Maternal transfer of RSV immunity in cotton rats vaccinated during pregnancy

Longitudinal humoral analysis in RSV-infected infants identifies pre-existing RSV strain-specific G and evolving cross-reactive F antibodies

Respiratory syncytial virus (RSV) is among the most common causes of lower respiratory tract infection (LRTI) and hospitalization in infants. However, the mechanisms of immune control in infants remain incompletely understood. Antibody profiling against attachment (G) and fusion (F) proteins in children less than 2 years of age, with mild (outpatients) or severe (inpatients) RSV disease, indicated substantial age-dependent differences in RSV-specific immunity. Maternal antibodies were detectable for the first 3 months of life, followed by a long window of immune vulnerability between 3 and 6 months and a rapid evolution of FcγR-recruiting immunity after 6 months of age. Acutely ill hospitalized children exhibited lower G-specific antibodies compared with healthy controls. With disease resolution, RSV-infected infants generated broad functional RSV strain-specific G-responses and evolved cross-reactive F-responses, with minimal maternal imprinting. These data suggest an age-independent RSV G-specific functional humoral correlate of protection, and the evolution of RSV F-specific functional immunity with disease resolution.

Pre-Clinical Development of an Adenovirus Vector Based RSV and Shingles Vaccine Candidate

Respiratory syncytial virus (RSV) infection and shingles are two viral diseases that affect older adults, and a combined vaccine to protect against both could be beneficial. RSV infection causes hospitalisations and significant morbidity in both children and adults and can be fatal in the elderly. The RSV fusion (F) envelope glycoprotein induces a strong RSV-neutralising antibody response and is the target of protective immunity in the first RSV vaccine for older adults, recently approved by the FDA. An initial childhood infection with the varicella zoster virus (VZV) results in chickenpox disease, but reactivation in older adults can cause shingles. This reactivation in sensory and autonomic neurons is characterized by a skin-blistering rash that can be accompanied by prolonged pain. The approved protein-in-adjuvant shingles vaccine induces VZV glycoprotein E (gE)-fspecific antibody and CD4+ T cell responses and is highly effective. Here we report the evaluation of RSV/shingles combination vaccine candidates based on non-replicating chimpanzee adenovirus (ChAd) vectors. We confirmed the cellular and humoral immunogenicity of the vaccine vectors in mice using T cell and antibody assays. We also carried out an RSV challenge study in cotton rats which demonstrated protective efficacy following a homologous prime-boost regimen with our preferred vaccine candidate.

Development of Respiratory Syncytial Virus Vaccine Candidates for the Elderly

Respiratory syncytial virus (RSV) is a significant threat to elderly populations and repeated infections that occur throughout life are poorly protective. To assess the role of prior RSV infections as well as elderly immune senescence on vaccine efficacy, we compared immune responses after virus-like particle (VLP) immunization of elderly cotton rats and young cotton rats, both previously RSV infected, in order to mimic the human population. We show that immunization of RSV-experienced young or elderly animals resulted in the same levels of anti-pre-F IgG, anti-G IgG, neutralizing antibody titers, and protection from challenge indicating that the delivery of F and G proteins in a VLP is equally effective in activation of protective responses in both elderly and young populations. Our results suggest that F and G protein-containing VLPs induce anti-RSV memory established in prior RSV infections equally well in both young and elderly animals and thus can be an effective vaccine for the elderly.

