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 a re poorly immunogenic but in vivo immune complexes elicit potent recal l responses. FDC trap Ag-Ab complexes and make immune complex mated bo dies or ''iccosomes''. B cells endocytose iccosomes, the Ag is process ed, and T-cell help is elicited. In vitro, addition of FDC bearing app ropriate Ag-Ab complex to memory T and B cells provoke potent recall r esponses (IgG and IgE). FDC also provide nonspecific costimulatory sig nals which augment B-cell proliferation and Ab production. B cell-FDC contact is important and interference with ICAM-1-LFA-1 interactions r educes FDC-mediated costimulation. Preliminary data suggest that a cos timulatory signal may be delivered via CR2L on FDC binding CR2 on B ce lls. FDC can also stimulate B cells to become chemotactically active a nd can protect lymphocytes from apoptosis. FDC also appear to be rich in thiol groups and may replace reducing compounds such as 2 mercaptoe thanol in cultures. In short, FDC-Ag specifically signals B cells thro ugh BCR, and FDC provide B cells with iccosomal-Ag necessary for proce ssing to elicit T-cell help. In addition, FDC provide nonspecific sign als that are important to promote B-cell proliferation, maintain viabi lity, and induce chemotactic responsiveness.