RSV-specific airway resident memory CD8+ T cells and differential disease severity after experimental human infection

T-Cell Responses in Adults During Natural Respiratory Syncytial Virus Infection

Background

The pathogenesis of respiratory syncytial virus (RSV) in older adults may be due to age-related T-cell immunosenescence. Thus, we evaluated CD4 and CD8 T-cell responses during RSV infection in adults across the age spectrum.

Methods

Peripheral blood mononuclear cells collected during RSV infection in adults, age 26-96 years, were stimulated with live RSV and peptide pools representing F, M, NP, and G proteins and analyzed by flow cytometry.

Results

There were no significant age-related differences in frequency of CD4+ T cells synthesizing interferon (IFN)γ, interleukin (IL)2, IL4, IL10, or tumor necrosis factor (TNF)α or in CD8+IFNγ+ T cells. IL4+CD4+ T-cell numbers were low, as were IL13 and IL17 responses. However, in univariate analysis, CD4 T-cell IFNγ, IL2, IL4, IL10, and TNFα responses and CD8+IFNγ+ T cells were significantly increased with more severe illness requiring hospitalization. In multivariate analysis, viral load was also associated with increased T-cell responses.

Conclusions

We found no evidence of diminished RSV-specific CD4 or CD8 T-cell responses in adults infected with RSV. However, adults with severe disease seemed to have more robust CD4 and CD8 T-cell responses during infection, suggesting that disease severity may have a greater association with T-cell responses than age.

Contribution of Resident Memory CD8+ T Cells to Protective Immunity Against Respiratory Syncytial Virus and Their Impact on Vaccine Design

Worldwide, human respiratory syncytial virus (RSV) is the most common etiological agent for acute lower respiratory tract infections (ALRI). RSV-ALRI is the major cause of hospital admissions in young children, and it can cause in-hospital deaths in children younger than six months old. Therefore, RSV remains one of the pathogens deemed most important for the generation of a vaccine. On the other hand, the effectiveness of a vaccine depends on the development of immunological memory against the pathogenic agent of interest. This memory is achieved by long-lived memory T cells, based on the establishment of an effective immune response to viral infections when subsequent exposures to the pathogen take place. Memory T cells can be classified into three subsets according to their expression of lymphoid homing receptors: central memory cells (T_CM), effector memory cells (T_EM) and resident memory T cells (T_RM). The latter subset consists of cells that are permanently found in non-lymphoid tissues and are capable of recognizing antigens and mounting an effective immune response at those sites. T_RM cells activate both innate and adaptive immune responses, thus establishing a robust and rapid response characterized by the production of large amounts of effector molecules. T_RM cells can also recognize antigenically unrelated pathogens and trigger an innate-like alarm with the recruitment of other immune cells. It is noteworthy that this rapid and effective immune response induced by T_RM cells make these cells an interesting aim in the design of vaccination strategies in order to establish T_RM cell populations to prevent respiratory infectious diseases. Here, we discuss the biogenesis of T_RM cells, their contribution to the resolution of respiratory viral infections and the induction of T_RM cells, which should be considered for the rational design of new vaccines against RSV.

Importance of Virus Characteristics in Respiratory Syncytial Virus-Induced Disease

Severe lower respiratory tract infection in infants and young children is most frequently caused by respiratory syncytial virus (RSV). RSV infects the smallest airways, making breathing difficult and in some infants requiring medical support. Severity is affected by viral dose, infant age, virus genotype, and effectiveness of the innate/adaptive immune responses. Severe disease correlates with later wheezing and asthma in some children. The adaptive immune response is protective but wanes after each infection, likely due to the ability of the RSV NS1/NS2 proteins to inhibit the innate immune response. Several vaccine approaches and candidates are currently in clinical trials.

Striking a Balance-Cellular and Molecular Drivers of Memory T Cell Development and Responses to Chronic Stimulation

Effective adaptive immune responses are characterized by stages of development and maturation of T and B cell populations that respond to disturbances in the host homeostasis in cases of both infections and cancer. For the T cell compartment, this begins with recognition of specific peptides by naïve, antigen-inexperienced T cells that results in their activation, proliferation, and differentiation, which generates an effector population that clears the antigen. Loss of stimulation eventually returns the host to a homeostatic state, with a heterogeneous memory T cell population that persists in the absence of antigen and is primed for rapid responses to a repeat antigen exposure. However, in chronic infections and cancers, continued antigen persistence impedes a successful adaptive immune response and the formation of a stereotypical memory population of T cells is compromised. With repeated antigen stimulation, responding T cells proceed down an altered path of differentiation that allows for antigen persistence, but much less is known regarding the heterogeneity of these cells and the extent to which they can become “memory-like,” with a capacity for self-renewal and recall responses that are characteristic of bona fide memory cells. This review focuses on the differentiation of CD4⁺ and CD8⁺ T cells in the context of chronic antigen stimulation, highlighting the central observations in both human and mouse studies regarding the differentiation of memory or “memory-like” T cells. The importance of both the cellular and molecular drivers of memory T cell development are emphasized to better understand the consequences of persisting antigen on T cell fates. Integrating what is known and is common across model systems and patients can instruct future studies aimed at further understanding T cell differentiation and development, with the goal of developing novel methods to direct T cells toward the generation of effective memory populations.

Human lung tissue resident memory T cells in health and disease

The human lung contains a heterogeneous population of immune cells which mediate protective responses, maintain tissue homeostasis, but can also promote immunopathology in disease. The majority of T cells in the human lung are tissue resident memory T cells (T_RM) which have been shown in mouse models to provide vital roles in the protection against multiple respiratory pathogens, and contribute to heterosubtypic protection in the context of vaccination. In this review, we will discuss recent studies in humans identifying lung TRM, their role in maintaining tissue homeostasis, and emerging evidence implicating T_RM in anti-tumor immunity and immune surveillance as well as their potential for immunopathology in chronic airway inflammation.

Human CD8+ T cell cross-reactivity across influenza A, B and C viruses

Influenza A, B and C viruses (IAV, IBV and ICV, respectively) circulate globally and infect humans, with IAV and IBV causing the most severe disease. CD8+ T cells confer cross-protection against IAV strains, however the responses of CD8+ T cells to IBV and ICV are understudied. We investigated the breadth of CD8+ T cell cross-recognition and provide evidence of CD8+ T cell cross-reactivity across IAV, IBV and ICV. We identified immunodominant CD8+ T cell epitopes from IBVs that were protective in mice and found memory CD8+ T cells directed against universal and influenza-virus-type-specific epitopes in the blood and lungs of healthy humans. Lung-derived CD8+ T cells displayed tissue-resident memory phenotypes. Notably, CD38+Ki67+CD8+ effector T cells directed against novel epitopes were readily detected in IAV- or IBV-infected pediatric and adult subjects. Our study introduces a new paradigm whereby CD8+ T cells confer unprecedented cross-reactivity across all influenza viruses, a key finding for the design of universal vaccines.

Tissue-Resident T Cells and Other Resident Leukocytes

Resident memory T (Trm) cells stably occupy tissues and cannot be sampled in superficial venous blood. Trm cells are heterogeneous but collectively constitute the most abundant memory T cell subset. Trm cells form an integral part of the immune sensing network, monitor for local perturbations in homeostasis throughout the body, participate in protection from infection and cancer, and likely promote autoimmunity, allergy, and inflammatory diseases and impede successful transplantation. Thus Trm cells are major candidates for therapeutic manipulation. Here we review CD8+ and CD4+ Trm ontogeny, maintenance, function, and distribution within lymphoid and nonlymphoid tissues and strategies for their study. We briefly discuss other resident leukocyte populations, including innate lymphoid cells, macrophages, natural killer and natural killer T cells, nonclassical T cells, and memory B cells. Lastly, we highlight major gaps in knowledge and propose ways in which a deeper understanding could result in new methods to prevent or treat diverse human diseases.

