Complement, viruses, and virus-infected cells

Induction of Fc-dependent functional antibodies against different variants of SARS-CoV-2 varies by vaccine type and prior infection

BACKGROUND

SARS-CoV-2 transmission and COVID-19 disease severity is influenced by immunity from natural infection and/or vaccination. Population-level immunity is complicated by the emergence of viral variants. Antibody Fc-dependent effector functions are as important mediators in immunity. However, their induction in populations with diverse infection and/or vaccination histories and against variants remains poorly defined.

METHODS

We evaluated Fc-dependent functional antibodies following vaccination with two widely used vaccines, AstraZeneca (AZ) and Sinovac (SV), including antibody binding of Fcγ-receptors and complement-fixation in vaccinated Brazilian adults (n = 222), some of who were previously infected with SARS-CoV-2, as well as adults with natural infection only (n = 200). IgG, IgM, IgA, and IgG subclasses were also quantified.

RESULTS

AZ induces greater Fcγ-receptor-binding (types I, IIa, and IIIa/b) antibodies than SV or natural infection. Previously infected individuals have significantly greater vaccine-induced responses compared to naïve counterparts. Fcγ-receptor-binding is highest among AZ vaccinated individuals with a prior infection, for all receptor types, and substantial complement-fixing activity is only seen among this group. SV induces higher IgM than AZ, but this does not drive better complement-fixing activity. Some SV responses are associated with subject age, whereas AZ responses are not. Importantly, functional antibody responses are well retained against the Omicron BA.1 S protein, being best retained for Fcγ-receptor-1 binding, and are higher for AZ than SV.

CONCLUSIONS

Hybrid immunity, from combined natural exposure and vaccination, generates strong Fc-mediated antibody functions which may contribute to immunity against evolving SARS-CoV-2 variants. Understanding determinants of Fc-mediated functions may enable future vaccines with greater efficacy against different variants.

Complement lectin pathway activation is associated with COVID-19 disease severity, independent of MBL2 genotype subgroups

Introduction

While complement is a contributor to disease severity in severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections, all three complement pathways might be activated by the virus. Lectin pathway activation occurs through different pattern recognition molecules, including mannan binding lectin (MBL), a protein shown to interact with SARS-CoV-2 proteins. However, the exact role of lectin pathway activation and its key pattern recognition molecule MBL in COVID-19 is still not fully understood.

Methods

We therefore investigated activation of the lectin pathway in two independent cohorts of SARS-CoV-2 infected patients, while also analysing MBL protein levels and potential effects of the six major single nucleotide polymorphisms (SNPs) found in the MBL2 gene on COVID-19 severity and outcome.

Results

We show that the lectin pathway is activated in acute COVID-19, indicated by the correlation between complement activation product levels of the MASP-1/C1-INH complex (p=0.0011) and C4d (p<0.0001) and COVID-19 severity. Despite this, genetic variations in MBL2 are not associated with susceptibility to SARS-CoV-2 infection or disease outcomes such as mortality and the development of Long COVID.

Conclusion

In conclusion, activation of the MBL-LP only plays a minor role in COVID-19 pathogenesis, since no clinically meaningful, consistent associations with disease outcomes were noted.

The complement system testing in clinical laboratory

With the advancement in research in the field of the complement system, a more comprehensive understanding developed about the complement system's role in the life process of an organism. It is a system of innate immune surveillance. This system plays a pivotal role in host defense against pathogens, inflammation, B and T cell homeostasis. Complement system analysis has a significant advantage in the assessment of the immune system status, diagnosis and prognosis of diseases, and medication guidelines. Currently, complement system testing is neither yet widely used across all clinical laboratoriesnor are the testing protocols yet systematic. Based on the current research, it is suggested that the analysis of complement activator-activated complement activity and total complement activity would be comprehensively assessed to evaluate the complement system's immunological function, and combine of the detection of its components to establish a systematic protocol for the complement system testing in the clinical laboratory. This article reviews the complement system's physiological role, disease relevance and the current testing status in clinical laboratories. Further more, some suggestions have also been provided for the preparation of complement standards i.e., the standardized preparation process for complement standards seems to be a feasible option given the easy inactivation of complement.

Complement Proteins as Soluble Pattern Recognition Receptors for Pathogenic Viruses

The complement system represents a crucial part of innate immunity. It contains a diverse range of soluble activators, membrane-bound receptors, and regulators. Its principal function is to eliminate pathogens via activation of three distinct pathways: classical, alternative, and lectin. In the case of viruses, the complement activation results in effector functions such as virion opsonisation by complement components, phagocytosis induction, virolysis by the membrane attack complex, and promotion of immune responses through anaphylatoxins and chemotactic factors. Recent studies have shown that the addition of individual complement components can neutralise viruses without requiring the activation of the complement cascade. While the complement-mediated effector functions can neutralise a diverse range of viruses, numerous viruses have evolved mechanisms to subvert complement recognition/activation by encoding several proteins that inhibit the complement system, contributing to viral survival and pathogenesis. This review focuses on these complement-dependent and -independent interactions of complement components (especially C1q, C4b-binding protein, properdin, factor H, Mannose-binding lectin, and Ficolins) with several viruses and their consequences.

A double edged-sword - The Complement System during SARS-CoV-2 infection

In the past 20 years, infections caused by coronaviruses SARS-CoV, MERS-CoV and SARS-CoV-2 have posed a threat to public health since they may cause severe acute respiratory syndrome (SARS) in humans. The Complement System is activated during viral infection, being a central protagonist of innate and acquired immunity. Here, we report some interactions between these three coronaviruses and the Complement System, highlighting the central role of C3 with the severity of these infections. Although it can be protective, its role during coronavirus infections seems to be contradictory. For example, during SARS-CoV-2 infection, Complement System can control the viral infection in asymptomatic or mild cases; however, it can also intensify local and systemic damage in some of severe COVID-19 patients, due to its potent proinflammatory effect. In this last condition, the activation of the Complement System also amplifies the cytokine storm and the pathogenicity of coronavirus infection. Experimental treatment with Complement inhibitors has been an enthusiastic field of intense investigation in search of a promising additional therapy in severe COVID-19 patients.

