Intranasal proteosome-based respiratory syncytial virus (RSV) vaccines protect BALB/c mice against challenge without eosinophilia or enhanced pathology

Unmasking the potential of secretory IgA and its pivotal role in protection from respiratory viruses

Mucosal immunity has regained its spotlight amidst the ongoing Coronavirus disease 19 (COVID-19) pandemic, with numerous studies highlighting the crucial role of mucosal secretory IgA (SIgA) in protection against Severe acute respiratory syndrome coronavirus-2 or SARS-CoV-2 infections. The observed limitations in the efficacy of currently authorized COVID-19 vaccines in inducing effective mucosal immune responses remind us of the limitations of systemic vaccination in promoting protective mucosal immunity. This resurgence of interest has motivated the development of vaccine platforms capable of enhancing mucosal responses, specifically the SIgA response, and the development of IgA-based therapeutics. Recognizing viral respiratory infections as a global threat, we would like to comprehensively review the existing knowledge on mucosal immunity, with a particular emphasis on SIgA, in the context of SARS-CoV-2, influenza, and Respiratory Syncytial Virus (RSV) infections. This review aims to describe the structural and functional specificities of SIgA, along with its nuanced role in combating influenza, RSV, and SARS-CoV-2 infections. Subsequent sections further elaborate promising vaccine strategies, including mucosal vaccines against Influenza, RSV, and SARS-CoV-2 respiratory viruses, currently undergoing preclinical and clinical development. Additionally, we address the challenges associated with mucosal vaccine development, concluding with a discussion on IgA-based therapeutics as a promising platform for the treatment of viral respiratory infections. This comprehensive review not only synthesizes current insights into mucosal immunity but also identifies critical knowledge gaps, strengthening the way for further advancements in our current understanding and approaches to combat respiratory viral threats.

Neonatal Immunity, Respiratory Virus Infections, and the Development of Asthma

Infants are exposed to a wide range of potential pathogens in the first months of life. Although maternal antibodies acquired transplacentally protect full-term neonates from many systemic pathogens, infections at mucosal surfaces still occur with great frequency, causing significant morbidity and mortality. At least part of this elevated risk is attributable to the neonatal immune system that tends to favor T regulatory and Th2 type responses when microbes are first encountered. Early-life infection with respiratory viruses is of particular interest because such exposures can disrupt normal lung development and increase the risk of chronic respiratory conditions, such as asthma. The immunologic mechanisms that underlie neonatal host-virus interactions that contribute to the subsequent development of asthma have not yet been fully defined. The goals of this review are (1) to outline the differences between the neonatal and adult immune systems and (2) to present murine and human data that support the hypothesis that early-life interactions between the immune system and respiratory viruses can create a lung environment conducive to the development of asthma.

Brief History and Characterization of Enhanced Respiratory Syncytial Virus Disease

In 1967, infants and toddlers immunized with a formalin-inactivated vaccine against respiratory syncytial virus (RSV) experienced an enhanced form of RSV disease characterized by high fever, bronchopneumonia, and wheezing when they became infected with wild-type virus in the community. Hospitalizations were frequent, and two immunized toddlers died upon infection with wild-type RSV. The enhanced disease was initially characterized as a "peribronchiolar monocytic infiltration with some excess in eosinophils." Decades of research defined enhanced RSV disease (ERD) as the result of immunization with antigens not processed in the cytoplasm, resulting in a nonprotective antibody response and CD4(+) T helper priming in the absence of cytotoxic T lymphocytes. This response to vaccination led to a pathogenic Th2 memory response with eosinophil and immune complex deposition in the lungs after RSV infection. In recent years, the field of RSV experienced significant changes. Numerous vaccine candidates with novel designs and formulations are approaching clinical trials, defying our previous understanding of favorable parameters for ERD. This review provides a succinct analysis of these parameters and explores criteria for assessing the risk of ERD in new vaccine candidates.

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.

A novel respiratory syncytial virus (RSV) F subunit vaccine adjuvanted with GLA-SE elicits robust protective TH1-type humoral and cellular immunity in rodent models

Background

Illness associated with Respiratory Syncytial Virus (RSV) remains an unmet medical need in both full-term infants and older adults. The fusion glycoprotein (F) of RSV, which plays a key role in RSV infection and is a target of neutralizing antibodies, is an attractive vaccine target for inducing RSV-specific immunity.

