Although knowledge of the etiology of SLE is incomplete, it is clear from the varied forms of tissue injury that a number of different effector mechanisms may act alone or in concert to produce the pleomorphic patterns of lupus nephritis. Autoantibodies may lead to cell and tissue injury by Fc receptor-mediated inflammation (8) as well as by direct cytotoxicity, which is usually complement-dependent, as has been shown for antibody-mediated hemolytic anemia or thrombocytopenia. In the kidney, intrinsic antigens such as extracellular matrix components or cell surface glycoproteins may serve as targets for autoantibody binding. In addition, renal injury in lupus nephritis may result from autoantibodies that bind to circulating antigens, forming circulating preformed immune complexes, or autoantibodies that bind to antigens deposited from the circulation in glomerular and vessel walls, causing in situ immune complex formation, as has been shown for nucleosomes and antidouble-stranded DNA autoantibodies (5). Subsequent Fc receptor and complement binding then initiates an inflammatory and cytotoxic reaction. Such cytotoxicity may be directed toward podocytes in the setting of membranous nephropathy, where in situ immune complex formation occurs along the subepithelial aspect of the glomerular basement membrane, or toward endocapillary cells in the case of the endocapillary proliferative and exudative inflammatory reaction that follows subendothelial immune complex formation.
In addition to direct immune complex-mediated cell and tissue injury, autoantibodies with antiphospholipid or cryoglobulin activity may also promote thrombotic and inflammatory vascular lesions in SLE (9). Antineutrophil cytoplasmic-antigen autoantibodies (ANCA) have been described in a subgroup of patients with lupus nephritis and may initiate vasculitis and glomerulonephritis by “pauci-immune” neutrophil-dependent mechanisms similar to those described for microscopic polyangiitis or Wegener’s granulomatosis (10). Finally, it is also likely that other poorly characterized autoantibodies of unknown specificity (such as anti-endothelial antibodies) may be operant in the pathogenesis of some forms of lupus nephritis.