TNFSF4 (also known as OX40L) is a cytokine that binds to its unique receptor, TNFRSF4 [].Cytokines can be grouped into a family on the basis of sequence, functional and structural similarities [, , ]. Tumor necrosis factor (TNF) (also known as TNF-alpha or cachectin) is a monocyte-derived cytotoxin that has been implicated in tumour regression, septic shock and cachexia [, ]. The protein is synthesised as a prohormone with an unusually long and atypical signal sequence, which is absent from the mature secreted cytokine []. A short hydrophobic stretch of amino acids serves to anchor the prohormone in lipid bilayers []. Both the mature protein and a partially-processed form of the hormone are secreted after cleavage of the propeptide [].There are a number of different families of TNF, but all these cytokines seem to form homotrimeric (or heterotrimeric in the case of LT-alpha/beta) complexes that are recognised by their specific receptors.
Tumor necrosis factor receptor superfamily member 4 (TNFRSF4), also known as OX40, ACT35, CD134, IMD16 or TXGP1L, activates NF-kappaB through its interaction with adaptor proteins TRAF2 and TRAF5 []. It also promotes the expression of apoptosis inhibitors BCL2 and BCL2lL1/BCL2-XL, and thus suppresses apoptosis []. It is primarily expressed on activated CD4+ and CD8+ T cells, where it is transiently expressed and upregulated on the most recently antigen-activated T cells within inflammatory lesions. This makes it an attractive target to modulate immune responses, i.e. TNFRSF4 (OX40) blocking agents to inhibit adverse inflammation or agonists to enhance immune responses [, ]. An artificially created biologic fusion protein, OX40-immunoglobulin (OX40-Ig), prevents OX40 from reaching the T-cell receptors, thus reducing the T-cell response. Some single nucleotide polymorphisms (SNPs) of its natural ligand OX40 ligand (OX40L, CD252), which is also found on activated T cells, have been associated with systemic lupus erythematosus [].This entry represents the N-terminal domain of TNFRSF4. TNF-receptors are modular proteins. The N-terminal extracellular part contains a cysteine-rich region responsible for ligand-binding. This region is composed of small modules of about 40 residues containing 6 conserved cysteines; the number of modules can vary in number and type in different members of the family [, , ].
