Respiratory syncytial virus nucleoprotein-specific cytotoxic T-cell epitopes in a South African population of diverse HLA types are conserved in circulating field strains

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.

Immunoproteomic Lessons for Human Respiratory Syncytial Virus Vaccine Design

Accurate antiviral humoral and cellular immune responses require prior recognition of antigenic peptides presented by human leukocyte antigen (HLA) class I and II molecules on the surface of antigen-presenting cells. Both the helper and the cytotoxic immune responses are critical for the control and the clearance of human respiratory syncytial virus (HRSV) infection, which is a significant cause of morbidity and mortality in infected pediatric, immunocompromised and elderly populations. In this article we review the immunoproteomics studies which have defined the general antigen processing and presentation rules that determine both the immunoprevalence and the immunodominance of the cellular immune response to HRSV. Mass spectrometry and functional analyses have shown that the HLA class I and II cellular immune responses against HRSV are mainly focused on three viral proteins: fusion, matrix, and nucleoprotein. Thus, these studies have important implications for vaccine development against this virus, since a vaccine construct including these three relevant HRSV proteins could efficiently stimulate the major components of the adaptive immune system: humoral, helper, and cytotoxic effector immune responses.

Aberrant T cell immunity triggered by human Respiratory Syncytial Virus and human Metapneumovirus infection

Human Respiratory syncytial virus (hRSV) and human metapneumovirus (hMPV) are the two major etiological viral agents of lower respiratory tract diseases, affecting mainly infants, young children and the elderly. Although the infection of both viruses trigger an antiviral immune response that mediate viral clearance and disease resolution in immunocompetent individuals, the promotion of long-term immunity appears to be deficient and reinfection are common throughout life. A possible explanation for this phenomenon is that hRSV and hMPV, can induce aberrant T cell responses, which leads to exacerbated lung inflammation and poor T and B cell memory immunity. The modulation of immune response exerted by both viruses include different strategies such as, impairment of immunological synapse mediated by viral proteins or soluble factors, and the induction of pro-inflammatory cytokines by epithelial cells, among others. All these viral strategies contribute to the alteration of the adaptive immunity in order to increase the susceptibility to reinfections. In this review, we discuss current research related to the mechanisms underlying the impairment of T and B cell immune responses induced by hRSV and hMPV infection. In addition, we described the role each virulence factor involved in immune modulation caused by these viruses.

Human CD8(+) T Cells Target Multiple Epitopes in Respiratory Syncytial Virus Polymerase

Respiratory syncytial virus (RSV) infection is a serious health problem in young children, immunocompromised patients, and the elderly. The development of novel prevention strategies, such as a vaccine to RSV, is a high priority. One strategy is to design a peptide-based vaccine that activates appropriate CD8(+) T-cell responses. However, this approach is limited by the low number of RSV peptide epitopes defined to date that activate CD8(+) T cells. We aimed to identify peptide epitopes that are presented by common human leukocyte antigen types (HLA-A*01, -A*02, and -B*07). We identify one novel HLA-A*02-restricted and two novel HLA-A*01-restricted peptide epitopes from RSV polymerase. Peptide-HLA multimer staining of specific T cells from healthy donor peripheral blood mononuclear cell, the memory phenotype of such peptide-specific T cells ex vivo, and functional IFNγ responses in short-term stimulation assays suggest that these peptides are recognized during RSV infection. Such peptides are candidates for inclusion into a peptide-based RSV vaccine designed to stimulate defined CD8(+) T-cell responses.

