The distribution, immune complex trapping ability and morphology of follicular dendritic cells in popliteal lymph nodes of aged rats.

Immune system function declines with age, and lymph nodes involute. The aims of the study were to describe the distribution of follicular dendritic cells (FDCs) in the lymphoid follicles of aged rats, and to determine whether these cells have reduced ability to trap immune complexes (ICs). Popliteal lymph nodes of rats aged 24-28 months were immunostained for S-100 protein as a marker of FDCs. Some rats were pretreated with peroxidase-anti-peroxidase complex (PAP) as an IC. FDCs were densely distributed in lymphoid follicles, which contained many primary follicles and a few secondary follicles. FDCs in primary follicles stained weakly for S-100 protein, and showed weak trapping, while those in secondary follicles stained strongly for both S-100 protein and trapping. Ultrastructurally, in involuted lymphoid follicles FDCs were atrophic. We conclude that FDCs in aged rats are densely distributed in involuted follicles and show reduced trapping ability and atrophic morphology. This seems to reflect the long life of FDCs and the reduced numbers of lymphoid cells in these follicles. We suggest that FDCs in aged rats may show some of their normal functions if fully developed germinal centers are induced, and may not play an important role in the process of involution of the follicles.