Persistence in Temporary Lung Niches: A Survival Strategy of Lung-Resident Memory CD8+ T Cells

Modulating cytokine microenvironment during T cell activation induces protective RSV-specific lung resident memory T cells in early life in mice

Maternal immunisation against respiratory viruses provides protection in early life, but as antibodies wane, there can be a gap in coverage. This immunity gap might be filled by inducing pathogen-specific lung tissue-resident T cells (TRM). However, the neonatal mouse lung has a different inflammatory environment to the adult lung which affects T cell recruitment. We compared the factors affecting viral-specific TRM recruitment in the lungs of adult or neonatal mice. In contrast to adulthood, we demonstrated that RSV or influenza infection in neonatal mice recruited fewer TRM to the lungs. This was associated with reduced lung levels of CCL5 and CXCL10. Co-administration of CCL5 or CXCL10 at the time of primary T cell activation significantly increased RSV-specific TRM in the lung, protecting mice upon reinfection. These chemokine differences were reflected in responses to infection in human cord blood. Here we show a critical role for CCL5 and CXCL10 in the induction of lung TRM and a possible strategy for boosting responses.

Microneedle-Mediated Immunization Promotes Lung CD8+ T-Cell Immunity

Microneedle array has proven more efficient in stimulating humoral immunity than intramuscular vaccination. However, its effectiveness in inducing pulmonary CD8+ T cells remains elusive, which is essential to the frontline defense against pulmonary viral infections such as influenza and COVID-19 viruses. The current investigation reveals that superior CD8+ T-cell responses are elicited by immunization with a microneedle array over intradermal or intramuscular immunization using the model antigen ovalbumin, irrespective of whether or not the antigen is provided in the lung. Mechanistically, microneedle array-mediated immunization targeted the epidermal layer and stimulated predominantly Langerhans cells, resulting in increased expression of α4β1 adhesion molecules on the CD8+ T-cell surface, which may play a role in T-cell homing to the lung, whereas CD8+ T cells induced by intramuscular immunization did not express the adhesion molecule sufficiently. CD8+ T cells with a lung-homing propensity were also seen after intradermal vaccination, yet to a much lesser extent. Accordingly, microneedle array immunization provided stronger protection against influenza viral infection than intradermal or intramuscular immunization. The observations offer insights into a strong cross-talk between epidermal immunization and lung immunity and are valuable for designing and delivering vaccines against respiratory viral infections.

IKK2/NFkB signaling controls lung resident CD8+ T cell memory during influenza infection

CD8⁺ T cell tissue resident memory (T_RM) cells are especially suited to control pathogen spread at mucosal sites. However, their maintenance in lung is short-lived. TCR-dependent NFkB signaling is crucial for T cell memory but how and when NFkB signaling modulates tissue resident and circulating T cell memory during the immune response is unknown. Here, we find that enhancing NFkB signaling in T cells once memory to influenza is established, increases pro-survival Bcl-2 and CD122 levels thus boosting lung CD8⁺ T_RM maintenance. By contrast, enhancing NFkB signals during the contraction phase of the response leads to a defect in CD8⁺ T_RM differentiation without impairing recirculating memory subsets. Specifically, inducible activation of NFkB via constitutive active IKK2 or TNF interferes with TGFβ signaling, resulting in defects of lung CD8⁺ T_RM imprinting molecules CD69, CD103, Runx3 and Eomes. Conversely, inhibiting NFkB signals not only recovers but improves the transcriptional signature and generation of lung CD8⁺ T_RM. Thus, NFkB signaling is a critical regulator of tissue resident memory, whose levels can be tuned at specific times during infection to boost lung CD8⁺ T_RM.

Specific in situ immuno-imaging of pulmonary-resident memory lymphocytes in human lungs

Introduction

Pulmonary-resident memory T cells (T_RM) and B cells (B_RM) orchestrate protective immunity to reinfection with respiratory pathogens. Developing methods for the in situ detection of these populations would benefit both research and clinical settings.

Methods

To address this need, we developed a novel in situ immunolabelling approach combined with clinic-ready fibre-based optical endomicroscopy (OEM) to detect canonical markers of lymphocyte tissue residency in situ in human lungs undergoing ex vivo lung ventilation (EVLV).

