Intranasal administration of RSV antigen-expressing MCMV elicits robust tissue-resident effector and effector memory CD8+ T cells in the lung

Lung Tissue Resident Memory T-Cells in the Immune Response to Mycobacterium tuberculosis

Despite widespread BCG vaccination and effective anti-TB drugs, Tuberculosis (TB) remains the leading cause of death from an infectious agent worldwide. Several recent publications give reasons to be optimistic about the possibility of a more effective vaccine, but the only full-scale clinical trial conducted failed to show protection above BCG. The immunogenicity of vaccines in humans is primarily evaluated by the systemic immune responses they generate, despite the fact that a correlation between these responses and protection from TB disease has not been demonstrated. A novel approach to tackling this problem is to study the local immune responses that occur at the site of TB infection in the human lung, rather than those detectable in blood. There is a growing understanding that pathogen specific T-cell immunity can be highly localized at the site of infection, due to the existence of tissue resident memory T-cells (Trm). Notably, these cells do not recirculate in the blood and thus may not be represented in studies of the systemic immune response. Here, we review the potential role of Trms as a component of the TB immune response and discuss how a better understanding of this response could be harnessed to improve TB vaccine efficacy.

RSV recombinant candidate vaccine G1F/M2 with CpG as an adjuvant prevents vaccine-associated lung inflammation, which may be associated with the appropriate types of immune memory in spleens and lungs

Respiratory syncytial virus (RSV) is a major respiratory pathogen in infants. The early formalin-inactivated RSV not only failed to protect infants against infection, but also was associated with enhanced pulmonary inflammatory disease upon natural infection. A safe and effective vaccine should prevent the inflammatory disease and provide protection. Immune memory is the cornerstone of vaccines. In this study, we evaluated three types of immune memory T cells, antibodies, and lung inflammation of a vaccine candidate G1F/M2, which includes a neutralizing epitope fragment of RSV G protein and a cytotoxic T lymphocyte epitope of M2 protein, with toll-like receptor 9 agonist CpG2006 as an adjuvant by intranasal (i.n.) and intraperitoneal (i.p.) immunization protocols. The results indicated that immunization of mice with G1F/M2 + CpG i.p. induced significantly higher level of CD4⁺ or CD8⁺ central memory (TCM), Th1-type effector memory (TEM), and balanced ratio of IgG1/IgG2a, but lower level of lung tissue-resident memory (TRM), compared with immunization with G1F/M2 + CpG i.n., G1F/M2 i.n., or G1F/M2 i.p. Following RSV challenge, the mice immunized with G1F/M2 + CpG i.p. showed higher level of Th1-type responses, remarkably suppressed inflammatory cytokines and histopathology in lungs, compared with mice immunized with G1F/M2 + CpG i.n., G1F/M2 i.n., or G1F/M2 i.p. These results suggested that high level of TCM and Th1 type of TEM in spleens may contribute to inhibition of lung inflammation, while high level of TRM in lungs and lack of or weak Th1-type immune memory in spleens may promote lung inflammation following RSV challenge.

Immunological goals for respiratory syncytial virus vaccine development

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

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.

Vaccine vectors: the bright side of cytomegalovirus

Cytomegaloviruses (CMVs) present singular features that are particularly advantageous for human vaccine development, a current medical need. Vaccines that induce neutralizing antibodies are among the most successful and efficacious available. However, chronic and persistent human infections, pathogens with high variability of exposed proteins, as well as tumors, highlight the need for developing novel vaccines inducing strong and long-lasting cellular immune responses mediated by effector or effector memory CD8⁺ cytotoxic T lymphocytes. CMVs induce the most potent CD8⁺ T lymphocyte response to a pathogen known in each of their hosts, maintain and even increase it for life for selected antigens, in what is known as the ever growing inflationary memory, and maintain an effector memory status due to recent and repeated antigen stimulation that endows these inflationary T lymphocytes with superior and faster protective potency. In addition to these CMV singularities, this family of viruses has two more common favorable features: they can superinfect an already infected host, which is needed in face of the high CMV prevalence, and they can harbor very large segments of foreign DNA at many different genomic sites. All these properties endow CMVs with a singular potential to be used as human vaccine vectors. Current developments with most of the recombinant CMV-based vaccine candidates that have been tested in animal models against clinically relevant viral and bacterial infections and for their use in tumor immunotherapy are reviewed herein. Since CMV vectors should be designed to avoid the risk of disease in immunocompromised individuals, special attention is also paid to attenuated vectors. Taken together, the results support the future use of CMV-based vaccine vectors to induce protective CD8⁺ T lymphocyte responses in humans, mainly against viral infections and as anti-tumor vaccines.

