The development of biology in this century has proceeded from the organismic level to
the molecular level. Retrovirology has followed this broad historical trend. In the
first six decades of the century, retrovirology dealt almost exclusively with
infection and disease in the animal host. This was followed in the 1960s and 1970s
by a dominant concern with the viral replication cycle and pathogenic effects at the
cellular level. Since the 1970s, studies at the molecular level have led the field.
The goal of current research is to explain the remarkably diverse pathogenic effects
of retroviruses in cellular and molecular terms.
Retroviruses as Natural Pathogens of Vertebrates
Retroviruses were discovered at the turn of the century in two investigations
devoted to neoplastic diseases in chickens. In 1908, the Danish
physician-veterinarian team of Vilhelm Ellermann and Oluf Bang showed that
chicken leukosis, a form of leukemia and of lymphoma, was caused by a virus. In
1911, Peyton Rous at the Rockefeller Institute in New York reported the
cell-free transmission of a sarcoma in chickens (
Ellermann and Bang 1908
;
Rous
1911
). The agents discovered by Ellermann and Bang are now known
under the collective term avian leukosis virus or ALV. The virus isolated by
Rous bears the name of its discoverer: Rous sarcoma virus. Together, these
viruses constitute the avian C-type virus genus, often referred to as avian
sarcoma/ leukosis viruses (ASLV).
In due time, the discovery of virus-induced tumors was extended to mammalian
species. In 1936, John Bittner established that mammary carcinoma in mice was
caused by a milk-transmitted, filterable agent, and Ludwik Gross in 1957
reported on successful efforts to select a potent leukemia virus in mice by a
combination of inbreeding and inoculation at an early age (
Bittner 1936
;
Gross
1957
). During the next two decades, many such viruses that cause
neoplastic disease in mice, cats, cattle, and monkeys were identified (see
). The number of virus-induced
tumors in fowl also greatly increased—by the early 1930s, about 20
viral isolates that cause histologically distinct tumors had been reported
(
Claude and Murphy 1933
). The list of
new avian Retroviruses is still growing (
Payne
1992
).
Principal Retroviruses and Their Origins.
Several of the viruses isolated during this period became important model
systems, actively studied at the cellular and molecular levels to this day. The
Friend and Rauscher murine leukemia viruses provided models for the study of
erythropoiesis (
Friend 1957
;
Rauscher 1962
). The rodent sarcoma viruses
of Kirsten, Harvey, and Moloney; the feline sarcoma virus of McDonough; the
avian erythroblastosis virus of Engelbreth-Holm and Rothe Meyer; and the avian
leukemia virus MC29, to name a few examples, yielded oncogenes that have
critical roles in cellular signal transduction and are also important in the
genesis of human tumors of nonviral origin (see
) (
Engelbreth-Holm and Rothe
Meyer 1932
;
Ivanov et al.
1962
;
Rauscher 1962
;
Harvey 1964
;
Moloney 1966
;
Kirsten and
Mayer 1967
;
McDonough et al.
1971
). Studies in animals also produced evidence for the occurrence
of endogenous retroviruses which initially revealed themselves by their
oncogenicity for uninfected animals. Otto Mühlbock found in 1955 that
MMTV-induced mammary carcinoma in mice could be inherited as well as virally
transmitted, and in 1958/1959, transmissible leukemogenic agents were obtained
from X-ray-induced murine leukemias (
Mühlbock 1955
;
Lieberman
and Kaplan 1959
;
Latarjet and Duplan
1962
).
The first description of what later turned out to be a lentiviral disease, equine
infectious anemia, goes back to before the turn of the century (
Vallée and Carré
1904
), although recognition of the viral origin of this disease came
later. Visna, a neurological disease in sheep caused by a lentivirus, was
described in 1957 and gave rise to the concept of slow (Latin:
lentus
, slow) viral infections (
Sigurdsson 1954
). Several lentiviruses that can induce
immunodeficiency in various species of mammals, including monkeys and cats, have
been isolated in recent years and serve as models for HIV infection (
Letvin et al. 1985
;
Pedersen et al. 1987
).
The history of the spumavirus discovery is unusual in that the isolates did not
come from sick animals but from cell cultures prepared from healthy monkeys.
