Plasmid DNA encoding the respiratory syncytial virus G protein is a promising vaccine candidate

Neonatal priming and infancy boosting with a novel respiratory syncytial virus vaccine induces protective immune responses without concomitant respiratory disease upon RSV challenge

Although respiratory syncytial virus (RSV) infection in infants and young children is a global public health issue, development of a safe RSV vaccine has been impeded by formalin-inactivated RSV-enhanced respiratory disease (ERD). In developing a safer yet effective RSV vaccine for children, a strategy to decrease over-reactive T cells and increase neutralizing anti-RSV antibodies should be considered. We previously demonstrated that adult mice immunized with RSV recombinant G protein plus low-dose Cyclosporine A (G+ CsA) could, upon subsequent RSV challenge, produce increased levels of antigen-specific T regulatory cells in lungs that overcame the ERD. Neutralizing anti-RSV antibodies that prevented viral infection were also elicited. In this study, we investigated if such a G+ CsA vaccine could provide infant mice with the same protection from RSV infection without ERD. The results showed that the G+ CsA vaccine could prevent RSV infection with only a mild loss of body weight. Importantly, there was nearly normal morphology and no mucus appearance in lung tissues after RSV challenge. These results demonstrate that the G+ CsA vaccine strategy achieved similar benefits in the neonatal prime and infancy boost model as in the adult mouse model. The G+ CsA immunization strategy is potentially safe and effective in neonates and infants because it suppresses the devastating ERD.

Will Attention by Vaccine Developers to the Host's Nuclear Hormone Levels and Immunocompetence Improve Vaccine Success?

Despite extraordinary advances in fields of immunology and infectious diseases, vaccine development remains a challenge. The development of a respiratory syncytial virus vaccine, for example, has spanned more than 50 years of research with studies of more than 100 vaccine candidates. Dozens of attractive vaccine products have entered clinical trials, but none have completed the path to licensing. Human immunodeficiency virus vaccine development has proven equally difficult, as there is no licensed product after more than 30 years of pre-clinical and clinical research. Here, we examine vaccine development with attention to the host. We discuss how nuclear hormones, including vitamins and sex hormones, can influence responses to vaccines. We show how nuclear hormones interact with regulatory elements of immunoglobulin gene loci and how the deletion of estrogen response elements from gene enhancers will alter patterns of antibody isotype expression. Based on these findings, and findings that nuclear hormone levels are often insufficient or deficient among individuals in both developed and developing countries, we suggest that failed vaccine studies may in some cases reflect weaknesses of the host rather than the product. We encourage analyses of nuclear hormone levels and immunocompetence among study participants in clinical trials to ensure the success of future vaccine programs.

Nucleic acid-based vaccines targeting respiratory syncytial virus: Delivering the goods

Respiratory syncytial virus (RSV) is a massive medical burden on a global scale. Infants, children and the elderly represent the vulnerable populations. Currently there is no approved vaccine to protect against the disease. Vaccine development has been hindered by several factors including vaccine enhanced disease (VED) associated with formalin-inactivated RSV vaccines, inability of target populations to raise protective immune responses after vaccination or natural viral infection, and a lack of consensus concerning the most appropriate virus-associated target antigen. However, with recent advances in the molecular understanding of the virus, and design of highly characterized vaccines with enhanced immunogenicity there is new belief a RSV vaccine is possible. One promising approach is nucleic acid-based vaccinology. Both DNA and mRNA RSV vaccines are showing promising results in clinically relevant animal models, supporting their transition into humans. Here we will discuss this strategy to target RSV, and the ongoing studies to advance the nucleic acid vaccine platform as a viable option to protect vulnerable populations from this important disease.

A Built-In CpG Adjuvant in RSV F Protein DNA Vaccine Drives a Th1 Polarized and Enhanced Protective Immune Response

Human respiratory syncytial virus (RSV) is the most significant cause of acute lower respiratory infection in children. However, there is no licensed vaccine available. Here, we investigated the effect of five or 20 copies of C-Class of CpG ODN (CpG-C) motif incorporated into a plasmid DNA vaccine encoding RSV fusion (F) glycoprotein on the vaccine-induced immune response. The addition of CpG-C motif enhanced serum binding and virus-neutralizing antibody responses in BALB/c mice immunized with the DNA vaccines. Moreover, mice vaccinated with CpG-modified vaccines, especially with the higher 20 copies, resulted in an enhanced shift toward a Th1-biased antibody and T-cell response, a decrease in pulmonary pathology and virus replication, and a decrease in weight loss after RSV challenge. This study suggests that CpG-C motif, cloned into the backbone of DNA vaccine encoding RSV F glycoprotein, functions as a built-in adjuvant capable of improving the efficacy of DNA vaccine against RSV infection.