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.

IL-33 Augments Virus-Specific Memory T Cell Inflation and Potentiates the Efficacy of an Attenuated Cytomegalovirus-Based Vaccine

Candidate vaccines designed to generate T cell-based immunity are typically vectored by nonpersistent viruses, which largely fail to elicit durable effector memory T cell responses. This limitation can be overcome using recombinant strains of CMV. Proof-of-principle studies have demonstrated the potential benefits of this approach, most notably in the SIV model, but safety concerns require the development of nonreplicating alternatives with comparable immunogenicity. In this study, we show that IL-33 promotes the accumulation and recall kinetics of circulating and tissue-resident memory T cells in mice infected with murine CMV. Using a replication-deficient murine CMV vector, we further show that exogenous IL-33 boosts vaccine-induced memory T cell responses, which protect against subsequent heterologous viral challenge. These data suggest that IL-33 could serve as a useful adjuvant to improve the efficacy of vaccines based on attenuated derivatives of CMV.

Demarcated thresholds of tumor-specific CD8 T cells elicited by MCMV-based vaccine vectors provide robust correlates of protection

BACKGROUND

The capacity of cytomegalovirus (CMV) to elicit long-lasting strong T cell responses, and the ability to engineer the genome of this DNA virus positions CMV-based vaccine vectors highly suitable as a cancer vaccine platform. Defined immune thresholds for tumor protection and the factors affecting such thresholds have not well been investigated in cancer immunotherapy. We here determined using CMV as a vaccine platform whether critical thresholds of vaccine-specific T cell responses can be established that relate to tumor protection, and which factors control such thresholds.

METHODS

We generated CMV-based vaccine vectors expressing the E7 epitope and tested these in preclinical models of HPV16-induced cancer. Vaccination was applied via different doses and routes (intraperitoneal (IP), subcutaneous (SC) and intranasal (IN)). The magnitude, kinetics and phenotype of the circulating tumor-specific CD8⁺ T cell response were determined. Mice were subsequently challenged with tumor cells, and the tumor protection was monitored.

RESULTS

Immunization with CMV-based vaccines via the IP or SC route eliciting vaccine-induced CD8⁺ T cell responses of > 0.3% of the total circulating CD8 T cell population fully protects mice against lethal tumor challenge. However, low dose inoculations via the IP or SC route or IN vaccination elicited vaccine-induced CD8⁺ T cell responses that did not reach protective thresholds for tumor protection. In addition, whereas weak pre-existing immunity did not alter the protective thresholds of the vaccine-specific T cell response following subsequent immunization with CMV-based vaccine vectors, strong pre-existing immunity inhibited the development of vaccine-induced T cells and their control on tumor progression.

CONCLUSIONS

This study highlight the effectiveness of CMV-based vaccine vectors, and shows that demarcated thresholds of vaccine-specific T cells could be defined that correlate to tumor protection. Together, these results may hold importance for cancer vaccine development to achieve high efficacy in vaccine recipients.

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.

