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

All Eyes on the Prefusion-Stabilized F Construct, but Are We Missing the Potential of Alternative Targets for Respiratory Syncytial Virus Vaccine Design?

Respiratory Syncytial Virus (RSV) poses a significant global health concern as a major cause of lower respiratory tract infections (LRTIs). Over the last few years, substantial efforts have been directed towards developing vaccines and therapeutics to combat RSV, leading to a diverse landscape of vaccine candidates. Notably, two vaccines targeting the elderly and the first maternal vaccine have recently been approved. The majority of the vaccines and vaccine candidates rely solely on a prefusion-stabilized conformation known for its highly neutralizing epitopes. Although, so far, this antigen design appears to be successful for the elderly, our current understanding remains incomplete, requiring further improvement and refinement in this field. Pediatric vaccines still have a long journey ahead, and we must ensure that vaccines currently entering the market do not lose efficacy due to the emergence of mutations in RSV's circulating strains. This review will provide an overview of the current status of vaccine designs and what to focus on in the future. Further research into antigen design is essential, including the exploration of the potential of alternative RSV proteins to address these challenges and pave the way for the development of novel and effective vaccines, especially in the pediatric population.

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

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

Utility of the Neonatal Calf Model for Testing Vaccines and Intervention Strategies for Use against Human RSV Infection

Respiratory syncytial virus (RSV) is a significant cause of pediatric respiratory tract infections. It is estimated that two-thirds of infants are infected with RSV during the first year of life and it is one of the leading causes of death in this age group worldwide. Similarly, bovine RSV is a primary viral pathogen in cases of pneumonia in young calves and plays a significant role in bovine respiratory disease complex. Importantly, naturally occurring infection of calves with bovine RSV shares many features in common with human RSV infection. Herein, we update our current understanding of RSV infection in cattle, with particular focus on similarities between the calf and human infection, and the recent reports in which the neonatal calf has been employed for the development and testing of vaccines and therapeutics which may be applied to hRSV infection in humans.

Bovine model of respiratory syncytial virus infection

Bovine respiratory syncytial virus (BRSV), which is an important cause of respiratory disease in young calves, is genetically and antigenically closely related to human (H)RSV. The epidemiology and pathogenesis of infection with these viruses are similar. The viruses are host-specific and infection produces a spectrum of disease ranging from subclinical to severe bronchiolitis and pneumonia, with the peak incidence of severe disease in individuals less than 6 months of age. BRSV infection in calves reproduces many of the clinical signs associated with HRSV in infants, including fever, rhinorrhoea, coughing, harsh breath sounds and rapid breathing. Although BRSV vaccines have been commercially available for decades, there is a need for greater efficacy. The development of effective BRSV and HRSV vaccines face similar challenges, such as the need to vaccinate at an early age in the presence of maternal antibodies, the failure of natural infection to prevent reinfection, and a history of vaccine-augmented disease. Neutralising monoclonal antibodies (mAbs) to the fusion (F) protein of HRSV, which can protect infants from severe HRSV disease, recognise the F protein of BRSV, and vice versa. Furthermore, bovine and human CD8(+) T-cells, which are known to be important in recovery from RSV infection, recognise similar proteins that are conserved between HRSV and BRSV. Therefore, not only can the bovine model of RSV be used to evaluate vaccine concepts, it can also be used as part of the preclinical assessment of certain HRSV candidate vaccines.

Nucleoprotein nanostructures combined with adjuvants adapted to the neonatal immune context: a candidate mucosal RSV vaccine

Background

The human respiratory syncytial virus (hRSV) is the leading cause of severe bronchiolitis in infants worldwide. The most severe RSV diseases occur between 2 and 6 months-of-age, so pediatric vaccination will have to be started within the first weeks after birth, when the immune system is prone to Th2 responses that may turn deleterious upon exposure to the virus. So far, the high risk to prime for immunopathological responses in infants has hampered the development of vaccine. In the present study we investigated the safety and efficacy of ring-nanostructures formed by the recombinant nucleoprotein N of hRSV (N^SRS) as a mucosal vaccine candidate against RSV in BALB/c neonates, which are highly sensitive to immunopathological Th2 imprinting.

