Inability to evoke a long-lasting protective immune response to respiratory syncytial virus infection in mice correlates with ineffective nasal antibody responses

Protective T cell immunity against respiratory syncytial virus is efficiently induced by recombinant BCG

Respiratory syncytial virus (RSV) is one of the leading causes of childhood hospitalization and a major health burden worldwide. Unfortunately, because of an inefficient immunological memory, RSV infection provides limited immune protection against reinfection. Furthermore, RSV can induce an inadequate Th2-type immune response that causes severe respiratory tract inflammation and obstruction. It is thought that effective RSV clearance requires the induction of balanced Th1-type immunity, involving the activation of IFN-gamma-secreting cytotoxic T cells. A recognized inducer of Th1 immunity is Mycobacterium bovis bacillus Calmette-Guérin (BCG), which has been used in newborns for decades in several countries as a tuberculosis vaccine. Here, we show that immunization with recombinant BCG strains expressing RSV antigens promotes protective Th1-type immunity against RSV in mice. Activation of RSV-specific T cells producing IFN-gamma and IL-2 was efficiently obtained after immunization with recombinant BCG. This type of T cell immunity was protective against RSV challenge and caused a significant reduction of inflammatory cell infiltration in the airways. Furthermore, mice immunized with recombinant BCG showed no weight loss and reduced lung viral loads. These data strongly support recombinant BCG as an efficient vaccine against RSV because of its capacity to promote protective Th1 immunity.

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.

Host immunity during RSV pathogenesis

Infection by respiratory syncytial virus (RSV) is the leading cause of childhood hospitalization as well as a major health and economic burden worldwide. Unfortunately, RSV infection provides only limited immune protection to reinfection, mostly due to inadequate immunological memory, which leads to an exacerbated inflammatory response in the respiratory tract promoting airway damage during virus clearance. This exacerbated and inefficient immune-inflammatory response triggered by RSV, has often been attributed to the induction of a Th2-biased immunity specific for some of the RSV antigens. These features of RSV infection suggest that the virus might possess molecular mechanisms to enhance allergic-type immunity in the host in order to prevent clearance by cytotoxic T cells and ensure survival and dissemination to other hosts. In this review, we discuss recent findings that contribute to explain the components of the innate and adaptive immune response that are involved in RSV-mediated disease exacerbation. Further, the virulence mechanisms used by RSV to avoid activation of protective immune responses are described.

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

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

The fusion protein of respiratory syncytial virus triggers p53-dependent apoptosis

Infection with respiratory syncytial virus (RSV) frequently causes inflammation and obstruction of the small airways, leading to severe pulmonary disease in infants. We show here that the RSV fusion (F) protein, an integral membrane protein of the viral envelope, is a strong elicitor of apoptosis. Inducible expression of F protein in polarized epithelial cells triggered caspase-dependent cell death, resulting in rigorous extrusion of apoptotic cells from the cell monolayer and transient loss of epithelial integrity. A monoclonal antibody directed against F protein inhibited apoptosis and was also effective if administered to A549 lung epithelial cells postinfection. F protein expression in epithelial cells caused phosphorylation of tumor suppressor p53 at serine 15, activation of p53 transcriptional activity, and conformational activation of proapoptotic Bax. Stable expression of dominant-negative p53 or p53 knockdown by RNA interference inhibited the apoptosis of RSV-infected A549 cells. HEp-2 tumor cells with low levels of p53 were not sensitive to RSV-triggered apoptosis. We propose a new model of RSV disease with the F protein as an initiator of epithelial cell shedding, airway obstruction, secondary necrosis, and consequent inflammation. This makes the RSV F protein a key target for the development of effective postinfection therapies.

Respiratory syncytial virus and innate immunity: a complex interplay of exploitation and subversion

Respiratory syncytial virus causes significant disease in infants, the elderly and select groups of immunocompromised patients. Healthy individuals are also naturally infected with respiratory syncytial virus repeatedly throughout life. Therefore, safe and effective vaccines and therapies are needed. However, a number of factors have prevented development of such antiviral interventions to date. These include a failed vaccine trial, the very young age of the primary target population (neonates), the inability of natural infection to induce long-term protective immunity, and an incomplete understanding of virus-host interactions. The identification of pattern recognition receptors has led to significant increases in our understanding of induction and regulation of innate immune responses. This review will address the impact of these findings on respiratory syncytial virus research.

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.

