It starts with a bite. Maybe a misjudged ice cream sundae. Or perhaps a cookie that was hiding a dangerous secret. Either way, once you ingest it, your body starts working against you. Once the proteins in the peanuts enter your blood stream, your immune system starts releasing volleys of chemicals. Your skin erupts in hives. Your body’s soft tissues and mucous membranes swell, constricting your airway and leaving you struggling for breath. Your stomach may start convulsing as your body tries to vomit out the offending substance. And if you’re one of a small percentage of people, your blood pressure may plummet and your heart could stop. It’s uncontrollable, often unexpected, and absolutely terrifying.
Severe allergies—especially of the sort that require people to carry EpiPens—are on the rise. Peanut allergies, for example, affected only 0.4% of children in the U.S. in 1997; in 2010, a study estimated that number had risen to 1.4%. So are autoimmune diseases, such as type I diabetes, multiple sclerosis, or rheumatoid arthritis, which, like severe allergies, occur when the immune system reacts forcefully to something harmless. They affect as many as 8% of Americans, according to the NIH. Both severe allergies and autoimmune diseases exact a toll, racking up treatment expenses and diminishing patients’ quality of life.
We don’t know exactly what’s causing the uptick in allergies or autoimmune disorders, but there’s hope of treating them by acclimating people’s bodies to whatever substance is setting off the alarm bells. A promising treatment may lie with our red blood cells. Or rather, what they can help us sneak into our bodies.
We’re a step closer to pulling off that subterfuge. Researchers at the Massachusetts Institute of Technology have recently manipulated red blood cells to carry virtually any molecule your doctor might prescribe. The technique, a collaborative effort by biology professors Harvey Lodish and Hidde Ploegh, is new, but the idea of using red blood cells to deliver therapeutic compounds has, surprisingly, been around for decades. It’s just been waiting for the right technology to come along.
Because your body is programmed to eliminate anything it identifies as “foreign,” even drugs that don’t attract the ire of the immune system are still quickly flushed out by other means, namely enzymes known as cytochrome P450s, which are tasked with neutralizing foreign molecules. But using red blood cells as couriers could allow drugs to sidestep both the immune system and P450s. Red blood cells have other advantages, too: They swish around the body for a long time—about 120 days in humans—and are invisible to the immune system’s scouts, thanks to what University of Chicago chemist Jeffrey Hubbell calls “don’t eat me” proteins on their surface.
There’s also a pragmatic reason that red blood cells are an appealing way to deliver drugs: we’re good at delivering new red blood cells to our bodies. “The blood transfusion is an ancient technology,” Ploegh points out. The first successful human-to-human blood transfusion took place almost 200 years ago; animal-to-human transfusions had been going on (with variable results) for about 150 years before that. “So as a cell type to focus attention on,” he says, “it makes perfect sense.”
An Old Approach, Revived
This isn’t the first time we’ve tried administering drugs using modified red blood cells, says Dr. Vladimir Muzykantov, a professor at the University of Pennsylvania’s medical school. In the early 1970s, researchers experimented with using enzyme-loaded red blood cells to treat Gaucher disease, a condition in which patients are missing an enzyme that helps digest a certain kind of fatty molecule, which then builds up in organs like the liver, spleen, or brain. Simply injecting the missing enzyme isn’t always effective: the immune system, unused to the molecule, sometimes reflexively unleashes the same type of dramatic assault that peanut proteins prompt in allergy sufferers.
So the researchers soaked red blood cells in an enzyme solution so salty that the cells burst open and swallowed the enzyme. Thus fortified, the red blood cells raised enzyme levels in cell cultures from Gaucher patients more effectively than the enzyme alone. But though the treatment seemed poised to take off, it was quickly dealt a blow when the HIV/AIDS epidemic hit in the 1980s. Suddenly, any treatment involving blood transfusions seemed too risky to pursue. Eventually, most researchers turned their attention to developing synthetic cell analogues: nanoscale medication packages made from polymers or fatty molecules.
But designing artificial carriers isn’t easy, says Elena Batrakova, a University of North Carolina pharmacy professor whose work uses white blood cells that can cross the blood-brain barrier to deliver treatments for neurodegenerative diseases like Parkinson’s. “You can never do better than nature,” she says. Nanoparticle delivery systems, Batrakova explains, present a catch-22: as soon as you encapsulate drugs in particles large enough to survive for extended periods in the body, their size attracts the attention of the immune system. To an immune system macrophage, a cancer medicine wrapped up in a little plastic shell looks a lot like an invading bacterium.
A few labs, including Dr. Muzykantov’s, kept pursuing red blood cell therapies. In the last decade or so, advances in medical technology have revived the possibility of using red blood cells to deliver drugs. Muzykantov, for example, has developed techniques to stick an anticoagulant to the outside of red blood cells and hitch a ride to dangerous blood clots, helping to break them down. The encapsulation method explored in the 1970s is alive and well, too, thanks in part to better screening for HIV-infected blood: several European companies have already started clinical trials.
Still, neither of these methods is perfect: Drug-loaded cells tend not to last very long in the body because forcing a cargo inside inevitably damages them. And compounds attached to the outside of a blood cell that’s already in circulation might not be able to hang on for the cell’s entire lifetime, reducing their efficacy.
