C5a and C5a(desArg) enhance the susceptibility of monocyte-derived macrophages to HIV infection

Flavonoid substituted polysaccharides from Tamarix chinensis Lour. alleviate H1N1-induced acute lung injury via inhibiting complement system

ETHNOPHARMACOLOGICAL RELEVANCE

Viral pneumonia is a highly pathogenic respiratory infectious disease associated with excessive activation of the complement system. Our previous studies found that the anticomplement polysaccharides from some medicinal plants could significantly alleviate H1N1-induced acute lung injury (H1N1-ALI). The leaves and twigs of Tamarix chinensis Lour. are traditionally used as a Chinese medicine Xiheliu for treating inflammatory disorders. Interestingly, its crude polysaccharides (MBAP90) showed potent anticomplement activity in vitro.

AIM OF THE STUDY

To evaluate the therapeutic effects and possible mechanism of MBAP90 on viral pneumonia and further isolate and characterize the key active substance of MBAP90.

MATERIALS AND METHODS

The protective effects of MBAP90 were evaluated by survival tests and pharmacodynamic experiments on H1N1-ALI mice. Histopathological changes, viral load, inflammatory markers, and complement deposition in lungs were analyzed by H&E staining, enzyme-linked immunosorbent assay (ELISA), and immunohistochemistry (IHC), respectively. An anticomplement homogenous polysaccharide (MBAP-3) was obtained from MBAP90 by bio-guided separation, and its structure was further characterized by methylation analysis and NMR spectroscopy.

RESULTS

Oral administration of MBAP90 at a dose of 400 mg/kg significantly increased the survival rate of mice infected with the lethal H1N1 virus. In H1N1-induced ALI, mice treated with MBAP90 (200 and 400 mg/kg) could decrease the lung index, lung pathological injury, the levels of excessive proinflammatory cytokines (IL-6, TNF-α, MCP-1, IL-18, and IL-1β), and complement levels (C3c and C5b-9). In addition, MBAP-3 was characterized as a novel homogenous polysaccharide with potent in vitro anticomplement activity (CH50: 0.126 ± 0.002 mg/mL), containing 10.51% uronic acids and 9.67% flavonoids, which were similar to the composition of MBAP90. The backbone of MBAP-3 consisted of →4)-α-D-Glcp-(1→, →3,4,6)-α-D-Glcp-(1→, and →3,4)-α-D-Glcp-(1→, with branches comprising α-L-Araf-(1→, α-D-GlcpA-(1→, →4,6)-α-D-Manp-(1→ and →4)-β-D-Galp-(1 → . Particularly, O-6 of →4)-β-D-Galp-(1→ was conjugated with a flavonoid, myricetin.

CONCLUSIONS

MBAP90 could ameliorate H1N1-ALI by inhibiting inflammation and over-activation of the complement system. These polysaccharides (MBAP90 and MBAP-3) with relative high contents of uronic acid and flavonoid substituent might be vital components of T. chinensis for treating viral pneumonia.

The Role of the Complement System in the Pathogenesis of Infectious Forms of Hemolytic Uremic Syndrome

Hemolytic uremic syndrome (HUS) is an acute disease and the most common cause of childhood acute renal failure. HUS is characterized by a triad of symptoms: microangiopathic hemolytic anemia, thrombocytopenia, and acute kidney injury. In most of the cases, HUS occurs as a result of infection caused by Shiga toxin-producing microbes: hemorrhagic Escherichia coli and Shigella dysenteriae type 1. They account for up to 90% of all cases of HUS. The remaining 10% of cases grouped under the general term atypical HUS represent a heterogeneous group of diseases with similar clinical signs. Emerging evidence suggests that in addition to E. coli and S. dysenteriae type 1, a variety of bacterial and viral infections can cause the development of HUS. In particular, infectious diseases act as the main cause of aHUS recurrence. The pathogenesis of most cases of atypical HUS is based on congenital or acquired defects of complement system. This review presents summarized data from recent studies, suggesting that complement dysregulation is a key pathogenetic factor in various types of infection-induced HUS. Separate links in the complement system are considered, the damage of which during bacterial and viral infections can lead to complement hyperactivation following by microvascular endothelial injury and development of acute renal failure.

Protective role of host complement system in Aspergillus fumigatus infection

Invasive aspergillosis (IA) is a life-threatening fungal infection for immunocompromised hosts. It is, therefore, necessary to understand the immune pathways that control this infection. Although the primary infection site is the lungs, aspergillosis can disseminate to other organs through unknown mechanisms. Herein we have examined the in vivo role of various complement pathways as well as the complement receptors C3aR and C5aR1 during experimental systemic infection by Aspergillus fumigatus, the main species responsible for IA. We show that C3 knockout (C3^-/-) mice are highly susceptible to systemic infection of A. fumigatus. Intriguingly, C4^-/- and factor B (FB)^-/- mice showed susceptibility similar to the wild-type mice, suggesting that either the complement pathways display functional redundancy during infection (i.e., one pathway compensates for the loss of the other), or complement is activated non-canonically by A. fumigatus protease. Our in vitro study substantiates the presence of C3 and C5 cleaving proteases in A. fumigatus. Examination of the importance of the terminal complement pathway employing C5^-/- and C5aR1^-/- mice reveals that it plays a vital role in the conidial clearance. This, in part, is due to the increased conidial uptake by phagocytes. Together, our data suggest that the complement deficiency enhances the susceptibility to systemic infection by A. fumigatus.

