perspective for developing live vector vaccines

Two severe porcine infectious diseases, pseudorabies (PR) and transmissible gastroenteritis (TGE) caused by pseudorabies virus (PRV) and transmissible gastroenteritis virus (TGEV) respectively often result in serious economic loss in animal husbandry worldwide. Vaccination is the important prevention means against both infections. To achieve a PRV genome-based virus live vector, aiming at further TGEV/PRV bivalent vaccine development, a recombinant plasmid pUG was constructed via inserting partial PK and full-length gG genes of PRV strain Bartha K-61 amplified into pUC119 vector. In parallel, another recombinant pHS was generated by introducing a fragment designated S1 encoding the major antigen sites of S gene from TGEV strain TH-98 into a prokaryotic expression vector pP_ROEX HTc. The SV40 polyA sequence was then inserted into the downstream of S1 fragment of pHS. The continuous region containing S1fragment, SV40 polyA and four single restriction enzyme sites digested from pHS was subcloned into the downstream of gG promoter of pUG. In addition, a LacZ reporter gene was introduced into the universal transfer vector named pUGS-LacZ. Subsequently, a PRV genome-based virus live vector was generated via homologous recombination. The functionally effective vector was purified and partially characterized. Moreover, the potential advantages of this system are discussed.

Transgenic plants in the biopharmaceutical market

Many of our 'small-molecule-drugs' are natural products from plants, or are synthetic compounds based on molecules found naturally in plants. However, the vast majority of the protein therapeutics (or biopharmaceuticals) we use are from animal or human sources, and are produced commercially in microbial or mammalian bioreactor systems. Over the last few years, it has become clear that plants have great potential for the production of human proteins and other protein-based therapeutic entities. Plants offer the prospect of inexpensive biopharmaceutical production without sacrificing product quality or safety, and following the success of several plant-derived technical proteins, the first therapeutic products are now approaching the market. In this review, the different plant-based production systems are discussed and the merits of transgenic plants are evaluated compared with other platforms. A detailed discussion is provided of the development issues that remain to be addressed before plants become an acceptable mainstream production technology. The many different proteins that have already been produced using plants are described, and a sketch of the current market and the activities of the key players is provided. Despite the currently unclear regulatory framework and general industry inertia, the benefits of plant-derived pharmaceuticals are now bringing the prospect of inexpensive veterinary and human medicines closer than ever before.

Expression of avian reovirus sigmaC protein in transgenic plants

Avian reovirus (ARV) structural protein, sigmaC encoded by S1 genome segment, is the prime candidate to become a vaccine against ARV infection. Two plant nuclear expression vectors with expression of sigmaC-encoding gene driven by CaMV 35S promoter and rice actin promoter were constructed, respectively. Agrobacterium containing the S1 expression constructs were used to transform alfalfa, and transformants were selected using hygromysin. The integration of S1 transgene in alfalfa chromosome was confirmed by PCR and histochemical GUS staining. Western blot analysis using antiserum against sigmaC was carried out to determine the expression of sigmaC protein in transgenic alfalfa cells. The highest expression levels of sigmaC protein in the cellular extracts of selected p35S-S1 and pAct1-S1 transgenic alfalfa lines were 0.008% and 0.007% of the total soluble protein, respectively. The transgenic alfalfa cells with expression of sigmaC protein pave the way for the development of edible vaccine.

Successful oral prime-immunization with VP60 from rabbit haemorrhagic disease virus produced in transgenic plants using different fusion strategies

Expression levels of vaccine antigens in transgenic plants have important consequences in their use as edible vaccines. The major structural protein VP60 from the rabbit haemorrhagic disease virus (RHDV) has been produced in transgenic plants using different strategies to compare its accumulation in plant tissues. The highest expressing plants were those presenting stable, complex, high-density structures formed by VP60, suggesting the importance of multisubunit structures for the stability of this protein in plant cells. Mice fed with leaves of transgenic plants expressing VP60 were primed to a subimmunogenic baculovirus-derived vaccine single dose. This indicates that plants expressing VP60 antigen may be a new means for oral RHDV immunization.

Cloning of Helicobacter pylori urease subunit B gene and its expression in tobacco (Nicotiana tabacum L.)

