David Kwan, University of British Columbia
In an emergency situation, when a blood transfusion can mean the difference between life and death, there might not be enough time to test a patientís blood type. Under circumstances like these, the universal donor blood type (group 0 negative) that can safely be given to everyone is whatís needed. But itís in short supply: only about 7% of the population have this blood type. Scientists at the University of British Columbia are tackling the problem by developing a way to turn blood of any type into the equivalent of group O negative, by stripping off the blood group markers, or antigens, from the outsides of the blood cells, as David Kwan explained to Chris Smith...
David - The cells in your body, including blood cells, all display certain markers on their surfaces. These could be proteins or carbohydrates - anything that kind of identifies the cell to other cells in the body. The immune system is able to recognise molecules on the surfaces of the bodyís own cells, but if you introduced other molecules then the immune system will potentially attack these. This is the problem with transfusing blood of different types.
Chris - Your strategy is to say, ďWell, if we can remove those markers from the surfaces of the cells we want to transfuse then the immune system would not have anything to recognise and therefore, it shouldnít react.
David - Precisely.
Chris - So, how on Earth can you remove those markers from the surfaces of cells?
David - We want to use enzymes which are like molecular scissors to cut off the antigen molecules from the surface of blood cells. Now, this idea has been around for some years, but the problem has been that the enzymes available have not been very efficient. So, what we wanted to do was to engineer these enzymes to better do the job.
Chris - Do these enzymes exist in the body naturally to strip off these antigens?
David - The enzyme that we have been working on comes from a pathogen. It comes from Streptococcus pneumoniae which is the causative agent of pneumonia. These enzymes chop off blood group antigens which are made up of sugar building blocks and this allows the organism to take up these and use them for its own food source.
Chris - What you're basically doing is stealing a pathogenís digestive system and you're going to use it to digest the antigens off of cells to make them more compatible for transfusion.
David - Exactly.
Chris - Why is the enzyme not good enough? Why can't you just use it as it is?
David - These molecules can be linked on to the cell surface in a number of different ways. Itís very good at cleaving off the most predominant linkage by which these molecules, these markers, are presented on the cell surface, and that job is good enough for Streptococcus pneumoniae. But the immune system is very sensitive to these markers, and so, we need almost complete removal of them.
Chris - How can you improve on what nature has given you in the form of the enzyme from Streptococcus then?
David - To improve on nature, we take lessons from nature. We actually do a process called directed evolution to engineer this enzyme. We take the gene for the enzyme and we put it into another system that allows us to produce large quantities of this enzyme. Once we have the gene, we can mutate it and this gives us a population of different mutants. We select the improved mutants, take the genes of those and continue the process of mutation and selection to get the best mutants.
Chris - So, in a nutshell, you are introducing genetic spelling mistakes into the gene that encodes this enzyme and then you do trial and error, do any of those spelling mistakes actually make the enzyme much better at stripping off these antigens, and if some of them do, you take the best performing gene and then just keep repeating that process until youíve got one thatís really good.
David - That's right.
Chris - Does it work?
David - It does. Weíve been able to improve the efficiency of this enzyme 170 fold compared to the parent ancestral enzyme for cleaving a specific linkage that the parent enzyme had difficult cutting.
Chris - I'm sensing there might be a but coming here - does that mean that it does one of these antigens quite well, but there are others that you can't yet chop off with this enzyme quite so well?
David - You're right. Our enzyme is good at cleaving some of the linkages by which these antigens are linked to cell surfaces but not all of them and weíre trying to improve that.
Chris - Does that mean then that you can do certain blood groups but not others or are you literally just at the stage where you can remove some of the antigen but not the whole lot? So itís not really ready even for one blood group quite yet?
David - The enzyme can completely remove the B antigen from B blood type individuals, but for the A antigen, itís not quite there yet. The problem is that there are many different ways by which the A antigen can be linked to cell surfaces and we can only cleave some of these linkages.
Chris - Have you actually demonstrated though that this blood, were it to be transfused having been stripped of its antigens using your strategy would be safe and wouldnít trigger a reaction if I for instance put group B blood treated in this way into a group A person?
David - Well, we haven't done the transfusion yet, but the typical clinical test for antigenicity of blood type B shows that these cells have the antigens completely removed from them.
Chris - How do you actually practically do this? Do you have a pot with some of this enzyme in and you just tip into the cells and let it work its magic, and then wash it off? Is that how it works?
David - Essentially, thatís the case. We want to take the donated blood, add a few drops of the enzyme to it, let it sit for a few hours and then wash the enzyme away.
Chris - Do the cells tolerate this? Are they harmed in any way?
David - All these tests that we have done donít show any indication that the cells are harmed in any way by this.
Chris - That's very reassuring. How long do you think it will be based on the present trajectory before youíve got something that you could consider for a clinical trial here?
David - Well, I can only give you an estimate but Iíd say something like 10 to 15 years perhaps...