Activated antigen-specific CD8+ T cells persist in the lungs following recovery from respiratory virus infections

Gradual changes within long-lived influenza virus-specific CD8+ T cells are associated with the loss of public TCR clonotypes in older adults

BACKGROUND

Susceptibility to life-threatening influenza increases with age, partly due to declining immunity. Frequency, phenotype and T-cell receptor (TCR) composition of influenza-specific CD8+ T-cells directed at the prominent A2/M158 influenza epitope change across the human lifespan.

METHODS

We investigated longevity and mechanisms underlying age-related changes in influenza-specific TCR repertoires by performing longitudinal analyses in young and older adults across 7-12 years within A2/M158+CD8+ T-cells using peptide-HLA tetramers directly ex vivo. Paired TCRαβ-chains were used to track clonotypes over time within individuals.

FINDINGS

Expanded public and private TCR clonotypes were long-lived but gradually declined over time. Loss of public clonotypes was initially compensated by expansions of clonotypes expressing public-associated features. Once these public-associated TCR clonotypes were abated in older adults, the void was filled by expansions of less similar private TCR clonotypes. Expanded older private TCR clonotypes also declined over time and were gradually replaced by other private TCR clonotypes with low similarity to public TCR clonotypes detected in adults.

INTERPRETATION

Despite our relatively small cohort, we provided conclusive evidence that CD8+ T-cells to a single HLA-A2-restricted influenza-epitope are long-lived. However, dynamic changes occur at the clonotypic level, which eventually result in loss of public clonotypes, indicating that T-cell-based influenza vaccines are likely more effective in adults than older adults.

FUNDING

This research was supported by the National Health and Medical Research Council (#1173871, #1159272), the Australian Research Council (#190102704), European Union's Horizon 2020 (#792532), the University of Melbourne. Funders had no role in design, analysis or reporting of the study.

Evolutionary diversity of CXCL16-CXCR6: Convergent substitutions and recurrent gene loss in sauropsids

The CXCL16-CXCR6 axis is crucial for regulating the persistence of CD8 tissue-resident memory T cells (TRM). CXCR6 deficiency lowers TRM cell numbers in the lungs and depletes ILC3s in the lamina propria, impairing mucosal defence. This axis is linked to diseases like HIV/SIV, cancer, and COVID-19. Together, these highlight that the CXCL16-CXCR6 axis is pivotal in host immunity. Previous studies of the CXCL16-CXCR6 axis found genetic variation among species but were limited to primates and rodents. To understand the evolution and diversity of CXCL16-CXCR6 across vertebrates, we compared approximately 400 1-to-1 CXCR6 orthologs spanning diverse vertebrates. The unique DRF motif of CXCR6 facilitates leukocyte adhesion by interacting with cell surface-expressed CXCL16 and plays a key role in G-protein selectivity during receptor signalling; however, our findings show that this motif is not universal. The DRF motif is restricted to mammals, turtles, and frogs, while the DRY motif, typical in other CKRs, is found in snakes and lizards. Most birds exhibit the DRL motif. These substitutions at the DRF motif affect the receptor-Gi/o protein interaction. We establish recurrent CXCR6 gene loss in 10 out of 36 bird orders, including Galliformes and Passeriformes, Crocodilia, and Elapidae, attributed to segmental deletions and/or frame-disrupting changes. Notably, single-cell RNA sequencing of the lung shows a drop in TRM cells in species with CXCR6 loss, suggesting a possible link. The concurrent loss of ITGAE, CXCL16, and CXCR6 in chickens may have altered CD8 TRM cell abundance, with implications for immunity against viral diseases and vaccines inducing CD8 TRM cells.

Lung Immune Cell Niches and the Discovery of New Cell Subtypes

Immune cells in the lungs are important for maintaining lung function. The importance of immune cells in defending against lung diseases and infections is increasingly recognized. However, a primary knowledge gaps in current studies of lung immune cells is the understanding of their subtypes and functional heterogeneity. Increasing evidence supports the existence of novel immune cell subtypes that engage in the complex crosstalk between lung-resident immune cells, recruited immune cells, and epithelial cells. Therefore, further studies on how immune cells respond to perturbations in the pulmonary microenvironment are warranted. This review explores the processes behind the formation of the immune cell niche during lung development, and the characteristics and cell interaction modes of several major lung-resident immune cells. It indicates that distinct lung microenvironments or inflammatory niches can mediate the formation of different cell subtypes. These findings summarize and clarify paths to identify new cell subtypes that originate from resident progenitor cells and recruited peripheral cells, which are remodeled by the pulmonary microenvironment. The development of new techniques combining transcriptome analysis and location information is essential for identifying new immune cell subtypes and their relative immune niches, as well as for uncovering the molecular mechanisms of immune cell-mediated lung homeostasis.

A vaccine platform targeting lung-resident memory CD4+ T-cells provides protection against heterosubtypic influenza infections in mice and ferrets

Lung tissue-resident memory T (TRM) cells induced by influenza vaccination are crucial for heterosubtypic immunity upon re-exposure to the influenza virus, enabling rapid and robust responses upon reactivation. To enhance the efficacy of influenza vaccines, we induce the generation of lung TRM cells following intranasal vaccination with a commercial influenza vaccine adjuvanted with NexaVant (NVT), a TLR3 agonist-based adjuvant. We demonstrate that intranasal immunization with the NVT-adjuvanted vaccine provides improved protection against influenza virus infections by inducing the generation of CD4+ TRM cells in the lungs in a type I interferon-dependent manner. These pulmonary CD4+ TRM cells provide potent mucosal immunity and cross-protection against heterosubtypic infections in both mouse and ferret models. This vaccine platform has the potential to significantly improve conventional intramuscular influenza vaccines by providing broader protection.

Passive immunization of mice with IgY anti-H5N1 protects against experimental influenza virus infection and allows development of protective immunity

Influenza virus contributes substantially to the global human and animal disease burden. To protect individuals against disease, strategies are needed to minimize the time an individual is at risk of developing disease symptoms. Passive immunization using avian IgY antibodies can protect individuals against a variety of pathogens, including influenza virus. Yet the effect of IgY administration on generation of protective immunity is largely unknown. To address the effect of passive immunization on the host immune response development, adult or aged, male and female C57BL/6NCrl mice received chicken IgY anti-H5N1, normal IgY or PBS intranasally four hours before, and 20 hours after intranasal infection with H1N1 influenza A virus (PR8). The mice receiving cross-reactive IgY anti-H5N1 were protected from disease and developed influenza virus-specific memory T cells similar to control-treated mice. When re-challenged with PR8 35 days post primary infection IgY anti-H5N1-treated mice were fully protected. Moreover, when challenged with heterologous H3N2 influenza A virus (X-31) or with PR8 three months post infection the mice were protected against severe disease and death, albeit a slight transient weight loss was noted. The results show that passive immunization with IgY anti-H5N1 is safe and protects mice against disease induced by influenza virus without inhibiting development of protective immunity after virus exposure. This indicate that passive immunization can be used as prophylactic therapy in combination with immunization to prevent disease.

