Cross-clade protective immunity of H5N1 influenza vaccines in a mouse model

Abstract

H5N1 highly pathogenic avian influenza viruses evolved into several clades, leading to appreciably distinct antigenicities of their hemagglutinins. As such, candidate H5N1 pre-pandemic vaccines for human use should be sought. Here, to evaluate fundamental immunogenic variations between H5N1 vaccines, we prepared four inactivated H5N1 test vaccines from different phylogenetic clades (clade 1, 2.1, 2.2, and 2.3.4) in accordance with the WHO recommendation, and tested their cross-clade immunity in a mouse model by vaccination followed by challenge with heterologous virulent viruses. All H5N1 vaccines tested provided full or partial cross-clade protective immunity, except one clade 2.2-based vaccine, which did not protect mice from clade 2.3.4 virus challenge. Among the test vaccines, a clade 2.1-based vaccine possessed the broadest-spectrum cross-immunity. These results suggest that currently stockpiled pre-pandemic vaccines, especially clade 2.1-based vaccines, will likely be useful as backup vaccines in a pandemic situation, even one involving antigenic-drifted viruses.

Introduction

Since the outbreak of H5N1 influenza A virus in humans in 1997, these viruses have continued to exert a growing toll, with more than 240 confirmed fatal human cases (http://www.who.int/csr/disease/avian_influenza/en/). With the re-emergence of H5N1 virus in humans in 2003, the epidemic regions have expanded from Asia to Europe, the Middle-East and Africa, raising concerns over a possible influenza pandemic [1]. In the event of such a pandemic, vaccination is one of the most potent methods of protection against this deadly threat. Outbreaks and the pandemic potential of H5N1 viruses have, for this reason, led to stockpiling of H5N1 pre-pandemic inactivated vaccines for human use in many countries.

The considerable diversity in hemagglutinin (HA) antigenicity among the H5N1 viruses, which transpired as a result of naturally occurring genetic alterations, has led to the creation of distinct clades and subclades [2]. It is difficult to predict which H5N1 virus could become a pandemic virus. Moreover, current H5N1 inactivated vaccines show low immunogenicity in humans [3], such that antigenic matching of vaccine viruses to a pandemic virus would be required for efficient protection. Therefore, the World Health Organization (WHO) now recommends the stockpiling of a panel of vaccines with HA antigenic variations, including clade 1 viruses, which have circulated mainly in Southeast Asia, as well as clade 2.1, 2.2, and 2.3.4 viruses, which are circulating predominantly in Indonesia, Asia, Europe and Africa, and Asian countries including China, respectively, as pre-pandemic vaccines [4]. Previously, the HA antigenicities of H5N1 viruses were characterized by using polyclonal and monoclonal antibodies, revealing clade/subclade-dependent antigenic variations, which included cross-reactivity to each other to various extents [4], [5], [6], [7], [8], [9]. However, systemic evaluation of the immunogenic relationship of H5N1 vaccines, including cross-clade protection, has not been carried out using animal models, with the exception of some attributive cross-protection analyses between limited clades, such as clade 1 and 2.1 viruses [6], [10], [11], [12], [13], [14], [15].

In this study, we prepared four H5N1 test vaccines, associated with clade 1, 2.1, 2.2, and 2.3.4 viruses, immunized mice with each vaccine and then challenged them with homologous as well as heterologous virulent viruses of different clades to evaluate fundamental immunogenic variations between H5N1 viruses in a mouse model.