Follicular dendritic cells and presentation of antigen and costimulatory signals to B cells.

This review focuses on how immunogens trapped by FDC in the form of Ag-Ab complexes productively signal B cells. In vitro. Ag-Ab complexes are poorly immunogenic but in vivo immune complexes elicit potent recall responses. FDC trap Ag-Ab complexes and make immune complex coated bodies or "iccosomes". B cells endocytose iccosomes, the Ag is processed, and T-cell help is elicited. In vitro, addition of FDC bearing appropriate Ag-Ab complex to memory T and B cells provoke potent recall responses (IgG and IgE). FDC also provide nonspecific costimulatory signals which augment B-cell proliferation and Ab production. B cell-FDC contact is important and interference with ICAM-1-LFA-1 interactions reduces FDC-mediated costimulation. Preliminary data suggest that a costimulatory signal may be delivered via CR2L on FDC binding CR2 on B cells. FDC can also stimulate B cells to become chemotactically active and can protect lymphocytes from apoptosis. FDC also appear to be rich in thiol groups and may replace reducing compounds such as 2 mercaptoethanol in cultures. In short, FDC-Ag specifically signals B cells through BCR, and FDC provide B cells with iccosomal-Ag necessary for processing to elicit T-cell help. In addition, FDC provide nonspecific signals that are important to promote B-cell proliferation, maintain viability, and induce chemotactic responsiveness.