Cross-protective immunity to influenza A viruses

Mucosal immunization with a candidate universal influenza vaccine reduces virus transmission in a mouse model

Pandemic influenza is a major public health concern, but conventional strain-matched vaccines are unavailable early in a pandemic. Candidate "universal" vaccines targeting the viral antigens nucleoprotein (NP) and matrix 2 (M2), which are conserved among all influenza A virus strains and subtypes, could be manufactured in advance for use at the onset of a pandemic. These vaccines do not prevent infection but can reduce disease severity, deaths, and virus titers in the respiratory tract. We hypothesized that such immunization may reduce virus transmission from vaccinated, infected animals. To investigate this hypothesis, we studied mouse models for direct-contact and airborne transmission of H1N1 and H3N2 influenza viruses. We established conditions under which virus transmission occurs and showed that transmission efficiency is determined in part at the level of host susceptibility to infection. Our findings indicate that virus transmission between mice has both airborne and direct-contact components. Finally, we demonstrated that immunization with recombinant adenovirus vectors expressing NP and M2 significantly reduced the transmission of virus to cohoused, unimmunized mice in comparison to controls. These findings have broad implications for the impact of conserved-antigen vaccines, not only in protecting the vaccinated individual but also in protecting others by limiting influenza virus transmission and potentially reducing the size of epidemics.

IMPORTANCE

Using a mouse model of influenza A virus transmission, we demonstrate that a candidate "universal" influenza vaccine both protects vaccinated animals from lethal infection and reduces the transmission of virus from vaccinated to nonvaccinated mice. This vaccine induces immunity against proteins conserved among all known influenza A virus strains and subtypes, so it could be used early in a pandemic before conventional strain-matched vaccines are available and could potentially reduce the spread of infection in the community.

Cross-reactive human B cell and T cell epitopes between influenza A and B viruses

Influenza A and B viruses form different genera, which were originally distinguished by antigenic differences in their nucleoproteins and matrix 1 proteins. Cross-protection between these two genera has not been observed in animal experiments, which is consistent with the low homology in viral proteins common to both viruses except for one of three polymerase proteins, polymerase basic 1 (PB1). Recently, however, antibody and CD4⁺ T cell epitopes conserved between the two genera were identified in humans. A protective antibody epitope was located in the stalk region of the surface glycoprotein, hemagglutinin, and a CD4⁺ T cell epitope was located in the fusion peptide of the hemagglutinin. The fusion peptide was also found to contain antibody epitopes in humans and animals. A short stretch of well-conserved peptide was also identified in the other surface glycoprotein, neuraminidase, and antibodies binding to this peptide were generated by peptide immunization in rabbits. Although PB1, the only protein which has relatively high overall sequence homology between influenza A and B viruses, is not considered an immunodominant protein in the T cell responses to influenza A virus infection, amino acid sequence comparisons show that a considerable number of previously identified T cell epitopes in the PB1 of influenza A viruses are conserved in the PB1 of influenza B viruses. These data indicate that B and T cell cross-reactivity exists between influenza A and B viruses, which may have modulatory effects on the disease process and recovery. Although the antibody titers and the specific T cell frequencies induced by natural infection or standard vaccination may not be high enough to provide cross protection in humans, it might be possible to develop immunization strategies to induce these cross-reactive responses more efficiently.

Protective CD8 T cell-mediated immunity against influenza A virus infection following influenza virus-like particle vaccination

The development of influenza A virus (IAV) vaccines capable of inducing cytotoxic CD8 T cell responses could potentially provide superior, long-term protection against multiple, heterologous strains of IAV. Although prior studies demonstrated the effectiveness of baculovirus-derived virus-like particle (VLP) vaccination in generating Ab-mediated protection, the role that CD8 T cell immunity plays in overall VLP-mediated protection is less-well understood. In this article, we demonstrate that intranasal vaccination of mice with a VLP containing the hemagglutinin and matrix 1 proteins of IAV/PR/8/34 leads to a significant increase in hemagglutinin 533-specific CD8 T cells in the lungs and protection following subsequent homologous challenge with IAV. VLP-mediated protection was significantly reduced by CD8 T cell depletion, indicating a critical role for CD8 T cells in protective immunity. Importantly, our results show that VLP vaccine-induced CD8 T cell-mediated protection is not limited to homologous IAV strains. VLP vaccination leads to an increase in protection following heterosubtypic challenge with a strain of IAV that avoids vaccine-induced neutralizing Abs but contains conserved, immunodominant CD8 T cell epitopes. Overall, our results demonstrate the ability of influenza protein-containing VLPs to prime IAV-specific CD8 T cell responses that contribute to protection from homo- and heterosubtypic IAV infections. These results further suggest that vaccination strategies focused on the development of cross-protective CD8 T cell responses may contribute to the development of "universal" IAV vaccines.

