It was while investigating a particular type of dendritic cell that Housseau noticed the outer membranes of these cells were studded with what were supposed to be hallmarks of NK cells, akin to finding feathers on a dog.
"We thought we were looking at dendritic cells, but we were wrong - they were some type of NK-dendritic cell blend," says Housseau. The blended cell turned out to be a newly identified actor on the immune system stage that retains all the molecular characteristics of both NK and dendritic cells.
Probing further, Housseau scoured the surface of IKDCs to create a sketch of its molecular profile. He found that it produces both types of interferon proteins, normally secreted independently by NK and dendritic cells. He also found both NK and dendriticlike molecules on the surface of IKDCs. Housseau calculated that they account for about 10 percent of conventional dendritic cells in the spleen.
IKDCs begin their lives behaving like an NK cell. After the cell encounters a pathogen, the cell switches roles from killer to dendriticlike messenger, and, according to the researchers, the swap occurs only once. Then, the cell dies and is replenished by the bone marrow.
"When an IKDC cell switches to its messenger function, the transformation is quite astonishing," says Pardoll. The cell sprouts long, hairy tentacles called dendrites. It uses its "arms" to increase the amount of surface area it reaches to communicate and interact with other immune cells.
In the next step of their investigation, the scientists tracked the location of fluorescent-tagged IKDCs and their corresponding stage of transformation after infecting mice with bacteria called listeria. In assassin-mode, the IKDCs were found in the blood, lining of the gut, liver and other organs - all areas where there is close contact with environmental pathogens. "Here, IKDCs are ready to sense invaders and spring into action," says
Contact: Vanessa Wasta
Johns Hopkins Medical Institutions