Combined local and systemic immunization is essential for durable T-cell mediated heterosubtypic immunity against influenza A virus

Mucosal tumor vaccination delivering endogenous tumor antigens protects against pulmonary breast cancer metastases

BACKGROUND

Generally, early-stage breast cancer has a good prognosis. However, if it spreads systemically, especially with pulmonary involvement, prospects worsen dramatically. Importantly, tumor-infiltrating T cells contribute to tumor control, particularly intratumoral T cells with a tissue-resident memory phenotype are associated with an improved clinical outcome.

METHODS

Here, we use an adenoviral vector vaccine encoding endogenous tumor-associated antigens adjuvanted with interleukin-1β to induce tumor-specific tissue-resident memory T cells (TRM) in the lung for the prevention and treatment of pulmonary metastases in the murine 4T1 breast cancer model.

RESULTS

The mucosal delivery of the vaccine was highly efficient in establishing tumor-specific TRM in the lung. Concomitantly, a single mucosal vaccination reduced the growth of pulmonary metastases and improved the survival in a prophylactic treatment. Vaccine-induced TRM contributed to these protective effects. In a therapeutic setting, the vaccination induced a pronounced T cell infiltration into metastases but resulted in only a minor restriction of the disease progression. However, in combination with stereotactic radiotherapy, the vaccine increased the survival time and rate of tumor-bearing mice.

CONCLUSION

In summary, our study demonstrates that mucosal vaccination is a promising strategy to harness the power of antitumor TRM and its potential combination with state-of-the-art treatments.

Both Humoral and Cellular Immunity Limit the Ability of Live Attenuated Influenza Vaccines to Promote T Cell Responses

One potential advantage of live attenuated influenza vaccines (LAIVs) is their ability to establish both virus-specific Ab and tissue-resident memory T cells (TRM) in the respiratory mucosa. However, it is hypothesized that pre-existing immunity from past infections and/or immunizations prevents LAIV from boosting or generating de novo CD8+ T cell responses. To determine whether we can overcome this limitation, we generated a series of drifted influenza A/PR8 LAIVs with successive mutations in the hemagglutinin protein, allowing for increasing levels of escape from pre-existing Ab. We also inserted a CD8+ T cell epitope from the Sendai virus nucleoprotein (NP) to assess both generation of a de novo T cell response and boosting of pre-existing influenza-specific CD8+ T cells following LAIV immunization. Increasing the level of escape from Ab enabled boosting of pre-existing TRM, but we were unable to generate de novo Sendai virus NP+ CD8+ TRM following LAIV immunization in PR8 influenza-immune mice, even with LAIV strains that can fully escape pre-existing Ab. As these data suggested a role for cell-mediated immunity in limiting LAIV efficacy, we investigated several scenarios to assess the impact of pre-existing LAIV-specific TRM in the upper and lower respiratory tract. Ultimately, we found that deletion of the immunodominant influenza NP366-374 epitope allowed for sufficient escape from cellular immunity to establish de novo CD8+ TRM. When combined, these studies demonstrate that both pre-existing humoral and cellular immunity can limit the effectiveness of LAIV, which is an important consideration for future design of vaccine vectors against respiratory pathogens.

Vaccine induced memory CD8+ T cells efficiently prevent viral transmission from the respiratory tract

Introduction

Mucosal immunization eliciting local T-cell memory has been suggested for improved protection against respiratory infections caused by viral variants evading pre-existing antibodies. However, it remains unclear whether T-cell targeted vaccines suffice for prevention of viral transmission and to which extent local immunity is important in this context.

Methods

To study the impact of T-cell vaccination on the course of viral respiratory infection and in particular the capacity to inhibit viral transmission, we used a mouse model involving natural murine parainfluenza infection with a luciferase encoding virus and an adenovirus based nucleoprotein targeting vaccine.

Results and discussion

Prior intranasal immunization inducing strong mucosal CD8+ T cell immunity provided an almost immediate shut-down of the incipient infection and completely inhibited contact based viral spreading. If this first line of defense did not operate, as in parentally immunized mice, recirculating T cells participated in accelerated viral control that reduced the intensity of inter-individual transmission. These observations underscore the importance of pursuing the development of mucosal T-cell inducing vaccines for optimal protection of the individual and inhibition of inter-individual transmission (herd immunity), while at the same time explain why induction of a strong systemic T-cell response may still impact viral transmission.

