Infection of mice with respiratory syncytial virus during neonatal life primes for enhanced antibody and T cell responses on secondary challenge

The Neonatal Immune System and Respiratory Pathogens

Neonates are more susceptible to some pathogens, particularly those that cause infection in the respiratory tract. This is often attributed to an incompletely developed immune system, but recent work demonstrates effective neonatal immune responses to some infection. The emerging view is that neonates have a distinctly different immune response that is well-adapted to deal with unique immunological challenges of the transition from a relatively sterile uterus to a microbe-rich world, tending to suppress potentially dangerous inflammatory responses. Problematically, few animal models allow a mechanistic examination of the roles and effects of various immune functions in this critical transition period. This limits our understanding of neonatal immunity, and therefore our ability to rationally design and develop vaccines and therapeutics to best protect newborns. This review summarizes what is known of the neonatal immune system, focusing on protection against respiratory pathogens and describes challenges of various animal models. Highlighting recent advances in the mouse model, we identify knowledge gaps to be addressed.

Respiratory syncytial virus infection during infancy and asthma during childhood in the USA (INSPIRE): a population-based, prospective birth cohort study

BACKGROUND

Early-life severe respiratory syncytial virus (RSV) infection has been associated with the onset of childhood wheezing illnesses. However, the relationship between RSV infection during infancy and the development of childhood asthma is unclear. We aimed to assess the association between RSV infection during infancy and childhood asthma.

METHODS

INSPIRE is a large, population-based, birth cohort of healthy infants with non-low birthweight born at term between June and December, 2012, or between June and December, 2013. Infants were recruited from 11 paediatric practices across middle Tennessee, USA. We ascertained RSV infection status (no infection vs infection) in the first year of life using a combination of passive and active surveillance with viral identification through molecular and serological techniques. Children were then followed up prospectively for the primary outcome of 5-year current asthma, which we analysed in all participants who completed 5-year follow-up. Statistical models, which were done for children with available data, were adjusted for child's sex, race and ethnicity, any breastfeeding, day-care attendance during infancy, exposure to second-hand smoke in utero or during early infancy, and maternal asthma.

FINDINGS

Of 1946 eligible children who were enrolled in the study, 1741 (89%) had available data to assess RSV infection status in the first year of life. The proportion of children with RSV infection during infancy was 944 (54%; 95% CI 52-57) of 1741 children. The proportion of children with 5-year current asthma was lower among those without RSV infection during infancy (91 [16%] of 587) than those with RSV infection during infancy (139 [21%] of 670; p=0·016). Not being infected with RSV during infancy was associated with a 26% lower risk of 5-year current asthma than being infected with RSV during infancy (adjusted RR 0·74, 95% CI 0·58-0·94, p=0·014). The estimated proportion of 5-year current asthma cases that could be prevented by avoiding RSV infection during infancy was 15% (95% CI 2·2-26·8).

INTERPRETATION

Among healthy children born at term, not being infected with RSV in the first year of life was associated with a substantially reduced risk of developing childhood asthma. Our findings show an age-dependent association between RSV infection during infancy and childhood asthma. However, to definitively establish causality, the effect of interventions that prevent, delay, or decrease the severity of the initial RSV infection on childhood asthma will need to be studied.

FUNDING

US National Institutes of Health.

Effect of Infant RSV Infection on Memory T Cell Responses at Age 2-3 Years

Background

It is unknown whether RSV infection in infancy alters subsequent RSV immune responses.

Methods

In a nested cohort of healthy, term children, peripheral blood mononuclear cells (PBMCs) were collected at ages 2-3 years to examine RSV memory T cell responses among children previously RSV infected during infancy (first year of life) compared to those RSV-uninfected during infancy. The presence vs. absence of infant RSV infection was determined through a combination of RSV molecular and serologic testing. Memory responses were measured in RSV stimulated PBMCs.

