A systemic neutrophil response precedes robust CD8(+) T-cell activation during natural respiratory syncytial virus infection in infants

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.

Pulmonary Dendritic Cell Subsets Shape the Respiratory Syncytial Virus-Specific CD8+ T Cell Immunodominance Hierarchy in Neonates

Young infants are generally more susceptible to viral infections and experience more severe disease than do adults. CD8⁺ T cells are important for viral clearance, and although often ineffective in neonates they can be protective when adequately stimulated. Using a murine CB6F1/J hybrid model of respiratory syncytial virus (RSV) infection, we previously demonstrated that the CD8⁺ T cell immunodominance hierarchy to two RSV-derived epitopes, K^dM2_82-90 and D^bM_187-195, was determined by the age at infection. To determine whether age-dependent RSV-specific CD8⁺ T cell responses could be modified through enhanced innate signaling, we used TLR4 or TLR9 agonist treatment at the time of infection, which remarkably changed the neonatal codominant response to an adult-like K^dM2_82-90 CD8⁺ T cell immunodominant response. This shift was associated with an increase in the number of conventional dendritic cells, CD11b⁺ and CD103⁺ dendritic cells, in the lung-draining lymph node, as well as increased expression of the costimulatory molecule CD86. The magnitude of the K^dM2_82-90 CD8⁺ T cell response in TLR agonist-treated neonates could be blocked with Abs against CD80 and CD86. These studies demonstrate the age-dependent function of conventional dendritic cells, their role in determining immunodominance hierarchy, and epitope-specific CD8⁺ T cell requirements for costimulation, all of which influence the immune response magnitude. The unique impact of TLR agonists on neonatal T cell responses is important to consider for RSV vaccines designed for young infants.

Baicalin from Scutellaria baicalensis blocks respiratory syncytial virus (RSV) infection and reduces inflammatory cell infiltration and lung injury in mice

The roots of Scutellaria baicalensis has been used as a remedy for inflammatory and infective diseases for thousands of years. We evaluated the antiviral activity against respiratory syncytial virus (RSV) infection, the leading cause of childhood infection and hospitalization. By fractionation and chromatographic analysis, we determined that baicalin was responsible for the antiviral activity of S. baicalensis against RSV infection. The concentration for 50% inhibition (IC₅₀) of RSV infection was determined at 19.9 ± 1.8 μM, while the 50% cytotoxic concentration (CC₅₀) was measured at 370 ± 10 μM. We then used a mouse model of RSV infection to further demonstrate baicalin antiviral effect. RSV infection caused significant lung injury and proinflammatory response, including CD4 and CD8 T lymphocyte infiltration. Baicalin treatment resulted in reduction of T lymphocyte infiltration and gene expression of proinflammatory factors, while the treatment moderately reduced RSV titers recovered from the lung tissues. T lymphocyte infiltration and cytotoxic T lymphocyte modulated tissue damage has been identified critical factors of RSV disease. The study therefore demonstrates that baicalin subjugates RSV disease through antiviral and anti-inflammatory effect.

Plant-based vaccines against respiratory diseases: current status and future prospects

INTRODUCTION

Respiratory infections have an enormous, worldwide epidemiologic impact on humans and animals. Among the prophylactic measures, vaccination has the potential to neutralize this impact. New technologies for vaccine production and delivery are of importance in this field since they offer the potential to develop new immunization approaches overriding the current limitations that comprise high cost, safety issues, and limited efficacy. Areas covered: In the present review, the state of the art in developing plant-based vaccines against respiratory diseases is presented. The review was based on the analysis of current biomedical literature. Expert commentary: Preclinical and clinical evaluations of several vaccine candidates against influenza, tuberculosis, respiratory syncytial virus, pneumonia, anthrax and asthma are discussed and placed in perspective.

Diagnostic value of plasma and bronchoalveolar lavage samples in acute lung allograft rejection: differential cytology

Diagnosis of acute lung allograft rejection is currently based on transbronchial lung biopsies. Additional methods to detect acute allograft dysfunction derived from plasma and bronchoalveolar lavage samples might facilitate diagnosis and ultimately improve allograft survival. This review article gives an overview of the cell profiles of bronchoalveolar lavage and plasma samples during acute lung allograft rejection. The value of these cells and changes within the pattern of differential cytology to support the diagnosis of acute lung allograft rejection is discussed. Current findings on the topic are highlighted and trends for future research are identified.

Determinants of early life immune responses to RSV infection

Respiratory syncytial virus causes significant morbidity and mortality in both developed and developing countries, and a vaccine that adequately protects from severe disease remains an important unmet need. RSV disease has an inordinate impact on the very young, and the physical and immunological immaturity of early life complicates vaccine design. Defining and targeting the functional capacities of early life immune responses and controlling responses during primary antigen exposure with selected vaccine delivery approaches will be important for protecting infants by active immunization. Alternatively, vaccination of older children and pregnant mothers may ameliorate disease burden indirectly until infants reach about six months of age, when they can generate more effective anti-RSV immune responses.

Bovine Gamma Delta T Cells Contribute to Exacerbated IL-17 Production in Response to Co-Infection with Bovine RSV and Mannheimia haemolytica

Human respiratory syncytial virus (HRSV) is a leading cause of severe lower respiratory tract infection in children under five years of age. IL-17 and Th17 responses are increased in children infected with HRSV and have been implicated in both protective and pathogenic roles during infection. Bovine RSV (BRSV) is genetically closely related to HRSV and is a leading cause of severe respiratory infections in young cattle. While BRSV infection in the calf parallels many aspects of human infection with HRSV, IL-17 and Th17 responses have not been studied in the bovine. Here we demonstrate that calves infected with BRSV express significant levels of IL-17, IL-21 and IL-22; and both CD4 T cells and γδ T cells contribute to this response. In addition to causing significant morbidity from uncomplicated infections, BRSV infection also contributes to the development of bovine respiratory disease complex (BRDC), a leading cause of morbidity in both beef and dairy cattle. BRDC is caused by a primary viral infection, followed by secondary bacterial pneumonia by pathogens such as Mannheimia haemolytica. Here, we demonstrate that in vivo infection with M. haemolytica results in increased expression of IL-17, IL-21 and IL-22. We have also developed an in vitro model of BRDC and show that co-infection of PBMC with BRSV followed by M. haemolytica leads to significantly exacerbated IL-17 production, which is primarily mediated by IL-17-producing γδ T cells. Together, our results demonstrate that calves, like humans, mount a robust IL-17 response during RSV infection; and suggest a previously unrecognized role for IL-17 and γδ T cells in the pathogenesis of BRDC.

Kinetics of Respiratory Syncytial Virus (RSV) Memphis Strain 37 (M37) Infection in the Respiratory Tract of Newborn Lambs as an RSV Infection Model for Human Infants

Rationale

Respiratory syncytial virus (RSV) infection in preterm and newborn infants can result in severe bronchiolitis and hospitalization. The lamb lung has several key features conducive to modeling RSV infection in human infants, including susceptibility to human strains of RSV such as the A2, Long, and Memphis Strain 37 (M37). In this study, the kinetics of M37 infection was investigated in newborn lambs in order to better define clinical, viral, physiological, and immunological parameters as well as the pathology and lesions.

Methods

Newborn lambs were nebulized with M37 hRSV (6 mL of 1.27 x 10⁷ FFU/mL), monitored daily for clinical responses, and respiratory tissues were collected from groups of lambs at days 1, 3, 4, 6, and 8 post-inoculation for the assessment of viral replication parameters, lesions and also cellular, immunologic and inflammatory responses.

