The generation of broadly neutralizing antibodies is a priority in the design of vaccines against HIV-1. Unfortunately, most antibodies to HIV-1 are narrow in their specificity, and a basic understanding of how to develop antibodies with broad neutralizing activity is needed. Designing methods to target antibodies to conserved HIV-1 epitopes may allow for the generation of broadly neutralizing antibodies and aid the global fight against AIDS by providing new approaches to block HIV-1 infection. Using a naturally occurring HIV-1 Envelope (Env) variant as a template, we sought to identify features of Env that would enhance exposure of conserved HIV-1 epitopes.
Methods and Findings
Within a cohort study of high-risk women in Mombasa, Kenya, we previously identified a subtype A HIV-1 Env variant in one participant that was unusually sensitive to neutralization. Using site-directed mutagenesis, the unusual neutralization sensitivity of this variant was mapped to two amino acid mutations within conserved sites in the transmembrane subunit (gp41) of the HIV-1 Env protein. These two mutations, when introduced into a neutralization-resistant variant from the same participant, resulted in 3- to >360-fold enhanced neutralization by monoclonal antibodies specific for conserved regions of both gp41 and the Env surface subunit, gp120, >780-fold enhanced neutralization by soluble CD4, and >35-fold enhanced neutralization by the antibodies found within a pool of plasmas from unrelated individuals. Enhanced neutralization sensitivity was not explained by differences in Env infectivity, Env concentration, Env shedding, or apparent differences in fusion kinetics. Furthermore, introduction of these mutations into unrelated viral Env sequences, including those from both another subtype A variant and a subtype B variant, resulted in enhanced neutralization susceptibility to gp41- and gp120-specific antibodies, and to plasma antibodies. This enhanced neutralization sensitivity exceeded 1,000-fold in several cases.
Two amino acid mutations within gp41 were identified that expose multiple discontinuous neutralization epitopes on diverse HIV-1 Env proteins. These exposed epitopes were shielded on the unmodified viral Env proteins, and several of the exposed epitopes encompass desired target regions for protective antibodies. Env proteins containing these modifications could act as a scaffold for presentation of such conserved domains, and may aid in developing methods to target antibodies to such regions.
Julie Overbaugh and colleagues analyze an HIV strain with high susceptibility to antibody neutralization and identify two gp41 envelope mutations that confer this sensitivity by exposing multiple neutralization epitopes.
In 1984 when scientists identified human immunodeficiency virus (HIV)—the cause of acquired immunodeficiency syndrome (AIDS)—many experts believed that a vaccine against HIV infection would soon be developed. Nearly 25 years later, there is still no such vaccine and with about 2.5 million new HIV infections in 2007, an effective vaccine is urgently needed to contain the AIDS epidemic. Vaccines provide protection against infectious diseases by priming the immune system to deal quickly and effectively with viruses and other pathogens. Vaccines do this by exposing the immune system to an immunogen—a fragment or harmless version of the pathogen. The immune system mounts a response against the immunogen and also “learns” from this experience so that if it is ever challenged with a virulent version of the same pathogen, it can quickly contain the threat. Many vaccines work by stimulating an antibody response. Antibodies are proteins made by the immune system that bind to molecules called antigens on the surface of pathogens. Antibodies that inactivate the invader upon binding to it are called “neutralizing” antibodies.
Why Was This Study Done?
Several characteristics of HIV have hampered the development of an effective vaccine. An “envelope” protein consisting of two subunits called gp120 and gp41 covers the outside of HIV. Many regions of this protein change rapidly, so the antibody response stimulated by a vaccine containing the envelope protein of one HIV variant provides little protection against other variants. However, other regions of the protein rarely change, so a vaccine that stimulates the production of antibodies to these “conserved” regions is likely to provide protection against many HIV variants. That is, it will stimulate the production of broadly neutralizing antibodies. Unfortunately, it has been difficult to find HIV vaccines that do this, because these conserved regions are often hidden from the immune system by other parts of the envelope protein. In this study, the researchers investigate the envelope protein of an HIV-1 variant they have isolated that is highly susceptible to inactivation by antibodies specific for these conserved regions. Comparing the envelope protein of this sensitive virus to closely related envelope proteins that are resistant to neutralization could identify features that might, if included in an envelope protein immunogen, produce a vaccine capable of generating broadly neutralizing antibodies.
What Did the Researchers Do and Find?
The researchers isolated a subtype A HIV-1 variant from a newly infected woman in Kenya that was efficiently neutralized by monoclonal antibodies (antibodies made by cells that have been cloned in the laboratory). These antibodies were specific for several different conserved regions of gp41 and gp120. The isolate was also neutralized by antibodies in blood from HIV-1-infected people. The envelope protein of the sensitive variant was the same as that of a resistant variant isolated at the same time from the woman, except for four amino acid changes in conserved regions of gp41 (proteins are made from long strings of amino acids). Using a technique called site-directed mutagenesis, the researchers introduced these amino acid changes into envelope proteins made in the laboratory and determined that just two of these changes were responsible for the neutralization sensitivity of the HIV-1 variant. The introduction of these two changes into the neutralization resistant variant and into the unrelated envelope sequences of another subtype A (common in Africa) HIV-1 variant and a subtype B HIV-1 (common in Europe and the Western Hemisphere) variant increased the sensitivity of all these viruses to antibody neutralization.
What Do These Findings Mean?
These findings show that two amino acid changes in gp41 of a neutralization-sensitive HIV-1 variant are responsible for the sensitivity of this variant to several neutralizing antibodies. The finding that the inclusion of these changes in the envelope protein of neutralization-resistant HIV-1 variants greatly increases their sensitivity to neutralizing antibodies indicates that the normally shielded regions of the protein are somehow made accessible to antibody by these changes. One possibility is that the amino acid changes might modify the overall shape of the envelope protein, thus exposing multiple, normally hidden regions in the HIV-1 envelope protein to antibodies. Importantly, these findings open up the possibility that the inclusion of these modifications in envelope-based immunogens might improve the ability of vaccines to generate broadly neutralizing antibodies against HIV-1.
Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.0050009.
Information is available from the US National Institute of Allergy and Infectious Diseases on HIV infection and AIDS
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The US Centers for Disease Control and prevention provides information on HIV/AIDS including information on its HIV vaccine unit (in English and some information in Spanish)
The AIDS Vaccine Clearinghouse provides clear information about HIV vaccine science, research and product development
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