RSV-specific airway resident memory CD8+ T cells and differential disease severity after experimental human infection

In animal models, resident memory CD8+ T (Trm) cells assist in respiratory virus elimination but their importance in man has not been determined. Here, using experimental human respiratory syncytial virus (RSV) infection, we investigate systemic and local virus-specific CD8+ T-cell responses in adult volunteers. Having defined the immunodominance hierarchy, we analyse phenotype and function longitudinally in blood and by serial bronchoscopy. Despite rapid clinical recovery, we note surprisingly extensive lower airway inflammation with persistent viral antigen and cellular infiltrates. Pulmonary virus-specific CD8+ T cells display a CD69+CD103+ Trm phenotype and accumulate to strikingly high frequencies into convalescence without continued proliferation. While these have a more highly differentiated phenotype, they express fewer cytotoxicity markers than in blood. Nevertheless, their abundance before infection correlates with reduced symptoms and viral load, implying that CD8+ Trm cells in the human lung can confer protection against severe respiratory viral disease when humoral immunity is overcome.

Recent advances in the development of subunit-based RSV vaccines

Respiratory syncytial virus (RSV) is a major cause of lower respiratory tract infections causing pneumonia and bronchiolitis in infants. RSV also causes serious illness in elderly populations, immunocompromised patients and individuals with pulmonary or cardiac problems. The significant morbidity and mortality associated with RSV infection have prompted interest in RSV vaccine development. In the 1960s, a formalin-inactivated vaccine trial failed to protect children, and indeed enhanced pathology when naturally infected later with RSV. Hence, an alternative approach to traditional killed virus vaccines, which can induce protective immunity without serious adverse events, is desired. Several strategies have been explored in attempts to produce effective vaccine candidates including gene-based and subunit vaccines. Subunit-based vaccine approaches have shown promising efficacy in animal studies and several have reached clinical trials. The current stage of development of subunit-based vaccines against RSV is reviewed in this article.

CD8 T-cell response to respiratory syncytial virus infection

ABSTRACT 

Respiratory syncytial virus (RSV) is the leading cause of lower respiratory tract infection and hospitalization among infants. Despite the significant healthcare burden, there is no licensed RSV vaccine currently available. This problem is further exacerbated as a natural RSV infection fails to elicit the development of long-lived immunity. It is well established that RSV-specific antibodies play a critical role in mediating protection from severe disease. The CD8 T-cell response is critical for mediating virus clearance following an acute RSV infection. However, the relative contribution of memory CD8 T cells in providing protection against secondary RSV infections remains unclear. In addition, data from animal models indicate that memory CD8 T-cell responses can be pathogenic under certain conditions. Herein, we provide an overview of the CD8 T-cell response elicited by RSV infection and how our current knowledge may impact future studies and vaccine development.

Serologic cross-reactions between nucleocapsid proteins of human respiratory syncytial virus and human metapneumovirus

Human respiratory syncytial virus (hRSV) and human metapneumovirus (hMPV) share virologic and epidemiologic features and cause clinically similar respiratory illness predominantly in young children. In a previous study of acute febrile respiratory illness in Bangladesh, we tested paired serum specimens from 852 children presenting fever and cough for diagnostic increases in titers of antibody to hRSV and hMPV by enzyme immunoassay (EIA). Unexpectedly, of 93 serum pairs that showed a ≥ 4-fold increase in titers of antibody to hRSV, 24 (25.8%) showed a concurrent increase in titers of antibody to hMPV; of 91 pairs showing an increase to hMPV, 13 (14.3%) showed a concurrent increase to hRSV. We speculated that common antigens shared by these viruses explain this finding. Since the nucleocapsid (N) proteins of these viruses show the greatest sequence homology, we tested hyperimmune antisera prepared for each virus against baculovirus-expressed recombinant N (recN) proteins for potential cross-reactivity. The antisera were reciprocally reactive with both proteins. To localize common antigenic regions, we first expressed the carboxy domain of the hMPV N protein that was the most highly conserved region within the hRSV N protein. Although reciprocally reactive with antisera by Western blotting, this truncated protein did not react with hMPV IgG-positive human sera by EIA. Using 5 synthetic peptides that spanned the amino-terminal portion of the hMPV N protein, we identified a single peptide that was cross-reactive with human sera positive for either virus. Antiserum prepared for this peptide was reactive with recN proteins of both viruses, indicating that a common immunoreactive site exists in this region.

