AIDS:

PART

I

immunodeficiency syndrome ABSTRACT .-Acquired (AIDS) is caused by infection with a pathogenic human retrovhus known as human immunodeficiency virus (HIV). Approximately 1 million people are currently infected with HIV in the United States, with 8 to 10 miilion infected individuals worldwide. The virus is transmitted predominantly through genital sexual contact, although orogenital spread has been rarely reported. Heterosexual transmission has been most corn&m in the Third World, whereas male homosexual transmission has predominated in the United States and western Europe. Transmission through homosexual contact has been steadily declinmg over the past 5 years as transmission through illicit intravenous drug use and promiscuous unprotected heterosexual activity has increased. Sexually transmitted diseases that cause hflammatory or ulcerative lesions of the genital tract act as important cofactors in increasing the risk of transmission through sexual contact. Perhratal transmission of HIV occurs in approximately 30% of infants born to infected mothers. Transmission to infants through breast-feeding has also been documented. Health care workers have been infected with HIV through accidental high-risk percutaneous or mucous membrane exposures, albeit at a low transmission rate of 0.3%. Infection of patients by infected health care professionals is a rare event, having been reported only once in 10 years of the epidemic. Infection with HIV results in a chronic lifelong infection. The major targets for HIV are CD4 + T-helper lymphocytes and cells of monocyte/macrophage lineage. Infection of the T-helper lymphocyte ultimately results hr the death of the cell. Over time (measured in years), a progressive destruction of the T-helper lymphocyte population occurs, which results in profound immune suppression. Infection of monocytes/macroDM, September

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phages is not tidal, but these cells do have functional alterations as a result of the infection, which may contribute to the immune deficiency. In addition, chronically infected tissue macrophages may act as an important reservoir for HIV, particularly in the central nervous system. Infection of the T-helper lymphoand is cytes monocytes/macrophages mediated through attachment of HIV through a specific binding interaction between CD4 expressed in the plasma membrane of these cells and a surface glycoprotein on the virus, gp120. Once the virus nucleocapsid (core particle) enters the cytoplasm of the target cell, the viral RNA genome is reverse transcribed by a reverse transcriptase enzyme into proviral DNA. This proviral DNA migrates into the nucleus where it integrates into the host cellular genome, which results in a chronically infected cell. Transcription of cellular DNA then results in the production of virion messenger RNA and genomic RNA, which ultimately leads to the production of new infectious viral particles. The diagnosis of HIV infection is based on both cllnical and laboratory indicators. Clinical manifestations of HIV infection range from an acute, selMmited, nonspecific, viral-like syndrome, through a prolonged asymptomatic phase, and then ultimately to signs and symptoms suggestive of profound immune suppression. Abnormalities of the skin and mucous membranes may be early clinical indicators of HIV infection. A history of high-risk behavior or sexually transmitted disease(s) should prompt an investigation for HIV infection. Laboratory diagnosis depends on the demonstration of viral-specific antibodies or direct evidence of viral infection. Serologic procedures for the detection of HIV antibodies and antigens have been developed. Direct viral culture for HIV is available predominantly through research laboratories. Newer molecular methods of diagnosis with enhanced sensitlvlty, such as the polymerase chain reaction, will soon be commercially available for routine diagnostic virology laboratories.

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IN BRIEF HUMAN

IMMUNODEFICIENW

VIRUS

Human immunodeficiency vir~~ses (HTV-1 and HTV-21 arti membws subfamily of the Retroviridae and are pathogenic of the Lentivirinae for humans. Infection with either HIV-l or HIV-Z ultimately results in the development of a progressive immune deficiency disorder known as the acquired immunodcficiency syndrome (AIDS). Infection with HIV-1 predominates worldwide. HIV infection is highly specific for cells expressing the CD4 surface antigen (i.e., T-helper lymphocytes [T,] and cells of monocyte/macrophage lineage). The HIV genome codes for a number of structural and regulatory pmteins important for replication of the virus. An envelope glycoprotein of HIV, pg120, binds specifical1.y to the CD4 antigen complex and is responsible for the specificity of the infection for the above cell types. Reverse transcriptase, an enzyme unique to the retroviruses, is critical to viral replication and is an important target for antiretroviral therapy. This enzyme mediates the conversion of &e HIV genome from single-stranded RNA into DNA (provirus), which integrates into the host cellular genome and results in chronic lifelong infection. The replicative cycle of HIV has been well characterized, starting with attachment of the virus to a CD4+ cell and ending with the “budding” of a mature virion from the plasma membrane of an infected T, lymphocyte. Transcription errors by reverse transcriptasc arc common during HIV replication, and this results in frequent mutations and a marked heterogenicity of different viral isolates, not only from person to person but also within the same host over time. This presents difficult challenges to the development of protective vaccines and effective long-term antiviral therapy. PATHOGENESIS Infection with HIV is cytopathic for T, lymphocytes and ultimately results in the depletion of this critical cell population. Decreasing numbers of CD4+ lymphocytes over time correlate with advancing immune suppression. T,-l.ymp hoc.yte counts less than 200/mm3 are associated with an increased risk of an AIDS-defining condition developing. As of April 1992, the Centers for Disease Control proposes to include a T,-lymphocyte count less than ZOO/mm” as an AIDS-deof the clinical slage of lhe palienl. fining event, irrespective The mechanism of the destruction of the CD4+ lymphocyte population is yet to be identified. Early in infection, as few as 1 in 1000 to 1 in 10,000 circulating T,, lymphocytes are infected with HIV. As the disease progresses, the T,-lymphocyte population not only deDM,

