Deposits of Terminal Complement Complex and S-Protein on Follicular Dendritic Cells in Human Lymphoid Tissue

Abstract

Follicular dendritic cells (FDC) retain C3-containing immune complexes (IC) (1,2) mainly by their complement receptors (CR) (3), Theoretically, membrane-bound IC might lead to C activation and eventually membrane damage of FDC in a type II hypersensitivity reaction. Complement activation splits C3 into the C3a and C3b. The latter binds covalently to target surfaces and initiate terminal pathway activation by splitting C5 into C5a and C5b. On biological membranes, the membranolytic terminal complement complex (TCC) C5b-9(m) is formed (A). By contrast, if C activation takes place in the extracellular fluid phase, S-protein binds to the terminal components and the hydrophilic non-lytic SC5b-9 is formed (5). Both C5b-9(m) and SC5b-9 expose TCC neoepitopes, which are not present in the native components. S-protein is produced by the liver and is a multi-functional 65-75 Kd protein with a serum concentration of 0.14-0.70 g/1 (6). In addition, S-protein, now known to be identical to vitronectin (7,8), is one of many proteins capable of promoting cell adhesion through the arg-gly-asp sequence and is found in various human tissues (9-11). In this study FDC of human lymphoid tissue were examined by immunohistochemistry for terminal pathway activation and S-protein distribution.