Human retroviruses

Human Retroviruses

B. J. Thomson,

A. G. Dalgleish

SUMMA R Y. After many unsuccessful years of searching, the first pathogenic human retrovirus, the human T-cell leukaemia lymphoma virus (HTLV-I), was reported as recently as 1980 and since that time has been causally linked to the adult T-cell leukaemia lymphoma syndrome. A second HTLV (HTLV-II) isolated shortly afterwards is less clearly linked to some leukaemic and chronic lymphoid malignancies. The second major family of human retroviruses are the human immunodeficiency viruses (HIV) the first group of isolates (HIV-I) of which cause the acquired deficiency syndrome (AIDS). A second group of these viruses (HIV-II), have recently been identified in West Africa. .They appear to be less clearly associated with disease and more similar in molecular structure to the Simian immunodeficiency viruses. AIDS has now become a major global pandemic, and vaccine and therapeutic strategies are urgently being investigated in an effort to control the disease. Unfortunately, current results are not very encouraging. In the meantime, preventative and educational measures are of utmost priority in order to prevent further spread. It is not unlikely that new human retroviruses will he discovered over the next few years.

Retroviruses are a distinct group of RNA viruses which replicate via a double stranded DNA intermediate, synthesised from the virion by an RNA dependent DNA polymerase (reverse transcriptase). This intermediate, known as the provirus, is integrated into host cell DNA and acts as a template for the production of viral genomic and messenger RNA by the host cell replicative machinery (Fig. 1). Retroviruses may be divided into three sub families: oncoviruses, lentiviruses and spumaviruses. The oncoviruses and lentiviruses in particular cause many important diseases in animals (Table 1) which have clear counterparts in man. The existence of such well defined animal models has stimulated an intensive search for retrovirally induced human disease, and provided the

A. G. Dalglekh, Clinical Research Centre, Watford Road. Harrow, HAI 3UJ, UK. B. J. Thomson, National Institute for Medical Research, Mill Hill, London NW7 IAA, UK. Blood Revrews (1988) 2. 21 l-221 1-8 1988 Longman Group UK Ltd

conceptual framework which led to the discovery of the human T-cell leukaemia viruses (HTLV-I and HTLV-II) and human immunodeficiency virus (HIV), the causative agent of the acquired immune deficiency syndrome (AIDS). The Search for Human Retroviruses Retroviruses may be detected by electron microscopy (EM), assays for reverse transcriptase (RT) activity and the detection of viral antigens. The presence of viral nucleic acid sequences is detectable by Southern Blotting, in situ hybridization and the more sensitive technique of the polymerase chain reaction.’ Normal human DNA, in common with other mammalian DNA, contains conserved cellular genes (c-one or proto-oncogenes) which are homologous to retroviral oncogenes. In addition, human DNA contains more complete retroviral genomes, known as endogenous retroviruses, which are transmitted in the germ line. These include genetic elements homologous to murine

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Fig. 1 Life cycle of a retrovirus. (1) Free virus (2) Binds to specific receptor and then fuses at surface (HIV) or enters by endocytosis (3) Virus is uncoated releasing RNA strands (4) Two of which are transcribed into DNA by reverse transciptase (coded by the virus) (5) Linear DNA becomes circular DNA (6) Circular DNA is integrated into host cell DNA (7) Transcription into 35s RNA (8) Transcripts are spliced (9) Translation (proteins made) (la) Assembly with genomic RNA and budding from cell surface (11) Free virus released (12) Retroviruses can also carry genes which can encode proteins which can interact with the host cell, i.e. oncogenes which, as their name implies, are genes which when mutated or allowed to produce in excess, lead to the development of cancer. It is not unlikely that some of the HIV genes may interfere with cellular gene expression by similar mechanisms, perhaps leading to cytooathic effects. HIV is known not to encode for any known oncogenic sequence neither is it immortalising or capable of transforming’ cells.

Table 1 Diseases caused by retrovirus in animals Disease

Species

Leukaemia

Avian, mouse, cat, primates Avian, mouse, cat, primates, fish Mouse (mammary) Chicken (renal) Mouse, cow, hamster, rat Avian, reptiles

Lymphoma Carcinoma Sarcoma Wasting and autoimmune disease Arthritis Degenerative disease of the nervous system Osteopetrosis

BAEV-SSAV-like agent has been isolated from a patient with acute myeloid leukaemia and hybridisation experiments have detected BAEV-like sequences in lymphocytes from patients with chronic myeloid leukaemia. These reports, although interesting, have not yet established a role for primate or other animal retroviruses in human disease (see Ref. 3 for full review). The first unambiguous link between retroviruses and human disease was not established until the discovery of the human T-cell leukaemia viruses.