Single mucosal vaccination targeting nucleoprotein provides broad protection against two lineages of influenza B virus

Nucleoprotein is highly conserved among each type of influenza viruses (A and B) and has received significant attention as a good target for universal influenza vaccine. In this study, we determined whether a recombinant adenovirus encoding nucleoprotein of type B influenza virus (rAd/B-NP) confers protection against influenza virus infection in mice. We also identified a cytotoxic T lymphocyte epitope in the nucleoprotein to determine B-NP-specific CD8 T-cell responses. We found that B-NP-specific CD8 T cells induced by rAd/B-NP immunization played a major role in protection following influenza B virus infection using CD8 knockout mice. To assess the effects of the administration routes on protective immunity, we immunized mice with rAd/B-NP via intranasal or intramuscular routes. Both groups showed strong NP-specific humoral and CD8 T-cell responses, but only intranasal immunization provided complete protection against both lineages of influenza B virus challenge. Intranasal but not intramuscular administration established resident memory CD8 T cells in the airway and lung parenchyma, which were required for efficient protection. Furthermore, rAd/B-NP in combination with rAd/A-NP protected mice against lethal infection with both influenza A and B viruses. These findings demonstrate that rAd/B-NP could be further developed as a universal vaccine against influenza.

Targeting CD40 enhances antibody- and CD8-mediated protection against respiratory syncytial virus infection

Respiratory Syncytial Virus (RSV) infects almost all children under the age of one and is the leading cause of hospitalization among infants. Despite several decades of research with dozens of candidate vaccines being vigorously evaluated in pre-clinical and clinical studies, there is no licensed vaccine available to date. Here, the RSV fusion protein (F) was fused with CD40 ligand and delivered by an adenoviral vector into BALB/c mice where the CD40 ligand serves two vital functions as a molecular adjuvant and an antigen-targeting molecule. In contrast to a formaldehyde-inactivated vaccine, the vectored vaccine effectively protected animals against RSV without inducing enhanced respiratory disease. This protection involved a robust induction of neutralizing antibodies and memory CD8 T cells, which were not observed in the inactivated vaccine group. Finally, the vectored vaccine was able to elicit long-lasting protection against RSV, one of the most challenging issues in RSV vaccine development. Further studies indicate that the long lasting protection elicited by the CD40 ligand targeted vaccine was mediated by increased levels of effector memory CD8 T cell 3 months post-vaccination.

Routing dependent immune responses after experimental R848-adjuvated vaccination

Most traditional vaccines are administered via the intramuscular route. Other routes of administration however, can induce equal or improved protective memory responses and might provide practical advantages such as needle-free immunization, dose sparing and induction of tissue-specific (mucosal) immunity. Here we explored the differences in immunological outcome after immunization with model antigens via two promising immunization routes (intradermal and intranasal) with or without the experimental adjuvant and TLR7/8-agonist R848. Because the adaptive immune response is largely determined by the local innate cells at the site of immunization, the effect of R848-adjuvation on local cellular recruitment, antigenic uptake by antigen-presenting cells and the initiation of the adaptive response were analyzed for the two routes of administration. We show a general immune-stimulating effect of R848 irrespective of the route of administration. This includes influx of neutrophils, macrophages and dendritic cells to the respective draining lymph nodes and an increase in antigen-positive antigen-presenting cells which leads for both intradermal and intranasal immunization to a mainly T_H1 response. Furthermore, both intranasal and intradermal R848-adjuvated immunization induces a local shift in DC subsets; frequencies of CD11b⁺DC increase whereas CD103⁺DC decrease in relative abundance in the draining lymph node. In spite of these similarities, the outcome of immune responses differs for the respective immunization routes in both magnitude and cytokine profile. Via the intradermal route, the induced T-cell response is higher compared to that after intranasal immunization, which corresponds with the local higher uptake of antigen by antigen-presenting cells after intradermal immunization. Furthermore, R848-adjuvation enhances ex vivo IL-10 and IL-17 production after intranasal, but not intradermal, T-cell activation. Quite the opposite, intradermal immunization leads to a decrease in IL-10 production by the vaccine induced T-cells. This knowledge may lead to a more rational development of novel adjuvanted vaccines administered via non-traditional routes.

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.

