Involvement of the complement system in the protection of mice from challenge with respiratory syncytial virus Long strain following passive immunization with monoclonal antibody 18A2B2

Protective Non-neutralizing anti-N-terminal Domain mAb Maintains Fc-mediated Function against SARS-COV-2 Variants up to BA.2.86-JN.1 with Superfluous In Vivo Protection against JN.1 Due to Attenuated Virulence

Substantial evidence supports that Fc-mediated effector functions of anti-spike Abs contribute to anti-SARS-Cov-2 protection. We have previously shown that two non-neutralizing but opsonic mAbs targeting the receptor-binding domain and N-terminal domain (NTD), Ab81 and Ab94, respectively, are protective against lethal Wuhan SARS-CoV-2 infection in K18-hACE2 mice. In this article, we investigated whether these protective non-neutralizing Abs maintain Fc-mediated function and Ag binding against mutated SARS-CoV-2 variants. Ab81 and Ab94 retained their nanomolar affinity and Fc-mediated function toward Omicron and its subvariants, such as BA.2, BA.4, BA.5, XBB, XBB1.5, and BQ1.1. However, when encountering the more heavily mutated BA.2.86, Ab81 lost its function, whereas the 10 new mutations in the NTD did not affect Ab94. In vivo experiments with Ab94 in K18-hACE2 mice inoculated with a stringent dose of 100,000 PFU of the JN.1 variant revealed unexpected results. Surprisingly, this variant exhibited low disease manifestation in this animal model with no weight loss or death in the control group. Still, assessment of mice using a clinical scoring system showed better protection for Ab94-treated mice, indicating that Fc-mediated functions are still beneficial. Our work shows that a protective anti-receptor-binding domain non-neutralizing mAb lost reactivity when BA.2.86 emerged, whereas the anti-NTD mAb was still functional. Finally, this work adds new insight into the evolution of the SARS-CoV-2 virus by reporting that JN.1 is substantially less virulent in vivo than previous strains.

A molecular perspective for the development of antibodies against the human respiratory syncytial virus

The human respiratory syncytial virus (hRSV) is the leading etiologic agent causing respiratory infections in infants, children, older adults, and patients with comorbidities. Sixty-seven years have passed since the discovery of hRSV, and only a few successful mitigation or treatment tools have been developed against this virus. One of these is immunotherapy with monoclonal antibodies against structural proteins of the virus, such as Palivizumab, the first prophylactic approach approved by the Food and Drug Administration (FDA) of the USA. In this article, we discuss different strategies for the prevention and treatment of hRSV infection, focusing on the molecular mechanisms against each target that underly the rational design of antibodies against hRSV. At the same time, we describe the latest results regarding currently approved therapies against hRSV and the challenges associated with developing new candidates.

New Developments and Challenges in Antibody-Based Therapies for the Respiratory Syncytial Virus

Since the discovery of the human respiratory syncytial virus (hRSV), multiple research efforts have been conducted to develop vaccines and treatments capable of reducing the risk of severe disease, hospitalization, long-term sequelae, and death from this pathogen in susceptible populations. In this sense, therapies specifically directed against hRSV are mainly based on monoclonal and polyclonal antibodies such as intravenous IgG (IVIG)-RSV and the monoclonal antibody palivizumab. However, these therapies are associated with significant limitations, including the need for the recruitment of a high number of convalescent volunteers who donate blood to procure IVIG-RSV and the costs associated with the need for repeated administrations of palivizumab. These limitations render this product not cost-effective for populations other than high-risk patients. These problems have underscored that it is still necessary to identify new safe and effective therapies for human use. However, these new therapies must benefit from a comparatively cheap production cost and the opportunity to be available to the high-risk population and anyone who requires treatment. Here, we review the different antibodies used to prevent the pathology caused by hRSV infection, highlighting therapies currently approved for human use and their clinical value. Also, the new, most promising candidates based on preclinical studies and clinical trial results are revised.

Correlates of Protection Against Respiratory Syncytial Virus Infection in Infancy

The highest morbidity and mortality from respiratory syncytial virus (RSV) infection occurs in young infants. Immunization of expectant mothers during pregnancy has the potential to substantially reduce the burden of RSV disease in a majority of infants. Correlates of protection (COP) are important in guiding the development of maternal RSV vaccines and the design of maternal RSV vaccine trials, as immune response to candidate vaccines should mirror protective RSV immunity at birth. Here, we review the literature reporting correlations between RSV immune measures at birth and clinical RSV outcomes during infancy. Less than a dozen studies have investigated immunological COP with RSV disease or related hospitalization, yielding inconsistent findings overall. The differences in findings between studies could be due to differences in inclusion/exclusion criteria (e.g., the inclusion of older infants who may benefit less from maternal antibodies or infants followed during inter-seasonal periods where RSV is absent), differences in semi-quantitative RSV antibody neutralization assays, or differences in RSV outcome measures such as the sensititivity/specificity of diagnostic tests. Future research in this field should seek to standardize RSV immunological measures and outcomes, expand the breadth of functional RSV measures beyond antibody neutralization, and consider infants' age and seasonality of RSV infection.