Results

Lambs had increased expiratory effort (forced expiration) at days 4, 6, and 8 post-inoculation. Nasal wash lacked RSV titers at day 1, but titers were present at low levels at days 3 (peak), 4, and 8. Viral titers in bronchoalveolar lavage fluid (BALF) reached a plateau at day 3 (4.6 Log₁₀ FFU/mL), which was maintained until day 6 (4.83 Log₁₀ FFU/mL), and were markedly reduced or absent at day 8. Viral RNA levels (detected by RT-qPCR) in BALF were indistinguishable at days 3 (6.22 ± 0.08 Log₁₀ M37 RNA copies/mL; mean ± se) and 4 (6.20 ± 0.16 Log₁₀ M37 RNA copies/mL; mean ± se) and increased slightly on day 6 (7.15 ± 0.2 Log₁₀ M37 RNA copies/mL; mean ± se). Viral antigen in lung tissue as detected by immunohistochemistry was not seen at day 1, was present at days 3 and 4 before reaching a peak by day 6, and was markedly reduced by day 8. Viral antigen was mainly present in airways (bronchi, bronchioles) at day 3 and was increasingly present in alveolar cells at days 4 and 6, with reduction at day 8. Histopathologic lesions such as bronchitis/bronchiolitis, epithelial necrosis and hyperplasia, peribronchial lymphocyte infiltration, and syncytial cells, were consistent with those described previously for lambs and infants.

Conclusion

This work demonstrates that M37 hRSV replication in the lower airways of newborn lambs is robust with peak replication on day 3 and sustained until day 6. These findings, along with the similarities of lamb lung to those of infants in terms of alveolar development, airway branching and epithelium, susceptibility to human RSV strains, lesion characteristics (bronchiolitis), lung size, clinical parameters, and immunity, further establish the neonatal lamb as a model with key features that mimic RSV infection in infants.

Immunology studies in non-human primate models of tuberculosis

Non-human primates, primarily macaques, have been used to study tuberculosis for decades. However, in the last 15 years, this model has been refined substantially to allow careful investigations of the immune response and host-pathogen interactions in Mycobacterium tuberculosis infection. Low-dose challenge with fully virulent strains in cynomolgus macaques result in the full clinical spectrum seen in humans, including latent and active infection. Reagents from humans are usually cross-reactive with macaques, further facilitating the use of this model system to study tuberculosis. Finally, macaques develop the spectrum of granuloma types seen in humans, providing a unique opportunity to investigate bacterial and host factors at the local (lung and lymph node) level. Here, we review the past decade of immunology and pathology studies in macaque models of tuberculosis.

How Neutrophils Shape Adaptive Immune Responses

Neutrophils are classically considered as cells pivotal for the first line of defense against invading pathogens. In recent years, evidence has accumulated that they are also important in the orchestration of adaptive immunity. Neutrophils rapidly migrate in high numbers to sites of inflammation (e.g., infection, tissue damage, and cancer) and are subsequently able to migrate to draining lymph nodes (LNs). Both at the site of inflammation as well as in the LNs, neutrophils can engage with lymphocytes and antigen-presenting cells. This crosstalk occurs either directly via cell–cell contact or via mediators, such as proteases, cytokines, and radical oxygen species. In this review, we will discuss the current knowledge regarding locations and mechanisms of interaction between neutrophils and lymphocytes in the context of homeostasis and various pathological conditions. In addition, we will highlight the complexity of the microenvironment that is involved in the generation of suppressive or stimulatory neutrophil phenotypes.

Neutrophils in respiratory syncytial virus infection: A target for asthma prevention

Lower respiratory tract infections by respiratory syncytial virus (RSV) are the foremost cause of infant hospitalization and are implicated in lasting pulmonary impairment and the development of asthma. Neutrophils infiltrate the airways of pediatric patients with RSV-induced bronchiolitis in vast numbers: approximately 80% of infiltrated cells are neutrophils. However, why neutrophils are recruited to the site of viral respiratory tract infection is not clear. In this review we discuss the beneficial and pathologic contributions of neutrophils to the immune response against RSV infection. Neutrophils can limit viral replication and spread, as well as stimulate an effective antiviral adaptive immune response. However, low specificity of neutrophil antimicrobial armaments allows for collateral tissue damage. Neutrophil-induced injury to the airways during the delicate period of infant lung development has lasting adverse consequences for pulmonary architecture and might promote the onset of asthma in susceptible subjects. We suggest that pharmacologic modulation of neutrophils should be explored as a viable future therapy for severe RSV-induced bronchiolitis and thereby prevent the inception of subsequent asthma. The antiviral functions of neutrophils suggest that targeting of neutrophils in patients with RSV-induced bronchiolitis is best performed under the umbrella of antiviral treatment.

Partial Attenuation of Respiratory Syncytial Virus with a Deletion of a Small Hydrophobic Gene Is Associated with Elevated Interleukin-1β Responses

The small hydrophobic (SH) gene of respiratory syncytial virus (RSV), a major cause of infant hospitalization, encodes a viroporin of unknown function. SH gene knockout virus (RSV ΔSH) is partially attenuated in vivo, but not in vitro, suggesting that the SH protein may have an immunomodulatory role. RSV ΔSH has been tested as a live attenuated vaccine in humans and cattle, and here we demonstrate that it protected against viral rechallenge in mice. We compared the immune response to infection with RSV wild type and RSV ΔSH in vivo using BALB/c mice and in vitro using epithelial cells, neutrophils, and macrophages. Strikingly, the interleukin-1β (IL-1β) response to RSV ΔSH infection was greater than to wild-type RSV, in spite of a decreased viral load, and when IL-1β was blocked in vivo, the viral load returned to wild-type levels. A significantly greater IL-1β response to RSV ΔSH was also detected in vitro, with higher-magnitude responses in neutrophils and macrophages than in epithelial cells. Depleting macrophages (with clodronate liposome) and neutrophils (with anti-Ly6G/1A8) demonstrated the contribution of these cells to the IL-1β response in vivo, the first demonstration of neutrophilic IL-1β production in response to viral lung infection. In this study, we describe an increased IL-1β response to RSV ΔSH, which may explain the attenuation in vivo and supports targeting the SH gene in live attenuated vaccines.

IMPORTANCE

There is a pressing need for a vaccine for respiratory syncytial virus (RSV). A number of live attenuated RSV vaccine strains have been developed in which the small hydrophobic (SH) gene has been deleted, even though the function of the SH protein is unknown. The structure of the SH protein has recently been solved, showing it is a pore-forming protein (viroporin). Here, we demonstrate that the IL-1β response to RSV ΔSH is greater in spite of a lower viral load, which contributes to the attenuation in vivo. This potentially suggests a novel method by which viruses can evade the host response. As all Pneumovirinae and some Paramyxovirinae carry similar SH genes, this new understanding may also enable the development of live attenuated vaccines for both RSV and other members of the Paramyxoviridae.

Challenges and opportunities in developing respiratory syncytial virus therapeutics

Two meetings, one sponsored by the Wellcome Trust in 2012 and the other by the Global Virology Foundation in 2013, assembled academic, public health and pharmaceutical industry experts to assess the challenges and opportunities for developing antivirals for the treatment of respiratory syncytial virus (RSV) infections. The practicalities of clinical trials and establishing reliable outcome measures in different target groups were discussed in the context of the regulatory pathways that could accelerate the translation of promising compounds into licensed agents. RSV drug development is hampered by the perceptions of a relatively small and fragmented market that may discourage major pharmaceutical company investment. Conversely, the public health need is far too large for RSV to be designated an orphan or neglected disease. Recent advances in understanding RSV epidemiology, improved point-of-care diagnostics, and identification of candidate antiviral drugs argue that the major obstacles to drug development can and will be overcome. Further progress will depend on studies of disease pathogenesis and knowledge provided from controlled clinical trials of these new therapeutic agents. The use of combinations of inhibitors that have different mechanisms of action may be necessary to increase antiviral potency and reduce the risk of resistance emergence.

