Circulating stem cells from bone marrow recognize tissues in distress and stimulate an innate immune response, according to findings published today in Cell. The researchers identified new pathways for these circulating hematopoietic cells, and propose that their travels contribute additional immune cells to tissues experiencing damage or infection.

"Stem cells are much more adventurous in a way than one would have expected previously," Ulrich von Andrian of Harvard University, an author of the study, told The Scientist. "They are constantly on the move, migrating through perhaps most tissues in the body looking for the presence of infectious agents or tissue damage."

Scientists were aware that hematopoietic stem cells could circulate around the body, but were unclear about the extent of their travels. Paul Kincade at the Oklahoma Medical Research Foundation, who did not participate in the study but wrote an editorial on the paper, said people had thought these cells circulated just to get from bone to bone and to balance stem cell distribution. But von Andrian and his colleagues found that hematopoietic stem cells can go from blood to numerous tissues to thoracic duct lymph back to blood and return to bone marrow. The study "defined new migration routes for hematopoietic stem cells," Kincade told The Scientist.

The researchers also found that the cells differentiate into innate immune cells in response to signals of infection, which likely explains why they travel so extensively throughout the body, said Amy Wagers, another author on the study.

The findings build upon evidence Kincade published in 2006 showing that hematopoietic stem cells express toll like receptors, which detect pathogens. Von Andrian and his colleagues showed that toll like receptor agonists prompt stem cell differentiation into innate immune cells. "It's a nice extension of our culture studies that we did to an in vivo situation," Kincade said.

Von Andrian said he's curious to know how these cells operate in a disease setting, and whether they might have any negative effects. "Is this surveillance by stem cells primarily beneficial or could there be instances where misguided differentiation contributes to pathology?"

Correction (Dec. 5): A previous version of this post misspelled Ulrich von Andrian's name. The Scientist regrets the error.