Results

Initially, cells from human lung digests (confirmed to contain T_RM/B_RM populations using flow cytometry) were stained with CD69 and CD103/CD20 fluorescent antibodies and imaged in vitro using KronoScan, demonstrating it's ability to detect antibody labelled cells. We next instilled these pre-labelled cells into human lungs undergoing EVLV and confirmed they could still be visualised using both fluorescence intensity and lifetime imaging against background lung architecture. Finally, we instilled fluorescent CD69 and CD103/CD20 antibodies directly into the lung and were able to detect T_RM/B_RM following in situ labelling within seconds of direct intra-alveolar delivery of microdoses of fluorescently labelled antibodies.

Discussion

In situ, no wash, immunolabelling with intra-alveolar OEM imaging is a novel methodology with the potential to expand the experimental utility of EVLV and pre-clinical models.

Mucosal tissue regulatory T cells are integral in balancing immunity and tolerance at portals of antigen entry

Foxp3+ regulatory T cells (Tregs) are a subset of CD4+ T cells that exert suppressive control over other immune cells. Tregs are critical for preventing systemic autoimmunity and maintaining peripheral tolerance, and yet they also assist in orchestration of immunity to pathogenic insult, wherein they limit collateral immunopathology and assist in facilitating a fine balance between immune tolerance and effector activity. Tregs have been extensively studied in lymphoid tissues, and a growing body of work has characterized phenotypically distinct Tregs localized in various nonlymphoid tissue compartments. These tissue Tregs can perform location-specific, alternative functions, highlighting their dynamic, context-dependent roles. Tregs have also been identified in mucosal tissues where specialized physiological functions are paramount, including helping the host to respond appropriately to pathogenic versus innocuous antigens that are abundant at mucosal portals of antigen entry. As in other tissue Treg compartments, mucosal Tregs in the respiratory, gastrointestinal, and genitourinary tracts are distinct from circulating counterparts and can carry out mucosa-specific functions as well as classic suppressive functions that are the hallmark of Tregs. In this review, we summarize current knowledge regarding mucosal Tregs in both health and disease.

Tissue-resident immunity in the lung: a first-line defense at the environmental interface

The lung is a vital organ that incessantly faces external environmental challenges. Its homeostasis and unimpeded vital function are ensured by the respiratory epithelium working hand in hand with an intricate fine-tuned tissue-resident immune cell network. Lung tissue-resident immune cells span across the innate and adaptive immunity and protect from infectious agents but can also prove to be pathogenic if dysregulated. Here, we review the innate and adaptive immune cell subtypes comprising lung-resident immunity and discuss their ontogeny and role in distinct respiratory diseases. An improved understanding of the role of lung-resident immunity and how its function is dysregulated under pathological conditions can shed light on the pathogenesis of respiratory diseases.

Gold-Nanostar-Chitosan-Mediated Delivery of SARS-CoV-2 DNA Vaccine for Respiratory Mucosal Immunization: Development and Proof-of-Principle

The COVID-19 pandemic is caused by the coronavirus SARS-CoV-2 (SC2). A variety of anti-SC2 vaccines have been approved for human applications, including those using messenger RNA (mRNA), adenoviruses expressing SC2 spike (S) protein, and inactivated virus. The protective periods of immunization afforded by these intramuscularly administered vaccines are currently unknown. An alternative self-administrable vaccine capable of mounting long-lasting immunity via sterilizing neutralizing antibodies would be hugely advantageous in tackling emerging mutant SC2 variants. This could also diminish the possibility of vaccinated individuals acting as passive carriers of COVID-19. Here, we investigate the potential of an intranasal (IN)-delivered DNA vaccine encoding the S protein of SC2 in BALB/c and C57BL/6J immunocompetent mouse models. The immune response to IN delivery of this SC2-spike DNA vaccine transported on a modified gold-chitosan nanocarrier shows a strong and consistent surge in antibodies (IgG, IgA, and IgM) and effective neutralization of pseudoviruses expressing S proteins of different SC2 variants (Wuhan, beta, and D614G). Immunophenotyping and histological analyses reveal chronological events involved in the recognition of SC2 S antigen by resident dendritic cells and alveolar macrophages, which prime the draining lymph nodes and spleen for peak SC2-specific cellular and humoral immune responses. The attainable high levels of anti-SC2 IgA in lung mucosa and tissue-resident memory T cells can efficiently inhibit SC2 and its variants at the site of entry and also provide long-lasting immunity.