Cooperativity between CD8+ T cells, non-neutralizing antibodies, and alveolar macrophages is important for heterosubtypic influenza virus immunity

Seasonal epidemics of influenza virus result in ∼36,000 deaths annually in the United States. Current vaccines against influenza virus elicit an antibody response specific for the envelope glycoproteins. However, high mutation rates result in the emergence of new viral serotypes, which elude neutralization by preexisting antibodies. T lymphocytes have been reported to be capable of mediating heterosubtypic protection through recognition of internal, more conserved, influenza virus proteins. Here, we demonstrate using a recombinant influenza virus expressing the LCMV GP33-41 epitope that influenza virus-specific CD8+ T cells and virus-specific non-neutralizing antibodies each are relatively ineffective at conferring heterosubtypic protective immunity alone. However, when combined virus-specific CD8 T cells and non-neutralizing antibodies cooperatively elicit robust protective immunity. This synergistic improvement in protective immunity is dependent, at least in part, on alveolar macrophages and/or other lung phagocytes. Overall, our studies suggest that an influenza vaccine capable of eliciting both CD8+ T cells and antibodies specific for highly conserved influenza proteins may be able to provide heterosubtypic protection in humans, and act as the basis for a potential “universal” vaccine.

Influenza virus continues to pose a significant risk to global health and is responsible for thousands of deaths each year in the United States. This threat is largely due to the ability of the influenza virus to undergo rapid changes, allowing it to escape from immune responses elicited by previous infections or vaccinations. Certain internal determinants of the influenza virus are largely conserved across different viral strains and represent attractive targets for potential “universal” influenza vaccines. Here, we demonstrated that cross-subtype protection against the influenza virus could be obtained through simultaneous priming of multiple arms of the immune response against conserved elements of the influenza virus. These results suggest a novel strategy that could potentially form a primary component of a universal influenza vaccine capable of providing long-lasting protection.

Vaccination to conserved influenza antigens in mice using a novel Simian adenovirus vector, PanAd3, derived from the bonobo Pan paniscus

Among approximately 1000 adenoviruses from chimpanzees and bonobos studied recently, the Pan Adenovirus type 3 (PanAd3, isolated from a bonobo, Pan paniscus) has one of the best profiles for a vaccine vector, combining potent transgene immunogenicity with minimal pre-existing immunity in the human population. In this study, we inserted into a replication defective PanAd3 a transgene expressing a fusion protein of conserved influenza antigens nucleoprotein (NP) and matrix 1 (M1). We then studied antibody and T cell responses as well as protection from challenge infection in a mouse model. A single intranasal administration of PanAd3-NPM1 vaccine induced strong antibody and T cell responses, and protected against high dose lethal influenza virus challenge. Thus PanAd3 is a promising candidate vector for vaccines, including universal influenza vaccines.

Cold-adapted X-31 live attenuated 2009 pandemic H1N1 influenza vaccine elicits protective immune responses in mice and ferrets

The 2009 pandemic influenza H1N1 (pdmH1N1) is characterized by rapid transmission among humans and disproportionate infection to children and young adults. Although the pdmH1N1 demonstrated less lethality than initially expected and has now moved into its post-pandemic period, it remains highly possible that through antigenic shift or antigenic drift the pdmH1N1 might re-emerge in the future as a more virulent strain than before, underscoring the need for vaccination prior to an outbreak. Using X-31 ca as a backbone strain, we generated a live attenuated pdmH1N1 vaccine and evaluated its potential as a safe and effective vaccine using mouse and ferret models. Despite an acceptable level of attenuation phenotypes, single dose of immunization with the vaccine efficiently stimulated both systemic and mucosal antibody responses and provided complete protection against lethal challenge with wild type pdmH1N1 virus, even at the lowest immunization dose of 10(3)PFU. The promising results of safety, immunogenicity, and protective efficacy of the vaccine not only contribute to expanding the repertoire of live vaccines as a judicious choice for pandemic H1N1 preparedness, but also suggest the great potential of X-31 ca donor strain to serve as reliable platform for generating diverse live vaccine constructs against seasonal influenza viruses and other pandemic strains.

