4-1BB

4-1BB (TNFRSF9, CD137, ILA) is an inducible co-stimulatory receptor of the TNF receptor superfamily, expressed on activated CD4+ and CD8+ T cells, regulatory T cells, NK cells, NKT cells, dendritic cells, monocytes and neutrophils. 4-1BB is a type I transmembrane glycoprotein with four cysteine-rich extracellular domains; ligand binding (4-1BBL, TNFSF9) recruits TRAF1/2 to activate canonical NF-κB, MAPK and ERK pathways, enhancing CD8+ T-cell proliferation, survival (via BCL-xL and BFL-1), mitochondrial biogenesis and cytotoxicity, and augmenting NK-cell function and antibody-dependent cellular cytotoxicity. Unlike CD28, 4-1BB signals are dispensable for initial activation but decisive for sustaining effector responses and generating durable CD8+ T-cell memory, especially under chronic antigen and hypoxic tumor conditions. Agonistic anti-4-1BB antibodies (urelumab, utomilumab) are in clinical development for solid tumors and lymphomas, particularly in combination with CAR-T cells and immune checkpoint blockade; hepatic toxicity of strong agonism necessitates carefully engineered formats.