Hepatitis C patient-derived glycoproteins exhibit marked differences in susceptibility to serum neutralizing antibodies: genetic subtype defines antigenic but not neutralization serotype

Neutralizing antibodies have a role in controlling hepatitis C virus (HCV) infection. A successful vaccine will need to elicit potently neutralizing antibodies that are capable of preventing the infection of genetically diverse viral isolates. However, the specificity of the neutralizing antibody response in natural HCV infection still is poorly understood. To address this, we examined the reactivity of polyclonal antibodies isolated from chronic HCV infection to the diverse patient-isolated HCV envelope glycoproteins E1 and E2 (E1E2), and we also examined the potential to neutralize the entry of pseudoparticles bearing these diverse E1E2 proteins. The genetic type of the infection was found to determine the pattern of the antibody recognition of these E1E2 proteins, with the greatest reactivity to homologous E1E2 proteins. This relationship was strongest when the component of the antibody response directed only to linear epitopes was analyzed. In contrast, the neutralization serotype did not correlate with genotype. Instead, serum-derived antibodies displayed a range of neutralization breadth and potency, while different E1E2 glycoproteins displayed different sensitivities to neutralization, such that these could be divided broadly into neutralization-sensitive and -resistant phenotypes. An important additional observation was that entry mediated by some E1E2 proteins was enhanced in the presence of some of the polyclonal antibody fractions isolated during chronic infection. These data highlight the need to use diverse E1E2 isolates, which represent extremes of neutralization sensitivity, when screening antibodies for therapeutic potential and for testing antibodies generated following immunization as part of vaccine development.     

Immunopathogenesis of hepatitis C virus infection

Hepatitis C virus, a recently identified member of the family Flaviviridae, is an important cause of chronic viral hepatitis and cirrhosis. There are similarities in the nature of the immune response to this pathogen with immunity in other flavivirus and hepatotropic virus infections, such as hepatitis B. However, the high rate of viral persistence after primary hepatitis C infection, and the observation that neutralizing antibodies are not protective, would suggest that there are a number of important differences between hepatitis C, other flaviviruses, and hepatitis B. The phenomenon of quasispecies evolution and other viral factors have been proposed to contribute to immune evasion by hepatitis C virus. In the face of established persistent infection, virus-specific cytotoxic T lymphocytes may exert some control over viral replication. However, these same effectors may also be responsible for the progressive liver damage characteristic of chronic hepatitis C infection. The nature of protective immunity, including the role of innate immune responses early after hepatitis C exposure, remains to be defined.     

Rapid assessment of adenovirus serum neutralizing antibody titer based on quantitative, morphometric evaluation of capsid binding and intracellular trafficking: population analysis of adenovirus capsid association with cells is predictive of adenovirus infectivity

Neutralizing antiviral antibodies are typically detected on the basis of inhibition of viral function, such as propagation of a viral infection or inhibition of viral gene expression. Evidence is presented that anti-adenovirus neutralizing antibodies can be evaluated by analysis of cell-associated capsids or by analysis of intracellular trafficking of the capsids within 1 h after infection. Quantitative analyses of these morphologic parameters represent rapid, broadly applicable, functional assays for the detection of anti-adenovirus neutralizing antibodies.     

Detection of antibody-dependent complement-mediated inactivation of both autologous and heterologous virus in primary human immunodeficiency virus type 1 infection

Specific CD8 T-cell responses to human immunodeficiency virus type 1 (HIV-1) are induced in primary infection and make an important contribution to the control of early viral replication. The importance of neutralizing antibodies in containing primary viremia is questioned because they usually arise much later. Nevertheless antienvelope antibodies develop simultaneously with, or even before, peak viremia. We determined whether such antibodies might control viremia by complement-mediated inactivation (CMI). In each of seven patients studied, antibodies capable of CMI appeared at or shortly after the peak in viremia, concomitantly with detection of virus-specific T-cell responses. The CMI was effective on both autologous and heterologous HIV-1 isolates. Activation of the classical pathway and direct viral lysis were at least partly responsible. Since immunoglobulin G (IgG)-antibodies triggered the CMI, specific memory B cells could also be induced by vaccination. Thus, consideration should be given to vaccination strategies that induce IgG antibodies capable of CMI.     

