Thymoma in Patient with Myasthenia Gravis Has Significantly Fewer Forkhead Box P3 Positive Lymphocytes than that without One.

Forkhead box P3 (FoxP3) is known as a pivotal and specific transcriptional factor of regulatory T cells, and are implicated in various immune diseases including myasthenia gravis (MG). The aim of this study was to investigate the relationships between FoxP3 expression of lymphocytes in thymoma and clinicopathological characteristics, particularly MG status in thymoma patients. We reviewed 83 thymoma specimens, including 22 from MG patients, and evaluated the FoxP3 expression of lymphocytes in thymoma using immunohistochemistry (IHC). Statistical association was evaluated using chi-square test and Fisher's exact test. Thirty-four cases (41.0%) were classified as FoxP3 positive. There were no statistical differences in sex (P=0.289), age (P=0.536), Masaoka stage (P=0.086), WHO histological classification (P=0.097), or Myasthenia Gravis Foundation of America (MGFA) Clinical Classification (P=0.117) between FoxP3 positive and negative cases. In contrast, thymoma in cases with MG showed significantly fewer FoxP3 positive lymphocytes than those in cases without MG (P=0.037). Moreover, cases with anti-acetylcholine receptor antibody titer equal to or greater than the normal limit also showed significantly fewer FoxP3 positive lymphocytes than cases within the normal limit (P<0.001). Our result indicated the possibility that the decrease of FoxP3 positive lymphocytes in thymoma may lead to the development of MG and to an increase in anti-acetylcholine receptor antibodies. In addition, FoxP3 positive lymphocytes might be a useful biomarker for evaluation of the risk of MG onset, and could open the way to more effective therapeutic strategies in thymoma patients.