A fragment of the virus that causes severe acute respiratory syndrome (SARS) has been expressed in transgenic tomatoes and tobacco plants. The work marks a key step towards developing a vaccine against the disease, claim US biotechnologists. 

'In view of a recent spread of SARS, there is a high demand for production of a vaccine to prevent this disease,' writes Hilary Koprowski, director of the biotechnology foundation laboratories at Thomas Jefferson University, US. Expressing a SARS immunogenic protein in plants would offer a safe, effective and cheap vaccine candidate, he says. 

Koprowski has a strong record in vaccine development, being recognised as creator of the live polio vaccine, developer of the rabies vaccine, and a pioneer of the diagnostic and therapeutic use of monoclonal antibodies.   

His team confirmed expression of the N-terminal fragment of the SARS coronavirus spike protein in tomato and tobacco plant genomes by PCR and RT-PCR.   

Feeding or injecting mice with material from these plants was found to trigger an anti-SARS immune response, suggesting that the fragment was immunogenic.   

The research community has ploughed considerable resources into the development of a SARS vaccine. The choice of the SARS coronavirus spike protein was made following studies by several other groups, which had concluded this would be the best candidate for recombinant vaccine development.   

Earlier vaccines developed in this way using related viruses were shown to induce strong protective immunity in animal models, says Koprowski. 

The study demonstrates the successful expression of the SARS virus S protein in transgenic plants in amounts sufficient to induce antibody response after feeding mice with transgenic material, concludes Koprowski. Mice that were injected with the plant-derived antigen developed an immune response after a booster immunisation. 

'Progress toward the goal of establishing a safe and inexpensive vaccination programme awaits further study with other animals and optimisation of feeding protocols that might include, for example, oral administration of subunit vaccine expressed in tomato fruit followed by a boost with tobacco-derived antigen,' his team concludes. Bea Perks