DNA vaccine encoding central conserved region of G protein induces Th1 predominant immune response and protection from RSV infection in mice

Human respiratory syncytial virus (RSV) can cause serious infection in the lower respiratory tract, especially in infants, young children, the elderly and the immunocompromised population worldwide. Previous study demonstrated the polypeptide (amino acids 148-198) of RSV attachment (G) glycoprotein, corresponding to the central conserved region and encompassing CX3C chemokine motif, could induce antibodies and protection from RSV challenge in mice [1,2]. In this study, we evaluated the immune efficacy of the recombinant DNA vaccine of pVAX1/3G_148-198 encoding RSV G protein polypeptide. RSV specific serum IgG antibodies with neutralizing activity were stimulated following prime-boost immunization of pVAX1/3G_148-198 intramuscularly, and the ratio of IgG2a/IgG1 was 4.93, indicating a Th1 biased immune response. After challenged intranasally with RSV Long, the vaccinated mice showed both decreased lung RSV titers, pulmonary inflammation and body weight loss. The results suggest that pVAX1/3G_148-198 DNA vaccine may be an effective RSV vaccine candidate, and deserves further exploration.

Chimeric virus-like particles containing a conserved region of the G protein in combination with a single peptide of the M2 protein confer protection against respiratory syncytial virus infection

To investigate the feasibility and efficacy of a virus-like particle (VLP) vaccine composed of the conserved antigenic epitopes of respiratory syncytial virus (RSV), the chimeric RSV VLPs HBcΔ-tG and HBcΔ-tG/M282-90 were generated based on the truncated hepatitis B virus core protein (HBcΔ). HBcΔ-tG consisted of HBcΔ, the conserved region (aa 144-204) of the RSV G protein. HBcΔ-tG was combined with a single peptide (aa 82-90) of the M2 protein to generate HBcΔ-tG/M282-90. Immunization of mice with the HBcΔ-tG or HBcΔ-tG/M282-90 VLPs elicited RSV-specific IgG and neutralizing antibody production and conferred protection against RSV infection. Compared with HBcΔ-tG, HBcΔ-tG/M282-90 induced decreased Th2 cytokine production (IL-4 and IL-5), increased Th1 cytokine response (IFN-γ, TNF-α, and IL-2), and increased ratios of IgG2a/IgG1 antibodies, thereby relieving pulmonary pathology upon subsequent RSV infection. Our results demonstrated that chimeric HBcΔ-tG/M282-90 VLPs represented an effective RSV subunit vaccine candidate.

A Recombinant G Protein Plus Cyclosporine A-Based Respiratory Syncytial Virus Vaccine Elicits Humoral and Regulatory T Cell Responses against Infection without Vaccine-Enhanced Disease

Respiratory syncytial virus (RSV) infection can cause severe disease in the lower respiratory tract of infants and older people. Vaccination with a formalin-inactivated RSV vaccine (FI-RSV) and subsequent RSV infection has led to mild to severe pneumonia with two deaths among vaccinees. The vaccine-enhanced disease (VED) was recently demonstrated to be due to an elevated level of Th2 cell responses following loss of regulatory T (Treg) cells from the lungs. To induce high levels of neutralizing Abs and minimize pathogenic T cell responses, we developed a novel strategy of immunizing animals with a recombinant RSV G protein together with cyclosporine A. This novel vaccine induced not only a higher level of neutralizing Abs against RSV infection, but, most importantly, also significantly higher levels of Treg cells that suppressed VED in the lung after RSV infection. The induced responses provided protection against RSV challenge with no sign of pneumonia or bronchitis. Treg cell production of IL-10 was one of the key factors to suppress VED. These finding indicate that G protein plus cyclosporine A could be a promising vaccine against RSV infection in children and older people.

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.

Induction of protective effector immunity to prevent pathogenesis caused by the respiratory syncytial virus. Implications on therapy and vaccine design

Human respiratory syncytial virus (hRSV) is the leading cause of respiratory illness in infants and young children around the globe. This pathogen, which was discovered in 1956, continues to cause a huge number of hospitalizations due to respiratory disease and it is considered a health and economic burden worldwide, especially in developing countries. The immune response elicited by hRSV infection leads to lung and systemic inflammation, which results in lung damage but is not efficient at preventing viral replication. Indeed, natural hRSV infection induces a poor immune memory that allows recurrent infections. Here, we review the most recent knowledge about the lifecycle of hRSV, the immune response elicited by this virus and the subsequent pathology induced in response to infection in the airways. Novel findings about the alterations that this virus causes in the central nervous system and potential therapies and vaccines designed to treat or prevent hRSV infection are discussed.

Baculovirus-expressed virus-like particle vaccine in combination with DNA encoding the fusion protein confers protection against respiratory syncytial virus

Respiratory syncytial virus (RSV) is a major viral agent causing significant morbidity and mortality in young infants and the elderly. There is no licensed vaccine against RSV and it is a high priority to develop a safe RSV vaccine. We determined the immunogenicity and protective efficacy of combined virus-like particle and DNA vaccines presenting RSV glycoproteins (Fd.VLP) in comparison with formalin inactivated RSV (FI-RSV). Immunization of mice with Fd.VLP induced higher ratios of IgG2a/IgG1 antibody responses compared to those with FI-RSV. Upon live RSV challenge, Fd.VLP and FI-RSV vaccines were similarly effective in clearing lung viral loads. However, FI-RSV immunized mice showed a substantial weight loss and high levels of T helper type 2 (Th2) cytokines as well as extensive lung histopathology and eosinophil infiltration. In contrast, Fd.VLP immunized mice did not exhibit Th2 type cytokines locally and systemically, which might contribute to preventing vaccine-associated RSV lung disease. These results indicate that virus-like particles in combination with DNA vaccines represent a potential approach for developing a safe and effective RSV vaccine.

