Immunization of cotton rats with the fusion (F) and large (G) glycoproteins of respiratory syncytial virus (RSV) protects against RSV challenge without potentiating RSV disease

Immunogenicity and protective efficacy of adenoviral and subunit RSV vaccines based on stabilized prefusion F protein in pre-clinical models

Respiratory Syncytial Virus (RSV) remains a leading cause of severe respiratory disease for which no licensed vaccine is available. We have previously described the derivation of an RSV Fusion protein (F) stabilized in its prefusion conformation (preF) as vaccine immunogen and demonstrated superior immunogenicity in naive mice of preF versus wild type RSV F protein, both as protein and when expressed from an Ad26 vaccine vector. Here we address the question if there are qualitative differences between the two vaccine platforms for induction of protective immunity. In naïve mice, both Ad26.RSV.preF and preF protein induced humoral responses, whereas cellular responses were only elicited by Ad26.RSV.preF. In RSV pre-exposed mice, a single dose of either vaccine induced cellular responses and strong humoral responses. Ad26-induced RSV-specific cellular immune responses were detected systemically and locally in the lungs. Both vaccines showed protective efficacy in the cotton rat model, but Ad26.RSV.preF conferred protection at lower virus neutralizing titers in comparison to RSV preF protein. Factors that may contribute to the protective capacity of Ad26.RSV.preF elicited immunity are the induced IgG2a antibodies that are able to engage Fcγ receptors mediating Antibody Dependent Cellular Cytotoxicity (ADCC), and the induction of systemic and lung resident RSV specific CD8 + T cells. These data demonstrate qualitative improvement of immune responses elicited by an adenoviral vector based vaccine encoding the RSV preF antigen compared to the subunit vaccine in small animal models which may inform RSV vaccine development.

Adenovector 26 encoded prefusion conformation stabilized RSV-F protein induces long-lasting Th1-biased immunity in neonatal mice

While RSV is a major cause of respiratory morbidity in infants, vaccine development is hindered by the immaturity and Th2-bias of the infant immune system and the legacy of enhanced respiratory disease (ERD) after RSV infection following immunization with formalin inactivated (FI)-RSV vaccine in earlier clinical trials. Preclinical studies have demonstrated that an adenoviral vector-based RSV F vaccine candidate (Ad26.RSV.FA2) induces Th1-biased protective immune responses, without signs of ERD upon subsequent RSV challenge. We here developed an Ad26 vector encoding the RSV F protein stabilized in its prefusion conformation (Ad26.RSV.preF). In adult mice, Ad26.RSV.preF induced superior, Th1-biased IgG2a-dominated humoral responses as compared to Ad26.RSV.FA2, while maintaining the strong Th1-biased cellular responses. Similar to adult mice, Ad26.RSV.preF induced robust and durable humoral immunity in neonatal mice, again characterized by IgG2a-dominated RSV F-binding antibodies, and high and stable virus-neutralizing titers. In addition, vaccine-elicited cellular immune responses were durable and characterized by IFN-γ-producing CD4+ and CD8+ T cells, with a profound Th1 bias. In contrast, immunization of neonatal mice with FI-RSV resulted in IgG1 RSV F-binding antibodies associated with a Th2 phenotype, no detectable virus-neutralizing antibodies, and a Th2-biased cellular response. These results are supportive for the clinical development of Ad26.RSV.preF for use in infants.

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.

Disruption of the airway epithelial barrier in a murine model of respiratory syncytial virus infection

Respiratory syncytial virus (RSV) is a major cause of hospitalization for infants and young children worldwide. RSV is known to infect epithelial cells and increase the permeability of model airway epithelial monolayers in vitro. We hypothesized that RSV infection also induces airway barrier dysfunction in vivo. C57BL/6 mice were intranasally inoculated with RSV, and on day 4 post-inoculation were examined for viral replication, lung inflammation, and barrier integrity as well as the structure and molecular composition of epithelial junctions. In parallel, primary mouse tracheal epithelial cells (mTEC) were cultured for in vitro studies. RSV-infected mice lost weight and showed significant peribronchial inflammation compared with noninfected controls and UV-inactivated RSV-inoculated animals. RSV infection increased the permeability of the airway epithelial barrier and altered the molecular composition of epithelial tight junctions. The observed RSV-induced barrier disruption was accompanied by decreased expression of several tight-junction proteins and accumulation of cleaved extracellular fragments of E-cadherin in bronchoalveolar lavage and mTEC supernatants. Similarly, in vitro RSV infection of mTEC monolayers resulted in enhanced permeability and disruption of tight-junction structure. Furthermore, incubation of mTEC monolayers with a recombinant fragment of E-cadherin caused tight-junction disassembly. Taken together, these data indicate that RSV infection leads to airway barrier dysfunction in vivo, mediated by either decreased expression or cleavage of junctional proteins. Our observations provide further insights into the pathophysiology of RSV infection and provide a rationale for development of barrier-protecting agents to alleviate the pathogenic effects of RSV infection.

Soluble F proteins exacerbate pulmonary histopathology after vaccination upon respiratory syncytial virus challenge but not when presented on virus-like particles

Respiratory syncytial virus (RSV) fusion (F) protein is suggested to be a protective vaccine target although its efficacy and safety concerns remain not well understood. We investigated immunogenicity, efficacy, and safety of F proteins in a soluble form or on virus-like particle (F-VLP). F VLP preferentially elicited IgG2a antibody and T helper type 1 (Th1) immune responses whereas F protein induced IgG1 isotype and Th2 responses. Despite lung viral clearance after prime or prime-boost and then RSV challenge, F protein immune mice displayed weight loss and lung histopathology and high mucus production and eosinophils. In contrast, prime or prime-boost vaccination of F VLP induced effective protection, prevented infiltration of eosinophils and vaccine- enhanced disease after challenge. This study provides insight into developing an effective and safe RSV vaccine candidate.