Pulmonary-Resident Memory Lymphocytes: Pivotal Orchestrators of Local Immunity Against Respiratory Infections

There is increasing evidence that lung-resident memory T and B cells play a critical role in protecting against respiratory reinfection. With a unique transcriptional and phenotypic profile, resident memory lymphocytes are maintained in a quiescent state, constantly surveying the lung for microbial intruders. Upon reactivation with cognate antigen, these cells provide rapid effector function to enhance immunity and prevent infection. Immunization strategies designed to induce their formation, alongside novel techniques enabling their detection, have the potential to accelerate and transform vaccine development. Despite most data originating from murine studies, this review will discuss recent insights into the generation, maintenance and characterisation of pulmonary resident memory lymphocytes in the context of respiratory infection and vaccination using recent findings from human and non-human primate studies.

Single-Dose Intranasal Administration of AdCOVID Elicits Systemic and Mucosal Immunity against SARS-CoV-2 and Fully Protects Mice from Lethal Challenge

The coronavirus disease 2019 (COVID-19) pandemic has highlighted the urgent need for effective prophylactic vaccination to prevent the spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Intranasal vaccination is an attractive strategy to prevent COVID-19 as the nasal mucosa represents the first-line barrier to SARS-CoV-2 entry. The current intramuscular vaccines elicit systemic immunity but not necessarily high-level mucosal immunity. Here, we tested a single intranasal dose of our candidate adenovirus type 5-vectored vaccine encoding the receptor-binding domain (RBD) of the SARS-CoV-2 spike protein (AdCOVID) in inbred, outbred, and transgenic mice. A single intranasal vaccination with AdCOVID elicited a strong and focused immune response against RBD through the induction of mucosal IgA in the respiratory tract, serum neutralizing antibodies, and CD4+ and CD8+ T cells with a Th1-like cytokine expression profile. A single AdCOVID dose resulted in immunity that was sustained for over six months. Moreover, a single intranasal dose completely protected K18-hACE2 mice from lethal SARS-CoV-2 challenge, preventing weight loss and mortality. These data show that AdCOVID promotes concomitant systemic and mucosal immunity and represents a promising vaccine candidate.

Immunity to Influenza Infection in Humans

This review discusses the human immune responses to influenza infection with some insights from studies using animal models, such as experimental infection of mice. Recent technological advances in the study of human immune responses have greatly added to our knowledge of the infection and immune responses, and therefore much of the focus is on recent studies that have moved the field forward. We consider the complexity of the adaptive response generated by many sequential encounters through infection and vaccination.

Legend of the Sentinels: Development of Lung Resident Memory T Cells and Their Roles in Diseases

SARS-CoV-2 is wreaking havoc around the world. To get the world back on track, hundreds of vaccines are under development. A deeper understanding of how the immune system responds to SARS-CoV-2 re-infection will certainly help. Studies have highlighted various aspects of T cell response in resolving acute infection and preventing re-infections. Lung resident memory T (TRM) cells are sentinels in the secondary immune response. They are mostly differentiated from effector T cells, construct specific niches and stay permanently in lung tissues. If the infection recurs, locally activated lung TRM cells can elicit rapid immune response against invading pathogens. In addition, they can significantly limit tumor growth or lead to pathologic immune responses. Vaccines targeting TRM cells are under development, with the hope to induce stable and highly reactive lung TRM cells through mucosal administration or "prime-and-pull" strategy. In this review, we will summarize recent advances in lung TRM cell generation and maintenance, explore their roles in different diseases and discuss how these cells may guide the development of future vaccines targeting infectious disease, cancer, and pathologic immune response.

Single-dose intranasal administration of AdCOVID elicits systemic and mucosal immunity against SARS-CoV-2 in mice

The coronavirus disease 2019 (COVID-19) pandemic has highlighted the urgent need for effective preventive vaccination to reduce burden and spread of severe acute respiratory syndrome (SARS) coronavirus 2 (SARS-CoV-2) in humans. Intranasal vaccination is an attractive strategy to prevent COVID-19 as the nasal mucosa represents the first-line barrier to SARS-CoV-2 entry before viral spread to the lung. Although SARS-CoV-2 vaccine development is rapidly progressing, the current intramuscular vaccines are designed to elicit systemic immunity without conferring mucosal immunity. Here, we show that AdCOVID, an intranasal adenovirus type 5 (Ad5)-vectored vaccine encoding the receptor binding domain (RBD) of the SARS-CoV-2 spike protein, elicits a strong and focused immune response against RBD through the induction of mucosal IgA, serum neutralizing antibodies and CD4+ and CD8+ T cells with a Th1-like cytokine expression profile. Therefore, AdCOVID, which promotes concomitant systemic and local mucosal immunity, represents a promising COVID-19 vaccine candidate.

