The Immunology of Aids Introduction Although HIV was first identified in 1983,
studies of previously stored blood samples indicate that the virus entered the
U.S. population sometime in the late 1970s. Worldwide, an estimated 27.9 million
people had become HIV-infected through mid-1996, and 7.7 million had developed
AIDS, according to the World Health Organization (WHO). AIDS is a disease of the
immune system, and is caused by Human Immuno deficiency Virus (HIV). HIV targets
and infects T-helper cells and macrophages. After infection, replication of the
virus occurs within the T-helper cells. The cells are lysed and the new viruses
are released to infect more T-helper cells. The course of the disease results in
the production of massive numbers of virus (1 billion/day) over the full course
of the disease. The T- helper cells are infected, and rapidly destroyed both by
virus and by cytotoxic T cells. T-helper cells are replaced with nearly a
billion produced per day. Over many years (average may be 10), the T-helper cell
population is depleted and the body loses its ability to mount an immune
response against infections. Thus, we mount a very strong immune response
against the virus for a long time, but the virus is produced at a very high rate
and ultimately overcomes the ability of the immune system to respond. Since HIV
belongs to a class of viruses called retroviruses, it has genes composed of
ribonucleic acid (RNA) molecules. Like all viruses, HIV can replicate only
inside host cells, commandeering the cell's machinery to reproduce. However,
only HIV and other retroviruses, once inside a cell, use an enzyme called
reverse transcriptase to convert their RNA into DNA, which can be incorporated
into the host cell's genes. HIV belongs to a subgroup of retroviruses known as
lenti-viruses, or "slow" viruses. The course of infection with these
viruses is characterized by a long interval, up to 12 years or more, between
initial infection and the onset of serious symptoms. Like HIV in humans, there
are animal viruses that primarily infect the immune system cells, often causing
immuno-deficiency and AIDS-like symptoms. Scientists use these and other viruses
and their animal hosts as models of HIV disease. The CDC currently defines AIDS
when one of 25 conditions indicative of severe immuno-suppression associated
with HIV infection, such as Pneumocystis carinii pneumonia (PCP) is present, or
HIV infection in an individual with a CD4+ T cell count less than 200 cells per
cubic millimeter (mm3) of blood. However, the question that now remains to be
answered is 'How does HIV effectively overcome the human immune system?' In this
paper I will try to answer this question. In the first chapter I will explain
how HIV is transmitted and what its life cycle looks like. This in order to
increase the understanding of how the virus operates. It can be seen as an
introductory chapter to the main body of the paper, chapter 2. In the second
chapter the specific interactions between the virus and the human immune system
will be discussed and shown why its is so threatening. In the last chapter I
will deal with certain promising treatments against AIDS. Chapter 1 The
Transmission of HIV Among adults, HIV is spread most commonly during sexual
intercourse with an infected partner. During sex, the virus can enter the body
through the mucosal linings of the vagina, vulva, penis, rectum or, very rarely,
via the mouth. The likelihood of transmission is increased by factors that may
damage these linings, especially other sexually transmitted diseases that cause
ulcers or inflammation. Research suggests that immune system cells called
dendritic cells, which reside in the mucosa, may begin the infection process
after sexual exposure by binding to and carrying the virus from the site of
infection to the lymph nodes where other cells of the immune system become
infected. HIV also can be transmitted by contact with infected blood, most often
by the sharing of drug needles or syringes contaminated with minute quantities
of blood containing the virus. The risk of acquiring HIV from blood transfusions
is now extremely small in Western countries, as all blood products in these
countries are screened routinely for evidence of the virus. Almost all
HIV-infected children acquire the virus from their mothers before or during
birth. The anatomy of HIV HIV has a diameter of 1/10,000 of a millimeter and is
spherical in shape. The outer coat of the virus, known as the viral envelope, is
composed of lipid bi-layer, taken from the membrane of a human cell when a newly
formed virus particle buds from the cell. Embedded in the viral envelope are
proteins from the host cell, as well as 72 copies (on average) of a complex HIV
protein that protrudes from the envelope surface. This protein, known as Env,
consists of a cap made of three or four molecules called glycoprotein (gp) 120,
and a stem consisting of three or four gp41 molecules that anchor the structure
in the viral envelope. Within the envelope of a mature HIV particle is a
bullet-shaped core or capsid, made of 2000 copies of another viral protein, p24.
