In the Quest for Universal Blood, Go With Your Gut

Science
سه شنبه 30 مرداد 1397 ساعت 10:31
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Smith sonian mag
Over 112.5 million donations of blood are collected each year around the world—but most of these contributions are unusable for some of the patients most in need.
Blood transfusions must match the blood type of a donor to that of the recipient; otherwise, the recipient’s immune system could attack the foreign blood, causing severe illness. Today, scientists at the 256th National Meeting & Exposition of the American Chemical Society report promising new steps towards hacking this system, using bacterial enzymes derived from the gut microbiome to convert restrictive blood types into more universal blood.
There are four main types of blood : AB, A, B and O blood, distinguished by the sugars red blood cells carry on their surface, called antigens.
AB is the selfish hoarder of the group, carrying both A antigen and B antigen. With all its bling, AB blood can only be transfused into others with the AB blood type—but people who have AB blood are universal recipients. A and B blood types carry just one of the two antigens, respectively, and people with these blood types can only receive blood that doesn’t sport the other sugar.
O blood, on the other hand, is the naked martyr that lacks the sugars that decorate its brethren. Its comparatively barren state makes it a friendly presence in almost all immune environments, and O type blood—the universal donor of the bunch—is in constant demand.
To meet the disproportionate need for universal blood, banks and donation centers are constantly on the lookout for these desirable donors. But even though around 40 percent of the population is type O, stocks always seem to fall short, partly because stored blood has a relatively short shelf life . In recent years, scientists have begun to experiment with generating type O in the lab—either by synthesizing red blood cells from scratch, or snipping the offensive sugars off of AB, A and B blood.
Last year, a group of researchers led by Jan Frayne made enormous strides with the former strategy, infecting a line of red blood cell precursors with cancerous genes to provoke them into replenishing themselves ad infinitum . However, this technique is far from entering the clinic —the synthetic cells have yet to be fully vetted for safety, and the cost of filling just one blood bag with these analogs remains astronomical.
On the other hand, converting blood types has been a work in progress for decades. This strategy is especially appealing because it could both create more universal blood while preventing harder-to-use donations from going to waste.
In 1982, a group of researchers took the first promising steps in artificially converting blood types. Using an enzyme isolated from unroasted green coffee beans , they snipped B antigens off red blood cells, effectively creating type O blood that could be transfused into human patients. But the coffee enzyme had its drawbacks. For one, it was finicky, requiring a very specific set of conditions to work—which meant putting the blood through the ringer before it could be used. Even when the experimental setup was just so, the enzyme was sluggish and inefficient, and the researchers had to use gobs of it to see an effect.
Still, the discovery of the coffee enzyme signaled to the rest of the world that blood conversion was possible—and, more importantly, the necessary tools likely already existed in nature.
By the early 2000s, an appreciation for the immense diversity of enzymes in the bacterial kingdom had begun to emerge, and researchers began to turn to microbes for their sugar-slicing needs. In 2007, researchers reported the discovery of two bacterial enzymes that, in combination, were capable of hacking both A and B sugars off of blood cells. The enzyme that sheared B antigens off blood was a thousand times more efficient than the coffee enzyme from 35 years prior. But the enzyme that targeted A antigen produced slightly more sobering results, requiring too high a dose of enzyme to be practical .
Several teams of researchers since have attempted to harness the power of microbes to “unsweeten” blood. But a few years ago, Peter Rahfeld and Stephen Withers , biochemists at the University of British Columbia, decided to turn to a yet-untapped resource: the gut microbiota —the teeming community of industrious microbes that live in the human intestine.
As it turns out, “gut microbes are professionals at breaking down sugars,” according to Katharine Ng , who studies the gut microbiome at Stanford University, but did not participate in this work. Sugar-laced proteins line the wall of the intestine—and some of these elaborate sugars resemble the same A and B antigens found on blood cells. What’s more, many gut microbes harvest these sugars by plucking them off the intestinal lining.
“I was excited when I found this out—[it meant we might be] able to use microbes to find new [tools],” says Rahfeld. “They’re all already in our guts, just waiting to be accessed. There’s so much potential.”
So far, most of the hunt for new blood-converting machines has involved painstakingly testing known bacterial enzymes one by one. Many members of the gut microbiota can now be grown in laboratory environments—but not all. To capture the full potential of the bacterial enzymes in the gut, Rahfeld and Withers chose a technique called metagenomics .
