Complement production and regulation by dendritic cells: molecular switches between tolerance and immunity

In recent years it has become clear that the innate and adaptive immune systems are highly integrated and interact at several levels. Dendritic cells (DCs) are on the one hand instrumental for directing and controlling adaptive immunity and on the other hand are specialized in detecting and integrating signals from the microenvironment. In view of the strong link between deficiencies in certain complement components and the development of autoimmunity, interaction between complement and DCs seems to be of fundamental importance. We will discuss the role of C1q, C3, as well as complement regulators in DC biology.

Complement activation by tubular cells is mediated by properdin binding

Activation of filtered complement products on the brush border of the tubular epithelium is thought to be a key factor underlying proteinuria-induced tubulointerstitial injury. However, the mechanism of tubular complement activation is still unclear. Recent studies on mechanisms of complement activation indicate a key role for properdin in the initiation of an alternative pathway. We hypothesized that properdin serves as a focal point for complement activation on the tubulus. We observed a strong staining for properdin on the luminal surface of the tubules in kidney biopsies from patients with proteinuric renal disease. In vitro experiments revealed dose-dependent binding of properdin to proximal tubular epithelial cells (PTEC), whereas no significant binding to endothelial cells was detected. Exposure of PTEC with normal human serum as a source of complement resulted in complement activation with deposition of C3 and generation of C5b-9. These effects were virtually absent with properdin-deficient serum. Preincubation of PTEC with properdin before addition of properdin-depleted serum fully restored complement activation on the cells, strongly suggesting a key role for properdin in the activation of complement at the tubular surface. In proteinuric renal disease, filtered properdin may bind to PTEC and act as a focal point for alternative pathway activation. We propose that this contribution of properdin is pivotal in tubular complement activation and subsequent damage. Interference with properdin binding to tubular cells may provide an option for the treatment of proteinuric renal disease.

Properdin binds to late apoptotic and necrotic cells independently of C3b and regulates alternative pathway complement activation

Cells that undergo apoptosis or necrosis are promptly removed by phagocytes. Soluble opsonins such as complement can opsonize dying cells, thereby promoting their removal by phagocytes and modulating the immune response. The pivotal role of the complement system in the handling of dying cells has been demonstrated for the classical pathway (via C1q) and lectin pathway (via mannose-binding lectin and ficolin). Herein we report that the only known naturally occurring positive regulator of complement, properdin, binds predominantly to late apoptotic and necrotic cells, but not to early apoptotic cells. This binding occurs independently of C3b, which is additional to the standard model wherein properdin binds to preexisting clusters of C3b on targets and stabilizes the convertase C3bBb. By binding to late apoptotic or necrotic cells, properdin serves as a focal point for local amplification of alternative pathway complement activation. Furthermore, properdin exhibits a strong interaction with DNA that is exposed on the late stage of dying cells. Our data indicate that direct recognition of dying cells by properdin is essential to drive alternative pathway complement activation.

Depressed activation of the lectin pathway of complement in hereditary angioedema

The possibility of simultaneous measurement of the classical pathway (CP), mannan-binding lectin (MBL)--lectin pathway (LP) and alternative pathway (AP) of complement activation by the recently developed Wielisa method allowed us to investigate the in vivo significance of the C1-inhibitor (C1INH) in three complement activation pathways. Functional activity of the CP, LP and AP were measured in the sera of 68 adult patients with hereditary angioedema (HAE) and 64 healthy controls. In addition, the level of C1q, MBL, MBL-associated serine protease-2 (MASP-2), C4-, C3- and C1INH was measured by standard laboratory methods. MBL-2 genotypes were determined by polymerase chain reaction. Besides the complement alterations (low CP and C1INH activity, low C4-, C1INH concentrations), which characterize HAE, the level of MASP-2 was also lower (P = 0.0001) in patients compared with controls. Depressed LP activity was found in patients compared with controls (P = 0.0008) in homozygous carriers of the normal MBL genotype (A/A), but not in carriers of variant genotypes (A/O, O/O). Activity of CP correlated with LP in patients (Spearman's r = 0.64; P < 0.0001), but no significant correlation was found in the control group and no correlation with AP was observed. In contrast, the activity of CP and AP correlated (Spearman's r = 0.47; P < 0.0001) in healthy controls, but there was no significant correlation in the HAE patients. We conclude that the activation of LP might also occur in subjects with C1INH deficiency, which is reflected by the low MASP-2 and C4 levels.