Targeting Resident Memory T Cells for Cancer Immunotherapy

A novel population of memory CD8⁺ T cells called resident memory T cells (T_RM) has been identified based on their phenotype (CD103, CD69) and on their local tissue residency without recirculating in the blood. These cells have been implicated in protective immune response against pathogens in both animal models and humans. Their role in cancer is just emerging as a key player in tumor immunosurveillance. Many properties of these cells suggest that they could control tumor growth: (i) they respond much faster to reexposure to cognate antigen than circulating memory cells, (ii) they express high levels of cytotoxic molecules, and (iii) they are enriched in tumor-specific T cells in close contact with tumor cells. T_RM are present in many human cancers and are associated with a good clinical outcome independently of the infiltration of CD8⁺ T cells. It has been recently shown that the efficacy of cancer vaccines depends on their ability to elicit T_RM. In adoptive cell therapy, the transfer of cells with the ability to establish T_RM at the tumor site correlates with the potency of this approach. Interestingly, T_RM express immune checkpoint molecules and preliminary data showed that they could expand early during anti-PD-1 treatment, and thus be considered as a surrogate marker of response to immunotherapy. Some cues to better expand these cells in vivo and improve the success of cancer immunotherapy include using mucosal routes of immunization, targeting subpopulations of dendritic cells as well as local signal at the mucosal site to recruit them in mucosal tissue.

Cytokines and CD8 T cell immunity during respiratory syncytial virus infection

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

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

Background

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

Objectives

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

Methods

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

Results

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

Conclusion

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

Pulmonary antigen encounter regulates the establishment of tissue-resident CD8 memory T cells in the lung airways and parenchyma

Resident memory CD8 T (TRM) cells in the lung parenchyma (LP) and airways provide heterologous protection against influenza virus challenge. However, scant knowledge exists regarding factors necessary to establish and maintain lung CD8 TRM. Here we demonstrate that, in contrast to mechanisms described for other tissues, airway, and LP CD8 TRM establishment requires cognate antigen recognition in the lung. Systemic effector CD8 T cells could be transiently pulled into the lung in response to localized inflammation, however these effector cells failed to establish tissue residency unless antigen was present in the pulmonary environment. The interaction of effector CD8 T cells with cognate antigen in the lung resulted in increased and prolonged expression of the tissue-retention markers CD69 and CD103, and increased expression of the adhesion molecule VLA-1. The inability of localized inflammation alone to establish lung TRM resulted in decreased viral clearance and increased mortality following heterosubtypic influenza challenge, despite equal numbers of circulating memory CD8 T cells. These findings demonstrate that pulmonary antigen encounter is required for the establishment of lung CD8 TRM and may inform future vaccine strategies to generate robust cellular immunity against respiratory pathogens.

Human respiratory syncytial virus: pathogenesis, immune responses, and current vaccine approaches

Respiratory syncytial virus continues to pose a serious threat to the pediatric populations worldwide. With a genomic makeup of 15,200 nucleotides, the virus encodes for 11 proteins serving as envelope spikes, inner envelope proteins, and non-structural and ribonucleocapsid complexes. The fusion (F) and attachment (G) surface glycoproteins are the key targets for neutralizing antibodies. The highly variable G with altered glycosylations and the conformational alternations of F create challenges for vaccine development. The metastable F protein is responsible for RSV-host cell fusion and thus infectivity. Novel antigenic sites were identified on this form following its stabilization and solving its crystal structure. Importantly, site ø displays neutralizing activity exceeding those of post-F-specific and shared antigenic sites, such as site II which is the target for Palivizumab therapeutic antibody. Induction of high neutralizing antibody responses by pre-F immunization in animal models promoted it as a major vaccine candidate. Since RSV infection is more serious at age extremities and in individuals with undermining health conditions, vaccines are being developed to target these populations. Infants below three months of age have a suppressive immune system, making vaccines' immunogenicity weak. Therefore, a suggested strategy to protect newborns from RSV infection would be through passive immunity of maternal antibodies. Hence, pregnant women at their third trimester have been selected as an ideal target for vaccination with RSV pre-F vaccine. This review summarizes the different modes of RSV pathogenesis and host's immune response to the infection, and illustrates on the latest updates of vaccine development and vaccination approaches.