HIV Associated Preeclampsia: A Multifactorial Appraisal

Introduction: This review explores angiogenesis, vascular dysfunction, the complement system, RAAS, apoptosis and NETosis as potential pathways that are dysregulated during preeclampsia, HIV infection and ART usage. Results: HIV-1 accessory and matrix proteins are protagonists for the elevation of oxidative stress, apoptosis, angiogenesis, and elevation of adhesion markers. Despite the immunodeficiency during HIV-1 infection, HIV-1 exploits our cellular defence arsenal by escaping cell-mediated lysis, yet HIV-1 infectivity is enhanced via C5a release of TNF-α and IL-6. This review demonstrates that PE is an oxidatively stressed microenvironment associated with increased apoptosis and NETosis, but with a decline in angiogenesis. Immune reconstitution in the duality of HIV-1 and PE by protease inhibitors, HAART and nucleoside reverse transcriptase, affect similar cellular pathways that eventuate in loss of endothelial cell integrity and, hence, its dysfunction. Conclusions: HIV-1 infection, preeclampsia and ARTs differentially affect endothelial cell function. In the synergy of both conditions, endothelial dysfunction predominates. This knowledge will help us to understand the effect of HIV infection and ART on immune reconstitution in preeclampsia.

In the Crosshairs: RNA Viruses OR Complement?

Complement, a part of the innate arm of the immune system, is integral to the frontline defense of the host against innumerable pathogens, which includes RNA viruses. Among the major groups of viruses, RNA viruses contribute significantly to the global mortality and morbidity index associated with viral infection. Despite multiple routes of entry adopted by these viruses, facing complement is inevitable. The initial interaction with complement and the nature of this interaction play an important role in determining host resistance versus susceptibility to the viral infection. Many RNA viruses are potent activators of complement, often resulting in virus neutralization. Yet, another facet of virus-induced activation is the exacerbation in pathogenesis contributing to the overall morbidity. The severity in disease and death associated with RNA virus infections shows a tip in the scale favoring viruses. Growing evidence suggest that like their DNA counterparts, RNA viruses have co-evolved to master ingenious strategies to remarkably restrict complement. Modulation of host genes involved in antiviral responses contributed prominently to the adoption of unique strategies to keep complement at bay, which included either down regulation of activation components (C3, C4) or up regulation of complement regulatory proteins. All this hints at a possible “hijacking” of the cross-talk mechanism of the host immune system. Enveloped RNA viruses have a selective advantage of not only modulating the host responses but also recruiting membrane-associated regulators of complement activation (RCAs). This review aims to highlight the significant progress in the understanding of RNA virus–complement interactions.

Rationale for targeting complement in COVID-19

A novel coronavirus, SARS-CoV-2, has recently emerged in China and spread internationally, posing a health emergency to the global community. COVID-19 caused by SARS-CoV-2 is associated with an acute respiratory illness that varies from mild to the life-threatening acute respiratory distress syndrome (ARDS). The complement system is part of the innate immune arsenal against pathogens, in which many viruses can evade or employ to mediate cell entry. The immunopathology and acute lung injury orchestrated through the influx of pro-inflammatory macrophages and neutrophils can be directly activated by complement components to prime an overzealous cytokine storm. The manifestations of severe COVID-19 such as the ARDS, sepsis and multiorgan failure have an established relationship with activation of the complement cascade. We have collected evidence from all the current studies we are aware of on SARS-CoV-2 immunopathogenesis and the preceding literature on SARS-CoV-1 and MERS-CoV infection linking severe COVID-19 disease directly with dysfunction of the complement pathways. This information lends support for a therapeutic anti-inflammatory strategy against complement, where a number of clinically ready potential therapeutic agents are available.

The role of the complement system in HIV infection and preeclampsia

BACKGROUND

The complement system is a key component of the innate immune system that plays a vital role in host defense, maintains homeostasis and acts as a mediator of the adaptive immune response. The complement system could possibly play a role in the pathogenesis of HIV infection and preeclampsia (PE), both of which represent major causes of maternal death in South Africa.

RECENT FINDINGS

The relationship between PE and HIV infection is unclear as PE represents an exaggerated immune response, while HIV infection is associated with a decline in immune activity. Although the complement system works to clear and neutralize HIV, it could also enhance the infectivity of HIV by various other mechanisms. It has been suggested that the dysregulation of the complement system is associated with the development of PE.

CONCLUSION

There is currently a paucity of information on the combined effect of the complement system in HIV-associated PE. This review highlights the role of the complement system in the duality of HIV infection and PE and provides new insights into this relationship whilst also elucidating potential therapeutic targets.

The role of complement activation in rhabdomyolysis-induced acute kidney injury

Rhabdomyolysis (RM) may cause kidney damage and results primarily in acute kidney injury (AKI). Complement is implicated in the pathogenesis of renal diseases and ischemia-reperfusion injury (IRI), but the role of complement, especially its activation pathway(s) and its effect in RM-induced AKI, is not clear. This study established a rat model of AKI induced by RM via intramuscular treatment with glycerol. Cobra venom factor (CVF) was administered via tail vein injection to deplete complement 12 h prior to intramuscular injection of glycerol. We found that the complement components, including complement 3 (C3), C1q, MBL-A, factor B(fB), C5a, C5b-9, and CD59, were significantly increased in rat kidneys after intramuscular glycerol administration. However, the levels of serum BUN and Cr, renal tubular injury scores, and the number of TUNEL-positive cells decreased significantly in the CVF+AKI group. These results suggest that complement plays an important role in RM-induced AKI and that complement depletion may improve renal function and decrease renal tissue damage by reducing the inflammatory response and apoptosis.

Effects of Well-Controlled HIV Infection on Complement Activation and Function

INTRODUCTION

Uncontrolled HIV infection is known to activate the complement system, leading to an increase in chronic inflammation. Whether or not this activation of complement persists and contributes to chronic inflammation in subjects with HIV infection that is well controlled through use of antiretroviral therapy has not been studied.

