CR1 stump peptide and terminal complement complexes are found in the glomeruli of lupus nephritis patients.

The number of CR1 on podocytes is reduced in nephropathies with severe glomerular damage, especially in the diffuse proliferative glomerulonephritis (DPGN) of systemic lupus erythematosus (SLE). Reduction of CR1 number on erythrocytes is due to proteolysis of CR1 by macrophage proteases activated by the reaction of their complement receptors, which leaves a 'CR1 stump peptide' on the erythrocyte. In the present study, we demonstrated the presence of the terminal complement complex (TCC) and the CR1 stump in histological sections of biopsies from patients with SLE by the indirect immunoperoxidase technique. Less severe glomerular lesions presented TCC deposits mainly in the mesangium (mesangial pattern). In lupus nephritis, with more severe glomerular damage, TCC deposits were detected both in the mesangium and in the capillary loops with podocyte involvement (mixed pattern). Patients with highly active DPGN presented a marked reduction of intact podocyte CR1 receptors in association with increased reactivity to the anti-CR1 stump antibody and with glomerular TCC deposits of mixed histological pattern. These results suggest that the decrease in the number of podocyte CR1 receptors in severe glomerular lesions of SLE may be due to a local proteolytic activity associated with activation and deposition of TCC.