Vaccination of squirrel monkeys (Saimiri spp.) with nanoparticle based-Toxoplasma gondii antigens: new hope for captive susceptible species

Squirrel monkeys (Saimiri spp.), new world primates from South America, are very susceptible to toxoplasmosis. Numerous outbreaks of fatal toxoplasmosis in zoos have been identified around the world, resulting in acute respiratory distress and sudden death. To date, preventive hygiene measures or available treatments are not able to significantly reduce this mortality in zoos. Therefore, vaccination seems to be the best long-term solution to control acute toxoplasmosis. Recently, we developed a nasal vaccine composed of total extract of soluble proteins of Toxoplasma gondii associated with muco-adhesive maltodextrin-nanoparticles. The vaccine, which generated specific cellular immune responses, demonstrated efficacy against toxoplasmosis in murine and ovine experimental models. In collaboration with six French zoos, our vaccine was used as a last resort in 48 squirrel monkeys to prevent toxoplasmosis. The full protocol of vaccination includes two intranasal sprays followed by combined intranasal and s.c. administration. No local or systemic side-effects were observed irrespective of the route of administration. Blood samples were collected to study systemic humoral and cellular immune responses up to 1 year after the last vaccination. Vaccination induced a strong and lasting systemic cellular immune response mediated by specific IFN-γ secretion by peripheral blood mononuclear cells. Since the introduction of vaccination, no deaths of squirrel monkeys due to T. gondii has been observed for more than 4 years suggesting the promising usage of our vaccine. Moreover, to explain the high susceptibility of naive squirrel monkeys to toxoplasmosis, their innate immune sensors were investigated. It was observed that Toll-like and Nod-like receptors appear to be functional following T. gondii recognition suggesting that the extreme susceptibility to toxoplasmosis may not be linked to innate detection of the parasite.

SARS-CoV-2 Spike Protein-Activated Dendritic Cell-Derived Extracellular Vesicles Induce Antiviral Immunity in Mice

The onset and spread of the SARS-CoV-2 virus have created an unprecedented universal crisis. Although vaccines have been developed against the parental SARS-CoV-2, outbreaks of the disease still occur through the appearance of different variants, suggesting a continuous need for improved and effective therapeutic strategies. Therefore, we developed a novel nanovesicle presenting Spike protein on the surface of the dendritic cell-derived extracellular vesicles (DEVs) for use as a potential vaccine platform against SARS-CoV-2. DEVs express peptide/MHC-I (pMHC-I) complexes, CCR-7, on their surface. The immunogenicity and efficacy of the Spike-activated DEVs were tested in mice and compared with free Spike protein. A 1/10 Spike equivalent dose of DEVs showed a superior potency in inducing anti-Spike IgG titers in blood of mice when compared to dendritic cells or free Spike protein treatment. Moreover, DEV-induced sera effectively reduced viral infection by 55-60% within 15 days of booster dose administration. Furthermore, a 1/10 Spike equivalent dose of DEV-treated mice was found to be equally effective in inducing CD19+CD38+ T-cells in the spleen and lymph node; CD8 cells in the bone marrow, spleen, and lymph node; and CD4+CD25+ T-cells in the spleen and lymph node after 90 days of treatment. Thus, our results support the immunogenic nature of DEVs, demonstrating that a low dose of DEVs induces antibodies to inhibit SARS-CoV-2 infection in vitro, therefore warranting further investigations.

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 cells in SARS-CoV-2 infection and vaccination

While antibodies garner the lion’s share of attention in SARS-CoV-2 immunity, cellular immunity (T cells) may be equally, if not more important, in controlling infection. Both CD8⁺ and CD4⁺ T cells are elicited earlier and are associated with milder disease, than antibodies, and T-cell activation appears to be necessary for control of infection. Variants of concern (VOCs) such as Omicron have escaped the neutralizing antibody responses after two mRNA vaccine doses, but T-cell immunity is largely intact. The breadth and patient-specific nature of the latter offers a formidable line of defense that can limit the severity of illness, and are likely to be responsible for most of the protection from natural infection or vaccination against VOCs which have evaded the antibody response. Comprehensive searches for T-cell epitopes, T-cell activation from infection and vaccination of specific patient groups, and elicitation of cellular immunity by various alternative vaccine modalities are here reviewed. Development of vaccines that specifically target T cells is called for, to meet the needs of patient groups for whom cellular immunity is weaker, such as the elderly and the immunosuppressed. While VOCs have not yet fully escaped T-cell immunity elicited by natural infection and vaccines, some early reports of partial escape suggest that future VOCs may achieve the dreaded result, dislodging a substantial proportion of cellular immunity, enough to cause a grave public health burden. A proactive, rather than reactive, solution which identifies and targets immutable sequences in SARS-CoV-2, not just those which are conserved, may be the only recourse humankind has to disarm these future VOCs before they disarm us.

Antiviral Effects of ABMA and DABMA against Influenza Virus In Vitro and In Vivo via Regulating the Endolysosomal Pathway and Autophagy

