Image: Prasad et al., Science CAPSID PROTEIN from the Norwalk virus forms particles that are candidates for vaccine development: they evoke an immune response without causing infection.

In July, Arntzen and his colleagues published their most recent clinical trial of an edible vaccine in theJournal of Infectious Diseases, using the capsid protein from the diarrhea-causing Norwalk virus as the test antigen. Ninety dimeric subunits of this protein form the outer shell of the virus and assemble spontaneously into viruslike particles. These particles, which lack genetic material, make an attractive candidate for a vaccine: unlike the virus, they do not cause disease, but they do evoke an immune response.

This new work is not the first to employ the viruslike particles from the Norwalk virus. Earlier immunization studies tested the efficacy of isolated particles, which had been produced in insect cells. But Arntzen's team took a novel approach: they engineered the gene for the capsid protein into potato plants, which then produced up to one milligram of viral protein in 200 grams of potatoes. In potato extracts, the scientists found that 25 to 50 percent of the viral protein had assembled into the particles presumed crucial for producing an immune response.

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