RSV vaccine-enhanced disease is orchestrated by the combined actions of distinct CD4 T cell subsets

There is no currently licensed vaccine for respiratory syncytial virus (RSV) despite being the leading cause of lower respiratory tract infections in children. Children previously immunized with a formalin-inactivated RSV (FI-RSV) vaccine exhibited enhanced respiratory disease following natural RSV infection. Subsequent studies in animal models have implicated roles for CD4 T cells, eosinophils and non-neutralizing antibodies in mediating enhanced respiratory disease. However, the underlying immunological mechanisms responsible for the enhanced respiratory disease and other disease manifestations associated with FI-RSV vaccine-enhanced disease remain unclear. We demonstrate for the first time that while CD4 T cells mediate all aspects of vaccine-enhanced disease, distinct CD4 T cell subsets orchestrate discrete and specific disease parameters. A Th2-biased immune response, but not eosinophils specifically, was required for airway hyperreactivity and mucus hypersecretion. In contrast, the Th1-associated cytokine TNF-α was necessary to mediate airway obstruction and weight loss. Our data demonstrate that individual disease manifestations associated with FI-RSV vaccine-enhanced disease are mediated by distinct subsets of CD4 T cells.

RSV is a significant healthcare burden and is the leading cause of bronchiolitis and pneumonia during childhood. The failure of the 1960's FI-RSV vaccine trial to not only elicit protection against RSV infection, but also provoke enhanced morbidity and mortality in vaccinees has significantly hampered development of new RSV vaccines for fear of disease potentiation. Therefore we sought to determine the specific immunological mechanisms that mediate FI-RSV VED to provide a framework to evaluate factors associated with disease exacerbation. Work presented herein demonstrate for the first time that individual disease manifestations associated with FI-RSV-immunization are mediated by distinct CD4 T cell subsets and not by eosinophils. Our results stress the need to evaluate multiple disease parameters for future RSV vaccine candidates. Failure to thoroughly assess the immune response and disease manifestations associated with new candidate vaccines may lead to undesired results in vaccine trials and further hinder future vaccine development.

Immunity to RSV in Early-Life

Respiratory Syncytial Virus (RSV) is the commonest cause of severe respiratory infection in infants, leading to over 3 million hospitalizations and around 66,000 deaths worldwide each year. RSV bronchiolitis predominantly strikes apparently healthy infants, with age as the principal risk factor for severe disease. The differences in the immune response to RSV in the very young are likely to be key to determining the clinical outcome of this common infection. Remarkable age-related differences in innate cytokine responses follow recognition of RSV by numerous pattern recognition receptors, and the importance of this early response is supported by polymorphisms in many early innate genes, which associate with bronchiolitis. In the absence of strong, Th1 polarizing signals, infants develop T cell responses that can be biased away from protective Th1 and cytotoxic T cell immunity toward dysregulated, Th2 and Th17 polarization. This may contribute not only to the initial inflammation in bronchiolitis, but also to the long-term increased risk of developing wheeze and asthma later in life. An early-life vaccine for RSV will need to overcome the difficulties of generating a protective response in infants, and the proven risks associated with generating an inappropriate response. Infantile T follicular helper and B cell responses are immature, but maternal antibodies can afford some protection. Thus, maternal vaccination is a promising alternative approach. However, even in adults adaptive immunity following natural infection is poorly protective, allowing re-infection even with the same strain of RSV. This gives us few clues as to how effective vaccination could be achieved. Challenges remain in understanding how respiratory immunity matures with age, and the external factors influencing its development. Determining why some infants develop bronchiolitis should lead to new therapies to lessen the clinical impact of RSV and aid the rational design of protective vaccines.

CD8+ T cell immunity against human respiratory syncytial virus

Human respiratory syncytial virus (HRSV) was first discovered in the 1950s, but despite decades of research, a licensed vaccine against it is not available. Epidemiological studies indicate that antibodies directed against the fusion protein (F) partially correlate with protection. In addition, an F-specific monoclonal antibody is licensed as a prophylactic treatment in children who are at high risk of developing complications following HRSV infection. Therefore, most HRSV-oriented vaccination strategies focus on inducing a humoral immune response against F. In the quest for the development of a safe HRSV vaccine, the induction of a T cell immune response has received a lot less attention. T cell immunity directed against HRSV has not been associated unequivocally with protection against HRSV and CD4(+) T helper cell responses may even worsen disease due to HRSV. However, many studies support a protective role for CD8(+) T cells in clearance of HRSV from the lungs. In this review we highlight the clinical and experimental evidence in favor of a CD8(+) T lymphocyte-based vaccination strategy to protect against HRSV. First, we describe how T cell responses and T cell memory are induced in the lungs upon respiratory viral infection. HRSV has evolved mechanisms that hamper CD8(+) T cell priming and effector functions. We appraise the information on HRSV-specific CD8(+) T cell immunity gained from laboratory mouse studies, taking into account the advantages and limitations of this animal model and, where possible, the accordance with clinical evidence. Finally, we focus on recent efforts to develop T cell based vaccines against HRSV.

Effects of anti-g and anti-f antibodies on airway function after respiratory syncytial virus infection

Respiratory syncytial virus (RSV) is the leading cause of lower respiratory tract illnesses in infants worldwide. Both RSV-G and RSV-F glycoproteins play pathogenic roles during infection with RSV. The objective of this study was to compare the effects of anti-RSV-G and anti-RSV-F monoclonal antibodies (mAbs) on airway hyperresponsiveness (AHR) and inflammation after primary or secondary RSV infection in mice. In the primary infection model, mice were infected with RSV at 6 weeks of age. Anti-RSV-G or anti-RSV-F mAbs were administered 24 hours before infection or Day +2 postinfection. In a secondary infection model, mice were infected (primary) with RSV at 1 week (neonate) and reinfected (secondary) 5 weeks later. Anti-RSV-G and anti-RSV-F mAbs were administered 24 hours before the primary infection. Both mAbs had comparable effects in preventing airway responses after primary RSV infection. When given 2 days after infection, anti-RSV-G-treated mice showed significantly decreased AHR and airway inflammation, which persisted in anti-RSV-F-treated mice. In the reinfection model, anti-RSV-G but not anti-RSV-F administered during primary RSV infection in neonates resulted in decreased AHR, eosinophilia, and IL-13 but increased levels of IFN-γ in bronchoalveolar lavage on reinfection. These results support the use of anti-RSV-G in the prevention and treatment of RSV-induced disease.

Development of an adenovirus-based respiratory syncytial virus vaccine: preclinical evaluation of efficacy, immunogenicity, and enhanced disease in a cotton rat model

The lack of a vaccine against respiratory syncytial virus (RSV) is a challenging and serious gap in preventive medicine. Herein, we characterize the immunogenicity of an adenovirus serotype 5-based RSV vaccine encoding the fusion (F) protein (Ad5.RSV-F) and the protection provided following immunization with Ad5.RSV-F and assess its potential for producing enhanced disease in a cotton rat (CR) model. Animals were immunized intranasally (i.n.) and/or intramuscularly (i.m.) and subsequently challenged with RSV/A/Tracy (i.n.) to assess protection. Robust immune responses were seen in CRs vaccinated with Ad5.RSV-F given i.m. or i.n., and these responses correlated with reduced replication of the virus in noses and lungs after challenge. Neutralizing antibody responses following immunization with a single dose of Ad5.RSV-F at 1 × 10(11) viral particles (v.p.) elicited antibody titers 64- to 256-fold greater than those seen after natural infection. CRs boosted with Ad5.RSV-F i.n. 28 days after an i.m. dose also had significant increases in neutralizing antibody titers. Antibody affinity for different F-protein antigenic sites revealed substantial differences between antibodies elicited by Ad5.RSV-F and those seen after RSV infection; differences in antibody profiles were also seen between CRs given Ad5.RSV-F i.m. and CRs given Ad5.RSV-F i.n. Ad5.RSV-F priming did not result in enhanced disease following live-virus challenge, in contrast to the histopathology seen in CRs given the formalin-inactivated RSV/A/Burnett vaccine.

