Analysis of gp41 Epitopes in Model Viral Membranes

Timothy Reichart
The Scripps Research Institute
Basic Biomedical Sciences

Despite 25 years of research and more than 30 clinical trials, no vaccine against HIV has been found to be effective.  Over the last decade, however, several antibodies that provide complete protection against HIV have been identified and isolated from human patients.  An exciting approach to develop a vaccine against HIV is to increase our knowledge of how these antibodies work in an attempt to enable the immune system to make similar antibodies.  Ideally, these antibodies would protect against infection.
Three antibodies of particular interest target the same region of a particular protein on the surface of HIV, the membrane proximal external region (MPER).  This is a region of potential vulnerability of the HIV virus and is the focus of my proposed study.  Previous studies have shown that merely injecting MPER into animal models was not sufficient to launch production of the antibodies that would bind to MPER and therefore prevent infection.  However, in natural HIV this protein is not simply floating in solution.  It is anchored in the viral membrane, making the MPER difficult for the immune system to reach.  This normally inaccessible part of HIV is potentially the most vulnerable to antibody attack.
My approach is to study the MPER using new methods.  Since the MPER is naturally just outside the membrane, I plan to study the MPER just outside a membrane.  Since natural viral particles are difficult to work with in this context, I plan to use an alternative membrane.  I will use Nanodiscs, which are membrane bilayers stabilized by a particular protein that forms discs 10 nanometers in size.  I will chemically synthesize several variants of the MPER in order to determine the natural structure, geometry, and orientation of the MPER in the presence of a membrane.  One variant will introduce chemical modifications to force the MPER to lie flat on the membrane exposing only one half or the other in order to determine what the structure of the MPER is when bound to neutralizing antibodies.  Another variant will physically link three copies of the MPER, in order to determine whether this configuration, which has been proposed in the literature but isn’t known, is the structure of the MPER when antibodies bind.  A third variant will assemble Nanodiscs with a modified membrane composition chosen to more closely resemble that of HIV in order to determine whether the unique composition of the HIV membrane is important in the action of neutralizing antibodies.  I will study the binding strength and kinetics of these several versions in order to determine which of them is most like the natural configuration, and also which is most likely to be effective in a vaccine. 

If we can identify the particular configuration of the MPER to which the HIV-neutralizing antibodies bind, we can then use that configuration in an attempt to generate similar antibodies in animal models that also protect against HIV.  Our hypothesis is that the membrane is an important component of antibody activity, and that this approach will be more likely to elicit neutralizing antibodies than in any previous approach.