Medmicro Chapter 62

Human Retroviruses


Miles W. Cloyd


General Concepts

Human Endogenous and Exogenous Retroviruses

Similar to other vertebrate animals, humans possess retroviruses that exist in two forms: as normal genetic elements in their chromosomal DNA (endogenous retroviruses) and as horizontally-transmitted infectious RNA-containing viruses which are transmitted from human-to-human (exogenous retroviruses, e.g. HIV and human T cell leukemia virus, HTLV). Endogenous retroviruses in animals and humans probably evolved from transposable elements, some of them gaining the ability to package themselves in a virion structure, leave the cell and infect another cell.

Clinical Manifestations

In general, endogenous human retroviruses are not pathogenic and many of them are not complete viruses. The human genome contains between 100­1,000 copies of such viruses and many of them have become pseudogenes or have various defects. However, some are complete viruses and the genes of some are transcribed and make virus-encoded proteins. Expression of such genes have been found in certain autoimmune diseases in humans such as systemic lupis erythematosis and Sjögren's syndrome. Endogenous virus gene expression has also been observed in human placentas and in reproductive tissues of humans without any apparent pathology.

Human T-Cell Leukemia viruses

Human T-cell leukemia viruses are horizonatally transmitted from human-to-human (i.e. exogenous virus) and are associated with development of some rare diseases. They are believed to have originated from highly-related simian viruses.

Clinical Manifestations

HTLVs are associated with three presentations: (1) asymptomatic infection (most common); (2) adult T-cell leukemia (ATL) (pre-adult T-cell leukemia, chronic, acute, and lymphoma forms); and (3) tropical spastic paraparesis, a neurologic disease.

Structure

HTLVs are spherical particles, l00 nm in diameter, made up of an external lipid bilayer/glycoprotein envelope covering an internal protein core. The core contains several copies of reverse transcriptase (the enzyme that transcribes RNA to DNA) bound to two identical single-stranded RNA molecules. The RNA codes for internal core proteins (gag), external envelope proteins (env), reverse transcriptase (polymerase) (pol), and regulatory proteins (tax, rex). The lengths of the HTLV-l and HTLV-2 proviral genomes are about 9.0 and 8.9 kilobases, respectively.

Classification and Antigenic Types

Three types of HTLV are recognized: HTLV- l, HTLV-2, and HTLV-5. HTLVs are classified on the basis of (l) isolation from and ability to infect mature T cells and (2) the presence of reverse transcriptase and cross-reacting internal core proteins. The coding regions of HTLV-l and HTLV-2 share about 60 percent homology.

Multiplication

After the virion enters the cell, viral reverse transcriptase transcribes the viral RNA into DNA, which integrates into the host cell genome. The integrated viral DNA may remain inactive or be transcribed into progeny viral RNA and into messenger RNA that is translated to produce the viral structural and regulatory proteins. Progeny virions assemble at the cell surface and acquire an envelope by budding.

Pathogenesis

HTLV- l and HTLV-2 infect T cells. The viral transactivator protein (tax) turns on the expression of cellular proteins and may lead to uncontrolled proliferation of target cells. The long incubation period (3O to 40 years) between infection and the development of leukemia suggests that additional events are required for leukemogenesis. The pathogenic mechanisms of tropical spastic paraparesis are unknown. Between 2-4% of infected people develop disease.

Host Defenses

HTLV-specific antibodies, cytotoxic T cells, and interferon have been identified in individuals with persistent infection, but their roles in protection and pathogenesis are unclear.

Epidemiology

HTLV-l is endemic in southwestern Japan, the Caribbean basin, southeastern United States, southern Italy, and sub-Saharan Africa. Up to 15 percent of normal blood donors in endemic areas of Japan and the Caribbean basin are positive for antibodies to HTLV-l; in nonendemic areas, fewer than 1 percent are positive. Data for HTLV-2 are incomplete. HTLVs can be transmitted by transfusion of blood from infected donors, by sexual contact, and from mothers to babies via breast milk.

Diagnosis

HTLV diseases are diagnosed by the presence of HTLV-specific antibodies and by clinical manifestations. The polymerase chain reaction detects the HTLV genome in infected cells and distinguishes between HTLV-l and HTLV-2 infection. Pre-adult T-cell leukemia is diagnosed on the basis of leukocytosis and abnormal lymphocytes. Acute adult T-cell leukemia-lymphoma is characterized by the presence of pleomorphic neoplastic cells with mature T-cell markers. Chronic adult T-cell leukemia is diagnosed on the basis of skin lesions, low levels of circulating leukemic cells, and the absence of visceral involvement. Tropical spastic paraparesis is characterized by a meningeal inflammatory process, largely limited to the spinal cord, with progressive weakness in the lower extremities.

Control

Infection is prevented by (1) screening blood products for HTLV antibodies and (2) education to prevent transmission by sexual contact or the sharing of needles. Glucocorticoids have been of limited value in treating some cases of tropical spastic paraparesis. Various combined chemotherapy regimens have been tried for adult T-cell leukemia with limited success.

Human Immunodeficiency Virus

Clinical Manifestations

Human immunodeficiency virus (HIV) is another horizontally-transmitted exogenous retrovirus and is associated with three presentations: (1) asymptomatic infection; (2) acute infection with symptoms that may include fever, sweats, myalgia or arthralgia, sore throat, lymphadenopathy, nausea, vomiting, diarrhea, headaches, and rash; and (3) acquired immune deficiency syndrome (AIDS), characterized by progressive immune deficiency accompanied by a wide range of opportunistic infections, neoplasms, and neurologic abnormalities, including progressive dementia and peripheral neuropathy.