METHODS

We conducted an observational, cross-sectional study using sera from 305 adults with well-controlled HIV infection and 30 healthy controls. Sera was tested for markers of complement activation (C3a and C5a levels), complement function (CH50 assay), and immunoglobulin levels (IgG1-IgG4) as IgG can activate complement. We evaluated the association of well-controlled HIV infection with C3a, C5a, CH50, IgG1-IgG4, and total IgG levels using both univariate and multivariate analyses, controlling for factors such as age, sex, race, comorbidities (including hepatitis C coinfection), smoking status, and statin use.

RESULTS

Well-controlled HIV infection was associated with a 54% increase in complement activation as measured by C3a levels compared with healthy controls (P < 0.0001). Hepatitis C coinfection was associated with a further 52% increase in complement activation, as measured by C3a levels, over HIV alone (P = 0.003).

CONCLUSION

These results suggest that complement activation may contribute to a proinflammatory state even in well-controlled HIV infection. Furthermore, hepatitis C virus coinfection may be even more proinflammatory, in complement activation, compared with HIV infection alone.

A Novel Role for the Receptor of the Complement Cleavage Fragment C5a, C5aR1, in CCR5-Mediated Entry of HIV into Macrophages

The complement system is an ancient pattern recognition system that becomes activated during all stages of HIV infection. Previous studies have shown that C5a can enhance the infection of monocyte-derived macrophages and T cells indirectly through the production of interleukin (IL)-6 and tumor necrosis factor (TNF)-α and the attraction of dendritic cells. C5a exerts its multiple biologic functions mainly through activation of C5a receptor 1 (C5aR1). Here, we assessed the role of C5aR1 as an enhancer of CCR5-mediated HIV infection. We determined CCR5 and C5aR1 heterodimer formation in myeloid cells and the impact of C5aR1 blockade on HIV entry and genomic integration. C5aR1/CCR5 heterodimer formation was identified by immunoprecipitation and western blotting. THP-1 cells and monocyte-derived macrophages (MDM) were infected by R5 laboratory strains or HIV pseudotyped for the vesicular stomatitis virus (VSV) envelope. Levels of integrated HIV were measured by quantitative PCR after targeting of C5aR1 by a C5aR antagonist, neutralizing C5aR1 monoclonal antibody (mAb) or hC5a. C5aR1 was also silenced by specific siRNA prior to viral entry. We found that C5aR1 forms heterodimers with the HIV coreceptor CCR5 in myeloid cells. Targeting C5aR1 significantly decreased integration by R5 viruses but not by VSV-pseudotyped viruses, suggesting that C5aR1 is critical for viral entry. The level of inhibition achieved with C5aR1-blocking reagents was comparable to that of CCR5 antagonists. Mechanistically, C5aR1 targeting decreased CCR5 expression. MDM from CCR5Δ32 homozygous subjects expressed levels of C5aR1 similar to CCR5 WT individuals, suggesting that mere C5aR1 expression is not sufficient for HIV infection. HIV appeared to preferentially enter THP-1 cells expressing high levels of both C5aR1 and CCR5. Targeted reduction of C5aR1 expression in such cells reduced HIV infection by ~50%. Our data thus suggest that C5aR1 acts as an enhancer of CCR5-mediated HIV entry into macrophages, the targeting of which may prove useful to reduce HIV infection by R5 strains.

The lectin pathway of complement: advantage or disadvantage in HIV pathogenesis?

The pattern recognition molecules of the lectin complement pathway are important components of the innate immune system with known functions in host-virus interactions. This paper summarizes current knowledge of how these intriguing molecules, including mannose-binding lectin (MBL), Ficolin-1, -2 and -3, and collectin-11 (CL-11) may influence HIV-pathogenesis. It has been demonstrated that MBL is capable of binding and neutralizing HIV and may affect host susceptibility to HIV infection and disease progression. In addition, MBL may cause variations in the host immune response against HIV. Ficolin-1, -2 and -3 and CL-11 could have similar functions in HIV infection as the ficolins have been shown to play a role in other viral infections, and CL-11 resembles MBL and the ficolins in structure and binding capacity.

Complement and HIV-I infection/HIV-associated neurocognitive disorders

The various neurological complications associated with HIV-1 infection, specifically HIV-associated neurocognitive disorders (HAND) persist as a major public health burden worldwide. Despite the widespread use of anti-retroviral therapy, the prevalence of HAND is significantly high. HAND results from the direct effects of an HIV-1 infection as well as secondary effects of HIV-1-induced immune reaction and inflammatory response. Complement, a critical mediator of innate and acquired immunity, plays important roles in defeating many viral infections by the formation of a lytic pore or indirectly by opsonization and recruitment of phagocytes. While the role of complement in the pathogenesis of HIV-1 infection and HAND has been previously recognized for over 15 years, it has been largely underestimated thus far. Complement can be activated through HIV-1 envelope proteins, mannose-binding lectins (MBL), and anti-HIV-1 antibodies. Complement not only fights against HIV-1 infection but also enhances HIV-1 infection. In addition, HIV-1 can hijack complement regulators such as CD59 and CD55 and can utilize these regulators and factor H to escape from complement attack. Normally, complement levels in brain are much lower than plasma levels and there is no or little complement deposition in brain cells. Interestingly, local production and deposition of complement are dramatically increased in HIV-1-infected brain, indicating that complement may contribute to the pathogenesis of HAND. Here, we review the current understanding of the role of complement in HIV-1 infection and HAND, as well as potential therapeutic approaches targeting the complement system for the treatment and eradications of HIV-1 infection.