Vaccines produced by transgenic plants would have the potential to change the traditional means of production and inoculation of vaccines, and to reduce the cost of vaccine production. In the present study, an UreB antigen gene from a new Helicobacter pylori strain ZJC02 was cloned into the binary vector pBI121 which contains a CaMV35S promoter and a kanamycin resistance gene, and then transformed UreB into tobacco leaf-disc by Agrobacterium-mediated method. A total of 50 regenerated plants with kanamycin resistance were obtained in the selection media. The 35 putative transgenic individuals were tested and verified the presence and integration of the UreB into the nuclear genome of tobacco plants by PCR, PCR-southern, and Southern analyses. Expression of UreB gene in the tobacco plants was confirmed by RT-PCR and Western Blot analysis using polyclonal human antiserum. To our knowledge, this is the first report of the expression of Helicobacter pylori UreB antigen gene in a plant system, suggesting a major step in the production of plant-based vaccines for Helicobacter pylori.

Intragastric administration of Lactobacillus casei expressing transmissible gastroentritis coronavirus spike glycoprotein induced specific antibody production

Lactobacillus casei strain Shirota was selected as a bacterial carrier for the development of live mucosal vaccines against coronavirus. A 75 kDa fragment of transmissible gastroenteritis coronavirus (TGEV) spike glycoprotein S was used as the model coronavirus antigen. The S glycoprotein was cloned into a Lactobacillus/E. coli shuttle vector (pLP500) where expression and secretion of the glycoprotein S from the recombinant lactobacilli was detected via immunoblotting. Oral immunization of BALB/c mice with recombinant LcS that constitutively expresses the 75 kDa fragment of the glycoprotein S, induced both local mucosal and systemic immune responses against TGEV. Maximum titers of IgG (8.38+/-0.19 ng/ml of serum) and IgA (64.82+/-2.9 ng/ml of intestinal water) were attained 32 days post oral inturbation. The induced antibodies demonstrated neutralizing effects on TGEV infection.

Expression of immunogenic VP2 protein of infectious bursal disease virus in Arabidopsis thaliana

VP2 protein is the major host-protective immunogen of infectious bursal disease virus (IBDV) of chickens. Transgenic lines of Arabidopsisthaliana expressing recombinant VP2 were developed. The VP2 gene of an IBDV antigenic variant E strain was isolated, amplified by RT-PCR and introduced into a plant expression vector, pE1857, having a strong promoter for plant expression. A resulting construct with a Bar gene cassette for bialaphos selection in plant (rpE-VP2) was introduced into Agrobacterium tumefaciensby electroporation. Agrobacterium containing the rpE-VP2 construct was used to transform Ar. thaliana and transgenic plants were selected using bialaphos. The presence of VP2 transgene in plants was confirmed by PCR and Southern blot analysis and its expression was confirmed by RT-PCR. Western blot analysis and antigen-capture ELISA assay using monoclonal anti-VP2 were used to determine the expression of VP2 protein in transgenic plants. The level of VP2 protein in the leaf extracts of selected transgenic plants varied from 0.5% to 4.8% of the total soluble protein. Recombinant VP2 protein produced in plants induced antibody response against IBDV in orally-fed chickens.

High-yield expression of recombinant SARS coronavirus nucleocapsid protein in methylotrophic yeast Pichia pastoris

AIM: Nucleocapsid (N) protein plays an important role in reproduction and pathological reaction of severe acute respiratory syndrome (SARS) coronavirus (SCoV), the antigenicity of the protein is better than spike (S) protein. This study was to find a highly specific and antigenic recombinant SCoV nucleocapsid (rSCoVN) protein, and to provide a basis for further researches on early diagnosis of SARS.

METHODS: Full length cDNA of SCoV nucleocapsid (SCoVN) protein was amplified through polymerase chain reaction (PCR) and cloned into yeast expression vector pPIC3.5K to construct plasmid of pPIC3.5K-SCoVN. The plasmid was linearized and then transformed into Pichia pastoris (P.pastoris) GS115 (His^-Mut⁺) by electroporation. His⁺Mut⁺ recombinant strains were identified by PCR and cultivated on MM/MD plates. The influence of different factors on biomass and rSCoVN protein production during induction phase, such as various induction media, dissolved oxygen (DO) and different final concentrations of methanol, was subsequently studied. The expression level and activation were detected by SDS-PAGE and Western-blot respectively.