Intranasal DNA Vaccine for Protection against Respiratory Infectious Diseases: The Delivery Perspectives

Intranasal delivery of DNA vaccines has become a popular research area recently. It offers some distinguished advantages over parenteral and other routes of vaccine administration. Nasal mucosa as site of vaccine administration can stimulate respiratory mucosal immunity by interacting with the nasopharyngeal-associated lymphoid tissues (NALT). Different kinds of DNA vaccines are investigated to provide protection against respiratory infectious diseases including tuberculosis, coronavirus, influenza and respiratory syncytial virus (RSV) etc. DNA vaccines have several attractive development potential, such as producing cross-protection towards different virus subtypes, enabling the possibility of mass manufacture in a relatively short time and a better safety profile. The biggest obstacle to DNA vaccines is low immunogenicity. One of the approaches to enhance the efficacy of DNA vaccine is to improve DNA delivery efficiency. This review provides insight on the development of intranasal DNA vaccine for respiratory infections, with special attention paid to the strategies to improve the delivery of DNA vaccines using non-viral delivery agents.

Maternal antibodies by passive immunization with formalin inactivated respiratory syncytial virus confer protection without vaccine-enhanced disease

Maternal immunization of mice with formalin inactivated respiratory syncytial virus (FI-RSV) resulted in the passive transfer of RSV antibodies but not cellular components to the offspring. The offspring born to FI-RSV immunized mothers showed serum RSV neutralizing activity, effectively controlled lung viral loads without vaccine-enhanced disease, did not induce pulmonary eosinophilia, and cytokine producing cells after live RSV infection. Therefore, this study provides evidence that maternal immunization provides an in vivo model in investigating the roles of antibodies independent of cellular components.

Recent advances in diagnosis, prevention, and treatment of human respiratory syncytial virus

Human respiratory syncytial virus (RSV) is a common cause of respiratory infection in infants and the elderly, leading to significant morbidity and mortality. The interdisciplinary fields, especially biotechnology and nanotechnology, have facilitated the development of modern detection systems for RSV. Many anti-RSV compounds like fusion inhibitors and RNAi molecules have been successful in laboratory and clinical trials. But, currently, there are no effective drugs for RSV infection even after decades of research. Effective diagnosis can result in effective treatment, but the progress in both of these facets must be concurrent. The development in prevention and treatment measures for RSV is at appreciable pace, but the implementation into clinical practice still seems a challenge. This review attempts to present the promising diverse research approaches and advancements in the area of diagnosis, prevention, and treatment that contribute to RSV management.

Gene-based vaccine approaches for respiratory syncytial virus

A respiratory syncytial virus (RSV) vaccine has remained elusive for decades, largely due to the failure of a formalin-inactivated RSV vaccine in the 1960s that resulted in enhanced disease upon RSV exposure in the immunized individuals. Vaccine development has also been hindered by the incomplete immunity conferred by natural infection allowing for re-infection at any time, and the immature immune system and circulating maternal antibodies present in the neonate, the primary target for a vaccine. This chapter will review the use of gene delivery, both nonviral and viral, as a potential vaccine approach for human RSV. Many of these gene-based vaccines vectors elicit protective immune responses in animal models. None of the RSV gene-based platforms have progressed into clinical trials, mostly due to uncertainty regarding the direct translation of animal model results to humans and the hesitancy to invest in costly clinical trials with the potential for unclear and complicated immune responses. The continued development of RSV vaccine gene-based approaches is warranted because of their inherent flexibility with regard to composition and administration. It is likely that multiple candidate vaccines will reach human testing in the next few years.

Non-propagating, recombinant vesicular stomatitis virus vectors encoding respiratory syncytial virus proteins generate potent humoral and cellular immunity against RSV and are protective in mice

Respiratory syncytial virus (RSV) is a major cause of severe lower respiratory tract illness in infants, the elderly, and other high-risk individuals. Despite years of research in this field, there is no effective licensed vaccine to prevent RSV infection. We have generated candidate RSV vaccines using a recombinant vesicular stomatitis virus (rVSV) replicon in which the attachment and fusion domains of the VSV glycoprotein (G) have been deleted (rVSV-Gstem), rendering the virus propagation-defective except in the presence of complementing VSV G provided in trans. A form of this vector encoding the RSV fusion protein (F) gene expressed high levels of F in vitro and elicited durable neutralizing antibody responses as well as complete protection against RSV challenge in vivo. Mice vaccinated with rVSV-Gstem-RSV-F replicons also developed robust cellular responses characterized by both primary and memory Th1-biased CD8+ and CD4+ T cells. Furthermore, a single high dose of the Gstem-RSV-F replicon was effective against challenge with both RSV A and B subgroup viruses. Finally, addition of an RSV glycoprotein (G)-expressing Gstem vector significantly improved the incomplete protection achieved with a single low dose of Gstem-RSV-F vector alone.

Respiratory syncytial virus vaccine development

Respiratory syncytial virus (RSV) is the leading cause of lower respiratory tract viral disease in infants and young children. Presently, there are no explicit recommendations for RSV treatment apart from supportive care. The virus is therefore responsible for an estimated 160,000 deaths per year worldwide. Despite half a century of dedicated research, there remains no licensed vaccine product. Herein are described past and current efforts to harness innate and adaptive immune potentials to combat RSV. A plethora of candidate vaccine products and strategies are reviewed. The development of a successful RSV vaccine may ultimately stem from attention to historical lessons, in concert with an integral partnering of immunology and virology research fields.

