We have shown here that apoJ/clusterin prevents a progressive glomerulopathy of aging in mice. apoJ/clusterin-deficient mice develop a pronounced mesangial expansion with the accumulation of electron-dense material in the matrix, which becomes highly organized into tubulo-fibrillary structures. As early as 4 weeks of age, immune material accumulates that includes IgG, IgM, and IgA. By 21 months, C1q, C3, and C9 are also deposited in the mesangium. The aging phenotype of the apoJ/clusterin-deficient mice could be accelerated in 6-month-old mice by unilateral nephrectomy combined with a high-protein diet, which increases trafficking of molecules through the mesangium (4
). The consequent increase in mesangial trafficking led, in particular, to exacerbation of the amount of deposited material in the mesangium compared to results with similarly aged control mice. Intriguingly, in both normal and accelerated kidney aging, immune material deposition occurs without concomitant inflammation or necrosis.
The mesangial deposits associated with apoJ/clusterin deficiency may represent the products of in vivo-formed immune complexes, or they may be plasma immunoglobulins that become trapped by the mesangium as they traffic through the kidney. At this point we lack direct evidence for antigen involvement in deposit accumulation, although it is a formal possibility that the deposits are selectively those of antigen-antibody complexes. Our evidence that the mesangial deposits represent immune complexes is that there are multiple immunoglobulin isotypes present and that in late-stage deposits, complement components also are present. In addition, we showed that in vitro-assembled, intravenously injected antigen-antibody complexes became localized to the mesangium of 5-month-old apoJ/clusterin-deficient mice but not in wild-type mice to a detectable level. Nonetheless, the temporal dissociation between immunoglobulin and complement deposition suggests that apoJ/clusterin deficiency may cause an initial problem in immunoglobulin management, per se. Moreover, the absence of an elicited inflammatory response or frank and severe kidney disease elicited by the deposits suggests that the complexes are primarily sterile and may be largely devoid of antigen. At this time, the exact origin of the deposits remains uncertain.
A number of factors can influence the inflammatory processing of immune complexes. These include the degree of antigen load, immune complex physicochemical properties, site of deposition, presence of molecules that facilitate or inhibit interactions between the immune complexes, and the relative activation of additional inflammatory pathways, such as through complement. For example, immune complexes isolated from synovial fluid of patients with juvenile rheumatoid arthritis differ in their ability to induce proinflammatory events (7
). The immune complexes in the aging deficient mice most likely form under conditions of low antigen load, which could also be a factor responsible for the lack of observed inflammatory sequelae.
Immune complexes can activate inflammation through Fc receptor-mediated events (19
). Signal transduction from Fc receptors, present on leukocytes and mesangial cells, is initiated by binding of Fc domains of immunoglobulins in immune complexes to the receptors, with their subsequent aggregation and induction of proinflammatory molecules, leukocyte recruitment, and mesangial cell activation (12
). This Fc receptor pathway is a critical determinant of immune complex-mediated glomerular injury, as evidenced by attenuated inflammation both in Fc receptor-deficient mice and by intravenously injected Fc receptor fragments in a number of experimental models of glomerulonephritis (1
). apoJ/clusterin, by binding immunoglobulin Fc domains, potentially enhances immune complex-Fc receptor-mediated inflammatory events. If this is the case, genetic ablation of apoJ/clusterin would dissociate inflammation from immune complex deposition.
A role for apoJ/clusterin in immune complex-mediated disease was first suggested by its interaction with immunoglobulins. apoJ/clusterin can bind to the Fc and Fab regions of all isotypes of IgG, IgM and IgA by a noncovalent mechanism (28
). The site of interaction is different than the Fc binding site of C1q and protein A (28
). apoJ/clusterin has been shown to preferentially bind to aggregated compared to monomeric IgG (28
). These results suggest that apoJ/clusterin has multiple potential binding sites through which it may interact and facilitate the clearance of polymeric IgG present in immune complexes.
In addition to its interaction with immunoglobulins, apoJ/clusterin has been found in conjunction with immune deposits in a number of immune-mediated glomerular diseases, including IgA nephropathy, membranous glomerulonephritis, and lupus nephritis (3
). In most cases apoJ/clusterin colocalizes with components of the membrane attack complex when these components are present with immunoglobulins but not when the membrane attack complex is found in the absence of immunoglobulins, suggesting a direct role for apoJ/clusterin in the processing of immune complexes (3
). apoJ/clusterin has also been localized to the glomerulus in such immune-mediated models of glomerulonephritis as Heymann nephritis and anti-Thy 1 nephritis (2
). In the latter model, upregulation of both clusterin mRNA and protein has been demonstrated in mesangial cells, providing evidence that clusterin can be synthesized by these cells following immune attack (29
). The association of apoJ/clusterin with immune deposits in experimental and human glomerulonephritis suggests it may modulate responses to immune-complex-induced injury. Further support for this view has been provided by Saunders et al. in the isolated rat kidney model of Heymann nephritis, where these investigators demonstrated increased proteinuria, greater deposition of complement components, and greater glomerular injury when these kidneys were perfused with apoJ/clusterin-depleted serum (22
An association between apoJ/clusterin and immune complex disease is found in patients with systemic lupus erythematosis (SLE). Levels of apoJ/clusterin in serum are lower in patients with SLE than in normal controls or patients with rheumatoid arthritis, osteoarthritis, or Sjogren's syndrome (17
). In this cross-sectional study of lupus patients, serum apoJ/clusterin levels were inversely correlated with disease activity, with the lowest levels observed in those patients with the most active disease. apoJ/clusterin levels were also lower in SLE patients with proteinuria than in those without proteinuria (17
). Based on these findings in SLE patients and the phenotype of apoJ/clusterin-deficient mice, apoJ/clusterin must be considered as a candidate disease modifier gene that may explain the different susceptibilities and clinical courses in human immune complex-mediated diseases.
Our results support the hypothesis that apoJ/clusterin prevents glomerular immune complex deposition of aging, and they support a role for circulating apoJ/clusterin in the clearance of immune complexes. Furthermore, these findings suggest that clusterin is part of a system that protects the mesangium from the continuous challenge of macromolecules trafficking through it. The phenotype of the apoJ/clusterin-deficient mouse may provide a clue as to the other responsibilities of apoJ/clusterin. We hypothesize that apoJ/clusterin is part of a metabolic pathway responsible for the identification and clearance of toxic and/or immunogenic macromolecules that arise during cell injury, death, and immune response processes. Disposal of bioactive material is critical for longevity in the setting of tissue turnover, inflammation, and immune response, particularly with age. This disposal function is necessary for the removal of foreign antigens, damaged endogenous macromolecules, and cellular debris generated during physiologic cell death (apoptosis) or necrosis. Failure to remove such material may contribute to progressive organ dysfunction and increase the severity of autoimmunity.