Download AIDS: Definition, Epidemiology, and Etiology

Symposium
AIDS: Definition, Epidemiology,
and Etiology
Jeffrey Laurence, MD
A
fatal new disorder characterized
by a rare malignancy, Kaposi's
sarcoma, Pneumocystis carinii pneumonia, and other severe opportunistic
infections was reported by the Centers for Disease Control (CDC), Atlanta, in 1981. The profound disruption
of cellular immune function underlying these illnesses led to their description as the acquired immunodeficiency syndrome (AIDS).
For the purposes of national reporting, a case definition of AIDS was
published in 1982.l These stringent
clinical criteria encompassed secondary conditions that reliably reflect
immune suppression. The etiologic
agent of AIDS, a retrovirus, was identified two years later. This discovery
was made independently by Luc Montagnier of the Pasteur Institute in
Paris and Robert C. Gallo at the National Cancer Institute. The French
group called the agent LAV (lymphadenopathy-associated virus), and the
American workers called it HTLV-III
(human T-cell lymphotropic virus type
III).
IgG antibodies to the human immunodeficiency virus (HIV, formerly
From Cornell University Medical Center, HematologyOncology Division, New York, New York 10021.
known as HTLV-III/LAV) structural
and envelope antigens were soon noted
in a large proportion of persons with
AIDS, as well as in individuals with
risk factors for the disease. Further
evidence for the primary role of this
virus came from its frequent isolation
from AIDS patients and its ability to
induce immune abnormalities typical
of AIDS in chimpanzees. The identification of anti-HIV antibodies and the
culturing of infectious virions refined
the specificity and broadened the scope
of the definition of AIDS.2
AIDS Risk Groups
By June 1986, twenty-one thousand five hundred seventeen cases of
AIDS had been reported to the CDC,
of which approximately 1.5% involved children. Over 11,700 persons
have died, including 71% of those diagnosed before July 1984. The number of cases continues to rise, although
the doubling time has decreased from
a mean of six months through 1983
to approximately 11 months in late
1985.3
AIDS was first noted among previously healthy homosexual men in
New York City, San Francisco, and
Los Angeles. Additional cases were
soon associated with other risk factors. Homosexual and bisexual men
continue to represent the dominant
risk category, with 73% of cases, but
the susceptible population has expanded to include heterosexual intravenous drug abusers (17%), recipients
of blood or blood components within
five years of diagnosis (1.5%), hemophiliacs treated with factor concentrates (0.7%), and the heterosexual
partners of AIDS patients or of persons at increased risk for AIDS (1.0%).
The remaining 6.8% represent people
born outside of the United States, in
countries such as Haiti, where many
cases have not been linked to specific
risk factors. In addition, several thousand cases have been noted in Europe
and in 23 African countries.
HIV-Related Disorders
A significant portion of HlV-associated disease lies outside of the CDC
criteria for AIDS. Persistent generalized lymphadenopathy has occurred
with increasing frequency among
otherwise asymptomatic members of
the risk groups previously outlined. A
spectrum of disorders, including nodal hyperplasia and systemic symptoms such as fever, night sweats, and
fatigue, is thought to represent the
first clinical indications of infection
in 20% to 40% of AIDS patients. The
term AIDS-related complex (ARC) has
LABORATORY MEDICINE • VOL. 17, NO. 11, NOVEMBER 1986 6 5 9
Table I: Diagnosis of the AIDS-Related Complex*
Fever >3 months
Weight loss ( -10% total body weight)
Lymphadenopathy, 3 months
Diarrhea
Fatigue
Night sweats
Decrease in T-helper cells
Decrease in T-helper T-suppressor ratio
Increase In serum globulins
Decrease in lymphocyte blastogenesis
Anergy
*Two clinical findings plus two laboratory abnormalities indicates an AIDS-related complex.
