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Ticket to ride for nanos
PHILADELPHIA—Researchers at the University of Pennsylvania School of Engineering and Applied Science and Penn's Institute for Translational Medicine and Therapeutics have figured out a way, they think, to provide a "passport" for therapeutic devices like nanoparticles, enabling them to get past the body's security system without being attacked as foreign invaders.
The research—published recently in the journal Science—was conducted by Prof. Dennis Discher, graduate students Pia Rodriguez, Takamasa Harada, David Christian and Richard K. Tsai and postdoctoral fellow Diego Pantano of the Molecular and Cell Biophysics Lab in Chemical and Biomolecular Engineering at Penn.
Unlike the learned response of the adaptive immune system, which includes the targeted antibodies that are formed after a vaccination, the innate immune system tries to destroy everything it doesn't recognize as being part of the body. For example, proteins in blood serum will adhere to objects in the blood stream to draw macrophages' attention; when macrophages determine these proteins are stuck to foreign invaders, they will consume them or signal other macrophages to form a barrier around them.
As the Penn researchers notes, drug-delivery nanoparticles naturally trigger this response, so researchers' earlier attempts to circumvent it involved coating the particles with polymer "brushes." These brushes stick out from the nanoparticle and attempt to physically block various blood serum proteins from sticking to its surface.
However, these brushes only slow down the macrophage- signaling proteins, so Discher and colleagues tried a different approach: Convincing the macrophages that the nanoparticles were part of the body and shouldn't be cleared.
In 2008, Discher's group showed that the human protein CD47, found on almost all mammalian cell membranes, binds to a macrophage receptor known as SIRPa in humans. As they put it at Penn, "Like a patrolling border guard inspecting a passport, if a macrophage's SIRPa binds to a cell's CD47, it tells the macrophage that the cell isn't an invader and should be allowed to proceed on."
"There may be other molecules that help quell the macrophage response," Discher said. "But human CD47 is clearly one that says, 'Don't eat me'."
Since the publication of that study, other researchers determined the combined structure of CD47 and SIRPa together. Using this information, Discher's group was able to computationally design the smallest sequence of amino acids that would act like CD47.
As this minimal peptide might one day be attached to a wide range of drug-delivery vehicles, the researchers also attached antibodies of the type that could be used in targeting cancer cells or other kinds of diseased tissue. Beyond a proof of concept for therapeutics, these antibodies also served to attract the macrophages' attention and ensure the minimal peptide's passport was being checked and approved.
"We're showing that the peptide actually does inhibit the macrophage's response," Discher said. "We force the interaction and then overwhelm it."
The test of this minimal peptide's efficacy was in mice that were genetically modified so their macophages had SIRPa receptors similar to the human version. The researchers injected two kinds of nanoparticles—ones carrying the peptide passport and ones without—and then measured how fast the mice's immune systems cleared them.
"We used different fluorescent dyes on the two kinds of nanoparticles, so we could take blood samples every 10 minutes and measure how many particles of each kind were left using flow cytometry," Rodriguez said. "We injected the two particles in a 1-to-1 ratio and 20 to 30 minutes later, there were up to four times as many particles with the peptide left."
While more research is necessary before such applications become a reality, reducing the peptide down to a sequence of only a few amino acids was a critical step, they say at Penn, adding that the relative simplicity of this passport molecule to be more easily synthesized makes it a more attractive component for future therapeutics.
"It can be made cleanly in a machine," Discher said, "and easily modified during synthesis in order to attach to all sorts of implanted and injected things, with the goal of fooling the body into accepting these things as 'self.'"
(Adapted from an article/new release at the Penn website)