IMPORTANCE

Respiratory syncytial virus (RSV) is the most common cause of acute lower respiratory infection in infants and young children and a serious health threat in the immunocompromised and the elderly. Infection severity increased in children in an immunization trial, hampering the over 4-decade-long quest for a successful RSV vaccine. In this study, we show that a genetically engineered RSV-F-encoding adenoviral vector provides protective immunity against RSV challenge without enhanced lung disease in cotton rats (CRs). CRs were vaccinated under a number of different regimens, and the immunity induced by the recombinant adenoviral RSV vaccine administered by use of an intramuscular prime-intranasal boost regimen may provide the best protection for young infants and children at risk of RSV infection, since this population is naive to adenoviral preformed immunity. Overall, this report describes a potential RSV vaccine candidate that merits further evaluation in a phase I clinical study 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.

Translational sciences approach to RSV vaccine development

Respiratory syncytial virus (RSV) is a major cause of lower respiratory tract infections in infants and the elderly. Despite its relatively low degree of antigenic variation, it causes frequent reinfection throughout life. Clinical manifestations of RSV disease and the immune response to infection differ in infants and the elderly, suggesting that vaccines designed to protect these two populations may require different attributes. Here, the authors describe the translational approach of utilizing data from epidemiology studies performed in these populations, the use of RSV diagnostics in clinical practice, lessons learned from previous vaccine clinical trials and the success of palivizumab in prevention of RSV disease in premature and high-risk infants to aid the development of safe and effective RSV vaccines.

Quantitative and qualitative deficits in neonatal lung-migratory dendritic cells impact the generation of the CD8+ T cell response

CD103+ and CD11b+ populations of CD11c+MHCIIhi murine dendritic cells (DCs) have been shown to carry antigens from the lung through the afferent lymphatics to mediastinal lymph nodes (MLN). We compared the responses of these two DC populations in neonatal and adult mice following intranasal infection with respiratory syncytial virus. The response in neonates was dominated by functionally-limited CD103+ DCs, while CD11b+ DCs were diminished in both number and function compared to adults. Infecting mice at intervals through the first three weeks of life revealed an evolution in DC phenotype and function during early life. Using TCR transgenic T cells with two different specificities to measure the ability of CD103+ DC to induce epitope-specific CD8+ T cell responses, we found that neonatal CD103+ DCs stimulate proliferation in a pattern distinct from adult CD103+ DCs. Blocking CD28-mediated costimulatory signals during adult infection demonstrated that signals from this costimulatory pathway influence the hierarchy of the CD8+ T cell response to RSV, suggesting that limited costimulation provided by neonatal CD103+ DCs is one mechanism whereby neonates generate a distinct CD8+ T cell response from that of adults.

Functional limitations of plasmacytoid dendritic cells limit type I interferon, T cell responses and virus control in early life

Infant mortality from viral infection remains a major global health concern: viruses causing acute infections in immunologically mature hosts often follow a more severe course in early life, with prolonged or persistent viral replication. Similarly, the WE strain of lymphocytic choriomeningitis virus (LCMV-WE) causes acute self-limiting infection in adult mice but follows a protracted course in infant animals, in which LCMV-specific CD8⁺ T cells fail to expand and control infection. By disrupting type I IFNs signaling in adult mice or providing IFN-α supplementation to infant mice, we show here that the impaired early life T cell responses and viral control result from limited early type I IFN responses. We postulated that plasmacytoid dendritic cells (pDC), which have been identified as one major source of immediate-early IFN-I, may not exert adult-like function in vivo in the early life microenvironment. We tested this hypothesis by studying pDC functions in vivo during LCMV infection and identified a coordinated downregulation of infant pDC maturation, activation and function: despite an adult-like in vitro activation capacity of infant pDCs, the expression of the E2-2 pDC master regulator (and of critical downstream antiviral genes such as MyD88, TLR7/TLR9, NF-κB, IRF7 and IRF8) is downregulated in vivo at baseline and during LCMV infection. A similar pattern was observed in response to ssRNA polyU, a model ligand of the TLR7 viral sensor. This suggests that the limited T cell-mediated defense against early life viral infections is largely attributable to / regulated by infant pDC responses and provides incentives for novel strategies to supplement or stimulate immediate-early IFN-α responses.

Respiratory syncytial virus--a comprehensive review

Respiratory syncytial virus (RSV) is amongst the most important pathogenic infections of childhood and is associated with significant morbidity and mortality. Although there have been extensive studies of epidemiology, clinical manifestations, diagnostic techniques, animal models and the immunobiology of infection, there is not yet a convincing and safe vaccine available. The major histopathologic characteristics of RSV infection are acute bronchiolitis, mucosal and submucosal edema, and luminal occlusion by cellular debris of sloughed epithelial cells mixed with macrophages, strands of fibrin, and some mucin. There is a single RSV serotype with two major antigenic subgroups, A and B. Strains of both subtypes often co-circulate, but usually one subtype predominates. In temperate climates, RSV infections reflect a distinct seasonality with onset in late fall or early winter. It is believed that most children will experience at least one RSV infection by the age of 2 years. There are several key animal models of RSV. These include a model in mice and, more importantly, a bovine model; the latter reflects distinct similarity to the human disease. Importantly, the prevalence of asthma is significantly higher amongst children who are hospitalized with RSV in infancy or early childhood. However, there have been only limited investigations of candidate genes that have the potential to explain this increase in susceptibility. An atopic predisposition appears to predispose to subsequent development of asthma and it is likely that subsequent development of asthma is secondary to the pathogenic inflammatory response involving cytokines, chemokines and their cognate receptors. Numerous approaches to the development of RSV vaccines are being evaluated, as are the use of newer antiviral agents to mitigate disease. There is also significant attention being placed on the potential impact of co-infection and defining the natural history of RSV. Clearly, more research is required to define the relationships between RSV bronchiolitis, other viral induced inflammatory responses, and asthma.

Targeting CD137 enhances vaccine-elicited anti-respiratory syncytial virus CD8+ T cell responses in aged mice

Respiratory syncytial virus (RSV) causes significant morbidity and mortality in children and the elderly. No vaccines for RSV are in use. Because of immunosenescence, the immunologic requirements for a successful RSV vaccine in the elderly might differ from a RSV vaccine for young children. Using an aged mouse model of RSV pathogenesis, we found that aged mice had impaired Ag-specific CD8(+) T cell responses and delayed RSV clearance compared with young mice. To study vaccine-elicited RSV-specific CD8(+) T cells in aged mice, we used a peptide vaccine approach. TriVax is a commixture of a peptide representing immunodominant RSV CD8(+) T cell epitope M282-90, a TLR agonist (polyinosinic-polycytidylic acid), and a costimulatory anti-CD40 Ab. TriVax vaccination generated robust, polyfunctional, and protective CD8(+) T cell responses in young BALB/c mice, but not in 18-mo-old (aged) BALB/c mice. We hypothesized that treatment of aged mice with agonistic anti-CD137 (41BB) mAb will partially restore T cell responses and TriVax efficacy in aged mice. We immunized 18-mo-old BALB/c mice twice with TriVax + anti-41BB mAb or TriVax + isotype control Ab. Coadministration of anti-41BB mAb with TriVax enhanced RSV-specific CD8(+) T cell responses and TriVax efficacy in challenge experiments. Triggering the 41BB costimulatory pathway may be a strategy for enhancing T cell responses to vaccines in the elderly.

