Phenotypic characteristics of thymic B lymphocytes in myasthenia gravis

Abstract

We have found prominent secretion of immunoglobulin and anti-acetylcholine antibodies by thymic lymphocytes (TL) of myasthenics despite a relative paucity of B (surface IgM⁺) cells in such TL. To determine whether there was a surface IgM⁻ B cell in the TL which could manifest such responses, we compared the frequency of cells expressing the B cell-specific phenotypic marker CD20 (B1⁺), surface IgM (SIgM⁺), surface IgG (SIgG⁺), and surface IgD (SIgD⁺) in TL and autologous blood mononuclear cells in 36 myasthenic patients. B1⁺ cells were significantly more frequent than SIgM⁺ cells in TL (3.2±0.6 vs 0.6±0.2). In double-labeling studies, less than 25% of the B1⁺ cells coexpressed SIgM. Only 0.3% of the TL were SIgD⁺. In contrast, the frequencies of B1⁺ and SIgM⁺ cells in autologous blood were not significantly different (10.7±1.3 vs 8.2±0.8%). About 75% of blood B1⁺ cells co-expressed SIgM. These findings suggest that mast B cells in these TL have undergone isotope switching during priorin vivo differentiation and could manifest the observed humoral responses.