Inducing broad-based immunity against viruses with pandemic potential

The brutal toll of another viral pandemic can be blunted by investing now in research that uncovers mechanisms of broad-based immunity so we may have vaccines and therapeutics at the ready. We do not know exactly what pathogen may trigger the next wave or next pandemic. We do know, however, that the human immune system must respond and must be bolstered with effective vaccines and other therapeutics to preserve lives and livelihoods. These countermeasures must focus on features conserved among families of pathogens in order to be responsive against something yet to emerge. Here, we focus on immunological approaches to mitigate the impact of the next emerging virus pandemic by developing vaccines that elicit both broadly protective antibodies and T cells. Identifying human immune mechanisms of broad protection against virus families with pandemic potential will be our best defense for humanity in the future.

Upper and lower respiratory tract correlates of protection against respiratory syncytial virus following vaccination of nonhuman primates

Respiratory syncytial virus (RSV) infection is a major cause of respiratory illness in infants and the elderly. Although several vaccines have been developed, none have succeeded in part due to our incomplete understanding of the correlates of immune protection. While both T cells and antibodies play a role, emerging data suggest that antibody-mediated mechanisms alone may be sufficient to provide protection. Therefore, to map the humoral correlates of immunity against RSV, antibody responses across six different vaccines were profiled in a highly controlled nonhuman primate-challenge model. Viral loads were monitored in both the upper and lower respiratory tracts, and machine learning was used to determine the vaccine platform-agnostic antibody features associated with protection. Upper respiratory control was associated with virus-specific IgA levels, neutralization, and complement activity, whereas lower respiratory control was associated with Fc-mediated effector mechanisms. These findings provide critical compartment-specific insights toward the rational development of future vaccines.

A morphological transformation in respiratory syncytial virus leads to enhanced complement deposition

The complement system is a critical host defense against infection, playing a protective role that can also enhance disease if dysregulated. Although many consequences of complement activation during viral infection are well established, mechanisms that determine the extent to which viruses activate complement remain elusive. Here, we investigate complement activation by human respiratory syncytial virus (RSV), a filamentous respiratory pathogen that causes significant morbidity and mortality. By engineering a strain of RSV harboring tags on the surface glycoproteins F and G, we are able to monitor opsonization of single RSV particles using fluorescence microscopy. These experiments reveal an antigenic hierarchy, where antibodies that bind toward the apex of F in either the pre- or postfusion conformation activate the classical pathway whereas other antibodies do not. Additionally, we identify an important role for virus morphology in complement activation: as viral filaments age, they undergo a morphological transformation which lowers the threshold for complement deposition through changes in surface curvature. Collectively, these results identify antigenic and biophysical characteristics of virus particles that contribute to the formation of viral immune complexes, and suggest models for how these factors may shape disease severity and adaptive immune responses to RSV.

Functional Features of the Respiratory Syncytial Virus G Protein

Respiratory syncytial virus (RSV) is a major cause of serious lower respiratory tract infections in children <5 years of age worldwide and repeated infections throughout life leading to serious disease in the elderly and persons with compromised immune, cardiac, and pulmonary systems. The disease burden has made it a high priority for vaccine and antiviral drug development but without success except for immune prophylaxis for certain young infants. Two RSV proteins are associated with protection, F and G, and F is most often pursued for vaccine and antiviral drug development. Several features of the G protein suggest it could also be an important to vaccine or antiviral drug target design. We review features of G that effect biology of infection, the host immune response, and disease associated with infection. Though it is not clear how to fit these together into an integrated picture, it is clear that G mediates cell surface binding and facilitates cellular infection, modulates host responses that affect both immunity and disease, and its CX3C aa motif contributes to many of these effects. These features of G and the ability to block the effects with antibody, suggest G has substantial potential in vaccine and antiviral drug design.

In the Crosshairs: RNA Viruses OR Complement?