Adjuvants and the vaccine response to the DS-Cav1-stabilized fusion glycoprotein of respiratory syncytial virus

Appropriate adjuvant selection may be essential to optimize the potency and to tailor the immune response of subunit vaccines. To induce protective responses against respiratory syncytial virus (RSV)-a highly prevalent childhood pathogen without a licensed vaccine-we previously engineered a pre-fusion-stabilized trimeric RSV F (pre-F) "DS-Cav1" immunogen, which induced high titer RSV-neutralizing antibodies, in mice and non-human primates, when formulated with adjuvants Poly (I:C) and Poly (IC:LC), respectively. To assess the impact of different adjuvants, here we formulated RSV F DS-Cav1 with multiple adjuvants and assessed immune responses. Very high RSV-neutralizing antibody responses (19,006 EC50) were observed in naïve mice immunized with 2 doses of DS-Cav1 adjuvanted with Sigma adjuvant system (SAS), an oil-in-water adjuvant, plus Carbopol; high responses (3658-7108) were observed with DS-Cav1 adjuvanted with Alum, SAS alone, Adjuplex, Poly (I:C) and Poly (IC:LC); and moderate responses (1251-2129) were observed with DS-Cav1 adjuvanted with the TLR4 agonist MPLA, Alum plus MPLA or AddaVax. In contrast, DS-Cav1 without adjuvant induced low-level responses (6). A balanced IgG1 and IgG2a (Th2/Th1) immune response was elicited in most of the high to very high response groups (all but Alum and Adjuplex). We also tested the immune response induced by DS-Cav1 in elderly mice with pre-existing DS-Cav1 immunity; we observed that DS-Cav1 adjuvanted with SAS plus Carbopol boosted the response 2-3-fold, whereas DS-Cav1 adjuvanted with alum boosted the response 5-fold. Finally, we tested whether a mixture of ISA 71 VG and Carbopol would enhanced the antibody response in DS-Cav1 immunized calves. While pre-F-stabilized bovine RSV F induced very high titers in mice when adjuvanted with SAS plus Carbopol, the addition of Carbopol to ISA 71 VG did not enhance immune responses in calves. The vaccine response to pre-F-stabilized RSV F is augmented by adjuvant, but the degree of adjuvant-induced enhancement appears to be both context-dependent and species-specific.

Immunization with Low Doses of Recombinant Postfusion or Prefusion Respiratory Syncytial Virus F Primes for Vaccine-Enhanced Disease in the Cotton Rat Model Independently of the Presence of a Th1-Biasing (GLA-SE) or Th2-Biasing (Alum) Adjuvant

Respiratory syncytial virus (RSV) infection of children previously immunized with a nonlive, formalin-inactivated (FI)-RSV vaccine has been associated with serious enhanced respiratory disease (ERD). Consequently, detailed studies of potential ERD are a critical step in the development of nonlive RSV vaccines targeting RSV-naive children and infants. The fusion glycoprotein (F) of RSV in either its postfusion (post-F) or prefusion (pre-F) conformation is a target for neutralizing antibodies and therefore an attractive antigen candidate for a pediatric RSV subunit vaccine. Here, we report the evaluation of RSV post-F and pre-F in combination with glucopyranosyl lipid A (GLA) integrated into stable emulsion (SE) (GLA-SE) and alum adjuvants in the cotton rat model. Immunization with optimal doses of RSV F antigens in the presence of GLA-SE induced high titers of virus-neutralizing antibodies and conferred complete lung protection from virus challenge, with no ERD signs in the form of alveolitis. To mimic a waning immune response, and to assess priming for ERD under suboptimal conditions, an antigen dose de-escalation study was performed in the presence of either GLA-SE or alum. At low RSV F doses, alveolitis-associated histopathology was unexpectedly observed with either adjuvant at levels comparable to FI-RSV-immunized controls. This occurred despite neutralizing-antibody titers above the minimum levels required for protection and with no/low virus replication in the lungs. These results emphasize the need to investigate a pediatric RSV vaccine candidate carefully for priming of ERD over a wide dose range, even in the presence of strong neutralizing activity, Th1 bias-inducing adjuvant, and protection from virus replication in the lower respiratory tract.IMPORTANCE RSV disease is of great importance worldwide, with the highest burden of serious disease occurring upon primary infection in infants and children. FI-RSV-induced enhanced disease, observed in the 1960s, presented a major and ongoing obstacle for the development of nonlive RSV vaccine candidates. The findings presented here underscore the need to evaluate a nonlive RSV vaccine candidate during preclinical development over a wide dose range in the cotton rat RSV enhanced-disease model, as suboptimal dosing of several RSV F subunit vaccine candidates led to the priming for ERD. These observations are relevant to the validity of the cotton rat model itself and to safe development of nonlive RSV vaccines for seronegative infants and children.

Human Respiratory Syncytial Virus: Role of Innate Immunity in Clearance and Disease Progression

Human respiratory syncytial virus (HRSV) infections have worldwide records. The virus is responsible for bronchiolitis, pneumonia, and asthma in humans of different age groups. Premature infants, young children, and immunocompromised individuals are prone to severe HRSV infection that may lead to death. Based on worldwide estimations, millions of cases were reported in both developed and developing countries. In fact, HRSV symptoms develop mainly as a result of host immune response. Due to inability to establish long lasting adaptive immunity, HRSV infection is recurrent and hence impairs vaccine development. Once HRSV attached to the airway epithelia, interaction with the host innate immune components starts. HRSV interaction with pulmonary innate defenses is crucial in determining the disease outcome. Infection of alveolar epithelial cells triggers a cascade of events that lead to recruitment and activation of leukocyte populations. HRSV clearance is mediated by a number of innate leukocytes, including macrophages, natural killer cells, eosinophils, dendritic cells, and neutrophils. Regulation of these cells is mediated by cytokines, chemokines, and other immune mediators. Although the innate immune system helps to clear HRSV infection, it participates in disease progression such as bronchiolitis and asthma. Resolving the mechanisms by which HRSV induces pathogenesis, different possible interactions between the virus and immune components, and immune cells interplay are essential for developing new effective vaccines. Therefore, the current review focuses on how the pulmonary innate defenses mediate HRSV clearance and to what extent they participate in disease progression. In addition, immune responses associated with HRSV vaccines will be discussed.

Respiratory syncytial virus protein structure, function and implications for subunit vaccine development

ABSTRACT 

Respiratory syncytial virus (RSV) is a leading cause of respiratory infections in infants and a significant cause of respiratory infections in the elderly and adults with chronic obstructive pulmonary disease and congestive heart failure. Despite more than 50 years of research, there are no RSV vaccines approved or in Phase III clinical trials. Key challenges include peak disease in infants less than 3 months of age and immunosenescence in the elderly. Due to advances in RSV immunology and structural biology, there is renewed interest in developing an RSV vaccine, with many vaccines in development. Here, we describe the epidemiology of RSV, the RSV virion structure, key neutralizing epitopes on the pre- and post-fusion forms of the fusion protein and implications for vaccine development.