Results

Compared to children not infected with RSV during the first year of life, children infected with RSV during infancy had lower memory T cell responses at ages 2-3 years to in vitro stimulation with RSV for most tested type-1 and type-17 markers for a number of memory T cell subsets.

Conclusions

RSV infection in infancy has long-term effects on memory T cell responses. This is the first study to show the potential for RSV infection in infancy to have long-term effects on the immune memory irrespective of the severity of the infection. Our results suggest a possible mechanism through which infant RSV infection may result in greater risk of subsequent childhood respiratory viral morbidity, findings also relevant to vaccine development.

Microbiota Signals during the Neonatal Period Forge Life-Long Immune Responses

The microbiota regulates immunological development during early human life, with long-term effects on health and disease. Microbial products include short-chain fatty acids (SCFAs), formyl peptides (FPs), polysaccharide A (PSA), polyamines (PAs), sphingolipids (SLPs) and aryl hydrocarbon receptor (AhR) ligands. Anti-inflammatory SCFAs are produced by Actinobacteria, Bacteroidetes, Firmicutes, Spirochaetes and Verrucomicrobia by undigested-carbohydrate fermentation. Thus, fiber amount and type determine their occurrence. FPs bind receptors from the pattern recognition family, those from commensal bacteria induce a different response than those from pathogens. PSA is a capsular polysaccharide from B. fragilis stimulating immunoregulatory protein expression, promoting IL-2, STAT1 and STAT4 gene expression, affecting cytokine production and response modulation. PAs interact with neonatal immunity, contribute to gut maturation, modulate the gut–brain axis and regulate host immunity. SLPs are composed of a sphingoid attached to a fatty acid. Prokaryotic SLPs are mostly found in anaerobes. SLPs are involved in proliferation, apoptosis and immune regulation as signaling molecules. The AhR is a transcription factor regulating development, reproduction and metabolism. AhR binds many ligands due to its promiscuous binding site. It participates in immune tolerance, involving lymphocytes and antigen-presenting cells during early development in exposed humans.

Lymphocyte-Specific Biomarkers Associated With Preterm Birth and Bronchopulmonary Dysplasia

Many premature babies who are born with neonatal respiratory distress syndrome (RDS) go on to develop Bronchopulmonary Dysplasia (BPD) and later Post-Prematurity Respiratory Disease (PRD) at one year corrected age, characterized by persistent or recurrent lower respiratory tract symptoms frequently related to inflammation and viral infection. Transcriptomic profiles were generated from sorted peripheral blood CD8+ T cells of preterm and full-term infants enrolled with consent in the NHLBI Prematurity and Respiratory Outcomes Program (PROP) at the University of Rochester and the University at Buffalo. We identified outcome-related gene expression patterns following standard methods to identify markers for oxygen utilization and BPD as outcomes in extremely premature infants. We further identified predictor gene sets for BPD based on transcriptomic data adjusted for gestational age at birth (GAB). RNA-Seq analysis was completed for CD8+ T cells from 145 subjects. Among the subjects with highest risk for BPD (born at <29 weeks gestational age (GA); n=72), 501 genes were associated with oxygen utilization. In the same set of subjects, 571 genes were differentially expressed in subjects with a diagnosis of BPD and 105 genes were different in BPD subjects as defined by physiologic challenge. A set of 92 genes could predict BPD with a moderately high degree of accuracy. We consistently observed dysregulation of TGFB, NRF2, HIPPO, and CD40-associated pathways in BPD. Using gene expression data from both premature and full-term subjects (n=116), we identified a 28 gene set that predicted the PRD status with a moderately high level of accuracy, which also were involved in TGFB signaling. Transcriptomic data from sort-purified peripheral blood CD8+ T cells from 145 preterm and full-term infants identified sets of molecular markers of inflammation associated with independent development of BPD in extremely premature infants at high risk for the disease and of PRD among the preterm and full-term subjects.