The pathogenesis of Ebola hemorrhagic fever

Ebola virus causes lethal hemorrhagic disease in humans, yet there are still no satisfactory biological explanations to account for its extreme virulence. This review focuses on recent findings relevant to understanding the pathogenesis of Ebola virus infection and developing vaccines and effective therapy. The available data suggest that the envelope glycoprotein and the interaction of some viral proteins with the immune system are likely to play important roles in the extraordinary pathogenicity of this virus. There are also indications that genetically engineered vaccines, including plasmid DNA and viral vectors expressing Ebola virus proteins, and passive transfer of neutralizing antibodies could be feasible options for the control of Ebola virus-associated disease.     

Induction of long-term protective antiviral endogenous immune response by short neutralizing monoclonal antibody treatment

Long-term immune control of viral replication still remains a major challenge in retroviral diseases. Several monoclonal antibodies (MAbs) have already shown antiviral activities in vivo, including in the clinic but their effects on the immune system of treated individuals are essentially unknown. Using the lethal neurodegeneration induced in mice upon infection of neonates by the FrCas(E) retrovirus as a model, we report here that transient treatment by a neutralizing MAb shortly after infection can, after an immediate antiviral effect, favor the development of a strong protective host immune response containing viral propagation long after the MAb has disappeared. In vitro virus neutralization- and complement-mediated cell lysis assays, as well as in vivo viral challenges and serum transfer experiments, indicate a clear and essential contribution of the humoral response to antiviral protection. Our observation may have important therapeutic consequences as it suggests that short antibody-based therapies early after infection should be considered, at least in the case of maternally infected infants, as adjunctive treatment strategies against human immunodeficiency virus, not only for a direct effect on the viral load but also for favoring the emergence of an endogenous antiviral immune response.     

In vitro inhibition of HIV-1 replication in autologous CD4~+ T cells indicates viral containment by multifactorial mechanisms

HIV-1-specific cytotoxic T lymphocytes(CTLs) and neutralizing antibodies(NAbs) are present during chronic infection, but the relative contributions of these effector mechanisms to viral containment remain unclear. Here, using an in vitro model involving autologous CD4+ T cells,primary HIV-1 isolates, HIV-1-specific CTLs, and neutralizing monoclonal antibodies, we show that b12, a potent and broadly neutralizing monoclonal antibody to HIV-1, was able to block viral infection when preincubated with virus prior to infection, but was much less effective than CTLs at limiting virus replication when added to infected cell cultures. However, the same neutralizing antibody was able to contain viruses by antibody-dependent cell-mediated virus inhibition in vitro,which was mediated by natural killer cells(NKs) and dependent on an Fc-Fc receptor interaction.Meanwhile, bulk CTLs from HIV-1 controllers were more effective in suppression of virus replication than those from progressors. These findings indicate that control of HIV-1 replication in activated CD4~+ T cells is ineffectively mediated by neutralizing antibodies alone, but that both CTLs and antibody-dependent NK-mediated immune mechanisms contribute to viral containment. Our study systemically compared three major players in controlling HIV-1 infection, CTLs, NAbs, and NKs, in an autologous system and highlighted the multifactorial mechanisms for viral containment and vaccine success.     

A Crucial Role for Infected-Cell/Antibody Immune Complexes in the Enhancement of Endogenous Antiviral Immunity by Short Passive Immunotherapy

Antiviral monoclonal antibodies (mAbs) represent promising therapeutics. However, most mAbs-based immunotherapies conducted so far have only considered the blunting of viral propagation and not other possible therapeutic effects independent of virus neutralization, namely the modulation of the endogenous immune response. As induction of long-term antiviral immunity still remains a paramount challenge for treating chronic infections, we have asked here whether neutralizing mAbs can, in addition to blunting viral propagation, exert immunomodulatory effects with protective outcomes. Supporting this idea, we report here that mice infected with the FrCas^E murine retrovirus on day 8 after birth die of leukemia within 4–5 months and mount a non-protective immune response, whereas those rapidly subjected to short immunotherapy with a neutralizing mAb survive healthy and mount a long-lasting protective antiviral immunity with strong humoral and cellular immune responses. Interestingly, the administered mAb mediates lysis of infected cells through an antibody-dependent cell cytotoxicity (ADCC) mechanism. In addition, it forms immune complexes (ICs) with infected cells that enhance antiviral CTL responses through FcγR-mediated binding to dendritic cells (DCs). Importantly, the endogenous antiviral antibodies generated in mAb-treated mice also display the same properties, allowing containment of viral propagation and enhancement of memory cellular responses after disappearance of the administered mAb. Thus, our data demonstrate that neutralizing antiviral mAbs can act as immunomodulatory agents capable of stimulating a protective immunity lasting long after the end of the treatment. They also show an important role of infected-cells/antibody complexes in the induction and the maintenance of protective immunity through enhancement of both primary and memory antiviral T-cell responses. They also indicate that targeting infected cells, and not just viruses, by antibodies can be crucial for elicitation of efficient, long-lasting antiviral T-cell responses. This must be considered when designing antiviral mAb-based immunotherapies.     