C5a receptor-dependent cell activation by physiological concentrations of desarginated C5a: insights from a novel label-free cellular assay

The complement anaphylatoxins C3a, C5a, and desarginated C5a (C5a(desArg)) play critical roles in the induction of inflammation and the modulation of innate and acquired immune responses after binding to their G protein-coupled receptors, C3a receptor and C5a receptor (C5aR). The role of C5a(desArg) in inducing cell activation has been often neglected, because the affinity of C5a(desArg) for C5aR has been reported to be much lower than that of C5a. We have used a novel label-free cellular assay to reassess the potential of C5a(desArg) to induce activation of transfected and primary immune cells. Our results indicate that physiological levels of C5a(desArg) induce significant levels of cell activation that are even higher than those achieved by stimulating cells with analogous concentrations of C5a. Such activation was strictly dependent on C5aR, because it was completely abrogated by PMX-53, a C5aR antagonist. Pharmacological inhibition of specific G proteins located downstream of C5aR indicated differential involvement of G(α) proteins upon C5aR engagement by C5a or C5a(desArg). Further, mass spectrometric characterization of plasma-derived C5a and C5a(desArg) provided important insight into the posttranslational modification pattern of these anaphylatoxins, which includes glycosylation at Asn(64) and partial cysteinylation at Cys(27). Although the context-specific physiological contribution of C5a(desArg) has to be further explored, our data suggest that C5a(desArg) acts as a key molecule in the triggering of local inflammation as well as the maintenance of blood surveillance and homeostatic status.

Complement Attack against Aspergillus and Corresponding Evasion Mechanisms

Invasive aspergillosis shows a high mortality rate particularly in immunocompromised patients. Perpetually increasing numbers of affected patients highlight the importance of a clearer understanding of interactions between innate immunity and fungi. Innate immunity is considered to be the most significant host defence against invasive fungal infections. Complement represents a crucial part of this first line defence and comprises direct effects against invading pathogens as well as bridging functions to other parts of the immune network. However, despite the potency of complement to attack foreign pathogens, the prevalence of invasive fungal infections is increasing. Two possible reasons may explain that phenomenon: First, complement activation might be insufficient for an effective antifungal defence in risk patients (due to, e.g., low complement levels, poor recognition of fungal surface, or missing interplay with other immune elements in immunocompromised patients). On the other hand, fungi may have developed evasion strategies to avoid recognition and/or eradication by complement. In this review, we summarize the most important interactions between Aspergillus and the complement system. We describe the various ways of complement activation by Aspergillus and the antifungal effects of the system, and also show proven and probable mechanisms of Aspergillus for complement evasion.

The good and evil of complement activation in HIV-1 infection

The complement system, a key component of innate immunity, is a first-line defender against foreign pathogens such as HIV-1. The role of the complement system in HIV-1 pathogenesis appears to be multifaceted. Although the complement system plays critical roles in clearing and neutralizing HIV-1 virions, it also represents a critical factor for the spread and maintenance of the virus in the infected host. In addition, complement regulators such as human CD59 present in the envelope of HIV-1 prevent complement-mediated lysis of HIV-1. Some novel approaches are proposed to combat HIV-1 infection through the enhancement of antibody-dependent complement activity against HIV-1. In this paper, we will review these diverse roles of complement in HIV-1 infection.

Plasma C3a-des-Arg levels in women with and without endometriosis

PROBLEM

The lack of a reliable method for early non-invasive detection of endometriosis often results in delayed diagnosis. The aim of this study was to test the hypothesis that the plasma concentration of complement factor C3a (anaphylatoxin) can be used as a non-invasive test in the diagnosis of endometriosis.

METHOD OF STUDY

The C3a concentration was analyzed using ELISA in 160 patients with (n = 109) or without (n = 51) endometriosis during menstruation (n = 49), follicular phase (n = 55), and luteal (n = 56) phase.

RESULTS

Plasma C3a concentration was comparable between patients with [102 (27-2213) ng/mL] and without [105 (32-2340) ng/mL] (P = 0.84) endometriosis, also when assessed separately during menstruation, follicular phase, and luteal phase.

CONCLUSION

We found no difference in C3a levels between women with and without endometriosis and did not confirm our hypothesis that plasma C3a levels can be used as diagnostic test for endometriosis.

Complement and its role in protection and pathogenesis of flavivirus infections

The complement system is a family of serum and cell surface proteins that recognize pathogen-associated molecular patterns, altered-self ligands, and immune complexes. Activation of the complement cascade triggers several antiviral functions including pathogen opsonization and/or lysis, and priming of adaptive immune responses. In this review, we will examine the role of complement activation in protection and/or pathogenesis against infection by Flaviviruses, with an emphasis on experiments with West Nile and Dengue viruses.

Complement and fungal pathogens: an update

Fungal infections are a serious complication in immunocompromised patients such as human immunodeficiency virus-infected individuals, patients with organ transplantations or with haematological neoplasia. The lethality of opportunistic fungal infection is high despite a growing arsenal of antimycotic drugs, implying the urgent need for supportive immunological therapies to strengthen the current inefficient antimicrobial defences of the immunocompromised host. Therefore, increasing effort has been directed to investigating the interplay between fungi and the host immunity and thus to find starting points for additional therapeutic approaches. In this article, we review the actual state of the art concerning the role of complement in the pathogenesis of fungal infections. Important aspects include the activation of the complement system by the fungal pathogen, the efficiency of the complement-associated antimicrobial functions and the arsenal of immune evasion strategies applied by the fungi. The twin functions of complement as an interactive player of the innate immunity and at the same time as a modulator of the adaptive immunity make this defence weapon a particularly interesting therapeutic candidate to mobilise a more effective immune response and to strengthen in one fell swoop a broad spectrum of different immune reactions. However, we also mention the 'Yin-Yang' nature of the complement system in fungal infections, as growing evidence assigns to complement a contributory part in the pathogenesis of fungus-induced allergic manifestations.