The capsid surrounds two single strands of HIV RNA, each of which has a copy of
the virus's nine genes. Three of these, gag, pol and env, contain information
needed to make structural proteins for new virus particles. The env gene, for
example, codes for a protein called gp160 that is broken down by a viral enzyme
to form gp120 and gp41, the components of Env. Three regulatory genes, tat, rev
and nef, and three auxiliary genes, vif, vpr and vpu, that contain the
information necessary for the production of proteins that control the ability of
HIV to infect a cell, produce new copies of virus or cause disease. The protein
encoded by nef, for instance, appears necessary for the virus to replicate
efficiently, and the vpu-encoded protein influences the release of new virus
particles from infected cells. The Life Cycle of HIV When HIV encounters its
target cell, the external glycoprotein portion of the viral envelope (GP120)
binds with high affinity to the extra cellular component of the receptor protein
CD 4, present on helper lymphocytes(Helper T cells). The membrane portion of the
viral envelope fuses to the lymphocyte membrane and the virus is expelled into
the cell. Then the reverse transcriptase of the virus copies the RNA into DNA.
Once the DNA is integrated into the host cell genome, the presence of HIV has
become a permanent part of the lymphocyte (Helper T). The viral production
proceeds through a complex set of highly regulated steps. First, messenger RNA
of the virus and viral proteins are produced. Proteins are then modified by a
viral protease to become mature viral proteins. Current efforts at anti-viral
therapy involve the use of reverse transcriptase inhibitors (notably AZT) and
newly developed inhibitors of the viral protease. AZT Chapter 2 The Immune
System and HIV The body's health is defended by the immune system. Lymphocytes
(B cells and T cells) protect the body from "germs" such as viruses,
bacteria, parasites, and fungi. When germs are detected, B cells and T cells are
activated to defend the body. This process is hindered in the case of the
acquired immuno-deficiency syndrome (AIDS). AIDS is a disease in which the
body's immune system breaks down. AIDS is caused by the human immuno-deficiency
virus (HIV). When HIV enters the body, it infects the CD4+ T cells, where the
virus grows. The virus kills these cells slowly. As more and more of the T cells
die, the body's ability to fight infection weakens. A person with HIV infection
may remain healthy for many years. People with HIV infection are said to have
AIDS when they are sick with serious illnesses and infections that can occur
with HIV. The illnesses tend to occur late in HIV infection, when only 200 T
cells per cubic millimeter remain. One reason HIV is unique is that despite the
body's aggressive immune responses, which are sufficient to clear most viral
infections, some HIV invariably escapes. One explanation is that the immune
system's best soldiers in the fight against HIV-certain subsets of killer T
cells- multiply rapidly following initial HIV infection and kill many
HIV-infected cells, but then appear to exhaust themselves and disappear,
allowing HIV to escape and continue replication. Additionally, in the few weeks
that they are detectable, these specific cells appear to accumulate in the
bloodstream rather than in the lymph nodes, where most HIV is sequestered. Viral
Variation Another reason for the uniqueness of HIV are the dynamics of HIV
replication. They also have profound implications for the generation of genetic
diversity of HIV quasispecies in individual patients. Virus isolates obtained
from patients at the time of initial infection show little genetic
heterogeneity. Over time, however, the population of viruses circulating in an
individual patient becomes increasingly diverse. The rapid replication kinetics
and high mutation rate of HIV reverse transcriptase drive the diversification of
the HIV quasispecies in response to selective pressure from the host immune
response. The rapid turnover of HIV also provides the ideal mechanism for
producing variants with mutations that confer drug resistance, or permit escape
from immunological control of HIV infection. When drugs that inhibit HIV-1
replication are partially or inappropriately administered, the resulting
evolutionary pressure selects for the emergence of resistant strains. In the
case of lamivudine (3TC) or nevirapine, a single nucleotide change in the HIV-1
RT gene is sufficient to produce high-level resistance. The entire virus