Immunological goals for respiratory syncytial virus vaccine development

Defining the immunological goals for respiratory syncytial virus (RSV) vaccination requires understanding of RSV biology and tropism, mechanisms of cell-to-cell spread and immunity, epidemiology, and transmission dynamics. The immunological goals for a particular vaccine would be product-specific based on antigen selection, delivery approach, and target population. There are many ways to achieve immunity against RSV infection involving innate and adaptive responses, humoral, and cellular effector mechanisms, and mucosal and systemic responses. Both protective and pathological immune response patterns have been demonstrated in animal models and humans. In this short commentary, the entire information matrix that may inform the design of particular vaccine candidates cannot be fully reviewed, but the rationale behind the major vaccine approaches in key target populations will be discussed.

Zymosan by-passes the requirement for pulmonary antigen encounter in lung tissue-resident memory CD8+ T cell development

Tissue-resident memory T cells (Trm) in the lung provide a frontline defence against respiratory pathogens. Vaccination models that lodge CD8⁺ Trm populations in the lung have been developed, all of which incorporate the local delivery of antigen plus adjuvant into the airways; a necessary approach as local cognate antigen recognition is required for optimal lung Trm development. Although pulmonary delivery of antigen is important for lung Trm development, the impact the co-administered adjuvant has on Trm differentiation is unclear. We show that while altering the adjuvant co-administered with the pulmonary delivered antigen does not impact the size of the lung Trm population, a particular adjuvant, zymosan, when administered into the airways without antigen can drive effector CD8⁺ T cells to differentiate into lung Trm. Zymosan signalling via dectin-1 receptor was sufficient to promote antigen-independent lung Trm development. When combined with an injectable influenza vaccination regime, intranasal zymosan delivery significantly boosted the size of the influenza virus-specific lung Trm population. Our results highlight that eliciting the appropriate local inflammatory milieu can by-pass the requirement for local antigen recognition in lung Trm development and emphasises that the appropriate selection of adjuvant can greatly improve vaccines that aim to elicit pulmonary Trm.

Novel genetically-modified chimpanzee adenovirus and MVA-vectored respiratory syncytial virus vaccine safely boosts humoral and cellular immunity in healthy older adults

OBJECTIVES

Respiratory syncytial virus (RSV) causes respiratory infection across the world, with infants and the elderly at particular risk of developing severe disease and death. The replication-defective chimpanzee adenovirus (PanAd3-RSV) and modified vaccinia virus Ankara (MVA-RSV) vaccines were shown to be safe and immunogenic in young healthy adults. Here we report an extension to this first-in-man vaccine trial to include healthy older adults aged 60-75 years.

METHODS

We evaluated the safety and immunogenicity of a single dose of MVA-RSV given by intra-muscular (IM) injection (n = 6), two doses of IM PanAd3-RSV given 4-weeks apart (n = 6), IM PanAd3-RSV prime and IM MVA-RSV boost 8-weeks later (n = 6), intra-nasal (IN) spray of PanAd3-RSV prime and IM MVA-RSV boost 8-weeks later (n = 6), or no vaccine (n = 6). Safety measures included all adverse events within one week of vaccination and blood monitoring. Immunogenicity measures included serum antibody responses (RSV- and PanAd3-neutralising antibody titres measured by plaque-reduction neutralisation and SEAP assays, respectively), peripheral B-cell immune responses (frequencies of F-specific IgG and IgA antibody secreting cells and memory B-cells by ex vivo and cultured dual-colour ELISpot assays respectively), and peripheral RSV-specific T-cell immune responses (frequencies of IFNγ-producing T-cells by ex vivo ELISpot and CD4+/CD8+/Tfh-like cell frequencies by ICS/FACS assay).

RESULTS

The vaccines were safe and well tolerated. Compared with each individual baseline immunity the mean fold-changes in serum RSV-neutralising antibody, appearance and magnitude of F-specific IgG and IgA ASCs and expansion of CD4+/CD8+ IFNγ-producing T-cells in peripheral circulation were comparable to the results seen from younger healthy adults who received the same vaccine combination and dose. There were little/no IgA memory B-cell responses in younger and older adults. Expansion of IFNγ-producing T-cells was most marked in older adults following IM prime, with balanced CD4+ and CD8+ T cell responses. The RSV-specific immune responses to vaccination did not appear to be attenuated in the presence of PanAd3 (vector) neutralising antibody.

CONCLUSIONS

PanAd3-RSV and MVA-RSV was safe and immunogenic in older adults and the parallel induction of RSV-specific humoral and cellular immunity merits further assessment in providing protection from severe disease.

Identification of Novel Respiratory Syncytial Virus CD4+ and CD8+ T Cell Epitopes in C57BL/6 Mice

Respiratory syncytial virus (RSV) is the most common cause of lower respiratory tract infection and hospitalization in infants. It is well established that both CD4⁺ and CD8⁺ T cells are critical for mediating viral clearance but also contribute to the induction of immunopathology following RSV infection. C57BL/6 mice are often used to study T cell responses following RSV infection given the wide variety of genetically modified animals available. To date, few RSV-derived CD4⁺ and CD8⁺ T cell epitopes have been identified in C57BL/6 mice. Using an overlapping peptide library spanning the entire RSV proteome, intracellular cytokine staining for IFN-γ was performed to identify novel CD4⁺ and CD8⁺ T cell epitopes in C57BL/6 mice. We identified two novel CD4⁺ T cell epitopes and three novel CD8⁺ T cell epitopes located within multiple RSV proteins. Additionally, we characterized the newly described T cell epitopes by determining their TCR Vβ expression profiles and MHC restriction. Overall, the novel RSV-derived CD4⁺ and CD8⁺ T cell epitopes identified in C57BL/6 mice will aid in future studies of RSV-specific T cell responses.

Tissue-resident lymphocytes: from adaptive to innate immunity

Efficient immune responses against invading pathogens often involve coordination between cells from both the innate and adaptive immune systems. For multiple decades, it has been believed that CD8⁺ memory T cells and natural killer (NK) cells constantly and uniformly recirculate. Only recently was the existence of noncirculating memory T and NK cells that remain resident in the peripheral tissues, termed tissue-resident memory T (T_RM) cells and tissue-resident NK (trNK) cells, observed in various organs owing to improved techniques. T_RM cells populate a wide range of peripheral organs, including the skin, sensory ganglia, gut, lungs, brain, salivary glands, female reproductive tract, and others. Recent findings have demonstrated the existence of T_RM in the secondary lymphoid organs (SLOs) as well, leading to revision of the classic theory that they exist only in peripheral organs. trNK cells have been identified in the uterus, skin, kidney, adipose tissue, and salivary glands. These tissue-resident lymphocytes do not recirculate in the blood or lymphatic system and often adopt a unique phenotype that is distinct from those of circulating immune cells. In this review, we will discuss the recent findings on the tissue residency of both innate and adaptive lymphocytes, with a particular focus on CD8⁺ memory T cells, and describe some advances regarding unconventional T cells (invariant NKT cells, mucosal-associated invariant T cells (MAIT), and γδ T cells) and the emerging family of trNK cells. Specifically, we will focus on the phenotypes and functions of these subsets and discuss their implications in anti-viral and anti-tumor immunity.

Utility of the Neonatal Calf Model for Testing Vaccines and Intervention Strategies for Use against Human RSV Infection

Respiratory syncytial virus (RSV) is a significant cause of pediatric respiratory tract infections. It is estimated that two-thirds of infants are infected with RSV during the first year of life and it is one of the leading causes of death in this age group worldwide. Similarly, bovine RSV is a primary viral pathogen in cases of pneumonia in young calves and plays a significant role in bovine respiratory disease complex. Importantly, naturally occurring infection of calves with bovine RSV shares many features in common with human RSV infection. Herein, we update our current understanding of RSV infection in cattle, with particular focus on similarities between the calf and human infection, and the recent reports in which the neonatal calf has been employed for the development and testing of vaccines and therapeutics which may be applied to hRSV infection in humans.

