Found in brain tumors:

https://www.nature.com/articles/s41591-024-03152-x?utm_source=Live+Audience&utm_cam...

"The human brain is usually considered to be beyond the reach of most immune cells. However, analysis of people who have a type of brain tumour called glioblastoma has revealed tumour-targeting T cells in the skull bone marrow adjacent to the tumour. Some of the characteristics of the cells hint that they’re something special. For example, their high expression of sphingosine-1-phosphate receptor 1 (S1PR1) protein, which signals to T cells to go out on the hunt, that the skull bone might serve as a reservoir for immune cells. And T cells isolated from skull bone marrow remain activated after rounds of restimulation — avoiding ‘exhaustion’, which is one of the main obstacles to immunotherapy."

Comment: just an article summary. Finding those cells is a surprise. S1PR1 protein implies design.