IgM and IgG Profiles Reveal Peculiar Features of Humoral Immunity Response to SARS-CoV-2 Infection

The emergence of coronavirus disease 2019 (COVID-19) is globally a major healthcare threat. There is little information regarding the mechanisms and roles of the humoral response in SARS-CoV-2 infection. The aim of this study was to analyze the antibody levels (IgM and IgG) by chemiluminescence immunoassay in 54 subjects positive to SARS-CoV-2 swab test in relation to their clinical status (whether asymptomatic, pauci-symptomatic or with mild, sever or critical symptoms), the time from the symptom onset, sex, age, and comorbidities. Overall, the presence of comorbidities and the age of subjects were associated with their clinical status. The IgG concentrations were significantly higher in patients who developed critical and severe symptoms and seemed to be independent from age, sex and comorbidities. IgG titers peaked around day 60, and then began gradually to drop, decreasing by approximately 50% on the 180th day, while the IgM titers progressively decreased as early as the tenth day, but they could be detected even at later time points. Despite the small number of individuals, some peculiar characteristics of the humoral response in COVID-19 emerged. We observed a high inter-individual variability, an ephemeral IgG half-life in several patients, and a persistence of IgM in others.

Complement C4 Prevents Viral Infection through Capsid Inactivation

The complement system is vital for anti-microbial defense. In the classical pathway, pathogen-bound antibody recruits the C1 complex (C1qC1r₂C1s₂) that initiates a cleavage cascade involving C2, C3, C4, and C5 and triggering microbial clearance. We demonstrate a C4-dependent antiviral mechanism that is independent of downstream complement components. C4 inhibits human adenovirus infection by directly inactivating the virus capsid. Rapid C4 activation and capsid deposition of cleaved C4b are catalyzed by antibodies via the classical pathway. Capsid-deposited C4b neutralizes infection independent of C2 and C3 but requires C1q antibody engagement. C4b inhibits capsid disassembly, preventing endosomal escape and cytosolic access. C4-deficient mice exhibit heightened viral burdens. Additionally, complement synergizes with the Fc receptor TRIM21 to block transduction by an adenovirus gene therapy vector but is partially restored by Fab virus shielding. These results suggest that the complement system could be altered to prevent virus infection and enhance virus gene therapy efficacy.

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Complement components C1 and C4 mediate potent neutralization of adenovirus

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Deposition of C4b on the viral capsid inactivates capsid disassembly

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C4 exerts direct antiviral functions independent from its role as a C3-convertase

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C4 antiviral functions synergize with TRIM21-mediated virus neutralization

Transchem project - Part II: Transgenerational effects of long-term exposure to pendimethalin at environmental concentrations on the early development and viral pathogen susceptibility of rainbow trout (Oncorhynchus mykiss)

In the Transchem project, rainbow trout genitors were exposed to environmental concentrations of pendimethalin over a period of 18 months and two new first generations of offspring, F1_2013 and F1_2014, were obtained. We investigated the impact of direct chemical exposure on juveniles as well as the potential cumulative transgenerational and direct effects on the larval development and on the pathogen susceptibility of offspring. Depending on the chemical treatment or not of the adults, their offspring were distributed in the tanks of our experimental system, in two batches i.e. juveniles from the control genitors (G-) and others from the contaminated ones (G+), and then, half of the tanks were exposed daily to pendimethalin (Off+) while the others were used as controls (Off-). Viral challenges were performed on the offspring, before and after three months of direct chemical exposure, with strains of infectious hematopoietic necrosis virus (IHNV), viral haemorrhagic septicemia virus (VHSV) and sleeping disease alphavirus (SDV). Direct and transgenerational macroscopic effects were observed on offspring, with a percentage of abnormalities in offspring derived from the genitors exposed to pendimethalin (G+) significantly higher compared to those from the genitors from non-exposed group (G-). Before the direct chemical exposure, similar kinetics of mortality was observed between the offspring from the contaminated or control genitors after VHSV infection. With IHNV, the G+ group died in a slightly larger proportion compared to the G- group and seroconversion was greater for the G- group. For the SDV challenge, the mortality was delayed for the G+ offspring compared to the G- and seroconversion reached 65% in the G+ group compared to 45% in the G-, with similar antibody titres. After three months of direct chemical exposure, kinetics of mortality induced by IHNV infection were similar for all groups studied. Infection with SDV resulted in a cumulative mortality of 40% for the G- groups (Off- and Off+), significantly higher than those observed from the contaminated genitors G+. Proportion of seropositivity for SDV varied from 24 to 47% depending on the group, with very low quantities of secreted antibodies. Lastly, the direct exposure of offspring could impact the capacity of fish to adapt their haematological parameters to environmental and physiological changes, and underlines the potential toxic effects on the next generations.

Complement Evasion Strategies of Viruses: An Overview

Being a major first line of immune defense, the complement system keeps a constant vigil against viruses. Its ability to recognize large panoply of viruses and virus-infected cells, and trigger the effector pathways, results in neutralization of viruses and killing of the infected cells. This selection pressure exerted by complement on viruses has made them evolve a multitude of countermeasures. These include targeting the recognition molecules for the avoidance of detection, targeting key enzymes and complexes of the complement pathways like C3 convertases and C5b-9 formation - either by encoding complement regulators or by recruiting membrane-bound and soluble host complement regulators, cleaving complement proteins by encoding protease, and inhibiting the synthesis of complement proteins. Additionally, viruses also exploit the complement system for their own benefit. For example, they use complement receptors as well as membrane regulators for cellular entry as well as their spread. Here, we provide an overview on the complement subversion mechanisms adopted by the members of various viral families including Poxviridae, Herpesviridae, Adenoviridae, Flaviviridae, Retroviridae, Picornaviridae, Astroviridae, Togaviridae, Orthomyxoviridae and Paramyxoviridae.

