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Introduction
HIV-1 has evolved extensive strategies to escape the human immune system. Thus, providing protection with traditional vaccines, such as inactivated viruses, against infection is a challenge. Eliciting broadly neutralizing antibodies (BnAbs) is a long-desired goal for a prophylactic HIV-1 vaccine. Although many attempts to induce BnAbs based on conserved protein subunit vaccines, plasmid DNA or Env recombinant viral vaccines have been made, none has been successful [1-3] .
2F5 and 4E10 are two promising BnAbs targeting the membrane proximal external region (MPER) of the HIV-1 Env protein [4] . Prior studies have shown that linear peptides of these epitopes fail to induce antibodies with neutralizing activity, suggesting that an effective immunogen must adopt a specific conformation [5,6] . Therefore, more attention has been paid to clarifying the core structure of these BnAbs and determining the best approach to presenting these epitopes accurately. It has become apparent that the conformation of the antigen must be considered in the rational design of HIV-1 vaccines for eliciting BnAbs. Crystal structures and nuclear magnetic resonance (NMR) studies have demonstrated that the monoclonal antibody (mAb) 2F5 contains a beta turn in complex with the gp41 peptide or alone, while mAb 4E10 forms a helical structure in complex with the 13-residue peptide which includes the 4E10 core epitope [7-10] .
Some studies have adopted live viruses as vehicles for presenting the 2F5 or 4E10 neutralizing antibody epitopes. In one previous study, guinea pigs immunized with recombinant human rhinoviruses (HRV) containing the 2F5 epitope elicited serum antibodies with broad but modestly neutralizing activity when boosted with corresponding 2F5 peptides [11] . In another study, a chimeric virus (influenza A/WSN/33), which contained the core epitope of 2F5 (ELDKWA) in the antigenic site B of HA, was able to elicit specific immunoreactivity in mice, inducing antisera that could neutralize the HIV-1 strains RF and MN [12] . As 2F5-specific IgA could be detected and persisted for more than 1 year in respiratory, intestinal and vaginal secretions of mice intranasally immunized with this chimeric virus, this mucosal immunization approach is a promising approach in preventing both vaginal and rectal transmission of HIV-1 [13] .
Several similarities exist between HIV-1 and influenza A virus [14] . (1) They are RNA viruses of approximately 80-120nm...