Virus genes and host correlates of pathology are markedly reduced during respiratory syncytial and influenza virus co-infection in BALB/c mice

Globally, influenza A virus (IAV) and respiratory syncytial virus (RSV) infection remain very high. There is also a high burden of IAV and RSV co-infection in developing countries. To develop universally protective vaccines against these infections, it is imperative that viral genes and immune correlates of pathology are elucidated. As such, we profiled virus genes expressions, histopathology and immunological responses of BALB/c mice infected with RSV and/or IAV in this study. RSV A2 and/or influenza A/H3N2/Perth/16/09 (Pr/H3N2) were induced over a seven-day period in BALB/c mice. Anaesthetized BALB/c mice (12-14 g) were divided into six groups (15-20 mice per group), inoculated with 32 μl each of 3LD50 Pr/H3N2 and/or 100 TCID50 RSV. Two groups (R or I) received RSV or Pr/H3N2 intranasally. Prior infection with either RSV or Pr/H3N2 was followed with a second challenge of the other virus 24 hours post inoculation in RI and IR groups. Another set was exposed to the two viruses simultaneously (I + R group) while the last group served as healthy controls. Five to seven mice per group were euthanized at days 2, 4 and 7. Lung and spleen organs were harvested for virus genes quantitation and immune cells phenotyping respectively. I + R group showed progressive downregulation of RSV F, G, NS1 and NS2 genes. IAV PB2 and M genes had high fold increase on day 2 and 4 post infections. However, by day 7 post infection, M and PB2 fold increase was lower. Also, increased proportions of NKT and T cell subsets were observed throughout the period in I + R group. Conversely, I group was characterized by reduced NKT cell counts and enhanced CD8 T cells levels while R group only showed an increased proportion of CD8 T cells towards the peak of infection. This study shows that RSV and IAV co-infection lead to reduced virulence and pathology compared to single infections. This information is very useful in combinatorial RSV/IAV vaccine design and development.

Integrative Physiology of Pneumonia

Pneumonia is a type of acute lower respiratory infection that is common and severe. The outcome of lower respiratory infection is determined by the degrees to which immunity is protective and inflammation is damaging. Intercellular and interorgan signaling networks coordinate these actions to fight infection and protect the tissue. Cells residing in the lung initiate and steer these responses, with additional immunity effectors recruited from the bloodstream. Responses of extrapulmonary tissues, including the liver, bone marrow, and others, are essential to resistance and resilience. Responses in the lung and extrapulmonary organs can also be counterproductive and drive acute and chronic comorbidities after respiratory infection. This review discusses cell-specific and organ-specific roles in the integrated physiological response to acute lung infection, and the mechanisms by which intercellular and interorgan signaling contribute to host defense and healthy respiratory physiology or to acute lung injury, chronic pulmonary disease, and adverse extrapulmonary sequelae. Pneumonia should no longer be perceived as simply an acute infection of the lung. Pneumonia susceptibility reflects ongoing and poorly understood chronic conditions, and pneumonia results in diverse and often persistent deleterious consequences for multiple physiological systems.

Small Animal Models of Respiratory Viral Infection Related to Asthma

Respiratory viral infections are strongly associated with asthma exacerbations. Rhinovirus is most frequently-detected pathogen; followed by respiratory syncytial virus; metapneumovirus; parainfluenza virus; enterovirus and coronavirus. In addition; viral infection; in combination with genetics; allergen exposure; microbiome and other pathogens; may play a role in asthma development. In particular; asthma development has been linked to wheezing-associated respiratory viral infections in early life. To understand underlying mechanisms of viral-induced airways disease; investigators have studied respiratory viral infections in small animals. This report reviews animal models of human respiratory viral infection employing mice; rats; guinea pigs; hamsters and ferrets. Investigators have modeled asthma exacerbations by infecting mice with allergic airways disease. Asthma development has been modeled by administration of virus to immature animals. Small animal models of respiratory viral infection will identify cell and molecular targets for the treatment of asthma.

Therapeutic targets for inflammation-mediated airway remodeling in chronic lung disease

INTRODUCTION

Acute exacerbations of chronic lung disease account for substantial morbidity and health costs. Repeated inflammatory episodes and attendant bronchoconstriction cause structural remodeling of the airway. Remodeling is a multicellular response to mucosal injury that results in epithelial cell-state changes, enhanced extracellular deposition, and expansion of pro-fibrotic myofibroblast populations. Areas covered: This manuscript overviews mechanistic studies identifying key sentinel cell populations in the airway and how pattern recognition signaling induces maladaptive mucosal changes and airway remodeling. Studies elucidating how NFκB couples with an atypical histone acetyltransferase, bromodomain-containing protein 4 (BRD4) that reprograms mucosal fibrogenic responses, are described. The approaches to development and characterization of selective inhibitors of epigenetic reprogramming on innate inflammation and structural remodeling in preclinical models are detailed. Expert commentary: Bronchiolar cells derived from Scgb1a1-expressing progenitors function as major sentinel cells of the airway, responsible for initiating antiviral and aeroallergen responses. In these sentinel cells, activation of innate inflammation is coupled to neutrophilic recruitment, mesenchymal transition and myofibroblast expansion. Therapeutics targeting the NFkB-BRD4 may be efficacious in reducing pathological effects of acute exacerbations in chronic lung disease.

The respiratory syncytial virus vaccine landscape: lessons from the graveyard and promising candidates

The global burden of disease caused by respiratory syncytial virus (RSV) is increasingly recognised, not only in infants, but also in older adults (aged ≥65 years). Advances in knowledge of the structural biology of the RSV surface fusion glycoprotein have revolutionised RSV vaccine development by providing a new target for preventive interventions. The RSV vaccine landscape has rapidly expanded to include 19 vaccine candidates and monoclonal antibodies (mAbs) in clinical trials, reflecting the urgency of reducing this global health problem and hence the prioritisation of RSV vaccine development. The candidates include mAbs and vaccines using four approaches: (1) particle-based, (2) live-attenuated or chimeric, (3) subunit, (4) vector-based. Late-phase RSV vaccine trial failures highlight gaps in knowledge regarding immunological protection and provide lessons for future development. In this Review, we highlight promising new approaches for RSV vaccine design and provide a comprehensive overview of RSV vaccine candidates and mAbs in clinical development to prevent one of the most common and severe infectious diseases in young children and older adults worldwide.

CD8+ Resident Memory T Cells and Viral Infection

Tissue-resident memory T (Trm) cells are a subset of recently identified memory T cells that mainly reside and serve as sentinels in non-lymphoid peripheral tissues. Unlike the well-characterized circulating central memory T (Tcm) cells and effector memory T (Tem) cells, Trm cells persist in the tissues, do not recirculate into blood, and offer immediate protection against pathogens upon reinfection. In this review, we focus on CD8⁺ Trm cells and briefly introduce their characteristics, development, maintenance, and function during viral infection. We also discuss some unresolved problems, such as how CD8⁺ Trm cells adapt to the local tissue microenvironment, how Trm cells interact with other immune cells during their development and maintenance, and the mechanisms by which CD8⁺ Trm cells confer immune protection. We believe that a better understanding of these problems is of great clinical and therapeutic value and may contribute to more effective vaccination and treatments against viral infection.