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creases, but the percentage of infected cells increases at least IO-fold and correlates with markers of increased viral replication. This suggests a direct relationship between HIV replication and accelerated destruction of T, lymphocytes. Cells of monocyte/macrophage lineage am also infected by HIV, but the infection is not cytopathic. Ongoing replication of HIV in these cells and their migration into virtually every tissue in the body may result in the monocyte/macrophage being an important vehicle for dissemination of and a reservoir for HIV. Although not cytopathic for monocytes/macrophages, HIV infection results in functional abnormalities of these cells that may contribute to the progressive immune deficiency. NATURAL

HISTORY

Infection with HIV is a biologic spectrum ranging from no symptoms to those of advanced immune suppression with opportunistic infections and malignancies. Acute infection with HIV may result in a self-limited, nonspecific, febrile viral syndrome that may be confused with influenza, mononucleosis, or rubella. The incubation period for this acute retroviral syndrome can range from 2 to 10 weeks, and the acute illness can last from 4 to 14 days. After recovery, patients become asymptomatic and can remain as such for years. The median incubation period from time of HIV infection until AIDS develops is almost 11 years. The decline in T, lymphocytes during the course of the infection is in the range of 50 to 100 cells/mm” per year, with accelerated rates of decline observed as the disease progresses from the asymptomatic to symptomatic stages. Progression to AIDS is unusual in the first 2 years of infection but ranges from 5% to 8% per year thereafter. The only recognized host cofactor for progression of disease is age. The immune response of the host plays an important role in the natural history of the progression of the disease. Early in the infection, after the maturation of the immune response, HIV replication is down-regulated to a low persistent level. Over time, this capacity of the immune system wanes, viral replication increases with an antecedent accelerated destruction of T, lymphocytes, and global immune dysfunction supersedes. The factor(s) responsible for this natural history is not completely understood. TRANSMISSION HIV is transmitted mucous membrane ucts, and perinatally. saliva, cerebrospinal 640

predominantly by sexual contact, parcnteral or exposure to contaminated blood or blood prodThe virus has been detected in blood, semen, fluid, breast milk, and amniotic fluid. Blood or DM, September

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plasma, sexual secretions, and breast milk are the most important body fluids in relationship to the spread of the virus. Male homoscxual activity has been the most common mode of transmission of HIV in Western societies, with heterosexual transmission predominating in Third World countries. The risk of HIV acquisition appears to be greater with receptive anal intercourse and from male to female. Orosexual contact has rarely been implicated in transmission. Heterosexual transmission of HIV has been increasing in the United States over the past 5 years, and this trend is projected to continue. Concomitant sexually transmitted diseases that cause ulcerative or inflammatory lesions of the genital tract increase the risk of transmission of the virus. Intravenous drug users (MXJ) are an important reservoir of Hw-infected individuals and constitute the largest pool of HIV-infected heterosexuals in both Europe and the United States. More than 50% of pediatric AIDS cases have been linked to an IVDU mother, with an additional 20% related to an IVDU sexual partner of the mother. Perinatal transmission can occur transplacentally, at birth, or through ingestion of infected breast milk. Ap roximately 30% of infants born to infected mothers are themselv s s infected. Other potential routes of transmission, such as transfusion, transplantation, or in the health care setting, currently account for less than 1% of all HIV infections in the United States. WORLDWIDE

EPIDEMIOLOGY

The World Health Organization estimates that 8 to 10 million people are currently infected with HIV and that 40 million people will be infected worldwide by the year 2000. More than 200,000 cases of AIDS have been reported in the United States as of the end of 1991. This is estimated to increase to 340,000 by the end of 1993. AIDS was projected to be the second leading cause of death for men between the ages of 25 and 44 years and among the top five causes of death in 15- to 44-year-old women in 1991. Newly reported cases of AIDS are increasing most rapidly among heterosexuals, among teenagers, among blacks and Hispanics, in the Southeast, and in cities with populations less than 100,000. These represent major demographic shifts in the epidemic. DIAGNOSIS The diagnosis of HIV infection can be established only by the demonstration of viral-specific antibodies, antigens, or nucleic acids or by the direct isolation of HIV from the blood of a patient. Laboratory evaluation of a patient for HIV infection should be considered if patients have a history of high-risk behaviors or exposures to HIV. DM,

