About a Virus
By Jason Socrates
Bardi
"The
first [precept] was never to accept anything for true which
I did not clearly know to be such; that is to say, carefully
to avoid precipitancy and prejudice, and to comprise nothing
more in my judgement than what was presented to my mind
so clearly and distinctly as to exclude all ground of doubt."
———Rene
Descartes, Discourse on the Method of Rightly Conducting
the Reason, and Seeking Truth in the Sciences, 1638
It is the early 1970s and a nurse named Johnny accidentally
sticks himself with a needle while doing a routine blood draw,
exposing himself to the hepatitis B virus (HBV). There are
at this time no screens for the virus, and aside from admonishing
himself for his clumsiness and washing the tiny wound with
antibiotic soap, Johnny thinks nothing of it.
A few weeks later, while the virus is replicating prolifically
in his liver, he transmits hepatitis B to his wife during
intercourse. Unbeknownst to either of them, she is already
pregnant. And when their baby is born, he is also infected.
The odds, in this fictionalized case, are that Johnny and
his wife both fully recover from their infection. Their immune
systems mount a strong defense against the virus and it is
eliminated from their systems. The odds do not favor their
son, however. His immature immune system will most likely
not be able to fight off the hepatitis B virus, and it will
establish a chronic, lifelong infection in his liver. Like
all other chronically infected people, he will have potentially
lethal health problems and an enormously increased risk of
developing liver cancer.
In the United States, there are 1,250,000 people living
with chronic hepatitis B, according to the Centers for Disease
Control and Prevention (CDC). The average American’s
lifetime chances of being infected by HBV are about five percent,
and about five percent of those infections will become chronic.
Globally, the situation is more dire. Nearly half the world’s
population lives in areas where more than eight percent of
people are chronically infected by hepatitis B. Anyone living
in those areas has a greater than 60 percent chance of being
infected by hepatitis B virus at some point in their lifetime.
Hepatitis B is the leading cause of liver cancer in the world.
“There are 350 million people in the world who are
chronically infected with hepatitis B,” says physician
Frank Chisari, who is professor in The Scripps Research Institute
(TSRI) Department of Molecular and Experimental Medicine.
“My lifelong dream is to contribute to the termination
of hepatitis B infection in all those chronically infected
people—that has been driving my research throughout my
career.”
Cells Can Cure Themselves
Hearing of goals, rather than of accomplishments, is strange
from someone who has recently received more than one lifetime
achievement award.
A few weeks ago, Chisari and TSRI Chemistry Professor Chi-Huey
Wong were elected to the National Academy of Sciences, becoming
two of the now 14 investigators at TSRI who have been admitted
to this august body. And just days ago, he was elected to
the American Academy of Microbiology, the highest honor the
American Society of Microbiology bestows upon its members.
Both honors recognize the work Chisari has done on hepatitis
since coming to TSRI in 1973.
In the last three decades, by studying infections in patients
and in a closely related species, and by developing transgenic
models to study the HBV immunobiology and pathogenesis, Chisari
and his collaborators have characterized the course of HBV
infection in the liver, the immune system’s response
to the virus, and the mechanisms whereby a chronic HBV infection
can lead to liver cancer. In recent years, he and his collaborators
completed a comprehensive analysis of the virological and
immunological features of HBV infection using liver biopsies
and blood samples they obtained from infected subjects every
week for six months after inoculation, describing the course
of infection with a level of detail that had never before
been attempted.
The insights they gained from these studies and their earlier
human and transgenic model experiments have revolutionized
the way we think the immune system can control a viral infection.
Furthermore, they also demonstrated that there’s a dark
side to the antiviral immune response, which can produce progressive
tissue damage and even trigger the development of cancer when
it goes awry.
Hepatitis is caused by one of several evolutionarily distinct
viruses (called A, B, C, D and E) that all target hepatocytes,
the parenchymal cells of the liver. Hepatocytes are the tiny
chemical factories in the liver that produce most of the proteins
present in blood, nurturing all the other organs of the body.
They also produce bile, a fluid used for digestion of fat
in the diet and for the elimination of waste.
Hepatitis B virus is a circular, double-stranded DNA virus
just over 3,000 base pairs long belonging to the Hepadnaviridae
family. The infectious particle, or virion, contains this
tiny genome and a viral polymerase enzyme in a protein capsid
shell surrounded by a lipid coat. HBV infection starts when
these virions are introduced into the bloodstream through
routes that are similar to those used by the human immunodeficiency
virus (HIV)—unprotected sex, contaminated needles, and
mother-infant transmission.
Once inside the bloodstream, the virions eventually pass
through the liver, and the process of disease starts when
HBV infects one or more liver hepatocytes. The initial number
of cells infected may be small—just a few—but within
six to eight weeks, the virus rapidly replicates and can infect
every hepatocyte cell in the liver.