Spumaviruses (Latin:
spuma
, foam) induce a characteristic foamy
appearance in the cytoplasm of cultured cells; the first isolate, simian foamy
virus, was made from macaque cultures in 1954 (
Enders and Peebles 1954
;
Rustigian
et al. 1955
).
The search for and isolation of animal retroviruses continue to this day. As
recent examples show, spontaneous tumors in animals form a rich and far from
exhausted resource for naturalists to cast their nets. The reward can be
significant for our knowledge of viral replication and of genes that have
important roles in cell growth and differentiation (
Maki et al. 1987
;
Mayer
et al. 1988
;
Souyri et al.
1990
;
Kawai et al. 1992
).
Quantitation of Infectivity and Oncogenicity
In the first five decades after the discovery of Retroviruses, infectivity and
oncogenicity were commonly titrated in the animal host, a costly,
time-consuming, and inaccurate procedure. A first step toward direct
quantitation was accomplished with the inoculation of RSV onto the
chorioallantoic membrane of the chick embryo (
Keogh 1938
). Individual small tumors appeared and their numbers
could be approximately correlated with viral concentrations, but great
variability in sensitivity existed between embryos. It was not until the 1950s
that cell culture became an effective and precise tool in animal virology,
allowing the kind of quantitative analysis that had been practiced successfully
with bacteriophage. In 1958, Howard Temin and Harry Rubin applied the powers of
cell culture to retrovirology by showing not only that RSV caused oncogenic
transformation of chick embryo fibroblasts in culture, but also that single
viral particles could induce discrete foci of transformed cells (
Temin and Rubin 1958
). The numbers of
these microtumors in culture are directly proportional to the viral inoculum: A
sensitive and quantitative assay for infectivity and tumorigenicity had been
created.
The focus assay facilitated important advances in the study of the viral life
cycle and of viral oncogenes and led to the discovery of the defectiveness in
replication of acutely transforming oncoviruses and their dependence on a
companion nontransforming helper virus. The analysis of defectiveness in turn
uncovered helper virus functions that reside in normal cells and produced early
evidence for the occurrence of Endogenous retroviral information in the genomes
of normal cells. The focus assay also allowed the isolation of conditional and
nonconditional viral mutants affecting replicative and oncogenic
characteristics. These mutants, in turn, prepared for the discovery and
isolation of oncogenes. Finally, the focus assay was instrumental in
demonstrating recombination between related Retroviruses. Similar quantal
assays, based on oncogenic transformation in cell culture, were developed for
all retroviruses that transduce an oncogene (
Baluda and Goetz 1961
;
Hartley and
Rowe 1966
;
Rosenberg et al.
1975
). Even some of the retroviruses that fail to transform cells in
culture can be assayed by their focal effects on a cell monolayer, such as
formation of fused giant cells, or syncytia, or plaques due to killing of
infected cells (
Klement et al. 1969
;
Graf 1972
).
Defectiveness in Replication
Under ideal conditions, a focus of transformed cells in culture originates from
the infection of a single cell by a single viral particle. In such single
infections, almost all acutely transforming oncogenic retroviruses fail to
produce infectious progeny, although they convert a normal cell into a cancer
cell. This defectiveness in replication was detected first with the Bryan
Hightiter strain of RSV (
Hanafusa et al.
1963
;
Temin 1963b
). It
results from the substitution of essential virus-coding sequences by a
cell-derived oncogene in the viral genome. Superinfection of the transformed
nonproducer cells with a related replication-competent but nontransforming
retrovirus leads to the synthesis of infectious oncogenic viral progeny. The
replicative functions missing in the acutely oncogenic virus are complemented at
the phenotypic level by the helper virus. The interaction between helper and
defective virus can be viewed as an extreme case of phenotypic mixing. The
oncogenic progeny virus remains genetically defective, and all viral proteins
for which genetic information is lacking are provided in
trans
by the helper (
Hanafusa 1965
). The model
provided by the defective oncogene-containing viruses provided the biological
precedent for the development of retroviral vectors (
Mann et al. 1983
).