Complement-HIV interactions during all steps of viral pathogenesis

Upon crossing the endothelial barrier of the host, HIV initiates immediate responses of the immunity system. Among its components, the complement system is one of the first the first elements, which are activated to affect HIV propagation. Complement participates not only in the early phase of the immune response, but its effects can be observed continuously and also concern the induction and modification of the adaptive immune response. Here we discuss the role of complement in early and late stages of HIV pathogenesis and review the escape mechanisms, which protect HIV from destruction by the complement system.

HIV-1 induced generation of C5a attracts immature dendriticcells and promotes infection of autologous T cells

For the recruitment of dendritic cells (DC) to the site of infection, DC express several sensors for danger signals, such as receptors for C5a. This anaphylatoxin is generated upon complement activation. As HIV-1 triggers the complement cascade, we determined whether C5a is generated by the virus and tested the functional activity of C5a in migration and infection assays. The immature (i)DC responded in migration assays to recombinant C5a and native C5a, which was generated in situ upon activation of the complement system by HIV-1. In combined migration and infection assays, a C5a-dependent enhancement of HIV-1 infection in DC-T cell cocultures was observed. These results indicate that HIV induces generation of C5a and thereby attracts iDC, which in turn promote the productive infection of autologous primary T cells.

HIV and complement: hijacking an immune defense

The complement system is a central player of the innate immune system. Activation of the complement system protects the host against pathogens. However, uncontrolled synthesis can be detrimental to host. This concise review summarizes the current understanding of the mechanism(s) of complement activation, the mechanism of C3 regulation, and the role of complement in human immunodeficiency virus (HIV) pathogenesis with emphasis on the cross-talk between HIV and complement system in NeuroAIDS and HIV-associated nephropathy (HIVAN).

Effect of an anti-C5a monoclonal antibody indicates a prominent role for anaphylatoxin in pulmonary xenograft dysfunction

BACKGROUND

In contrast to renal or cardiac xenografts, the inhibition of complement using cobra venom factor (CVF) accelerates pulmonary xenograft failure. By activating C3/C5 convertase, CVF depletes complement while additionally generating C5a and other anaphylatoxins, to which pulmonary xenografts may be uniquely susceptible. The current study investigates the role of C5a in pulmonary xenograft failure in baboons.

METHODS

Left orthotopic pulmonary xenografts using swine lungs expressing human CD46 were performed in baboons receiving: I) no other treatment (n=4), II) immunodepletion (n=5), and III) immunodepletion plus a single dose of mouse anti-human C5a monoclonal antibody (anti-C5a, 0.6 mg/kg administered intravenously) (n=3). The extent to which anti-C5a inhibits baboon C5a was assessed in vitro using a hemolytic reaction involving baboon serum and porcine red blood cells and by ELISA.

RESULTS

Baboons in Group III exhibited significantly prolonged xenograft survival (mean=722+/-121 min, P=0.02) compared to baboons in Group I (mean=202+/-24 min) and Group II (mean=276+/-79 min). Furthermore, baboons in Groups I and II experienced pronounced hemodynamic compromise requiring inotropic support whereas those in Group III remained hemodynamically stable throughout experimentation without the need for additional pharmacologic intervention.

CONCLUSIONS

These findings indicate that C5a exacerbates pulmonary xenograft injury and compromises recipient hemodynamic status. Moreover, blockade of anaphylatoxins, such as C5a, offers a promising approach for future investigations aimed at preventing pulmonary xenograft injury in baboons.

Multifunctional glyceraldehyde-3-phosphate dehydrogenase of Streptococcus pyogenes is essential for evasion from neutrophils

Streptococcus pyogenes is an important pathogen that causes pharyngitis, sepsis, and rheumatic fever. Cell-associated streptococcal C5a peptidase (ScpA) protects S. pyogenes from phagocytosis and has been suggested to interrupt host defenses by enzymatically cleaving complement C5a, a major factor in the accumulation of neutrophils at sites of infection. How S. pyogenes recognizes and binds to C5a, however, is unclear. We detected a C5a-binding protein in 8 M urea extracts of S. pyogenes by ligand blotting using biotinylated C5a. Searching of genome databases showed that the C5a-binding protein is identical to the streptococcal plasmin receptor (Plr), also known as streptococcal surface dehydrogenase (SDH) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH). In the present study we identified a novel function of this multifunctional protein. Western blotting and immunofluorescence microscopy with anti-Plr/SDH/GAPDH showed that Plr/SDH/GAPDH is located on the bacterial surface and released into the culture supernatant. Next, we examined whether the streptococcal Plr/SDH/GAPDH inhibits the biological effects of C5a on human neutrophils. We found that soluble Plr/SDH/GAPDH inhibits C5a-activated chemotaxis and H2O2 production. Furthermore, our results suggested that soluble Plr/SDH/GAPDH captures C5a, inhibiting its chemotactic function. Also, cell-associated Plr/SDH/GAPDH and ScpA were both necessary for the cleavage of C5a on the bacterial surface. Together, these results indicate that the multifunctional protein Plr/SDH/GAPDH has additional functions that help S. pyogenes escape detection by the host immune system.

