The 4-1BB ligand (4-1BBL), also known as CD137L, is a type II transmembrane protein that is part of the TNF superfamily. 4-1BBL is the high affinity ligand of 4-1BB. Its receptor, CD137/4-1BB, is found on a variety of cells, including inflamed endothelial cells, where its expression enhances extravasation of CD137L expressing monocytes. 4-1BB Ligand is expressed by activated B cells, monocytes, macrophages, dendritic cells (DC), T cells, lymphoma and multiple myeloma cells, hematopoietic stem cells, early myeloid progenitors, neurons, and astrocytes. 4-1BBL induces the proliferation of activated peripheral blood T-cells. Also, 4-1BBL may have a role in activation-induced cell death (AICD).
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