A costimulatory signal from the tumor necrosis factor receptor (TNFR) family molecule OX40 (CD134), which is induced on activated T cells, is important for Tcell immunity. Aberrant OX40 cosignaling has been implicated in autoimmune and inflammatory disorders. However, the molecular mechanism by which the OX40 cosignaling regulates the Tcell response remains obscure. We found that OX40 associated with a scaffold protein, IQ motifcontaining GTPaseactivating protein 1 (IQGAP1) after ligation by its ligand OX40L. Nave CD4+ T cells from Iqgap1/ mice displayed enhanced proliferation and cytokine secretion upon receiving OX40 cosignaling. A Cterminal IQGAP1 region was responsible for its association with OX40, and TNFRassociated factor 2 (TRAF2) bridged these two proteins. The enhanced cytokine response in Iqgap1/ T cells was restored by the expression of the Cterminal IQGAP1. Thus, the IQGAP1 binding limits the OX40 cosignaling. Disease severity of experimental autoimmune encephalomyelitis (EAE) was significantly exacerbated in Iqgap1/ mice as compared to wildtype mice. Additionally, recipient mice with Iqgap1/ donor CD4+ T cells exhibited significantly higher EAE scores than those with their wildtype counterparts, and OX40 blockade led to a significant reduction in the EAE severity. Thus, our study defines an important component of the OX40 cosignaling that restricts inflammation driven by antigenactivated T cells.