Immune-mediated Liver Injury in Hepatitis B Virus Infection

Hepatitis B virus (HBV) is responsible for approximately 350 million chronic infections worldwide and is a leading cause of broad-spectrum liver diseases such as hepatitis, cirrhosis and liver cancer. Although it has been well established that adaptive immunity plays a critical role in viral clearance, the pathogenetic mechanisms that cause liver damage during acute and chronic HBV infection remain largely known. This review describes our current knowledge of the immune-mediated pathogenesis of HBV infection and the role of immune cells in the liver injury during hepatitis B.

IgM and its receptors: structural and functional aspects

This review combines the data obtained before the beginning of the 1990s with results published during the last two decades. The predominant form of the IgM molecule is a closed ring composed of five 7S subunits and a J chain. The new model of spatial structure of the pentamer postulates nonplanar mushroom-shaped form of the molecule with the plane formed by a radially-directed Fab regions and central protruding portion consisting of Cµ4 domains. Up to the year 2000 the only known Fc-receptor for IgM was pIgR. Interaction of IgM with pIgR results in secretory IgM formation, whose functions are poorly studied. The receptor designated as Fcα/µR is able to bind IgM and IgA. It is expressed on lymphocytes, follicular dendritic cells, and macrophages. A receptor binding IgM only named FcµR has also been described. It is expressed on T- and B-lymphocytes. The discovery of new Fc-receptors for IgM requires revision of notions that interactions between humoral reactions involving IgM and the cells of the immune system are mediated exclusively by complement receptors. In the whole organism, apart from IgM induced by immunization, natural antibodies (NA) are present and comprise in adults a considerable part of the circulating IgM. NA are polyreactive, germ-line-encoded, and emerge during embryogenesis without apparent antigenic stimuli. They demonstrate a broad spectrum of antibacterial activity and serve as first line of defense against microbial and viral infections. NA may be regarded as a transitional molecular form from invariable receptors of innate immunity to highly diverse receptors of adaptive immunity. By means of interaction with autoantigens, NA participate in maintenance of immunological tolerance and in clearance of dying cells. At the same time, NA may act as a pathogenic factor in atherosclerotic lesion formation and in development of tissue damage due to ischemia/reperfusion.

Disabling complement regulatory activities of vaccinia virus complement control protein reduces vaccinia virus pathogenicity

Poxviruses encode a repertoire of immunomodulatory proteins to thwart the host immune system. One among this array is a homolog of the host complement regulatory proteins that is conserved in various poxviruses including vaccinia (VACV) and variola. The vaccinia virus complement control protein (VCP), which inhibits complement by decaying the classical pathway C3-convertase (decay-accelerating activity), and by supporting inactivation of C3b and C4b by serine protease factor I (cofactor activity), was shown to play a role in viral pathogenesis. However, the role its individual complement regulatory activities impart in pathogenesis, have not yet been elucidated. Here, we have generated monoclonal antibodies (mAbs) that block the VCP functions and utilized them to evaluate the relative contribution of complement regulatory activities of VCP in viral pathogenesis by employing a rabbit intradermal model for VACV infection. Targeting VCP by mAbs that inhibited the decay-accelerating activity as well as cofactor activity of VCP or primarily the cofactor activity of VCP, by injecting them at the site of infection, significantly reduced VACV lesion size. This reduction however was not pronounced when VCP was targeted by a mAb that inhibited only the decay-accelerating activity. Further, the reduction in lesion size by mAbs was reversed when host complement was depleted by injecting cobra venom factor. Thus, our results suggest that targeting VCP by antibodies reduces VACV pathogenicity and that principally the cofactor activity of VCP appears to contribute to the virulence.

Immunobiology and pathogenesis of viral hepatitis

Among the many viruses that are known to infect the human liver, hepatitis B virus (HBV) and hepatitis C virus (HCV) are unique because of their prodigious capacity to cause persistent infection, cirrhosis, and liver cancer. HBV and HCV are noncytopathic viruses and, thus, immunologically mediated events play an important role in the pathogenesis and outcome of these infections. The adaptive immune response mediates virtually all of the liver disease associated with viral hepatitis. However, it is becoming increasingly clear that antigen-nonspecific inflammatory cells exacerbate cytotoxic T lymphocyte (CTL)-induced immunopathology and that platelets enhance the accumulation of CTLs in the liver. Chronic hepatitis is characterized by an inefficient T cell response unable to completely clear HBV or HCV from the liver, which consequently sustains continuous cycles of low-level cell destruction. Over long periods of time, recurrent immune-mediated liver damage contributes to the development of cirrhosis and hepatocellular carcinoma.

Viral mimicry of the complement system

The complement system is a potent innate immune mechanism consisting of cascades of proteins which are designed to fight against and annul intrusion of all the foreign pathogens. Although viruses are smaller in size and have relatively simple structure, they are not immune to complement attack. Thus, activation of the complement system can lead to neutralization of cell-free viruses, phagocytosis of C3b-coated viral particles, lysis of virus-infected cells, and generation of inflammatory and specific immune responses. However, to combat host responses and succeed as pathogens, viruses not only have developed/adopted mechanisms to control complement, but also have turned these interactions to their own advantage. Important examples include poxviruses, herpesviruses, retroviruses, paramyxoviruses and picornaviruses. In this review, we provide information on the various complement evasion strategies that viruses have developed to thwart the complement attack of the host. A special emphasis is given on the interactions between the viral proteins that are involved in molecular mimicry and the complement system.

