Expression of OX40 and OX40 ligand (gp34) in the normal and myasthenic thymus

Objectives - To examine the expression of OX40, an activated memory T-cell marker, and its ligand (OX40L), a set of molecules for T-cell-B-cell interaction, and other lymphocyte activation markers in the thymuses of myasthenia gravis (MG) and controls. Material and methods - We studied the expression of OX40, OX40L, IL-2Rα and HLA-DR in the thymic tissues of MG and controls using immunocytochemistry and flowcytometry. Results - In both hyperplastic thymus of MG and control thymus, OX40⁺ cells were scattered mainly in the medulla with much fewer OX40L⁺ cells being distributed in the corticomedullary junctions. IL-2Rα and HLA-DR were expressed in the medulla at higher frequencies as compared with OX40 in controls as well as MG. In contrast, the numbers of OX40⁺ cells around the germinal centers (GC) were significantly greater than those of control thymuses, and some mononuclear cells in GC were OX40L⁺. A considerable number of OX40⁺ cells were seen in the thymic tissues adjacent to thymomas. OX40⁺ cells were CD4⁺CD8^- or CD4⁺CD8⁺ and were mostly HLA-DR^-. (The coexpression of OX40 and IL-2Rα on activated CD4⁺ T cells was previously reported.) Conclusion - OX40, expressed in a fraction of activated CD4⁺ T cells, may be upregulated in thymic tissues adjacent to GC and thymoma in MG, and OX40 may interact with OX40L in GC to enhance anti-acetylcholine receptor antibody production in MG.