T cells in the brain enhance neonatal mortality during peripheral LCMV infection

In adult mice the severity of disease from viral infections is determined by the balance between the efficiency of the immune response and the magnitude of viral load. Here, the impact of this dynamic is examined in neonates. Newborns are highly susceptible to infections due to poor innate responses, lower numbers of T cells and Th2-prone immune responses. Eighty-percent of 7-day old mice, immunologically equivalent to human neonates, succumbed to extremely low doses (5 PFU) of the essentially non-lethal lymphocytic choriomeningitis virus (LCMV-Armstrong) given intraperitoneally. This increased lethality was determined to be dependent upon poor early viral control, as well as, T cells and perforin as assessed in knockout mice. By day 3, these neonatal mice had 400-fold higher viral loads as compared to adults receiving a 10,000-fold (5X104 PFU) higher dose of LCMV. The high viral load in combination with the subsequent immunological defect of partial CD8 T cell clonal exhaustion in the periphery led to viral entry and replication in the brain. Within the brain, CD8 T cells were protected from exhaustion, and thus were able to mediate lethal immunopathology. To further delineate the role of early viral control, neonatal mice were infected with Pichinde virus, a less virulent arenavirus, or LCMV was given to pups of LCMV-immune mothers. In both cases, peak viral load was at least 29-fold lower, leading to functional CD8 T cell responses and 100% survival.

Preliminary Trichinella spiralis Infection Ameliorates Subsequent RSV Infection-Induced Inflammatory Response

Respiratory syncytial virus (RSV) infection affects the lives of neonates throughout the globe, causing a high rate of mortality upon hospital admission. Yet, therapeutic options to deal with this pulmonary pathogen are currently limited. Helminth therapy has been well received for its immunomodulatory role in hosts, which are crucial for mitigating a multitude of diseases. Therefore, in this study, we used the helminth Trichinella spiralis and assessed its capabilities for modulating RSV infection as well as the inflammatory response induced by it in mice. Our results revealed that RSV-specific antibody responses were enhanced by pre-existing T. spiralis infection, which also limited pulmonary viral replication. Diminished lung inflammation, indicated by reduced pro-inflammatory cytokines and inflammatory cell influx was confirmed, as well as through histopathological assessment. We observed that inflammation-associated nuclear factor kappa-light-chain enhancement of activated B cells (NF-κB) and its phosphorylated forms were down-regulated, whereas antioxidant-associated nuclear factor erythroid 2-related factor 2 (Nrf2) protein expression was upregulated in mice co-infected with T. spiralis and RSV. Upregulated Nrf2 expression contributed to increased antioxidant enzyme expression, particularly NQO1 which relieved the host of oxidative stress-induced pulmonary inflammation caused by RSV infection. These findings indicate that T. spiralis can mitigate RSV-induced inflammation by upregulating the expression of antioxidant enzymes.

Recombinant BCG Vaccines Reduce Pneumovirus-Caused Airway Pathology by Inducing Protective Humoral Immunity

The Human Respiratory Syncytial Virus (hRSV) and the Human Metapneumovirus (hMPV) are two pneumoviruses that are leading agents causing acute lower respiratory tract infections (ALRTIs) affecting young infants, the elderly, and immunocompromised patients worldwide. Since these pathogens were first discovered, many approaches for the licensing of safe and effective vaccines have been explored being unsuccessful to date. We have previously described that immunization with recombinant strains of Mycobacterium bovis Bacillus Calmette-Guérin (rBCG) expressing the hRSV nucleoprotein (rBCG-N) or the hMPV phosphoprotein (rBCG-P) induced immune protection against each respective virus. These vaccines efficiently promoted viral clearance without significant lung damage, mainly through the induction of a T helper 1 cellular immunity. Here we show that upon viral challenge, rBCG-immunized mice developed a protective humoral immunity, characterized by production of antibodies specific for most hRSV and hMPV proteins. Further, isotype switching from IgG1 to IgG2a was observed in mice immunized with rBCG vaccines and correlated with an increased viral clearance, as compared to unimmunized animals. Finally, sera obtained from animals immunized with rBCG vaccines and infected with their respective viruses exhibited virus neutralizing capacity and protected naïve mice from viral replication and pulmonary disease. These results support the notion that the use of rBCG strains could be considered as an effective vaccination approach against other respiratory viruses with similar biology as hRSV and hMPV.

