FIG 5.

Hemagglutinin (HA) of influenza virus: subdominant sites as targets for protective antibodies. (A) Models of influenza virus and hemagglutinin (HA). (Left) In a viral particle, the glycoproteins HA and neuraminidase (NA) are anchored in a lipid envelope surrounding the core, which contains the eight RNA segments of the genome. (Right) Top and side views of an HA trimer of subtype H1, showing the immunodominant head and the subdominant stem (149). The location of the five highly variable and immunodominant sites in the head are indicated (Sa, blue; Sb, gold; Ca1, purple; Ca2, orange; Cb, red). Right panel adapted from reference 149 with permission from Springer Nature. (B) Cartoon showing the location of conserved sites (red) that are targets for protective antibodies in HA. These sites are of at least five types and include the RBS, a partially occluded site located at the monomer interface in the head, other conserved sites in the head, and two sites in the stem, one of which is occluded on native HA, i.e., not accessible to antibodies. Except for this occluded site in the stem, which has unique properties, the various sites may be described as subdominant. Of note, the whole head is variable and immunodominant compared to the stem, but it includes subdominant sites that are conserved and protective. In contrast, the entire stem is subdominant but contains protective sites. Panels C to F, and the corresponding legends, describe procedures that may allow an antibody response to be targeted to a subdominant site. (C) A stem-only construct (“mini-HA”), derived from the subdominant stem of H1 HA, elicits broadly protective antibodies (191). Figure based on PDB 5CJQ and produced with VMD (235). Of note, the structure of mini-HA differs slightly from that for the stem of intact HA, as it adopts a more open splayed conformation (191). (D) Use of chimeric HA proteins to focus responses on the subdominant stem (33, 196). (Left) Adults have preexisting antibodies to the head (top) and the stem (bottom) of H1 HA, although the response to the stem is very limited. (Middle) Immunization with a chimeric H8/1 protein may elicit a memory response to the H1 stem but only a primary response to the “exotic” H8 head. (Right) Boosting with an H5/1 chimera may further boost the response to the H1 stem while again eliciting a primary response to the “exotic” head. As in Fig. 1, antibodies directed against subdominant and immunodominant regions are shown in light blue and dark blue, respectively. (E) Schematic representation of “breathing” in an HA molecule, resulting in the exposure of a site in the head that is located at the monomer interface and is a target for protective antibodies (157, 159, 160). Formation of antibodies to this site may be favored by immunization with hyperglycosylated HA (197). (F) Use of mosaic nanoparticles to focus antibody responses on sites in the HA head that are subdominant and conserved (198). In this procedure, nanoparticles were covered with monomeric HA heads. (Left) A nanoparticle covered with heads from a single virus strain will elicit antibodies directed almost exclusively against strain-specific immunodominant sites, which promote very strong binding to cognate B cells, thereby diverting the antigen from B cells recognizing any other site(s). (Right) Use of a mosaic nanoparticle covered with several different heads will reduce the local concentration of strain-specific immunodominant sites, potentially favoring a response to a conserved and shared subdominant site, which may promote binding of relatively high avidity to cognate B cells.