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risks and safety of transgenic plant systems (part i)

There are several advantages to using transgenic plant systems rather than bacteria or mammalian cells in the production of immune and immune-related compounds. Foremost among these is the fact that transgenic plants can produce recombinant proteins at high yields and lower costs^[1]. This is usually achieved through the genetic "tweaking" of these proteins. For example Ansari et al. increased the production of a G surface glycoprotein from the rabies virus in one study by replacing the signalling sequence that is normally found in the rabies virus with one that is used by plants^[2]. The G protein was expressed in up to 30% of the leaves in the plant. This strategy has also proven successful in antibody production^[3,4]. Schouten et al. demonstrated that the insertion of a C-terminal KDEL sequence into the single chain antibody fragment increased antibody expression in transgenic tobacco plants^[4]. Therefore, transgenic plants systems are capable of producing proteins at high yields or can be altered to do so with genetic engineering.

Plant systems do not transmit animal pathogens and glycosylate their proteins

The second advantage of transgenic plant systems is that they are similar enough to mammalian cells to generate glycosylated proteins, but not similar enough to transmit human pathogens^[5]. The transmission of small nonenveloped viruses, such as parvoviruse B19 and hepatitis A virus (HAV), and prions⁶. B19 is a virus that infects human red blood cells. Though infections are associated with flu-like or no symptoms, current research has shown an association between active B19 infection and cardiac disease^[7]. HAV is caused by a small RNA virus that destroys liver cells and is transmitted through contaminated food or water^[8]. Prions cause neurodegenerative diseases such as mad cow disease or bovine spongiform encephaloapthy (BSE) in which brain matter is destroyed to form holes within the brain^[9].

Oral application of plant-produced proteins can prevent unwanted immune reactions

But differences in glycosylation between plant and mammalian systems^[10], however, raise questions about the safety of these transgenic plant systems since these improperly glycosylated proteins may induce unwanted immune reactions in the host when used in immune therapy^[11]. Research on ways to "humanize" these plant antibodies is, however, sparse. One technique that is currently used to minimize unwanted immune reactions to transgenic plant proteins is the oral application of these recombinant proteins. An interesting example of this is the oral use of glutamic acid decarboxylase (GAD) produced in transgenic tobacco and carrot plants in the treatment of type 1 diabetes (T1D) in non-obese diabetic (NOD) mice^[12,13]. GAD is one of the antigens postulated to be involved in type 1 diabetes, in which the individual's immune system recognizes GAD as being foreign, mounts an immune response against it and destroys the β-cells of the pancreas^[13]. GAD is used in TD1 patients to prime the immune system, which will gradually induce tolerance to GAD. The oral application of GAD is important in the treatment of T1D because it reduces the severity of the immune response: had GAD been injected into the patient, a more severe and systemic immune response would have been mounted. In general, however, more research on the glycosylation of plantibodies should be done to further improve the safety of these antibodies.

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