Influenza virus is an acute and highly contagious respiratory pathogen that causes great concern to public health and for which there is a need for extensive drug discovery. The small chemical compound ABMA and its analog DABMA, containing an adamantane or a dimethyl-adamantane group, respectively, have been demonstrated to inhibit multiple toxins (diphtheria toxin, Clostridium difficile toxin B, Clostridium sordellii lethal toxin) and viruses (Ebola, rabies virus, HSV-2) by acting on the host’s vesicle trafficking. Here, we showed that ABMA and DABMA have antiviral effects against both amantadine-sensitive influenza virus subtypes (H1N1 and H3N2), amantadine-resistant subtypes (H3N2), and influenza B virus with EC₅₀ values ranging from 2.83 to 7.36 µM (ABMA) and 1.82 to 6.73 µM (DABMA), respectively. ABMA and DABMA inhibited the replication of influenza virus genomic RNA and protein synthesis by interfering with the entry stage of the virus. Molecular docking evaluation together with activity against amantadine-resistant influenza virus strains suggested that ABMA and DABMA were not acting as M2 ion channel blockers. Subsequently, we found that early internalized H1N1 virions were retained in accumulated late endosome compartments after ABMA treatment. Additionally, ABMA disrupted the early stages of the H1N1 life cycle or viral RNA synthesis by interfering with autophagy. ABMA and DABMA protected mice from an intranasal H1N1 challenge with an improved survival rate of 67%. The present study suggests that ABMA and DABMA are potential antiviral leads for the development of a host-directed treatment against influenza virus infection.

A Novel H-2d Epitope for Influenza A Polymerase Acidic Protein

Understanding the complexity of the T-cell epitope hierarchy in humans through mouse models can be difficult. In particular, using only one murine strain, the C57BL/6 mouse, to investigate the immune response to influenza virus infection limits our understanding. In the present study, by immunizing C57BL/6 mice with an adenoviral vector encoding the polymerase acidic (AdIiPA) protein of influenza A virus, we were able to induce a high number of PA-specific T cells. However, upon challenge, these cells were only partly protective. When instead immunizing BALB/c mice with AdIiPA, we found that the immunized mice were fully protected against challenge. We found that this protection was dependent on CD8 T cells, and we identified a novel H-2Dd-restricted epitope, PA33. These findings provide a new tool for researchers to study PA-specific immunity in mice with an H-2d haplotype. Additionally, our findings underscore the importance of critically evaluating important limitations of using a single inbred mouse strain in vaccine studies.

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.

Improved immunologic responses to heterologous influenza strains in children with low preexisting antibody response vaccinated with MF59-adjuvanted influenza vaccine

Vaccination is the most effective approach to reduce the substantial morbidity and mortality caused by influenza infection. Vaccine efficacy is highly sensitive to antigenic changes causing differences between circulating and vaccine viruses. Adjuvants such as MF59 increase antibody-mediated cross-reactive immunity and therefore may provide broader seasonal protection. A recent clinical trial showed that an MF59-adjuvanted vaccine was more efficacious than a nonadjuvanted comparator in subjects < 2 years of age, although not in those ≥ 2 years, during influenza seasons in which the predominant circulating virus was an A/H3N2 strain that was antigenically different from the vaccine virus. This finding suggested that the increased efficacy of the adjuvanted vaccine in younger subjects may be mediated by strain cross-reactive antibodies. A subset of the trial population, representing subjects with distinct age and/or immunological history, was tested for antibody responses to the vaccine A/H3N2 strain as well as A/H3N2 drifted strains antigenically matching the viruses circulating during the trial seasons. The neutralizing tests showed that, compared with nonadjuvanted vaccine, the adjuvanted vaccine improved not only the neutralizing antibody response to the vaccine strain but also the cross-reactive antibody response to the drifted strains in subjects with lower preexisting antibody titers, regardless of their age or vaccine history. The results demonstrated an immunological benefit and suggested a potential efficacy benefit by adjuvanted vaccine in subjects with lower preexisting antibody responses.

Effect of an Adenovirus-Vectored Universal Influenza Virus Vaccine on Pulmonary Pathophysiology in a Mouse Model

Current influenza vaccines, live attenuated or inactivated, do not protect against antigenically novel influenza A viruses (IAVs) of pandemic potential, which has driven interest in the development of universal influenza vaccines. Universal influenza vaccine candidates targeting highly conserved antigens of IAV nucleoprotein (NP) are promising as vaccines that induce T cell immunity, but concerns have been raised about the safety of inducing robust CD8 T cell responses in the lungs. Using a mouse model, we systematically evaluated effects of recombinant adenovirus vectors (rAd) expressing IAV NP (A/NP-rAd) or influenza B virus (IBV) NP (B/NP-rAd) on pulmonary inflammation and function after vaccination and following live IAV challenge. After A/NP-rAd or B/NP-rAd vaccination, female mice exhibited robust systemic and pulmonary vaccine-specific B cell and T cell responses and experienced no morbidity (e.g., body mass loss). Both in vivo pulmonary function testing and lung histopathology scoring revealed minimal adverse effects of intranasal rAd vaccination compared with unvaccinated mice. After IAV challenge, A/NP-rAd-vaccinated mice experienced significantly less morbidity, had lower pulmonary virus titers, and developed less pulmonary inflammation than unvaccinated or B/NP-rAd-vaccinated mice. Based on analysis of pulmonary physiology using detailed testing not previously applied to the question of T cell damage, mice protected by vaccination also had better lung function than controls. Results provide evidence that, in this model, adenoviral universal influenza vaccine does not damage pulmonary tissue. In addition, adaptive immunity, in particular, T cell immunity in the lungs, does not cause damage when restimulated but instead mitigates pulmonary damage following IAV infection.IMPORTANCE Respiratory viruses can emerge and spread rapidly before vaccines are available. It would be a tremendous advance to use vaccines that protect against whole categories of viruses, such as universal influenza vaccines, without the need to predict which virus will emerge. The nucleoprotein (NP) of influenza virus provides a target conserved among strains and is a dominant T cell target. In animals, vaccination to NP generates powerful T cell immunity and long-lasting protection against diverse influenza strains. Concerns have been raised, but not evaluated experimentally, that potent local T cell responses might damage the lungs. We analyzed lung function in detail in the setting of such a vaccination. Despite CD8 T cell responses in the lungs, lungs were not damaged and functioned normally after vaccination alone and were protected upon subsequent infection. This precedent provides important support for vaccines based on T cell-mediated protection, currently being considered for both influenza and SARS-CoV-2 vaccines.

