Backpacks for Cells

Polymer patches hitched to the surfaces of immune cells can transport a variety of cargo.

Living cells wearing microscopic "backpacks"--nanostructured polymer patches loaded with chemical cargo--might one day be able to ferry drugs or imaging agents to diseased tissue. MIT researchers say that they have successfully constructed such backpacks, filled them with magnetic particles, and tethered them to the surfaces of immune cells without disrupting the cells' ability to interact with their environment. The work is described in a recent issue of Nano Letters.

Geared up: Two immune cells (gray) wear polymer "backpacks" (green). The attached backpacks have two layers: a cell adhesion layer that grabs on to the cell surface, and a payload layer that carries some chemical cargo--in this case, green fluorescent dye. Researchers hope that the backpacks can one day be adapted to deliver drugs or imaging agents to specific regions in the body. Credit: Nano Letters

Multimedia   See the immune cells in action.

"Overall, this is a very significant piece of work," says Michael Sailor, a professor of chemistry and biochemistry at the University of California, San Diego, who was not involved in the study. "There are many possible variations on this theme for a host of different diseases. I think it could start an entirely new subdiscipline."

The backpacks are built from three thin layers of polymer film. The bottom layer anchors the backpack to a surface during construction and loading. The middle layer carries the backpack's cargo. And the top layer acts as a hook that latches on to a cell's surface.

Once they had synthesized the backpacks, the researchers added a solution containing living immune cells, which were immediately hooked by the backpacks' top layers. Then, by lowering the temperature, they triggered the bottom polymer layers to dissolve, releasing the backpack-wearing cells from the surface.

This process allows for incredible versatility in the backpacks' cargo, says Michael Rubner, director of MIT's Center for Materials Science and Engineering and senior author of the paper. Because the cells aren't added until the very end, there's no danger in using toxic chemicals and harsh conditions to build and load the backpacks. "You can use all the harsh chemistry you want, because the cell isn't there to be killed," says Rubner. "It's only in the last step of the process that the cell attaches to the surface, grabs its backpack, and lifts it off."

To test how tightly the backpacks attached, the researchers filled them with magnetic nanoparticles, loaded them onto immune cells, and placed the cells near a magnet. Under a microscope, the cells could be seen migrating toward the magnet--tugged along by their backpacks, which stayed firmly anchored in place.

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Usually, particles incorporated into a cell's surface are internalized in a matter of seconds, says Mauro Ferrari, director of the division of nanomedicine at the University of Texas, who was not involved in the work. "The fact that this thing stays there for longer than seconds is remarkable," he says.

Sailor cautions that while the technology is promising, the real challenge will be getting it to work inside the body. There's no way of knowing at this stage how the backpack-wearing cells would fare as they circulated in the bloodstream. They might engulf or shed their packs, or lodge in tight spaces. Initial studies suggest that the backpacks don't pose any danger to the immune cells' health, but much more work is needed before the system can be tested inside a living animal, says Rubner.