population evolves from wild-type to resistant in a matter of weeks when these
drugs are given as single agents. Little or no viral variation emerges in
patients with complete suppression of plasma HIV-1 RNA in response to potent
combination therapy. The Role of Immune Activation in HIV Disease During HIV
infection, however, the immune system may be chronically activated, with
negative consequences. For HIV replication and spread are much more efficient in
activated CD4+ cells. Chronic immune system activation during HIV disease may
also result in a massive stimulation of a person's B cells, impairing the
ability of these cells to make antibodies against other pathogens. Chronic
immune activation also can result in apoptosis, and an increased production of
cytokines that may not only increase HIV replication but also have other
deleterious effects. Increased levels of TNF-alpha , for example, may be at
least partly responsible for the severe weight loss or wasting syndrome seen in
many HIV-infected individuals. The persistence of HIV and HIV replication
probably plays an important role in the chronic state of immune activation seen
in HIV-infected people. In addition, researchers have shown that infections with
other organisms activate immune system cells and increase production of the
virus in HIV-infected people. Chronic immune activation due to persistent
infections, or the cumulative effects of multiple episodes of immune activation
and bursts of virus production, likely contribute to the progression of HIV
disease. The Role of CD8+ T Cells CD8+ T cells are important in the immune
response to HIV during the acute infection and the clinically latent stage of
disease. These cells attack and kill infected cells that are producing virus.
CD8+ T cells also appear to secrete soluble factors that suppress HIV
replication. Three of these molecules-RANTES, MIP-1alpha and
MIP-1beta-apparently block HIV replication by occupying receptors necessary for
the entry of certain strains of HIV into their target cells. Researchers have
hypothesized that an abundance of RANTES, MIP-1alpha or MIP-1beta, or a relative
lack of receptors, notably CCR-5, for these molecules, block the entry of HIV.
This may help explain why some individuals have not become infected with HIV,
despite repeated exposure to the virus. A possible explanation for that is that
some people have a mutation in the allele coding for that receptor. Figure 2.
New Co-receptors for HIV-1. T-cell-tropic strains of HIV-1, which are usually
syncytium-inducing, require CXCR-4 as co-receptor. This receptor is found on T
lymphocytes, but not monocytes. Mono-cytotropic strains, which are usually non-syncytium-inducing,
require the CCR-5 receptor, which is found on both monocytes and T lymphocytes.
This illustrates why these isolates can infect monocytes and primary
lymphocytes, both of which express CCR-5, but not T-cell lines, which lack this
co-receptor. By contrast, T-cell-tropic strains cannot infect monocytes because
they lack the CXCR-4 co-receptor. CD8+ T cells are thought to also secrete other
soluble factors-as yet unidentified-that suppress HIV replication. The Loss of
Cells of the Immune System Researchers around the world are studying how HIV
destroys or disables CD4+ T cells, and it is thought that a number of mechanisms
may occur simultaneously in an HIV-infected individual. Recent data suggest that
billions of CD4+ T cells may be destroyed every day, eventually overwhelming the
immune system's regenerative capacity. Infected CD4+ T cells may be killed
directly when large amounts of virus are produced and bud off from the cell
surface, disrupting the cell membrane, or when viral proteins and nucleic acids
collect inside the cell, interfering with cellular machinery. Infected CD4+ T
cells may be killed when cellular regulation is distorted by HIV proteins,
probably leading to their suicide by a process known as programmed cell death or
apoptosis. Recent reports indicate that apoptosis occurs to a greater extent in
HIV-infected individuals, both in the bloodstream and lymph nodes. Normally,
when CD4+ T cells mature in the thymus gland, a small proportion of these cells
is unable to distinguish self from non-self. Because these cells would otherwise
attack the body's own tissues, they receive a biochemical signal from other
cells that results in apoptosis. Investigators have shown in cell cultures that
gp120 alone or bound to gp120 antibodies sends a similar but inappropriate
signal to CD4+ T cells causing them to die even if not infected by HIV.