Structure

HIV is, in general, similar to HTLV, although the genome is larger (approximately 10 kilobases). As with HTLV, the HIV genome codes for core structural proteins, envelope proteins, reverse transcriptase, and a set of regulatory proteins more complex than those of HTLV.

Classification and Antigenic Types

Techniques of identification are similar to those used for HTLV. Two antigenic types (HIV-l and HIV-2) are distinguished by antibody reactivity to envelope glycoproteins.

Multiplication

HIV recognizes host cells by binding to the CD4 cell membrane receptor. Multiplication within the host cell is similar to that of HTLV.

Pathogenesis

HIV infects CD4-bearing cells (T4 lymphocytes and monocyte-macrophages). Infection may be latent or chronic low level. Activation leads to the development of immune dysfunction as a result of direct and indirect killing of T4 cells and functional impairment of viable T4 cells. Macrophages appear to be the primary target cells in brain infection. The pathogenic mechanisms of neurologic disease are not known.

Host Defenses

Persistent infection stimulates humoral and cell-mediated immune responses and interferon production, but the role of these responses in preventing pathogenesis or disease progression is not known.

Epidemiology

HIV infection occurs worldwide and is endemic in central Africa. In the United States and other developed countries, infection is found primarily in homosexual and bisexual men, intravenous drug users, hemophiliacs, transfusion recipients, sexual partners of infected persons, and infants born to infected mothers. Transmission occurs through sexual contact, exposure to contaminated blood or blood products, and perinatally.

Diagnosis

HIV infection is determined by demonstrating HIV-specific antibodies, which usually appear at some interval of time after infection. The polymerase chain reaction detects the HIV genome in infected cells. Detection of the p24 core antigen in serum also indicates HIV infection. AIDS is diagnosed mainly on the basis of specific opportunistic infections or cancers coinciding with a T4 cell defect in the absence of other known causes of immunodeficiency. In addition, HIV infections with neuropsychiatric manifestations or severe wasting are classified as AIDS.

Control

Infection is prevented by screening blood products for HIV and by education to prevent transmission by sexual contact and sharing of needles. Azidothymidine (AZT) and other antiviral agents are used for prophylaxis against progression to disease and for treatment. Opportunistic infections and neoplasms are treated individually.

INTRODUCTION

Types of Endogenous and Exogenous Retroviruses

Retroviruses, as virions contain RNA genomes, but the infectious cycle requires copying the RNA genome into DNA from which transcription occurs. The virus therefore encodes and carries within the virion an enzyme called RNA dependent DNA polymerase or reverse transcriptase which will transcribe the RNA genome into a double-stranded DNA intermediate. This DNA intermediate usually covalently integrates into the chromosomal DNA of the cell and therefore becomes a permanent genetic element within that host cell. Retroviruses exist, therefore, in two forms; as RNA-containing virions which bud from a producing cell and can infect another cell, and as DNA proviruses which may be active or silent. Proviruses exist naturally in most vertebrates, as well as some non-vertebrates, and are present in the germ line as "normal" genes. Retroviruses are unique in this respect. Humans contain a number of retroviral-like or retroviral sequences within their genomic DNA but most of these are silent or have become pseudogenes. Nevertheless, one or more of these genes can be expressed during normal or pathological conditions, and such "endogenous retroviruses" are known to play roles in naturally-occurring leukemias and certain immunological diseases in non-human animals. The endogenous retroviruses are believed to have evolved from transposable elements which today are still present in lower life forms (yeast, maze, drosophila).

Some retroviruses, however, have evolved to be transmitted horizontally from animal to animal or person-to-person and these are the ones which usually cause diseases. In man, three such horizontally-transmitted "exogenous" viruses are associated with disease: HTLV-1 is associated with adult T-cell leukemia and tropical spastic paraparesis; HTLV-2 is associated with hairy cell leukemia; and human immunodeficiency virus (HIV) is associated with the acquired immunodeficiency syndrome (AIDS). Retroviruses are classified into three families: Oncoviruses, Lentiviruses, and Spumaviruses. Retroviruses are also classified based on their morphological types in the electron microscope as A-type, B-type, C-type, and D-type. The A-type viruses bud intracellularly, either into the cytoplasm or within endoplasmic reticulum, are not considered to be infectious, and have an electron lucent core. These are endogenous viruses and some animal species have thousands of copies of these A-type viruses in their chromosomal DNA. Their function remains unknown. B-type viruses have an eccentric core and the mammary tumor viruses exclusively have this structure. These viruses exist as endogenous and exogenous viruses in some animals and when expressed can cause mammary tumors. C-type viruses have a central electron-dense core, and most of the oncoviruses and endogenous viruses are of this type. The D-type viruses have a rod-shaped core and Lentiviruses are of this type.

While most of the endogenous retroviruses are not pathogenic, the exogenous horizontally-transmitted viruses usually are. Diseases in animals or humans which are induced by or associated with horizontally-transmitted exogenous retroviruses, include feline leukemias or sarcomas, chicken leukemias or sarcomas, mouse leukemias or sarcomas, equine infectious anemia, bovine leukemia, caprine arthritis-encephalitis, human adult T-cell leukemia, human tropic spastic paraparesis, and AIDS. Spumavirus (human foamy virus) has not yet been definitely associated with disease in humans.