Fc gamma receptors in respiratory syncytial virus infections: implications for innate immunity

RSV infections are a major burden in infants less than 3 months of age. Newborns and infants express a distinct immune system that is largely dependent on innate immunity and passive immunity from maternal antibodies. Antibodies can regulate immune responses against viruses through interaction with Fc gamma receptors leading to enhancement or neutralization of viral infections. The mechanisms underlying the immunomodulatory effect of Fc gamma receptors on viral infections have yet to be elucidated in infants. Herein, we will discuss current knowledge of the effects of antibodies and Fc gamma receptors on infant innate immunity to RSV. A better understanding of the pathogenesis of RSV infections in young infants may provide insight into novel therapeutic strategies such as vaccination.

Local IL-17A potentiates early neutrophil recruitment to the respiratory tract during severe RSV infection

Respiratory syncytial virus (RSV) bronchiolitis triggers a strong innate immune response characterized by excessive neutrophil infiltration which contributes to RSV induced pathology. The cytokine IL-17A enhances neutrophil infiltration into virus infected lungs. IL-17A is however best known as an effector of adaptive immune responses. The role of IL-17A in early immune modulation in RSV infection is unknown. We aimed to elucidate whether local IL-17A facilitates the innate neutrophil infiltration into RSV infected lungs prior to adaptive immunity. To this end, we studied IL-17A production in newborns that were hospitalized for severe RSV bronchiolitis. In tracheal aspirates we measured IL-17A concentration and neutrophil counts. We utilized cultured human epithelial cells to test if IL-17A regulates RSV infection-induced IL-8 release as mediator of neutrophil recruitment. In mice we investigated the cell types that are responsible for early innate IL-17A production during RSV infection. Using IL-17A neutralizing antibodies we tested if IL-17A is responsible for innate neutrophil infiltration in mice. Our data show that increased IL-17A production in newborn RSV patient lungs correlates with subsequent neutrophil counts recruited to the lungs. IL-17A potentiates RSV-induced production of the neutrophil-attracting chemokine IL-8 by airway epithelial cells in vitro. Various lung-resident lymphocytes produced IL-17A during early RSV infection in Balb/c mice, of which a local population of CD4 T cells stood out as the predominant RSV-induced cell type. By removing IL-17A during early RSV infection in mice we showed that IL-17A is responsible for enhanced innate neutrophil infiltration in vivo. Using patient material, in vitro studies, and an animal model of RSV infection, we thus show that early local IL-17A production in the airways during RSV bronchiolitis facilitates neutrophil recruitment with pathologic consequences to infant lungs.

The respiratory syncytial virus fusion protein and neutrophils mediate the airway mucin response to pathogenic respiratory syncytial virus infection

Respiratory syncytial virus (RSV) is the leading cause of death due to a viral etiology in infants. RSV disease is characterized by epithelial desquamation, neutrophilic bronchiolitis and pneumonia, and obstructive pulmonary mucus. It has been shown that infection of BALB/cJ mice with RSV clinical isolate A2001/2-20 (2-20) results in a higher early viral load, greater airway necrosis, and higher levels of interleukin-13 (IL-13) and airway mucin expression than infection with RSV laboratory strain A2. We hypothesized that the fusion (F) protein of RSV 2-20 is a mucus-inducing viral factor. In vitro, the fusion activity of 2-20 F but not that of A2 F was enhanced by expression of RSV G. We generated a recombinant F-chimeric RSV by replacing the F gene of A2 with the F gene of 2-20, generating A2-2-20F. Similar to the results obtained with the parent 2-20 strain, infection of BALB/cJ mice with A2-2-20F resulted in a higher early viral load and higher levels of subsequent pulmonary mucin expression than infection with the A2 strain. A2-2-20F infection induced greater necrotic airway damage and neutrophil infiltration than A2 infection. We hypothesized that the neutrophil response to A2-2-20F infection is involved in mucin expression. Antibody-mediated depletion of neutrophils in RSV-infected mice resulted in lower tumor necrosis factor alpha levels, fewer IL-13-expressing CD4 T cells, and less airway mucin production in the lung. Our data are consistent with a model in which the F and attachment (G) glycoprotein functional interaction leads to enhanced fusion and F is a key factor in airway epithelium infection, pathogenesis, and subsequent airway mucin expression.

Benefit and harm from immunity to respiratory syncytial virus: implications for treatment

PURPOSE OF REVIEW

Human respiratory syncytial virus (RSV) infection is a major cause of morbidity in children and of morbidity and mortality in elderly or immunocompromised adults. Given prophylactically, antibody can protect against infection, but natural levels are poorly protective. Vaccination may enhance disease, and there is no well tolerated and effective vaccine or antiviral treatment. Despite over 50 years of research, therapy remains nonspecific and supportive.

RECENT FINDINGS

Experimental human challenge in adult volunteers is beginning to elucidate the dynamics of viral shedding and causes of disease, but investigations of naturally infected children remain logistically challenging. RSV was known to bind several surface ligands, but the recent demonstration that nucleolin acts as a receptor for the RSV fusion protein was unexpected. Recent studies increasingly emphasize the relevance of innate immune responses and the dysregulation of inflammation as key factors in causing the pathological effects of infection. Studies in both human infants and mice indicate that interleukin-17 plays a role in some forms of RSV disease and regulatory T cells may be important in controlling inflammation.

SUMMARY

Improved understanding of the human immune response to RSV infection continues to be needed in order to accelerate the development of vaccines and new treatments for bronchiolitis.

Signal inhibitory receptor on leukocytes-1 (SIRL-1) negatively regulates the oxidative burst in human phagocytes

ROS production is an important effector mechanism mediating intracellular killing of microbes by phagocytes. Inappropriate or untimely ROS production can lead to tissue damage, thus tight regulation is essential. We recently characterized signal inhibitory receptor on leukocytes-1 (SIRL-1) as an inhibitory receptor expressed by human phagocytes. Here, we demonstrate that ligation of SIRL-1 dampens Fc receptor-induced ROS production in primary human phagocytes. In accordance, SIRL-1 engagement on these cells impairs the microbicidal activity of neutrophils, without affecting phagocytosis. The inhibition of ROS production may result from reduced ERK activation, since co-ligation of Fc receptors and SIRL-1 on phagocytes inhibited phosphorylation of ERK. Importantly, we demonstrate that microbial and inflammatory stimuli cause rapid downregulation of SIRL-1 expression on the surface of primary neutrophils and monocytes. In accordance, SIRL-1 expression levels on neutrophils in bronchoalveolar lavage fluid from patients with neutrophilic airway inflammation are greatly reduced. We propose that SIRL-1 on phagocytes sets an activation threshold to prevent inappropriate production of oxygen radicals. Upon infection, SIRL-1 expression is downregulated, allowing microbial killing and clearance of the pathogen.