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These include male homosexual activity, a history of sexual promiscuity, the use of injectable illicil drugs, a history of any sexually transmitted disease (in particular, syphilis, gonorrhea, or genital herpes), sexual contact with an IVDU or other individual at risk for HIV infection, transfusion of blood before April 1985, and transfusion of blood clotting factors before January 1987. In addition, associated signs and symptoms of HIV infection should alert the physician to inquire about specific risk exposures and initiate testing for HIV. These include the occurrence of an unexplained febrile, viral-like syndrome compatible with the acute retroviral syndrome; herpes zoster (shingles) in an individual less than 40 to 50 years of age; oral or unexplained vaginal candidiasis; generalized lymphadenopathy; severe gingival disease; human papillomavirus infection involving the genital tract, especially in women with cervical intraepithelial neoplasia; and certain common dermatologic conditions if they are unusually severe or poorly responsive LO therapy. Also, unexplained routine laboratory abnormalities such as anemia, leukopenia, lymphocytopenia, thrombocytopenia, proteinuria, hypocholesterolemia, hypertriglyceridemia, or hyperglobulinemia should stimulate the physician to at least obtain a history for any risk factors for HIV infection and to consider HIV testing.

SPECIFIC

HIV TESTING

Enzyme immunoassay (EIA) is the most widely used method for screening large numbers of individuals either for antibody to HIV or for direct detection of HIV antigen. The sensitivity and specificity of commercially available antibody kits approved by the Food and Drug Administration are generally in excess of 99%. False-positive results occur, particularly when testing low-risk populations. The Western blot assay is used to confirm the results of EL4 testing. The Western blot test allows for the direct visualization of antibodies to specific HIV proteins. A positive Weslern blot test depends on the presence of two or three antibody bands to the proteins of the env, gag, and/or pal genes of HIV. The absence of bands is considered negative, whereas the presence of one or two bands is considered indeterminate. The management of the patient with a repeatedly reactive EL4 and an indeterminate Western blot test result is problematic. Patients with no risk faclors for HIV generally lose the antibody bands on repeat testing or remain stable. Patients with high-risk behavior or other risk factors for HIV infection usually have additional evidence of HIV infection on repeat testing, with the development of additional bands on the Western blot assay. Therefore in counseling the low-risk or no-risk patient concerning an indeterminate Western blot test result, the physician can be encouraging. Follow-up EIA 642

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and Western blot testing for all patients is still indicated at 3 or 6 months, or both, as is counseling regarding safe sex practices during the monitoring period. Additional HIV tests include the HIV antigen EIA, HIV culture, and the polymerase chain reaction. The HIV antigen EL4 is useful in diagnosing the acute retroviral syndrome before the development of HIV antibodies, in monitoring the clinical stage of the biologic spectrum of HIV infection, and in evaluating and monitoring antiviral therapy. Culture of HIV from the blood, body fluids, and tissues of patients is the most specific way to establish a diagnosis of HIV infection. Unfortunately, the procedure is of limited sensitivity during the early stages of the infection; the coculture technique is expensive and time consuming; and the procedure is limited primarily to research laboratories. Although culture of HIV in vitro is of limited usefulness clinically, it has been of value in helping to understand the immunobiologv of HIV infection and also in monitoring therapies directed against the virus. The polymerase chain reaction is a new and exciting technology that allows for the direct detection of HIV DNA or RNA sequences in the tissues or body fluids*of infected patients. This method has a remarkable sensitivity, which is a result of a series of reactions (cycles) that result in the doubling of the number of copies of the HIV nucleic acid present in a particular patient sample. Repeated cycles exponentially increase the amount of nucleic acid in the reaction mixture so that after 20 to 35 cycles, approximately a 1 million-fold increase in the number of viral nucleic acid copies is seen. HIV polymerase chain reaction technology has been used most widely in the diagnosis of neonatal HIV infection. Other potential uses include individuals with indeterminate Western blot assay results; seronegative, high-risk individuals who may have delayed development of antibodies after HIV infection; sexual partners of known HIV-infected people; and the monitoring of individuals with high-risk, nonsexual exposures to the blood and body fluids of HIV-infected patients (Le., health care workers). In addition, quantitative polymerase chain reaction techniques have been useful in monitoring the efficacy of antiviral chemotherapy.

