Properdin: New roles in pattern recognition and target clearance

Alternative complement pathway assessment in patients with atypical HUS

The atypical Hemolytic Uremic Syndrome (aHUS) is a rare thrombotic microangiopathy leading to end stage renal disease in approximately 60% of patients. Over the last decade, a clear link has been demonstrated between this disease and defective complement regulation. The hallmark of the aHUS is the association with mutations in complement alternative pathway genes. Endothelial damage is related to complement dysregulation, but the exact mechanism is just starting to be elucidated. Screening for and characterization of mutations in the components of the C3 convertase (C3 and FB) or its regulators (FH, FI, MCP, and Thrombomodulin) or anti-FH antibodies has become an indispensable part of the disease's diagnostic. This review will initially summarize current knowledge on the understanding of complement activation and regulation, followed by a description on the genetic analysis as well as the methods used for complement protein quantification. Another part of this review will focus on the mechanisms of action of aHUS-associated mutations. We will emphasize on when and why some mutations lead to protein deficiency, while others result in - to dysfunctional but normally expressed proteins. Finally, we will discuss how the therapy of aHUS patients can be modified according to the functional consequences of each particular genetic defect.

Identification of tubular heparan sulfate as a docking platform for the alternative complement component properdin in proteinuric renal disease

Properdin binds to proximal tubular epithelial cells (PTEC) and activates the complement system via the alternative pathway in vitro. Cellular ligands for properdin in the kidney have not yet been identified. Because properdin interacts with solid-phase heparin, we investigated whether heparan sulfate proteoglycans (HSPG) could be the physiological ligands of properdin. Kidneys from proteinuric rats showed colocalization of syndecan-1, a major epithelial HSPG, and properdin in the apical membranes of PTEC, which was not seen in control renal tissue. In vitro, PTEC did not constitutively express properdin. However, exogenous properdin binds to these cells in a dose-dependent fashion. Properdin binding was prevented by heparitinase pretreatment of the cells and was dose-dependently inhibited by exogenous heparin. ELISA and surface plasmon resonance spectroscopy (BIAcore) showed a strong dose-dependent interaction between heparan sulfate (HS) and properdin (K(d) = 128 nm). Pretreatment of HSPG with heparitinase abolished this interaction in ELISA. Competition assays, using a library of HS-like polysaccharides, showed that sulfation pattern, chain length, and backbone composition determine the interaction of properdin with glycosaminoglycans. Interestingly, two nonanticoagulant heparin derivatives inhibited properdin-HS interaction in ELISA and BIAcore. Incubation of PTEC with human serum as complement source led to complement activation and deposition of C3 on the cells. This C3 deposition is dependent on the binding of properdin to HS as shown by heparitinase pretreatment of the cells. Our data identify tubular HS as a novel docking platform for alternative pathway activation via properdin, which might play a role in proteinuric renal damage. Our study also suggests nonanticoagulant heparinoids may provide renoprotection in complement-dependent renal diseases.

Genetic and therapeutic targeting of properdin in mice prevents complement-mediated tissue injury

The alternative pathway (AP) of complement activation is constitutively active and must be regulated by host proteins to prevent autologous tissue injury. Dysfunction of AP regulatory proteins has been linked to several human inflammatory disorders. Properdin is a positive regulator of AP complement activation that has been shown to extend the half-life of cell surface–bound C3 convertase C3bBb; it may also initiate AP complement activation. Here, we demonstrate a critical role for properdin in autologous tissue injury mediated by AP complement activation. We identified myeloid lineage cells as the principal source of plasma properdin by generating mice with global and tissue-specific knockout of Cfp (which encodes properdin) and by generating BM chimeric mice. Properdin deficiency rescued mice from AP complement–mediated embryonic lethality caused by deficiency of the membrane complement regulator Crry and markedly reduced disease severity in the K/BxN model of arthritis. Ab neutralization of properdin in WT mice similarly ameliorated arthritis development, whereas reconstitution of properdin-null mice with exogenous properdin restored arthritis sensitivity. These data implicate systemic properdin as a key contributor to AP complement–mediated injury and support its therapeutic targeting in complement-dependent human diseases.