Resident memory T cells and cancer

Tissue-resident memory T (TRM) cells positively correlate with cancer survival, but the anti-tumor mechanisms underlying this relationship are not understood. This review reconciles these observations, summarizing concepts of T cell immunosurveillance, fundamental TRM cell biology, and clinical observations on the role of TRM cells in cancer and immunotherapy outcomes. We also discuss emerging strategies that utilize TRM-phenotype cells for patient diagnostics, staging, and therapy. Current challenges are highlighted, including a lack of standardized T cell nomenclature and our limited understanding of relationships between T cell markers and underlying tumor biology. Existing findings are integrated into a summary of the field while emphasizing opportunities for future research.

Cognate antigen-independent differentiation of resident memory T cells in chronic kidney disease

Resident memory T cells (TRMs), which are memory T cells that are retained locally within tissues, have recently been described as antigen-specific frontline defenders against pathogens in barrier and nonbarrier epithelial tissues. They have also been noted for perpetuating chronic inflammation. The conditions responsible for TRM differentiation are still poorly understood, and their contributions, if any, to sterile models of chronic kidney disease (CKD) remain a mystery. In this study, we subjected male C57BL/6J mice and OT-1 transgenic mice to five consecutive days of 2 mg/kg aristolochic acid (AA) injections intraperitoneally to induce CKD or saline injections as a control. We evaluated their kidney immune profiles at 2 wk, 6 wk, and 6 mo after treatment. We identified a substantial population of TRMs in the kidneys of mice with AA-induced CKD. Flow cytometry of injured kidneys showed T cells bearing TRM surface markers and single-cell (sc) RNA sequencing revealed these cells as expressing well-known TRM transcription factors and receptors responsible for TRM differentiation and maintenance. Although kidney TRMs expressed Cd44, a marker of antigen experience and T cell activation, their derivation was independent of cognate antigen-T cell receptor interactions, as the kidneys of transgenic OT-1 mice still harbored considerable proportions of TRMs after injury. Our results suggest a nonantigen-specific or antigen-independent mechanism capable of generating TRMs in the kidney and highlight the need to better understand TRMs and their involvement in CKD.NEW & NOTEWORTHY Resident memory T cells (TRMs) differentiate and are retained within the kidneys of mice with aristolochic acid (AA)-induced chronic kidney disease (CKD). Here, we characterized this kidney TRM population and demonstrated TRM derivation in the kidneys of OT-1 transgenic mice with AA-induced CKD. A better understanding of TRMs and the processes by which they can differentiate independent of antigen may help our understanding of the interactions between the immune system and kidneys.

Resident memory T cells in nonlesional skin and healed lesions of patients with chronic inflammatory diseases: Appearances can be deceptive

Tissue-resident memory T (TRM) cells serve as a first line of defense in peripheral tissues to protect the organism against foreign pathogens. However, autoreactive TRM cells are increasingly implicated in autoimmunity, as evidenced in chronic autoimmune and inflammatory skin conditions. This highlights the need to characterize their phenotype and understand their role for the purpose of targeting them specifically without affecting local immunity. To date, the investigation of TRM cells in human skin diseases has focused mainly on lesional tissues of patients. Accumulating evidence suggests that self-reactive TRM cells are still present in clinically healed lesions of patients and play a role in disease flares, but TRM cells also populate skin that is apparently normal. This review discusses the ontogeny of TRM cells in the skin as well as recent insights regarding the presence of self-reactive TRM cells in both clinically healed skin and nonlesional skin of patients with autoimmune and inflammatory skin conditions, with a particular focus on psoriasis, atopic dermatitis, and vitiligo.

Egress of resident memory T cells from tissue with neoadjuvant immunotherapy: Implications for systemic anti-tumor immunity

INTRODUCTION

Resident memory T (TRM) cells are embedded in peripheral tissue and capable of acting as sentinels that can respond quickly to repeat pathogen exposure as part of an endogenous anti-microbial immune response. Recent evidence suggests that chronic antigen exposure and other microenvironment cues may promote the development of TRM cells within solid tumors as well, and that this TRM phenotype can sequester tumor-specific T cells into tumors and out of circulation resulting in limited systemic antitumor immunity. Here, we perform a review of the published English literature and describe tissue-specific mediators of TRM cell differentiation in states of infection and malignancy with special focus on the role of TGF-β and how targeting TGF-β signaling could be used as a therapeutical approach to promote tumor systemic immunity.

DISCUSSION

The presence of TRM cells with antigen specificity to neoepitopes in tumors associates with positive clinical prognosis and greater responsiveness to immunotherapy. Recent evidence indicates that solid tumors may act as reservoirs for tumor specific TRM cells and limit their circulation - possibly resulting in impaired systemic antitumor immunity. TRM cells utilize specific mechanisms to egress from peripheral tissues into circulation and other peripheral sites, and emerging evidence indicates that immunotherapeutic approaches may initiate these processes and increase systemic antitumor immunity.

CONCLUSIONS

Reversing tumor sequestration of tumor-specific T cells prior to surgical removal or radiation of tumor may increase systemic antitumor immunity. This finding may underlie the improved recurrence free survival observed with neoadjuvant immunotherapy in clinical trials.

Assessing the generation of tissue resident memory T cells by vaccines

Vaccines have been a hugely successful public health intervention, virtually eliminating many once common diseases of childhood. However, they have had less success in controlling endemic pathogens including Mycobacterium tuberculosis, herpesviruses and HIV. A focus on vaccine-mediated generation of neutralizing antibodies, which has been a successful approach for some pathogens, has been complicated by the emergence of escape variants, which has been seen for pathogens such as influenza viruses and SARS-CoV-2, as well as for HIV-1. We discuss how vaccination strategies aimed at generating a broad and robust T cell response may offer superior protection against pathogens, particularly those that have been observed to mutate rapidly. In particular, we consider here how a focus on generating resident memory T cells may be uniquely effective for providing immunity to pathogens that typically infect (or become reactivated in) the skin, respiratory mucosa or other barrier tissues.