Immunity against heterosubtypic influenza virus induced by adenovirus and MVA expressing nucleoprotein and matrix protein-1

Alternate prime/boost vaccination regimens employing recombinant replication-deficient adenovirus or MVA, expressing Influenza A virus nucleoprotein and matrix protein 1, induced antigen-specific T cell responses in intradermally (ID) vaccinated mice; with the strongest responses resulting from Ad/MVA immunization. In BALB/C mice the immunodominant response was shifted from the previously identified immunodominant epitope to a novel epitope when the antigen was derived from A/Panama/2007/1999 rather than A/PR/8. Alternate immunization routes did not affect the magnitude of antigen-specific systemic IFN-γ response, but higher CD8(+) T-cell IFN-γ immune responses were seen in the bronchoalveolar lavage following intransal (IN) boosting after intramuscular (IM) priming, whilst higher splenic antigen-specific CD8(+) T cell IFN-γ was seen following IM boosting. Partial protection against heterologous influenza virus challenge was achieved following either IM/IM or IM/IN but not ID/ID immunization. These data may be of relevance for the design of optimal immunization regimens for human influenza vaccines, especially for influenza-naïve infants.

Targeting the skin for microneedle delivery of influenza vaccine

Influenza infection represents a major socioeconomic burden worldwide. Skin represents a new target that has gained much attention in recent years for delivery of influenza vaccine as an alternative to the conventional intramuscular route of immunization. In this review we describe different microneedle vaccination approaches used in vivo, including metal and dissolving microneedle patches that have demonstrated promising results. Additionally we analyze the immunological basis for microneedle skin immunization and targeting of the skin's dense population of antigen presenting cells, their role, characterization, and function. Additionally we analyze the importance of inflammatory signaling in the skin after microneedle delivery.

Advances and future challenges in recombinant adenoviral vectored H5N1 influenza vaccines

The emergence of a highly pathogenic avian influenza virus H5N1 has increased the potential for a new pandemic to occur. This event highlights the necessity for developing a new generation of influenza vaccines to counteract influenza disease. These vaccines must be manufactured for mass immunization of humans in a timely manner. Poultry should be included in this policy, since persistent infected flocks are the major source of avian influenza for human infections. Recombinant adenoviral vectored H5N1 vaccines are an attractive alternative to the currently licensed influenza vaccines. This class of vaccines induces a broadly protective immunity against antigenically distinct H5N1, can be manufactured rapidly, and may allow mass immunization of human and poultry. Recombinant adenoviral vectors derived from both human and non-human adenoviruses are currently being investigated and appear promising both in nonclinical and clinical studies. This review will highlight the current status of various adenoviral vectored H5N1 vaccines and will outline novel approaches for the future.

Novel viral vectored vaccines for the prevention of influenza

Influenza represents a substantial global healthcare burden, with annual epidemics resulting in 3-5 million cases of severe illness with a significant associated mortality. In addition, the risk of a virulent and lethal influenza pandemic has generated widespread and warranted concern. Currently licensed influenza vaccines are limited in their ability to induce efficacious and long-lasting herd immunity. In addition, and as evidenced by the H1N1 pandemic in 2009, there can be a significant delay between the emergence of a pandemic influenza and an effective, antibody-inducing vaccine. There is, therefore, a continued need for new, efficacious vaccines conferring cross-clade protection-obviating the need for biannual reformulation of seasonal influenza vaccines. Development of such a vaccine would yield enormous health benefits to society and also greatly reduce the associated global healthcare burden. There are a number of alternative influenza vaccine technologies being assessed both preclinically and clinically. In this review we discuss viral vectored vaccines, either recombinant live-attenuated or replication-deficient viruses, which are current lead candidates for inducing efficacious and long-lasting immunity toward influenza viruses. These alternate influenza vaccines offer real promise to deliver viable alternatives to currently deployed vaccines and more importantly may confer long-lasting and universal protection against influenza viral infection.

