Understanding immunity to influenza: implications for future vaccine development

ABSTRACT

Introduction

Influenza virus changes its genotype through antigenic drift or shift making it difficult to develop immunity to infection or vaccination. Zoonotic influenza A virus (IAV) strains can become established in humans. Several impediments to human infection and transmission include sialic acid expression, host anti-viral factors (including interferons), and other elements that govern viral replication. Controlling influenza infection, replication, and transmission is important because IAVs cause annual epidemics and occasional pandemics. Effective seasonal influenza vaccines exist, but these vaccines do not fully protect against novel or pandemic strains.

Areas covered

With new vaccine production technology, vaccines can be produced rapidly. Universal IAV vaccines are being developed to protect against seasonal, novel, and zoonotic IAVs. These efforts are being enhanced and accelerated by a better understanding the host immune response to influenza viruses.

Expert opinion

This review discusses several implications for future influenza vaccine development. Host immune responses to influenza virus infection or vaccination can guide vaccine development as anti-influenza immunity is affected by responses influenced by the previous immune history including first and subsequent exposures to influenza virus infections and vaccinations.

KEYWORDS:

• Influenza
• immunity
• host immunity
• vaccines
• human

1. Introduction

The importance of understanding immunity to influenza and its role in future influenza vaccine development is well understood because influenza viruses are the causative agent of major respiratory infections in humans, and annually, up to 650,000 deaths are associated with seasonal influenza virus epidemics [1]. This paper was collected from a PubMed, EndNote 20, and Google literature search.

1. Influenza viruses belong to the Orthomyxoviridae family, which are enveloped RNA viruses with a segmented genome of 8 single-stranded negative-sense RNA segments. Two types of influenza virus (A and B) are endemic in humans and responsible for annual epidemics. Influenza viruses are divided into subtypes based on their hemagglutinin (HA) and neuraminidase (NA) genes. Influenza A viruses (IAVs) are categorized as subtypes, while influenza B viruses (IBVs) are classified into one of two lineages, i.e. B/Yamagata or B/Victoria. IAVs and IBVs are further classified into clades/subclades or groups/sub-groups [2]. There are 18 different HA subtypes and 11 different NA subtypes (H1 - H18 and N1 - N11) with ~ 130 IAV combinations (primarily in birds) [3]. The potential for numerous IAV combinations exists because of viral gene reassortment. Reassortments may occur when different IAVs infect the same host cell at the same time and exchange genetic information [4]. In addition to epidemics, IAVs may cause pandemics. Examples are 1918 pandemic (H1N1 virus), 1957-1958 pandemic (H2N2 virus), 1968 pandemic (H3N2 virus), and 2009 H1N1 pandemic (H1N1pdm09 virus) [5]. These events have raised worldwide concern about vaccine preparedness.

2. The effectiveness of influenza vaccines is limited even when the vaccines match circulating strains in the population. Typically, the vaccines only reduce disease by 40-60%, and their efficacy is lower when the strains are not well-matched [6]. Influenza viruses have the ability to genetically and antigenically drift which helps them avoid neutralizing immunity. Neutralizing immunity is primarily mediated by antibodies that react to the HA globular head [7]. Unfortunately, this attribute drives genetic and antigenic drift which can lead to mutations that eventually reduce neutralization [8]. As a result, regular updates to seasonal influenza vaccines are necessary to match the circulating viruses [9]. Although antigenic mismatch is often blamed for the inadequate efficacy of seasonal influenza vaccines, the effectiveness may still be low even when there is a good match [10]. Factors such as age, immune and vaccination status, and immunological imprinting may contribute to vaccine variability [11,12]. Therefore, there is a need for a better understanding of immune history regarding influenza vaccines in influenza-experienced populations.

There are generally two types of influenza vaccines available: inactivated influenza vaccines (IIV) and live attenuated influenza vaccines (LAIV) [13]. IIVs are created using a whole virus that is chemically inactivated, concentrated, and purified to produce split virus vaccines [14]. These are treated to separate the viral envelope from viral proteins. Inactivated or live attenuated, virus-vectored, and subunit vaccine platforms have been developed along with non-viral vaccine technologies such as virus-like particle vaccines, nucleic acid vaccines, and structural vaccine design to provide alternatives to influenza virus vaccine development [10,11]. While these vaccine platforms can typically trigger a strong adaptive immune response, their longevity is limited [12]. As a result, various vaccine adjuvants [13] and new vaccine strategies are being developed, such as the self-amplifying mRNA vaccine [15,16].

Among the various methods used to produce influenza vaccines, the traditional manufacturing process uses fertilized chicken eggs which takes about six months to allow for virus propagation after egg adaptation of the vaccine seed viruses. However, adaptation can result in changes in viral antigens that may reduce the vaccine’s effectiveness due to antigenic mismatch. On the other hand, cell-culture technology does not require fertilized eggs, and it has been associated with increased vaccine effectiveness [14]. Vaccination against seasonal influenza viruses is recommended for people aged 65 and over, young children, and high-risk individuals with certain health conditions.

