HIV-1?envelope?(Env) trimers, stabilized in a prefusion-closed conformation, can elicit humoral?responses?capable of neutralizing?HIV-1strains closely matched in sequence to the immunizing strain. One strategy to increase elicited?neutralization?breadth?involves vaccine priming of immune?responses?against a target site of vulnerability followed by vaccine boosting of these?responses?with prefusion-closed Env trimers. This strategy has succeeded at the?fusion?peptide (FP)-site of vulnerability in eliciting cross-clade neutralizing?responses?in standard vaccine-test animals. However, the?breadth?and potency of the elicited?responses?have been less than optimal. Here we identify three mutations... More
HIV-1?envelope?(Env) trimers, stabilized in a prefusion-closed conformation, can elicit humoral?responses?capable of neutralizing?HIV-1strains closely matched in sequence to the immunizing strain. One strategy to increase elicited?neutralization?breadth?involves vaccine priming of immune?responses?against a target site of vulnerability followed by vaccine boosting of these?responses?with prefusion-closed Env trimers. This strategy has succeeded at the?fusion?peptide (FP)-site of vulnerability in eliciting cross-clade neutralizing?responses?in standard vaccine-test animals. However, the?breadth?and potency of the elicited?responses?have been less than optimal. Here we identify three mutations (3mut) - Met302, Leu320, and Pro329 - that stabilize the apex of the Env?trimer?in a prefusion-closed conformation and show antigenically, structurally and immunogenically that combining 3mut with other approaches (e.g. repair-and-stabilize and glycine-helix breaking) yields well-behaved clade C-Env trimers capable of boosting the?breadth?of FP-directed?responses. Crystal structures of these trimers confirmed prefusion-closed apexes stabilized by hydrophobic patches contributed by Met302 and Leu320, with Pro329 assuming canonically restricted dihedral angles. We substituted the N-terminal eight residues of FP (FP8, residues 512-519) of these trimers with the second most prevalent FP8 sequence (FP8v2, AVGLGAVF), and observed a 3mut-stabilized consensus clade C-Env?trimer?with FP8v2 to?boost?the?breadth?elicited in guinea pigs of FP-directed?responses?induced by immunogens containing the most prevalent FP8 sequence (FP8v1, AVGIGAVF). Overall, 3mut can stabilize the Env-trimer?apex, and the resultant apex-stabilized Env trimers can be used to expand the?neutralization?breadth?elicited against the FP-site of vulnerability.IMPORTANCE?A major hurdle to the?development?of an effective?HIV-1vaccine is the elicitation of serum?responses?capable of neutralizing circulating strains of HIV, which are extraordinarily diverse in sequence and often highly?neutralization?resistant. Recently, we showed how sera with 20-30%?neutralization?breadth?could, nevertheless, be elicited in standard vaccine-test animals by priming with the most prevalent N-terminal 8 residues of the?HIV-1?fusion?peptide (FP8) followed by boosting with a stabilized BG505-envelope?(Env)?trimer. Here we show that subsequent boosting with a '3mut' apex-stabilized consensus C-Env?trimer, modified to have the second most prevalent FP8 sequence, elicits higher?neutralization?breadth?than induced by continued boosting with the stabilized BG505-Env?trimer. With increased neutralizing?breadth?elicited by boosting with a heterologous?trimer?containing the second most prevalent FP8 sequence, the?fusion?peptide-directed?immune-focusing approach moves a step closer toward realizing an effective?HIV-1?vaccine regimen.
