Infection of mice with a human influenza A/H3N2 virus induces protective immunity against lethal infection with influenza A/H5N1 virus

The frequency and function of nucleoprotein-specific CD8+ T cells are critical for heterosubtypic immunity against influenza virus infection

Cytotoxic T lymphocytes (CTLs) mediate host defense against viral and intracellular bacterial infections and tumors. However, the magnitude of CTL response and their function needed to confer heterosubtypic immunity against influenza virus infection are unknown. We addressed the role of CD8+ T cells in the absence of any cross-reactive antibody responses to influenza viral proteins using an adenoviral vector expressing a 9mer amino acid sequence recognized by CD8+ T cells. Our results indicate that both CD8+ T cell frequency and function are crucial for heterosubtypic immunity. Low morbidity, lower viral lung titers, low to minimal lung pathology, and better survival upon heterosubtypic virus challenge correlated with the increased frequency of NP-specific CTLs. NP-CD8+ T cells induced by differential infection doses displayed distinct RNA transcriptome profiles and functional properties. CD8+ T cells induced by a high dose of influenza virus secreted significantly higher levels of IFN-γ and exhibited higher levels of cytotoxic function. The mice that received NP-CD8+ T cells from the high-dose virus recipients through adoptive transfer had lower viral titers following viral challenge than those induced by the low dose of virus, suggesting differential cellular programming by antigen dose. Enhanced NP-CD8+ T-cell functions induced by a higher dose of influenza virus strongly correlated with the increased expression of cellular and metabolic genes, indicating a shift to a more glycolytic metabolic phenotype. These findings have implications for developing effective T cell vaccines against infectious diseases and cancer.

IMPORTANCE

Cytotoxic T lymphocytes (CTLs) are an important component of the adaptive immune system that clears virus-infected cells or tumor cells. Hence, developing next-generation vaccines that induce or recall CTL responses against cancer and infectious diseases is crucial. However, it is not clear if the frequency, function, or both are essential in conferring protection, as in the case of influenza. In this study, we demonstrate that both CTL frequency and function are crucial for providing heterosubtypic immunity to influenza by utilizing an Ad-viral vector expressing a CD8 epitope only to rule out the role of antibodies, single-cell RNA-seq analysis, as well as adoptive transfer experiments. Our findings have implications for developing T cell vaccines against infectious diseases and cancer.

Runx3 Regulates CD8+ T Cell Local Expansion and CD43 Glycosylation in Mice by H1N1 Influenza A Virus Infection

We have recently reported that transcription factor Runx3 is required for pulmonary generation of CD8+ cytotoxic T lymphocytes (CTLs) that play a crucial role in the clearance of influenza A virus (IAV). To understand the underlying mechanisms, we determined the effects of Runx3 knockout (KO) on CD8+ T cell local expansion and phenotypes using an inducible general Runx3 KO mouse model. We found that in contrast to the lungs, Runx3 general KO promoted enlargement of lung-draining mediastinal lymph node (mLN) and enhanced CD8+ and CD4+ T cell expansion during H1N1 IAV infection. We further found that Runx3 deficiency greatly inhibited core 2 O-glycosylation of selectin ligand CD43 on activated CD8+ T cells but minimally affected the cell surface expression of CD43, activation markers (CD44 and CD69) and cell adhesion molecules (CD11a and CD54). Runx3 KO had a minor effect on lung effector CD8+ T cell death by IAV infection. Our findings indicate that Runx3 differently regulates CD8+ T cell expansion in mLNs and lungs by H1N1 IAV infection. Runx3 is required for CD43 core 2 O-glycosylation on activated CD8+ T cells, and the involved Runx3 signal pathway may mediate CD8+ T cell phenotype for pulmonary generation of CTLs.

Severe respiratory viral infections: T-cell functions diverging from immunity to inflammation

Respiratory viral infections such as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and influenza A virus (IAV) trigger distinct clinical outcomes defined by immunity-based viral clearance or disease associated with exaggerated and prolonged inflammation. The important role of T cells in shaping both antiviral immunity and inflammation has revived interest in understanding the host-pathogen interactions that lead to the diverse functions of T cells in respiratory viral infections. Inborn deficiencies and acquired insufficiency in immunity can prolong infection and shift the immune response towards exacerbated inflammation, which results from persistent innate immune activation and bystander T-cell activation that is nonspecific to the pathogen but is often driven by cytokines. This review discusses how virus variants, exposure doses, routes of infection, host genetics, and immune history can modulate the activation and function of T cells, thus influencing clinical outcomes. Knowledge of virus-host interaction can inform strategies to prevent immune dysfunction in respiratory viral infection and help in the treatment of associated diseases.

Heterotypic Influenza Infections Mitigate Susceptibility to Secondary Bacterial Infection

Influenza-associated bacterial superinfections have devastating impacts on the lung and can result in increased risk of mortality. New strains of influenza circulate throughout the population yearly, promoting the establishment of immune memory. Nearly all individuals have some degree of influenza memory before adulthood. Due to this, we sought to understand the role of immune memory during bacterial superinfections. An influenza heterotypic immunity model was established using influenza A/Puerto Rico/8/34 and influenza A/X31. We report in this article that influenza-experienced mice are more resistant to secondary bacterial infection with methicillin-resistant Staphylococcus aureus as determined by wasting, bacterial burden, pulmonary inflammation, and lung leak, despite significant ongoing lung remodeling. Multidimensional flow cytometry and lung transcriptomics revealed significant alterations in the lung environment in influenza-experienced mice compared with naive animals. These include changes in the lung monocyte and T cell compartments, characterized by increased expansion of influenza tetramer-specific CD8+ T cells. The protection that was seen in the memory-experienced mouse model is associated with the reduction in inflammatory mechanisms, making the lung less susceptible to damage and subsequent bacterial colonization. These findings provide insight into how influenza heterotypic immunity reshapes the lung environment and the immune response to a rechallenge event, which is highly relevant to the context of human infection.

Influenza Vaccines: Successes and Continuing Challenges

Influenza vaccines have been available for over 80 years. They have contributed to significant reductions in influenza morbidity and mortality. However, there have been limitations in their effectiveness, in part due to the continuous antigenic evolution of seasonal influenza viruses, but also due to the predominant use of embryonated chicken eggs for their production. The latter furthermore limits their worldwide production timelines and scale. Therefore today, alternative approaches for their design and production are increasingly pursued, with already licensed quadrivalent seasonal influenza vaccines produced in cell cultures, including based on a baculovirus expression system. Next-generation influenza vaccines aim at inducing broader and longer-lasting immune responses to overcome seasonal influenza virus antigenic drift and to timely address the emergence of a new pandemic influenza virus. Tailored approaches target mechanisms to improve vaccine-induced immune responses in individuals with a weakened immune system, in particular older adults.

Impact of Pre-Existing Immunity to Influenza on Live-Attenuated Influenza Vaccine (LAIV) Immunogenicity

During the previous influenza seasons, between 2010 and 2016, the live attenuated influenza vaccine (LAIV) provided variable efficacy against influenza in the U.S., causing the recommendation against the use of the LAIV. In striking contrast, pre-clinical studies have repeatedly demonstrated superior efficacy of LAIV against mismatched influenza viruses, compared to inactivated influenza vaccines (IIV). This disparity in reported vaccine efficacies between pre-clinical and clinical studies may in part be explained by limitations of the animal models of influenza. In particular, the absence of pre-existing immunity in animal models has recently emerged as a potential explanation for the discrepancies between preclinical findings and human studies. This commentary focuses on the potential impact of pre-existing immunity on LAIV induced immunogenicity with an emphasis on cross-protective immunity.