Correlative outcomes of maternal immunization against RSV in cotton rats

Maternal anti-respiratory syncytial virus (RSV) antibodies protect neonates from RSV disease throughout first weeks of life. Previous studies of maternal immunization in cotton rats showed that a single immunization during pregnancy of RSV-primed dams with virus-like particles (VLPs) assembled with pre-fusion F protein and the wild type G protein boosted their RSV serum antibody concentration and protected pups early in life against RSV challenge. We extended these findings by evaluating responses to RSV infection in litters from two consecutive pregnancies of immunized dams. Using an RSV-primed population of VLP-vaccinated and unvaccinated dams, we defined correlations between dams' and litters' RSV neutralizing antibodies (NA); between litters' NA and protection; and between litter's NA and their lung expression of selected cytokines, of a first or of a second pregnancy. Lung pathology was also evaluated. We found positive correlation between the NA titers in the dams at delivery and the NA in their first and second litters and negative correlations between the litters' NA and protection from RSV challenge. Vaccination of dams modulated the mRNA expression for IFNγ and IL-6 and lung pathology in the first and in the second litter at different times after birth, even in the absence of detectable NA. Maternal RSV vaccination enhanced the levels of antibodies transferred to offspring and their protection from challenge. Importantly, maternal vaccination also impacted the immunological and inflammatory response of the offspring's lungs well into maturity, and after the antiviral effect of maternally transferred NA waned or was no longer detectable.

From animal studies into clinical trials: the relevance of animal models to develop vaccines and therapies to reduce disease severity and prevent hRSV infection

INTRODUCTION

Human respiratory syncytial virus (hRSV) is an important cause of lower respiratory tract infections in the pediatric and the geriatric population worldwide. There is a substantial economic burden resulting from hRSV disease during winter. Although no vaccines have been approved for human use, prophylactic therapies are available for high-risk populations. Choosing the proper animal models to evaluate different vaccine prototypes or pharmacological treatments is essential for developing efficient therapies against hRSV.

AREAS COVERED

This article describes the relevance of using different animal models to evaluate the effect of antiviral drugs, pharmacological molecules, vaccine prototypes, and antibodies in the protection against hRSV. The animal models covered are rodents, mustelids, bovines, and nonhuman primates. Animals included were chosen based on the available literature and their role in the development of the drugs discussed in this manuscript.

EXPERT OPINION

Choosing the correct animal model is critical for exploring and testing treatments that could decrease the impact of hRSV in high-risk populations. Mice will continue to be the most used preclinical model to evaluate this. However, researchers must also explore the use of other models such as nonhuman primates, as they are more similar to humans, prior to escalating into clinical trials.

Evolution of protection after maternal immunization for respiratory syncytial virus in cotton rats

Maternal anti-respiratory syncytial virus (RSV) antibodies acquired by the fetus through the placenta protect neonates from RSV disease through the first weeks of life. In the cotton rat model of RSV infections, we previously reported that immunization of dams during pregnancy with virus-like particles assembled with mutation stabilized pre-fusion F protein as well as the wild type G protein resulted in robust protection of their offspring from RSV challenge. Here we describe the durability of those protective responses in dams, the durability of protection in offspring, and the transfer of that protection to offspring of two consecutive pregnancies without a second boost immunization. We report that four weeks after birth, offspring of the first pregnancy were significantly protected from RSV replication in both lungs and nasal tissues after RSV challenge, but protection was reduced in pups at 6 weeks after birth. However, the overall protection of offspring of the second pregnancy was considerably reduced, even at four weeks of age. This drop in protection occurred even though the levels of total anti-pre-F IgG and neutralizing antibody titers in dams remained at similar, high levels before and after the second pregnancy. The results are consistent with an evolution of antibody properties in dams to populations less efficiently transferred to offspring or the less efficient transfer of antibodies in elderly dams.

Transplacental Antibody Transfer of Respiratory Syncytial Virus Specific IgG in Non-Human Primate Mother-Infant Pairs

One approach to protect new-borns against respiratory syncytial virus (RSV) is to vaccinate pregnant women in the last trimester of pregnancy. The boosting of circulating antibodies which can be transferred to the foetus would offer immune protection against the virus and ultimately the disease. Since non-human primates (NHPs) have similar reproductive anatomy, physiology, and antibody architecture and kinetics to humans, we utilized this preclinical species to evaluate maternal immunization (MI) using an RSV F subunit vaccine. Three species of NHPs known for their ability to be infected with human RSV in experimental challenge studies were tested for RSV-specific antibodies. African green monkeys had the highest overall antibody levels of the old-world monkeys evaluated and they gave birth to offspring with anti-RSV titers that were proportional to their mother. These higher overall antibody levels are associated with greater durability found in their offspring. Immunization of RSV seropositive AGMs during late pregnancy boosts RSV titers, which consequentially results in significantly higher titers in the vaccinated new-borns compared to the new-borns of unvaccinated mothers. These findings, accomplished in small treatment group sizes, demonstrate a model that provides an efficient, resource sparing and translatable preclinical in vivo system for evaluating vaccine candidates for maternal immunization.

