The OX-40 cost1mulatory receptor is expressed on the most recently activated CD4⁺ T cells found within autoimmune lesions in EAE

The OX-40 protein, a member of the newly described TNF-receptor gene family, provides a costimulatory signal to anti-CD3 activated CD4+ T cells. In naive animals, peripheral blood and spleen T cells are OX-40" and OX-40 protein expression is found only on activated T cells (preferentially CD4⁺). Lewis rats were actively immunized with myelin basic protein (MBP) in CFA and the spinal cord T cells were analyzed for OX-40 expression. At the onset of disease 13-20% of the spinal cord lymphocytes were OX-40⁺, while 70-80% were IL-2 receptor positive. We sorted for the OX-40⁺ T cells isolated from the spinal cord and found they were highly enriched for the encephalitogenic associated T cell receptor Vβ8.2 compared to the OX-40⁺ or unsorted populations. OX-40+ T cells isolated from the spinal cord had an enhanced proliferative response to MBP, whereas OX-40⁺ cells isolated from the lymph nodes responded to both MBP and purified protein derivative (PPD). The T cell receptor CDR3 sequences of OX-40⁺/Vβ8.2⁺ spinal cord T cells were compared to OX-40/Vβ8.2⁺ spinal cord T cells. 16/17 sequences showed the binding motif for MBP in the OX-40⁺ fraction while only 5/17 displayed the motif in the OX-40⁺ fraction. The autoreactive T cells express OX-40 at the site of inflammation and this costimulatory molecule may be responsible for the activation and progression of CD4⁺ T cell autoimmunity.