Human 4-1BB Ligand/TNFSF9 APC-conjugated Antibody

4-1BB Ligand (4-1BBL; also TNFSF9 and CD137L) is a 32 kDa type II transmembrane protein that belongs to the TNF superfamily (TNFSF) molecules (1‑4). The human 4-1BBL cDNA encodes a 254 amino acid (aa) protein that contains a 25 aa N-terminal cytoplasmic domain, a 23 aa transmembrane segment, and a 206 aa C-terminal extracellular region (5). The extracellular domain (ECD) of 4-1BBL has a jelly-roll, beta-sandwich tertiary structure that is similar to other TNFSF members. There is only one cysteine in the human ECD, and no potential N-linked glycosylation sites. The potential exists, however, for O-linked glycosylation. The human 4-1BBL ECD shares 32% and 35% aa identity with mouse and rat ECD, respectively. In the cytoplasmic domain, human 4-1BBL is 55 aa shorter than the equivalent region in rodents. 4-1BBL is expressed by activated B cells, macrophages, dendritic cells, activated T cells, neurons and astrocytes (2, 3, 6). A bioactive 26 kDa soluble form of 4-1BBL, presumably generated by MMP cleavage, occurs in humans (4). Human 4-1BBL signals through both CD137/4-1BB and itself. Its cytoplasmic tail participates in reverse signaling that induces apoptosis in T cells and cytokine secretion (IL-6; TNF-alpha) by monocytes (7, 8). 4-1BBL binding to CD137/4-1BB produces a number of effects. It seems to play a key role in the T cell recall response. It maintains T cell numbers at the end of a primary response, and induces CD4⁺ and CD8⁺ T cells to proliferate and secrete cytokines such as IL-2 and IFN-gamma in CD4⁺ cells, and IFN-gamma in CD8⁺ cells (9, 10).