A study of the effects of age on the dynamics of RSV in animal models

Respiratory syncytial virus can cause severe illness and even death, particularly in infants. The increased severity of disease in young children is thought to be due to a lack of previous exposure to the virus as well as the limited immune response in infants. While studies have examined the clinical differences in disease between infants and adults, there has been limited examination of how the viral dynamics differ as infants develop. In this study, we apply a mathematical model to data from cotton rats and ferrets of different ages to assess how viral kinetics parameters change as the animals age. We find no clear trend in the viral decay rate, infecting time, and basic reproduction number as the animals age. We discuss possible reasons for the null result including the limited data, lack of detail of the mathematical model, and the limitations of animal models.

Cotton Rat Placenta Anatomy and Fc Receptor Expression and Their Roles in Maternal Antibody Transfer

Respiratory syncytial virus (RSV) is the leading cause of bronchiolitis and viral pneumonia in infants and young children worldwide. Currently no vaccine is available to prevent RSV infection, but virus-neutralizing monoclonal antibodies can be given prophylactically, emphasizing the protective potential of antibodies. One concept of RSV vaccinology is mothers' immunization to induce high antibody titers, leading to passive transfer of high levels of maternal antibody to the fetus through the placenta and to the neonate through colostrum. Cotton rats are an excellent small animal model for RSV infection and have been used to test maternal immunization. To mechanistically understand antibody transfer in the cotton rat model, we characterized the cotton rat placenta and Fc receptor localization. Placentas from cotton rats at midgestation (approximately day 14) and at late gestation (approximately day 25) and neonatal (younger than 1 wk) gastrointestinal tracts were collected for light microscopy, immunohistochemistry, and transmission electron microscopy. The cotton rat placenta is hemotrichorial and has 5 distinct layers: decidua, junctional zone, labyrinth, chorionic plate, and yolk sac. Consistent with the transfer of maternal antibodies, the majority of the Fc receptors are present in the yolk sac endoderm and fetal capillary endothelium of the chorionic plate, involving 10% of the cells within the labyrinth. In addition, Fc receptors are present on duodenal and jejunal enterocytes in cotton rats, similar to humans, mice, and rats. These findings provide the structural basis for the pre- and postnatal transfer of maternal antibodies described in cotton rats.

Recombinant subtype A and B human respiratory syncytial virus clinical isolates co-infect the respiratory tract of cotton rats

Human respiratory syncytial virus (HRSV) is an important respiratory pathogen causing a spectrum of illness, from common cold-like symptoms, to bronchiolitis and pneumonia requiring hospitalization in infants, the immunocompromised and the elderly. HRSV exists as two antigenic subtypes, A and B, which typically cycle biannually in separate seasons. There are many unresolved questions in HRSV biology regarding the interactions and interplay of the two subtypes. Therefore, we generated a reverse genetics system for a subtype A HRSV from the 2011 season (A11) to complement our existing subtype B reverse genetics system. We obtained the sequence (HRSVA11) directly from an unpassaged clinical sample and generated the recombinant (r) HRSVA11. A version of the virus expressing enhanced green fluorescent protein (EGFP) from an additional transcription unit in the fifth (5) position of the genome, rHRSVA11EGFP(5), was also generated. rHRSVA11 and rHRSVA11EGFP(5) grew comparably in cell culture. To facilitate animal co-infection studies, we derivatized our subtype B clinical isolate using reverse genetics toexpress the red fluorescent protein (dTom)-expressing rHRSVB05dTom(5). These viruses were then used to study simultaneous in vivo co-infection of the respiratory tract. Following intranasal infection, both rHRSVA11EGFP(5) and rHRSVB05dTom(5) infected cotton rats targeting the same cell populations and demonstrating that co-infection occurs in vivo. The implications of this finding on viral evolution are important since it shows that inter-subtype cooperativity and/or competition is feasible in vivo during the natural course of the infection.

