Scientists have discovered a protein that plays a central role in promoting immunity to viruses and cancer, an advance that may open the door to new therapies. Researchers from Imperial College London and colleagues in experiments in mice and human cells found that the protein promotes the proliferation of cytotoxic T cells, which kill cancer cells and cells infected with viruses. The discovery was unexpected because the new protein had no known function and does not resemble any other protein. The team, which also included researchers from Queen Mary University of London, ETH Zurich and Harvard Medical School, is now developing a gene therapy designed to boost the infection-fighting cells, and hope to begin human trials in three years. Cytotoxic T cells are an important component of the immune system, but when faced with serious infections or advanced cancer, they are often unable to proliferate in large enough quantities to fight the disease. By screening mice with genetic mutations, the team discovered a strain of mice that produced 10 times as many cytotoxic T cells when infected with a virus compared with normal mice. These mice suppressed the infection more effectively, and were more resistant to cancer. They also produced more of a second type of T cells, memory cells, enabling them to recognise infections they have encountered previously and launch a rapid response. The mice with enhanced immunity produced high levels of a hitherto unknown protein, which the researchers named lymphocyte expansion molecule, or LEM. They went on to show that LEM modulates the proliferation of human T cells as well as in mice. "Cancer cells have ways to suppress T cell activity, helping them to escape the immune system. Genetically engineering T cells to augment their ability to fight cancer has been a goal for some time and techniques for modifying them already exist," said Professor Philip Ashton-Rickardt from the Section of Immunobiology in the Department of Medicine at Imperial, who led the study. "By introducing an active version of the LEM gene into the T cells of cancer patients, we hope we can provide a robust treatment for patients," he said. The study is published in the journal Science.