Evolution of anaphylatoxins, their diversity and novel roles in innate immunity: Insights from the study of fish complement

Anaphylatoxins are small molecules ( approximately 9 kDa) that are generated as a result of the activation of the complement system. These molecules play an important role in inflammation, and they are responsible for the activation of various innate and adaptive immune processes. The study of these important inflammatory molecules has been restricted to mammalian species so far. Recent studies have shown that teleost fish, unlike any other known animal species, contain multiple forms of the C3a anaphylatoxin, all of which are functionally active and play a prominent role in inducing superoxide production in fish leukocytes. The C5a anaphylatoxin has also been characterized in these animals, and like in mammals, it plays an important role in leukocyte chemotaxis and in triggering the respiratory burst of leukocytes. Interestingly, it has been shown that rainbow trout anaphylatoxins play an unexpected role in enhancing phagocytosis of particles. C5a and C3a receptors have recently been cloned and characterized in rainbow trout, suggesting that the duplication of these receptors from a common ancestor occurred before the emergence of teleosts. The studies derived from these molecules in teleost fish indicate that the basic structure and function of anaphylatoxins and their receptors, have been conserved for more than 300 million years.

Airway smooth muscle cells enhance C3a-induced mast cell degranulation following cell-cell contact

Growing evidence suggests that anaphylatoxins, C3a and C5a, play important roles in innate immunity and may also participate in the pathogenesis of asthma. Previous studies with animal models and immunohistochemistry analysis of lung tissue indicated that anaphylatoxins may regulate airway hyperresponsiveness (AHR) in asthma via the activation of their cell surface G protein-coupled receptors (C3aR and C5aR) in airway smooth muscle (ASM) cells. Using RT-PCR, flow cytometry, and confocal microscopy, we made the surprising observation that while C3aR and C5aR were expressed in human mast cells, they were not present in cultured primary human or murine ASM cells. Furthermore, we could not detect C3aR in smooth muscle-positive cells of human trachea or bronchus. Interestingly, incubation of human mast cells with ASM cells, but not its culture supernatant, caused a significant enhancement of C3a-induced mast cell degranulation. Although stem cell factor (SCF) and its receptor c-kit are constitutively expressed on ASM cells and mast cells, respectively, neutralizing antibodies to SCF and c-kit failed to inhibit ASM cell-mediated enhancement of mast cell degranulation. However, dexamethasone-treated ASM cells were normal for cell surface SCF expression but were significantly less effective in enhancing C3a-induced mast cell degranulation when compared with untreated cells. These findings suggest that cell-cell interaction between ASM cells and mast cells, via a SCF-c-kit-independent but dexamethasone-sensitive mechanism, enhances C3a-induced mast cell degranulation, which likely regulates ASM function, thus contributing to the pathogenesis of asthma.

C5a differentially stimulates the ERK1/2 and p38 MAPK phosphorylation through independent signaling pathways to induced chemotactic migration in RAW264.7 macrophages

We elucidate the roles of various protein kinases involved in complement 5a (C5a)-induced cell migration. Results showed that extracellular signal-regulated kinase1/2 (ERK1/2), p38 mitogen-activated protein kinase (p38 MAPK) and phosphatidylinositol 3-kinase (P13K) were necessary for C5a-induced migration, whereas protein kinase C and c-Jun N-terminal kinase (JNK) were nonessential. C5a-induced migration was also suppresses by phospholipase C (PLC) inhibitor U73122 and pertussis toxin (PTX). We found that C5a-induced, time-dependent (1) ERK1/2 phosphorylation was markedly diminished by PTX, U73122, P13K inhibitors wortmannin and LY294002 and ERK1/2 inhibitor PD98059; (2) Akt phosphorylation was also attenuated by the above inhibitors except PD98059; (3) p38 MAPK phosphorylation was only affected by PTX. Furthermore, C5a also stimulated PLCbeta(2) membrane translocation in a time-dependent manner that occurred early prior to Akt phosphorylation and could be abolished only by PTX and U73122. These results suggest that C5a, through the activation of PTX-sensitive G protein, to differentially stimulate ERK1/2 and p38 MAPK phosphorylation and evoke cell migration. That is, ERK1/2 but not p38 MAPK phosphorylation is down stream of P13K/Akt and modulated by PLC. Additionally, beta(2) isoform may be one of the participates in C5a signal and acts more upstream of P13K/Akt.

Complement-opsonized HIV: the free rider on its way to infection

The complement system (C) is one of the main humoral components of innate immunity. Three major tasks of C against invading pathogens are: (i) lysis of pathogens by the formation of the membrane attack complex (MAC); (ii) opsonization of pathogens with complement fragments to favor phagocytosis; and (iii) attraction of inflammatory cells by chemotaxis. Like other particles, HIV activates C and becomes opsonized. To escape complement-mediated lysis, HIV has adopted various properties, which include the acquisition of HIV-associated molecules (HAMs) belonging to the family of complement regulators, such as CD46, CD55, CD59, and the interaction with humoral regulatory factors like factor H (fH). Opsonized virus may bind to complement receptor positive cells to infect them more efficiently or to remain bound on the surface of such cells. In the latter case HIV can be transmitted to cells susceptible for infection. This review discusses several aspects of C-HIV interactions and provides a model for the dynamics of this process.

HIV and human complement: inefficient virolysis and effective adherence

Both, HIV envelope proteins gp120 and gp41 can directly activate complement system, even in the absence of HIV-specific antibodies. During the budding process HIV acquires host membrane-associated molecules among these complement regulatory proteins (CRPs). The presence of CRPs on the viral surface rescues HIV from complement-mediated virolysis. The inefficient virolysis results in the deposition of complement-fragments on the viral surface allowing interactions of HIV with complement receptor expressing cells. In this review, the interaction of HIV with the complement system and the consequences of complement opsonisation on virus infection will be discussed.