Comparisons of Antibody Populations in Different Pre-Fusion F VLP-Immunized Cotton Rat Dams and Their Offspring

Respiratory syncytial virus (RSV) infection poses a significant risk for infants. Since the direct vaccination of infants is problematic, maternal vaccination may provide a safer, more effective approach to their protection. In the cotton rat (CR) model, we have compared the immunization of pregnant CR dams with virus-like particles assembled with the prototype mutation stabilized pre-fusion F protein, DS-Cav1, as well two alternative mutation stabilized pre-fusion proteins (UC-2 F, UC-3 F) and showed that the alternative pre-fusion F VLPs protected the offspring of immunized dams significantly better than DS-Cav1 F VLPs (Blanco, et al. J. Virol. 93: e00914). Here, we have addressed the reasons for this increased protection by characterizing the specificities of antibodies in the sera of both immunized dams and their offspring. The approach was to measure the levels of total anti-pre-F IgG serum antibodies that would block the binding of representative pre-fusion specific monoclonal antibodies to soluble pre-fusion F protein targets. Strikingly, we found that the sera in most offspring of DS-Cav1 F VLP-immunized dams had no mAb D25-blocking antibodies, although their dams had robust levels. In contrast, all offspring of UC-3 F VLP-immunized dams had robust levels of these D25-blocking antibodies. Both sets of pup sera had significant levels of mAb AM14-blocking antibodies, indicating that all pups received maternal antibodies. A lack of mAb D25-blocking antibodies in the offspring of DS-Cav1 F VLP-immunized dams may account for the lower protection of their pups from challenge compared to the offspring of UC-3 F VLP-immunized dams.

Alternative Virus-Like Particle-Associated Prefusion F Proteins as Maternal Vaccines for Respiratory Syncytial Virus

Maternal vaccination may be the most effective and safest approach to the protection of infants from respiratory syncytial virus (RSV) infection, a severe acute lower respiratory tract disease in infants and young children worldwide. We previously compared five different virus-like particle (VLP)-associated, mutation-stabilized prefusion F (pre-F) proteins, including the prototype DS-Cav1 F VLPs. We showed that alternative versions of prefusion F proteins have different conformations and induce different populations of anti-F protein antibodies. Two of these alternative pre-F VLPs, the UC-2 F and UC-3 F VLPs, stimulated in mice higher titers of neutralizing antibodies than DS-Cav1 F VLPs (M. L. Cullen, R. M. Schmidt, M. G. Torres, A. A. Capoferri, et al., Vaccines 7:21-41, 2019, https://doi.org/10.3390/vaccines7010021). Here we describe a comparison of these two pre-F VLPs with DS-Cav1 F VLPs as maternal vaccines in cotton rats and report that UC-3 F VLPs significantly increased the neutralizing antibody (NAb) titers in pregnant dams compared to DS-Cav1 F VLPs. The neutralizing antibody titers in the sera of the offspring of the dams immunized with UC-3 F VLPs were significantly higher than those in the sera of the offspring of dams immunized with DS-Cav1 VLPs. This increase in serum NAb titers translated to a 6- to 40-fold lower virus titer in the lungs of the RSV-challenged offspring of dams immunized with UC-3 F VLPs than in the lungs of the RSV-challenged offspring of dams immunized with DS-Cav1 F VLPs. Importantly, the offspring of UC-3 F VLP-immunized dams showed significant protection from lung pathology and from induction of inflammatory lung cytokine mRNA expression after RSV challenge. Immunization with UC-3 F VLPs also induced durable levels of high-titer neutralizing antibodies in dams.IMPORTANCE Respiratory syncytial virus (RSV) is a significant human pathogen severely impacting neonates and young children, but no vaccine exists to protect this vulnerable population. Furthermore, direct vaccination of neonates is likely ineffective due to the immaturity of their immune system, and neonate immunization is potentially unsafe. Maternal vaccination may be the best and safest approach to the protection of neonates through the passive transfer of maternal neutralizing antibodies in utero to the fetus after maternal immunization. Here we report that immunization of pregnant cotton rats, a surrogate model for human maternal immunization, with novel RSV virus-like particle (VLP) vaccine candidates containing stabilized prefusion RSV F proteins provides significant levels of protection of the offspring of immunized dams from RSV challenge. We also found that antibodies induced by VLPs containing different versions of the prefusion F protein varied by 40-fold in the extent of protection provided to the offspring of vaccinated dams upon RSV challenge.