Type I Interferon Potentiates IgA Immunity to Respiratory Syncytial Virus Infection During Infancy

Respiratory syncytial virus (RSV) infection is the most frequent cause of hospitalization in infants and young children worldwide. Although mucosal RSV vaccines can reduce RSV disease burden, little is known about mucosal immune response capabilities in children. Neonatal or adult mice were infected with RSV; a subset of neonatal mice received interferon alpha (IFN-α) (intranasal) prior to RSV infection. B cells, B cell activating factor (BAFF) and IgA were measured by flow cytometry. RSV specific IgA was measured in nasal washes. Nasal associated lymphoid tissue (NALT) and lungs were stained for BAFF and IgA. Herein, we show in a mouse model of RSV infection that IFN-α plays a dual role as an antiviral and immune modulator and age-related differences in IgA production upon RSV infection can be overcome by IFN-α administration. IFN-α administration before RSV infection in neonatal mice increased RSV-specific IgA production in the nasal mucosa and induced expression of the B-cell activating factor BAFF in NALT. These findings are important, as mucosal antibodies at the infection site, and not serum antibodies, have been shown to protect human adults from experimental RSV infection.

Siglec-1 inhibits RSV-induced interferon gamma production by adult T cells in contrast to newborn T cells

Interferon gamma (IFN‐γ) plays an important role in the antiviral immune response during respiratory syncytial virus (RSV) infections. Monocytes and T cells are recruited to the site of RSV infection, but it is unclear whether cell‐cell interactions between monocytes and T cells regulate IFN‐γ production. In this study, micro‐array data identified the upregulation of sialic acid‐binding immunoglobulin‐type lectin 1 (Siglec‐1) in human RSV‐infected infants. In vitro, RSV increased expression of Siglec‐1 on healthy newborn and adult monocytes. RSV‐induced Siglec‐1 on monocytes inhibited IFN‐γ production by adult CD4⁺ T cells. In contrast, IFN‐γ production by RSV in newborns was not affected by Siglec‐1. The ligand for Siglec‐1, CD43, is highly expressed on adult CD4⁺ T cells compared to newborns. Our data show that Siglec‐1 reduces IFN‐γ release by adult T cells possibly by binding to the highly expressed CD43. The Siglec‐1‐dependent inhibition of IFN‐γ in adults and the low expression of CD43 on newborn T cells provides a better understanding of the immune response against RSV in early life and adulthood.

Cysteinyl leukotriene receptor 1 expression identifies a subset of neutrophils during the antiviral response that contributes to postviral atopic airway disease

BACKGROUND

Viral respiratory tract infections increase the risk of development and exacerbation of atopic disease. Previously, we demonstrated the requirement for a neutrophil (PMN) subset expressing CD49d to drive development of postviral atopic airway disease in mice.

OBJECTIVE

We sought to determine whether human CD49d⁺ PMNs are present in the nasal mucosa during acute viral respiratory tract infections and further characterize this PMN subset in human subjects and mice.

METHODS

Sixty subjects (5-50 years old) were enrolled within 4 days of acute onset of upper respiratory symptoms. Nasal lavage for flow cytometry and nasal swabs for viral PCR were performed at enrollment and during convalescence. The Sendai virus mouse model was used to investigate the phenotype and functional relevance of CD49d⁺ PMNs.

RESULTS

CD49d⁺ PMN frequency was significantly higher in nasal lavage fluid during acute respiratory symptoms in all subjects (2.9% vs 1.0%, n = 42, P < .001). In mice CD49d⁺ PMNs represented a "proatopic" neutrophil subset that expressed cysteinyl leukotriene receptor 1 (CysLTR1) and produced TNF, CCL2, and CCL5. Inhibition of CysLTR1 signaling in the first days of a viral respiratory tract infection was sufficient to reduce accumulation of CD49d⁺ PMNs in the lungs and development of postviral atopic airway disease. Similar to the mouse, human CD49d⁺ PMNs isolated from nasal lavage fluid during a viral respiratory tract infection expressed CysLTR1.

