An unusual regressive germinal center, the 'FDC-only lymphoid follicle,' in lymph nodes of organ transplant recipients.

Follicular lesions include germinal center (GC) hyperplasia, regressive transformation of GCs, and follicle lysis. The present histologic, electron microscopic, and immunohistochemical study of six autopsy cases after organ transplantation accompanied by the administration of immunosuppressive drugs revealed a peculiar regression of lymph node GCs in two cases, which has not been noted previously. The histologic findings of the regressive GCs were classified into three patterns. In pattern A, the GCs had few or no lymphocytes and were surrounded by a poorly developed mantle zone-like structure. Apoptotic cell death of GC lymphocytes was found in a few GCs, but most GCs lacked tingible body macrophages. In pattern B, the GC lymphocytes and tingible body macrophages were absent, showing crowded follicular dendritic cells (FDCs) in a corpuscular shape. In pattern C, the lymphocytic mantle was absent. The GCs were smaller than those in the other patterns, and the shape was irregular because of disintegration of FDCs. The immunostaining for FDC markers revealed dispersed growth of FDCs. On electron microscopy, the lesions were composed of a dense mass of elliptical and oval cells without prominent cytoplasmic processes, a labyrinthlike structure, and emperipolesis of lymphocytes. The distinct desmosomelike adhesive junctions, specific electron microscopic features of FDCs, were evident. We propose to call these follicular lesions "FDC-only lymphoid follicles." It is speculated that this follicle may be evoked after preceding follicular hyperplasia with a complicated mechanism including increased apoptosis of GC lymphocytes and decreased lymphocyte migration to lymph node GCs caused by immunosuppressive drugs.