Identification of non-HIV-derived (poly)peptides as primary immunogens for HIV-1 vaccine development and localization of two dominant ADCC epitopes on hemagglutinin antigen of pandemic H1N1 influenza virus

Abstract

Development of effective vaccines against mutable viruses (i.e HIV-1 and influenza) remains a big challenge. Antibody-dependent cell-mediated cytotoxicity (ADCC) has been found to be a key component of immune protection against viral infections in vivo. Therefore, vaccine immunogens that elicit broadly neutralizing antibodies with high ADCC are desired for vaccine development. This study is to identify primary immunogens that can initiate somatic maturation of germline antibodies of known broadly neutralizing HIV-1 antibodies (bnAbs) for HIV vaccine development and to localize dominant ADCC epitopes on hemagglutinin (HA) of pandemic H1N1 influenza virus for development of a flu vaccine.

Based on the observations that known HIV-1 bnAbs have extensive somatic mutation compared to their germline versions and that HIV-1 envelope (Env) glycoprotein lacks measurable binding to putative germline antibodies of known bnAbs, we hypothesized that non-HIV-derived (poly)peptides may serve as primary immunogens to trigger somatic maturation of germline antibodies of bnAbs, leading to elicitation of intermediate antibodies (iAbs) that can further mature to HIV-1 bnAbs upon Env vaccination or HIV-1 infection. Using b12 as a model bnAb, we identified five non-HIV-derived (poly)peptides that bound to putative b12 germline and iAbs, and immunized rabbits with the (poly)peptide priming followed by Env boosting. Rabbit immunization with (poly)peptides alone induced high titers of antibodies that were cross-reactive with gp140SF162 trimer and resurfaced Env RSC3, and the serum IgGs neutralized SF162 and JRFL. These results suggest that the (poly)peptides might structurally mimic CD4bs of Env. Priming rabbits with (poly)peptides followed by boosts with gp140SF162 and RSC3 resulted in antibodies capable of competing with b12 for binding to gp140SF162 trimer and neutralizing cross-clade isolates, while control rabbits without priming produced antibodies that were unable to compete with b12 for gp140SF162 trimer binding, and the serum IgGs neutralized only 3 clade B isolates. Our results provide proof of concept that non-HIV-derived (poly)peptides may serve as primary vaccine immunogens to initiate guided immune responses towards bnAbs.

HA protein has high level of immunogenicity and considered the most important target for immune protection against influenza virus infection. Several potent HA-specific bnAbs have been reported with their conserved neutralizing epitopes revealed, but there has been no report so far about ADCC epitopes on HA. Using yeast display and flow cytometry assisted cell sorting, we mapped the epitope of convalescent plasma IgGs with different ADCC activity, we identified two dominant ADCC epitopes, designated HA-E1 [AA92-117] and HA-E2 (AA 124-159), on HA of 2009 pandemic H1N1 influenza virus. E1 and E2 overlapped with the immunodominant epitopes of HA. Depletion of purified patient plasma IgGs with yeast cells expressing E1 or E2 peptides decreased ADCC activity of the IgGs. E1 and E2 sequences are highly conserved in H1N1 strains, but less so in other subtypes of influenza A viruses. Our study may aid in designing immunogens that can elicit antibodies with high ADCC activity. Vaccine immunogens designed to include the structural determinants of potent bnAbs and ADCC epitopes may confer a comprehensive immune protection against influenza virus infection.