Surface expression of the hRSV nucleoprotein impairs immunological synapse formation with T cells

Human respiratory syncytial virus (hRSV) is the leading cause of bronchiolitis and pneumonia in young children worldwide. The recurrent hRSV outbreaks and reinfections are the cause of a significant public health burden and associate with an inefficient antiviral immunity, even after disease resolution. Although several mouse- and human cell-based studies have shown that hRSV infection prevents naïve T-cell activation by antigen-presenting cells, the mechanism underlying such inhibition remains unknown. Here, we show that the hRSV nucleoprotein (N) could be at least partially responsible for inhibiting T-cell activation during infection by this virus. Early after infection, the N protein was expressed on the surface of epithelial and dendritic cells, after interacting with trans-Golgi and lysosomal compartments. Further, experiments on supported lipid bilayers loaded with peptide-MHC (pMHC) complexes showed that surface-anchored N protein prevented immunological synapse assembly by naive CD4(+) T cells and, to a lesser extent, by antigen-experienced T-cell blasts. Synapse assembly inhibition was in part due to reduced T-cell receptor (TCR) signaling and pMHC clustering at the T-cell-bilayer interface, suggesting that N protein interferes with pMHC-TCR interactions. Moreover, N protein colocalized with the TCR independently of pMHC, consistent with a possible interaction with TCR complex components. Based on these data, we conclude that hRSV N protein expression at the surface of infected cells inhibits T-cell activation. Our study defines this protein as a major virulence factor that contributes to impairing acquired immunity and enhances susceptibility to reinfection by hRSV.

Gold nanorod vaccine for respiratory syncytial virus

Respiratory syncytial virus (RSV) is a major cause of pneumonia and wheezing in infants and the elderly, but to date there is no licensed vaccine. We developed a gold nanorod construct that displayed the major protective antigen of the virus, the fusion protein (F). Nanorods conjugated to RSV F were formulated as a candidate vaccine preparation by covalent attachment of viral protein using a layer-by-layer approach. In vitro studies using ELISA, electron microscopy and circular dichroism revealed that conformation-dependent epitopes were maintained during conjugation, and transmission electron microscopy studies showed that a dispersed population of particles could be achieved. Human dendritic cells treated with the vaccine induced immune responses in primary human T cells. These results suggest that this vaccine approach may be a potent method for immunizing against viruses such as RSV with surface glycoproteins that are targets for the human immune response.

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.

Immunoprotectivity of HLA-A2 CTL peptides derived from respiratory syncytial virus fusion protein in HLA-A2 transgenic mouse

Identification of HLA-restricted CD8⁺ T cell epitopes is important to study RSV-induced immunity and illness. We algorithmically analyzed the sequence of the fusion protein (F) of respiratory syncytial virus (RSV) and generated synthetic peptides that can potentially bind to HLA-A*0201. Four out of the twenty-five 9-mer peptides tested: peptides 3 (F33–41), 13 (F214–222), 14 (F273–281), and 23 (F559–567), were found to bind to HLA-A*0201 with moderate to high affinity and were capable of inducing IFN-γ and IL-2 secretion in lymphocytes from HLA-A*0201 transgenic (HLA-Tg) mice pre-immunized with RSV or recombinant adenovirus expressing RSV F. HLA-Tg mice were immunized with these four peptides and were found to induce both Th1 and CD8⁺ T cell responses in in vitro secondary recall. Effector responses induced by these peptides were observed to confer differential protection against live RSV challenge. These peptides also caused better recovery of body weight loss induced by RSV. A significant reduction of lung viral load was observed in mice immunized with peptide 23, which appeared to enhance the levels of inflammatory chemokines (CCL17, CCL22, and IL-18) but did not increase eosinophil infiltration in the lungs. Whereas, significant reduction of infiltrated eosinophils induced by RSV infection was found in mice pre-immunized with peptide 13. Our results suggest that HLA-A2-restricted epitopes of RSV F protein could be useful for the development of epitope-based RSV vaccine.

