Stem cell shift may lead to infections, leukemia.

Researchers have long wondered why elderly people suffer more infections and have a greater chance of developing myeloid leukemia, a type of blood cancer, than younger people do. Now, research in mice suggests that the aging of blood-producing stem cells could be responsible for both conditions.

With age, the body of a person or other animal loses its capacity to sustain its tissues and organs. "Since we know the cells mediating this maintenance are stem cells, it doesn't take a great leap of faith to think that stem cells are at the heart of that failure," says Derrick Rossi of Stanford University.

To examine whether the aging of stem cells contributes to infections and leukemia, Rossi and his colleagues irradiated young and old mice to kill off their blood-making stem cells. The scientists then transplanted such stem cells from young donor mice into elderly irradiated animals and from old donors into young irradiated animals.

After several weeks, the researchers found that young animals' stem cells transplanted into the old mice produced the different types of blood cells in ratios much like those in young mice that haven't been irradiated. However, the young animals that received old animals' stem cells had significantly fewer new lymphoid blood cells--which make cells that battle infections--than normal young animals do.

After examining gene activity in the stem cells transplanted from old animals, Rossi's team found a boost in activity among genes responsible for creating myeloid cells. These create red blood cells and some other blood components. Many myeloid-production genes have been associated with myeloid leukemia in people.

The scientists conclude in the June 28 Proceedings of the National Academy of Sciences that a shift from lymphoid-cell production to myeloid-cell production could be responsible for the increases both in infections and in risk of leukemia that come with old age.