Memory CD4+ T cells protect against influenza through multiple synergizing mechanisms

Lung influenza virus-specific memory CD4 T cell location and optimal cytokine production are dependent on interactions with lung antigen-presenting cells

Influenza A virus (IAV) infection leads to the formation of mucosal memory CD4 T cells that can protect the host. An in-depth understanding of the signals that shape memory cell development is required for more effective vaccine design. We have examined the formation of memory CD4 T cells in the lung following IAV infection of mice, characterizing changes to the lung landscape and immune cell composition. IAV-specific CD4 T cells were found throughout the lung at both primary and memory time points. These cells were found near lung airways and in close contact with a range of immune cells including macrophages, dendritic cells, and B cells. Interactions between lung IAV-specific CD4 T cells and major histocompatibility complex (MHC)II+ cells during the primary immune response were important in shaping the subsequent memory pool. Treatment with an anti-MHCII blocking antibody increased the proportion of memory CD4 T cells found in lung airways but reduced interferon-γ expression by IAV-specific immunodominant memory CD4 T cells. The immunodominant CD4 T cells expressed higher levels of programmed death ligand 1 (PD1) than other IAV-specific CD4 T cells and PD1+ memory CD4 T cells were located further away from MHCII+ cells than their PD1-low counterparts. This distinction in location was lost in mice treated with anti-MHCII antibodies. These data suggest that sustained antigen presentation in the lung impacts the formation of memory CD4 T cells by regulating their cytokine production and location.

Opportunities and challenges for T cell-based influenza vaccines

Vaccination remains our main defence against influenza, which causes substantial annual mortality and poses a serious pandemic threat. Influenza virus evades immunity by rapidly changing its surface antigens but, even when the vaccine is well matched to the current circulating virus strains, influenza vaccines are not as effective as many other vaccines. Influenza vaccine development has traditionally focused on the induction of protective antibodies, but there is mounting evidence that T cell responses are also protective against influenza. Thus, future vaccines designed to promote both broad T cell effector functions and antibodies may provide enhanced protection. As we discuss, such vaccines present several challenges that require new strategic and economic considerations. Vaccine-induced T cells relevant to protection may reside in the lungs or lymphoid tissues, requiring more invasive assays to assess the immunogenicity of vaccine candidates. T cell functions may contain and resolve infection rather than completely prevent infection and early illness, requiring vaccine effectiveness to be assessed based on the prevention of severe disease and death rather than symptomatic infection. It can be complex and costly to measure T cell responses and infrequent clinical outcomes, and thus innovations in clinical trial design are needed for economic reasons. Nevertheless, the goal of more effective influenza vaccines justifies renewed and intensive efforts.

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

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

Cytomegalovirus vaccine vector-induced effector memory CD4 + T cells protect cynomolgus macaques from lethal aerosolized heterologous avian influenza challenge

An influenza vaccine approach that overcomes the problem of viral sequence diversity and provides long-lived heterosubtypic protection is urgently needed to protect against pandemic influenza viruses. Here, to determine if lung-resident effector memory T cells induced by cytomegalovirus (CMV)-vectored vaccines expressing conserved internal influenza antigens could protect against lethal influenza challenge, we immunize Mauritian cynomolgus macaques (MCM) with cynomolgus CMV (CyCMV) vaccines expressing H1N1 1918 influenza M1, NP, and PB1 antigens (CyCMV/Flu), and challenge with heterologous, aerosolized avian H5N1 influenza. All six unvaccinated MCM died by seven days post infection with acute respiratory distress, while 54.5% (6/11) CyCMV/Flu-vaccinated MCM survived. Survival correlates with the magnitude of lung-resident influenza-specific CD4 + T cells prior to challenge. These data demonstrate that CD4 + T cells targeting conserved internal influenza proteins can protect against highly pathogenic heterologous influenza challenge and support further exploration of effector memory T cell-based vaccines for universal influenza vaccine development.

A targeted single mutation in influenza A virus universal epitope transforms immunogenicity and protective immunity via CD4+ T cell activation

CD4+ T cells are central to adaptive immunity. Their role in cross-protection in viral infections such as influenza and severe acute respiratory syndrome (SARS) is well documented; however, molecular rules governing T cell receptor (TCR) engagement of peptide-human leukocyte antigen (pHLA) class II are less understood. Here, we exploit an aspect of HLA class II presentation, the peptide-flanking residues (PFRs), to "tune" CD4+ T cell responses within an in vivo model system of influenza. Using a recombinant virus containing targeted substitutions at immunodominant HLA-DR1 epitopes, we demonstrate limited weight loss and improved clinical scores after heterosubtypic re-challenge. We observe enhanced protection linked to lung-derived influenza-specific CD4+ and CD8+ T cells prior to re-infection. Structural analysis of the ternary TCR:pHLA complex identifies that flanking amino acids influence side chains in the core 9-mer peptide, increasing TCR affinity. Augmentation of CD4+ T cell immunity is achievable with a single mutation, representing a strategy to enhance adaptive immunity that is decoupled from vaccine modality.

Polyanhydride nanovaccine against H3N2 influenza A virus generates mucosal resident and systemic immunity promoting protection

Influenza A virus (IAV) causes significant morbidity and mortality worldwide due to seasonal epidemics and periodic pandemics. The antigenic drift/shift of IAV continually gives rise to new strains and subtypes, aiding IAV in circumventing previously established immunity. As a result, there has been substantial interest in developing a broadly protective IAV vaccine that induces, durable immunity against multiple IAVs. Previously, a polyanhydride nanoparticle-based vaccine or nanovaccine (IAV-nanovax) encapsulating H1N1 IAV antigens was reported, which induced pulmonary B and T cell immunity and resulted in cross-strain protection against IAV. A key feature of IAV-nanovax is its ability to easily incorporate diverse proteins/payloads, potentially increasing its ability to provide broad protection against IAV and/or other pathogens. Due to human susceptibility to both H1N1 and H3N2 IAV, several H3N2 nanovaccines were formulated herein with multiple IAV antigens to examine the "plug-and-play" nature of the polyanhydride nanovaccine platform and determine their ability to induce humoral and cellular immunity and broad-based protection similar to IAV-nanovax. The H3N2-based IAV nanovaccine formulations induced systemic and mucosal B cell responses which were associated with antigen-specific antibodies. Additionally, systemic and lung-tissue resident CD4 and CD8 T cell responses were enhanced post-vaccination. These immune responses corresponded with protection against both homologous and heterosubtypic IAV infection. Overall, these results demonstrate the plug-and-play nature of the polyanhydride nanovaccine platform and its ability to generate immunity and protection against IAV utilizing diverse antigenic payloads.