CONCLUSION

CD49d and CysLTR1-coexpressing PMNs are present during symptoms of an acute viral respiratory tract infection in human subjects. Further study is needed to examine selective targeting of proatopic neutrophils as a potential therapeutic strategy to prevent development of postviral atopic airway disease.

Immune and inflammatory response in bronchiolitis due to respiratory Syncytial Virus and Rhinovirus infections in infants

Bronchiolitis is a common disease in infancy, mostly due to Respiratory Syncytial Virus and Rhinovirus. In addition to acute infection, viral bronchiolitis is responsible for sequelae including recurrent wheezing and asthma. The analysis of the viral characteristics and of the pathogenesis of the infection shows differences between the two viruses that may be helpful for the development of therapies and preventive strategies.

Neonatal mucosal immunology

Although largely deprived from exogenous stimuli in utero, the mucosal barriers of the neonate after birth are bombarded by environmental, nutritional, and microbial exposures. The microbiome is established concurrently with the developing immune system. The nature and timing of discrete interactions between these two factors underpins the long-term immune characteristics of these organs, and can set an individual on a trajectory towards or away from disease. Microbial exposures in the gastrointestinal and respiratory tracts are some of the key determinants of the overall immune tone at these mucosal barriers and represent a leading target for future intervention strategies. In this review, we discuss immune maturation in the gut and lung and how microbes have a central role in this process.

Contribution of Fcγ receptors to human respiratory syncytial virus pathogenesis and the impairment of T-cell activation by dendritic cells

Human respiratory syncytial virus (hRSV) is the leading cause of infant hospitalization related to respiratory disease. Infection with hRSV produces abundant infiltration of immune cells into the airways, which combined with an exacerbated pro-inflammatory immune response can lead to significant damage to the lungs. Human RSV re-infection is extremely frequent, suggesting that this virus may have evolved molecular mechanisms that interfere with host adaptive immunity. Infection with hRSV can be reduced by administering a humanized neutralizing antibody against the virus fusion protein in high-risk infants. Although neutralizing antibodies against hRSV effectively block the infection of airway epithelial cells, here we show that both, bone marrow-derived dendritic cells (DCs) and lung DCs undergo infection with IgG-coated virus (hRSV-IC), albeit abortive. Yet, this is enough to negatively modulate DC function. We observed that such a process is mediated by Fcγ receptors (FcγRs) expressed on the surface of DCs. Remarkably, we also observed that in the absence of hRSV-specific antibodies FcγRIII knockout mice displayed significantly less cellular infiltration in the lungs after hRSV infection, compared with wild-type mice, suggesting a potentially harmful, IgG-independent role for this receptor in hRSV disease. Our findings support the notion that FcγRs can contribute significantly to the modulation of DC function by hRSV and hRSV-IC. Further, we provide evidence for an involvement of FcγRIII in the development of hRSV pathogenesis.

Postnatal Infections and Immunology Affecting Chronic Lung Disease of Prematurity

Premature infants suffer significant respiratory morbidity during infancy with long-term negative consequences on health, quality of life, and health care costs. Enhanced susceptibility to a variety of infections and inflammation play a large role in early and prolonged lung disease following premature birth, although the mechanisms of susceptibility and immune dysregulation are active areas of research. This article reviews aspects of host-pathogen interactions and immune responses that are altered by preterm birth and that impact chronic respiratory morbidity in these children.

