At the recent CROI in Seattle, antibody expert Dennis Burton compared HIV’s envelope structure to an assemblage of Tootsie Pops. In this analogy, the parts of HIV’s envelope that are susceptible to neutralizing antibodies are represented by the chocolate contained inside the Tootsie Pop, and the carbohydrate molecules that act as decoys (by inducing ineffective non-neutralizing antibody responses) are represented by the coating of candy that surrounds the chocolate filling. Last year on the blog I wrote about two studies exploring a new strategy for dealing with HIV’s shielded envelope: using certain enzymes to digest—essentially eat away—the carbohydrate covering so that neutralizing antibody targets are revealed. One of the research groups pursuing this idea is that of James Binley at Torrey Pines Institute for Molecular Studies, and in a new paper in the Journal of Virology they report that they have now refined the approach sufficiently to create unshielded envelope proteins (known as trimers) that can be tested in animal models. The hope is that these envelope proteins will be able to reliably induce neutralizing antibodies, unlike the carbohydrate-clad versions that have been tested to date.
First published February 2012, doi: 10.1128/JVI.06938-11
Tommy Tong1, Ema T. Crooks1, Keiko Osawa1 and James M. Binley1
1Torrey Pines Institute for Molecular Studies, 3550 General Atomics Court, San Diego, CA 92121, USA
Hypothetically, since native HIV-1 Env trimers are exclusively recognized by neutralizing antibodies, they might induce them in a vaccine setting. This idea has not been evaluated, due to the difficulty of separating trimers from non-functional Env (uncleaved gp160 and gp41 stumps). The latter are immunodominant and induce non-neutralizing antibodies. We previously showed that non-functional Env can be selectively cleared from virus-like particle (VLP) surfaces by enzyme digests (Crooks ET, Tong T, Osawa K, Binley JM. J.Virol. 85: 5825 (2011)). Here, we investigated the effects of these digests on the antigenicity of VLPs and their sensitivity to neutralization. Before digestion, WT-VLPs (bearing wild type Env) and UNC-VLPs (bearing uncleaved gp160) were recognized by various Env-specific mAbs, irrespective of their neutralizing activity, consistent with the presence of non-functional Env. After digestion, only neutralizing mAbs recognized WT-VLPs, consistent with selective removal of non-functional Env (i.e. “trimer VLPs”). Digests eliminated the binding of all mAbs to UNC-VLPs, again consistent with removal of non-functional Env. An exception was mAb 2F5 that weakly bound to digested UNC-VLPs and bald VLPs (bearing no Env), perhaps due to lipid cross-reactivity. Trimer VLPs were infectious and their neutralization sensitivity was largely comparable to undigested WT-VLPs. However, they were ∼100-fold more sensitive to mAbs 4E10 and Z13e1, suggesting increased exposure of the gp41 base. Importantly, a scatterplot analysis revealed a strong correlation between mAb binding and neutralization of trimer VLPs. This suggests that trimer VLPs bear essentially pure native trimer that should allow its unfettered evaluation in a vaccine setting.