A human monoclonal heterosubtypic antibody, MAb 3.1, with its heavy chain encoded by V H 3-30, was isolated using phage display with immobilized hemagglutinin (HA) from influenza virus A/Japan/305/1957(H2N2) as the target. Antibody 3.1 potently neutralizes influenza viruses from the H1a clade (i.e., H1, H2, H5, H6) but has little neutralizing activity against the H1b clade. Its crystal structure in complex with HA from a pandemic H1N1 influenza virus, A/South Carolina/1/1918(H1N1), revealed that like other heterosubtypic anti-influenza virus antibodies, MAb 3.1 contacts a hydrophobic groove in the HA stem, primarily using its heavy chain. However, in contrast to the closely related monoclonal antibody (Mab) FI6 that relies heavily on HCDR3 for binding, MAb 3.1 utilizes residues from HCDR1, HCDR3, and framework region 3 (FR3). Interestingly, HCDR1 of MAb 3.1 adopts an ␣-helical conformation and engages in hydrophobic interactions with the HA very similar to those of the de novo in silico-designed and affinity-matured synthetic protein HB36.3. These findings improve our understanding of the molecular requirements for binding to the conserved epitope in the stem of the HA protein and, therefore, aid the development of more universal influenza vaccines targeting these epitopes. IMPORTANCE Influenza viruses rapidly evade preexisting immunity by constantly altering the immunodominant neutralizing antibody epitopes (antigenic drift) or by acquiring new envelope serotypes (antigenic shift). As a consequence, the majority of antibodies elicited by immunization or infection protect only against the immunizing or closely related strains.Here, we describe a novel monoclonal antibody that recognizes the conserved heterosubtypic epitope in the stem of influenza A virus hemagglutinin. This antibody, referred to as MAb 3.1, recognizes its epitope in a manner that resembles recognition of a similar epitope by the de novo in silico-designed and affinity-matured synthetic protein HB36.3. Thus, besides providing novel insights into the molecular interactions between heterosubtypic antibodies and influenza virus hemagglutinin, MAb 3.1 demonstrates that de novo in silico-designed and affinity-matured synthetic proteins can foretell naturally selected antibody binding. This knowledge will aid development of a pan-influenza virus vaccine.

The majority of influenza virus-specific antibodies elicited by vaccination or natural infection are effective only against the eliciting or closely related viruses. Rare stem-specific heterosubtypic monoclonal antibodies (hMAbs) can neutralize multiple strains and subtypes by preventing hemagglutinin (HA)-mediated fusion of the viral membrane with the endosomal membrane. The epitopes recognized by these hMAbs are therefore considered promising targets for the development of pan-influenza virus vaccines. Here, we report the isolation of a novel human HA stem-reactive monoclonal antibody, hMAb 1.12, with exceptionally broad neutralizing activity encompassing viruses from 15 distinct HA subtypes. Using MAb 1.12 and two other monoclonal antibodies, we demonstrate that neutralization by hMAbs is virtually irreversible but becomes severely impaired following virus attachment to cells. In contrast, no interference by human anti-influenza virus serum antibodies was found, indicating that apically binding antibodies do not impair access to the membrane-proximal heterosubtypic epitopes. Our findings therefore encourage development of new vaccine concepts aiming at the induction of stem-specific heterosubtypic antibodies, as we provide support for their effectiveness in individuals previously exposed to influenza virus. IMPORTANCEThe influenza A virus hemagglutinin (HA) can easily accommodate changes in its antigenic structures to escape preexisting immunity. This variability restricts the breadth and long-term efficacy of influenza vaccines. Only a few heterosubtypic antibodies (hMAbs), i.e., antibodies that can neutralize more than one subtype of influenza A virus, have been identified. The molecular interactions between these heterosubtypic antibodies and hemagglutinin are well characterized, yet little is known about the functional properties of these antibodies. Using a new, extraordinarily broad hMAb, we show that virus neutralization by hMAbs is virtually irreversible and that efficient neutralization is possible only if stem-specific hMAbs bind to HA before the virus attaches to the cell surface. No interference between strain-specific human serum immunoglobulin and hMAbs was found, indicating that preexisting humoral immunity to influenza virus does not limit the efficacy of stem-reactive heterosubtypic antibodies. This knowledge supports the development of a pan-influenza virus vaccine. H emagglutinin (HA), the major surface antigen of influenza A virus, exists in 18 subtypes and is responsible for virus entry into the host cell. Influenza virus vaccines are usually effective against seasonal influenza (1-3), but currently available vaccines elicit antibodies of limited breadth that neutralize only the inoculated and closely related seasonal strains. This strain-specific (or homotypic) nature of the antibody response implies that seasonal vaccines have to be regularly reformulated to reflect antigenic changes acquired by drifting. Furthermore, vaccines have to precisely match the antigenic outfit of the ...

Background. The effectiveness of trivalent influenza vaccination has been confirmed in several studies. To date, it is not known whether repeated exposure and vaccination to influenza promote production of cross-reactive antibodies. Furthermore, how strains encountered earlier in life imprint the immune response is currently poorly understood.Methods. To determine the prevalence for human homo-and heterosubtypic antibody responses, we scrutinized serum samples from 305 healthy volunteers for hemagglutinin-binding and -neutralizing antibodies against several strains and subtypes of influenza A. Statistical analyses were then performed to establish the association of measured values with potential predictors.Results. It was found that vaccination not only promoted higher binding and neutralizing antibody titers to homosubtypic influenza isolates but also increased heterosubtypic human immune responses. Both binding and neutralizing antibody titers in relation with age of the donors mirrored the course of the different influenza strain circulation during the last century. Advanced age appeared to be of advantage for both binding and neutralizing titers to most subtypes. In contrast, the first virus subtype encountered was found to imprint to some degree subsequent antibody responses. Antibodies to recent strains, however, primarily seemed to be promoted by vaccination.Conclusions. We provide evidence that vaccinations stimulate both homo-and heterosubtypic immune responses in young and middle-aged as well as more senior individuals. Our analyses suggest that influenza vaccinations not only prevent infection against currently circulating strains but can also stimulate broader humoral immune responses that potentially attenuate infections with zoonotic or antigenically shifted strains.

Our results indicate that repetitive vaccinations in HIV-positive individuals enhance antibody titers to human isolates. Interestingly, development of antibody titers to conserved heterosubtypic epitopes paradoxically appeared to profit from HIV-induced immune damage, as did the ratio of binding to neutralizing antibodies.