Hereditary deficiency of C3 in animals and humans

The Complement-Targeted Inhibitor Mini-FH Protects against Experimental Periodontitis via Both C3-Dependent and C3-Independent Mechanisms

A minimized version of complement factor H (FH), designated mini-FH, was previously engineered combining the N-terminal regulatory domains (short consensus repeat [SCR]1-4) and C-terminal host-surface recognition domains (SCR19-20) of the parent molecule. Mini-FH conferred enhanced protection, as compared with FH, in an ex vivo model of paroxysmal nocturnal hemoglobinuria driven by alternative pathway dysregulation. In the current study, we tested whether and how mini-FH could block another complement-mediated disease, namely periodontitis. In a mouse model of ligature-induced periodontitis (LIP), mini-FH inhibited periodontal inflammation and bone loss in wild-type mice. Although LIP-subjected C3-deficient mice are protected relative to wild-type littermates and exhibit only modest bone loss, mini-FH strikingly inhibited bone loss even in C3-deficient mice. However, mini-FH failed to inhibit ligature-induced bone loss in mice doubly deficient in C3 and CD11b. These findings indicate that mini-FH can inhibit experimental periodontitis even in a manner that is independent of its complement regulatory activity and is mediated by complement receptor 3 (CD11b/CD18). Consistent with this notion, a complement receptor 3-interacting recombinant FH segment that lacks complement regulatory activity (specifically encompassing SCRs 19 and 20; FH19-20) was also able to suppress bone loss in LIP-subjected C3-deficient mice. In conclusion, mini-FH appears to be a promising candidate therapeutic for periodontitis by virtue of its ability to suppress bone loss via mechanisms that both include and go beyond its complement regulatory activity.

Molecular pharmacology in complement-mediated hemolytic disorders

In the last decade, a deeper understanding of the pathogenesis of complement mediated hemolytic disorders, such as paroxysmal nocturnal hemoglobinuria (PNH), cold agglutinin disease (CAD), warm type autoimmune hemolytic anemia (AIHA) with complement activation (wAIHA), and atypical hemolytic uremic syndrome (aHUS), paved the way to the therapeutic shift from purely supportive approaches to complement-targeted therapies. This resulted in a significant improvement in disease management, survival, and quality of life. In this review, we will provide a snapshot of novel therapies for complement-mediated hemolytic anemias with a focus on those ready to use in clinical practice. C5 inhibitors eculizumab and the long-acting ravulizumab, are the established gold standard for untreated PNH patients, whilst the C3 inhibitor pegcetacoplan should be considered for suboptimal responders to anti-C5 drugs. Several additional compounds targeting the complement cascade at different levels (other C5 inhibitors, factor B and D inhibitors) are under active investigation with promising results. In CAD, immunosuppression with rituximab remains the first-line. However, recently FDA and EMA approved the anti-C1s monoclonal antibody, sutimlimab, that showed dramatic responses and whose regulatory approval is soon awaited in many countries. Other drugs under investigation in AIHA include the C3 inhibitor pegcetacoplan, and the anti-C1q ANX005 for warm AIHA with complement activation. Finally, aHUS is an indication for complement inhibitors. Eculizumab and ravulizumab have been approved, whilst other C5 inhibitors, and novel lectin pathway inhibitors are under active investigation in this disease.

Complement inhibition: A possible therapeutic approach in the fight against Covid-19

The complement system, as a vital part of innate immunity, has an important role in the clearance of pathogens; however, unregulated activation of this system probably has a key role in the pathogenesis of acute lung injury, which is induced by highly pathogenic viruses (i.e. influenza A viruses and severe acute respiratory syndrome [SARS] coronavirus). The novel coronavirus SARS-CoV-2, which is the causal agent for the ongoing global pandemic of the coronavirus disease 2019 (Covid-19), has recently been spread to almost all countries around the world. Although most people are immunocompetent to SARS-CoV-2, a small group develops hyper-inflammation that leads to complications like acute respiratory distress syndrome, disseminated intravascular coagulation, and multi-organ failure. Emerging evidence demonstrates that the complement system exerts a crucial role in this inflammatory reaction. Additionally, patients with the severe form of Covid-19 show over-activation of the complement in their skin, sera, and lungs. This study aims to summarise current knowledge concerning the interaction of SARS-CoV-2 with the complement system and to critically appraise complement inhibition as a potential new approach for Covid-19 treatment.

Expanding horizons in complement drug discovery: challenges and emerging strategies

The complement system is best known for its role in innate immunity, providing a first line of defence against infection, maintaining tissue homeostasis by flagging apoptotic cells and debris for removal, and orchestrating crosstalk between adaptive and innate immunity. In a growing number of diseases, complement is known to drive pathogenesis or to contribute as an inflammatory amplifier of a disease trigger. Association of complement with common and devastating diseases has driven an upsurge in complement drug discovery, but despite a wealth of knowledge in the complexities of the cascade, and many decades of effort, very few drugs have progressed to late-stage clinical studies. The reasons for this are becoming clear with difficulties including high target concentration and turnover, lack of clarity around disease mechanism and unwanted side effects. Lessons learnt from drugs which are either approved, or are currently in late-stage development, or have failed and dropped off the drug development landscape, have been invaluable to drive a new generation of innovative drugs which are progressing through clinical development. In this review, the challenges associated with complement drug discovery are discussed and the current drug development landscape is reviewed. The latest approaches to improve drug characteristics are explored and those agents which employ these technologies to improve accessibility to patients are highlighted.

