CD8A and carcinoma: As a member of the tumor necrosis factor (TNF) ligand family, 4-1BBL is endogenously expressed on activated APCs, T cells, and several carcinoma cell lines.8, 9, 10 Engagement of 4-1BBL with its receptor, 4-1BB, promotes costimulatory signal transduction in CD4+ and CD8+ T cells, leading to their activation and expansion of CD8+ T cells with enhanced IFN-γ production.8