Activation of the alternative complement pathway by enveloped viruses containing limited amounts of sialic acid

Role of Defective Interfering Particles in Complement-Mediated Lysis of Parainfluenza Virus-Infected Cells

RNA viruses pose a significant global public health burden due to their high mutation rates, zoonotic potential, and ability to evade immune responses. A common aspect of their replication is the generation of defective interfering particles (DIPs), which contain truncated defective viral genomes (DVGs) that depend on full-length standard (STD) virus for replication. DVGs have gained recognition as they are increasingly detected in clinical samples from natural infections. While their role in modulating type I interferon (IFN-I) responses is well established, their impact on the complement (C') system is not understood. In this study, we examined how DVGs influence C'-mediated lysis during parainfluenza virus 5 (PIV5) infection using real-time in vitro cell viability assays. Our results demonstrated that C' effectively killed human lung epithelial cells infected with STD PIV5, whereas co-infection with DIP-enriched stocks significantly suppressed C'-mediated killing through mechanisms that were dependent on DVG replication but independent of IFN-I production. The titration of DI units in co-infection with STD PIV5 showed a strong linear relationship between DIP-mediated decreases in surface viral glycoprotein expression and the inhibition of C'-mediated lysis. Our findings reveal a previously unrecognized function of DVGs in modulating C' pathways, shedding light on their potential role in viral persistence and immune evasion.

Resistance to complement-mediated lysis of parainfluenza virus 5-infected cells is acquired after transition from acute to persistent infection

Persistent viral infections can be an important medical problem, with persistently infected (PI) cells extending viral shedding, maintaining inflammation, and providing potential sources for new viral variants. Given that PI cells can acquire resistance to some innate immune pathways, we tested the hypothesis that complement (C')-mediated lysis of parainfluenza virus 5 (PIV5)-infected cells would differ between acute-infected and PI cells. Biochemical and real-time cell viability assays showed effective C'-mediated lysis of A549 lung cells acutely infected with PIV5, through pathways that depended on C3 and C5, but largely independent of C6. A PIV5 PI cell line established by long-term culturing of acutely infected A549 cells showed a high-level persistent expression of PIV5 proteins and infectious virus. Under conditions that led to effective lysis of acute PIV5-infected cells, the PI cells were nearly completely resistant to C'-mediated killing. This lack of C' killing was not due to failure to activate C', since C'-treated PIV5 PI cells had extensive C3 and membrane attack complex deposition, as well as production of C3a and C5a. Transcriptomics analysis revealed the C' cascade as the most significantly upregulated pathway in PIV5 PI cells versus acute infection. Biochemical analyses showed that resistance to C' killing correlated with increased expression in PI cells of two major C' inhibitors: complement factor H and Vitronectin. The finding of acquisition of C' resistance after the transition from acute PIV5 infection to PI cells raises the potential to inform therapeutics for PIs based on modulating C' pathways.

IMPORTANCE

A persistent infection (PI) with RNA viruses can extend virus shedding, prolong inflammation, and be a source of new viral variants. Since profound changes to innate immune pathways can occur in PI cells, it was important to test PI cells for changes in sensitivity to the complement (C') system, powerful innate immune pathways capable of lysing infected cells. Using parainfluenza virus 5 (PIV5) as a model system, we show that PI cells are nearly completely resistant to C'-mediated lysis, in stark contrast to high sensitivity of acute PIV5-infected cells to C' killing. A key finding was the upregulated expression in PI cells of two C' inhibitors: Vitronectin and complement factor H. These are important results with strong potential to inform therapeutics, given that polymorphisms in C' genes can correlate with severity of viral infections, and clinical trials are underway with new drugs that modulate C' responses.

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.

A Factor I-Like Activity Associated with Chikungunya Virus Contributes to Its Resistance to the Human Complement System

Chikungunya virus (CHIKV) is an emerging pathogen capable of causing explosive outbreaks. Prior studies showed that exacerbation in arthritogenic alphavirus-induced pathogenesis is attributed to its interaction with multiple immune components, including the complement system. Viremia concomitant to CHIKV infection makes exposure of the virus to complement unavoidable, yet very little is known about CHIKV-complement interactions. Here, we show that CHIKV activated serum complement to modest levels in a concentration- and time-dependent manner, but the virus effectively resisted complement-mediated neutralization. Heat-inactivated serum from seropositive donors could actively neutralize CHIKV due to the presence of potent anti-CHIKV antibodies. Deposition of key complement components C3 and C4 did not alter the resistance of CHIKV to complement. Further, we identified a factor I-like activity in CHIKV that limited complement by inactivating C3b into inactive C3b (iC3b), the complement component known to significantly contribute to disease severity in vivo, but this activity had no effect on C4b. Inactivation of C3b by CHIKV was largely dependent on the concentration of the soluble host cofactor factor H and the virus concentration. A factor I function-blocking antibody had only a negligible effect on the factor I-like activity associated with CHIKV, suggesting that this activity is independent of host factor I and could be of viral origin. Thus, our findings suggest a complement modulatory action of CHIKV which not only helps the virus to evade human complement but may also have implications in alphavirus-induced arthritogenic symptoms.IMPORTANCE Chikungunya virus is a vector-borne pathogen of global significance. The morbidity associated with chikungunya virus (CHIKV) infection, neurovirulence and adaptability to Aedes albopictus, necessitates a deeper understanding of the interaction of CHIKV with the host immune system. Here, we demonstrate that CHIKV is resistant to neutralization by one of the potent barriers of the innate immune arm, the complement system. Chikungunya virus showed marked resistance to complement despite activation and deposition of complement proteins. Interestingly the C3 component associated with the virion was found to be inactive C3b (iC3b), a key factor implicated in the pathogenesis and disease severity in the mouse model of Ross River virus infection. CHIKV also had an associated unique factor I-like activity that mediated the inactivation of C3b into iC3b. We have unraveled a smart strategy adopted by CHIKV to limit complement which has serious implications in viral dissemination, pathogenesis, and disease.