Uninfected cells may die in an innocent bystander scenario: HIV particles may
bind to the cell surface, giving them the appearance of an infected cell and
marking them for destruction by killer T cells. Killer T cells also may
mistakenly destroy uninfected CD4+ T cells that have consumed HIV particles and
that display HIV fragments on their surfaces. Alternatively, because HIV
envelope proteins bear some resemblance to certain molecules that may appear on
CD4+ T cells, the body's immune responses may mistakenly damage such cells as
well. Studies suggest that HIV also destroys precursor cells that mature to have
special immune functions, as well as the parts of the bone marrow and the thymus
needed for the development of such cells. These organs probably lose the ability
to regenerate, further compounding the suppression of the immune system. HIV is
Active in the Lymph Nodes Although HIV-infected individuals often exhibit an
extended period of clinical latency with little evidence of disease, the virus
is never truly latent. NIAID researchers have shown that even early in disease,
HIV actively replicates within the lymph nodes and related organs, where large
amounts of virus become trapped in networks of specialized cells with long,
tentacle-like extensions. These cells are called follicular dendritic cells (FDCs).
FDCs are located in hot spots of immune activity called germinal centers. They
act like flypaper, trapping invading pathogens (including HIV) and holding them
until B cells come along to initiate an immune response. Close on the heels of B
cells are CD4+ T cells, which rush into the germinal centers to help B cells
fight the invaders. CD4+ T cells, the primary targets of HIV, probably become
infected in large numbers as they encounter HIV trapped on FDCs. Research
suggests that HIV trapped on FDCs remains infectious, even when coated with
antibodies. Once infected, CD4+ T cells may leave the germinal center and infect
other CD4+ cells that congregate in the region of the lymph node surrounding the
germinal center. However, over a period of years, even when little virus is
readily detectable in the blood, significant amounts of virus accumulate in the
germinal centers, both within infected cells and bound to FDCs. In and around
the germinal centers, numerous CD4+ T cells are probably activated by the
increased production of cytokines such as TNF-alpha and IL-6, possibly secreted
by B cells. Activation allows uninfected cells to be more easily infected and
increases replication of HIV in already infected cells. While greater quantities
of certain cytokines such as TNF-alpha and IL-6 are secreted during HIV
infection, others with key roles in the regulation of normal immune function may
be secreted in decreased amounts. For example, CD4+ T cells may lose their
capacity to produce interleukin 2 (IL-2), a cytokine that enhances the growth of
other T cells and helps to stimulate other cells' response to invaders. Infected
cells also have low levels of receptors for IL-2, which may reduce their ability
to respond to signals from other cells. Ultimately, accumulated HIV overwhelms
the FDC networks. As these networks break down, their trapping capacity is
impaired, and large quantities of virus enter the bloodstream. The destruction
of the lymph node structure seen late in HIV disease may prevent a successful
immune response against not only HIV but other pathogens as well. This
devastation heralds the onset of the opportunistic infections and cancers that
characterize AIDS. HIV's Strategy Researchers have discovered a devious strategy
used by the human immuno-deficiency virus (HIV) to undermine the immune system.
