Intercellular contacts between germinal center cells. Mechanisms of adhesion between lymphoid cells and follicular dendritic cells.

Intercellular connections exist between germinal center cells especially between lymphoid cells and follicular dendritic cells (FDC). Even after isolation, FDC remain associated to lymphocytes and are able, in a cell suspension, to establish new connections with others. Using human tonsillar cells or mouse lymph node cells we analysed these connections which were shown to be species-specific. Low temperature as well as absence of Ca++ and Mg++ in the culture medium reduced the adherence of fluorochrome-labeled lymphoid cells to FDC. Colchicine treatment did not impair the adherence, whereas cytochalasin B dit it; this was the first observation underlining the importance of microfibrils in FDC. Antibodies directed towards integrin molecules (LFA-1 alpha or beta chain, CD11a and CD18 respectively) reduced the adherence, others (anti-CR3 or anti-gp 150/95, CD11b and c respectively) did not influence it. Antibodies directed against MHC class II exerted no inhibitory action on the lymphoid cell adhesion to FDC. As, at ultrastructural level, gold-labeled immune complexes can be found between FDC and lymphoid cells, we examined the effect on cell adhesion of the addition of immune complexes to the cell suspensions. It only impaired the lymphoid cell adhesion when complement components were present. IgM complexes were then more inhibitory than IgG complexes. When antibodies against Fc IgG receptors (CD16) were added, the adhesion was strongly reduced whereas antibodies to Fc IgE (CD23) receptors had no influence. The antibody DRC1, specifically recognizing an antigen on human FDC reduced the attachment of cells to FDC. This antigen thus seems to play a role in the intercellular contacts; this is the first function ascribed to this FDC specific antigen.