Human T-Cell Leukemia Viruses

Clinical Manifestations

Infection with human T-cell leukemia virus type 1 (HTLV-l), an exogenous C-type oncovirus, results in a spectrum of clinical manifestations ranging from asymptomatic infection to lymphoproliferative and neurologic disorders (Fig. 62-1). Most HTLV-l-infected individuals are asymptomatic, with normal white blood cell and differential counts. However, some asymptomatic individuals may present with moderate lymphocytosis and aberrant lymphocytes. Approximately half of these latter individuals will progress to a chronic form of adult T-cell leukemia (ATL), characterized by small numbers of leukemic cells in the peripheral blood, by skin lesions, and by a lack of involvement of other organ systems. Only 2-4% of HTLV-I infected individuals eventually develop ATL. Approximately 15 to 20 percent of adult T-cell leukemia cases follow a chronic course. T cell lymphomas occur and may involve a variety of organ systems. The most common malignant sequela of HTLV-l infection is acute adult T-cell leukemia, which follows an aggressive course characterized by polylobular malignant T cells, hypercalcemia, leukemic cell infiltrates of the dermis and epidermis, and immunosuppression leading to opportunistic infections.

FIGURE 62-1 Clinical manifestations and pathogenesis of HTLV-1 and HTLV-2 infections.

Infection with HTLV-l may also result in a slowly progressive encephalomyelopathy called tropical spastic paraparesis or HTLV-1-associated myelopathy. The predominant clinical manifestation is progressive weakness and partial paralysis of the lower extremities. The clinical features may mimic those of multiple sclerosis. In fact, there is some speculation that an HTLV-related virus is involved in multiple sclerosis. There have also been reports that HTLV-l is associated with large granular lymphocytic leukemia and malignant hypereosinophilic syndrome. However, these claims have not been substantiated.

Two other HTLVs, HTLV-2 and HTLV-5, have been identified. The relation of these viruses to human disease is unclear. HTLV-2 has been associated with two cases of hairy-cell leukemia and has been reported in one case of T-cell prolymphocytic leukemia and one of T-cell chronic lymphocytic leukemia. HTLV-5 has been isolated from a patient with Tac-antigen-negative cutaneous T cell lymphoma-leukemia (mycosis fungoides). HTLV-5 DNA sequences have been found in the tumor cells of an additional seven patients with mycosis fungoides.

Structure

HTLVs are members of the oncovirus family of retroviruses, which are distinguished from other viruses by the presence of reverse transcriptase, an enzyme that transcribes RNA into DNA. Most retroviruses are spherical particles, approximately 100 nm in diameter, consisting of an internal protein core surrounded by an envelope of glycoproteins embedded in a lipid bilayer. The core contains several copies of reverse transcriptase bound to two identical single-stranded RNA molecules. The DNA copy (provirus) of genomic viral RNA (Fig. 62-2) is flanked on each end by a long terminal repeat (LTR) that contains sequences for the integration of the virus into the cellular DNA and the regulation of virus expression. In the viruses that cause chronic leukemias, the viral genes encode the core proteins (gag), envelope proteins (env), replication enzymes (pol), and regulatory proteins (tax and rex). Some animal retroviruses, in contrast, are acutely transforming. The env gene of these viruses is truncated and replaced with a cellular gene (onc) that causes acute transformation of the target cells.

FIGURE 62-2 HTLV genome (proteins encoded).

The envelope of HTLV consists of two glycoproteins of molecular weights approximately 20,000 and 46,000 daltons. Three gag proteins (molecular weights of 9,000, 15,000, 24,000 to 26,000 daltons) constitute the viral core. Unlike most animal retroviruses, the genome of HTLV codes for two nonstructural proteins, tax and rex, which are involved in the regulation of HTLV expression. Tax may also be indirectly involved in HTLV transformation of lymphocytes.

Classification and Antigenic Types

A virus is classified as an HTLV by the presence of reverse transcriptase and by its isolation from and infection of mature T cells. In vitro, HTLV infection of T cells results in their immortilization (i.e., in the continuous proliferation of the cells in the absence of exogenous growth factors). The genomes of HTLV hybridize only weakly with the genomes of other animal retroviruses. HTLV-1, HTLV-2, and HTLV-5 are the antigenic types recognized currently. The HTLVs are only remotely related to human immunodeficiency virus (HIV). The HTLVs share cross-reacting internal core proteins which differ slightly in molecular weight. The HTLV-1 and HTLV-2 proviral genomes are approximately 9.0 and 8.9 kilobases in length, respectively, and have approximately 60 percent overall homology. Although the long terminal repeat sequences of these two viruses are quite dissimilar, specific regulatory regions in the long terminal repeat are highly conserved. The other highly (more than 80 percent) homologous region of the genomes of HTLV-l and HTLV-2 is the tax gene. The genome of HTLV-5 is less well characterized. The DNA of HTLV-5 hybridizes only weakly to HTLV-1 and does not hybridize to HTLV-2. The different HTLVs are distinguished by using polymerase chain reaction (PCR) techniques to amplify HTLV sequences and by hybridization with specific viral probes.

