Finding a needle in a haystack: Activation-induced CD137 expression accurately identifies naturally occurring tumor-reactive T cells in cancer patients

We have recently identified tumor necrosis factor receptor superfamily, member 9 (TNFRSF9, best known as CD137 or 4–1BB) as a biomarker of tumor-reactive T cells naturally occurring in cancer patients, and developed a rapid, accurate system to comprehensively isolate lymphocytes with tumor-rejecting properties from human biopsies. Our findings reveal a previously unappreciated role for CD137, a co-stimulatory TNFR family member, in the immunobiology of human cancer.