Microalgae-made human vaccines and therapeutics: A decade of advances

Microalgal emergence is a promising platform with two-decade historical background for producing vaccines and biopharmaceuticals. During that period, microalgal-based vaccines have reported successful production for various diseases. Thus, species selection is important for genetic transformation and delivery methods that have been developed. Although many vaccine prototypes have been produced for infectious and non-infectious diseases, fewer studies have reached immunological and immunoprotective evaluations. Microalgae-made vaccines for Staphylococcus aureus, malaria, influenza, human papilloma, and Zika viruses have been explored in their capacity to induce humoral or cellular immune responses and protective efficacies against experimental challenges. Therefore, specific pathogen antigens and immune system role are important and addressed in controlling these infections. Regarding non-communicable diseases, these vaccines have been investigated for breast cancer; microalgal-produced therapeutic molecules and microalgal-made interferon-α have been explored for hypertension and potential applications in treating viral infections and cancer, respectively. Thus, conducting immunological trials is emphasized, discussing the promising results observed in terms of immunogenicity, desired immune response for controlling affections, and challenges for achieving the desired protection levels. The potential advantages and hurdles associated with this innovative approach are highlighted, underlining the relevance of assessing immune responses in preclinical and clinical trials to validate the efficacy of these biopharmaceuticals. The promising future of this healthcare technology is also envisaged.

Ex Pluribus Unum: The CD4 T Cell Response against Influenza A Virus

Current Influenza A virus (IAV) vaccines, which primarily aim to generate neutralizing antibodies against the major surface proteins of specific IAV strains predicted to circulate during the annual 'flu' season, are suboptimal and are characterized by relatively low annual vaccine efficacy. One approach to improve protection is for vaccines to also target the priming of virus-specific T cells that can protect against IAV even in the absence of preexisting neutralizing antibodies. CD4 T cells represent a particularly attractive target as they help to promote responses by other innate and adaptive lymphocyte populations and can also directly mediate potent effector functions. Studies in murine models of IAV infection have been instrumental in moving this goal forward. Here, we will review these findings, focusing on distinct subsets of CD4 T cell effectors that have been shown to impact outcomes. This body of work suggests that a major challenge for next-generation vaccines will be to prime a CD4 T cell population with the same spectrum of functional diversity generated by IAV infection. This goal is encapsulated well by the motto 'ex pluribus unum': that an optimal CD4 T cell response comprises many individual specialized subsets responding together.

CD4 memory has a hierarchical structure created by requirements for infection-derived signals at an effector checkpoint

Our recent studies reveal that the persistence, location, and amount of both antigen and signals that induce pathogen recognition responses determine the number of CD4 memory cells, the subsets that develop, their location, and hence their protective efficacy. Non-replicating vaccines provide antigen that is short-lived and generate low levels of only some memory subsets that are mostly restricted to secondary lymphoid tissue. In contrast, exposure to long-lived replicating viruses and bacteria provides high levels of diverse antigens in sites of infection and induces strong pathogen recognition signals for extended periods of time, resulting in much higher levels of memory cells of diverse subsets in both lymphoid and nonlymphoid sites. These include memory subsets with highly potent functions such as T follicular helpers and cytotoxic CD4 effectors at sites of infection, where they can most effectively combat the pathogen early after re-infection. These effectors also do not develop without antigen and pathogen recognition signals at the effector stage, and both subsets must receive these signals in the tissue sites where they will become resident. We postulate that this leads to a hierarchical structure of memory, with the strongest memory induced only by replicating pathogens. This paradigm suggests a likely roadmap for markedly improving vaccine design.

Pathogenicity and virulence of influenza

Influenza viruses, including four major types (A, B, C, and D), can cause mild-to-severe and lethal diseases in humans and animals. Influenza viruses evolve rapidly through antigenic drift (mutation) and shift (reassortment of the segmented viral genome). New variants, strains, and subtypes have emerged frequently, causing epidemic, zoonotic, and pandemic infections, despite currently available vaccines and antiviral drugs. In recent years, avian influenza viruses, such as H5 and H7 subtypes, have caused hundreds to thousands of zoonotic infections in humans with high case fatality rates. The likelihood of these animal influenza viruses acquiring airborne transmission in humans through viral evolution poses great concern for the next pandemic. Severe influenza viral disease is caused by both direct viral cytopathic effects and exacerbated host immune response against high viral loads. Studies have identified various mutations in viral genes that increase viral replication and transmission, alter tissue tropism or species specificity, and evade antivirals or pre-existing immunity. Significant progress has also been made in identifying and characterizing the host components that mediate antiviral responses, pro-viral functions, or immunopathogenesis following influenza viral infections. This review summarizes the current knowledge on viral determinants of influenza virulence and pathogenicity, protective and immunopathogenic aspects of host innate and adaptive immune responses, and antiviral and pro-viral roles of host factors and cellular signalling pathways. Understanding the molecular mechanisms of viral virulence factors and virus-host interactions is critical for the development of preventive and therapeutic measures against influenza diseases.

Characterization of non-neutralizing human monoclonal antibodies that target the M1 and NP of influenza A viruses

IMPORTANCE

Currently, many groups are focusing on isolating both neutralizing and non-neutralizing antibodies to the mutation-prone hemagglutinin as a tool to treat or prevent influenza virus infection. Less is known about the level of protection induced by non-neutralizing antibodies that target conserved internal influenza virus proteins. Such non-neutralizing antibodies could provide an alternative pathway to induce broad cross-reactive protection against multiple influenza virus serotypes and subtypes by partially overcoming influenza virus escape mediated by antigenic drift and shift. Accordingly, more information about the level of protection and potential mechanism(s) of action of non-neutralizing antibodies targeting internal influenza virus proteins could be useful for the design of broadly protective and universal influenza virus vaccines.

Multifunctional cytokine production marks influenza A virus-specific CD4 T cells with high expression of survival molecules

Cytokine production by memory T cells is a key mechanism of T cell mediated protection. However, we have limited understanding of the persistence of cytokine producing T cells during memory cell maintenance and secondary responses. We interrogated antigen-specific CD4 T cells using a mouse influenza A virus infection model. Although CD4 T cells detected using MHCII tetramers declined in lymphoid and non-lymphoid organs, we found similar numbers of cytokine+ CD4 T cells at days 9 and 30 in the lymphoid organs. CD4 T cells with the capacity to produce cytokines expressed higher levels of pro-survival molecules, CD127 and Bcl2, than non-cytokine+ cells. Transcriptomic analysis revealed a heterogeneous population of memory CD4 T cells with three clusters of cytokine+ cells. These clusters match flow cytometry data and reveal an enhanced survival signature in cells capable of producing multiple cytokines. Following re-infection, multifunctional T cells expressed low levels of the proliferation marker, Ki67, whereas cells that only produce the anti-viral cytokine, interferon-γ, were more likely to be Ki67+ . Despite this, multifunctional memory T cells formed a substantial fraction of the secondary memory pool. Together these data indicate that survival rather than proliferation may dictate which populations persist within the memory pool.

T cell control of SARS-CoV-2: When, which, and where?

Efficient immune protection against viruses such as SARS-CoV-2 requires the coordinated activity of innate immunity, B and T cells. Accumulating data point to a critical role for T cells not only in the clearance of established infection, but also for aborting viral replication independently of humoral immunity. Here we review the evidence supporting the contribution of antiviral T cells and consider which of their qualitative features favour efficient control of infection. We highlight how studies of SARS-CoV-2 and other coronaviridae in animals and humans have provided important lessons on the optimal timing (When), functionality and specificity (Which), and location (Where) of antiviral T cells. We discuss the clinical implications, particularly for the development of next-generation vaccines, and emphasise areas requiring further study.