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Harold A. Kessler, M.D., is an Associate Professor of Medicine and Immunology/Microbiology at Rush Medical College/Rush-Presbyterian-St. Luke’s Medical Center and Director of the HIV Treatment Program and the Coordinated AIDS Resource Center at Rush. He completed his internal medicine residency and infectious disease training at Rush, followed by a Research Fellowship in Jlirology at the London School of Hygiene and Tropical Medicine. Dr. Kessler specializes in infectious diseases, with a focus on viral infections. He has been active in the treatment of patients with HIV infection and clinical and laboratory research relpted to HIVsince 1983.

Joseph Bick, M.D., is an Instructor in Medicine at Rush Medical College/Rush-Presbyterian-St. Luke’s Medical Center. He completed his residency in internal medicine at Rush and is currently a Fellow in the Section of Znfectious Disease. Dr. Bick is actively involved in his fellowship in the clinical and laboratory research program in HIV infection at Rush. 644

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John C. Pottage, Jr., M.D., is an Assistant Professor uf Medicine and Immunology/Microbiology at Rush Medical College/Rush-Presbyterian-St. Luke’s Medical Center. He is a member of the Section of Infectious Disease. Dr. Pottage completed his residency in internal medicine and clinical fellowship in infectious disease at Rush. He spent a third year of fellowship in laboratory research on studies related to hepatitis and herpes simplex infections. Dr. Pottage focuses his clinical research on the development of new therapies for HIV infection and, in addition, on opportunistic fungal infections associated with immunocompromised patients.

Constance A. Benson, M.D., is an Assistant Professor of Medicine at Rush Medical College/Rush-Presbyterian-St. Luke’s Medical Center and is a member of the Section of Infectious Disease. She completed her internal medicine residency and infectious disease training at Rush. She currently serves as the Medical Director of the inpatient AIDS care cluster at Rush. Dr. Benson has focused her clinical research eflorts on the prophylwis and treatment of Pneumocystis carinii pneumonia, cytomegalovirus retiniMycobacterium avium complex distis, and disseminated ease associated with advanced HIV infection. DM,

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64.5

AIDS:

PART

I

INTRODUCTION As the human immunodeficiency virus (HIV) epidemic enters its second decade, we continue to be faced with challenges that appear to be insurmountable. The worldwide spread of HIV infection continues at an alarming rate, with the World Health Organization (WHO) now predicting that 40 million people will be infected by the year 2000.* This is in contrast to the approximately 10 million people currently infected in the world and 1 million estimated to be infected in the United States.’ Most of these new infections will occur in developing Third World countries, especially those areas that have high rates of sexually transmitted diseases, known to be important cofactors in the transmission of HIV.2-4 The prospects for the control of the spread of the epidemic, either thmugh changes in sexual and other high-risk behaviors or through the development of an effective vaccine, are not encouraging for the next 10 years. Health care providers in the United States must not assume that because the epidemic appears to have peaked in the mid-1980s, HIV infection and disease will not therefore continue to have a serious impact on our health care delivery system. The numbers of individuals infected will continue to increase, as will changes in the populations affected by the epidemic. Although homosexual and bisexual men constituted most of the individuals in whom the acquired immunodeficiency syndrome (AIDS) was diagnosed in the early 198Os, by 1993 only 49% of newly diagnosed cases of AIDS will be in this risk group, with 28% occurring in heterosexuals who use intravenous drugs and 12% occurring through heterosexual transmission.5 AIDS is currently the ninth leading cause of death in the United States but is second in mean number of potential years of life lost per death.6 Only infant mortality has a greater economic impact on our society as measured by this indicator, which is a direct reflection of the age of the populations most affected by HIV infection. In addition, the epidemic will continue to shift its demographics from predominantly white middle-class men to socially disadvantaged African-American and Hispanic women and children. Is there any good news in this evolving story? Clearly there is. In 646

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terms of reducing the transmission of HIV, we know that education can have a positive influence in changing high-risk sexual behaviors, as has been evidenced in the homosexual and bisexual population. Drugs for the treatment of HIV infection and the opportunistic infections associated with advanced HIV disease have decreased the rate of progression of HIV infection to AIDS and have significantly increased the life expectancy of persons with AIDS.7 Drug development is progressing at a rapid pace, with at least four new drugs having been approved in the past 18 months: fluconazole, ganciclovir, foscarnet, and most recently, didanosine, a second HIV nucleoside reverse transcriptase inhibitor, which will complement zidovudine, the first approved compound in this class of drugs for the treatment of HIV infection. Does the primary care physician have a role in the management of people with HIV infection, or do the perceived complexities of the care of these patients relegate them to the care of the “AIDS treater” or subspecialists in infectious disease or hematology-oncology? Based on our understanding of the natural history of HIV infection, the primary care physician can be the most important chregiver for the major portion of time an individual is infected with HIV. Although HIV infection is currently not curable, it is a chronic, manageable medical condition, and treatment, if initiated early in the asymptomatic stages of the infection, can have a significant benefit on the overall course of the illness.‘, ’ In this way, HIV infection is similar to hypertension, diabetes, or chronic congestive heart failure, all diseases that the primary care provider treats on a daily basis, In this issue of Disease-a-Month we will focus on those aspects of HIV infection that we hope will provide both the generalist and the specialist with basic information for the care of HIV-infected individuals, particularly those in the earlier stages of the spectrum of the HIV disease process.

