Elevated p21 expression blocks key steps in viral life cycle
14-Mar-2011 - Elevated levels of p21, a protein best known as a cancer fighter, may be involved in the ability
of a few individuals to control HIV infection with their immune system alone. In a paper in the April edition of the Journal of
Clinical Investigation, researchers from the Massachusetts General Hospital (MGH) Infectious Disease Division and the Ragon
Institute of MGH, MIT and Harvard report that CD4 T cells from HIV controllers, while capable of being infected, can
effectively suppress key aspects of the viral life cycle, an ability that may be associated with increased
expression of the p21 protein. Preliminary results of this study were presented at the October 2010
Infectious Diseases Society of America meeting.
"Finding that p21 - best known for its role as a tumor suppressor - is strongly upregulated in CD4 cells from elite
controllers and that experimentally knocking out the protein substantially increases those cells' susceptibility to HIV was quite surprising
and suggests there may be shared mechanisms of immune defense against cancer and HIV," says Mathias Lichterfeld, MD, of the MGH
Infectious Disease Division, the paper's corresponding author.
About one in 300 individuals infected with HIV are naturally able to suppress viral replication without
antiviral drug treatment, an ability that keeps viral levels extremely low and prevents the immune system breakdown that
characterizes AIDS. In those individuals termed elite controllers, viral levels cannot be detected with standard
assays, while in other viremic controllers, levels remain below 2000 copies/ml. In 2006 Ragon Institute
investigators established the International HIV Controllers Study (http://www.hivcontrollers.org) to
investigate immunologic characteristics that may underlie this rare ability. To date, more than
1,500 controllers worldwide have been enrolled in the study.
In November 2010, Ragon investigators reported in the journal Science that mutations in an important immune system
protein explain some but not all of the ability to control HIV infection. The current study was designed to investigate additional mechanisms
influencing natural HIV control and focused on factors that interfere with viral growth within CD4 T cells, the virus's primary target.
The researchers first confirmed that HIV growth is much less efficient in cells from elite controllers than in those from individuals
with progressive HIV infection or uninfected blood donors. CD4 cells from viremic controllers also had reduced viral replication,
although not to the extent seen in elite controllers.
Because previous studies have found that elevated levels of the tumor suppressor protein p21 - which blocks a cellular enzyme required
for viral replication - reduced HIV replication in other immune cells, the researchers examined the protein's expression in CD4 T cells.
The experiments revealed that levels of p21 and its associated mRNA were 10 to 20 times higher in CD4 cells from HIV controllers.
In addition, experimentally knocking out the protein significantly increased viral replication in CD4 cells from HIV controllers.
"We found that p21 expression affects at least two stages in the HIV life cycle inside CD4 T cells. It blocks reverse transcription - one
of the earliest stages at which viral RNA is converted into DNA, a necessary step in making the building blocks for new viruses - and it
inhibits the production of new viral RNA molecules derived from the viral DNA integrated into the cells'
chromosomes," says Lichterfeld. "This ability to independently block two different events
seems to enhance p21's effect in limiting the production of new viruses.
"The challenge now is to understand why p21 is more strongly expressed in HIV controllers," he continues. "Once we know that,
we may be able to induce increased p21 expression in a broader patient population, which may produce greater resistance to HIV. It looks
like immune protection against HIV is mediated by a combination of different mechanisms, and p21-mediated inhibition of HIV replication
may be an important piece of the puzzle." Lichterfeld is an instructor in Medicine at Harvard Medical School.
Additional co-authors of the Journal of Clinical Investigation report are Huabiao Chen, PhD, Chun Li, Jinghe Huang, PhD,
Thai Cung, Katherine Seiss, Jill Beamon, Mary Carrington, Lindsay Porter, Patrick Burke, Yue Yang, MD, Bethany Ryan, Florencia
Pereyra, MD, Abraham Brass, MD, PhD, Bruce Walker, MD, and Xu Yu, MD, Ragon Institute and Massachusetts General Hospital;
Eric Rosenberg, MD, MGH Infectious Disease Division; Ruiwu Liu, PhD, and Robert Weiss, MD, University of California,
Davis; and William Cress, PhD, Moffitt Cancer Center, Tampa, Florida.
Original support for the International HIV Controllers study came through a 2006 grant from the Mark and Lisa Schwartz Foundation,
and the study was expanded in 2008 through the support of the Bill and Melinda Gates Foundation. Additional support for this study
includes grants from the National Institutes of Health, the Doris Duke Charitable Foundation, the Department of Veteran's Affairs
and the Florida Department of Health.
The Ragon Institute of MGH, MIT and Harvard was established in 2009 with a gift from the Philip T. and Susan M. Ragon Foundation,
creating a collaborative scientific mission among these institutions to harness the immune system to combat and cure human
diseases. The primary initial focus of the institute is to contribute to the development of an effective AIDS vaccine.
The Ragon Institute draws scientists and engineers from diverse backgrounds and areas of expertise across the
Harvard and MIT communities and throughout the world, in order to apply the full arsenal of scientific
knowledge to understanding mechanisms of immune control and immune failure and to apply these
advances to directly benefit patients. For further information visit www.ragoninstitute.org .
Massachusetts General Hospital