Subunit and virus-like particle vaccine approaches for respiratory syncytial virus

Despite its impact on global health, there is no vaccine available for the prevention of respiratory syncytial virus (RSV) infection. Failure to develop a licensed vaccine is not due to lack of effort, as numerous vaccine candidates have been characterized in preclinical and clinical studies spanning five decades. The vaccine candidates thus far explored can be generally divided into four categories: (1) whole inactivated virus, (2) replication competent, attenuated virus including recombinant viruses, (3) gene-based vectors, and (4) subunit and particulate forms of RSV antigens. The first clinically tested RSV vaccine candidate was a formalin-inactivated purified virus preparation administered to infants and children in the late 1960s. Due to the disastrous outcome of these trials and results of animal models investigating the mechanisms involved, there have been no further studies with inactivated RSV vaccines. Rather, efforts have focused on development of other approaches. In this chapter, we review the history and status of purified proteins, peptides, virus-like particles, virosomes, and nanoparticles and discuss their future potential.

Nanoparticle vaccines encompassing the respiratory syncytial virus (RSV) G protein CX3C chemokine motif induce robust immunity protecting from challenge and disease

Nanoparticle vaccines were produced using layer-by-layer fabrication and incorporating respiratory syncytial virus (RSV) G protein polypeptides comprising the CX3C chemokine motif. BALB/c mice immunized with G protein nanoparticle vaccines produced a neutralizing antibody response that inhibited RSV replication in the lungs following RSV challenge. ELISPOT analysis showed that G nanoparticle vaccinated mice had increased levels of RSV G protein-specific IL-4 and IFN-γ secreting cells compared to controls following RSV challenge. Remarkably, RSV challenge of G protein nanoparticle vaccinated mice resulted in increased RSV M2-specific IL-4 and IFN-γ secreting T cells, and increased M2-specific H-2Kd-tetramer positive CD8⁺ T cells in the lungs compared to controls. Cell type analysis showed vaccination was not associated with increased pulmonary eosinophilia following RSV challenge. These results demonstrate that vaccination of mice with the RSV G protein nanoparticle vaccines induces a potent neutralizing antibody response, increased G protein- and M2- specific T cell responses, and a reduction in RSV disease pathogenesis.

A novel influenza virus hemagglutinin-respiratory syncytial virus (RSV) fusion protein subunit vaccine against influenza and RSV

Influenza A virus and respiratory syncytial virus (RSV) cause substantial morbidity and mortality afflicting the ends of the age spectrum during the autumn through winter months in the United States. The benefit of vaccination against RSV and influenza using a subunit vaccine to enhance immunity and neutralizing antibody was investigated. Influenza virus hemagglutinin (HA) and RSV fusion (F) protein were tested as vaccine components alone and in combination to explore the adjuvant properties of RSV F protein on HA immunity. Mice vaccinated with HA and F exhibited robust immunity that, when challenged, had reduced viral burden for both influenza and RSV. These studies show an enhancing and cross-protective benefit of F protein for anti-HA immunity.

Genetic vaccine for respiratory syncytial virus provides protection without disease potentiation

Respiratory syncytial virus (RSV) is a major cause of infectious lower respiratory disease in infants and the elderly. As there is no vaccine for RSV, we developed a genetic vaccine approach that induced protection of the entire respiratory tract from a single parenteral administration. The approach was based on adenovirus vectors derived from newly isolated nonhuman primate viruses with low seroprevalence. We show for the first time that a single intramuscular (IM) injection of the replication-deficient adenovirus vectors expressing the RSV fusion (F0) glycoprotein induced immune responses that protected both the lungs and noses of cotton rats and mice even at low doses and for several months postimmunization. The immune response included high titers of neutralizing antibody that were maintained ≥ 24 weeks and RSV-specific CD8+ and CD4+ T cells. The vectors were as potently immunogenic as a human adenovirus 5 vector in these two key respiratory pathogen animal models. Importantly, there was minimal alveolitis and granulocytic infiltrates in the lung, and type 2 cytokines were not produced after RSV challenge even under conditions of partial protection. Overall, this genetic vaccine is highly effective without potentiating immunopathology, and the results support development of the vaccine candidate for human testing.

Need for a safe vaccine against respiratory syncytial virus infection

Human respiratory syncytial virus (HRSV) is a major cause of severe respiratory tract illnesses in infants and young children worldwide. Despite its importance as a respiratory pathogen, there is currently no licensed vaccine for HRSV. Following failure of the initial trial of formalin-inactivated virus particle vaccine, continuous efforts have been made for the development of safe and efficacious vaccines against HRSV. However, several obstacles persist that delay the development of HRSV vaccine, such as the immature immune system of newborn infants and the possible Th2-biased immune responses leading to subsequent vaccine-enhanced diseases. Many HRSV vaccine strategies are currently being developed and evaluated, including live-attenuated viruses, subunit-based, and vector-based candidates. In this review, the current HRSV vaccines are overviewed and the safety issues regarding asthma and vaccine-induced pathology are discussed.

Antibody response to the central unglycosylated region of the respiratory syncytial virus attachment protein in mice

We examined the humoral immune response to the unglycosylated central region of the respiratory syncytial virus (RSV) attachment (G) protein in mice following intranasal challenge at day 0 (primary) and day 21 (secondary) with subtype A (A2 strain) or B (B1 strain) RSV preparations. Our serological screening reagents included bacterially derived glutathione S-transferase (GST) fusion proteins, each bearing a portion of the RSV G central core (CC; residues 151-190), proximal central core (PCC; residues 151-172), and the distal central core (DCC; residues 173-190) and purified RSV G proteins from subtype A and B viruses. Convalescent sera collected on day 21 following primary RSV infection bore robust IgG response primarily against the homosubtypic RSV G DCC with relatively modest antigen affinity/avidity as demonstrated by brief incubation with 6M urea. In contrast, sera collected on day 42 following secondary homosubtypic RSV infection bore IgG titers of higher magnitudes and antigen affinity/avidity against the homosubtypic RSV G CC, PCC, and/or the DCC regions and full-length RSV G protein but not against the heterosubtypic RSV G protein or recombinant CC subdomains. In contrast, heterosubtypic secondary RSV infection elicits a broad array of IgG responses with titers of varying magnitudes to homo- and heterosubtypic RSV G CC regions as well as to purified F, Ga, and Gb proteins with the notable exception of minimal response to the RSV G DCC domain associated with the secondary RSV challenge. Our results have implications for RSV G-based serological assays as well as prophylactic immunotherapy and RSV vaccine development.