Endogenous Retroviruses
Some nonproducer cells infected and transformed by the Bryan Hightiter strain of
RSV can spontaneously begin to release infectious virus without the addition of
an exogenous helper. An analysis of this viral production showed that certain
types of normal cells can supply helper functions endogenously, indicating not
only the presence, but also the expression of retroviral genetic information
(
Dougherty et al. 1967
;
Vogt 1967
;
Weiss 1967
;
Hanafusa et
al. 1970
;
Weiss and Payne
1971
). Endogenous retroviral genomes have indeed been found in all
vertebrates where they have been seriously looked for, including humans (
Aaronson and Stephenson 1976
;
Hughes et al. 1981
;
Repaske et al. 1985
;
Larsson et al. 1989
;
Löwer
et al. 1993
). They occur in expressed and in silent forms, as
complete or as partially deleted defective viral genomes. Endogenous proviruses
are present in somatic and germ cells alike as evidenced by genetically defined
chicken and mouse lines in which inheritance of specific endogenous proviruses
has been followed (
Payne and Chubb 1968
;
Robinson 1978
;
Pincus 1980
). They are transmitted from parent to
offspring in the form of single dominant Mendelian loci. Complete retroviral
genomes Endogenous to normal chicken or mouse cells can be induced by
irradiation of the cell or exposure to demethylating agents; these genomes then
direct the synthesis of infectious virus (
Rowe
et al. 1971
;
Weiss et al.
1971
). Production and release of endogenous virus can also occur
spontaneously (
Vogt and Friis 1971
;
Levy 1978
;
Pincus 1980
). All endogenous retroviral genomes belong to
the simple category. So far, no endogenous lenti-, spuma-, or HTLV-like viruses
have been identified.
The Virion
The adaptation of modern cell culture techniques to retrovirology was also
accompanied by a steady improvement in the methods of viral growth and virion
purification. Analysis of viral proteins from infected cells and from mature
viral particles and information generated by the study of viral mutants defined
three principal groups of virion proteins (
Fleissner 1971
;
Oroszlan et al.
1971
;
Schäfer et al.
1972
;
August et al. 1974
).
Located at the virion surface are glycosylated envelope (Env) proteins. The
structural proteins of the matrix and nucleocapsid core are nonglycosylated and
are referred to as Gag proteins. The acronym Gag is derived from
g
roup-specific
a
nti
g
en,
in reference to the cross-reactive immunological tests in which a single
antiserum was able to detect related retroviruses infecting the same host
species. Associated with nucleocapsid and RNA are the polymerase (Pol) proteins
that are responsible for reverse transcription and integration.
An important advance in the understanding of viral gene expression and assembly
of progeny virus came with the discovery that the primary translational Products
of retroviruses are three polyproteins translated from polycistronic messages
corresponding to Gag, Pro, Pol, and Env sequences (
von der Helm 1977
;
Eisenman and Vogt 1978
). These are cleaved proteolytically to
generate the functional virion components. The processed Gag proteins are
referred to as MA (matrix), CA (capsid), and NC (nucleocapsid). The viral
protease, PR, cleaves Gag, Pol, and sometimes Env precursors. It is encoded by
the
pro
gene and maps between
gag
and
pol
. The Pol cleavage products are RT (reverse
transcriptase) and IN (integrase). The Env precursor is processed by a cellular
enzyme into SU (surface) and TM (transmembrane) proteins that are linked by
disulfide bonds or noncovalent interactions (
Leis et al. 1988
;
Hunter and
Swanstrom 1990
;
Oroszlan and Luftig
1990
). Translation of the polycistronic messages of retroviruses is
subject to controls that include suppression of termination and ribosomal
frameshifting at the intersections between the
gag
,
pro
, and
pol
genes (
Jacks 1990
; and
Chapter 7
).
Virion RNA sediments as a complex of 60–70 Svedberg (S) units
(corresponding to about 20–30 kb of RNA) in the ultracentrifuge; it
can be dissociated by heat into 30 S (7–12 kb) components (
Robinson et al. 1965
;
Duesberg 1968
). This observation
immediately raised the question of whether the smaller components are
functionally and genetically identical or whether they are different, performing
distinct, nonoverlapping tasks in the viral growth cycle. The answer came from a
determination of the genetic complexity of retroviruses by RNA fingerprinting.
The results favored the lower 30 S limit for the whole genome, corresponding to
7–12 kb in size, and provided evidence for diploidy of Retroviruses
(
Beemon et al. 1974
;
Billeter et al. 1974
). RNA fingerprinting
of viral genomes with specific deletions and of recombinants defined the basic
gene order as
5′-
gag
-
pol
-
env
-3′
(
Joho et al. 1975
;
Coffin and Billeter 1976
;
Wang et al. 1976a
,
b
,
c
).