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

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

Induction of vaginal-resident HIV-specific CD8 T cells with mucosal prime-boost immunization

Tissue-resident memory (T_RM) CD8 T cells survey a range of non-lymphoid mucosal tissues where they rapidly mediate clearance of viral infections at the entry portals. Vaccines that establish CD8 T_RM cells in the cervicovaginal mucosa hold promise for effective immunity against sexually transmitted HIV. We demonstrate that HIV-specific CD8 T_RM cells can be established in the murine vaginal mucosa using a combined intranasal and intravaginal mucosal immunization with recombinant influenza-HIV vectors. Using in situ tetramer immunofluorescence microscopy, we found that this mucosally administered prime-boost immunization also resulted in the durable seeding of CD8 T cells in the frontline vaginal epithelial compartment as opposed to the vaginal submucosa. Upon cognate antigen recognition within the vaginal mucosa, these HIV-specific CD8 T_RM cells rapidly initiated a tissue-wide state of immunity. The activation of HIV-specific CD8 T_RM cells resulted in the upregulation of endothelial vessel addressin expression and substantial recruitment of both adaptive and innate immune cells in the vaginal mucosa. These findings suggest that the epithelial localization of HIV-specific CD8 T_RM cell populations and their capacity to rapidly activate both arms of the immune system could significantly augment frontline defenses against vaginal HIV infection.

The CD8 T Cell Response to Respiratory Virus Infections

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

Features of Effective T Cell-Inducing Vaccines against Chronic Viral Infections

For many years, the focus of prophylactic vaccines was to elicit neutralizing antibodies, but it has become increasingly evident that T cell-mediated immunity plays a central role in controlling persistent viral infections such as with human immunodeficiency virus, cytomegalovirus, and hepatitis C virus. Currently, various promising prophylactic vaccines, capable of inducing substantial vaccine-specific T cell responses, are investigated in preclinical and clinical studies. There is compelling evidence that protection by T cells is related to the magnitude and breadth of the T cell response, the type and homing properties of the memory T cell subsets, and their cytokine polyfunctionality and metabolic fitness. In this review, we evaluated these key factors that determine the qualitative and quantitative properties of CD4+ and CD8+ T cell responses in the context of chronic viral disease and prophylactic vaccine development. Elucidation of the mechanisms underlying T cell-mediated protection against chronic viral pathogens will facilitate the development of more potent, durable and safe prophylactic T cell-based vaccines.

Biased Generation and In Situ Activation of Lung Tissue-Resident Memory CD4 T Cells in the Pathogenesis of Allergic Asthma

Asthma is a chronic inflammatory disease mediated by allergen-specific CD4 T cells that promote lung inflammation through recruitment of cellular effectors into the lung. A subset of lung T cells can persist as tissue-resident memory T cells (TRMs) following infection and allergen induction, although the generation and role of TRM in asthma persistence and pathogenesis remain unclear. In this study, we used a mouse model of chronic exposure to intranasal house dust mite (HDM) extract to dissect how lung TRMs are generated and function in the persistence and pathogenesis of allergic airway disease. We demonstrate that both CD4⁺ and CD8⁺ T cells infiltrate into the lung tissue during acute HDM exposure; however, only CD4⁺ TRMs, and not CD8⁺ TRMs, persist long term following cessation of HDM administration. Lung CD4⁺ TRMs are localized around airways and are rapidly reactivated upon allergen re-exposure accompanied by the rapid induction of airway hyperresponsiveness independent of circulating T cells. Lung CD4⁺ TRM activation to HDM challenge is also accompanied by increased recruitment and activation of dendritic cells in the lungs. Our results indicate that lung CD4⁺ TRMs can perpetuate allergen-specific sensitization and direct early inflammatory signals that promote rapid lung pathology, suggesting that targeting lung CD4⁺ TRMs could have therapeutic benefit in alleviating recurrent asthma episodes.