Robust induction of TRMs by combinatorial nanoshells confers cross-strain sterilizing immunity against lethal influenza viruses

Antigen-specific lung-resident memory T cells (T_RMs) constitute the first line of defense that mediates rapid protection against respiratory pathogens and inspires novel vaccine designs against infectious pandemic threats, yet effective means of inducing T_RMs, particularly via non-viral vectors, remain challenging. Here, we demonstrate safe and potent induction of lung-resident T_RMs using a biodegradable polymeric nanoshell that co-encapsulates antigenic peptides and TLR9 agonist CpG-oligodeoxynucleotide (CpG-ODN) in a virus-mimicking structure. Through subcutaneous priming and intranasal boosting, the combinatorial nanoshell vaccine elicits prominent lung-resident CD4⁺ and CD8⁺ T cells that surprisingly show better durability than live viral infections. In particular, nanoshells containing CpG-ODN and a pair of conserved class I and II major histocompatibility complex-restricted influenza nucleoprotein-derived antigenic peptides are demonstrated to induce near-sterilizing immunity against lethal infections with influenza A viruses of different strains and subtypes in mice, resulting in rapid elimination of replicating viruses. We further examine the pulmonary transport dynamic and optimal composition of the nanoshell vaccine conducive for robust T_RM induction as well as the benefit of subcutaneous priming on T_RM replenishment. The study presents a practical vaccination strategy for inducing protective T_RM-mediated immunity, offering a compelling platform and critical insights in the ongoing quest toward a broadly protective vaccine against universal influenza as well as other respiratory pathogens.

Adenovirus Vectors: Excellent Tools for Vaccine Development

Adenovirus was originally used as a vector for gene therapy. In recent years, with the development of the next-generation vectors with increased safety and high immunogenicity to transgene products, its utility as a vaccine vector has continued to increase. Adenovirus-based vaccines are currently being tested not only to prevent various infectious diseases but also to be applied as cancer vaccines. In this review, I discuss the innate and adaptive aspects of the immunological characteristics of adenovirus vectors and further examine the current status of advanced adenovirus-based vaccine development. Various methods that can overcome the limitations of currently used adenoviruses as vaccine vehicles are also discussed. Through this study, I hope that vaccine development using adenovirus vectors will be expedited and more successful.

Simultaneous Aerosol and Intramuscular Immunization with Influenza Vaccine Induces Powerful Protective Local T Cell and Systemic Antibody Immune Responses in Pigs

A vaccine providing both powerful Ab and cross-reactive T cell immune responses against influenza viruses would be beneficial for both humans and pigs. In this study, we evaluated i.m., aerosol (Aer), and simultaneous systemic and respiratory immunization (SIM) by both routes in Babraham pigs, using the single cycle candidate influenza vaccine S-FLU. After prime and boost immunization, pigs were challenged with H1N1pdm09 virus. i.m.-immunized pigs generated a high titer of neutralizing Abs but poor T cell responses, whereas Aer induced powerful respiratory tract T cell responses but a low titer of Abs. SIM pigs combined high Ab titers and strong local T cell responses. SIM showed the most complete suppression of virus shedding and the greatest improvement in pathology. We conclude that SIM regimes for immunization against respiratory pathogens warrant further study.

T Cell Immunity against Influenza: The Long Way from Animal Models Towards a Real-Life Universal Flu Vaccine

Current flu vaccines rely on the induction of strain-specific neutralizing antibodies, which leaves the population vulnerable to drifted seasonal or newly emerged pandemic strains. Therefore, universal flu vaccine approaches that induce broad immunity against conserved parts of influenza have top priority in research. Cross-reactive T cell responses, especially tissue-resident memory T cells in the respiratory tract, provide efficient heterologous immunity, and must therefore be a key component of universal flu vaccines. Here, we review recent findings about T cell-based flu immunity, with an emphasis on tissue-resident memory T cells in the respiratory tract of humans and different animal models. Furthermore, we provide an update on preclinical and clinical studies evaluating T cell-evoking flu vaccines, and discuss the implementation of T cell immunity in real-life vaccine policies.