Identification of potential human respiratory syncytial virus and metapneumovirus T cell epitopes using computational prediction and MHC binding assays

Human respiratory syncytial virus (RSV) and human metapneumovirus (MPV) are two of the most common causes of serious viral lower respiratory tract illness in humans. CD8+ T cells have been shown to be important in animal models and human clinical studies for the clearance of viral infection, and they may contribute in part to protection against severe disease during reinfections. Precise enumeration and accurate phenotyping of RSV- or MPV-specific CD8+ T cells in humans is currently limited by the relatively small number of T cell epitopes that have been mapped with accompanying identification of MHC restriction patterns. We sought to expand the number of potential RSV and MPV epitopes for use in clinical and translational studies by identifying an expanded set of MHC-binding peptides based on RSV and MPV wild-type virus strain protein sequences. We interrogated the full protein sequences of all 9 or 11 proteins of MPV or RSV respectively using four established epitope prediction algorithms for human HLA A*0101, A*0201, or B*0702 binding and attempted to synthesize the top-scoring 150-152 peptides for each of the two viruses. Synthesis resulted in 442 synthesized and soluble peptides of the 452 predicted epitopes for MPV or RSV. We then determined the binding of the synthetic peptides to recombinant human HLA A*0101, A*0201 or B*0702 molecules with the predicted restriction using a commercially available plate-based assay, iTopia. A total of 230 of the 442 peptides tested exhibited binding to the appropriate MHC molecule. The binding results suggested that existing algorithms for prediction of MHC A*0201 binding are particularly robust. The binding results also provided a large benchmarking data collection for comparison of new prediction algorithms.

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.

Conserved cysteine residues within the attachment G glycoprotein of respiratory syncytial virus play a critical role in the enhancement of cytotoxic T-lymphocyte responses

The cytotoxic T-lymphocyte (CTL) response plays an important role in the control of respiratory syncytial virus (RSV) replication and the establishment of a Th1-CD4+ T cell response against the virus. Despite lacking Major Histocompatibility Complex I (MHC I)-restricted epitopes, the attachment G glycoprotein of RSV enhances CTL activity toward other RSV antigens, and this effect depends on its conserved central region. Here, we report that RSV-G can also improve CTL activity toward antigens from unrelated pathogens such as influenza, and that a mutant form of RSV-G lacking four conserved cysteine residues at positions 173, 176, 182, and 186 fails to enhance CTL responses. Our results indicate that these conserved residues are essential for the wide-spectrum pro-CTL activity displayed by the protein.

Unusual viral ligand with alternative interactions is presented by HLA-Cw4 in human respiratory syncytial virus-infected cells

Short viral antigens bound to human major histocompatibility complex (HLA) class I molecules are presented on infected cells. Vaccine development frequently relies on synthetic peptides to identify optimal HLA class I ligands. However, when natural peptides are analyzed, more complex mixtures are found. By immunoproteomics analysis, we identify in this study a physiologically processed HLA ligand derived from the human respiratory syncytial virus matrix protein that is very different from what was expected from studies with synthetic peptides. This natural HLA-Cw4 class I ligand uses alternative interactions to the anchor motifs previously described for its presenting HLA-Cw4 class I molecule. Finally, this octameric peptide shares its C-terminal core with the H-2D(b) nonamer ligand previously identified in the mouse model. These data have implications for the identification of antiviral cytotoxic T lymphocyte responses and for vaccine development.

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.

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.