Trivalent inactivated influenza vaccines induce broad immunological reactivity to both internal virion components and influenza surface proteins

There are a number of related goals of influenza vaccination, including elicitation of protective antibodies and induction of cellular CD4 and CD8+ T cell responses. Because CD4+ T cell expansion and functionality are influenced by peptide specificity and T cell gene expression can be modified by repeated re-stimulations, it is important to evaluate how frequent influenza vaccinations affect CD4+ T cell dependent functions in protective immunity to influenza. Trivalent influenza vaccines (TIV) have production of neutralizing antibodies to HA as their primary goal and main criteria for efficacy. Accordingly, they are not characterized for any other viral components. In the current study, we evaluated whether other influenza virus proteins were present in commercial TIV at levels sufficient for immunogenicity in vivo. Mice that differed with regard to their expressed class II molecules were used in concert with peptide-stimulated cytokine ELISPOT assays to comprehensively evaluate the CD4+ T cell antigen specificity induced by the TIV. Our studies revealed that NA, NP, M1 and NS1 were present in sufficient quantities in the TIV to prime and boost CD4+ T cells. These results suggest that in humans, the broad CD4+ T cell repertoire induced by live infection is continually boosted and maintained throughout life by regular vaccination with licensed intramuscular split vaccines. The implications raised by our findings on CD4+ T cell functionality in influenza are discussed.

Immunity to the conserved influenza nucleoprotein reduces susceptibility to secondary bacterial infections

Influenza causes >250,000 deaths annually in the industrialized world, and bacterial infections frequently cause secondary illnesses during influenza outbreaks, including pneumonia, bronchitis, sinusitis, and otitis media. In this study, we demonstrate that cross-reactive immunity to mismatched influenza strains can reduce susceptibility to secondary bacterial infections, even though this fails to prevent influenza infection. Specifically, infecting mice with H3N2 influenza before challenging with mismatched H1N1 influenza reduces susceptibility to either Gram-positive Streptococcus pneumoniae or Gram-negative Klebsiella pneumoniae. Vaccinating mice with the highly conserved nucleoprotein of influenza also reduces H1N1-induced susceptibility to lethal bacterial infections. Both T cells and Abs contribute to defense against influenza-induced bacterial diseases; influenza cross-reactive T cells reduce viral titers, whereas Abs to nucleoprotein suppress induction of inflammation in the lung. These findings suggest that nonneutralizing influenza vaccines that fail to prevent influenza infection may nevertheless protect the public from secondary bacterial diseases when neutralizing vaccines are not available.

Pseudotyped influenza A virus as a vaccine for the induction of heterotypic immunity

There is a need for vaccines that can protect broadly across all influenza A strains. We have produced a pseudotyped influenza virus based on suppression of the A/PR/8/34 hemagglutinin signal sequence (S-FLU) that can infect cells and express the viral core proteins and neuraminidase but cannot replicate. We show that when given by inhalation to mice, S-FLU is nonpathogenic but generates a vigorous T cell response in the lung associated with markedly reduced viral titers and weight loss after challenge with H1 and H3 influenza viruses. These properties of S-FLU suggest that it may have potential as a broadly protective A virus vaccine, particularly in the setting of a threatened pandemic before matched subunit vaccines become available.

Viva la revolución: rethinking influenza a virus antigenic drift

Rapid antigenic evolution of the influenza A virus hemagglutinin has precluded developing vaccines that provide durable protection. The yearly costs of influenza (circa $10(11) in the USA alone) easily justify investments in better understanding the interaction of influenza with antibodies and other inducible elements of the immune system that potentially limit or circumvent antigenic variation. Here, I summarize exciting new findings that offer the possibility of a quantum improvement in vaccine efficacy, focusing on studies clearly documenting robust neutralizing antibody responses to the conserved stem region of the hemagglutinin.

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

Memory CD4+ T cells combat viral infection and contribute to protective immune responses through multiple mechanisms, but how these pathways interact is unclear. We found that several pathways involving memory CD4+ T cells act together to effectively clear influenza A virus (IAV) in otherwise unprimed mice. Memory CD4+ T cell protection was enhanced through synergy with naive B cells or CD8+ T cells and maximized when both were present. However, memory CD4+ T cells protected against lower viral doses independently of other lymphocytes through production of IFN-γ. Moreover, memory CD4+ T cells selected for epitope-specific viral escape mutants via a perforin-dependent pathway. By deconstructing protective immunity mediated by memory CD4+ T cells, we demonstrated that this population simultaneously acts through multiple pathways to provide a high level of protection that ensures eradication of rapidly mutating pathogens such as IAV. This redundancy indicates the need for reductionist approaches for delineating the individual mechanisms of protection mediated by memory CD4+ T cells responding to pathogens.