Current progress on development of respiratory syncytial virus vaccine

Human respiratory syncytial virus (HRSV) is a major cause of upper and lower respiratory tract illness in infants and young children worldwide. Despite its importance as a respiratory pathogen, there is currently no licensed vaccine for prophylaxis of HRSV infection. There are several hurdles complicating the development of a RSV vaccine: 1) incomplete immunity to natural RSV infection leading to frequent re-infection, 2) immature immune system and maternal antibodies of newborn infants who are the primary subject population, and 3) imbalanced Th2-biased immune responses to certain vaccine candidates leading to exacerbated pulmonary disease. After the failure of an initial trial featuring formalin-inactivated virus as a RSV vaccine, more careful and deliberate efforts have been made towards the development of safe and effective RSV vaccines without vaccine-enhanced disease. A wide array of RSV vaccine strategies is being developed, including live-attenuated viruses, protein subunit-based, and vector-based candidates. Though licensed vaccines remain to be developed, our great efforts will lead us to reach the goal of attaining safe and effective RSV vaccines in the near future.

Viral infections of the respiratory tract: prevention and treatment

The rapid discovery of specific viral agents as the cause of many acute respiratory diseases was accompanied by considerable optimism that vaccines or other control measures could be developed quickly. Subsequent experience has demonstrated that effective control of these important public health problems has been an elusive goal. However, recent exciting developments in our understanding of the molecular biology and immunology of these viruses may provide the basis for more effective strategies in the future.

Respiratory syncytial virus vaccine development

Respiratory syncytial virus (RSV) is a clinically significant cause of respiratory tract disease, especially among high-risk infants and immunocompromised and elderly adults. Despite the burden of disease, there is no licensed prophylactic RSV vaccine. The initial efforts to develop an RSV vaccine involved formalin-inactivated virus preparations that unexpectedly caused vaccine-enhanced disease in clinical trials in RSV-naive children. Over the last 40 years, cautious and deliberate progress has been made toward RSV vaccine development using various experimental approaches, including live attenuated strains and vector-based and viral protein subunit/DNA-based candidates. The scientific rationale, preclinical testing, and clinical development of each of these approaches are reviewed.

Vaccination to induce antibodies blocking the CX3C-CX3CR1 interaction of respiratory syncytial virus G protein reduces pulmonary inflammation and virus replication in mice

Respiratory syncytial virus (RSV) infection causes substantial morbidity and some deaths in the young and elderly worldwide. There is no safe and effective vaccine available, although it is possible to reduce the hospitalization rate for high-risk children by anti-RSV antibody prophylaxis. RSV has been shown to modify the immune response to infection, a feature linked in part to RSV G protein CX3C chemokine mimicry. This study determined if vaccination with G protein polypeptides or peptides spanning the central conserved region of the G protein could induce antibodies that blocked G protein CX3C-CX3CR1 interaction and disease pathogenesis mediated by RSV infection. The results show that mice vaccinated with G protein peptides or polypeptides containing the CX3C motif generate antibodies that inhibit G protein CX3C-CX3CR1 binding and chemotaxis, reduce lung virus titers, and prevent body weight loss and pulmonary inflammation. The results suggest that RSV vaccines that induce antibodies that block G protein CX3C-CX3CR1 interaction may offer a new, safe, and efficacious RSV vaccine strategy.

Lack of antibody affinity maturation due to poor Toll-like receptor stimulation leads to enhanced respiratory syncytial virus disease

Respiratory syncytial virus (RSV) is a leading cause of hospitalization in infants. A formalin-inactivated RSV vaccine was used to immunize children and elicited nonprotective, pathogenic antibody. Immunized infants experienced increased morbidity after subsequent RSV exposure. No vaccine has been licensed since that time. A widely accepted hypothesis attributed the vaccine failure to formalin disruption of protective antigens. Here we show that the lack of protection was not due to alterations caused by formalin but instead to low antibody avidity for protective epitopes. Lack of antibody affinity maturation followed poor Toll-like receptor (TLR) stimulation. This study explains why the inactivated RSV vaccine did not protect the children and consequently led to severe disease, hampering vaccine development for 42 years. It also suggests that inactivated RSV vaccines may be rendered safe and effective by inclusion of TLR agonists in their formulation, and it identifies affinity maturation as a key factor for the safe immunization of infants.

Immunogenicity and efficacy of two candidate human metapneumovirus vaccines in cynomolgus macaques

Human metapneumovirus (HMPV) is an important cause of acute respiratory tract disease for which the development of vaccine candidates is warranted. We have previously described the generation of an iscom matrix-adjuvanted HMPV fusion protein subunit vaccine (Fsol) and a live-attenuated vaccine (HMPVM11). Here, we evaluate the immunogenicity and efficacy of these vaccines in cynomolgus macaques. Immunization with Fsol induced HMPV F-specific antibody responses, virus neutralizing antibody titers, and cellular immune responses, but the induced humoral immune response waned rapidly over time. HMPVM11 was strongly attenuated and displayed limited immunogenicity, although immunization with this virus primed for a good secondary HMPV-specific lymphoproliferative response after challenge infection. The duration of virus shedding in HMPVM11-immunized animals was reduced compared to sham-immunized animals. Both vaccines induced HMPV-specific immune responses, but the rapid waning of immunity is a challenging obstacle for vaccine development.

Single intranasal immunization with recombinant adenovirus-based vaccine induces protective immunity against respiratory syncytial virus infection

Respiratory syncytial virus (RSV) is a major cause of severe lower respiratory tract disease in infancy and early childhood. Despite its importance as a pathogen, there is no licensed vaccine against RSV. The G glycoprotein of RSV, a major attachment protein, is a potentially important target for protective antiviral immune responses. Here, a recombinant replication-deficient adenovirus-based vaccine, rAd/3xG, expressing the soluble core domain of G glycoprotein (amino acids 130 to 230) engineered by codon optimization and tandem repetition for higher-level expression, was constructed and evaluated for its potential as an RSV vaccine in a murine model. A single intranasal immunization with rAd/3xG provided potent protection against RSV challenge which lasted for more than 10 weeks. Strong mucosal immunoglobulin A responses were also induced by a single intranasal immunization but not by intramuscular or oral administration of rAd/3xG. Interestingly, neither gamma interferon- nor interleukin-4-producing CD4 T cells directed to I-E(d)-restricted epitope were detected in the lungs of rAd/3xG-immune mice upon challenge, whereas priming with vaccinia virus expressing RSV G (vvG) elicited strong Th1/Th2 mixed CD4 T-cell responses. Lung eosinophilia and vaccine-induced weight loss were significantly lower in the rAd/3xG-immune group than in the vvG-primed group. Together, our data demonstrate that a single intranasal administration of rAd/3xG elicits beneficial protective immunity and represents a promising vaccine regimen against RSV infection.

