Phenotypic characteristics of thymic B lymphocytes in myasthenia gravis

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 prior in vivo differentiation and could manifest the observed humoral responses.