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

Innate Immune Components that Regulate the Pathogenesis and Resolution of hRSV and hMPV Infections

The human respiratory syncytial virus (hRSV) and human Metapneumovirus (hMPV) are two of the leading etiological agents of acute lower respiratory tract infections, which constitute the main cause of mortality in infants. However, there are currently approved vaccines for neither hRSV nor hMPV. Moreover, despite the similarity between the pathology caused by both viruses, the immune response elicited by the host is different in each case. In this review, we discuss how dendritic cells, alveolar macrophages, neutrophils, eosinophils, natural killer cells, innate lymphoid cells, and the complement system regulate both pathogenesis and the resolution of hRSV and hMPV infections. The roles that these cells play during infections by either of these viruses will help us to better understand the illnesses they cause. We also discuss several controversial findings, relative to some of these innate immune components. To better understand the inflammation in the lungs, the role of the respiratory epithelium in the recruitment of innate immune cells is briefly discussed. Finally, we review the main prophylactic strategies and current vaccine candidates against both hRSV and hMPV.

Double-edged role of natural killer cells during RSV infection

Natural killer cells play a vital role in the rejection of tumors and pathogen-infected cells. NK cells are indispensable in the early immune response against viral infections by directly targeting infected cells. Furthermore, NK cells influence adaptive immunity by driving virus-specific T-cell responses. Respiratory syncytial virus, a highly contagious virus that causes bronchiolitis, is the main reason for mortality in infants and elderly patients. RSV infection triggers both innate and adaptive immune responses. However, immunity against RSV is ephemeral due to the impaired development of immunological memory. The role of NK cells during RSV infection remains ambiguous. NK cells play a dual role in RSV infection; initially, their role is a protective one as they utilize their intrinsic cytotoxicity, followed by a detrimental one that induces lung injury due to the inhibition of antibody responses and the secretion of pro-inflammatory factors. Noteworthy, IFN-γ released from NK cells play a critical role in promoting a shift to adaptive responses and inhibiting antibody responses in neonates. Indeed, NK cells have a pro-inflammatory and inhibitory role rather than a cytotoxic one that contributes to the severity of the disease. Therapeutic options, including DNA-protein-based vaccines, synthetic peptides, and attenuated strains, are presently under tests. However, there is a need for effective strategies to augment NK cell activity and circumvent the pro-inflammatory activity to benefit the host. In this review, we focused on the role played by NK cells in the immune response and its outcome on the immunopathogenesis of RSV disease.

Fc-Mediated Antibody Effector Functions During Respiratory Syncytial Virus Infection and Disease

Respiratory syncytial virus (RSV) is a major cause of severe lower respiratory tract infections and hospitalization in infants under 1 year of age and there is currently no market-approved vaccine available. For protection against infection, young children mainly depend on their innate immune system and maternal antibodies. Traditionally, antibody-mediated protection against viral infections is thought to be mediated by direct binding of antibodies to viral particles, resulting in virus neutralization. However, in the case of RSV, virus neutralization titers do not provide an adequate correlate of protection. The current lack of understanding of the mechanisms by which antibodies can protect against RSV infection and disease or, alternatively, contribute to disease severity, hampers the design of safe and effective vaccines against this virus. Importantly, neutralization is only one of many mechanisms by which antibodies can interfere with viral infection. Antibodies consist of two structural regions: a variable fragment (Fab) that mediates antigen binding and a constant fragment (Fc) that mediates downstream effector functions via its interaction with Fc-receptors on (innate) immune cells or with C1q, the recognition molecule of the complement system. The interaction with Fc-receptors can lead to killing of virus-infected cells through a variety of immune effector mechanisms, including antibody-dependent cell-mediated cytotoxicity (ADCC) and antibody-dependent cellular phagocytosis (ADCP). Antibody-mediated complement activation may lead to complement-dependent cytotoxicity (CDC). In addition, both Fc-receptor interactions and complement activation can exert a broad range of immunomodulatory functions. Recent studies have emphasized the importance of Fc-mediated antibody effector functions in both protection and pathogenesis for various infectious agents. In this review article, we aim to provide a comprehensive overview of the current knowledge on Fc-mediated antibody effector functions in the context of RSV infection, discuss their potential role in establishing the balance between protection and pathogenesis, and point out important gaps in our understanding of these processes. Furthermore, we elaborate on the regulation of these effector functions on both the cellular and humoral side. Finally, we discuss the implications of Fc-mediated antibody effector functions for the rational design of safe and effective vaccines and monoclonal antibody therapies against RSV.

Patterns of antibody response during natural hRSV infection: insights for the development of new antibody-based therapies

INTRODUCTION

The human respiratory syncytial virus (hRSV) is the main cause of acute lower respiratory tract infection in susceptible population worldwide, such as young children and the elderly. Although hRSV is a major public health burden, there are no licensed vaccines and the only available therapy is palivizumab. During life, reinfections with hRSV are common, suggesting that the virus can impair the development of an efficient host immune response. This feature has hindered the development of efficient therapies.

