Shark antibodies may lead to new diagnostic

The structure of a novel class of shark antibodies known as immunoglobulin new antibody receptors (IgNARs) isolated from wobbegong sharks (Orectolobus maculatus) has provided some clues to the evolutionary history of antibodies, and may lead to a new tool for detecting a range of human pathogens.

Researchers at Melbourne's CRC for Diagnostics at CSIRO Health Science and Nutrition have determined the three-dimensional crystal structure of the antibodies showing that, unlike conventional antibodies, which have a variable heavy chain and a variable light chain; IgNARs have a single variable region. In fact, although the antibody proteins display characteristics of the so-called immunoglobulin superfamily, which includes antibodies and T cell receptors, they also resemble cell surface adhesion molecules.

Comparisons of the IgNAR structure with other immune system molecules by the researchers suggests that the shark IgNARs evolved as a separate lineage to conventional antibodies, which are also found in sharks and other cartilaginous fish, some 450 million years ago.

"Over the last 500 million years, the shark immune system appears to have co-opted a particular type of cell-surface protein to its own use, creating a novel additional class of antibodies completely unlike those seen in humans," said Dr Victor Streltsov, who along with Dr Stewart Nuttall, led the project.

The unique structure of the IgNARs has also been reproduced in the laboratory where the researchers are using them to create libraries of potential clinical reagents for the detection of pathogens, with applications including the detection of biowarfare agents such as anthrax, pathogens like SARS, and other biotechnology and environmental applications.

"There is great interest in them from a diagnostic point of view," Nuttall said. "They are half the size of antibodies and very stable, which makes them useful for arrays, biosensors and diagnostics."

The molecules also tend to bind antigens in a different way to conventional antibody-antigen binding, with loops that can penetrate into clefts and cavities on the antigen surface, allowing them to access hidden epitopes and cryptic sites.

"We think we can develop a range of novel antigen binding molecules with different features. We're designing novel libraries using these structures that can be screened against a wide range of targets," Nuttall said.

So far, he says, the group has engineered several libraries based on the IgNARs structure, each containing 10-100 million different proteins. Now the CRC researchers are working on applications for use of the libraries in a number of diagnostic platforms including molecular array systems and rapid, point-of-care diagnostics.

Streltsov and Nuttall's paper describing the structure of the IgNARs was published in the online edition of the Proceedings of the National Academy of Sciences on August 9.