Histochemical contributions to the binding mechanism of complement (CR1, CR2) receptors

Complement receptors (CR1, CR2, CR3), and their ligands (C3b, C3d, iC3b) are essentially involved in germinal center development and in binding, trapping, and retaining immunocomplexes. Methods studying complement receptor (CR1/CR2)-ligand (C3b/C3d) interactions mostly involve coating of sheep erythrocytes (E), sheep erythrocyte-antisheep erythrocyte antibody (EA complexes) and whole human (h) or mouse (m) sera as a source of complement, EACh/m complexes, as reagents. The observation of Dukor et al. (1970), that EACm complexes in native cryostat sections bind selectively and very strongly to the B lymphocyte regions of lymphoid organs allowed the topo-histochemical analysis of receptor (CR1/CR2)-ligand (C3b/C3d) interactions in such an immunologically important area as the germinal centers. The main finding of this study is, that periodic acid pretreatment of unfixed cryostat tonsil sections-oxidizing vicinal glycol groups of polysaccharide chains into dialdehydes-completely abolished the binding of all EAC/EC complexes to germinal center area. It may suggest the involvement of receptor carbohydrate in C3 receptor/ligand binding. In addition to, the subsequent sodium borohydride reduction-converting aldehydes (produced by periodic acid oxidation) into primary alcohols-restored selectively the binding of all applied EAC/EC complexes to follicular centers. These in vitro topo-histochemical studies give a strong hint for the participation of-OH groups of sugar residues in CR1/CR2 ligand (C3b/C3d) binding.