Cat, primates, mouse Goat

HTLV-I Sheep, goat, mouse, rat Chicken

leukaemia virus (MLV), mouse mammary tumour virus (MMTV) and baboon endogenous virus (BAEV). The MLV-like sequences have recently been implicated in the development of autoimmune disease in man.2 The endogenous retroviruses are not otherwise considered pathogenic. In contrast, a plethora of reports have linked exogenous animal retroviruses to human diseases. Retroviral particles have been detected by EM in the tissues and cultures of patients with leukaemia, sarcoma and melanoma. RT activity has been reported in cell cultures of patients with acute and chronic leukaemia, myelofibrosis and breast cancer. Viral antigens detected by antibodies raised to the gibbon ape leukaemia virus-simian sarcoma associated virus (GALV-SSAV), BAEV and Mason-Pfizer monkey virus have been found in a variety of human malignancies. In addition, a

Two coincident approaches led to the identification of the human T-cell leukaemia viruses. The first was the development by Gallo and colleagues of novel methods for the isolation of human retroviruses, and in particular the discovery of IL-2, a cytokine which selectively prolongs T-cell growth in vitro. These advances led to the isolation of a new retrovirus, known as HTLV-I, from OKT4 (CD4) positive Tlymphocytes of two patients with cutaneous T-cell lymphomas. The second approach stemmed from the recognition of an aggressive leukaemia of CD4+ Tlymphocytes in Southern Japan. The marked clustering and distinctive clinical features of this disease, known as adult T-cell leukaemiajymphoma (ATLL), strongly suggested a viral aetiology. In addition, the sera of 90% of patients with ATLL contained antibodies which reacted with cytoplasmic antigens in an ATLL cell line. These antibodies were subsequently shown to be directed against HTVL-I. Further epidemiological, clinical and molecular evidence have

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clearly established that HTLV-I is the causative agent of ATLL.4 Cell and Molecular Biology

HTLV-I has a single stranded genome of 9032 bases. In common with all retroviruses the genome contains a gag region which encodes the core proteins, a pol region which codes for the (magnesium dependent) reverse transciptase and the env gene encoding the envelope polypeptides (Fig. 2). These structural genes are flanked by long terminal repeat sequences (LTR’s). The LTR’s contain promotor, enhancer and regulatory elements for the control of viral gene expression. In addition, HTLV-I contains a unique region which lies between the 3’ end of the env gene and the adjacent LTR. This region contains several open reading frames (i.e. sequences capable of encoding a polypeptide) which code for at least 3 distinct but overlapping proteins known as ~21, p27 and ~40. The p40 protein, known as tat-I, acts on upstream promotor elements to increase transcription of viral tar-1 can also genes, i.e. it is a transactivator. enhance expression of cellular genes, and in particular upregulates the expression of both IL-2 and the IL-2 receptor. These molecular properties are reflected in the biological activity of the virus. HTLV-I is capable of immortalising T lymphocytes in vitro and induces

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Epidemiology and Disease

HTLV-I is endemic in Japan, the Caribbean, South Eastern USA, South America, the Arctic, many parts of Africa and in recent immigrants from at least some of these areas. The rates of sero prevalence vary between and within endemic areas. As many as 15% of the populations in areas of south-west Japan carry antibodies to HTLV-I and there is clear evidence of intrafamilial clustering.’ Seroprevalence rates in other communities in which the virus is endemic vary between 1% and 6%. The modes of transmission of HTLV-I are not yet clearly established. The virus is likely to be transmitted by sexual contact and may be transferred from mother to child both in utero and in

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constitutive expression of IL-2 and its receptor in addition to upregulating other markers of cell activation. Although HTLV-I does not’ contain sequences conventionally regarded as oncogenes, introduction of the tat-1 gene into the germ line of mice induces mesenchymal tumours in the transgenic animals.5 HTLV-I is therefore correctly regarded as an oncogeneic virus. Patients with ATLL have high titres of antibody to all HTLV-I structural and functional proteins, including tat-I.6 HTLV-I isolates from Japan, UK and USA have all expressed the same serotype.

I

HTLV-

REVIEWS

LTR GAG

POL

‘TAT

ENV

3’

HIV-1 TAT

56 C

t

GPl20

GP41

(Carboxy)

Fusion sequence

Areas of homology

to immunoglobulin

genes

Areas of homology

to HIA - class II genes

246 296 406 517

-

280 320 445 550

Group neutralising site Isolate specific neutralising CD4 binding site Putative fusion site

site

Fig. 2 Genomic which

structure of HTLV-I showing the classical gag.pol.env structure with the extra long open reading frame (LOR) has been shown to control replication by transactivation (TAT) on the 5’ LTR. The genomic structure of HIV-1 is more

complicated with at least five extra genes, two of which are subdivided being at either end of the envelope gene. These are the tat gene similar to the /or/ratof HTLV-I and the art.also known as TRS gene, which is required for post transcriptional regulation. The sor gene appears necessary for full infectivity of the assembled virion. 3’ otfappears to down regulate replication. The R gene and a new ninth gene have not yet been associated with putative functions. The envelope gene is detailed showing the external glycoprotein 120 and the transmembrane gp41. The gene is numbered O-856 to represent the amino acid numbering system. Areas of homology to the immunoglobulin supergene family and to the HLA class II genes are marked as well as the CD4 binding site, the group neutralisation site, the type specific neutratisation site and the putative fusion site (on gp41).