Complement-Mediated Neutralization of a Potent Neurotropic Human Pathogen, Chandipura Virus, Is Dependent on C1q

Among the innate immune sentinels, the complement system is a formidable first line of defense against pathogens, including viruses. Chandipura virus (CHPV), a neurotropic vesiculovirus of the family Rhabdoviridae, is a deadly human pathogen known to cause fatal encephalitis, especially among children. The nature of interaction and the effect of human complement on CHPV are unknown. Here, we report that CHPV is a potent activator of complement and, thus, is highly sensitive to complement proteins in normal human serum (NHS). Utilizing a panel of specific complement component depleted/reconstituted human serum, we have demonstrated that CHPV neutralization is C3, C4, and C1q dependent and independent of factor B, suggesting the importance of the classical pathway in limiting CHPV. Employing a range of biochemical approaches, we showed (i) a direct association of C1q to CHPV, (ii) deposition of complement proteins C3b, C4b, and C1q on CHPV, and (iii) virus aggregation. Depletion of C8, an important component of the pore-forming complex of complement, had no effect on CHPV, further supporting the finding that aggregation and not virolysis is the mechanism of virus neutralization. With no approved vaccines or treatment modalities in place against CHPV, insights into such interactions can be exploited to develop potent vaccines or therapeutics targeting CHPV.IMPORTANCE Chandipura virus is a clinically important human pathogen of the Indian subcontinent. The rapidity of death associated with CHPV infection in addition to the absence of an effective vaccine or therapeutics results in poor clinical prognosis. The biology of the virus and its interaction with the host immune system, including the complement system, are understudied. Our investigation reveals the susceptibility of CHPV to fluid phase complement and also dissects the pathway involved and the mechanism of virus neutralization. Direct binding of C1q, an important upstream component of the classical pathway of complement to CHPV, and the strong dependency on C1q for virus neutralization highlight the significance of identifying such interactions to better understand CHPV pathogenesis and devise strategies to target this deadly pathogen.

Relative Contribution of Cellular Complement Inhibitors CD59, CD46, and CD55 to Parainfluenza Virus 5 Inhibition of Complement-Mediated Neutralization

The complement system is a part of the innate immune system that viruses need to face during infections. Many viruses incorporate cellular regulators of complement activation (RCA) to block complement pathways and our prior work has shown that Parainfluenza virus 5 (PIV5) incorporates CD55 and CD46 to delay complement-mediated neutralization. In this paper, we tested the role of a third individual RCA inhibitor CD59 in PIV5 interactions with complement pathways. Using a cell line engineered to express CD59, we show that small levels of functional CD59 are associated with progeny PIV5, which is capable of blocking assembly of the C5b-C9 membrane attack complex (MAC). PIV5 containing CD59 (PIV5-CD59) showed increased resistance to complement-mediated neutralization in vitro comparing to PIV5 lacking regulators. Infection of A549 cells with PIV5 and RSV upregulated CD59 expression. TGF-beta treatment of PIV5-infected cells also increased cell surface CD59 expression and progeny virions were more resistant to complement-mediated neutralization. A comparison of individual viruses containing only CD55, CD46, or CD59 showed a potency of inhibiting complement-mediated neutralization, which followed a pattern of CD55 > CD46 > CD59.

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.

The α2,3-sialyltransferase encoded by myxoma virus is a virulence factor that contributes to immunosuppression

Myxoma virus (MYXV) induces a lethal disease called Myxomatosis in European rabbits. MYXV is one of the rare viruses that encodes an α2,3-sialyltransferase through its M138L gene. In this study, we showed that although the absence of the enzyme was not associated with any in vitro deficit, the M138L deficient strains are highly attenuated in vivo. Indeed, while all rabbits infected with the parental and the revertant strains died within 9 days post-infection from severe myxomatosis, all but one rabbit inoculated with the M138L deficient strains survived the infection. In primary lesions, this resistance to the infection was associated with an increased ability of innate immune cells, mostly neutrophils, to migrate to the site of virus replication at 4 days post-infection. This was followed by the development of a better specific immune response against MYXV. Indeed, at day 9 post-infection, we observed an important proliferation of lymphocytes and an intense congestion of blood vessels in lymph nodes after M138L knockouts infection. Accordingly, in these rabbits, we observed an intense mononuclear cell infiltration throughout the dermis in primary lesions and higher titers of neutralizing antibodies. Finally, this adaptive immune response provided protection to these surviving rabbits against a challenge with the MYXV WT strain. Altogether, these results show that expression of the M138L gene contributes directly or indirectly to immune evasion by MYXV. In the future, these results could help us to better understand the pathogenesis of myxomatosis but also the importance of glycans in regulation of immune responses.