Production of recombinant C5a from rainbow trout (Oncorhynchus mykiss): role in leucocyte chemotaxis and respiratory burst

Activation of the complement system can lead to the formation of the membrane attack complex, in which the component C5 is cleaved into C5a and C5b fragments. The C5a anaphylatoxin is a very potent pro-inflammatory molecule that induces chemotaxis and respiratory burst processes in a variety of mammalian leucocytes. While C5a has been well studied in mammals, little is known about the structure and function of C5a in teleost fish or other non-mammalian species. In the present study, we have produced and purified recombinant rainbow trout C5a (rtC5a), and we have shown that it plays an important role in inducing leucocyte migration as well as in triggering the respiratory burst of peripheral blood (PBLs) and head kidney leucocytes (HKLs). When the carboxy-terminal Arg was removed from rtC5a, its ability to induce cell migration and superoxide production remained intact. Interestingly, we show that leucocytes migrating towards rtC5a attached to the plate with a well-spread circular morphology, whereas those migrating towards activated trout serum displayed more irregular and dendritic-like shapes. Our data suggest that the basic mechanisms of action of the C5a anaphylotoxin have remained conserved for more than 300 million years.

Anaphylatoxin-like molecules generated during complement activation induce a dramatic enhancement of particle uptake in rainbow trout phagocytes

Here we have identified a serum fraction containing approximately 8-kDa molecules with an unexpected capacity to greatly enhance particle uptake in trout head kidney leukocytes (HKLs). This approximately 8-kDa particle-uptake enhancing fraction (PUEF-8) was purified from complement-activated serum by gel filtration chromatography. Mass spectrometric analysis and reactivity of anti-trout C3-1 and C4 antibodies, indicated the presence of C3a, C4a and C5a molecules in PUEF-8. Using a newly developed flow cytometric assay that measures the capacity of cells to ingest fluorescent beads, we showed that PUEF-8 induced a striking enhancement (344+/-50% higher than the PBS control value) in the number of HKLs ingesting three or more beads. In contrast, the effect of PUEF-8 on peripheral blood leukocytes (PBLs) was almost negligible. Interestingly, PUEF-8 acted as a strong chemoattractant for both HKLs and PBLs. These findings suggest a novel role for the anaphylatoxins generated during complement activation in teleost fish.

Human monocyte-derived dendritic cells are chemoattracted to C3a after up-regulation of the C3a receptor with interferons

The anaphylatoxin C3a is an important inflammatory mediator in the innate and adaptive immune systems. Recent reports in various animal models have fostered the role of C3a in mediating allergic reactions such as pulmonary allergies. However, data in humans are limited and the cellular targets for C3a are not fully understood. We sought to explore human dendritic cells as a new target for C3a, because C3a receptor (C3aR) expression has been described on myeloid cells, and dendritic cells are likely make contact with C3a at sites of inflammatory reactions. In this study, we demonstrated the expression of the C3aR on human monocyte-derived dendritic cells (MoDC) and its up-regulation by interferon (IFN)-alpha, IFN-gamma and prostaglandin E2 (PGE2). The strongest up-regulation was yielded by the combination of IFN-alpha+ IFN-gamma. Tumour necrosis factor-alpha (TNF-alpha) down-regulated the C3aR. After up-regulation of the C3aR by IFN-alpha+ IFN-gamma, C3a significantly up-regulated the surface expression of CD54, CD83 and CD86, but not of CD40, CD80 or human leucocyte antigen (HLA)-DR. C3a had no effect on the production of interleukin (IL)-10 or IL-12p70, or on the capacity of MoDC to stimulate autologous T-cell proliferation. However, C3a had a direct migratory effect on MoDC, as indicated by the induction of F-actin polymerization and migration in Boyden chamber experiments, which was pronounced after up-regulation of the C3aR with IFN-alpha+ IFN-gamma. Therefore, dendritic cells represent another group of target cells that might be recruited by C3a to areas of inflammation, in particular under conditions where IFNs are increased in the surrounding environment.

The role of complement in invasive fungal infections. Die Rolle des Komplements bei invasiven Pilzinfektionen

New therapeutic approaches enable organ transplantations and guarantee longer survival for AIDS patients or patients with haematological neoplasia. The price for these medical advances is immunosuppression and thus enhanced susceptibility to opportunistic fungal infections. As a consequence invasive fungal infections are on the march in modern medicine. Therapeutic limitations and difficulties strongly demand for a deeper understanding of the interaction between the various fungi and the hosts' innate and adaptive immune defence system. This understanding is the essential prerequisite for a potential therapeutic approach, which may support specifically the insufficient antifungal attack of the host. In the present article, we therefore review the current knowledge of the role of the complement system as a central part of innate immunity and as a fine tuner of adaptive immunity in the pathogenesis of invasive fungal infections, such as aspergillosis, candidosis, cryptococcosis, paracoccidioidomycosis, blastomycosis and histoplasmosis.

Cloning, Expression, Cellular Distribution, and Role in Chemotaxis of a C5a Receptor in Rainbow Trout: The First Identification of a C5a Receptor in a Nonmammalian Species

C3a, C4a, and C5a anaphylatoxins generated during complement activation play a key role in inflammation. C5a is the most potent of the three anaphylatoxins in eliciting biological responses. The effects of C5a are mediated by its binding to C5a receptor (C5aR, CD88). To date, C5aR has only been identified and cloned in mammalian species, and its evolutionary history remains ill-defined. To gain insights into the evolution, conserved structural domains, and functions of C5aR, we have cloned and characterized a C5aR in rainbow trout, a teleost fish. The isolated cDNA encoded a 350-aa protein that showed the highest sequence similarity to C5aR from other species. Genomic analysis revealed the presence of one continuous exon encoding the entire open reading frame. Northern blot analysis showed significant expression of the trout C5a receptor (TC5aR) message in PBLs and kidney. Flow cytometric analysis showed that two Abs generated against two different areas of the extracellular N-terminal region of TC5aR positively stained the same leukocyte populations from PBLs. B lymphocytes and granulocytes comprised the majority of cells recognized by the anti-TC5aR. More importantly, these Abs inhibited chemotaxis of PBLs toward a chemoattractant fraction purified from complement-activated trout serum. Our data suggest that the split between C5aR and C3aR from a common ancestral molecule occurred before the emergence of teleost fish. Moreover, we demonstrate that the overall structure of C5aR as well as its role in chemotaxis have remained conserved for >300 million years.