Brain-Resident T Cells Following Viral Infection

Activated CD8⁺ lymphocytes infiltrate the brain in response to many viral infections; where some remain stationed long term as memory T cells. Brain-resident memory T cells (bT_RM) are positioned to impart immediate defense against recurrent or reactivated infection. The cytokine and chemokine milieu present within a tissue is critical for T_RM generation and retention; and reciprocal interactions exist between brain-resident glia and bT_RM. High concentrations of TGF-β are found within brain and this cytokine has been shown to induce CD103 (integrin αeβ7) expression. The majority of T cells persisting within brain express CD103, which aids in retention through interaction with E-cadherin. Likewise, cytokines produced by T cells also modulate microglia. The anti-inflammatory cytokine IL-4 has been shown to preferentially polarize microglial cells toward an M2 phenotype, with a corresponding increase in E-cadherin expression. These findings demonstrate that the brain microenvironment, both during and following inflammation, prominently contributes to the role of CD103 in T cell persistence. Further evidence shows that microglia, and astrocytes, upregulate programmed death (PD) ligand 1 during neuroinflammation, likely to limit neuropathology, and the PD-1: PD-L1 pathway also aids in bT_RM generation and retention. Upon reactivation of quiescent neurotropic viruses, bT_RM may respond to small amounts of de novo-produced viral antigen by rapidly releasing IFN-γ, resulting in interferon-stimulated gene expression in surrounding glia, thereby amplifying activation of a small number of adaptive immune cells into an organ-wide innate antiviral response. While advantageous from an antiviral perspective; over time, recall response-driven, organ-wide innate immune activation likely has cumulative neurotoxic and neurocognitive consequences.

Memory Inflation Drives Tissue-Resident Memory CD8+ T Cell Maintenance in the Lung After Intranasal Vaccination With Murine Cytomegalovirus

Tissue-resident memory T (T_RM) cells provide first-line defense against invading pathogens encountered at barrier sites. In the lungs, T_RM cells protect against respiratory infections, but wane more quickly than T_RM cells in other tissues. This lack of a sustained T_RM population in the lung parenchyma explains, at least in part, why infections with some pathogens, such as influenza virus and respiratory syncytial virus (RSV), recur throughout life. Intranasal (IN) vaccination with a murine cytomegalovirus (MCMV) vector expressing the M protein of RSV (MCMV-M) has been shown to elicit robust populations of CD8⁺ T_RM cells that accumulate over time and mediate early viral clearance. To extend this finding, we compared the inflationary CD8⁺ T cell population elicited by MCMV-M vaccination with a conventional CD8⁺ T cell population elicited by an MCMV vector expressing the M2 protein of RSV (MCMV-M2). Vaccination with MCMV-M2 induced a population of M2-specific CD8⁺ T_RM cells that waned rapidly, akin to the M2-specific CD8⁺ T_RM cell population elicited by infection with RSV. In contrast to the natural immunodominance profile, however, coadministration of MCMV-M and MCMV-M2 did not suppress the M-specific CD8⁺ T cell response, suggesting that progressive expansion was driven by continuous antigen presentation, irrespective of the competitive or regulatory effects of M2-specific CD8⁺ T cells. Moreover, effective viral clearance mediated by M-specific CD8⁺ T_RM cells was not affected by the coinduction of M2-specific CD8⁺ T cells. These data show that memory inflation is required for the maintenance of CD8⁺ T_RM cells in the lungs after IN vaccination with MCMV.

Cytokines and CD8 T cell immunity during respiratory syncytial virus infection

Respiratory syncytial virus (RSV) is the leading cause of lower respiratory tract infection and hospitalization in infants. In spite of the enormous clinical burden caused by RSV infections, there remains no efficacious RSV vaccine. CD8 T cells mediate viral clearance as well as provide protection against a secondary RSV infection. However, RSV-specific CD8 T cells may also induce immunopathology leading to exacerbated morbidity and mortality. Many of the crucial functions performed by CD8 T cells are mediated by the cytokines they produce. IFN-γ and TNF are produced by CD8 T cells following RSV infection and contribute to both the acceleration of viral clearance and the induction of immunopathology. To prevent immunopathology, regulatory mechanisms are in place within the immune system to inhibit CD8 T cell effector functions after the infection has been cleared. The actions of a variety of cytokines, including IL-10 and IL-4, play a critical role in the regulation of CD8 T cell effector activity. Herein, we review the current literature on CD8 T cell responses and the functions of the cytokines they produce following RSV infection. Additionally, we discuss the regulation of CD8 T cell activation and effector functions through the actions of various cytokines.

Beyond Passive Immunity: Is There Priming of the Fetal Immune System Following Vaccination in Pregnancy and What Are the Potential Clinical Implications?

Infection is responsible for over half a million neonatal deaths worldwide every year, and vaccination in pregnancy is becoming increasingly recognized as an important strategy for the protection of young infants. Increasing evidence suggests that exposure to maternal infection in utero may "prime" the developing immune system, even in the absence of infant infection. It is also possible that in utero priming may occur following maternal vaccination, with antigen-specific cellular immune responses detectable in utero and at birth. However, this remains a topic of some controversy. This review focuses on the evidence for in utero priming and the clinical implications for vaccination in pregnancy, considering whether in utero priming following vaccination could provide protection independent of antibody-mediated passive immunity, the possible effects of vaccination on subsequent infant vaccinations, their potential "non-specific" effects, and how the design and timing of vaccination might affect prenatal priming. Looking forward, we describe other possible options for quantifying antigen-specific cellular responses, including MHC tetramers, novel proliferation and cytokine-based assays, and animal models. Together, these may help us address future research questions and establish more robust evidence of fetal immune system priming.

A Systematic Review: The Role of Resident Memory T Cells in Infectious Diseases and Their Relevance for Vaccine Development

Background

Resident memory T cells have emerged as key players in the immune response generated against a number of pathogens. Their ability to take residence in non-lymphoid peripheral tissues allows for the rapid deployment of secondary effector responses at the site of pathogen entry. This ability to provide enhanced regional immunity has gathered much attention, with the generation of resident memory T cells being the goal of many novel vaccines.

Objectives

This review aimed to systematically analyze published literature investigating the role of resident memory T cells in human infectious diseases. Known effector responses mounted by these cells are summarized and key strategies that are potentially influential in the rational design of resident memory T cell inducing vaccines have also been highlighted.

Methods

A Boolean search was applied to Medline, SCOPUS, and Web of Science. Studies that investigated the effector response generated by resident memory T cells and/or evaluated strategies for inducing these cells were included irrespective of published date. Studies must have utilized an established technique for identifying resident memory T cells such as T cell phenotyping.

Results

While over 600 publications were revealed by the search, 147 articles were eligible for inclusion. The reference lists of included articles were also screened for other eligible publications. This resulted in the inclusion of publications that studied resident memory T cells in the context of over 25 human pathogens. The vast majority of studies were conducted in mouse models and demonstrated that resident memory T cells mount protective immune responses.

Conclusion

Although the role resident memory T cells play in providing immunity varies depending on the pathogen and anatomical location they resided in, the evidence overall suggests that these cells are vital for the timely and optimal protection against a number of infectious diseases. The induction of resident memory T cells should be further investigated and seriously considered when designing new vaccines.