Complement alternative pathway acts as a positive feedback amplification of neutrophil activation

Complement alternative pathway plays an important, but not clearly understood, role in neutrophil-mediated diseases. We here show that neutrophils themselves activate complement when stimulated by cytokines or coagulation-derived factors. In whole blood, tumor necrosis factor/formyl-methionyl-leucyl-phenylalanine or phorbol myristate acetate resulted in C3 fragments binding on neutrophils and monocytes, but not on T cells. Neutrophils, stimulated by tumor necrosis factor, triggered the alternative pathway on their surface in normal and C2-depleted, but not in factor B-depleted serum and on incubation with purified C3, factors B and D. This occurred independently of neutrophil proteases, oxidants, or apoptosis. Neutrophil-secreted properdin was detected on the cell surface and could focus "in situ" the alternative pathway activation. Importantly, complement, in turn, led to further activation of neutrophils, with enhanced CD11b expression and oxidative burst. Complement-induced neutrophil activation involved mostly C5a and possibly C5b-9 complexes, detected on tumor necrosis factor- and serum-activated neutrophils. In conclusion, neutrophil stimulation by cytokines results in an unusual activation of autologous complement by healthy cells. This triggers a new amplification loop in physiologic innate immunity: Neutrophils activate the alternative complement pathway and release C5 fragments, which further amplify neutrophil proinflammatory responses. This mechanism, possibly required for effective host defense, may be relevant to complement involvement in neutrophil-mediated diseases.

Proteomic profiling of blood-dialyzer interactome reveals involvement of lectin complement pathway in hemodialysis-induced inflammatory response

PURPOSE

dialysis-induced inflammatory response including leukocyte and complement activation is considered a significant cofactor of chronic morbidity in long-term hemodialysis (HD) patients. The aim of this study was to provide better insight into its molecular background.

EXPERIMENTAL DESIGN

in 16 patients, basic biocompatibility markers, i.e. leukocyte counts and C5a levels, were monitored during HD on a polysulfone membrane. Proteins adsorbed to dialyzers were eluted and separated by 2-DE. Selected proteins were identified by MS; ficolin-2 plasma levels were assessed. Data are given as medians (quartile ranges).

RESULTS

in total, 7.2 (34.7) mg proteins were retrieved from dialyzer eluates and were resolved into 217 protein spots. The proteins most enriched in eluates (and hence selectively adsorbed) were those involved in complement activation (C3c, ficolin-2, mannan-binding lectin serine proteases, properdin) and cell adhesion (actin, caldesmon, tropomyosin, vitronectin, vinculin). A significant decrease of plasma ficolin-2 (41% [4.7], p<0.001) was evidenced during one HD session, associated with leukopenia (r=0.73, p=0.001) and C5a production (r=-0.62, p=0.01) at 15 min.

CONCLUSIONS AND CLINICAL RELEVANCE

ficolin-2 adsorption to polysulfone dialyzer initiates the lectin pathway of complement activation, mediates dialysis-induced leukopenia, and results in a significant depletion of ficolin-2, an essential component of innate immunity.

Innate immunity as orchestrator of stem cell mobilization

Hematopoietic stem and progenitor cells (HSPCs), as well as other types of stem cells, circulate under steady-state conditions at detectable levels in peripheral blood (PB), with their numbers increasing in response to stress, inflammation and tissue/organ injury. This mobilization process may be envisioned as a danger-sensing response mechanism triggered by hypoxia or mechanical or infection-induced tissue damage that recruits into PB different types of stem cells that have a role in immune surveillance and organ/tissue regeneration. Mobilization is also significantly enhanced by the administration of pharmacological agents, which has been exploited in hematological transplantology as a means to obtain HSPCs for hematopoietic reconstitution. In this review we will present mounting evidence that innate immunity orchestrates this evolutionarily conserved mechanism of HSPC mobilization.