A mosaic influenza virus-like particles vaccine provides broad humoral and cellular immune responses against influenza A viruses

The development of a universal influenza vaccine to elicit broad immune responses is essential in reducing disease burden and pandemic impact. In this study, the mosaic vaccine design strategy and genetic algorithms were utilized to optimize the seasonal influenza A virus (H1N1, H3N2) hemagglutinin (HA) and neuraminidase (NA) antigens, which also contain most potential T-cell epitopes. These mosaic immunogens were then expressed as virus-like particles (VLPs) using the baculovirus expression system. The immunogenicity and protection effectiveness of the mosaic VLPs were compared to the commercial quadrivalent inactivated influenza vaccine (QIV) in the mice model. Strong cross-reactive antibody responses were observed in mice following two doses of vaccination with the mosaic VLPs, with HI titers higher than 40 in 15 of 16 tested strains as opposed to limited cross HI antibody levels with QIV vaccination. After a single vaccination, mice also show a stronger level of cross-reactive antibody responses than the QIV. The QIV vaccinations only elicited NI antibodies to a small number of vaccine strains, and not even strong NI antibodies to its corresponding vaccine components. In contrast, the mosaic VLPs caused robust NI antibodies to all tested seasonal influenza virus vaccine strains. Here, we demonstrated the mosaic vaccines induces stronger cross-reactive antibodies and robust more T-cell responses compared to the QIV. The mosaic VLPs also provided protection against challenges with ancestral influenza A viruses of both H1 and H3 subtypes. These findings indicated that the mosaic VLPs were a promising strategy for developing a broad influenza vaccine in future.

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

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

SARS-CoV-2-specific T cell therapy for severe COVID-19: a randomized phase 1/2 trial

Despite advances, few therapeutics have shown efficacy in severe coronavirus disease 2019 (COVID-19). In a different context, virus-specific T cells have proven safe and effective. We conducted a randomized (2:1), open-label, phase 1/2 trial to evaluate the safety and efficacy of off-the-shelf, partially human leukocyte antigen (HLA)-matched, convalescent donor-derived severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-specific T cells (CoV-2-STs) in combination with standard of care (SoC) in patients with severe COVID-19 compared to SoC during Delta variant predominance. After a dose-escalated phase 1 safety study, 90 participants were randomized to receive CoV-2-ST+SoC (n = 60) or SoC only (n = 30). The co-primary objectives of the study were the composite of time to recovery and 30-d recovery rate and the in vivo expansion of CoV-2-STs in patients receiving CoV-2-ST+SoC over SoC. The key secondary objective was survival on day 60. CoV-2-ST+SoC treatment was safe and well tolerated. The study met the primary composite endpoint (CoV-2-ST+SoC versus SoC: recovery rate 65% versus 38%, P = 0.017; median recovery time 11 d versus not reached, P = 0.052, respectively; rate ratio for recovery 1.71 (95% confidence interval 1.03-2.83, P = 0.036)) and the co-primary objective of significant CoV-2-ST expansion compared to SοC (CoV-2-ST+SoC versus SoC, P = 0.047). Overall, in hospitalized patients with severe COVID-19, adoptive immunotherapy with CoV-2-STs was feasible and safe. Larger trials are needed to strengthen the preliminary evidence of clinical benefit in severe COVID-19. EudraCT identifier: 2021-001022-22 .

CD4 Effector TCR Avidity for Peptide on APC Determines the Level of Memory Generated

Initial TCR affinity for peptide Ag is known to impact the generation of memory; however, its contributions later, when effectors must again recognize Ag at 5-8 d postinfection to become memory, is unclear. We examined whether the effector TCR affinity for peptide at this "effector checkpoint" dictates the extent of memory and degree of protection against rechallenge. We made an influenza A virus nucleoprotein (NP)-specific TCR transgenic mouse strain, FluNP, and generated NP-peptide variants that are presented by MHC class II to bind to the FluNP TCR over a broad range of avidity. To evaluate the impact of avidity in vivo, we primed naive donor FluNP in influenza A virus-infected host mice, purified donor effectors at the checkpoint, and cotransferred them with the range of peptides pulsed on activated APCs into second uninfected hosts. Higher-avidity peptides yielded higher numbers of FluNP memory cells in spleen and most dramatically in lung and draining lymph nodes and induced better protection against lethal influenza infection. Avidity determined memory cell number, not cytokine profile, and already impacted donor cell number within several days of transfer. We previously found that autocrine IL-2 production at the checkpoint prevents default effector apoptosis and supports memory formation. Here, we find that peptide avidity determines the level of IL-2 produced by these effectors and that IL-2Rα expression by the APCs enhances memory formation, suggesting that transpresentation of IL-2 by APCs further amplifies IL-2 availability. Secondary memory generation was also avidity dependent. We propose that this regulatory pathway selects CD4 effectors of highest affinity to progress to memory.

STAT1 Controls the Functionality of Influenza-Primed CD4 T Cells but Therapeutic STAT4 Engagement Maximizes Their Antiviral Impact

It is generally accepted that influenza A virus (IAV) infection promotes a Th1-like CD4 T cell response and that this effector program underlies its protective impact. Canonical Th1 polarization requires cytokine-mediated activation of the transcription factors STAT1 and STAT4 that synergize to maximize the induction of the "master regulator" Th1 transcription factor, T-bet. Here, we determine the individual requirements for these transcription factors in directing the Th1 imprint primed by influenza infection in mice by tracking virus-specific wild-type or T-bet-deficient CD4 T cells in which STAT1 or STAT4 is knocked out. We find that STAT1 is required to protect influenza-primed CD4 T cells from NK cell-mediated deletion and for their expression of hallmark Th1 attributes. STAT1 is also required to prevent type I IFN signals from inhibiting the induction of the Th17 master regulator, Rorγt, in Th17-prone T-bet-/- cells responding to IAV. In contrast, STAT4 expression does not appreciably impact the phenotypic or functional attributes of wild-type or T-bet-/- CD4 T cell responses. However, cytokine-mediated STAT4 activation in virus-specific CD4 T cells enhances their Th1 identity in a T-bet-dependent manner, indicating that influenza infection does not promote maximal Th1 induction. Finally, we show that the T-bet-dependent protective capacity of CD4 T cell effectors against IAV is optimized by engaging both STAT1 and STAT4 during Th1 priming, with important implications for vaccine strategies aiming to generate T cell immunity.

Next-Generation Vaccines Against COVID-19 Variants: Beyond the Spike Protein

Vaccines are among the most effective medical countermeasures against infectious diseases. The current Coronavirus disease 2019 (COVID-19) pandemic, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has spurred the scientific strategies to fight against the disease. Since 2020, a great number of vaccines based on different platforms have been in development in response to the pandemic, among which mRNA, adenoviral vector, and subunit vaccines have been clinically approved for use in humans. These first-generation COVID-19 vaccines largely target the viral spike (S) protein and aim for eliciting potent neutralizing antibodies. With the emergence of SARS-CoV-2 variants, especially the highly transmissible Omicron strains, the S-based vaccine strategies have been faced constant challenges due to strong immune escape by the variants. The coronavirus nucleocapsid (N) is one of the viral proteins that induces strong T-cell immunity and is more conserved across different SARS-CoV-2 variants. Inclusion of N in the development of COVID-19 vaccines has been reported. Here, we briefly reviewed and discussed COVID-19 disease, current S-based vaccine strategies, and focused on the immunobiology of N protein in SARS-CoV-2 host immunity, as well as the next-generation vaccine strategies involving N protein, to combat current and emerging SARS-CoV-2 variants.

