NAction! How Can Neuraminidase-Based Immunity Contribute to Better Influenza Virus Vaccines?

Abstract

Neuraminidase is one of the two surface glycoproteins of influenza A and B viruses. It has enzymatic activity that cleaves terminal sialic acid from glycans, and that activity is essential at several points in the virus life cycle. While neuraminidase is a major target for influenza antivirals, it is largely ignored in vaccine development. Current inactivated influenza virus vaccines might contain neuraminidase, but the antigen quantity and quality are varied and not standardized. While there are data that show a protective role of anti-neuraminidase immunity, many questions remain unanswered. These questions, among others, concern the targeted epitopes or antigenic sites, the potential for antigenic drift, and, connected to that, the breadth of protection, differences in induction of immune responses by vaccination versus infection, mechanisms of protection, the role of mucosal antineuraminidase antibodies, stability, and the immunogenicity of neuraminidase in vaccine formulations. Reagents for analysis of neuraminidase-based immunity are scarce, and assays are not widely used for clinical studies evaluating vaccines. However, efforts to better understand neuraminidase-based immunity have been made recently. A neuraminidase focus group, NAction!, was formed at a Centers of Excellence for Influenza Research and Surveillance meeting at the National Institutes of Health in Bethesda, MD, to promote research that helps to understand neuraminidase-based immunity and how it can contribute to the design of better and broadly protective influenza virus vaccines. Here, we review open questions and knowledge gaps that have been identified by this group and discuss how the gaps can be addressed, with the ultimate goal of designing better influenza virus vaccines.

Keywords: influenza vaccines; neuraminidase; universal influenza virus vaccine.

FIG 1

Influenza virus neuraminidase structure and phylogeny. (A) Top-down view of the NA tetramer, with one of the monomers indicated in gray. (B) Side view of the molecule. The structure of the variable stalk domain has so far not been solved and is indicated by the 4 bars. Panels A and B are based on the structure of the N2 NA of A/Tanzania/205/2010 (PDB number 4GZO [102]), visualized in PyMOL (Schrödinger, Inc.). (C) Tree of known influenza virus NAs and NA-like proteins (N10 and N11). Influenza NAs cluster into group 1 (N1, N4, N5, N8) and group 2 NAs (N2, N3, N6, N7, N9). Influenza B NAs as well as the NA-like proteins (from sequences found in bats) form their own clusters. The tree was generated using Clustal Omega and was visualized in FigTree. The scale bar represents a 7% amino acid difference. NA sequences from the following strains were used: A/California/04/2009 (N1-Cal09), A/New Caledonia/20/1999 (N1-NC99), A/Puerto Rico/8/1934 (N1-PR8), A/Vietnam/1203/2004 (N1-H5N1), A/Michigan/45/2015 (N1-Mi15), A/Brevig Mission/1/1918 (N1-1918), A/Hong Kong/1/1968 (N2-HK68), A/Hong Kong/4801/2014 (N2-HK14), A/Singapore/1/1957 (N2-Sing57), A/chicken/Hong Kong/G9/1997 (N2-H9N2), A/Philippines/2/1982 (N2-Phil82), A/Beijing/353/1989 (N2-BJ89), A/Perth/16/2009 (N2-Perth09), A/swine/Missouri/4296424/2006 (N3), A/mallard/Sweden/24/2002 (N4), A/mallard/Sweden/86/2003 (N5), A/Caspian seal/Russia/T1/2012 (N6), A/harbor seal/Germany/1/2014 (N7), A/Jiangxi-Donghu/346/2013 (N8), A/Hong Kong/125/2017 (N9), A/yellow-shouldered bat/Guatemala/060/2010 (N10), A/bat/Peru/33/2010 (N11), B/Lee/1940 (Lee40), B/Florida/04/2006 (Flor06), B/Malaysia/2506/2004 (Mal04).