Many drugs for many targets: novel treatments for complement-mediated glomerular disease

There is a large body of experimental and clinical evidence that complement activation contributes to glomerular injury in multiple different diseases. However, the underlying mechanisms that trigger complement activation vary from disease to disease. Immune complexes activate the classical pathway of complement in many types of glomerulonephritis, whereas the alternative pathway and mannose-binding lectin pathways are directly activated in some diseases. Eculizumab is an inhibitory antibody to C5 that has been approved for the treatment of atypical hemolytic uremic syndrome, and case reports suggest that it is also effective in other types of glomerulonephritis. Furthermore, new complement-inhibitory drugs are being developed that target additional proteins within the complement cascade, raising the possibility of blocking the specific complement proteins involved in a given disease. This review examines the rationale for targeting different proteins within the complement cascade, the new anti-complement drugs currently in development and some of the challenges that investigators will face in bringing these drugs to the clinic.

Generation of complement protein C3 deficient pigs by CRISPR/Cas9-mediated gene targeting

Complement protein C3 is the pivotal component of the complement system. Previous studies have demonstrated that C3 has implications in various human diseases and exerts profound functions under certain conditions. However, the delineation of pathological and physiological roles of C3 has been hampered by the insufficiency of suitable animal models. In the present study, we applied the clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated (Cas) system to target the C3 gene in porcine fetal fibroblasts. Our results indicated that CRISPR/Cas9 targeting efficiency was as high as 84.7%, and the biallelic mutation efficiency reached at 45.7%. The biallelic modified colonies were used as donor for somatic cell nuclear transfer (SCNT) technology to generate C3 targeted piglets. A total of 19 C3 knockout (KO) piglets were produced and their plasma C3 protein was undetectable by western blot analysis and ELISA. The hemolytic complement activity and complement-dependent cytotoxicity assay further confirmed that C3 was disrupted in these piglets. These C3 KO pigs could be utilized as a valuable large animal model for the elucidation of the roles of C3.

Paroxysmal nocturnal hemoglobinuria: a complement-mediated hemolytic anemia

Paroxysmal nocturnal hemoglobinuria is manifests with a chronic hemolytic anemia from uncontrolled complement activation, a propensity for thrombosis and marrow failure. The hemolysis is largely mediated by the alternative pathway of complement. Clinical manifestations result from the lack of specific cell surface proteins, CD55 and CD59, on PNH cells. Complement inhibition by eculizumab leads to dramatic clinical improvement. While this therapeutic approach is effective, there is residual complement activity resulting from specific clinical scenarios as well as from upstream complement components that can account for suboptimal responses in some patients. Complement inhibition strategies are an area of active research.

The complement receptor 2/factor H fusion protein TT30 protects paroxysmal nocturnal hemoglobinuria erythrocytes from complement-mediated hemolysis and C3 fragment

Paroxysmal nocturnal hemoglobinuria (PNH) is characterized by complement-mediated intravascular hemolysis because of the lack from erythrocyte surface of the complement regulators CD55 and CD59, with subsequent uncontrolled continuous spontaneous activation of the complement alternative pathway (CAP), and at times of the complement classic pathway. Here we investigate in an in vitro model the effect on PNH erythrocytes of a novel therapeutic strategy for membrane-targeted delivery of a CAP inhibitor. TT30 is a 65 kDa recombinant human fusion protein consisting of the iC3b/C3d-binding region of complement receptor 2 (CR2) and the inhibitory domain of the CAP regulator factor H (fH). TT30 completely inhibits in a dose-dependent manner hemolysis of PNH erythrocytes in a modified extended acidified serum assay, and also prevents C3 fragment deposition on surviving PNH erythrocytes. The efficacy of TT30 derives from its direct binding to PNH erythrocytes; if binding to the erythrocytes is disrupted, only partial inhibition of hemolysis is mediated by TT30 in solution, which is similar to that produced by the fH moiety of TT30 alone, or by intact human fH. TT30 is a membrane-targeted selective CAP inhibitor that may prevent both intravascular and C3-mediated extravascular hemolysis of PNH erythrocytes and warrants consideration for the treatment of PNH patients.

The dual role of complement in the progression of renal disease in NZB/W F(1) mice and alternative pathway inhibition

Complement plays a dual role in the progression of systemic lupus erythematosus since it has important protective functions, such as the clearance of immune complexes and apoptotic cells, but is also a mediator of renal inflammation. To investigate this balance in a clinically relevant setting, we investigated how targeted inhibition of all complement pathways vs. targeted inhibition of only the alternative pathway impacts immune and therapeutic outcomes in NZB/W F(1) mice. Following onset of proteinuria, mice were injected twice weekly with CR2-fH (inhibits alternative pathway), CR2-Crry (inhibits all pathways at C3 activation step), sCR2 (C3d targeting vehicle) or saline. Sera were analyzed every 2 weeks for anti-dsDNA antibody levels, and urinary albumin excretion was determined. Kidneys were collected for histological evaluation at 32 weeks. Compared to the control group, all CR2-fH, CR2-Crry and sCR2 treated groups showed significantly reduced serum anti-dsDNA antibody levels and strong trends towards reduced glomerular IgG deposition levels. Glomerular C3 deposition levels were also significantly reduced in all three-treated groups. However, significant reductions of disease activity (albuminuria and glomerulonephritis) were only seen in the CR2-fH treated group. These data highlight the dual role played by complement in the pathogenesis of lupus, and demonstrate a benefit of selectively inhibiting the alternative complement pathway, presumably because of protective contributions from the classical and/or lectin pathways. The sCR2 targeting moiety appears to be contributing to therapeutic outcome via modulation of autoimmunity. Furthermore, these results are largely consistent with our previous data using the MRL/lpr lupus model, thus broadening the significance of these findings.