Generation of antigen-specific memory CD4 T cells by heterologous immunization enhances the magnitude of the germinal center response upon influenza infection

Current influenza vaccine strategies have yet to overcome significant obstacles, including rapid antigenic drift of seasonal influenza viruses, in generating efficacious long-term humoral immunity. Due to the necessity of germinal center formation in generating long-lived high affinity antibodies, the germinal center has increasingly become a target for the development of novel or improvement of less-efficacious vaccines. However, there remains a major gap in current influenza research to effectively target T follicular helper cells during vaccination to alter the germinal center reaction. In this study, we used a heterologous infection or immunization priming strategy to seed an antigen-specific memory CD4+ T cell pool prior to influenza infection in mice to evaluate the effect of recalled memory T follicular helper cells in increased help to influenza-specific primary B cells and enhanced generation of neutralizing antibodies. We found that heterologous priming with intranasal infection with acute lymphocytic choriomeningitis virus (LCMV) or intramuscular immunization with adjuvanted recombinant LCMV glycoprotein induced increased antigen-specific effector CD4+ T and B cellular responses following infection with a recombinant influenza strain that expresses LCMV glycoprotein. Heterologously primed mice had increased expansion of secondary Th1 and Tfh cell subsets, including increased CD4+ TRM cells in the lung. However, the early enhancement of the germinal center cellular response following influenza infection did not impact influenza-specific antibody generation or B cell repertoires compared to primary influenza infection. Overall, our study suggests that while heterologous infection/immunization priming of CD4+ T cells is able to enhance the early germinal center reaction, further studies to understand how to target the germinal center and CD4+ T cells specifically to increase long-lived antiviral humoral immunity are needed.

Tumor-resident memory T cells as a biomarker of the response to cancer immunotherapy

Tumor-infiltrating lymphocytes (TIL) often include a substantial subset of CD8+ tissue-resident memory T (TRM) cells enriched in tumor-specific T cells. These TRM cells play a major role in antitumor immune response. They are identified on the basis of their expression of the CD103 (αE(CD103)β7) and/or CD49a (α1(CD49a)β1) integrins, and the C-type lectin CD69, which are involved in tissue residency. TRM cells express several T-cell inhibitory receptors on their surface but they nevertheless react strongly to malignant cells, exerting a strong cytotoxic function, particularly in the context of blocking interactions of PD-1 with PD-L1 on target cells. These TRM cells form stable conjugates with autologous tumor cells and interact with dendritic cells and other T cells within the tumor microenvironment to orchestrate an optimal in situ T-cell response. There is growing evidence to indicate that TGF-β is essential for the formation and maintenance of TRM cells in the tumor, through the induction of CD103 expression on activated CD8+ T cells, and for the regulation of TRM effector functions through bidirectional integrin signaling. CD8+ TRM cells were initially described as a prognostic marker for survival in patients with various types of cancer, including ovarian, lung and breast cancers and melanoma. More recently, these tumor-resident CD8+ T cells have been shown to be a potent predictive biomarker of the response of cancer patients to immunotherapies, including therapeutic cancer vaccines and immune checkpoint blockade. In this review, we will highlight the major characteristics of tumor TRM cell populations and the possibilities for their exploitation in the design of more effective immunotherapy strategies for cancer.

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.

Memories that last: Dynamics of memory T cells throughout the body

Memory T cells form an essential part of immunological memory, which can last for years or even a lifetime. Much experimental work has shown that the individual cells that make up the memory T-cell pool are in fact relatively short-lived. Memory T cells isolated from the blood of humans, or the lymph nodes and spleen of mice, live about 5-10 fold shorter than naive T cells, and much shorter than the immunological memory they convey. The commonly accepted view is, therefore, that long-term T-cell memory is maintained dynamically rather than by long-lived cells. This view is largely based on memory T cells in the circulation, identified using rather broad phenotypic markers, and on research in mice living in overly clean conditions. We wondered to what extent there may be heterogeneity in the dynamics and lifespans of memory T cells. We here review what is currently known about the dynamics of memory T cells in different memory subsets, locations in the body and conditions of microbial exposure, and discuss how this may be related to immunometabolism and how this knowledge can be used in various clinical settings.

Memory B-cell diversity: From early generation to tissue residency and reactivation

Memory B cells (MBCs) have a crucial function in providing an enhanced response to repeated infections. Upon antigen encounter, MBC can either rapidly differentiate to antibody secreting cells or enter germinal centers (GC) to further diversify and affinity mature. Understanding how and when MBC are formed, where they reside and how they select their fate upon reactivation has profound implications for designing strategies to improve targeted, next-generation vaccines. Recent studies have crystallized much of our knowledge on MBC but also reported several surprising discoveries and gaps in our current understanding. Here, we review the latest advancements in the field and highlight current unknowns. In particular, we focus on timing and cues leading to MBC generation before and during the GC reaction, discuss how MBC become resident in mucosal tissues, and finally, provide an overview of factors shaping MBC fate-decision upon reactivation in mucosal and lymphoid tissues.

Remodeling and Restraining Lung Tissue Damage Through the Regulation of Respiratory Immune Responses

Tissue damage caused by various stimuli under certain conditions, such as biological and environmental cues, can actively induce systemic and/or local immune responses. Therefore, understanding the immunological perspective would be critical to not only regulating homeostasis of organs and tissues but also to restrict and remodel their damage. Lungs serve as one of the key immunological organs, and thus, in the present article, we focus on the innate and adaptive immune systems involved in remodeling and engineering lung tissue. Innate immune cells are known to react immediately to damage. Macrophages, one of the most widely studied types of innate immune cells, are known to be involved in tissue damage and remodeling, while type 2 innate lymphoid cells (ILC2s) have recently been revealed as an important cell type responsible for tissue remodeling. On the other hand, adaptive immune cells are also involved in damage control. In particular, resident memory T cells in the lung prevent prolonged disease that causes tissue damage. In this review, we first outlined the structure of the respiratory system with biological and environmental cues and the innate/adaptive immune responses in the lung. It is our hope that understanding an immunological perspective for tissue remodeling and damage control in the lung will be beneficial for stakeholders in this area.

Lung tissue-resident memory T cells: the gatekeeper to respiratory viral (re)-infection

The discovery of lung tissue-resident memory T (TRM) cells and the elucidation of their function in antiviral immunity have inspired considerable efforts to leverage the power of TRM cells, in defense to the infections and reinfections by respiratory viruses. Here, we have reviewed lung TRM cell identification, molecular regulation, and function after influenza and SARS-CoV-2 infections. Furthermore, we have discussed emerging data on TRM responses induced by systemic and mucosal vaccination strategies. We hope that our current outstanding of TRM cells in this review could provide insights toward the development of vaccines capable of inducing highly efficacious mucosal TRM responses for protection against respiratory viral infections.

Characterization of natural killer and T cells in bronchoalveolar lavage and peripheral blood of sarcoidosis patients

The characterization of frequency and phenotypes of natural killer (NK) cells and T cells in BAL and peripheral blood of patients with sarcoidosis was evaluated, to discriminate the differential status of these cells in these two compartments. The analysis revealed that CD56brightCD16neg resulted higher in BAL than PB of sarcoidosis and healthy subjects, while CD56dimCD16+ showed a different proportion between BAL and PB of both Sarcoidosis patients and HC. Moreover, in comparison with autologous PB, BAL was characterized by a higher expression of activated NK cell markers NKp44, CD69 and CD25. Significantly increased levels of PD-1+ NK cells in the BAL of patients were detected. Regarding the maturation of CD4 and CD8, an increase of Effector Memory T cells (TEM) was reported in BAL compared to PB. A better characterization of NK and T cells may lead to an improvement of the pathogenetic mechanisms in sarcoidosis.

