Recombinant TAT-CD137 Ligand Cytoplasmic Domain Fusion Protein Induces the Production of IL-6 and TNF-α in Peritoneal Macrophages

The ligand for CD137 (CD137L; also called 4-1BBL) is mainly expressed on activated APCs such as dendritic cells, B cells and macrophages. Even though CD137L functions as a trigger of the CD137 signaling pathway for T cell activation and expansion, engagement of CD137L can deliver a signal leading to the production of proinflammatory cytokines in macrophages. We generated cell-permeable TAT-CD137L cytoplasmic domain fusion protein (TAT-CD137Lct) and examined its ability to initiate the CD137L reverse signaling pathway. Treatment of TAT-CD137Lct induced the production of high levels of IL-6 and TNF-α mRNAs and proteins in peritoneal macrophages. TAT-CD137Lct increased phosphorylation of Erk, p38 MAPK and Jnk, and activated transcription factors C/EBP and CREB. However, TAT-CD137Lct did not visibly affect the degradation of the inhibitor of NF-kB (IkBα). We further demonstrated that JNK activation was required for TAT-CD137Lct-induced production of TNF-α, while activation of Erk and p38 MAPK were involved in IL-6 and TNF-α production. Our results suggest that TAT-CD137Lct is an effective activator for the CD137L reverse signaling pathway.