The benefit of targeted and selective inhibition of the alternative complement pathway for modulating autoimmunity and renal disease in MRL/lpr mice

OBJECTIVE

Complement has both protective and pathogenic functions in lupus due to a balance between its role in the clearance of immune complexes (ICs) and apoptotic cells and its role in inflammation. The classical pathway contributes to IC and apoptotic cell clearance, whereas the alternative pathway is a key mediator of renal inflammation. The aim of this study was to investigate the effect of a new targeted inhibitor of the alternative pathway, CR2-fH, on lupus-like renal disease in MRL/lpr mice.

METHODS

Mice were treated with either saline, CR2-fH, CR2-Crry (which inhibits all complement pathways), or soluble CR2 (sCR2; C3d-binding targeting vehicle). Sera were analyzed every 2 weeks for autoantibodies, circulating ICs, and C3. Urinary excretion of albumin was also determined, and kidneys were collected at 23 weeks for histologic evaluation.

RESULTS

Treatment with CR2-fH or CR2-Crry improved survival and significantly reduced proteinuria, glomerular C3 deposition, and the level of circulating ICs. CR2-fH, but not CR2-Crry, also significantly reduced glomerulonephritis, expression of serum anti-double-stranded DNA (anti-dsDNA) antibodies, and glomerular IgG and C1q deposition. Interestingly, sCR2 also significantly reduced the levels of anti-dsDNA antibodies and circulating ICs and reduced glomerular deposition of IgG, C1q, and C3, although there was no significant reduction in glomerulonephritis, proteinuria, or mortality.

CONCLUSION

Targeted and selective inhibition of the alternative complement pathway is an effective treatment of murine lupus and is more effective than blockade of all pathways. The data demonstrate benefits to leaving the classical/lectin pathways intact and indicate distinct roles for the classical and alternative pathways of complement in disease progression. The sCR2-targeting vehicle contributes to therapeutic activity, possibly via modulation of autoimmunity.

C3b and factor H: key components of the complement system

In all three complement pathways, the central molecule is C3, which, upon activation cleavage, forms the major opsonin C3b - the key component of complement. C3b is also essential for propagation of the complement cascade to the stage of the lytic terminal complement complexes. In order to prevent damage to self cells and tissues and restrict overconsumption of the complement components, C3b molecules need to be controlled by factor H. Defect in C3 functions leads to compromised microbial defence and increased susceptibility to certain autoimmune diseases. Deficiency of factor H, or a functional defect in its N terminus, often leads to membranoproliferative glomerulonephritis and complement depletion, owing to continuous overconsumption of C3. Defect in the factor H C terminus leads to a dramatically increased risk of atypical hemolytic uremic syndrome. In addition, recently, a polymorphism in the middle part of factor H (Y402H) has been shown to be the major risk factor for the most common cause of blindness in the industrialized world: age-related macular degeneration. In future, analysis of patient samples for defects in these key complement components may prove useful in diagnosis of these diseases and new therapeutic targets will certainly be the aim for use in the recently recognized factor H-related diseases.

Cobra venom factor: Structure, function, and humanization for therapeutic complement depletion

Cobra venom factor (CVF) is the complement-activating protein in cobra venom. This manuscript reviews the structure and function of CVF, how it interacts with the complement system, the structural and functional homology to complement component C3, and the use of CVF as an experimental tool to decomplement laboratory animals to study the functions of complement in host defense and immune response as well as in the pathogenesis of diseases. This manuscript also reviews the recent progress in using the homology between CVF and C3 to study C3 structure and function, and to develop human C3 derivatives with the complement-depleting function of CVF. These human C3 derivatives represent humanized CVF, and are a conceptually different concept for pharmacological intervention of the complement system, therapeutic complement depletion. The use of humanized CVF for therapeutic complement depletion in several pre-clinical models of human diseases is also reviewed.

C1q and anti-C1q antibody levels are correlated with disease severity in Chinese pediatric systemic lupus erythematosus

The aim of this study was to measure the level of serum complement lq(Clq) and anti-C1q antibodies (ClqAb) in pediatric systemic lupus erythematosus (PSLE), and then analyze the correlation between the levels of serum Clq and ClqAb with disease activity and kidney damage. We investigated 90 PSLE patients, including 43 patients in active stage and 47 patients in remission. Our results showed that the level of serum Clq of PSLE patients was significantly lower than the level of healthy children and children with other rheumatic diseases. In contrast, the level of ClqAb of PSLE patients was significantly higher than the other groups. The level of serum Clq was negatively correlated with systemic lupus erythematosus (SLE) disease active index (SLEDAI) and the level of ClqAb was positively correlated with SLEDAI and kidney damage. The sensitivity of using ClqAb levels to diagnose PSLE was 95.6% and the specificity was 97.5%. 93.0% SLE patients who had high C1qAb levels also showed kidney damage. Therefore, the levels of C1q and C1qAb in serum reflect the disease severity of SLE. Decreased Clq and increased C1qAb is closely correlated with lupus nephritis (LN) in children and may have diagnosis values for child LN.

