Neonatal Immune Responses to Respiratory Viruses

Phillyrin counters β2 integrin-mediated neutrophil adhesion and chemotaxis to alleviate endotoxin-induced acute lung injury in neonatal rats

Acute lung injury (ALI) in neonates presents a grave threat to infant health, characterized by a heightened risk of mortality. Phillyrin, an extraordinary constituent derived from a traditional Chinese medicinal herb Forsythia suspensa, has garnered considerable attention for its pronounced anti-inflammatory properties. However, its therapeutic potential for acute inflammatory diseases in neonates remains unclear. Therefore, our current study endeavors to assess the protective effects of phillyrin against lipopolysaccharide (LPS)-induced ALI in neonates and elucidate the underlying mechanisms. Phillyrin exhibited significant amelioration of lung damage in neonatal rats with LPS-induced ALI, accompanied by reductions in the total cell counts, neutrophil counts, and total protein level in bronchoalveolar lavage fluid (BALF). Notably, phillyrin substantially attenuated proinflammatory cytokine secretion and suppressed NF-κB activation in the lungs of neonatal ALI rats; however, it demonstrated inefficacy in mitigating LPS-induced cytokine secretion and NF-κB activation in vitro. Notably, phillyrin effectively inhibited β2 integrin-mediated neutrophil adhesion, migration, and chemotaxis. Moreover, phillyrin robustly suppressed β2 integrin engagement-induced actin polymerization and the Vav1/Rac1/PAK1/LIMK1/cofilin pathway. From a mechanistic standpoint, phillyrin exhibited direct interaction with β2 integrin, effectively antagonizing its function and significantly disrupting its binding affinity to intercellular adhesion molecule 1 (ICAM-1). This investigation unveils the promising therapeutic prospects of phillyrin as a novel compound against neonatal ALI.

Reframing the paradigm-rethinking placental and fetal immunity

Preterm birth (PTB), defined as delivery before 37 weeks of gestation, is a leading cause of infant mortality worldwide, with nearly 10 % of births in the U.S. being preterm. PTB leads to various complications, including respiratory, neurological, and immunological issues, along with long-term health problems. It also costs the U.S. healthcare system approximately $25 billion annually, impacting families with lost productivity and long-term care needs. Understanding fetal and placental immunity is critical in addressing PTB. The immune system plays a vital role in maternal tolerance to the fetus and fetal immune development. Research into immune cells and signaling within the placenta may help prevent complications leading to PTB. Additionally, the interaction between maternal and fetal immune systems could reveal therapeutic targets to reduce preterm labor. Although animal models, especially mice, have advanced our understanding of fetal immunity, their differences from humans limit their applicability. Human studies, particularly those examining cord blood, had shown that neonatal immune cells are naïve at birth. However closer examination of preterm infants' blood demonstrated that they exhibit memory T cells linked to preterm labor. Building on this discovery, data demonstrates that fetal memory T cells exist in numerous fetal organs including the placenta. As such, research indicates that the fetus actively shapes the immune environment within the placenta. Disruptions in this process may contribute to PTB. Future investigations into fetal trained immunity and how to improve fetal immune responses could enhance neonatal protection. Understanding immune development in utero could lead to interventions that optimize neonatal health and prevent infections.

The impact of respiratory syncytial virus on asthma development and exacerbation

Asthma is a chronic inflammatory disorder of the lower airways clinically characterized by recurrent wheezing, breathlessness, cough, and dyspnea and the most prevalent chronic disease among children and adolescents. Respiratory viral infections are implicated in asthma inception and exacerbation, with respiratory syncytial virus (RSV) emerging as a key contributor. RSV is a leading cause of acute lower respiratory tract infections, particularly infant bronchiolitis, and is associated with a type 2-biased immune response, diminished interferon activity, epithelial barrier dysfunction, and altered airway microbiome. Although the causal relationship between RSV and asthma remains debated, early life RSV lower respiratory tract infections are increasingly recognized as a significant risk factor for recurrent wheezing and asthma-like symptoms in childhood. This review comprehensively evaluates existing evidence on the long-term respiratory outcomes of infant RSV infection, elucidates the pathophysiological mechanisms connecting RSV infection to asthma development-such as immune dysregulation, chronic airway inflammation, and gene-environment interplay-and highlights novel preventive strategies. Recent advancements, such as maternal RSV vaccines and long-acting monoclonal antibodies, demonstrate efficacy in reducing severe RSV disease burden and subsequent wheeze in high-risk infants. By bridging clinical observations with mechanistic insights, this review underpins the development of future clinical therapies.