Table II: Estimated No. of Adults Infected With HIV in the United States*
Group
Homosexuals or bisexuals
Intravenous drug abusers
Hemophiliacs
Haitians^
Heterosexual contacts of persons
at high risk
Recipients of blood or blood products
Persons at no known risk
Total
No. of Persons
Living With AIDSf
Estimated No. of
Persons With HIV
4,485
923
31
139
48
1,345,500
270,000
8,970
41,700
14,400
69
214
5,909
20,700
64,200
1,765,470
'Data taken from reference 16.
tCenters for Disease Control, surveillance data, July 22, 1985.
t-Or persons born in a country where AIDS cases have not been associated with known high risk.
been proposed to describe a constellation of signs, symptoms, and laboratory abnormalities that might be
predictive of AIDS (Table I). An alternative classification scheme, employing symptoms, HIV serology, and
in vitro and in vivo immunologic
measurements, has been offered by a
group at Walter Reed Army Medical
Center, Washington, DC."
Critical to an understanding of AIDS
epidemiology is a prospective survey
of individuals with ARC or other
manifestations of early HIV infection.
Estimates of the number of such individuals vary widely, depending upon
assumptions of risk group size and efficiency of viral transmission.
On the basis of one such study, calculations of the number of asymptomatic viral carriers in various risk
groups have been given (Table II).
One problem with these estimates
is the uncertainty of the interval required for seroconversion following
infection. In terms of the two main
routes by which HIV is spread—parenteral exposure and sexual intercourse—anecdotal reports document
antibody development within 40 days
of inoculation. The range is wide, extending to over nine months. Certain
asymptomatic viral carriers appear not
to generate viral-specific antibody.
These seronegative carriers represent
an obvious problem in terms of delineating epidemiology of viral trans-
mission, as well as in the screening of
blood products solely by tests for antibody.
Serologic Assays for HIV
The assays used to determine seroconversion—the enzyme-linked immunosorbent assay (ELISA), Western
blot, ELISA competition with HIV
components, and immunofluorescence—differ in their sensitivity and
specificity. The Western blot is considered by many to be the most specific, with electrophoretic separation
of viral proteins (p) and glycoproteins
(gp) yielding a profile of bands characteristic of HIV when antibody-positive sera are applied.
Antibodies to the viral core protein
(p24) and its group antigen (gag) precursor peptide (p55) occur earliest in
the course of HIV infection. They may
disappear as the clinical syndrome
becomes manifest and advances. The
primary envelope (env) gene product
(gpl60) and its processed fragments
(gpl20) and the transmembrane protein (gp41) react with most sera from
HIV-infected individuals regardless
of clinical stage. Antibodies that recognize these antigens on Western blot
analysis or by immunofluorescence
using HIV-infected T cells appear the
most stable indicators of infection, at
least among cohorts in the United
States. A diagram of the HIV virion
shows the interrelationship of these
6 6 0 LABORATORY MEDICINE • VOL. 17, NO. 11, NOVEMBER 1986
proteins (Fig 1). Also shown are radioimmunoprecipitations of certain
envelope and structural components
of HIV in sera of two representative
AIDS patients.
Other immunogenic regions of HIV
include tatm {trans-acting, type III)
protein pl4; p27 of the 3'-orf (open
reading frame); p23 of sor (short open
reading frame); pl5, p9, p7 of gag; and
p55, p61, and p31, portions of the polymerase (pol) gene product. The
functions of these antigens will be
discussed later.
The previously described protean
manifestations of HIV infection are a
challenge in the clarification of an incubation period for this agent. HIV is
commonly referred to as the "AIDS
virus," but this is clearly a misnomer.
Clinical presentations consonant with
the CDC surveillance definition are
quite different; other disorders linked
to HIV infection are even more varied
(Table III).
An acute mononucleosis-like illness characterized by fever, malaise,
pharyngitis, and erythematous cutaneous lesions may occur within days
to weeks following nosocomial inoculation with blood from an HlV-seropositive person. Lymphadenopathy
and, less frequently, splenomegaly
may develop some weeks to months
thereafter and persist indefinitely. The
interval to appearance of an opportunistic infection, Kaposi's sarcoma,
or B-cell lymphoma ("clinical AIDS")
is currently being defined. In one study
of hemophilia A patients in Pennsylvania, recent seroconvertors had a 12.8
± 4.8% incidence of AIDS within three
years. The syndrome developed a median of 27.5 months after seroconversion.5 The incidence of AIDS among a
cohort of homosexual males in Manhattan (New York City) followed for
the same period was 34.2 ± 8.0%.5 No
plateau was apparent for either group.
Infection with HIV is probably a lifelong event. In animal models of nononcogenic retroviral disorders, persistence of viral genome and
continuous or episodic viral replication occurs uniformly, even in the immunocompetent host.