Complement C3 in Bernese Mountain dogs

BACKGROUND

Previous research suggests that low serum concentrations of the third component of complement (C3) are associated with both the susceptibility to infectious agents such as Borrelia burgdorferi and the development of glomerular disease. We hypothesized that low levels of C3 are associated with the coincident occurrence of B. burgdorferi infection and glomerulonephritis in Bernese Mountain dogs.

OBJECTIVES

The aims of this study were to evaluate the serum concentration of C3 in Bernese Mountain dogs with and without antibodies against B. burgdorferi and to compare this concentration with that of healthy control dogs.

METHODS

Eighty-three clinically healthy Bernese Mountain dogs and 46 control dogs were included. Antibodies against B. burgdorferi were determined using an ELISA with a whole cell sonicate as antigen. Results were confirmed using Western blot. C3 was measured using a single radial immunodiffusion test. Results were reported as the percentage concentration of C3 compared with that in pooled preserved canine serum (100% C3 concentration).

RESULTS

Median C3 concentration was 128.5% in Bernese Mountain dogs with antibodies against B. burgdorferi, 133.5% in B. burgdorferi-negative Bernese Mountain dogs, 87.8% in positive control dogs, and 102.2% in negative control dogs. Within Bernese Mountain and control groups, C3 was lower in dogs with antibodies against B. burgdorferi compared with those without. Percentage concentration of C3 was higher in healthy Bernese Mountain dogs compared with control dogs.

CONCLUSION

Low C3 concentration is not an explanation for the high prevalence of B. burgdorferi infections and glomerular disease in Bernese Mountain dogs.

Heavily isotype-dependent protective activities of human antibodies against vaccinia virus extracellular virion antigen B5

Antibodies against the extracellular virion (EV or EEV) form of vaccinia virus are an important component of protective immunity in animal models and likely contribute to the protection of immunized humans against poxviruses. Using fully human monoclonal antibodies (MAbs), we now have shown that the protective attributes of the human anti-B5 antibody response to the smallpox vaccine (vaccinia virus) are heavily dependent on effector functions. By switching Fc domains of a single MAb, we have definitively shown that neutralization in vitro--and protection in vivo in a mouse model--by the human anti-B5 immunoglobulin G MAbs is isotype dependent, thereby demonstrating that efficient protection by these antibodies is not simply dependent on binding an appropriate vaccinia virion antigen with high affinity but in fact requires antibody effector function. The complement components C3 and C1q, but not C5, were required for neutralization. We also have demonstrated that human MAbs against B5 can potently direct complement-dependent cytotoxicity of vaccinia virus-infected cells. Each of these results was then extended to the polyclonal human antibody response to the smallpox vaccine. A model is proposed to explain the mechanism of EV neutralization. Altogether these findings enhance our understanding of the central protective activities of smallpox vaccine-elicited antibodies in immunized humans.

Renal disease associated with inherited disorders of the complement system

The human complement system is vital for host defense and plays a role in a number of inflammatory disorders. Inherited deficiency or dysfunction of most of the individual complement components occurs uncommonly. The phenotype displayed by such patients varies with the specific component deficiency and ranges from recurrent infections to autoimmune diseases. Most of the latter are associated with glomerulonephritis. The onset of severe lupus erythematosus in a young child, with prominent cutaneous and renal manifestations, especially if a similar disorder is present in another family member, is a clue to the presence of a complement component deficiency. The distinguishing of acquired deficiencies from inherited deficiencies in complement components is sometimes difficult and may require sophisticated laboratory testing.

Ageing is associated with diminished apoptotic cell clearance in vivo

Ageing leads to immune system dysfunction and the accumulation of autoantibodies. Because the rapid phagocytic clearance of apoptotic cells is required to prevent the development of autoimmunity, we examined the relative clearance of apoptotic material in young and aged mice using two independent assays. First, 2-year-old mice were found to be impaired in their ability to clear apoptotic keratinocytes following ultraviolet irradiation of the skin. Secondly, peritoneal macrophages exposed to apoptotic Jurkat T cells in vivo displayed diminished phagocytic activity in aged mice compared with 8-week-old mice. Consistent with these findings, aged mice exhibited signs of autoimmunity with the appearance of anti-nuclear antibodies and increased kidney glomerular size as well as complement deposits within the glomeruli. In vitro assays revealed that the pretreatment of macrophages with the serum from aged mice led to a reduction in their ability to phagocytose apoptotic bodies compared with macrophages treated with serum from young mice. These data show that the ageing process is accompanied by a diminished ability to clear apoptotic debris. This accumulation of apoptotic debris could contribute to immune system dysfunction that occurs in aged organisms.

New boundaries for complement in renal disease

Complement is an important component of the innate immune system whose function is integrated with the adaptive immune response. Since complement proteins are produced in virtually any cell in the body, it is important to question which pools of complement are responsible for what actions. This is particularly so in the case of complement-mediated renal disease, where distinct sites may require individualized approaches for therapy. From experimental and clinical evidence to date, it seems that the circulating pool of complement underlies much of the pathology traditionally associated with glomerular disease, including capillary wall injury. In contrast, the renal tubulointerstitium is the domain of local synthesis of complement, notably the axial component C3, principally expressed by the tubular epithelium. This means that therapeutic targeting will have to ensure penetration of the interstitial space in certain disorders. Likewise, monitoring of disease activity may benefit from evaluating this extravascular pool. Therapeutic and diagnostic applications in human disease are already taking this into account, with transplantation leading the way.