Multiple, non-conserved, internal viral ligands naturally presented by HLA-B27 in human respiratory syncytial virus-infected cells

Cytotoxic T lymphocyte (CTL)-mediated death of virus-infected cells requires prior recognition of short viral peptide antigens that are presented by human leukocyte antigen (HLA) class I molecules on the surface of infected cells. The CTL response is critical for the clearance of human respiratory syncytial virus (HRSV) infection. Using mass spectrometry analysis of complex HLA-bound peptide pools isolated from large amounts of HRSV-infected cells, we identified nine naturally processed HLA-B27 ligands. The isolated peptides are derived from six internal, not envelope, proteins of the infective virus. The sequences of most of these ligands are not conserved between different HRSV strains, suggesting a mechanism to explain recurrent infection with virus of different HRSV antigenic subgroups. In addition, these nine ligands represent a significant fraction of the proteome of this virus, which is monitored by the same HLA class I allele. These data have implications for vaccine development as well as for analysis of the CTL response.

Pulmonary immunity and immunopathology: lessons from respiratory syncytial virus

Respiratory syncytial virus (RSV) is the leading cause of severe respiratory disease in infants and is an important source of morbidity and mortality in the elderly and immunocompromised. This review will discuss the humoral and cellular adaptive immune responses to RSV infection and how these responses are shaped in the immature immune system of the infant and the aged environment of the elderly. Furthermore, we will provide an overview of our current understanding of the role the various arms of the adaptive immune response play in mediating the delicate balance between the successful elimination of the virus from the host and the induction of immunopathology. Efficacious immunization against RSV remains a high priority within the field and we will highlight recent advances made in vaccine design.

Enhancement of the CD8+ T cell response to a subdominant epitope of respiratory syncytial virus by deletion of an immunodominant epitope

Cytotoxic T lymphocytes (CTLs) are critical for the control of respiratory syncytial virus infection (RSV) in humans and mice. Recently, we identified a new H-2K(d)-restricted subdominant epitope in the respiratory syncytial virus M2 protein. In this study, we investigated if modification of anchor residues at positions 2 and 9 in the dominant M2(82-90) epitope in the M2 protein would alter the CTL epitope dominance hierarchy following immunization with plasmid DNA encoding M2 proteins. We showed that immunogenicity of the subdominant epitope M2(127-135) was enhanced when the anchor residues of the dominant epitope were mutated, suggesting that the immunodominant epitope induces a suppression of response to the subdominant epitope.

Major histocompatibility complex class I cytotoxic T lymphocyte immunity to human metapneumovirus (hMPV) in individuals with previous hMPV infection and respiratory disease

Recently identified human metapneumovirus (hMPV) is an important respiratory pathogen in children and adults worldwide. Little is known about cytotoxic T lymphocyte (CTL) responses that may control hMPV infection in humans. To address this, we evaluated major histocompatibility complex (MHC) class I T cell immunity in 7 patients with previous hMPV respiratory disease. CTL responses were evident in most patients and to most proteins of hMPV. Individual patients had responses to at least 2 hMPV proteins (particularly the M protein) and had multiallele responses. In addition, we identified 9 CTL epitopes that are presented by human leukocyte antigen alleles of the most common MHC "supertypes." Many of these CTL epitopes are conserved across hMPV types, and there is epitope similarity between hMPV and human respiratory syncytial virus. This study provides the first report of MHC class I T cell immunity to hMPV in humans. These findings have significance for understanding cellular immunity to hMPV infection and for future vaccine development.

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.

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.

Tissue-specific regulation of CD8+ T-lymphocyte immunodominance in respiratory syncytial virus infection