Noncytolytic control of viral infections by the innate and adaptive immune response

This review describes the contribution of noncytolytic mechanisms to the control of viral infections with a particular emphasis on the role of cytokines in these processes. It has long been known that most cell types in the body respond to an incoming viral infection by rapidly secreting antiviral cytokines such as interferon alpha/beta (IFN-alpha/beta). After binding to specific receptors on the surface of infected cells, IFN-alpha/beta has the potential to trigger the activation of multiple noncytolytic intracellular antiviral pathways that can target many steps in the viral life cycle, thereby limiting the amplification and spread of the virus and attenuating the infection. Clearance of established viral infections, however, requires additional functions of the immune response. The accepted dogma is that complete clearance of intracellular viruses by the immune response depends on the destruction of infected cells by the effector cells of the innate and adaptive immune system [natural killer (NK) cells and cytotoxic T cells (CTLs)]. This notion, however, has been recently challenged by experimental evidence showing that much of the antiviral potential of these cells reflects their ability to produce antiviral cytokines such as IFN-gamma and tumor necrosis factor (TNF)-alpha at the site of the infection. Indeed, these cytokines can purge viruses from infected cells noncytopathically as long as the cell is able to activate antiviral mechanisms and the virus is sensitive to them. Importantly, the same cytokines also control viral infections indirectly, by modulating the induction, amplification, recruitment, and effector functions of the immune response and by upregulating antigen processing and display of viral epitopes at the surface of infected cells. In keeping with these concepts, it is not surprising that a number of viruses encode proteins that have the potential to inhibit the antiviral activity of cytokines.

Role and significance of the complement system in mucosal immunity: particular reference to the human breast milk complement

The complement system plays an important role in a host's defence mechanisms, such as in immune bacteriolysis, neutralization of viruses, immune adherence, immunoconglutination and in enhancement of phagocytosis. The possible role of this important biological system in biological fluids on the mucosal surfaces, including breast milk, has however been largely neglected. Its contribution to the 'common' mucosal immunity is still enigmatic and largely speculative. Assessment of the complement system in human breast milk, which has so far largely been limited to different assays of the individual component proteins, is reviewed. A brief review of the classical and the alternative pathways of complement activation is presented. The potential physiological roles of various complement components and their activation fragments in human milk in particular, and other mucosal surfaces in general, are also presented. It was concluded that the complement system might play a complementary role to other immunological and non-immunological protective mechanisms on the mucosal surfaces.

Neutralizing antiviral antibody responses

Neutralizing antibodies are evolutionarily important effectors of immunity against viruses. Their evaluation has revealed a number of basic insights into specificity, rules of reactivity (tolerance), and memory—namely, (1) Specificity of neutralizing antibodies is defined by their capacity to distinguish between virus serotypes; (2) B cell reactivity is determined by antigen structure, concentration, and time of availability in secondary lymphoid organs; and (3) B cell memory is provided by elevated protective antibody titers in serum that are depending on antigen stimulation. These perhaps slightly overstated rules are simple, correlate with in vivo evidence as well as clinical observations, and appear to largely demystify many speculations about antibodies and B cell physiology. The chapter also considers successful vaccines and compares them with those infectious diseases where efficient protective vaccines are lacking, it is striking to note that all successful vaccines induce high levels of neutralizing antibodies (nAbs) that are both necessary and sufficient to protect the host from disease. Successful vaccination against infectious diseases such as tuberculosis, leprosy, or HIV would require induction of additional long-lasting T cell responses to control infection.

Novel mechanism of antibody-independent complement neutralization of herpes simplex virus type 1

The envelope surface glycoprotein C (gC) of HSV-1 interferes with the complement cascade by binding C3 and activation products C3b, iC3b, and C3c, and by blocking the interaction of C5 and properdin with C3b. Wild-type HSV-1 is resistant to Ab-independent complement neutralization; however, HSV-1 mutant virus lacking gC is highly susceptible to complement resulting in > or =100-fold reduction in virus titer. We evaluated the mechanisms by which complement inhibits HSV-1 gC null virus to better understand how gC protects against complement-mediated neutralization. C8-depleted serum prepared from an HSV-1 and -2 Ab-negative donor neutralized gC null virus comparable to complement-intact serum, indicating that C8 and terminal lytic activity are not required. In contrast, C5-depleted serum from the same donor failed to neutralize gC null virus, supporting a requirement for C5. EDTA-treated serum did not neutralize gC null virus, indicating that complement activation is required. Factor D-depleted and C6-depleted sera neutralized virus, suggesting that the alternative complement pathway and complement components beyond C5 are not required. Complement did not aggregate virus or block attachment to cells. However, complement inhibited infection before early viral gene expression, indicating that complement affects one or more of the following steps in virus replication: virus entry, uncoating, DNA transport to the nucleus, or immediate early gene expression. Therefore, in the absence of gC, HSV-1 is readily inhibited by complement by a C5-dependent mechanism that does not require viral lysis, aggregation, or blocking virus attachment.

Complement can neutralize HIV-1 plasma virus by a C5-independent mechanism

A previous study showed a portion of HIV-1 plasma virus was lysed by the addition of exogenous human AB+ seronegative complement. The current study was performed to determine whether infectious plasma virus was inactivated by complement. Incubation of plasma virus with AB+-seronegative serum resulted in substantial decreases in infectious titers, demonstrating that infectious plasma virus is susceptible to complement-mediated inactivation. Although complement also induced some lysis of plasma virus samples, virus was neutralized to a significantly higher degree, suggesting neutralization did not occur solely by lysis. Additionally, C5-deficient complement substantially neutralized virus, indicating coating of virus by early complement components was an important mechanism of neutralization. A portion of some freshly isolated plasma virus samples bound to complement receptor 2 in the absence of exogenous complement, indicating that early complement components bound virus in vivo. Furthermore, plasma virus samples that had less C3 deposited on their surface in vivo had higher infectious titers than samples with a larger fraction with surface C3. These findings suggest that complement can neutralize HIV-1 plasma virus in vivo by coating with complement proteins. This is the first study to provide evidence that coating by complement leads to functional inactivation of a virus in vivo.