AREAS COVERED

This article focuses on research about the natural development of antibodies in humans after the exposure to hRSV. The difficulties of developing anti-hRSV therapies based on monoclonal antibodies have been recently associated to the relationship between the disease outcome and the pattern of antibody response.

EXPERT OPINION

Development of monoclonal antibodies is a potentially successful approach to prevent the population from suffering severe respiratory diseases caused by hRSV infection, for which there are no available vaccines. Although the use of palivizumab is safe, its effectiveness is controversial. Recent data have prompted research to develop therapies targeting alternative viral antigens, rather than focusing only on the F protein, as well as the development of antibodies with a cell-mediated function.

Monoclonal Antibody against G Glycoprotein Increases Respiratory Syncytial Virus Clearance In Vivo and Prevents Vaccine-Enhanced Diseases

Respiratory syncytial virus (RSV) is a common cause of lower respiratory tract illness in infants, young children, and the elderly. The G glycoprotein plays a role in host cell attachment and also modulates the host immune response, thereby inducing disease pathogenesis. We generated two monoclonal antibodies (mAbs; 5H6 and 3A5) against G protein core fragment (Gcf), which consisted of amino acid residues 131 to 230 from RSV A2 G protein. Epitope mapping study revealed that 5H6 specifically binds to the G/164-176 peptide that includes conserved sequences shared by both RSV A and B subtypes, and 3A5 binds to the G/190-204 peptide. Studies with mutant Gcf proteins in which cysteine residues were substituted with alanine revealed that 5H6 requires four cysteines for binding and 3A5 binds to Gcf variants with alanine substitutions better than wild-type. To determine if these mAbs reduce pulmonary viral infection, BALB/c mice were administered mAb and subsequently challenged with RSV. On day 4 post-infection, lung viral titers were reduced by up to 93% with the 5H6 injection and 90% with the 3A5 injection, indicating that prophylactic injection of these mAbs contributes to RSV clearance in vivo. Importantly, 5H6 injection reduced vaccine-enhanced diseases. Overall, our results suggest that this novel anti-G mAb could be used as a prophylactic regimen against RSV diseases.

In Hot Pursuit of the First Vaccine Against Respiratory Syncytial Virus

Human respiratory syncytial virus (RSV) is the leading cause of severe lower respiratory tract infection, such as bronchiolitis, bronchitis, or pneumonia, in both infants and the elderly. Despite the global burden of diseases attributable to RSV infection, no clinically approved vaccine is available, and a humanized monoclonal antibody for prophylaxis is not readily affordable in developing countries. There are several hurdles to the successful development of RSV vaccines: immune-vulnerable target populations such as premature infants, pregnant women, and immunocompromised people; safety concerns associated with vaccine-enhanced diseases; repeated infection; and waning memory. To develop successful strategies for the prevention of RSV infection, it is necessary to understand the protective and pathologic roles of host immune responses to RSV infection. In this review, we will summarize the positive and negative relationship between RSV infection and host immunity and discuss strategies for the development of the first successful RSV vaccine.

Functions of Antibodies

Antibodies can impact pathogens in the presence or in the absence of effector cells or effector molecules such as complement, and experiments can often sort out with precision the mechanisms by which an antibody inhibits a pathogen in vitro . In addition, in vivo models, particularly those engineered to knock in or knock out effector cells or effector molecules, are excellent tools for understanding antibody functions. However, it is highly likely that multiple antibody functions occur simultaneously or sequentially in the presence of an infecting organism in vivo . The most critical incentive for measuring antibody functions is to provide a basis for vaccine development and for the development of therapeutic antibodies. In this respect, some functions, such as virus neutralization, serve to inhibit the acquisition of a pathogen or limit its pathogenesis. However, antibodies can also enhance replication or contribute to pathogenesis. This review emphasizes those antibody functions that are potentially beneficial to the host. In addition, this review will focus on the effects of antibodies on organisms themselves, rather than on the toxins the organisms may produce.

Generation and epitope mapping of a sub-group cross-reactive anti-respiratory syncytial virus G glycoprotein monoclonal antibody which is protective in vivo

Passively administered antibodies to conserved epitopes on the attachment (G) glycoprotein of human respiratory syncytial virus (hRSV) have potential in the immunoprophylaxis of human infections. This study set out to generate monoclonal antibodies (MAbs) recognizing all prevalent lineages of HRSV and capable of immunoprophylaxis in mice. Two murine MAbs of broad specificity for prevalent virus strains were generated by immunization of mice with hRSV of sub-group A followed by selection of hybridomas on recombinant G glycoprotein from a sub-group B virus. The anti-G hybridomas generated secreted antibody of high affinity but negligible neutralizing capacity one of which was tested in mice and found to be protective against live virus challenge. Western blotting and partial epitope mapping on transiently expressed G-glycoprotein fragments indicate that these antibodies recognize a complex epitope on the protein backbone of the molecule involving residues both C'- and N-terminal to the central conserved motif.