New insights for development of a safe and protective RSV vaccine

Respiratory Syncytial Virus (RSV) is the leading cause of pneumonia and bronchiolitis in infants and children <1 year old, resulting in significant morbidity and mortality worldwide. There is currently no RSV vaccine. In the 1960s, a formalin-inactivated RSV (FI-RSV) vaccine trial led to exacerbated disease upon natural infection of vaccinees, including two deaths. The causes involved in the disastrous results of these vaccine trials are still unclear but they remain the engine for searching new avenues to develop a safe vaccine that can provide long-term protection against this important pathogen. This article reviews some of the early history of RSV vaccine development,as well as more recent information on the interaction between RSV and the host innate and adaptive immune responses. A safe and efficacious vaccine for RSV will require "re-education" of the host immune response against RSV to prevent vaccine-enhanced or severe RSV disease.

Prospects for defined epitope vaccines for respiratory syncytial virus

The history of vaccines for respiratory syncytial virus (RSV) illustrates the complex immunity and immunopathology to this ubiquitous virus, starting from the failed formalin-inactivated vaccine trials performed in the 1960s. An attractive alternative to traditional live or killed virus vaccines is a defined vaccine composed of discrete antigenic epitopes for which immunological activities have been characterized as comprehensively as possible. Here we present cumulative data on murine and human CD4, CD8 and neutralization epitopes identified in RSV proteins along with information regarding their associated immune responses and host-dependent variability. Identification and characterization of RSV epitopes is a rapidly expanding topic of research with potential contributions to the tailored design of improved safe and effective vaccines.

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.

Murine host responses to respiratory syncytial virus (RSV) following intranasal administration of a Protollin-adjuvanted, epitope-enhanced recombinant G protein vaccine

BACKGROUND

Immunization of mice with the G protein of respiratory syncytial virus (RSV) characteristically induces an immune response that is partially protective, but which can prime for pulmonary eosinophilia. We have shown previously that the N191A mutation in a recombinant RSV G protein fragment is associated with reduced pulmonary eosinophilic infiltration when administered with alum subcutaneously in BALB/c mice followed by RSV challenge. We hypothesize that the performance of this "epitope enhanced" recombinant G protein fragment may be further improved by combining with the newly developed adjuvant, Protollin, coupled with intranasal delivery.

OBJECTIVES

To investigate efficacy of an intranasally delivered, Protollin-adjuvanted, epitope-enhanced recombinant G protein vaccine in BALB/c mice.

STUDY DESIGN

Recombinant protein, designated Trx-G128-229, consisted of a bacterially expressed central fragment (amino acids 128-229) of the RSV Long strain G protein fused to a fragment of thioredoxin (Trx). BALB/c mice were chosen to evaluate the effectiveness of wild type and epitope-enhanced Trx-G128-229 as a nasal vaccine with the adjuvant Protollin.

RESULTS

The intranasal administration of Trx-G128-229 with Protollin conferred similar protection against RSV challenge as subcutaneously administered Trx-G128-229 with alum, but with markedly reduced eosinophilia and the Th2 cytokine IL-13.

CONCLUSIONS

These results support the concept of an RSV vaccine optimized by combined strategies, including epitope enhancement and judicious selection of adjuvants.

Vaccination of calves using the BRSV nucleocapsid protein in a DNA prime-protein boost strategy stimulates cell-mediated immunity and protects the lungs against BRSV replication and pathology

Respiratory syncytial virus (RSV) is a major cause of respiratory disease in both cattle and young children. Despite the development of vaccines against bovine (B)RSV, incomplete protection and exacerbation of subsequent RSV disease have occurred. In order to circumvent these problems, calves were vaccinated with the nucleocapsid protein, known to be a major target of CD8⁺ T cells in cattle. This was performed according to a DNA prime–protein boost strategy. The results showed that DNA vaccination primed a specific T-cell-mediated response, as indicated by both a lymphoproliferative response and IFN-γ production. These responses were enhanced after protein boost. After challenge, mock-vaccinated calves displayed gross pneumonic lesions and viral replication in the lungs. In contrast, calves vaccinated by successive administrations of plasmid DNA and protein exhibited protection against the development of pneumonic lesions and the viral replication in the BAL fluids and the lungs. The protection correlated to the cell-mediated immunity and not to the antibody response.

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.

Polylactide-co-glycolide (PLG) microparticles modify the immune response to DNA vaccination

Priming with the major surface glycoprotein G of respiratory syncytial virus (RSV) expressed by recombinant vaccinia leads to strong Th2 responses and lung eosinophilia during viral challenge. We now show that DNA vaccination in BALB/c mice with plasmids encoding G attenuated RSV replication but also enhanced disease with lung eosinophilia and increased IL-4/5 production. However, formulating the DNA with PLG microparticles reduced the severity of disease during RSV challenge without significantly lessening protection against viral replication. PLG formulation greatly reduced lung eosinophilia and prevented the induction of IL-4 and IL-5 during challenge, accompanied by a less marked CD4+ T cell response and a restoration of the CD8+ T cell recruitment seen during infection of non-vaccinated animals. After RSV challenge, lung eosinophilia was enhanced and prolonged in mice vaccinated with DNA encoding a secreted form of G; this effect was virtually prevented by PLG formulation. Therefore, PLG microparticulate formulation modifies the pattern of immune responses induced by DNA vaccination boosts CD8+ T cell priming and attenuates Th2 responses. We speculate that PLG microparticles affect antigen uptake and processing, thereby influencing the outcome of DNA vaccination.

