Proliferation of germinal center B lymphocytes in vitro by direct membrane contact with follicular dendritic cells.

In secondary lymphoid organs, follicular dendritic cells (FDC) are located within B cell follicles and germinal centers. Through their cytoplasmic extensions they come into contact with a large number of neighboring lymphocytes. Using an enzyme cocktail to digest human tonsils followed by ultracentrifugation on bovine serum albumin gradients, single cell suspensions were obtained. Immunocytochemistry revealed that 7% of the cells were FDC, 5% T cells, and 5% macrophages. The remaining population were B cells with greater than 95% being of the germinal center phenotype (i.e. CD19-positive, CD39/sIgD negative). After 24 h of culture up to 44% of the lymphocytes were found in clusters centered around FDC. At the start of the culture as well as 24 and 72 h later, between 31 and 55% of the B cells within FDC associated clusters were in late G1 to M phase of the cell cycle. In contrast, less than 10% of the B cells not in contact with FDC (i.e. outside the clusters) were in an activated state. Autoradiography revealed that after three days of incubation the rate of proliferation was 26.2 times higher for the lymphocytes involved in cluster formation as compared to those cells not associated with FDC. Furthermore, the number of viable B cells after a 72 h mitogen-free culture period was determined. By adding FDC to these preparations, 31.9% of the lymphocytes were rescued from dying. These data show that FDC provide a microenvironment which can maintain the viability, activation and proliferation of germinal center B cells in vitro.