Properdin in childhood and its association with wheezing and atopy

Properdin, a serum glycoprotein, is an important component of innate immunity, the only known positive regulator of complement, acting as an initiation point for alternative pathway activation. As an X-linked protein, we hypothesized that properdin may play a modulatory role in the pathogenesis of viral wheeze in children, which tends to be more common and more severe in boys. We aimed to determine properdin levels in a community-based paediatric sample, and to assess whether levels of properdin were associated with childhood wheeze phenotypes and atopy. We studied 137 school-children aged 8-12 yrs, a nested sample from a cohort study. Properdin was measured by a commercial enzyme-linked immunoabsorbant assay. We assessed wheeze by questionnaire, validated it by a nurse-led interview and performed skin prick tests and a methacholine challenge in all children. Forty children (29%) reported current wheeze. Serum properdin levels ranged between 18 and 40 microg/ml. Properdin was not associated with age, gender, atopy, bronchial responsiveness, current wheeze (neither the viral wheeze nor multiple-trigger wheeze phenotype) or severity of wheeze, but was slightly lower in south Asian (median 21.8 microg/ml) compared with white children (23.3 microg/ml; p = 0.006). Our data make it unlikely that properdin deficiency is common in healthy children or that levels of properdin are a major risk factor for wheeze or atopy.

A complex role for complement in allergic asthma

Allergic asthma is a chronic inflammatory disease of the upper airway. It is well appreciated that maladaptive Th2 immunity promotes the allergic phenotype, the underlying mechanisms of which remain elusive. The disease is associated with activation of complement, an ancient danger-sensing component of the innate immune system. Different models of experimental allergic asthma suggest that the small complement fragments of C3 and C5, the anaphylatoxins C3a and C5a, not only promote proallergic effector functions during the allergic effector phase but regulate the development of Th2 immunity during allergen sensitization. The available data support a concept in which C5a is dominant during allergen sensitization and protects against the development of maladaptive Th2 immunity. By contrast, C3a and C5a appear to act synergistically and drive allergic inflammation during the effector phase. In this article, we will review the recent findings in the field to judge the benefit of complement targeting in allergic asthma.

Screening of DNA-variants in the properdin gene (CFP) in age-related macular degeneration (AMD)

Several variants in the complement cascade genes (complement factor H [CFH], C2, C3, CFB, and Serping1) have been reported to associate with age-related macular degeneration (AMD). Of these, a member of the complement alternative pathway, CFH, represents the highest risk. As properdin (P) is also an important protein in this pathway, we analysed whether variants in the properdin gene (CFP) at Xp11.4 are associated with AMD. Ten exons of CFP were sequenced in a total of 222 Finnish patients with AMD (150 sporadic cases and 72 familial cases). The controls were 86 age-matched non-AMD patients with no large drusen and no or minimal focal pigmentary abnormalities. A total of four single nucleotide polymorphisms (SNPs) were detected in CFP, three of them infrequent (in 5 patients and controls in total). The fourth SNP, rs1048118 in exon 10, was more frequent, but was not associated with AMD, either alone (p=0.33) or in conjunction with other risk factors. Thus, CFP does not seem to confer any risk for AMD.

Therapeutic potential of complement modulation

The complement system is an essential component of innate immunity that has been more recently recognized as an unexpected player in various pathological states. These include age-related macular degeneration, atypical haemolytic uraemic syndrome, allergy, foetal loss, and axonal and myelin degradation after trauma. Its importance has also been recognized in physiological processes including haematopoietic stem cell homing to the bone marrow, liver regeneration and modulation of adaptive immune responses. Although the complement system has long been known to be involved in autoimmune and inflammatory diseases, few agents that target the complement system are currently approved for clinical use. However, renewed interest in modulating this system in various pathological conditions has emerged, and several agents are now in development.