Persistence of circulating memory B cell clones with potential for dengue virus disease enhancement for decades following infection

Symptomatic dengue virus infection ranges in disease severity from an influenza-like illness to life-threatening shock. One model of the mechanism underlying severe disease proposes that weakly neutralizing, dengue serotype cross-reactive antibodies induced during a primary infection facilitate virus entry into Fc receptor-bearing cells during a subsequent secondary infection, increasing viral replication and the release of cytokines and vasoactive mediators, culminating in shock. This process has been termed antibody-dependent enhancement of infection and has significantly hindered vaccine development. Much of our understanding of this process has come from studies using mouse monoclonal antibodies (MAbs); however, antibody responses in mice typically exhibit less complexity than those in humans. A better understanding of the humoral immune response to natural dengue virus infection in humans is sorely needed. Using a high-efficiency human hybridoma technology, we isolated 37 hybridomas secreting human MAbs to dengue viruses from 12 subjects years or even decades following primary or secondary infection. The majority of the human antibodies recovered were broadly cross-reactive, directed against either envelope or premembrane proteins, and capable of enhancement of infection in vitro; few exhibited serotype-specific binding or potent neutralizing activity. Memory B cells encoding enhancing antibodies predominated in the circulation, even two or more decades following infection. Mapping the epitopes and activity of naturally occurring dengue antibodies should prove valuable in determining whether the enhancing and neutralizing activity of antibodies can be separated. Such principles could be used in the rational design of vaccines that enhance the induction of neutralizing antibodies, while lowering the risk of dengue shock syndrome.     

Viral Escape from HIV-1 Neutralizing Antibodies Drives Increased Plasma Neutralization Breadth through Sequential Recognition of Multiple Epitopes and Immunotypes

Identifying the targets of broadly neutralizing antibodies to HIV-1 and understanding how these antibodies develop remain important goals in the quest to rationally develop an HIV-1 vaccine. We previously identified a participant in the CAPRISA Acute Infection Cohort (CAP257) whose plasma neutralized 84% of heterologous viruses. In this study we showed that breadth in CAP257 was largely due to the sequential, transient appearance of three distinct broadly neutralizing antibody specificities spanning the first 4.5 years of infection. The first specificity targeted an epitope in the V2 region of gp120 that was also recognized by strain-specific antibodies 7 weeks earlier. Specificity for the autologous virus was determined largely by a rare N167 antigenic variant of V2, with viral escape to the more common D167 immunotype coinciding with the development of the first wave of broadly neutralizing antibodies. Escape from these broadly neutralizing V2 antibodies through deletion of the glycan at N160 was associated with exposure of an epitope in the CD4 binding site that became the target for a second wave of broadly neutralizing antibodies. Neutralization by these CD4 binding site antibodies was almost entirely dependent on the glycan at position N276. Early viral escape mutations in the CD4 binding site drove an increase in wave two neutralization breadth, as this second wave of heterologous neutralization matured to recognize multiple immunotypes within this site. The third wave targeted a quaternary epitope that did not overlap any of the four known sites of vulnerability on the HIV-1 envelope and remains undefined. Altogether this study showed that the human immune system is capable of generating multiple broadly neutralizing antibodies in response to a constantly evolving viral population that exposes new targets as a consequence of escape from earlier neutralizing antibodies.     

Cytomegalovirus Vaccine Strain Towne-Derived Dense Bodies Induce Broad Cellular Immune Responses and Neutralizing Antibodies That Prevent Infection of Fibroblasts and Epithelial Cells

