Mild respiratory COVID can cause multi-lineage neural cell and myelin dysregulation.

Many people who've recovered from COVID-19 continue to experience lingering issues with thinking, focus, and memory — often described as "brain fog." Interestingly, this pattern of symptoms looks a lot like the cognitive difficulties some cancer patients experience after treatment, a condition where inflammation involving the brain's immune cells, called microglia, plays a central role. This study set out to investigate whether a similar underlying process might explain lingering brain symptoms after COVID. Using mouse models, researchers studied what happens in the brain after a mild respiratory COVID infection — one that stays in the lungs and airways rather than directly infecting the brain. They found that even this mild, non-brain infection triggered lasting activation of microglia, but specifically within the brain's white matter (the tissue responsible for connecting different brain regions). This matched what researchers separately observed in human brain tissue as well. The consequences of this brain immune activation were long-lasting. Weeks after the initial infection, the mice showed ongoing problems: reduced production of new neurons in the hippocampus (a brain region critical for memory and learning), fewer oligodendrocytes (the cells that maintain the protective myelin coating around nerve fibers), and a loss of myelin itself. Alongside these changes, the researchers found elevated levels of inflammatory signaling molecules in the fluid surrounding the brain and spinal cord, including one called CCL11. To test whether CCL11 itself might be driving these brain changes, the researchers administered it directly into mice and found it was enough, on its own, to trigger microglial activation in the hippocampus and impair the growth of new neurons there. This pointed to CCL11 as a key player in the process. Supporting this idea, humans who continued to experience cognitive symptoms after COVID also had elevated levels of CCL11 in their systems, mirroring what was seen in the mice. The researchers then compared these effects to a different respiratory illness: mild influenza. Early on, influenza caused a very similar pattern of brain changes as COVID — the same kind of white-matter microglial activation, oligodendrocyte loss, impaired neuron growth, and elevated CCL11. However, over time, most of these effects faded after influenza. The notable exception was that the elevated CCL11 and the neuron-generation problems in the hippocampus persisted, much like what was seen with COVID. Taken together, these findings suggest that a mild, purely respiratory infection with either COVID or the flu can trigger inflammation-driven brain changes similar to those seen after cancer treatment. But COVID appeared to have a broader and more durable impact on the brain compared to influenza. This research helps explain why lingering cognitive symptoms are common after even mild COVID infections, and it points to CCL11 and brain immune cell activation as potential targets for future treatments aimed at helping patients recover their full cognitive function.