Long-term maintenance of lung resident memory T cells is mediated by persistent antigen

Tissue-resident memory T cells (T_RM) in the lungs are pivotal for protection against repeated infection with respiratory viruses. However, the gradual loss of these cells over time and the associated decline in clinical protection represent a serious limit in the development of efficient T cell based vaccines against respiratory pathogens. Here, using an adenovirus expressing influenza nucleoprotein (AdNP), we show that CD8 T_RM in the lungs can be maintained for at least 1 year post vaccination. Our results reveal that lung T_RM continued to proliferate in situ 8 months after AdNP vaccination. Importantly, this required airway vaccination and antigen persistence in the lung, as non-respiratory routes of vaccination failed to support long-term lung T_RM maintenance. In addition, parabiosis experiments show that in AdNP vaccinated mice, the lung T_RM pool is also sustained by continual replenishment from circulating memory CD8 T cells that differentiate into lung T_RM, a phenomenon not observed in influenza-infected parabiont partners. Concluding, these results demonstrate key requirements for long-lived cellular immunity to influenza virus, knowledge that could be utilized in future vaccine design.

A Systemic Prime-Intrarectal Pull Strategy Raises Rectum-Resident CD8+ T Cells for Effective Protection in a Murine Model of LM-OVA Infection

As the entry sites of many pathogens such as human immunodeficiency virus (HIV), mucosal sites are defended by rapidly reacting resident memory T cells (TRM). TRMs represent a special subpopulation of memory T cells that persist long term in non-lymphoid sites without entering the circulation and provide the “sensing and alarming” role in the first-line defense against infection. The rectum and vagina are the two primary mucosal portals for HIV entry. However, compared to vaginal TRM, rectal TRM is poorly understood. Herein, we investigated the optimal vaccination strategy to induce rectal TRM. We identified an intranasal prime–intrarectal boost (pull) strategy that is effective in engaging rectal TRM alongside circulating memory T cells and demonstrated its protective efficacy in mice against infection of Listeria monocytogenes. On the contrary, the same vaccine delivered via either intranasal or intrarectal route failed to raise rectal TRM, setting it apart from vaginal TRM, which can be induced by both intranasal and intrarectal immunizations. Moreover, intramuscular prime was also effective in inducing rectal TRM in combination with intrarectal pull, highlighting the need of a primed systemic T cell response. A comparison of different pull modalities led to the identification that raising rectal TRM is mainly driven by local antigen presence. We further demonstrated the interval between prime and boost steps to be critical for the induction of rectal TRM, revealing circulating recently activated CD8+ T cells as the likely primary pullable precursor of rectal TRM. Altogether, our studies lay a new framework for harnessing rectal TRM in vaccine development.

Adenoviral Vector-Based Vaccine Platforms for Developing the Next Generation of Influenza Vaccines

Ever since the discovery of vaccines, many deadly diseases have been contained worldwide, ultimately culminating in the eradication of smallpox and polio, which represented significant medical achievements in human health. However, this does not account for the threat influenza poses on public health. The currently licensed seasonal influenza vaccines primarily confer excellent strain-specific protection. In addition to the seasonal influenza viruses, the emergence and spread of avian influenza pandemic viruses such as H5N1, H7N9, H7N7, and H9N2 to humans have highlighted the urgent need to adopt a new global preparedness for an influenza pandemic. It is vital to explore new strategies for the development of effective vaccines for pandemic and seasonal influenza viruses. The new vaccine approaches should provide durable and broad protection with the capability of large-scale vaccine production within a short time. The adenoviral (Ad) vector-based vaccine platform offers a robust egg-independent production system for manufacturing large numbers of influenza vaccines inexpensively in a short timeframe. In this review, we discuss the progress in the development of Ad vector-based influenza vaccines and their potential in designing a universal influenza vaccine.

A Vaccine Displaying a Trimeric Influenza-A HA Stem Protein on Capsid-Like Particles Elicits Potent and Long-Lasting Protection in Mice

Due to constant antigenic drift and shift, current influenza-A vaccines need to be redesigned and administered annually. A universal flu vaccine (UFV) that provides long-lasting protection against both seasonal and emerging pandemic influenza strains is thus urgently needed. The hemagglutinin (HA) stem antigen is a promising target for such a vaccine as it contains neutralizing epitopes, known to induce cross-protective IgG responses against a wide variety of influenza subtypes. In this study, we describe the development of a UFV candidate consisting of a HA_stem trimer displayed on the surface of rigid capsid-like particles (CLP). Compared to soluble unconjugated HA_stem trimer, the CLP-HA_stem particles induced a more potent, long-lasting immune response and were able to protect mice against both homologous and heterologous H1N1 influenza challenge, even after a single dose.

Memory T Cell Dynamics in the Lung during Influenza Virus Infection

Influenza A virus is highly contagious, infecting 5-15% of the global population every year. It causes significant morbidity and mortality, particularly among immunocompromised and at-risk individuals. Influenza virus is constantly evolving, undergoing continuous, rapid, and unpredictable mutation, giving rise to novel viruses that can escape the humoral immunity generated by current influenza virus vaccines. Growing evidence indicates that influenza-specific T cells resident along the respiratory tract are highly effective at providing potent and rapid protection against this inhaled pathogen. As these T cells recognize fragments of the virus that are highly conserved and less prone to mutation, they have the potential to provide cross-strain protection against a wide breadth of influenza viruses, including newly emerging strains. In this review, we will discuss how influenza-specific memory T cells in the lung are established and maintained and how we can harness this knowledge to design broadly protective influenza A virus vaccines.