A Single-Dose Intranasal ChAd Vaccine Protects Upper and Lower Respiratory Tracts against SARS-CoV-2

The coronavirus disease 2019 pandemic has made deployment of an effective vaccine a global health priority. We evaluated the protective activity of a chimpanzee adenovirus-vectored vaccine encoding a prefusion stabilized spike protein (ChAd-SARS-CoV-2-S) in challenge studies with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and mice expressing the human angiotensin-converting enzyme 2 receptor. Intramuscular dosing of ChAd-SARS-CoV-2-S induces robust systemic humoral and cell-mediated immune responses and protects against lung infection, inflammation, and pathology but does not confer sterilizing immunity, as evidenced by detection of viral RNA and induction of anti-nucleoprotein antibodies after SARS-CoV-2 challenge. In contrast, a single intranasal dose of ChAd-SARS-CoV-2-S induces high levels of neutralizing antibodies, promotes systemic and mucosal immunoglobulin A (IgA) and T cell responses, and almost entirely prevents SARS-CoV-2 infection in both the upper and lower respiratory tracts. Intranasal administration of ChAd-SARS-CoV-2-S is a candidate for preventing SARS-CoV-2 infection and transmission and curtailing pandemic spread.

Tissue-resident lymphocytes: weaponized sentinels at barrier surfaces

Tissue-resident immune cells stably localize in tissues largely independent of the circulatory system. While initial studies have focused on the recognition of CD8 ⁺ tissue-resident memory T (CD8 T _RM) cells, it is now clear that numerous cell types such as CD4 ⁺ T cells, gd T cells, innate lymphoid cells and mucosal-associated invariant T (MAIT) cells form stable populations in tissues. They are enriched at the barrier surfaces and within non-lymphoid compartments. They provide an extensive immune network capable of sensing local perturbations of the body’s homeostasis. This positioning enables immune cells to positively influence immune protection against infection and cancer but paradoxically also augment autoimmunity, allergy and chronic inflammatory diseases. Here, we highlight the recent studies across multiple lymphoid immune cell types that have emerged on this research topic and extend our understanding of this important cellular network. In addition, we highlight the areas that remain gaps in our knowledge of the regulation of these cells and how a deeper understanding may result in new ways to ‘target’ these cells to influence disease outcome and treatments.

Highly pathogenic coronaviruses: thrusting vaccine development in the spotlight

Coronaviruses (CoVs) are a large family of viruses that cause illness ranging from the common cold to more severe diseases such as Severe Acute Respiratory Syndrome (SARS) and Middle East Respiratory Syndrome (MERS). Severe Acute Respiratory Syndrome coronavirus 2 (SARS-CoV-2) disease (COVID-19) has caused major public health crises. There have been more than 4,400,000 reported cases of COVID-2019 and more than 300,000 reported deaths to date (16/05/2020). SARS-CoV, MERS-CoV and SARS-CoV-2 have attracted widespread global attention due to their high infectivity and pathogenicity. To date, there is no specific treatment proven effective against these viral infectious diseases. Vaccination is considered one of the most effective strategies to prevent viral infections. Therefore, the development of effective vaccines against highly pathogenic coronaviruses is essential. In this review, we will briefly describe coronavirus vaccine design targets, summarize recent advances in the development of coronavirus vaccines, and highlight current adjuvants for improving the efficacy of coronavirus vaccines.

Conserved T-cell epitopes of respiratory syncytial virus (RSV) delivered by recombinant live attenuated influenza vaccine viruses efficiently induce RSV-specific lung-localized memory T cells and augment influenza-specific resident memory T-cell responses