Immune Defects
Immunologic sequelae of HIV infection are also variable. The most
prominent abnormalities in clinical
AIDS are an absolute lymphopenia, a
Mojor glycoprotein (gpl20)
'
160—
120—
-
64—
m
31—
—
.Tronsmembrone protein (gp41)
ID
Reverse tronscnptose
POL
Table III: HIV-Related Disorders
AIDS: opportunistic infections, Kaposi's
sarcoma, B-cell lymphomas, as defined
by the Centers for Disease Control
Lymphadenopathy syndrome
Acute mononucleosis-like disease: fever,
malaise, pharyngitis, rash
Neuropsychiatric disorders: aseptic meningitis, myelitis, dementia, psychosis
Tetratogenesis, spontaneous abortion
Thrombocytopenia
Malignancy: carcinomas; lymphoproliferative disorders, including lymphoid interstitial pneumonitis
Dermatologic disease: atopic dermatitis,
leukoplakia
gp!20
gp41
3'
LTR
-Mojor core protein ( p 2 4 )
ENV-gpl60
Fig 1. At left, Schematic of radioimmunoprecipitation (RIP-SDSPAGE) analysis of serum samples from two AIDS patients. T cells
were infected with HTV, metabolically labeled with 36S-methionine
and disrupted and electrophoresed on polyacrylamide slab gels
together with immune complexes formed with serum and Staph
protein A. HIV -specific proteins, conforming to structural and envelope regions of HTV, are apparent. At right, Structure of HTV.
The virion has two 35S single-stranded pieces of RNA noncovalently linked at their 5' ends to form the genomic 70S RNA.
profound decrement in peripheral and
tissue helper/inducer T 4 lymphocytes,
i n v e r s i o n of t h e T 4 :T 8 p h e n o t y p i c
helper-suppressor T-cell ratio, diminished in vitro lymphocyte responses
to antigen, and cutaneous anergy. The
antigen-responsive T4 lymphocyte appears to be selectively decreased early
in the course of HIV infection, even
while normal proliferative responses
to nonspecific lectins may occur.6
Other frequent aberrations include
elevated serum immunoglobulin and
acid labile alpha-interferon levels,
depressed production of or response to
lymphokines such as interleukin-2 and
gamma-interferon, and deficient antibody response to in vivo immunization. Natural killer cell and cytotoxic
T-lymphocyte functions may also be
impaired, as well as antigen recognition in the autologous and allogeneic m i x e d l y m p h o c y t e c u l t u r e
reactions. These latter changes appear secondary to defects in both the
EN0OINT
3 -ORF
Fig 2. A schematic representation of the HIV genome. The six
known genes of HTV: gag (group antigen), pol (polymerase, including reverse transcriptase, protease, and endonuclease), sor (short
open reading frame), tat (trans-activating), env (envelope), and
3'-ort(open reading frame) are depicted, together with their translational products. Tat is a bipartite gene, represented by the two
exons connected by dashed lines. Post-translational processing of
the protein (p) and glycoprotein (gp) products required for virion
assembly are given together with their apparent molecular weights.
A seventh gene, art, partly overlaps the tat/7/ and env genes.
T-lymphocyte and antigen-presenting
cell. Alterations in chemotaxis and
d i m i n u t i o n of m e m b r a n e HLA-DR
antigens of antigen presenting cells
have been observed. The pattern of
infectious complications in AIDS, primarily involving intracellular parasites, is likewise compatible with HTVmediated defects among T lymphocytes and cells of the monocyte lineage.
It is clear that HIV can infect several components of the immune system beside the T cell, including B
lymphocytes, monocytes, and possibly
neuroglial cells. Yet central to the
pathogenesis of HIV is an alteration
of the T 4 lymphocyte. How is it that
by damaging a single link, HTV causes
the immune system as a whole to unravel? The answer lies in the complex
series of interactions among the different classes of cells and their secreted p r o d u c t s t h a t t a k e p a r t i n
immunity. The T4 lymphocyte, through
its surface receptors, activation antigens, and lymphokines, plays the prim a r y c o o r d i n a t i n g role i n t h i s
network. 7 The T 4 molecule itself appears to be part of the receptor for HTV.
