Improving Vaccine Potency through Particle Formation or by Using Live Attenuated Vectors to Generate Vaccines for HIV and Select Agents

Principal Investigator: Ira Berkower, MD, PhD
Office / Division / Lab: OVRR / DVP / LI

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Overview

Public Health Issue: For HIV vaccines CBER is faced with challenging questions, such as: what should an effective HIV vaccine look like? Which viral proteins should it contain and in what form? Which type of host immune response is needed and how can it be enhanced to reach a level needed for protection? How can we make HIV vaccines that are feasible for delivery in the developing world, where they are needed most?

Regulatory Contribution: In response to the AIDS epidemic, we have evaluated a number of HIV vaccine candidates. Each one must be assessed in terms of its composition and the strength and type of immune response elicited. Up to now, the search for a safe and effective vaccine has been a long process based on trial and error. By applying rational vaccine design, this laboratory facilitates more efficient development and evaluation of new HIV vaccines.

Research Approach: We have focused on the two envelope glycoproteins of HIV as targets of immunity: gp120 and the transmembrane protein gp41. The virus appears to use a number of strategies to conceal important gp120 epitopes from neutralizing antibodies. One strategy is to hide the protein between two conformations. One is the open conformation preferred by the CD4 receptor and accessible to most broadly reactive neutralizing antibodies. The other is the closed form as found on the virus, in which antibody binding is greatly reduced. Because most gp120 vaccines have faithfully reproduced viral gp120, they have been in the closed form, and it is not surprising that they were unable to elicit these antibodies. We are working on gp120 modifications that favor the open form, as shown by improved accessibility to antibodies. We have identified loop structures that interfere with antibody binding: removing the loops improved binding by a panel of neutralizing monoclonal antibodies. However, further modifications are needed before we can produce a gp120 that spends most of its time in the open conformation and is capable of eliciting antibodies of this type. Our studies are guided by molecular modeling, which has identified internal residues that hold the protein in the two conformations, as well as surface exposed residues that contribute to the binding affinity for CD4 receptor and neutralizing antibodies. We have also focused on the transmembrane protein gp41. This protein is the target of broadly reactive antibodies that can neutralize nearly all HIV strains in circulation. The major neutralizing determinant on gp41 is called the "membrane proximal region" MPR, because it is located right above the spot where the transmembrane protein crosses the lipid bilayer. We have expressed this MPR determinant linked to a carrier protein that is capable of self assembly into virus-like particles with a lipid surface. Recently, we made the surprising observation that the membrane spanning domain of gp41 could be swapped for one of the membrane spanning domains of the HBsAg carrier, and the resulting protein would still assemble into particles. This allows us to anchor the MPR determinant to lipid particles using its own membrane spanning domain, and to display the MPR in the most immunogenic site on our particles. Finally, we have made double MPR particles, which contain MPR substitutions at both sites on HBsAg. These particles shed light on the essential features of particle formation, and they may indicate the conditions needed to present MPR in the most immunogenic form. These studies will help CBER evaluate a number of vaccine candidates that are based on gp120 or the MPR determinant of gp41. These vaccine antigens can be compared for openness of structure, accessibility to antibody binding, and for the quantity and quality of antibodies they elicit. We are also developing rubella as a live recombinant vector for HIV antigens. So far, we have succeeded in expressing a reporter gene in rubella and have demonstrated stable expression for at least 10 generations. If successful in expressing HIV antigens, this could provide a safe and inexpensive way to immunize children of the developing world.

Mission Relevance & Outcomes: These studies will provide new scientific information and tools for developing and evaluating vaccines for HIV/AIDS, e.g., by demonstrating the need for envelope proteins in their native conformation and whether incorporation into virus-like particles on a lipid layer can enhance vaccine potency. Further work will identify the correlates of immunity that can be applied to the evaluation of a broad range of HIV vaccines.

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Publications

Virology 2008 Aug 1;377(2):330-8
Targeted deletion in the beta20-beta21 loop of HIV envelope glycoprotein gp120 exposes the CD4 binding site for antibody binding.
Berkower I, Patel C, Ni Y, Virnik K, Xiang Z, Spadaccini A

Virology 2008 Mar 30;373(1):72-84
Analysis of the human immunodeficiency virus type 1 gp41 membrane proximal external region arrayed on hepatitis B surface antigen particles.
Phogat S, Svehla K, Tang M, Spadaccini A, Muller J, Mascola J, Berkower I, Wyatt R

J Infect Dis 2007 Oct 1;196(7):1026-32
Antibodies to the A27 Protein of Vaccinia Virus Neutralize and Protect against Infection but Represent a Minor Component of Dryvax Vaccine-Induced Immunity.
He Y, Manischewitz J, Meseda CA, Merchlinsky M, Vassell RA, Sirota L, Berkower I, Golding H, Weiss CD

Virology 2004 Mar 30;321(1):75-86
Assembly, structure, and antigenic properties of virus-like particles rich in HIV-1 envelope gp120.
Berkower I, Raymond M, Muller J, Spadaccini A, Aberdeen A