Oral respiratory syncytial virus (RSV) DNA vaccine expressing RSV F protein delivered by attenuated Salmonella typhimurium

Human respiratory syncytial virus (RSV) is a major viral pathogen of the lower respiratory tract of infants and young children worldwide. No effective prevention measure is available. Attenuated Salmonella strains expressing heterologous antigens can be delivered by the oral route, triggering efficient antigen-specific humoral, cellular, and mucosal immunity. In this study, we orally administered attenuated Salmonella strain SL7207, carrying the plasmid pcDNA3.1/F expressing the RSV F gene, to BALB/c mice and showed significant elevations of serum anti-RSV IgG and bronchoalveolar lavage secretory IgA as compared with the control group carrying empty plasmid (p<0.001). The ratio of IgG1 and IgG2a was 0.96. The experimental group also showed a stronger cytotoxic T cell response (p<0.01 at effector:target ratios of 100:1 and 50:1) and a higher stimulation index value of T cell proliferation (p<0.05) than the respective control group. RSV titers in the lung homogenates of the experimental group on day 3 and day 5 postchallenge were lower than in the control group (p<0.05). Histopathological analysis showed obvious differences in infiltration of inflammatory cells and pulmonary alveolar wall thickness (p<0.01) between the two groups. In summary, our results demonstrate the potential of orally administered SL7207-based DNA vaccines against RSV infection.

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.

Immunogenicity and efficacy of codon optimized DNA vaccines encoding the F-protein of respiratory syncytial virus

Respiratory syncytial virus F-protein (RSV-F) is poorly expressed from DNA expression plasmids containing the wild type RSV-F open reading frame. By codon optimization, premature polyadenylation signals were deleted and a striking enhancement of RSV-F expression levels was achieved. Therefore, the immunogenicity and efficacy of wild type DNA vaccines were compared to codon optimized expression plasmids encoding full-length RSV-F or its ectodomain. Mice were immunized twice with the different DNA vaccines followed by an RSV challenge. Only codon optimized DNA vaccines and in particular the one encoding the ectodomain of RSV-F induced substantial antibody levels and reduced viral load 13-170-fold. Thus, codon optimization enhances the immunogenicity and efficacy of RSV encoding DNA vaccines.

DNA immunization with plasmids encoding fusion and nucleocapsid proteins of bovine respiratory syncytial virus induces a strong cell-mediated immunity and protects calves against challenge

Respiratory syncytial viruses (RSV) are one of the most important respiratory pathogens of humans and cattle, and there is currently no safe and effective vaccine prophylaxis. In this study, we designed two codon-optimized plasmids encoding the bovine RSV fusion (F) and nucleocapsid (N) proteins and assessed their immunogenicity in young calves. Two administrations of both plasmids elicited low antibody levels but primed a strong cell-mediated immunity characterized by lymphoproliferative response and gamma interferon production in vitro and in vivo. Interestingly, this strong cellular response drastically reduced viral replication, clinical signs, and pulmonary lesions after a highly virulent challenge. Moreover, calves that were further vaccinated with a killed-virus vaccine developed high levels of neutralizing antibody and were fully protected following challenge. These results indicate that DNA vaccination could be a promising alternative to the classical vaccines against RSV in cattle and could therefore open perspectives for vaccinating young infants.

Immunopathology of RSV infection: prospects for developing vaccines without this complication

Respiratory syncytial virus is the most important cause of lower respiratory tract infection in infants and young children. RSV clinical disease varies from rhinitis and otitis media to bronchiolitis and pneumonia. An increased incidence of asthma later in life has been associated with the more severe lower respiratory tract infections. Despite its importance as a pathogen, there is no licensed vaccine against RSV. This is due to a number of factors complicating the development of an effective and safe vaccine. The immunity to natural RSV infection is incomplete as re-infections occur in all age groups, which makes it challenging to design a protective vaccine. Second, the primary target population is the newborn infant, which has a relatively immature immune system and maternal antibodies that can interfere with vaccination. Finally, some vaccines have resulted in a predisposition for exacerbated pulmonary disease in infants, which was attributed to an imbalanced Th2-biased immune response, although the exact cause has not been elucidated. This makes it difficult to proceed with vaccine testing in infants. It is likely that an effective and safe vaccine needs to elicit a balanced immune response, including RSV-specific neutralising antibodies, CD8 T-cells, Th1/Th2 CD4 T-cells and preferably secretory IgA. Subunit vaccines formulated with appropriate adjuvants may be adequate for previously exposed individuals. However, intranasally delivered genetically engineered attenuated or vectored vaccines are currently most promising for newborns, as they are expected to induce a balanced immune response similar to that elicited to natural infection and not be subject to interference from maternal antibodies. Maternal vaccination may be the optimal strategy to protect the very young infants.

