Date Published: September 14, 2016
Publisher: Public Library of Science
Author(s): P. J. Klasse, Celia C. LaBranche, Thomas J. Ketas, Gabriel Ozorowski, Albert Cupo, Pavel Pugach, Rajesh P. Ringe, Michael Golabek, Marit J. van Gils, Miklos Guttman, Kelly K. Lee, Ian A. Wilson, Salvatore T. Butera, Andrew B. Ward, David C. Montefiori, Rogier W. Sanders, John P. Moore, Ronald C. Desrosiers.
We have investigated the immunogenicity in rabbits of native-like, soluble, recombinant SOSIP.664 trimers based on the env genes of four isolates of human immunodeficiency virus type 1 (HIV-1); specifically BG505 (clade A), B41 (clade B), CZA97 (clade C) and DU422 (clade C). The various trimers were delivered either simultaneously (as a mixture of clade A + B trimers) or sequentially over a 73-week period. Autologous, Tier-2 neutralizing antibody (NAb) responses were generated to the clade A and clade B trimers in the bivalent mixture. When delivered as boosting immunogens to rabbits immunized with the clade A and/or clade B trimers, the clade C trimers also generated autologous Tier-2 NAb responses, the CZA97 trimers doing so more strongly and consistently than the DU422 trimers. The clade C trimers also cross-boosted the pre-existing NAb responses to clade A and B trimers. We observed heterologous Tier-2 NAb responses albeit inconsistently, and with limited overall breath. However, cross-neutralization of the clade A BG505.T332N virus was consistently observed in rabbits immunized only with clade B trimers and then boosted with clade C trimers. The autologous NAbs induced by the BG505, B41 and CZA97 trimers predominantly recognized specific holes in the glycan shields of the cognate virus. The shared location of some of these holes may account for the observed cross-boosting effects and the heterologous neutralization of the BG505.T332N virus. These findings will guide the design of further experiments to determine whether and how multiple Env trimers can together induce more broadly neutralizing antibody responses.
Multiple recombinant, soluble envelope glycoprotein (Env) trimers from human immunodeficiency virus type 1 (HIV-1) are currently being produced as immunogens for induction of neutralizing antibody (NAb) responses [1–5]. As NAbs act by recognizing native Env trimers on the HIV-1 surface, trimer-based immunogens intended to induce NAbs should mimic the native structure as closely as possible [6–12]. The SOSIP.664 design of soluble native-like trimers is based on the natural cleavage of the gp140 precursor protein into its gp120 and gp41ECTO subunits and stabilization of the metastable trimer by engineered sequence changes . Multiple native-like SOSIP.664 trimers based on env sequences from clades A, B and C have now been described [7, 9, 10, 14–16]. When tested as individual immunogens in rabbits, two such trimers (BG505, clade A; B41, clade B) induced consistently high titers of NAbs against the autologous viruses, which are classified as Tier-2 on the neutralization sensitivity spectrum [5, 16]. While the generation of autologous Tier-2 NAbs is likely to be a necessary step in a vaccine-development program, it is clearly not sufficient [17–20]. The diversity of circulating HIV-1 strains is so extensive that, for immunogens to be practically useful, they must be able to induce broadly active NAbs (bNAbs) that can counter a wide range of viruses. The key unanswered question is how such bNAbs can be induced.
Native-like soluble trimers, exemplified by SOSIP.664 and next-generation derivatives, are now being used to explore how bNAbs may eventually be induced by immunization [24, 26]. Studies of the immune responses to these trimers in various animals serve to identify which approaches are the more promising. In other words, how can the tool kit provided by the creation of native-like trimers be used most efficiently? This rabbit study was designed to generate information on the immunogenicity of multiple trimers, delivered sequentially or simultaneously. As noted above, the original design of some study groups was compromised by contamination that we detected while the initial set of immunizations was in progress. Other groups were unaffected, and we believe that there remains considerable value in the information derived from this study. One key point relating to the robustness of any conclusions is the limitation imposed by using group sizes of only 5 rabbits. With autologous NAb titers that vary in magnitude from undetectable to >10,000, random skews are inevitable. Moreover, the non-responsiveness of some rabbits, for reasons that are not understood, can also complicate data interpretations.