Methodology and Principal Findings

BALB/c mice and cotton rats, two well-characterized rodent models of RSV infection, were used to evaluate the immunogenicity of intramuscularly administered RSV vaccine candidates consisting of purified soluble F (sF) protein formulated with TLR4 agonist glucopyranosyl lipid A (GLA), stable emulsion (SE), GLA-SE, or alum adjuvants. Protection from RSV challenge, serum RSV neutralizing responses, and anti-F IgG responses were induced by all of the tested adjuvanted RSV sF vaccine formulations. However, only RSV sF + GLA-SE induced robust F-specific T_H1-biased humoral and cellular responses. In mice, these F-specific cellular responses include both CD4 and CD8 T cells, with F-specific polyfunctional CD8 T cells that traffic to the mouse lung following RSV challenge. This RSV sF + GLA-SE vaccine formulation can also induce robust RSV neutralizing titers and prime IFNγ-producing T cell responses in Sprague Dawley rats.

Conclusions/Significance

These studies indicate that a protein subunit vaccine consisting of RSV sF + GLA-SE can induce robust neutralizing antibody and T cell responses to RSV, enhancing viral clearance via a T_H1 immune-mediated mechanism. This vaccine may benefit older populations at risk for RSV disease.

Respiratory Syncytial Virus: pathology, therapeutic drugs and prophylaxis

Human Respiratory Syncytial Virus (hRSV) is the leading cause of lower respiratory tract diseases, affecting particularly newborns and young children. This virus is able to modulate the immune response, generating a pro-inflammatory environment in the airways that causes obstruction and pulmonary alterations in the infected host. To date, no vaccines are available for human use and the first vaccine that reached clinical trials produced an enhanced hRSV-associated pathology 50 years ago, resulting in the death of two children. Currently, only two therapeutic approaches have been used to treat hRSV infection in high risk children: 1. Palivizumab, a humanized antibody against the F glycoprotein that reduces to half the number of hospitalized cases and 2. Ribavirin, which fails to have a significant therapeutic effect. A major caveat for these approaches is their high economical cost, which highlights the need of new and affordable therapeutic or prophylactic tools to treat or prevents hRSV infection. Accordingly, several efforts are in progress to understand the hRSV-associated pathology and to characterize the immune response elicited by this virus. Currently, preclinical and clinical trials are being conducted to evaluate safety and efficacy of several drugs and vaccines, which have shown promising results. In this article, we discuss the most important advances in the development of drugs and vaccines, which could eventually lead to better strategies to treat or prevent the detrimental inflammation triggered by hRSV infection.

Inhibition of STAT6 during vaccination with formalin-inactivated RSV prevents induction of Th2-cell-biased airway disease

The pattern of immune response to a vaccine antigen can influence both efficacy and adverse events. Th2-cell-deviated responses have been implicated in both human and murine susceptibility to enhanced disease following formalin-inactivated (FI) vaccines for measles and RSV. In this study, we used the Th2-cell-deviated murine model of FI-RSV vaccination to test the ability of a dominant negative, cell-penetrating peptide inhibitor of STAT6 (STAT6 inhibitory peptide (IP)) to modulate the vaccine-induced predisposition to exaggerated inflammation during later RSV infection. Intranasal delivery of STAT6-IP in BALB/c mice at the time of distal intramuscular FI-RSV vaccination (Early Intervention) markedly decreased vaccine-enhanced, Th2-cell-dependent pathology upon subsequent RSV challenge. Administration of the STAT6-IP at the time of RSV challenge (Late Intervention) had no effect. Following RSV challenge, the STAT6-IP-treated mice in the Early Intervention group had lower airway eosinophils, increased lung IFN-γ levels, as well as increased IFN-γ-secreting CD4(+) and CD8(+) cells in the lungs. Our findings demonstrate the feasibility of targeting intracellular signaling pathways as a new way to modulate vaccine-induced responses.

Mucosal vaccines against respiratory syncytial virus

Respiratory syncytial virus (RSV) is a leading cause of severe respiratory disease in infants, young children, immune-compromised and elderly populations worldwide. Natural RSV infection in young children does not elicit long-lasting immunity and individuals remain susceptible to repeated RSV infections throughout life. Because RSV infection is restricted to the respiratory tract, an RSV vaccine should elicit mucosal immunity at upper and lower respiratory tracts in order to most effectively prevent RSV reinfection. Although there is no safe and effective RSV vaccine available, significant progress has been recently made in basic RSV research and vaccine development. This review will discuss recent advances in the identification of a new neutralizing antigenic site within the RSV fusion (F) protein, understanding the importance of mucosal immune responses against RSV infection, and the development of novel mucosal vaccination strategies.