Effect of maternal exposure to carbon black nanoparticle during early gestation on the splenic phenotype of neonatal mouse

Maternal exposure to environmental factors is implicated as a major factor in the development of the immune system in newborns. Newborns are more susceptible to microbial infection because their immune system is immature. Development of lymphocytes reflects an innate program of lymphocyte proliferation. The aim of this study was to investigate the effects of maternal exposure to carbon black nanoparticle (CB-NP) during early gestation on the development of lymphoid tissues in infantile mice. Pregnant ICR mice were treated with a suspension of CB-NP (95 μg kg(-1) time(-1)) by intranasal instillation on gestational day 5 and 9. Spleen tissues were collected from offspring mice at 1, 3, 5, and 14 days postpartum. Splenocyte phenotypes were examined by investigating the pattern of surface molecules using flow cytometry. Gene expression in the spleen was examined by quantitative RT-PCR. CD3(+) (T), CD4(+) and CD8(+) cells were decreased in the spleen of 1-5-day-old offspring in the treated group. Expression level of Il15 was significantly increased in the spleen of newborn male offspring, and Ccr7 and Ccl19 were increased in the spleen of female offspring in the CB-NP group. Splenic mRNA change profiles by CBNP were similar between male and female offspring. This article concluded that exposure of pregnant mothers to CB-NP partially suppressed the development of the immune system of offspring mice. The decrease in splenic T cells in the treated group recovered at 14 days after birth. This is the first report of developmental effect of nanoparticle on the lymphatic phenotype.

Immunological, Viral, Environmental, and Individual Factors Modulating Lung Immune Response to Respiratory Syncytial Virus

Respiratory syncytial virus is a worldwide pathogen agent responsible for frequent respiratory tract infections that may become severe and potentially lethal in high risk infants and adults. Several studies have been performed to investigate the immune response that determines the clinical course of the infection. In the present paper, we review the literature on viral, environmental, and host factors influencing virus response; the mechanisms of the immune response; and the action of nonimmunological factors. These mechanisms have often been studied in animal models and in the present review we also summarize the main findings obtained from animal models as well as the limits of each of these models. Understanding the lung response involved in the pathogenesis of these respiratory infections could be useful in improving the preventive strategies against respiratory syncytial virus.

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.

Neonatal immunology: responses to pathogenic microorganisms and epigenetics reveal an "immunodiverse" developmental state

Neonatal animals have heightened susceptibility to infectious agents and are at increased risk for the development of allergic diseases, such as asthma. Experimental studies using animal models have been quite useful for beginning to identify the cellular and molecular mechanisms underlying these sensitivities. In particular, results from murine neonatal models indicate that developmental regulation of multiple immune cell types contributes to the typically poor responses of neonates to pathogenic microorganisms. Surprisingly, however, animal studies have also revealed that responses at mucosal surfaces in early life may be protective against primary or secondary disease. Our understanding of the molecular events underlying these processes is less well developed. Emerging evidence indicates that the functional properties of neonatal immune cells and the subsequent maturation of the immune system in ontogeny may be regulated by epigenetic phenomena. Here, we review recent findings from our group and others describing cellular responses to infection and developmentally regulated epigenetic processes in the newborn.

Rapid proliferation and differentiation impairs the development of memory CD8+ T cells in early life

Neonates often generate incomplete immunity against intracellular pathogens, although the mechanism of this defect is poorly understood. An important question is whether the impaired development of memory CD8+ T cells in neonates is due to an immature priming environment or lymphocyte-intrinsic defects. In this article, we show that neonatal and adult CD8+ T cells adopted different fates when responding to equal amounts of stimulation in the same host. Whereas adult CD8+ T cells differentiated into a heterogeneous pool of effector and memory cells, neonatal CD8+ T cells preferentially gave rise to short-lived effector cells and exhibited a distinct gene expression profile. Surprisingly, impaired neonatal memory formation was not due to a lack of responsiveness, but instead because neonatal CD8+ T cells expanded more rapidly than adult cells and quickly became terminally differentiated. Collectively, these findings demonstrate that neonatal CD8+ T cells exhibit an imbalance in effector and memory CD8+ T cell differentiation, which impairs the formation of memory CD8+ T cells in early life.