Mathematical modelling identifies the role of adaptive immunity as a key controller of respiratory syncytial virus in cotton rats

Respiratory syncytial virus (RSV) is a common virus that can have varying effects ranging from mild cold-like symptoms to mortality depending on the age and immune status of the individual. We combined mathematical modelling using ordinary differential equations (ODEs) with measurement of RSV infection kinetics in primary well-differentiated human bronchial epithelial cultures in vitro and in immunocompetent and immunosuppressed cotton rats to glean mechanistic details that underlie RSV infection kinetics in the lung. Quantitative analysis of viral titre kinetics in our mathematical model showed that the elimination of infected cells by the adaptive immune response generates unique RSV titre kinetic features including a faster timescale of viral titre clearance than viral production, and a monotonic decrease in the peak RSV titre with decreasing inoculum dose. Parameter estimation in the ODE model using a nonlinear mixed effects approach revealed a very low rate (average single-cell lifetime > 10 days) of cell lysis by RSV before the adaptive immune response is initiated. Our model predicted negligible changes in the RSV titre kinetics at early times post-infection (less than 5 dpi) but a slower decay in RSV titre in immunosuppressed cotton rats compared to that in non-suppressed cotton rats at later times (greater than 5 dpi) in silico. These predictions were in excellent agreement with the experimental results. Our combined approach quantified the importance of the adaptive immune response in suppressing RSV infection in cotton rats, which could be useful in testing RSV vaccine candidates.

RSV Vaccine Based on Rhabdoviral Vector Protects after Single Immunization

The respiratory syncytial virus (RSV) is one major cause of lower respiratory tract infections in childhood and an effective vaccine is still not available. We previously described a new rhabdoviral vector vaccine, VSV-GP, a variant of the vesicular stomatitis virus (VSV), where the VSV glycoprotein G is exchanged by the glycoprotein GP of the lymphocytic choriomeningitis virus. Here, we evaluated VSV-GP as vaccine vector for RSV with the aim to induce RSV neutralizing antibodies. Wild-type F (Fwt) or a codon optimized version (Fsyn) were introduced at position 5 into the VSV-GP genome. Both F versions were efficiently expressed in VSV-GP-F infected cells and incorporated into VSV-GP particles. In mice, high titers of RSV neutralizing antibodies were induced already after prime and subsequently boosted by a second immunization. After challenge with RSV, viral loads in the lungs of immunized mice were reduced by 2-3 logs with no signs of an enhanced disease induced by the vaccination. Even a single intranasal immunization significantly reduced viral load by a factor of more than 100-fold. RSV neutralizing antibodies were long lasting and mice were still protected when challenged 20 weeks after the boost. Therefore, VSV-GP is a promising candidate for an effective RSV vaccine.