Secretory immunoglobulin A (IgA) responses in IgA nephropathy patients after mucosal immunization, as part of a polymeric IgA response

Secretory immunoglobulin A (SIgA), although generated at mucosal surfaces, is also found in low concentrations in the circulation. Recently, SIgA was demonstrated in mesangial deposits of patients with immunoglobulin A nephropathy (IgAN), suggesting a role in the pathogenesis. This finding is in line with the belief that high molecular weight (HMW) immunoglobulin A (IgA) is deposited in the kidney. However, there is little information on the size distribution of antigen-specific IgA in circulation upon mucosal challenge. In this study we measured antigen-specific IgA, including SIgA, in serum following challenge of IgAN patients and controls via intranasal vaccination with a neoantigen, cholera toxin subunit B (CTB). We size-fractionated serum and nasal washes to study the size distribution of total IgA, SIgA and CTB-specific IgA. Finally, we compared the size distribution of antigen-specific IgA after mucosal immunization with the distribution upon systemic immunization. A significant induction of antigen-specific SIgA was detectable in serum of both patients with IgAN and controls after mucosal immunization with CTB. Independent of the route of immunization, in both groups the antigen-specific IgA response was predominantly in the polymeric IgA fractions. This is in contrast to total IgA levels in serum that are predominantly monomeric. We conclude that mucosal challenge results in antigen-specific SIgA in the circulation, and that the antigen-specific IgA response in both IgAN patients and in controls is of predominantly HMW in nature. No differences between IgAN patients and controls were detected, suggesting that the size distribution of antigen-specific IgA in the circulation is not disturbed specifically in IgAN patients.

The role of complement in innate, adaptive and eosinophil-dependent immunity to the nematode Nippostrongylus brasiliensis

Complement may be important for immunity to infection with parasitic helminths, by promoting the recruitment of leukocytes to infected tissues and by modulating the function of cytotoxic effector leukocytes. However, the importance of complement in vivo during helminth infection is poorly understood. In this study, mice lacking classical (C1q-deficient), alternative (factor B-deficient) or all pathways of complement activation (C3-deficient) were used to assess the role of complement in immunity to the nematode Nippostrongylus brasiliensis. Double-mutant complement-deficient/IL-5 transgenic (Tg) mice were used to determine if complement is required for the strong eosinophil-dependent resistance to this parasite. Complement activation on larvae (C3 deposition), extracellular eosinophil peroxidase activity, larval aggregation and eosinophil recruitment to the skin 30 min post-injection (p.i.) of larvae were reduced in factor B-deficient mice. Inhibition of the C5a receptor with the antagonist PMX53 impaired eosinophil and neutrophil recruitment to the skin. C3 deposition on larvae was minimal by 150 min p.i. and at this time cell adherence, larval aggregation, eosinophil recruitment and degranulation were complement-independent. Factor B and C3 deficiency were associated with higher lung larval burdens in primary infections. Complement-deficient/IL-5 Tg mice were highly resistant to N. brasiliensis, suggesting that eosinophils can limit infection in a complement-independent manner. Potent secondary immunity was similarly complement-independent. In conclusion, although the alternative pathway is important for parasite recognition and leukocyte recruitment early in N. brasiliensis infections, the parasite soon becomes resistant to complement and other factors can compensate to promote eosinophil-dependent immunity.

Myeloperoxidase is critically involved in the induction of organ damage after renal ischemia reperfusion

In this study the role of myeloperoxidase (MPO) in a murine (C57BL/6) model of ischemia and reperfusion (I/R)-induced renal failure was investigated. The renal function after I/R was analyzed in MPO-deficient (Mpo(-/-)) mice and compared with wild-type (WT) controls. A significant reduction in renal function loss (blood urea nitrogen) was observed after 24 hours of reperfusion of ischemically damaged kidneys in Mpo(-/-) mice compared with I/R WT controls (I/R Mpo(-/-) = 31.3 +/- 1.7 mmol/L versus I/R WT = 42.8 +/- 2.1 mmol/L, sham = 7.0 +/- 0.5 mmol/L; P = 0.003). The early reperfusion phase (2 hours of reperfusion) was characterized by a substantial increase in apoptosis and early complement activation, surprisingly similar in Mpo(-/-) and WT mice. Improved renal function in Mpo(-/-) mice after extended reperfusion was accompanied by a reduced neutrophil influx (P = 0.017) compared with WT controls. Activation and deposition of complement was not significantly reduced in Mpo(-/-) mice compared with WT controls after 24 hours of reperfusion, indicating no specific in vivo role for MPO in activating complement after renal I/R. Taken together, these results demonstrated an important contribution of MPO in the induction of organ damage after renal I/R by influencing critical factors such as neutrophil extravasation but not complement activation.