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

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

The Chemokine Receptor CXCR3 Promotes CD8+ T Cell Accumulation in Uninfected Salivary Glands but Is Not Necessary after Murine Cytomegalovirus Infection

Recent work indicates that salivary glands are able to constitutively recruit CD8⁺ T cells and retain them as tissue-resident memory T cells, independently of local infection, inflammation, or Ag. To understand the mechanisms supporting T cell recruitment to the salivary gland, we compared T cell migration to the salivary gland in mice that were infected or not with murine CMV (MCMV), a herpesvirus that infects the salivary gland and promotes the accumulation of salivary gland tissue-resident memory T cells. We found that acute MCMV infection increased rapid T cell recruitment to the salivary gland but that equal numbers of activated CD8⁺ T cells eventually accumulated in infected and uninfected glands. T cell recruitment to uninfected salivary glands depended on chemokines and the integrin α₄ Several chemokines were expressed in the salivary glands of infected and uninfected mice, and many of these could promote the migration of MCMV-specific T cells in vitro. MCMV infection increased the expression of chemokines that interact with the receptors CXCR3 and CCR5, but neither receptor was needed for T cell recruitment to the salivary gland during MCMV infection. Unexpectedly, however, the chemokine receptor CXCR3 was critical for T cell accumulation in uninfected salivary glands. Together, these data suggest that CXCR3 and the integrin α₄ mediate T cell recruitment to uninfected salivary glands but that redundant mechanisms mediate T cell recruitment after MCMV infection.

Adenoviral vaccine induction of CD8+ T cell memory inflation: Impact of co-infection and infection order

The efficacies of many new T cell vaccines rely on generating large populations of long-lived pathogen-specific effector memory CD8 T cells. However, it is now increasingly recognized that prior infection history impacts on the host immune response. Additionally, the order in which these infections are acquired could have a major effect. Exploiting the ability to generate large sustained effector memory (i.e. inflationary) T cell populations from murine cytomegalovirus (MCMV) and human Adenovirus-subtype (AdHu5) 5-beta-galactosidase (Ad-lacZ) vector, the impact of new infections on pre-existing memory and the capacity of the host's memory compartment to accommodate multiple inflationary populations from unrelated pathogens was investigated in a murine model. Simultaneous and sequential infections, first with MCMV followed by Ad-lacZ, generated inflationary populations towards both viruses with similar kinetics and magnitude to mono-infected groups. However, in Ad-lacZ immune mice, subsequent acute MCMV infection led to a rapid decline of the pre-existing Ad-LacZ-specific inflating population, associated with bystander activation of Fas-dependent apoptotic pathways. However, responses were maintained long-term and boosting with Ad-lacZ led to rapid re-expansion of the inflating population. These data indicate firstly that multiple specificities of inflating memory cells can be acquired at different times and stably co-exist. Some acute infections may also deplete pre-existing memory populations, thus revealing the importance of the order of infection acquisition. Importantly, immunization with an AdHu5 vector did not alter the size of the pre-existing memory. These phenomena are relevant to the development of adenoviral vectors as novel vaccination strategies for diverse infections and cancers. (241 words).

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

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

CXCR3 Signaling Is Required for Restricted Homing of Parenteral Tuberculosis Vaccine-Induced T Cells to Both the Lung Parenchyma and Airway

Although most novel tuberculosis (TB) vaccines are designed for delivery via the muscle or skin for enhanced protection in the lung, it has remained poorly understood whether systemic vaccine-induced memory T cells can readily home to the lung mucosa prior to and shortly after pathogen exposure. We have investigated this issue by using a model of parenteral TB immunization and intravascular immunostaining. We find that systemically induced memory T cells are restricted to the blood vessels in the lung, unable to populate either the lung parenchymal tissue or the airway under homeostatic conditions. We further find that after pulmonary TB infection, it still takes many days before such T cells can enter the lung parenchymal tissue and airway. We have identified the acquisition of CXCR3 expression by circulating T cells to be critical for their entry to these lung mucosal compartments. Our findings offer new insights into mucosal T cell biology and have important implications in vaccine strategies against pulmonary TB and other intracellular infections in the lung.