Protection by recombinant viral proteins against a respiratory challenge with virulent avian metapneumovirus

Protection by recombinant avian metapneumovirus (aMPV) N or M proteins against a respiratory challenge with virulent aMPV was examined. N, M or N+M proteins were administered intramuscularly (IM) with incomplete Freund's adjuvant (IFA) or by the oculonasal (ON) route with cholera toxin-B (CTB). Each turkey received 40 or 80 microg of each recombinant protein. Birds were considered protected against challenge if the challenge virus was not detectable in the choanal swabs by RT-PCR. At a dose of 40 microg/bird, N protein given with IFA by the IM route protected eight out of nine birds. M protein at the same dose protected three out of seven birds, while a combination of N+M proteins (40 microg each) protected three out of four birds. At a dose of 80 microg of each of N and M proteins per bird given with IFA by the IM route, 100% protection was achieved. ON immunization with a mixture of N and M proteins induced partial protection when the proteins were given with CTB; no detectable protection was noted without CTB. N and M proteins induced anti-aMPV antibodies, although protection against virulent virus challenge did not appear to be associated with the level or presence of antibodies.

Carriers for the delivery of a vaccine against respiratory syncytial virus

Respiratory syncytial virus (RSV) is a major cause of bronchiolitis and pneumonia in young children and the elderly. Despite its clinical importance, there is no licensed vaccine available at present. Vaccine development has been hampered by observations of increased pathology after RSV infection in infants vaccinated with formalin-inactivated RSV; incomplete immunity following natural infection; and the need to be effective during the neonatal period when levels of maternal antibody are high. Four categories of RSV vaccine carriers--live-attenuated RSVs, recombinant vectors expressing the protective antigens of RSV, DNA vaccines and subunit vaccines--have been evaluated in animal models and/or clinical trials. So far, studies with live-attenuated virus vaccines highlight the need to improve immunogenicity whilst maintaining a suitable level of attenuation. Studies with recombinant vectors, DNA and subunit vaccines illustrate the pivotal nature of the vaccine carrier in determining the balance between immune-mediated protection against infection and the induction of immune-mediated pulmonary pathology.

Pulmonary DNA vaccination: concepts, possibilities and perspectives

Mucosal immunity establishes the first line of defence against pathogens entering the body via mucosal surfaces. Besides eliciting both local and systemic immunity, mucosal vaccination strategies that are non-invasive in nature may increase patient compliance and reduce the need for vaccine application by trained personnel. A relatively new concept is mucosal immunization using DNA vaccines. The advantages of DNA vaccines, such as the opportunity to combine the genetic information of various antigen epitopes and stimulatory cytokines, the enhanced stability and ease of production make this class of vaccines attractive and suitable for mucosal application. In contrast to the area of intranasal vaccination, only a few recent studies have focused on pulmonary immunization and the involvement of the pulmonary immune system in eliciting protective immune responses against inhaled pathogens. This review focuses on DNA vaccine delivery to the lung as a promising approach to prevent pulmonary-associated diseases caused by inhaled pathogens. Attractive immunological features of the lung as a site for immunization, the mechanisms of action of DNA vaccines and the pulmonary application of such vaccines using novel delivery systems will be discussed. We also examine pulmonary diseases prone to prevention or therapeutical intervention by application of DNA vaccines.

Comparison of affinity chromatography and adsorption to vaccinia virus recombinant infected cells for depletion of antibodies directed against respiratory syncytial virus glycoproteins present in a human immunoglobulin preparation

Antibodies directed against human respiratory syncytial virus (HRSV) glycoproteins were depleted from a commercial immunoglobulin preparation (RespiGam) by two different methods. The first method consisted of repeated adsorption of RespiGam to Sepharose beads with covalently bound soluble forms of the two major viral glycoproteins (F or G). The second method consisted of adsorption of immunoglobulins to live cells expressing F or G glycoproteins on their surfaces after infection with vaccinia virus recombinants. While the first method removed efficiently antibodies that reacted with F and/or G glycoproteins by ELISA, it was inefficient in the elimination of anti-HRSV neutralizing antibodies. In contrast, the second method removed efficiently anti-HRSV antibodies that both reacted by ELISA and neutralized virus infectivity. These results confirm that human neutralizing antibodies are directed exclusively against HRSV F and G glycoproteins, and, they raise the possibility that F and G glycoproteins inserted into cell membranes differ antigenically from their soluble forms linked covalently to Sepharose beads.

DNA immunization against respiratory syncytial virus (RSV) in infant rhesus monkeys

A DNA vaccine was tested in infant Rhesus macaques to evaluate its safety, immunogenicity and protective efficacy. Monkeys were vaccinated and challenged with a clinical isolate of human RSV. Vaccinated animals developed humoral and cellular responses following inoculation with plasmid DNA encoding the fusion (F) and nucleoprotein (N), from closely related bovine RSV. Vaccinated monkeys had decreased RSV in their lungs post-infection, and there was a qualitative difference in histopathology observed between vaccinated and unvaccinated animals. The combined result of safety and immunogenicity in a neonatal primate model is encouraging, suggesting the feasibility of DNA vaccines against RSV in infants.