Immune modulation of human dendritic cells by complement

Deficiency in complement proteins such as C1q is associated with the development of systemic lupus erythematosus (SLE). Here, we show that the differentiation of dendritic cells (DC) in the presence of C1q (C1qDC) gives rise to CD1a(+)/DC-SIGN(+) cells with high phagocytic capacity and low expression of CD80, CD83 and CD86. Further, when C1qDC were exposed to LPS, a significant reduction in the production of IL-6, TNF-alpha and IL-10 occurred with a limited up-regulation of CD80, CD83 and CD86. In addition, C1qDC were less responsive to activation by CD40L in terms of IL-12p70 secretion and CD86 expression. C1qDC showed an impaired ability to stimulate alloreactive T cells, with a reduced production of IFN-gamma. In conclusion, we have shown that C1q is a potent modulator of DC, resulting in cells characterized by an impaired capacity of cytokine production and an impaired up-regulation of costimulatory molecules, leading to a limited T cell response. Therefore, we hypothesize that, next to a pivotal role in the safe clearance of apoptotic cells, C1q regulates the threshold of DC activation and thereby prevents hyperactivation of the overall immune response.

Immuno-histological analysis of dendritic cells in nasal biopsies of IgA nephropathy patients

BACKGROUND

IgA nephropathy (IgAN) is the most common primary glomerulonephritis worldwide. Intranasal vaccination of patients with IgAN has shown mucosal and systemic IgA hyporesponsiveness. Here, we investigated whether this IgA hyporesponse in IgAN patients can be explained by reduced numbers or altered subset distribution of dendritic cells (DCs) in nasal mucosa.

METHODS

Eighteen IgAN patients and 18 healthy volunteers were recruited for this study. Nasal biopsies were taken, after local anaesthesia, from the lower edge of the inferior turbinate. Staining for different subsets of DCs was performed using specific monoclonal antibodies. To detect myeloid DCs, we used CD1a, DC-SIGN and blood dendritic cell antigen-1 (BDCA-1) as a marker and for plasmacytoid DCs we used BDCA-2. DC-cell numbers in the epithelium and in lamina propria were counted separately and expressed as positively stained cells per mm(2).

RESULTS

Both myeloid and plasmacytoid DC could be demonstrated in nasal biopsies. Quantification showed that IgAN patients contained significantly more DC-SIGN-positive cells in the lamina propria compared to controls. In addition, in IgAN patients, we observed more CD1a-positive cells in the epithelium. No differences in BDCA-1 and BDCA-2-positive cells were found between patients and controls. The number of positively stained cells in the epithelial layer correlated strongly with the number of positively stained cells in the lamina propria.

CONCLUSIONS

Patients with IgAN have higher numbers of CD1a-positive cells in the epithelial layer and more DC-SIGN-positive cells in the lamina propria. Therefore, the earlier observed IgA hyporesponsiveness in IgAN patients after mucosal vaccination cannot be explained by lower numbers of nasal DCs.

Complement Factor H polymorphism Y402H associates with inflammation, visual acuity, and cardiovascular mortality in the elderly population at large

The haplotype tagging Y402H polymorphism in the Complement Factor H gene (CFH) has consistently been associated with age-related macular degeneration, whereas conflicting results have been reported on its relationship with cardiovascular disease. CFH plays a role in inflammation, which is causal to both diseases and both are highly prevalent in old age. Therefore, we investigated whether or not Y402H associated with inflammation, visual acuity, and cardiovascular disease in old age. Within the Leiden 85-plus Study, a prospective population-based study of participants aged 85 years and older, we found that carriers of the CFH 402HH variant had a higher production of IL-6 in whole blood samples compared to those carrying the 402YY variant (P=0.029). Carriers of the 402HH genotype also had a steeper increase in circulating C-reactive protein levels during follow-up (P=0.009), lower visual acuity (P=0.020), and an increased risk of cardiovascular mortality (P=0.004). Subjects in the lowest tertile of visual acuity had a twofold increased risk of cardiovascular mortality compared to those in the highest tertile (P=0.001). We conclude that the CFH Y402H polymorphism associates with inflammation, visual impairment, and cardiovascular mortality in the elderly population at large. Visual impairment identifies elderly with an increased risk of cardiovascular disease.