CD8+ T cells induced by adenovirus-vectored vaccine are capable of preventing establishment of latent murine γ-herpesvirus 68 infection

CD8+ T cells are known to control infections, but their role in preventing latent infection from establishing has not been thoroughly investigated. We hypothesized that a potent CD8+ T cell response patrolling the mucosal viral entry points could kill the first infected cells and thereby abrogate the infection before latency is established. To investigate this, replication deficient adenovirus serotype 5 vectors encoding murine γ-herpesvirus-68 CD8+ T cell epitopes linkedto the T cell adjuvant Invariant chain, were developed. We show that intranasal vaccination of mice reduces the risk of establishment of latent infection from multiple intranasal ID50 challenges with murine γ-herpesvirus-68 by 81% per exposure at 14 days post vaccination. Protection waned over time, but immune responses were extended by heterologous prime-boost vaccination applied simultaneously intramuscularly and intranasally, and animals vaccinated 66 days prior to challenge showed a strong trend of long-term protection. Our data provides evidence that CD8+ T cells are able to protect against establishment of latent infection. Although the protective efficacy is difficult to maintain over time, this proof-of-concept study suggests a role for a CD8+ T cell arm in future vaccine strategies against latent human viral infections caused by pathogens such as HIV and multiple herpes virus.

Local Antigen Encounter Is Essential for Establishing Persistent CD8+ T-Cell Memory in the CNS

While the brain is considered an immune-privileged site, the CNS may nevertheless be the focus of immune mediated inflammation in the case of infection and certain autoimmune diseases, e.g., multiple sclerosis. As in other tissues, it has been found that acute T-cell infiltration may be followed by establishment of persistent local T-cell memory. To improve our understanding regarding the regulation of putative tissue resident memory T (Trm) cells in CNS, we devised a new model system for studying the generation of Trm cells in this site. To this purpose, we exploited the fact that the CNS may be a sanctuary for adenoviral infection, and to minimize virus-induced disease, we chose replication-deficient adenoviruses for infection of the CNS. Non-replicating adenoviruses are known to be highly immunogenic, and our studies demonstrate that intracerebral inoculation causes marked local T-cell recruitment, which is followed by persistent infiltration of the CNS parenchyma by antigen specific CD8⁺ T cells. Phenotypical analysis of CNS infiltrating antigen specific CD8⁺ T cells was consistent with these cells being Trms. Regarding the long-term stability of the infiltrate, resident CD8⁺ T cells expressed high levels of the anti-apoptotic molecule Bcl-2 as well as the proliferation marker Ki-67 suggesting that the population is maintained through steady homeostatic proliferation. Functionally, memory CD8⁺ T cells from CNS matched peripheral memory cells with regard to capacity for ex vivo cytotoxicity and cytokine production. Most importantly, our experiments revealed a key role for local antigen encounter in the establishment of sustained CD8⁺ T-cell memory in the brain. Inflammation in the absence of cognate antigen only led to limited and transient infiltration by antigen specific CD8⁺ T cells. Together these results indicate that memory CD8⁺ T cells residing in the CNS predominantly mirror previous local infections and immune responses to local autoantigens.

A single cycle influenza virus coated in H7 haemagglutinin generates neutralizing antibody responses to haemagglutinin and neuraminidase glycoproteins and protection from heterotypic challenge

A non-replicating form of pseudotyped influenza virus, inactivated by suppression of the haemagglutinin signal sequence (S-FLU), can act as a broadly protective vaccine. S-FLU can infect for a single round only, and induces heterotypic protection predominantly through activation of cross-reactive T cells in the lung. Unlike the licensed live attenuated virus, it cannot reassort a pandemic haemagglutinin (HA) into seasonal influenza. Here we present data on four new forms of S-FLU coated with H7 HAs from either A/Anhui/1/2013, A/Shanghai/1/2013, A/Netherlands/219/2003 or A/New York/107/2003 strains of H7 virus. We show that intranasal vaccination induced a strong local CD8 T cell response and protected against heterosubtypic X31 (H3N2) virus and highly virulent PR8 (H1N1), but not influenza B virus. Intranasal vaccination also induced a strong neutralizing antibody response to the encoded neuraminidase. If given at higher dose in the periphery with intraperitoneal administration, H7 S-FLU induced a specific neutralizing antibody response to H7 HA coating the particle. Polyvalent intraperitoneal vaccination with mixed H7 S-FLU induced a broadly neutralizing antibody response to all four H7 strains. S-FLU is a versatile vaccine candidate that could be rapidly mobilized ahead of a new pandemic threat.