Viral Mutants
The focus assay also provided the technical basis for the first genetic
experiments with retroviruses. Much of the early work was done with nondefective
RSV, which is both strongly transforming-competent and replication-competent.
The ability to clone virus by picking single foci of transformed cells and to
work with the progeny of single viral particles was a prerequisite for the
isolation and characterization of conditional and nonconditional viral mutants
(
Temin 1961
;
Toyoshima and Vogt 1969
;
Martin 1970
;
Hanafusa et al.
1972
;
Wyke 1973
). RSV mutants
defined viral gene functions, provided markers for construction of the genetic
map, and was used to reveal the cellular origin of retroviral oncogenes. The
mutants could be grouped into two basic categories, one—in the
oncogene—affecting only virus-induced transformation and oncogenesis
and the other—in
gag
,
pol
, or
env
—interfering primarily with viral growth. The
characteristics of these mutants allowed the important conclusion that
transformation and replication were separately coded functions and provided the
first genetic definition of retroviral oncogenes.
Mutants of RSV that were temperature-sensitive for focus formation in cultures of
chicken embryo fibroblasts demonstrated unequivocally that both the induction
and maintenance of the transformed phenotype depended on the activity of a viral
gene (
Toyoshima and Vogt 1969
;
Martin 1970
;
Kawai and Hanafusa 1971
). Nonconditional
transformation-defective mutants of RSV were found to have suffered a deletion
of the
src
oncogene. These mutants were still able to produce
infectious nontransforming progeny, marking the oncogene as dispensable for
viral growth and survival (
Duesberg and Vogt
1970
;
Vogt 1971a
). The
congenic pair of RSV and its derivative
src
deletion mutant
later provided the raw material for the synthesis of a specific
src
cDNA probe, the tool that traced the origin of the
src
gene to the genome of the normal cell.
Genetic Recombination
Biological cloning in cell culture and viral host range markers defined through
the use of genetically resistant cells played a decisive part in the discovery
of stable genetic exchanges between related retroviruses (
Vogt 1971b
;
Kawai and
Hanafusa 1972
). The nature of these genetic interactions was not
immediately clear. When the initial experiments were done, the complexity of the
genome had not been resolved, and there was still a possibility that it
consisted of independently replicating segments that could reassort at the
observed high frequencies. Only later did molecular analysis of recombinant RNA
reveal true crossing over as the mechanism of retroviral recombination, favored
by the diploidy of the nonsegmented retroviral genome (
Beemon et al. 1974
;
Joho
et al. 1975
).
Provirus and Reverse Transcriptase
The molecular tools available throughout the 1960s were primitive by today's
standards. Many conclusions concerning the replication of animal viruses were
based on the effects of metabolic inhibitors. For retroviruses, the use of such
inhibitors created a paradox: The replication of these RNA viruses was widely
assumed to follow the model of other single-stranded RNA viruses such as
poliovirus, where replication involves RNA-dependent RNA synthesis carried out
by a virus-coded enzyme that creates partially double-stranded RNA replicative
forms. Yet no double-stranded viral RNA species could be detected in
retrovirus-infected cells. Moreover, retroviral replication was found to be
sensitive to inhibitors of DNA synthesis and DNA-directed RNA synthesis (
Temin 1963a
;
Bader 1965
). In 1964, Howard Temin provided a coherent
explanation for these puzzling data by proposing the provirus hypothesis, which
postulates the generation of a DNA copy of the viral genome and its subsequent
integration into the cell genome (
Temin
1964
). This radical idea gained general acceptance only with the
discovery of reverse transcriptase in retroviral virions (
Baltimore 1970
;
Temin and
Mizutani 1970
). Further evidence in support of the provirus
hypothesis came from DNA transfection experiments. Total DNA extracted from
RSV-transformed cells and introduced into uninfected recipients induced
oncogenic transformation and, under appropriate conditions, synthesis of
complete RSV (
Hill and Hillova
1972
).