Multiplication

Retroviruses have a common mode of viral replication that is based on the function of the reverse transcriptase molecules (Fig. 62-3). The initial event in retroviral infection is the attachment of the virus to the cell membrane. The specific cellular receptor(s) for HTLV has not yet been identified, but recent experiments have shown that a receptor is encoded by a gene on human chromosome 17. After binding to the cell membrane, the virion enters the cell and is uncoated to release the viral RNA. The reverse transcriptase, which is complexed to the viral RNA, transcribes the RNA into DNA. The virion-associated integrase then enables the viral DNA to integrate into the host cell genome. The integrated viral DNA, now called a provirus, can either remain inactive or be transcribed into viral RNA and into messenger RNA that is translated into viral structural and regulatory proteins. Progeny viral RNA and structural proteins assemble at the cell surface and bud from the cell membrane. Since the provirus is duplicated along with the cellular DNA during the replication cycle of the cell, infection of the cell will persist throughout the lifespan of the clone.

FIGURE 62-3 Multiplication of human retroviruses.

Pathogenesis

The HTLVs appear to induce disease in a fundamentally different way from other oncogenic animal retroviruses. The acutely transforming animal retroviruses cause malignant transformation by their onc genes which were originally derived from cellular growth genes, but are no longer under cellular control and rather are under the control of the viral promoter. The chronic oncogenic animal retroviruses induce leukemia after a long latent period. Often the provirus inserts near a cellular onc gene. As a result, the cellular gene comes under the control of the viral promoter, thereby enhancing its expression. Therefore, the virally transformed animal leukemic cells have proviral copies integrated at specific sites in the cellular DNA. In contrast, the HTLVs do not possess an onc gene, nor do they integrate in the same site in different tumors (i.e., they show random integration).

HTLVs possess two unique genes, tax and rex, which are required for viral replication and efficient transcription of the HTLV genome. The rex gene encodes a protein of molecular weight 26,000 to 27,000 daltons that is localized in the nucleus and is required for the accumulation of unspliced gag mRNA and for efficient expression of the gag gene products. The tax gene product is a nuclear protein of molecular weight 37,000 to 40,000 daltons that transactivates viral gene expression. The exact mechanism of tax-induced transactivation is unknown, but it has been suggested that the tax gene product may activate a constitutively expressed cellular transcription factor that binds to the viral promoter and indirectly enhances viral gene expression. Although expression of the tax gene can lead to cancer in some strains of transgenic mice, the resulting tumors are neurofibromas and do not demonstrate a direct effect of tax on leukemogenesis.

One mechanism by which the HTLV tax gene product may play a role in leukemogenesis is by activating cellular genes that are involved in T-cell growth (Fig.62-4). Mitogen- or antigen-stimulated T cells produce a factor, interleukin-2 (IL-2), that binds to a receptor on the surface of activated T cells and induces T-cell multiplication. Primary tumor cells from patients infected with HTLV-1 and from T-cell lines that have been transformed in vitro with HTLV-1 or HTLV-2 express large numbers of receptors for IL-2 on their surface. In addition, cells transfected with HTLV-1 gag-tax genes release high levels of IL-2 receptors constitutively. Therefore, it has been hypothesized that the tax gene product is responsible for stimulating the production of IL-2 receptors through the induction of host transcription factors that bind to the IL-2 receptor gene. Research has identified in the IL-2 receptor gene a region that is essential for tax-mediated regulation of gene expression. It has also been suggested that the tax protein can transactivate the IL-2 gene itself, although less strongly than the IL-2 receptor gene. HTLV can also stimulate T-cell growth without actually infecting the cell. Exposure of T cells to inactivated HTLV-1 or to partially purified viral proteins can result in mitogenic stimulation and proliferation of the T cells in the absence of exogenous IL-2.

FIGURE 62-4 Pathogenesis of HTLV-induced leukemia.

Two features of infection indicate that HTLV-induced leukemogenesis must involve additional pathogenic events. The first is the long incubation period between infection with HTLV and the appearance of leukemic cells: it is thought that leukemia may take several decades to develop. The second feature is the monoclonal nature of the malignant cells in adult T-cell leukemia patients. Although HTLV integrates randomly into the cellular DNA of target T cells, the adult T-cell leukemia cells in an individual patient originate from a single HTLV-infected cell. Therefore, although HTLV may polyclonally stimulate the growth of T cells via either tax-mediated mechanisms or mitogenic stimulation, the outgrowth of leukemic cells presumably depends on as yet unidentified secondary events.

Infection with HTLV-1 also results in immunosuppression. In vitro, HTLV-1 infection not only alters helper T-cell function by causing increased proliferation, but also induces nonspecific polyclonal immunoglobulin production by B cells regardless of the type of antigen-presenting cell. Infection of cytotoxic T-cell clones leads to reduction or loss of cytotoxic function. Cells expressing HTLV-1 have abnormal expression of major histocompatibility complex (MHC) antigens (cell surface proteins essential for the functional interaction of immune cells). Inappropriate expression of these antigens on infected T-cells would impede or abolish their normal function. In addition, the HTLV-1 envelope and major histocompatibility complex proteins share certain antigenic determinants, so the immune system may be tricked into thinking that the HTLV-infected cell is self and need not be eliminated by the cytotoxic T-cell response.

The pathogenesis of the second major clinical manifestation of HTLV-1 infection, tropical spastic paraparesis, is unknown. Several animal retroviruses, particularly members of the lentivirus family, can infect the central nervous system and cause chronic neurologic disease in animals. The damage to the central nervous system is generally considered to result not from direct infection of neuronal cells but rather from the release of toxic factors by cells infected with HTLV-1. Another hypothesis is that neural tissue is damaged by the host immune system via an autoimmune mechanism. Like adult T-cell leukemia, tropical spastic paraparesis occurs with very low frequency in HTLV-1-infected individuals (~1-3%). Even rarer is the occurrence of both diseases in the same individual. Possible explanations for these observations include the existence of distinct adult T-cell leukemia-inducing strains and tropical spastic paraparesis-inducing strains of HTLV-1 or genetic determinants or host cell factors that contribute to variable disease expression.

