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The HIV fusion peptide (FP) is a promising vaccine target. FP-directed monoclonal antibodies from vaccinated macaques have been identified that neutralize up to ∼60% of HIV strains; these vaccinations, however, have involved ∼1 year with an extended neutralization-eclipse phase without measurable serum neutralization. Here, in 32 macaques, we test seven vaccination regimens, each comprising multiple immunizations of FP-carrier conjugates and HIV envelope (Env) trimers. Comparisons of vaccine regimens reveal FP-carrier conjugates to imprint cross-clade neutralizing responses and a cocktail of FP conjugate and Env trimer to elicit the earliest broad responses. We identify a signature, appearing as early as week 6 and involving the frequency of B cells recognizing both FP and Env trimer, predictive of vaccine-elicited breadth ∼1 year later. Immune monitoring of B cells in response to vaccination can thus enable vaccine insights even in the absence of serum neutralization, here identifying FP imprinting, cocktail approach, and early signature as means to improve FP-directed vaccine responses., Competing Interests: Declaration of Interests The authors declare no competing interests., (Published by Elsevier Inc.)

The vaccine-mediated elicitation of antibodies (Abs) capable of neutralizing diverse HIV-1 strains has been a long-standing goal. To understand how broadly neutralizing antibodies (bNAbs) can be elicited, we identified, characterized, and tracked five neutralizing Ab lineages targeting the HIV-1-fusion peptide (FP) in vaccinated macaques over time. Genetic and structural analyses revealed two of these lineages to belong to a reproducible class capable of neutralizing up to 59% of 208 diverse viral strains. B cell analysis indicated each of the five lineages to have been initiated and expanded by FP-carrier priming, with envelope (Env)-trimer boosts inducing cross-reactive neutralization. These Abs had binding-energy hotspots focused on FP, whereas several FP-directed Abs induced by immunization with Env trimer-only were less FP-focused and less broadly neutralizing. Priming with a conserved subregion, such as FP, can thus induce Abs with binding-energy hotspots coincident with the target subregion and capable of broad neutralization., (Published by Elsevier Inc.)

The vaccine elicitation of broadly neutralizing responses is a central goal of HIV research. Recently, we elicited cross-clade neutralizing responses against the N terminus of the fusion peptide (FP), a critical component of the HIV-entry machinery. While the consistency of the elicited cross-clade neutralizing responses was good in mice, it was poor in guinea pigs: after seven immunizations comprising either envelope (Env) trimer or FP coupled to a carrier, serum from only one of five animals could neutralize a majority of a cross-clade panel of 19 wild-type strains. Such a low response rate-only 20%-made increasing consistency an imperative. Here, we show that additional Env-trimer immunizations could boost broad FP-directed neutralizing responses in a majority of immunized animals. The first boost involved a heterologous Env trimer developed from the transmitted founder clade C strain of donor CH505, and the second boost involved a cocktail that combined the CH505 trimer with a trimer from the BG505 strain. After boosting, sera from three of five animals neutralized a majority of the 19-strain panel and serum from a fourth animal neutralized 8 strains. We demonstrate that cross-reactive serum neutralization targeted the FP by blocking neutralization with soluble fusion peptide. The FP competition revealed two categories of elicited responses: an autologous response to the BG505 strain of high potency (~10,000 ID50), which was not competed by soluble FP, and a heterologous response of lower potency, which was competed by soluble FP. While the autologous response could increase rapidly in response to Env-trimer boost, the heterologous neutralizing response increased more slowly. Overall, repetitive Env-trimer immunizations appeared to boost low titer FP-carrier primed responses to detectable levels, yielding cross-clade neutralization. The consistent trimer-boosted neutralizing responses described here add to accumulating evidence for the vaccine utility of the FP site of HIV vulnerability., Competing Interests: The authors have declared that no competing interests exist.