Levels of urinary complement factor H in patients with IgA nephropathy are closely associated with disease activity

Factor H plays a key inhibitory role in control of the activation of alternative pathway of complement system. The aim of the study was to investigate the predictive value of factor H as a biomarker of renal injury in IgA nephropathy (IgAN). Urine factor H concentration from 202 patients was measured and compared with that of 60 healthy volunteers. Forty-eight patients fulfilled Haas-I or II (group 1), 60 fulfilled Haas-III (group 2) and 94 fulfilled Haas-IV or V (group 3). Co-deposition of factor H and C3b in kidneys were investigated using confocal microscope. The levels of urinary factor H, when expressed as a ratio of urinary creatinine, were significantly higher in groups 3 than group 1 and 2, also significantly higher in group 2 than group 1. In addition, the levels of urinary factor H were significantly higher in those with factor H deposition in the kidney than those without deposition. The levels of urinary factor H may be a useful biomarker to evaluate kidney injury in IgAN.

Complement deficiencies in humans and animals: links to autoimmunity

Complement is involved in the pathogenesis of systemic lupus erythematosus (SLE) in multiple ways and may act as both friend and foe. Inherited homozygous deficiency of one of the earliest components of the classical pathway is strongly associated with susceptibility to the development of SLE. However, complement is also implicated in the effector inflammatory phase of the autoimmune response that characterizes the disease. A further paradox in the links between complement and SLE is the observation that autoantibodies to some complement proteins, especially to C1q, develop as part of the autoantibody response. In this chapter, the role of the complement system in SLE is reviewed and hypotheses advanced to explain the complex relationships between complement and lupus.

Macrophages from C3-deficient mice have impaired potency to stimulate alloreactive T cells

Impaired T-cell reactivity is a feature of C3-deficient mice in several disease models. The mechanism behind the reduced T-cell response is, however, poorly understood. We explored the hypothesis that antigen-presenting cells (APCs) from C3-/- mice have impaired potency to stimulate antigen-specific T cells, in an alloantigen-dependent model. Our results show that C3-/- macrophages have reduced ability to elicit alloreactive T-cell responses in vitro and in vivo, affecting both the primary and secondary responses. The C3 status of donor macrophages had a major impact on the CD4 T-cell response. The impaired CD4 T-cell response was associated with reduced expression of MHC class II on the surface of C3-/- macrophages, without loss of class II gene expression. Furthermore, inhibition of C3 gene expression in C3+/+ macrophages reduced their ability to stimulate alloreactive T cells, suggesting that endogenous production of C3 could in part contribute to the potency of APCs. Our data provide compelling evidence that C3 deficiency modulates the potency of APCs to stimulate the T-cell response, suggesting a critical role for complement in the maintenance of APC function. This could offer a partial explanation as to why the T-cell response is impaired in C3-/- mice. (Blood. 2006;107:2461-2469)

Complement Component C3d-Antigen Complexes Can Either Augment or Inhibit B Lymphocyte Activation and Humoral Immunity in Mice Depending on the Degree of CD21/CD19 Complex Engagement

C3d can function as a molecular adjuvant by binding CD21 and thereby enhancing B cell activation and humoral immune responses. However, recent studies suggest both positive and negative roles for C3d and the CD19/CD21 signaling complex in regulating humoral immunity. To address whether signaling through the CD19/CD21 complex can negatively regulate B cell function when engaged by physiological ligands, diphtheria toxin (DT)-C3d fusion protein and C3dg-streptavidin (SA) complexes were used to assess the role of CD21 during BCR-induced activation and in vivo immune responses. Immunization of mice with DT-C3d3 significantly reduced DT-specific Ab responses independently of CD21 expression or signaling. By contrast, SA-C3dg tetramers dramatically enhanced anti-SA responses when used at low doses, whereas 10-fold higher doses did not augment immune responses, except in CD21/35-deficient mice. Likewise, SA-C3dg (1 microg/ml) dramatically enhanced BCR-induced intracellular calcium concentration ([Ca2+]i) responses in vitro, but had no effect or inhibited [Ca2+]i responses when used at 10- to 50-fold higher concentrations. SA-C3dg enhancement of BCR-induced [Ca2+]i responses required CD21 and CD19 expression and resulted in significantly enhanced CD19 and Lyn phosphorylation, with enhanced Lyn/CD19 associations. BCR-induced CD22 phosphorylation and Src homology 2 domain-containing protein tyrosine phosphatase-1/CD22 associations were also reduced, suggesting abrogation of negative regulatory signaling. By contrast, CD19/CD21 ligation using higher concentrations of SA-C3dg significantly inhibited BCR-induced [Ca2+]i responses and inhibited CD19, Lyn, CD22, and Syk phosphorylation. Therefore, C3d may enhance or inhibit Ag-specific humoral immune responses through both CD21-dependent and -independent mechanisms depending on the concentration and nature of the Ag-C3d complexes.

