Mutations in the Hemagglutinin Stalk Domain Do Not Permit Escape from a Protective, Stalk-Based Vaccine-Induced Immune Response in the Mouse Model

Current seasonal influenza virus vaccines target regions of the hemagglutinin (HA) head domain that undergo constant antigenic change, forcing the painstaking annual reformulation of vaccines. The development of broadly protective or universal influenza virus vaccines that induce cross-reactive, protective immune responses could circumvent the need to reformulate current seasonal vaccines. Many of these vaccine candidates target the HA stalk domain, which displays epitopes conserved within and across influenza virus subtypes, including those with pandemic potential. While HA head-mediated antigenic drift is well understood, the potential for antigenic drift in the stalk domain is understudied. Using a panel of HA stalk-specific monoclonal antibodies (MAbs), we applied selection pressure to the stalk domain of A/Netherlands/602/2009 (pdmH1N1) to determine fitness and phenotypes of escape mutant viruses (EMVs). We found that HA stalk MAbs with lower cross-reactivity caused single HA stalk escape mutations, whereas MAbs with broader cross-reactivity forced multiple mutations in the HA. Each escape mutant virus greatly decreased mAb neutralizing activity, but escape mutations did not always ablate MAb binding or Fc-Fc receptor-based effector functions. Escape mutant viruses were not attenuated in vitro but showed attenuation in an in vivo mouse model. Importantly, mice vaccinated with a chimeric HA universal vaccine candidate were protected from lethal challenge with EMVs despite these challenge viruses containing escape mutations in the stalk domain. Our study indicates that while the HA stalk domain can mutate under strong MAb selection pressure, mutant viruses may have attenuated phenotypes and do not evade a polyclonal, stalk-based vaccine-induced response.