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 tra nsformation of GCs, and follicle lysis. The present histologic, electron mi croscopic, and immunohistochemical study of six autopsy cases after organ t ransplantation accompanied by the administration of immunosuppressive drugs revealed a peculiar regression of lymph node GCs in two cases, which has n ot been noted previously. The histologic findings of the regressive GCs wer e classified into three patterns. In pattern A, the GCs had few or no lymph ocytes 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 G Cs lacked tingible body macrophages. In pattern B, the GC lymphocytes and t ingible body macrophages were absent, showing crowded follicular dendritic cells (FDCs) in a corpuscular shape. In pattern C, the lymphocytic mantle w as absent. The GCs were smaller than those in the other patterns, and the s hape was irregular because of disintegration of FDCs. The immunostaining fo r FDC markers revealed dispersed growth of FDCs. On electron microscopy, th e lesions were composed of a dense mass of elliptical and oval cells withou t prominent cytoplasmic processes, a labyrinthlike structure, and emperipol esis of lymphocytes. The distinct desmosomelike adhesive junctions, specifi c electron microscopic features of FDCs, were evident. We propose to call t hese follicular lesions "FDC-only lymphoid follicles." It is speculated tha t 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 immunosuppressiv e drugs.