Recombinant single-cycle influenza virus with exchangeable pseudotypes allows repeated immunization to augment anti-tumour immunity with immune checkpoint inhibitors

Virus-based tumour vaccines offer many advantages compared to other antigen-delivering systems. They generate concerted innate and adaptive immune response, and robust CD8⁺ T cell responses. We engineered a non-replicating pseudotyped influenza virus (S-FLU) to deliver the well-known cancer testis antigen, NY-ESO-1 (NY-ESO-1 S-FLU). Intranasal or intramuscular immunization of NY-ESO-1 S-FLU virus in mice elicited a strong NY-ESO-1-specific CD8⁺ T cell response in lungs and spleen that resulted in the regression of NY-ESO-1-expressing lung tumour and subcutaneous tumour, respectively. Combined administration with anti-PD-1 antibody, NY-ESO-1 S-FLU virus augmented the tumour protection by reducing the tumour metastasis. We propose that the antigen delivery through S-FLU is highly efficient in inducing antigen-specific CD8⁺ T cell response and protection against tumour development in combination with PD-1 blockade.

Unique properties of tissue-resident memory T cells in the lungs: implications for COVID-19 and other respiratory diseases

Tissue-resident memory T (T_RM) cells were originally identified as a tissue-sequestered population of memory T cells that show lifelong persistence in non-lymphoid organs. That definition has slowly evolved with the documentation of T_RM cells having variable terms of tissue residency combined with a capacity to return to the wider circulation. Nonetheless, reductionist experiments have identified an archetypical population of T_RM cells showing intrinsic permanent residency in a wide range of non-lymphoid organs, with one notable exception: the lungs. Despite the fact that memory T cells generated during a respiratory infection are maintained in the circulation, local T_RM cell numbers in the lung decline concomitantly with a decay in T cell-mediated protection. This Perspective describes the mechanisms that underpin long-term T cell lodgement in non-lymphoid tissues and explains why residency is transient for select T_RM cell subsets. In doing so, it highlights the unusual nature of memory T cell egress from the lungs and speculates on the broader disease implications of this process, especially during infection with SARS-CoV-2.

IRF4 expression by lung dendritic cells drives acute but not Trm cell-dependent memory Th2 responses

Expression of the transcription factor interferon regulatory factor 4 (IRF4) is required for the development of lung conventional DCs type 2 (cDC2s) that elicit Th2 responses, yet how IRF4 functions in lung cDC2s throughout the acute and memory allergic response is not clear. Here, we used a mouse model that loses IRF4 expression after lung cDC2 development to demonstrate that mice with IRF4-deficient DCs display impaired memory responses to allergen. This defect in the memory response was a direct result of ineffective Th2 induction and impaired recruitment of activated effector T cells to the lung after sensitization. IRF4-deficient DCs demonstrated defects in their migration to the draining lymph node and in T cell priming. Finally, T cells primed by IRF4-competent DCs mediated potent memory responses independently of IRF4-expressing DCs, demonstrating that IRF4-expressing DCs are not necessary during the memory response. Thus, IRF4 controlled a program in mature DCs governing Th2 priming and effector responses, but IRF4-expressing DCs were dispensable during tissue-resident memory T cell-dependent memory responses.

Persistent Antigen Harbored by Alveolar Macrophages Enhances the Maintenance of Lung-Resident Memory CD8+ T Cells

Lung tissue-resident memory T cells are crucial mediators of cellular immunity against respiratory viruses; however, their gradual decline hinders the development of T cell-based vaccines against respiratory pathogens. Recently, studies using adenovirus (Ad)-based vaccine vectors have shown that the number of protective lung-resident CD8+ TRMs can be maintained long term. In this article, we show that immunization of mice with a replication-deficient Ad serotype 5 expressing influenza (A/Puerto Rico/8/34) nucleoprotein (AdNP) generates a long-lived lung TRM pool that is transcriptionally indistinct from those generated during a primary influenza infection. In addition, we demonstrate that CD4+ T cells contribute to the long-term maintenance of AdNP-induced CD8+ TRMs. Using a lineage tracing approach, we identify alveolar macrophages as a cell source of persistent NP Ag after immunization with AdNP. Importantly, depletion of alveolar macrophages after AdNP immunization resulted in significantly reduced numbers of NP-specific CD8+ TRMs in the lungs and airways. Combined, our results provide further insight to the mechanisms governing the enhanced longevity of Ag-specific CD8+ lung TRMs observed after immunization with recombinant Ad.

T cell kinetics reveal expansion of distinct lung T cell subsets in acute versus in resolved influenza virus infection

Influenza virus infection is restricted to airway-associated tissues and elicits both cellular and humoral responses ultimately resulting in generation of memory cells able to initiate a rapid immune response against re-infections. Resident memory T cells confer protection at the site of infection where lung-resident memory T cells are important for protecting the host against homologous and heterologous influenza virus infections. Mapping kinetics of local and systemic T cell memory formation is needed to better understand the role different T cells have in viral control and protection. After infecting BALB/c mice with influenza virus strain A/Puerto Rico/8/1934 H1N1 the main proportion of activated T cells and B cells expressing the early activation marker CD69 was detected in lungs and lung-draining mediastinal lymph nodes. Increased frequencies of activated cells were also observed in the peripheral lymphoid organs spleen, inguinal lymph nodes and mesenteric lymph nodes. Likewise, antigen-specific T cells were most abundant in lungs and mediastinal lymph nodes but present in all organs studied. CD8+CD103-CD49a+ lung-resident T cells expanded simultaneously with timing of viral clearance whereas CD8+CD103+CD49a+ lung-resident T cells was the most abundant subset after resolution of infection and antigen-specific, lung-resident T cells were detected up to seven months after infection. In conclusion, the results in this detailed kinetic study demonstrate that influenza virus infection elicits adaptive immune responses mainly in respiratory tract-associated tissues and that distinct subsets of lung-resident T cells expand at different time points during infection. These findings contribute to the understanding of the adaptive immune response locally and systemically following influenza virus infection and call for further studies on the roles of the lung-resident T cell subsets.

