Metformin promotes the expansion of CD8+ central memory T cells by offsetting Pentose phosphate pathway

The immune system relies on a specialized squad of cells called T cells to remember past infections and fight off new threats. Among these, a group known as central memory T cells acts as a long-term reserve, capable of multiplying rapidly when the body encounters a familiar enemy again. These cells are crucial for lasting protection, but their numbers and effectiveness tend to decline as people grow older, leaving the body more vulnerable. Scientists have long known that these cells need energy to survive and function, and that the way they generate this energy changes as they age. While a common diabetes medication called metformin has shown promise in helping the immune system in animal studies, it was unclear whether it could actually help restore these vital memory cells in middle-aged humans, or how it might change the way those cells burn fuel.

Researchers at Sun Yat-sen University set out to answer this question by studying blood samples from 138 healthy adults between the ages of 45 and 65. They took these samples and treated them in a laboratory dish with a specific dose of metformin for 24 hours, comparing the results to untreated samples. The team found that the medication caused a significant shift in the immune landscape. Specifically, the number of central memory T cells increased by more than double in the treated samples. This expansion did not happen by chance; the medication appeared to reprogram the cells' internal energy systems to support this growth.

To understand how this happened, the scientists looked inside the cells to see how they were producing energy. They discovered that metformin encouraged these memory cells to switch their primary fuel source. Instead of relying heavily on breaking down sugar, which is a common strategy for rapidly dividing cells, the treated cells began to burn fat more efficiently. The researchers observed a rise in the machinery responsible for this fat-burning process, along with an increase in the number of mitochondria, the tiny power plants inside the cell. At the same time, the cells showed a boost in their ability to generate energy through a process called oxidative phosphorylation, which is a more efficient way to produce power than sugar breakdown.

This shift in fuel usage triggered a chain reaction within the cell. The increased activity in the mitochondria produced a small, controlled amount of reactive oxygen, which acted as a signal rather than a harmful toxin. In response, the cells activated a specific pathway known as the pentose phosphate pathway. This pathway helps the cell manage its internal balance and build the materials needed for growth. The study showed that the medication increased the activity of an enzyme that drives this pathway while simultaneously reducing the activity of enzymes that push the cell toward sugar breakdown. Essentially, the drug guided the cell to use its resources in a way that favored long-term survival and expansion rather than short-term, rapid action.

The researchers also noted that this effect was specific to certain types of cells. While the central memory T cells expanded and changed their metabolism, other types of T cells did not respond in the same way. For instance, the medication did not increase the numbers of effector memory cells, which are designed for immediate, short-term attacks. This suggests that metformin does not simply boost all immune cells indiscriminately but selectively supports the specific subset responsible for long-term immunity. The study confirmed that this metabolic reprogramming occurred in human cells from middle-aged donors, indicating that the immune system retains the ability to adapt its energy use even as people age.

By mapping these changes, the study reveals a clear link between the medication, the way cells burn fat, and the growth of long-lasting immune cells. The findings suggest that metformin helps these cells maintain their energy balance and redox stability, allowing them to persist and multiply. While the research was conducted in a laboratory setting and does not yet prove that taking the drug will prevent disease in people, it provides a detailed look at the biological mechanism behind the effect. The work highlights that the immune system of middle-aged adults remains flexible and responsive to metabolic signals, offering a potential avenue for supporting immune health as we age.