The researchers had wiped out the adaptive immune systems of the mice with radiation, prior to infecting them, to prevent those "memory" cells from complicating their analysis. Monitoring blood counts, the team found a virus-specific natural killer cell family grew and multiplied in numbers 100 times higher than normal in the spleen, and 1,000 times higher-than-normal in the liver, of the mice. "That's more like the adaptive immune system, than the innate," notes Lanier. Some of these virus-specific natural killer cells lived for at least 70 days and when transferred into another just-infected mouse delivered an immune response 10 times more powerful than regular natural killer cells. Just like adaptive immune cells, "NK cells have the ability to 'remember' previously encountered pathogens and are able to mediate more efficient protective immunity against subsequent infection," concludes the study.
"The textbooks are wrong, or at least, need to be amended," says von Andrian, who was not part of the new study. "Even this study could be just the tip of the iceberg, given the many subsets of natural killers cell that exist." Past studies his group has submitted suggesting results along these lines have met resistance from reviewers skeptical that "innate" immune cells could demonstrate memory, von Andrian adds.
The trick to demonstrating the "memory" of natural killer cells was using the right kind of virus, ones the cells were predisposed to destroy, to test their longevity, Lanier says.
A whole new approach to vaccination might result from the natural killer cell finding, suggests Lanier, knowing now that they 'remember' past invaders. "It won't work against influenza, but might against the virus families that the cells are primed to respond to already," Lanier says. The finding might explain some things about the reappearance of shingles, where chicken pox hidden in the nervous system can reappear to strike often-elderly patients as well.
For immunologists, the finding moves natural killer cells from the innate immune system to "kind of an intermediate" place in the evolution of the immune system, Lanier says, with natural killer cells representing a transitory cell type between the innate and adaptive immune system. "Natural killer cells might be a 'missing link,' in that sense," Lanier concludes.