Identification and characterization of the C3 binding domain of the Staphylococcus aureus extracellular fibrinogen-binding protein (Efb)

The secreted Staphylococcus aureus extracellular fibrinogen-binding protein (Efb) is a virulence factor that binds to both the complement component C3b and fibrinogen. Our laboratory previously reported that by binding to C3b, Efb inhibited complement activation and blocked opsonophagocytosis. We have now located the Efb binding domain in C3b to the C3d fragment and determined a disassociation constant (Kd) of 0.24 microM for the Efb-C3d binding using intrinsic fluorescence quenching assays. Using truncated, recombinant forms of Efb, we also demonstrate that the C3b binding region of Efb is located within the C terminus, in contrast to the fibrinogen binding domains that are located at the N-terminal end of the protein. Enzyme-linked immunosorbent assay-type binding assays demonstrated that recombinant Efb could bind to both C3b and fibrinogen simultaneously, forming a trimolecular complex and that the C-terminal region of Efb could inhibit complement activity in vitro. In addition, secondary structure analysis using circular dichroism spectroscopy revealed that the C-terminal, C3b binding region of Efb is composed primarily of alpha-helices, suggesting that this domain of Efb represents a novel type of C3b-binding protein.

BF, HP, DQB and DRB are associated with haemolytic complement activity, acute phase protein reaction and antibody response in the pig

In order to examine the loci factor B (BF), C3, corticotropin releasing hormone (CRH), DQB, DRB, haptoglobin (HP) and transforming growth factor beta 1 (TGFB1) for association with traits of humoral, specific and unspecific defence F2-animals of a porcine resource family were genotyped at single nucleotide polymorphisms within these loci. Haemolytic complement activity in the alternative and classical pathway, C3c and haptoglobin serum concentration and antibody titres were determined immediately prior and at days 4 and 10 after vaccinations against Mycoplasma hyopneumoniae (Mh), Aujeszky's disease virus, and porcine reproductive and respiratory syndrome virus at 6, 14 and 16 weeks of age, respectively. Analysis of variance revealed association of BF, HP and DRB with C3c serum concentration. The trend of haemolytic complement activity and C3c serum concentration during the experiment was affected by the interaction of DQB genotype and time of measurement. Association with antibody titres were found for BF, DQB and DRB. Results of the mixed model analyses were confirmed by quantitative transmission disequilibrium test that showed linkage and association with antibody titres, complement activity and acute phase reaction at certain times of measurement. The findings promote the importance of the candidate genes for humoral mechanisms of unspecific and specific defence that provide natural resistance against many pathogens.

Cutting Edge: C3d Functions as a Molecular Adjuvant in the Absence of CD21/35 Expression

Complement component C3 covalently attaches to Ags following activation, where the C3d cleavage fragment can function as a molecular adjuvant to augment humoral immune responses. C3d is proposed to exert its adjuvant-like activities by targeting Ags to the C3d receptor (CD21/35) expressed by B cells and follicular dendritic cells. To directly assess the importance of CD21/35 in mediating the immunostimulatory effects of C3d, CD21/35-deficient (CD21/35(-/-)) mice were immunized with streptavidin (SA), SA-C3dg tetramers, recombinant HIV gp120 (gp120), or gp120 fused with linear multimers of C3d. Remarkably, SA- and gp120-specific Ab responses were significantly augmented in CD21/35(-/-) mice when these Ags were complexed with C3d in comparison to Ag alone. In fact, primary and secondary Ab responses and Ab-forming cell responses of CD21/35(-/-) mice approached those of wild-type mice immunized with SA-C3dg and gp120-C3d. Thus, C3d can function as a molecular adjuvant in the absence of CD21/35 expression.

BAFF/BLyS can potentiate B-cell selection with the B-cell coreceptor complex

The tumor necrosis factor (TNF)–like ligand BAFF/BLyS (B-cell activating factor of the TNF family/B-lymphocyte stimulator) is a potent B-cell survival factor, yet its functional relationship with other B-cell surface molecules such as CD19 and CD40 is poorly understood. We found that follicular dendritic cells (FDCs) in human lymph nodes expressed BAFF abundantly. BAFF up-regulated a B cell–specific transcription factor Pax5/BSAP (Pax5/B cell–specific activator protein) activity and its target CD19, a major component of the B-cell coreceptor complex, and synergistically enhanced CD19 phosphorylation by B-cell antigen receptor (BCR). BAFF further enhanced B-cell proliferation, immunoglobulin G (IgG) production, and reactivity to CD154 by BCR/CD19 coligation and interleukin-15 (IL-15). Our results suggest that BAFF may play an important role in FDC–B-cell interactions through the B-cell coreceptor complex and a possibly sequential link between the T cell–independent and –dependent B-cell responses in the germinal centers.

A role for complement in feedback enhancement of antibody responses by IgG3

IgG1, IgG2a, and IgG2b, passively administered with soluble Ags, enhance specific Ab responses. The effect of IgG3 in this type of feedback regulation has not been studied previously. We immunized mice with trinitrophenyl (TNP)-coupled carrier proteins (bovine serum albumin [BSA] or ovalbumin [OVA]) alone or complexed to monoclonal TNP-specific IgG3. The carrier-specific Ab responses were enhanced by several hundred-fold by IgG3. Enhancement was significantly impaired in mice depleted of complement factor C3 and in mice lacking complement receptors 1 and 2 (Cr2-/-). In contrast, mice lacking the common Fc-receptor gamma chain (FcR gamma -/-), resulting in reduced expression of Fc gamma RI and lack of Fc gamma RIII, and mice lacking Fc gamma RIIB (Fc gamma RIIB-/-), responded equally well to immunization with IgG3-complexed Ag as wild-type controls. These findings demonstrate that IgG3 can induce feedback enhancement and that IgG3, in analogy with IgM, uses the complement system for this function.