Cytotoxic T lymphocytes (CTLs) are critical for control of respiratory syncytial virus (RSV) infection in humans and mice. To investigate cellular immune responses to infection, it is important to identify major histocompatibility complex (MHC) class I-restricted CTL epitopes. In this study, we identified a new RSV-specific, H-2K(d)-restricted subdominant epitope in the M2 protein, M2(127-135) (amino acids 127 to 135). This finding allowed us to study the frequency of T lymphocytes responding to two H-2K(d)-presented epitopes in the same protein following RSV infection by enzyme-linked immunospot (ELISPOT) and intracellular cytokine assays for both lymphoid and nonlymphoid tissues. For the subdominant epitope, we identified an optimal nine-amino-acid peptide, VYNTVISYI, which contained an H-2K(d) consensus sequence with Y at position 2 and I at position 9. In addition, an MHC class I stabilization assay using TAP-2-deficient RMA-S cells transfected with K(d) or L(d) indicated that the epitope was presented by K(d). The ratios of T lymphocytes during the peak CTL response to RSV infection that were specific for M2(82-90) (dominant) to T lymphocytes specific for M2(127-135) (subdominant) were approximately 3:1 in the spleen and 10:1 in the lung. These ratios were observed consistently in primary or secondary infection by the ELISPOT assay and in secondary infection by MHC/peptide tetramer staining. The number of antigen-specific T lymphocytes dropped in the 6 weeks after infection; however, the proportions of T lymphocytes specific for the immunodominant and subdominant epitopes were maintained to a remarkable degree in a tissue-specific manner. These studies will facilitate investigation of the regulation of immunodominance of RSV-specific CTL epitopes.

T Cell Responses to Respiratory Syncytial Virus Fusion and Attachment Proteins in Human Peripheral Blood Mononuclear Cells

The cellular immune response to respiratory syncytial virus (RSV) is considered important in both protection and immunopathogenesis. We have studied the HLA class I- and class II-restricted T cell responses to RSV fusion (F) and attachment (G) proteins in peripheral blood mononuclear cells (PBMCs) obtained from healthy young adults. PBMCs were stimulated with autologous cells infected with recombinant modified vaccinia virus Ankara (rMVA) expressing RSV F (rMVA-F) or G (rMVA-G). In rMVA-F-stimulated bulk cultures F-specific CD4(+) and CD8(+) T cell responses were demonstrated, whereas in rMVA-G-stimulated cultures only G-specific CD4(+) T cell responses were detected. Using a set of overlapping peptides spanning the F protein, a number of the F-specific T cell responses could be mapped to different antigenic regions, whereas for the G protein only CD4(+) T cell responses recognizing the central conserved domain could be detected. These results suggest that the RSV glycoprotein-specific T cell response is directed to a number of different epitopes. Further studies must be performed to confirm the apparent inability of the RSV G protein to induce CD8(+) T cell responses. The rMVA-based in vitro stimulation protocol will be useful to define protein-specific T cell responses in different viral systems.

The cysteine-rich region and secreted form of the attachment G glycoprotein of respiratory syncytial virus enhance the cytotoxic T-lymphocyte response despite lacking major histocompatibility complex class I-restricted epitopes

The cytotoxic T-lymphocyte (CTL) response is important for the control of viral replication during respiratory syncytial virus (RSV) infection. The attachment glycoprotein (G) of RSV does not encode major histocompatibility complex class I-restricted epitopes in BALB/c mice (H-2(d)). Furthermore, studies to date have described an absence of significant CTL activity directed against this protein in humans. Therefore, G previously was not considered necessary for the generation of RSV-specific CTL responses. In this study, we demonstrate that, despite lacking H-2(d)-restricted epitopes, G enhances the generation of an effective CTL response against RSV. Furthermore, we show that this stimulatory effect is independent of virus titers and RSV-induced inflammation; that it is associated primarily with the secreted form of G; and that the effect depends on the cysteine-rich region of G (GCRR), a segment conserved in wild-type isolates worldwide. These findings reveal a novel function for the GCRR with potential implications for the generation of protective cellular responses and vaccine development.