HUMAN

IMMUNODEFICIENCY

VIRUS

Human immunodeficiency viruses are members of the Lentivirinae subfamily of the Retroviridae.‘“’ l1 Two other Retroviricfae subfamilies have been identified: the Spumavirinae and the Oncovirinae. Spumavirinae, or “foamy viruses,” have been isolated from human, simian, and other mammalian species. l’hey cause vacuolization in infected cells, result in persistent infection in vitro, and have not been associated with any human disease. The Oncovirinae consist of five groups of tumor-inducing viruses. Of these, only the human T-cell leukemia/lymphoma viruses, HTLV-I and HTLV-II, are known to be associated with any human diseases. The Lentivirinae include two viruses that infect humans: HIV types DM,

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Lentivirinae include the simian 1 and 2 (HIV-l, HIVY~).~ Nonhuman and feline immunodeficiency viruses, the bovine leukemia virus, the visna-maedi virus of sheep, the equine infectious anemia virus, and the caprine arthritis-encephalitis virus. kntivirinae differ from Spumavirinae and Oncovirinae in that they induce cytopathic changes in infected cel1s.l” Of the two lentiviruses that infect humans, HIV-l predominates worldwide. HIV-2 has PI+ marily been identified in western Africa, with importation predominantly into western Europe.‘4 In recent years, however, several cases of HIV-2 infection have been identified in the United States, primarily in patients of western African origin.‘” Interestingly, although HIV-2 and simian immunodeficiency virus share approximately 75% homology in nucleotide sequence, each is only roughly 40% homologous to HIV-l.'" All three viruses, however, can cause an AIDS-like illness in their respective species.‘7 HIV-l and HIV-2 are similar to HTLV-I/II in that they also infect predominantly T lymphocytes that express a surface antigen known as CD4. Additionally, like HTLV, they establish latent infections. However, HIV is more similar genetically, morphologically, and immunologically to the Lentivirinae than to the Oncovirinae to which HTLV belongs. Similar to other retroviruses, HIV is approximately 100 nm in diameter and has a single-stranded RNA genome. The virion has a cylindric core (nucleocapsid), within which is contained the virion RNA and viral enzymes, including a proteinase (protease), integrase, and reverse transcriptase. Surrounding the core is a lipid envelope derived from the infected host cell, which has viral-encoded glycoproteins associated with it (Figure 1). The genome structure of HIV is similar to that of other retroviruses, with a long terminal repeat sequence at each end that is important for expression of HIV genes. Additionally, there are three major structural genes: gag, pal, and env. The gag region codes for a precursor polyprotein (~551, which is cleaved to form a matrix protein (~171, capsid protein (~241, and nucleic acid binding pro’tein (~7). The pal region encodes a precursor protein that is cleaved to yield three enzymes: endonuclease (~311, proteinase (~101, and reverse transcriptase (~66151). The env gene region codes for an 850-amino acid glycosylated precursor protein (gp160). Gpl60 is subsequently cleaved by an endopeptidase to a 41,000~Mw transmembrane glycoprotein (gp41) and a I~O,OOO-Mw extracellular surface membrane glycoprotein @31201. Although HIV-l and HIV-2 are genetically similar in the gag and pal regions, they are quite distinct in the env region and in their regulatory proteins. Variation in the env region results in substantial differences in the envelope glycoproteins. These heterogeneities lead to distinct immune

94s

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gag

HIV genome: -7Y-

I?“”

PO/

P55

r-+---l ~15 p24

P6w51

P31

p17

gp:60 gp41

gp120

Major HIV-1 Proteins

in Western Blot Analysis

gp160

- env precursor

gpl20

-Envelope

p66 - Reverse

trenscriptase

p55 - Core precursor p51 - Reverse gp41

glycoprotein

glycoproteln

protein

transcriptase

- Transmembrane

glycoproteln

p31 - Endonucleaee ~24 - Major core protein p17

Core protein

FIG 1. Baw

structural

characteristics

of HIV-l.

