Marked increase in CD44-highly positive cells in hyperplastic thymuses from patients with Myasthenia gravis

Antigen-specific T-cell activation in hyperplastic thymus in myasthenia gravis

In order to determine whether antigen-specific T-cell activation by dendritic cells (DCs) is accelerated in thymuses exhibiting lymphofollicular hyperplasia (TLFH) among patients with early-onset myasthenia gravis (EOMG), we investigated the expression levels of phosphorylated protein kinase C (PKC)theta and the local relationship between the presence of phosphorylated PKCtheta and the homing receptor CD44 or CD83, a marker for mature DCs, in samples taken from EOMG patients with early improvement following thymectomy, in remnant thymuses from late-onset MG patients, and in non-MG control thymuses. Antigen-specific T-cell activation was markedly accelerated in TLFH from EOMG patients. Activated T cells and adjacent DCs appeared to be components of a CD44(high) cell population circulating from the blood to the thymus. Although there is no convincing evidence that thymectomy is of benefit in MG, in some EOMG patients with early improvement following thymectomy, blockade of CD44-associated circulation mechanisms is probably the cause for the early benefits of thymectomy and is a potential alternative to thymectomy.

Thymic myoid cells as a myasthenogenic antigen and antigen-presenting cells

We investigated immune property of a myoid cell line, established from Fisher rat thymus. Immunization of syngeneic rats with the myoid cells induced anti-rat acetylcholine receptor (AChR). Implantation of them into the thymus failed to induce typical thymic pathology of human myasthenia gravis (MG) or anti-AChR responses. We also demonstrated that the myoid cells were able to present exogenous antigens to T cells and induce antigen-specific T cell proliferation. These results suggest that myoid cells have the potential antigenicity to induce anti-AChR and the functions of antigen-presenting cells, but their expansion in the thymus may not directly cause MG.

Dendritic cells in hyperplastic thymuses from patients with myasthenia gravis

To investigate the role of dendritic cells (DCs) in the hyperplastic myasthenia gravis (MG) thymus, we studied the frequency and distribution of three mature DC phenotypes (CD83(+)CD11c(+), CD86(+)CD11c(+), and HLA-DR(+)CD11c(+)) in samples from patients with MG whose symptoms dramatically improved following thymectomy and in non-MG control thymuses. In hyperplastic MG thymuses, mature DCs were much more numerous in nonmedullary areas, such as the subcapsular/outer cortex; around the germinal centers; and in extralobular connective tissue, particularly around blood vessels. Mature DCs strongly coexpressed CD44 and appeared to be components of a CD44-highly positive (CD44(high)) cell population migrating from the vascular system. Furthermore, in the hyperplastic MG thymus, the expression of secondary lymphoid-tissue chemokine (SLC) markedly increased especially around extralobular blood vessels, where the CD44(high) cell population accumulated. These findings suggest that DCs may migrate into the hyperplastic thymus from the vascular system via mechanisms that involve CD44 and SLC. DCs may present self-antigens, thereby promoting the priming and/or boosting of potentially autoreactive T cells against the acetylcholine receptor.

NKT-associated markers and perforin in hyperplastic thymuses from patients with Myasthenia gravis

Immunohistochemical expression of natural killer T (NKT) cell-associated markers (Valpha24 and CD56) and perforin in relation to CD44-highly positive (CD44(high)) cells was studied in hyperplastic thymuses from patients with myasthenia gravis (MG) whose symptoms dramatically improved after thymectomy and compared with non-MG control thymuses. In the control thymuses, Valpha24-positive (Valpha24(+)) and CD56-positive (CD56(+)) cells were sparsely distributed in the medullary area only. In contrast, in hyperplastic MG thymus, Valpha24(+) and CD56(+) cells were more frequent in connective tissue, appeared to have penetrated the thymic parenchyma, and most coexpressed CD44(high). Perforin-positive cells were not present in the control thymus, but were in the connective tissue and perilobular cortical areas in the hyperplastic MG thymus. Most of these perforin-positive cells were CD44(high) and were located near blood vessels. They appeared to have migrated directly from the vascular system and penetrated the thymic parenchyma. Some perforin-positive cells coexpressed Valpha24, CD56, or both. These findings suggest that in this particular type of MG thymus, NKT-like cells may have increased via a CD44- and perforin-mediated mechanism, leading to an imbalance in the immune system that favored an antibody-mediated autoimmunity against the acetylcholine receptor.