A New Take
That’s where the work of Ploegh and Lodish come in. The pair, who work at MIT’s Whitehead Institute in immunology and red-blood-cell biology, respectively, think they have come up with a better way to use red blood cells to smuggle foreign substances past the body’s defenses.
It works like this: Ploegh, Lodish, and their team begin by fiddling with the red blood cell genome so one of the cell’s surface proteins contain an additional five amino acids. Those added amino acids, known as a tag, attract specific a bacterial enzyme called sortase, which kicks off one specific amino acid in the tag, glycine, and latches on in its place. The beauty of sortase is it can then be exchanged for just about any other molecule you want the cell to carry, provided it has a glycine available: small drug molecules, other proteins, even pieces of DNA. “You’re essentially limited only by what your friend the organic chemist can come up with,” Ploegh says. (Normally, there’s a risk of cancer associated with treatments using genetically-modified cells, but since red blood cells have jettisoned their DNA by the time they’re mature, there is no risk in this case, Ploegh says.)
“You’re essentially limited only by what your friend the organic chemist can come up with.”
Because sortase will only bind to that five-amino-acid tag but is unscrupulous about the molecules it will exchange with, you end up with a method that combines the innate specificity of large proteins with the versatility of well-understood small-molecule chemistry: any molecule you want, exactly where you want it.
The attached compounds can do their work for as long as the red blood cell is circulating, which can be quite a long time compared with other treatments. That longevity could be a major advantage when trying to neutralize toxins, for example, like those in some biological weapons. Injecting an antitoxin using traditional techniques might buy you a couple weeks of prophylactic treatment, but using one bound to a red blood cell could soak up dangerous molecules for up to four months. This method, which Ploegh and his colleagues call “Sortagging,” after the tag and its associated bacterial enzyme, could also be used to develop treatments for chronic inflammation and other systemic problems, providing long-term exposure to medication throughout all 150 miles of the circulatory system.
Taming the Immune System
Besides longevity, red blood cells have another superpower—one that suggests a whole different suite of intriguing applications. Millions of red blood cells get recycled every second, and the body is an expert at digesting them. Last year, Hubbell, the University of Chicago chemist, and his colleagues published a paper suggesting that anything attached to a red blood cell when it dies—even if it’s something that would normally cause an allergic reaction—gets marked as “friendly” by the cells responsible for cleanup. When that happens, the white blood cells that were previously programmed to attack it are destroyed—after all, since their original target is now considered harmless, they no longer serve any purpose. The next time the body encounters that allergen, the immune system doesn’t bat an eye. The therapy has extinguished a single, problematic immune response without dismantling the rest of the body’s defenses.
That means that modified red blood cells might be able to treat allergies like the ones that make parents fear rogue peanut-butter crackers at preschool. No more panicked searches for EpiPens—just a few transfusions of allergen-flavored red cells could train your immune system to love peanut-butter, shellfish, or even the explosion of pollen in the spring.
There are other possibilities, too. Some protein-based treatments, like the antibody therapies, have proven very successful in fighting certain cancers, though their utility is limited since they can also trigger allergic reactions. But give the patient a few shots of antibody-fortified red blood cells and, Ploegh says, “perhaps you might be able to blunt a possible immune response when you start the therapy.” Inducing a protein-specific tolerance could also dramatically change the way autoimmune diseases are treated, shutting down the pathway that causes the body to attack the cells that produce, say, insulin like in type I diabetes or myelin like in multiple sclerosis.
A Long Journey
Pleogh is careful to stress that these applications are “fairly fanciful and very much in the distance,” but both he and Hubbell contend that using modified red blood cells to create a less excitable immune system could one day be a realistic possibility.
Whether or not Ploegh and Lodish’s method will someday be a most practical way to do that isn’t clear yet. For one thing, it requires modifying cells’ DNA, which, Hubbell points out, “is a lot easier to do in the laboratory than it is to turn it into a pharmaceutical process.” That’s why Hubbell’s work currently focuses on designing antibodies that will bind with red blood cells inside the body—no genetic engineering or transfusion needed. In practice, “sortagging” would be, says Batrakova, the pharmacy professor, “basically personalized medicine,” which is challenging to implement on a grand scale. That’s not to say it’s impractical, Hubbell says. “One would just have to find the applications that warrant the complexity.”
“The more approaches we have, the better.”
Those applications are still far in the future, though. Ploegh and Lodish have only transplanted their modified cells in mice, and haven’t tested how efficiently they might carry drugs or suppress immune responses. Plus, the long-lived nature of these cells requires scientists be cautious when choosing applications to investigate. As Hubbell points out, “If you embark on a 120 day long journey without a stop, you make a big bet that everything’s going to turn out fine.” Clinical trials of this sort are complicated, so it will take a long time before we know whether sortagging or red blood cell encapsulation or even simple protein-binding strategies are best for any particular disease.
Ultimately, though, Muzykantov thinks all these approaches are complementary, not mutually exclusive. Batrakova agrees: “I think that the more approaches we have, the better.”
Using the body’s own cells as drug mules has the potential to change the way we deliver medicine. Scientists are just beginning to glimpse what that strategic shift could accomplish. Could we eliminate rheumatoid arthritis? Stop multiple sclerosis in its tracks? Reduce peanut allergies to a faint memory? “Your imagination is the limit, basically,” Ploegh says.
Image credits: NSF/DARPA, NIH