Blood Stem Cells Remember Every Infection, Study Finds

Blood Stem Cells Remember Every Infection — and a team of Toronto scientists says it finally understands how. A study led by Dr. Stephanie Xie and colleagues at UHN's Princess Margaret Cancer Centre, published in the journal Nature, found that a distinct subset of blood-forming stem cells carries a lasting molecular memory of past inflammatory stress — changes that linger in the bone marrow long after the illness itself is gone. Put simply: Blood Stem Cells Remember Every Infection, and the traces accumulate with age. The work was honored this week with one of Canada's most respected stem cell research prizes.

Deep inside bone marrow, a small pool of blood stem cells quietly manufactures every red blood cell and immune cell a person will ever make. These cells are rare and self-renewing, and they keep blood production running for a lifetime. Inflammation — the body's response to infections and other stressors — is known to wear these stem cells down over time, contributing to an age-related decline in their function. What was not clear was how repeated bouts of inflammation leave a durable mark on cells that spend most of their lives in a quiet, resting state.

A Hidden Memory Written in the Bone Marrow

To find out, the researchers built specialized laboratory models of inflammation and recovery using human cells, exposing them to conditions that mimic severe infection and the inflammatory background of aging, then analyzing the stem cells at the molecular and single-cell level, according to the hospital research group's write-up of the study. The experiment revealed how Blood Stem Cells Remember Every Infection: a previously unrecognized group of stem cells differed in how their genes worked, and even after the inflammation faded, they retained long-lasting shifts in their behavior and in how they communicated. These memory-carrying cells stayed in a resting state and produced fewer blood cells than usual, and they handed their inflammatory instructions down to the immune cells they generated.

The memory appears to be a feature of aging. The inflammatory memory cells accumulated as the body got older, and they were enriched in people with clonal hematopoiesis — an aging-related phenomenon in which mutated blood stem cell clones expand, often considered a precursor to blood cancer and heart disease. Cancer-associated mutations affected these memory cells more than they did other stem cells. Extending the work beyond the lab, the researchers then turned to the Ontario Health Study, one of Canada's largest long-term health studies, to look for consequences in real people. There they found that in young adults, the presence of an inherited inflammatory signature in immune cells was associated with a higher mortality risk score, according to the university's account of the research.

What the Researchers Will Do Next

The discovery opens a clear line of follow-up questions. Xie's group is expanding the research into bigger population cohorts to pin down whether the inflammatory signatures can serve as biomarkers for identifying people at elevated risk. The team is also investigating whether reducing inflammation — for example, with a class of newer metabolic medicines — can decrease the occurrence of clonal hematopoiesis and prevent blood cancers. In a twist, the researchers are equally interested in the other subset of blood stem cells, the ones that showed no response to inflammatory stimulation, and whether that ability to resist inflammation can be harnessed as a therapy.

The recognition this week underscores how significant the field considers the finding. The Stem Cell Network named Xie the recipient of the 2026 Till & McCulloch Award, which honors the most impactful peer-reviewed article of the year by a stem cell or regenerative medicine researcher working in Canada, according to the network's announcement.

"Now it's about taking these findings to the population level to really see if we have signatures that can perhaps serve as biomarkers," Xie said in the university's write-up. She added that the findings underscore how environment and everyday actions can shape health: "Not all of it is out of our control. Small things, like walking to work, can matter. Decreasing inflammation could have huge benefits for our health."

That reader-facing angle is the point. Nobody can avoid every infection, and the researchers are not suggesting that ordinary illness dooms anyone to disease. But the study adds a concrete mechanism to a growing picture: the inflammatory experiences a body accumulates do not simply wash away, and managing them — through movement, sleep, and the medical tools already in use — may pay dividends decades later. The work was supported by the Canadian Institutes of Health Research, the Princess Margaret Cancer Foundation, and the Terry Fox Research Institute.

For more on how everyday habits shape long-term health, see how breakfast, sleep and exercise shape resilience, why doctors may soon prescribe nature walks for loneliness, and when health apps actually work.