Host Defenses

A hallmark of HTLV infection is its persistence. Once an individual is infected with HTLV, the virus remains in that individual for life. Specific antibodies are produced against a variety of HTLV proteins, including env, gag, and tax. Cytotoxic lymphocytes that recognize viral proteins have also been identified. In patients with tropical spastic paraparesis, antibody against HTLV-1 is synthesized intrathecally. The role of HTLV-specific antibodies and T-cell cytotoxicity in the prevention of new infections or in disease progression is unclear. It is also unclear whether interferon, which is produced by HTLV-1-infected cells in culture, is beneficial in HTLV infections. HTLV-1 is not lysed by human serum, although human complement can lyse other animal retroviruses. The persistence of HTLV infection in vivo indicates that completely effective immune responses do not occur naturally.

Epidemiology

HTLV-l infection occurs, with varying degrees of prevalence, in several regions of the world (Table 62-1). The highest prevalence is in southwestern Japan, where six endemic areas have been identified. The prevalence of anti-HTLV-1 antibodies in inhabitants of these areas ranges from 6 to 37 percent. The Caribbean is another region where HTLV-1 is endemic, with an overall seropositivity rate of 4 percent. Between 19 and 48 percent of family members of HTLV-1 -infected individuals in these two endemic areas are seropositive. In both regions, the prevalence of anti-HTLV-1 antibodies increases with age, from 2 percent in young children to 30 percent in adults in their mid-forties. Although fewer data are available for Africa, HTLV- 1 infection is also endemic there, with seropositivity rates ranging from 4 to 7 percent. HTLV infection is also found in defined populations (e.g., predominantly in intravenous drug users and homosexuals in developed countries such as the United States and Italy). It is not clear whether the HTLV type in these populations is HTLV-1 or HTLV-2.

The predominant modes of transmission of HTLV infection are by sexual contact, via contaminated blood or blood products, and from mother to child via breast milk. Studies in Japan have shown that between 48 and 82 percent of recipients of seropositive blood seroconverted. The rising prevalence with age of antibodies to HTLV-1 is consistent with sexual transmission of HTLV-1.

One unique characteristic of HTLV-1 infection is the extremely long incubation period between initial infection and the occurrence of disease. The incubation period for adult T-cell leukemia can range from years to decades and may be as long as 40 years; the incubation period for tropical spastic paraparesis is thought to be shorter but is still several years. Antibodies to HTLV-1 are found in the vast majority of patients with adult T-cell leukemia and tropical spastic paraparesis. However, the incidence of T-cell cancers or tropical spastic paraparesis in HTLV-1-infected individuals is extremely low. Studies on the incidence of adult T-cell leukemia in Japan estimate that fewer than 0.1 percent of HTLV-1-infected individuals develop adult T-cell leukemia each year. Adult T-cell leukemia is usually found in individuals older than 40 years, and mostly in individuals who were infected as infants.

Diagnosis

The determination of HTLV infection is usually made on the basis of a positive anti-HTLV antibody test. The enzyme-linked immunosorbent assay (ELISA) is the most common screening test for the presence of anti-HTLV antibodies. A positive assay is confirmed by direct radioimmunoprecipitation assays or Western immunoblots. Owing to immunogenic cross-reactivity, these tests do not distinguish between HTLV-1 and HTLV-2 infection. The polymerase chain reaction assay, which is based on the amplification of viral DNA segments, is used to distinguish between these two viruses. Since the interval between HTLV infection and the appearance of antibodies is not known, the polymerase chain reaction can also be used to diagnose HTLV infection in antibody-negative individuals.

The presence of modest leukocytosis with abnormal-appearing lymphocytes in an asymptomatic, HTLV-infected individual is indicative of pre-adult T-cell leukemia. Acute adult T-cell leukemia-lymphoma is characterized by the presence of pleomorphic neoplastic cells with mature T lymphocyte markers. Since adult T-cell leukemia cells express high levels of Tac antigen (a protein chain that is part of the IL-2 receptor), diagnosis should include staining for the Tac antigen. Clinical features may include dermal or epidermal infiltrates, hypercalcemia, osteolytic bone lesions, hepatosplenomegaly, and increased susceptibility to opportunistic infections. Chronic (smoldering) adult T-cell leukemia is diagnosed on the basis of skin lesions, low levels of circulating leukemic cells, and an absence of visceral involvement. Not all cases of adult T-cell leukemia are associated with HTLV-1 infection.

Tropical spastic paraparesis is characterized by a meningeal inflammatory process largely limited to the spinal cord, with progressive weakness in the lower extremities and sensory abnormalities. HTLV-1 can be isolated from the blood and cerebrospinal fluid of many patients with tropical spastic paraparesis.

