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Complement activation in human lymphoid germinal centres.

The presence of complement activation products has been studied in morphologically normal human lymphatic tissue from tonsil, spleen and lymph node. Newly established monoclonal antibodies (mAbs) with reactivity against the C4 cleavage fragments C4a, C4b, C4c and C4d were applied on cyrostat sections in the indirect immunoperoxidase staining technique. Irrespective of organ type, C4d activation product could be detected in germinal centres of all secondary lymphoid follicles. To substantiate this finding, the complete sequence of complement activation products was investigated by a series of mono- and polyclonal antibodies to the complement proteins C1, C2, C3, factor B, C5, C9 to C5b-9 neoantigens and to the regulatory complement proteins C4 binding protein (C4bp), factor I, factor H and properdin. Similar to C4d, all secondary follicles exhibited a strong staining reaction for C3d antigens restricted to germinal centres. At the same site, albeit with distinctly weaker intensity, components of the membrane attack complex (MAC) C5b-9 were found. The simultaneous deposition of C1, C4b and C4bp in certain germinal centres indicates that complement activation is induced via the classical pathway. Concomitant deposition of IgM suggests IgM-antigen complexes that have been trapped on follicular dendritic cells (FDC) during normal immune response as the most likely candidates for activators of the classical pathway. Our data demonstrate that human lymphoid germinal centres as important sites of immune regulation closely interrelate with the complete cascade of complement-activation products, including the membrane attack complex (MAC).