CD134, also known as OX40, is a member of the TNFR-superfamily of receptors which is not constitutively expressed on resting naïve T cells, unlike CD28. OX40 is a secondary costimulatory molecule, expressed after 24 to 72 hours following activation; its ligand, OX40L, is also not expressed on resting antigen presenting cells, but is following their activation. Expression of OX40 is dependent on full activation of the T cell; without CD28, expression of OX40 is delayed and of fourfold lower levels.
OX40 has no effect on the proliferative abilities of CD4+ cells for the first three days, however after this time proliferation begins to slow and cells die at a greater rate, due to an inability to maintain a high level of PKB activity and expression of Bcl-2, Bcl-XL and survivin. OX40 binds to receptors on T-cells, preventing them from dying and subsequently increasing cytokine production. OX40 has a critical role in the maintenance of an immune response beyond the first few days and onwards to a memory response due to its ability to enhance survival. OX40 also plays a crucial role in both Th1 and Th2 mediated reactions in vivo.
OX40 binds TRAF2, 3 and 5 as well as PI3K by an unknown mechanism. TRAF2 is required for survival via NF-κB and memory cell generation whereas TRAF5 seems to have a more negative or modulatory role, as knockouts have higher levels of cytokines and are more susceptible to Th2-meditated inflammation. TRAF3 may play a critical role in OX40-mediated signal transduction. CTLA-4 is down-regulated following OX40 engagement in vivo and the OX40-specific TRAF3 DN defect was partially overcome by CTLA-4 blockade in vivo. TRAF3 may be linked to OX40-mediated memory T cell expansion and survival, and point to the down-regulation of CTLA-4 as a possible control element to enhance early T cell expansion through OX40 signaling.
OX40 has been implicated in the pathologiccytokine storm associated with certain viral infections, including the H5N1bird flu. An artificially created biologic fusion protein, OX40-immunoglobulin (OX40-Ig), prevents OX40 from reaching the T-cell receptors, thus reducing the T-cell response. Experiments in mice have demonstrated that OX40-Ig can reduce the symptoms associated with the cytokine storm (an immune overreaction) while allowing the immune system to fight off the virus successfully.
^Kawamata S, Hori T, Imura A, Takaori-Kondo A, Uchiyama T (Mar 1998). "Activation of OX40 signal transduction pathways leads to tumor necrosis factor receptor-associated factor (TRAF) 2- and TRAF5-mediated NF-kappaB activation". J. Biol. Chem.273 (10): 5808–14. doi:10.1074/jbc.273.10.5808. PMID9488716.
So T, Salek-Ardakani S, Nakano H, Ware CF, Croft M (2004). "TNF receptor-associated factor 5 limits the induction of Th2 immune responses". J. Immunol.172 (7): 4292–7. doi:10.4049/jimmunol.172.7.4292. PMID15034043.
Song J, Salek-Ardakani S, Rogers PR, Cheng M, Van Parijs L, Croft M (2004). "The costimulation-regulated duration of PKB activation controls T cell longevity". Nat. Immunol.5 (2): 150–8. doi:10.1038/ni1030. PMID14730361.
Croft M (2003). "Co-stimulatory members of the TNFR family: keys to effective T-cell immunity?". Nat. Rev. Immunol.3 (8): 609–20. doi:10.1038/nri1148. PMID12974476.
Rogers PR, Song J, Gramaglia I, Killeen N, Croft M (2001). "OX40 promotes Bcl-xL and Bcl-2 expression and is essential for long-term survival of CD4 T cells". Immunity15 (3): 445–55. doi:10.1016/S1074-7613(01)00191-1. PMID11567634.