Human cytomegalovirus (HCMV), a betaherpesvirus, can cause severe disease in immunosuppressed patients and following congenital infection. A vaccine that induces both humoral and cellular immunity may be required to prevent congenital infection. Dense bodies (DBs) are complex, noninfectious particles produced by HCMV-infected cells and may represent a vaccine option. As knowledge of the antigenicity and immunogenicity of DB is incomplete, we explored characterization methods and defined DB production methods, followed by systematic evaluation of neutralization and cell-mediated immune responses to the DB material in BALB/c mice. DBs purified from Towne-infected cultures treated with the viral terminase inhibitor 2-bromo-5,6-dichloro-1-beta-d-ribofuranosyl benzimidazole riboside (BDCRB) were characterized by nanoparticle tracking analysis (NTA), two-dimensional fluorescence difference gel electrophoresis (2D-DIGE), immunoblotting, quantitative enzyme-linked immunosorbent assay, and other methods. The humoral and cellular immune responses to DBs were compared to the immunogenicity of glycoprotein B (gB) administered with the adjuvant AddaVax (gB/AddaVax). DBs induced neutralizing antibodies that prevented viral infection of cultured fibroblasts and epithelial cells and robust cell-mediated immune responses to multiple viral proteins, including pp65, gB, and UL48. In contrast, gB/AddaVax failed to induce neutralizing antibodies that prevented infection of epithelial cells, highlighting a critical difference in the humoral responses induced by these vaccine candidates. Our data advance the potential for the DB vaccine approach, demonstrate important immunogenicity properties, and strongly support the further evaluation of DBs as a CMV vaccine candidate.     

Antiidiotypic vaccination against murine coronavirus infection.

Rabbit polyclonal antiidiotypic antibodies were generated against a neutralizing mAb specific for a conformational epitope on the S glycoprotein of murine hepatitis virus, strain A59 (MHV-A59). These anti-Id were directed predominantly against an Id that was undetectable in rabbit and rat anti-MHV-A59 sera and weakly represented in syngeneic and allogeneic antiviral sera. However, some partial idiotypic sharing was observed between the Id-bearing antibody and a mAb with a similar antigenic site specificity. The anti-Id inhibited the virus-binding and neutralizing activities of the immunizing antibody, demonstrating that they recognize paratope-associated idiotopes. Mice immunized with affinity-purified anti-Id developed MHV-A59-specific antibodies that neutralized viral infectivity to high titers. Moreover, these animals survived an otherwise lethal challenge with viral murine hepatitis virus, unlike control mice immunized with normal rabbit Ig. These results indicate that at least a subpopulation of the polyclonal anti-Id could induce a protective immune response directed toward a biologically important MHV-A59 epitope, and demonstrate the feasibility of antiidiotypic vaccination against a coronavirus infection.     

Delay of HIV-1 rebound after cessation of antiretroviral therapy through passive transfer of human neutralizing antibodies

To determine the protective potential of the humoral immune response against HIV-1 in vivo we evaluated the potency of three neutralizing antibodies (2G12, 2F5 and 4E10) in suppressing viral rebound in six acutely and eight chronically HIV-1-infected individuals undergoing interruption of antiretroviral treatment (ART). Only two of eight chronically infected individuals showed evidence of a delay in viral rebound during the passive immunization. Rebound in antibody-treated acutely infected individuals upon cessation of ART was substantially later than in a control group of 12 individuals with acute infection. Escape mutant analysis showed that the activity of 2G12 was crucial for the in vivo effect of the neutralizing antibody cocktail. By providing further direct evidence of the potency, breadth and titers of neutralizing antibodies that are required for in vivo activity, these data underline both the potential and the limits of humoral immunity in controlling HIV-1 infection.     

A 64,000 dalton matrix protein of human cytomegalovirus induces in vitro immune responses similar to those of whole viral antigen

Human cytomegalovirus contains approximately 30 to 35 structural polypeptides. Although antibodies to several of these proteins are made during natural infection, their relationship to T cell recognition of this virus and subsequent control of infection is poorly understood. We have purified one of these proteins (HCMVgp64) that is found in abundance in infected cell lysates in order to delineate the relationship of single viral proteins to the immune response caused by the virus. HCMVgp64 induced T cell reactivity only in individuals with serologic evidence of past infection. In addition, HCMVgp64 elicited similar in vitro immune reactions as the whole virus including T cell proliferation, interleukin 2 production, and receptor expression as well as interferon production. These studies suggest that single proteins of HCMV such as HCMVgp64 are capable of inducing T cell responses and may be important in the development of immune reactivity to HCMV.     

