An edible vaccine for Hepatitis B - part 2

Part 2: from 2000 to the present.

You might like to read Part 1 of this story first, but no matter if you aren't that keen. I left you with an experiment to try and produce an edible Hepatitis B virus vaccine in potatoes. The principle was proved successful, but there wasn't enough antigen in the potatoes to make it a serious contender at this stage.

The first option that might strike you as sensible is to try a different plant. Not just because it might work better, but because raw potato isn't generally considered an appetising food. Apart from the lettuce experiment, different teams have done similar trials in tomatillo (a Mexican fruit), banana, rice and tomato, although I'm not sure what the rice team were thinking, given that cooking the rice would destroy most of the protein. None of them tried feeding their plants to humans, but the team that wrote 'my' paper had a go with potato.

Most, but not all of the human volunteers who ate the transgenic potato had the desired immune response, and they didn't even bother with the adjuvant this time (if you remember, the adjuvant is something to make sure the immune system doesn't ignore your vaccine). Still, none of these foods made enough antigen yet to make a viable edible vaccine.

There were two other paths still being explored, both of them involving tinkering with the transgene (the virus DNA fragment that is put into the plant) by a) choosing a different bit of the virus as the antigen, in case the bit they first chose isn't the best bit - if they could get a better immune response from another fragment, then they wouldn't need as much protein in the vaccine, or b) putting different promoters and targeting constructs in it.

The promoter is the bit of DNA that tells the plant cell machinery to "start transcribing DNA into mRNA from here". You may know that there's an awful lot of random DNA that we don't think is being used - 'junk' DNA. One way the cell knows which bits to use and which bits to leave alone because of certain sequences called promoter regions. Sometimes these promoters are permanently blocked so that the DNA is never transcribed and the protein is never made. For example, every cell in the body contains the DNA that could make the milk protein casein or the blood protein haemoglobin, but we only really want them to be made in mammary cells or bone marrow cells respectively, so it's permanently blocked everywhere else.

Of course, I'm writing as if the cell 'knows' what to do. That's one of the miracles of biology - of course it knows nothing. At molecular levels, which is where this is all going on, it's all about electrical charges and attractions between individual atoms based on where their electrons are. Molecules float about in cells, and randomly bump into other molecules, which give them a push in one direction or other in the same way that comets float about the universe getting pushed about by stars' gravitational fields. Billions of years of evolution (and magic) means that this random motion is directed into interactions that make life possible. We're all about chemistry inside our cells, and chemistry is all about physics.

Where was I? Oh yes, promoters and targeting constructs. If promoters are at the beginning of a gene saying 'start making your mRNA here', then targeting constructs are at the end of the gene, and tell the cell where to make the protein, or what to do with it after it's made. This is where you try to direct the vaccine protein to the fruit or tuber rather than the leaves or flowers, or where you could try and get it secreted, or stored in chloroplasts or the vacuole or other internal areas of the cells. This bit is quite technical, but they've tried a few different ones with variable results. The trouble is that storing a lot of foreign protein carries the risk of poisoning the plant, so there's an amazingly clever technique called inducible expression.

I've mentioned that there are promoters that are permanently blocked in some cells so that their associated gene DNA can't be transcribed. Transcription factors are molecules or complexes that do this blocking, by attaching themselves to the promoter region of DNA. It's possible to treat the plant with something that changes the configuration of the transcription factor so it stops being attached to the promoter. That means that a gene that previously was not transcribed becomes unblocked, and can start making mRNA and then protein. The thing that can change the transcription factor could be as simple as cold, or ethanol, or 'wounding'.

So you could grow your normal potato or tomato or whatever, harvest it normally, then chill it, expose it to ethanol fumes or pound it to a pulp. All the transcription factors would detach from the promoters in every cell, and the DNA would get busy and make your Hepatitis B antigen protein. Brilliant. This has been tried out in tobacco leaves, but not in an edible plant yet.

So we are now at the stage when we've proved the principle of the edible vaccine, but we’re struggling to get the concentration high enough in the edible part of the plant without killing it in the process. There's one other alternative, but for that you'll have to wait for Part 3.

The picture of the potatoes came from www.bbc.co.uk