Fas ligand expression in the germinal centre.

Whereas the importance of the Fas/FasL system in the regulation of T-cell homeostasis is well established, it is not yet clear if FasL is involved in B-cell regulation, especially in the clonal selection of B lymphocytes in the germinal centre (GC). This study therefore investigated the expression of FasL protein in tonsils and lymph nodes with lymphofollicular hyperplasia by western blotting and immunohistochemistry. In all the samples examined, western blot analysis showed FasL proteins of 33 and 52 kD, which presumably correspond to membrane-bound and soluble forms of the FasL protein. Immunohistochemically, FasL was found in a limited number of cells confined to a cluster in the light zone of the GC. The signal showed a delicate meshwork-like pattern of branching processes enmeshing the centrocytes and the few centroblasts of the light zone. In serial sections, the immunostaining pattern for FasL was found largely to coincide with the CD23 staining of follicular dendritic cells (FDCs), which are typically located in the light zone. In contrast, the FasL signal did not correspond to the distribution of the CD4-positive GC T-cells. In conclusion, expression of FasL in lymphofollicular hyperplasia seems to be largely confined to the light zone of the GCs, where selection of FDC-associated centrocytes is known to occur. These observations thus suggest that FasL is involved in selection processes of the B-cell system.