Scientists have found what appears to be a whole new way for immune cells to communicate with one another: long, narrow tubes that enable them to connect and exchange molecules. In this month's issue of Immunity, they report that such tubes, called tunneling nanotubules (TNT), may help explain how immune responses can be initiated so rapidly.
This kind of structure was first observed in mammals last year by a team of scientists in Germany and Norway in several types of cell cultures (Science, 13 February 2004, p. 1007). Now, Simon Watkins and Russell Salter of the University of Pittsburgh, Pennsylvania, say they have, for the first time, caught the tubules in action transmitting signals from cell to cell. Using state-of-the-art techniques, they watched human dendritic cells and macrophages, immune cells that circulate in blood. They saw waves of calcium ions--which tell immune cells to kick into action--and small molecules spread from one to another via a network of membrane tubules.
The tubes can extend hundreds of micrometers--the cells themselves are just 15 micrometers wide--and each cell can have up to 75 of these transient extensions, the researchers say. "It's nothing short of amazing," Salter says. Antigens--foreign substances that elicit an immune response--may also be distributed among cells this way, he says.
Watkins calls this "clearly a third form of intercellular communication"--distinct from synapses between nerve cells and from gap junctions, which are involved in transfer of small molecules between adjacent cells. The study shows that neurons are not the only cells capable of long-distance communication, he says, and it means immune system cells have an additional way of communicating beyond the secretion of substances such as cytokines.
Cell biologist Hans-Hermann Gerdes of the University of Bergen in Norway, one of the original researchers to show the existence of TNTs in mammals, calls the study "important" because it shows that a physiologically significant signaling molecule--calcium--travels through the tubes. He predicts that nanotubules will be found in a wide variety of cell types.