Immunization of Syrian golden hamsters with F subunit vaccine of human metapneumovirus induces protection against challenge with homologous or heterologous strains

Human metapneumovirus (hMPV), a newly discovered paramyxovirus, is associated with acute respiratory-tract illness, primarily in young children, individuals with underlying disease and the elderly. Two genetic lineages of hMPV circulate around the world, and viruses from these two lineages demonstrate antigenic differences. The clinical impact of hMPV warrants the development of vaccines. Recombinant soluble fusion (F) proteins of prototype viruses of the two main lineages of hMPV that can be produced in high yields have been constructed. In this study, the antigenicity, immunogenicity and protective efficacy of these soluble F subunit vaccines were evaluated in Syrian golden hamsters (Mesocricetus auratus). Immunization of hamsters with the soluble F proteins, adjuvanted with Specol or iscom matrix, induced high virus-neutralization titres, with higher titres against the homologous than the heterologous virus. The neutralizing antibodies protected from subsequent infection of the lungs with both homologous and heterologous virus. Upon challenge, viral titres in the nasal turbinates of immunized animals were reduced significantly compared with those of PBS-immunized animals. In conclusion, a soluble F subunit vaccine for hMPV that induces cross-protective immunity for infection of the lower respiratory tract in Syrian golden hamsters has been generated.

Human and bovine respiratory syncytial virus vaccine research and development

Human (HRSV) and bovine (BRSV) respiratory syncytial viruses (RSV) are two closely related viruses, which are the most important causative agents of respiratory tract infections of young children and calves, respectively. BRSV vaccines have been available for nearly 2 decades. They probably have reduced the prevalence of RSV infection but their efficacy needs improvement. In contrast, despite decades of research, there is no currently licensed vaccine for the prevention of HRSV disease. Development of a HRSV vaccine for infants has been hindered by the lack of a relevant animal model that develops disease, the need to immunize immunologically immature young infants, the difficulty for live vaccines to find the right balance between attenuation and immunogenicity, and the risk of vaccine-associated disease. During the past 15 years, intensive research into a HRSV vaccine has yielded vaccine candidates, which have been evaluated in animal models and, for some of them, in clinical trials in humans. Recent formulations have focused on subunit vaccines with specific CD4+ Th-1 immune response-activating adjuvants and on genetically engineered live attenuated vaccines. It is likely that different HRSV vaccines and/or combinations of vaccines used sequentially will be needed for the various populations at risk. This review discusses the recent advances in RSV vaccine development.

Atypical measles and enhanced respiratory syncytial virus disease (ERD) made simple

Atypical measles and enhanced respiratory syncytial virus disease (ERD) were serious diseases that resulted from exposure of children immunized with inactivated vaccines against measles virus (MV) and respiratory syncytial virus (RSV) to the respective wild-type agents in the 1960s. Although the clinical manifestations of both illnesses were different, the immune responses elicited and primed for by the vaccines shared important similarities. Both vaccines failed to elicit long-lived protective antibody and to promote cytotoxic T lymphocyte responses. In both cases, postvaccination exposure to wild type virus during community outbreaks was associated with immune complex deposition in affected tissues, vigorous CD4 T lymphocyte proliferative responses, and a Th2 bias of the immune response. No relapses of atypical measles or ERD were ever reported. In this manuscript, the pathogeneses of both enhanced diseases and the requirements for the generation of protective antibodies against MV and RSV are discussed, to contribute to the development of newer safe and effective vaccines against these important pathogens.

Involvement of antibody, complement and cellular immunity in the pathogenesis of enhanced respiratory syncytial virus disease

In 1966, infants and children in the USA were immunized with a formalin-inactivated vaccine against respiratory syncytial virus. The vaccine was immunogenic but elicited mainly nonprotective antibody. Upon exposure to respiratory syncytial virus in the community, immunized children developed severe pulmonary disease characterized by bronchoconstriction and pneumonia. Two immunized infants died as toddlers after respiratory syncytial virus infection. Exploration of the mechanisms of disease has dominated the literature for decades. In this review, the pathogenesis of enhanced respiratory disease is discussed and the characteristics of protective and pathogenic respiratory syncytial virus vaccines are examined.

Human metapneumovirus fusion protein vaccines that are immunogenic and protective in cotton rats

Human metapneumovirus (hMPV) is a recently described paramyxovirus that is a major cause of upper and lower respiratory infection in children and adults worldwide. A safe and effective vaccine could decrease the burden of disease associated with this novel pathogen. We previously reported the development of the cotton rat model of hMPV infection and pathogenesis (J. V. Williams et al., J. Virol. 79:10944-10951, 2005). We report here the immunogenicity of an hMPV fusion (F) protein in this model. We constructed DNA plasmids that exhibited high levels of expression of hMPV F in mammalian cells (DNA-F). These constructs were used to develop a novel strategy to produce highly pure, soluble hMPV F protein lacking the transmembrane domain (FDeltaTM). We then immunized cotton rats at 0 and 14 days with either control vector, DNA-F alone, DNA-F followed by FDeltaTM protein, or FDeltaTM alone. All groups were challenged intranasally at 28 days with live hMPV. All three groups that received some form of hMPV F immunization mounted neutralizing antibody responses and exhibited partial protection against virus shedding in the lungs compared to controls. The FDeltaTM-immunized animals showed the greatest degree of protection (>1,500-fold reduction in lung virus titer). All three immunized groups showed a modest reduction of nasal virus shedding. Neither evidence of a Th2-type response nor increased lung pathology were present in the immunized animals. We conclude that sequence-optimized hMPV F protein protects against hMPV infection when delivered as either a DNA or a protein vaccine in cotton rats.

Effects of alveolar macrophage depletion on liposomal vaccine protection against respiratory syncytial virus (RSV)

Little is known about the identities and roles of antigen-presenting cells upon exposure to antigens of respiratory syncytial virus (RSV). Here, we focused on elucidating the importance of alveolar macrophages in conferring protective immunity in mice administered a liposome-encapsulated recombinant fragment of the RSV G protein. Mice were depleted of alveolar macrophages by intranasal inoculation of liposome-encapsulated dichloromethylenediphosphonic acid (DMDP). Mice depleted of alveolar macrophages prior to immunization developed reduced levels of serum RSV-neutralizing antibody and showed dramatically impaired protection against RSV challenge. The severity of interstitial inflammation was also markedly reduced in macrophage-depleted mice. In conclusion, this study demonstrates a pivotal role for alveolar macrophages during exposure to liposome-encapsulated RSV antigen in initiating both protective and histopathological responses against RSV.

Vaccines for the prevention of respiratory viral infections: problems and current status

Acute respiratory virus infections cause the majority of lower respiratory tract illnesses and hospitalisations of infants and the elderly. The emergence of new respiratory viruses and a high probability that influenza will cause further pandemics highlights the necessity for developing better preventative strategies. Although there is a clear and pressing need for vaccines to prevent respiratory syncytial virus, rhinoviruses, coronaviruses, parainfluenza and human metapneumovirus, progress has been extremely slow. This review presents the current status of vaccine development for respiratory viral diseases and outlines novel approaches for the future.

