CD137 is expressed on blood vessel walls at sites of inflammation and enhances monocyte migratory activity.

The cytokine receptor CD137 is a member of the TNF receptor family and a potent T cell costimulatory molecule. Its ligand is expressed on antigen presenting cells as a transmembrane protein and it too can deliver signals into the cells it is expressed on (reverse signaling). In monocytes, immobilized CD137 protein induces activation, prolongation of survival and proliferation. Here we show that recombinant immobilized CD137 protein enhances migration of monocytes in vitro. Further, CD137 expression on spheroids leads to a significantly enhanced infiltration by monocytes. The migration-inducing activity of CD137 could be confirmed in vivo. Matrigel, which was coated with recombinant CD137 protein and was inserted into the flanks of mice attracted large numbers of monocytes and was heavily infiltrated by these cells. In vivo, expression of CD137 by blood vessel walls at sites of inflammation was detectable by immunohistochemistry. CD137 expression is inducible by proinflammatory cytokines in endothelial cells, suggesting that a physiological function of CD137 may be the facilitation of monocyte extravasation in inflammatory tissues.

Expression of CD137 on Hodgkin and Reed-Sternberg cells inhibits T-cell activation by eliminating CD137 ligand expression.

Hodgkin lymphoma is caused by a minority population of malignant Hodgkin and Reed-Sternberg (HRS) cells that recruit an abundance of inflammatory cells. The long-term survival of HRS cells among the vast majority of immune cells indicates that they have developed potent immune escape mechanisms. We report that the TNF receptor family member CD137 (TNFRSF9) is expressed on HRS cells, while normal B cells, from which HRS cells are most often derived, do not express CD137. In 48 of 53 cases of classical Hodgkin lymphoma, CD137 was detected on HRS cells. Ectopically expressed CD137 transferred by trogocytosis from HRS cells to neighboring HRS and antigen-presenting cells, which constitutively express the CD137 ligand (CD137L and TNFSF9), became associated with CD137L and the CD137-CD137L complex was internalized. Disappearance of CD137L from the surface of HRS and antigen-presenting cells led to reduced costimulation of T cells through CD137, reducing IFN-γ release and proliferation. Our results reveal a new regulatory mechanism for CD137L expression that mediates immune escape by HRS cells, and they identify CD137 as a candidate target for immunotherapy of Hodgkin lymphoma.