Extracellular enveloped vaccinia virus is resistant to complement because of incorporation of host complement control proteins into its envelope

Vaccinia virus (VV) produces two antigenically and structurally distinct infectious virions, intracellular mature virus (IMV) and extracellular enveloped virus (EEV). Here we have investigated the resistance of EEV and IMV to neutralization by complement in the absence of immune antibodies. When EEV is challenged with complement from the same species as the cells used to grow the virus, EEV is resistant to neutralization by complement, whereas IMV is not. EEV resistance was not a result of EEV protein B5R, despite its similarity to proteins of the regulators of complement activation (RCA) family, or to any of the other EEV proteins tested (A34R, A36R, and A56R gene products). EEV was sensitive to complement when the virus was grown in one species and challenged with complement from a different species, suggesting that complement resistance might be mediated by host RCA incorporated into the EEV outer envelope. This hypothesis was confirmed by several observations: (i) immunoblot analysis revealed that cellular membrane proteins CD46, CD55, CD59, CD71, CD81, and major histocompatibility complex class I antigen were detected in purified EEV but not IMV; (ii) immunoelectron microscopy revealed cellular RCA on the surface of EEV retained on the cell surface; and (iii) EEV derived from rat cells expressing the human RCA CD55 or CD55 and CD59 were more resistant to human complement than EEV derived from control rat cells that expressed neither CD55 nor CD59. These data justify further analysis of the roles of these (and possible other) cellular proteins in EEV biology.

Genetic deficiencies of complement

Genetic deficiencies of proteins of the complement system are associated with diverse clinical phenotypes. These clinical manifestations vary as a function of the specific component that is missing. Molecular and cellular biological methods, coupled with more intensive clinical studies, have defined the pathophysiological basis for this set of genetic disorders. Insights into the normal function of complement and its role in immunopathology have been derived from the extensive work in this field during the past few years.

Physiology and pathophysiology of complement: progress and trends

The complement system comprises a family of at least 20 plasma and membrane proteins that interact in a tightly regulated cascade system to destroy invading bacteria and prevent the deposition of immune complexes in the tissues. This brief review addresses the basic mechanisms of complement activation and control and describes the active fragments produced during complement activation. The biological importance of the complement system is amply illustrated in patients with complement deficiencies, who are susceptible to bacterial infections and immune complex diseases. The involvement of complement in other immunological diseases is an expanding area of clinical research, supported by the development of new assays for the identification of complement activation. This area is discussed here with particular reference to neurological diseases. A promising new prospect involves the use of complement inhibitory molecules in therapy of complement-mediated disease and this exciting area is also discussed. Novel physiological roles of complement also are being revealed and new evidence that complement and complement receptors play an important role in reproduction is summarized. It is hoped that this brief overview will convey some of the enthusiasm currently pervading research in this underappreciated area of immunology.

Regulation of C3 deposition on gp120 coated CD4 positive cells by decay accelerating factor and factor H

We investigated complement activation by recombinant gp120 (rgp120) treated CD4 cells and the role host complement regulatory proteins play in controlling C3 deposition. Complement activation was determined by detection of C3 on rgp120 coated cells in the presence and absence of HIV seropositive sera using flow cytometry. Treatment of rgp120 coated cells with complement resulted in C3 deposition only if HIV positive sera was included. Examination of C3 fragments on these cells demonstrated rapid cleavage of C3b to iC3b. The role of the regulatory proteins was examined by pretreating cells with mAb to block decay accelerating factor (DAF) or membrane cofactor protein (MCP) or by using factor H depleted sera as a complement source. Inhibition of DAF or use of factor H depleted sera significantly increased C3 deposition on rgp120 coated cells. In contrast, C3 deposition on rgp120 coated cells was not increased after blocking MCP. The sensitivity of rgp120 coated cells to complement lysis was unchanged after inhibition of the regulatory proteins, despite the increase in C3 deposited. These results indicate that in a model of virus infected cells, C3 deposition is regulated by DAF and factor H but MCP appears to have no role.

Membrane-bound complement regulatory activity is decreased on vaccinia virus-infected cells

Decay accelerating factor (DAF), membrane cofactor protein (MCP), complement receptor 1 and mouse Crry are cell surface-bound complement regulatory proteins capable of inhibiting C3 convertase activity on cell membranes, and therefore provide a substantial protection from attack by homologous complement activated either by the classical or by the alternative pathway. Decrease in complement regulatory activity might lead to spontaneous complement deposition and subsequent cell injury. MoAb 5I2 can inhibit the complement regulatory activity of molecules on rat cells, resulting in deposition of homologous complement. The antigen recognized by 5I2 MoAb in rats is homologous to mouse Crry. Fifteen to 20 h after infection with vaccinia virus, in vitro cultured KDH-8 rat hepatoma cells show a strong decrease in expression of Crry-like antigen, and proved to be sensitive to complement deposition when 1:5 diluted normal rat serum was added to the culture medium as a source of complement. Addition of complement to the cultured KDH-8 cells infected with a very low dose of vaccinia virus (1 plaque-forming unit (PFU)/1000 cells) substantially reduced spreading of virus infection in the cell culture, while inactivation of complement by heat or zymosan treatment abrogated the protective effect.

