Researchers have found why the natural killer cells that act as major weapons in the body's immune system are powerless against HIV.

The finding by researchers at Rush University Medical Center could potentially help in developing new drug therapies against the virus. The study is published in journal Cell Host and Microbe. The problem had dogged researchers for over two decades.

It marks the "beginning of a fascinating story that will shed new light on an important but still poorly understood aspect of the interaction of HIV with natural killer cells," according to an editorial in the journal.

"With this information, we now have a major new target for drug therapies that could potentially stop HIV and allow the body's natural killer cells to do what they are designed to do – protect the body from this lethal virus," said Edward Barker, PhD, associate professor of immunology and microbiology at Rush University and lead author of the study.

The researchers found that a protein called NTB-A (for Natural killer T-cell and B-cell Antigen), has virtually disappeared from the surface of the infected cell. Without NTB-A in place, the natural killer cells won't attack.

HIV infects a cell, replicates itself and spreads through the body by taking over the machinery of the cells and deflects the immunological cells that might destroy it.If all worked as it should to protect the body from HIV, the natural killer cells would start firing their lethal pellets. But they don't, and that is what has puzzled scientists for so long. "The barrel of the shotgun is loaded, but the trigger still has to be pulled," said Barker.

The finding is extremely exciting not only because it resolves a longstanding puzzle in how HIV is able to evade the body's innate immune response but also because it opens up new possibilities for combating HIV, he said.