PHILADELPHIA—Researchers at the University ofPennsylvania School of Engineering and Applied Science and Penn's Institute forTranslational Medicine and Therapeutics have figured out a way, they think, toprovide a "passport" for therapeutic devices like nanoparticles, enabling themto get past the body's security system without being attacked as foreigninvaders.
The research—published recently in the journal Science—was conducted by Prof. DennisDischer, graduate students Pia Rodriguez, Takamasa Harada, David Christian andRichard K. Tsai and postdoctoral fellow Diego Pantano of the Molecular and CellBiophysics Lab in Chemical and Biomolecular Engineering at Penn.
Unlike the learned response of the adaptive immunesystem, which includes the targeted antibodies that are formed after avaccination, the innate immune system tries to destroy everything it doesn'trecognize as being part of the body. For example, proteins in blood serum willadhere to objects in the blood stream to draw macrophages' attention; when macrophagesdetermine these proteins are stuck to foreign invaders, they will consume themor signal other macrophages to form a barrier around them.
As the Penn researchers notes, drug-deliverynanoparticles naturally trigger this response, so researchers' earlier attemptsto circumvent it involved coating the particles with polymer "brushes." Thesebrushes stick out from the nanoparticle and attempt to physically block variousblood serum proteins from sticking to its surface.
However, these brushes only slow down themacrophage-signaling proteins, so Discher and colleagues tried a differentapproach: Convincing the macrophages that the nanoparticles were part of thebody and shouldn't be cleared.
In 2008, Discher's group showed that the humanprotein CD47, found on almost all mammalian cell membranes, binds to amacrophage receptor known as SIRPa in humans. As they put it at Penn, "Like apatrolling border guard inspecting a passport, if a macrophage's SIRPa binds toa cell's CD47, it tells the macrophage that the cell isn't an invader andshould be allowed to proceed on."
"There may be other molecules that help quell themacrophage response," Discher said. "But human CD47 is clearly one that says,'Don't eat me'."
Since the publication of that study, otherresearchers determined the combined structure of CD47 and SIRPa together. Usingthis information, Discher's group was able to computationally design thesmallest sequence of amino acids that would act like CD47.
As this minimal peptide might one day be attached to a wide range ofdrug-delivery vehicles, the researchers also attached antibodies of thetype that could be used in targeting cancer cells or other kinds ofdiseased tissue. Beyond a proof of concept for therapeutics, theseantibodies also served to attract the macrophages' attention and ensurethe minimal peptide's passport was being checked and approved.
"We're showing that the peptide actually does inhibit the macrophage'sresponse," Discher said. "We force the interaction and then overwhelmit."
The test of this minimal peptide's efficacy was in micethat were genetically modified so their macophages had SIRPa receptorssimilar to the human version. The researchers injected two kinds ofnanoparticles—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 manyparticles 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 minuteslater, there were up to four times as many particles with the peptideleft."
While more research is necessary before such applications become areality, reducing the peptide down to a sequence of only a few aminoacids was a critical step, they say at Penn, adding that the relative simplicity of this passportmolecule to be more easily synthesized makes it a more attractivecomponent for future therapeutics.
"It can be made cleanly ina machine," Discher said, "and easily modified during synthesis inorder to attach to all sorts of implanted and injected things, with thegoal of fooling the body into accepting these things as 'self.'"
(Adapted from an article/new release at the Penn website)