Influenza virosomes are an efficient delivery system for respiratory syncytial virus-F antigen inducing humoral and cell-mediated immunity

In the present study we investigated the efficacy of a new potential vaccine constituted of the respiratory syncytial virus (RSV)-F protein associated with influenza virosomes (RSV-F/IRIV) in combination with the mucosal adjuvant Escheriagen (Escherichia coli heat-labile toxin), administered intranasally (i.n.) to BALB/c mice. After an intramuscular "priming" with influenza virus vaccine, group A of mice was i.n. immunized with of RSV-F/IRIV+heat-labile toxin (HLT), groups B and C were inoculated i.n. with F-RSV+HLT and IRIV+HLT, respectively. The results showed that the virosomal delivery system greatly potentiate immune responses in animals. All mice immunized with the RSV-F/IRIV+HLT developed a mucosal IgA response and a high level of serum IgG. A balanced Th1/Th2 cytokine profile was observed in mice immunized with RSV-F/IRIV+HLT, while a Th2 response was observed in mice immunized with RSV-F+HLT. Histological analysis of lung tissue of RSV challenged mice did not reveal a vaccine-enhanced pulmonary eosinophilia. These results show that i.n. immunization of BALB/c mice with RSV-F/IRIV in combination with HLT can be considered a promising approach for the development of an efficacious human vaccine.

A role for immune complexes in enhanced respiratory syncytial virus disease

Respiratory syncytial virus (RSV) is the leading cause of bronchiolitis and viral pneumonia in infants and young children. Administration of a formalin inactivated vaccine against RSV to children in the 1960s resulted in increased morbidity and mortality in vaccine recipients who subsequently contracted RSV. This incident precluded development of subunit RSV vaccines for infants for over 30 years, because the mechanism of illness was never clarified. An RSV vaccine for infants is still not available. Here, we demonstrate that enhanced RSV disease is mediated by immune complexes and abrogated in complement component C3 and B cell-deficient mice but not in controls. Further, we show correlation with the enhanced disease observed in children by providing evidence of complement activation in postmortem lung sections from children with enhanced RSV disease.

Current concepts on active immunization against respiratory syncytial virus for infants and young children

Respiratory syncytial virus (RSV) is the most important causative agent of viral respiratory tract infections in infants and young children. Passive immunization against RSV became available recently, but this does not apply to an effective vaccine as a result of dramatic adverse results of immunization with a RSV candidate vaccine in the 1960s and the lack of full knowledge of the immune response induced by RSV. Nonetheless intensive research during the past two decades has resulted in several interesting candidate vaccines, of which some have gone through testing in humans. These include the subunit vaccines PFP-1, PFP-2, BBG2Na and cold-passaged/temperature-sensitive mutants. The development of candidate vaccines against RSV is discussed. Because of questions, uncertainties and difficulties with the development of effective vaccines against RSV, it will probably be at least another 5 to 10 years before routine immunization against RSV becomes available.

Perflubron reduces lung inflammation in respiratory syncytial virus infection by inhibiting chemokine expression and nuclear factor-kappa B activation

Airway mucosa inflammation plays a critical role in the pathogenesis of lower respiratory tract infections caused by respiratory syncytial virus (RSV), the major etiologic agent of bronchiolitis in infancy. Type and intensity of cellular infiltration are dictated by inflammatory chemokines, which are rapidly and abundantly induced in lung tissue by RSV. This process is, to a large extent, transcriptionally regulated by RSV-mediated activation of the nuclear factor-kappa B. The administration of a perfluorocarbon (PFC) liquid, such as perflubron, during partial liquid ventilation improves lung function and also reduces inflammation. In this study we demonstrate that treatment of BALB/c mice with perflubron intranasally 6 hours after RSV infection significantly inhibited lung cellular inflammation as well as the expression of the chemokines RANTES, MIP-1 alpha, MIP-1 beta, and MIP-2, compared with phosphate-buffered saline-treated control mice. However, perflubron treatment did not affect RSV replication. Strikingly, treatment with perflubron abrogated nuclear factor-kappa B activation in lung of RSV-infected mice. These results demonstrate a novel mechanism by which PFC may exert antiinflammatory activity and suggest that partial liquid ventilation with PFC may be considered in future clinical trials for infants with severe RSV infections requiring mechanical ventilation.

Immunostimulatory DNA sequences inhibit respiratory syncytial viral load, airway inflammation, and mucus secretion

BACKGROUND

Immunostimulatory DNA sequences (ISS) activate the innate immune system to generate antiviral cytokines, such as IFN-gamma.

OBJECTIVE

This study investigated whether ISS could reduce viral load, mucus secretion, airway inflammation, and airway hyperreactivity to methacholine in a mouse model of respiratory syncytial virus (RSV) infection.

METHODS

Mice were pretreated with ISS 6 days before RSV infection, and lung indices of RSV viral load (viral titer and PCR), bronchoalveolar lavage fluid cytokines (IFN-gamma), airway inflammation (peribronchial inflammation and periodic acid-Schiff-positive mucus cells), and airway hyperreactivity (methacholine responsiveness) were assessed 4 to 6 days after RSV infection.

RESULTS

ISS induced the expression of the antiviral cytokine IFN-gamma in the lung, and this was associated with significantly reduced RSV viral titers, mucus secretion, and peribronchial inflammation. ISS reduced, but did not significantly inhibit, RSV-induced airway hyperreactivity to methacholine.

CONCLUSION

Because ISS induced significant levels of lung IFN-gamma, an immunization strategy based solely on the administration of IFN-gamma may be insufficient to inhibit RSV-induced airway hyperreactivity to methacholine, an endpoint important in the subset of RSV-infected subjects with asthma.

Construction and characterization of recombinant vaccinia viruses co-expressing a respiratory syncytial virus protein and a cytokine

Recombinant vaccinia viruses are well-characterized tools that can be used to define novel approaches to vaccine formulation and delivery. While vector co-expression of immune mediators has enormous potential for optimizing the composition of vaccine-induced immune responses, the impact on antigen expression and vector antigenicity must also be considered. Co-expression of IL-4 increased vaccinia virus vector titres, while IFN-gamma co-expression reduced vaccinia virus replication in BALB/c mice and in C57BL/6 mice infected with some recombinant viruses. Protection against respiratory syncytial virus (RSV) challenge was similar in mice immunized with vaccinia virus expressing RSV G glycoprotein and IFN-gamma, even though the replication efficiency of the vector was diminished. These data demonstrate the ability of vector-expressed cytokine to influence the virulence of the vector and to direct the development of selected immune responses. This suggests that the co-expression of cytokines and other immunomodulators has the potential to improve the safety of vaccine vectors while improving the immunogenicity of vaccine antigens.