Non-sterilizing, Infection-Permissive Vaccination With Inactivated Influenza Virus Vaccine Reshapes Subsequent Virus Infection-Induced Protective Heterosubtypic Immunity From Cellular to Humoral Cross-Reactive Immune Responses

Conventional influenza vaccines aim at the induction of virus-neutralizing antibodies that provide with sterilizing immunity. However, influenza vaccination often confers protection from disease but not from infection. The impact of infection-permissive vaccination on the immune response elicited by subsequent influenza virus infection is not well-understood. Here, we investigated to what extent infection-permissive immunity, in contrast to virus-neutralizing immunity, provided by a trivalent inactivated virus vaccine (TIV) modulates disease and virus-induced host immune responses after sublethal vaccine-matching H1N1 infection in a mouse model. More than one TIV vaccination was needed to induce a serum HI titer and provide sterilizing immunity upon homologous virus infection. However, single TIV administration provided infection-permissive immunity, characterized by lower viral lung titers and faster recovery. Despite the presence of replicating virus, single TIV vaccination prevented induction of pro-inflammatory cyto- and chemokines, alveolar macrophage depletion as well as the establishment of lung-resident B and T cells after infection. To investigate virus infection-induced cross-protective heterosubtypic immune responses in vaccinated and unvaccinated animals, mice were re-infected with a lethal dose of H3N2 virus 4 weeks after H1N1 infection. Single TIV vaccination did not prevent H1N1 virus infection-induced heterosubtypic cross-protection, but shifted the mechanism of cross-protection from the cellular to the humoral branch of the immune system. These results suggest that suboptimal vaccination with conventional influenza vaccines may still positively modulate disease outcome after influenza virus infection, while promoting humoral heterosubtypic immunity after virus infection.

Immune Imprinting in the Influenza Ferret Model

The initial exposure to influenza virus usually occurs during childhood. This imprinting has long-lasting effects on the immune responses to subsequent infections and vaccinations. Animal models that are used to investigate influenza pathogenesis and vaccination do recapitulate the pre-immune history in the human population. The establishment of influenza pre-immune ferret models is necessary for understanding infection and transmission and for designing efficacious vaccines.

The Quest for a Truly Universal Influenza Vaccine

There is an unmet public health need for a universal influenza vaccine (UIV) to provide broad and durable protection from influenza virus infections. The identification of broadly protective antibodies and cross-reactive T cells directed to influenza viral targets present a promising prospect for the development of a UIV. Multiple targets for cross-protection have been identified in the stalk and head of hemagglutinin (HA) to develop a UIV. Recently, neuraminidase (NA) has received significant attention as a critical component for increasing the breadth of protection. The HA stalk-based approaches have shown promising results of broader protection in animal studies, and their feasibility in humans are being evaluated in clinical trials. Mucosal immune responses and cross-reactive T cell immunity across influenza A and B viruses intrinsic to live attenuated influenza vaccine (LAIV) have emerged as essential features to be incorporated into a UIV. Complementing the weakness of the stand-alone approaches, prime-boost vaccination combining HA stalk, and LAIV is under clinical evaluation, with the aim to increase the efficacy and broaden the spectrum of protection. Preexisting immunity in humans established by prior exposure to influenza viruses may affect the hierarchy and magnitude of immune responses elicited by an influenza vaccine, limiting the interpretation of preclinical data based on naive animals, necessitating human challenge studies. A consensus is yet to be achieved on the spectrum of protection, efficacy, target population, and duration of protection to define a “universal” vaccine. This review discusses the recent advancements in the development of UIVs, rationales behind cross-protection and vaccine designs, and challenges faced in obtaining balanced protection potency, a wide spectrum of protection, and safety relevant to UIVs.

Why Are CD8 T Cell Epitopes of Human Influenza A Virus Conserved?

Universal influenza vaccines against the conserved epitopes of influenza A virus have been proposed to minimize the burden of seasonal outbreaks and prepare for the pandemics. However, it is not clear how rapidly T cell-inducing vaccines will select for viruses that escape these T cell responses. Our mathematical models explore the factors that contribute to the conservation of CD8 T cell epitopes and how rapidly the virus will evolve in response to T cell-inducing vaccines. We identify the key biological parameters to be measured and questions that need to be addressed in future studies.

The high degree of conservation of CD8 T cell epitopes of influenza A virus (IAV) may allow for the development of T cell-inducing vaccines that provide protection across different strains and subtypes. This conservation is not fully explained by functional constraint, since an additional mutation(s) can compensate for the replicative fitness loss of IAV escape variants. Here, we propose three additional mechanisms that contribute to the conservation of CD8 T cell epitopes of IAV. First, influenza-specific CD8 T cells may protect predominantly against severe pathology rather than infection and may have only a modest effect on transmission. Second, polymorphism of the human major histocompatibility complex class I (MHC-I) gene restricts the advantage of an escape variant to only a small fraction of the human population who carry the relevant MHC-I alleles. Finally, infection with CD8 T cell escape variants may result in a compensatory increase in the responses to other epitopes of IAV. We use a combination of population genetics and epidemiological models to examine how the interplay between these mechanisms affects the rate of invasion of IAV escape variants. We conclude that for a wide range of biologically reasonable parameters, the invasion of an escape variant virus will be slow, with a timescale of a decade or more. The results suggest T cell-inducing vaccines do not engender the rapid evolution of IAV. Finally, we identify key parameters whose measurement will allow for more accurate quantification of the long-term effectiveness and impact of universal T cell-inducing influenza vaccines.

IMPORTANCE Universal influenza vaccines against the conserved epitopes of influenza A virus have been proposed to minimize the burden of seasonal outbreaks and prepare for the pandemics. However, it is not clear how rapidly T cell-inducing vaccines will select for viruses that escape these T cell responses. Our mathematical models explore the factors that contribute to the conservation of CD8 T cell epitopes and how rapidly the virus will evolve in response to T cell-inducing vaccines. We identify the key biological parameters to be measured and questions that need to be addressed in future studies.

Efficacy of a Recombinant Turkey Herpesvirus AI (H5) Vaccine in Preventing Transmission of Heterologous Highly Pathogenic H5N8 Clade 2.3.4.4b Challenge Virus in Commercial Broilers and Layer Pullets

Outbreaks caused by the highly pathogenic avian influenza virus (HPAIV) H5N8 subtype clade 2.3.4.4 were first reported in 2014 in South Korea then spread very rapidly in Asia, to Europe, and for the first time, to North America. Efficacy of a recombinant HVT-AI (H5) vaccine (rHVT-H5) to provide clinical protection as well as to significantly reduce the shedding of an H5N8 challenge virus has already been demonstrated in SPF chickens. The aim of our studies was to test the efficacy of the same rHVT-H5 vaccine in controlling the transmission of a recent Hungarian HPAIV H5N8 challenge virus in commercial chickens. Broilers and layers were vaccinated at day old according to the manufacturer's recommendation and then challenged with a 2017 Hungarian HPAIV H5N8 (2.3.4.4b) isolate at 5 or 7 weeks of age, respectively. Evaluation of clinical protection, reduction of challenge virus shedding, and transmission to vaccinated contact birds was done on the basis of clinical signs/mortality, detection, and quantitation of challenge virus in oronasal and cloacal swabs (regularly between 1 and 14 days postchallenge). Measurement of seroconversion to AIV nucleoprotein was used as an indicator of infection and replication of challenge virus. Our results demonstrated that rHVT-H5 vaccination could prevent the development of clinical disease and suppress shedding very efficiently, resulting in the lack of challenge virus transmission to vaccinated contact chickens, regardless the type of birds. Single immunization with the tested rHVT-H5 vaccine proved to be effective to stop HPAIV H5N8 (2.3.4.4b) transmission within vaccinated poultry population under experimental conditions.

