Follicular dendritic cell-dependent adhesion and proliferation of B cells in vitro

In response to an antigenic challenge, B cells proliferate in germinal centers within secondary lymphoid tissue. Specialized accessory cells, follicular dendritic cells (FDC), and T cells are necessary to drive this reaction. Indirect evidence suggests that FDC provide signals which not only induce B cell proliferation but can rescue B cells programmed to die by apoptosis. An in vitro system was developed to: 1) define the role of FDC and 2) identify molecules involved in this response. Activated, low density B cells and T cells were coisolated with FDC from immune mouse lymph nodes. Upon culturing, large cellular aggregates formed, composed of 1 to 3 FDC interdigitating between 30 to 90 B cells and 1 to 5 T cells. Many of these B cells were undergoing DNA synthesis. Depleting FDC or T cells from the cultures immediately stopped cluster formation and proliferation. Separating clustered vs nonclustered cells revealed that the FDC-associated population remained viable, whereas cells in suspension became apoptotic. The adhesion/activation molecules ICAM-1, LFA-1, and CD44 supported both cluster formation and proliferation. In addition, anti-class II and anti-kappa L chain mAb interfered dramatically with DNA synthesis. This model mimics many of the features of a germinal center and can be used to further study B cell activation, proliferation, and differentiation in vitro.