PD-1 and ICOS counter-regulate tissue resident regulatory T cell development and IL-10 production during flu

Regulatory T cells that express the transcription factor Foxp3 (Treg cells) are a highly heterogenous population of immunoregulatory cells critical for maintaining immune homeostasis and preventing immunopathology during infections. Tissue resident Treg (TR-Treg) cells are maintained within nonlymphoid tissues and have been shown to suppress proinflammatory tissue resident T cell responses and promote tissue repair. Human populations are repetitively exposed to influenza infections and lung tissue resident effector T cell responses are associated with flu-induced long-term pulmonary sequelae. The kinetics of TR-Treg cell development and molecular features of TR-Treg cells during repeated and/or long-term flu infections are unclear. Utilizing a Foxp3^RFP/IL-10^GFP dual reporter mouse model along with intravascular fluorescent in vivo labeling, we characterized the TR-Treg cell responses to repetitive heterosubtypic influenza infections. We found lung tissue resident Treg cells accumulated and expressed high levels of co-inhibitory and co-stimulatory receptors post primary and secondary infections. Blockade of PD-1 or ICOS signaling reveals that PD-1 and ICOS signaling pathways counter-regulate TR-Treg cell expansion and IL-10 production, during secondary influenza infection. Furthermore, the virus-specific TR-Treg cell response displayed distinct kinetics, when compared to conventional CD4⁺ tissue resident memory T cells, during secondary flu infection. Our results provide insight into the tissue resident Foxp3⁺ regulatory T cell response during repetitive flu infections, which may be applicable to other respiratory infectious diseases such as tuberculosis and COVID.

Tissue immunity to SARS-CoV-2: Role in protection and immunopathology

The SARS-CoV-2 pandemic has demonstrated the importance of studying antiviral immunity within sites of infection to gain insights into mechanisms for immune protection and disease pathology. As SARS-CoV-2 is tropic to the respiratory tract, many studies of airway washes, lymph node aspirates, and postmortem lung tissue have revealed site-specific immune dynamics that are associated with the protection or immunopathology but are not readily observed in circulation. This review summarizes the growing body of work identifying immune processes in tissues and their interplay with immune responses in circulation during acute SARS-CoV-2 infection, severe disease, and memory persistence. Establishment of tissue resident immunity also may have implications for vaccination and the durability of immune memory and protection.

Regulatory T Cells Decreased during Recovery from Mild COVID-19

Depending on the intensity and duration of SARS-CoV-2 infection, the host immune response plays a significant role in immunological protection. Here, we studied the regulatory T-cell (Treg) response in relation to kinetic change and cytokine production in patients with mild COVID-19. Nineteen SARS-CoV-2-positive patients were recruited, and blood was collected at four time points, i.e., seven days after admission, after discharge, and one and three months after recovery. CD3⁺CD4⁺CD25⁺CD127^low was marked as the Treg population, with IL-10 and TGF-β used to study cytokine-producing Tregs. IFN-γ-producing CD8⁺ T cells were observed for an effector response. The Treg percentage in patients with mild COVID-19 increased during hospitalization compared to during the recovery period. Peripheral blood mononuclear cells (PBMCs) were quantified, and the T-cell response was characterized by re-stimulation with S1 and N peptides. IL-10 and TGF-β were produced by CD25⁺CD127^low T cells during the active infection phase, especially with N peptide stimulation. Compared to N peptide stimulation, S1 peptide stimulation provided superior IFN-γ-secreting CD8⁺ T-cell responses. Our results suggest that while IFN-γ⁺CD8⁺ T cells confer antiviral immunity, cytokine-producing Tregs may have a substantial role in regulating inflammatory responses in mild SARS-CoV-2 infection. Novel vaccine development may also consider enhancing T-cell repertoires.

Tissue-resident memory T cells in gastrointestinal cancer immunology and immunotherapy: ready for prime time?

Tissue-resident memory T (T_RM) cells have emerged as immune sentinels that patrol the tissue microenvironment and orchestrate localized antitumor immunity in various solid cancers. Recent studies have revealed that T_RM cells are key players in cancer immunosurveillance, and their involvement has been linked to favorable responses to immunotherapy as well as general better clinical outcome in cancer patients. In this review, we provide an overview of the major advances and recent findings regarding T_RM cells phenotype, transcriptional and epigenetic regulation in cancer with a special focus on gastrointestinal tumors. Finally, we highlight the exciting clinical implication of T_RM cells in these types of tumors.

Recent and advanced nano-technological strategies for COVID-19 vaccine development

The outbreak of the COVID-19 pandemic in 2019 has been one of the greatest challenges modern medicine and science has ever faced. It has affected millions of people around the world and altered human life and activities as we once knew. The high prevalence as well as an extended period of incubations which usually does not present with symptoms have played a formidable role in the transmission and infection of millions. A lot of research has been carried out on developing suitable treatment and effective preventive measures for the control of the pandemic. Preventive strategies which include social distancing, use of masks, washing of hands, and contact tracing have been effective in slowing the spread of the virus; however, the infectious nature of the SARS-COV-2 has made these strategies unable to eradicate its spread. In addition, the continuous increase in the number of cases and death, as well as the appearance of several variants of the virus, has necessitated the development of effective and safe vaccines in a bid to ensure that human activities can return to normalcy. Nanotechnology has been of great benefit in the design of vaccines as nano-sized materials have been known to aid the safe and effective delivery of antigens as well as serve as suitable adjuvants to potentiate responses to vaccines. There are only four vaccine candidates currently approved for use in humans while many other candidates are at various levels of development. This review seeks to provide updated information on the current nano-technological strategies employed in the development of COVID-19 vaccines.

Two sides of the same coin: Protective versus pathogenic CD4+ resident memory T cells

The ability of the adaptive immune system to form memory is key to providing protection against secondary infections. Resident memory T cells (T_RM) are specialized T cell populations that reside within tissue sites where they await reencounter with their cognate antigen. T_RM are distinct from circulating memory cells, including central and effector memory T cells, both functionally and transcriptionally. Since the discovery of T_RM, most research has focused on CD8⁺ T_RM, despite that CD4⁺ T_RM are also abundant in most tissues. In the past few years, more evidence has emerged that CD4⁺ T_RM can contribute both protective and pathogenic roles in disease. A complexity inherent to the CD4⁺ T_RM field is the ability of CD4⁺ T cells to polarize into a multitude of distinct subsets and recognize not only viruses and intracellular bacteria but also extracellular bacteria, fungi, and parasites. In this review, we outline the key features of CD4⁺ T_RM in health and disease, including their contributions to protection against SARS-CoV-2 and potential contributions to immunopathology associated with COVID-19.

Respiratory Syncytial Virus Provides Protection against a Subsequent Influenza A Virus Infection

Respiratory infections are a leading cause of morbidity and mortality. The presence of multiple heterologous virus infections is routinely observed in a subset of individuals screened for the presence of respiratory viruses. However, the impact overlapping infections has on disease severity and the host immune response is not well understood. Respiratory syncytial virus (RSV) and influenza A virus (IAV) are two of the most common respiratory infections observed in hospitalized patients, particularly in the very young and aged populations. In this study, we examined how the order in which BALB/c mice were infected with both RSV and IAV impacts disease severity. RSV infection prior to an IAV infection was associated with decreased weight loss and increased survival as compared with IAV infection alone. In contrast, IAV infection prior to an RSV infection was associated with similar morbidity and mortality as compared with an IAV infection alone. Our results suggest that the order in which viral infections are acquired plays a critical role in the outcome of disease severity and the host immune response.