Cotton rat model for testing vaccines and antivirals against respiratory syncytial virus

Respiratory syncytial virus is the leading cause of pneumonia and bronchiolitis in infants and is a serious health risk for elderly and immunocompromised individuals. No vaccine has yet been approved to prevent respiratory syncytial virus infection and the only available treatment is immunoprophylaxis of severe respiratory syncytial virus disease in high-risk infants with Palivizumab (Synagis^®). The development of respiratory syncytial virus vaccine has been hampered by the phenomenon of enhanced respiratory syncytial virus disease observed during trials of a formalin-inactivated respiratory syncytial virus in 1960s. A search for effective respiratory syncytial virus therapeutics has been complicated by the fact that some of the most advanced respiratory syncytial virus antivirals, while highly effective in a prophylactic setting, had not demonstrated clinical efficacy when given after infection. A number of respiratory syncytial virus vaccines and antivirals are currently under development, including several vaccines proposed for maternal immunization. The cotton rat Sigmodon hispidus is an animal model of respiratory syncytial virus infection with demonstrated translational value. Special cohort scenarios, such as infection under conditions of immunosuppression and maternal immunization have been modeled in the cotton rat and are summarized here. In this review, we focus on the recent use of the cotton rat model for testing respiratory syncytial virus vaccine and therapeutic candidates in preclinical setting, including the use of special cohort models. An overview of published studies spanning the period of the last three years is provided. The emphasis, where possible, is made on candidates in the latest stages of preclinical development or currently in clinical trials.

Current Animal Models for Understanding the Pathology Caused by the Respiratory Syncytial Virus

The human respiratory syncytial virus (hRSV) is the main etiologic agent of severe lower respiratory tract infections that affect young children throughout the world, associated with significant morbidity and mortality, becoming a serious public health problem globally. Up to date, no licensed vaccines are available to prevent severe hRSV-induced disease, and the generation of safe-effective vaccines has been a challenging task, requiring constant biomedical research aimed to overcome this ailment. Among the difficulties presented by the study of this pathogen, it arises the fact that there is no single animal model that resembles all aspects of the human pathology, which is due to the specificity that this pathogen has for the human host. Thus, for the study of hRSV, different animal models might be employed, depending on the goal of the study. Of all the existing models, the murine model has been the most frequent model of choice for biomedical studies worldwide and has been of great importance at contributing to the development and understanding of vaccines and therapies against hRSV. The most notable use of the murine model is that it is very useful as a first approach in the development of vaccines or therapies such as monoclonal antibodies, suggesting in this way the direction that research could have in other preclinical models that have higher maintenance costs and more complex requirements in its management. However, several additional different models for studying hRSV, such as other rodents, mustelids, ruminants, and non-human primates, have been explored, offering advantages over the murine model. In this review, we discuss the various applications of animal models to the study of hRSV-induced disease and the advantages and disadvantages of each model, highlighting the potential of each model to elucidate different features of the pathology caused by the hRSV infection.

A multifaceted approach to RSV vaccination

Respiratory Syncytial Virus (RSV) is the leading cause of pneumonia and bronchiolitis in infants, resulting in significant morbidity and mortality worldwide. In addition, RSV infections occur throughout different ages, thus, maintaining the virus in circulation, and increasing health risk to more susceptible populations such as infants, the elderly, and the immunocompromised. To date, there is no vaccine approved to prevent RSV infection or minimize symptoms of infection. Current clinical trials for vaccines against RSV are being carried out in four very different populations. There are vaccines that target two different pediatric populations, infants 2 to 6 month of age and seropositive children over 6 months of age, as well as women (non-pregnant or pregnant in their third trimester). There are vaccines that target adult and elderly populations. In this review, we will present and discuss RSV vaccine candidates currently in clinical trials. We will describe the preclinical studies instrumental for their advancement, with the goal of introducing new preclinical models that may more accurately predict the outcome of clinical vaccine studies.

