The human brain takes in large numbers of immune cells from the rest of the body as it ages, according to a Stanford study published in Nature. That finding runs against the long-standing textbook view that the brain runs its own separate immune operation, protected by the blood-brain barrier and patrolled by its own specialized resident immune cells.
"We usually think of the brain as a closed system," said Julia Belk, a postdoctoral scholar in pathology at Stanford Medicine and the study's first author. "What we found is that actually a lot of immune cells enter the human brain during aging." The work was supported in part by the Knight Initiative for Brain Resilience at Stanford's Wu Tsai Neurosciences Institute.
The implication is that brain aging may be less of an isolated process than assumed. If immune cells circulating elsewhere in the body are arriving in brain tissue in significant numbers over a lifetime, then the body-wide immune system becomes a plausible player in what happens to the aging brain - and, potentially, a target that drugs could reach more easily than cells sitting behind the blood-brain barrier. The Stanford team suggests the discovery could eventually open new avenues for treating neurological disease.
Belk came to the question by an unusual route, training in computer science at Stanford before moving into biology through the Chemistry/Biology Interface Predoctoral Training Program at Sarafan ChEM-H, an interdisciplinary background she has credited for the approach behind the work. What these infiltrating cells actually do once inside the brain - whether they contribute to damage, respond to it, or both - is the obvious next question, and one this study opens rather than settles.