Annual influenza vaccination affects the development of heterosubtypic immunity

Annual vaccination of healthy children >6 months of age against seasonal influenza has been recommended by public health authorities of some countries. However, currently used seasonal vaccines provide only limited protection against (potentially) pandemic influenza viruses. Furthermore, we recently hypothesized that annual vaccination may hamper the development of cross-reactive immunity against influenza A viruses of novel subtypes, that would otherwise be induced by natural infection. Here we summarize our findings in animal models in which we demonstrated that vaccination against influenza A/H3N2 virus reduced the induction of heterosubtypic immunity against highly pathogenic avian influenza A/H5N1 virus, otherwise induced by a prior infection with influenza A/H3N2 virus. The reduction of heterosubtypic immunity correlated with reduced virus-specific CD8+ T cell responses. An additional study was performed in humans, in which we collected peripheral blood mononuclear cells from annually vaccinated children with cystic fibrosis (CF) and age-matched unvaccinated healthy control children to study the virus-specific T cell response. An age-related increase of the virus-specific CD8+ T cell response was observed in unvaccinated children that was absent in vaccinated children with CF. These findings highlight the importance of the development of vaccines that provide protection against influenza A viruses of all subtypes.

Impact of cross-protective vaccines on epidemiological and evolutionary dynamics of influenza

Large-scale immunization has profoundly impacted control of many infectious diseases such as measles and smallpox because of the ability of vaccination campaigns to maintain long-term herd immunity and, hence, indirect protection of the unvaccinated. In the case of human influenza, such potential benefits of mass vaccination have so far proved elusive. The central difficulty is a considerable viral capacity for immune escape; new pandemic variants, as well as viral escape mutants in seasonal influenza, compromise the buildup of herd immunity from natural infection or deployment of current vaccines. Consequently, most current influenza vaccination programs focus mainly on protection of specific risk groups, rather than mass prophylactic protection. Here, we use epidemiological models to show that emerging vaccine technologies, aimed at broad-spectrum protection, could qualitatively alter this picture. We demonstrate that sustained immunization with such vaccines could--through potentially lowering transmission rates and improving herd immunity--significantly moderate both influenza pandemic and seasonal epidemics. More subtly, phylodynamic models indicate that widespread cross-protective immunization could slow the antigenic evolution of seasonal influenza; these effects have profound implications for a transition to mass vaccination strategies against human influenza, and for the management of antigenically variable viruses in general.

Influenza viruses: from birds to humans

Avian influenza viruses are the precursors of human influenza A viruses. They may be transmitted directly from avian reservoirs, or infect other mammalian species before subsequent transmission to their human host. So far, avian influenza viruses have caused sporadic-yet increasingly more frequently recognized-cases of infection in humans. They have to adapt to and circulate efficiently in human populations, before they may trigger a worldwide human influenza outbreak or pandemic. Cross-species transmission of avian influenza viruses from their reservoir hosts-wild waterbirds-to terrestrial poultry and to humans is based on different modes of transmission and results in distinctive pathogenetic manifestations, which are reviewed in this paper.

Profiling of humoral immune responses to influenza viruses by using protein microarray

The emergence of pandemic A(H1N1) 2009 influenza showed the importance of rapid assessment of the degree of immunity in the population, the rate of asymptomatic infection, the spread of infection in households, effects of control measures, and ability of candidate vaccines to produce a response in different age groups. A limitation lies in the available assay repertoire: reference standard methods for measuring antibodies to influenza virus are haemagglutination inhibition (HI) assays and virus neutralization tests. Both assays are difficult to standardize and may be too specific to assess possible partial humoral immunity from previous exposures. Here, we describe the use of antigen-microarrays to measure antibodies to HA1 antigens from seven recent and historical seasonal H1, H2 and H3 influenza viruses, the A(H1N1) 2009 pandemic influenza virus, and three avian influenza viruses. We assessed antibody profiles in 18 adult patients infected with A(H1N1) 2009 influenza virus during the recent pandemic, and 21 children sampled before and after the pandemic, against background reactivity observed in 122 persons sampled in 2008, a season dominated by seasonal A(H1N1) influenza virus. We show that subtype-specific and variant-specific antibody responses can be measured, confirming serological responses measured by HI. Comparison of profiles from persons with similar HI response showed that the magnitude and broadness of response to individual influenza subtype antigens differs greatly between individuals. Clinical and vaccination studies, but also exposure studies, should take these findings into consideration, as they may indicate some level of humoral immunity not measured by HI assays.