Control

Control of HTLV infection and disease involves three approaches: therapy, education and public health, and vaccination (Table 62-2). Treatment of HTLV-1 infection and its sequelae is extremely difficult. Current treatment regimens for adult T cell leukemia involve combination chemotherapy protocols which have not significantly increased survival. The elimination of Tac-positive cells by anti-Tac antibodies coupled to toxin is being considered as an experimental therapy. Glucocorticoids have been used in treating tropical spastic paraparesis, but with only limited success. In vitro, azidothymidine (AZT) and dideoxycytidine (DDC) inhibit the infectivity of HTLV-1 in T4 cells and reduce the growth of HTLV-1-infected T cells. The effect of these agents on HTLV in vivo is unknown.

Although there is no treatment for HTLV infection, asymptomatic HTLV-infected individuals should be routinely screened for evidence of disease progression and counseled on ways to avoid spreading the infection. Methods to prevent the spread of HTLV infection include screening of blood products for antibodies to HTLV-1 and HTLV-2, educating people about prevention of HTLV transmission via sexual intercourse and the sharing of needles, and advising mothers to refrain from breast-feeding.

There is no vaccine against HTLV infection or disease. The development of vaccines for retrovirus infections in general has proven extremely difficult. Several experimental HTLV-1 vaccines have been designed that use the envelope portion of the virus as the immunogen. One HTLV-1 recombinant hybrid envelope product has been tested in cynomolgus monkeys. The monkeys that developed anti-HTLV-1 envelope antibody titers of 20 or higher were protected from challenge with HTLV-1 producing cells. Control monkeys and those with antibody titers below 20 developed signs of infection between 2 and 6 weeks after inoculation. Rabbits have been successfully immunized with vaccinia virus-env constructs. These products have not been tested in humans.

Because the development of adequate therapeutic approaches and vaccines may take many years, the most effective strategy to reduce the number of individuals with HTLV infection and disease is education .

Human Immunodeficiency Virus

Clinical Manifestations

Initial infection with HIV may result in an acute syndrome with symptoms including fever, sweats, myalgia or arthralgia, sore throat, lymphadenopathy, nausea, vomiting, diarrhea, headaches, and rash (Fig. 62-5). During the acute infection there may be a drop in the number of circulating T4 lymphocytes. It is not known what proportion of new HIV infections present with an acute syndrome and it is not clear that these symptoms are due to HIV, perse. Often other infectious agents are co-transmitted with HIV. Following initial infection, there is a long and variable asymptomatic period. During this period there may be a slow, progressive decline in T4-cell numbers and an increase in T8 cells. Some individuals may maintain relatively constant, normal levels of T4 cells during the asymptomatic period.

FIGURE 62-5 Pathogenesis of HIV infection.

Acquired immune deficiency syndrome (AIDS) is a late manifestation of infection with HIV (Fig. 62-6). AIDS is characterized by a marked depletion of T4 cells, resulting in a reversal of the T4/T8-cell ratio (normally 1.5:1 to 2.0:1) (Fig. 62-7). The progressive immune deficiency is accompanied by a wide range of life-threatening opportunistic infections and neoplasms, the most common being Pneumocystis carinii pneumonia and Kaposi sarcoma, respectively. Another manifestation of HIV infection is the AIDS-dementia complex (AIDS encephalopathy). This syndrome is characterized by neurologic abnormalities, including progressive dementia and peripheral neuropathy, and may occur in the absence of known opportunistic diseases.

 

FIGURE 62-6 Clinical manifestations and pathogenesis of HIV infection.

FIGURE 62-7 Pathogenic mechanisms of HIV infection.

Structure

The general structure of HIV is similar to that of HTLV (see above); the virus consists of an external lipid bilayer glycoprotein envelope (including envelope proteins gp 120 and gp 41), an internal protein core (proteins p15, p17, and p24), a viral RNA complexed with reverse transcriptase. The HIV genome is approximately 10 kilobases, which is larger than that of HTLV. In addition to the structural gag, pol, and env genes and the regulatory tat (analogous to HTLV tax) and rev (analogous to HTLV rex) genes, the HIV-l genome contains at least four regulatory genes (nef, vif, vpu, and vpr). HIV-2 does not have sequences for vpu, but does encode a novel gene, vpx, that is also found in the simian immunodeficiency virus.

Classification and Antigenic Types

HIV is classified as a retrovirus because it contains reverse transcriptase. It is a D-type virus in the Lentivirus family. Infection of cultured T4 cells with HIV usually results in cell death. Two major antigenic types (HIV-l and HIV-2) have been identified and are readily distinguished by differences in antibody reactivity to the envelope glycoprotein. The two HIV types share approximately 40 percent genetic identity. There is some disagreement about whether they are equally pathogenic. Both apparently cause AIDS, but some researchers think that HIV-2 is less efficient in causing disease.

Different isolates of HIV-l and HIV-2 exhibit considerable genomic variation and antigenic heterogeneity. The most variable regions of the genome are in the env gene. This type of variation is also observed in HIV isolates obtained from individuals over the course of their infection. HIV strains often display differences in replicative capacity and cytopathicity. The significance of these variations for the disease process is unclear.

Multiplication

The first step in HIV infection is the high-affinity binding of the gp 120 envelope glycoprotein to the CD4 receptor. The CD4 receptor is present on the surface of several cell types, including T4 cells and monocyte-macrophages. Following attachment to the CD4 receptor, HIV multiplication proceeds in a manner similar to that of HTLV (see above). In contrast to HTLV infection, however, the final step in HIV replication often involves the budding of massive numbers of virions from the cell surface, resulting in cell lysis.