Complement receptors CD21/35 link innate and protective immunity during Streptococcus pneumoniae infection by regulating IgG3 antibody responses

The CD21/35 receptor provides an important link between innate and adaptive immunity. Its importance during protective immune responses to encapsulated extracellular bacteria was assessed using a new line of mice completely deficient in CD21/35 expression (CD21/35(-/-)). CD21/35 expression was essential for the rapid trapping of C3dg-antigen complexes by B cells in vivo, especially in splenic marginal zones. Despite normal B cell development in CD21/35(-/-) mice, T cell-independent and -dependent antibody responses to low-dose antigens were significantly decreased, with a striking impairment in IgG3 responses. Accordingly, CD21/35(-/-) mice were more susceptible to acute lethal Streptococcus pneumoniae infection. Thus, CD21/35 expression is critical for early protective antibody responses to lethal pathogens that rapidly multiply and quickly overwhelm the immune system.

Complement and systemic lupus erythematosus

Complement is implicated in the pathogenesis of systemic lupus erythematosus (SLE) in several ways and may act as both friend and foe. Homozygous deficiency of any of the proteins of the classical pathway is causally associated with susceptibility to the development of SLE, especially deficiency of the earliest proteins of the activation pathway. However, complement is also implicated in the effector inflammatory phase of the autoimmune response that characterizes the disease. Complement proteins are deposited in inflamed tissues and, in experimental models, inhibition of C5 ameliorates disease in a murine model. As a further twist to the associations between the complement system and SLE, autoantibodies to some complement proteins, especially to C1q, develop as part of the autoantibody response. The presence of anti-C1q autoantibodies is associated with severe illness, including glomerulonephritis. In this chapter the role of the complement system in SLE is reviewed and hypotheses are advanced to explain the complex relationships between complement and lupus.

Complement component C3 is not required for full expression of immune complex glomerulonephritis in MRL/lpr mice

Complement activation and tissue deposition of complement fragments occur during disease progression in lupus nephritis. Genetic deficiency of some complement components (e.g., Factor B) and infusion of complement inhibitors (e.g., Crry, anti-C5 Ab) protect against inflammatory renal disease. Paradoxically, genetic deficiencies of early components of the classical complement pathway (e.g., C1q, C4, and C2) are associated with an increased incidence of lupus in humans and lupus-like disease in murine knockout strains. Complement protein C3 is the converging point for activation of all three complement pathways and thus plays a critical role in biologic processes mediated by complement activation. To define the role of C3 in lupus nephritis, mice rendered C3 deficient by targeted deletion were backcrossed for eight generations to MRL/lpr mice, a mouse strain that spontaneously develops lupus-like disease. We derived homozygous knockout (C3(-/-)), heterozygous (C3(+/-)), and C3 wild-type (C3(+/+)) MRL/lpr mice. Serum levels of autoantibodies and circulating immune complexes were similar among the three groups. However, there was earlier and significantly greater albuminuria in the C3(-/-) mice compared with the other two groups. Glomerular IgG deposition was also significantly greater in the C3(-/-) mice than in the other two groups, although overall pathologic renal scores were similar. These results indicate that C3 and/or activation of C3 is not required for full expression of immune complex renal disease in MRL/lpr mice and may in fact play a beneficial role via clearance of immune complexes.

Glomerulonephritis in a patient with complement factor I deficiency

Complement factor I deficiency is known to be associated with recurrent pyogenic infections. The patient described here had recurrent attacks of otitis, sinusitis, and bronchopneumonia since childhood. At the age of 24 years, he had an acute episode of systemic vasculitis with purpura, but no nephritis. A factor I deficiency was diagnosed when he was 36 years old. Because of the uncontrolled activation of the alternative pathway of complement, several other components were depleted, in particular C3, which explained the predisposition for pyogenic infections. A progressive loss of renal function accompanied by proteinuria and hematuria started after the age of 40 years. Renal biopsy showed a focal segmental glomerulonephritis (GN) with glomerular deposits of immunoglobulins and complement C3 and C4 fragments. The glomerular podocytes showed an almost complete loss of complement receptor 1 (CR1; CD35). The expression of CR1 was very low on erythrocytes, as well. Thus, CR1, the most efficient cell-bound cofactor for the inactivation of C4b/C3b by factor I, appears to be consumed when factor I is missing. Although this is the first report of factor I deficiency associated with GN, it is unlikely that the development of the nephritis was fortuitous because GN has been found in many other diseases characterized by uncontrolled activation of the alternative pathway.

