Trimeric Class I Viral Fusion Protein Vaccine Immunogens Using a Trimeric Autotransporter in a Killed Whole-Cell Bacteria Vaccine Platform: Applications to HIV MPER.

BACKGROUND: Trimeric envelope-proximal domains in viral class I fusion proteins are conserved targets of broadly neutralizing antibodies (bNAbs), but it has proven difficult to develop vaccines against those targets. The HIV-1 gp41 membrane-proximal external region (MPER) is one such target. Induction of a neutralizing response likely depends on the immunogen having a close-to-native structure. METHODS: Native sequence MPER was displayed on genome-reduced bacteria as a coiled-coil homotrimer using a Haemophilus influenzae Hia trimeric autotransporter. Vaccine designs incorporated additional features, including trimerization domains to stabilize MPER, tandem MPER repeats to increase antigen valency, and immunomodulatory elements. Antigen exposure was assessed by flow cytometry, antibody responses were evaluated by ELISA, and functional activity was measured using HIV-1 pseudovirus neutralization assays. RESULTS: Trimer stabilization improved MPER exposure, but immunogen visibility alone did not predict neutralization. After three immunizations, neutralizing activity was detected only in the most extensively engineered vaccine, which neutralized tier 2 virus CNE55. After five immunizations, the same vaccine also neutralized the tier 2 virus 25710-2.43. A further design modification that included an extended Hia-derived spacer increased MPER exposure and antibody binding, with neutralization detected against MN.3, X1632_S2_B10, and 25710-2.43 viruses in subsets of animals. CONCLUSIONS: As a proof-of-concept, native-sequence MPER can induce detectable, though modest and virus-dependent, HIV-1 neutralizing activity when displayed in a carefully controlled trimeric bacterial surface-display platform. The results show that MPER vaccine performance depends not only on antigen exposure, but also on multimeric organization, immunomodulatory context, and antigen-scaffold geometry. Analogous coiled-coil trimeric bacterial surface display immunogens may inform vaccine development for stem/stalk regions of other Class I fusion protein viruses.