Species selective interaction of Alphaherpesvirinae with the "unspecific" immune system of the host

During evolution Herpesviridae have developed glycoproteins, which interact with essential components of the immune system. Besides immunoglobulin-binding proteins (= Fc-receptors), expressed by several members of the herpesfamily, the interaction with the complement system plays a role in the pathogenicity of herpes simplex virus. Here we report that the ability to interact with the third complement component (C3), the central mediator of complement activation, was also found among several animal alphaherpesviruses. This interaction appeared to be species-selective as the viral proteins preferentially bound to the C3 originated from the respective host. That could provide a possible explanation for the evolution of a variety of herpesviruses as the species tropism observed among Herpesviridae may be influenced by specific adaptation of protective virus-proteins to the immune system of the different hosts. The data have critical implications for the studies of virus host interactions in heterologous systems and support a role for the C3-binding proteins in pathogenesis. Since the C3-binding proteins are conserved among different herpesviruses they could serve as suitable subunit-vaccine candidates.

Idiopathic membranoproliferative glomerulonephritis in childhood

Membranoproliferative glomerulonephritis (MPGN), recognized since 1965, is now known to have three forms, designated types I, II, and III. The types are similar in the frequency of hypocomplementemia and clinical course but are dissimilar in glomerular ultrastructure, pathogenesis, mechanisms of complement activation, predisposition to recur in the renal transplant, and, to some extent, in clinical presentation. Although glomerular proliferation is usually diffuse, it may be focal and segmental particularly in mild cases of MPGN I. Hypocomplementemia, present in about 80% of patients, is the result of hypercatabolism of C3 by three mechanisms as well as of diminished C3 synthesis. The hypocomplementemia is unrelated to clinical course or prognosis. Although MPGN I and III both have a high frequency of an extended haplotype on chromosome 6, which has known associations with autoimmune phenomena, and both have a high frequency of inherited complement defects, they are nevertheless dissimilar in glomerular ultrastructure, complement profile, and immunohistology in ways which suggest a wide difference in pathogenesis. Abnormalities in humoral immunity appear not to be involved in MPGN III. Treatment with anticoagulant, antiplatelet and cytotoxic drugs have, in controlled trials, been either ineffective or marginally effective. Long-term use of alternate-day prednisone in high dosage appears to be the most efficacious regimen in both controlled and uncontrolled studies.

Vaccine protection of rhesus macaques against simian immunodeficiency virus infection

Rhesus macaques (Macaca mulatta) immunized with an inactivated whole SIVmac vaccine and muramyl dipeptide (MDP), incomplete Freund's adjuvant (IFA), or aqueous suspension were challenged intravenously with 0.1 TCID50 of cell-free SIVmac. Whereas virus was readily recovered from the peripheral blood lymphocytes of 10 of 10 nonvaccinated controls following this challenge dose, virus was not recovered from the three animals that received the vaccine with MDP nor from one of two animals that received the vaccine with IFA and one of three animals that received the aqueous vaccine. The animals that were protected against challenge were those that had detectable SIV antibody response to the envelop, both the outer glycoprotein (gp120) and the truncated transmembrane glycoprotein (gp31). Protected monkeys tended to have higher titers of syncytial inhibition antibody prior to challenge. An anamnestic response after challenge was observed only in the vaccinated monkeys that became infected. Vaccinated animals that became challenge-infected tended to live longer than infected controls. These results confirm those at two other primate centers and indicate that killed whole SIV vaccines can protect against low challenge doses of SIV and prevent early death in those monkeys that do become infected. The mechanism of this protection remains undetermined. This finding adds optimism to the possibility of an eventual AIDS vaccine.

Low incidence of null alleles of the fourth component of complement (C4) in elderly people

Allotype frequencies of three complement proteins (C2, factor B, and C4) encoded on the sixth chromosome were tested in 150 "young" (under 53 yrs) and 131 "old" (over 62 yrs) healthy Hungarian individuals. In women, we found significant differences in BF*F allotype frequencies between the two age groups. A marked decrease of null alleles for C4 was observed especially in men. In the "old" age group, the total frequency of the non-expressed, "null" (Q0) alleles of the C4 protein dropped to almost one third of the "young" ones. The decrease was even larger (a 5.5-fold decrease in the incidence) among men. The relative risk of those having C4B*Q0 allele not to survive a "critical age period" of 53-62 yrs for males and 58-62 yrs for females was calculated to be 12.4 and 2.85, respectively. Our data substantiate the view that the expected life time is genetically determined and it can be forecast partly by the analysis of the antigens encoded on the sixth chromosome.

Complement activation in patients with renal failure as detected through the quantitation of fragments of the complement proteins C3, C5, and factor B

Using sensitive and highly specific enzyme-linked immunosorbent assays fragments of the complement proteins C3, C5, and factor B were quantitated in patients with renal failure. During hemodialysis on new cuprophan membranes raised levels not only of C3a, but in addition of activated C3, C5a, and Ba were demonstrated. In patients with chronic renal failure and end-stage renal disease plasma concentrations of Ba and activated C3 were markedly elevated independent of hemodialysis. This finding is taken as an indication of a continuous recruitment of the alternative pathway of complement in these patients. As the detected complement protein fragments are known to exert immune regulatory functions these findings may imply that these peptides are involved in the maintenance of the immune suppressed state in renal failure.