responses to infection, necessitating specific HIV-l and HIV-2 immunoassays or Western blot procedures for serologic diagnosis.16,18 HIV genome structure departs from that of other retroviruses by the presence of six additional regulatory genes. These include tat, rev, ne$ v$ vpr, and vu. The M (transactivator) gene encodes for an 866amino acid 14-kD nuclear protein. This protein is involved in enhancing viral replication, as evidenced by tat-negative mutants that are replication incompetent.” This regulatory protein is a potent transactivator of all viral genes and may be an important target for antiretroviral therapy. At least one drug capable of inhibiting tat has already been described,” and human clinical trials may soon begin. The rev (regulator of viral proteins) gene encodes a 19&D intranuclear protein, which is important in the transition from early regulatory gene expression to the expression of structural proteins.‘l Similar to tat, it is also critical for competent replication. The ra$ (negative regulator) gene product, located in the cytoplasm, is a 27-W protein that previously had been thought to function as a downregulator of viral expression and replication.” Subsequent studies have failed to corroborate these early observations, and in fact the true function of net-protein is yet to be identified. Similar to nef; the vpr gene product is not essential for viral replication, and its specific contribution to the viral life cycle remains unidentified. Vif (viral infectivity factor) and vpu encode proteins that are important in the

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later maturation stages of HIV. Mutants lacking vifreplicate to normal levels and are infectious through cell-to-cell contact but are deficient in terms of cell-free virus transmissionz3 Vif may, therefore, be involved in regulation of final maturation of the virus at the host The vpu gene product protein is also important in cell membrane.24 final maturation through promotion of release of mature virions from the surface of an infected cell.‘l Unique to HIV has been the marked heterogeneity of viral isolates. This heterogeneity exists on multiple levels: continent to continent, infected individual to individual, and even within the same infected can be hastened under the selective preshost.“’ Genetic divergence sure of antiviral therapy but also occurs without such stimulus. Some of this variability derives from the high spontaneous mutation rate found in retroviruses, which may be related to the frequent error rate of the reverse transcriptase enzyme.‘” Variability of viral genomes is more pronounced in the env region and the nefgene than in the gag, pal, vif; tat, rev, or long terminal repeat genes. Divergence of amino acid sequences of over 30% has been detected in viral envelope proteins.14’27 Notably, isolates from Europe and America are generally more similar to each other than to African isolates, suggesting that the virus may have existed longer genetic heterogeneity has far-reaching ramin Africa.z7 This marked ifications. These include differing tissue and cell tropisms, variations in pathogenesis, contrasting viral virulences, disparate responses to therapy, and potential challenges to creating a broadly cross-reactive protective vaccine. Crucial to the development of effective therapies for HIV infection is an understanding of the replicative cycle of the virus (Figure 2). The initial event in HIV infection involves attachment of the virus to a susceptible cell. This process is receptor mediated via HIV gp1.20 binding to the CD4 membrane antigen complex of a susceptible ce11.28-3o This receptor is found on most T-helper/inducer (T,) lymphocytes and cells of monocyte/macrophage lineage, and appears to be the major receptor involved in HIV infection. Proof for this includes the following: (1) lymphocytes expressing CD4 can support viral replication in vitro; (2) antibodies to CD4 can block infection of susceptible cells; 13) fusion of CD4 to cells that are not usually susceptible to HIV results in productive infection; and (4) addition of soluble CD4 to cocultures of HIV and CD4-positive cells results in decreased viral replication in vitro because of competitive binding of the soluble CD4 to viral g~120.~~’ ‘*, 32 Other cell surface receptors for HIV infection have been postulated. These include Fc receptors on monocyte/macrophages, as well as yet-to-be-described receptors on astrocytes, Langerhans cells, glial cells, and colonic epithelium.33-35 The complement re650

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1 Attachment 2 Uncoating 3 Reverse Transcription

4 Circularization 5 Integration 6 Transcription

7 Translation 8 Core Particle Assembly 9 Final Assembly I Budding

FIG 2. HIV-1 replication drugs.

in T-helper

lymphocyte

and points

of potential

inhibitlon

by antiretroviral 5.

ceptor CR2 has also been recently shown to act as an alternative binding molecule for HIV gp120.“” After attachment, the lipid membrane of the virus fuses with that of the target cell, allowing entry of the core particle into the host cell cytoplasm. This process is possibly a function of HIV gp41 and may be similar to the fusion process described for Paramy)toviruses.37 Once the virus core has entered the host cell cytoplasm, it is uncoated, allowing transcription of virion RNA. Transcription is mediated by a reverse transcriptase enzyme unique to Retroviridae. This enzyme allows the transcription of virion RNA into a complementary strand of DNA (cDNA). The cDNA becomes double stranded via cellular enzymes and subsequently circularizes before migration into the cell nucleus. Once intranuclear, the proviral DNA integrates into the host cellular genome using a viral endonuclease (integrase). After integration, the cell is chronically infected. Integration results in either a latently infected cell, in which no viral RNA is produced, or a productive infection, in which mature virions are synthesized and released. What determines whether a cell is latently or productively infected is not yet understood. This variability may depend on the particular HIV strain involved, the type of cell infected, and the degree of immune deficiency of the host. A variety of host cell regulatory factors are also likely involved. A recent study has suggested that in HIV-infected T, lymphocytes that are in a quiescent state, HIV cDNA remains in a linear episomal form in the cytoplasm of the infected cell. On immunologic stimulation/activation of