The neutralizing capacity of antibodies elicited by parainfluenza virus infection of African Green Monkeys is dependent on complement

The African Green Monkey (AGM) model was used to analyze the role of complement in neutralization of parainfluenza virus. Parainfluenza virus 5 (PIV5) and human parainfluenza virus type 2 were effectively neutralized in vitro by naïve AGM sera, but neutralizing capacity was lost by heat-inactivation. The mechanism of neutralization involved formation of massive aggregates, with no evidence of virion lysis. Following inoculation of the respiratory tract with a PIV5 vector expressing HIV gp160, AGM produced high levels of serum and tracheal antibodies against gp120 and the viral F and HN proteins. However, in the absence of complement these anti-PIV5 antibodies had very poor neutralizing capacity. Virions showed extensive deposition of IgG and C1q with post- but not pre-immune sera. These results highlight the importance of complement in the initial antibody response to parainfluenza viruses, with implications for understanding infant immune responses and design of vaccine strategies for these pediatric pathogens.

Paramyxovirus activation and inhibition of innate immune responses

Paramyxoviruses represent a remarkably diverse family of enveloped nonsegmented negative-strand RNA viruses, some of which are the most ubiquitous disease-causing viruses of humans and animals. This review focuses on paramyxovirus activation of innate immune pathways, the mechanisms by which these RNA viruses counteract these pathways, and the innate response to paramyxovirus infection of dendritic cells (DC). Paramyxoviruses are potent activators of extracellular complement pathways, a first line of defense that viruses must face during natural infections. We discuss mechanisms by which these viruses activate and combat complement to delay neutralization. Once cells are infected, virus replication drives type I interferon (IFN) synthesis that has the potential to induce a large number of antiviral genes. Here we describe four approaches by which paramyxoviruses limit IFN induction: by limiting synthesis of IFN-inducing aberrant viral RNAs, through targeted inhibition of RNA sensors, by providing viral decoy substrates for cellular kinase complexes, and through direct blocking of the IFN promoter. In addition, paramyxoviruses have evolved diverse mechanisms to disrupt IFN signaling pathways. We describe three general mechanisms, including targeted proteolysis of signaling factors, sequestering cellular factors, and upregulation of cellular inhibitors. DC are exceptional cells with the capacity to generate adaptive immunity through the coupling of innate immune signals and T cell activation. We discuss the importance of innate responses in DC following paramyxovirus infection and their consequences for the ability to mount and maintain antiviral T cells.

Point mutations in the paramyxovirus F protein that enhance fusion activity shift the mechanism of complement-mediated virus neutralization

Parainfluenza virus 5 (PIV5) activates and is neutralized by the alternative pathway (AP) in normal human serum (NHS) but not by heat-inactivated (HI) serum. We have tested the relationship between the fusion activity within the PIV5 F protein, the activation of complement pathways, and subsequent complement-mediated virus neutralization. Recombinant PIV5 viruses with enhanced fusion activity were generated by introducing point mutations in the F fusogenic peptide (G3A) or at a distal site near the F transmembrane domain (S443P). In contrast to wild-type (WT) PIV5, the mutant G3A and S443P viruses were neutralized by both NHS and HI serum. Unlike WT PIV5, hyperfusogenic G3A and S443P viruses were potent C4 activators, C4 was deposited on NHS-treated mutant virions, and the mutants were neutralized by factor B-depleted serum but not by C4-depleted serum. Antibodies purified from HI human serum were sufficient to neutralize both G3A and S443P viruses in vitro but were ineffective against WT PIV5. Electron microscopy data showed greater deposition of purified human antibodies on G3A and S443P virions than on WT PIV5 particles. These data indicate that single amino acid changes that enhance the fusion activity of the PIV5 F protein shift the mechanism of complement activation in the context of viral particles or on the surface of virus-infected cells, due to enhanced binding of antibodies. We present general models for the relationship between enhanced fusion activity in the paramyxovirus F protein and increased susceptibility to antibody-mediated neutralization.