The amplification loop of the complement pathways

The C3 amplification loop lies at the core of all the complement pathways, rather than the alternative pathway alone. It is, in evolutionary terms, the oldest part of the complement system and its antecedents can be seen in insects and in echinoderms. The amplification loop is the balance between two competing cycles both acting on C3b: the C3 feedback cycle which enhances amplification and the C3 breakdown cycle which downregulates it. It is solely the balance between their rates of reaction on which amplification depends. The C3 breakdown cycle generates iC3b as its primary reaction product. iC3b, through its reaction with the leukocyte integrins (and complement receptors) CR3 (CD11b/CD18) and CR4 (CD11c/CD18), is the most important mechanism by which complement mediates inflammation. A variety of genetic polymorphisms in components of the amplification loop have been shown to predispose to two kidney diseases-dense deposit disease and atypical haemolytic uraemic syndrome-and to age-related macular degeneration. All predisposing alleles enhance amplification, whereas protective alleles downregulate amplification. This leads to the conclusion that there is a "hyperinflammatory complement phenotype" determined by these polymorphisms. This hyperinflammatory phenotype protects against bacterial infections in early life but in later life is associated with immunopathology. Besides the diseases already mentioned, there is evidence that this hyperinflammatory complement phenotype may predispose to accelerated atherosclerosis and also shows an association with Alzheimer's disease. Downregulation of the amplification loop therefore constitutes an important therapeutic target.

Critical roles of complement and antibodies in host defense mechanisms against Neisseria meningitidis as revealed by human complement genetic deficiencies

Certain complement defects are associated with an increased propensity to contract Neisseria meningitidis infections. We performed detailed analyses of complement-mediated defense mechanisms against N. meningitidis 44/76 with whole blood and serum from two adult patients who were completely C2 or C5 deficient. The C5-deficient patient and the matched control were also deficient in mannose-binding lectin (MBL). The proliferation of meningococci incubated in freshly drawn whole blood was estimated by CFU and quantitative DNA real-time PCR. The serum bactericidal activity and opsonophagocytic activity by granulocytes were investigated, including heat-inactivated postvaccination sera, to examine the influence of antimeningococcal antibodies. The meningococci proliferated equally in C2- and C5-deficient blood, with a 2 log(10) increase of CFU and 4- to 5-log(10) increase in DNA copies. Proliferation was modestly decreased in reconstituted C2-deficient and control blood. After reconstitution of C5-deficient blood, all meningococci were killed, which is consistent with high antibody titers being present. The opsonophagocytic activity was strictly C2 dependent, appeared with normal serum, and increased with postvaccination serum. Serum bactericidal activity was strictly dependent on C2, C5, and high antibody titers. MBL did not influence any of the parameters observed. Complement-mediated defense against meningococci was thus dependent on the classical pathway. Some opsonophagocytic activity occurred despite low levels of antimeningococcal antibodies but was more efficient with immune sera. Serum bactericidal activity was dependent on C2, C5, and immune sera. MBL did not influence any of the parameters observed.

Urinary properdin excretion is associated with intrarenal complement activation and poor renal function

BACKGROUND

Proteinuria predicts progressive renal failure. Next to being a progression marker, non-selective proteinuria itself is thought to be toxic to the tubulointerstitium. In proteinuric states, activation of filtered or locally produced complement is toxic for renal tubular cells and likely contributes to the progression of renal failure. Recent experimental evidence suggests an important role for properdin in promoting intrarenal complement activation. We measured properdin in proteinuric urine and assessed its relation with urinary SC5b-9 levels, the soluble form of the effector phase of complement activation.

METHODS

Seventy patients with renal disease of different origin but all with a protein excretion of at least 1 g/day were studied. Urinary properdin and SC5b-9 levels were measured using an ELISA technique.

RESULTS

Properdin was detectable in the urine of 37 patients (53%). These subjects had higher urinary SC5b-9 levels {median 0.50 U/ml [interquartile range (IQR) 0.13-1.81] versus 0.049 U/ml (IQR 0.024-0.089), P < 0.001}. When adjusted for proteinuria and renal function, properdin excretion was strongly associated with increased urinary SC5b-9 levels (odds ratio 16.2, 95% confidence interval 3.6-74.4). Properdin excretion was associated with worse renal function.