Intermediate levels of vaccination coverage may minimize seasonal influenza outbreaks

For most pathogens, vaccination reduces the spread of the infection and total number of cases; thus, public policy usually advocates maximizing vaccination coverage. We use simple mathematical models to explore how this may be different for pathogens, such as influenza, which exhibit strain variation. Our models predict that the total number of seasonal influenza infections is minimized at an intermediate (rather than maximal) level of vaccination, and, somewhat counter-intuitively, further increasing the level of the vaccination coverage may lead to higher number of influenza infections and be detrimental to the public interest. This arises due to the combined effects of: competition between multiple co-circulating strains; limited breadth of protection afforded by the vaccine; and short-term strain-transcending immunity following natural infection. The study highlights the need for better quantification of the components of vaccine efficacy and longevity of strain-transcending cross-immunity in order to generate nuanced recommendations for influenza vaccine coverage levels.

Influenza vaccines: 'tailor-made' or 'one fits all'

Currently used inactivated influenza vaccines aim at the induction of virus-neutralizing antibodies directed to the variable head domain of the viral hemagglutinin. Although these vaccines are effective against antigenically matching virus strains, they offer little protection against antigenically distinct drift variants or potentially pandemic viruses of alternative subtypes. In the last decades, the threat of novel influenza pandemics has sparked research efforts to develop vaccines that induce more broadly protective immunity. Here, we discuss the immune responses induced by conventional 'tailor-made' inactivated and live influenza vaccines and novel 'one fits all' candidate vaccines able to induce cross-reactive virus-specific antibody and T cell responses and to afford protection to a wider range of influenza viruses.

Extensive T cell cross-reactivity between diverse seasonal influenza strains in the ferret model

Influenza virus causes widespread, yearly epidemics by accumulating surface protein mutations to escape neutralizing antibodies established from prior exposure. In contrast to antibody epitopes, T cell mediated immunity targets influenza epitopes that are more highly conserved and have potential for cross-protection. The extent of T cell cross-reactivity between a diverse array of contemporary and historical influenza strains was investigated in ferrets challenged with 2009 pandemic H1N1 influenza or the seasonal H3N2 strain, A/Perth/16/2009. Post-challenge cell-mediated immune responses demonstrated extensive cross-reactivity with a wide variety of contemporary and historical influenza A strains as well as influenza B. Responses in peripheral blood were undetectable by 36d post-challenge, but cross-reactivity persisted in spleen. The strongest responses targeted peptides from the NP protein and demonstrated cross-reactivity in both the CD4+ and CD8+ T cell populations. Cross-reactive CD4+ T cells also targeted HA and NA epitopes, while cross-reactive CD8+ T cells targeted internal M1, NS2, and PA. T cell epitopes demonstrated extensive cross-reactivity between diverse influenza strains in outbred animals, with NP implicated as a significant antigenic target demonstrating extensive cross-reactivity for both CD4+ and CD8+ T cells.

Harnessing the Power of T Cells: The Promising Hope for a Universal Influenza Vaccine

Next-generation vaccines that utilize T cells could potentially overcome the limitations of current influenza vaccines that rely on antibodies to provide narrow subtype-specific protection and are prone to antigenic mismatch with circulating strains. Evidence from animal models shows that T cells can provide heterosubtypic protection and are crucial for immune control of influenza virus infections. This has provided hope for the design of a universal vaccine able to prime against diverse influenza virus strains and subtypes. However, multiple hurdles exist for the realisation of a universal T cell vaccine. Overall primary concerns are: extrapolating human clinical studies, seeding durable effective T cell resident memory (Trm), population human leucocyte antigen (HLA) coverage, and the potential for T cell-mediated immune escape. Further comprehensive human clinical data is needed during natural infection to validate the protective role T cells play during infection in the absence of antibodies. Furthermore, fundamental questions still exist regarding the site, longevity and duration, quantity, and phenotype of T cells needed for optimal protection. Standardised experimental methods, and eventually simplified commercial assays, to assess peripheral influenza-specific T cell responses are needed for larger-scale clinical studies of T cells as a correlate of protection against influenza infection. The design and implementation of a T cell-inducing vaccine will require a consensus on the level of protection acceptable in the community, which may not provide sterilizing immunity but could protect the individual from severe disease, reduce the length of infection, and potentially reduce transmission in the community. Therefore, increasing the standard of care potentially offered by T cell vaccines should be considered in the context of pandemic preparedness and zoonotic infections, and in combination with improved antibody vaccine targeting methods. Current pandemic vaccine preparedness measures and ongoing clinical trials under-utilise T cell-inducing vaccines, reflecting the myriad questions that remain about how, when, where, and which T cells are needed to fight influenza virus infection. This review aims to bring together basic fundamentals of T cell biology with human clinical data, which need to be considered for the implementation of a universal vaccine against influenza that harnesses the power of T cells.

Cross-Protective Immune Responses Induced by Sequential Influenza Virus Infection and by Sequential Vaccination With Inactivated Influenza Vaccines

Sequential infection with antigenically distinct influenza viruses induces cross-protective immune responses against heterologous virus strains in animal models. Here we investigated whether sequential immunization with antigenically distinct influenza vaccines can also provide cross-protection. To this end, we compared immune responses and protective potential against challenge with A(H1N1)pdm09 in mice infected sequentially with seasonal A(H1N1) virus followed by A(H3N2) virus or immunized sequentially with whole inactivated virus (WIV) or subunit (SU) vaccine derived from these viruses. Sequential infection provided solid cross-protection against A(H1N1)pdm09 infection while sequential vaccination with WIV, though not capable of preventing weight loss upon infection completely, protected the mice from reaching the humane endpoint. In contrast, sequential SU vaccination did not prevent rapid and extensive weight loss. Protection correlated with levels of cross-reactive but non-neutralizing antibodies of the IgG2a subclass, general increase of memory T cells and induction of influenza-specific CD4+ and CD8+ T cells. Adoptive serum transfer experiments revealed that despite lacking neutralizing activity, serum antibodies induced by sequential infection protected mice from weight loss and vigorous virus growth in the lungs upon A(H1N1)pdm09 virus challenge. Antibodies induced by WIV vaccination alleviated symptoms but could not control virus growth in the lung. Depletion of T cells prior to challenge revealed that CD8+ T cells, but not CD4+ T cells, contributed to cross-protection. These results imply that sequential immunization with WIV but not SU derived from antigenically distinct viruses could alleviate the severity of infection caused by a pandemic and may improve protection to unpredictable seasonal infection.

Heat-killed Lactobacillus casei confers broad protection against influenza A virus primary infection and develops heterosubtypic immunity against future secondary infection

Lactic acid bacteria (LAB) are the common probiotics. Here, we investigated the antiviral protective effects of heat-killed LAB strain Lactobacillus casei DK128 (DK128) on influenza viruses. Intranasal treatment of mice with DK128 conferred protection against different subtypes of influenza viruses by lessening weight loss and lowering viral loads. Protection via heat-killed DK128 was correlated with an increase in alveolar macrophage cells in the lungs and airways, early induction of virus specific antibodies, reduced levels of pro-inflammatory cytokines and innate immune cells. Importantly, the mice that were protected against primary viral infection as a result of heat-killed DK128 pretreatment developed subsequent heterosubtypic immunity against secondary virus infection. For protection against influenza virus via heat-killed DK128 pretreatment, B cells and partially CD4 T cells but not CD8 T cells were required as inferred from studies using knockout mouse models. Our study provides insight into how hosts can be equipped with innate and adaptive immunity via heat-killed DK128 treatment to protect against influenza virus, supporting that heat-killed LAB may be developed as anti-virus probiotics.