Vaccine-induced antigen-specific regulatory T cells attenuate the antiviral immunity against acute influenza virus infection

Peptide-based T cell vaccines targeting the conserved epitopes of influenza virus can provide cross-protection against distantly related strains, but they are generally not immunogenic. Foreign antigen-specific regulatory T (Treg) cells are induced under subimmunogenic conditions peripherally, although their development and role in vaccine-mediated antiviral immunity is unclear. Here, we demonstrated primary vaccination with peptides alone significantly induced antigen-specific Foxp3⁺ Treg cells, which were further expanded by repeated vaccination with unadjuvanted peptides. Certain adjuvants, including CpG, suppressed the induction and expansion of antigen-specific Treg cells by peptide vaccination. Interestingly, secondary influenza virus infection significantly increased the frequency of preexisting antigen-specific Treg cells, although primary infection barely induced them. Importantly, specific depletion of vaccine-induced antigen-specific Treg cells promoted influenza viral clearance, indicating their inhibitory role in vivo. Immunization with CpG-adjuvanted peptides by the subcutaneous prime-intranasal-boost strategy restricted the recruitment and accumulation of antigen-specific Treg cells in lung, and stimulated robust T cell immunity. Finally, subcutaneous prime-intranasal-boost immunization with CpG-adjuvanted peptides or whole-inactivated influenza vaccines protected mice from heterosubtypic influenza virus infection. In conclusion, antigen-specific Treg cells induced by peptide vaccines attenuate the antiviral immunity against influenza virus infection. CpG-adjuvanted peptide vaccines provide heterosubtypic influenza protection probably by inhibiting Treg development and enhancing T cell immunity.

Analysis of adenovirus-induced immunity to infection with Listeria monocytogenes: Fading protection coincides with declining CD8 T cell numbers and phenotypic changes

Defining correlates of T cell mediated protection is important in order to accelerate the development of efficient T cell based vaccines conferring long-term immunity. Extensive studies have provided important insight regarding the characteristics and functional properties of the effector and memory CD8 T cells induced by viral vector based vaccines. However, long-term protection has been difficult to achieve with T cell inducing vaccines, and the determinants underlying this loss in protection over time are still not fully defined. In this study we analyzed different parameters of the CD8 T cell response as a function of time after vaccination with a human serotype 5 adenovector expressing the glycoprotein (GP) of LCMV tethered to the MHC class II-associated invariant chain. Using this vector we have previously found that CD8 T cells mediate protection from challenge with GP-expressing Listeria monocytogenes at 60 days post vaccination, but only little protection after further 60 days, and we now confirm this observation. A comparison of vaccine-primed CD8 T cells early and late after vaccination revealed a minor decline in the overall numbers of antigen specific memory CD8 T cells during this interval. More importantly, we also observed phenotypic changes over time with a distinct decline in the frequency and number of KLRG1⁺ CD8 T cells, and, notably, adoptive transfer studies confirmed that memory CD8 T cells expressing KLRG1 are central to protection from systemic L. monocytogenes infection. Together these findings imply that multiple factors including changes in memory T cell numbers and phenotypic composition over time influence the longevity of CD8 T-cell mediated protection.

Seasonal Influenza Split Vaccines Confer Partial Cross-Protection against Heterologous Influenza Virus in Ferrets When Combined with the CAF01 Adjuvant

Influenza epidemics occur annually, and estimated 5–10% of the adult population and 20–30% of children will become ill from influenza infection. Seasonal vaccines primarily work through the induction of neutralizing antibodies against the principal surface antigen hemagglutinin (HA). This important role of HA-specific antibodies explains why previous pandemics have emerged when new HAs have appeared in circulating human viruses. It has long been recognized that influenza virus-specific CD4(+) T cells are important in protection from infection through direct effector mechanisms or by providing help to B cells and CD8(+) T cells. However, the seasonal influenza vaccine is poor at inducing CD4(+) T-cell responses and needs to be combined with an adjuvant facilitating this response. In this study, we applied the ferret model to investigate the cross-protective efficacy of a heterologous trivalent influenza split-virion (TIV) vaccine adjuvanted with the CAF01 adjuvant, with proven ability to induce CD4(+) T-cell and antibody responses in mice, ferrets, pigs, primates, and humans. Our results indicate that CAF01-adjuvanted vaccine induces HA inhibition (HAI)-independent protection after heterologous challenge, manifested as reduced viral load and fever. On the other hand, we observe increased inflammation in the airways and more neutrophil and mononuclear cell infiltration in these ferrets when compared with optimally protected animals, i.e., ferrets receiving the same vaccine but a homologous challenge. This suggest that HAI-independent immunity induced by TIV + CAF01 can reduce viral shedding and systemic disease symptoms, but does not reduce local inflammation in the nasal cavity.

