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Training for hired killers
Last February, Christian Münz reported that natural killer cells — the assassins of the body’s immune system — go through a period of “training” in the tonsils, lymph nodes and spleen before they are released into the bloodstream. It’s during this period that immune system cells called dendritic cells activate the natural killers in one of two modes. Either the cells become full-fledged executioners or they become dispatchers, secreting cytokines, a type of chemical messenger protein that influences the strategies the body uses to fight infection.
“We saw that dendritic cells were able to make the natural killer cells proliferate, secrete cytokines, and increase their killing ability,” Münz says. “We then got curious about where this interaction takes place.”
In the November 23 issue of Proceedings of the National Academy of Sciences, Münz and colleagues show that dendritic cells and natural killer cells are found together in a specific area of human lymph nodes. Two proteins that the dendritic cells secrete ensure the survival and proliferation of the natural killer cells and increase natural killer cell production of the protein interferon-gamma.
It’s the interferon-gamma that’s the key. Natural killer cells are part of the innate immune response, broadly battling all types of infection. But interferon-gamma signaling can sway the adaptive immune response, which tailors its attack to a specific type of intruder. When they interact with dendritic cells in the lymph nodes, the natural killer cells become a bridge to the adaptive immune system, helping direct its response.
“Our new findings change the view of natural killer cells,” says Münz. “Initially it was thought that as part of the innate immune response, they only limit viral infection or tumor cell mass until the adaptive immune response kicks in. Now we can see that natural killer cells aren’t just effectors, but helper cells for the adaptive immune response.”

January 28, 2005



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