Increased Protein Degradation Improves Influenza Virus Nucleoprotein-Specific CD8+ T Cell Activation In Vitro but Not in C57BL/6 Mice

Due to antigenic drift of influenza viruses, seasonal influenza vaccines need to be updated annually. These vaccines are based on predictions of strains likely to circulate in the next season. However, vaccine efficacy is greatly reduced in the case of a mismatch between circulating and vaccine strains. Furthermore, novel antigenically distinct influenza viruses are introduced into the human population from animal reservoirs occasionally and may cause pandemic outbreaks. To dampen the impact of seasonal and pandemic influenza, vaccines that induce broadly protective and long-lasting immunity are preferred. Because influenza virus-specific CD8⁺ T cells are directed mainly against relatively conserved internal proteins, like nucleoprotein (NP), they are highly cross-reactive and afford protection against infection with antigenically distinct influenza virus strains, so-called heterosubtypic immunity. Here, we used modified vaccinia virus Ankara (MVA) as a vaccine vector for the induction of influenza virus NP-specific CD8⁺ T cells. To optimize the induction of CD8⁺ T cell responses, we made several modifications to NP, aiming at retaining the protein in the cytosol or targeting it to the proteasome. We hypothesized that these strategies would increase antigen processing and presentation and thus improve the induction of CD8⁺ T cell responses. We showed that NP with increased degradation rates improved CD8⁺ T cell activation in vitro if the amount of antigen was limited or if CD8⁺ T cells were of low functional avidity. However, after immunization of C57BL/6 mice, no differences were detected between modified NP and wild-type NP (NPwt), since NPwt already induced optimal CD8⁺ T cell responses.

IMPORTANCE

Due to the continuous antigenic drift of seasonal influenza viruses and the threat of a novel pandemic, there is a great need for the development of novel influenza vaccines that offer broadly protective immunity against multiple subtypes. CD8⁺ T cells can provide immunity against multiple subtypes of influenza viruses by the recognition of relatively conserved internal antigens. In this study, we aimed at optimizing the CD8⁺ T cell response to influenza A virus by making modifications to influenza A virus nucleoprotein (NP) expressed from the modified vaccinia virus Ankara (MVA) vaccine vector. These modifications resulted in increased antigen degradation, thereby producing elevated levels of peptides that can be presented on major histocompatibility complex (MHC) class I molecules to CD8⁺ T cells. Although we were unable to increase the NP-specific immune response in the mouse strain used, this approach may have benefits for vaccine development using less-immunogenic proteins.

A plant-derived quadrivalent virus like particle influenza vaccine induces cross-reactive antibody and T cell response in healthy adults

Recent issues regarding efficacy of influenza vaccines have re-emphasized the need of new approaches to face this major public health issue. In a phase 1-2 clinical trial, healthy adults received one intramuscular dose of a seasonal influenza plant-based quadrivalent virus-like particle (QVLP) vaccine or placebo. The hemagglutination inhibition (HI) titers met all the European licensure criteria for the type A influenza strains at the 3μg/strain dose and for all four strains at the higher dosages 21days after immunization. High HI titers were maintained for most of the strains 6months after vaccination. QVLP vaccine induced a substantial and sustained increase of hemagglutinin-specific polyfunctional CD4 T cells, mainly transitional memory and TEMRA effector IFN-γ(+) CD4 T cells. A T cells cross-reactive response was also observed against A/Hong-Kong/1/1968 H3N2 and B/Massachusetts/2/2012. Plant-based QVLP offers an attractive alternative manufacturing method for producing effective and HA-strain matching seasonal influenza vaccines.

Mathematical Model Reveals the Role of Memory CD8 T Cell Populations in Recall Responses to Influenza

The current influenza vaccine provides narrow protection against the strains included in the vaccine, and needs to be reformulated every few years in response to the constantly evolving new strains. Novel approaches are directed toward developing vaccines that provide broader protection by targeting B and T cell epitopes that are conserved between different strains of the virus. In this paper, we focus on developing mathematical models to explore the CD8 T cell responses to influenza, how they can be boosted, and the conditions under which they contribute to protection. Our models suggest that the interplay between spatial heterogeneity (with the virus infecting the respiratory tract and the immune response being generated in the secondary lymphoid organs) and T cell differentiation (with proliferation occurring in the lymphoid organs giving rise to a subpopulation of resident T cells in the respiratory tract) is the key to understand the dynamics of protection afforded by the CD8 T cell response to influenza. Our results suggest that the time lag for the generation of resident T cells in the respiratory tract and their rate of decay following infection are the key factors that limit the efficacy of CD8 T cell responses. The models predict that an increase in the level of central memory T cells leads to a gradual decrease in the viral load, and, in contrast, there is a sharper protection threshold for the relationship between the size of the population of resident T cells and protection. The models also suggest that repeated natural influenza infections cause the number of central memory CD8 T cells and the peak number of resident memory CD8 T cells to reach their plateaus, and while the former is maintained, the latter decays with time since the most recent infection.

Universal influenza vaccines: a realistic option?

The extensive antigenic drift displayed by seasonal influenza viruses and the risk of pandemics caused by newly emerging antigenically distinct influenza A viruses of novel subtypes has raised considerable interest in the development of so-called universal influenza vaccines. We review options for the development of universal flu vaccines and discuss progress that has been made recently.

Long-Lasting Cross-Protection Against Influenza A by Neuraminidase and M2e-based immunization strategies

There is mounting evidence that in the absence of neutralizing antibodies cross-reactive T cells provide protection against pandemic influenza viruses. Here, we compared protection and CD8+ T cell responses following challenge with H1N1 2009 pandemic and H3N2 viruses of mice that had been immunized with hemagglutinin (HA), neuraminidase (NA) and the extracellular domain of matrix protein 2 (M2e) fused to a virus-like particle (VLP). Mice were challenged a first time with a sublethal dose of H1N1 2009 pandemic virus and, four weeks later, challenged again with an H3N2 virus. Mice that had been vaccinated with HA, NA, NA + M2e-VLP and HA + NA + M2e-VLP were protected against homologous H1N1 virus challenge. Challenged NA and NA + M2e-VLP vaccinated mice mounted CD8+ T cell responses that correlated with protection against secondary H3N2 challenge. HA-vaccinated mice were fully protected against challenge with homologous H1N1 2009 virus, failed to mount cross-reactive CD8+ T cells and succumbed to the second challenge with heterologous H3N2 virus. In summary, NA- and M2e-based immunity can protect against challenge with (homologous) virus without compromising the induction of robust cross-reactive CD8+ T cell responses upon exposure to virus.

Human influenza viruses and CD8(+) T cell responses

Influenza A viruses (IAVs) cause significant morbidity and mortality worldwide, despite new strain-specific vaccines being available annually. As IAV-specific CD8(+) T cells promote viral control in the absence of neutralizing antibodies, and can mediate cross-reactive immunity toward distinct IAVs to drive rapid recovery from both mild and severe influenza disease, there is great interest in developing a universal T cell vaccine. However, despite detailed studies in mouse models of influenza virus infection, there is still a paucity of data on human epitope-specific CD8(+) T cell responses to IAVs. This review focuses on our current understanding of human CD8(+) T cell immunity against distinct IAVs and discusses the possibility of achieving a CD8(+) T cell mediated-vaccine that protects against multiple, distinct IAV strains across diverse human populations. We also review the importance of CD8(+) T cell immunity in individuals highly susceptible to severe influenza infection, including those hospitalised with influenza, the elderly and Indigenous populations.