Developmental Regulation of Effector and Resident Memory T Cell Generation during Pediatric Viral Respiratory Tract Infection

Viral respiratory tract infections (VRTI) remain a leading cause of morbidity and mortality among infants and young children. In mice, optimal protection to VRTI is mediated by recruitment of effector T cells to the lungs and respiratory tract, and subsequent establishment of tissue resident memory T cells (Trm), which provide long-term protection. These critical processes of T cell recruitment to the respiratory tract, their role in disease pathogenesis, and establishment of local protective immunity remain undefined in pediatric VRTI. In this study, we investigated T cell responses in the upper respiratory tract (URT) and lower respiratory tract (LRT) of infants and young children with VRTI, revealing developmental regulation of T cell differentiation and Trm generation in situ. We show a direct concurrence between T cell responses in the URT and LRT, including a preponderance of effector CD8⁺ T cells that was associated with disease severity. During infant VRTI, there was an accumulation of terminally differentiated effector cells (effector memory RA⁺ T cells) in the URT and LRT with reduced Trm in the early neonatal period, and decreased effector memory RA⁺ T cell and increased Trm formation with age during the early years of childhood. Moreover, human infant T cells exhibit increased expression of the transcription factor T-bet compared with adult T cells, suggesting a mechanism for preferential generation of effector over Trm. The developmental regulation of respiratory T cell responses as revealed in the present study is important for diagnosing, monitoring, and treating VRTI in the critical early life stages.

Human respiratory syncytial virus and hospitalization in young children in Italy

BACKGROUND

Human respiratory syncytial virus (hRSV) is ubiquitous and causes respiratory diseases in both children and adults. Worldwide, hRSV pneumonia is the second cause of postnatal infant death after malaria. Given the high impact in terms of morbidity, mortality and costs, especially in the pediatric population, hRSV is recognized as a global health problem and the WHO, in view of the availability of new vaccines, has urged an active surveillance program of virus-related infections. The aim of this study has been to evaluate the impact of hRSV infections in the Italian population, particularly the pediatric one, in terms of hospitalizations.

METHODS

In the period 2001-2014, Hospital Discharge Records (HDRs) with the following diagnosis codes included in the primary diagnosis were evaluated: 466.11 (hRSV bronchiolitis), 480.1 (hRSV pneumonia) and 796 (hRSV). HDRs were supplied by the National Archive of HDRs data, Ministry of Health.

RESULTS

During the period 2001-2014, 57,656 hospital admissions due to hRSV pathologies were performed. Most hospitalizations (88.8%) involved patients with less than 1 year of age. Considering only primary diagnosis, 93% of the admissions were due to bronchiolitis, 5% to pneumonia and 2% to not otherwise specified hRSV infections. In the period 2001-2014, the hospitalization rate in 0-2 years old children, was equal to 224.8, 9.6 and 4.6/100,000 for hRSV bronchiolitis, hRSV pneumonia and not otherwise specified hRSV infection, respectively.

CONCLUSIONS

This study confirms the high impact of hRSV on the pediatric population in the age class 0-4 years, with a peak in the first 12 months of life. Most hospitalizations were urgent, although the duration of the hospital stay was for the most part less than a week, with ordinary discharge at home. Pending the conclusion of ongoing clinical trials on different hRSV vaccine types, it is extremely important to have updated data on the impact of hRSV-related pathologies in the various age groups.

The function and dysfunction of memory CD8+ T cells in tumor immunity

The generation and maintenance of CD8+ T cell memory is crucial to long-term host survival, yet the basic tenets of CD8+ T cell immunity are still being established. Recent work has led to the discovery of tissue-resident memory cells and refined our understanding of the transcriptional and epigenetic basis of CD8+ T cell differentiation and dysregulation. In parallel, the unprecedented clinical success of immunotherapy has galvanized an intense, global research effort to decipher and de-repress the anti-tumor response. However, the progress of immunotherapy is at a critical juncture, since the efficacy of immuno-oncology agents remains confined to a fraction of patients and often fails to provide durable benefit. Unlocking the potential of immunotherapy requires the design of strategies that both induce a potent effector response and reliably forge stable, functional memory T cell pools capable of protecting from recurrence or relapse. It is therefore essential that basic and emerging concepts of memory T cell biology are rapidly and faithfully transposed to advance therapeutic development in cancer immunotherapy. This review highlights seminal and recent reports in CD8+ T cell memory and tumor immunology, and evaluates recent data from solid cancer specimens in the context of the key paradigms from preclinical models. We elucidate the potential significance of circulating effector cells poised downstream of neoantigen recognition and upstream of T cell dysfunction and propose that cells in this immunological 'sweet spot' may be key anti-tumor effectors.

Comparison of Heterosubtypic Protection in Ferrets and Pigs Induced by a Single-Cycle Influenza Vaccine

Influenza is a major health threat, and a broadly protective influenza vaccine would be a significant advance. Signal Minus FLU (S-FLU) is a candidate broadly protective influenza vaccine that is limited to a single cycle of replication, which induces a strong cross-reactive T cell response but a minimal Ab response to hemagglutinin after intranasal or aerosol administration. We tested whether an H3N2 S-FLU can protect pigs and ferrets from heterosubtypic H1N1 influenza challenge. Aerosol administration of S-FLU to pigs induced lung tissue-resident memory T cells and reduced lung pathology but not the viral load. In contrast, in ferrets, S-FLU reduced viral replication and aerosol transmission. Our data show that S-FLU has different protective efficacy in pigs and ferrets, and that in the absence of Ab, lung T cell immunity can reduce disease severity without reducing challenge viral replication.

Respiratory virus-induced heterologous immunity: Part of the problem or part of the solution?

Purpose

To provide current knowledge on respiratory virus-induced heterologous immunity (HI) with a focus on humoral and cellular cross-reactivity. Adaptive heterologous immune responses have broad implications on infection, autoimmunity, allergy and transplant immunology. A better understanding of the mechanisms involved might ultimately open up possibilities for disease prevention, for example by vaccination.

Methods

A structured literature search was performed using Medline and PubMed to provide an overview of the current knowledge on respiratory-virus induced adaptive HI.

Results

In HI the immune response towards one antigen results in an alteration of the immune response towards a second antigen. We provide an overview of respiratory virus-induced HI, including viruses such as respiratory syncytial virus (RSV), rhinovirus (RV), coronavirus (CoV) and influenza virus (IV). We discuss T cell receptor (TCR) and humoral cross-reactivity as mechanisms of HI involving those respiratory viruses. Topics covered include HI between respiratory viruses as well as between respiratory viruses and other pathogens. Newly developed vaccines, which have the potential to provide protection against multiple virus strains are also discussed. Furthermore, respiratory viruses have been implicated in the development of autoimmune diseases, such as narcolepsy, Guillain-Barré syndrome, type 1 diabetes or myocarditis. Finally, we discuss the role of respiratory viruses in asthma and the hygiene hypothesis, and review our recent findings on HI between IV and allergens, which leads to protection from experimental asthma.

Conclusion

Respiratory-virus induced HI may have protective but also detrimental effects on the host. Respiratory viral infections contribute to asthma or autoimmune disease development, but on the other hand, a lack of microbial encounter is associated with an increasing number of allergic as well as autoimmune diseases. Future research might help identify the elements which determine a protective or detrimental outcome in HI-based mechanisms.

The CD8 T Cell Response to Respiratory Virus Infections

Humans are highly susceptible to infection with respiratory viruses including respiratory syncytial virus (RSV), influenza virus, human metapneumovirus, rhinovirus, coronavirus, and parainfluenza virus. While some viruses simply cause symptoms of the common cold, many respiratory viruses induce severe bronchiolitis, pneumonia, and even death following infection. Despite the immense clinical burden, the majority of the most common pulmonary viruses lack long-lasting efficacious vaccines. Nearly all current vaccination strategies are designed to elicit broadly neutralizing antibodies, which prevent severe disease following a subsequent infection. However, the mucosal antibody response to many respiratory viruses is not long-lasting and declines with age. CD8 T cells are critical for mediating clearance following many acute viral infections in the lung. In addition, memory CD8 T cells are capable of providing protection against secondary infections. Therefore, the combined induction of virus-specific CD8 T cells and antibodies may provide optimal protective immunity. Herein, we review the current literature on CD8 T cell responses induced by respiratory virus infections. Additionally, we explore how this knowledge could be utilized in the development of future vaccines against respiratory viruses, with a special emphasis on RSV vaccination.

