Dependence of germinal center B cells on expression of CD21/CD35 for survival

Evidence for an Important Interaction Between a Complement-Derived CD21 Ligand on Follicular Dendritic Cells and CD21 on B Cells in the Initiation of IgG Responses

The addition of Ags to mononuclear leukocyte cultures typically elicits modest Ab responses, implying that cosignals beyond those provided by T cells and macrophages may be needed. Recently, we reported that Ab responses could be dramatically enhanced (10–1000-fold) by the addition of follicular dendritic cells (FDC), suggesting that FDC may provide an important costimulatory signal. This result prompted a study of molecules involved in FDC-mediated enhancement of Ab responses stimulated by specific Ag with memory T and B cells or nonspecifically by the addition of LPS. In this study, we report evidence supporting the concept that FDC bear a ligand that engages complement receptor II (CR2 or CD21) on B cells and provides a critical cosignal for both Ag-specific and polyclonal responses. A blockade of the CR2 ligand on FDC by the use of soluble CR2 or a blockade of CR2 on B cells by use of CR2 knockout mice (or B cells with CR2 blocked) reduced Ab responses from the μg/ml to the ng/ml range (10–1000-fold reductions). FDC from C3 knockout mice, which cannot generate the CR2-binding fragments (iC3b, C3d, and C3dg), were unable to provide costimulatory activity, suggesting the CR2 ligand on FDC consists of C3 fragments. FDC trap complement-activating Ag-Ab complexes, and it appears that FDC present B cells with both specific Ag to engage B cell receptors and a CR2 ligand to engage B cell-CR2. In short, optimal induction of specific Ab responses appears to require the combination of specific Ag and costimulatory molecules from both T cells and FDC.

Germinal centers: form and function

Our understanding of the formation of germinal centers has advanced considerably during the past year. Highlights include the elucidation of the role of cytokines and chemokines in splenic organization and lymphocyte migration and their roles in germinal center development. The functional consequences of recombination-activating gene re-expression in the germinal center have also been reported, as have in vitro models of somatic mutation. Finally the resolution of the germinal center reaction is being addressed by analysis of the individual cell types produced.