CD4(+) T cell frequencies and Th1/Th2 cytokine patterns expressed in the acute and memory response to respiratory syncytial virus I-E(d)-restricted peptides

The respiratory syncytial virus (RSV)-specific frequencies and cytokine expression patterns of acute and memory CD4(+) T cells from RSV strain-A- and strain-B-infected BALB/c mice were determined following restimulation with a panel of 14 predicted RSV I-E(d) peptides from NSP-2, M, SH, F, and L proteins. Ten of fourteen peptides stimulated intracellular Th1 and/or Th2 cytokines in CD4(+) T cells from the mediastinal lymph nodes (MLN) and spleens of RSV strain-A- or strain-B-immune BALB/c mice. Spleen cells exhibited a predominant Th2 cytokine expression pattern after peptide stimulation, whereas MLN cells exhibited a mixed Th1/Th2 cytokine pattern. For a few peptides, there were differences in the Th1/Th2 cytokine response to peptides from the homologous versus heterologous RSV group. None of the 10 peptides induced both Th1 and Th2 cytokines in cells from similarly immunized mice. The frequency and breadth of cytokine expression by I-E(d)-restricted CD4(+) T cells to peptide stimulation was diminished in the memory response.

Inducible expression of inflammatory chemokines in respiratory syncytial virus-infected mice: role of MIP-1alpha in lung pathology

Lower respiratory tract disease caused by respiratory syncytial virus (RSV) is characterized by profound airway mucosa inflammation, both in infants with naturally acquired infection and in experimentally inoculated animal models. Chemokines are central regulatory molecules in inflammatory, immune, and infectious processes of the lung. In this study, we demonstrate that intranasal infection of BALB/c mice with RSV A results in inducible expression of lung chemokines belonging to the CXC (MIP-2 and IP-10), CC (RANTES, eotaxin, MIP-1beta, MIP-1alpha, MCP-1, TCA-3) and C (lymphotactin) families. Chemokine mRNA expression occurred as early as 24 h following inoculation and persisted for at least 5 days in mice inoculated with the highest dose of virus (10(7) PFU). In general, levels of chemokine mRNA and protein were dependent on the dose of RSV inoculum and paralleled the intensity of lung cellular inflammation. Immunohisthochemical studies indicated that RSV-induced expression of MIP-1alpha, one of the most abundantly expressed chemokines, was primarily localized in epithelial cells of the alveoli and bronchioles, as well as in adjoining capillary endothelium. Genetically altered mice with a selective deletion of the MIP-1alpha gene (-/- mice) demonstrated a significant reduction in lung inflammation following RSV infection, compared to control littermates (+/+ mice). Despite the paucity of infiltrating cells, the peak RSV titer in the lung of -/- mice was not significantly different from that observed in +/+ mice. These results provide the first direct evidence that RSV infection may induce lung inflammation via the early production of inflammatory chemokines.

Primary respiratory syncytial virus infection: pathology, immune response, and evaluation of vaccine challenge strains in a new mouse model

Respiratory syncytial virus (RSV) is the primary cause of lower respiratory tract illness in young children. Vaccine development has been hampered by the experience of the formalin-inactivated vaccine tested in the 1960's. Currently, several vaccine candidates are under development and immune response to these candidate vaccines must be evaluated closely. We introduce a novel low-dose murine model of RSV infection and a new pathologic scoring system for the resultant pulmonary disease. We have also developed new sensitive methods for measuring cytokine expression. We then used this new model to test vaccine challenge strains of RSV in order to determine their pathogenicity.

A RhoA-derived peptide inhibits syncytium formation induced by respiratory syncytial virus and parainfluenza virus type 3

The fusion glycoproteins of human respiratory syncytial virus (RSV) and human parainfluenza virus type-3 (PIV-3) mediate virus entry and syncytium formation. Interaction between the fusion protein of RSV and RhoA, a small GTPase, facilitates virus-induced syncytium formation. We show here a RhoA-derived peptide inhibits RSV and syncytium formation induced by RSV and PIV-3, both in vitro by inhibition of cell-to-cell fusion and in vivo by reduction of peak titer by 2 log10 in RSV-infected mice. These findings indicate that the interaction between these two paramyxovirus fusion proteins and RhoA is an important target for new antiviral strategies.

The efficacy of modified-live bovine respiratory syncytial virus vaccines in experimentally infected calves

The efficacy of modified-live (MLV) bovine respiratory syncytial virus (BRSV) vaccines and the correlates of vaccine-induced immunity were investigated in calves using a virulent experimental infection. Clinical disease and pulmonary pathology were significantly reduced, relative to unvaccinated controls, in calves vaccinated according to label directions with commercial multivalent MLV BRSV vaccines. In vitro assays of cellular immunity were more consistent correlates of vaccine associated protection than presence of post vaccination serum antibody. Most vaccinated calves shed virus, but peak virus titre was suppressed compared to unvaccinated controls, with clearance coincident with the simultaneous appearance of mucosal antibody, cytotoxic cells in the lung and anamnestic or primary serum antibody responses. Virus clearance in unvaccinated calves was coincident with the appearance of BRSV specific cytotoxic cells, before mucosal antibody was detected.

The effect of formalin-inactivated vaccine on respiratory disease associated with bovine respiratory syncytial virus infection in calves

The effect of vaccination with a formalin-inactivated, alum-precipitated (FI), bovine respiratory syncytial virus (BRSV) vaccine on BRSV induced respiratory disease in calves was investigated. Six month old BRSV-naive calves were vaccinated with either a FI, a modified live virus (MLV), or virus antigen negative control vaccine (n = 4 per group). One month after the second vaccination, the calves were aerosol challenged with lung wash from a newborn calf infected with a field isolate of BRSV. Moderate to severe clinical disease occurred in all calves. Calves that received FI vaccine had a significantly earlier (day 2 vs. day 4-5) onset of pyrexia and dyspnea (P < 0.05). Pulmonary lesions, consisting of cranioventral atelectasis and dorsal emphysema, occurred in all groups. Two calves that received MLV, and three that received FI vaccine, had reduced pneumonic lung area relative to controls. Vaccination with the FI vaccine resulted in more rapid onset of clinical disease, but ultimately, reduced pulmonary pathology in most recipients.