Respiratory syncytial virus (RSV) can cause recurrent infection in people because it does not stimulate a long-lived immunological memory. There is an urgent need to develop a safe and efficacious vaccine against RSV that would induce immunological memory without causing immunopathology following natural RSV infection. We have previously generated two recombinant live attenuated influenza vaccine (LAIV) viruses that encode immunodominant T-cell epitopes of RSV M2 protein in the neuraminidase or NS1 genes. These chimeric vaccines afforded protection against influenza and RSV infection in mice, without causing pulmonary eosinophilia or inflammatory RSV disease. The current study assessed the formation of influenza-specific and RSV-specific CD4 and CD8 T-cell responses in the lungs of mice, with special attention to the lung tissue-resident memory T cell subsets (T_RM). The RSV epitopes did not affect influenza-specific CD4 effector memory T cell (Tem) levels in the lungs. The majority of these cells formed by LAIV or LAIV-RSV viruses had CD69⁺CD103^- phenotype. Both LAIV+NA/RSV and LAIV+NS/RSV recombinant viruses induced significant levels of RSV M2₈₂ epitope-specific lung-localized CD8 Tem cells expressing both CD69 and CD103 T_RM markers. Surprisingly, the CD69⁺CD103⁺ influenza-specific CD8 Tem responses were augmented by the addition of RSV epitopes, possibly as a result of the local microenvironment formed by the RSV-specific memory T cells differentiating to T_RM in the lungs of mice immunized with LAIV-RSV chimeric viruses. This study provides evidence that LAIV vector-based vaccination can induce robust lung-localized T-cell immunity to the inserted T-cell epitope of a foreign pathogen, without altering the immunogenicity of the viral vector itself.

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Two LAIV-RSV vaccine viruses induced RSV M2₈₂-specific effector memory CD8 T cells producing both IFNγ and TNFα cytokines.

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The inserted RSV epitopes did not affect influenza-specific CD4 Tem levels in the lungs of immunized mice.

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LAIV-RSV viruses induced RSV M2₈₂-specific lung-localized CD8 Tem cells expressing both CD69 and CD103 T_RM markers.

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The magnitude of RSV M2₈₂-specific CD8 Tem responses correlates with protection against RSV-induced lung pathology.

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The addition of RSV epitopes into the LAIV strain augmented CD69⁺CD103⁺ influenza-specific CD8 Tem responses in the lungs.

PD-1hi CD8+ resident memory T cells balance immunity and fibrotic sequelae

CD8⁺ tissue-resident memory T (T_RM) cells provide frontline immunity in mucosal tissues. The mechanisms regulating CD8⁺ T_RM maintenance, heterogeneity, and protective and pathological functions are largely elusive. Here, we identify a population of CD8⁺ T_RM cells that is maintained by major histocompatibility complex class I (MHC-I) signaling, and CD80 and CD86 costimulation after acute influenza infection. These T_RM cells have both exhausted-like phenotypes and memory features and provide heterologous immunity against secondary infection. PD-L1 blockade after the resolution of primary infection promotes the rejuvenation of these exhausted-like T_RM cells, restoring protective immunity at the cost of promoting postinfection inflammatory and fibrotic sequelae. Thus, PD-1 serves to limit the pathogenic capacity of exhausted-like T_RM cells at the memory phase. Our data indicate that T_RM cell exhaustion is the result of a tissue-specific cellular adaptation that balances fibrotic sequelae with protective immunity.

The Way Forward: Potentiating Protective Immunity to Novel and Pandemic Influenza Through Engagement of Memory CD4 T Cells

Potentially pandemic strains of influenza pose an undeniable threat to human populations. Therefore, it is essential to develop better strategies to enhance vaccine design and predict parameters that identify susceptible humans. CD4 T cells are a central component of protective immunity to influenza, delivering direct effector function and potentiating responses of other lymphoid cells. Humans have highly diverse influenza-specific CD4 T-cell populations that vary in stimulation history, specificity, and functionality. These complexities constitute a formidable obstacle to predicting immune responses to pandemic strains of influenza and derivation of optimal vaccine strategies. We suggest that more precise efforts to identify and enumerate both the positive and negative contributors of immunity in the CD4 T-cell compartment will aid in both predicting susceptible hosts and in development of vaccination strategies that will poise most human subjects to respond to pandemic influenza strains with protective immune responses.