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breast milk. HTLV-I is certainly transmitted by both blood8 and blood products9 Seroconversion has been recorded after transfusion of whole blood, red cell concentrates and platelets, but not of cell-free extracts.” Indirect evidence has also suggested that HTLV-I may be transmitted by an insect vector.” ATLL develops in less than 1% of individuals infected by HTLV-I. The disease appears after a characteristically long latent period-sometimes several decades after seroconversion-which suggests that an unidentified ‘second hit’ is necessary to precipitate malignancy. ATLL is a particularly aggressive neoplasm often associated with hypercalcemia and bone lesions. Few patients survive longer than 6 months despite combination chemotherapy. Treatment utilising anti-IL-2 receptor antibodies to selectively direct toxins to ATLL cells may be of benefit.’ 2 A link has recently been established between HTLV-I and a chronic demyelinating disease known as tropical spastic paraparesis (TSP) or, in Japan, as HTLV-I associated myelopathy (HAM). 100% of patients with TSP investigated in the UK have antibodies to HTLV-I and peripheral blood lymphocytes have the same features and express the same viral antigens as those from patients with ATLL.i3 The onset of TSP occurs within a few years of seroconversion. TSP and ATLL seldom coexist in the same patient. TSP is strikingly similar to multiple sclerosis (MS). Both are slowly progressive, neurological diseases characterised by demyelinisation and perivascular lymphocyte cuffing of the small vessels in infected areas. The lesions of TSP, however, are confined to the spinal cord and are associated with marked meningeal irritation and an inexorable progress to paraparesis. There is some evidence that both TSP and HAM may respond to steroids early in the course of the disease.

HTLV-II A second human T-lymphotrophic virus (HTLV-II) has been isolated from 2 patients with hairy cell leukaemia, and a drug addict and haemophiliac with no evidence of malignancy.14 Antibodies to HTLV-II have been found in the serum of London drug addicts. Despite these findings HTLV-II has not yet been causally linked to disease and little is known of its epidemiology, although an increasing number of seropositive cases in the USA have been associated with low grade lymphomas or leukaemias.

Human Immunodeficiency Viruses and the Acquired Immune Deficiency Syndrome The acquired immune deficiency syndrome was first recognised as a distinct clinical entity in 1981. The major reservoirs for HIV in the West are promiscuous homosexuals and intravenous drug abusers. It is now likely, however, that on a global scale the majority of

patients infected with HIV have acquired the virus by heterosexual transmission. The first isolation of the new human retrovirus subsequently shown to be the causative agent of AIDS was reported in 1983 by Montagnier and colleagues who detected reverse transcriptase activity and retro-virus-like particles in the IL-2 stimulated peripheral blood lymphocytes of a patient with generalised lymphadenopathy. The virus was named lymphadenopathy associated virus (LAV-I). The association of LAV to AIDS remained obscure until Gallo reported the isolation and characterisation of a retrovirus from patients with or at risk of AIDS. This virus was named human T-lymphotrophic virus III (HTLV-III). Subsequent studies have shown LAV and HTLV-III to be virtually identical. In addition, similar viruses known as AIDS related viruses (ARV) were isolated from homosexuals in San Francisco. All isolates of these related retroviruses are now termed human immunodeficiency virus type I (HIV-I). The origins of HIV-I are unknown. Studies on stored blood samples indicate the presence of HIV in Zaire in 1959. In addition, HIV appears able to exist in rural African populations without epidemic spread. 1’ This suggests that the virus may have been endemic in parts of Central Africa and that its epidemiology has recently been transformed by modern population movements and exposure to hosts more likely to have multiple sexual partners and abuse intravenous drugs. Epidemiology

More than 70000 cases of AIDS occurring in 129 countries have been reported to the WHO. 55000 of these cases have occurred in the USA. Reportage of AIDS in other parts of the world, particularly Africa, is likely to be incomplete and the true number of AIDS cases globally is estimated to approach 150 000. These figures are rising so rapidly that they will be greatly surpassed by the time this is read. 65% of the total numbers with AIDS in the USA are homosexual or bisexual men without a history of drug abuse and a further 8% have occurred in homosexuals who were also drug abusers. Heterosexual drug abusers constituted 17% of the total. 2.5% of those with AIDS in the United States in 1986 contracted the disease by heterosexual transmission. In over 60% of these cases, the index partner was an intravenous drug abuser. The number of heterosexually acquired AIDS cases is now equivalent to the total number of AIDS cases in 1983 and continues to rise exponentially. 1% of adults and 5% of children with AIDS have haemophilia. 2% of the adult total had developed the disease following transfusion with infected blood.“j The most powerful predictor of the future incidence of AIDS is the current prevalence of HIV infection. More than 200 studies have addressed prevalence of HIV antibody In acknowledged high risk groups. Between 10% and 70% of homosexuals studied are