Respiratory virus-induced heterologous immunity: Part of the problem or part of the solution?

Purpose

To provide current knowledge on respiratory virus-induced heterologous immunity (HI) with a focus on humoral and cellular cross-reactivity. Adaptive heterologous immune responses have broad implications on infection, autoimmunity, allergy and transplant immunology. A better understanding of the mechanisms involved might ultimately open up possibilities for disease prevention, for example by vaccination.

Methods

A structured literature search was performed using Medline and PubMed to provide an overview of the current knowledge on respiratory-virus induced adaptive HI.

Results

In HI the immune response towards one antigen results in an alteration of the immune response towards a second antigen. We provide an overview of respiratory virus-induced HI, including viruses such as respiratory syncytial virus (RSV), rhinovirus (RV), coronavirus (CoV) and influenza virus (IV). We discuss T cell receptor (TCR) and humoral cross-reactivity as mechanisms of HI involving those respiratory viruses. Topics covered include HI between respiratory viruses as well as between respiratory viruses and other pathogens. Newly developed vaccines which have the potential to provide protection against multiple virus strains are also discussed. Furthermore, respiratory viruses have been implicated in the development of autoimmune diseases, such as narcolepsy, Guillain–Barré syndrome, type 1 diabetes or myocarditis. Finally, we discuss the role of respiratory viruses in asthma and the hygiene hypothesis, and review our recent findings on HI between IV and allergens, which leads to protection from experimental asthma.

Conclusion

Respiratory-virus induced HI may have protective but also detrimental effects on the host. Respiratory viral infections contribute to asthma or autoimmune disease development, but on the other hand, a lack of microbial encounter is associated with an increasing number of allergic as well as autoimmune diseases. Future research might help identify the elements which determine a protective or detrimental outcome in HI-based mechanisms.

Tissue-Resident Memory CD8+ T Cells: From Phenotype to Function

Tissue-resident memory CD8⁺ T cells are an important first line of defense from infection in peripheral non-lymphoid tissues, such as the mucosal tissues of the respiratory, digestive, and urogenital tracts. This memory T cell subset is established late during resolution of primary infection of those tissues, has a distinct genetic signature, and is often defined by the cell surface expression of CD69, CD103, CD49a, and CD44 in both mouse and human studies. The stimuli that program or imprint the unique gene expression and cell surface phenotypes on T_RM are beginning to be defined, but much work remains to be done. It is not clear, for example, when and where the T_RM precursors receive these signals, and there is evidence that supports imprinting in both the lymph node and the peripheral tissue sites. In most studies, expression of CD49a, CD103, and CD69 on T cells in the tissues appears relatively late in the response, suggesting there are precise environmental cues that are not present at the height of the acute response. CD49a and CD103 are not merely biomarkers of T_RM, they confer substrate specificities for cell adhesion to collagen and E-cadherin, respectively. Yet, little attention has been paid to how expression affects the positioning of T_RM in the peripheral tissues. CD103 and CD49a are not mutually exclusive, and not always co-expressed, although whether they can compensate for one another is unknown. In fact, they may define different subsets of T_RM in certain tissues. For instance, while CD49a⁺CD8⁺ memory T cells can be found in almost all peripheral tissues, CD103 appears to be more restricted. In this review, we discuss the evidence for how these hallmarks of T_RM affect positioning of T cells in peripheral sites, how CD49a and CD103 differ in expression and function, and why they are important for immune protection conferred by T_RM in mucosal tissues such as the respiratory tract.

Induction and Subversion of Human Protective Immunity: Contrasting Influenza and Respiratory Syncytial Virus

Respiratory syncytial virus (RSV) and influenza are among the most important causes of severe respiratory disease worldwide. Despite the clinical need, barriers to developing reliably effective vaccines against these viruses have remained firmly in place for decades. Overcoming these hurdles requires better understanding of human immunity and the strategies by which these pathogens evade it. Although superficially similar, the virology and host response to RSV and influenza are strikingly distinct. Influenza induces robust strain-specific immunity following natural infection, although protection by current vaccines is short-lived. In contrast, even strain-specific protection is incomplete after RSV and there are currently no licensed RSV vaccines. Although animal models have been critical for developing a fundamental understanding of antiviral immunity, extrapolating to human disease has been problematic. It is only with recent translational advances (such as controlled human infection models and high-dimensional technologies) that the mechanisms responsible for differences in protection against RSV compared to influenza have begun to be elucidated in the human context. Influenza infection elicits high-affinity IgA in the respiratory tract and virus-specific IgG, which correlates with protection. Long-lived influenza-specific T cells have also been shown to ameliorate disease. This robust immunity promotes rapid emergence of antigenic variants leading to immune escape. RSV differs markedly, as reinfection with similar strains occurs despite natural infection inducing high levels of antibody against conserved antigens. The immunomodulatory mechanisms of RSV are thus highly effective in inhibiting long-term protection, with disturbance of type I interferon signaling, antigen presentation and chemokine-induced inflammation possibly all contributing. These lead to widespread effects on adaptive immunity with impaired B cell memory and reduced T cell generation and functionality. Here, we discuss the differences in clinical outcome and immune response following influenza and RSV. Specifically, we focus on differences in their recognition by innate immunity; the strategies used by each virus to evade these early immune responses; and effects across the innate-adaptive interface that may prevent long-lived memory generation. Thus, by comparing these globally important pathogens, we highlight mechanisms by which optimal antiviral immunity may be better induced and discuss the potential for these insights to inform novel vaccines.

Adjuvanted influenza vaccines

In spite of current influenza vaccines being immunogenic, evolution of the influenza virus can reduce efficacy and so influenza remains a major threat to public health. One approach to improve influenza vaccines is to include adjuvants; substances that boost the immune response. Adjuvants are particularly beneficial for influenza vaccines administered during a pandemic when a rapid response is required or for use in patients with impaired immune responses, such as infants and the elderly. This review outlines the current use of adjuvants in human influenza vaccines, including what they are, why they are used and what is known of their mechanism of action. To date, six adjuvants have been used in licensed human vaccines: Alum, MF59, AS03, AF03, virosomes and heat labile enterotoxin (LT). In general these adjuvants are safe and well tolerated, but there have been some rare adverse events when adjuvanted vaccines are used at a population level that may discourage the inclusion of adjuvants in influenza vaccines, for example the association of LT with Bell's Palsy. Improved understanding about the mechanisms of the immune response to vaccination and infection has led to advances in adjuvant technology and we describe the experimental adjuvants that have been tested in clinical trials for influenza but have not yet progressed to licensure. Adjuvants alone are not sufficient to improve influenza vaccine efficacy because they do not address the underlying problem of mismatches between circulating virus and the vaccine. However, they may contribute to improved efficacy of next-generation influenza vaccines and will most likely play a role in the development of effective universal influenza vaccines, though what that role will be remains to be seen.

Memory CD8 T cells mediate severe immunopathology following respiratory syncytial virus infection

Memory CD8 T cells can provide protection from re-infection by respiratory viruses such as influenza and SARS. However, the relative contribution of memory CD8 T cells in providing protection against respiratory syncytial virus (RSV) infection is currently unclear. To address this knowledge gap, we utilized a prime-boost immunization approach to induce robust memory CD8 T cell responses in the absence of RSV-specific CD4 T cells and antibodies. Unexpectedly, RSV infection of mice with pre-existing CD8 T cell memory led to exacerbated weight loss, pulmonary disease, and lethal immunopathology. The exacerbated disease in immunized mice was not epitope-dependent and occurred despite a significant reduction in RSV viral titers. In addition, the lethal immunopathology was unique to the context of an RSV infection as mice were protected from a normally lethal challenge with a recombinant influenza virus expressing an RSV epitope. Memory CD8 T cells rapidly produced IFN-γ following RSV infection resulting in elevated protein levels in the lung and periphery. Neutralization of IFN-γ in the respiratory tract reduced morbidity and prevented mortality. These results demonstrate that in contrast to other respiratory viruses, RSV-specific memory CD8 T cells can induce lethal immunopathology despite mediating enhanced viral clearance.