Immune Protection by a Cytomegalovirus Vaccine Vector Expressing a Single Low-Avidity Epitope

Experimental CMV-based vaccine vectors expressing a single MHC class I-restricted high-avidity epitope provided strong, T cell-dependent protection against viruses or tumors. In this study we tested the low-avidity epitope KCSRNRQYL, and show that a mouse CMV (MCMV) vector provides complete immune control of recombinant vaccinia virus expressing the same epitope if KCSRNRQYL is expressed within the immediate-early MCMV gene ie2 The same epitope expressed within the early M45 gene provided no protection, although MCMV vectors expressing the high-avidity epitope SSIEFARL induced protective immunity irrespective of gene expression context. Immune protection was matched by Ag-induced, long-term expansion of effector memory CD8 T cells, regardless of epitope avidity. We explained this pattern by observing regularities in Ag competition, where responses to high-avidity epitopes outcompeted weaker ones expressed later in the replicative cycle of the virus. Conversely, robust and early expression of a low-avidity epitope compensated its weak intrinsic antigenicity, resulting in strong and sustained immunity and immune protection.

Cytomegalovirus vector expressing RAE-1γ induces enhanced anti-tumor capacity of murine CD8+ T cells

Designing CD8⁺ T-cell vaccines, which would provide protection against tumors is still considered a great challenge in immunotherapy. Here we show the robust potential of cytomegalovirus (CMV) vector expressing the NKG2D ligand RAE-1γ as CD8⁺ T cell-based vaccine against malignant tumors. Immunization with the CMV vector expressing RAE-1γ, delayed tumor growth or even provided complete protection against tumor challenge in both prophylactic and therapeutic settings. Moreover, a potent tumor control in mice vaccinated with this vector can be further enhanced by blocking the immune checkpoints TIGIT and PD-1. CMV vector expressing RAE-1γ potentiated expansion of KLRG1⁺ CD8⁺ T cells with enhanced effector properties. This vaccination was even more efficient in neonatal mice, resulting in the expansion and long-term maintenance of epitope-specific CD8⁺ T cells conferring robust resistance against tumor challenge. Our data show that immunomodulation of CD8⁺ T-cell responses promoted by herpesvirus expressing a ligand for NKG2D receptor can provide a powerful platform for the prevention and treatment of CD8⁺ T-cell sensitive tumors.

Intratumoral infection by CMV may change the tumor environment by directly interacting with tumor-associated macrophages to promote cancer immunity

Cytomegalovirus (CMV) is a herpesvirus that induces an extremely robust and sustained immune response. For this reason, CMV has been proposed as a vaccine vector to promote immunity to both pathogens and cancer. However, exploration of CMV as a vaccine vector is at an early stage and there are many questions. Using a mouse melanoma model, we recently found that a CMV-based vaccine induced large populations of melanoma-specific T cells, but was not effective at slowing tumor growth unless it was injected directly into the tumor. These surprising results have led us to hypothesize that CMV may be adept at modulating the tumor micro-environment through its infection of macrophages. Importantly, injection of CMV into the growing tumor synergized with blockade of the PD-1 checkpoint to clear well-established tumors. Here, we discuss our results in the context of CMV-based vaccines for pathogens and cancer.

Vaccine development for respiratory syncytial virus

Respiratory syncytial virus (RSV) is an important and ubiquitous respiratory pathogen for which no vaccine is available notwithstanding more than 50 years of effort. It causes the most severe disease at the extremes of age and in settings of immunodeficiency. Although RSV is susceptible to neutralizing antibody, it has evolved multiple mechanisms of immune evasion allowing it to repeatedly infect people despite relatively little genetic diversity. Recent breakthroughs in determining the structure and antigenic content of the fusion (F) glycoprotein in its metastable untriggered prefusion form (pre-F) and the stable rearranged postfusion form (post-F) have yielded vaccine strategies that can induce potent neutralizing antibody responses and effectively boost pre-existing neutralizing activity. In parallel, novel live-attenuated and chimeric virus vaccine candidates and other novel approaches to deliver vaccine antigens have been developed. These events and activities have aroused optimism and a robust pipeline of potential vaccine products that promise to provide a means to reduce the public health burden of RSV infection.