Induction of virus-specific cytotoxic T lymphocytes as a basis for the development of broadly protective influenza vaccines

There is considerable interest in the development of broadly protective influenza vaccines because of the continuous emergence of antigenic drift variants of seasonal influenza viruses and the threat posed by the emergence of antigenically distinct pandemic influenza viruses. It has been recognized more than three decades ago that influenza A virus-specific cytotoxic T lymphocytes recognize epitopes located in the relatively conserved proteins like the nucleoprotein and that they cross-react with various subtypes of influenza A viruses. This implies that these CD8+ T lymphocytes may contribute to protective heterosubtypic immunity induced by antecedent influenza A virus infections. In the present paper, we review the evidence for the role of virus-specific CD8+ T lymphocytes in protective immunity against influenza virus infections and discuss vaccination strategies that aim at the induction of cross-reactive virus-specific T-cell responses.

Annual vaccination against influenza virus hampers development of virus-specific CD8⁺ T cell immunity in children

Infection with seasonal influenza A viruses induces immunity to potentially pandemic influenza A viruses of other subtypes (heterosubtypic immunity). We recently demonstrated that vaccination against seasonal influenza prevented the induction of heterosubtypic immunity against influenza A/H5N1 virus induced by infection with seasonal influenza in animal models, which correlated with the absence of virus-specific CD8(+) T cell responses. Annual vaccination of all healthy children against influenza has been recommended, but the impact of vaccination on the development of the virus-specific CD8(+) T cell immunity in children is currently unknown. Here we compared the virus-specific CD8(+) T cell immunity in children vaccinated annually with that in unvaccinated children. In the present study, we compared influenza A virus-specific cellular and humoral responses of unvaccinated healthy control children with those of children with cystic fibrosis (CF) who were vaccinated annually. Similar virus-specific CD4(+) T cell and antibody responses were observed, while an age-dependent increase of the virus-specific CD8(+) T cell response that was absent in vaccinated CF children was observed in unvaccinated healthy control children. Our results indicate that annual influenza vaccination is effective against seasonal influenza but hampers the development of virus-specific CD8(+) T cell responses. The consequences of these findings are discussed in the light of the development of protective immunity to seasonal and future pandemic influenza viruses.

Live attenuated influenza vaccine (LAIV) impacts innate and adaptive immune responses

Influenza A infection induces a massive inflammatory response in the lungs that leads to significant illness and increases the susceptibility to secondary bacterial pneumonia. The most efficient way to prevent influenza infection is through vaccination. While inactivated vaccines induce protective levels of serum antibodies to influenza hemaglutinin (HA) and neuraminidase (NA) surface proteins, these are strain specific and offer little protection against heterosubtypic influenza viruses. In contrast, live attenuated influenza vaccines (LAIVs) induce a T cell response in addition to antibody responses against HA and NA surface proteins. Importantly, LAIV vaccination induces a response in a mouse model that protects against illness due to heterosubtypic influenza strains. While it is not completely clear what is the mechanism of action of LAIV heterosubtypic protection in humans, it has been shown that LAIV induces heterosubtypic protection in mice that is dependent upon a Type 1 immune response and requires CD8 T cells. In this study, we show that LAIV-induced immunity leads to significantly reduced viral titers and inflammatory responses in the lungs of mice following heterosubtypic infection. Not only are viral titers reduced in LAIV vaccinated mice, the amounts of inflammatory cytokines and chemokines in lung tissue are significantly lower. Additionally, we show that LAIV vaccination of healthy adults also induces a robust Type 1 memory response including the production of chemokines and cytokines involved in T cell activation and recruitment. Thus, our results indicate that LAIV vaccination functions by inducing immune memory which can act to modulate the immune response to subsequent heterosubtypic challenge by influencing both innate and adaptive responses.