SARS-CoV-2: Immunity, Challenges with Current Vaccines, and a Novel Perspective on Mucosal Vaccines

The global rollout of COVID-19 vaccines has played a critical role in reducing pandemic spread, disease severity, hospitalizations, and deaths. However, the first-generation vaccines failed to block severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and transmission, partially due to the limited induction of mucosal immunity, leading to the continuous emergence of variants of concern (VOC) and breakthrough infections. To meet the challenges from VOC, limited durability, and lack of mucosal immune response of first-generation vaccines, novel approaches are being investigated. Herein, we have discussed the current knowledge pertaining to natural and vaccine-induced immunity, and the role of the mucosal immune response in controlling SARS-CoV2 infection. We have also presented the current status of the novel approaches aimed at eliciting both mucosal and systemic immunity. Finally, we have presented a novel adjuvant-free approach to elicit effective mucosal immunity against SARS-CoV-2, which lacks the safety concerns associated with live-attenuated vaccine platforms.

Protective Efficacy of a Mucosal Influenza Vaccine Formulation Based on the Recombinant Nucleoprotein Co-Administered with a TLR2/6 Agonist BPPcysMPEG

Current influenza vaccines target highly variable surface glycoproteins; thus, mismatches between vaccine strains and circulating strains often diminish vaccine protection. For this reason, there is still a critical need to develop effective influenza vaccines able to protect also against the drift and shift of different variants of influenza viruses. It has been demonstrated that influenza nucleoprotein (NP) is a strong candidate for a universal vaccine, which contributes to providing cross-protection in animal models. In this study, we developed an adjuvanted mucosal vaccine using the recombinant NP (rNP) and the TLR2/6 agonist S-[2,3-bispalmitoyiloxy-(2R)-propyl]-R-cysteinyl-amido-monomethoxyl-poly-ethylene-glycol (BPPcysMPEG). The vaccine efficacy was compared with that observed following parenteral vaccination of mice with the same formulation. Mice vaccinated with 2 doses of rNP alone or co-administered with BPPcysMPEG by the intranasal (i.n.) route showed enhanced antigen-specific humoral and cellular responses. Moreover, NP-specific humoral immune responses, characterized by significant NP-specific IgG and IgG subclass titers in sera and NP-specific IgA titers in mucosal territories, were remarkably increased in mice vaccinated with the adjuvanted formulation as compared with those of the non-adjuvanted vaccination group. The addition of BPPcysMPEG also improved NP-specific cellular responses in vaccinated mice, characterized by robust lymphoproliferation and mixed Th1/Th2/Th17 immune profiles. Finally, it is notable that the immune responses elicited by the novel formulation administered by the i.n. route were able to confer protection against the influenza H1N1 A/Puerto Rico/8/1934 virus.

Intermediate Levels of Pre-Existing Protective Antibody Allow Priming of Protective T Cell Immunity against Influenza

Overcoming interfering impacts of pre-existing immunity to generate universally protective influenza A virus (IAV)-specific T cell immunity through vaccination is a high priority. In this study, we passively transfer varied amounts of H1N1-IAV-specific immune serum before H1N1-IAV infection to determine how different levels of pre-existing Ab influence the generation and protective potential of heterosubtypic T cell responses in a murine model. Surprisingly, IAV nucleoprotein-specific CD4 and CD8 T cell responses are readily detected in infected recipients of IAV-specific immune serum regardless of the amount transferred. When compared with responses in control groups and recipients of low and intermediate levels of convalescent serum, nucleoprotein-specific T cell responses in recipients of high levels of IAV-specific serum, which prevent overt weight loss and reduce peak viral titers in the lungs, are, however, markedly reduced. Although detectable at priming, this response recalls poorly and is unable to mediate protection against a lethal heterotypic (H3N2) virus challenge at later memory time points. A similar failure to generate protective heterosubtypic T cell immunity during IAV priming is seen in offspring of IAV-primed mothers that naturally receive high titers of IAV-specific Ab through maternal transfer. Our findings support that priming of protective heterosubtypic T cell responses can occur in the presence of intermediate levels of pre-existing Ab. These results have high relevance to vaccine approaches aiming to incorporate and evaluate cellular and humoral immunity towards IAV and other viral pathogens against which T cells can protect against variants escaping Ab-mediated protection.

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

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

Flagellin-Fused Protein Targeting M2e and HA2 Induces Innate and T-Cell Responses in Mice of Different Genetic Lines

Efficient control of influenza A infection can potentially be achieved through the development of broad-spectrum vaccines. Recombinant proteins incorporating conserved influenza A virus peptides are one of the platforms for the development of cross-protective influenza vaccines. We constructed a recombinant protein Flg-HA2-2-4M2ehs, in which the extracellular domain of the M2 protein (M2e) and the sequence (aa76-130) of the second subunit of HA (HA2) were used as target antigens. In this study, we investigated the ability of the Flg-HA2-2-4M2ehs protein to activate innate immunity and stimulate the formation of T-cell response in mice of different genetic lines after intranasal immunization. Our studies showed that the Flg-HA2-2-4M2ehs protein was manifested in an increase in the relative content of neutrophils, monocytes, and interstitial macrophages, against the backdrop of a decrease in the level of dendritic cells and increased expression in the CD86 marker. In the lungs of BALB/c mice, immunization with the Flg-HA2-2-4M2ehs protein induced the formation of antigen-specific CD4+ and CD8+ effector memory T cells, producing TNF-α. In mice C57Bl/6, the formation of antigen-specific effector CD8+ T cells, predominantly producing IFN-γ+, was demonstrated. The data obtained showed the formation of CD8+ and CD4+ effector memory T cells expressing the CD107a.

Protection from infection or disease? Re-evaluating the broad immunogenicity of inactivated SARS-CoV-2 vaccines

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Viral lineages that escape antibodies have changed the landscape of the pandemic.

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SARS-CoV-2 specific T cells effectively protect against disease development.

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Vaccine efficacy should be evaluated by its ability to protect from severe disease.

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Inactivated SARS-CoV-2 vaccine induces a multi-protein specific T cell response.

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The superior breadth of the vaccine T cell response could be a protective asset.