Pathogenesis

The high-affinity binding of the HIV envelope glycoprotein to the CD4 receptor is a crucial step in the pathogenesis of HIV, since the major cell expressing CD4 is the T4 lymphocyte (often a helper cell). The T4 cell plays a central role in all aspects of immune system function, so that death or impairment of this cell results in widespread immune dysfunction. There are several potential ways that HIV can damage T4 cells. (1) HIV replication may kill T4 cells as a result of destruction of the cell membrane by viral proteins. (2) The production of large quantities of viral genetic material and proteins may interfere with normal cell metabolism. (3) HIV may also infect and destroy the progenitor cells that are responsible for the propagation of the lymphoid cell pool.

In addition to direct cytopathicity, HIV infection may indirectly cause T4-cell death. One mechanism of indirect cell killing may involve autoimmune phenomena in which anti-HIV immune responses are targeted to uninfected T4 cells that either have free envelope protein bound to their membrane or present processed envelope antigens. In addition, since both the HIV envelope protein and the class II major histocompatibility complex antigens bind to the CD4 receptor, their common binding sites may represent cross-reacting antigens. Therefore, anti-HIV antibodies may react with uninfected T4 cells that express class II major histocompatibility complex molecules. Also, it is very likely that anti-HIV immune effectors kill many infected cells.

HIV-infected individuals usually exhibit immune dysfunction prior to a depletion of their T4 cells. HIV may induce these functional abnormalities by a variety of pathways not necessarily involving a spreading infection of T4 cells. For example, by binding to the CD4 receptor, HIV or its envelope protein can interfere with the CD4-mediated monocyte-T-cell interactions that are necessary for antigen-specific responses. In addition, crosslinking of the CD4 molecules by the envelope protein may render the cell nonresponsive to subsequent antigenic stimulation.

During the long asymptomatic period of HIV infection, the virus resides predominantly in a latent state and at low-level chronic form within lymph node T4 cells and, to a lesser extent, within monocytes-macrophages. The mechanisms by which the virus is maintained in this relatively quiescent state are unclear. Similarly, little is known about the events that provoke activation. In vitro, a number of factors have been identified that are associated with the activation of HIV expression. These factors include antigens, mitogens, cotransfection, or coinfection of heterologous viruses, and cytokines. It is thought that HIV upregulation by these factors involves the induction of cellular proteins that bind to the promotor region of the HIV DNA and boost its expression.

The monocyte-macrophage is a target cell for HIV infection both in vivo and in vitro. Infection of these cells may occur through the CD4 receptor or via phagocytosis. In contrast to T4 cells, monocyte-macrophages appear to be resistant to cell lysis. Moreover, the virus can replicate intracellularly in monocyte-macrophages, with virions budding into intracytoplasmic vesicles. As a result, viral antigens may not be expressed on the cell surface, potentially enabling the monocyte-macrophages to escape immune surveillance and to transport the infection to other organ systems, particularly the lungs and brain. Although the mechanisms by which HIV induces neuropsychiatric abnormalities are unknown, the macrophage is thought to play an important role. HIV infection in the brain appears to be largely restricted to macrophages, which may indirectly damage neuronal tissue by releasing neurotoxic factors or factors that induce inflammation. HIV may also interfere with the binding of neurotropic factors to their receptors on neurons. Another mechanism of HIV-induced neuropathology may involve autoimmune phenomena.

Clearly, some fraction of the neurologic abnormalities in HIV-induced disease is due to the wide range of opportunistic infections and tumors that afflict AIDS patients. Although other retroviruses can directly cause cancers, HIV does not transform cells in vivo or in vitro. The multitude of tumors found in AIDS patients may result from widespread immunosuppression and/or the induction of factors that induce cellular proliferation, as may be the case with Kaposi sarcoma. HIV proviral DNA has not been found in any of these tumors.

Host Defenses

A broad range of immune responses to HIV has been observed in all stages of HIV infection. Antibodies to both the structural and regulatory proteins of HIV generally appear several weeks to months after initial infection. During the progression of HIV infection, antibody titers to the p24 core antigen decline, while antibody titers to the envelope protein remain relatively constant. HIV-infected individuals produce virus-neutralizing antibodies, and these decline in late stages of AIDS. Antibodies that mediate antibody-dependent cellular cytotoxicity (ADCC) have also been observed in the sera of AIDS patients. Cell-mediated responses to HIV include T-cell proliferation and cytotoxicity mediated by both T-cells and natural killer (NK) cells. Interferon has been found in some AIDS patients, but coinfecting opportunistic viruses may be responsible. Since it is thought that HIV ultimately causes a slowly progressive, persistent infection in all infected individuals, the role of immune responses in preventing HIV-induced disease is unclear. The emergence of genetic variants of HIV in vivo during the disease may be one way that HIV evades both humoral and cellular immune responses.

Epidemiology

HIV-1 infection has been reported throughout the world in both developed and developing countries. In the United States and other developed countries, HIV infection is found predominantly in homosexual and bisexual men and intravenous drug users. Hemophiliacs, transfusion recipients, sexual partners of infected persons, and infants born to infected mothers are also at high risk. In many parts of Africa and the Caribbean, HIV-1 is found predominantly in heterosexuals, transfusion recipients, and infants born to infected mothers. HIV-2 is found predominantly in West Africa, Portugal, and Brazil. Both HIV-1 and HIV-2 are spread through sexual contact, exposure to contaminated blood or blood products, and perinatally from an infected mother to her offspring. It is estimated that as of 1990, between 800,000 and 1.3 million individuals in the United States are infected with HIV. It is not known what proportion of these individuals will develop AIDS. The median incubation period is thought to be approximately 10 years.