Tissue-Resident Macrophages Limit Pulmonary CD8 Resident Memory T Cell Establishment

Tissue resident memory CD8 T cells (T_RM) serve as potent local sentinels and contribute significantly to protective immunity against intracellular mucosal pathogens. While the molecular and transcriptional underpinnings of T_RM differentiation are emerging, how T_RM establishment is regulated by other leukocytes in vivo is largely unclear. Here, we observed that expression of PPAR-γ in the myeloid compartment was a negative regulator of CD8 T_RM establishment following influenza virus infection. Interestingly, myeloid deficiency of PPAR-γ resulted in selective impairment of the tissue-resident alveolar macrophage (AM) compartment during primary influenza infection, suggesting that AM are likely negative regulators of CD8 T_RM differentiation. Indeed, influenza-specific CD8 T_RM cell numbers were increased following early, but not late ablation of AM using the CD169-DTR model. Importantly, these findings were specific to the parenchyma of infected tissue as circulating memory T cell frequencies in lung and T_CM and T_EM in spleen were largely unaltered following macrophage ablation. Further, the magnitude of the effector response could not explain these observations. These data indicate local regulation of pulmonary T_RM differentiation is alveolar macrophage dependent. These, findings could aid in vaccine design aimed at increasing T_RM density to enhance protective immunity, or deflating their numbers in conditions where they cause overt or veiled chronic pathologies.

Tissue-resident memory T cells and their biological characteristics in the recurrence of inflammatory skin disorders

The skin is the largest organ of the body. The establishment of immunological memory in the skin is a crucial component of the adaptive immune response. Once naive T cells are activated by antigen-presenting cells, a small fraction of them differentiate into precursor memory T cells. These precursor cells ultimately develop into several subsets of memory T cells, including central memory T (T_CM) cells, effector memory T (T_EM) cells, and tissue resident memory T (T_RM) cells. T_RM cells have a unique transcriptional profile, and their most striking characteristics are their long-term survival (longevity) and low migration in peripheral tissues, including the skin. Under physiological conditions, T_RM cells that reside in the skin can respond rapidly to pathogenic challenges. However, there is emerging evidence to support the vital role of T_RM cells in the recurrence of chronic inflammatory skin disorders, including psoriasis, vitiligo, and fixed drug eruption, under pathological or uncontrolled conditions. Clarifying and characterizing the mechanisms that are involved in skin T_RM cells will help provide promising strategies for reducing the frequency and magnitude of skin inflammation recurrence. Here, we discuss recent insights into the generation, homing, retention, and survival of T_RM cells and share our perspectives on the biological characteristics of T_RM cells in the recurrence of inflammatory skin disorders.

Interstitial-resident memory CD8+ T cells sustain frontline epithelial memory in the lung

Populations of CD8⁺ lung-resident memory T (T_RM) cells persist in the interstitium and epithelium (airways) following recovery from respiratory virus infections. While it is clear that CD8⁺ T_RM cells in the airways are dynamically maintained via the continuous recruitment of new cells, there is a vigorous debate about whether tissue-circulating effector memory T (T_EM) cells are the source of these newly recruited cells. Here we definitively demonstrate that CD8⁺ T_RM cells in the lung airways are not derived from T_EM cells in the circulation, but are seeded continuously by T_RM cells from the lung interstitium. This process is driven by CXCR6 that is expressed uniquely on T_RM cells but not T_EM cells. We further demonstrate that the lung interstitium CD8⁺ T_RM cell population is also maintained independently of T_EM cells via a homeostatic proliferation mechanism. Taken together, these data show that lung memory CD8⁺ T_RM cells in the lung interstitium and airways are compartmentally separated from T_EM cells and clarify the mechanisms underlying their maintenance.

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.

Establishment and Maintenance of Conventional and Circulation-Driven Lung-Resident Memory CD8+ T Cells Following Respiratory Virus Infections

Antigen-specific CD8⁺ tissue-resident memory T cells (T_RM cells) persist in the lung following resolution of a respiratory virus infection and provide first-line defense against reinfection. In contrast to other memory T cell populations, such as central memory T cells that circulate between lymph and blood, and effector memory T cells (T_EM cells) that circulate between blood and peripheral tissues, T_RM cells are best defined by their permanent residency in the tissues and their independence from circulatory T cell populations. Consistent with this, we recently demonstrated that CD8⁺ T_RM cells primarily reside within specific niches in the lung (Repair-Associated Memory Depots; RAMD) that normally exclude CD8⁺ T_EM cells. However, it has also been reported that circulating CD8⁺ T_EM cells continuously convert into CD8⁺ T_RM cells in the lung interstitium, helping to sustain T_RM numbers. The relative contributions of these two mechanisms of CD8⁺ T_RM cells maintenance in the lung has been the source of vigorous debate. Here we propose a model in which the majority of CD8⁺ T_RM cells are maintained within RAMD (conventional T_RM) whereas a small fraction of T_RM are derived from circulating CD8⁺ T_EM cells and maintained in the interstitium. The numbers of both types of T_RM cells wane over time due to declines in both RAMD availability and the overall number of T_EM in the circulation. This model is consistent with most published reports and has important implications for the development of vaccines designed to elicit protective T cell memory in the lung.

