TNFRSF9 (CD137) expression is limited to a subset of the DLBCL tumor microenvironment. (A) Within sorted subpopulations, TNFRSF9 is more highly expressed in nontumor (CD19⁻) than tumor cells (CD19⁺).¹⁸  (B) Immunohistochemical analysis of CD137 expression among rare infiltrating cells in normal tonsil (40×, left) and a representative DLBCL from 75 tumors (200×, right). Insets show high magnification of individual positive infiltrating cells (600×). (C) Flow cytometric evaluation of CD137 expression in a representative DLBCL tumor showing lack of expression among tumor cells but detectable expression in a small proportion (5.9%) of CD4⁺ T cells and a larger proportion (28.6%) of CD8⁺ T cells. (D) Analysis of T cells in tonsils, PBMCs, and DLBCL tumors demonstrates CD137 expression on CD4⁺ and CD8⁺ T cells in the tumor microenvironment, but not on healthy lymphoid counterparts. (E) Fraction of CD137-expressing T cells is not correlated with total T-cell frequency in DLBCL tumors.