The distribution of parenchyma, follicles, and lymphocyte subsets in thymus of patients with myasthenia gravis, with special reference to remission after thymectomy

Abstract

We sought to examine the distribution of parenchyma, follicles, and lymphocyte subsets in the thymus of patients with myasthenia gravis and to identify determinants of remission after thymectomy. Sixty patients with myasthenia gravis who underwent thymectomy were examined. The thymus was divided into upper, middle, and lower parts. The upper part was defined as the superior horn, the lower part as the inferior horn, and the middle part as tissue located between the 2 horns. The percentage of parenchyma was measured morphometrically. The degree of follicular hyperplasia was classified into 5 grades. The densities of CD3+, CD4+, and CD8+ lymphocytes were classified into 5 grades. The remission of myasthenia gravis after thymectomy was examined with those variables in each part of the thymus. The middle part had the highest percentage of parenchyma, the highest grade of follicular hyperplasia, and the highest density of CD3+, CD4+, and CD8+ lymphocytes among the 3 parts (P < .001-.05). The grades of follicular hyperplasia in the middle and lower parts were significantly higher in patients with improvement of myasthenia gravis than in those without (P < .05). The densities of CD3+, CD4+, and CD8+ lymphocytes in the cortex of the middle part were significantly higher in patients with improvement than in those without improvement (P < .01-.05). The thymus has a heterogeneous distribution of parenchyma, follicles, and lymphocyte subsets. The middle part had the largest parenchyma, the highest grade of follicular hyperplasia, and the highest densities of CD3+, CD4+, and CD8+ lymphocytes among the 3 parts of the thymus. The grade of follicular hyperplasia and the density of these lymphocyte subsets are predictive of improvement in myasthenia gravis after thymectomy.