RESULTS: All of the recombinants were His⁺Mut⁺ after transformation of P.pastoris with linearized plasmids. The BMMY medium was optimal for recombinant ScoVN (rSCoVN) protein expression and growth of the recombinant strains. The final optimal concentration of methanol was 20 mL/L, the DO had a significant effect on rSCoVN protein expression and growth of recombinant strains. The rSCoVN protein expressed in recombinant strains was about 8% of the total cell protein, 520 mg/L of rSCoVN protein was achieved, and a maximum cell A at 600 nm of 62 was achieved in shake flask culture. The rSCoVN protein had a high specificity against mouse-anti-SARS-CoVN-mAb and SARS positive sera, but had no cross-reaction with normal human serum. The biological activity of rSCoVN expressed in P.pastoris was about 4-fold higher than that expressed in E.coli when the same rSCoVN protein quantity was used.

CONCLUSION: Active recombinant severe acute respiratory syndrome (SARS) coronavirus nucleocapsid (rSCoVN) protein can be successfully expressed in recombinant methylotrophic yeast P.pastoris GS115. The rSCoVN protein has a high specificity against SARS-CoVN-mAb and SARS positive sera, but has no cross-reaction with normal human serum. This provides a basis for further researches on the early diagnosis of SARS and the mechanism of SCoV.

Evaluation of a safe and sensitive Spike protein-based immunofluorescence assay for the detection of antibody responses to SARS-CoV

Previously, we have identified a truncated antigenic fragment named protein C [441 to 700 amino acids (a.a.)] as the immunodominant fragment of Spike (S) protein of severe acute respiratory syndrome (SARS) coronavirus (SARS-CoV). We have now successfully expressed protein C using the baculovirus system in S. frugiperda (Sf-9) cells. This recombinant baculovirus expressing protein C was first characterized using five SARS convalescent human sera and five normal human sera. The results showed that protein C is an authentic antigen against SARS-CoV antibody. Our Spike protein-based immunoflourescence assay (IFA) based on this recombinant baculovirus-Sf-9 system was further assessed with a panel of 163 clinical samples collected during the SARS epidemic in Singapore, which include samples from 21 clinically confirmed SARS, 42 non-SARS patient sera, and 100 normal sera. The results were compared to a commercial SARS IFA kit (EUROIMMUN, Germany) and a conventional IFA test performed in Singapore General Hospital. All of the 21 SARS-positive serum samples could be recognized by our IFA, giving a specificity and sensitivity of 100%, which was compatible with both whole virus-based IFA assays. No cross-reactivity with serum samples against infectious bronchitis virus (IBV) and transmissible gastroenteritis virus (TGEV) were detected in our assays. Thus, our Spike protein-based IFA could offer a safer procedure which can be performed in a BSL-2 laboratory as it could mimic the whole virus based-IFA without any loss of sensitivity and specificity. It is also more user-friendly and cost-effective than the whole virus-based IFA.

Expression of non-toxic mutant of Escherichia coli heat-labile enterotoxin in tobacco chloroplasts

Chloroplast transformation systems offer unique advantages in biotechnology, including high level of foreign gene expression, maternal inheritance, and polycistronic expression. We studied chloroplast expression of LTK63 (change Ser-->Lys at position 63 in the A subunit) which is the mutant of Escherichia coli heat-labile toxin. LTK63 is devoid of any toxic activity, but still retains its mucosal adjuvanticity. The LTK63 was cloned into chloroplast targeting vector and transformed to tobacco chloroplasts by particle bombardment. PCR and Southern blot analyses confirmed stable homologous recombination of the LTK63 gene into the chloroplast genome. The amount of LTK63 protein detected in tobacco chloroplasts was approximately 3.7% of the total soluble protein. The GM1-ganglioside binding assay confirmed that chloroplast-synthesized LTB of LTK63 binds to the intestinal membrane GM1-ganglioside receptor. Thus, the expression of LTK63 in chloroplasts provides a potential route toward the development of a plant-based edible vaccine for high expression system and environmentally friendly approach.

