HIV-1 neutralizing antibodies induced by native-like envelope trimers

Steps in the right direction HIV-1 mutates rapidly, making it difficult to design a vaccine that will protect people against all of the virus' iterations. A potential successful vaccine design might protect by eliciting broadly neutralizing antibodies (bNAbs), which target specific regions on HIV-1's trimeric envelope glycoprotein (Env) (see the Perspective by Mascola). Jardine et al. used mice engineered to express germline-reverted heavy chains of a particular bNAb and immunized them with an Env-based immunogen designed to bind to precursors of that bNAb. Sanders et al. compared rabbits and monkeys immunized with Env trimers that adopt a nativelike conformation. In both cases, immunized animals produced antibodies that shared similarities with bNAbs. Boosting these animals with other immunogens may drive these antibodies to further mutate into the longsought bNAbs. Chen et al. report that retaining the cytoplasmic domain of Env proteins may be important to attract bNAbs. Removing the cytoplasmic domain may distract the immune response and instead generate antibodies that target epitopes on Env that would not lead to protection. Science, this issue p. 139, 10.1126/science.aac4223, p. 156; see also p. 191 Recombinant, native-like, HIV-1 envelope trimers induce neutralizing antibody responses in animal models. [Also see Perspective by Mascola] INTRODUCTION A major goal of HIV-1 vaccine development is to identify immunogens capable of inducing protective titers of broadly neutralizing antibodies (bNAbs) against circulating, neutralization-resistant (tier 2) viruses. The envelope glycoprotein (Env) trimer on the virus surface is the only bNAb target and accordingly serves as the basis for recombinant protein immunogens intended to induce bNAbs. We have engineered soluble, recombinant trimers based on the BG505 clade A and B41 clade B tier 2 viruses. These proteolytically cleaved and stabilized trimers, designated BG505 and B41 SOSIP.664 gp140, display multiple bNAb epitopes and have antigenic properties that mimic viral Env. High-resolution structures of the BG505 trimers reinforce this perspective. In contrast, other trimer designs (uncleaved gp140) that lack the SOSIP modifications and are based on eliminating the intersubunit proteolytic cleavage site adopt non-native configurations. RATIONALE To determine the NAb response elicited by native-like trimers, we immunized rabbits and macaques with BG505 SOSIP.664 gp140 proteins, and rabbits with their B41 counterparts. For comparison with the BG505 trimers, we also tested gp120 monomers and uncleaved gp140 proteins of the same genotype. We characterized the resulting antibody responses by quantifying their ability to neutralize the autologous tier 2 viruses and multiple heterologous tier 1 (neutralization-sensitive) and tier 2 viruses. We mapped the BG505 NAb responses using various techniques, including a large panel of Env-pseudotyped virus mutants. RESULTS BG505 SOSIP.664 trimers consistently induced potent NAbs against the autologous tier 2 virus in rabbits and similar but weaker responses in macaques. Cross-reactive NAbs against the more sensitive tier 1 viruses were also induced in all the immunized animals, but heterologous neutralization of tier 2 viruses was seen only weakly and sporadically. The rabbit responses to B41 SOSIP.664 trimers were qualitatively similar to those seen in the BG505 trimer recipients. The autologous tier 2 NAb titers to both SOSIP.664 trimers were greater than any such responses previously observed using various uncleaved gp140 proteins, including the BG505 version used in this study. The rabbit tier 2 and tier 1 NAb responses to the BG505 SOSIP.664 trimers were uncorrelated. Mapping studies showed the tier 2 NAbs recognized conformational epitopes that differed between animals. In some cases, these NAbs targeted components of the glycan shield previously associated with bNAb epitopes. In contrast, the tier 1 NAbs targeted linear epitopes in the gp120 V3 region. CONCLUSIONS Although an autologous tier 2 NAb response is not sufficient for vaccine protection against HIV-1, it may be a necessary step in various strategies intended to induce bNAbs. Thus, our findings indicate that native-like trimers represent a promising starting point for the further development of recombinant Env immunogens intended to broaden the NAb response. The high-resolution structures of SOSIP.664 trimers allow improvements to be rationally designed. Relevant strategies include the further stabilization of trimer-associated bNAb epitopes while reducing the antigenicity of V3 and other non-neutralizing epitopes that may be distractive; immunizing with longitudinal series of SOSIP.664 trimers from infected individuals who generate bNAbs; using trimer cocktails (e.g., from clades A, B, and C); and priming with trimer variants that trigger desirable germline responses before boosting with wild-type trimers. Autologous tier 2 NAb responses in animals immunized with native-like (SOSIP.664) trimers or non-native uncleaved gp140s. (Left) Representative reference-free 2D class averages of negative-stain electron microscopy (EM) images of the uncleaved BG505 gp140 and the native-like BG505 SOSIP.664 trimers. The illustrations (orange) of the two categories of Env protein are based on negative-stain EM. (Right) Autologous tier 2 serum NAb titers (IC50) in 50 animals immunized with various uncleaved gp140 proteins, compared to titers in the 30 rabbits given the BG505 or B41 SOSIP.664 trimers described here (see legend of Fig. 2 in the full article for details; P < 0.0001, two-tailed Mann-Whitney test). A challenge for HIV-1 immunogen design is the difficulty of inducing neutralizing antibodies (NAbs) against neutralization-resistant (tier 2) viruses that dominate human transmissions. We show that a soluble recombinant HIV-1 envelope glycoprotein trimer that adopts a native conformation, BG505 SOSIP.664, induced NAbs potently against the sequence-matched tier 2 virus in rabbits and similar but weaker responses in macaques. The trimer also consistently induced cross-reactive NAbs against more sensitive (tier 1) viruses. Tier 2 NAbs recognized conformational epitopes that differed between animals and in some cases overlapped with those recognized by broadly neutralizing antibodies (bNAbs), whereas tier 1 responses targeted linear V3 epitopes. A second trimer, B41 SOSIP.664, also induced a strong autologous tier 2 NAb response in rabbits. Thus, native-like trimers represent a promising starting point for the development of HIV-1 vaccines aimed at inducing bNAbs.