Herpes simplex virus glycoproteins gC-1 and gC-2 bind to the third component of complement and provide protection against complement-mediated neutralization of viral infectivity

Cells infected with herpes simplex virus type 1 (HSV-1) form rosettes with C3b-coated erythrocytes, whereas cells infected with herpes simplex virus type 2 (HSV-2) or other herpes viruses do not. It was reported that glycoprotein C of HSV-1 (gC-1) mediates the binding of C3b-coated erythrocytes to infected cells and has regulatory (decay-accelerating) activity for the alternative pathway C3 convertase of human complement. We show here that solubilized gC-1 binds to iC3-Sepharose affinity columns. We also report that solubilized gC-2, the genetically related glycoprotein specified by HSV-2, binds to iC3-Sepharose. mAb specific for gC-1 or gC-2 and mutant viral strains were used to identify the C3-binding glycoproteins. In other experiments, HSV-1 mutant strains and recombinants, differing only in their expression of gC, were tested for sensitivity to neutralization by human complement in the presence or absence of antibodies specific for HSV gD. In either case the gC- strain was most sensitive. Expression of gC-1 or gC-2 by isogenic insertion mutants provided protection against complement-mediated neutralization. These results indicate that the genetically and structurally related gC-1 and gC-2 share the functional activity of binding to human C3 and enhance viral infectivity.

Retrovirus-induced feline pure red cell aplasia. Hematopoietic progenitors are infected with feline leukemia virus and erythroid burst-forming cells are uniquely sensitive to heterologous complement

Feline leukemia virus subgroup C/Sarma (FeLV-C) induces pure red cell aplasia (PRCA) in cats. Just before the onset of anemia, erythroid colony-forming cells (CFU-E) become undetectable in marrow culture, yet normal frequencies of erythroid burst-forming cells (BFU-E)- and granulocyte-macrophage colony-forming cells (CFU-GM) persist. To determine if erythroid progenitors were uniquely infected with retrovirus, marrow mononuclear cells from cats viremic with FeLV-C were labeled with monoclonal antibodies to gp70 and then analyzed with a fluorescence-activated cell sorter. Both erythroid and granulocyte-macrophage progenitors were among cells sorting positively, suggesting that infection of BFU-E alone did not result in PRCA. The results were confirmed by complement (C') lysis studies using baby rabbit or guinea pig sera as sources of C'. These studies also suggested that BFU-E from cats with PRCA were unusually sensitive to C' alone, without the addition of antibody. In further studies, we demonstrated that C' activation was via the classical pathway and that C' sensitivity was unique to BFU-E and not a property of CFU-E, CFU-GM, or progenitors that were capable of giving rise to BFU-E in suspension culture. As BFU-E from cats viremic with FeLV-A/Glasgow-1 or the Rickard strain of feline leukemia virus were not sensitive to C', this finding may relate to the pathogenesis of feline PRCA. We hypothesize that, in cats viremic with FeLV-C, the abnormal C' sensitivity of BFU-E leads to the absence of CFU-E and anemia.

Influence of C3 level on the determination of C3d in plasma and synovial fluid by radial immunodiffusion

The influence of C3 levels on the determination of C3d in plasma and synovial fluid by radial immunodiffusion was investigated. In the method used, C3 is precipitated by 11% polyethylene glycol (PEG), and C3d is measured in the supernatant. In 51 healthy donors, a weak though significant correlation between C3 and C3d levels was found. The mean concentration of C3d was 1.6% of that in aged serum from healthy donors. So, small amounts of C3 (i.e., 1-2% of the normal plasma level) in the 11% PEG supernatants may contribute significantly to the C3d levels measured. A radioimmunoassay that detects C3, C3b, iC3b and C3c was used to measure C3 levels in the PEG supernatants. In PEG supernatants of 4 plasma samples, 0.3-0.6% of the C3 level in normal plasma was found, whereas in those of 2 synovial fluids much higher levels were found (4-10% of the normal plasma level). When purified 125I-labeled antibodies against C3c were added to the gel of the radial immunodiffusion, C3c antigen was detected in the precipitation rings obtained with all PEG supernatants of plasma samples from patients. Therefore, the quantitative contribution of C3 to the precipitation rings in the C3d radial immunodiffusion was analyzed after the addition of an excess of anti-C3c antibodies to the gel. No effect on the size of the C3d-precipitation rings obtained with plasma samples from patients was observed. However, the C3d precipitation rings obtained with synovial fluids were significantly smaller when the gel used in the radial immunodiffusion contained an excess of anti-C3c antibodies together with the anti-C3d serum. We conclude that it is necessary to add an excess of anti-C3c antibodies to the gel used for the radial immunodiffusion, for the determination of C3d levels in synovial fluid. An antiserum against human C3b, which contains both anti-C3c and anti-C3d antibodies, can be used for this purpose.

Complement subcomponent C1q secreted by cultured human monocytes has subunit structure identical with that of serum C1q

An enzyme-linked immunosorbent assay (e.l.i.s.a.) that is capable of quantifying C1q concentrations as low as 2 ng/ml and a sensitive haemolytic assay were used to study the appearance of material that cross-reacts with human serum C1q as well as C1q haemolytic activity in human monocyte culture media. This material was detected in the medium after 10-14 days and continued to be secreted through to day 28 of culture, at which time the cultures were terminated. Material specifically immunoabsorbed with Sepharose-anti-C1q antibody from a culture medium of cells that was metabolically labelled with [3H] proline or [35S] methionine demonstrated a polypeptide pattern identical with that of serum C1q on SDS/polyacrylamide-gel electrophoresis. Under non-reducing conditions two protein bands were detected migrating with the same Rf values as the serum C1q A-B and C-C dimers. On reduction three bands were evident, which migrated identically with the A, B and C chains of serum C1q. The amount of radioactivity in these bands increased with time in culture, consistent with the e.l.i.s.a. and haemolytic C1q assays. These bands were reactive with monospecific anti-C1q antibody after transfer to nitrocellulose.