DNA vaccination against respiratory syncytial virus in young calves

A DNA vaccine encoding the fusion (F) gene (DNA-F) of bovine respiratory syncytial virus (BRSV) induced significant protection against BRSV infection in young calves. However, serum antibody to RSV developed more slowly in animals vaccinated with DNA-F when compared with those previously infected with BRSV. Furthermore, protection against BRSV infection was not as great as that induced by prior BRSV infection. Although there was little difference in the level of protection induced in calves vaccinated with DNA-F by either the intramuscular (i.m.) or intradermal (i.d.) routes, only the i.m. route primed for a rapid BRSV-specific IgA response after BRSV challenge. These results indicate that a DNA vaccination may be effective against RSV infection even in very young infants and calves.

In vitro expression of full-length and truncated bovine respiratory syncytial virus G proteins and their antibody responses in BALB/c mice

Bovine respiratory syncytial virus (BRSV) is a primary cause of lower respiratory tract disease in calves. Protection is incomplete following vaccination or natural infection, as re-infections are common. The objectives of this study were to create plasmid DNA constructs encoding the full-length, secreted, or conserved region of the BRSV G glycoprotein, and to compare and evaluate their expression in cell culture and potential to induce antibody responses in BALB/c mice. Transfection of COS-7 cells with plasmid DNA resulted in expression of the BRSV G region from each of the plasmid DNA constructs. Following inoculation of BALB/c mice with plasmid DNA, a significant and equivalent anti-BRSV G IgG response was elicited to the full-length and truncated BRSV G proteins. These constructs may be used to study host pathological and immunological responses.

The cotton rat: an underutilized animal model for human infectious diseases can now be exploited using specific reagents to cytokines, chemokines, and interferons

The cotton rat represents the best or only animal model for a large number of human infectious diseases, and it may be unique among small laboratory animals in its susceptibility to several potential agents of bioterrorism. Although the cotton rat is a reliable model to define pathologic changes produced during infection with human pathogens, the lack of specific reagents has precluded a more extensive analysis of the molecular basis of pathogenesis. Here, we report the cloning of 24 cotton rat genes encoding various cytokines, chemokines, and interferons (IFNs). Analysis of the expression of most of these genes was performed by RT-PCR in cotton rat macrophages during treatment with lipopolysaccharide (LPS) and in cotton rat lungs after infection with influenza virus. The availability of these reagents will provide the tools for molecular analysis of pathogenesis and immune responses to a wide variety of pathogens and set the basis for the development of new prophylactic and therapeutic strategies against human infectious diseases.

Immunization of turkeys with a DNA vaccine expressing either the F or N gene of avian metapneumovirus

In this study we compared protection by DNA vaccination with the F (pCMV-F) or N (pCMV-N) gene from avian metapneumovirus (aMPV) in turkeys. One-week-old turkey poults received two intramuscular injections 2 wk apart. Birds were challenged with a turkey-embryo-adapted aMPV at 5 wk of age. Birds vaccinated with pCMV-F had decreased clinical signs of disease as well as significantly reduced virus load in tracheal swabs compared with birds vaccinated with pCMV-N or unvaccinated control birds. Serum neutralizing antibodies were significantly higher in birds receiving pCMV-F compared with all other groups. These results indicate that DNA vaccination with the F, but not N, gene of aMPV can induce significant protection against aMPV infection.

Respiratory syncytial virus vaccine: Is it coming?

Respiratory syncytial virus (RSV) is recognized as an important cause of childhood morbidity and mortality. Vaccine development has been challenging in young infants and has required the advent of molecular technologies to optimize the safety profile, while maintaining immunogenicity of live-attenuated vaccines. Protein-based vaccines have been evaluated in clinical trials and are promising candidates for RSV-primed populations or for maternal vaccination to provide early life protection. This review provides a summary of the need for an RSV vaccine, as well as the challenges and progress in the vaccine's development.

CD40 ligand (CD154) improves the durability of respiratory syncytial virus DNA vaccination in BALB/c mice

Respiratory syncytial virus (RSV) infection is the single most important cause of serious acute respiratory illness in children <1 year of age worldwide, and is associated with life-threatening pneumonia or bronchiolitis in the elderly. Current vaccine strategies include live, attenuated virus, subunit and DNA vaccines, however, none have been sufficiently safe, or shown to induce satisfactory long-term immunity, thus immune modulators are being considered to enhance the effectiveness of RSV vaccines. In this study, we examine CD40 ligand (CD40L) as an immune modulator to enhance the durability of DNA vaccines encoding RSV F and/or G glycoproteins in BALB/c mice. The addition of CD40L to DNA vaccines encoding the F glycoprotein enhanced virus clearance and some aspects of the immune response to RSV challenge, suggesting that CD40L may enhance the durability of RSV DNA vaccines.

Mucosal gene expression vaccine: a novel vaccine strategy for respiratory syncytial virus

A number of approaches have been used in attempts to develop a safe and effective vaccine for respiratory syncytial virus (RSV) infection. This article describes an effective prophylactic intranasal gene transfer strategy utilizing chitosan-DNA nanospheres [the mucosal gene expression vaccine (MGXV)], containing a mixture of plasmid DNAs encoding RSV antigens. In a mouse model of RSV infection, a single administration of MGXV (25 microg/mouse) results in a significant reduction of viral titers and viral antigen load after acute RSV infection of these mice. MGXV-treated mice show no significant change in airway reactivity to methacholine and no apparent pulmonary inflammation. Together these results demonstrate the potential of MGXV against acute RSV infection.

