4-1BB (CD137), a member of the tumor necrosis factor receptor superfamily, is an inducible co-stimulatory receptor expressed on activated T cells and natural killer (NK) cells. First identified in 1989, 4-1BB triggers a signaling cascade upon ligation, leading to upregulation of antiapoptotic molecules, cytokine secretion, and enhanced effector functions. It is expressed on various cell types, including T cells, NK cells, dendritic cells (DCs), B cells, monocytes, and neutrophils. Interaction between 4-1BB and its ligand, 4-1BBL, positively regulates T-cell immunity, and several antibodies targeting 4-1BB have been shown to replicate this agonism, providing co- stimulatory signaling to T cells in vitro.

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