Virion-associated complement regulator CD55 is more potent than CD46 in mediating resistance of mumps virus and vesicular stomatitis virus to neutralization

Enveloped viruses can incorporate host cell membrane proteins during the budding process. Here we demonstrate that mumps virus (MuV) and vesicular stomatitis virus (VSV) assemble to include CD46 and CD55, two host cell regulators which inhibit propagation of complement pathways through distinct mechanisms. Using viruses which incorporated CD46 alone, CD55 alone, or both CD46 and CD55, we have tested the relative contribution of these regulators in resistance to complement-mediated neutralization. Virion-associated CD46 and CD55 were biologically active, with VSV showing higher levels of activity of both cofactors, which promoted factor I-mediated cleavage of C3b into iC3b as well as decay-accelerating factor (DAF) activity against the C3 convertase, than MuV. Time courses of in vitro neutralization with normal human serum (NHS) showed that both regulators could delay neutralization, but viruses containing CD46 alone were neutralized faster and more completely than viruses containing CD55 alone. A dominant inhibitory role for CD55 was most evident for VSV, where virus containing CD55 alone was not substantially different in neutralization kinetics from virus harboring both regulators. Electron microscopy showed that VSV neutralization proceeded through virion aggregation followed by lysis, with virion-associated CD55 providing a delay in both aggregation and lysis more substantial than that conferred by CD46. Our results demonstrate the functional significance of incorporation of host cell factors during virion envelope assembly. They also define pathways of virus complement-mediated neutralization and suggest the design of more effective viral vectors.

Interactions of human complement with virus particles containing the Nipah virus glycoproteins

Complement is an innate immune response system that most animal viruses encounter during natural infections. We have tested the role of human complement in the neutralization of virus particles harboring the Nipah virus (NiV) glycoproteins. A luciferase-expressing vesicular stomatitis virus (VSV) pseudotype that contained the NiV fusion (F) and attachment (G) glycoproteins (NiVpp) showed dose- and time-dependent activation of human complement through the alternative pathway. In contrast to our findings with other paramyxoviruses, normal human serum (NHS) alone did not neutralize NiVpp infectivity in vitro, and electron microscopy demonstrated no significant deposition of complement component C3 on particles. This lack of NiVpp neutralization by NHS was not due to a global inhibition of complement pathways, since complement was found to significantly enhance neutralization by antibodies specific for the NiV F and G glycoproteins. Complement components C4 and C1q were necessary but not sufficient by themselves for the enhancement of antibody neutralization. Human complement also enhanced NiVpp neutralization by a soluble version of the NiV receptor EphrinB2, and this depended on components in the classical pathway. The ability of complement to enhance neutralization fell into one of two profiles: (i) anti-F monoclonal antibodies showed enhancement only at high and not low antibody concentrations, and (ii) anti-G monoclonal antibodies and EphrinB2 showed enhancement at both high and very low levels of antibody (e.g., 3.1 ng) or EphrinB2 (e.g., 2.5 ng). Together, these data establish the importance of human complement in the neutralization of particles containing the NiV glycoproteins and will help guide the design of more effective therapeutics that harness the potency of complement pathways.

Virus–complement interactions: an assiduous struggle for dominance

The complement system is a major component of the innate immune system that recognizes invading pathogens and eliminates them by means of an array of effector mechanisms, in addition to using direct lytic destruction. Viruses, in spite of their small size and simple composition, are also deftly recognized and neutralized by the complement system. In turn, as a result of years of coevolution with the host, viruses have developed multiple mechanisms to evade the host complement. These complex interactions between the complement system and viruses have been an area of focus for over three decades. In this article, we provide a broad overview of the field using key examples and up-to-date information on the complement-evasion strategies of viruses.

The paramyxoviruses simian virus 5 and mumps virus recruit host cell CD46 to evade complement-mediated neutralization

The complement system is a critical component of the innate immune response that all animal viruses must face during natural infections. Our previous results have shown that treatment of the paramyxovirus simian virus 5 (SV5) with human serum results in deposition of complement C3-derived polypeptides on virion particles. Here, we show that the virion-associated C3 component includes the inactive form iC3b, suggesting that SV5 may have mechanisms to evade the host complement system. Electron microscopy, gradient centrifugation, and Western blot analysis indicated that purified SV5 virions derived from human A549 cells contained CD46, a plasma membrane-expressed regulator of complement that acts as a cofactor for cleavage and inactivation of C3b into iC3b. In vitro cleavage assays with purified complement components showed that SV5 virions had C3b cofactor activity, resulting in specific factor I-mediated cleavage of C3b into inactive iC3b. SV5 particles generated in CHO cells, which do not express CD46, did not have cofactor activity. Conversely, virions derived from a CHO cell line that was engineered to overexpress human CD46 contained elevated levels of virion-associated CD46 and displayed enhanced C3b cofactor activity. In comparison with C3b, purified SV5 virions had very low cofactor activity against C4b, consistent with the known preference of CD46 for C3b versus C4b. Similar results were obtained for the closely related mumps virus (MuV), except that MuV particles derived from CHO-CD46 cells had higher C4b cofactor activity than SV5 virions. In neutralization assays with human serum, SV5 and MuV containing CD46 showed slower kinetics and more resistance to neutralization than SV5 and MuV that lacked CD46. Our results support a model in which the rubulaviruses SV5 and MuV incorporate cell surface complement inhibitors into progeny virions as a mechanism to limit complement-mediated neutralization.