Influence of respiratory syncytial virus strain differences on pathogenesis and immunity

Molecular epidemiology studies have provided convincing evidence of antigenic and sequence variability among respiratory syncytial virus (RSV) isolates. Circulating viruses have been classified into two antigenic groups (A and B) that correlate with well-delineated genetic groups. Most sequence and antigenic differences (both inter- and intra-groups) accumulate in two hypervariable segments of the G-protein gene. Sequences of the G gene have been used for phylogenetic analyses. These studies have shown a worldwide distribution of RSV strains with both local and global replacement of dominant viruses with time. Although data are still limited, there is evidence that strain variation may contribute to differences in pathogenicity. In addition, there is some but limited evidence that RSV variation may be, at least partially, immune (antibody) driven. However, there is the paradox in RSV that, in contrast to other viruses (e.g., influenza viruses) the epitopes recognized by the most effective RSV-neutralizing antibodies are highly conserved. In contrast, antibodies that recognize strain-specific epitopes are poorly neutralizing. It is likely that this apparent contradiction is due to the lack of a comprehensive knowledge of the duration and specificities of the human antibody response against RSV antigens. Since there are some data supporting a group- (or clade-) specific antibody response after a primary infection in humans, it may be wise to consider the incorporation of strains representative of groups A and B (or their antigens) in future RSV vaccine development.

Newcastle disease virus-like particles containing respiratory syncytial virus G protein induced protection in BALB/c mice, with no evidence of immunopathology

Respiratory syncytial virus (RSV) is the leading cause of serious respiratory infections in children as well as a serious cause of disease in elderly and immunosuppressed populations. There are no licensed vaccines available to prevent RSV disease. We have developed a virus-like particle (VLP) vaccine candidate for protection from RSV. The VLP is composed of the NP and M proteins of Newcastle disease virus (NDV) and a chimeric protein containing the cytoplasmic and transmembrane domains of the NDV HN protein and the ectodomain of the human RSV G protein (H/G). Immunization of mice with 10 or 40 microg total VLP-H/G protein by intraperitoneal or intramuscular inoculation stimulated antibody responses to G protein which were as good as or better than those stimulated by comparable amounts of UV-inactivated RSV. Immunization of mice with two doses or even a single dose of these particles resulted in the complete protection of mice from RSV replication in the lungs. Immunization with these particles induced neutralizing antibodies with modest titers. Upon RSV challenge of VLP-H/G-immunized mice, no enhanced pathology in the lungs was observed, although lungs of mice immunized in parallel with formalin-inactivated RSV (FI-RSV) showed the significant pathology that has previously been documented after immunization with FI-RSV. Thus, the VLP-H/G candidate vaccine was immunogenic in BALB/c mice and prevented replication of RSV in murine lungs, with no evidence of immunopathology. These data support further development of virus-like particle vaccine candidates for protection against RSV.

Treatment with respiratory syncytial virus G glycoprotein monoclonal antibody or F(ab')2 components mediates reduced pulmonary inflammation in mice

Therapeutic treatment with a non-neutralizing monoclonal antibody (mAb) (131-2G) specific to respiratory syncytial virus (RSV) G glycoprotein mediates virus clearance and decreases leukocyte trafficking and interferon gamma (IFN-gamma) production in the lungs of RSV-infected mice. Its F(ab')(2) component only mediates decreased leukocyte trafficking and IFN-gamma production without reducing virus replication. Thus, this mAb has two independent actions that could facilitate treatment and/or prevention of RSV infection by reducing both virus replication and virus-induced pulmonary inflammation.

Protection of mice against Human respiratory syncytial virus by wild-type and aglycosyl mouse–human chimaeric IgG antibodies to subgroup-conserved epitopes on the G glycoprotein

Monoclonal antibodies (mAbs) to conserved epitopes on the G glycoprotein of human respiratory syncytial virus (HRSV) subgroup A fail to neutralize the virus in cell culture in the absence of complement, but are protective in rodent models of infection. They may have potential as prophylactic agents in human infants. In order to investigate the role of Fc-dependent pathways in protection by one such antibody, 1C2, the VH and VL genes were isolated by RT-PCR and assembled with human κ light-chain and human γ1 heavy-chain constant-region genes to form two mouse–human chimaeras, which were expressed in NS0 cells. One of the chimaeras carried a wild-type γ1 chain, whilst the other had an aglycosyl mutation in the CH2 domain rendering the antibody defective in complement activation and FcγR binding. Whilst both chimaeric antibodies exhibited similar avidity for HRSV in ELISA, only the fully glycosylated wild type was capable of neutralizing the virus in the presence of complement. In mice passively immunized with either murine or wild-type γ1 chimaeric antibody, no virus could be recovered from the lungs 4 days after intranasal inoculation of HRSV. In mice immunized with the aglycosyl γ1 chimaera, however, virus was present in the lungs following challenge, although virus titres were significantly reduced compared with controls (P<0·005). These results indicate that the protective effect of this antibody is mediated by both Fc-dependent and Fc-independent pathways.

