Scientists Locate Immune
System Switch Which
Cuts Off Tumours
By Patricia Reaney

LONDON (Reuters) - Scientists believe they have found a ``master switch'' which allows tumours to use various tricks to evade the body's immune system. It's a protein called PML. If it works, killer T-cells in the body's immune system go into battle against cancerous cells. If it doesn't, the tumour takes over with devastating consequences. ``The master switch controls whether killer cells can recognise the tumour,'' Dr Yang Liu, an immunologist at Ohio State University Medical Center in Columbus, said in a telephone interview. PML works by controlling several genes that produce and transport something called a major histocompatibility complex (MHC) -- comprising a peptide and molecules -- to the surface of the cell. They act as a marker for the T-cells, alerting them to the tumour. ``PML is like a master switch that controls whether the cell can or cannot put the peptide on the cell surface,'' Liu added. The finding is another piece in the puzzle of how cancer and viruses invade the body's natural defences. If there is enough PML the immune system can keep the tumour in check, but if there isn't the tumour evades the T-cells and takes over. Liu thinks the protein could be a good focus for new cancer therapies if scientists find a way to induce the body to produce PML. In a report published in the science journal Nature, Liu, Pan Zheng and their colleagues described how they tested PML in leukaemia cells but they believe the research could apply to other cancers. ``We know that this problem of switching off the cell surface molecule is a general phenomena of many, many cancers. In a high percentage of malignant tumours you have this process going on. We need to prove that it is PML that controls it,'' said Liu. If every cell type needs PML to produce MHC, Liu believes the finding could be revolutionary. His guess is that PML may be the first of many such master switches.