Micronodular thymoma with lymphoid stroma: an immunohistochemical study of the distribution of Langerhans cells and mature dendritic cells in six patients

Aims: Micronodular thymoma with lymphoid stroma (MNT) is an uncommon variant of thymoma, characterized by multiple small nodules consisting of type A thymoma-like cells, which are separated by abundant B lymphocytes. The aim of the study was to elucidate the pathogenesis of the stromal lymphoid hyperplasia, which is currently unclear.

Methods and results: We retrieved six cases of MNT, and immunohistochemically examined the number and distribution of Langerhans cells (LCs) and mature dendritic cells (DCs), and compared them with those in type A and type AB thymomas. Many LCs were present within the small tumour nests, but LCs were rarely seen in the stroma (75.5/HPF versus 6.1/HPF, P < 0.0001). In contrast, mature DCs were present mainly in the surrounding stroma rather than within the tumour nodules (63.5/HPF versus 6.0/HPF, P < 0.0001), forming clusters with mature T lymphocytes adjacent to lymphoid follicles. In large nodules, as well as in type A and type AB thymomas, a few scattered LCs and DCs were identified. All patients were still alive and well.

Conclusions: Our results suggest that LCs take up tumour antigens and migrate to the stroma, where they mature and form clusters with T lymphocytes to activate them, resulting in lymphoid follicle formation. The favourable clinical behaviour may be attributable to the immune response induced by LCs.