Measurement of antibody against contemporary virus lineages of human respiratory syncytial virus sub-group A in infants and their mothers

BACKGROUND

Human respiratory syncytial virus (hRSV) infects the majority of infants in their first year of life. Maternal antibodies offer some protection although a small proportion of infected infants develop bronchiolitis and require admission to hospital. A number of lineages of the virus co-circulate in the population and the prevalent virus lineage changes from epidemic to epidemic. The effect of antigenic variation between virus lineages upon the protection offered by maternal antibodies has not been assessed.

OBJECTIVES

To explore the possibility that infants may develop bronchiolitis because of a virus lineage-specific deficiency in their maternal antibodies.

STUDY DESIGN

Virus isolates from infants admitted to hospital in Newcastle upon Tyne with hRSV infection during two consecutive winter epidemics were classified into lineages by genotypic analysis. Antibodies to the surface glycoproteins of contemporary sub-group A lineages and to the A2 virus strain were assayed in the acute sera of infected infants, in a group of uninfected infants and in the mothers of both groups.

RESULTS

Four lineages of sub-group A hRSV were found circulating during the study period. Antibody titres measured against all virus lineages in the acute serum of infants with hRSV bronchiolitis were similar. In the uninfected infants and in the mothers of both infected and uninfected groups antibody titres to all four contemporary virus lineages were also similar. However, in these groups antibodies to the A2 virus strain were four-fold lower than those to contemporary isolates.

CONCLUSIONS

Infants admitted to hospital with hRSV bronchiolitis exhibited no apparent selective deficiency in maternal antibodies to the viral glycoproteins of the infecting virus strain or lineage.

Virus-neutralizing activity mediated by the Fab fragment of a hemagglutinin-specific antibody is sufficient for the resolution of influenza virus infection in SCID mice

Antibodies (Abs) contribute to the control of influenza virus infection in vivo by reducing progeny virus yield from infected cells (yield reduction [YR]) and by inhibiting progeny virus from spreading the infection to new host cells (virus neutralization [VN]). Previous studies showed that the infection could be resolved in severe combined immunodeficiency (SCID) mice by treatment with hemagglutinin (HA)-specific monoclonal antibodies (MAbs) that exhibit both VN and YR activities but not by MAbs that exhibited only YR activity. To determine whether virus clearance requires both activities, we measured the therapeutic activity of an HA-specific MAb (VN and YR) and its Fab fragment (VN) by intranasal (i.n.) administration to infected SCID mice. Immunoglobulin G (IgG) and Fab cleared the infection with i.n. 50% effective doses (ED(50)s) of 16 and 90 pmol, respectively. To resolve an established infection solely by VN activity, Fab must be present in the respiratory tract at an effective threshold concentration until all infected cells have died and production of virus has ceased. Because IgG and Fab had different half-lives in the respiratory tract (22 and 8 h, respectively) and assuming that both operated mainly or solely by VN, it could be estimated that clearance was achieved 24 h after Ab treatment when both reagents were present in the respiratory tract at approximately 10 pmol. This dose was approximately 200 times larger than the respiratory tract-associated Ab dose resulting from administration of the intraperitoneal ED(50) (270 pmol) of IgG. This indicated that our procedure of i.n. administration of Ab did not make optimal use of the Ab's therapeutic activity.

Passive transfer of serum antibodies induced by BBG2Na, a subunit vaccine, in the elderly protects SCID mouse lungs against respiratory syncytial virus challenge

Respiratory syncytial virus (RSV) is responsible for severe low respiratory tract infections in young infants and the elderly. To investigate whether BBG2Na, a recombinant subunit vaccine comprising aa 130-230 of the RSV G protein, induced protective Abs in subjects over 60 years during phase II clinical trial, pre- and postimmunization sera of individuals immunized with BBG2Na or placebo were transferred into SCID mice before RSV challenge. These sera dose-dependently reduced lung RSV titers. However at some points of serial dilutions, postimmunization sera of BBG2Na-immunized subjects only were significantly more efficient than the corresponding preimmunization sera, in agreement with the induction of an increased Ab response against multiple epitopes on RSV-A G protein. Thus, BBG2Na is immunogenic in the elderly and confers passive protection in mice after serum transfer. To our knowledge, this is the first description of protective Abs induced by a subunit vaccine in human.