Maternal immunization with a recombinant adenovirus-expressing fusion protein protects neonatal cotton rats from respiratory syncytia virus infection by transferring antibodies via breast milk and placenta

We evaluated the efficacy of a recombinant adenovirus that expresses a membrane-truncated respiratory syncytial virus (RSV) fusion protein (Ad-F0ΔTM) in newborns via maternal immunization (MI) of pregnant cotton rats. Intranasal Ad-F0ΔTM immunization was given to pregnant female rats, and MI-newborn rats were then challenged intranasally with RSV. Anti-RSV IgGs were observed in the serum of MI-newborn rats after birth. The pulmonary viral loads in Ad-F0ΔTM vs. control vector, Ad-LacZ, and MI-newborns on day 3 post-challenge were reduced by 4 log₁₀/g lung. The neutralizing antibody remained for up to 3 weeks in the serum of MI-newborns, which is when weaning began. Ad-F0ΔTM protected MI-newborns from RSV challenge for 1 week. Vertical-transferred protective antibodies were examined in the breast milk and placenta as well. Finally, anti-RSV immunity was not boosted but was only primed during the next RSV exposure in Ad-F0ΔTM-MI-newborns. Maternal Ad-F0ΔTM immunization provides acute protection against RSV infection in neonates.

Efficacy of a respiratory syncytial virus vaccine candidate in a maternal immunization model

Respiratory syncytial virus (RSV) is the most common cause of bronchiolitis in infants. Maternal immunization is an option to increase maternal antibody levels and protect infants from infection. Here we assess the efficacy of virus-like particle (VLP) vaccine candidates containing stabilized pre-fusion (pre-F) or post-fusion (post-F) conformations of the RSV F protein and the attachment RSV G protein in a maternal immunization model using cotton rats. VLP vaccines containing RSV F and G proteins strongly boost pre-existing RSV immunity in dams preventing their perinatal drop in immunity. Boosting is stronger for the pre-F VLP than for the post-F VLP or purified subunit F protein vaccines, giving an advantage on mothers’ protection. VLP immunization of dams provides significant protection to pups from RSV challenge and reduced pulmonary inflammation. Collectively, our results show that a VLP vaccine with RSV F and G proteins is safe and effective for maternal and adult vaccination.

RSV infection is a major cause of bronchiolitis in infants and maternal vaccination is a potential preventive option. Here, Blanco et al. show efficacy of a Newcastle disease virus-based virus-like particle vaccine candidate in naive and pre-exposed cotton rat dams and their offspring.

Preclinical assessment of safety of maternal vaccination against respiratory syncytial virus (RSV) in cotton rats

Maternal immunization directed to control RSV infection in newborns and infants is an appealing vaccination strategy currently under development. In this work we have modeled maternal vaccination against RSV in cotton rats (CR) to answer two fundamental questions on maternal vaccine safety. We tested (i), whether a known, unsafe RSV vaccine (i.e., FI-RSV Lot 100 vaccine) induces vaccine enhanced disease in the presence of passively transferred, RSV maternal immunity, and (ii) whether the same FI-RSV vaccine could induce vaccine enhanced disease in CR litters when used to immunize their RSV-primed mothers. Our data show that FI-RSV immunization of pups with subsequent RSV infection results in vaccine-enhanced disease independent of whether the pups were born to RSV-seropositive or RSV-seronegative mothers, and that FI-RSV immunization of RSV-seropositive mothers does not present a health risk to either the mother or the infant. Our study also raises a novel concern regarding infant immunization, namely that "safe" RSV vaccines (e.g., live RSV administered intramuscularly) may induce vaccine-enhanced disease in RSV-infected pups born to seropositive mothers. Finally, we describe for the first time a sharp decrease in RSV neutralizing antibody titers in immunized seropositive CR at the time of delivery. This decline may reflect maternal immune suppression, potentially pinpointing a window of increased vulnerability to RSV infection that could be alleviated by effective immunization of expectant mothers.