Neonatal antibody responses are attenuated by interferon-γ produced by NK and T cells during RSV infection

Respiratory syncytial virus (RSV) infects most children in the first year of life and is a major single cause of hospitalization in infants and young children. There is no effective vaccine, and antibody generated by primary neonatal infection is poorly protective against reinfection even with antigenically homologous viral strains. Studying the immunological basis of these observations in neonatal mice, we found that antibody responses to infection were low and unaffected by CD4 depletion, in contrast with adult mice, which had stronger CD4-dependent antibody responses. Natural killer cell depletion or codepletion of CD4(+) and CD8(+) cells during neonatal RSV infection caused a striking increase in anti-RSV antibody titer. These cells are major sources of the cytokine IFN-γ, and blocking IFN-γ also enhanced RSV-specific antibody responses in neonates. In addition, infection with a recombinant RSV engineered to produce IFN-γ reduced antibody titer, confirming that IFN-γ plays a pivotal role in inhibition of antibody responses after neonatal infection. These unexpected findings show that the induction of a strong cellular immune response may limit antibody responses in early life and that vaccines that induce IFN-γ-secreting cells might, in some situations, be less protective than those that do not.

Nucleoprotein nanostructures combined with adjuvants adapted to the neonatal immune context: a candidate mucosal RSV vaccine

Background

The human respiratory syncytial virus (hRSV) is the leading cause of severe bronchiolitis in infants worldwide. The most severe RSV diseases occur between 2 and 6 months-of-age, so pediatric vaccination will have to be started within the first weeks after birth, when the immune system is prone to Th2 responses that may turn deleterious upon exposure to the virus. So far, the high risk to prime for immunopathological responses in infants has hampered the development of vaccine. In the present study we investigated the safety and efficacy of ring-nanostructures formed by the recombinant nucleoprotein N of hRSV (N^SRS) as a mucosal vaccine candidate against RSV in BALB/c neonates, which are highly sensitive to immunopathological Th2 imprinting.

Methodology and Principal Findings

A single intranasal administration of N^SRS with detoxified E.coli enterotoxin LT(R192G) to 5–7 day old neonates provided a significant reduction of the viral load after an RSV challenge at five weeks of age. However, neonatal vaccination also generated an enhanced lung infiltration by neutrophils and eosinophils following the RSV challenge. Analysis of antibody subclasses and cytokines produced after an RSV challenge or a boost administration of the vaccine suggested that neonatal vaccination induced a Th2 biased local immune memory. This Th2 bias and the eosinophilic reaction could be prevented by adding CpG to the vaccine formulation, which, however did not prevent pulmonary inflammation and neutrophil infiltration upon viral challenge.

Conclusions/Significance

In conclusion, protective vaccination against RSV can be achieved in neonates but requires an appropriate combination of adjuvants to prevent harmful Th2 imprinting.

Susceptibility to acute mouse adenovirus type 1 respiratory infection and establishment of protective immunity in neonatal mice

There is an incomplete understanding of the differences between neonatal immune responses that contribute to the increased susceptibility of neonates to some viral infections. We tested the hypothesis that neonates are more susceptible than adults to mouse adenovirus type 1 (MAV-1) respiratory infection and are impaired in the ability to generate a protective immune response against a second infection. Following intranasal infection, lung viral loads were greater in neonates than in adults during the acute phase but the virus was cleared from the lungs of neonates as efficiently as it was from adult lungs. Lung gamma interferon (IFN-γ) responses were blunted and delayed in neonates, and lung viral loads were higher in adult IFN-γ(-/-) mice than in IFN-γ(+/+) controls. However, administration of recombinant IFN-γ to neonates had no effect on lung viral loads. Recruitment of inflammatory cells to the airways was impaired in neonates. CD4 and CD8 T cell responses were similar in the lungs of neonates and adults, although a transient increase in regulatory T cells occurred only in the lungs of infected neonates. Infection of neonates led to protection against reinfection later in life that was associated with increased effector memory CD8 T cells in the lungs. We conclude that neonates are more susceptible than adults to acute MAV-1 respiratory infection but are capable of generating protective immune responses.