In planta Production and Validation of Neuraminidase Derived from Genotype 4 Reassortant Eurasian Avian-like H1N1 Virus as a Vaccine Candidate

Influenza viruses are a major public health threat that causes repetitive outbreaks. In recent years, genotype 4 (G4) reassortant Eurasian avian-like (EA) H1N1 (G4 EA H1N1) has garnered attention as a potential novel pandemic strain. The necessity of developing vaccines against G4 EA H1N1 is growing because of the increasing cases of human infection and the low cross-reactivity of the strain with current immunity. In this study, we produced a G4 EA H1N1-derived neuraminidase (G4NA) as a vaccine candidate in Nicotiana benthamiana. The expressed G4NA was designed to be accumulated in the endoplasmic reticulum (ER). The M-domain of the human receptor-type tyrosine-protein phosphatase C was incorporated into the expression cassette to enhance the translation of G4NA. In addition, the family 3 cellulose-binding module and Brachypodium distachyon small ubiquitin-like modifier sequences were used to enable the cost-effective purification and removal of unnecessary domains after purification, respectively. The G4NA produced in plants displayed high solubility and assembled as a tetramer, which is required for the efficacy of an NA-based vaccine. In a mouse immunization model, the G4NA produced in plants could induce significant humoral immune responses. The plant-produced G4NA also stimulated antigen-specific CD4 T cell activation. These G4NA vaccine-induced immune responses were intensified by the administration of the antigen with a vaccine adjuvant. These results suggest that G4NA produced in plants has great potential as a vaccine candidate against G4 EA H1N1.

SARS-CoV-2-specific T cells in the changing landscape of the COVID-19 pandemic

Since the onset of the coronavirus disease 2019 (COVID-19) pandemic, multiple severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants with increasing ability to evade neutralizing antibodies have emerged. Thus, earlier interest in defining the correlates of protection from infection, mainly mediated by humoral immunity, has shifted to correlates of protection from disease, which require a more comprehensive analysis of both humoral and cellular immunity. In this review, we summarized the evidence that supports the role of SARS-CoV-2-specific T cells induced by infection, by vaccination or by their combination (defined as hybrid immunity) in disease protection. We then analyzed the different epidemiological and virological variables that can modify the magnitude, function, and anatomical localization of SARS-CoV-2-specific T cells and their influence in the possible ability of T cells to protect the host from severe COVID-19 development.

A comparative characterization of SARS-CoV-2-specific T cells induced by mRNA or inactive virus COVID-19 vaccines

Unlike mRNA vaccines based only on the spike protein, inactivated severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) vaccines should induce a diversified T cell response recognizing distinct structural proteins. Here, we perform a comparative analysis of SARS-CoV-2-specific T cells in healthy individuals following vaccination with inactivated SARS-CoV-2 or mRNA vaccines. Relative to spike mRNA vaccination, inactivated vaccines elicit a lower magnitude of spike-specific T cells, but the combination of membrane, nucleoprotein, and spike-specific T cell response is quantitatively comparable with the sole spike T cell response induced by mRNA vaccine, and they efficiently tolerate the mutations characterizing the Omicron lineage. However, this multi-protein-specific T cell response is not mediated by a coordinated CD4 and CD8 T cell expansion but by selective priming of CD4 T cells. These findings can help in understanding the role of CD4 and CD8 T cells in the efficacy of the different vaccines to control severe COVID-19 after Omicron infection.

Preclinical immunogenicity assessment of a cell-based inactivated whole-virion H5N1 influenza vaccine

In influenza vaccine development, Madin–Darby canine kidney (MDCK) cells provide multiple advantages, including large-scale production and egg independence. Several cell-based influenza vaccines have been approved worldwide. We cultured H5N1 virus in a serum-free MDCK cell suspension. The harvested virus was manufactured into vaccines after inactivation and purification. The vaccine effectiveness was assessed in the Wuhan Institute of Biological Products BSL2 facility. The pre- and postvaccination mouse serum titers were determined using the microneutralization and hemagglutination inhibition tests. The immunological responses induced by vaccine were investigated using immunological cell classification, cytokine expression quantification, and immunoglobulin G (IgG) subtype classification. The protective effect of the vaccine in mice was evaluated using challenge test. Antibodies against H5N1 in rats lasted up to 8 months after the first dose. Compared with those of the placebo group, the serum titer of vaccinated mice increased significantly, Th1 and Th2 cells were activated, and CD8+ T cells were activated in two dose groups. Furthermore, the challenge test showed that vaccination reduced the clinical symptoms and virus titer in the lungs of mice after challenge, indicating a superior immunological response. Notably, early after vaccination, considerably increased interferon-inducible protein-10 (IP-10) levels were found, indicating improved vaccine-induced innate immunity. However, IP-10 is an adverse event marker, which is a cause for concern. Overall, in the case of an outbreak, the whole-virion H5N1 vaccine should provide protection.

Act Early and at the Right Location: SARS-CoV-2 T Cell Kinetics and Tissue Localization

The emergence of new SARS-CoV-2 lineages able to escape antibodies elicited by infection or vaccination based on the Spike protein of the Wuhan isolates has reduced the ability of Spike-specific antibodies to protect previously infected or vaccinated individuals from infection. Therefore, the role played by T cells in the containment of viral replication and spread after infection has taken a more central stage. In this brief review, we will discuss the role played by T cells in the protection from COVID-19, with a particular emphasis on the kinetics of the T cell response and its localization at the site of primary infection.

Correlates of protection against SARS-CoV-2 infection and COVID-19 disease

Antibodies against epitopes in S1 give the most accurate CoP against infection by the SARS-CoV-2 coronavirus. Measurement of those antibodies by neutralization or binding assays both have predictive value, with binding antibody titers giving the highest statistical correlation. However, the protective functions of antibodies are multiple. Antibodies with multiple functions other than neutralization influence efficacy. The role of cellular responses can be discerned with respect to CD4+ T cells and their augmentation of antibodies, and with respect to CD8+ cells with regard to control of viral replication, particularly in the presence of insufficient antibody. More information is needed on mucosal responses.

The role of cell-mediated immunity against influenza and its implications for vaccine evaluation

Influenza vaccines remain the most effective tools to prevent flu and its complications. Trivalent or quadrivalent inactivated influenza vaccines primarily elicit antibodies towards haemagglutinin and neuraminidase. These vaccines fail to induce high protective efficacy, in particular in older adults and immunocompromised individuals and require annual updates to keep up with evolving influenza strains (antigenic drift). Vaccine efficacy declines when there is a mismatch between its content and circulating strains. Current correlates of protection are merely based on serological parameters determined by haemagglutination inhibition or single radial haemolysis assays. However, there is ample evidence showing that these serological correlates of protection can both over- or underestimate the protective efficacy of influenza vaccines. Next-generation universal influenza vaccines that induce cross-reactive cellular immune responses (CD4+ and/or CD8+ T-cell responses) against conserved epitopes may overcome some of the shortcomings of the current inactivated vaccines by eliciting broader protection that lasts for several influenza seasons and potentially enhances pandemic preparedness. Assessment of cellular immune responses in clinical trials that evaluate the immunogenicity of these new generation vaccines is thus of utmost importance. Moreover, studies are needed to examine whether these cross-reactive cellular immune responses can be considered as new or complementary correlates of protection in the evaluation of traditional and next-generation influenza vaccines. An overview of the assays that can be applied to measure cell-mediated immune responses to influenza with their strengths and weaknesses is provided here.

