After increase immunization, the M2e-B stalk group was challenged with influenza B/Florida virus and monitored for body weight changes and survival rates (Fig. A computer virus supplemented with M2e-B stalk effectively induced hemagglutination inhibiting antibodies, humoral and cellular M2e immune responses, and enhanced heterosubtypic protection. This study demonstrates the importance of Pou5f1 HA functional integrity in influenza vaccine efficacy and that supplementation of influenza vaccines with M2e-B stalk protein could be a feasible strategy of improving cross-protection against influenza viruses. Keywords: Hemagglutinin integrity, M2e immunity, Protection 1.?Introduction Influenza computer virus causes 5 million cases of severe disease and a range of 290,000 and 650,000 deaths worldwide annually (Iuliano et al., 2018). Influenza vaccines, which include inactivated whole or split computer virus, live attenuated computer virus, and purified protein vaccines, are based on inducing immunity to hemagglutinin (HA). However, current influenza vaccines require annual updates and are not effective in providing protection against drifted seasonal or pandemic viruses due to high antigenic variations in the HA receptor binding domain name. Previous studies have reported conserved antigenic targets in vaccine design, which include M2 ectodomain (M2e), HA stalk domains, and T cell epitope peptides (M001, FLU-v) (Atsmon et al., 2014; Kim et al., 2013; Pleguezuelos et al., 2020; Yassine et al., 2015). However, vaccines targeting conserved domains or epitopes alone present limitations due to intrinsic immunorecessive properties and non-neutralizing immunity. In addition, they induce weaker protection than neutralizing immunity of HA based vaccines, despite broadening the windows of cross protection (Jegerlehner et al., 2004; Lee et al., 2016; Mallajosyula et al., 2014; Sutton et al., 2017). HA globular head region contains receptor-binding site that specifically interacts with sialic acid moieties on the target cell surface glycoproteins, whereas the relatively conserved HA stalk domain name mediates viral and host cell membrane fusion activity. We sought to induce additional immunity to conserved epitopes for broader cross protection while retaining HA-based seasonal protection. Seasonal HA vaccination supplemented with M2e-based vaccine was reported to induce strain-specific HA immunity and M2e immunity-mediated cross protection (Kim et al., 2014; Lee et al., 2015, 2019). In our previous studies, reassortant influenza viruses displaying M2e epitopes on the surface of virion particles with chimeric M2e and functional HA (M2e-HA) molecules were generated and shown to induce cross protection (Kim et al., 2017; Park et al., 2021). However, one drawback was that administering live virus platforms of M2e-HA recombinant viruses would not be effective in adults. Chemical cross-linking was previously utilized to produce immunogenic protein nanoparticle vaccines inducing cross protection (Deng et al., 2018a, 2018b) and might provide an approach of conjugating M2e conserved epitopes onto influenza virus particles. PKR Inhibitor In this study, we tested whether chemical cross-linking of inactivated influenza virus particles with M2e PKR Inhibitor peptides would enhance cross protection. However, treatment of influenza virus with chemical cross-linker resulted in a loss of HA receptor binding activity and lower efficacy of homologous and heterologous protection. We then further investigated an alternative strategy of supplementing influenza vaccination with a new chimeric M2e-B stalk protein derived from influenza B virus. This strategy was effective in inducing immunity to M2e and HA, as well as the stalk region of influenza B virus, and it resulted in enhanced cross protection. 2.?Materials and methods 2.1. Viruses Influenza viruses PKR Inhibitor such as A/Puerto Rico/8/34 (A/PR8 H1N1) and A/Philippines/2/82 (A/Phil H3N2) were amplified in 10-day old embryonated chicken eggs (Hy-Line N.A., Mansfield, GA, USA), and the viruses were purified from allantoic fluid. A/PR8 viruses were inactivated by treatment with 37% formalin at 1:4000 (v/v) dilutions, as described previously (Quan et al., 2008). 2.2. Generation of vaccine constructs DTSSP [3,3′-dithiobis (sulfosuccinimidyl propionate)] reacts with primary amines and was previously utilized to produce immunogenic protein nanoparticle vaccines inducing cross protection (Deng et al., 2018a, 2018b), showing its potential as a chemical cross-linker in influenza vaccines. To conjugate M2e peptides to the surface of inactivated A/PR8 (iPR8) virus, iPR8 H1N1 virus (300 g) was mixed with 170 g of M2e peptide (SLLTEVETPIRNEWGSRSNDSSDKKK, GenScript) (iPR8-M2e*, Fig. 1A). DTSSP cross-linker (Sigma, Saint Louis, MO, USA) was added to a final.