Immunological characterization of the spike protein of the severe acute respiratory syndrome coronavirus

Severe acute respiratory syndrome (SARS) is a novel infectious disease caused by the SARS-associated coronavirus (SARS-CoV). There are four major structural proteins in the SARS-CoV, including the nucleocapsid, spike, membrane, and small envelope proteins. In this study, two sets of truncated fragments of spike protein were generated, the first were approximately 210-bp nonoverlapping fragments and the second were overlapping segments of 750 to 900 bp. From these 23 fragments, we identified a fragment of 259 amino acids (amino acids 441 to 700) that is a major immunodominant epitope. This fragment was highly expressed, and the purified fragment C could detect all 33 SARS patient serum samples tested, collected from 7 to 60 days after the onset of fever, but had no reactivity with all 66 healthy human serum samples tested. Thus, fragment C of spike protein was identified as an immunodominant antigen and could be used for serological detection of SARS-CoV infection.

Expression of the hepatitis B surface S and preS2 antigens in tubers of Solanum tuberosum

In an attempt to develop an edible vaccine, we transformed a recombinant hepatitis B virus (HBV) gene encoding the middle protein of HBV that contains the surface S and preS2 antigen into potato by Agrobacterium-mediated transformation. The HBV gene was under control of either the CaMV 35S promoter, the double 35S promoter with the AlMV 5' non-translated leader sequence, or the tuber-specific patatin promoter. HBV mRNA levels were higher with the 35S promoter than with the double 35S and patatin promoters; however, the levels of the S and preS2 antigen in the transformed tubers were higher with the patatin promoter than with the CaMV 35S and double promoters. The levels of preS2 antigen produced are the highest reported to date. Transgenic potato tubers were fed to mice, and the mice showed an immune response against the HBV S antigen.

Protective lactogenic immunity conferred by an edible peptide vaccine to bovine rotavirus produced in transgenic plants

Vaccines produced in transgenic plants constitute a promising alternative to conventional immunogens, presenting the possibility of stimulating secretory and systemic immunity against enteric pathogens when administered orally. Protection against enteric pathogens affecting newborn animals requires, in most cases, the stimulation of lactogenic immunity. Here, the group presents the development of an experimental immunogen based on expression of an immunorelevant peptide, eBRV4, of the VP4 protein of bovine rotavirus (BRV), which has been described as harbouring at least one neutralizing epitope as well as being responsible for the adsorption of the virus to epithelial cells. The eBRV4 epitope was efficiently expressed in transgenic alfalfa as a translational fusion protein with the highly stable reporter enzyme β-glucuronidase (βGUS), which served as a carrier, stabilized the synthesized peptide and facilitated screening for the higher expression levels in plants. Correlation of expression of the eBRV4 epitope in plants with those presenting the highest βGUS activities was confirmed by a Western blot assay specific for the BRV peptide. The eBRV4 epitope expressed in plants was effective in inducing an anti-rotavirus antibody response in adult female mice when administered either intraperitoneally or orally and, more importantly, suckling mice born from immunized female mice were protected against oral challenge with virulent rotavirus. These results demonstrate the feasibility of inducing lactogenic immunity against an enteric pathogen using an edible vaccine produced in transgenic plants.

Systemic and oral immunogenicity of hemagglutinin protein of rinderpest virus expressed by transgenic peanut plants in a mouse model

Rinderpest causes a devastating disease, often fatal, in wild and domestic ruminants. It has been eradicated successfully using a live, attenuated vaccine from most part of the world leaving a few foci of disease in parts of Africa, the Middle East, and South Asia. We have developed transgenic peanut (Arachis hypogaea L.) plants expressing hemagglutinin (H) protein of rinderpest virus (RPV), which is antigenically authentic. In this work, we have evaluated the immunogenicity of peanut-expressed H protein using mouse model, administered parenterally as well as orally. Intraperitoneal immunization of mice with the transgenic peanut extract elicited antibody response specific to H. These antibodies neutralized virus infectivity in vitro. Oral immunization of mice with transgenic peanut induced H-specific serum IgG and IgA antibodies. The systemic and oral immunogenicity of plant-derived H in absence of any adjuvant indicates the potential of edible vaccine for rinderpest.