Possible Impact of Yearly Childhood Vaccination With Trivalent Inactivated Influenza Vaccine (TIV) on the Immune Response to the Pandemic Strain H1N1

BACKGROUND

Annual vaccination of children against seasonal influenza with trivalent inactivated influenza vaccine (TIV) has shown to be beneficial. However, this yearly practice may have unintended effect. Studies have shown that infection with wild type influenza A viruses can stimulate protective heterotypic immunity against unrelated or new influenza subtypes. We hypothesized that a consequence of yearly TIV vaccination is lack of induction of heterotypic immunity against the recent H1N1 pandemic.

METHODS

This was a retrospective case-control study. We reviewed the medical records of polymerase chain reaction-confirmed cases of 2009 H1N1 influenza infection in children 6 months to 18 years and a matched control group seen during the pandemic.

RESULTS

We identified 353 polymerase chain reaction-confirmed H1N1 cases and 396 matching control subjects. Among the H1N1 group, 202/353 (57%) cases received a total of 477 doses of seasonal TIV compared with 218/396 (55%) in the control group who received a total of 435 doses. Seasonal TIV uptake was significantly higher in the H1N1 group 477/548 (87%) than in the control group, 435/532 (81%) (P = .017).

CONCLUSION

Seasonal TIV uptake was significantly higher in H1N1-infected group. The finding suggests that the practice of yearly vaccination with TIV might have negatively affected the immune response against the novel pandemic H1N1 strain. Given the rarity of pandemic novel influenza viruses, and the high predictability of seasonal influenza occurrence, the practice of yearly influenza vaccination should be continued. However, the use of live attenuated intranasal vaccine, as opposed to TIV, may allow for the desirable development of a vigorous heterotypic immune response against future pandemics.

Balancing Immune Protection and Immune Pathology by CD8(+) T-Cell Responses to Influenza Infection

Influenza A virus (IAV) is a significant human pathogen causing annual epidemics and periodic pandemics. CD8⁺ cytotoxic T lymphocyte (CTL)-mediated immunity contributes to the clearance of virus-infected cells, and CTL immunity targeting the conserved internal proteins of IAVs is a key protection mechanism when neutralizing antibodies are absent during heterosubtypic IAV infection. However, CTL infiltration into the airways, its cytotoxicity, and the effects of produced proinflammatory cytokines can cause severe lung tissue injury, thereby contributing to immunopathology. Studies have discovered complicated and exquisite stimulatory and inhibitory mechanisms that regulate CTL magnitude and effector activities during IAV infection. Here, we review the state of knowledge on the roles of IAV-specific CTLs in immune protection and immunopathology during IAV infection in animal models, highlighting the key findings of various requirements and constraints regulating the balance of immune protection and pathology involved in CTL immunity. We also discuss the evidence of cross-reactive CTL immunity as a positive correlate of cross-subtype protection during secondary IAV infection in both animal and human studies. We argue that the effects of CTL immunity on protection and immunopathology depend on multiple layers of host and viral factors, including complex host mechanisms to regulate CTL magnitude and effector activity, the pathogenic nature of the IAV, the innate response milieu, and the host historical immune context of influenza infection. Future efforts are needed to further understand these key host and viral factors, especially to differentiate those that constrain optimally effective CTL antiviral immunity from those necessary to restrain CTL-mediated non-specific immunopathology in the various contexts of IAV infection, in order to develop better vaccination and therapeutic strategies for modifying protective CTL immunity.

Influenza B viruses: not to be discounted

In contrast to influenza A viruses, which have been investigated extensively, influenza B viruses have attracted relatively little attention. However, influenza B viruses are an important cause of morbidity and mortality in the human population and full understanding of their biological and epidemiological properties is imperative to better control this important pathogen. However, some of its characteristics are still elusive and warrant investigation. Here, we review evolution, epidemiology, pathogenesis and immunity and identify gaps in our knowledge of influenza B viruses. The divergence of two antigenically distinct influenza B viruses is highlighted. The co-circulation of viruses of these two lineages necessitated the development of quadrivalent influenza vaccines, which is discussed in addition to possibilities to develop universal vaccination strategies.

Heterosubtypic immunity to H7N9 influenza virus in isogenic guinea pigs after infection with pandemic H1N1 virus

Heterosubtypic immunity is defined as immune-mediated (partial) protection against an influenza virus induced by an influenza virus of another subtype to which the host has not previously been exposed. This cross-protective effect has not yet been demonstrated to the newly emerging avian influenza A viruses of the H7N9 subtype. Here, we assessed the induction of protective immunity to these viruses by infection with A(H1N1)pdm09 virus in a newly developed guinea pig model. To this end, ten female 12-16 week old strain 2 guinea pigs were inoculated intratracheally with either A(H1N1)pdm09 influenza virus or PBS (unprimed controls) followed 4 weeks later with an A/H7N9 influenza virus challenge. Nasal swabs were taken daily and animals from both groups were sacrificed on days 2 and 7 post inoculation (p.i.) with A/H7N9 virus and full necropsies were performed. Nasal virus excretion persisted until day 7 in unprimed control animals, whereas only two out of seven H1N1pdm09-primed animals excreted virus via the nose. Infectious virus was recovered from nasal turbinates, trachea and lung of all animals at day 2 p.i., but titers were lower for H1N1pdm09-primed animals, especially in the nasal turbinates. By day 7 p.i., relatively high virus titers were found in the nasal turbinates of all unprimed control animals but infectious virus was isolated from the nose of only one of four H1N1pdm09-primed animals. Animals of both groups developed inflammation of variable severity in the entire respiratory tract. Viral antigen positive cells were demonstrated in the nasal epithelium of both groups at day 2. The bronchi(oli) and alveoli of unprimed animals showed a moderate to strong positive signal at day 2, whereas H1N1pdm09-primed animals showed only minimal positivity. By day 7, only viral antigen positive cells were found after H7N9 virus infection in the nasal turbinates and the lungs of unprimed controls. Thus infection with H1N1pdm09 virus induced partially protective heterosubtypic immunity to H7N9 virus in (isogenic) guinea pigs that could not be attributed to cross-reactive virus neutralizing antibodies.

Computational Characterization of Transient Strain-Transcending Immunity against Influenza A

The enigmatic observation that the rapidly evolving influenza A (H3N2) virus exhibits, at any given time, a limited standing genetic diversity has been an impetus for much research. One of the first generative computational models to successfully recapitulate this pattern of consistently constrained diversity posits the existence of a strong and short-lived strain-transcending immunity. Building on that model, we explored a much broader set of scenarios (parameterizations) of a transient strain-transcending immunity, ran long-term simulations of each such scenario, and assessed its plausibility with respect to a set of known or estimated influenza empirical measures. We evaluated simulated outcomes using a variety of measures, both epidemiological (annual attack rate, epidemic duration, reproductive number, and peak weekly incidence), and evolutionary (pairwise antigenic diversity, fixation rate, most recent common ancestor, and kappa, which quantifies the potential for antigenic evolution). Taking cumulative support from all these measures, we show which parameterizations of strain-transcending immunity are plausible with respect to the set of empirically derived target values. We conclude that strain-transcending immunity which is milder and longer lasting than previously suggested is more congruent with the observed short- and long-term behavior of influenza.