Neutralization resistance of virological synapse-mediated HIV-1 Infection is regulated by the gp41 cytoplasmic tail

Human immunodeficiency virus type 1 (HIV-1) infection can spread efficiently from infected to uninfected T cells through adhesive contacts called virological synapses (VSs). In this process, cell-surface envelope glycoprotein (Env) initiates adhesion and viral transfer into an uninfected recipient cell. Previous studies have found some HIV-1-neutralizing patient sera to be less effective at blocking VS-mediated infection than infection with cell-free virus. Here we employ sensitive flow cytometry-based infection assays to measure the inhibitory potency of HIV-1-neutralizing monoclonal antibodies (MAb) and HIV-1-neutralizing patient sera against cell-free and VS-mediated infection. To various degrees, anti-Env MAbs exhibited significantly higher 50% inhibitory concentration (IC(50)s) against VS-mediated infection than cell-free infection. Notably, the MAb 17b, which binds a CD4-induced (CD4i) epitope on gp120, displayed a 72-fold reduced efficacy against VS-mediated inocula compared to cell-free inocula. A mutant with truncation mutation in the gp41 cytoplasmic tail (CT) which is unable to modulate Env fusogenicity in response to virus particle maturation but which can still engage in cell-to-cell infection was tested for the ability to resist neutralizing antibodies. The ΔCT mutation increased cell surface staining by neutralizing antibodies, significantly enhanced neutralization of VS-mediated infection, and had reduced or no effect on cell-free infection, depending upon the antibody. Our results suggest that the gp41 CT regulates the exposure of key neutralizing epitopes during cell-to-cell infection and plays an important role in immune evasion. Vaccine strategies should consider immunogens that reflect Env conformations exposed on the infected cell surface to enhance protection against VS-mediated HIV-1 spread.     

Neutralization of HIV-1: a paradox of humoral proportions.

The production of immunoglobulin capable of neutralizing the infectivity of a virus represents one of the most remarkable molecular accomplishments of the host's available immune defenses. It should be no surprise that a virus that has existed in the parenchyma of the immune system has evolved as an equally dynamic molecule (i.e., viral envelope) for survival. Neutralizing immunoglobulin (Ig) can best serve the host under conditions where the invading pathogen requires a well-defined cell-free state for establishing an infection or transmission. Evidence for a controlling and therefore protective role of neutralizing Ig against lentiviruses has been defined in natural and experimental infections with equine infectious anemia virus of ungulate members in the family equidae. Rapid replication of the virus immediately after infection and its release in a cell-free state leads to the production of neutralizing Ig and subsequent control of the primary viremia. A similar cause-effect relationship exists in humans between the high-titered viremia, observed shortly after HIV-1 infection, and the subsequent production of neutralizing Ig. Partially controlling this acute stage of viral replication by neutralizing Ig and thus preventing an otherwise acute form of immunosuppression or immune complex disease may be viewed paradoxically as a survival property of the virus. Immunologically mediated control, in a Darwinian sense, selects for viruses that have optimized the parameters of longevity and transmission from host to host. This paradox of neutralization in HIV-1 infection appears to be mediated by the convergence of structural and functional roles of the third variable domain (V3) of the external envelope glycoprotein. During infection or envelope-based vaccination, antibody to this cross-reactive, immuno-dominant epitope dominates the antigenic repertoire. Once this occurs, the host is less able to respond to emerging viruses containing closely related V3 structures. Thus a relatively restricted clonal-dominance of the neutralization response results. The V3 domain, apparently in concert with the rest of the molecule, provides an epitope that can tolerate and utilize its conformational flexibility to allow immune escape while maintaining its functional role in infectivity. Sixteen other putative epitopes have been described as being involved in the induction of neutralizing Ig. Currently the biologically functional role of neutralizing Ig to these other epitopes are complicated by a prior lack of knowledge and appreciation of the in vitro variables affecting their measurements.(ABSTRACT TRUNCATED AT 400 WORDS)     

Lack of Protection following Passive Transfer of Polyclonal Highly Functional Low-Dose Non-Neutralizing Antibodies

Recent immune correlates analysis from the RV144 vaccine trial has renewed interest in the role of non-neutralizing antibodies in mediating protection from infection. While neutralizing antibodies have proven difficult to induce through vaccination, extra-neutralizing antibodies, such as those that mediate antibody-dependent cellular cytotoxicity (ADCC), are associated with long-term control of infection. However, while several non-neutralizing monoclonal antibodies have been tested for their protective efficacy in vivo, no studies to date have tested the protective activity of naturally produced polyclonal antibodies from individuals harboring potent ADCC activity. Because ADCC-inducing antibodies are highly enriched in elite controllers (EC), we passively transferred highly functional non-neutralizing polyclonal antibodies, purified from an EC, to assess the potential impact of polyclonal non-neutralizing antibodies on a stringent SHIV-SF162P3 challenge in rhesus monkeys. Passive transfer of a low-dose of ADCC inducing antibodies did not protect from infection following SHIV-SF162P3 challenge. Passively administered antibody titers and gp120-specific, but not gp41-specific, ADCC and antibody induced phagocytosis (ADCP) were detected in the majority of the monkeys, but did not correlate with post infection viral control. Thus these data raise the possibility that gp120-specific ADCC activity alone may not be sufficient to control viremia post infection but that other specificities or Fc-effector profiles, alone or in combination, may have an impact on viral control and should be tested in future passive transfer experiments.     