Additive effects characterize the interaction of antibodies involved in neutralization of the primary dualtropic human immunodeficiency virus type 1 isolate 89.6

Human immunodeficiency virus-type I (HIV-1) infection elicits antibodies (Abs) directed against several regions of the gp120 and gp41 envelope glycoproteins. Many of these Abs are able to neutralize T-cell-line-adapted strains (TCLA) of HIV-1, but only a few effectively neutralize primary HIV-1 isolates. The nature of HIV-1 neutralization has been carefully studied using human monoclonal Abs (MAbs), and the ability of such MAbs to act in synergy to neutralize HIV-1 has also been extensively studied. However, most synergy studies have been conducted using TCLA strains. To determine the nature of Ab interaction in HIV-1 primary isolate neutralization, a panel of 12 anti-HIV-1 human immunoglobulin G (IgG) MAbs, specific for epitopes in gp120 and gp41, were used. Initial tests showed that six of these MAbs, as well as sCD4, used individually, were able to neutralize the dualtropic primary isolate HIV-1(89.6); MAbs giving significant neutralization at 2 to 10 microg/ml included 2F5 (anti-gp41), 50-69 (anti-gp41), IgG1b12 (anti-gp120(CD4bd)), 447-52D (anti-gp120(V3)), 2G12 (anti-gp120), and 670-D (anti-gp120(C5)). For studies of reagent interaction, 16 binary combinations of reagents were tested for their ability to neutralize HIV-1(89.6). Reagent combinations tested included one neutralizing MAb with sCD4, six pairs consisting of two neutralizing MAbs, and nine pairs consisting of one neutralizing MAb with another non-neutralizing MAb. To assess the interaction of the latter type of combination, a new mathematical treatment of reagent interaction was developed since previously used methods could be used only when both reagents neutralize. Synergy was noted between sCD4 and a neutralizing anti-gp120(V3) MAb. Antagonism was noted between two pairs of anti-gp41 MAbs (one neutralizing and one non-neutralizing). All of the other 13 pairs of MAbs tested displayed only additive effects. These studies suggest that Abs rarely act in synergy to neutralize primary isolate HIV-1(89.6); many anti-HIV-1 Abs act additively to mediate this biological function.

The antiviral activity of antibodies in vitro and in vivo

This chapter discusses in vitro and in vivo antiviral activities of antibody. Since experimentation is far easier in vitro , researchers have been sought to develop in vitro assays that are expected to predict activity in vivo . This could be important in both vaccine design and in passive antibody administration. The proposed mechanisms of in vitro neutralization range from those requiring binding of a single antibody molecule to virus to those requiring substantially complete antibody coating of virus. In vitro, antiviral activity can be separated into activity against virions and activity against infected cells. The activity against virions most often considered is neutralization that can be defined as the loss of infectivity, which ensues when antibody molecule(s) bind to a virus particle, and occurs without the involvement of any other agency. In vivo, it is conventional to distinguish phenomenologically between two types of antibody antiviral activity. One of them is the ability of antibody to protect against infection when it is present before or immediately following infection. Evidence for a number of viruses in vitro indicates that lower antibody concentrations are required to inhibit infection propagated by free virus than are required to inhibit infection propagated by cell-to-cell spread.

Preventing infectious disease with passive immunization

Antibodies can prevent infectious diseases by providing passive immune protection. Here we review successful clinical trials of passive immunization and consider some of the unique qualities monoclonal antibodies are now beginning to offer for developing methods for passive immunization against a wide range of infectious diseases.

Human antibody responses to mature and immature forms of viral envelope in respiratory syncytial virus infection: significance for subunit vaccines

A number of antibodies generated during human respiratory syncytial virus (RSV) infection have been cloned by the phage library approach. Antibodies reactive with an immunodominant epitope on the F glycoprotein of this virus have a high affinity for affinity-purified F antigen. These antibodies, however, have a much lower affinity for mature F glycoprotein on the surface of infected cells and are nonneutralizing. In contrast, a potent neutralizing antibody has a high affinity for mature F protein but a much lower affinity for purified F protein or F protein in viral lysates. The data indicate that at least two F protein immunogens are produced during natural RSV infection: immature F, found in viral lysates, and mature F, found on infected cells or virions. Binding studies with polyclonal human immunoglobulin G suggest that the antibody responses to the two immunogens are of similar magnitudes. Competitive binding studies suggest that overlap between the responses is relatively limited. A mature envelope with an antigenic configuration different from that of the immature envelope has an evolutionary advantage in that the infecting virus is less subject to neutralization by the humoral response to the immature envelope that inevitably arises following lysis of infected cells. Subunit vaccines may be at a disadvantage because they most often resemble immature envelope molecules and ignore this aspect of viral evasion.

