Administration of IgG Fc fragments prevents glomerular injury in experimental immune complex nephritis.

Most human nephritis is due to glomerular deposition and/or formation of immune complexes (IC). In cultured mesangial cells, Fc receptor stimulation induces proliferation, matrix synthesis, and release of several mediators implicated in the initiation and progression of glomerular injury. Since Ig Fc fragments in vitro modified these phenomena, we studied the effects of systemic administration of IgG Fc fragments on the evolution of experimental IC nephritis. Fc fragment injection (1 mg/day i.p.) to rats with ongoing nephritis (proteinuria 20-50 mg/24 h vs 9 +/- 0.2 mg/24 h in controls) markedly ameliorates proteinuria, renal function, and morphological renal lesions. This was accompanied by a reduction in the renal synthesis of chemokines (monocyte chemoattractant protein-1, IFN-inducible protein-10, and cytokine-induced neutrophil chemoattractant-1), matrix proteins, and growth factors (platelet-derived growth factor, and TGF-beta), and in the activity of transcription factors. The treatment did not affect the glomerular deposition of IgG IC and complement C1q. In contrast, a decrease in the renal expression and production of C3 was observed without changes in serum complement levels. In vitro, very low complement consumption and no C3b covalent interaction were observed with Fc fragments, confirming that they did not modify systemic complement activity. These results indicate that the administration of Fc fragments prevents the development of glomerular damage in an aggressive model of proliferative glomerulonephritis through mechanisms involving a reduced local generation of complement, chemokines and growth factors. Modulation of IC-mesangial cell interaction by Fc fragment administration could represent a new approach to the treatment of severe immune nephritis.

The C(H)1 domain of IgG is not essential for C3 covalent binding: importance of the other constant domains as targets for C3.

The covalent binding of C3 to antigen-antibody complexes [immune complexes (IC)] plays a pivotal role in the elimination of antigens. C3 prevents the formation of large IC lattices promoting their solubilization. Subsequently, bound C3 fragments determine the efficacy of antigen presentation, and the generation of antibody responses and immunological memory. C3 binding to IgG-IC generates IgG-C3b-C3b complexes which are detected by SDS-PAGE as two major bands: C3alpha65-heavy chain and C3alpha65-C3alpha43 covalent complexes. Using human heat-aggregated IgG1 as a model of IC, a C3b binding site was localized only in the Cgamma1 domain. However, with true IC of ovalbumin and rabbit IgG anti-ovalbumin, C3b binds to both the Fab and Fc regions of IgG. To study the binding of C3b to the different domains of IgG and particularly to evaluate the involvement of the Cgamma1 domain, we have constructed recombinant single-chain antibodies without Cgamma1, which have the structure: V(H)-linker-V(L)-hinge-Cgamma2-Cgamma3 (scAb). The variable domains were from a mouse mAb anti-HSA and the constant region (hinge-C(H)2-C(H)3) from human IgG1 or rabbit IgG. C3 binds very efficiently to IC formed with human (h-scAb) or rabbit (r-scAb) recombinant antibodies (scAb-HSA) and generates also two bands on SDS-PAGE (C3alpha65-scAb and C3alpha65-C3alpha43), which are the counterparts of those of the complete antibody. In addition, IC formed with scAb activate the alternative pathway to a similar extent as IC of the entire IgG. These data indicate that the Cgamma1 domain is a dispensable region for C3b binding and that the remaining constant domains are as efficient as Cgamma1 in C3b binding. Overall these results support the view that C3 does not specifically recognize a unique site in the Cgamma1 domain. Rather it seems to be able to attach along the antibody molecule. Probably this implies an advantage for effective processing of C3b-IC and elimination of antigens in vivo.

Analysis of complement C3 deposition and degradation on Klebsiella pneumoniae.

The majority of Klebsiella pneumoniae serum-resistant strains activate complement and bind C3b, the opsonic fragment of C3, without C5b-9 formation and bacterial killing. The mechanisms leading to C3b deposition without cell death were studied, and the results indicate that serum-resistant strains activate principally the alternative pathway and that serum-sensitive strains activate both the alternative and classical pathways. Bacterial molecules implicated in C3b deposition are the outer membrane porin proteins and smooth and rough lipopolysaccharides. Porins activate both complement pathways, and the rough lipopolysaccharide activates the classical pathway, causing deposition of C3b in serum-sensitive strains. The smooth lipopolysaccharide of serum-resistant strains activates only the alternative pathway, impeding the binding of C1q to porins (S. Albertí, G. Marqués, S. Camprubí, S. Merino, J. M. Tomás, F. Vivanco, and V. J. Benedí, Infect. Immun. 61:852-860, 1993; S. Albertí, F. Rodríguez-Quinónes, T. Schirmer, G. Rummel, J. M. Tomás, J. P. Rosenbusch, and V. J. Benedí, Infect. Immun. 63:903-910, 1995) and rough lipopolysaccharide molecules and thereby preventing activation of the classical pathway. After its deposition, C3b is quickly degraded to iC3b on both types of strains, but the higher-level deposition of C3b on serum-sensitive strains, resulting from activation of both the alternative and classical complement pathways, supports further complement activation and killing of serum-sensitive strains.