Immunologic characterization of a novel inactivated nasal mumps virus vaccine adjuvanted with Protollin

An inactivated, mucosal mumps virus (MuV) vaccine would address many of the problems associated with current live-attenuated formulations. Protollin (Prl)-based adjuvants (containing TLR2 and TLR4 ligands) are well-suited for nasal administration. We sought to develop an inactivated whole-virus nasal vaccine for MuV using the Prl adjuvant/delivery vehicle and to test tolerability and immunogenicity in a mouse model. BALB/c mice exhibited signs of transient reactogenicity (hunched posture, erect fur, weight loss ≤10% of total body weight) following administration of intranasal MuV-Prl vaccines, though most of these manifestations resolved within 24h. Compared to high-dose unadjuvanted vaccine (8μgMuV), administration of high-dose adjuvanted formulation (8μgMuV-Prl) induced greater MuV-specific serum IgG (3.26E6ng/mL vs. 2.2E5ng/mL, 8μgMuV-Prl vs. 8μgMuV, p<0.001) and mucosal IgA (128ng/mL vs. 45ng/mL, 8μgMuV-Prl vs. 8μgMuV, p<0.05). Serum IgG isotypes and splenocyte cytokine secretion induced by MuV-Prl suggested a predominant T helper cell (Th)1-type immune response. This response was characterized by: (1) ≥four-fold increase of IgG2a levels compared to IgG1; and (2) high IL-2 (644pg/mL)/IFN-γ (228pg/mL) and low IL-5 (31pg/mL) secretion in MuV-restimulated splenocytes from animals receiving MuV-Prl formulations. MuV-Prl vaccination induced higher levels of serum antibodies capable of neutralizing MuV in vitro than MuV alone, particularly for high-dose 8μg formulations (357 neutralizing units (NU)/mL vs. 32NU/mL, 8μgMuV-Prl vs. 8μgMuV, p<0.001). Thus, nasal MuV-Prl vaccines are fairly well-tolerated and highly immunogenic in mice.

Pattern recognition receptors for respiratory syncytial virus infection and design of vaccines

Respiratory syncytial virus (RSV) is the leading cause of lower respiratory tract illness in infants and young children. Host immune response has been implicated in both the protection and immunopathological mechanisms. Pattern recognition receptors (PRRs) expressed on innate immune cells during RSV infection recognize the RSV-associated molecular patterns and activate innate immune cells as well as mediate airway inflammation, protective immune response, and pulmonary immunopathology. The resident and recruited innate immune cells play important roles in the protection and pathogenesis of an RSV disease by expressing these PRRs. Agonist-binding PRRs are the basis of many adjuvants that are essential for most vaccines. In the present review, we highlight recent advances in the innate immune recognition of and responses to RSV through PRRs, including toll-like receptors (TLRs), retinoic acid-inducible gene (RIG)-I-like receptors (RLRs), and nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs). We also describe the role of PRRs in the design of RSV vaccines.

Mucosal vaccine design and delivery

Mucosal surfaces are a major portal of entry for many human pathogens that are the cause of infectious diseases worldwide. Vaccines capable of eliciting mucosal immune responses can fortify defenses at mucosal front lines and protect against infection. However, most licensed vaccines are administered parenterally and fail to elicit protective mucosal immunity. Immunization by mucosal routes may be more effective at inducing protective immunity against mucosal pathogens at their sites of entry. Recent advances in our understanding of mucosal immunity and identification of correlates of protective immunity against specific mucosal pathogens have renewed interest in the development of mucosal vaccines. Efforts have focused on efficient delivery of vaccine antigens to mucosal sites that facilitate uptake by local antigen-presenting cells to generate protective mucosal immune responses. Discovery of safe and effective mucosal adjuvants are also being sought to enhance the magnitude and quality of the protective immune response.

Pulmonary eosinophils and their role in immunopathologic responses to formalin-inactivated pneumonia virus of mice