Diagnosis

HIV infection is determined by the presence of HIV-specific antibodies, which usually occur weeks after infection. An HIV enzyme-linked immunosorbent assay is commonly used as the initial screening assay, but the current FDA-approved assays may miss many early-infected people. There are sporadic reports of individuals in whom anti-HIV antibodies could not be detected for several years after infection. An individual is considered to be infected with HIV if a positive assay is confirmed by a positive Western blot or a similar, more specific assay. Diagnosis of HIV-2 infections requires HIV-2-specific assays. Detection of virus nucleic acid by the polymerase chain reaction may be used to diagnose HIV infection in antibody-negative individuals.

AIDS is diagnosed predominantly on the basis of opportunistic infections or cancers indicative of a T4 cell defect in the absence of a known cause of immunodeficiency. Neuropsychiatric manifestations are also indicative of AIDS. Although neurologic complications may arise from opportunistic infections of the central nervous system, tumors, and vascular problems, AIDS may present in the absence of these conditions as an HIV encephalopathy that is diagnosed by clinical findings of cognitive or motor dysfunction that interferes with daily activities. AIDS may also be diagnosed in an HIV-infected individual on the basis of unexplained weight loss with either chronic diarrhea or chronic weakness and fever, known as HIV wasting syndrome.

Control

Strategies for the control of HIV infection and disease are similar to those for HTLV infection: therapy, education and public health, and vaccination. Therapeutic categories for HIV include antiviral therapies, immune modulators, and therapies to treat and prevent opportunistic diseases. Serveral nucleoside analogs, are currently the only anti-HIV agents that have been shown to significantly improve survival in AIDS patients. Additional nucleoside analogs, as well as other potential antiviral agents, are undergoing clinical trials. A combination of antiviral agents may prove to be the most effective treatment for AIDS since it may allow the use of lower doses of drugs to minimize toxicity. Owing to the persistence of HIV infection, lifelong antiviral treatment will probably be necessary. In addition to the antiviral agents, several immunomodulatory agents are undergoing clinical trials. With regard to treatment of opportunistic infections, the use of aerosolized pentamidine has brought about a major advance in preventing P carinii pneumonia. Since P carinii pneumonia is the most common cause of death in AIDS patients, aerosolized pentamidine can greatly improve survival in these patients.

Intensive educational efforts are needed to prevent transmission of HIV by sexual intercourse, intravenous drug use, and exposure to blood or blood products. Sexual abstinence, monogamous sexual relationships with uninfected partners, the use of condoms, and abstinence from intravenous drug use or the use of clean needles are necessary for the control of HIV through sexual intercourse and intravenous drug use. Transmission of HIV can be prevented in health care workers and other medical settings by the application of universal precautions (i.e., treating all blood as potentially infected by HIV). HIV infection via blood products is prevented by screening donated blood for anti-HIV antibodies and by excluding donors at high risk of HIV infection. HIV-infected women of childbearing age should be counseled about the 20 to 30 percent risk of transmitting HIV infection perinatally.

One important obstacle to the development of a vaccine against HIV is that the relation of immunity to HIV infection is not understood. The experience gained in developing a vaccine against an animal retrovirus, feline leukemia virus, suggests that immunity to the viral envelope protein is crucial for prevention of infection. Efforts have therefore been made to develop HIV vaccines that use the precursor HIV glycoprotein (gpl60) or its external subunit (gpl20) as immunogens. These envelope subunit vaccines are being evaluated in phase 1 clinical trials for safety and immunogenicity in uninfected volunteers. In addition, phase 1 trials are being conducted with inactivated HIV preparations in symptomatic HIV-infected individuals. Although the usual use of a vaccine is to prevent infection with a microorganism, it is hoped that an HIV vaccine will also prevent the development of AIDS in individuals already infected with HIV.

REFERENCES

Blattner WA: Retroviruses. In Evan AS (ed): Viral Infections of Humans. Plenum, New York, 1989

Bour S, Geleziunas R, Wainberg MA: The human immunodeficiency virus type 1 (HIV-1) CD4 receptor and its central role in promotion of HIV-1 infection. Microbiol. Rev. 49(1):63-93, 1995

Centers for Disease Control: Revision of the CDC surveillance case definition for acquired immunodeficiency syndrome. MMWR 36:3S, 1987

Ehrlich GD, Poiesz BJ: Clinical and molecular parameters of HTLV-l infection. Clin Lab Med 8:65, 1988

Lower R, Boller K, Hasenmaier B, Kormacher C, Muller-Lantzsch N, Kurth R: Identification of human endogenous retroviruses with complex mRNA expression and particle formation. Proc. Natl. Acad of Science USA. 90:4480-4484, 1993

Nerurkar LS, Wong-Staal F, Callo RC: Human retroviruses, leukemia, and AIDS. In

Henderson ES, Lister TA (ed): Leukemia. 5th Ed. WB Saunders, Philadelphia, 1990

Rosenblatt JD, Chen ISY, Wachsman W: Infection with HTLV-I and HTLV-II: evolving concepts. Semin Hematol 25:230, 1988

Rosenberg ZF, Fauci AS: The immunopathogenesis of HIV infection. Adv Immunol 47:377, 1989

Williams LM, Cloyd MW: Polymorphic human gene(s) determines differential susceptibility of CD4 lymphocytes to infection by certain HIV-1 isolates. Virology 184:723-728, 1991