Cloaked blood hides from immune system.

For people with blood disorders such as thalassemia or sickle-cell anemia, repeated blood transfusions become a part of life. The introduction of so many foreign cells often makes transfusion recipients sensitive to the hundreds of antigen proteins on the surface of red blood cells. Over time, it becomes increasingly difficult to find suitable blood for them.

Now, researchers have found a way to hide those cell surface proteins, thus rendering the cells invisible to the immune system. Mark D. Scott of Albany (N.Y.) Medical College and his colleagues link long molecules of polyethylene glycol (PEG), a nonimmunogenic substance, to the proteins, creating what they call "fuzzy little red cells." The PEG fuzz keeps out large proteins like antibodies but allows small molecules such as oxygen and glucose to diffuse in. That way, the cells are protected from attack by immune cells yet can still function normally. The scientists describe their work in the July 8 Proceedings of the National Academy of Sciences.

In mice, treated blood cells survive just as long as untreated ones. Furthermore, Scott says, "as an extreme test, we transfused blood between two different species." Untreated sheep blood cells injected into mice lasted only about 5 minutes, but PEG-treated cells lasted for hours.

It will be a few years before the technique can be tested in people. In test-tube studies, PEG-treated human blood cells were protected from antibodies and functioned normally. Scott suggests that this treatment could be combined with a method that converts blood types A and B into type 0 by trimming characteristic cell surface proteins (SN: 1/11/97, p. 24). PEG could disguise any cell surface proteins not affected by the conversion.

The technique may prove useful not only for human blood transfusion but also for veterinary medicine. "You don't have centralized blood banks, yet you still have to match blood. This is very low-tech, so it can be easily translated into a veterinary practice," Scott says.