The discovery of reverse transcriptase was a watershed event. The specific
enzymatic and mechanistic problems posed by reverse transcription of the RNA
viral genome could now be intensely studied. The primer for initiation of DNA
synthesis was identified as a species of cellular tRNA (
Harada et al. 1975
;
Peters and Dahlberg 1979
). Synthesis of the viral DNA was shown to
start near the 5′end of the virion RNA template (
Taylor and Ilmensee 1975
). To continue reverse
transcription, the polymerase-nascent DNA complex must jump to the
3′end of virion RNA, a feat made possible by the RNase H activity of
reverse transcriptase (
Moelling et al.
1971
) and by the existence of short direct repeats at the
5′and 3′ends of the genome (
Coffin and Haseltine 1977
;
Stoll et
al. 1977
). The initial product of reverse transcription is an RNA-DNA
duplex; RNase H digests the RNA template, allowing newly synthesized DNA to base
pair with the repeat at the 3′end of the genome. A second similar
template jump was found to take place later in the process of reverse
transcription (
Gilboa et al. 1979
) to
produce a complete proviral copy with two LTRs. Integration was found to take
place into a very large number of sites in the host genome, without circular
permutation of the proviral sequence (
Hughes et
al. 1978
;
Steffen and Weinberg
1978
). Structure and function of proviral RNA transcripts were
determined as well as their mechanism of synthesis, the essential features being
the synthesis of full-length and spliced RNA copies with characteristics of host
mRNAs (
Hayward 1977
;
Weiss et al. 1977
). The former serve as
progeny genomes and as mRNA for Gag, Pro, and Pol proteins; the latter function
as messages for Env proteins and, in the case of the complex retroviruses, for
regulatory proteins. Cloning and sequencing of DNA in the early 1980s greatly
expanded the technical possibilities of the field, and the first complete
sequence of a retrovirus, Moloney murine leukemia virus, was published in 1981
(
Shinnick et al. 1981
).
The Cellular Origin of Oncogenes
Early studies on reverse transcriptase had shown that cDNA copies of the viral
genome could be synthesized in preparations of purified virions. Although the
DNA transcripts were much smaller than the virion RNA template, they were
representative of all sequences in the genome. Being able to generate short cDNA
copies that contained sequences from the entire genome allowed the construction
of probes for specific genes by the use of defined deletion mutants as selection
agents. A probe specific for the
src
oncogene was obtained in
this way
(
Stehelin et al. 1976a
) and was found to
hybridize to sequences in normal cellular DNA (
Stehelin et al. 1976b
). This surprising result showed that
src
was not originally a retroviral gene, but a gene of
cellular origin carried in the viral genome and transduced by the virus from
cell to cell. In short order, all retroviral oncogenes were found to be recent
acquisitions from the cell. Many were eventually identified as cellular genes
with normal functions in mitogenic signal transduction: coding for peptide
growth factors, growth factor receptors, protein kinases, G proteins, and
transcription factors (
Bishop 1983
;
Varmus 1984
;
Weinberg 1989
;
Cooper
1990
).
The discoveries at the nucleic acid level were complemented by important advances
at the protein level. The product of the
src
oncogene was
identified as a 60-kD phosphoprotein by immunoprecipitation using sera from
rabbits bearing RSV-induced tumors, and translation in vitro from
src
mRNA (
Brugge and
Erickson 1977
;
Beemon and Hunter
1978
;
Purchio et al. 1978
).
The
src
gene product was soon found to be a protein kinase,
specifically one that targets tyrosine (
Hunter
and Sefton 1980
). Tyrosine kinases are now known to have a central
role in cellular signal tranduction (
Chapter 10
).
The discovery of the cellular origin of retroviral oncogenes also proved to be
key to unlocking the origins of human cancer in the absence of viruses. The
question was: If normal cells contain Protooncogenes, can they be made oncogenic
by mutation alone, without being acquired by a virus? The first suggestion that
this might be the case came from the analysis of tumors induced by ALV, a virus
that causes lymphoma in a high percentage of infected chickens but does not
contain an oncogene. Tumors induced by this virus were found to carry a provirus
inserted within a common region of the protooncogene c-
myc
,
previously identified as the cellular counterpart of the oncogene of several
avian retroviruses (
Hayward et al. 1981
;
Payne et al. 1982
). The insertions
were quickly shown to lead to deregulated expression of c-
myc
,
demonstrating that oncogenes could be activated by juxtaposition of
protooncogene and regulatory sequences without the requirement of incorporation
into a virus. This precedent led other investigators to look among chromosomal
rearrangements in human tumors for evidence of DNA rearrangements involving
c-
myc
, and several examples were rapidly uncovered (
Dalla-Favera et al. 1982
;
Marcu et al. 1983
). Indeed, chromosomal
rearrangements that place Protooncogenes under strong cellular transcriptional
control signals or create oncogenic fusion proteins are a common feature of many
naturally occurring malignancies (
Gale and
Canaani 1984
;
Groffen et al.
