Differential characteristics of immune-bound antibodies in diffuse proliferative and membranous forms of lupus glomerulonephritis.

Diffuse proliferative (PGN) and membranous (MGN) glomerulonephritis represent contrasting histologic lesions in systemic lupus erythematosus (SLE). Serum, cryoglobulins, and renal biopsies in 8 SLE patients with PGN and 8 with MGN were studied in order to determine whether variations in the properties of immune-bound antibodies correlate with the pattern of glomerular involvement. Several immunologic parameters suggested differences in complement activation between the two groups. PGN cryoglobulins demonstrated immunoglobulin G (IgG) anti-native DNA (nDNA) subclass heterogeneity with highest titers of IgG3. These findings contrasted with the observation that MGN was characterized by a predominance of IgG4 in cryoglobulins. The major glomerular IgG subclasses in PGN were IgG3 and IgG1, while MGN biopsies demonstrated IgG4 in largest amount. Serum C1q was lower in PGN than in MGN. Serum anti-nDNA antibodies, solid-phase C1q-binding IgG immune complexes, and cryoglobulin protein concentrations were higher in PGN sera. Cryoglobulin complement component and control protein concentrations were greater in PGN than in MGN, while cryoglobulin Ig and immune-bound anti-nDNA were not different. In vitro C3 fixation by cryoglobulin anti-nDNA was greater in PGN than in MGN. Glomerular C1q, C4-binding protein (C4bp), and C3c were present in comparable amounts to IgG deposits in PGN biopsies, while in MGN IgG was demonstrable in greater quantities than C1q, C4bp, and C3c. In contrast, glomerular C3d (alpha 2d), C5, C6, P, and H were comparable in the two groups. It was concluded that immune-bound antibodies in cryoglobulins and in glomerular immune deposits in SLE PGN appear to activate complement via the classical and alternative pathways, while complement activation in MGN appears to occur predominantly via the alternative pathway. These differences in IgG subclass composition may account for the differential complement activation and may explain the contrasting histologic expression of immune aggregate localization in glomerular capillaries in these variants of lupus nephritis.