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the T, lymphocytes, the HIV cDNA circularizes and then migrates into the nucleus to complete its replication cycle.38 If corroborated, this observation may have an important impact on the design of future strategies for the treatment of HIV infection. For cells that produce virus, transcription of proviral DNA results in virion and messenger RNA. In the cytoplasm, messenger RNA is translated into HIV-specific structural proteins. Nucleocapsids (core particles) are assembled and migrate to the plasma membrane of the infected cell to areas where HIV-specific gp41 and gplZ0 outer membrane glycoproteins have been inserted. Final maturation occurs by a process of reverse endocytosis, or budding. Dissemination occurs either via free infectious particles or, more likely, by cell-to-cell transfer. The level of viral replication or total body viral burden has been measured by several surrogate markers including serum HIV ~24 antigen levels, the quantitation of free infectious virus in plasma, quantitative peripheral blood mononuclear cell cultures, or quantitative polymerase chain reaction techniques. Irrespective of the marker used, the level of viral replication in the early stages of HIV infection is several powers of magnitude lower than in the advanced stages of the infection when immune suppression is severe.3s-4ci The factor(s) responsible for increasing viral replication over time and the associated accelerated destruction of T, lymphocytes is as yet undetermined. PATHOGEh’ESIS An early laboratory observation of the AIDS epidemic was the marked reversal in the ratio of T, CD4+ lymphocytes to T-suppressor/cytotoxic (T,) CD8+ lymphocytes in patients manifesting AIDS.46 This reversal was found to be due to a marked decrease in’ the number of T, lymphocytes, while the number of T, lymphocytes was either normal or slightly increased. In vitro studies subsequently demonstrated that HIV is cytopathic for T, lymphocytes. The normal range for total T lymphocytes is 750 to 2400/mm3, of which 36% to 67%, or 250 to 1600/mm3, are T,. Patients with advanced HIV infection generally have fewer than 200 T, cells per mm3. Although the mechanism(s) responsible for death of an HIV-infected T, lymphocyte has not yet been elucidated, the cell must be actively producing virus for the virus to be cytopathic.47,48 Latently infected T, lymphocytes remain viable unless viral replication is stimulated. Interestingly, studies have shown that as few as 1 in 1000 to 1 in 10,000 circulating T, lymphocytes are infected with Hn/ in the early asymptomatic stages of HIV infection.49 As the disease progresses to the symptomatic stages with progressive decline in the T,-lymphocyte population, the percentage of infected T, lymphocytes increases Io-fold.40-45 This increase in the percentage of infected T, lympho652

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cytes is in concert with the observed increase in markers of viral activity or burden previously outlined, indicating a direct relationship between amount of replicating HIV and the spread of infection within the susceptible T,-cell population. The factors that lead to the progressive decline in T,-lymphocyte population and the upregulation of viral replication as individuals move from asymptomatic to symptomatic infection are as yet undefined (Figure 3). Possibilities include autoimmune destruction of T, lymphocytes; the formation of large syncytia (fusion of multiple cell membranes mediated by gp41) of infected T, lymphocytes with uninfected T, lymphocytes; the production of viral or host cellular products that are toxic to lymphocytes; a decrease in the production of substances necessary for normal growth and function; or infection of early progenitor cells, which then limits the ability of the host to repopulate depleted T, lymphocytes.47J 48 Whatever the mechanism(s), the depletion of the T,-lymphocyte population is the critical event that leads to the profound immune deficiency associated with HIV infection. Quantitation of the T,-cell population, either as an abhlute number or as a percentage of the total T-lymphocyte population, serves as an excellent marker of the degree of immune suppression. Data from the Multicenter AIDS Cohort Study, an ongoing epidemiologic investigation of the natural history of HIV infection in homosexual

WeeksMonths

Years lime

FIG 3. Natural history of course of HIV-1 infection as it relates to viral replication, responses, and clinical manifestations. DM,