Influenza B virus-specific CD8+ T-lymphocytes strongly cross-react with viruses of the opposing influenza B lineage

Influenza B viruses fall in two antigenically distinct lineages (B/Victoria/2/1987 and B/Yamagata/16/1988 lineage) that co-circulate with influenza A viruses of the H3N2 and H1N1 subtypes during seasonal epidemics. Infections with influenza B viruses contribute considerably to morbidity and mortality in the human population. Influenza B virus neutralizing antibodies, elicited by natural infections or vaccination, poorly cross-react with viruses of the opposing influenza B lineage. Therefore, there is an increased interest in identifying other correlates of protection which could aid the development of broadly protective vaccines. blast analysis revealed high sequence identity of all viral proteins. With two online epitope prediction algorithms, putative conserved epitopes relevant for study subjects used in the present study were predicted. The cross-reactivity of influenza B virus-specific polyclonal CD8+ cytotoxic T-lymphocyte (CTL) populations obtained from HLA-typed healthy study subjects, with intra-lineage drift variants and viruses of the opposing lineage, was determined by assessing their in vitro IFN-γ response and lytic activity. Here, we show for the first time, to the best of our knowledge, that CTLs directed to viruses of the B/Victoria/2/1987 lineage cross-react with viruses of the B/Yamagata/16/1988 lineage and vice versa.

Developing Universal Influenza Vaccines: Hitting the Nail, Not Just on the Head

Influenza viruses have a huge impact on public health. Current influenza vaccines need to be updated annually and protect poorly against antigenic drift variants or novel emerging subtypes. Vaccination against influenza can be improved in two important ways, either by inducing more broadly protective immune responses or by decreasing the time of vaccine production, which is relevant especially during a pandemic outbreak. In this review, we outline the current efforts to develop so-called “universal influenza vaccines”, describing antigens that may induce broadly protective immunity and novel vaccine production platforms that facilitate timely availability of vaccines.

Diverse heterologous primary infections radically alter immunodominance hierarchies and clinical outcomes following H7N9 influenza challenge in mice

The recent emergence of a novel H7N9 influenza A virus (IAV) causing severe human infections in China raises concerns about a possible pandemic. The lack of pre-existing neutralizing antibodies in the broader population highlights the potential protective role of IAV-specific CD8+ cytotoxic T lymphocyte (CTL) memory specific for epitopes conserved between H7N9 and previously encountered IAVs. In the present study, the heterosubtypic immunity generated by prior H9N2 or H1N1 infections significantly, but variably, reduced morbidity and mortality, pulmonary virus load and time to clearance in mice challenged with the H7N9 virus. In all cases, the recall of established CTL memory was characterized by earlier, greater airway infiltration of effectors targeting the conserved or cross-reactive H7N9 IAV peptides; though, depending on the priming IAV, each case was accompanied by distinct CTL epitope immunodominance hierarchies for the prominent K^bPB1₇₀₃, D^bPA₂₂₄, and D^bNP₃₆₆ epitopes. While the presence of conserved, variable, or cross-reactive epitopes between the priming H9N2 and H1N1 and the challenge H7N9 IAVs clearly influenced any change in the immunodominance hierarchy, the changing patterns were not tied solely to epitope conservation. Furthermore, the total size of the IAV-specific memory CTL pool after priming was a better predictor of favorable outcomes than the extent of epitope conservation or secondary CTL expansion. Modifying the size of the memory CTL pool significantly altered its subsequent protective efficacy on disease severity or virus clearance, confirming the important role of heterologous priming. These findings establish that both the protective efficacy of heterosubtypic immunity and CTL immunodominance hierarchies are reflective of the immunological history of the host, a finding that has implications for understanding human CTL responses and the rational design of CTL-mediated vaccines.

The emergence of human infections with a novel strain of avian-origin H7N9 virus in China raises a pandemic concern. The introduction of a new subtype in humans makes people at all ages susceptible due to the lack of population-wide neutralizing antibodies. However, cross-subtype protection from existing host immunity might provide important protection that can limit severe disease. Our study found that previous infection with non-H7N9 subtype viruses such as H9N2 viruses or H1N1 viruses could provide protection against lethal H7N9 challenge to varying degrees in mice. The virus-specific memory CD8+ T cells generated by the previous infection but targeting conserved or related portions of the internal proteins (epitopes) of the H7N9 viruses were selectively expanded and recruited at very early time points after H7N9 challenge, contributing to protective efficacy. The magnitude of the priming virus-generated memory CD8+ T cells was the best predictor of the protective efficacy of the heterosubtypic immunity against subsequent H7N9 challenge in these animals, when compared to the magnitude of the challenge response or the degree of epitope conservation between the priming and challenge strains. These results demonstrate the complexity of cross-reactive CD8+ T cell dynamics and suggest that significant protective immunity can be present even when few epitopes are conserved during heterosubtypic influenza infection.

Influenza virus-like particle vaccines made in Nicotiana benthamiana elicit durable, poly-functional and cross-reactive T cell responses to influenza HA antigens

Cell-mediated immunity plays a major role in long-lived, cross-reactive protection against influenza virus. We measured long-term poly-functional and cross-reactive T cell responses to influenza hemagglutinin (HA) elicited by a new plant-made Virus-Like Particle (VLP) vaccine targeting either H1N1 A/California/7/09 (H1) or H5N1 A/Indonesia/5/05 (H5). In two independent clinical trials, we characterized the CD4(+) and CD8(+) T cell homotypic and heterotypic responses 6 months after different vaccination regimens. Responses of VLP-vaccinated subjects were compared with placebo and/or a commercial trivalent inactivated vaccine (TIV:Fluzone™) recipients. Both H1 and H5 VLP vaccines elicited significantly greater poly-functional CD4(+) T cell responses than placebo and TIV. Poly-functional CD8(+) T cell responses were also observed after H1 VLP vaccination. Our results show that plant-made HA VLP vaccines elicit both strong antibody responses and poly-functional, cross-reactive memory T cells that persist for at least 6 months after vaccination.

Infection of influenza virus neuraminidase-vaccinated mice with homologous influenza virus leads to strong protection against heterologous influenza viruses

Vaccination is the best measure to prevent influenza pandemics. Here, we studied the protective effect against heterologous influenza viruses, including A/reassortant/NYMC X-179A (pH1N1), A/Chicken/Henan/12/2004 (H5N1), A/Chicken/Jiangsu/7/2002 (H9N2) and A/Guizhou/54/89×A/PR/8/34 (A/Guizhou-X) (H3N2), in mice first vaccinated with a DNA vaccine of haemagglutinin (HA) or neuraminidase (NA) of A/PR/8/34 (PR8) and then infected with the homologous virus. We showed that PR8 HA or NA vaccination both protected mice against a lethal dose of the homologous virus; PR8 HA or NA DNA vaccination and then PR8 infection in mice offered poor or excellent protection, respectively, against a second, heterologous influenza virus challenge. In addition, before the second heterologous influenza infection, the highest antibody level against nucleoprotein (NP) and matrix (M1 and M2) proteins was found in the PR8 NA-vaccinated and PR8-infected group. The level of induced cellular immunity against NP and M1 showed a trend consistent with that seen in antibody levels. However, PR8 HA+NA vaccination and then PR8 infection resulted in limited protection against heterologous influenza virus challenge. Results of the present study demonstrated that infection of the homologous influenza virus in mice already immunized with a NA vaccine could provide excellent protection against subsequent infection of a heterologous influenza virus. These findings suggested that NA, a major antigen of influenza virus, could be an important candidate antigen for universal influenza vaccines.