Antibody-dependent cell-mediated viral inhibition emerges after simian immunodeficiency virus SIVmac251 infection of rhesus monkeys coincident with gp140-binding antibodies and is effective against neutralization-resistant viruses

Antibody-dependent cell-mediated viral inhibition (ADCVI) is an attractive target for vaccination because it takes advantage of both the anamnestic properties of an adaptive immune response and the rapid early response characteristics of an innate immune response. Effective utilization of ADCVI in vaccine strategies will depend on an understanding of the natural history of ADCVI during acute and chronic human immunodeficiency virus type 1 (HIV-1) infection. We used the simian immunodeficiency virus (SIV)-infected rhesus monkey as a model to study the kinetics of ADCVI in early infection, the durability of ADCVI through the course of infection, and the effectiveness of ADCVI against viruses with envelope mutations that are known to confer escape from antibody neutralization. We demonstrate the development of ADCVI, capable of inhibiting viral replication 100-fold, within 3 weeks of infection, preceding the development of a comparable-titer neutralizing antibody response by weeks to months. The emergence of ADCVI was temporally associated with the emergence of gp140-binding antibodies, and in most animals, ADCVI persisted through the course of infection. Highly evolved viral envelopes from viruses isolated at late time points following infection that were resistant to plasma neutralization remained susceptible to ADCVI, suggesting that the epitope determinants of neutralization escape are not shared by antibodies that mediate ADCVI. These findings suggest that despite the ability of SIV to mutate and adapt to multiple immunologic pressures during the course of infection, SIV envelope may not escape the binding of autologous antibodies that mediate ADCVI.     

Impaired Antibody Response Causes Persistence of Prototypic T Cell–Contained Virus

CD8 T cells are recognized key players in control of persistent virus infections, but increasing evidence suggests that assistance from other immune mediators is also needed. Here, we investigated whether specific antibody responses contribute to control of lymphocytic choriomeningitis virus (LCMV), a prototypic mouse model of systemic persistent infection. Mice expressing transgenic B cell receptors of LCMV-unrelated specificity, and mice unable to produce soluble immunoglobulin M (IgM) exhibited protracted viremia or failed to resolve LCMV. Virus control depended on immunoglobulin class switch, but neither on complement cascades nor on Fc receptor γ chain or Fc γ receptor IIB. Cessation of viremia concurred with the emergence of viral envelope-specific antibodies, rather than with neutralizing serum activity, and even early nonneutralizing IgM impeded viral persistence. This important role for virus-specific antibodies may be similarly underappreciated in other primarily T cell–controlled infections such as HIV and hepatitis C virus, and we suggest this contribution of antibodies be given consideration in future strategies for vaccination and immunotherapy.     

Intranasal Administration of Antibody-Bound Respiratory Syncytial Virus Particles Efficiently Primes Virus-Specific Immune Responses in Mice

Infants are protected from a severe respiratory syncytial virus (RSV) infection in the first months of life by maternal antibodies or by prophylactically administered neutralizing antibodies. Efforts are under way to produce RSV-specific antibodies with increased neutralizing capacity compared to the currently licensed palivizumab. While clearly beneficial during primary infections, preexisting antibodies might affect the onset of adaptive immune responses and the ability to resist subsequent RSV infections. Therefore, we addressed the question of how virus neutralizing antibodies influence the priming of subsequent adaptive immune responses. To test a possible role of the neonatal Fc receptor (FcRn) in this process, we compared the responses in C57BL/6 wild-type (WT) and FcRn^−/− mice. We observed substantial virus-specific T-cell priming and B-cell responses in mice primed with RSV IgG immune complexes resulting in predominantly Th1-type CD4⁺ T-cell and IgG2c antibody responses upon live-virus challenge. RSV-specific CD8⁺ T cells were primed as well. Activation of these adaptive immune responses was independent of FcRn. Thus, neutralizing antibodies that localize to the airways and prevent infection-related routes of antigen processing can still facilitate antigen presentation of neutralized virus particles and initiate adaptive immune responses against RSV.