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men, has shown a direct correlation between the number of T, cells and the risk that an AIDS-defining opportunistic infection will deMasur et a15’ from the National Institutes of velop.50 Additionally, Health have reported on the uncommon occurrence of Pneumocystis carinii pneumonia (the most common AIDS-defining opportunistic infection in individuals with more than 250 T, cells per cubic millimeter). In addition, the results of studies from the AIDS Clinical Trials Group of the National Institute of Allergy and Infectious Diseases evaluating the efficacy of antiretroviral therapy in delaying progression to AIDS in either asymptomatic or mildly symptomatic HWinfected individuals indicated a benefit in initiating antiretroviral therapy when the absolute To-lymphocyte count declines below 500/mm”.” ’ Furthermore, a recent study by Yarchoan et al.“’ from the National Cancer Institute indicates that individuals with advanced HIV infection, that is, AIDS, who have fewer than 50 T, lymphocytes per cubic millimeter have a significantly increased risk of death from an AIDS-related complication. Therefore, sequential monitoring of the T,-lymphocyte population is a crucial component in the management of the HIV-infected individual. In the future, evaluation of the functional aspects of T, lymphocytes, as measured by T-cell blastogenesis assays, may be as important in assessing the degree of immune dysfunction as are the current quantitative assays. Several studies have recently demonstrated that these types of functional assessments may be important in the clinical staging of the degree of immune suppression, the risk for progression to advanced HIV disease, and the response to antiretroviral therapy.53’ 54 As mentioned previously, cells of monocyte/macrophage lineage are also important immune effector cells expressing the CD4 antigen.55 HIV has been isolated from purified populations of circulating monocytes, and in vitro monocytes are susceptible to ‘HIV infection.5”’ 57 Additionally, in situ DNA hybridization has demonstrated HIV infection of macrophages from bone marrow, lung, lymph nodes, skin, and notably, the brain.13 HIV infection of monocytes is not cytopathic but results in a chronically infected viable ce11.58 Some of the ditferende in biologic effect of HIV on lymphocytes as opposed to monocytes may be explained by a difference in replication. Final maturation of HIV in monocytes occurs at the endoplasmic reticulum membrane rather than the plasma membrane, as it does in lymphocytes. Thus, the cytoplasm of infected monocytes contains numerous HIV particles within intracytoplasmic vesicles, sparing the integrity of the monocyte plasma membrane as compared with that of the lymphocyte. The noncytopathic character of HIV infection on tissue macrophages suggests that these cells are also important reservoirs of infection, particularly in the brain, where they are the predominant cell type infected. Isolation of HIV from circulating purified mono654

DA4) Septemhl?r

1992

cyte populations has suggested that this cell population may act as the vehicle for dissemination of HIV throughout the body.5n However, subsequent studies using polymerase chain reaction analysis to detect HIV proviral DNA in circulating monocytes from HIVinfected individuals in different stages of HIV infection have failed to demonstrate infection of these cells using this technique.59 This raises important questions as to when monocytes/macrophages become infected in the host and whether, in fact, these cells are important in the viral dissemination process. It is possible that macrophages become infected after the migration of circulating monocytes into the tissue by interaction with infected T, lymphocytes. This would not explain the observation of the isolation of HIV from circulating purified monocyte populations. A more likely explanation is that reverse transcription of HIV genomic RNA into proviral DNA (cDNA) occurs when monocytes go through their terminal differentiation into tissue macrophages after their migration from the intravascular space. Therefore, it remains likely that the monocyte does act as the vehicle for dissemination of HIV. Although not cytotoxic for monocytes/macrophage$, HIV does have an effect on a number of functional aspects of these important immune effector cells. These defects have included (1) decreased migration responses to chemoattractants; (2) defective intracellular kiling of various microorganisms such as Toxoplasma go&ii and Candida; (3) reduced expression of the membrane class II molecule HLADR, which may impair the ability of the monocyte/macmphage to properly piwcess and present antigens to T, lymphocytes; and (4) prostaglandin-mediated suppression by monocytes of T-cell blastogenesis as measured by interleukin-2 production6’ m63 In addition, excessive production of tumor necrosis factor-o by monocytes from patients with advanced stages of HIV infection may contribute to the development of the AIDS dementia complex and the wasting syndrome with unexplained fevers, which commonly affect patients in these later stages of the disease process.64J6B

NATURAL

HISTORY

Infection with HIV is a biologic spectrum that may begin with an acute, self-limited, nonspecific, febrile, viral syndrome, progresses through a prolonged, clinically asymptomatic stage, and subsequently results in clinical signs and symptoms indicative of profound immune suppression. The time course of this progression is highly variable. Some cases of HIV infection have progressed to AIDS in less than a year, whereas the majority of individuals remains clinically asymptomatic 7 to 8 years after infection (Table 1). The proportion of those infected with HIV in whom AIDS will eventually deLAW, September

1~'d.z

665

TABLE 1. Clinical Stages Clinical

Acute

of HIV Infection,

CD4 Lymphocyte Duration

Stage

HN infection

1 2 0 0 0

Asymptnmatic

Symptomatic Advanced IIIV disease End-stage HIV disease HN

~ lwumr~

immun~sleti~.ir,r~:v

to

to to to to

weeks >10 yeas 5+ years Df ,yea* 2+ years

Counts,

and Time

of Progression

CD4 Lymphocyte per m m 3

2

Count,

,750

-750 -500

to 200 to 100