Characterisation of Neutralising and Functional Antibody Responses to Different HIV-1 Env Vaccines in Bovines

Abstract

Two main challenges have impeded the development of an effective HIV-1 envelope (Env) vaccine, with antibodies eliciting neutralisation of virions as well as Fc-effector functions, such as antibody-dependent cytotoxicity (ADCC), phagocytosis (ADP) or complement deposition (ADCD). On one hand, designing the right Env vaccine to elicit humoral or cellular protection has been challenging and, to date, SOSIP-Env trimers which are covalently constrained in the closed, pre-fusion conformation are the best vaccine candidate over uncleaved (Unc), open-structured trimers. On the other hand, eliciting heterologous neutralising antibodies in several animal models (including humans) has been difficult. Cows nevertheless produce unique antibodies with long CDRH3 regions, capable of accessing neutralising epitopes beneath the glycan shield, inaccessible for other animals. We tested how differences in clade and/or structure of HIV-1 Env vaccines affect the neutralising activity and Fc-effector functions of antibodies elicited, using recombinant trimers of clades A (KNH1,BG505), B (AD8, PSC89) and C (MW), which exposed either an open structure (Unc gp140) or a closed structure (SOSIP gp140). KNH1/BG505 SOSIP gp140 vaccine elicited the best neutralising IgGs against heterologous tier-2 pseudoviruses with high potency and breadth. While AD8 Unc gp140 also induced neutralisation, it was against only tier-1 pseudoviruses. Nevertheless, it was the only vaccine able to elicit IgGs that engaged CD32 (FcgRIIa), induced phagocytosis and complement-activation. The different antibody profile observed with both vaccines was explained by the Env immunogen structure, as KNH1/BG505 SOSIP gp140 induced mostly IgGs targeting the V1/V2 loop, whereas AD8 Unc gp140 induced antibodies targeting CD4-binding site and CD4-induced epitopes. In addition, analysis of IgG repertoires from animals of KNH1/BG505 SOSIP 100 and AD8 Unc 500 groups showed that KNH1/BG505 SOSIP gp140 induced higher rates of somatic hypermutation in germline genes compared to AD8 Unc gp140, with each animal presenting a unique antibody profile, and with germline antibodies already presenting high affinity towards HIV-1 Env trimers, as high levels of affinity maturation were not required to obtain antibodies with high neutralising activity. Overall, the results in this work show that open structured trimers elicit antibodies which highly activate antibody-effector functions, while SOSIP trimers focus antibody responses to concealed neutralising epitopes. The high neutralising responses observed in bovines against HIV-1 Env are due to antibodies which do not need high levels of somatic hypermutations and, in particular for KNH1/BG505 SOSIP, this antigen induced high levels of affinity maturation, probably favouring the improvement of both binding and neutralisation. Our study suggests that an effective vaccine regimen may include both uncleaved gp140 and SOSIP gp140, in order to target epitopes required for antibody-dependent effector functions as well as neutralisation, or a new trimeric structure with flexibility in the gp120-gp41 interface, exposing both epitopes involved in Fc-effector functions as well as neutralising ones.