The 1AH2 monoclonal antibody specifically binds to CD137. CD137 is a member of the TNFR/NGFR superfamily that is likewise known as Tnfrsf9, 4-1BB, Ly-63, or ILA. Monomers or multimeric forms of CD137 are expressed, upon activation, on the surface of splenic T lymphocytes, thymocytes, intestinal intraepithelial T lymphocytes (IEL), and some T cell lines and clones. While stimulating T cells by IL-2, IL-4, or anti-CD28 alone does not result in the expression of CD137; addition of IL-2, IL-4, anti-CD28, or syngeneic accessory cells to splenic T cells stimulated via TCR/CD3 can result in a high level CD137 expression. CD137 is also reportedly expressed on IL-2 activated NK cells, but not on freshly isolated NK cells. CD137 physically associates with p56 [lck] through a Cys-Arg-Cys-Pro binding site in its cytoplasmic domain; the same motif in the cytoplasmic tail of the CD4 and CD8a molecules is responsible for association with p56 [lck]. A signaling function for the CD137 molecule in mouse T cells is indicated by reports in which cross-linking of CD137 with 1AH2 mAb resulted in enhanced proliferation of CD3e-activated splenic T cells and IEL and in enhanced cytolytic activity of IEL in response to immobilized anti-CD3e. In addition to extracellular matrix proteins which bind to CD137, the CD137L (4-1BBL) serves as a ligand for CD137. This molecule has also been detected on LPS-activated macrophages, and anti-IgM antibody-activated splenic B cells. Interaction between T and B cells through CD137/CD137L is reported to play a role in antigen presentation, further supporting a costimulatory role for CD137 in the immune response of T lymphocytes.