T cells in severe childhood asthma

Severe asthma in children is a debilitating condition that accounts for a disproportionately large health and economic burden of asthma. Reasons for the lack of a response to standard anti-inflammatory therapies remain enigmatic. Work in the last decade has shed new light on the heterogeneous nature of asthma, and the varied immunopathologies of severe disease, which are leading to new treatment approaches for the individual patient. However, most studies to date that explored the immune landscape of the inflamed lower airways have focused on adults. T cells are pivotal to the inception and persistence of inflammatory processes in the diseased lungs, despite a contemporary shift in focus to immune events at the epithelial barrier. This article outlines current knowledge on the types of T cells and related cell types that are implicated in severe asthma. The potential for environmental exposures and other inflammatory cues to condition the immune environment of the lung in early life to favour pathogenic T cells and steroid resistance is discussed. The contributions of T cells and their cytokines to inflammatory processes and treatment resistance are also considered, with an emphasis on new observations in children that argue against conventional type 1 and type 2 T cell paradigms. Finally, the ability for new technologies to revolutionize our understanding of T cells in severe childhood asthma, and to guide future treatment strategies that could mitigate this disease, is highlighted.

Fuel and brake of memory T cell inflation

Memory T cell inflation is a process in which a large number of effector memory T cells accumulates in peripheral tissues. This phenomenon is observed upon certain low level persistent virus infections, but it is most commonly described upon infection with the β-herpesvirus Cytomegalovirus. Due to the induction of this large pool of functional effector CD8 T cells in peripheral tissues, the interest in using CMV-based vaccine vectors for vaccination purposes is rising. However, the exact mechanisms of memory T cell inflation are not yet fully understood. It is clear that repetitive exposure to antigen is a key determinant for memory inflation, and therefore the viral inoculum dose and the subsequent number of viral reactivation events strongly impact on the magnitude of the inflationary T cell pool. In addition, the number of CMV-specific CD8 T cells that is able to sense these reactivation events affects the size of the inflationary T cell pool. In the following, we will discuss factors that either promote or limit T cell inflation from both the virus and host perspective. These factors mostly operate by influencing the amount of available antigen or by affecting the T cell pool that is able to respond to the antigen. Furthermore, we will discuss the recent use of CMV-based vaccines in pre-clinical experimental settings, where these vectors have shown promising results by inducing prolonged effector memory T cell responses to foreign-introduced epitopes and thereby provided protection from subsequent virus or tumour challenges.

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.

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.

Distinct and complementary roles of CD4 T cells in protective immunity to influenza virus

CD4 T cells play a multiplicity of roles in protective immunity to influenza. Included in these functions are help for high affinity antibody production, enhancement of CD8 T cell expansion, function and memory, acceleration of the early innate response to infection and direct cytotoxicity. The influenza-specific CD4 T cell repertoire in humans established through exposures to infection and vaccination has been found to be highly variable in abundance, specificity and functionality. Deficits in particular subsets of CD4 T cells recruited into the response result in diminished antibody responses and protection from infection. Therefore, improved strategies for vaccination should include better methods to identify deficiencies in the circulating CD4 T cell repertoire, and vaccine constructs that increase the representation of CD4 T cells of the correct specificity and functionality.

CD4 T Cell Epitope Specificity and Cytokine Potential Are Preserved as Cells Transition from the Lung Vasculature to Lung Tissue following Influenza Virus Infection

