LONDON - A new way of introducing tumor antigens into cells could help to improve an experimental treatment for cancer that involves stimulating the body's own immune system to attack malignant cells. The new technique makes it easy to program cells of the immune system to trigger the required immune responses.

A clinical trial to evaluate the safety of the approach in human patients with malignant melanoma is expected to begin next year.

The study is described in a paper in the December issue of Nature Biotechnology titled, "Efficient nonviral transfection of dendritic cells and their use for in vivo immunization."

Much research into immunotherapy for cancer has focused on a type of antigen-presenting cell called the dendritic cell. These cells ingest foreign particles, break down the proteins and display short fragments of protein called peptides on their surfaces. This allows them to stimulate other cells of the immune system to kill cells bearing the same peptide, or to produce antibodies that bind to them.

For this type of cancer immunotherapy to work, researchers need to find a way of inducing dendritic cells to take up disease-associated antigens in a manner that enables the cells to display peptides derived from these antigens. In most preclinical and clinical studies with dendritic cells to date, this has been achieved by exposing the cells directly to the antigen peptides that need to be displayed. For most types of cancer, however, the particular peptides required have not been identified.

An alternate means of achieving display of the appropriate antigen peptides is to deliver the entire antigen to the dendritic cells in the form of a gene encoding it. However, methods for delivering genes encoding antigens into dendritic cells are not very efficient and have some drawbacks. For example, using adenovirus as a vector can give efficient delivery but it also delivers numerous viral antigens, which may divert the immune response away from the antigen associated with the disease.

Researchers working in the UK now say they have developed a new and more effective way of inserting DNA into dendritic cells. Furthermore, the success of this method does not rely on knowledge of which particular peptides are needed to trigger an immune response that will fight cancer cells.

Andy Mountain, research director of Cobra Therapeutics, based at the University of Keele in Keele, UK, told BioWorld International: "All our data suggest that this is going to be a better method of loading dendritic cells with antigen. It has lots of advantages over using adenoviral vectors for antigen gene delivery. The method promises to speed up research into many applications of dendritic cells, and that includes fundamental research, preclinical research and clinical research."

Mountain added that the method could be used for any life-threatening disease - including many infectious diseases - in which an immune response would be clinically beneficial and where a suitable antigen, specific to the disease state in question, has been identified. He said Cobra is interested in licensing the new system to other companies keen to manipulating dendritic cells and/or collaborating with companies that have proprietary antigens with potential clinical benefit if introduced into dendritic cells.

The study published in Nature Biotechnology describes how the researchers identified a short peptide only 35 amino acids long called CL22. This molecule, because it is positively charged, interacts well with DNA, which is negatively charged.

"It has the effect of condensing the DNA into a very small particle, with the result that there is enhanced uptake by dendritic cells," Mountain explained. The cell processes the incoming genetic material and synthesizes a protein from it, which is then dealt with by the cell in the same way as foreign proteins derived from invading pathogens, breaking it down into peptides that then are displayed on HLA Class I and HLA Class II molecules on the cell's surface.

Experiments carried out by the team showed that CL22 was a much more efficient method of loading human dendritic cells with antigen than any other nonviral system. Dendritic cells manipulated in this way retained their ability to present antigens as normal, and continued to display all the proteins on their surfaces that researchers believe are associated with this ability.

Mountain and his colleagues went on to show that the manipulated dendritic cells were able to stimulate in vitro a type of T cell known as CD8, and to do so in a way specific to the antigen with which they were loaded.

Other experiments were carried out in mice. First, the researchers transfected mouse dendritic cells with an antigen, and used these cells to vaccinate mice. "We found that this generated both a cellular immune response, of the type believed to be required for therapeutic vaccination in humans, as well as a humoral or antibody response, which is usually associated with preventive vaccination," Mountain said. "This suggests that this approach could be used both for therapeutic vaccination and preventive vaccination."

Finally, the group tried to mimic the clinical situation in human cancer with an experiment involving mice. They transfected mouse dendritic cells with DNA encoding a melanoma-associated antigen and inoculated these cells into mice. They then measured to what extent this procedure prevented the mice from developing tumors, using a mouse model of malignant melanoma. "We showed that this leads to antitumor effects, and found that this technique prevented tumors from forming in about 80 percent of the mice," Mountain said.