mRNA-LNP HIV-1 trimer boosters elicit precursors to broad neutralizing antibodies

INTRODUCTION Broadly neutralizing antibodies (bnAbs) isolated from HIV-1-infected patients demonstrate that the humoral immune system can develop effective antibody responses to HIV, even if those antibodies are rare and the result of a somewhat winding journey. Germline-targeting (GT) vaccination seeks to induce bnAbs through sequential immunization. To overcome the relatively low affinity of many germline precursors to bnAbs to the HIV-1 envelope protein (Env), precursors to bnAbs are identified and first-round immunogens developed to prime those responses; later-stage boost immunogens with an increasing number of native-Env features are then meant to continue guiding the evolution of B cell receptors (BCRs) to bnAb development. However, previous work has found that high-affinity, epitope-focused responses to primes may inhibit later boost stages, thus potentially undercutting the GT approach. RATIONALE The development of a vaccine to induce bnAbs similar to the V3-glycan-targeting bnAb BG18 has been a key goal of much GT vaccine work. Previously, our group developed a mouse cell line with B cell receptors bearing the heavy chain of a human BG18 precursor. B cells from this cell line were adoptively transferred into wild-type (WT) mice to produce a stringent preclinical model, which was used to validate a series of GT priming immunogens. In this study, that BG18 precursor model was used to investigate a next-generation priming immunogen (N332-GT5), followed by one of two new boost immunogens (B11 and B16) designed to limit off-target responses to the V1 loop. As mRNA-lipid nanoparticle (LNP) immunogens were found to be highly effective during the COVID-19 pandemic, protein trimer and mRNA-LNP regimens were compared. RESULTS We found that both new boost immunogens (B11 and B16) could drive further maturation of BG18 precursors in a stringent humanized mouse model when delivered after an N332-GT5 protein trimer prime. An mRNA-LNP delivery of both the prime and boost phases also provided long-term activation and was observed to drive somatic hypermutation. Both the protein trimer and mRNA-LNP regimens facilitated boost-stage responses, which may be the result of either germinal center (GC) refueling or of memory B cell re-recruitment to germinal centers. CONCLUSION Our prime-boost regimen has demonstrated on-target activation and boosting of V3-glycan-class responses in a high-bar preclinical model, revealing that boosting can occur after a GT prime. The effectiveness of both protein and mRNA prime-boost regimens opens the route to the clinical development of a sequential HIV vaccine centered on the V3-glycan epitope. Preclinical validation of V3-glycan-targeting prime-boost regimens. (Top left) Germline (gl) BG18 heavy-chain knockin mouse model and adoptive transfer. (Bottom left) Design of the germline-targeting N332-GT5 priming immunogen and the B11 and B16 boosters. (Top right) Effective GC recruitment of BG18 precursors after prime-boost by protein trimer or mRNA-LNP regimens. (Bottom right) Prime-boost increased somatic hypermutation, produced on-target binding, and drove virus neutralization.