Complement in the pathophysiology and diagnosis of human diseases

Complement is a humoral effector system composed of 21 plasma proteins that was identified initially because of its cytolytic effects. In addition to cytolysis, complement has a number of different functions related to inflammatory and other host defense processes. The description of the reaction mechanism includes: (1) activation of the classical pathway through recognition of IgG and IgM antibodies by C1q, (2) activation of the alternative pathway which is usually achieved without participation of immunoglobulins, (3) generation of proteolytic enzymes composed of heteropolymers that cleave certain precursor proteins, (4) formation of the membrane attack complex (MAC), and (5) participation of control mechanisms. Methodologies for studying protein concentration and functional activities of complement components include not only the classical hemolytic techniques but also the extremely sensitive new radioimmunoassays and enzyme immunoassays for measuring the products of complement activation that are generated in vivo. Examples of genetically controlled complement deficiencies have been published for most complement components. The symptomatology of some of these patients serves to emphasize the protective role of complement. Acquired deficiencies are significant not only as laboratory aids in diagnosis and to evaluate the course of certain diseases, but also to indicate possible pathogenic disease mechanisms. Recently, it has been recognized that the complement proteins with genes located in the HLA region are polymorphic. Certain variants of proteins C2, C4, and factor B occur with higher frequencies in certain diseases than in the general population, which appears to be of great practical importance in laboratory medicine.

Complement effector mechanisms in health and disease

Complement is an effector system able to mediate a number of biological activities in vitro and in vivo. Most familiar is the ability of the system to mediate the lytic destruction of numerous kinds of cells and pathogenic organisms including bacteria, viruses, and virus-infected cells. In addition, the complement system also activates neutrophils, monocytes, basophils, mast cells, and lymphocytes to perform specialized functions. While generally considered to be confined to the effector side of immune reactions, recent evidence indicates that the complement system also directly recognizes and is triggered by a number of bacteria and viruses as well as virus-infected cells in the absence of antibody. In such reactions, complement fulfills the recognition role normally associated with the antibody molecule or immune lymphocyte. The complement system may thus also function as a natural surveillance system operative prior to the induction of specific immunity. Involvement of the complement system in biological reactions has been ascertained by several techniques over the years. These include quantitation of individual complement components in human sera and demonstration of complement deposition in diseased tissues in human diseases and in experimental diseases in animals. Such techniques, however, have limitations in specificity and sensitivity. Assays which detect specific features of the complement activation process have become available in recent years. These tests detect the physical, chemical, or antigenic changes characteristic of the complement activation process. These assays are extremely specific and quantitative; furthermore, most are usable with samples from patients. Three general approaches have been utilized to develop such specific quantitative assays for complement activation. The first includes assays which quantitate activation-specific limited proteolysis of the complement components. The second type of assay includes tests which detect and quantitate new antigens or other activation-specific antigenic changes. The third category is represented by assays which detect and quantitate the protein-protein complexes characteristic of the activation process. Examples of tests presenting each of these approaches are given.(ABSTRACT TRUNCATED AT 400 WORDS)

Hemolytic plate assay for quantification of active human complement component C3 using methylamine-treated plasma as complement source

A hemolytic plate assay specific for active human complement component C3 is described. The method is well suited for tracing active C3 during preparative purification or for screening of plasma samples. The assay is based on activation of the alternative pathway of complement by unmodified rabbit erythrocytes. Plasma treated with methylamine supplies the essential complement components other than C3. The lytic reaction is complete in 5 h at 37 degrees C and is unchanged by incubation overnight. The dose-response curve, i.e., lysis diameter versus logarithm of C3 concentration, is linear within 0.1-10 times normal plasma concentrations of C3. The standard deviation is below 10%. The hemolytic agarose plates are easy and inexpensive to prepare, and they can be stored at 4 degrees C for 2 weeks before use. This paper describes the optimal conditions of the assay and proves its specificity. Its use in C3 preparation and plasma screening for C3 is discussed.

The role of antibody and complement in the control of viral infections

Host defense against viral infection is extremely complex and includes both humoral and cellular immune mechanisms. This contribution examines the mechanisms by which antibody (Ab) and the complement (C) system, major constituents of the humoral immune system, inactivate viruses and block viral maturation in virus-infected cells in vitro. Ab and C may neutralize viruses by envelopment in a coating of protein, by aggregation by lysis, or by facilitating interactions with various effector cells. Ab and C molecules deposited on the surfaces of viruses may physically interfere with the ability of the virus to infect a potentially susceptible cell. This appears to be the most common mechanism by which Ab and C neutralize viruses. In rare instances, Ab and/or C may aggregate viruses; aggregation reduces the net number of infectious particles and thus is manifest as neutralization. C may lyse enveloped viruses, resulting in irreversible viral inactivation. However, this does not appear to be a major mechanism of viral neutralization. Finally, the Fc portions of bound Ab molecules as well as bound C molecules may interact with effector cells with specific receptors for these factors and thereby facilitate viral destruction. In regard to virus-infected cells, the deposition of Ab or C on the cell surface may prevent the maturation or release of viral particles and alter normal cellular functions. Ab and C may also lyse virus-infected cells, abruptly stopping further viral maturation. Such lytic events require only the F(ab')2 portion of the Ab molecule and proceed via activation of the alternative C pathway. Effector cells may also interact with Ab and/or C molecules deposited on virus-infected cells, leading to cytotoxic reactions and/or ingestion depending on the type of effector cell involved. The activated C system has the ability to produce an acute inflammatory response leading to alterations in vessel permeability, edema, changes in smooth-muscle contractility, and the influx of leukocytes. Such inflammatory responses occurring in tissues, including the skin, as a result of C activation not only retard the spread of the infection and facilitate the destruction of the infectious agent, but also in all likelihood damage normal tissues in the vicinity. In addition, C activation in tissues also has the ability to stimulate arachidonic acid metabolism and induce the release of histamine and other mediators as well as pyrogens from appropriate cell types. A number of the systemic symptoms characteristic of viral infections, such as headaches, myalgias, and fever, likely result from such processes.