The complement system is controlled by soluble and cell-bound regulators. The C1 inhibitor (C1-INH), which belongs to the serpin family, is only one strong inhibitor in circulation for early components such as C1r, C1s, and MASP (Fig. 7.2) [7, 8]. AP is controlled by two types of inhibition mechanisms, termed decay-accelerating activity and cofactor-mediated cleavage. Decay-accelerating activity dissociates C3 and C5 convertases. Cofactor activity cleaves C3b into iC3b (inactivated form) and C3f with factor I. Plasma factor H (fH) and its truncated form factor H-like protein-1 (FHL-1), plasma C4-binding protein (C4bp), cell-bound complement receptor 1 (CR1, CD35), decay-accelerating factor (DAF, CD55), and membrane cofactor protein (MCP, CD46) belong to regulators of the complement activation family [9]. Cell-bound regulators, CD59, soluble clusterin, and vitronectin, prevent MAC formation. Carboxypeptidase N (CPN) is a soluble protease that digests anaphylatoxin, C3a, and C5a and inactivates them into C3a desArg and C5a desArg, respectively [4, 8].

A detailed analysis of the role of the complement system in the pathogenesis of glomerulonephritis would discriminate a subpopulation of patients from others. It is emphasized that we need to have concern about the production, consumption, and deficiencies of complement components. Complement deficiencies are common genetic disorders. In most cases, heterozygotes produce one-half of the normal plasma level of a specific complement protein. The frequency of deficiency of C1-INH, MBL, C4, and C9 is approximately 1:50,000, 1:10, 1:3 (including partial deletion), and 1:1000 (in Japanese healthy blood donors), respectively [10, 11]. In an analysis of a patient with low CH50, we were first concerned with the possibility of complement deficiencies. Next, we should take notice of overproduction of complement components. The liver is the main source of these components. Because the complement components belong to the acute reactive proteins, like C-reactive protein, infections and chronic inflammation enhance the production of complement components by hepatocytes [12]. Intriguingly, the target organ can also produce the components in situ [13, 14]. For example, C3 is synthesized by kidney resident cells and a single donor kidney is able to produce 5 % of circulating C3 [13, 15]. Actually, we can see the fluctuations of complement levels in the context of many clinical situations. The breakdown product of C3, C3a desArg or, in another term, “acylation-stimulating protein (ASP),” is recognized as one type of adipocytokine. Adipocytes also produce C3, fB, and fD, which are spontaneously activated by the nearby adipocytes themselves. CPN splits C3a into C3a desArg and arginine. C3a desArg has been recognized as an inactivated form of C3a, but energetic studies identified it as a potent stimulator of glucose transport and triglyceride synthesis in adipocytes [16, 17]. Then, the level of C3 increases in an overnutrition state, such as metabolic syndrome, diabetes, or obesity (Fig. 7.3a, b). Body mass index (BMI) and serum complement parameters such as C3, C4, and CH50 have a positive correlation. Especially, the serum level of C3 has a strong correlation with BMI and can fluctuate with insulin resistance. Cryoglobulin-containing serum easily activates CP in test tubes (the so-called cold activation), with the results of the serum level of components and the titer of CH50 being overestimated [18]. Generally, in autoimmune diseases, such as lupus nephritis (LN) and IgG4-related tubulointerstitial nephritis, IC activates CP. In these diseases, IC leads to the consumption of early components of CP in the fluid phase, and laboratory data presents a low serum level of C4 and a low titer of CH50 [19]. Simultaneously, renal depositions of early components of CP, C1q, and C4 were observed. On the other hand, in the cases of post-streptococcal acute glomerulonephritis (PSAGN), most patients presented a low serum level of C3, a low titer of CH50, and glomerular depositions of C3. Thus, it is recognized that AP is the main route of complement activation in PSAGN. From the viewpoint of excess of AP activation, a newly defined clinical entity “C3 glomerulopathy,” which comprises of glomerular lesions with predominant C3 staining, was proposed. Although this category contains cases of membranoproliferative glomerulonephritis (MPGN) and dense deposit disease, many pathogenic abnormalities, such as fH mutations, C3 gene variants, nephritic factors, anti-complement regulatory proteins, were reported [20]. The histological damage of these cases was deduced due to uncontrolled AP activation.

