Expression of Fas in B cells of the mouse germinal center and Fas-dependent killing of activated B cells.

Fas is a cell-surface protein of 45 kDa. Binding of the Fas ligand (FasL) to Fas induces apoptosis in Fas-bearing cells. Analysis of mouse mutants in either Fas or FasL have indicated that the Fas system is involved in apoptosis of lymphocytes. To identify the cells expressing Fas, mouse spleen sections were analyzed by in situ hybridization and immunohistochemistry. Fas mRNA was detected in the T cells of the inner region of the periarterial lymphatic sheath and the inner lumen of the marginal zone. The cells in the outer region of the periarterial lymphatic sheath weakly expressed Fas mRNA, whereas it was abundant in the B cells of germinal centers. Immunizing mice with keyhole limpet hemocyanin induced formation of many germinal centers in the spleen. The B cells in the activated germinal center expressed abundant Fas and underwent apoptosis. The in vitro activation of splenocytes with lipopolysaccharide induced Fas expression in B cells and the B cells became sensitive to the Fas-mediated apoptosis. These results suggest that Fas is involved in the activation-induced death of B cells.