Th17 lymphocytes in respiratory syncytial virus infection

Infection by respiratory syncytial virus (RSV) affects approximately 33 million infants annually worldwide and is a major cause of hospitalizations. Helper T lymphocytes (Th) play a central role in the immune response during such infections. However, Th lymphocytes that produce interleukin 17 (IL-17), known as Th17 lymphocytes, in addition to been protective can also cause pathology that accompany this type of infection. The protective effects of Th17 is associated with better prognosis in most infected individuals but heightened Th17 responses causes inflammation and pathology in others. Studies employing animal models haves shown that activated Th17 lymphocytes recruit neutrophils and facilitate tertiary lymphoid structure development in infected lungs. However, IL-17 also inhibits the ability of CD8+ lymphocytes to clear viral particles and acts synergistically with the innate immune system to exacerbate inflammation. Furthermore, IL-17 enhances IL-13 production which, in turn, promotes the activation of Th2 lymphocytes and excessive mucus production. Studies of these animal models have also shown that a lack of, or inadequate, responses by the Th1 subset of T lymphocytes enhances Th17-mediated responses and that this is detrimental during RSV co-infection in experimental asthma. The available evidence, therefore, indicates that Th17 can play contradictory roles during RSV infections. The factors that determine the shift in the balance between beneficial and adverse Th17 mediated effects during RSV infection remains to be determined.

Viral shedding and immune responses to respiratory syncytial virus infection in older adults

BACKGROUND

Comprehensive analyses of host, viral, and immune factors associated with severe respiratory syncytial virus (RSV) infection in adults have not been performed.

METHODS

Adults with RSV infection identified in both outpatient and inpatient settings were evaluated. Upper and lower respiratory tract virus load, duration of virus shedding, select mucosal chemokine and cytokine levels, humoral and mucosal immunoglobulin responses, and systemic T-cell responses were measured.

RESULTS

A total of 111 RSV-infected adults (61 outpatients and 50 hospitalized patients) were evaluated. Hospitalized subjects shed virus in nasal secretions at higher titers and for longer durations than less ill outpatients, had greater mucosal interleukin 6 (IL-6) levels throughout infection, and had higher macrophage inflammatory protein 1α (MIP-1α) levels early in infection. Persons >64 years old and those with more severe disease had a higher frequency of activated T cells in the blood than younger, less ill subjects at infection. Multivariate analysis found that the presence of underlying medical conditions, female sex, increased mucosal IL-6 level, and longer duration of virus shedding were associated with severe disease. Older age and increased nasal MIP-1α levels were of borderline statistical significance.

CONCLUSIONS

Multiple factors, but not older age, are independently associated with severe RSV infection in adults. The presence of underlying medical conditions had the greatest influence on disease severity.

Recombinant influenza virus carrying the respiratory syncytial virus (RSV) F85-93 CTL epitope reduces RSV replication in mice

Respiratory syncytial virus (RSV) is the leading cause of lower respiratory tract infections in infants worldwide. Despite decades of research, there is still no registered vaccine available for this major pathogen. We investigated the protective efficacy of a recombinant influenza virus, PR8/NA-F(85-93), that carries the RSV CD8(+) T cell epitope F(85-93) in its neuraminidase stalk. F(85-93)-specific cytotoxic T lymphocytes (CTLs) were induced in mice after a single intranasal immunization with PR8/NA-F(85-93) virus, and these CTLs provided a significant reduction in the lung viral load upon a subsequent challenge with RSV. To avoid influenza-induced morbidity, we treated mice with matrix protein 2 (M2e)-specific monoclonal antibodies before PR8/NA-F(85-93) virus infection. Treatment with anti-M2e antibodies reduced the infiltration of immune cells in the lungs upon PR8/NA-F(85-93) infection, whereas the formation of inducible bronchus-associated lymphoid tissue was not affected. Moreover, this treatment prevented body weight loss yet still permitted the induction of RSV F-specific T cell responses and significantly reduced RSV replication upon challenge. These results demonstrate that it is possible to take advantage of the infection-permissive protection of M2e-specific antibodies against influenza A virus to induce heterologous CD8(+) T cell-mediated immunity by an influenza A virus vector expressing the RSV F(85-93) epitope.

The adaptive immune response to respiratory syncytial virus

Respiratory syncytial virus (RSV) causes severe respiratory disease in children, the elderly and immunocompromised individuals. The combined actions of CD4 and CD8 T cells play a critical role in terminating an acute RSV infection whereas antibodies can provide protection from re-infection. Despite eliciting an immune response that mediates clearance of the virus, immunity to the virus appears to wane over time and individuals remain susceptible to reinfection with RSV throughout their lifetime. The ineffectiveness of the natural infection to induce long-term immunity has hampered vaccine efforts and there is currently no licensed RSV vaccine. In this review, we summarize our current understanding of the adaptive immune response to RSV and its contribution to disease.

Consequences of immature and senescent immune responses for infection with respiratory syncytial virus

Infants in the first 6 months of life and older adults (>65 years of age) are disproportionately burdened with respiratory syncytial virus (RSV)-associated morbidity and mortality. While other factors play a role in the risk these groups assume, shortcomings of the immune response make a substantial contribution to the predisposition to severe disease. Ineffectual antibody production with misdirected cytokine responses and excess inflammation in the airways are common to both groups. However, the mechanisms underlying these immune responses differ between infants and older adults and need to be better understood. Preventative approaches to decreasing the burden of disease are preferable to therapeutic intervention and effective vaccination strategies will need to target the strengths of the immune responses in these populations.

Use of MMP-8 and MMP-9 to assess disease severity in children with viral lower respiratory tract infections

Matrix metalloproteinases (MMPs) play an important role in respiratory inflammatory diseases, such as asthma and chronic obstructive pulmonary disease. It was hypothesized that MMP-8 and MMP-9 may function as biological markers to assess disease severity in viral lower respiratory tract infections in children. MMP-8 and MMP-9 mRNA expression levels in peripheral blood mononuclear cells (PBMCs) and granulocytes obtained in both the acute and recovery phase from 153 children with mild, moderate, and severe viral lower respiratory tract infections were determined using real-time PCR. In addition, MMP-8 and MMP-9 concentrations in blood and nasopharyngeal specimens were determined during acute mild, moderate, and severe infection, and after recovery using ELISA. Furthermore, PBMCs and neutrophils obtained from healthy volunteers were stimulated with RSV, LPS (TLR4 agonist), and Pam3Cys (TLR2 agonist) in vitro. Disease severity of viral lower respiratory tract infections in children is associated with increased expression levels of the MMP-8 and MMP-9 genes in both PBMCs and granulocytes. On the contrary, in vitro experiments showed that MMP-8 and MMP-9 mRNA and protein expression in PBMCs and granulocytes is not induced by stimulation with RSV, the most frequent detected virus in young children with viral lower respiratory tract infections. These data indicate that expression levels of the MMP-8 and MMP-9 genes in both PBMCs and neutrophils are associated with viral lower respiratory tract infections disease severity. These observations justify future validation in independent prospective study cohorts of the usefulness of MMP-8 and MMP-9 as potential markers for disease severity in viral respiratory infections.