1984
).
The second landmark discovery in this area was made with an extension of the
technique which had demonstrated that the DNA of RSV is biologically active:
Transfection of normal cells with DNA extracted from cells infected with RSV
produced foci (
Hill and Hillova 1972
;
Shih et al. 1979
;
Krontiris and Cooper 1981
). Now the
sources of DNA were nonviral tumors, including human cancers and tumor cell
lines. In recipient mouse cells, such DNA induced neoplastic transformation. The
effect was traced to a single, dominant gene that in serial DNA transfections
retained the ability to confer the malignant phenotype upon the recipient cell.
In several types of human tumors, this gene was identified as the mutated
cellular form of the retroviral oncogene
ras
, previously
studied in a murine sarcoma virus (
Der et al.
1982
;
Parada et al. 1982
;
Santos et al. 1982
).
With these discoveries, oncogenes appeared on the scene as candidate causative
factors of human cancer. Transfection studies characterized oncogenes as
dominant effectors of neoplastic transformation. Further analysis of individual
oncogenes and their transforming potential revealed cooperative effects between
oncogenes, such as
myc
and
ras,
introduced in
the same cell (
Land et al. 1983
). The
often inefficient transformation of normal cells freshly cultured from an animal
organism and the enhanced oncogenicity of certain combinations of oncogenes
probably reflect the multistep nature of carcinogenesis. For the induction of
full oncogenic transformation, a change in a single gene is generally
insufficient; multiple genetic events that promote growth-promoting genetic
events are required for the development of a malignant tumor. Several human
tumors show a high incidence of genetic change in specific oncogenes, and in
some cancers, such as Burkitt's lymphoma or chronic myelogenous leukemia, a
specific change involving an oncogene occurs in virtually all cases (
Haluska et al. 1987
). Such consistent
deregulation of growth-promoting genes very likely has an etiological role in
carcinogenesis.
The study of oncogenes has blossomed into a major field of biology and has moved
far from its retroviral roots. The production of transgenic animals and the germ
line inactivation of genes by homologous recombination added new dimensions to
oncogene research (
Palmiter and Brinster
1986
;
Sinn et al. 1987
;
Mansour et al. 1988
). Investigations with
these techniques firmly established the causative role of mutated and
ectopically expressed cellular oncogenes in cancer and uncovered often
unexpected functions of these genes in embryonal development and
differentiation.
Human Infections: The Challenge to Retrovirology
The isolation of retroviruses from tumors in higher mammals in the 1960s
intensified the search for human cancer viruses. These efforts were supported by
the Special Virus Cancer Program of the U.S. National Cancer Institute,
initiated in 1971. This program greatly enhanced basic knowledge of animal tumor
viruses at the organismic, cellular, and molecular levels but was not
immediately successful in one of its primary missions, namely, finding an
oncogenic human retrovirus. Although several retroviral isolates from human
material were described, closer scrutiny relegated all of them to the category
of contaminants from animal sources (
Weiss
1982
). By the end of the 1970s, Retroviruses had been searched for
without success in most types of human tumors. Despite the universal occurrence
of these viruses in a number of mammalian species, including some primates, it
seemed questionable whether a human retrovirus existed at all. However,
persistence in the face of declining odds was eventually rewarded.
The initial clue came in 1977 from clinical and epidemiological studies that
revealed an unusually high frequency and a suspicious clustering of adult T-cell
leukemia in areas of the Kyushu and Shikoku islands of Japan (
Takatsuki et al. 1977
;
Uchiyama et al. 1977
). These
epidemiological patterns were suggestive of a transmissible leukemogenic agent.