Methodology and Principal Findings

A single intranasal administration of N^SRS with detoxified E.coli enterotoxin LT(R192G) to 5–7 day old neonates provided a significant reduction of the viral load after an RSV challenge at five weeks of age. However, neonatal vaccination also generated an enhanced lung infiltration by neutrophils and eosinophils following the RSV challenge. Analysis of antibody subclasses and cytokines produced after an RSV challenge or a boost administration of the vaccine suggested that neonatal vaccination induced a Th2 biased local immune memory. This Th2 bias and the eosinophilic reaction could be prevented by adding CpG to the vaccine formulation, which, however did not prevent pulmonary inflammation and neutrophil infiltration upon viral challenge.

Conclusions/Significance

In conclusion, protective vaccination against RSV can be achieved in neonates but requires an appropriate combination of adjuvants to prevent harmful Th2 imprinting.

Efficient lung recruitment of respiratory syncytial virus-specific Th1 cells induced by recombinant bacillus Calmette-Guérin promotes virus clearance and protects from infection

Infection by the respiratory syncytial virus (RSV) can cause extensive inflammation and lung damage in susceptible hosts due to a Th2-biased immune response. Such a deleterious inflammatory response can be enhanced by immunization with formalin- or UV-inactivated RSV, as well as with vaccinia virus expressing the RSV-G protein. Recently, we have shown that vaccination with rBCG-expressing RSV Ags can prevent the disease in the mouse. To further understand the immunological mechanisms responsible for protection against RSV, we have characterized the T cell populations contributing to virus clearance in mice immunized with this BCG-based vaccine. We found that both CD4(+) and CD8(+) T cells were recruited significantly earlier to the lungs of infected mice that were previously vaccinated. Furthermore, we observed that simultaneous adoptive transfer of CD8(+) and CD4(+) RSV-specific T cells from vaccinated mice was required to confer protection against virus infection in naive recipients. In addition, CD4(+) T cells induced by vaccination released IFN-γ after RSV challenge, indicating that protection is mediated by a Th1 immune response. These data suggest that vaccination with rBCG-expressing RSV Ags can induce a specific effector/memory Th1 immune response consisting on CD4(+) and CD8(+) T cells, both necessary for a fully protective response against RSV. These results support the notion that an effective induction of Th1 T cell immunity against RSV during childhood could counteract the unbalanced Th2-like immune response triggered by the natural RSV infection.

A new subunit vaccine based on nucleoprotein nanoparticles confers partial clinical and virological protection in calves against bovine respiratory syncytial virus

Human and bovine respiratory syncytial viruses (HRSV and BRSV) are two closely related, worldwide prevalent viruses that are the leading cause of severe airway disease in children and calves, respectively. Efficacy of commercial bovine vaccines needs improvement and no human vaccine is licensed yet. We reported that nasal vaccination with the HRSV nucleoprotein produced as recombinant ring-shaped nanoparticles (N(SRS)) protects mice against a viral challenge with HRSV. The aim of this work was to evaluate this new vaccine that uses a conserved viral antigen, in calves, natural hosts for BRSV. Calves, free of colostral or natural anti-BRSV antibodies, were vaccinated with N(SRS) either intramuscularly, or both intramuscularly and intranasally using Montanide ISA71 and IMS4132 as adjuvants and challenged with BRSV. All vaccinated calves developed anti-N antibodies in blood and nasal secretions and N-specific cellular immunity in local lymph nodes. Clinical monitoring post-challenge demonstrated moderate respiratory pathology with local lung tissue consolidations for the non-vaccinated calves that were significantly reduced in the vaccinated calves. Vaccinated calves had lower viral loads than the non-vaccinated control calves. Thus N(SRS) vaccination in calves provided cross-protective immunity against BRSV infection without adverse inflammatory reaction.