CONCLUSION

Our results suggest that urinary properdin excretion enhances intrarenal complement activation and thus may contribute to the progression of renal damage in proteinuric states.

Complement activation pathways in murine immune complex-induced arthritis and in C3a and C5a generation in vitro

The alternative pathway (AP) of complement alone is capable of mediating immune complex-induced arthritis in the collagen antibody-induced arthritis (CAIA) model in mice. Whether the classical pathway (CP) or lectin pathway (LP) alone can mediate CAIA is not known. Using mice genetically deficient in different complement components, our results reported herein establish that the CP and LP alone are each incapable of mediating CAIA. A lower level or absence of C3 and/or C5 activation by the CP may be possible explanations for the importance of the AP in CAIA and in many murine models of disease. In addition, other investigators have reported that CP C5 convertase activity is absent in mouse sera. To address these questions, we employed an in vitro system of adherent immunoglobulin (Ig)G-induced complement activation using plates coated with murine anti-collagen monoclonal antibody (mAb). These experiments used complement-deficient mouse sera and wild-type mouse or normal human sera under conditions inactivating either the CP (Ca(++) deficiency) or the AP (mAb inhibitory to factor B). Robust generation of both C3a and C5a by either the AP or CP alone were observed with both mouse and human sera, although there were some small differences between the species of sera. We conclude that neither the CP nor LP alone is capable of mediating CAIA in vivo and that mouse sera exhibits a high level of IgG-induced C5a generation in vitro through either the CP or AP.

Complement regulators and inhibitory proteins

The complement system is important for cellular integrity and tissue homeostasis. Complement activation mediates the removal of microorganisms and the clearance of modified self cells, such as apoptotic cells. Complement regulators control the spontaneously activated complement cascade and any disturbances in this delicate balance can result in damage to tissues and in autoimmune disease. Therefore, insights into the mechanisms of complement regulation are crucial for understanding disease pathology and for enabling the development of diagnostic tools and therapies for complement-associated diseases.

The role of the anaphylatoxins in health and disease

The anaphylatoxin (AT) C3a, C5a and C5a-desArg are generally considered pro-inflammatory polypeptides generated after proteolytic cleavage of C3 and C5 in response to complement activation. Their well-appreciated effector functions include chemotaxis and activation of granulocytes, mast cells and macrophages. Recent evidence suggests that ATs are also generated locally within tissues by pathogen-, cell-, or contact system-derived proteases. This local generation of ATs is important for their pleiotropic biologic effects beyond inflammation. The ATs exert most of the biologic activities through ligation of three cognate receptors, i.e. the C3a receptor, the C5a receptor and the C5a receptor-like, C5L2. Here, we will discuss recent findings suggesting that ATs regulate cell apoptosis, lipid metabolism as well as innate and adaptive immune responses through their impact on antigen-presenting cells and T cells. As we will outline, such regulatory functions of ATs and their receptors play important roles in the pathogenesis of allergy, autoimmunity, neurodegenerative diseases, cancer and infections with intracellular pathogens.

Approaching the asymptote: 20 years later

The pattern recognition theory proposed by the late Charles Janeway, Jr. 20 years ago provided a conceptual framework for our current understanding of the innate immune recognition and its role in the activation of adaptive immunity. Discovery of several families of pattern recognition receptors and their roles in mammalian immunity provided experimental support for the Janeway's theory. In addition to pattern recognition, there are other forms of innate immune sensing, which presumably work in specific combinations depending on the pathogen class and the type of the immune response they elicit. Here, the development of the Janeway's theory is discussed in the context of the advances made in field of innate immune recognition over the past two decades.

