Sunday, 18 January 2009

An edible vaccine for Hepatitis B - part 3

Part 3 - 2008 and into the future

If you're keen you could go back to read part 1 and part 2. They describe progress on vaccines against Hepatitis B virus from genetic modification of yeast to the production of vaccine in edible plants like potato, lettuce and tomato.

The technique for genetic modification I've described so far produces 'stably transformed' plants. This is enabled by the wonderful ability of the plant kingdom to grow a whole new plant out of a mature cell of an adult plant instead of doing the pollen-egg-seed sexual fertilisation thing.

The fragment of virus DNA (the antigen) plus its promoters and targeting sequences that you've constructed are packaged up and fired into plant cells. You can then pick out just those plant cells that have successfully taken up the new DNA and integrated it into their own, and grow new plants that incorporate your gene for virus antigen in every cell, including the pollen and egg cells. So your transformed plants produce a new generation of transformed plants of their own, however you choose to propagate, by cuttings or transferring pollen or whatever.

There's another way to get your carefully constructed DNA into a plant, and that's by using plant viruses. The way that viruses work is that they hijack a host's cells in order to reproduce and spread, so if you put your Hepatitis antigen DNA into a plant virus and then infect a plant with it, the plant produces your desired protein as well as propagating the virus. It's called 'transient transformation' because unless the virus gets to every cell in the plant (which would probably kill it), the viral DNA plus your new DNA isn't incorporated into the whole plant and isn't passed on to the next generation.

The advantage of this transient transformation technique using plant viruses is that you get lots of protein very quickly. To start with, they found that the levels dropped off very rapidly, as if the plant had worked out that something was going on and mobilised defences against it. This leads us to an area of genetic research called 'gene silencing' and a whole new topic that I won't go into here (and in the end, I left it out of my critique too). A way to combat this gene silencing effect was found, and let's leave it at that.

Virus-infected leafMost of this viral transformation work has been done with non-food plants, like our friend tobacco. One study, though, used a virus that can infect edible plants like cucumber, and perhaps we'll be able to eat the virus-infected plant to get immunised (the study hasn't reached that conclusion yet). Another used a fancy new viral transfection technique, admittedly on tobacco leaves again, but managed to produce extraordinary high levels of Hepatitis B antigen in only 14 days.

The interesting thing about this last study is that it was done by the same team that published the original paper I was reviewing, when they were expressing the antigen in potato and feeding it to mice. They are still working on an edible vaccine, but I think the concentrations they are getting are so low and it takes so long to grow the different plants that this parallel line of research is quite attractive, Perhaps producing Hepatitis B antigen by this viral transformation might form a bridge between the yeast-derived vaccine and a future stably transformed plant supplying an edible vaccine.

Then there are one or two other issues that will need to be sorted out for an edible vaccine. In the paper describing this success with viral transformation, they write: "Although our original research goal was to provide inexpensive, easily administered vaccines in the form of edible transgenic plant material, it is realized that plant-derived vaccines, like any other drug, will be subject to the strict regulations of the US Food and Drug Administration or other similar national agencies. Requirements, including dose standardization, must be met before a plant-derived vaccine is approved. To ensure dosing consistency, vaccine antigen-expressing plant materials must be subjected to downstream processing to some extent, including steps ranging from simple freeze-drying to chromatography purification."

For my coursework I had to put all of this into 2000 words. Obviously I left out some of the explanation, but I had to put in quite a lot more technical detail. I read more than 30 scientific papers, and referenced 20 of them in the final version. I have absolutely no idea whether I will get high, middling or low marks for this bit of work, because it will depend on whether I've interpreted the assignment correctly and written about what they expected me to write about with the correct amount of detail and accurate referencing. I've found it thoroughly fascinating, though, and it all stands me in good stead for the exam.

The last lecture in this module was delivered by one of the team who was responsible for Dolly the cloned sheep. It's been the only lecture that touched on genetic modification of animals; all the rest has been about plants. It's been the best module of the six I've been doing this semester, and isn't even part of the core syllabus. Psychology has been very interesting too, and I've blogged a little bit about that. The modules about the effects of nutrition, exercise and hormones on metabolism have also been good.

The other two modules have only been OK. Immunology is an interesting subject but was taught very badly, and what was described as Mammalian Biochemistry turned out to be about embryology and early organ development. If you think that viral vectors, transcription factors and manipulating DNA are complicated, it's nothing compared to the complexity of complement cascades in the immune system, or the factors that affect the early development of your heart, liver, kidneys and the rest of your innards, starting with one single cell - the fertilised egg. Miraculous.

1 comment:

Amy said...

Good heavens! I'm so glad I chose journalism!