An in vitro selected binding protein (affibody) shows conformation-dependent recognition of the respiratory syncytial virus (RSV) G protein

Using phage-display technology, a novel binding protein (Z-affibody) showing selective binding to the RSV (Long strain) G protein was selected from a combinatorial library of a small alpha-helical protein domain (Z), derived from staphylococcal protein A (SPA). Biopanning of the Z-library against a recombinant fusion protein comprising amino acids 130-230 of the G protein from RSV-subgroup A, resulted in the selection of a Z-affibody (Z(RSV1)) which showed G protein specific binding. Using biosensor technology, the affinity (K(D)) between Z(RSV1) and the recombinant protein was determined to be in the micromolar range (10(-6) M). Interestingly, the Z(RSV1) affibody was demonstrated to also recognize the partially (54%) homologous G protein of RSV subgroup B with similar affinity. Using different recombinant RSV G protein derived fragments, the binding was found to be dependent on the presence of the cysteinyl residues proposed to be involved in the formation of an intramolecular disulfide-constrained loop structure, indicating a conformation-dependent binding. Results from epitope mapping studies, employing a panel of monoclonal antibodies directed to different RSV G protein subfragments, suggest that the Z(RSV1) affibody binding site is located within the region of amino acids 164-186 of the G protein. This region contains a 13 amino acid residue sequence which is totally conserved between subgroups A and B of RSV and extends into the cystein loop region (amino acids 173-186). The potential use of the RSV G protein-specific Z(RSV1) affibody in diagnostic and therapeutic applications is discussed.

Immunization with a peptide derived from the G glycoprotein of bovine respiratory syncytial virus (BRSV) reduces the incidence of BRSV-associated pneumonia in the natural host

Previous reports demonstrate that synthetic peptides corresponding to the amino acid region 174-187 of G glycoprotein from subgroups A and B human respiratory syncytial virus (HRSV), containing a Cys-->Ser substitution at position 186, confer complete resistance to immunized BALB/c mice against infection with the respective virus. In this report, we show that a Cys186-->Ser substituted peptide (BG/174-187) representing the corresponding region of the bovine (B) RSV G glycoprotein conferred complete protection of mice against BRSV challenge, suggesting that the 174-187 region of RSV G glycoproteins constitutes a dominant protective epitope which has been maintained throughout evolution. Furthermore, immunization of calves with peptide BG/174-187 efficiently induced the production of antibodies capable of recognizing both the parental G glycoprotein and peptide BG/174-187. Following challenge with live BRSV, although none of the animals were protected from upper respiratory tract disease, there were little or no gross pneumonic lesions in the four peptide-immunized calves. In contrast, moderate to extensive pneumonic lesions were observed in 2 out of 3 calves in the control group. Our results thus suggest that peptide BG/174-187 efficiently prevented BRSV-associated pneumonia in the natural host. The use of this system as a model is quite promising with regard to the development of a human synthetic vaccine.

Subgroup specific protection of mice from respiratory syncytial virus infection with peptides encompassing the amino acid region 174-187 from the G glycoprotein: the role of cysteinyl residues in protection

We identified subgroup specific protective epitopes represented by the amino acid regions 174-187 and 171-187 of the G glycoproteins from respiratory syncytial virus (RSV), subgroups A and B. Mice immunized with coupled synthetic peptides corresponding to either the region 174-187 containing a Cys186-->Ser substitution or to the native region 171-187 were completely resistant to RSV infection but only to the respective virus. The protective activities of the peptides 174-187 were dependent on the Cys186-->Ser substitution. In addition, a recombinant protein representing the region 125-203 of the A subgroup G glycoprotein expressed in Escherichia coli was capable without further treatment to completely protect animals against RSV subgroup A infection. We show that the combinations of cysteinyl residues (positions 173, 176, 182, and 186) retained within either synthetic peptides or the recombinant protein G125-203 greatly influenced their protective activities. This indicates that the region 171-187 is essential for the protection conferred by the G125-203 protein. Furthermore, our results strongly suggest that the peptides' and recombinant protein's potencies are a function of a loop-like structure which is stabilized by intramolecular disulfide linkages between Cys176-Cys182 and Cys173-Cys186. This is further supported by the observation that chemical blocking of the sulfidryl groups in synthetic peptides completely eliminated their protective activity.