Characterization of the resistance of SJL/J mice to pneumonia virus of mice, a model for infantile bronchiolitis due to a respiratory syncytial virus

Respiratory syncytial virus (RSV), a prominent cause of airway morbidity in children, maintains an excessive hospitalization rate despite decades of research. Host factors are assumed to influence the disease severity. As a first step toward identifying the underlying resistance mechanisms, we recently showed that inbred mouse strains differ dramatically as regards their susceptibility to pneumonia virus of mice (PVM), the murine counterpart of RSV. PVM infection in mice has been shown to faithfully mimic the severe RSV disease in human infants. This study aimed at dissecting the remarkable PVM-resistance shown by the SJL/J strain. To characterize its genetic component, we assessed clinical, physiopathological, and virological resistance/susceptibility traits in large first (F1) and second (F2) generations obtained by crossing the SJL/J (resistant) and 129/Sv (susceptible) strains. Then, to acquire conclusive in vivo evidence in support of the hypothesis that certain radiosensitive hematopoietic cells might play a significant role in PVM-resistance, we monitored the same resistance/susceptibility traits in mock- and γ-irradiated SJL/J mice. Segregation analysis showed that (i) PVM-resistance is polygenic, (ii) the resistance alleles are recessive, and (iii) all resistance-encoding alleles are concentrated in SJL/J. Furthermore, there was no alteration of SJL/J PVM-resistance after immunosuppression by γ-irradiation, which suggests that adaptive immunity is not involved. We conclude that host resistance to pneumoviruses should be amenable to genetic dissection in this mouse model and that radioresistant lung epithelial cells and/or alveolar macrophages may control the clinical severity of pneumovirus-associated lung disease.

Vaccine-elicited CD8+ T cells protect against respiratory syncytial virus strain A2-line19F-induced pathogenesis in BALB/c mice

CD8(+) T cells may contribute to vaccines for respiratory syncytial virus (RSV). Compared to CD8(+) T cells responding to RSV infection, vaccine-elicited anti-RSV CD8(+) T cells are less well defined. We used a peptide vaccine to test the hypothesis that vaccine-elicited RSV-specific CD8(+) T cells are protective against RSV pathogenesis. BALB/c mice were treated with a mixture (previously termed TriVax) of an M2(82-90) peptide representing an immunodominant CD8 epitope, the Toll-like receptor (TLR) agonist poly(I·C), and a costimulatory anti-CD40 antibody. TriVax vaccination induced potent effector anti-RSV CD8(+) cytotoxic T lymphocytes (CTL). Mice were challenged with RSV strain A2-line19F, a model of RSV pathogenesis leading to airway mucin expression. Mice were protected against RSV infection and against RSV-induced airway mucin expression and cellular lung inflammation when challenged 6 days after vaccination. Compared to A2-line19F infection alone, TriVax vaccination followed by challenge resulted in effector CD8(+) T cells with greater cytokine expression and the more rapid appearance of RSV-specific CD8(+) T cells in the lung. When challenged 42 days after TriVax vaccination, memory CD8(+) T cells were elicited with RSV-specific tetramer responses equivalent to TriVax-induced effector CD8(+) T cells. These memory CD8(+) T cells had lower cytokine expression than effector CD8(+) T cells, and protection against A2-line19F was partial during the memory phase. We found that vaccine-elicited effector anti-RSV CD8(+) T cells protected mice against RSV infection and pathogenesis, and waning protection correlated with reduced CD8(+) T cell cytokine expression.

Progress in understanding and controlling respiratory syncytial virus: still crazy after all these years

Human respiratory syncytial virus (RSV) is a ubiquitous pathogen that infects everyone worldwide early in life and is a leading cause of severe lower respiratory tract disease in the pediatric population as well as in the elderly and in profoundly immunosuppressed individuals. RSV is an enveloped, nonsegmented negative-sense RNA virus that is classified in Family Paramyxoviridae and is one of its more complex members. Although the replicative cycle of RSV follows the general pattern of the Paramyxoviridae, it encodes additional proteins. Two of these (NS1 and NS2) inhibit the host type I and type III interferon (IFN) responses, among other functions, and another gene encodes two novel RNA synthesis factors (M2-1 and M2-2). The attachment (G) glycoprotein also exhibits unusual features, such as high sequence variability, extensive glycosylation, cytokine mimicry, and a shed form that helps the virus evade neutralizing antibodies. RSV is notable for being able to efficiently infect early in life, with the peak of hospitalization at 2-3 months of age. It also is notable for the ability to reinfect symptomatically throughout life without need for significant antigenic change, although immunity from prior infection reduces disease. It is widely thought that re-infection is due to an ability of RSV to inhibit or subvert the host immune response. Mechanisms of viral pathogenesis remain controversial. RSV is notable for a historic, tragic pediatric vaccine failure involving a formalin-inactivated virus preparation that was evaluated in the 1960s and that was poorly protective and paradoxically primed for enhanced RSV disease. RSV also is notable for the development of a successful strategy for passive immunoprophylaxis of high-risk infants using RSV-neutralizing antibodies. Vaccines and new antiviral drugs are in pre-clinical and clinical development, but controlling RSV remains a formidable challenge.

Differential pathogenesis of respiratory syncytial virus clinical isolates in BALB/c mice

Airway mucus is a hallmark of respiratory syncytial virus (RSV) lower respiratory tract illness. Laboratory RSV strains differentially induce airway mucus production in mice. Here, we tested the hypothesis that RSV strains differ in pathogenesis by screening six low-passage RSV clinical isolates for mucogenicity and virulence in BALB/cJ mice. The RSV clinical isolates induced variable disease severity, lung interleukin-13 (IL-13) levels, and gob-5 levels in BALB/cJ mice. We chose two of these clinical isolates for further study. Infection of BALB/cJ mice with RSV A2001/2-20 (2-20) resulted in greater disease severity, higher lung IL-13 levels, and higher lung gob-5 levels than infection with RSV strains A2, line 19, Long, and A2001/3-12 (3-12). Like the line 19 RSV strain, the 2-20 clinical isolate induced airway mucin expression in BALB/cJ mice. The 2-20 and 3-12 RSV clinical isolates had higher lung viral loads than laboratory RSV strains at 1 day postinfection (p.i.). This increased viral load correlated with higher viral antigen levels in the bronchiolar epithelium and greater histopathologic changes at 1 day p.i. The A2 RSV strain had the highest peak viral load at day 4 p.i. RSV 2-20 infection caused epithelial desquamation, bronchiolitis, airway hyperresponsiveness, and increased breathing effort in BALB/cJ mice. We found that RSV clinical isolates induce variable pathogenesis in mice, and we established a mouse model of clinical isolate strain-dependent RSV pathogenesis that recapitulates key features of RSV disease.

The use of a neonatal mouse model to study respiratory syncytial virus infections

Respiratory syncytial virus (RSV) infection is the most significant cause of viral death in infants worldwide. The significant morbidity and mortality associated with this disease underscores the urgent need for the development of an RSV vaccine. The development of an RSV vaccine has been hampered by our limited understanding of the human host immune system, which plays a significant role in RSV pathogenesis, susceptibility and vaccine efficacy. As a result, animal models have been developed to better understand the mechanisms by which RSV causes disease. Within the past few years, a revolutionary variation on these animal models has emerged--age at time of initial infection--and early studies in neonatal mice (aged <7 days at time of initial infection) indicate the validity of this model to understand RSV infection in infants. This article reviews available information on current murine and emerging neonatal mouse RSV models.

Local cytokine response upon respiratory syncytial virus infection

Respiratory syncytial virus (RSV) is the leading cause of childhood hospitalization and respiratory distress and has been recognized for several decades as a major health and economic burden worldwide. This virus has developed several virulence mechanisms to impair the establishment of a protective immune response to re-infection. Accordingly, inefficient immunological memory is usually generated after exposure to this pathogen. Furthermore, it has been shown that RSV can actively promote the induction of an inadequate cellular immune response at the site of infection that causes exacerbated inflammation in the respiratory tract. Such an inflammatory response is both inefficient for clearing the virus and can be responsible for detrimental symptoms, such as asthma and wheezing. Recent data suggest that RSV possesses molecular mechanisms to induce the secretion of pro-inflammatory cytokines that modulate the immune response and impair viral clearance by reducing IFN-γ production. Here, we discuss recent research leading to the identification of RSV virulence factors that are responsible of promoting a pro-inflammatory environment at the airways and their implications on pathogenicity.