Recent developments in the use of transgenic plants for the production of human therapeutics and biopharmaceuticals

In recent years there has been a dramatic increase in the application of plant biotechnology for the production of a variety of commercially valuable simple and complex biological molecules (biologics) for use in human and animal healthcare. Transgenic whole plants and plant cell culture systems have been developed that have the capacity to economically produce large-scale quantities of antibodies and antibody fragments, antigens and/or vaccine epitopes, metabolic enzymes, hormones, (neuro)peptides and a variety of biologically active complexes and secondary metabolites for direct use as therapeutic agents or diagnostic tools in the medical healthcare industry. As the products of genetically modified plants make their way from concept to commercialization the associated risks and acceptance by the public has been become a focal point. In this paper, we summarize the recent advances made in the use of transgenic plants and plant cell cultures as biological factories for the production of human therapeutics and biopharmaceuticals and discuss the long-term potential of `molecular farming' as a low-cost, efficient method for the production of biological materials with demonstrated utility to the pharmaceutical industry or medical community.

Expression of the B subunit of E. coli heat-labile enterotoxin in the chloroplasts of plants and its characterization

Transgenic chloroplasts have become attractive systems for heterologous gene expressions because of unique advantages. Here, we report a feasibility study for producing the nontoxic B subunit of Escherichia coli heat-labile enterotoxin (LTB) via chloroplast transformation of tobacco. Stable site-specific integration of the LTB gene into chloroplast genome was confirmed by PCR and genomic Southern blot analysis in transformed plants. Immunoblot analysis indicated that plant-derived LTB protein was oligomeric, and dissociated after boiling. Pentameric LTB molecules were the dominant molecular species in LTB isolated from transgenic tobacco leaf tissues. The amount of LTB protein detected in transplastomic tobacco leaf was approximately 2.5% of the total soluble plant protein, approximately 250-fold higher than in plants generated via nuclear transformation. The GM1-ELISA binding assay indicated that chloroplast-synthesized LTB protein bound to GM1-ganglioside receptors. LTB protein with biochemical properties identical to native LTB protein in the chloroplast of edible plants opens the way for inexpensive, safe, and effective plant-based edible vaccines for humans and animals.

Expression of immunogenic S1 glycoprotein of infectious bronchitis virus in transgenic potatoes

The expression of infectious bronchitis virus (IBV) S1 glycoprotein in potatoes and its immunogenicity in mice and chickens were investigated. Potato plants were genetically transformed with a cDNA construct encoding the IBV S1 glycoprotein with the Agrobacterium system. Genomic DNA and mRNA analyses of the transformed plantlets confirmed the integration of the foreign cDNA into the potato genome, as well as its transcription. Mice and chickens vaccinated with the expressed IBV S1 glycoprotein produced antibodies that neutralized IBV infectivity. After three immunizations, vaccinated chickens were completely protected from virulent IBV infection. These results demonstrate that transgenic potatoes expressing IBV S1 glycoprotein can be used as a source of recombinant antigen for vaccine production.

Comparative full-length genome sequence analysis of 14 SARS coronavirus isolates and common mutations associated with putative origins of infection

BACKGROUND

The cause of severe acute respiratory syndrome (SARS) has been identified as a new coronavirus. Whole genome sequence analysis of various isolates might provide an indication of potential strain differences of this new virus. Moreover, mutation analysis will help to develop effective vaccines.

METHODS

We sequenced the entire SARS viral genome of cultured isolates from the index case (SIN2500) presenting in Singapore, from three primary contacts (SIN2774, SIN2748, and SIN2677), and one secondary contact (SIN2679). These sequences were compared with the isolates from Canada (TOR2), Hong Kong (CUHK-W1 and HKU39849), Hanoi (URBANI), Guangzhou (GZ01), and Beijing (BJ01, BJ02, BJ03, BJ04).

FINDINGS

We identified 129 sequence variations among the 14 isolates, with 16 recurrent variant sequences. Common variant sequences at four loci define two distinct genotypes of the SARS virus. One genotype was linked with infections originating in Hotel M in Hong Kong, the second contained isolates from Hong Kong, Guangzhou, and Beijing with no association with Hotel M (p<0.0001). Moreover, other common sequence variants further distinguished the geographical origins of the isolates, especially between Singapore and Beijing.

INTERPRETATION

Despite the recent onset of the SARS epidemic, genetic signatures are emerging that partition the worldwide SARS viral isolates into groups on the basis of contact source history and geography. These signatures can be used to trace sources of infection. In addition, a common variant associated with a non-conservative aminoacid change in the S1 region of the spike protein, suggests that immunological pressures might be starting to influence the evolution of the SARS virus in human populations.