Cold-adapted influenza and recombinant adenovirus vaccines induce cross-protective immunity against pH1N1 challenge in mice

Background

The rapid spread of the 2009 H1N1 pandemic influenza virus (pH1N1) highlighted problems associated with relying on strain-matched vaccines. A lengthy process of strain identification, manufacture, and testing is required for current strain-matched vaccines and delays vaccine availability. Vaccines inducing immunity to conserved viral proteins could be manufactured and tested in advance and provide cross-protection against novel influenza viruses until strain-matched vaccines became available. Here we test two prototype vaccines for cross-protection against the recent pandemic virus.

Methodology/Principal Findings

BALB/c and C57BL/6 mice were intranasally immunized with a single dose of cold-adapted (ca) influenza viruses from 1977 or recombinant adenoviruses (rAd) expressing 1934 nucleoprotein (NP) and consensus matrix 2 (M2) (NP+M2-rAd). Antibodies against the M2 ectodomain (M2e) were seen in NP+M2-rAd immunized BALB/c but not C57BL/6 mice, and cross-reacted with pH1N1 M2e. The ca-immunized mice did not develop antibodies against M2e. Despite sequence differences between vaccine and challenge virus NP and M2e epitopes, extensive cross-reactivity of lung T cells with pH1N1 peptides was detected following immunization. Both ca and NP+M2-rAd immunization protected BALB/c and C57BL/6 mice against challenge with a mouse-adapted pH1N1 virus.

Conclusion/Significance

Cross-protective vaccines such as NP+M2-rAd and ca virus are effective against pH1N1 challenge within 3 weeks of immunization. Protection was not dependent on recognition of the highly variable external viral proteins and could be achieved with a single vaccine dose. The rAd vaccine was superior to the ca vaccine by certain measures, justifying continued investigation of this experimental vaccine even though ca vaccine is already available. This study highlights the potential for cross-protective vaccines as a public health option early in an influenza pandemic.

Influenza virus-like particle can accommodate multiple subtypes of hemagglutinin and protect from multiple influenza types and subtypes

Despite existing vaccines and specific therapies, epidemics of seasonal influenza annually claim 200,000-500,000 lives worldwide. Pandemic influenza represents an even greater threat, with numerous potentially pandemic viruses circulating in nature. Development of multi-specific vaccines against multiple pandemic or seasonal strains is important for human health and the global economy. Here we report a novel virus-like particle (VLP) platform that contains three hemagglutinin (HA) subtypes. This recombinant vaccine design resulted in the expression of three HA subtypes co-localized within a VLP. Experimental triple-HA VLPs containing HA proteins derived from H5N1, H7N2, and H2N3 viruses were immunogenic and protected ferrets from challenge from all three potentially pandemic viruses. Similarly, VLPs containing HA subtypes derived from seasonal H1N1, H3N2, and type B influenza viruses protected ferrets from three seasonal influenza viruses. We conclude that this technology may represent a novel strategy for rapid development of trivalent seasonal and pandemic vaccines.

Seasoned adaptive antibody immunity for highly pathogenic pandemic influenza in humans

Fundamentally new approaches are required for the development of vaccines to pre-empt and protect against emerging and pandemic influenzas. Current strategies involve post-emergent homotypic vaccines that are modelled upon select circulating 'seasonal' influenzas, but cannot induce cross-strain protection against newly evolved or zoonotically introduced highly pathogenic influenza (HPI). Avian H5N1 and the less-lethal 2009 H1N1 and their reassortants loom as candidates to seed a future HPI pandemic. Therefore, more universal 'seasoned' vaccine approaches are urgently needed for heterotypic protection ahead of time. Pivotal to this is the need to understand mechanisms that can deliver broad strain protection. Heterotypic and heterosubtypic humoral immunities have largely been overlooked for influenza cross-protection, with most 'seasoned' vaccine efforts for humans focussed on heterotypic cellular immunity. However, 5 years ago we began to identify direct and indirect indicators of humoral-herd immunity to protein sites preserved among H1N1, H3N2 and H5N1 influenzas. Since then the evidence for cross-protective antibodies in humans has been accumulating. Now proposed is a rationale to stimulate and enhance pre-existing heterotypic humoral responses that, together with cell-mediated initiatives, will deliver pre-emptive and universal human protection against emerging epidemic and pandemic influenzas.