There is substantial efforts in developing universal influenza vaccines that provide robust and long-lasting protection against all circulating and emerging influenza types including pandemic strains [17]. These goals require the identification and validation of correlates of protection that center on the generation of protective antibodies [18]. Examples include antibody targeting the influenza HA globular head, the HA stalk region, the NA, the M1/M2 matrix proteins, and the nuclear protein (NP) [11]. It is generally accepted that antibodies directed against the HA globular head are major correlates of protection [7]. The NA is the other major influenza envelope protein, and antibodies directed against NA can block its function and can contribute to protective immunity [19]. In contrast to HA-specific antibodies, NA-specific antibodies cannot prevent infection, but limit virus release and spread. It has been shown in animal models that influenza-specific T cells, particularly CD8+ T cells, are an important correlate of protection against influenza and contribute to heterosubtypic immunity [20]. Virus-specific CD4+ T cells have also been shown to correlate with reduced disease severity [21].

Researchers are exploring different methods to enhance natural and acquired immunity to increase the effectiveness of influenza vaccines. Typically, vaccines require multiple doses to produce lasting protection against the disease [22]. These repeated immunogenic stimulations can improve the quality, strength, and durability of the acquired immune response [23–25]. Factors that impact the vaccine’s effectiveness include the type and number of immunizations and the interval between doses [26]. It’s crucial that the vaccine triggers both the innate and adaptive B and T cell responses [22]. Adjuvants have been employed to amplify the adaptive immune response to vaccines [25,27]. Adjuvants can heighten the potency and quality of the adaptive response to provide the highest degree of protection. They can also help reduce the vaccine dosage and the number of doses required for immunization.

The efficacy of mRNA vaccines has been well documented for SARS-CoV-2 [28–30]. The BNT162b2 and mRNA-1273 mRNA vaccines were shown to be highly effective under real-world conditions in preventing symptomatic COVID-19 including those at risk for severe COVID-19 and those in racial and ethnic groups that have been disproportionately affected by the pandemic [28]. The efficacy of the mRNA vaccines can be increased in several ways including adding 5’ Kozak and cap sequences and 3’ poly-A sequences, modified nucleosides to increase mRNA stability, codon optimization, and by thermostable mRNA [31–33]. Immune imprinting proposes that memory B cells generated during the initial influenza virus encounter lead to a recall of B cells to those strain-specific epitopes despite subsequent encounters with different influenza virus strains. This phenomenon can affect the efficacy of multi-HA (universal) mRNA vaccines, however, improvements to HA stalk immunogenicity may hold the promise of improving mRNA influenza vaccination by improving strain match, vaccine production, and immunity to multivalent formulations [34].

Although seasonal influenza vaccines may offer some protection against influenza which are similar to those included in the vaccine, they do not provide comprehensive and enduring immunity against drifted influenza viruses [35]. Among the most frequently used influenza vaccines are the inactivated vaccines produced in embryonated chicken eggs. There are three vaccine types: whole-virus, split-virus, and HA and NA subunits [36]. In general, current seasonal influenza vaccines work better against influenza A(H1N1) viruses but provide less protection against influenza A(H3N2) viruses [37]. Future vaccine development and the next generation of seasonal influenza vaccines need to address the issues surrounding antigenic drift to contribute to better address pandemic preparedness. One new way being investigated is mRNA-based vaccines against all HA subtypes, i.e. H1 through H18 for IAV [38]. This vaccine has been shown to elicit high levels of cross-reactive and subtype-specific antibodies in mice and ferrets, protecting these animals from matched and mismatched influenza virus strains. These studies show the promise of mRNA vaccines and indicate that protection against antigenically variable viruses can be induced by simultaneously inducing antibodies against multiple antigens. The findings also show that protection against antigenically matched strains is mediated by neutralizing antibodies, whereas protection against mismatched viral strains may occur through non-neutralizing mechanisms [39,40]. However, it is important to investigate how continual exposure to the immune system to influenza viruses affects imprinting by the first infection or vaccination and how the immune response is further refined at each subsequent exposure. In addition, how the various vaccine approaches modify broad heterosubtypic responses as well as contribute to longer-lasting protective immune responses. Finally, it is important to study the use of adjuvants to improve the vaccine response by enhancing and modulating the immune response [41,42].

2. Conclusion

Being vaccinated against influenza can significantly decrease the number of people who get sick or die from the infection. However, there remains a need to enhance vaccine effectiveness to provide more robust, comprehensive, and longer-lasting protection while ensuring vaccine safety. Addressing factors that decrease vaccine efficacy, especially in vulnerable populations such as the elderly, is needed. Moreover, there is a need to improve vaccine design and delivery for the development of the next generation of influenza vaccines. It is unclear how much negative interference exists between old and new vaccine responses, although adjuvants are known to attenuate the effects of imprinting [41,43]. It is important to evaluate new vaccines, such as mRNA vaccines, to ensure that they can induce protection that aligns with stronger and more comprehensive immunity.