Protection against respiratory syncytial virus infection by DNA immunization

Respiratory syncytial virus (RSV) remains a major cause of morbidity and mortality in infants and the elderly and is a continuing challenge for vaccine development. A murine T helper cell (Th) type 2 response associates with enhanced lung pathology, which has been observed in past infant trials using formalin-inactivated RSV vaccine. In this study, we have engineered an optimized plasmid DNA vector expressing the RSV fusion (F) protein (DNA-F). DNA-F was as effective as live RSV in mice at inducing neutralizing antibody and cytotoxic T lymphocyte responses, protection against infection, and high mRNA expression of lung interferon gamma after viral challenge. Furthermore, a DNA-F boost could switch a preestablished anti-RSV Th2 response towards a Th1 response. Critical elements for the optimization of the plasmid constructs included expression of a secretory form of the F protein and the presence of the rabbit beta-globin intron II sequence upstream of the F-encoding sequence. In addition, anti-F systemic immune response profile could be modulated by the route of DNA-F delivery: intramuscular immunization resulted in balanced responses, whereas intradermal immunization resulted in a Th2 type of response. Thus, DNA-F immunization may provide a novel and promising RSV vaccination strategy.

The immunostimulating complex (ISCOM) is an efficient mucosal delivery system for respiratory syncytial virus (RSV) envelope antigens inducing high local and systemic antibody responses

ISCOM is an efficient mucosal delivery system for RSV envelope proteins as measured by antibody responses in respiratory tract secretions and in sera of mice following two intranasal (i.n.) administrations. Intranasally administered RSV ISCOMs induced high levels of IgA antibodies both in the upper respiratory tract and in the lungs. In the lungs, a prominent and long-lasting IgA response was recorded, which still persisted 22 weeks after the second i.n. immunization when the experiment ended. Subcutaneous (s.c.) immunization only induced low IgA titres in the upper respiratory tract and no measurable response to RSV was found in the lungs. Differences were also noticed in serum between the i.n. and s.c. modes of immunization. ISCOMs given intranasally induced earlier, higher and longer lasting IgM and IgG1 serum anti-RSV antibody responses than those induced by the s.c. mode of administration. A low serum IgE response was only detectable at 2 weeks after i.n. immunization with ISCOMs and after s.c. immunization with an inactivated virus, but no IgE response was detectable after s.c. injection of ISCOMs. The serum IgA response was more pronounced following s.c. injection of inactivated virus than after i.n. application of ISCOMs, and a clear-cut booster effect was obtained with a second immunization. Virtually no serum IgA response was detected after the s.c. administration of ISCOMs. In conclusion, the high immune responses induced by RSV ISCOMs in the respiratory tract and serum after i.n. administration indicate prominent mucosal delivery and adjuvant properties of the ISCOMs, warranting further studies.

Respiratory syncytial virus vaccines

Respiratory syncytial virus (RSV) is the most important cause of viral lower respiratory tract illness (LRI) in infants and children worldwide and causes significant LRI in the elderly and in immunocompromised patients. The goal of RSV vaccination is to prevent serious RSV-associated LRI. There are several obstacles to the development of successful RSV vaccines, including the need to immunize very young infants, who may respond inadequately to vaccination; the existence of two antigenically distinct RSV groups, A and B; and the history of disease enhancement following administration of a formalin-inactivated vaccine. It is likely that more than one type of vaccine will be needed to prevent RSV LRI in the various populations at risk. Although vector delivery systems, synthetic peptide, and immune-stimulating complex vaccines have been evaluated in animal models, only the purified F protein (PFP) subunit vaccines and live attenuated vaccines have been evaluated in recent clinical trials. PFP-2 appears to be a promising vaccine for the elderly and for RSV-seropositive children with underlying pulmonary disease, whereas live cold-passaged (cp), temperature-sensitive (ts) RSV vaccines (denoted cpts vaccines) would most probably be useful in young infants. The availability of cDNA technology should allow further refinement of existing live attenuated cpts candidate vaccines to produce engineered vaccines that are satisfactorily attenuated, immunogenic, and phenotypically stable.

Immunological response of mice to the bovine respiratory syncytial virus fusion glycoprotein expressed in recombinant baculovirus infected insect cells

Bovine respiratory syncytial virus (BRSV) is a major cause of respiratory disease in calves. The BRSV genome encodes two major glycoproteins, G and F, which are the major targets for the host antibody response. We have expressed the F glycoprotein in insect cells (Sf9) using a recombinant baculovirus vector. A comparison of the F protein expressed in mammalian and insect cells by SDS-PAGE showed that only part of the baculovirus-produced protein was soluble and processed like the native protein. The antigenicity of the soluble form of the F protein expressed in insect cells was identical to that of the F protein expressed in mammalian cells. Immunization with the F protein expressed in insect cells induced neutralizing antibodies in mice. This antigenic preparation adjuvanted with Quil-A produced an increased neutralizing antibody titer and induced protection.

Induction of protective immunity in rodents by vaccination with a prokaryotically expressed recombinant fusion protein containing a respiratory syncytial virus G protein fragment

A subunit approach to the development of a respiratory syncytial virus (RSV) vaccine was investigated. It involved the production, in Escherichia coli, of an RSV (Long) G protein fragment (G2Na) as a C-terminal fusion partner to an albumin binding region (BB) of streptococcal protein G. G2Na incorporated amino acid residues 130-230 and was specifically recognized by murine anti-RSV-A polyclonal serum. In mice, intraperitoneal immunization with BBG2Na induced high anti-RSV-A serum ELISA titers and low to moderate neutralization activity. The immune response induced by BBG2Na demonstrated a potent protective efficacy against upper and lower respiratory tract RSV-A infection. The immunogenicity and protective efficacy of BBG2Na was maintained for at least 47 and 48 weeks, respectively, and was as potent and durable as live RSV-A administered in a similar fashion. Intramuscular immunization of cotton rats with BBG2Na protected lungs from both homologous and heterologous virus challenge. In contrast to mice, however, cotton rat nasal tracts were not protected after BBG2Na immunization. Consistent with antibody-mediated protection, virus was cleared within 24 hr from the lungs of BBG2Na-immunized mice. The anti-RSV-A antibodies induced in mice were exclusively of the IgG1 isotype and were detected in the serum, lungs, and nasal tracts. Passive transfer of these antibodies prevented acute, and eliminated chronic, RSV-A lung infection in normal and immunodeficient mice, respectively, confirming that such antibodies are important and sufficient for BBG2Na-induced pulmonary protection. Our results clearly demonstrate that BBG2Na contains an important immunogenic domain of the RSV G protein. The prokaryotic origin of this protein indicates that glycosylation of the RSV G protein is not necessary for protective efficacy. Thus, BBG2Na has potential as an RSV subunit vaccine.