Follicular dendritic cells in the regulation and maintenance of immune responses.

The objectives of this report are to explore known and potential regulatory roles of follicular dendritic cells (FDCs) in immune responses. FDCs are found in B-cell areas of lymphatic tissue and are especially rich around germinal centers where B-cell proliferation is occurring. Recent advances in isolating FDCs in our laboratories made it possible to begin investigating interactions of FDCs with other cells in vitro. In a series of experiments, we sought to determine whether a low concentration of FDC-enriched cells (2-10%) could enhance B-cell and/or T-cell mitogenesis. The results showed that an enriched population of FDCs markedly enhanced lipopolysaccharide-induced stimulation of nude mouse spleen cells, while an equivalent number of resident peritoneal cells or thymocytes had no effect. Depletion of Thy 1.2 positive cells or an exposure to 2400 rads of gamma radiation did not abrogate the enhancing capacity of FDC-enriched cells. FDC-enriched cells also increased phytohemagglutinin-induced proliferation. Scanning electron microscopy of FDC preparations showed that antigen-bearing dendritic processes of Type 2 FDCs have a "beaded" appearance. It was proposed that these immune complex coated microspheres may represent an important mechanism for exposing persisting antigen to the immune system. The gradual shedding of these microspheres may play an important role in regulating and maintaining immune responses within lymphoid tissues.