Cytopathic Effects
HIV causes the premature maturation and death of the T 4 cell. This
effect is mediated by several novel regions of the viral genome. Like all retroviruses, HIV has a group antigen
or gag region describing its core or
structural proteins; pol coding for the
polymerase associated with reverse
transcriptase, endonuclease, and protease activities; and env representing
envelope glycoproteins. The four novel
genes of HIV are tatm, sor, S'-orf, and
art (Fig 2). These sequences are presumably responsible for the unique
ecologic niche that HIV h a s acquired:
the ability to uncouple the requirement for integration with replication,
permitting infection of nondividing
cells.
HIV is highly cytopathic for the T 4
cell. Subsequent to activation of an
infected lymphocyte, rapid viral transcription and cell death occur. This is
partly a consequence of the virus-associated trans-acting
factor that
greatly elevates t h e level of gene
expression directed by t h e viral long
terminal repeat (LTR). The elements
of this autostimulatory pathway include an effector, the tatj,, p l 4 protein
encoded by the bipartite tatm Sene>an^
a responder element, the trans-activating response sequence (TAR), located w i t h i n t h e LTR. 8 Art ( a n t i repression tams-activator) encodes a
second fra/is-acting region t h a t partly
overlaps t h e tatm and envelope genes.
It acts post-transcriptionally to relieve cis-acting negative regulation of
the messenger RNAs for viral capsid
and envelope proteins. 9 In terms of
genome d i s t r i b u t i o n w i t h i n a n infected cell, the unusual ability of HIV
to persist in a cytoplasmic proviral
state is also found in other agents, including spleen necrosis and visna viruses, which have strong cytopathic
potential.
The feedback m e c h a n i s m important to HIV replication is regulated
by an increased t r a n s l a t i o n a l efficiency of virus-specific messenger RNA
LABORATORY MEDICINE • VOL. 17, NO. 11, NOVEMBER 1986 6 6 1
lipid - QCI've cgent,
antibody
anti-sense
oligomers,
interferons
Fig. 3. A schematic diagram of possible mechanisms of action of drugs with anti-HTV
activity. Four major sites for interference include virus attachment, reverse transcription,
viral gene translation, and virion assembly.
species and possibly by other pathways of post-transcriptional or transcriptional control. It is also
conceivable that alteration of the
translational efficiency of cellular
messenger RNAs associated with endogenous TAR-like sequences could
augment production of normal cellular proteins in physiologic modulation of T-cell function. Molecules
capable of inhibiting T-cell-dependent immune responses have been
identified in short-term cultures of
mononuclear cells derived from AIDS
and ARC patients, in virus-free supernatants of HIV-infected cells, and
in the sera of AIDS patients. Potent
suppressor lymphokines secreted by
T4 lymphocytes early in the course of
HIV infection could serve to amplify
the overall depression in immune responsiveness.10 This reaction would be
analogous to suppressor factors identified in several mammalian models
of retroviral infection, including feline and murine leukemia viruses.11
Viral Latency
Another important control region
of HTV is the negative regulatory element (NRE) located within the LTR.
Gene deletion experiments have determined the inhibitory effect of this
sequence on viral transcription.12 This
suggests an active state for the maintenance of viral latency in infected,
nonreplicating cells. In addition, lack
of tatm or art function might lead to
accumulation of viral RNA without
synthesis of viral structural proteins,
thereby establishing a latent state.9
Models for latency have been established in vitro, as T lymphocytes can
be induced to express viral proteins
and release infectious virions following exposure to certain chemicals,
steroids, or antigen. If this model reflects the situation in vivo, it could
account for the ostensible clinical importance of cofactors in the development and progression of AIDS. It is
possible that antigenic stimulation by
immunization or infection with certain lymphotropic viruses, including
cytomegalovirus, Epstein-Barr virus,
and hepatitis B virus could activate a
T cell, rendering it more susceptible
to productive infection by HIV, or induce viral expression among a population of latently infected cells.
