After centuries of medical advancement, it seems logical to assume that we would by now have a complete list of all the organs in the human body. Yet, scientists continue to discover new ones—organs that operate in the background, performing vital functions such as pain perception or saliva production.
In a new study published in the journal Nature, biologists from the Washington University School of Medicine (WashU) in St. Louis have discovered a new immune organ inside the skull. It appears to act as a “security checkpoint” for fighting brain cancer in its early stages.
So far, this organ has only been identified in mice, but researchers say there are indications that it exists in humans as well. If we can boost these structures, we might gain a new weapon against deadly forms of brain cancer.
“This study shows that skull bone marrow is much more than just a structural framework; it houses previously unknown centers for immune responses specific to the brain,” says study co-author Jonathan Kipnis of Washington University. “Identifying this localized immune niche changes our understanding of neuro-immune interactions and opens up exciting new possibilities for treating brain tumors and other neurological diseases.”
The human body operates with not just one, but two distinct immune systems: one for the central nervous system—including the brain and spinal cord—and another for the rest of the body.
For a long time, these two systems were thought to be completely separate; indeed, immune cells from the peripheral system can cause a variety of problems if they enter the brain. Some are known to contribute to neurodegenerative diseases, as the delicate tissues of the nervous system can be damaged by the “reckless” tactics employed elsewhere in the body. However, a growing body of research suggests that the connection between these processes is somewhat broader than previously thought. In earlier work, the team behind this new study identified tiny channels running from brain tissue, through the dura mater (the outermost protective membrane surrounding the brain), and into the skull itself.
This time, the researchers used fluorescent tracer proteins to observe the movement of molecules through these channels, tracking their path from neurons to the skull’s bone marrow.
Ultimately, they traced them to locations strikingly similar to germinal centers—structures that form in lymph nodes and serve as training grounds for immune cells.
Essentially, naive B cells enter these structures, proliferate rapidly, and arm themselves; other immune cells provide them with “wanted posters” identifying specific pathogens, and they head off to battle.
In this instance, these training camps are strategically positioned near the brain, allowing for frontline combat operations without the need to recruit soldiers from more distant parts of the body.
“We had never seen structures like this in healthy bone marrow before,” says study co-author Chan Hyun Park of the University of Washington. “It is an exciting discovery, indicating that the complex brain requires its own specialized immune structures for protection.”
The researchers tested this concept using mouse models of glioma, a particularly dangerous form of brain cancer. They modulated the activity of these lymphoid structures by applying a hydrogel containing either immunostimulatory or immunosuppressive compounds beneath the scalp.
Mice with compromised skull lymphoid structures exhibited significantly more aggressive tumors and lower survival rates compared to control mice. However, mice injected with a mixture of three immune-boosting proteins exhibited a stronger immune response from the skull’s lymphoid tissue, enabling them to fight cancer more effectively and significantly increasing their survival rates.
Of course, results obtained in mice do not necessarily apply to humans, but researchers note that follicular T cells have previously been found in the bone marrow of human skulls, suggesting that these structures may also be present in our own heads.
If this is the case, it could ultimately lead to significant improvements in the fight against brain cancer: injecting a hydrogel under the scalp is far less invasive than full-scale brain surgery to remove tumors.
This discovery fundamentally changes our current understanding of neuroimmunology,” says Kipnis. “Knowing that the brain relies on defense mechanisms located in the surrounding skull could reshape how we approach developing treatments for many neurological conditions—including Alzheimer’s disease, Parkinson’s disease, schizophrenia, long COVID, and others with an immune component. Such therapies could target these immune hubs directly through the skull, avoiding serious side effects affecting peripheral organs.”
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