Differential mechanisms of complement-mediated neutralization of the closely related paramyxoviruses simian virus 5 and mumps virus

The complement system is an important component of the innate immune response to virus infection. The role of human complement pathways in the in vitro neutralization of three closely related paramyxoviruses, Simian Virus 5 (SV5), Mumps virus (MuV) and Human Parainfluenza virus type 2 (HPIV2) was investigated. Sera from ten donors showed high levels of neutralization against HPIV2 that was largely complement-independent, whereas nine of ten donor sera were found to neutralize SV5 and MuV only in the presence of active complement pathways. SV5 and MuV neutralization proceeded through the alternative pathway of the complement cascade. Electron microscopy studies and biochemical analyses showed that treatment of purified SV5 with human serum resulted in C3 deposition on virions and the formation of massive aggregates, but there was relatively little evidence of virion lysis. Treatment of MuV with human serum also resulted in C3 deposition on virions, however in contrast to SV5, MuV particles were lysed by serum complement and there was relatively little aggregation. Assays using serum depleted of complement factors showed that SV5 and MuV neutralization in vitro was absolutely dependent on complement factor C3, but was not dependent on downstream complement factors C5 or C8. Our results indicate that even though antibodies exist that recognize both SV5 and MuV, they are mostly non-neutralizing and viral inactivation in vitro occurs through the alternative pathway of complement. The implications of our work for development of paramyxovirus vectors and vaccines are discussed.

Natural antibody and complement mediate neutralization of influenza virus in the absence of prior immunity

Early control of virus replication by the innate immune response is essential to allow time for the generation of a more effective adaptive immune response. As an important component of innate immunity, complement has been shown to be necessary for protection against numerous microbial infections. This study was undertaken to investigate the role of complement in neutralizing influenza virus. Results demonstrated that the classical pathway of complement mediated serum neutralization of influenza virus. Although nonimmune serum neutralized influenza virus, the mechanism of virus neutralization (VN) required antibody, as sera from RAG1-deficient mice lacked VN activity; moreover, purified natural immunoglobulin M (IgM) restored VN activity to antibody-deficient sera. The mechanism of VN by natural IgM and complement was associated with virion aggregation and coating of the viral hemagglutinin receptor; however, viral lysis did not significantly contribute to VN. Additionally, reconstitution of RAG1-deficient mice with natural IgM resulted in delayed morbidity during influenza virus infection. Collectively, these results provide evidence that natural IgM and the early components of the classical pathway of complement work in concert to neutralize influenza virus and that this interaction may have a significant impact on the course of influenza viral pneumonia.

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.

Complement-mediated binding of naturally glycosylated and glycosylation-modified human immunodeficiency virus type 1 to human CR2 (CD21)

Particulate glycoproteins lacking sialic acid, such as desialylated enveloped viruses, readily activate complement through the alternative pathway. Human immunodeficiency virus type 1 (HIV-1) contains two heavily glycosylated and partially sialylated envelope glycoproteins: a surface gp120 and a transmembrane gp41. The abilities of naturally glycosylated HIV-1 and glycosylation-modified HIV-1 to interact with the complement system were examined with a biological assay which measured the binding of whole virus particles to cells expressing CR2 (CD21), the complement receptor found naturally in abundance on follicular dendritic cells and immature B cells. HIV-1 IIIB was synthesized in the presence or absence of the mannosidase II inhibitor, swainsonine, to give rise to high-mannose-type, nonsialylated, nonfucosylated carbohydrate moieties. The virus also was treated with neuraminidase or endo-beta-galactosidase to remove terminal sialic acids. An enzyme immunoassay specific for HIV-1 p24 core protein was used to quantitate the amount of virus bound to cell surfaces. Virus particles incubated with 1:3-diluted, fresh HIV-1-negative human serum as a source of complement readily bound to MT-2 (CD4+ CR2+) and Raji-3 (CD4- CR2+) cells but not to CEM (CD4+ CR2-) cells, suggesting that the virus bound to CR2 independently of CD4. Compared with heat-inactivated or C3-deficient sera, fresh complement increased binding by as much as 62 times for naturally glycosylated virus, and 5 times more than this for glycosylation-modified virus. Similar observations were made with freshly isolated, non-mitogen-stimulated peripheral blood mononuclear cells. Additional evidence that HIV-1 bound to CR2 independently of CD4 was provided by the fact that binding was blocked by monoclonal antibody OKB7 (anti-CR2) but not by OKT4a (anti-CD4). Also, the virus bound to transfected K562 cells (CD4-) which expressed recombinant human CR2 but did not bind to untransfected K562 cells. Results obtained with complement component-deficient sera indicated that binding required the alternative complement pathway. Raji-3 and transfected K562 cells could not be infected with HIV-1 in the presence of complement, suggesting that utilization of CR2 as a receptor in the absence of CD4 does not allow virus entry. The demonstration of CR2 as a receptor for HIV-1 in the presence of complement, together with the ability to enhance binding by desialylation, provides new insights into mechanisms of HIV-1-induced immunity and immunopathogenesis.