Memory CD8 T cells have been shown to provide protection against many respiratory viruses. However, the ability of memory CD8 T cells to provide protection against RSV has not been extensively examined. Unexpectedly, mice with pre-existing CD8 T cell memory, in the absence of memory CD4 T cells and antibodies, exhibited exacerbated morbidity and mortality following RSV infection. We demonstrate that the immunopathology is the result of early and excessive production of IFN-γ by memory CD8 T cells in the lung. Our research provides important new insight into the mechanisms of how memory T cells induce immunopathology. In addition, our findings serve as an important cautionary tale against the use of epitope-based T cell vaccines against RSV.

Airway T cells protect against RSV infection in the absence of antibody

Tissue resident memory T (Trm) cells act as sentinels and early responders to infection. Respiratory syncytial virus (RSV)-specific Trm cells have been detected in the lungs after human RSV infection, but whether they have a protective role is unknown. To dissect the protective function of Trm cells, BALB/c mice were infected with RSV; infected mice developed antigen-specific CD8⁺ Trm cells (CD103⁺/CD69⁺) in the lungs and airways. Intranasally transferring cells from the airways of previously infected animals to naïve animals reduced weight loss on infection in the recipient mice. Transfer of airway CD8 cells led to reduced disease and viral load and increased interferon-γ in the airways of recipient mice, while CD4 transfer reduced tumor necrosis factor-α in the airways. Because DNA vaccines induce a systemic T-cell response, we compared vaccination with infection for the effect of memory CD8 cells generated in different compartments. Intramuscular DNA immunization induced RSV-specific CD8 T cells, but they were immunopathogenic and not protective. Notably, there was a marked difference in the induction of Trm cells; infection but not immunization induced antigen-specific Trm cells in a range of tissues. These findings demonstrate a protective role for airway CD8 against RSV and support the need for vaccines to induce antigen-specific airway cells.

Inferior immunogenicity and efficacy of respiratory syncytial virus fusion protein-based subunit vaccine candidates in aged versus young mice

Respiratory syncytial virus (RSV) is recognized as an important cause of lower and upper respiratory tract infections in older adults, and a successful vaccine would substantially lower morbidity and mortality in this age group. Recently, two vaccine candidates based on soluble purified glycoprotein F (RSV F), either alone or adjuvanted with glucopyranosyl lipid A formulated in a stable emulsion (GLA-SE), failed to reach their primary endpoints in clinical efficacy studies, despite demonstrating the desired immunogenicity profile and efficacy in young rodent models. Here, one of the RSV F vaccine candidates (post-fusion conformation, RSV post-F), and a stabilized pre-fusion form of RSV F (RSV pre-F, DS-Cav1) were evaluated in aged BALB/c mice. Humoral and cellular immunogenicity elicited after immunization of naïve, aged mice was generally lower compared to young animals. In aged mice, RSV post-F vaccination without adjuvant poorly protected the respiratory tract from virus replication, and addition of GLA-SE only improved protection in the lungs, but not in nasal turbinates. RSV pre-F induced higher neutralizing antibody titers compared to RSV post-F (as previously reported) but interestingly, RSV F-specific CD8 T cell responses were lower compared to RSV post-F responses regardless of age. The vaccines were also tested in RSV seropositive aged mice, in which both antigen forms similarly boosted neutralizing antibody titers, although GLA-SE addition boosted neutralizing activity only in RSV pre-F immunized animals. Cell-mediated immune responses in the aged mice were only slightly boosted and well below levels induced in seronegative young mice. Taken together, the findings suggest that the vaccine candidates were not able to induce a strong anti-RSV immune response in recipient mice with an aged immune system, in agreement with recent human clinical trial results. Therefore, the aged mouse model could be a useful tool to evaluate improved vaccine candidates, targeted to prevent RSV disease in older adults.

Formation and function of tissue-resident memory T cells during viral infection

Memory T cells are an important component of the adaptive immune response. Tissue-resident memory T cells (T_RM) are a recently described subset of memory T cells that reside in peripheral tissues and are maintained independently of circulating subsets of memory T cells. Importantly, T_RM are frequently found in barrier tissues that commonly serve as entry portals for pathogens such as viruses. Mounting evidence shows that T_RM are superior to their circulating counterparts in conferring protective immunity against a wide range of viruses. In this review, we will discuss the role of T_RM in controlling viral infection with a focus on CD8+ T_RM, the factors that regulate differentiation and a potential role for T_RM in future vaccine development.

Respiratory syncytial virus elicits enriched CD8+ T lymphocyte responses in lung compared with blood in African green monkeys

Respiratory syncytial virus (RSV) is a leading cause of serious lower respiratory tract disease in young children and older adults throughout the world. Prevention of severe RSV disease through active immunization is optimal but no RSV vaccine has been licensed so far. Immune mechanisms of protection against RSV infection in humans have not been fully established, thus a comprehensive characterization of virus-specific immune responses in a relevant animal model will be beneficial in defining correlates of protection. In this study, we infected juvenile naive AGMs with RSV A2 strain and longitudinally assessed virus-specific humoral and cellular immune responses in both peripheral blood and the respiratory tract. RSV viral loads at nasopharyngeal surfaces and in the lung peaked at around day 5 following infection, and then largely resolved by day 10. Low levels of neutralizing antibody titers were detected in serum, with similar kinetics as RSV fusion (F) protein-binding IgG antibodies. RSV infection induced CD8+, but very little CD4+, T lymphocyte responses in peripheral blood. Virus-specific CD8+ T cell frequencies were ~10 fold higher in bronchoaveolar lavage (BAL) compared to peripheral blood and exhibited effector memory (CD95+CD28-) / tissue resident memory (CD69+CD103+) T (TRM) cell phenotypes. The kinetics of virus-specific CD8+ T cells emerging in peripheral blood and BAL correlated with declining viral titers, suggesting that virus-specific cellular responses contribute to the clearance of RSV infection. RSV-experienced AGMs were protected from subsequent exposure to RSV infection. Additional studies are underway to understand protective correlates in these seropositive monkeys.

Tuberculosis Infectiousness and Host Susceptibility

The transmission of tuberculosis is complex. Necessary factors include a source case with respiratory disease that has developed sufficiently for Mycobacterium tuberculosis to be present in the airways. Viable bacilli must then be released as an aerosol via the respiratory tract of the source case. This is presumed to occur predominantly by coughing but may also happen by other means. Airborne bacilli must be capable of surviving in the external environment before inhalation into a new potential host-steps influenced by ambient conditions and crowding and by M. tuberculosis itself. Innate and adaptive host defenses will then influence whether new infection results; a process that is difficult to study owing to a paucity of animal models and an inability to measure infection directly. This review offers an overview of these steps and highlights the many gaps in knowledge that remain.