The Emerging Role of Tissue-Resident Memory CD8+ T Lymphocytes in Human Digestive Tract Cancers

Malignant digestive tract tumors are a great threat to human public health. In addition to surgery, immunotherapy brings hope for the treatment of these tumors. Tissue-resident memory CD8⁺ T (Trm) cells are a focus of tumor immunology research and treatment due to their powerful cytotoxic effects, ability to directly kill epithelial-derived tumor cells, and overall impact on maintaining mucosal homeostasis and antitumor function in the digestive tract. They are a group of noncirculating immune cells expressing adhesion and migration molecules such as CD69, CD103, and CD49a that primarily reside on the barrier epithelium of nonlymphoid organs and respond rapidly to both viral and bacterial infection and tumorigenesis. This review highlights new research exploring the role of CD8⁺ Trm cells in a variety of digestive tract malignant tumors, including esophageal cancer, gastric cancer, colorectal cancer, and hepatocellular carcinoma. A summary of CD8⁺ Trm cell phenotypes and characteristics, tissue distribution, and antitumor functions in different tumor environments is provided, illustrating how these cells may be used in immunotherapies against digestive tract tumors.

Islet Lymphocytes Maintain a Stable Regulatory Phenotype Under Homeostatic Conditions and Metabolic Stress

T cells and B cells have been identified in human and murine islets, but the phenotype and role of islet lymphocytes is unknown. Resident immune populations set the stage for responses to inflammation in the islets during homeostasis and diabetes. Thus, we sought to identify the phenotype and effector function of islet lymphocytes to better understand their role in normal islets and in islets under metabolic stress. Lymphocytes were located in the islet parenchyma, and were comprised of a mix of naïve, activated, and memory T cell and B cell subsets, with an enrichment for regulatory B cell subsets. Use of a Nur77 reporter indicated that CD8 T cells and B cells both received local antigen stimulus, indicating that they responded to antigens present in the islets. Analysis of effector function showed that islet T cells and B cells produced the regulatory cytokine IL-10. The regulatory phenotype of islet T cells and B cells and their response to local antigenic stimuli remained stable under conditions of metabolic stress in the diet induced obesity (DIO) model. T cells present in human islets retained a similar activated and memory phenotype in non-diabetic and T2D donors. Under steady-state conditions, islet T cells and B cells have a regulatory phenotype, and thus may play a protective role in maintaining tissue homeostasis.

Tissue resident memory T cells in the respiratory tract

Owing to their capacity to rapidly spread across the population, airborne pathogens represent a significant risk to global health. Indeed, several of the past major global pandemics have been instigated by respiratory pathogens. A greater understanding of the immune cells tasked with protecting the airways from infection will allow for the development of strategies that curb the spread and impact of these airborne diseases. A specific subset of memory T-cell resident in both the upper and lower respiratory tract, termed tissue-resident memory (Trm), have been shown to play an instrumental role in local immune responses against a wide breadth of both viral and bacterial infections. In this review, we discuss factors that influence respiratory tract Trm development, longevity, and immune surveillance and explore vaccination regimes that harness these cells, such approaches represent exciting new strategies that may be utilized to tackle the next global pandemic.

A Combination Adjuvant for the Induction of Potent Antiviral Immune Responses for a Recombinant SARS-CoV-2 Protein Vaccine

Several SARS-CoV-2 vaccines have received EUAs, but many issues remain unresolved, including duration of conferred immunity and breadth of cross-protection. Adjuvants that enhance and shape adaptive immune responses that confer broad protection against SARS-CoV-2 variants will be pivotal for long-term protection as drift variants continue to emerge. We developed an intranasal, rationally designed adjuvant integrating a nanoemulsion (NE) that activates TLRs and NLRP3 with an RNA agonist of RIG-I (IVT DI). The combination adjuvant with spike protein antigen elicited robust responses to SARS-CoV-2 in mice, with markedly enhanced TH1-biased cellular responses and high virus-neutralizing antibody titers towards both homologous SARS-CoV-2 and a variant harboring the N501Y mutation shared by B1.1.7, B.1.351 and P.1 variants. Furthermore, passive transfer of vaccination-induced antibodies protected naive mice against heterologous viral challenge. NE/IVT DI enables mucosal vaccination, and has the potential to improve the immune profile of a variety of SARS-CoV-2 vaccine candidates to provide effective cross-protection against future drift variants.

Inhaled vaccine delivery in the combat against respiratory viruses: a 2021 overview of recent developments and implications for COVID-19

INTRODUCTION

As underlined by the late 2019 outbreak of severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2), vaccination remains the cornerstone of global health-care. Although vaccines for SARS-CoV-2 are being developed at a record-breaking pace, the majority of those that are licensed or currently registered in clinical trials are formulated as an injectable product, requiring a tightly regulated cold-chain infrastructure, and primarily inducing systemic immune responses.

AREAS COVERED

Here, we shed light on the status of inhaled vaccines against viral pathogens, providing background to the role of the mucosal immune system and elucidating what factors determine an inhalable vaccine's efficacy. We also discuss whether the development of an inhalable powder vaccine formulation against SARS-CoV-2 could be feasible. The review was conducted using relevant studies from PubMed, Web of Science and Google Scholar.

EXPERT OPINION

We believe that the scope of vaccine research should be broadened toward inhalable dry powder formulations since dry vaccines bear several advantages. Firstly, their dry state can tremendously increase vaccine stability and shelf-life. Secondly, they can be inhaled using disposable inhalers, omitting the need for trained health-care personnel and, therefore, facilitating mass-vaccination campaigns. Thirdly, inhalable vaccines may provide improved protection since they can induce an IgA-mediated mucosal immune response.

Longitudinal Analysis of COVID-19 Patients Shows Age-Associated T Cell Changes Independent of Ongoing Ill-Health

Objectives

The immunological and inflammatory changes following acute COVID-19 are hugely variable. Persistent clinical symptoms following resolution of initial infection, termed long COVID, are also hugely variable, but association with immunological changes has not been described. We investigate changing immunological parameters in convalescent COVID-19 and interrogate their potential relationships with persistent symptoms.

Methods

We performed paired immunophenotyping at initial SARS-CoV-2 infection and convalescence (n=40, median 68 days) and validated findings in 71 further patients at median 101 days convalescence. Results were compared to 40 pre-pandemic controls. Fatigue and exercise tolerance were assessed as cardinal features of long COVID using the Chalder Fatigue Scale and 6-minute-walk test. The relationships between these clinical outcomes and convalescent immunological results were investigated.

Results

We identify persistent expansion of intermediate monocytes, effector CD8+, activated CD4+ and CD8+ T cells, and reduced naïve CD4+ and CD8+ T cells at 68 days, with activated CD8+ T cells remaining increased at 101 days. Patients >60 years also demonstrate reduced naïve CD4+ and CD8+ T cells and expanded activated CD4+ T cells at 101 days. Ill-health, fatigue, and reduced exercise tolerance were common in this cohort. These symptoms were not associated with immune cell populations or circulating inflammatory cytokines.