CD4+ Cytotoxic T cells - Phenotype, Function and Transcriptional Networks Controlling Their Differentiation Pathways

The two major subsets of peripheral T cells are classically divided into the CD4⁺ T helper cells and the cytotoxic CD8⁺ T cell lineage. However, the appearance of some effector CD4⁺ T cell populations displaying cytotoxic activity, in particular during viral infections, has been observed, thus breaking the functional dichotomy of CD4⁺ and CD8⁺ T lymphocytes. The strong association of the appearance of CD4⁺ cytotoxic T lymphocytes (CD4 CTLs) with viral infections suggests an important role of this subset in antiviral immunity by controlling viral replication and infection. Moreover, CD4 CTLs have been linked with anti-tumor activity and might also cause immunopathology in autoimmune diseases. This raises interest into the molecular mechanisms regulating CD4 CTL differentiation, which are poorly understood in comparison to differentiation pathways of other Th subsets. In this review, we provide a brief overview about key features of CD4 CTLs, including their role in viral infections and cancer immunity, and about the link between CD4 CTLs and immune-mediated diseases. Subsequently, we will discuss the current knowledge about transcriptional and epigenetic networks controlling CD4 CTL differentiation and highlight recent data suggesting a role for histone deacetylases in the generation of CD4 CTLs.

A single-shot adenoviral vaccine provides hemagglutinin stalk-mediated protection against heterosubtypic influenza challenge in mice

Conventional influenza vaccines fail to confer broad protection against diverse influenza A viruses with pandemic potential. Efforts to develop a universal influenza virus vaccine include refocusing immunity towards the highly conserved stalk domain of the influenza virus surface glycoprotein, hemagglutinin (HA). We constructed a non-replicating adenoviral (Ad) vector, encoding a secreted form of H1 HA, to evaluate HA stalk-focused immunity. The Ad5_H1 vaccine was tested in mice for its ability to elicit broad, cross-reactive protection against homologous, heterologous, and heterosubtypic lethal challenge in a single-shot immunization regimen. Ad5_H1 elicited hemagglutination inhibition (HI+) active antibodies (Abs), which conferred 100% sterilizing protection from homologous H1N1 challenge. Furthermore, Ad5_H1 rapidly induced H1-stalk-specific Abs with Fc-mediated effector function activity, in addition to stimulating both CD4+ and CD8+ stalk-specific T cell responses. This phenotype of immunity provided 100% protection from lethal challenge with a head-mismatched, reassortant influenza virus bearing a chimeric HA, cH6/1, in a stalk-mediated manner. Most importantly, 100% protection from mortality following lethal challenge with a heterosubtypic avian influenza virus, H5N1, was observed following a single immunization with Ad5_H1. In conclusion, Ad-based influenza vaccines can elicit significant breadth of protection in naive animals and could be considered for pandemic preparedness and stockpiling.

Pneumonia Severity and Phase Linked to Virus-Specific T Cell Responses with Distinct Immune Checkpoints during pH1N1 Infection

The detailed features and the longitudinal variation of influenza-specific T cell responses within naturally infected patients and the relationship with disease severity remain uncertain. In this study, we characterized the longitudinal influenza-specific CD4⁺ and CD8⁺ T cell responses, T cell activation, and migration-related cytokine/chemokine secretion in pH1N1-infected patients with or without viral pneumonia with human PBMCs. Both the influenza-specific CD4⁺ and CD8⁺ T cells presented higher responses in patients with severe infection than in mild ones, but with distinct longitudinal variations, phenotypes of memory markers, and immune checkpoints. At 7 ± 3 d after onset of illness, effector CD8⁺ T cells (CD45RA⁺CCR7^-) with high expression of inhibitory immune receptor CD200R dominated the specific T cell responses. However, at 21 ± 3 d after onset of illness, effector memory CD4⁺ T cells (CD45RA^-CCR7^-) with high expression of PD1, CTLA4, and LAG3 were higher among the patients with severe disease. The specific T cell magnitude, T cell activation, and migration-related cytokines/chemokines possessed a strong connection with disease severity. Our findings illuminate the distinct characteristics of immune system activation during dynamic disease phases and its correlation with lung injury of pH1N1 patients.

Bona Fide Th17 Cells without Th1 Functional Plasticity Protect against Influenza

Optimal transcriptional programming needed for CD4 T cells to protect against influenza A virus (IAV) is unclear. Most IAV-primed CD4 T cells fit Th1 criteria. However, cells deficient for the Th1 "master regulator," T-bet, although marked by reduced Th1 identity, retain robust protective capacity. In this study, we show that T-bet's paralog, Eomesodermin (Eomes), is largely redundant in the presence of T-bet but is essential for the residual Th1 attributes of T-bet-deficient cells. Cells lacking both T-bet and Eomes instead develop concurrent Th17 and Th2 responses driven by specific inflammatory signals in the infected lung. Furthermore, the transfer of T-bet- and Eomes-deficient Th17, but not Th2, effector cells protects mice from lethal IAV infection. Importantly, these polyfunctional Th17 effectors do not display functional plasticity in vivo promoting gain of Th1 attributes seen in wild-type Th17 cells, which has clouded evaluation of the protective nature of Th17 programming in many studies. Finally, we show that primary and heterosubtypic IAV challenge is efficiently cleared in T-bet- and Eomes double-deficient mice without enhanced morbidity despite a strongly Th17-biased inflammatory response. Our studies thus demonstrate unexpectedly potent antiviral capacity of unadulterated Th17 responses against IAV, with important implications for vaccine design.

Elimination of receptor binding by influenza hemagglutinin improves vaccine-induced immunity

The binding of influenza hemagglutinin (HA) to sialic acid (SA) receptors plays a well-defined role in shaping infection but the impact of such binding on vaccine responses has not yet been explored. We generated a virus-like particle (VLP) vaccine bearing the HA of H1N1 A/California/07/09 that is unable to bind to its α(2,6)-linked SA receptor (H1_Y98F-VLP) and compared its immunogenicity and efficacy to a wild-type H1-VLP (H1_WT-VLP) in mice. The H1_Y98F-VLP elicited significantly stronger and more durable antibody responses (hemagglutination inhibition and microneutralization titers) and greater avidity maturation, likely attributable to improved germinal center formation. H1_Y98F-VLP also resulted in a robust population of IL-2⁺TNFα⁺IFNγ⁻ CD4⁺ T cells that correlated with antibody responses. Compared to H1_WT-VLP vaccination, mice immunized with H1_Y98F-VLP had 2.3-log lower lung viral loads and significantly lower pulmonary inflammatory cytokine levels 5 days post-challenge. These findings suggest that abrogation of HA-SA interactions may be a promising strategy to improve the quality and durability of influenza vaccine-induced humoral responses.

Isolated specific IgA against respiratory viruses, Influenza or SARS-CoV-2, present in the saliva of a fraction of healthy and asymptomatic volunteers

OBJECTIVES

Defense against respiratory viruses depends on an immune response present in the mucosa, as saliva IgA secretes antibodies. During the pandemic, such as influenza or SARS-CoV-2, most infected patients are asymptomatic but retain specific antibodies post-infection. The authors evaluated IgG and IgA antibodies against SARS-CoV-2 and influenza in the saliva of asymptomatic volunteers, validated with controls or vaccinated individuals.

METHODS

The authors detected specific antibodies by validated conventional ELISA using natural SARS-CoV-2 antigens from infected Vero cells or capture-ELISA for influenza using natural antigens of the influenza vaccine.