Complement in glomerular injury

In recent years, research into the role of complement in the immunopathogenesis of renal disease has broadened our understanding of the fragile balance between the protective and harmful functions of the complement system. Interventions into the complement system in various models of immune-mediated renal disease have resulted in both favourable and unfavourable effects and will allow us to precisely define the level of the complement cascade at which a therapeutic intervention will result in an optimal effect. The discovery of mutations of complement regulatory molecules has established a role of complement in the haemolytic uremic syndrome and membranoproliferative glomerulonephritis, and genotyping for mutations of the complement system are already leaving the research laboratory and have entered clinical practice. These clinical discoveries have resulted in the creation of relevant animal models which may provide crucial information for the development of highly specific therapeutic agents. Research into the role of complement in proteinuria has helped to understand pathways of inflammation which ultimately lead to renal failure irrespective of the underlying renal disease and is of major importance for the majority of renal patients. Complement science is a highly exciting area of translational research and hopefully will result in meaningful therapeutic advances in the near future.

A novel role of complement factor C1q in augmenting the presentation of antigen captured in immune complexes to CD8+ T lymphocytes

Ag-IgG immune complexes (IC) are efficiently taken up, and Ag-derived peptides are subsequently processed and presented by APC. In vitro experiments indicate that IgG Fc Receptors (FcgammaR) facilitate the efficient uptake of IC by dendritic cells. Previous experiments showed that the cross-presentation of Ag-derived peptides after s.c. administration of IC is FcgammaR-dependent. To study the role of different FcgammaR and complement in MHC class I Ag presentation after i.v. administration, we used mice deficient for FcgammaRs and complement components. These mice were injected with CFSE-labeled OVA-specific CD8+ T cells followed by administration of IC composed of OVA and rabbit anti-OVA IgG i.v. to measure MHC class I presentation of OVA-derived peptides. The Ag presentation was partly reduced in FcRgamma-chain-deficient mice, but not affected in FcgammaRI/II/III-deficient mice, complement factor C3-deficient mice, or FcgammaRI/II/III x C3-deficient mice. Importantly, CD8+ T cell proliferation was significantly reduced in mice deficient for C1q. This proliferation could be restored when IC were incubated with purified human C1q before injection. Likewise, purified C1q could strongly enhance the uptake and presentation of IC by dendritic cells in vitro. Heat inactivation abrogated the C1q-mediated uptake of IC. In addition, in vivo uptake of OVA-IC in the spleen was significantly reduced in C1q-deficient mice compared with wild-type mice. Together, these results indicate a novel function of C1q, which is present in high levels in the bloodstream, by directly enhancing the uptake and MHC class I presentation of Ag captured in IC by APC to CD8+ T cells.

Low pretransplantation mannose-binding lectin levels predict superior patient and graft survival after simultaneous pancreas-kidney transplantation

Simultaneous pancreas-kidney transplantation (SPKT) is the treatment of choice for patients with type 1 diabetes and renal failure. However, this procedure is characterized by a high rate of postoperative infections, acute rejection episodes, and cardiovascular mortality. The lectin pathway of complement activation contributes to cardiovascular disease in diabetes and may play an important role in inflammatory damage after organ transplantation. This study therefore sought to determine how mannose-binding lectin (MBL), a major recognition molecule of the lectin pathway of complement activation, influences outcome after SPKT. MBL serum levels were determined in 99 and MBL genotypes in 97 consecutive patients who received an SPKT from 1990 through 2000 and related to patient and graft survival. At 12 yr, cumulative death-censored kidney graft survival was 87.5% in patients with an MBL level <400 ng/ml and 74.8% in the group with MBL levels >400 ng/ml (P = 0.021). Pancreas graft survival was significantly better in patients with low MBL levels (P = 0.016). MBL levels >400 ng/ml were associated with a hazard ratio of 6.28 for patient death (95% confidence interval 1.8 to 20.3; P = 0.003). Accordingly, survival was significantly better in recipients with MBL gene polymorphisms associated with low MBL levels. These findings identify MBL as a potential risk factor for graft and patient survival in SPKT. It is hypothesized that MBL contributes to the pathogenesis of inflammation-induced vascular damage both in the transplanted organs and in the recipient's native blood vessels.