Antibody-dependent and antibody-independent complement-mediated enhancement of human immunodeficiency virus type 1 infection in a human, Epstein-Barr virus-transformed B-lymphocytic cell line

A human Epstein-Barr virus-transformed B-cell line (IC.1) was characterized for cell surface antigen profile and permissivity to immunodeficiency virus (HIV) infection. According to cocultivation assay with MT2 cells, P24 release, and immunofluorescence assay, complement-sufficient serum enhanced in vitro infection of IC.1 cells. Enhancement occurs independently of the presence of HIV type 1-specific antibodies, although more efficiently when they are present. Blocking experiments with monoclonal antibodies demonstrated that complement receptor type 2 mediates this phenomenon and that the CD4 molecule is required for infection. Enhancement of in vitro infection on IC.1 cells appears closely related to previously described complement-mediated, antibody-dependent enhancement of HIV infection on the T-lymphoblastoid cell line MT2 (W. E. Robinson, Jr., D. C. Montefiori, and W. M. Mitchell, Lancet i:790-794, 1988).

Complement activation is associated with the presence of specific human immunodeficiency virus (HIV)-anti-HIV immune complexes in patients with acquired immunodeficiency syndrome-related complex or lymphoadenopathy syndrome

The complement system was examined in a group of eight patients (six with lymphoadenopathy syndrome (LAS); two with acquired immunodeficiency syndrome (AIDS)-related complex (ARC], who were found to be human immunodeficiency virus (HIV)-positive, for the presence of specific HIV-anti-HIV complexes. A significant impairment of the classical and/or alternative pathway was found associated with the presence of cleavage fragments of C3 and/or B and a significant reduction in the complement factors studied. Ultracentrifugation fractions of serum samples obtained from one of the patients were assessed for the detection of specific HIV-anti-HIV (GP41-anti-GP41) complexes and were incubated with normal human serum to determine their complement activation capacity. A clear complement activation was found with the fraction in which a clear peak of HIV-anti-HIV (GP41-anti-GP41) immune complexes was present. The results demonstrate that specific immune complexes and complement activation are sometimes concomitantly present in patients with AIDS-related disease and that specific immune complexes may be one of the causal factors of the pathogenesis of complement activation in these patients. Possible consequences for the severe immune regulation with relevance to the dramatic failure in treating the virus effectively are discussed.

Antibody-independent, complement-mediated enhancement of HIV-1 infection by mannosidase I and II inhibitors

Human immunodeficiency virus type 1 (HIV-1) infectivity and cytopathic effect require proper maturation of the viral envelope glycoprotein carbohydrate moieties. We have found that fresh human serum enhances the infectivity of HIV-1 in MT-2 cell infection assays when virus is synthesized in the presence of the mannosidase I inhibitor, 1-deoxymannojirimycin, or the mannosidase II inhibitor, swainsonine, but has no enhancing effect on virus synthesized in the presence of the glucosidase I inhibitors, castanospermine and 1-deoxynojirimycin, or the glucosidase II inhibitor, bromoconduritol. Enhanced infections were characterized by cytopathic effect, antigen synthesis and reverse transcriptase release, all which occurred sooner than in control-infected cultures. This enhancement of infection was also observed in C1q-deficient serum but was not observed in serum that was heat-inactivated or depleted of complement components C3 or factor B, thus suggesting a requirement for the alternate pathway of complement.

Complement activation during an active cytomegalovirus infection after renal transplantation: due to circulating immune complexes or alternative pathway activation?

In 32 patients with a renal allograft, serial determinations after transplantation were made of C3d, the stable conversion product of the complement factor C3, as well as serial measurements of the anaphylatoxin C3a des arg. Furthermore, serial determinations were made on the presence of circulating immune complexes using three different assays (C1q binding assay, polyethylene glycol precipitation test, and indirect granulocyte phagocytosis test). Twenty patients were studied during an active cytomegalovirus (CMV) infection, and 12 patients were studied during allograft rejection or during stable phase after renal transplantation. In 12 patients with a CMV infection serial measurements were made of AP50 (alternative pathway of complement). During an active CMV infection elevated C3d as well as elevated C3a des arg levels were found and not in the control group (P less than 0.01). In 8 out of the 12 patients tested, with CMV infection, a decreased hemolytic activity of the alternative pathway (AP50) was found, together with the elevated levels of C3d and C3a des arg. Serum C4 levels were normal or high during CMV infection. Furthermore, circulating immune complexes were found to be positive in 15 out of the 20 patients with a CMV infection (both primary and secondary infections), and in 2 out of 12 patients of the control group. The complement activation found in the CMV group was not related to the presence of circulating immune complex-like material, since complement activation was present in advance of the appearance of the immune complexes, suggesting that complement activation was not due to classical pathway activation by those complexes. We conclude that our data are consistent with complement activation and the formation of biologically active peptides like C3a des arg in patients with an active CMV infection. The decreased hemolytic activity of the alternative pathway (AP50) together with the normal or high C4 levels suggest involvement of the alternative pathway, although further studies of the alternative pathway of C are warranted to confirm this hypothesis.

