BioWorld International Correspondent

LONDON - An artificial molecule that can activate a cell-surface receptor that plays a key role in coordinating immune responses could one day lead to new treatments to help fight infections. Ultimately, researchers also hope to develop the approach to trigger immune responses that would recognize and destroy tumor cells.

The molecule interacts with the receptor CD40, but the researchers who developed it say that the same method could be used to produce molecules capable of controlling other receptors from the same family.

Gilles Guichard, a chemist at the Centre National de la Recherche Scientifique based at the Institute for Molecular and Cellular Biology in Strasbourg, France, told BioWorld International: "This strategy allows us to mimic important molecules that control the immune response. One day, it is possible that this method could be applied to develop immunotherapy to treat cancer, although it should be emphasised that this is a long way in the future."

Guichard worked jointly with Sylvie Fournel, an associate professor of immunology at the Louis Pasteur University in Strasbourg. Together with collaborators, including groups in Belgium, Switzerland and the Netherlands, they reported their findings in the Nov. 6, 2005, issue of Nature Chemical Biology in a paper titled: "C₃-symmetric peptide scaffolds are functional mimetics of trimeric CD40L."

CD40 is a member of the superfamily of tumor necrosis factor receptors (TNFR). Together with the molecule that activates it - the CD40 ligand, or CD40L - they help to control the cellular and humoral immune responses.

The CD40 molecule is found on the surface of dendritic cells, macrophages and B cells, while CD40L is expressed on activated T cells. Once activated, CD40 can stimulate proliferation of B cells, production of antibodies and development of B-cell memory. On dendritic cells, activation of CD40 stimulates the cells to present antigens to T cells, transforming them into cytotoxic cells that can destroy cells infected by a virus or tumor cells.

Studies also have shown that antibodies to CD40 that act as agonists can boost the immune response both to infection and against cancer cells.

Researchers have therefore been searching for small molecules that could act as agonists on CD40. Fournel, Guichard and their collaborators set out to determine both a target peptide capable of triggering CD40 and a way of presenting that peptide so that it efficiently activates CD40.

They knew that the natural CD40L associates as a homotrimer - that is, in complexes comprising three identical molecules linked together. Other groups already had obtained the X-ray crystallographic structure of the CD40L homotrimer, so the French-led team knew the dimensions and geometry of the structure they were trying to emulate.

Guichard said, "The idea was to develop a molecule with three CD40 binding motifs on it in order to bring three CD40 cell-surface receptors into proximity with each other, sufficient to induce signaling within the cell."

They first designed a "molecular scaffold" that was made of small, flat, rigid cyclic peptides to which they joined three peptides, each comprising four amino acid residues from the binding site of CD40L. The scaffold allowed them to attach the peptide chains in a radial orientation, at 12, 4 and 8 o'clock.

Initial tests showed the artificial molecule was able to bind to CD40, and it could inhibit binding of CD40L to its receptor.

The group then embarked on experiments to find out what kind of activity the artificial molecule had. First, they took human B-cell lymphoma cells. Those cells express CD40 and undergo apoptosis when CD40L binds CD40 on their surface.

"We thought this would be a good model to check if our molecule could mimic the natural effect of CD40L," Guichard said. "When we incubated B cell lymphoma cells in the presence of our molecule, we observed concentration-dependent apoptosis."

The researchers also replaced individual amino acids from the peptide chains that represented the binding site, with other amino acids. They found three out of the four amino acids on the peptide chains were required for binding to CD40 and for induction of apoptosis in B-cell lymphoma cells.

Further tests confirmed the artificial molecule could mimic the action of CD40L in a range of ways.

The team now wants to find out more about how the artificial molecule works and discover whether it has similar effects when used in animal models. They also want to apply the method to other members of the TNFR superfamily.