On any given day, hospitals across the US burn through some 16,500 litres of donated blood for emergency surgeries, scheduled operations, and routine transfusions. But recipients can’t take just any blood: For a transfusion to be successful, the patient and donor blood types must be compatible. Now, researchers analysing bacteria in the human gut have discovered that microbes there produce two enzymes that can convert the common type A into a more universally accepted type. If the process pans out, blood specialists suggest it could revolutionise blood donation and transfusion.
“This is a first, and if these data can be replicated, it is certainly a major advance,” says Harvey Klein, a blood transfusion expert at the National Institutes of Health’s Clinical Centre in Bethesda, Maryland, who was not involved with the work.
People typically have one of four blood types—A, B, AB, or O—defined by unusual sugar molecules on the surfaces of their red blood cells. If a person with type A receives type B blood, or vice versa, these molecules, called blood antigens, can cause the immune system to mount a deadly attack on the red blood cells. But type O cells lack these antigens, making it possible to transfuse that blood type into anyone. That makes this “universal” blood especially important in emergency rooms, where nurses and doctors may not have time to determine an accident victim’s blood type.
“Around the US and the rest of the world, there is a constant shortage,” says Mohandas Narla, a red blood cell physiologist at the New York Blood Centre in New York City.
To up the supply of universal blood, scientists have tried transforming the second most common blood, type A, by removing its “A-defining” antigens. But they’ve met with limited success, as the known enzymes that can strip the red blood cell of the offending sugars aren’t efficient enough to do the job economically.
After 4 years of trying to improve on those enzymes, a team led by Stephen Withers, a chemical biologist at the University of British Columbia (UBC) in Vancouver, Canada, decided to look for a better one among human gut bacteria. Some of these microbes latch onto the gut wall, where they “eat” the sugar-protein combos called mucins that line it. Mucins’ sugars are similar to the type-defining ones on red blood cells.
So UBC postdoc Peter Rahfeld collected a human stool sample and isolated its DNA, which in theory would include genes that encode the bacterial enzymes that digest mucins. Chopping this DNA up and loading different pieces into copies of the commonly used lab bacterium Escherichia coli, the researchers monitored whether any of the microbes subsequently produced proteins with the ability to remove A-defining sugars.
At first, they didn’t see anything promising. But when they tested two of the resulting enzymes at once – adding them to substances that would glow if the sugars were removed – the sugars came right off. The enzymes also worked their magic in human blood. The enzymes originally come from a gut bacterium called Flavonifractor plautii, Rahfeld, Withers, and their colleagues report. Tiny amounts added to a unit of type A blood could get rid of the offending sugars, they found.
“The findings are very promising in terms of their practical utility,” Narla says. In the US, type A blood makes up just under one-third of the supply, meaning the availability of “universal” donor blood could almost double.
But Narla says more work is needed to ensure that all the offending A antigens have been removed, a problem in previous efforts. And Withers says researchers need to make sure the microbial enzymes have not inadvertently altered anything else on the red blood cell that could produce problems. For now, the researchers are focusing on only converting type A, as it’s more common than type B blood. Having the ability to transform type A to type O, Withers says, “would broaden our supply of blood and ease these shortages.”
Access to efficient enzymes that can convert A and B type red blood cells to ‘universal’ donor O would greatly increase the supply of blood for transfusions. Here we report the functional metagenomic screening of the human gut microbiome for enzymes that can remove the cognate A and B type sugar antigens. Among the genes encoded in our library of 19,500 expressed fosmids bearing gut bacterial DNA, we identify an enzyme pair from the obligate anaerobe Flavonifractor plautii that work in concert to efficiently convert the A antigen to the H antigen of O type blood, via a galactosamine intermediate. The X-ray structure of the N-acetylgalactosamine deacetylase reveals the active site and mechanism of the founding member of an esterase family. The galactosaminidase expands activities within the CAZy family GH36. Their ability to completely convert A to O of the same rhesus type at very low enzyme concentrations in whole blood will simplify their incorporation into blood transfusion practice, broadening blood supply.
Peter Rahfeld, Lyann Sim, Haisle Moon, Iren Constantinescu, Connor Morgan-Lang, Steven J Hallam, Jayachandran N Kizhakkedathu, Stephen G Withers