Distinction between processing of normal and mutant complement C3 within human skin fibroblasts

Inherited C3 deficiency may result from mutations in the C3 gene affecting transcription or translation (type I deficiency). We described a type II C3 deficiency caused by a mutation yielding an abnormal non-secreted C3. The post-translational processing of mutant and normal C3 was analyzed in fibroblasts grown from skin biopsies. Mutant C3 is located mainly in the endoplasmic reticulum (ER), whereas normal C3 is seen evenly distributed throughout the cytoplasm. Most of the mutant C3 is degraded within the cell, and only a small fraction (around 8%) is secreted after 20 h chase. Processing of C3 at 19 degrees C was reduced in normal fibroblasts but completely blocked in mutant fibroblasts. ATP depletion blocked processing of normal proC3 to C3. In contrast, the mutant proC3 was partly degraded in ATP-depleted cells, yet its complete degradation and secretion were blocked. Intracellular degradation of the mutant C3 was not inhibited by NH4Cl, thus excluding cleavage within lysosomes. These results demonstrate that the type II mutant C3 studied here is retained in the ER probably by a quality contol machinery that identifies abnormal protein folding. Consequently, it is destined to undergo a two-step intracellular degradation; an initial ATP-independent step followed by an ATP-dependent step.

C1q and systemic lupus erythematosus

In this chapter we review the association between SLE and C1q. In the first part of the chapter we discuss the clinical associations of C1q deficiency, and tabulate the available information in the literature relating to C1q deficiency and autoimmune disease. Other clinical associations of C1q deficiency are then considered, and we mention briefly the association between other genetically determined complement deficiencies and lupus. In the review we explore the relationship between C1q consumption and lupus and we discuss the occurrence of low molecular weight (7S) C1q in lupus, which raises the possibility that increased C1q turnover in the disease may result in unbalanced chain synthesis of the molecule. Anti-C1q antibodies are also strongly associated with severe SLE affecting the kidney, and with hypocomplementaemic urticarial vasculitis, and these associations are also examined. We address the question of how C1q deficiency may cause SLE, discussing the possibility that this may be due to abnormalities of immune complex processing, which have been well characterised in a umber of different human models. There is clear evidence that immune complex processing is abnormal in patients with hypocomplementaemia, and this is compatible with the hypothesis that ineffective immune complex clearance could cause tissue injury, and this may in turn stimulate an autoantibody response. We have also considered the possibility that C1q-C1q receptor interactions are critical in the regulation of apoptosis, and we explore the hypothesis that dysregulation of apoptosis could explain important features in the development of autoimmune disease associated with C1q deficiency. An abnormally high rate of apoptosis, or defective clearance of apoptotic cells, could promote the accumulation of abnormal cellular products that might drive an autoimmune response. Anti-C1q antibodies have been described in a number of murine models of lupus, and these are also briefly discussed. We focus on the recently developed C1q "knockout" mice, which have been developed in our laboratory. Amongst the C1q deficient mice of a mixed genetic background high titres of antinuclear antibodies were detected in approximately half the animals, and around 25% of the mice, aged eight months had evidence of a glomerulonephritis with immune deposits. Large numbers of apoptotic bodies were also present in diseased glomeruli, and this supports the hypothesis that C1q may have a critical role to play in the physiological clearance of apoptotic cells.

Immunomodulatory properties of Nocardia lysozyme digest (NLD) in complement normal and C5-deficient mice

The constantly increasing number of substances with adjuvant activity outpaces the elucidation of their mode of action. This problem is of great importance as the immunomodulatory action of an adjuvant is time- and route-dependent, which implies that administration at a different moment or site may result in a reduced immune response. In the present work the possibility to achieve dual effect (stimulatory or inhibitory) is regarded in the light of the complement system. The object of the study is a preparation obtained by lysozyme digestion of Nocardia opaca cell walls (NLD). According to the results, the administration of NLD to mice (i.p. at a daily dose of 0.5 mg kg-1) during 3 days prior to the antigen resulted in an inhibition of serum antibody level against sheep red blood cells (SRBC) and lipopolysaccharide (LPS). At the same time, the preparation stimulated the antibody response to SRBC if it was applied after the antigen. The ability of NLD to ensure protection against experimental infection with Klebsiella pneumoniae was comparatively studied in complement-normal mice (strain ICR) and in C5-deficient mice (strain DBA/2). Firstly, it was established that complement-deficient mice were more resistant to infection than complement-normal. Secondly, the preparation expressed a protective effect in C5-deficient animals; nevertheless the inoculation was done s.c. or i.v. The departure of the infection depended on the rate of opsonization of K. pneumoniae. Under certain conditions NLD can provoke excessive C3 activation, which might aggravate the course of the infection. The preparation augmented the host response to second challenge with K. pneumoniae of complement-normal and C5-deficient mice.

Molecular Analysis of the Third Component of Canine Complement (C3) and Identification of the Mutation Responsible for Hereditary Canine C3 Deficiency

Genetically determined deficiency of the third component of complement (C3) in the dog is characterized by a predisposition to recurrent bacterial infections and to type 1 membranoproliferative glomerulonephritis. The current studies were undertaken to characterize the cDNA for wild-type canine C3 and identify the molecular basis for hereditary canine C3 deficiency. Amplification, cloning, and sequence analysis indicated that canine C3 is highly conserved in comparison with human, mouse, and guinea pig C3. Southern blot analysis failed to show any gross deletions or rearrangements of DNA from C3-deficient animals. Northern blot analysis indicated that the livers of these animals contain markedly reduced quantities of a normal length C3 mRNA. The full-length 5.1-kb canine C3 cDNA was amplified in overlapping PCR fragments. Sequence analysis of these fragments has shown a deletion of a cytosine at position 2136 (codon 712), leading to a frameshift that generates a stop codon 11 amino acids downstream. The deletion has been confirmed in genomic DNA, and its inheritance has been demonstrated by allele-specific oligonucleotide hybridization.