Th1 differentiation and function are inhibited in neonates following human metapneumovirus infection

Human metapneumovirus (HMPV) is a leading cause of lower respiratory tract infection in children accounting for 7% of acute care visits and hospitalizations. In particular, neonates and infants have worse outcomes with HMPV infection. The neonatal immune system is regulated to favor anti-inflammatory and tolerogenic responses compared to adults, including prior work demonstrating epigenetic factors in neonatal CD4+ T cells promoting Th2 formation rather than antiviral Th1 differentiation. To interrogate the neonatal immune response to HMPV, 4-to-6 day-old mice or adult 6-to-8 week-old mice were infected with HMPV. Neonates had a decreased Th1 population and increased Th2 and regulatory T-cell (Treg) populations compared to adults. Neonatal Th1 function, but not cell number, was restrained by surface PD-1 expression. To assess if neonatal Th1 formation was intrinsically inhibited after HMPV, neonatal and adult CD4s were transferred into immunocompetent or immunodeficient neonates. Both adult and neonatal CD4s demonstrated reduced Th1 differentiation in the immunocompetent neonates, but robust Th1 differentiation in immunodeficient neonates and immunocompetent adults, suggesting an extrinsic mechanism. Loss of neonatal Tregs led to increased Th1 differentiation after HMPV infection. Neonatal Tregs had increased TGF-β production compared to adult Tregs, and disruption of TGF-β signaling increased Th1 induction. These data demonstrate Tregs provide extrinsic regulation of Th1 formation in the context of respiratory viral infections, rather than an intrinsic limitation of neonatal CD4s. Collectively, these findings identify a nuanced neonatal response to respiratory viruses limiting Th1 formation and function.

Age-Stratified Treg Responses During Viral Infections of the Central Nervous System: A Literature Review

Regulatory T cells (Tregs) play a vital role in limiting inflammation and resolving the immune response after a viral infection. Within the central nervous system (CNS), Tregs are especially important for the protection of neurons, which have limited regenerative capacity, and the preservation of myelin sheaths, which support neuronal function and survival. Nevertheless, viral infections of the CNS often result in enduring neurological dysfunction, especially in more vulnerable age groups such as newborns and the elderly. Although it is appreciated that Treg activity changes with age, it is unclear how these age-dependent changes impact viral CNS infections. In this review, we explore Treg development over the life of the host and discuss evidence for age-dependent Treg responses to peripheral viral infections. We also discuss the CNS-specific roles of Tregs, where both immunomodulatory and neuroprotective functions can contribute to preservation of brain cells. Finally, we examine the current evidence for Treg activity in neurotropic infections in the context of age, and highlight gaps in our understanding of Treg function in younger and older hosts. Overall, a better understanding of age-dependent Treg activity in the CNS may reveal opportunities for therapeutic interventions tailored to the most vulnerable ages.

Enterovirus and Parechovirus Neurologic Infections in Children: Clinical Presentations and Neuropathogenesis

Enteroviruses (EVs) and parechoviruses (PeVs) are common pathogens of childhood. Enteroviral infections cause a range of clinical syndromes from mild illness to neurologic manifestations of meningitis, encephalitis, and acute flaccid myelitis. Disease manifestations are driven by a combination of viral replication and host immune response. Despite ubiquitousness and clinical importance, there are no approved targeted therapies for these viruses and most are without an available vaccine. Studies of EV neuropathogenesis began with poliovirus and are ongoing for other nonpolio EVs and PeVs. Many unanswered questions remain with regard to cellular tropism, mechanisms of dissemination, receptor usage, immunologic control, and cellular death. This review describes what is known about epidemiology, clinical presentations, and neuropathogenesis of these important pathogens.

Method to Assess Immunosenescent CD8+ T Cells in Respiratory Viral Infections

Immunosenescence is a well-characterized phenomenon that occurs with increasing age in all immune and somatic cells. In order to best study immunosenescence, it is imperative to develop methods to accurately identify immunosenescent cells. Elderly patients are known to have impaired immune responses to respiratory viruses, and it is hypothesized that this is due, in part, to immunosenescent, terminally exhausted CD8+ T cells. To test this hypothesis, we developed an aged mouse model and a flow cytometry protocol using the Cytek® Aurora to assess the CD8+ T-cell response during respiratory viral infection and identify immunosenescent CD8+ T cells. This protocol and our aged mouse model have great potential to be incredibly valuable for future studies elucidating how to rejuvenate and possibly reverse immunosenescent CD8+ T cells, which could improve the immune response to respiratory viruses in this at-risk population.

