Terminal complement complex (TCC) and S-protein (vitronectin) on follicular dendritic cells in human lymphoid tissues

Follicular dendritic cells (FDC) present in germinal centres trap considerable amounts of C3-containing immune complexes (IC). Activation of the terminal pathway of complement (C) on biological membranes normally generates C5b-9(m), which is the membranolytic form of the terminal complement complex (TCC). By contrast, when C activation takes place in the extracellular fluid phase, S-protein binds to C5b-7 and the non-lytic soluble SC5b-9 is formed. In this study deposits of C3d, C5, C9 TCC neoepitope and S-protein were demonstrated by immunohistochemistry on FDC in human tonsils, lymph nodes, spleens, appendices and colonic mucosae. TCC and S-protein were likewise observed on FDC in imprints from tonsils. The identical spatial distribution revealed by paired staining suggested that TCC had been generated in situ. The staining intensity for TCC and S-protein varied in parallel, which suggested that the S-protein may be incorporated in the membrane-bound TCC--perhaps rendering it non-lytic. However, the actual mechanism and function of the observed TCC and associated S-protein deposition needs further elucidation.