Antibodies against conserved antigens provide opportunities for reform in influenza vaccine design

Abstract

High-performance neutralizing antibody against influenza virus typically recognizes the globular head region of its hemagglutinin (HA) envelope glycoprotein. To-date, approved human vaccination strategies have been designed to induce such antibodies as a sole means of preventing the consequences of this infection. However, frequent amino-acid changes in the HA globular head allow for efficient immune evasion. Consequently, vaccines inducing such neutralizing antibodies need to be annually re-designed and re-administered at a great expense. These vaccines furthermore provide little-to-no immunity against antigenic-shift strains, which arise from complete replacement of HA or of neuraminidase genes, and pose pandemic risks. To address these issues, laboratory research has focused on inducing immunity effective against all strains, regardless of changes in the HA globular head. Despite prior dogma that such cross-protection needs to be induced by cellular immunity alone, several advances in recent years demonstrate that antibodies of other specificities are capable of cross-strain protection in mice. This review discusses the reactivity, induction, efficacy, and mechanisms of antibodies that react with poorly accessible epitopes in the HA stalk, with the matrix 2 membrane ion channel, and even with the internal nucleoprotein. These advances warrant further investigation of the inducibility and efficacy of such revolutionary antibody strategies in humans.

Keywords: antibody; hemagglutinin; influenza virus; matrix 2 external domain; nucleoprotein; vaccine.

Figure 1

Neutralizing antibody binding to hemagglutinin. (A) Gross structure of the hemagglutinin (HA). HA1 (light blue) makes-up most of the globular head domain as well as a polypeptide extension into the stalk area. HA2 (dark blue) makes-up the membrane-proximal stalk domain. (B) Schematized secondary structure of HA stalk monomer (HA2) in the pre-fusion conformation (adapted from Han and Marasco, 2011). Cylinders represent α helices and broad blue arrow represents a β sheet. Middle red line represents N-terminal fusion peptide. Previously defined subdomains are labeled with capital letters. Gray arrows point to epitope locations for the indicated broadly neutralizing antibody clones (nAb, see Table 1).

Figure 2

Influenza virus infection and mechanisms of HA-specific neutralizing antibodies. Unimpeded influenza virus binds to receptors on the host-cell plasma membrane (A) and is internalized. The resulting vesicles become acidified via the M2 proton channel. This pH reduction results in HA conformational change that catalyzes the fusion of the host vesicle membrane with the viral envelope (B). The virion is subsequently dissociated into the cytoplasm (C), followed by transport of the ribonucleoprotein viral genome segments into the nucleus for replication and transcription (not shown). HAI-competent antibodies that bind to HA globular head effectively inhibit virion binding to host cells, preventing virus entry (D). HAI-independent neutralizing antibodies that react with the HA stalk region can prevent the conformational changes of this antigen and prevent fusion of viral envelope with host membrane (E). Not drawn to scale.

Figure 3

Influenza virus release and possible mechanisms for antibodies against NA and M2e. Infected host cells package eight ribonucleoprotein genome segments into viral buds formed by M1 capsid monomers (gray diamonds) and host plasma membrane containing HA, NA, and M2 (A). M2 catalyzes membrane fusion to pinch-off newly budded virions, which initially remain tethered to sialic acid on the host-cell surface (B). NA enzymatic activity cleaves the interactions of tethered HA (not depicted) and NA with sialic acid, effectively releasing the new virions (C). Antibodies against M2e may interfere with the pinching-off stage of viral budding (D), whereas antibodies against NA may prevent final release of the virion (E). See Figure 2 for symbol legend. Not drawn to scale.

Figure 4

Proposed mechanisms for antibody against influenza nucleoprotein. Virus-infected cells (typically epithelia) release NP protein (A), which would then be available to NP-specific antibody. Immune complexes formed by antigen and antibody (B) could thus engage Fc receptors on leukocytes (C,F) triggering antiviral reactions. These reactions may include enhanced antigen presentation by dendritic cells or other antigen-presenting cells (C,D), which could then in turn enhance antiviral T cell responses capable of eliminating virally infected epithelia (E). Alternatively or additionally, Fc receptor engagement on macrophages or other leukocytes (F) may execute antiviral reactions directly (G) or more indirectly by enhancing the antiviral T cell response at various levels (H). Anti-NP antibody engagement of plasma membrane-associated NP could also trigger complement-mediated lysis of infected cells (I).