TNFSF9

TNFSF9 consists of an extracellular domain responsible for receptor binding, a transmembrane region, and a short intracellular domain, and can also exist in a soluble form when cleaved from the membrane. This structural organization enables TNFSF9 to function as a bidirectional signal transducer, facilitating costimulatory signaling crucial for T cell activation and immune response modulation.

Receptor/ligand complex

TNFSF9 forms a trimeric complex on the cell surface, which interacts with the 4-1BB (CD137) receptor on activated T lymphocytes. Each 4-1BB monomer binds to two 4-1BBL subunits via cysteine-rich domains (CRDs), with the CRD2 and CRD3 regions of 4-1BB engaging specific loops on 4-1BBL to stabilize the interaction through multiple hydrogen bonds.

The 4-1BB/4-1BBL complex consists of three monomeric 4-1BBs bound to a trimeric 4-1BBL. Each 4-1BB monomer binds to two 4-1BBLs via cysteine-rich domains (CRDs). The interaction between 4-1BB and the second 4-1BBL is required to stabilize their interactions. The link with 4-1BBL is largely made up of amino acids from the dynamic loops of the CRD2 and the β sheet of CRD3 of 4-1BB, according to a detailed study of the binding between the 4-1BB and 4-1BBL interface. CRD2 amino acids (T61, Q67, and K69) interact with the AA′ loop (Y110 and G114) and the intra-H-strand loop (Q227 and Q230) of 4-1BBL to form various hydrogen bond interactions.

TNFSF9 plays a key role in immune cell interactions. TNFSF9 forms a trimeric complex on the cell surface, which interacts with the 4-1BB (CD137) receptor on activated T lymphocytes. Complex formation with its receptor enables TNFSF9 to function as a bidirectional signal transducer, facilitating costimulatory signaling crucial for T cell activation and immune response modulation.

Early studies using the poorly immunogenic Ag104A sarcoma and highly tumorigenic P815 mastocytoma models provided the first systematic evidence that anti-4-1BB antibodies exert strong anti-tumor effects. These antibodies were found to significantly suppress tumor growth by enhancing cytotoxic T lymphocyte (CTL) activity. Subsequent research has consistently confirmed the role of 4-1BB signaling in promoting anti-tumor immunity.

The 4-1BB/4-1BBL interaction delivers costimulatory signals that enhance T-cell responses, a mechanism with significant implications for cancer immunotherapy. When combined with T-cell receptor signaling, this interaction stimulates both CD4⁺ and CD8⁺ T cells, contributing to effective anti-tumor responses. However, in human CD28⁻ T cells, 4-1BB signaling can promote expansion of this subset, which is associated with adverse outcomes in cancer and other diseases. As a result, modulating this pathway represents a promising therapeutic strategy.