Activation of the alternative complement pathway by mumps infected cells: relationship to viral neuraminidase activity

An inverse relationship exists between the sialic acid content of a particle and its ability to activate the alternative complement pathway. The present studies were performed to determine if the neuraminidase (NANase) activities of different mumps virus strains could influence the ability of mumps virus infected cells to activate the alternative pathway. CV-1 cells were infected with three different mumps virus strains (RW, O'Take, and Kilham) and after 24 hours, 10 percent guinea pig serum (GPS) treated with EGTA/MgCl2 or GPS lacking the 4th component of complement (C4DGPS) was added to the cell monolayers. After 30 minutes, the percentage C3 consumed was determined by a functional hemolytic assay. Cells infected with RW (high NANase) consumed significantly more C3 (23.2 per cent) than cells infected with Kilham (5.7 percent, low NANase). Cells infected with O'Take were intermediate in their ability to activate C3. The degree of C3 deposition on the surface of infected cells, detected by fluorescence microscopy, was also greater for cells infected with the RW than the Kilham strain of mumps virus. These studies suggest that the NANase activity of mumps virus can influence the ability of infected cells to activate the alternative pathway and thereby, the ability of complement to participate in host defense against mumps virus infection.

Surface-associated sialic acid is an immunological adjuvant

The influence of neuraminidase on the immunogenicity of heterologous erythrocytes as determined by serum haemagglutination titres was investigated in mice. For this study sheep and rabbit erythrocytes were selected because of their high and low N-acetylneuraminic (sialic) acid content, respectively. Preincubation with neuraminidase resulted in a ten-fold reduction of the immunogenicity of sheep erythrocytes (ShE). By contrast, the immune response to rabbit erythrocytes appeared to be resistant to sialidase treatment. Addition of the extrinsic adjuvant dimethyldioctadecylammonium bromide largely restored the immunogenicity of neuraminidase-treated ShE, but did not change the response to control-treated ShE. The maximal antibody level induced by neuraminidase-treated ShE was lower than that provoked by control ShE. These results suggest that sialic acid is both an intrinsic immunological adjuvant and an antigenic determinant of ShE. The adjuvant effect of sialic acid does not depend on complement component C3 as judged by the response of cobra venom factor-pretreated animals. In genetically C5-deficient and in nude mice, however, sialic acid showed diminished and absent adjuvant activity, respectively.

Lung alterations in guinea-pigs infected with influenza virus

Guinea-pigs were infected intranasally with influenza A Hong Kong 68 (H3N2) virus. Infective particles were re-isolated from lung homogenates up to 3 days after inoculation and indicated local replication. The subsequent lung inflammatory stages were studied by light microscopy, scanning and transmission electron microscopy (TEM). Lung alterations appeared after 24 h and intensified up to 7 days after virus inoculation, progressively decreasing until 3 weeks thereafter. The damage was reversible and complete restoration of structure was obtained within 5 weeks. The lesions commenced with the infiltration of bronchiolar and alveolar walls by polymorphonuclear cells, histiocytes and macrophages. A purulent exudate was seen to occupy the bronchiolar lumen. Cilia disappeared from tracheal and bronchiolar epithelia. Tracheal epithelium desquamated in some animals. TEM examination showed deterioration in type I pneumocytes, an increase in type II pneumocytes and concomitant damage to alveolar capillaries. Alveolar oedema and fibrinous deposits were seen. The pleura presented slight modifications. These results show that infection of guinea-pigs with influenza virus is a useful model for the study of lung pathology associated with a non-lethal respiratory viral infection.

Development and application of an enzyme-linked immunosorbent assay for the quantitation of alternative complement pathway activation in human serum

We have developed a new, specific, and highly sensitive enzyme-linked immunosorbent assay (ELISA) which quantitates activation of the alternative pathway in human serum, plasma, or on the surface of activators. The ELISA detects the third component of complement (C3b), proteolytic fragment of complement Factor B (Bb), and properdin (P) complex or its derivative product, C3b,P. In the method, activator-plasma mixtures, plasma containing an activated alternative pathway, or other samples are added to the wells of microtitration plates precoated with antibody to P. C3b, Bb,P or C3b,P complexes which become bound are quantitated by subsequently added, enzyme-labeled, anti-C3. The resulting hydrolysis of the chromogenic substrate is expressed as nanograms of C3b by reference to a C3 standard curve. In addition to absolute specificity for activation of the pathway because of the nature of the complex detected by the assay, the ELISA is highly sensitive and able to reproducibly detect 10-20 ng/ml of C3b,P complexes in serum. This value corresponds to 0.0015% of the C3 in serum. In a series of studies to validate the parameters of the ELISA, reactivity was found to be dependent on the presence of alternative pathway proteins, the functional integrity of the pathway, and on the presence of magnesium. Sheep erythrocytes were converted to activators by treatment with neuraminidase. By using a variety of activators, the kinetics of activation and the numbers of bound C3b molecules quantitated by the ELISA were very similar to those measured by C3b deposition. The ELISA also detected identical activation kinetics when MgEGTA-serum and a mixture of the purified alternative pathway proteins were used as sources of the pathway. ELISA reaction kinetics also correlated with the restriction index, a measure of alternative pathway-activating ability. These studies cumulatively validate the ELISA as a direct and quantitative assay for alternative pathway activation. The sensitivity of the ELISA has permitted its use to detect direct alternative pathway activation by several viruses. The ELISA has also shown that certain classical pathway activators trigger the amplification loop of the alternative pathway while others do not. In addition, stable ELISA reactive complexes appeared in the supernatant of mixtures of serum with certain, but not other activators. The ability of the ELISA to detect activation which has already occurred and the stability of the reactive complexes permits studies of clinical sera. Normal human sera (20) contained low levels (5-20 ng/ml) of ELISA-reactive complexes. A proportion of sera from individuals with the adult respiratory distress syndrome (9-10), typhoid fever (8-10), malaria (3-5), gram-negative sepsis (9 of 47), acute trauma and shock (6 f 25), and systemic lupus erythematosus (3 of 29) showed elevated levels of complexes reactive in the alternative pathway ELISA. In contrast, nine sera from patients with circulating C3 nephritic factor were not reactive in the ELISA.

