4-1BB is a member of the tumor necrosis factor (TNF) receptor family. Its alternative names are tumor necrosis factor receptor superfamily member 9 (TNFRSF9), CD137 and induced by lymphocyte activation (ILA). It is currently of interest to immunologists as a co-stimulatory immune checkpoint molecule._x000D_
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4-1BB can be expressed by activated T cells, but to a larger extent on CD8 than on CD4 T cells. In addition, CD137_x000D_
expression is found on dendritic cells, follicular dendritic cells, natural killer cells, granulocytes and cells of blood vessel walls at sites of inflammation._x000D_
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The best characterized activity of 4-1BB is its costimulatory activity for activated T cells. Crosslinking of 4-1BB enhances T cell proliferation, IL-2 secretion, survival and cytolytic activity. Further, it can enhance immune activity to eliminate tumors in mice.