RESULTS

Saliva from influenza-vaccinated individuals had more IgA than paired serum, contrary to the findings for specific IgG. In COVID-19-vaccinated samples, specific IgA in saliva increased after vaccination, but IgG levels were high after the first dose. In saliva from the asymptomatic population (226), anti-Influenza IgG was found in 57.5% (130) of samples, higher than IgA, found in 35% (79) of samples. IgA results were similar for SARS-CoV-2, with IgA present in 30% (68) of samples, while IgG was less present, in 44.2% (100) of samples. The proportion of influenza IgG responders was higher than that for SARS-CoV-2 IgG, but both populations presented similar proportions of IgA responders, possibly due to variable memory B cell survival. For both viruses, the authors found an important proportion (> 10%) of IgA+IgG- samples, suggesting the occurrence of humoral immunity directed to the mucosa.

CONCLUSION

Specific antibodies for respiratory viruses in saliva are found in either infection or vaccination and are a convenient and sensitive diagnostic tool for host immune response.

Clonotypic analysis of protective influenza M2e-specific lung resident Th17 memory cells reveals extensive functional diversity

The fate of tissue-resident memory CD4 T cells (Trm) has been incompletely investigated. Here we show that intranasal, but not parenteral, immunization with CTA1-3M2e-DD stimulated M2e-specific Th17 Trm cells, which conferred strong protection against influenza virus infection in the lung. These cells rapidly expanded upon infection and effectively restricted virus replication as determined by CD4 T cell depletion studies. Single-cell RNAseq transcriptomic and TCR VDJ-analysis of M2e-tetramer-sorted CD4 T cells on day 3 and 8 post infection revealed complete Th17-lineage dominance (no Th1 or Tregs) with extensive functional diversity and expression of gene markers signifying mature resident Trm cells (Cd69, Nfkbid, Brd2, FosB). Unexpectedly, the same TCR clonotype hosted cells with different Th17 subcluster functions (IL-17, IL-22), regulatory and cytotoxic cells, suggesting a tissue and context-dependent differentiation of reactivated Th17 Trm cells. A gene set enrichment analysis demonstrated up-regulation of regulatory genes (Lag3, Tigit, Ctla4, Pdcd1) in M2e-specific Trm cells on day 8, indicating a tissue damage preventing function. Thus, contrary to current thinking, lung M2e-specific Th17 Trm cells are sufficient for controlling infection and for protecting against tissue injury. These findings will have strong implications for vaccine development against respiratory virus infections and influenza virus infections, in particular.

The Chemokine Receptor CCR5 Links Memory CD4+ T Cell Metabolism to T Cell Antigen Receptor Nanoclustering

The inhibition of anabolic pathways, such as aerobic glycolysis, is a metabolic cornerstone of memory T cell differentiation and function. However, the signals that hamper these anabolic pathways are not completely known. Recent evidence pinpoints the chemokine receptor CCR5 as an important player in CD4⁺ T cell memory responses by regulating T cell antigen receptor (TCR) nanoclustering in an antigen-independent manner. This paper reports that CCR5 specifically restrains aerobic glycolysis in memory-like CD4⁺ T cells, but not in effector CD4⁺ T cells. CCR5-deficient memory CD4⁺ T cells thus show an abnormally high glycolytic/oxidative metabolism ratio. No CCR5-dependent change in glucose uptake nor in the expression of the main glucose transporters was detected in any of the examined cell types, although CCR5-deficient memory cells did show increased expression of the hexokinase 2 and pyruvate kinase M2 isoforms, plus the concomitant downregulation of Bcl-6, a transcriptional repressor of these key glycolytic enzymes. Further, the TCR nanoclustering defects observed in CCR5-deficient antigen-experienced CD4⁺ T cells were partially reversed by incubation with 2-deoxyglucose (2-DG), suggesting a link between inhibition of the glycolytic pathway and TCR nanoscopic organization. Indeed, the treatment of CCR5-deficient lymphoblasts with 2-DG enhanced IL-2 production after antigen re-stimulation. These results identify CCR5 as an important regulator of the metabolic fitness of memory CD4⁺ T cells, and reveal an unexpected link between T cell metabolism and TCR organization with potential influence on the response of memory T cells upon antigen re-encounter.

Cytotoxic CD4+ T cells in cancer: Expanding the immune effector toolbox

Cytotoxic T cells are important effectors of anti-tumor immunity. While tumor killing is ascribed to CD8⁺ T cell function, pre-clinical and clinical studies have identified intra-tumoral CD4⁺ T cells that possess cytotoxic programs and can directly kill cancer cells. Cytotoxic CD4⁺ T cells are found in other disease settings including infection and autoimmunity. Here, we review the phenotypic and functional characteristics of cytotoxic CD4⁺ T cells in non-cancer and cancer contexts. We conduct a comparative examination of cytolytic mechanisms of cytotoxic CD4⁺ T cells across disease states and synthesize features that define these cells independent of context. We discuss regulatory mechanisms driving ontogeny and effector function and evidence for the clinical relevance of cytotoxic CD4⁺ T cells in cancer. In this context, we highlight important gaps in understanding in the biology of cytotoxic CD4⁺ T cells as well as the potential use of these cells in immunotherapies for specific cancers.

Durable CD4 T-Cell Memory Generation Depends on Persistence of High Levels of Infection at an Effector Checkpoint that Determines Multiple Fates

We have discovered that the determination of CD4 effector and memory fates after infection is regulated not only by initial signals from antigen and pathogen recognition, but also by a second round of such signals at a checkpoint during the effector response. Signals to effectors determine their subsequent fate, inducing further progression to tissue-restricted follicular helpers, cytotoxic CD4 effectors, and long-lived memory cells. The follicular helpers help the germinal center B-cell responses that give rise to high-affinity long-lived antibody responses and memory B cells that synergize with T-cell memory to provide robust long-lived protection. We postulate that inactivated vaccines do not provide extended signals from antigen and pathogen beyond a few days, and thus elicit ineffective CD4 T- and B-cell effector responses and memory. Defining the mechanisms that underlie effective responses should provide insights necessary to develop vaccine strategies that induce more effective and durable immunity.

Synergistic effect of non-neutralizing antibodies and interferon-γ for cross-protection against influenza

Current influenza vaccines do not typically confer cross-protection against antigenically mismatched strains. To develop vaccines conferring broader cross-protection, recent evidence indicates the crucial role of both cross-reactive antibodies and viral-specific CD4+ T cells; however, the precise mechanism of cross-protection is unclear. Furthermore, adjuvants that can efficiently induce cross-protective CD4+ T cells have not been identified. Here we show that CpG oligodeoxynucleotides combined with aluminum salts work as adjuvants for influenza vaccine and confer strong cross-protection in mice. Both cross-reactive antibodies and viral-specific CD4+ T cells contributed to cross-protection synergistically, with each individually ineffective. Furthermore, we found that downregulated expression of Fcγ receptor IIb on alveolar macrophages due to IFN-γ secreted by viral-specific CD4+ T cells improves the activity of cross-reactive antibodies. Our findings inform the development of optimal adjuvants for vaccines and how influenza vaccines confer broader cross-protection.