Immunological Features of Respiratory Syncytial Virus-Caused Pneumonia-Implications for Vaccine Design

The human respiratory syncytial virus (hRSV) is the causative agent for high rates of hospitalizations due to viral bronchiolitis and pneumonia worldwide. Such a disease is characterized by an infection of epithelial cells of the distal airways that leads to inflammation and subsequently to respiratory failure. Upon infection, different pattern recognition receptors recognize the virus and trigger the innate immune response against the hRSV. Further, T cell immunity plays an important role for virus clearance. Based on animal studies, it is thought that the host immune response to hRSV is based on a biased T helper (Th)-2 and Th17 T cell responses with the recruitment of T cells, neutrophils and eosinophils to the lung, causing inflammation and tissue damage. In contrast, human immunity against RSV has been shown to be more complex with no definitive T cell polarization profile. Nowadays, only a humanized monoclonal antibody, known as palivizumab, is available to protect against hRSV infection in high-risk infants. However, such treatment involves several injections at a significantly high cost. For these reasons, intense research has been focused on finding novel vaccines or therapies to prevent hRSV infection in the population. Here, we comprehensively review the recent literature relative to the immunological features during hRSV infection, as well as the new insights into preventing the disease caused by this virus.

Preclinical evaluation of bacterially produced RSV-G protein vaccine: Strong protection against RSV challenge in cotton rat model

In current study, we evaluated the safety and protective efficacy of recombinant unglycosylated RSV G protein ectodomain produced in E. coli (in presence and absence of oil-in-water adjuvant) in a preclinical RSV susceptible cotton rat challenge model compared to formaldehyde inactivated RSV (FI-RSV) and live RSV experimental infection. The adjuvanted G protein vaccine induced robust neutralization antibody responses comparable to those generated by live RSV infection. Importantly, adjuvanted G protein significantly reduced viral loads in both the lungs and nose at early time points following viral challenge. Antibody kinetics determined by Surface Plasmon Resonance showed that adjuvanted G generated 10-fold higher G-binding antibodies compared to non-adjvuanted G vaccine and live RSV infection, which correlated strongly with both neutralization titers and viral load titers in the nose and lungs post-viral challenge. Antibody diversity analysis revealed immunodominant antigenic sites in the N- and C-termini of the RSV-G protein, that were boosted >10-fold by adjuvant and inversely correlated with viral load titers. Enhanced lung pathology was observed only in animals vaccinated with FI-RSV, but not in animals vaccinated with unadjuvanted or adjuvanted RSV-G vaccine after viral challenge. The bacterially produced unglycosylated G protein could be developed as a protective vaccine against RSV disease.

Enterovirus D-68 Infection, Prophylaxis, and Vaccination in a Novel Permissive Animal Model, the Cotton Rat (Sigmodon hispidus)

In recent years, there has been a significant increase in detection of Enterovirus D-68 (EV-D68) among patients with severe respiratory infections worldwide. EV-D68 is now recognized as a re-emerging pathogen; however, due to lack of a permissive animal model for EV-D68, a comprehensive understanding of the pathogenesis and immune response against EV-D68 has been hampered. Recently, it was shown that EV-D68 has a strong affinity for α2,6-linked sialic acids (SAs) and we have shown previously that α2,6-linked SAs are abundantly present in the respiratory tract of cotton rats (Sigmodon hispidus). Thus, we hypothesized that cotton rats could be a potential model for EV-D68 infection. Here, we evaluated the ability of two recently isolated EV-D68 strains (VANBT/1 and MO/14/49), along with the historical prototype Fermon strain (ATCC), to infect cotton rats. We found that cotton rats are permissive to EV-D68 infection without virus adaptation. The different strains of EV-D68 showed variable infection profiles and the ability to produce neutralizing antibody (NA) upon intranasal infection or intramuscular immunization. Infection with the VANBT/1 resulted in significant induction of pulmonary cytokine gene expression and lung pathology. Intramuscular immunization with live VANBT/1 or MO/14/49 induced strong homologous antibody responses, but a moderate heterologous NA response. We showed that passive prophylactic administration of serum with high content of NA against VANBT/1 resulted in an efficient antiviral therapy. VANBT/1-immunized animals showed complete protection from VANBT/1 challenge, but induced strong pulmonary Th1 and Th2 cytokine responses and enhanced lung pathology, indicating the generation of exacerbated immune response by immunization. In conclusion, our data illustrate that the cotton rat is a powerful animal model that provides an experimental platform to investigate pathogenesis, immune response, anti-viral therapies and vaccines against EV-D68 infection.