Conclusion

We demonstrate myeloid recovery but persistent T cell abnormalities in convalescent COVID-19 patients more than three months after initial infection. These changes are more marked with age and are independent of ongoing subjective ill-health, fatigue and reduced exercise tolerance.

A Combination Adjuvant for the Induction of Potent Antiviral Immune Responses for a Recombinant SARS-CoV-2 Protein Vaccine

Several SARS-CoV-2 vaccines have received EUAs, but many issues remain unresolved, including duration of conferred immunity and breadth of cross-protection. Adjuvants that enhance and shape adaptive immune responses that confer broad protection against SARS-CoV-2 variants will be pivotal for long-term protection. We developed an intranasal, rationally designed adjuvant integrating a nanoemulsion (NE) that activates TLRs and NLRP3 with an RNA agonist of RIG-I (IVT DI). The combination adjuvant with spike protein antigen elicited robust responses to SARS-CoV-2 in mice, with markedly enhanced T _H 1-biased cellular responses and high virus-neutralizing antibody titers towards both homologous SARS-CoV-2 and a variant harboring the N501Y mutation shared by B1.1.7, B.1.351 and P.1 variants. Furthermore, passive transfer of vaccination-induced antibodies protected naive mice against heterologous viral challenge. NE/IVT DI enables mucosal vaccination, and has the potential to improve the immune profile of a variety of SARS-CoV-2 vaccine candidates to provide effective cross-protection against future drift variants.

Enhanced generation of influenza-specific tissue resident memory CD8 T cells in NK-depleted mice

Natural Killer (NK) cells are among the first effectors to directly contact influenza and influenza-infected cells and their activation affects not only their intrinsic functions, but also subsequent CD8+ T cell responses. We utilized a NK cell depletion model to interrogate the contribution of NK cells to the development of anti-influenza CD8+ T cell memory. NK cell ablation increased the number of influenza-specific memory CD8+ T cells in the respiratory tract and lung-draining lymph node. Interestingly, animals depleted of NK cells during primary influenza infection were protected as well as their NK-intact counterparts despite significantly fewer reactivated CD8+ T cells infiltrating the respiratory tract after lethal, heterosubtypic challenge. Instead, protection in NK-deficient animals seems to be conferred by rapid reactivation of an enlarged pool of lung tissue-resident (TRM) memory cells within two days post challenge. Further interrogation of how NK cell ablation enhances respiratory TRM indicated that TRM development is independent of global and NK cell derived IFN-γ. These data suggest that reduction in NK cell activation after vaccination with live, non-lethal influenza virus increases compartmentalized, broadly protective memory CD8+ T cell generation and decreases the risk of CD8+ T cell-mediated pathology following subsequent influenza infections.

Prefusion F-Based Polyanhydride Nanovaccine Induces Both Humoral and Cell-Mediated Immunity Resulting in Long-Lasting Protection against Respiratory Syncytial Virus

Respiratory syncytial virus (RSV) is a leading cause of lower respiratory tract infection in both young children and in older adults. Despite the morbidity, mortality, and high economic burden caused by RSV worldwide, no licensed vaccine is currently available. We have developed a novel RSV vaccine composed of a prefusion-stabilized variant of the fusion (F) protein (DS-Cav1) and a CpG oligodeoxynucleotide adjuvant encapsulated within polyanhydride nanoparticles, termed RSVNanoVax. A prime-boost intranasal administration of RSVNanoVax in BALB/c mice significantly alleviated weight loss and pulmonary dysfunction in response to an RSV challenge, with protection maintained up to at least 6 mo postvaccination. In addition, vaccinated mice exhibited rapid viral clearance in the lungs as early as 2 d after RSV infection in both inbred and outbred populations. Vaccination induced tissue-resident memory CD4 and CD8 T cells in the lungs, as well as RSV F-directed neutralizing Abs. Based on the robust immune response elicited and the high level of durable protection observed, our prefusion RSV F nanovaccine is a promising new RSV vaccine candidate.

Antigen-Specific Tissue-Resident Memory T Cells in the Respiratory System Were Generated following Intranasal Vaccination of Mice with BCG

Tissue-resident memory T cells (T_RM) are different from effector memory T cells (T_EM) and central memory T cells (T_CM) and contribute to the protective immunity against local challenges. Currently, we found that CD4⁺ and CD8⁺ T_RM cells in the nasal mucosa, trachea, lungs, and lavage fluids were heterogeneous on the expression of CD69 and CD103 as well as the production of cytokines including IFN-γ, IL-2, and TNF-α. After intranasal vaccination of mice with BCG, respiratory tissues expressed higher levels of the chemokine CXCL16 and T_RM cells expressed CXCR6 to CXCL16. In addition, antigen-specific CD4⁺ and CD8⁺ T_RM cells expressed cytokines following the stimulation with BCG and persisted in the nasal mucosa, trachea, and lungs for more than a hundred days. At the same time, mice were infected intranasally with live BCG and the results showed that vaccinated mice cleared up live BCG faster than nonvaccinated mice in the respiratory system. Taken together, our data demonstrated that intranasal vaccination of mice with BCG could induce antigen-specific CD4⁺ and CD8⁺ T_RM cells in the respiratory system and have the ability to provide protection against pulmonary reinfection.

Retrograde migration supplies resident memory T cells to lung-draining LN after influenza infection

Numerous observations indicate that resident memory T cells (TRM) undergo unusually rapid attrition within the lung. Here we demonstrate that contraction of lung CD8+ T cell responses after influenza infection is contemporized with egress of CD69+/CD103+ CD8+ T cells to the draining mediastinal LN via the lymphatic vessels, which we term retrograde migration. Cells within the draining LN retained canonical markers of lung TRM, including CD103 and CD69, lacked Ly6C expression (also a feature of lung TRM), maintained granzyme B expression, and did not equilibrate among immunized parabiotic mice. Investigations of bystander infection or removal of the TCR from established memory cells revealed that the induction of the TRM phenotype was dependent on antigen recognition; however, maintenance was independent. Thus, local lung infection induces CD8+ T cells with a TRM phenotype that nevertheless undergo retrograde migration, yet remain durably committed to the residency program within the draining LN, where they provide longer-lived regional memory while chronicling previous upstream antigen experiences.

