New role for the blood protein serum amyloid P in the body's response to medical materials

Biomedical engineers at Washington University in St. Louis have found a new role for the blood protein serum amyloid P in the body's response to medical materials, which may help to explain a variety of problems associated with heart-lung bypass, hemodialysis and the use of artificial vascular grafts.

Using a technique called proteomics, the researchers identified many of the blood proteins that stick (adsorb) to the surfaces of medical devices. Blood proteins that adsorb to the surfaces of materials unfold and can be recognized by the body, which then mounts a response against the device. The body's response to adsorbed proteins contributes to a variety of problems, including the formation of small clots that may close off small diameter vascular grafts or break away to end up in the lungs, kidney or brain.

Previously, the study of blood proteins on the surfaces of medical devices has been limited by the large number of unique proteins in the blood -- greater than 150 -- as well as the extremely small amounts of proteins adsorbed on the materials. For example, the amount of protein that might adsorb to one side of a quarter is about a millionth of a gram.

Donald Elbert, Ph.D., Washington University assistant professor of biomedical engineering, used advanced protein separations and mass spectrometry to track the proteins on the surfaces of various polymers used in medical devices. The analysis techniques, collectively called 'proteomics,' are most often used to study protein expression in cells.

"The techniques are extremely sensitive and are really well-suited to studying proteins on surfaces," said Elbert. "Using these techniques, we can in principle identify a protein given only a billionth of a gram of the protein, even if the protein were mixed with many other types of proteins."

Elbert and his colleagues Jinku Kim and Evan Scott were able to follow the adsorption of multiple blood proteins on the surface of a biomaterial over time.

"Traditionally, most studies were limited to the 'big three' proteins in blood - albumin, fibrinogen and IgG", Elbert said. The results were published in the Oct. 1, 2005 issue of the Journal of Biomedical Materials Research.

By casting a wider net, they found one particular protein on the materials in large amounts, serum amyloid P. Serum amyloid P is very similar in structure to C-reactive protein, a well-known marker for cardiovascular disease. Normally, serum amyloid P is involved in the clearance of DNA that is released from dying cells, protecting the individual from auto-immune disorders.

"No one had ever observed serum amyloid P on biomedical materials before, because, unless you were specifically looking for them, the technology wasn't around to easily identify proteins present in such small amounts," Elbert said. "No one, including us, had ever posed the hypothesis that this protein might be important in the biocompatibility of materials. Our results show the importance of large-scale techniques that emphasize discovery of new knowledge, rather than just hypothesis-testing."

The Washington University researchers also found that leukocytes -- white blood cells -- adhered to the serum amyloid P after it adsorbed to surface. Leukocyte adhesion and activation on biomaterials is an important part of the body's response to medical devices. For example, large numbers of activated leukocytes are found stuck to heart-lung bypass machines, and these cells can activate blood clotting. This in turn may contribute to neurocognitive impairment following the use of these devices, possibly due to small clots that break away from the device.

"It's really exciting that even after over 60 years of research in the area, there is more to learn about how blood interacts with materials," Elbert said.