Follicular dendritic cells: beyond the necessity of T-cell help

Follicular dendritic cells (FDCs) are potent accessory cells for B cells, b ut the molecular basis of their activity is not understood. Several importa nt molecules involved in FDC-B-cell interactions are indicated by blocking the ligands and receptors on FDCs and/or B cells. The engagement of CD21 in the B-cell coreceptor complex by complement-derived CD21 ligand on FDCs de livers a crucial signal that dramatically augments the stimulation delivere d by the binding of antigen to the B-cell receptor (BCR). The engagement of Fc gamma receptor IIB (Fc gamma RIIB) by the Ig crystallizable fragment (F c) in antigen-antibody complexes held on FDCs decreases the activation of i mmunoreceptor tyrosine-based inhibition motifs (ITIMs), mediated by the cro sslinking of BCR and Fc gamma RIIB. Thus, FDCs minimize a negative B-cell s ignal. In short, these ligand-receptor interactions help to signal to B cel ls and meet a requirement for B-cell stimulation that goes beyond the neces sity of T-cell help.