COVID-19 vaccines: The status and perspectives in delivery points of view

Due to the high prevalence and long incubation periods often without symptoms, the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) has infected millions of individuals globally, causing the coronavirus disease 2019 (COVID-19) pandemic. Even with the recent approval of the anti-viral drug, remdesivir, and Emergency Use Authorization of monoclonal antibodies against S protein, bamlanivimab and casirimab/imdevimab, efficient and safe COVID-19 vaccines are still desperately demanded not only to prevent its spread but also to restore social and economic activities via generating mass immunization. Recent Emergency Use Authorization of Pfizer and BioNTech's mRNA vaccine may provide a pathway forward, but monitoring of long-term immunity is still required, and diverse candidates are still under development. As the knowledge of SARS-CoV-2 pathogenesis and interactions with the immune system continues to evolve, a variety of drug candidates are under investigation and in clinical trials. Potential vaccines and therapeutics against COVID-19 include repurposed drugs, monoclonal antibodies, antiviral and antigenic proteins, peptides, and genetically engineered viruses. This paper reviews the virology and immunology of SARS-CoV-2, alternative therapies for COVID-19 to vaccination, principles and design considerations in COVID-19 vaccine development, and the promises and roles of vaccine carriers in addressing the unique immunopathological challenges presented by the disease.

T cell responses in respiratory viral infections and chronic obstructive pulmonary disease

ABSTRACT

Respiratory viruses are major human pathogens that cause approximately 200 million pneumonia cases annually and induce various comorbidities with chronic obstructive pulmonary disease (COPD), resulting in significant health concerns and economic burdens. Clinical manifestations in respiratory viral infections and inflammations vary from asymptomatic, mild, to severe, depending on host immune cell responses to pathogens and interactions with airway epithelia. We critically review the activation, effector, and regulation of T cells in respiratory virus infections and chronic inflammations associated with COPD. Crosstalk among T cells, innate immune cells, and airway epithelial cells is discussed as essential parts of pathogenesis and protection in viral infections and COPD. We emphasize the specificity of peptide antigens and the functional heterogeneity of conventional CD4+ and CD8+ T cells to shed some light on potential cellular and molecular candidates for the future development of therapeutics and intervention against respiratory viral infections and inflammations.

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.

Influenza-Specific Lung-Resident Memory CD8+ T Cells

Despite the availability of seasonal vaccines, influenza A (IAV) prevails as a leading cause of respiratory infection worldwide. Current vaccination efforts aim at increasing protection against heterologous and potentially pandemic IAV strains. Lung-resident CD8⁺ T cells (Trm) generated upon IAV infection are vital for heterosubtypic immunity to IAV reexposure and provide quick and robust responses upon reactivation. Yet, protection wanes with time as lung Trm cell numbers decline, a contrasting feature with Trm cells at other mucosal sites such as the skin. In this review, we discuss current data on lung Trm compared to Trm cells in other tissues. Furthermore, major knowledge gaps in the generation and maintenance of IAV-specific lung Trm are addressed and mechanisms that may contribute to their decline are discussed. Further understanding in the mechanisms that govern effector function versus immunopathology is paramount for future IAV vaccine design in enhancing durability of lung Trm cells.

Induction of SARS-CoV-2 Protein S-Specific CD8+ T Cells in the Lungs of gp96-Ig-S Vaccinated Mice

Given the aggressive spread of COVID-19-related deaths, there is an urgent public health need to support the development of vaccine candidates to rapidly improve the available control measures against SARS-CoV-2. To meet this need, we are leveraging our existing vaccine platform to target SARS-CoV-2. Here, we generated cellular heat shock chaperone protein, glycoprotein 96 (gp96), to deliver SARS-CoV-2 protein S (spike) to the immune system and to induce cell-mediated immune responses. We showed that our vaccine platform effectively stimulates a robust cellular immune response against protein S. Moreover, we confirmed that gp96-Ig, secreted from allogeneic cells expressing full-length protein S, generates powerful, protein S polyepitope-specific CD4+ and CD8+ T cell responses in both lung interstitium and airways. These findings were further strengthened by the observation that protein-S -specific CD8+ T cells were induced in human leukocyte antigen HLA-A2.1 transgenic mice thus providing encouraging translational data that the vaccine is likely to work in humans, in the context of SARS-CoV-2 antigen presentation.

Total Recall: Intestinal TRM Cells in Health and Disease

Tissue-resident memory T cells (TRM cells) have crucial functions in host defense in mucosal tissues. They provide local adaptive immune surveillance and allow the fast initiation of targeted adaptive immune responses in case of antigen re-exposure. Recently, an aberrant activation in the case of immunologically mediated diseases has been increasingly acknowledged. As the organ with the largest interface to the environment, the gastrointestinal tract faces billions of antigens every day. Tightly balanced processes are necessary to ensure tolerance towards non-hazardous antigens, but to set up a powerful immune response against potentially dangerous ones. In this complex nexus of immune cells and their mediators, TRM cells play a central role and have been shown to promote both physiological and pathological events. In this review, we will summarize the current knowledge on the homeostatic functions of TRM cells and delineate their implication in infection control in the gut. Moreover, we will outline their commitment in immune dysregulation in gastrointestinal chronic inflammatory conditions and shed light on TRM cells as current and potential future therapeutic targets.

Characterizing T cell subsets in the nasal mucosa of children with acute respiratory symptoms

BACKGROUND

In infants admitted to an ICU with respiratory failure, there is an association between the ratio of CD8⁺ to CD4⁺ T cells within the upper respiratory tract and disease severity. Whether this ratio is associated with respiratory disease severity within children presenting to a pediatric emergency department is not known.

METHODS

We studied a convenience sample of 63 children presenting to a pediatric emergency department with respiratory symptoms. T cell subsets in the nasal mucosa were analyzed by flow cytometry. We compared CD4⁺ and CD8⁺ T cells subsets in these samples and analyzed the proportion of these subsets that expressed markers associated with tissue residency.

RESULTS

We were able to identify major subsets of CD8 and CD4 T cells within the nasal mucosa using flocked swabs. We found no difference in the ratio CD8⁺ to CD4⁺ T cells in children with upper or lower respiratory illness. A positive association between tissue-resident memory T cell frequency and patient age was identified.

CONCLUSIONS

In our patient populations, the CD8⁺:CD4⁺ ratio was not associated with disease severity. The majority of T cells collected on nasal swabs are antigen experienced, and there is an association between the frequency of tissue-resident T cells and age.

IMPACT

Immune cell populations from the nasal mucosa can be captured using flocked nasal swabs and analyzed by flow cytometry. Nasal CD8⁺:CD4⁺ ratio does not predict respiratory illness severity in children presenting to the emergency department. The frequency of CD8⁺ and CD4⁺ resident memory T cells within the nasal mucosa increases with age.

Divergence of Tissue-Memory T Cells: Distribution and Function-Based Classification

Tissue-resident memory T cells (Trm) comprise the majority of memory cells in nonlymphoid tissues and play a predominant role in immunity at barrier surfaces. A better understanding of Trm cell maintenance and function is essential for the development of vaccines that confer frontline protection. However, it is currently challenging to precisely distinguish Trm cells from other T cells, and this has led to confusion in the literature. Here we highlight gaps in our understanding of tissue memory and discuss recent advances in the classification of Trm cell subsets based on their distribution and functional characteristics.