Serum antibodies critically affect virus-specific CD4+/CD8+ T cell balance during respiratory syncytial virus infections

Following infection with respiratory syncytial virus (RSV), reinfection in healthy individuals is common and presumably due to ineffective memory T cell responses. In peripheral blood of healthy adults, a higher CD4(+)/CD8(+) memory T cell ratio was observed compared with the ratio of virus-specific effector CD4(+)/CD8(+) T cells that we had found in earlier work during primary RSV infections. In mice, we show that an enhanced ratio of RSV-specific neutralizing to nonneutralizing Abs profoundly enhanced the CD4(+) T cell response during RSV infection. Moreover, FcγRs and complement factor C1q contributed to this Ab-mediated enhancement. Therefore, the increase in CD4(+) memory T cell response likely occurs through enhanced endosomal Ag processing dependent on FcγRs. The resulting shift in memory T cell response was likely amplified by suppressed T cell proliferation caused by RSV infection of APCs, a route important for Ag presentation via MHC class I molecules leading to CD8(+) T cell activation. Decreasing memory CD8(+) T cell numbers could explain the inadequate immunity during repeated RSV infections. Understanding this interplay of Ab-mediated CD4(+) memory T cell response enhancement and infection mediated CD8(+) memory T cell suppression is likely critical for development of effective RSV vaccines.

Respiratory syncytial virus is associated with an inflammatory response in lungs and architectural remodeling of lung-draining lymph nodes of newborn lambs

Respiratory syncytial virus (RSV) is the leading cause of lower respiratory tract infection in children worldwide. The understanding of neonatal RSV pathogenesis depends on using an animal model that reproduces neonatal RSV disease. Previous studies from us and others demonstrated that the neonatal lamb model resembles human neonatal RSV infection. Here, we provide an extensive and detailed characterization of the histopathology, viral load, cellular infiltration, and cytokine production in lungs and tracheobronchial lymph nodes of lambs inoculated with human RSV strain A2 over the course of infection. In the lung, RSV titers were low at day 3 postinfection, increased significantly by day 6, and decreased to baseline levels at day 14. Infection in the lung was associated with an accumulation of macrophages, CD4(+) and CD8(+) T cells, and a transcriptional response of genes involved in inflammation, chemotaxis, and interferon response, characterized by increased IFNγ, IL-8, MCP-1, and PD-L1, and decreased IFNβ, IL-10, and TGF-β. Laser capture microdissection studies determined that lung macrophage-enriched populations were the source of MCP-1 but not IL-8. Immunoreactivity to caspase 3 occurred within bronchioles and alveoli of day 6-infected lambs. In lung-draining lymph nodes, RSV induced lymphoid hyperplasia, suggesting an ability of RSV to enhance lymphocytic proliferation and differentiation pathways. This study suggests that, in lambs with moderate clinical disease, RSV enhances the activation of caspase cell death and Th1-skewed inflammatory pathways, and complements previous observations that emphasize the role of inflammation in the pathogenesis of RSV disease.

Responses against a subdominant CD8+ T cell epitope protect against immunopathology caused by a dominant epitope

CD8(+) T cell responses are critical for the control of virus infections. Following infection, epitope-specific responses establish an unpredictable but reproducible pattern of dominance that is dictated by a large number of both positive and negative factors. Immunodomination, or diminution of subdominant epitope-specific responses by dominant epitopes, can play a substantial role in the establishment of epitope hierarchy. To determine the role of a dominant (K(d)M2(82-90)) and a subdominant (D(b)M(187-195)) epitope of respiratory syncytial virus in viral control and immunodomination, MHC-binding anchor residues in the two epitopes were mutated individually in recombinant infectious viruses, greatly reducing or deleting the epitope-specific CD8(+) T cell responses. Neither mutation negatively affected viral clearance in mice, and compensation by the unmutated epitope was seen in both cases, whereas compensation by five other subdominant epitopes was minimal. Mutation of the dominant K(d)M2(82-90) response resulted in effective viral clearance by the subdominant epitope with less illness, whereas mutation of the subdominant D(b)M(187-195) response resulted in overcompensation of the already dominant K(d)M2(82-90) epitope, and increased severity of illness. Increased illness was associated with poor functionality of the abundant population of CD8(+) T cells specific to the dominant K(d)M2(82-90) epitope, as measured by the percentage and magnitude of IFN-γ production. These data demonstrate efficient viral clearance, and a protective effect of subdominant CD8(+) T cell responses.

Molecular quantification of respiratory syncytial virus in respiratory samples: reliable detection during the initial phase of infection

Quantitative real-time PCR for the detection of respiratory syncytial virus (RSV) RNA is increasingly used to study the causal role of RSV in lower airway disease. The objective of our study was to evaluate variations in RSV RNA loads at different steps in the RNA quantification process: (i) variation in RSV RNA load within one sample (step 1), (ii) variation in the load in samples from patients who were sampled twice on the same day (step 2), and (iii) variation in the load between simultaneously taken nasopharyngeal aspirate (NPA) samples and tracheal aspirate (TA) samples (step 3). Thirty-two infants with RSV infection at the pediatric intensive care unit (PICU) were included. NPA and TA samples were taken three times a week during ventilation and were not diluted. Intrasample variation (step 1) was shown to be minimal (<0.5 log(10) particles/ml). Intraday variation (step 2) was the lowest for samples with high viral loads (95% limits of agreement, -1.3 to +0.9 log(10)), whereas it increased for samples with relatively lower viral loads (viral load, <6.0 log(10) particles/ml; n = 138 sample pairs from 20 patients). RSV loads in NPA and TA samples (step 3) were found to be the most comparable during the early phase of infection (95% limits of agreement, -1.5 to +1.4 log(10)). The variation increased during the late phase of infection (i.e., in follow-up samples), with the loads in NPA samples remaining significantly higher than the loads in TA samples (n = 138 sample pairs from 31 patients). In conclusion, quantitative detection of RSV RNA in undiluted mucus is a reliable method to quantify viral loads. Nasopharyngeal aspirate samples collected in the initial phase of infection can be used to predict RSV RNA loads in the lower airways.

Age-dependent differences in the pathogenesis of bovine respiratory syncytial virus infections related to the development of natural immunocompetence

The severity of respiratory syncytial virus (RSV) infections appears to differ with age in both humans and bovines. A primary RSV infection in naïve infants and in young calves runs a more severe course when they are 1-6 months old than in their first month of life. The relative lack of clinical signs in the first month of age may be due to high levels of maternally derived neutralizing antibodies or low exposure to infectious virus. This study examined whether age-dependent differences in the pathogenesis of bovine RSV (bRSV) between neonatal and young calves may be due to differences in age-dependent immunocompetence. To study the effect of age and immune parameters on bRSV disease in neonatal and young calves, neonatal (1-day-old) calves without maternally derived antibodies were infected experimentally with bRSV and the severity of disease and immune responses were evaluated in comparison with disease in similar 6-week-old infected calves. Neonatal calves had more extensive virus replication and lung consolidation, but lower pro-inflammatory [in particular tumour necrosis factor alpha (TNF-α)] responses, specific humoral immune responses, lung neutrophilic infiltration and clinical signs of disease than 6-week-old calves. The lack of correlation between virus replication and clinical signs suggests an important role of pro-inflammatory cytokines, in particular TNF-α, in the disease. The capacity to produce pro-inflammatory TNF-α appeared to increase with age, and may explain the age-dependent differences in RSV pathogenesis.