Intranasal immunisation with inactivated RSV and bacterial adjuvants induces mucosal protection and abrogates eosinophilia upon challenge

We have previously shown that following intranasal exposure to influenza virus, specific plasma cells are generated in the nasal-associated lymphoid tissue (NALT) and maintained for the life of the animal. However, we also showed that following infection with respiratory syncytial virus (RSV), specific plasma cells are generated in the NALT but wane quickly and are not maintained even after challenge, even though RSV-specific serum antibody responses remain robust. Only infection with influenza virus generated sterilising immunity, implying a role for these long-lived plasma cells in protection. We show here that the RSV-specific IgA NALT plasma cell population and lung antibody levels can be substantially boosted, both at acute and memory time points, by intranasal immunisation with inactivated RSV (iRSV) in combination with bacterial outer membrane vesicles (OMV) compared to live RSV alone. Finally, challenge with live RSV showed that immunisation with iRSV and OMV protect against both virus replication in the lung and the eosinophil infiltrate generated by either live RSV or iRSV alone. These data show that immunisation with iRSV and OMV maintains a NALT RSV-specific plasma cell population and generates an efficient protective immune response following RSV infection.

Neutralizing antibody response and SARS severity

Using the Taiwan nationwide laboratory-confirmed severe acute respiratory syndrome (SARS) database, we analyzed neutralizing antibody in relation to clinical outcomes. With a linear mixed model, neutralizing antibody titer was shown to peak between week 5 and week 8 after onset and to decline thereafter, with a half-life of 6.4 weeks. Patients with a longer illness showed a lower neutralizing antibody response than patients with a shorter illness duration (p = 0.008). When early responders were compared with most patients, who seroconverted on and after week 3 of illness, the small proportion (17.4%) of early responders (antibody detectable within 2 weeks) had a higher death rate (29.6% vs. 7.8%) (Fisher exact test, p = 0.004), had a shorter survival time of <2 weeks (Fisher exact test, p = 0.013), and were more likely to be > 60 years of age (Fisher exact test, p = 0.01). Our findings have implications for understanding the pathogenesis of SARS and for SARS vaccine research and development.

Formalin-inactivated bovine RSV vaccine influences antibody levels in bronchoalveolar lavage fluid and disease outcome in experimentally infected calves

Respiratory syncytial virus (RSV) causes severe respiratory disease in calves and human infants. In response to outbreaks, formalin inactivated (FI)-RSV vaccines were developed and found to exacerbate disease following a live RSV infection. We have reproduced vaccination induced disease enhancement in calves and screened various antibody isotypes in bronchoalveolar lavage fluid (BALF) from two studies: one with disease enhancement and another where moderate protection resulted from FI-bovine RSV (BRSV) vaccination. Semi-protected vaccinated calves produced BRSV-specific BALF IgG1, but not IgA and IgG2 prior to infection; whereas, calves with enhanced disease failed to develop BRSV-specific IgG1 in BALF. Ultimately, the formulation and delivery of RSV vaccines influences protective antibody levels in respiratory secretions.

Antigen-dependent proliferation and cytokine induction in respiratory syncytial virus-infected cotton rats reflect the presence of effector-memory T cells

Respiratory syncytial virus (RSV) is a major cause of lower airway disease in infants and children. Immunity to RSV is not long lasting, resulting in re-occurring infections throughout life. Effective long-lived immunity results when central-memory T cells that proliferate vigorously and secrete IL-2 are present. In contrast, effector-memory T cells that mainly produce IFN-gamma, facilitate virus clearance but are not long lived. To identify the type of memory response induced after RSV-A (Long) infection, we characterized the kinetics of the antigen-specific immune response and identified the types of cytokines induced. RSV-specific lymphocytic proliferation following primary and secondary infection was similar, and in both cases responses waned within a short period of time. In addition, mRNA for IFN-gamma but not IL-2 was induced in RSV-specific CD4(+) T cells. This supports the idea that the presence of effector-memory rather than central-memory T cells contributes to the ineffectiveness of the immune response to RSV.

Respiratory syncytial virus-induced immunoprotection and immunopathology

Respiratory syncytial virus (RSV), a member of the Paramyxoviridae family, is a major clinical problem causing yearly epidemics of severe lower airway disease in both infants and the elderly. Attempts at vaccination have been frustrated by both the poor immunogenicity of this virus, and the severe immunopathology observed in early vaccine trials. Primary infection generally occurs in infancy, with approximately 5% of infected infants requiring hospitalization. Equally problematic is the apparent link between severe RSV disease and the later development of allergy and asthma. While there is no evidence that natural infection promotes Th2 predominance, development of enhanced eosinophilic disease in children receiving inactivated virus administered with a commonly used adjuvant demonstrated how easily the balance between immune-mediated protection and immune-mediated pathology can be perturbed. In this review we have focused on studies carried out in the mouse model aimed at determining the correlates of RSV protection and explaining the mechanism of vaccine enhanced immunopathology.