CSF1R inhibition during cranial radiotherapy reshapes the immune environment and glial dynamics

When doctors treat brain tumors or cancer that has spread to the brain, they often use high-energy beams of radiation to destroy the disease. This treatment, known as cranial radiotherapy, is a powerful tool, but it comes with a heavy cost. Many patients who survive the cancer later face a slow, progressive decline in their thinking and memory. This happens because the radiation, while killing the tumor, also injures the healthy brain tissue around it. Inside the brain, there are tiny immune cells called microglia. These cells act as the brain's own security force, constantly patrolling to clean up debris and repair damage. When radiation hits the brain, these security cells often become overactive and inflamed, which can lead to further injury. Scientists have long wondered if turning off these cells during treatment might protect the brain, allowing it to heal better. The idea was that if you could temporarily remove the security force, you might stop the inflammation that causes long-term cognitive problems.

A team of researchers at the Netherlands Cancer Institute decided to test this idea with a specific drug that blocks the signal microglia need to survive. They gave this drug to mice before, during, and shortly after the mice received a course of radiation to their heads that mimicked a high-dose human treatment. The goal was to see if clearing out the microglia would shield the brain from the worst effects of the radiation. The results, however, told a more complicated story than the scientists expected. Instead of simply protecting the brain, removing the microglia created a vacuum that was quickly filled by a different type of immune cell from the bloodstream. While the original brain security cells were gone, these new invaders took their place, but they did not behave exactly the same way. The study found that this swap did not stop the radiation from damaging other critical parts of the brain, such as the cells responsible for making new brain connections or the cells that wrap nerves in protective insulation. In fact, the absence of the original microglia seemed to make the brain's support cells react faster and more intensely to the injury.

The researchers began by feeding the mice a special diet containing the drug pexidartinib for three weeks before the radiation treatment started, continuing for ten days after the treatment ended. This drug works by blocking a receptor that microglia need to survive, effectively starving them. In the mice that received only the drug, the number of microglia dropped significantly, but not completely. However, when the researchers combined the drug with radiation, the effect was much stronger. The radiation made the remaining microglia even more vulnerable to the drug, leading to a near-total disappearance of these cells from the brain. This was a dramatic success in removing the target cells, but it set the stage for an unexpected consequence.

Once the mice stopped eating the special diet, the empty space left by the vanished microglia did not stay empty for long. The brain, sensing a lack of its usual security force, began to pull in immune cells from the blood. These are called monocytes, and they are the body's general-purpose immune cells that usually stay outside the brain. In the mice that had received both the drug and radiation, these monocytes flooded into the brain and took up residence. The researchers could tell these were not the original microglia because they lacked a specific marker that healthy, resting microglia always carry. Instead, these new cells were a different kind of immune cell that had migrated in from the periphery. Over time, these intruders changed their shape to look more like the microglia they replaced, but they never fully regained the same identity or function as the original cells.

The study also looked at how this change affected other parts of the brain. The researchers examined the cells that create new brain cells and the cells that produce myelin, the fatty coating that helps nerves send signals quickly. They found that the radiation had destroyed many of these cells, and the drug did not stop this damage. The loss of new brain cells happened just as severely in the mice that received the drug as in those that did not. Similarly, the protective myelin coating on the nerves was damaged by the radiation, and the drug offered no rescue. In fact, the brain cells that provide structural support, known as astrocytes, reacted much more quickly and aggressively in the mice that had received the drug. These support cells usually swell up and become active when the brain is injured, but in the absence of microglia, this reaction happened faster and was more pronounced.

By six months after the treatment, the researchers observed a partial recovery of the myelin-producing cells in some areas of the brain in the mice that had received the drug. This suggests that the new environment created by the drug and the invading monocytes might eventually help some of these specific cells to repopulate. However, this was a limited recovery. The overall health of the brain tissue, including the integrity of the nerve coatings and the ability to generate new brain cells, did not return to normal. The study concluded that while removing microglia during radiation does change the brain's immune landscape, it does not provide the broad protection scientists had hoped for. Instead, it leads to a permanent shift in the types of immune cells living in the brain, replacing the original security force with a different group that behaves differently over the long term.

The findings challenge the simple idea that removing microglia is a straightforward way to protect the brain from radiation. The brain is a complex ecosystem where different cell types rely on each other. When one group is removed, others move in to fill the gap, but they do not always perform the same duties. The study suggests that the timing and the extent of such interventions are critical. Simply wiping out the microglia during treatment might not be enough to prevent the long-term cognitive decline seen in patients. The researchers noted that future strategies might need to be more precise, perhaps allowing some microglia to remain so they can guide the repair process, rather than leaving the brain to be repopulated by cells from the blood. This work highlights that the brain's response to injury is a delicate balance, and altering one part of the system can have ripple effects that are difficult to predict.