Pulmonary CD4 T cells are critical in respiratory virus control, both by delivering direct effector function and through coordinating responses of other immune cells. Recent studies have shown that following influenza virus infection, virus-specific CD4 T cells are partitioned between pulmonary vasculature and lung tissue. However, very little is known about the peptide specificity or functional differences of CD4 T cells within these two compartments. Using a mouse model of influenza virus infection in conjunction with intravascular labeling in vivo, the cell surface phenotype, epitope specificity, and functional potential of the endogenous polyclonal CD4 T cell response was examined by tracking nine independent CD4 T cell epitope specificities. These studies revealed that tissue-localized CD4 cells were globally distinct from vascular cells in expression of markers associated with transendothelial migration, residency, and micropositioning. Despite these differences, there was little evidence for remodeling of the viral epitope specificity or cytokine potential as cells transition from vasculature to the highly inflamed lung tissue. Our studies also distinguished cells in the pulmonary vasculature from peripheral circulating CD4 T cells, providing support for the concept that the pulmonary vasculature does not simply reflect circulating cells that are trapped within the narrow confines of capillary vessels but rather is enriched in transitional cells primed in the draining lymph node that have specialized potential to enter the lung tissue.IMPORTANCE CD4 T cells convey a multitude of functions in immunity to influenza, including those delivered in the lymph node and others conveyed by CD4 T cells that leave the lymph node, enter the blood, and extravasate into the lung tissue. Here, we show that the transition of recently primed CD4 cells detected in the lung vasculature undergo profound changes in expression of markers associated with tissue localization as they establish residence in the lung. However, this transition does not edit CD4 T cell epitope specificity or the cytokine potential of the CD4 T cells. Thus, CD4 T cells that enter the infected lung can convey diverse functions and have a sufficiently broad viral antigen specificity to detect the complex array of infected cells within the infected tissue, offering the potential for more effective protective function.

Niches for the Long-Term Maintenance of Tissue-Resident Memory T Cells

Tissue-resident memory T cells (T_RM cells) are a population of immune cells that reside in the lymphoid and non-lymphoid organs without recirculation through the blood. These important cells occupy and utilize unique anatomical and physiological niches that are distinct from those for other memory T cell populations, such as central memory T cells in the secondary lymphoid organs and effector memory T cells that circulate through the tissues. CD8⁺ T_RM cells typically localize in the epithelial layers of barrier tissues where they are optimally positioned to act as sentinels to trigger antigen-specific protection against reinfection. CD4⁺ T_RM cells typically localize below the epithelial layers, such as below the basement membrane, and cluster in lymphoid structures designed to optimize interactions with antigen-presenting cells upon reinfection. A key feature of T_RM populations is their ability to be maintained in barrier tissues for prolonged periods of time. For example, skin CD8⁺ T_RM cells displace epidermal niches originally occupied by γδ T cells, thereby enabling their stable persistence for years. It is also clear that the long-term maintenance of T_RM cells in different microenvironments is dependent on multiple tissue-specific survival cues, although the specific details are poorly understood. However, not all T_RM persist over the long term. Recently, we identified a new spatial niche for the maintenance of CD8⁺ T_RM cells in the lung, which is created at the site of tissue regeneration after injury [termed repair-associated memory depots (RAMD)]. The short-lived nature of RAMD potentially explains the short lifespans of CD8⁺ T_RM cells in this particular tissue. Clearly, a better understanding of the niche-dependent maintenance of T_RM cells will be important for the development of vaccines designed to promote barrier immunity. In this review, we discuss recent advances in our understanding of the properties and nature of tissue-specific niches that maintain T_RM cells in different tissues.

Diverse Epitope Specificity, Immunodominance Hierarchy, and Functional Avidity of Effector CD4 T Cells Established During Priming Is Maintained in Lung After Influenza A Virus Infection

One of the major contributions to protective immunity to influenza viruses that is provided by virus-specific CD4 T cells is delivery of effector function to the infected lung. However, there is little known about the selection and breadth of viral epitope-specific CD4 T cells that home to the lung after their initial priming. In this study, using a mouse model of influenza A infection and an unbiased method of epitope identification, the viral epitope-specific CD4 T cells elicited after infection were identified and quantified. We found that a very diverse specificity of CD4 T cells is primed by infection, including epitopes from hemagglutinin, neuraminidase, matrix protein, nucleoprotein, and non-structural protein-1. Using peptide-specific cytokine EliSpots, the diversity and immunodominance hierarchies established in the lung-draining lymph node were compared with specificities of CD4 T cells that home to the lung. Our studies revealed that CD4 T cells of all epitope specificities identified in peripheral lymphoid tissue home back to the lung and that most of these lung-homing cells are localized within the tissue rather than the pulmonary vasculature. There is a striking shift of CD4 T cell functionality that enriches for IFN-γ production as cells are primed in the lymph node, enter the lung vasculature, and finally establish residency in the tissue, but with no apparent shifts in their functional avidity. We conclude that CD4 T cells of broad viral epitope specificity are recruited into the lung after influenza infection, where they then have the opportunity to encounter infected or antigen-bearing antigen-presenting cells.