Novel G3/DT adjuvant promotes the induction of protective T cells responses after vaccination with a seasonal trivalent inactivated split-virion influenza vaccine

Vaccines used against seasonal influenza are poorly effective against influenza A viruses of novel subtypes that may have pandemic potential. Furthermore, pre(pandemic) influenza vaccines are poorly immunogenic, which can be overcome by the use of adjuvants. A limited number of adjuvants has been approved for use in humans, however there is a need for alternative safe and effective adjuvants that can enhance the immunogenicity of influenza vaccines and that promote the induction of broad-protective T cell responses. Here we evaluated a novel nanoparticle, G3, as an adjuvant for a seasonal trivalent inactivated influenza vaccine in a mouse model. The G3 adjuvant was formulated with or without steviol glycosides (DT, for diterpenoid). The use of both formulations enhanced the virus-specific antibody response to all three vaccine strains considerably. The adjuvants were well tolerated without any signs of discomfort. To assess the protective potential of the vaccine-induced immune responses, an antigenically distinct influenza virus strain, A/Puerto Rico/8/34 (A/PR/8/34), was used for challenge infection. The vaccine-induced antibodies did not cross-react with strain A/PR/8/34 in HI and VN assays. However, mice immunized with the G3/DT-adjuvanted vaccine were partially protected against A/PR/8/34 infection, which correlated with the induction of anamnestic virus-specific CD8(+) T cell responses that were not observed with the use of G3 without DT. Both formulations induced maturation of human dendritic cells and promoted antigen presentation to a similar extent. In conclusion, G3/DT is a promising adjuvant formulation that not only potentiates the antibody response induced by influenza vaccines, but also induces T cell immunity which could afford broader protection against antigenically distinct influenza viruses.

Humans and ferrets with prior H1N1 influenza virus infections do not exhibit evidence of original antigenic sin after infection or vaccination with the 2009 pandemic H1N1 influenza virus

The hypothesis of original antigenic sin (OAS) states that the imprint established by an individual's first influenza virus infection governs the antibody response thereafter. Subsequent influenza virus infection results in an antibody response against the original infecting virus and an impaired immune response against the newer influenza virus. The purpose of our study was to seek evidence of OAS after infection or vaccination with the 2009 pandemic H1N1 (2009 pH1N1) virus in ferrets and humans previously infected with H1N1 viruses with various antigenic distances from the 2009 pH1N1 virus, including viruses from 1935 through 1999. In ferrets, seasonal H1N1 priming did not diminish the antibody response to infection or vaccination with the 2009 pH1N1 virus, nor did it diminish the T-cell response, indicating the absence of OAS in seasonal H1N1 virus-primed ferrets. Analysis of paired samples of human serum taken before and after vaccination with a monovalent inactivated 2009 pH1N1 vaccine showed a significantly greater-fold rise in the titer of antibody against the 2009 pH1N1 virus than against H1N1 viruses that circulated during the childhood of each subject. Thus, prior experience with H1N1 viruses did not result in an impairment of the antibody response against the 2009 pH1N1 vaccine. Our data from ferrets and humans suggest that prior exposure to H1N1 viruses did not impair the immune response against the 2009 pH1N1 virus.

Building and optimizing a virus-specific T cell receptor library for targeted immunotherapy in viral infections

Restoration of antigen-specific T cell immunity has the potential to clear persistent viral infection. T cell receptor (TCR) gene therapy can reconstitute CD8 T cell immunity in chronic patients. We cloned 10 virus-specific TCRs targeting 5 different viruses, causing chronic and acute infection. All 10 TCR genetic constructs were optimized for expression using a P2A sequence, codon optimization and the addition of a non-native disulfide bond. However, maximum TCR expression was only achieved after establishing the optimal orientation of the alpha and beta chains in the expression cassette; 9/10 TCRs favored the beta-P2A-alpha orientation over alpha-P2A-beta. Optimal TCR expression was associated with a significant increase in the frequency of IFN-gamma+ T cells. In addition, activating cells for transduction in the presence of Toll-like receptor ligands further enhanced IFN-gamma production. Thus, we have built a virus-specific TCR library that has potential for therapeutic intervention in chronic viral infection or virus-related cancers.

Human cytotoxic T lymphocytes directed to seasonal influenza A viruses cross-react with the newly emerging H7N9 virus

In February 2013, zoonotic transmission of a novel influenza A virus of the H7N9 subtype was reported in China. Although at present no sustained human-to-human transmission has been reported, a pandemic outbreak of this H7N9 virus is feared. Since neutralizing antibodies to the hemagglutinin (HA) globular head domain of the virus are virtually absent in the human population, there is interest in identifying other correlates of protection, such as cross-reactive CD8(+) T cells (cytotoxic T lymphocytes [CTLs]) elicited during seasonal influenza A virus infections. These virus-specific CD8(+) T cells are known to recognize conserved internal proteins of influenza A viruses predominantly, but it is unknown to what extent they cross-react with the newly emerging H7N9 virus. Here, we assessed the cross-reactivity of seasonal H3N2 and H1N1 and pandemic H1N1 influenza A virus-specific polyclonal CD8(+) T cells, obtained from HLA-typed study subjects, with the novel H7N9 virus. The cross-reactivity of CD8(+) T cells to H7N9 variants of known influenza A virus epitopes and H7N9 virus-infected cells was determined by their gamma interferon (IFN-γ) response and lytic activity. It was concluded that, apart from recognition of individual H7N9 variant epitopes, CD8(+) T cells to seasonal influenza viruses display considerable cross-reactivity with the novel H7N9 virus. The presence of these cross-reactive CD8(+) T cells may afford some protection against infection with the new virus.

Assessment of influenza virus hemagglutinin stalk-based immunity in ferrets

Therapeutic monoclonal antibodies that target the conserved stalk domain of the influenza virus hemagglutinin and stalk-based universal influenza virus vaccine strategies are being developed as promising countermeasures for influenza virus infections. The pan-H1-reactive monoclonal antibody 6F12 has been extensively characterized and shows broad efficacy against divergent H1N1 strains in the mouse model. Here we demonstrate its efficacy against a pandemic H1N1 challenge virus in the ferret model of influenza disease. Furthermore, we recently developed a universal influenza virus vaccine strategy based on chimeric hemagglutinin constructs that focuses the immune response on the conserved stalk domain of the hemagglutinin. Here we set out to test this vaccination strategy in the ferret model. Both strategies, pretreatment of animals with a stalk-reactive monoclonal antibody and vaccination with chimeric hemagglutinin-based constructs, were able to significantly reduce viral titers in nasal turbinates, lungs, and olfactory bulbs. In addition, vaccinated animals also showed reduced nasal wash viral titers. In summary, both strategies showed efficacy in reducing viral loads after an influenza virus challenge in the ferret model.

IMPORTANCE

Influenza virus hemagglutinin stalk-reactive antibodies tend to be less potent yet are more broadly reactive and can neutralize seasonal and pandemic influenza virus strains. The ferret model was used to assess the potential of hemagglutinin stalk-based immunity to provide protection against influenza virus infection. The novelty and significance of the findings described in this report support the development of vaccines stimulating stalk-specific antibody responses.

H1N1, but not H3N2, influenza A virus infection protects ferrets from H5N1 encephalitis

Seasonal influenza causes substantial morbidity and mortality because of efficient human-to-human spread. Rarely, zoonotic strains of influenza virus spread to humans, where they have the potential to mediate new pandemics with high mortality. We studied systemic viral spread after intranasal infection with highly pathogenic avian influenza virus (H5N1 [A/Viet Nam/1203/2004]) in ferrets with or without prior pandemic H1N1pdm09 (A/Mexico/4108/2009) or H3N2 (A/Victoria/361/2011) infection. After intranasal challenge with H5N1 influenza virus, naive ferrets rapidly succumbed to systemic infection. Animals challenged with H5N1 influenza virus greater than 3 months after recovering from an initial H1N1pdm09 infection survived H5N1 virus challenge and cleared virus from the respiratory tract 4 days after infection. However, a prolonged low-level infection of hematopoietic elements in the small bowel lamina propria, liver, and spleen was present for greater than 2 weeks postinfection, raising the potential for reassortment of influenza genes in a host infected with multiple strains of influenza. Animals previously infected with an H3N2 influenza virus succumbed to systemic disease and encephalitis after H5N1 virus challenge. These results indicate prior infection with different seasonal influenza strains leads to radically different protection from H5N1 challenge and fatal encephalitis.