Enhanced disease is the term used to describe the aberrant Th2-skewed responses to naturally acquired human respiratory syncytial virus (hRSV) infection observed in individuals vaccinated with formalin-inactivated viral Ags. Here we explore this paradigm with pneumonia virus of mice (PVM), a pathogen that faithfully reproduces features of severe hRSV infection in a rodent host. We demonstrate that PVM infection in mice vaccinated with formalin-inactivated Ags from PVM-infected cells (PVM Ags) yields Th2-skewed hypersensitivity, analogous to that observed in response to hRSV. Specifically, we detect elevated levels of IL-4, IL-5, IL-13, and eosinophils in bronchoalveolar lavage fluid of PVM-infected mice that were vaccinated with PVM Ags, but not among mice vaccinated with formalin-inactivated Ags from uninfected cells (control Ags). Interestingly, infection in PVM Ag-vaccinated mice was associated with a approximately 10-fold reduction in lung virus titer and protection against weight loss when compared with infected mice vaccinated with control Ags, despite the absence of serum-neutralizing Abs. Given recent findings documenting a role for eosinophils in promoting clearance of hRSV in vivo, we explored the role of eosinophils in altering the pathogenesis of disease with eosinophil-deficient mice. We found that eosinophil deficiency had no impact on virus titer in PVM Ag-vaccinated mice, nor on weight loss or levels of CCL11 (eotaxin-1), IFN-gamma, IL-5, or IL-13 in bronchoalveolar lavage fluid. However, levels of both IL-4 and CCL3 (macrophage inflammatory protein-1alpha) in bronchoalveolar lavage fluid were markedly diminished in PVM Ag-vaccinated, PVM-infected eosinophil-deficient mice when compared with wild-type controls.

Pulmonary V gamma 4+ gamma delta T cells have proinflammatory and antiviral effects in viral lung disease

Host defenses, while effecting viral clearance, contribute substantially to inflammation and disease. This double action is a substantial obstacle to the development of safe and effective vaccines against many agents, particularly respiratory syncytial virus (RSV). RSV is a common cold virus and the major cause of infantile bronchiolitis worldwide. The role of alphabeta T cells in RSV-driven immunopathology is well studied, but little is known about the role of "unconventional" T cells. During primary RSV challenge of BALB/c mice, some Vgamma7+ gammadelta T cells were present; however, immunization with a live vaccinia vector expressing RSV F protein substantially enhanced Vgamma4+ gammadelta T cell influx after RSV infection. Harvested early, these cells produced IFN-gamma, TNF, and RANTES after ex vivo stimulation. By contrast, those recruited 5 days after challenge made IL-4, IL-5, and IL-10. Depletion of gammadelta T cells in vivo reduced lung inflammation and disease severity and slightly increased peak viral replication but did not prevent viral clearance. These studies demonstrate a novel role for gammadelta T cells in the development of immunopathology and cellular influx into the lungs after immunization and RSV challenge. Though a minor population, gammadelta T cells have a critical influence on disease and are an attractive interventional target in the alleviation of viral lung disease.

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.

Emerging vaccines for influenza

BACKGROUND

Influenza remains one of the leading causes of morbidity and mortality worldwide. The available vaccines are least effective in the populations at greatest risk--children, the elderly, and the immunocompromised. Furthermore, avian influenza and other novel strains have the potential to cause the next influenza pandemic. Research efforts have accelerated worldwide to develop new vaccines to provide better immunity against annual epidemics and a potential pandemic.

OBJECTIVE

To summarize the global research efforts at developing new influenza vaccines, adjuvants, and delivery devices.

METHOD

MEDLINE and Pharmaprojects databases were searched for publications and continuing research on new influenza vaccine technologies.

RESULTS/CONCLUSIONS

Technologies such as DNA vaccines, live recombinant viral vector vaccines, and virus-like particles have shown significant promise for immunogenicity and protection from experimental challenge to influenza. New modalities for vaccine delivery and methods for rapid vaccine production are also being investigated. With the possibility of an influenza pandemic increasing the need to develop new vaccines, the global research community has made large strides to meet this challenge.

Mouse models for the study of mucosal vaccination against otitis media

Otitis media (OM) is one of the most common infectious diseases in humans. The pathogenesis of OM involves nasopharyngeal (NP) colonization and retrograde ascension of the pathogen up the Eustachian tube into the middle ear (ME). Due to increasing rates of antibiotic resistance, there is an urgent need for vaccines to prevent infections caused by the most common causes of bacterial OM, including nontypeable Haemophilus influenzae, Streptococcus pneumoniae and Moraxella catarrhalis. Current vaccine strategies aim to diminish bacterial NP carriage, thereby reducing the likelihood of developing acute OM. To be effective, vaccination should induce local mucosal immunity both in the ME and in the NP. Studies in animal models have demonstrated that the intranasal route of vaccination is particularly effective at inducing immune responses in the nasal passage and ME for protection against OM. The mouse is increasingly used in these models, because of the availability of murine reagents and the existence of technology to manipulate murine models of disease immunologically and genetically. Previous studies confirmed the suitability of the mouse as a model for inflammatory processes in acute OM. Here, we discuss various murine models of OM and review the applicability of these models to assess the efficacy of mucosal vaccination and the mechanisms responsible for protection. In addition, we discuss various mucosal vaccine antigens, mucosal adjuvants and mucosal delivery systems.