With metagenomics, scientists can pool a community of microbes—like those in a fecal sample—and simply study the DNA within en masse . Even if the bacteria don’t survive well outside the human body, their DNA is far hardier, and can still give researchers a sense of what enzymes each microbe is capable of churning out. “[Metagenomics] a way to get a snapshot of all the DNA [in the human gut] at one point in time,” explains Rahfeld.
After isolating bacterial genomes from human feces, Rahfeld and his colleagues broke the DNA into small chunks and put them into E. coli, a common strain of bacteria that can be easily manipulated to express foreign genes, such as those that code for enzymes. The researchers tested about 20,000 different fragments of genetic material against simple sugar proxies mimicking A and B antigens; candidates that passed this first round of screening were then exposed to more complicated analogs that better resembled human blood.
In the end, the team was left with 11 possible enzymes that were active against A antigen and one against B antigen—including one extraordinarily promising enzyme that was 30 times more effective against A antigen than the one discovered in 2007. Encouragingly, the new enzyme was a low-maintenance worker, able to perform at a variety of temperatures and salt concentrations—meaning that blood cells could be converted without compromising additives.
When the researchers next tested their powerful new enzyme against real type A human blood, the results were the same—and only a minute quantity of the protein was needed to wipe the blood clean of the offending sugars. Additionally, the researchers were thrilled to find that they could combine their new enzyme, active against type A blood, with previously discovered enzymes that snip away B antigens. By consolidating decades of work, the team now had the tools to efficiently convert AB, A and B blood into universally accepted O.
“It worked beautifully,” says Jay Kizhakkedathu , a professor of chemistry at the University of British Columbia’s Centre for Blood Research who is collaborating with Rahfeld and Withers on their studies.
The researchers are now testing their enzymes on a larger scale. In the future, Withers plans to use genetic tools to tinker with their newfound enzyme to further increase its trimming power. Eventually, the team hopes such blood conversion technology could be a mainstay in hospitals, where the need for O-type blood is always dire.
Even with such promising results, the blood-converting enzymes discovered so far are likely only the tip of the iceberg, says Zuri Sullivan , an immunologist at Yale University who did not participate in the research. Given the immense diversity found in different individuals’ gut microbiomes, screening more donors and other bacterial communities could yield even more exciting results.
“The premise here is really powerful,” Sullivan says. “There’s an untapped genetic resource in the [genes] encoded by the gut microbiome.”
Of course, safety remains of primary concern going forward. Modifying human cells, even with natural enzymes, is a tricky business. So far, Rahfeld and Withers report, it has been fairly trivial to wash the enzymes away after treatment—but the researchers will have to be sure that all traces of their enzyme are removed before blood can be transfused into a sick patient.
That's partly because sugar antigens appear on countless cells throughout the body, explains Jemila Caplan Kester , a microbiologist at the Massachusetts Institute of Technology. Although the enzyme in this study appears to be pretty precise in targeting A antigens on blood cells, there’s always a small chance it could do some damage if a small quantity were to slip through the cracks. Additionally, the recipient’s immune system could also react to these bacterial enzymes, interpreting them as signals of an infectious attack. However, Kizhakkedathu believes such a scenario is probably unlikely, since our bodies are supposedly already exposed to these enzymes in the intestine.
“Even with all these considerations, there are more problems we maybe [can’t anticipate]—we’ll see them when we actually test [the blood in a real body],” says Kester. “The human body often finds ways to make [our experiments] not work.”
Additionally, the science of blood typing goes far beyond just A and B antigens. One other common mismatch occurs when Rh antigen is considered. The presence or absence of Rh is what makes someone’s blood type “positive” or “negative,” respectively—and only negative blood can go into both positive and negative recipients.
This means that, despite the power of Rahfeld and Withers’ system, it can’t generate truly universal blood every time. And because Rh antigen is actually a protein, not a sugar, an entirely different set of enzymes will have to be explored to create the most widely accepted universal blood type: O negative.
Still, the team’s technique has immense potential—and not just for the clinic. According to Ng, a better understanding of these bacterial enzymes could also shed light on the complex relationship between humans and the microbes that live within our bodies. In truth, scientists still don’t fully understand the purpose behind the presence of these antigens on blood cells—much less on the lining of our intestines. But bacteria have been privy to this knowledge for millennia—and have been evolving to take advantage of them, Ng says, and learning more about these microbes could answer questions humans haven’t yet thought to ask.
In the meantime, Withers is simply pleased to see progress in any direction. “It’s always surprising when things work well,” he reflects with a laugh. “It gives you hope that you’ve made a real leap forward.
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