3. Expert opinion

Recent advancements in vaccine technology have made it possible to quickly produce and distribute new influenza vaccines within months as shown in the response to the 2009 IAV pandemic [44] and the SARS-CoV-2 pandemic mRNA vaccine response [45]. Typically, vaccines are not immediately available during the early stages of an endemic or pandemic, and it usually takes government intervention to prompt vaccine manufacturers to shift production to address a novel zoonotic virus [46]. A current objective is to develop a safe and universal influenza vaccine that can prime against seasonal IAV, and potentially novel or zoonotic viruses. One possible strategy is to prime with a multiepitope mRNA vaccine and then boost with a protein-based vaccine specific to the novel virus, or revaccinate with the multiepitope mRNA vaccine plus adjuvant to prime B and T cells. This approach could help alleviate the pressure on the national stockpile of IAV vaccines [47,48].

It is important to note that the innate immune and T cell responses significantly contribute to protection against influenza viruses and are essential for the induction of robust antibody responses [49,50]. It is important to better understand what induces broadly neutralizing antibodies, and how broadly neutralizing antibodies that target influenza are maintained as this could be key in the development of universal vaccines [51]. It is also important to better understand how cross-reactive memory B cell pools may complicate the vaccine-induced memory B cell response by immune imprinting [52], and understand the bias toward the expansion of preexisting B cell clones and a predominance of antibodies of lower potency directed toward highly conserved regions of HA [53]. Other issues are that the molecular basis of immunodominance is largely unknown, and, without understanding this, we cannot design vaccines that refocus antibody responses to our epitopes of choice [39].

To improve influenza vaccines, it’s important to utilize existing clinical and preclinical data in order to identify new ways to protect against the virus. This information is key in mRNA vaccine development that focuses on epitopes retained by multiple strains of influenza as the key target of a broadly neutralizing antibody. One possibility to improve vaccines is to redirect immune responses toward the conserved viral antigens using adjuvants [54]. Antibodies to conserved epitopes may be cross-reactive, but typically conserved epitopes result from relatively low immune pressure. However, through adjuvants it may be possible to promote the induction of protective and long-lasting responses, including antibodies targeting the conserved HA and NA epitopes. Antibodies can neutralize viruses by blocking attachment to the host cell, preventing virus penetration of the host cell, or interfere with uncoating of the virus within the cell. Additionally, adjuvants can enhance the immunogenicity of the vaccine, which could potentially allow for a lower vaccine dose [55,56].

One challenge we must address is the negative interference or immune imprinting caused by previous influenza infections or vaccinations [57]. This is particularly important for infants and toddlers in the United States and Europe who can now receive the influenza vaccine before exposure to the virus [38]. Additionally, clinical trials are currently underway to examine the effectiveness of maternal immunization against seasonal influenza in protecting young infants (clinical trials: NCT01797029, NCT01430689, NCT01306669). Immune imprinting has a lasting impact on the B and T cell response as it initiates a cascade of innate and adaptive immune responses leading to lifelong immunological memory [58]. Continuous exposure of the immune system to influenza effects imprinting as the immune response becomes more refined with each subsequent exposure. To improve influenza vaccine efficacy, we may be able to capitalize on the immune specificity of infants and toddlers by identifying specific Influenza epitopes critical to protective immunity and maintaining CD4 T cell help. Moreover, we need to more thoroughly investigate influenza NA as a vaccine target. The NA has a critical role in the virus life cycle, and is an important target of the host immune system [59]. Despite significant HA sequence variation between IAV subtypes [60], the catalytic site residues in NAs are highly conserved [61]. Studies have shown that supplementing influenza vaccines with NA can result in a clear reduction in pulmonary virus titer challenged with a mismatched HA [61–63]. Exploring NA in or as vaccines for children and toddlers is worthwhile as that strategy may broaden the immune response and improve protection against drifted or emerging strains, and potentially complement HA-based influenza vaccine strategies including as mRNA vaccines.

Article highlights

• Influenza disease is vaccine-preventable.

• Influenza vaccines need yearly reformulation and come with variable vaccine efficacy.

• Influenza vaccines may contribute to reductions in influenza morbidity and mortality.

• Influenza vaccines have limited effectiveness in part due to antigenic evolution and immune imprinting.

• Influenza vaccines predominantly use embryonated chicken eggs for vaccine production.

• Alternative influenza vaccine approaches are being investigated to try to induce more robust and longer-lasting immune responses with the hope of overcoming antigenic drift, immune imprinting, and addressing the potential of emerging/novel influenza viruses.

• New approaches are being examined to boost innate and/or adaptive immunity and to improve vaccine-induced immune responses in individuals with diminished immunity, including older adults.

• Evidence indicates that universal influenza vaccine efficacy will be difficult if not unachievable.

• mRNA influenza vaccines could become an armament for universal influenza vaccines.

Declaration of interests

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or material discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or mending, or royalties.

Reviewer disclosures

Peer reviewers on this manuscript have no relevant financial or other relationships to disclose.

Author contribution

Ralph Tripp contributed to the conception, review, interpretation of the relevant literature, and writing/revision of the article.

Additional information

Funding

This manuscript was funded in part by the Georgia Research Alliance.