Imaging Techniques Yield New Information
on How HIV Infects Cells and Provides Clues to Vaccine Design
The use of advanced imaging techniques has allowed researchers
to visualize how a key part of the human immunodeficiency virus
(HIV) changes shape after binding to immune system cells or to
infection-fighting antibodies. Although scientists had been able
to visualize individual components of this part of the virus, called
the HIV spike, the new research characterizes, for the first time,
the structure of the intact spike on virus particles, which is
a crucial piece of knowledge that may aid the design of new vaccines
or drugs to fight HIV infection. The research was conducted by
scientists at the National Cancer Institute (NCI), part of the
National Institutes of Health. The results were published online
July 30, 2008, in Nature.
"Understanding the structure and design of the HIV spike
has been the subject of intensive effort for over a decade because
of its potential importance in understanding mechanisms of viral
entry into immune system cells and designing treatment and prevention
approaches," said NCI Director John E. Niederhuber, M.D. "The
sophisticated imaging techniques used in this HIV/AIDS study have
the potential to advance our understanding of not only HIV, but
many other diseases, including cancer."
"Previous research showing how HIV interacts with immune
system cells and antibodies has been important in vaccine design," said
Sriram Subramaniam, Ph.D., of NCI’s Center for Cancer Research,
and head of the research team that carried out the study. "However,
understanding the complete structure of the viral spike may reveal
other vulnerable targets. This knowledge will be crucial to solving
the puzzles associated with strategies at the heart of virus invasion."
The HIV virus binds to and infects target cells through the interaction
of viral spikes and receptors on the surface of the cells. The
spike is composed of two types of proteins, called gp120 and gp41,
which interact with one another to form a protein pair. The final
structure of the spike is achieved by the interaction of three
of these protein pairs on the surface of the virus, forming a structure
called a trimer. The head of the spike consists of gp120, and it
is responsible for binding to the receptor on the target cell,
which is a protein called CD4; gp41 spans the virus’ outer membrane,
forming the stalk of the spike. gp41 is responsible for events
in which HIV injects its genetic material into the host cells.
The trimeric spike is held together by strong contacts among the
gp41 proteins at the bottom but by only a few contacts among the
gp120 proteins at the top.
The Subramaniam team used imaging techniques, including cryo-electron
tomography, to produce three-dimensional renderings of the spike.
Basically, the researchers froze the virus and took pictures of
it from different angles. Then, they used advanced computer image-processing
methods to average thousands of high-resolution images of individual
spikes, which enabled them to interpret the three-dimensional images
in terms of atomic structures. The researchers found that, upon
interaction with CD4 receptors, the gp120 proteins rearrange, causing
the spike to spring open, exposing the gp41 stalk and other structures
that are required for the virus to infect target cells. This rearrangement
also draws the virus and the target cell closer together, which
may help gp41 penetrate the target cell, allowing the virus’ genetic
material to be injected.
Although most of the antibodies that the body produces to fight
HIV are ineffective, some antibodies are produced that can neutralize
HIV. To gain a better understanding of how these antibodies work,
the researchers visualized the HIV spike bound to an antibody called
b12 that can neutralize a broad range of HIV particles. The team
showed that, in response to b12, the gp120 proteins rearrange in
a similar manner as with CD4, but b12 prevents the spike from opening
completely. The antibody locks the spike in a semi-open position,
preventing the series of actions that enable HIV to infect the
"The insights gained by understanding the binding of one
of the most effective broadly neutralizing antibodies lays the
foundation for designing more effective strategies for blocking
HIV infection," said Subramaniam. "We are working actively
to increase the resolution of our structural analyses, and to understand
the differences in binding between antibodies that neutralize,
which are very rare in HIV-infected individuals, and non-neutralizing
antibodies that are found in almost all AIDS patients. Knowing
what these differences are will be critical to designing better
strategies to neutralize HIV and to providing a new addition to
the arsenal of strategies to combat HIV/AIDS."
The Subramaniam lab has been pioneering advances in electron tomography
and related methods for three-dimensional electron microscopy,
and is applying these emerging technologies to understanding not
only virus-host interactions but also structures inside the cell
that distinguish cancer cells from normal cells.
For more information on Dr. Subramaniam’s research, please go
to http://ccr.cancer.gov/staff/staff.asp?profileid=5614 and http://electron.nci.nih.gov.
For more information about cancer, please visit the NCI website
at http://www.cancer.gov, or
call NCI’s Cancer Information Service at 1-800-4-CANCER (1-800-422-6237).
The National Institutes of Health (NIH) — The Nation's
Medical Research Agency — includes 27 Institutes and Centers
and is a component of the U.S. Department of Health and Human Services.
It is the primary federal agency for conducting and supporting basic,
clinical and translational medical research, and it investigates
the causes, treatments, and cures for both common and rare diseases.
For more information about NIH and its programs, visit www.nih.gov.
Reference: Liu J, Bartesaghi A, Borgnia M, Sapiro G, and Subramaniam
S. Molecular architecture of native HIV-1 gp120 trimers. Nature.
Online July 30, 2008.