Researchers Find Way to Convert Type A Blood to Type O

Ricki Lewis, PhD

June 17, 2019

Researchers have found a way to use a pair of enzymes from a human gut bacterium to convert type A to the universal donor type O blood, according to a report published June 10 in Nature Microbiology.

The researchers say the high activity and specificity of these enzymes "make these very promising candidates for cost-efficient implementation into the already existing automated routines of blood collection, processing and storage, with major implications for the flexibility of our blood supply and possible applications in organ transplantation."

A shortage of human blood of all blood types persists. In mid-May, the American Red Cross warned that without more blood donations some surgeries will need to be delayed — and the agency offered $5 gift cards to entice donors.

At the time, less than a 2-day supply of type O blood was available for emergency rooms; a 5-day supply is ideal. The shortage is critical because O-negative is a universal blood type, able to be transfused into any patient, buying time. The situation is equally dire in Canada.

Mismatched blood leads to potentially fatal transfusion reactions that activate complement and lyse red blood cells (RBCs).

Blood types arise from groups of cell surface antigens. In the ABO system, type A RBCs are distinguished by an amino sugar (alpha-1,3-linked-N-acetylgalactosamine) antigen, whereas type B antigen has a sugar (galactose). Type O blood has neither antigen.

Peter Rahfeld, PhD, from the University of British Columbia in Canada, and colleagues scrutinized the human gut microbiome for enzymes that might effectively remove the A antigen.

The idea to deploy enzymes to strip sugars from RBCs is not new. "The concept goes back to the 1980s, when the only alpha galactosidase you could buy as a reagent was from green coffee beans. Since then, much faster ways to discover new enzymes have been developed, such as genome sequencing," coauthor Stephen Withers, PhD, professor of chemistry and biochemistry at the University of British Columbia, told Medscape Medical News.

A report from 1982 described using the coffee enzyme to convert type B into type O blood and transfusing it successfully. But converting type A to type O is more challenging chemically because the A antigen has subtypes. The coffee approach couldn't attain the gram quantities needed for clinical use.

In 2007, researchers identified enzymes from two bacterial species that could dismantle complex sugars, such as blood antigens. But they too needed large supplies of the enzymes and specific buffer conditions, especially to convert the recalcitrant type A.

Other efforts did not remove enough antigens to prevent mismatch reactions. Even a directed evolution approach, in which researchers brainstormed possible gene variants encoding an enzyme in Streptococcus pneumoniae, was not efficient or specific enough for clinical use.

So instead of starting with a bacterial species and tweaking its enzymes, Withers and colleagues took a metagenomic approach, screening libraries of human gut inhabitants. ("Metagenome" refers to all the DNA in an area.) They zeroed in on the glycoprotein-rich mucin that lines intestinal interiors.

"From the bacterial perspective, mucin provides a nice anchor to stop them from getting flushed out of the GI tract, and these bacteria feed upon the glycan (polysaccharide or complex sugar) layer," Withers explained.

Inspired by blood-sucking leeches and vampire bats, he added, the team knew that sugars similar to the ABO antigens dot the gut wall, in mucin. Might bacterial enzymes that degrade these antigens also dismantle the A and B antigens on RBCs, creating type O cells? "We reasoned the human gut would be a good place to look for enzymes that degrade the A and B antigens, in the context of mucin."

Two bizarre situations also suggested to the researchers that gut bacteria might be efficient blood type converters.

In the so-called "acquired B phenomenon," the blood of a person with sepsis can transiently change type, reverting when the infection dissipates.

The second instance was the case of a dismembered body found in England's River Thames. "When forensic scientists pulled out body parts, depending on where they sampled tissue, they got different blood types. The bacteria in the Thames were doing the same thing as the bacteria causing sepsis," Withers explained.

The researchers built a metagenomic library of DNA pieces from feces from a man with type AB+ blood, so that blood cells in the stool would sport both antigens. (Withers and coauthor Steven Hallam, PhD, recently reported a similar conceptual approach, analyzing actions of the fecal microbiome of beavers in processing wood to build dams and extract nutrients.)

Of the approximately 19,500 entrants in the gut bacterial library, the researchers zeroed in on the obligate anaerobe Flavonifractor plautii. Its N-acetylgalactosamine deacetylase converts type A to type O blood. Not much is known about the anaerobe because it is rarely isolated from clinical specimens.

The researchers scaled up enzyme production using recombinant E coli. An ultraviolet assay marked the jettisoning of the antigen that converts type A to type O.

The researchers tested the efficiency of the conversion using fluorescence-activated cell sorting and measuring agglutination times with anti-A and anti-H antibodies. (RBCs of all ABO types have a short antigen, called H, to which the other parts attach.) Testing revealed that just 5 micrograms per milliliter of enzyme is enough to convert blood types.

In addition, testing on type A blood from 26 people showed no detectable antigen, and the technique converted an entire unit of blood. The bacterial enzymes are easily removed during red blood cell centrifugation.

Ongoing work is investigating whether the bacterial enzymes might expose other antigens from within the cell membrane. The researchers are continuing to look for better enzymes.

To ready the technique for application to blood banking, "the main goal is safety, safety, safety. We must show that the enzymes are removing all traces of the A and B antigens. We also have to show that the enzymes are not causing any other changes to the blood," Withers told Medscape Medical News.

He cautioned that standard blood typing reagents might not be sensitive enough to detect very low levels of non-conversion.

The University of British Columbia has filed a patent connected with this work, on which Withers, Rahfeld, and study coauthor Jayachandran Kizhakkedathu are named as authors. The remaining study coauthors have disclosed no relevant financial relationships.

Nat Micro. Published online June 10, 2019. Abstract

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