Scabies mite inactivated serine protease paralogs inhibit the human complement system

Infestation of skin by the parasitic itch mite Sarcoptes scabiei afflicts 300 million people worldwide and there is a need for novel and efficient therapies. We have previously identified a multigene family of serine proteases comprising multiple catalytically inactive members (scabies mite-inactivated protease paralogs (SMIPPs)), which are secreted into the gut of S. scabiei. SMIPPs are located in the mite gut and in feces excreted into the upper epidermis. Scabies mites feed on epidermal protein, including host plasma; consequently, they are exposed to host defense mechanisms both internally and externally. We found that two recombinantly expressed SMIPPs inhibited all three pathways of the human complement system. Both SMIPPs exerted their inhibitory action due to binding of three molecules involved in the three different mechanisms which initiate complement: C1q, mannose-binding lectin, and properdin. Both SMIPPs bound to the stalk domains of C1q, possibly displacing or inhibiting C1r/C1s, which are associated with the same domain. Furthermore, we found that binding of both SMIPPs to properdin resulted in prevention of assembly of the alternative pathway convertases. However, the SMIPPs were not able to dissociate already formed convertases. Immunohistochemical staining demonstrated the presence of C1q in the gut of scabies mites in skin burrows. We propose that SMIPPs minimize complement-mediated gut damage and thus create a favorable environment for the scabies mites.

MyD88 expression by CNS-resident cells is pivotal for eliciting protective immunity in brain abscesses

MyD88 KO (knockout) mice are exquisitely sensitive to CNS (central nervous system) infection with Staphylococcus aureus, a common aetiological agent of brain abscess, exhibiting global defects in innate immunity and exacerbated tissue damage. However, since brain abscesses are typified by the involvement of both activated CNS-resident and infiltrating immune cells, in our previous studies it has been impossible to determine the relative contribution of MyD88-dependent signalling in the CNS compared with the peripheral immune cell compartments. In the present study we addressed this by examining the course of S. aureus infection in MyD88 bone marrow chimaera mice. Interestingly, chimaeras where MyD88 was present in the CNS, but not bone marrow-derived cells, mounted pro-inflammatory mediator expression profiles and neutrophil recruitment equivalent to or exceeding that detected in WT (wild-type) mice. These results implicate CNS MyD88 as essential in eliciting the initial wave of inflammation during the acute response to parenchymal infection. Microarray analysis of infected MyD88 KO compared with WT mice revealed a preponderance of differentially regulated genes involved in apoptotic pathways, suggesting that the extensive tissue damage characteristic of brain abscesses from MyD88 KO mice could result from dysregulated apoptosis. Collectively, the findings of the present study highlight a novel mechanism for CNS-resident cells in initiating a protective innate immune response in the infected brain and, in the absence of MyD88 in this compartment, immunity is compromised.

Target pattern recognition by complement proteins of the classical and alternative pathways

The complement system is a major component of the innate defence of animals against invading microorganisms, and is also essential for the recognition and clearance of damaged or structurally-altered host cells or macromolecules. The system is activated by three different pathways, each of which responds, using different recognition molecules, to a very wide range of activators. The recognition protein of the complement classical pathway, C1q is described in detail here, with comparisons to the alternative pathway.

Age-related macular degeneration: activation of innate immunity system via pattern recognition receptors

Age-related macular degeneration (AMD) is the most common cause of irreversible loss of central vision. Histopathological studies have demonstrated that inflammation is the key player in the pathogenesis of AMD. Genetic studies have revealed that complement factor H is a strong risk factor for the development of AMD. However, innate immunity defence involves several other pattern recognition receptors (PRRs) which can trigger inflammatory responses. Retinal pigment epithelial (RPE) cells have the main role in the immune defence in macula. In this study, we examine in detail the endogenous danger signals which can activate different PRRs in RPE cells, such as Toll-like, NOD-like and scavenger receptors along with complement system. We also characterise the signalling pathways triggered by PRRs in evoking inflammatory responses. In addition, we will discuss whether AMD pathology could represent the outcome of chronic activation of the innate immunity defence in human macula.