Intervention Strategies
Vaccines
Intervention in this tangle of pathology, either to prevent infection,
mitigate its effects, or cure the disease, will be difficult. Preventive
measures, including behavior modification and education, hold some hope
of retarding the spread of HrV infection. Simple reduction in the number
of sexual partners may in itself provide little protection, however. This
relates to the extraordinary seroprevalence of HrV in blood samples of homosexual males from major urban
areas in the United States and Europe. It has reached a level as high as
60% in New York City and 73% in
San Francisco among asymptomatic
homosexual men.13 Potential spread
by heterosexual contact with bisexuals and intravenous drug abusers is
another problem, with documented
transmission of HIV from male to female as well as female to male.13 Thus,
there is a pressing need for a vaccine
against HIV.
6 6 2 LABORATORY MEDICINE • VOL. 17, NO. 11, NOVEMBER 1986
Current strategies for vaccine development focus on identification of
gpl20 epitopes conserved among HIV
isolates. These could be used to engineer a vaccine genetically. The heterogeneity of HIV isolates, based on
restriction fragment analysis and primary nucleotide sequence data, will
hamper this quest. Other difficulties
include the low viral neutralization
titers of most serum samples from
AIDS and ARC patients, and the
scarcity of data correlating potent viral
neutralization in vitro with a favorable clinical course.
The discovery of a series of T-cell
tropic retroviruses in monkeys has
opened up a new strategy for vaccine
development. Present data on the biology of the HIV-related simian Tlymphotropic virus (STLV-IIIAGM) isolated from the African Green monkey
indicate that virus- and antibody-positive monkeys are healthy. This finding is in direct contrast to the biology
of STLV-IIImac in the macaque host,
where this virus is associated with an
immune deficiency syndrome resembling human AIDS.14 There is serologic evidence that a small proportion
of persons in west Africa are healthy
yet infected with an STLV-IIIAGM-related virus. It has been hypothesized
that infection with such an agent may
be protective for subsequent HIV infection and disease, and therefore
might offer clues to the identity of immunogenic and protective viral epitopes.
Drugs
All retroviruses require reverse
transcriptase in order to replicate. This
enzyme is thus an obvious target for
drug therapy. Several compounds have
shown an effect in vitro against HIV,
including ribavirin, azido-deoxythymidine, suramin, phosphonoformate, and ansamycin. Many of these
drugs are currently in limited phase
I or phase II trials. Other points at
which the viral life cycle may be interrupted are depicted in Fig 3. For
example, over 40% of the viral capsid
is lipid. Agents capable of altering the
phospholipid composition of the viral
envelope might block viral adsorption
or penetration.
Inhibition of viral translation is also
attractive. This could be accomplished by interferons, or oligodeoxynucleotide "anti-sense" sequences
complementary to specific regions of
the viral genome. The unique genes
of HIV-related to irans-activation
present special targets. Anti-sense oligomers, modified to permit direct entry into cells while maintaining
resistance to cellular degredative enzymes, or antibiotics which block
translational processes, might affect
HIV growth through this mechanism.
It is hoped that the knowledge
gained through work in the serologic
and culture identification of HIV and
strategies to block its infectivity and
replication will be applicable to the
study of HTLV-I and II as well. A high
incidence of HTLV-I (9%) and HTLVII (18%) was noted among HIV-infected intravenous drug abusers15 and
of HTLV-I (6%) among HIV seropositive homosexual men. These related
retroviruses are probably transmitted
by the same routes as HIV. Their dissemination may be a harbinger of future epidemics of T-cell leukemia,
lymphoma, and other syndromes
linked to HTLV-I and II with much
longer incubation periods than the
HIV-related disorders.
ery of HIV as the cause of AIDS. New
information is beginning to uncover
the basic mechanisms by which this
agent interferes with the cellular immune system. Longitudinal studies
have shown that HIV infection may
have a long incubation period and is
manifest by myriad clinical signs and
symptoms, from an acute mononucleosis-like illness to generalized
lymphadenopathy or development of
an opportunistic infection or neoplasm defining clinical AIDS. Cofactors for the outcome of HIV infection
are being defined. In the absence of a
vaccine or antiviral drug, the worldwide incidence of AIDS will continue
to increase but, because of education,
alterations in life-style among risk
group members, and the screening of
blood and blood products for HIV antibody, at a slower rate. The rapid development of specific immunologic and
virologic reagents as well as preventative and therapeutic strategies offer
the promise of future successful interventions in this tragic disease.
References
Conclusion
Data from several disciplines, including epidemiology, virology, and
immunology, have led to the discov-
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