[Fas antigen gene in thymuses from patients with myasthenia gravis]

The Fas antigen is a cell surface protein that can mediate apoptosis, and plays a major role in functional maturity in clonal deletion of autoreactive T cells in the thymus. Recently a cDNA encoding the human Fas antigen was isolated. It was suggested that a signal-transducing domain, along with inhibitory one, was presented in the cytoplasmic domain of the Fas antigen. We examined whether any abnormality in Fas antigen gene may breakdown the immunotolerance in patients with myasthenia gravis (MG). We used single-strand conformational polymorphism (SSCP) analysis, generally used to screen for unknown mutation, to examine the cytoplasmic cDNA of Fas antigen obtained by reverse transcription (rt)-PCR from mRNA of MG thymuses. Furthermore, we studied in situ expression of Fas antigen mRNA in thymuses from MG patients. Fragments from all the 12 (3 control subjects, and 9 MG patients) thymuses produced by rt-PCR showed equally stained two bands of the two single strands. In the thymuses from both controls and MG patients, the Fas antigen mRNA was mainly expressed in small thymic cells. These cells were located near clustered cells with relatively large cytoplasms in the cortex and sometimes surrounded them, but were also founded in clusters in the follicules. In situ expressions of Fas antigen mRNA were more remarkable in MG thymuses than in control subjects, and in hyperplastic thymuses than in normoplastic ones. These results suggest that mRNA for the cytoplasmic domain of the Fas antigen have no mutation, and its expression is not reduced in thymuses from patients with MG.