By 1995 researchers attempting to answer the many questions before them had established that plants could indeed manufacture foreign antigens in their proper conformations. For instance, Arntzen and his colleagues had introduced into tobacco plants the gene for a protein derived from the hepatitis B virus and had gotten the plants to synthesize the protein. When they injected the antigen into mice, it activated the same immune system components that are activated by the virus itself. (Hepatitis B can damage the liver and contribute to liver cancer.)
Green Lights on Many Fronts
BUT INJECTION is not the aim; feeding is. In the past 10 years experiments conducted by Arntzen, now at Arizona State University, and his collaborators and by my group at Loma Linda University have demonstrated that tomato or potato plants can synthesize antigens from the Norwalk virus, enterotoxigenic E. coli, V. cholerae, rotavirus, HIV, anthrax, shigella and the hepatitis B virus. Moreover, feeding antigen-laced tubers or fruits to test animals can evoke mucosal and systemic immune responses that fully or partly protect animals from subsequent exposure to the real pathogens or, in the case of V. cholerae and enterotoxigenic E. coli, to microbial toxins. Edible vaccines have also provided laboratory animals with some protection against challenge by the rabies virus, Helicobacter pylori (a bacterial cause of ulcers) and the mink enteric virus (which does not affect humans).
It is not entirely surprising that antigens delivered in plant foods survive the trip through the stomach well enough to reach and activate the mucosal immune system. The tough outer wall of plant cells apparently serves as temporary armor for the antigens, keeping them relatively safe from gastric secretions. When the wall finally begins to break up in the intestines, the cells gradually release their antigenic cargo.
Of course, the key question is whether food vaccines can be useful in people. The era of clinical trials for this technology is just beginning. Nevertheless, Arntzen and his collaborators obtained reassuring results in the first published human trial, involving about a dozen subjects. In 1997 volunteers who ate pieces of peeled, raw potatoes containing a benign segment of the E. coli toxin (the part called the B subunit) displayed both mucosal and systemic immune responses. Since then, the group has also seen immune reactivity in 19 of 20 people who ate a potato vaccine aimed at the Norwalk virus. Similarly, after Hilary Koprowski of Thomas Jefferson University fed transgenic lettuce carrying a hepatitis B antigen to three volunteers, two of the subjects displayed a good systemic response. Whether edible vaccines actually can protect against human disease remains to be determined, however.
Still to Be Accomplished
IN SHORT, the studies completed so far in animals and people have provided a proof of principle; they indicate that the strategy is feasible. Yet many issues must still be addressed. For one, the amount of vaccine made by a plant is low. Production can be increased in different ways--for instance, by linking antigen genes with regulatory elements known to help switch on the genes more readily or by engineering chloroplasts to manufacture more vaccine. As researchers solve that challenge, they will also have to ensure that any given amount of a vaccine food provides a predictable dose of antigen.
Additionally, workers could try to enhance the odds that antigens will activate the immune system instead of passing out of the body unused. General stimulators (adjuvants) and better targeting to the immune system might compensate in part for low antigen production.
One targeting strategy involves linking antigens to molecules that bind well to immune system components known as M cells in the intestinal lining. M cells take in samples of materials that have entered the small intestine (including pathogens) and pass them to other cells of the immune system, such as antigen-presenting cells. Macrophages and other antigen-presenting cells chop up their acquisitions and display the resulting protein fragments on the cell surface. If white blood cells called helper T lymphocytes recognize the fragments as foreign, they may induce B lymphocytes (B cells) to secrete neutralizing antibodies and may also help initiate a broader attack on the perceived enemy.