Elsevier

Vaccine

Volume 26, Issues 29–30, 4 July 2008, Pages 3626-3633
Vaccine

Heterologous HA DNA vaccine prime—inactivated influenza vaccine boost is more effective than using DNA or inactivated vaccine alone in eliciting antibody responses against H1 or H3 serotype influenza viruses

https://doi.org/10.1016/j.vaccine.2008.04.073Get rights and content

Abstract

The trivalent inactivated vaccine (TIV) is used to prevent seasonal influenza virus infection in humans, however, the immunogenicity of this vaccine may be influenced by the priming effect of previous influenza vaccinations or exposure to antigenically related influenza viruses. The current study examines the immunogenicity of a clinically licensed TIV in rabbits naïve to influenza antigens. Animals were immunized with either the licensed TIV, a bivalent (H1 and H3) HA DNA vaccine or the combination of both. Temporal and peak level serum anti-influenza virus IgG responses were determined by enzyme-linked immunosorbent assay (ELISA). Functional antibody responses were measured by hemagglutination inhibition and microneutralization against either A/NewCaledonia//20/99 (H1N1) or A/Panama/2007/99 (H3N2) influenza viruses. Our results demonstrate that the immunogenicity of the TIV is low in sero-negative animals. More significantly, the heterologous DNA prime-TIV boost regimen was more immunogenic than the homologous prime-boost using either TIV or DNA vaccines alone. This finding justifies further investigation of HA DNA vaccines as a priming immunogen for the next generation of vaccines against seasonal or pandemic influenza virus infections.

Introduction

Influenza A virus infection continues to be a major public health threat. Seasonal influenza virus infections can produce high morbidity and the emerging threat of pandemic influenza, particularly from an avian source, has become a new concern to the health of the worldwide human population [1], [2], [3]. The best option for reducing the impact of influenza virus infection in humans is vaccination [4].

Currently, the main form of licensed human influenza virus vaccines is the traditional trivalent inactivated influenza vaccines (TIV). This vaccine modality incorporates circulating human viral strains: an H1 subtype and an H3 subtype of the influenza A virus plus an influenza B virus. Another type of influenza vaccine is the cold-adapted live influenza virus vaccine (CAIV) which has been shown to be more immunogenic than TIV in inducing protective immunity and may be associated with a longer lasting and more cross-protective immune response than is elicited by TIV [5]. A new technology termed ‘reverse genetics’ has been developed to generate high growth reassortants [6], [7], [8], [9], [10], [11] and combines viral genes from the high growth yield laboratory strain of influenza A virus A/PR/8/34 (H1N1) with genes encoding protective antigens of the target viral strains [7]. However, this technology does not change the subsequent manufacturing process needed to produce large stocks of vaccine viruses to make the final TIV or CAIV formulations. For TIV, additional steps of inactivation and purification of the protective HA antigens are needed. Due to safety concerns, CAIV is not indicated for very young children, the elderly or people with a compromised immune system. Therefore, TIV continues to be the main vaccine used annually to prevent seasonal influenza. Furthermore, the recently licensed pandemic vaccines against H5N1 influenza viruses are also of the inactivated form [12].

Inactivated vaccines are known to have weak immunogenicity but have been used effectively in preventing seasonal influenza virus infection [13]. Usually one injection with TIV can induce satisfactory levels of protective antibody responses [5], as required by regulatory approval for licensing. However, during the testing of inactivated avian influenza virus vaccines to prevent pandemic influenza, it is clear that at least two immunizations are needed and a strong adjuvant may be required, in order to elicit the same magnitude of protective antibody responses as are seen with TIV for seasonal influenza [14], [15]. One possible reason for this difference in immunogenicity between seasonal TIV and an inactivated avian influenza virus vaccine is the pre-existing immunity against the human virus strains vs. a lack of such immunity for the avian virus strains. Human populations, in general, have been exposed to early circulating H1 and H3 serotype influenza viruses and some people have received previous TIV immunizations. These events serve a “priming” effect to the host immune system and thus, one shot of TIV can easily “boost” antibody responses against seasonal influenza viruses. On the other hand, human populations, at least at this point, are still naïve to avian influenza virus strains (such as H5, H7 and H9 serotypes), and one immunization of an inactivated avian influenza vaccine may not be immunogenic to the point where they are able to elicit high level protective antibody responses in subjects naïve to avian influenza virus antigens.

In the current study, we tested the immunogenicity of the licensed, split virus TIV in a naïve rabbit model to study the HA-specific binding and functional antibody responses. The immunogenicity of TIV was compared to a DNA vaccine expressing HA antigens from the same or closely related H1 and H3 viruses. In addition, the relative immunogenicity between the homologous and heterologous prime-boost approaches was determined. Our results indicate that DNA prime-TIV boost was, in fact, the most immunogenic immunization regimen. This finding suggests a new option in the effort to develop the next generation influenza vaccines.

Section snippets

Inactivated influenza vaccine

The inactivated trivalent influenza vaccine used in this study was the licensed split virus TIV, Fluzone, used during the 2004–2005 influenza season. This particular formulation consists of 15 μg of each HA protein from A/NewCaledonia/20/99 (H1N1), A/Wyoming/03/2003 (H3N2) and B/Jiansu/10/2003 in 500 μl of vaccine (one adult human dose). The product was stored at UMMS clinical pharmacy according to manufacturer's recommendation and the product was used before its expiration date.

Codon optimized HA DNA vaccines

The codon

Design of immunization studies in NZW rabbits

Two immunogenicity studies were organized in the current report (Fig. 1). For the first study, three groups of NZW rabbits were immunized twice with one of the following immunization regimens. Animals in Group 1 received 1/2 of the normal human dose of a clinically licensed TIV influenza vaccine for the 2004–2005 flu season by intramuscular (IM) injection. This TIV includes HA antigens from influenza virus isolates A/NewCaledonia/20/99 (H1N1) and A/Wyoming/03/2003 (H3N2). Group 2 received a

Discussion

In the current study, we used the licensed influenza vaccine against the H1 and H3 serotypes as a model system to study the immunogenicity of an inactivated influenza virus vaccine in animals that were sero-negative to influenza virus antigens. Then, we compared the relative levels of protective antibodies between homologous (DNA + DNA or TIV + TIV) and heterologous (DNA + TIV) prime-boost vaccination strategies. In this study, New Zealand White rabbits were used to study the immunogenicity of

Acknowledgements

This work was partially supported by NIH/NIAID U01 AI 056536 (S. Lu) and CIVIA, a human immunology center supported by NIH/NIAID U19 grant AI62623 (A.G.-S.). The project also used core facility resources at the University of Massachusetts Medical School supported by NIH grant 5P30DK32520 from the NIDDK. We thank Dr. Jill M. Grimes-Serrano for critical reading of the manuscript and Richard Cadagan for excellent technical support.

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