Role of complement in the control of HIV dynamics and pathogenesis

In all ex vivo preparations of HIV tested so far, C3 fragments and, after seroconversion, antibodies were detected on the viral surface. This indicates that HIV survives complement-mediated lysis. The virus has adopted different protection mechanisms to keep complement activation under the threshold necessary to induce virolysis. Among them are complement regulatory proteins that remain functionally active on the surface of HIV and turn down the complement cascade and serum proteins with complement regulatory activities. Therefore, opsonized virions accumulate in HIV-infected individuals, and subsequently adhere to complement receptor (CR) expressing cells. Among them are B cells, which bind opsonized virus. Such bound virus is efficiently transferred to autologous T cells, which subsequently are infected. Other cells interacting via CR with opsonized HIV are follicular dendritic cells (FDC). As shown by ex vivo experiments, up to 80% of virus is bound to follicular dendritic cells through C3-CR interactions. In the brain, HIV is not only interacting with complement proteins, but is able to induce their expression. Thus, interaction of HIV with the complement system is a main mechanism for pathogenesis to AIDS, since retention of (complement-resistant) opsonized viral particles on cell surfaces via CRs occurs in different compartments in HIV-infected individuals, thereby promoting transmission of virus to other permissive cells.

HUVEC take up opsonized zymosan particles and secrete cytokines IL-6 and IL-8 in vitro

Uptake of zymosan A particles by human umbilical vein endothelial cells (HUVEC) and its effect on cellular cytokine and oxygen radical production was examined. HUVEC took up more serum-opsonized than -unopsonized zymosan as demonstrated by flow cytometry with fluorescence-labeled particles. The former uptake was inhibited in the presence of anti-C3c antibodies and thus complement-mediated. It probably occurred via CR1 (CD35), although participation of other receptors cannot be ruled out. Scanning electron microscopy indicated that HUVEC with fully internalized zymosan particles were damaged. Prolonged incubation of both serum-opsonized and -unopsonized zymosan particles with HUVEC induced increased secretion of the proinflammatory cytokines IL-6 and IL-8 to the cell culture supernatants, but had no effect on production of oxygen radicals. The results confirm previous reports that EC can internalize yeast and other pathogens and points to complement as a mechanism of uptake, but illustrates that the cells may be damaged in the process. Moreover, EC may participate in the anti-infection defense effort by secreting proinflammatory and chemotactic cytokines in response to the contact with pathogens.

Enhanced transcytosis of R5-tropic human immunodeficiency virus across tight monolayer of polarized human endometrial cells under pro-inflammatory conditions

Most cases of human immunodeficiency virus (HIV) infection worldwide occur following sexual contact, implying that the virus may breach the protective epithelial barrier lining the genital tract. HIV infection is known to preferentially occur when the genital epithelial integrity is altered, particularly when epithelial micro-ulcerations occur during heterosexual intercourse or ulcerations appear, due to sexually transmitted infections or else in the context of ectopy of the endocervical mucosa, which may leave the genital tissue. We report that R5-tropic infectious HIV-1 isolates are capable of in vitro transcytosis through a tight and polarized monolayer of human endometrial HEC-1 cells. Transcytosis of HIV particles was increased 2-fold within a pro-inflammatory micro-environment. Our findings suggest that transcytosis may be a relevant mechanism for the passage of virus through the genital mucosa in vivo, particularly when inflammatory cells and mediators are present in the vicinity of the mucosal surface.

Complement C4 monitoring in the follow-up of chronic hepatitis C treatment

The overall role of complement in the host--pathogen relationship is now well understood. However, its involvement at a chronic stage of infection, such as chronic hepatitis C, is less well documented. Here, results are reported which point to the use of specific C4 monitoring in the follow-up of HCV patients. This study concerns 66 patients with chronic HCV infection, treated with interferon alpha 2b alone or with interferon alpha 2b + ribavirin, and 50 healthy adults as controls. Complement blood tests were performed to measure C1q, C3, C4, mannan binding lectin (MBL), C1s-C1 inhibitor complexes, total (CH50) and C4 (C4H) haemolytic activity; specific C4 activity was taken as the C4H/C4 protein ratio. Rheumatoid factor (RF) levels were also measured. A significant reduction in CH50 and specific C4 activity in HCV patients, compared with the healthy controls, was observed before the onset of treatment; the other parameters were not affected and no C1s-C1 inhibitor complexes were detected. At the same time, a significant reduction in specific C4 activity was observed in relapsers compared with sustained responders. These results point to a potential predictive function of C4 specific activity to monitor the response to therapy. Restoration of specific C4 activity at 6 months was better in responders than in non-responders. Complement activation in chronic hepatitis C does not seem to involve the C1 stage of the classical pathway. A negative correlation between specific C4 activity and RF titres suggests a possible involvement of RF in C4 activation, via the lectin pathway. Specific C4 monitoring appears to be a valuable tool for the follow-up of chronic hepatitis C treatment, together with the other conventional investigations.