PD-1 signaling in neonates restrains CD8+ T cell function and protects against respiratory viral immunopathology

Respiratory viral infections, including human metapneumovirus (HMPV), remain a leading cause of morbidity and mortality in neonates and infants. However, the mechanisms behind the increased sensitivity to those respiratory viral infections in neonates are poorly understood. Neonates, unlike adults, have several anti-inflammatory mechanisms in the lung, including elevated baseline expression of programmed death ligand 1 (PD-L1), a ligand for the inhibitory receptor programmed cell death protein 1 (PD-1). We thus hypothesized that neonates would rely on PD-1:PD-L1 signaling to restrain antiviral CD8 responses. To test this, we developed a neonatal primary HMPV infection model using wild-type C57BL/6 (B6) and Pdcd1-/- (lacking PD-1) mice. HMPV-infected neonatal mice had increased PD-L1/PD-L2 co-expression on innate immune cells but a similar number of antigen-specific CD8+ T cells and upregulation of PD-1 to that of adult B6 mice. Neonatal CD8+ T cells had reduced interferon-gamma (IFN-γ), granzyme B, and interleukin-2 production compared with B6 adults. Pdcd1-/- neonatal CD8+ T cells had markedly increased production of IFN-γ and granzyme B compared with B6 neonates. Pdcd1-/- neonates had increased acute pathology with HMPV or influenza. Pdcd1-/- neonates infected with HMPV had long-term changes in pulmonary physiology with evidence of immunopathology and a persistent CD8+ T-cell response with increased granzyme B production. Using single-cell ribonucleic acid sequencing from a child lacking PD-1 signaling, a similar activated CD8+ T-cell signature with increased granzyme B expression was observed. These data indicate that PD-1 signaling critically limits CD8+ T-cell effector functions and prevents immunopathology in response to neonatal respiratory viral infections.

Life-threatening infections in human newborns: Reconciling age-specific vulnerability and interindividual variability

In humans, the interindividual variability of clinical outcome following exposure to a microorganism is immense, ranging from silent infection to life-threatening disease. Age-specific immune responses partially account for the high incidence of infection during the first 28 days of life and the related high mortality at population level. However, the occurrence of life-threatening disease in individual newborns remains unexplained. By contrast, inborn errors of immunity and their immune phenocopies are increasingly being discovered in children and adults with life-threatening viral, bacterial, mycobacterial and fungal infections. There is a need for convergence between the fields of neonatal immunology, with its in-depth population-wide characterization of newborn-specific immune responses, and clinical immunology, with its investigations of infections in patients at the cellular and molecular levels, to facilitate identification of the mechanisms of susceptibility to infection in individual newborns and the design of novel preventive and therapeutic strategies.

Liposomal Glutathione Augments Immune Defenses against Respiratory Syncytial Virus in Neonatal Mice Exposed in Utero to Ethanol

We previously reported that maternal alcohol use increased the risk of sepsis in premature and term newborns. In the neonatal mouse, fetal ethanol (ETOH) exposure depleted the antioxidant glutathione (GSH), which promoted alveolar macrophage (AM) immunosuppression and respiratory syncytial virus (RSV) infections. In this study, we explored if oral liposomal GSH (LGSH) would attenuate oxidant stress and RSV infections in the ETOH-exposed mouse pups. C57BL/6 female mice were pair-fed a liquid diet with 25% of calories from ethanol or maltose-dextrin. Postnatal day 10 pups were randomized to intranasal saline, LGSH, and RSV. After 48 h, we assessed oxidant stress, AM immunosuppression, pulmonary RSV burden, and acute lung injury. Fetal ETOH exposure increased oxidant stress threefold, lung RSV burden twofold and acute lung injury threefold. AMs were immunosuppressed with decreased RSV clearance. However, LGSH treatments of the ETOH group normalized oxidant stress, AM immune phenotype, the RSV burden, and acute lung injury. These studies suggest that the oxidant stress caused by fetal ETOH exposure impaired AM clearance of infectious agents, thereby increasing the viral infection and acute lung injury. LGSH treatments reversed the oxidative stress and restored AM immune functions, which decreased the RSV infection and subsequent acute lung injury.