Human neutrophil peptide-1 inhibits both the classical and the lectin pathway of complement activation

Human neutrophil peptide-1 (HNP-1) is a member of the alpha-defensin family. Defensins are cationic antimicrobial peptides, which play an important role in the antimicrobial response to microorganisms. In addition, recent studies have revealed the involvement of defensins in inflammation, immunity and wound repair. Defensins are present in the azurophilic granules of neutrophils and are released upon neutrophil stimulation. Previous studies showed that HNP-1 binds to C1q and inhibits the classical complement pathway. In view of the structural and functional similarity between C1q and MBL, we have now examined the interactions between HNP-1 and MBL. We observed a dose-dependent binding of HNP-1 to MBL in calcium-free buffer, indicating that HNP-1 binds to MBL most likely via the collagenous domains. To identify the binding sites in HNP-1 involved in the binding to C1q and MBL, we used a series of overlapping synthetic linear peptides that spanned the entire HNP-1 sequence. Both MBL and C1q showed a dose-dependent binding to the same set of peptides, suggesting a similar binding site in HNP-1 for both MBL and C1q. Strongest binding was observed to peptides containing the C- or N-terminal part of the HNP-1 molecule. Using an ELISA based system, we demonstrated that HNP-1 inhibits activation of both the classical pathway and lectin pathway of complement. Furthermore, we demonstrated that C1q and MBL can form complexes with HNP-1 in solution. Together, the data indicate that HNP-1 interacts with both C1q and MBL efficiently resulting in inhibition of both the classical and the lectin pathway of complement. We conclude that HNP-1 may play a role in protection against tissue injury during inflammatory conditions by inhibiting the early phase of complement activation.

Demonstration of secretory IgA in kidneys of patients with IgA nephropathy

BACKGROUND

Recently we reported a possible role for secretory IgA (SIgA) in IgA nephropathy (IgAN), as suggested by increased serum levels in patients with active disease and accumulation of SIgA in a glomerular eluate. Therefore, we attempted to find support for these findings by analysis of the presence of SIgA in biopsies of IgAN patients.

METHODS

Renal biopsies of 26 patients with biopsy-proven IgAN were analysed for the presence of SIgA and complement proteins.

RESULTS

In 15% mesangial deposition of SIgA was demonstrated, using a specific staining for secretory component (SC) and colocalization with IgA. The presence of SIgA in these biopsies showed a strong correlation with deposition of mannose-binding lectin (MBL) and C4d. Moreover, we observed a strong colocalization between SIgA and MBL or C4d. This local complement activation has previously been linked to more severe renal disease.

CONCLUSIONS

Therefore, these data provide additional evidence for a pathogenic role for SIgA in IgA nephropathy.

Human peritoneal macrophages show functional characteristics of M-CSF-driven anti-inflammatory type 2 macrophages

We have recently shown that in vitro polarized M-CSF-driven anti-inflammatory macrophages (MPhi2) have the unique capacity to preferentially bind and ingest early apoptotic cells. However, these data are based on in vitro polarized cells and it is unclear whether MPhi2-like cells exist in vivo. Here we used CD163 as a cell surface marker to distinguish MPhi2 from the pro-inflammatory MPhi1. We show that human peritoneal MPhi (pMPhi) freshly isolated from patients on peritoneal dialysis have the phenotypical characteristics of MPhi2, including CD163 surface expression and lack of CD16. Like MPhi2, pMPhi have the capacity for endocytosis and macropinocytosis, are able to preferentially bind and ingest early apoptotic cells, and produce large amounts of IL-10 upon stimulation with LPS. Moreover, upon LPS stimulation both pMPhi and MPhi2 down-regulate CD86, resulting in a reduced capacity to stimulate proliferation of allogeneic T cells and an inhibition of Th1 cytokine release of these T cells. Our data provide the evidence for the first time that in vitro polarized MPhi2 exist in vivo, and human pMPhi resemble the anti-inflammatory MPhi2. We propose that pMPhi have the potential to maintain an anti-inflammatory condition in the peritoneal cavity.

Factor B structure provides insights into activation of the central protease of the complement system

Factor B is the central protease of the complement system of immune defense. Here, we present the crystal structure of human factor B at 2.3-A resolution, which reveals how the five-domain proenzyme is kept securely inactive. The canonical activation helix of the Von Willebrand factor A (VWA) domain is displaced by a helix from the preceding domain linker. The two helices conformationally link the scissile-activation peptide and the metal ion-dependent adhesion site required for binding of the ligand C3b. The data suggest that C3b binding displaces the three N-terminal control domains and reshuffles the two central helices. Reshuffling of the helices releases the scissile bond for final proteolytic activation and generates a new interface between the VWA domain and the serine protease domain. This allosteric mechanism is crucial for tight regulation of the complement-amplification step in the immune response.