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

Follicular dendritic cells (FDCs) are potent accessory cells for B cells, but the molecular basis of their activity is not understood. Several important 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 delivers a crucial signal that dramatically augments the stimulation delivered by the binding of antigen to the B-cell receptor (BCR). The engagement of Fc γ receptor IIB (FcγRIIB) by the Ig crystallizable fragment (Fc) in antigen–antibody complexes held on FDCs decreases the activation of immunoreceptor tyrosine-based inhibition motifs (ITIMs), mediated by the crosslinking of BCR and FcγRIIB. Thus, FDCs minimize a negative B-cell signal. In short, these ligand–receptor interactions help to signal to B cells and meet a requirement for B-cell stimulation that goes beyond the necessity of T-cell help.