Isolation and characterization of type III group B streptococcal mutants defective in biosynthesis of the type-specific antigen

Four classes of mutants of type III group B streptococcus were isolated by serial subculture of the wild-type strain in the presence of type III-specific rabbit antiserum. Class I mutants no longer synthesized sialic acid but still elaborated the core antigen. Class II mutants maintained the ability to synthesize sialic acid but could not attach it to the core antigen. Class III mutants did not produce the core antigen but still synthesized intracellular sialic acid. Class IV mutants synthesized the complete antigen; however, only approximately 4% of the antigen synthesized was found associated with the cell wall peptidoglycan (in the wild-type strain greater than 85% of the antigen synthesized is covalently attached to the cell wall peptidoglycan), whereas greater than 90% of the antigen was secreted into the growth medium. Production of other components (CAMP factor, group B antigen, beta-hemolysin, neuraminidase) by these mutants appeared similar to those of the wild-type strain. Mouse lethality studies of these strains indicated that all four classes have greater than 3 log10-higher 50% lethal dose values than that of the wild-type strain. To understand the basis for this variation, the invasive ability of the wild-type strain and the sialic acid-deficient mutant strain M-10 (class I) was examined. Mice received 10(5) CFU of each organism; they were then sacrificed at various times postinoculation, and viable group B streptococci from different organs were enumerated. Mice were able to clear M-10 more efficiently, with greater than 80% of M-10 cells being phagocytized by macrophages within 1 h, whereas the wild-type strain was able to evade phagocytic killing and disseminate to other tissues. These data, therefore, strongly indicate that the sialic acid moiety greatly enhances the virulence of the type III antigen. In addition, the level of cell-associated type-specific antigen appears to contribute significantly to the pathogenicity of the organism.

Natural immunity to Sindbis virus is influenced by host tissue sialic acid content

Recent studies have shown that the sialic acid content of Sindbis virus influences both its ability to active the alternative pathway in vitro and its susceptibility to complement dependent clearance from the bloodstream in vivo. Other studies have shown that the sialic acid content of Sindbis virus is determined by the host in which it is propagated. Because individuals vary in their cell surface sialic acid content, it is possible they also vary in their ability to defend themselves against Sindbis virus infection by virtue of their ability to modify the virus sialic acid content and thereby the capacity of the virus to activate the alternative pathway. To test this hypothesis, outbred Swiss mice were injected subcutaneously with Sindbis virus. There was a significant positive correlation between the level of viremia 18 hr after infection and the sialic acid content of the host's erythrocytes. In addition, animals with erythrocyte sialic acid levels equal to or greater than the mean had a higher level of viremia than animals with erythrocyte sialic acid levels less than the mean. Finally, animals that had muscle sialic acid levels equal to or greater than the mean had a higher incidence of viremia than animals with muscle sialic acid levels less than the mean. These studies suggest that the amount of tissue sialic acid in an individual host influences its ability to resist Sindbis virus infection.

Complement, viruses, and virus-infected cells

The attachment of specific antibody to viral glycoproteins and other structures on the surface of a virus or virus-infected cell has a number of potential consequences to the virus or virus-infected cell. Antibody is multivalent and thus able to redistribute or patch surface viral proteins or virus-encoded structures within the lipid bilayer of the viral envelope or the cell membrane. In certain instances, antibody may agglutinate viruses or virus-infected cells. The physical presence of antibody molecules on the virus surface may interfere with the ability of the virus to infect potentially susceptible cells. Antibody on the surface of virus-infected cells may prevent the maturation and release of virus particles; antibody also can alter certain normal cell functions. The Fc portions of antibody molecules bound to virus-infected cells facilitate interactions with effector cells bearing Fc receptors. In the case of lymphocytes and perhaps phagocytic cells, this interaction may lead to antibody-dependent cellular cytotoxicity (ADCC) [51, 58]. The exposed Fc regions may also facilitate attempts at ingestion by monocytes, macrophages, and polymorphonuclear leukocytes.