Early IL-6 signalling promotes IL-27 dependent maturation of regulatory T cells in the lungs and resolution of viral immunopathology

Interleukin-6 is a pleiotropic, pro-inflammatory cytokine that can promote both innate and adaptive immune responses. In humans with respiratory virus infections, such as Respiratory Syncytial Virus (RSV), elevated concentrations of IL-6 are associated with more severe disease. In contrast the polymorphisms in the Il6 promoter which favour lower IL-6 production are associated with increased risk of both RSV and Rhinovirus infections. To determine the precise contribution of IL-6 to protection and pathology we used murine models of respiratory virus infection. RSV infection resulted in increased IL-6 production both in the airways and systemically which remained heightened for at least 2 weeks. IL-6 depletion early, but not late, during RSV or Influenza A virus infection resulted in significantly increased disease associated with an influx of virus specific T_H1 and cytotoxic CD8⁺ T cells, whilst not affecting viral clearance. IL-6 acted by driving production of the immunoregulatory cytokine IL-27 by macrophages and monocytes, which in turn promoted the local maturation of regulatory T cells. Concordantly IL-27 was necessary to regulate T_H1 responses in the lungs, and sufficient to limit RSV induced disease. Overall we found that during respiratory virus infection the prototypic inflammatory cytokine IL-6 is a critical anti-inflammatory regulator of viral induced immunopathology in the respiratory tract through its induction of IL-27.

In clearing a respiratory virus, the host must strike a careful balance between the need to clear the infection and the potential of the immune response to damage the delicate structure of the lungs. Here we show that Interleukin-6, a soluble mediator commonly associated with inflammation and seen in humans with severe respiratory infection, is actually critical in promoting the resolution of the host response to respiratory virus infection and limiting disease. We have found that the early production of IL-6 after infection promotes the production of the regulatory mediator Interleukin-27 by lung resident immune cells, which in turn drives suppression of otherwise damaging inflammation. Removal of either IL-6 or IL-27 enhances disease during viral infection, while restoration of IL-27 is sufficient to allow faster recovery. Thus we have identified a novel immunological network within the respiratory tract which accelerates recovery after respiratory virus infection.

Tissue compartmentalization of T cell responses during early life

The immune system in early life is tasked with transitioning from a relatively protected environment to one in which it encounters a wide variety of innocuous antigens and dangerous pathogens. The immaturity of the developing immune system, and particularly the distinct functionality of T lymphocytes in early life, has been implicated in increased susceptibility to infection. Previous work has demonstrated that immune responses in early life are skewed toward limited inflammation and atopy; however, there is mounting evidence that such responses are context- and tissue-dependent. The regulation, differentiation, and maintenance of infant T cell responses, particularly as it relates to tissue compartmentalization, remains poorly understood. How the tissue environment impacts early-life immune responses and whether the development of localized protective immune memory cell subsets are established is an emerging area of research. As infectious diseases affecting the respiratory and digestive tracts are a leading cause of morbidity and mortality worldwide in infants and young children, a deeper understanding of site-specific immunity is essential to addressing these challenges. Here, we review the current paradigms of T cell responses during infancy as they relate to tissue localization and discuss implications for the development of vaccines and therapeutics.

Expression and role of VLA-1 in resident memory CD8 T cell responses to respiratory mucosal viral-vectored immunization against tuberculosis

Lung resident memory T cells (T_RM) characterized by selective expression of mucosal integrins VLA-1 (α1β1) and CD103 (α_Eβ7) are generated following primary respiratory viral infections. Despite recent progress, the generation of lung T_RM and the role of mucosal integrins following viral vector respiratory mucosal immunization still remains poorly understood. Here by using a replication-defective viral vector tuberculosis vaccine, we show that lung Ag-specific CD8 T cells express both VLA-1 and CD103 following respiratory mucosal immunization. However, VLA-1 and CD103 are acquired in differential tissue sites with the former acquired during T cell priming in the draining lymph nodes and the latter acquired after T cells entered the lung. Once in the lung, Ag-specific CD8 T cells continue to express VLA-1 at high levels through the effector/expansion, contraction, and memory phases of T cell responses. Using a functional VLA-1 blocking mAb, we show that VLA-1 is not required for trafficking of these cells to the lung, but it negatively regulates them in the contraction phase. Furthermore, VLA-1 plays a negligible role in the maintenance of these cells in the lung. Our study provides new information on vaccine-inducible lung T_RM and shall help develop effective viral vector respiratory mucosal tuberculosis vaccination strategies.

Systemic and Terminal Ileum Mucosal Immunity Elicited by Oral Immunization With the Ty21a Typhoid Vaccine in Humans

BACKGROUND & AIMS

Systemic cellular immunity elicited by the Ty21a oral typhoid vaccine has been extensively characterized. However, very limited data are available in humans regarding mucosal immunity at the site of infection (terminal ileum [TI]). Here we investigated the host immunity elicited by Ty21a immunization on terminal ileum-lamina propria mononuclear cells (LPMC) and peripheral blood in volunteers undergoing routine colonoscopy.

METHODS

We characterized LPMC-T memory (T_M) subsets and assessed Salmonella enterica serovar Typhi (S Typhi)-specific responses by multichromatic flow cytometry.

RESULTS

No differences were observed in cell yields and phenotypes in LPMC CD8⁺-T_M subsets following Ty21a immunization. However, Ty21a immunization elicited LPMC CD8⁺ T cells exhibiting significant S Typhi-specific responses (interferon-γ, tumor necrosis factor-α, interleukin-17A, and/or CD107a) in all major T_M subsets (T-effector/memory [T_EM], T-central/memory, and T_EM-CD45RA⁺), although each T_M subset exhibited unique characteristics. We also investigated whether Ty21a immunization elicited S Typhi-specific multifunctional effectors in LPMC CD8⁺ T_EM. We observed that LPMC CD8⁺ T_EM responses were mostly multifunctional, except for those cells exhibiting the characteristics associated with cytotoxic responses. Finally, we compared mucosal with systemic responses and made the important observation that LPMC CD8⁺S Typhi-specific responses were unique and distinct from their systemic counterparts.

CONCLUSIONS

This study provides the first demonstration of S Typhi-specific responses in the human terminal ileum mucosa and provides novel insights into the generation of mucosal immune responses following oral Ty21a immunization.

Group B streptococcus and respiratory syncytial virus immunisation during pregnancy: a landscape analysis

Group B streptococcus and respiratory syncytial virus are leading causes of infant morbidity and mortality worldwide. No licensed vaccines are available for either disease, but vaccines for both are under development. Severe respiratory syncytial virus disease can be prevented by passively administered antibody. The presence of maternal IgG antibody specific to respiratory syncytial virus is associated with reduced prevalence and severity of respiratory syncytial virus disease in the first few weeks of life, whereas maternal serotype-specific anticapsular antibody is associated with protection against both early-onset and late-onset group B streptococcus disease. Therefore, vaccination in pregnancy might protect infants against both diseases. This report describes what is known about immune protection against group B streptococcus and respiratory syncytial virus, identifies knowledge gaps regarding the immunobiology of both diseases, and aims to prioritise research directions in maternal immunisation.

Pulmonary immunity to viruses

Mucosal surfaces, such as the respiratory epithelium, are directly exposed to the external environment and therefore, are highly susceptible to viral infection. As a result, the respiratory tract has evolved a variety of innate and adaptive immune defenses in order to prevent viral infection or promote the rapid destruction of infected cells and facilitate the clearance of the infecting virus. Successful adaptive immune responses often lead to a functional state of immune memory, in which memory lymphocytes and circulating antibodies entirely prevent or lessen the severity of subsequent infections with the same virus. This is also the goal of vaccination, although it is difficult to vaccinate in a way that mimics respiratory infection. Consequently, some vaccines lead to robust systemic immune responses, but relatively poor mucosal immune responses that protect the respiratory tract. In addition, adaptive immunity is not without its drawbacks, as overly robust inflammatory responses may lead to lung damage and impair gas exchange or exacerbate other conditions, such as asthma or chronic obstructive pulmonary disease (COPD). Thus, immune responses to respiratory viral infections must be strong enough to eliminate infection, but also have mechanisms to limit damage and promote tissue repair in order to maintain pulmonary homeostasis. Here, we will discuss the components of the adaptive immune system that defend the host against respiratory viral infections.