Upon reaching this widespread infection, there is a rapid
spike of viral DNA in the bloodstream and the initiation of
an immune response, which is evident by the appearance of
T cells in the liver and a reduction by several orders of
magnitude in the amount of virus in the blood.
Most adults who are infected with hepatitis B suffer an
acute infection. After the HBV activity peaks, the body mounts
an immune response and the virus disappears. The immune response
is so effective that the liver goes from having all its hepatocytes
infected to having none of them infected.
Over a decade ago, Chisari suspected that the immune system
must be using an unexpected way of clearing the virus from
infected liver cells during an HBV infection, because in many
cases it was clearing the virus without killing off all the
infected cells.
For years, scientists had recognized that one of the principal
ways that the immune system deals with a viral infection like
HBV is to unleash cytotoxic T lymphocytes (CTL), also called
killer T cells, which carry a receptor on their surface that
specifically recognizes tell-tale viral markers on the surface
of infected cells that indicate these “target” cells
should be eliminated. CTLs then kill these infected cells
by inducing them to undergo apoptosis, the cellular equivalent
of suicide. Until recently, however, this destructive process
was thought to be the only antiviral mechanism that CTLs had
at their disposal.
But Chisari realized that this couldn’t be the primary
mechanism for clearing HBV because killing requires direct
contact between a CTL and an infected hepatocyte, and there
are simply not enough killer T cells to kill off every hepatocyte
in the liver. And even if there were, killing your liver is
the last thing your body would want to do, because it is impossible
to live without this vital organ.
So, in the early 1990s he started looking into the possibility
that CTLs might be able to coax infected cells into curing
themselves without being destroyed. The mechanism of this
clearance occupied nearly a decade of Chisari’s time,
and a few years ago he and his colleague Luca Guidotti, an
Associate Professor at TSRI, demonstrated that the immune
system can indeed help cure infected cells and how this “intracellular
effector” function may actually be the primary way that
the immune system controls HBV infection—something that
took most people by surprise.
This unprecedented concept established a new paradigm in
our understanding of the host-virus relationship, and like
many revolutionary ideas, was initially met with surprise
and skepticism. In the past several years, however, it was
independently confirmed for HBV and it has been extended to
a number of other infections as well.
The Immune System Helps Cells that Help Themselves
Basically, in addition to the direct killing of infected
hepatocytes, the activated killer T cell will start to produce
and secrete chemicals, called cytokines, that bind to surrounding
cells that are also infected and that carry specific markers
to which the cytokines bind.
Once these cytokines bind to an infected cell, that binding
event activates genes within the infected cell that produce
proteins that intercept the lifecycle of the pathogen, leading
to an internal elimination of the virus without destroying
the cell. In hepatitis, the primary cytokine that drives this
process—which also occurs in other cells that are infected
with other pathogens—is called interferon-gamma (IFN-g).
In hepatitis, Chisari and Staff Scientist Stefan Wieland
in his group demonstrated that the first defense mechanism
of INF-g involves interrupting the assembly of the viral RNA
and associated proteins into infectious capsids.
“Assembly of [capsid] is very rapidly abolished by
signals that are delivered by IFN-g,” says Chisari. In
recent years, he has worked to categorize the molecules that
are produced by HBV-infected liver cells after they are activated
by IFN-g.
One candidate class he and his postdoctoral fellow Michael
Robek have found to be upregulated in response to the cytokines
are proteins of the proteasome, the cell organelle responsible
for degrading protein in the cell’s cytoplasm. Chisari
has demonstrated that treating HBV-infected cells with inhibitors
of these proteasome proteins blocks the antiviral activity
of IFN-g.
Clearance is not limited to this one mechanism. A second,
slower mechanism that HBV-infected cells engage after they
are turned on by IFN-g is to remove all the viral RNA from
the cell by destroying a cellular protein that protects the
viral RNA. Without the protection of this cellular protein,
the viral RNA is susceptible to ribonuclease enzymes in the
cytosol, which destroy it.
Nor is the production of cytokines during such an immune
response limited to one type of immune cell. Multiple cells
of the immune system, including cytotoxic T lymphocytes, helper
T cells, natural killer cells, macrophages, and dendritic
cells all release such cytokines. And when these cells are
activated to produce IFN-g in the liver, the infection will
be cleared.
“We think this is what happens in most of the acute
infections in adult patients,” says Chisari. “[Cytokine-induced
viral clearance] is the dominant effector force for the control
of HBV infection.”
Not every intracellular event involved in this clearance
is known, and a large portion of Chisari’s laboratory
is busy mapping all the details. Nevertheless, the usefulness
of purging the infection while preserving the integrity of
the cells is obvious when one compares acutely infected patients
to the more serious, chronic cases: acute infections are rapidly
controlled by the immune system and chronic infections are
not.
1 | 2 |
|