Influenza infection fortifies local lymph nodes to promote lung-resident heterosubtypic immunity

Influenza infection generates tissue-resident memory T cells (TRMs) that are maintained in the lung and can mediate protective immunity to heterologous influenza strains, but the precise mechanisms of local T cell-mediated protection are not well understood. In a murine heterosubtypic influenza challenge model, we demonstrate that protective lung T cell responses derive from both in situ activation of TRMs and the enhanced generation of effector T cells from the local lung draining mediastinal lymph nodes (medLNs). Primary infection fortified the medLNs with an increased number of conventional dendritic cells (cDCs) that mediate enhanced priming of T cells, including those specific for newly encountered epitopes; cDC depletion during the recall response diminished medLN T cell generation and heterosubtypic immunity. Our study shows that during a protective recall response, cDCs in a fortified LN environment enhance the breadth, generation, and tissue migration of effector T cells to augment lung TRM responses.

Intranasal Nanoparticle Vaccination Elicits a Persistent, Polyfunctional CD4 T Cell Response in the Murine Lung Specific for a Highly Conserved Influenza Virus Antigen That Is Sufficient To Mediate Protection from Influenza Virus Challenge

Lung-localized CD4 T cells play a critical role in the control of influenza virus infection and can provide broadly protective immunity. However, current influenza vaccination strategies primarily target influenza hemagglutinin (HA) and are administered peripherally to induce neutralizing antibodies. We have used an intranasal vaccination strategy targeting the highly conserved influenza nucleoprotein (NP) to elicit broadly protective lung-localized CD4 T cell responses. The vaccine platform consists of a self-assembling nanolipoprotein particle (NLP) linked to NP with an adjuvant. We have evaluated the functionality, in vivo localization, and persistence of the T cells elicited. Our study revealed that intranasal vaccination elicits a polyfunctional subset of lung-localized CD4 T cells that persist long term. A subset of these lung CD4 T cells localize to the airway, where they can act as early responders following encounter with cognate antigen. Polyfunctional CD4 T cells isolated from airway and lung tissue produce significantly more effector cytokines IFN-γ and TNF-α, as well as cytotoxic functionality. When adoptively transferred to naive recipients, CD4 T cells from NLP:NP-immunized lung were sufficient to mediate 100% survival from lethal challenge with H1N1 influenza virus. IMPORTANCE Exploiting new, more efficacious strategies to potentiate influenza virus-specific immune responses is important, particularly for at-risk populations. We have demonstrated the promise of direct intranasal protein vaccination to establish long-lived immunity in the lung with CD4 T cells that possess features and positioning in the lung that are associated with both immediate and long-term immunity, as well as demonstrating direct protective potential.

A safe and effective mucosal RSV vaccine in mice consisting of RSV phosphoprotein and flagellin variant

Respiratory syncytial virus (RSV) is a major cause of serious acute lower respiratory tract infection in infants and the elderly. The lack of a licensed RSV vaccine calls for the development of vaccines with other targets and vaccination strategies. Here, we construct a recombinant protein, designated P-KFD1, comprising RSV phosphoprotein (P) and the E.-coli-K12-strain-derived flagellin variant KFD1. Intranasal immunization with P-KFD1 inhibits RSV replication in the upper and lower respiratory tract and protects mice against lung disease without vaccine-enhanced disease (VED). The P-specific CD4+ T cells provoked by P-KFD1 intranasal (i.n.) immunization either reside in or migrate to the respiratory tract and mediate protection against RSV infection. Single-cell RNA sequencing (scRNA-seq) and carboxyfluorescein succinimidyl ester (CFSE)-labeled cell transfer further characterize the Th1 and Th17 responses induced by P-KFD1. Finally, we find that anti-viral protection depends on either interferon-γ (IFN-γ) or interleukin-17A (IL-17A). Collectively, P-KFD1 is a promising safe and effective mucosal vaccine candidate for the prevention of RSV infection.

The interplay between airway epithelium and the immune system - A primer for the respiratory clinician

The respiratory epithelium is one of the primary interfaces between the body's immune system and the external environment. This review discusses the innate and adaptive immunomodulatory effects of the respiratory epithelium, highlighting the physiologic immune responses associated with health and the disease-causing sequelae when these physiologic responses go awry. Airway macrophages, dendritic cells, and innate lymphoid cells are discussed as orchestrators of physiological and pathological innate immune responses and T cells, B cells, mast cells, and granulocytes (eosinophils and neutrophils) as orchestrators of physiologic and pathologic adaptive immune responses. The interplay between the airway epithelium and the varied immune cells as well as the interplay between these immune cells is discussed, highlighting the importance of the dose of noxious stimuli and pathogens in immune programming and the timing of their interaction with the immune cells that determine the pattern of immune responses. Although each cell type has been researched individually, this review highlights the need for simultaneous temporal investigation of immune responses from these varied cells to noxious stimuli and pathogens.

Repeated Exposure to Subinfectious Doses of SARS-CoV-2 May Promote T Cell Immunity and Protection against Severe COVID-19

Europe is experiencing a third wave of COVID-19 due to the spread of highly transmissible SARS-CoV-2 variants. A number of positive and negative factors constantly shape the rates of COVID-19 infections, hospitalization, and mortality. Among these factors, the rise in increasingly transmissible variants on one side and the effect of vaccinations on the other side create a picture deeply different from that of the first pandemic wave. Starting from the observation that in several European countries the number of COVID-19 infections in the second and third pandemic wave increased without a proportional rise in disease severity and mortality, we hypothesize the existence of an additional factor influencing SARS-CoV-2 dynamics. This factor consists of an immune defence against severe COVID-19, provided by SARS-CoV-2-specific T cells progressively developing upon natural exposure to low virus doses present in populated environments. As suggested by recent studies, low-dose viral particles entering the respiratory and intestinal tracts may be able to induce T cell memory in the absence of inflammation, potentially resulting in different degrees of immunization. In this scenario, non-pharmaceutical interventions would play a double role, one in the short term by reducing the detrimental spreading of SARS-CoV-2 particles, and one in the long term by allowing the development of a widespread (although heterogeneous and uncontrollable) form of immune protection.

Protein transduction domain-mediated influenza NP subunit vaccine generates a potent immune response and protection against influenza virus in mice

The nucleoprotein (NP) is a highly conserved internal protein of the influenza virus, a major target for universal influenza vaccine. Our previous studies have proven NP-based subunit vaccine can provide partial protection in mice. It is reported that the protein transduction domain (PTD) TAT protein from human immunodeficiency virus-1 (HIV-1) is able to penetrate cells when added exogenous protein and could effectively enhance the immune response induced by the exogenous protein. In present study, the recombinant protein TAT-NP, a fusion of TAT and NP was effectively expressed in Escherichia coli and purified as a candidate component for an influenza vaccine. We evaluated the immunogenicity and protective efficacy of recombinant influenza TAT-NP vaccine by intranasal immunization. In vitro experiments showed that TAT-NP could efficiently penetrate into cells. Animal results showed that mice vaccinated with TAT-NP could not only induce higher levels of IgG and mucosal IgA, but also elicit a robust cellular immune response. Moreover, the TAT-NP fusion protein could significantly increase the protection of mice against lethal doses of homologous influenza virus PR8 and could also provide mice protection against a lethal dose challenge against heterosubtypic H9N2 and H3N2 influenza virus. In conclusion, the recombinant TAT-NP might be a universal vaccine candidate against influenza virus.