Mucosal Vaccination with Heterologous Viral Vectored Vaccine Targeting Subdominant SIV Accessory Antigens Strongly Inhibits Early Viral Replication

Conventional HIV T cell vaccine strategies have not been successful in containing acute peak viremia, nor in providing long-term control. We immunized rhesus macaques intramuscularly and rectally using a heterologous adenovirus vectored SIV vaccine regimen encoding normally weakly immunogenic tat, vif, rev and vpr antigens fused to the MHC class II associated invariant chain. Immunizations induced broad T cell responses in all vaccinees. Following up to 10 repeated low-dose intrarectal challenges, vaccinees suppressed early viral replication (P = 0.01) and prevented the peak viremia in 5/6 animals. Despite consistently undetectable viremia in 2 out of 6 vaccinees, all animals showed evidence of infection induced immune responses indicating that infection had taken place. Vaccinees, with and without detectable viremia better preserved their rectal CD4 + T cell population and had reduced immune hyperactivation as measured by naïve T cell depletion, Ki-67 and PD-1 expression on T cells. These results indicate that vaccination towards SIV accessory antigens vaccine can provide a level of acute control of SIV replication with a suggestion of beneficial immunological consequences in infected animals of unknown long-term significance.

In conclusion, our studies demonstrate that a vaccine encoding subdominant antigens not normally associated with virus control can exert a significant impact on acute peak viremia.

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Mucosal heterologousvirus-vectored vaccine used with MHC class II associated invariant chain linked SIV accessory antigens

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Mucosal vaccination targeting subdominant antigens delay SIV mac251 replication in rhesus macaques.

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Longterm reduction of immune hyperactivation following SIV infection of vaccinated macaques.

Mucosal immunization is used with heterologous viral vectors and a genetic adjuvant to raise responses against poorly immunogenic SIV antigens. Following repeated low-dose challenge we observed delayed establishment of chronic phase viremia and reduced immune hyperactivation 6 months after established infection. Vaccination was found to strongly reduce viremia at early, but not late time points, after detected infection and in 2 out of 6 animals infection could only observed as virus induced T cell responses. Subdominant antigen vaccines may thus be used to delay SIV mac251 infection and can enable control of chronic viremia in a minority of cases.

PB1 as a potential target for increasing the breadth of T-cell mediated immunity to Influenza A

Recently, we showed that combined intranasal and subcutaneous immunization with a non-replicating adenoviral vector expressing NP of influenza A, strain PR8, induced long-standing protection against a range of influenza A viruses. However, H-2^b mice challenged with an influenza A strain mutated in the dominant NP₃₆₆ epitope were not efficiently protected. To address this problem, we envision the use of a cocktail of adenovectors targeting different internal proteins of influenza A virus. Consequently, we investigated the possibility of using PB1 as a target for an adenovector-based vaccine against influenza A. Our results showed that PB1 is not as immunogenic as the NP protein. However, by tethering PB1 to the murine invariant chain we were able to circumvent this problem and raise quite high numbers of PB1-specific CD8⁺ T cells in the circulation. Nevertheless, mice immunized against PB1 were not as efficiently protected against influenza A challenge as similarly NP-vaccinated animals. The reason for this is not a difference in the quality of the primed cells, nor in functional avidity. However, under similar conditions of immunization fewer PB1-specific cells were recruited to the airways, and surface expression of the dominant PB1 peptide, PB1₇₀₃, was less stable than in the case of NP₃₆₆.

Environmental cues orchestrate regional immune surveillance and protection by pulmonary CTLs

Tissue-resident memory CD8 T cells (T_RM) provide preemptive immunity against infections that begin in peripheral tissues by guarding the site of initial pathogen exposure. Their role in immunity to respiratory virus infection is particularly important because severe damage to the alveoli can be avoided when local populations of T_RM cells reduce viral burdens and dampen the responses of effector CD8 T cells in the lungs. Although a connection between rapid immune activation and early viral control is well established, the signals that keep T_RM cells poised for action in the local tissues remain poorly defined. Recent studies have shown that environmental cues influence the fate decisions of activated CTLs during memory formation. Manipulation of these signaling pathways could provide new ways to capitalize on protection from T_RM cells in mucosal tissues, while reducing collateral damage and pathology during vaccination.

Heterosubtypic T-Cell Immunity to Influenza in Humans: Challenges for Universal T-Cell Influenza Vaccines

Influenza A virus (IAV) remains a significant global health issue causing annual epidemics, pandemics, and sporadic human infections with highly pathogenic avian or swine influenza viruses. Current inactivated and live vaccines are the mainstay of the public health response to influenza, although vaccine efficacy is lower against antigenically distinct viral strains. The first pandemic of the twenty-first century underlined the urgent need to develop new vaccines capable of protecting against a broad range of influenza strains. Such “universal” influenza vaccines are based on the idea of heterosubtypic immunity, wherein immune responses to epitopes conserved across IAV strains can confer protection against subsequent infection and disease. T-cells recognizing conserved antigens are a key contributor in reducing viral load and limiting disease severity during heterosubtypic infection in animal models. Recent studies undertaken during the 2009 H1N1 pandemic provided key insights into the role of cross-reactive T-cells in mediating heterosubtypic protection in humans. This review focuses on human influenza to discuss the epidemiological observations that underpin cross-protective immunity, the role of T-cells as key players in mediating heterosubtypic immunity including recent data from natural history cohort studies and the ongoing clinical development of T-cell-inducing universal influenza vaccines. The challenges and knowledge gaps for developing vaccines to generate long-lived protective T-cell responses is discussed.