They found that even when HIV does not enter a cell, proteins in the outer
envelope of the virus can bind to CCR5 receptor on the cell's surface and
initiate a biochemical cascade that sends a signal to the cell's interior. This
signaling process may activate the cell, making it more vulnerable to HIV
infection. It also may cause cells to migrate to sites of HIV replication,
thereby increasing their vulnerability to infection. If the cell is already
infected with HIV, activation may boost the production of the virus. HIV
generally requires two receptors (as discussed in 'The Role of CD8+ T Cells') to
enter a target cell: CD4, and either CCR5 or CXCR4, depending on the strain of
virus. The strains of HIV most commonly seen early in HIV disease, known as
macrophage-tropic (M-tropic) viruses, use CD4 and CCR5 for cell entry. Many
strains of the simian immuno-deficiency virus (SIV), a cousin of HIV that
infects non-human primates such as monkeys, also use these receptors for
cellular entry. Researchers found that envelope proteins from four different
M-tropic HIV strains and one M-tropic SIV strain induced a signal through CCR5
that caused cells to migrate in culture. In contrast, envelope proteins from
other strains of the viruses, known as T-cell tropic (T-tropic) strains, did not
cause signaling. Chapter 3 Immunological Treatments for HIV/AIDS HRG 214: A
joint effort between scientists and industry has resulted in the development of
a new drug to treat patients in the advanced stages of AIDS. Dr. Frank Gelder,
director of Immuno-diagnostic Testing Laboratories, Department of Surgery at
Louisiana State University Medical Center in Shreveport, Louisiana, invented the
drug, HRG214. HRG214 is formulated as an immuno-chemically-engineered group of
antibodies that neutralize and inactivate essential steps in the life cycle of
HIV. HRG214 is the first immunology based pharmaceutical to show successful
treatment of HIV infection. When HRG214 is used in conjunction with two
additional drugs, one to initiate and one to control cytokine pathways, (the
chemical signals by which cells communicate). CD8 lymphocytes and other cells,
which fight infection, (present but not functioning normally in AIDS patients),
are rapidly restored to normal function. This drug regime opens new therapeutic
options for the care of HIV patients, including those in advanced stages of
AIDS. In addition, CD4 and CD8 lymphocyte numbers have statistically increased,
and marked clinical improvements have been observed in all patients receiving
treatment with HRG214. These improvements include increase in appetite and
stamina, as well as marked improvements in AIDS-related conditions such as
chronic fatigue syndrome, diarrhea, malabsorption, and other HIV-related
diseases. Cytolin Unlike current AIDS drugs, which attack HIV directly, Cytolin
would help the body's immune system by correcting the immune system's
self-destruct mechanism that is triggered by an HIV infection. Cytolin is a
monoclonal antibody designed to prevent one part of the immune system-a
particular type of "killer" CD8 cells-from attacking another part-CD4
cells, the destruction of which results in AIDS. Cytolin is designed to protect
the immune system's natural defenses while antiviral drugs take the offensive
against HIV. Cytolin is to be given in a doctor's office, most often as an
adjunct to a combination of antiviral drugs. Combinations, or
"cocktails," of antiviral drugs have helped some patients
significantly reduce the level of their HIV infection, improving their health.
However, the side effects of antiviral drugs can be so significant that at least
15 percent of patients cannot take them. Even some patients who can tolerate
antiviral therapy have continued to face declining health. Following injection
with Cytolin, the patients demonstrated significantly reduced levels of HIV
infection and clinical signs of immune system recovery, including increased
levels of disease fighting CD4 cells. Conclusion First of all, HIV attacks the
very cells that are responsible for the defense of the human body against
invaders, the CD4+ T cells. However, HIV also targets other immune system cells
with CD4 on their surface. Not only are HIV replication and the spread of the
virus more efficient in activated cells, but chronic immune activation during
HIV disease may result in a massive stimulation of a person's B cells, impairing
the ability of these cells to make antibodies against other pathogens. Chronic
immune activation also can result in a form of cellular suicide known as
apoptosis, and in the increased production of signaling molecules called
cytokines that can themselves increase HIV replication. This strategy shows that
HIV does not to invade the CD4+ cells to inflict damage to the immune system.
The chronic immune activation not only impairs the ability of B cells to make
pathogens against other cells, but it also results in apoptosis, and an
increased production of cytokines that may not only increase the HIV replication
but also have other deleterious effects, such as the severe weight loss caused
by increased levels of TNF-alpha. Now, finally researchers have found a two
potentially successful immunological treatments, HRG 214 and Cytolin. HRG 214
neutralizes and inactivates essential steps in the replication cycle of HIV.
Cytolin helps the immune system by correcting its self-destruct mechanism that
is triggered by an HIV infection.

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