Because serum levels of C3 and C4 fluctuate within the normal range, complement activation in the fluid phase has not been focused on in clinical practice. However, the ratio of the serum level of IgA and C3 (IgA/C3 ratio, more than 3.01) has become a candidate biomarker for the diagnosis of IgAN [21, 22]. In this clinical parameter, a low serum level of C3 contributes to the increase of the IgA/C3 ratio. A cross-sectional study has also indicated that serum C3 levels do not increase despite the elevated levels of other complement components in IgAN at the time of biopsy (Table 7.1). These evidences suggest the possibility that the consumption of serum C3 and MBL via AP and LP might be increased compared with hepatic production of C3 and MBL [23]. Aggregated IgA, polymeric IgA, and chemically deglycosylated IgA1 can activate directly with AP; however, the molecular basis for this difference is not fully understood [24, 25]. To the best of our knowledge, galactose-deficient IgA1 itself is able to activate C3 directly in fluid phase [26].

There is accumulating evidence to support a hypothesis of the occurrence of LP activation in the pathogenesis of IgAN. Under-O-glycosylated IgA leads to the polymerization of IgA [27], and purified polymeric IgA from patients with IgAN can activate LP [28]. On the other hand, prominent ligands for MBL are mannose and N-acetylglucosamine (GlcNAc), and ligands for ficolins are GlcNAc. Although such an under-O-glycosylated IgA1 (N-acetylgalactosamine, GalNAc) from patients with IgAN could potentially interact with plant lectins [29], GalNAc residues cannot be recognized by MBL and ficolins. So far, certain ligands of MBL and ficolin are yet to be defined. Secretory IgA (SIgA) is the first line of defense in protecting the respiratory and intestinal tract and is a credible candidate. Since the N-glycans on the heavy chains of both IgA1 and SIgA2 present terminal GlcNAc and mannose residues, they could be recognized by MBL and ficolins [30].

Tomino et al. deduced that the polymeric form of IgA1 is predominantly eluted from the renal tissues of IgAN patients [44]. Tomino and Conley et al. also confirmed the predominantly glomerular deposition of IgA1, rather than that of IgA2 [45, 46]. A subsequent report showed that patients with mesangial deposits of IgA1 and C3c showed no deposits of C4, MBL, and MASP-1. AP is certainly activated in glomerulus [47]. Endo et al. first demonstrated that the glomerular deposition of MBL/MASP-1 occurred in 25 % of cases of IgAN and that this was not observed in the normal kidney and that the frequency was higher than that found in the other forms of glomerulonephritis [48]. Patients with glomerular deposits of MBL/MASP-1 were young, and the duration of the disease prior to renal biopsy was short compared with that in patients without MBL/MASP-1 deposition. Therefore, Roos et al. concluded that the glomerular activation of LP was associated with more severe renal damage, as demonstrated by proteinuria, decreased renal function, and more severe histological findings, such as mesangial proliferation, crescent formation, glomerular sclerosis, and interstitial fibrosis [26]. Because a recent cohort study also confirmed the significance of MBL deposition in IgAN, the glomerular deposition of MBL is a prognostic marker of IgAN [49].

In an experimental model, Hashimoto et al. analyzed the grouped ddY mouse, which presents spontaneous IgAN [50]. They found that the complement components belonging to LP and CP were deposited in glomeruli; furthermore, the serum level of IgA–MBL complex and IgA–IgG2a complex was significantly higher (Fig. 7.6). Further analysis of this model might unravel the pathogenic obscurities between complement activation and IgAN.

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