Systemic lupus erythematosus (SLE), an autoimmune disease characterized by chronic nephritis, arthritis and dermatitis, and the presence of antinuclear autoantibodies, is associated with complement factor deficiencies in the classical activation pathway. In addition, IFN-a seems to be a key cytokine in SLE as an activated IFN-a system is regularly observed in patients with SLE. Here, we demonstrate that in lupus-susceptible, complement C4-deficient mice the lack of complement results in elevated intravascular levels of apoptotic DNA. The apoptotic DNA is targeted to the splenic marginal zone where it accumulates and induces IFN-a. As such, we present here a unifying hypothesis for the induction of SLE that incorporates the role of complement deficiency and elevated levels of IFN-a. IntroductionSystemic lupus erythematosus (SLE) is a chronic autoimmune disease characterized by autoantibodies against various nuclear antigens, as well as the generation of immune complexes (IC) leading to inflammatory responses in skin, kidneys, joints, the vasculature and the heart [1]. Susceptibility genes for SLE include those involved in the deficiency of early complement factors of the classical pathway (C1, C4, C2) [2], which has been confirmed in animal models for complement deficiency [3,4]. Current explanations for the loss of tolerance in states of complement deficiency include impaired elimination of apoptotic bodies [4,5] or dying cells [6] as a source of autoantigens [7,8], and inadequate regulation of follicular B cells via the complement receptor CD21/35 [3], although the absence of CD21, or its most important ligand (C3d), does not seem to be associated with SLE [9,10]. As such, neither model alone can adequately account for the association of complement deficiency and SLE [11].IFN-a is another important factor in the development of SLE, and elevated levels of this antiviral cytokine or IFN-a-induced genes are frequently observed in patients with SLE [12][13][14][15]. Activation of the class I IFN system has been confirmed by recent studies using microarray approaches [16][17][18][19]. Moreover, the administration of IFN-a for therapeutic purposes induces typical SLE autoantibodies as a side effect [20,21]. Both these phenomena are poorly understood and no unifying hypothesis, incorporating the role of complement deficiency and elevated IFN-a levels in the induction of SLE, exists. Recently, we described an unusual deposition of IgMcontaining immune complexes (IgM-IC) in the splenic marginal zone (MZ) of complement C4-deficient (C4 null ) mice. This IgM-IC deposition led to restoration of a humoral immune response against foreign antigens within those IC, when compared to mice receiving antigen only [22]. Murine MZ macrophages are known to produce IFN-a [23] and we hypothesized that intravascular nuclear antigen levels would be elevated as a result of complement deficiency and that natural IgM anti-DNA antibodies would mediate the IC deposition in the splenic MZ. Here, the nuclear antigens would induce IFN-a, which i...
SummaryDeficiencies in early complement components are associated with the development of systemic lupus erythematosus (SLE) and therefore early complement components have been proposed to influence B lymphocyte activation and tolerance induction. A defect in apoptosis is a potential mechanism for breaking of peripheral B cell tolerance, and we hypothesized that the lack of the early complement component C4 could initiate autoimmunity through a defect in peripheral B lymphocyte apoptosis. Previous studies have shown that injection of a high dose of soluble antigen, during an established primary immune response, induces massive apoptotic death in germinal centre B cells. Here, we tested if the antigen-induced apoptosis within germinal centres is influenced by early complement components by comparing complement C4-deficient mice with C57BL/6 wild-type mice. We demonstrate that after the application of a high dose of soluble antigen in wild-type mice, antibody levels declined temporarily but were restored almost completely after a week. However, after antigen-induced apoptosis, B cell memory was severely limited. Interestingly, no difference was observed between wild-type and complement C4-deficient animals in the number of apoptotic cells, restoration of antibody levels and memory response.
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