Terminally exhausted CD8+ T cells contribute to age-dependent severity of respiratory virus infection

BACKGROUND

Lower respiratory infections are a leading cause of severe morbidity and mortality among older adults. Despite ubiquitous exposure to common respiratory pathogens throughout life and near universal seropositivity, antibodies fail to effectively protect the elderly. Therefore, we hypothesized that severe respiratory illness in the elderly is due to deficient CD8+ T cell responses.

RESULTS

Here, we establish an aged mouse model of human metapneumovirus infection (HMPV) wherein aged C57BL/6 mice exhibit worsened weight loss, clinical disease, lung pathology and delayed viral clearance compared to young adult mice. Aged mice generate fewer lung-infiltrating HMPV epitope-specific CD8+ T cells. Those that do expand demonstrate higher expression of PD-1 and other inhibitory receptors and are functionally impaired. Transplant of aged T cells into young mice and vice versa, as well as adoptive transfer of young versus aged CD8+ T cells into Rag1-/- recipients, recapitulates the HMPV aged phenotype, suggesting a cell-intrinsic age-associated defect. HMPV-specific aged CD8+ T cells exhibit a terminally exhausted TCF1/7- TOX+ EOMES+ phenotype. We confirmed similar terminal exhaustion of aged CD8+ T cells during influenza viral infection.

CONCLUSIONS

This study identifies terminal CD8+ T cell exhaustion as a mechanism of severe disease from respiratory viral infections in the elderly.

Surveillance of Viral Respiratory Infections in the Neonatal Intensive Care Unit-Evolution in the Last 5 Years

Viral respiratory infections (VRIs) in very low birthweight infants can be associated with high rates of morbidity. The COVID-19 pandemic has exerted a strong impact on viral circulation. The purpose of this study is to report on VRIs during NICU admission in infants below 32 weeks' gestation and compare data collected between the pre-and post-COVID-19 pandemic periods. A prospective surveillance study was conducted at a tertiary NICU between April 2016 and June 2022. The COVID-19 post-pandemic period was established as being from March 2020 onwards. Respiratory virus detection was performed by real-time multiplex PCR assays in nasopharyngeal aspirates (NPAs). A total of 366 infants were enrolled. There were no statistical differences between periods regarding infants' birth weight, gestational age, gender distribution, or rates of bronchopulmonary dysplasia. Among the 1589 NPA collected during the pre-COVID-19 period, 8.9% were positive, and among the 1147 NPA collected during the post-pandemic period, only 3% were positive (p < 0.005). The type of viruses detected did not differ according to the study period (pre-COVID19 vs. post-COVID-19): rhinovirus (49.5% vs. 37.5%), adenovirus (22.6% vs. 25%), and human coronavirus (12.9% vs. 16.7%). SARS-CoV-2 was only detected in one patient. In conclusion, the viral profile causing VRI during the pre-COVID-19 and post-COVID-19 era was similar. However, the total number of VRI dropped significantly, most probably due to the global increase in infection prevention measures.

Shaping of the alveolar landscape by respiratory infections and long-term consequences for lung immunity

Respiratory infections and especially viral infections, along with other extrinsic environmental factors, have been shown to profoundly affect macrophage populations in the lung. In particular, alveolar macrophages (AMs) are important sentinels during respiratory infections and their disappearance opens a niche for recruited monocytes (MOs) to differentiate into resident macrophages. Although this topic is still the focus of intense debate, the phenotype and function of AMs that recolonize the niche after an inflammatory insult, such as an infection, appear to be dictated in part by their origin, but also by local and/or systemic changes that may be imprinted at the epigenetic level. Phenotypic alterations following respiratory infections have the potential to shape lung immunity for the long-term, leading to beneficial responses such as protection against allergic airway inflammation or against other infections, but also to detrimental responses when associated with the development of immunopathologies. This review reports the persistence of virus-induced functional alterations in lung macrophages, and discusses the importance of this imprinting in explaining inter-individual and lifetime immune variation.

Respiratory viruses: New frontiers-a Keystone Symposia report

Respiratory viruses are a common cause of morbidity and mortality around the world. Viruses like influenza, RSV, and most recently SARS-CoV-2 can rapidly spread through a population, causing acute infection and, in vulnerable populations, severe or chronic disease. Developing effective treatment and prevention strategies often becomes a race against ever-evolving viruses that develop resistance, leaving therapy efficacy either short-lived or relevant for specific viral strains. On June 29 to July 2, 2022, researchers met for the Keystone symposium "Respiratory Viruses: New Frontiers." Researchers presented new insights into viral biology and virus-host interactions to understand the mechanisms of disease and identify novel treatment and prevention approaches that are effective, durable, and broad.