IMPORTANCE

Seasonal influenza is efficiently transmitted from human to human, causing substantial morbidity and mortality. Rarely, zoonotic strains of influenza virus spread to humans, where they have the potential to mediate new pandemics with high mortality. Infection of naive ferrets with H5N1 avian influenza virus causes a rapid and lethal systemic disease. We studied systemic H5N1 viral spread after infection of ferrets with or without prior exposure to either of two seasonal influenza virus strains, H1N1 and H3N2. Ferrets previously infected with H1N1 survive H5N1 challenge while those previously infected with H3N2 die of encephalitis. However ferrets protected from lethal H5N1 infection develop persistent low-level infection of the small intestine, liver, or spleen, providing a nidus for future viral strain recombination. The mechanism by which prior infection with specific strains of seasonal influenza virus protect from lethal H5N1 challenge needs to be elucidated in order to design effective immunization and treatments.

Cellular immune correlates of protection against symptomatic pandemic influenza

The role of T cells in mediating heterosubtypic protection against natural influenza illness in humans is uncertain. The 2009 H1N1 pandemic (pH1N1) provided a unique natural experiment to determine whether crossreactive cellular immunity limits symptomatic illness in antibody-naive individuals. We followed 342 healthy adults through the UK pandemic waves and correlated the responses of pre-existing T cells to the pH1N1 virus and conserved core protein epitopes with clinical outcomes after incident pH1N1 infection. Higher frequencies of pre-existing T cells to conserved CD8 epitopes were found in individuals who developed less severe illness, with total symptom score having the strongest inverse correlation with the frequency of interferon-γ (IFN-γ)(+) interleukin-2 (IL-2)(-) CD8(+) T cells (r = -0.6, P = 0.004). Within this functional CD8(+)IFN-γ(+)IL-2(-) population, cells with the CD45RA(+) chemokine (C-C) receptor 7 (CCR7)(-) phenotype inversely correlated with symptom score and had lung-homing and cytotoxic potential. In the absence of crossreactive neutralizing antibodies, CD8(+) T cells specific to conserved viral epitopes correlated with crossprotection against symptomatic influenza. This protective immune correlate could guide universal influenza vaccine development.

Cross-protective immunity against influenza A/H1N1 virus challenge in mice immunized with recombinant vaccine expressing HA gene of influenza A/H5N1 virus

Background

Influenza virus undergoes constant antigenic evolution, and therefore influenza vaccines must be reformulated each year. Time is necessary to produce a vaccine that is antigenically matched to a pandemic strain. A goal of many research works is to produce universal vaccines that can induce protective immunity to influenza A viruses of various subtypes. Despite intensive studies, the precise mechanisms of heterosubtypic immunity (HSI) remain ambiguous.

Method

In this study, mice were vaccinated with recombinant virus vaccine (rL H5), in which the hemagglutinin (HA) gene of influenza A/H5N1 virus was inserted into the LaSota Newcastle disease virus (NDV) vaccine strain. Following a challenge with influenza A/H1N1 virus, survival rates and lung index of mice were observed. The antibodies to influenza virus were detected using hemagglutination inhibition (HI). The lung viral loads, lung cytokine levels and the percentages of both IFN-γ⁺CD4⁺ and IFN-γ⁺CD8⁺ T cells in spleen were detected using real-time RT-PCR, ELISA and flow cytometry respectively.

Results

In comparison with the group of mice given phosphate-buffered saline (PBS), the mice vaccinated with rL H5 showed reductions in lung index and viral replication in the lungs after a challenge with influenza A/H1N1 virus. The antibody titer in group 3 (H1N1-H1N1) was significantly higher than that in other groups which only low levels of antibody were detected. IFN-γ levels increased in both group 1 (rL H5-H1N1) and group 2 (rL H5 + IL-2-H1N1). And the IFN-γ level of group 2 was significantly higher than that of group 1. The percentages of both IFN-γ⁺CD4⁺ and IFN-γ⁺CD8⁺ T cells in group 1 (rL H5-H1N1) and group 2 (rL H5 + IL-2-H1N1) increased significantly, as measured by flow cytometry.

Conclusion

After the mice were vaccinated with rL H5, cross-protective immune response was induced, which was against heterosubtypic influenza A/H1N1 virus. To some extent, cross-protective immune response can be enhanced by IL-2 as an adjuvant. Cellular immune responses may play an important role in HSI against influenza virus.

Towards a universal influenza vaccine: volunteer virus challenge studies in quarantine to speed the development and subsequent licensing

There are now more than 5 experimental vaccine formulations which induce T and B cell immunity towards the internally situated virus proteins matrix (M1 and M2e) and nucleoprotein (NP), and towards stem and stalk regions of the HA which have a shared antigenic structure amongst many of the 17 influenza A virus sub types. Such 'universal vaccines' could be used, at least in theory, as a prophylactic stockpile vaccine for newly emerged epidemic and novel pandemic influenza A viruses or as a supplement to conventional HA/NA vaccines. My own laboratory has approached the problem from the clinical viewpoint by identifying CD4(+) cells which are present in influenza infected volunteers who resist influenza infection. We have established precisely which peptides in M and NP proteins react with these immune CD4 cells. These experimental vaccines induce immunity in animal models but with a single exception no data have been published on protection against influenza virus infection in humans. The efficacy of the latter vaccine is based on vaccinia virus (MVA) as a carrier and was analyzed in a quarantine unit. Given the absence of induced HI antibody in the new universal vaccines a possible licensing strategy is a virus challenge model in quarantine whereby healthy volunteers can be immunized with the new vaccine and thereafter deliberately infected and clinical signs recorded alongside quantities of virus excreted and compared with unvaccinated controls.

Immune responses to infection with H5N1 influenza virus

Influenza A H5N1 viruses remain a substantial threat to global public health. In particular, the expanding genetic diversity of H5N1 viruses and the associated risk for human adaptation underscore the importance of better understanding host immune responses that may protect against disease or infection. Although much emphasis has been placed on investigating early virus-host interactions and the induction of innate immune responses, little is known of the consequent adaptive immune response to H5N1 virus infection. In this review, we describe the H5N1 virus-specific and cross-reactive antibody and T cell responses in humans and animal models. Data from limited studies suggest that although initially robust, there is substantial waning of the serum antibody responses in survivors of H5N1 virus infection. Characterization of monoclonal antibodies generated from memory B cells of survivors of H5N1 virus infection has provided an understanding of the fine specificity of the human antibody response to H5N1 virus infection and identified strategies for immunotherapy. Human T cell responses induced by infection with seasonal influenza viruses are directed to relatively conserved internal proteins and cross-react with the H5N1 subtype. A role for T cell-based heterosubtypic immunity against H5N1 viruses is suggested in animal studies. Further studies on adaptive immune responses to H5N1 virus infection in both humans and animals are needed to inform the design of optimal immunological treatment and prevention modalities.

Clearance of influenza virus infections by T cells: risk of collateral damage?

Influenza A viruses are a major cause of respiratory infections in humans. To protect against influenza, vaccines mainly aim at the induction of antibodies against the two surface proteins and do not protect against influenza A viruses from other subtypes. There is an increasing interest in heterosubtypic immunity that does protect against different subtypes. CD8 and CD4 T cells have a beneficial effect on the course of influenza A virus infection and can recognize conserved IAV epitopes. The T cell responses are tightly regulated to avoid collateral damage due to overreaction. Different studies have shown that an aberrant T cell response to an influenza virus infection could be harmful and could contribute to immunopathology. Here we discuss the recent findings on the balance between the beneficial and detrimental effects of T cell responses in influenza virus infections.