Vaccination with recombinant alphavirus or immune-stimulating complex antigen against respiratory syncytial virus

Respiratory syncytial virus (RSV) causes severe respiratory diseases in infants and young children. Inappropriate immunity to the virus can lead to disease enhancement upon subsequent infection. In this study, we have characterized the antiviral immunity elicited by the recombinant Semliki Forest virus (SFV) encoding the RSV fusion (F) and attachment (G) protein, and compared with that induced by the immune-stimulating complex (ISCOM)-incorporated FG proteins. Antiviral immunity against RSV elicited nasally or parentally by either of the immunogen having divergent profiles could reduce lung RSV titers upon challenge. However, resistance to RSV without disease enhancement was only observed in those vaccinated with SFV recombinants via nasal route. Presence of postvaccination pulmonary IFN-gamma response to the H-2K(d)-restricted T cell epitope (F(85-93); KYKNAVTEL) was found to be associated with absence of enhanced pulmonary disease and goblet cell hyperplasia as well as reduced Th2-cytokine expression. This result demonstrates that the SFV recombinants can result in enhanced clearance of RSV without enhancing the RSV-associated disease, and underlines the importance in priming pulmonary MHC class I-restricted T cells when RSV FG-based vaccines are used.

Intranasal gene transfer by chitosan-DNA nanospheres protects BALB/c mice against acute respiratory syncytial virus infection

Respiratory syncytial virus (RSV) infection is often associated in infancy with life-threatening bronchiolitis, which is also a major risk factor for the development of asthma. At present, no effective prophylaxis is available against RSV infection. Herein, we describe an effective prophylactic intranasal gene transfer strategy utilizing chitosan-DNA nanospheres (IGT), containing a cocktail of plasmid DNAs encoding all RSV antigens, except L. A single administration of IGT (25 microg/mouse) induces expression of the mRNA and proteins of all antigens in the lung and results in a significant reduction of viral titers and viral antigen load after acute RSV infection of these mice. IGT-administered mice show no significant change in airway reactivity to methacholine and no apparent pulmonary inflammation. Furthermore, IGT results in significant induction of RSV-specific IgG antibodies, nasal IgA antibodies, cytotoxic T lymphocytes, and interferon-gamma production in the lung and splenocytes compared with controls. Together, these results demonstrate the potential of IGT against acute RSV infection.

Pathogenesis of RSV lower respiratory tract infection: implications for vaccine development

Respiratory syncytial virus (RSV) infection is the most prevalent cause of severe respiratory disease in infants. It also causes considerable morbidity in older children and adults with underlying risk factors. RSV vaccine development has been complicated by the need to administer the vaccine at a very young age and by enhanced disease observed after vaccination with formalin inactivated RSV. For infants live attenuated vaccines, which may not be expected to predispose for vaccine induced enhanced pathology, hold the greatest promise. However, the balance between attenuation and immunogenicity appears to be delicate. For older risk groups, results with subunit vaccines are most promising.

Monophosphoryl lipid A adjuvant reverses a principal histologic parameter of formalin-inactivated respiratory syncytial virus vaccine-induced disease

The mechanisms by which administration of a formalin-inactivated respiratory syncytial virus vaccine resulted in enhanced disease among children after they later became naturally infected with the virus remains largely undefined. After immunization and live virus challenge, the cotton rat demonstrated the histopathologic marker of the enhanced disease, polymorphonuclear leukocyte infiltration of lung alveolar spaces. We now report that immunization with formalin-inactivated vaccine formulated with the adjuvant, 3-deacylated monophosphoryl lipid A, dramatically reduces or eliminates the polymorphonuclear leukocyte infiltration within the alveoli of cotton rats post-challenge. We suggest, that this or similar adjuvants may be beneficial components of candidate non-replicating respiratory syncytial virus vaccines, whose development has been hampered by safety concerns.

Protection against respiratory syncytial virus (RSV) elicited in mice by plasmid DNA immunisation encoding a secreted RSV G protein-derived antigen

Plasmid vectors encoding two different variants, one cytoplasmic and one secreted version, of a candidate vaccine BBG2Na to respiratory syncytial virus (RSV), were constructed and evaluated in a nucleic acid vaccination study. The two different vectors, which employed the Semliki Forest virus gene amplification system, were found to express BBG2Na appropriately in in vitro cell cultures. Immunisation of mice with the plasmid vectors elicited significant serum anti-BBG2Na IgG responses only in the mice receiving the plasmid encoding the secreted version of BBG2Na. Consistent with antibody induction data, sterilising lung protection against RSV-A challenge was also only observed in this group. These results indicate that the targeting of antigen expression (intracellular versus secreted) would be an important factor to consider in the design of nucleic acid vaccines.

Efficacy and safety studies of a recombinant chimeric respiratory syncytial virus FG glycoprotein vaccine in cotton rats

Several formulations of a recombinant chimeric respiratory syncytial virus (RSV) vaccine consisting of the extramembrane domains of the F and G glycoproteins (FG) were tested in cotton rats to evaluate efficacy and safety. The FG vaccine was highly immunogenic, providing nearly complete resistance to pulmonary infection at doses as low as 25 ng in spite of inducing relatively low levels of serum neutralizing antibody at low vaccine doses. Upon RSV challenge animals primed with FG vaccine showed quite mild alveolitis and interstitial pneumonitis, which were eliminated by the addition of monophosphoryl lipid A to the formulation.