The isolation and identification of that agent, however, depended on a technical
breakthrough, the long-term cultivation in vitro of human T lymphocytes (
Morgan et al. 1976
;
Ruscetti et al. 1977
). Success in the culture of T cells
depended on the discovery of the T-cell growth factor, now known as
interleukin-2 (IL-2). Cultures of leukemic T cells then were the source of the
first human oncogenic retrovirus, human T-cell leukemia virus 1 (HTLV-1) (
Poiesz et al. 1980
;
Yoshida et al. 1982
). HTLV-1 is the causative agent of the
aggressive T-cell leukemia of southern Japan and parts of the Caribbean.
HTLV-1 soon became the prototype of the complex retroviruses. Cloning and
sequencing of its viral genome showed that HTLV-1 lacked a cell-derived
oncogene, yet it was more complex than other oncogenic retroviruses (see
Seiki et al. 1983
). Coding information for
new types of retroviral proteins, then referred to as X proteins and now termed
Tax and Rex, was discovered. Subsequent analysis revealed several nonvirion
regulatory proteins in the HTLV-1 genome which thus became the first example of
a complex retrovirus (see
Fig. 1
).
HTLV-1 infection is strongly linked to adult T-cell leukemia epidemiologically. A
similar link of HTLV-1 was discovered to a disease of the central nervous
system, tropical spastic paraparesis (TSP) (
Gessain et al. 1985
;
Osame et al.
1986
). Adult T-cell leukemia is seen in less than 1% of individuals
who show immunological evidence of past infection with HTLV-1; the percentage
for TSP is even smaller. HTLV-1 infection therefore appears to be necessary but
not sufficient to cause hematopoietic or neurological disorders. In adult T-cell
leukemia, the HTLV-1 provirus was found to be integrated at the same chromosomal
site in all leukemic lymphocytes from a given patient, but at different sites in
cells from different patients (
Wong-Staal et
al. 1983
;
Yoshida et al.
1983
). This integration pattern demonstrated both the clonal origin of
the disease and absence of insertional activation of a cellular oncogene. The
two well-known mechanisms of retroviral Oncogenesis, transduction and
cis
-activation of an oncogene, therefore did not apply to
HTLV-1. A plausible scenario, in accord with available data, is that
Leukemogenesis is caused by expression of a viral regulatory protein, probably
Tax. Tax can act as a transcriptional regulator and has been shown to affect
expression levels of several host genes (
Sodroski et al. 1984
;
Feuer and Chen
1992
). However, the low incidence and long latency of this disease
implies collaboration of other factors or genetic changes.
About the time of the isolation of HTLV-1, an epidemic of AIDS arose in several
developed countries. Again, the ability to culture human T cells played a
critical part in isolating a candidate retrovirus variously referred to by its
discoverers as lymphadenopathy-associated virus (LAV), HTLV-3, or AIDS-related
virus (ARV) (
Barré-Sinoussi et al.
1983
;
Gallo et al. 1984
;
Levy et al. 1984b
), but now known as
human immunodeficiency virus (HIV). Cloning and sequencing of the HIV genome
showed that this virus was of the complex variety, capable of coding for several
regulatory proteins in addition to the standard set of Retroviral virion
proteins. HIV was rapidly and firmly linked to the causation of AIDS:
Epidemiological data showed a strong correlation with the disease (
Gallo 1988
,
1990
). Sexual intercourse was found to be the major mode
of transmission, and promiscuity defined major risk groups. Blood-borne
transmission was also recognized as an important factor in the spread of HIV and
responsible for the high rates of infection among intravenous drug users and
hemophiliacs. Blood-borne infections provided dramatic proof of the etiological
role of HIV in AIDS: Transfusions given in the course of surgery at the
beginning of the AIDS epidemic but before contamination with HIV could be tested
led directly to the transmission of the disease, as did administration of
HIV-tainted coagulation factors to hemophiliacs. Such accidental infections had
invariably tragic consequences. Screening for HIV virtually eliminated such
iatrogenic infections. Key elements of AIDS pathogenicity were recognized early:
drastic depletion of CD4-positive lymphocytes, and the consequent heightened
susceptibility to opportunistic microbial and parasitic infections, severe
neurological complications, and high incidences of lymphoma and Kaposi's sarcoma
(
Gottlieb et al. 1981
;
Safai and Koziner 1985
;
Navia et al. 1986
). At this time, neither
protective vaccine nor curative therapy is available for HIV infection. The
immense human suffering caused by the continued spread of HIV constitutes the
greatest challenge to retrovirology.