Distinct role of follicular dendritic cells and T cells in the proliferation, differentiation, and apoptosis of a centroblast cell line, L3055.

Germinal center (GC) B cells undergo complex interactions with follicular dendritic cells (FDC) and T cells in the course of differentiation into memory B and plasma cells. To delineate the individual roles of FDC and T cells at each stage of GC B cell differentiation at the clonal level and to analyze the signals involved, we adopted a unique experimental model using an FDC line, HK, and a lymphoma cell line, L3055, that resembles centroblasts. A detailed phenotypic analysis revealed L3055 cells to be a clonal population originating from the GC. Like freshly isolated centroblasts, L3055 cells underwent spontaneous apoptosis when cultured in the absence of fresh FDC or HK cells. L3055 cells proliferated continuously in the presence of HK cells, while they differentiated into a population with the phenotype of centrocytes after stimulation with CD40 ligand (CD40L) and IL-4. The CD40L-stimulated L3055 cells underwent CD95-mediated apoptosis, which was reminiscent of the feature of CD40L-stimulated tonsillar GC B cells. In contrast to HK cells that did not protect L3055 cells from anti-Ig killing, CD40L plus IL-2, IL-4, and IL-10 prevented anti-Ig-induced apoptosis. These experimental results demonstrate a distinct function of FDC and activated T cells, in that FDC provide signals for rapid proliferation of centroblasts, whereas T cells confer signals for differentiation of centroblasts into centrocytes and resistance to B cell receptor-mediated apoptosis. T cells collaborate with FDC in the protection and expansion of the Ag-specific GC B cells.