Gene optimization leads to robust expression of human respiratory syncytial virus nucleoprotein and phosphoprotein in human cells and induction of humoral immunity in mice

Human respiratory syncytial virus (HRSV) is the major pathogen leading to respiratory disease in infants and neonates worldwide. An effective vaccine has not yet been developed against this virus, despite considerable efforts in basic and clinical research. HRSV replication is independent of the nuclear RNA processing constraints, since the virus genes are adapted to the cytoplasmic transcription, a process performed by the viral RNA-dependent RNA polymerase. This study shows that meaningful nuclear RNA polymerase II dependent expression of the HRSV nucleoprotein (N) and phosphoprotein (P) proteins can only be achieved with the optimization of their genes, and that the intracellular localization of N and P proteins changes when they are expressed out of the virus replication context. Immunization tests performed in mice resulted in the induction of humoral immunity using the optimized genes. This result was not observed for the non-optimized genes. In conclusion, optimization is a valuable tool for improving expression of HRSV genes in DNA vaccines.

Overcoming T cell-mediated immunopathology to achieve safe RSV vaccination

Respiratory syncytial virus (RSV) is the leading cause of lower respiratory tract disease in young children. Premature infants, immunocompromised individuals, and the elderly exhibit an increased risk for the development of severe disease after RSV infection. Currently, there is not a safe and effective RSV vaccine available, in part due to our incomplete understanding of how severe immunopathology was induced following RSV infection of children previously immunized with a formalin-inactivated RSV vaccine. Much of our current understanding of RSV vaccine-enhanced disease can be attributed to the establishment of multiple mouse models of RSV vaccination. Studies analyzing the RSV-specific immune response in mice have clearly demonstrated that both CD4 and CD8 memory T cells contribute to RSV-induced immunopathology. In this review we will focus our discussion on data generated from the mouse models of RSV immunization that have advanced our understanding of how virus-specific T cells mediate immunopathology and RSV vaccine-enhanced disease.

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.

Sub-nucleocapsid nanoparticles: a nasal vaccine against respiratory syncytial virus

BACKGROUND

Bronchiolitis caused by the respiratory syncytial virus (RSV) in infants less than two years old is a growing public health concern worldwide, and there is currently no safe and effective vaccine. A major component of RSV nucleocapsid, the nucleoprotein (N), has been so far poorly explored as a potential vaccine antigen, even though it is a target of protective anti-viral T cell responses and is remarkably conserved between human RSV A and B serotypes. We recently reported a method to produce recombinant N assembling in homogenous rings composed of 10-11 N subunits enclosing a bacterial RNA. These nanoparticles were named sub-nucleocapsid ring structure (N SRS).

METHODOLOGY AND PRINCIPAL FINDINGS

The vaccine potential of N SRS was evaluated in a well-characterized and widely acknowledged mouse model of RSV infection. BALB/c adult mice were immunized intranasally with N SRS adjuvanted with the detoxified E. coli enterotoxin LT(R192G). Upon RSV challenge, vaccinated mice were largely protected against virus replication in the lungs, with a mild inflammatory lymphocytic and neutrophilic reaction in their airways. Mucosal immunization with N SRS elicited strong local and systemic immunity characterized by high titers of IgG1, IgG2a and IgA anti-N antibodies, antigen-specific CD8(+) T cells and IFN-gamma-producing CD4(+) T cells.

CONCLUSIONS/SIGNIFICANCE

This is the first report of using nanoparticles formed by the recombinant nucleocapsid protein as an efficient and safe intra-nasal vaccine against RSV.

Sequence-optimised E2 constructs from BVDV-1b and BVDV-2 for DNA immunisation in cattle

We report DNA immunisation experiments in cattle using plasmid constructs that encoded glycoprotein E2 from bovine viral diarrhoea virus (BVDV)-1 (E2.1) and BVDV-2 (E2.2). The coding sequences were optimised for efficient expression in mammalian cells. A modified leader peptide sequence from protein gD of BoHV1 was inserted upstream of the E2 coding sequences for efficient membrane export of the proteins. Recombinant E2 were efficiently expressed in COS7 cells and they presented the native viral epitopes as judged by differential recognition by antisera from cattle infected with BVDV-1 or BVDV-2. Inoculation of pooled plasmid DNA in young cattle elicited antibodies capable of neutralising viral strains representing the major circulating BVDV genotypes.

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.