Amino acid variation within the fusion protein of respiratory syncytial virus subtype A and B strains during annual epidemics in South Africa

Recent evidence of positive selection within the cytotoxic T-cell (CTL) epitopes of the highly conserved nucleoprotein of influenza virus raised the question of whether the CTL epitopes of Respiratory syncytial virus (RSV) are also affected by immune driven change over annual epidemics. The fusion protein (F-protein) of RSV is highly conserved within the two subtypes (A and B) and the most important target for the protective response. The position of various neutralizing epitopes has been mapped and characterized between RSV subtypes. CTL epitopes have also recently been mapped for the F-protein of subtype A, however variation within these epitopes between and within the subtypes has not been determined. To address this question, the F-proteins of 18 strains representative of all subgroup A and B genotypes identified in South Africa over a period of 5 years were sequenced. F-protein sequences were highly conserved within and between South African genotypes, with most variability occurring at the nucleotide level. Most of the amino acid differences identified within neutralizing and CTL epitopes were conserved within the subtypes, and therefore does not indicate immune selection. However, out of three CTL epitopes previously identified in subtype A, two (restricted to HLA B*57 and HLA A *01) were conserved only within subtype A, while the third (restricted to Cw*12) contained both subtype- and genotype-specific changes. These results suggest that most of the identified CTL epitopes are subtype A-specific and may not be recognized in subtype B viruses, while the HLA Cw*12 restricted epitope may also not be recognized efficiently in GA5 strains.

Human CD8(+) T cell responses against five newly identified respiratory syncytial virus-derived epitopes

CD8(+) T lymphocytes play a major role in the clearance of respiratory syncytial virus (RSV) infections. To be able to study the primary CTL response in RSV-infected children, epitopes presented by a set of commonly used HLA alleles (HLA-A1, -A3, -B44 and -B51) were searched for. Five epitopes were characterized derived from the matrix (M), non-structural (NS2) and second matrix (M2) proteins of RSV. All epitopes were shown to be processed and presented by RSV-infected antigen-presenting cells. HLA-A1 tetramers for one of these epitopes derived from the M protein were constructed and used to quantify and phenotype the memory CD8(+) T cell pool in a panel of healthy adult donors. In about 60 % of the donors, CD8(+) T cells specific for the M protein could be identified. These cells belonged to the memory T cell subset characterized by expression of CD27 and CD28, and down-regulation of CCR7 and CD45RA. The frequency of tetramer-positive cells varied between 0.4 and 3 per 10(4) CD8(+) T cells in PBMC of healthy asymptomatic adult donors.

Identification of a common HLA-DP4-restricted T-cell epitope in the conserved region of the respiratory syncytial virus G protein

The cellular immune response to respiratory syncytial virus (RSV) is important in both protection and immunopathogenesis. In contrast to HLA class I, HLA class II-restricted RSV-specific T-cell epitopes have not been identified. Here, we describe the generation and characterization of two human RSV-specific CD4(+)-T-cell clones (TCCs) associated with type 0-like cytokine profiles. TCC 1 was specific for the matrix protein and restricted over HLA-DPB1*1601, while TCC 2 was specific for the attachment protein G and restricted over either HLA-DPB1*0401 or -0402. Interestingly, the latter epitope is conserved in both RSV type A and B viruses. Given the high allele frequencies of HLA-DPB1*0401 and -0402 worldwide, this epitope could be widely recognized and boosted by recurrent RSV infections. Indeed, peptide stimulation of peripheral blood mononuclear cells from healthy adults resulted in the detection of specific responses in 8 of 13 donors. Additional G-specific TCCs were generated from three of these cultures, which recognized the identical (n = 2) or almost identical (n = 1) HLA-DP4-restricted epitope as TCC 2. No significant differences were found between the capacities of cell lines obtained from infants with severe (n = 41) or mild (n = 46) RSV lower respiratory tract infections to function as antigen-presenting cells to the G-specific TCCs, suggesting that the severity of RSV disease is not linked to the allelic frequency of HLA-DP4. In conclusion, we have identified an RSV G-specific human T helper cell epitope restricted by the widely expressed HLA class II alleles DPB1*0401 and -0402. Its putative role in protection and/or immunopathogenesis remains to be determined.