Neonatal CD8 T-cell hierarchy is distinct from adults and is influenced by intrinsic T cell properties in respiratory syncytial virus infected mice

Following respiratory syncytial virus infection of adult CB6F1 hybrid mice, a predictable CD8+ T cell epitope hierarchy is established with a strongly dominant response to a K^d-restricted peptide (SYIGSINNI) from the M2 protein. The response to K^dM2_82-90 is ∼5-fold higher than the response to a subdominant epitope from the M protein (NAITNAKII, D^bM_187-195). After infection of neonatal mice, a distinctly different epitope hierarchy emerges with codominant responses to K^dM2_82-90 and D^bM_187-195. Adoptive transfer of naïve CD8+ T cells from adults into congenic neonates prior to infection indicates that intrinsic CD8+ T cell factors contribute to age-related differences in hierarchy. Epitope-specific precursor frequency differs between adults and neonates and influences, but does not predict the hierarchy following infection. Additionally, dominance of K^dM2_82-90 –specific cells does not correlate with TdT activity. Epitope-specific Vβ repertoire usage is more restricted and functional avidity is lower in neonatal mice. The neonatal pattern of codominance changes after infection at 10 days of age, and rapidly shifts to the adult pattern of extreme K^dM2_{82- 90} -dominance. Thus, the functional properties of T cells are selectively modified by developmental factors in an epitope-specific and age-dependent manner.

RSV causes yearly winter epidemics of respiratory disease with peak hospitalization rates at 2.5 months of age. Clearance of virus-infected cells depends on CD8 T-cells, and defining mechanisms of CD8 T-cell regulation is essential for understanding RSV disease pathogenesis and guiding therapeutic interventions. CD8 T-cells recognize a virus-infected cell by detecting peptides cleaved from viral proteins that are presented in host cell MHC molecules. The strength of CD8 T-cell response to processed peptide epitopes from the virus commonly assumes a predictable response hierarchy. In adult hybrid mice that have both H-2^b and H-2^d MHC alleles, the majority of the CD8 T-cell response is targeted at a peptide from the M2 protein and presented by the K^d MHC molecule, K^dM2_82-90, and a smaller subset of CD8 T-cells respond to a peptide from the M protein, D^bM_187-195. Interestingly, when infecting neonatal hybrid mice the dominance pattern is not seen and the D^bM_187-195 response is equal to or greater than the K^dM2_82-90 response. We show that the adult dominance pattern emerges at 10 days of age and that T cells are modified by developmental factors in an epitope-specific and age-dependent manner. These observations may influence future vaccine design.

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.

Respiratory viruses and eosinophils: exploring the connections

In this review, we consider the role played by eosinophilic leukocytes in the pathogenesis and pathophysiology of respiratory virus infection. The vast majority of the available information on this topic focuses on respiratory syncytial virus (RSV; Family Paramyxoviridae, genus Pneumovirus), an important pediatric pathogen that infects infants worldwide. There is no vaccine currently available for RSV. A formalin-inactivated RSV vaccine used in a trial in the 1960s elicited immunopathology in response to natural RSV infection; this has been modeled experimentally, primarily in inbred mice and cotton rats. Eosinophils are recruited to the lung tissue in response to formalin-inactivated RSV vaccine antigens in humans and in experimental models, but they may or may not be involved in promoting the severe clinical sequelae observed. Pulmonary eosinophilia elicited in response to primary RSV infection has also been explored; this response is particularly evident in the youngest human infants and in neonatal mouse models. Although pulmonary eosinophilia is nearly always perceived in a negative light, the specific role played by virus-elicited eosinophils - negative, positive or neutral bystander - remain unclear. Lastly, we consider the data that focus on the role of eosinophils in promoting virus clearance and antiviral host defense, and conclude with a recent study that explores the role of eosinophils themselves as targets of virus infection.