Whitehead Institute scientists and associates have modified red blood cells (RBCs) to carry a range of valuable therapeutic and diagnostic payloads — such as drugs, vaccines, and disease-detecting imaging agents  — for delivery to specific sites throughout the body.

“We wanted to create high-value red cells that do more than simply carry oxygen,” says Whitehead Founding Member Harvey Lodish, who collaborated with Whitehead Member Hidde Ploegh in this pursuit. So they modified the genes and enzymes in mouse and human RBCs in culture (in the lab).

The work, published this week in the Proceedings of the National Academy of Sciences (PNAS), combines Lodish’s expertise in the biology of red blood cells (RBCs) with biochemical methods developed in Ploegh’s lab.

RBCs are an attractive vehicle for potential therapeutic applications, the researchers say. They are more numerous than any other cell type in the body. They have a long lifespan (up to 120 days in circulation). And during RBC production, the progenitor cells that eventually mature to become RBCs jettison their nuclei and all DNA. Without a nucleus, a mature RBC lacks any genetic material or any signs of earlier genetic manipulation that could result in tumor formation or other adverse effects.*

Wide range of medical uses

The researchers suggest that the applications are potentially vast, including:

• Bind and remove bad cholesterol from the bloodstream.
• Carry clot-busting proteins to treat ischemic strokes or deep-vein thrombosis.
• Deliver anti-inflammatory antibodies to alleviate chronic inflammation.
• Suppress the unwanted immune response that often accompanies treatment with protein-based therapies.
• Neutralize a toxin. “Because the modified human red blood cells can circulate in the body for up to four months, one could envision a scenario in which the cells are used to introduce antibodies that neutralize a toxin,” says Ploegh. “The result would be long-lasting reserves of antitoxin antibodies.” That’s why the U.S. military and its Defense Advanced Research Projects Agency (DARPA) is supporting the research at Whitehead in the interest of developing treatments or vaccines effective against biological weapons.

* Exploiting this feature, Lodish and his lab introduced genes coding for specific slightly modified normal red cell surface proteins into early-stage RBC progenitors. As the RBCs approach maturity and enucleate, the proteins remain on the cell surface, where they are modified by Ploegh’s protein-labeling technique. This approach, called “sortagging,” relies on the bacterial enzyme sortase A to establish a strong chemical bond between the surface protein and a substance of choice, be it a small-molecule therapeutic or an antibody capable of binding a toxin. The modifications leave the cells and their surfaces unharmed.

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Abstract of Proceedings of the National Academy of Sciences paper

We developed modified RBCs to serve as carriers for systemic delivery of a wide array of payloads. These RBCs contain modified proteins on their plasma membrane, which can be labeled in a sortase-catalyzed reaction under native conditions without inflicting damage to the target membrane or cell. Sortase accommodates a wide range of natural and synthetic payloads that allow modification of RBCs with substituents that cannot be encoded genetically. As proof of principle, we demonstrate site-specific conjugation of biotin to in vitro-differentiated mouse erythroblasts as well as to mature mouse RBCs. Thus modified, RBCs remain in the bloodstream for up to 28 d. A single domain antibody attached enzymatically to RBCs enables them to bind specifically to target cells that express the antibody target. We extend these experiments to human RBCs and demonstrate efficient sortase-mediated labeling of in vitro-differentiated human reticulocytes.