Identification of immunodominant CD8 epitope in the stalk domain of influenza B viral hemagglutinin

https://doi.org/10.1016/j.bbrc.2018.05.148Get rights and content

Highlights

  • The HA2 subunit of influenza hemagglutinin is highly conserved and could be a potential candidate for a universal vaccine.

  • CD8 T cells are important for influenza viral clearance from the lungs.

  • Assessment of CD8 T cell-induced immunity is critical for vaccine evaluation.

  • Immunodominant H-2Kd-restricted peptide, YYSTAASSL, was found to effectively induce cell-mediated immune responses.

  • YYSTAASSL induces high levels of IL-2, TNF-α and cytotoxic CD8 T cells.

Abstract

Human infections by type B influenza virus constitute about 25% of all influenza cases. The viral hemagglutinin is comprised of two subunits, HA1 and HA2. While HA1 is constantly evolving in an unpredictable fashion, the HA2 subunit is highly conserved, making it a potential candidate for a universal vaccine. However, immunodominant epitopes in the HA2 subunit remain largely unknown. To delineate MHC Class I epitopes, we first identified 9-mer H-2Kd-restricted CD8 T cell epitopes in the HA2 domain by in silico analyses, followed by evaluating the immunodominance of these peptides in mice challenged with the virus. Of three peptides selected through in silico analysis, the universally conserved peptide, YYSTAASSL (B/HA2-190), possessed the highest predicted binding affinity to MHC Class I and was most effective in inducing IL-2 and TNF-α in mouse splenocytes. Importantly, the peptide demonstrated best capability of stimulating peptide-specific ex-vivo cytotoxicity against target cells. Taken together, this finding would be of value for assessment of cell-mediated immune responses elicited by vaccines based on the highly conserved HA2 stalk domain.

Introduction

Influenza B virus (IBV) on average causes 20–30% of all influenza cases but it can be the dominant strain in a given flu season [1] [2] [3] [4] [5] [6] [7] [8]. Of particular concern is the increasing influenza B-related mortality rate among infants and children under the age of 10 [9]. During the 2010–2011 epidemic, 25% of all influenza cases were caused by IBV with 38% of all pediatric deaths [5] [10], [11] [12], [13]. Furthermore, in 2017-18 flu season 46.8% of all reported cases worldwide were positive for influenza A whereas 53.2% were positive for influenza B with more severe symptoms [14]. These data indicate that efforts should be strengthened to prevent and contain IBV infections.

The most effective means against influenza is annual vaccination of susceptible populations. The current seasonal influenza vaccines are produced using the strains recommended by the World Health Organization (WHO) 6–8 months ahead of the targeted season [15]. However, there are inherent disadvantages associated with the preparation of conventional influenza vaccines such as the uncertainty of the actual circulating strains, the need for annual updating of the manufacturing process. Furthermore, mismatches between the strains selected for vaccine preparation and the circulating viruses can cause a marked reduction in the efficacy of seasonal influenza A/B vaccines [4] [16] [17].

IBV is broadly classified into two genetic lineages, i.e. Victoria and Yamagata; these two antigenically different groups of virus are known to co-circulate within the human population [16] [6] [9] [18]. It is noted that vaccine effectiveness in the 2017-18 flu season in the United States was estimated to be only 42% against influenza B viruses [19]. All these problems concerning the influenza vaccines are largely due to the frequent mutations of the virus surface proteins, particularly the hemagglutinin (HA) [20]. The mutations often take place in the HA1 subunit of the HA protein in both IAV and IBV, resulting in highly variable antigenicity of the HA protein whereas the other subunit, HA2, is highly conserved among all strains analyzed [21].

Viral infection or vaccination mainly induce antibodies targeting the variable HA1 subunit (head) whereas the HA2 subunit (stalk) is not the primary target of neutralizing antibodies, given that the latter is largely shielded by the head [22] [23]. Moreover, while antibodies targeting the stalk of HA can be broadly protective since it is highly conserved [1] [24], the role of cell mediated immune responses targeting the stalk region is not well studied, particularly in response to IBV infection or vaccination. In this study, we used in silico analyses to predict candidate epitopes in the HA2 stalk region of IBV and subsequently validated their immunodominance using H-2Kd-restricted mice.

Section snippets

Materials and methods

The detailed methods have been presented as supplementary information.

H-2Kd-restricted epitope prediction in silico

The highly conserved HA2 subunit of Influenza B/Victoria/2/87 was subjected to in silico analyses using three different online MHC class I binding prediction programs, NeTMHC3.4, syfpeithi. de and IEDB Analysis Resource (Supp Table S1) [35]. A snapshot of the outputs from each program consisting of 9mer peptides is summarized in Supp Table S1 in descending order of MHC I binding affinity. All three programs predicted the same two peptides to have the highest binding affinity to H-2Kd-restricted

Acknowledgements

We thank Dr. Jessie Lavoie and Dr. Michael Johnston for their critical review of the manuscript. The authors declare no conflict of interest. This work is funded by the Canadian Regulatory Strategy for Biotechnology (XL).

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