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infected, with the highest prevalence in East coast cities in the USA and San Francisco. Up to 70% of intravenous drug abusers are HIV positive, with considerable interpopulation variation. Figures for both populations are likely to be overestimates due to the nature of selection of the study populations. 15-90% of haemophiliacs in the United States are seropositive. This rate varies with the severity of the disease, and hence the requirement for coagulation factors, and the amount of factor concentrate received before screening of all blood donations became compulsory in 1985. In the UK, 60% of ‘severe’ haemophiliacs, 23% of ‘moderate’ cases and 9% of ‘mild’ haemophiliacs are seropositive.i7 The prevalence of antibody to HIV in patients with haemophilia A is much higher than those with haemophilia B. These differences, which are also found in the United States, are likely to be due to differences in the methods of manufacture of factor VIII and factor IX. Seroprevalence in the general population cannot yet be accurately determined. Screening of military recruits has yielded figures of between 0.29% and 1.2% . I8 These figures underestimate the contribution from homosexuals and drug addicts who are discouraged from applying for military service. A recent survey of over 4000 patients attending a clinic for sexually transmitted diseases found that 6.3% of males and 3% of females were HIV positive (of whom almost 50% did not acknowledge high risk behaviour). i9 An average of 2.1 women per thousand in Massachusetts gave birth to an HIV positive child; an estimate which predicts that between 1600 and 4800 HIV infected children will be born per year in the USA.20 In the UK a total of 1400 AIDS cases have been reported and an estimated 30-100000 carry the virus. Transmission of HIV

The risk of transmission of HIV within the homosexual population clearly increases with the number of partners and probably with the frequency of receptive anal intercourse and other practices which induce rectal trauma. The spread of the virus among intravenous abusers is likely to be facilitated by the sharing of needles and possibly by heterosexual contact between addicts. There is a crucial and incompletely understood difference between the heterosexual spread of HIV in Africa and Haiti and the much more limited transmission by this route in the West. Between 7.5% and 23% of the sexual partners of infected Western haemophiliacs become HIV seropositive. 21~22 A recent study of the partners of patients with transfusion associated AIDS found that 2 out of the 25 husbands of infected wives and 10 of the 55 wives and infected husbands became HIV seropositive.23 This study showed no correlation between the frequency of sexual contact and the risk of transmitting HIV. It is likely that the ability to transmit HIV to a sexual partner varies with the

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course of the disease and may correlate with falling T4/T8 ratio of peripheral blood lymphocytes. The consensus of a number of small studies indicates that heterosexual transmission probability of HIV in the West is approximately 0.1 female/male and 0.2 male/ female. In contrast the sex ratio of AIDS cases in most parts of Africa is equal and the principal risk factor appears to be multiple sexual partners. There is no convincing evidence of a difference in genetic susceptibility to the disease. The higher prevalence of sexually transmitted disease in African ‘at risk’ populations may, however, be important. Cross-sectional and prospective studies in Kenya have confirmed that genital ulceration and chlamydia trichomatis infection in woman and genital ulceration, particularly chancroid, in men greatly increase the risk of acquiring HIV-I infection. Infection with Neisseria gonorrhoea carries no increased risk.24 Perinatal Transmission

In populations in which HIV-I is heterosexually transmitted many women of child bearing age are now infected with the virus. Seroprevalence among pregnant women in Central and Eastern Africa varies between 3% and 15%. In the West more than 70% of all children with AIDS are born to mothers who are intravenous drug abusers. HIV can be transmitted in utero and has been isolated from a 15 week fetus. It may also cause a distinctive dysmorphic syndrome suggesting infection can occur in the first trimester. HIV is also likely to be spread by exchange of maternal and fetal blood at child birth and may be transmitted by breast milk. The rate of transmission from mother to child is uncertain, and rates of between 25% and 60% for first and subsequent births have been reported (see Ref. 25 for full review). Transmission by Blood and Blood Products

Almost all those transfused with HIV positive blood become infected. The voluntary deferral of blood donation by high risk groups and the compulsory screening of all blood donations for HIV antibody have greatly reduced the incidence of post-transfusion HIV infection. However, infection by transfusion of blood screened as HIV negative can still occur.26 Transmission of HIV to health care staff and others coming into contact with the virus nosocomially is likely to be exceedingly rare but nevertheless dictates that rigorous adherence to careful protocol should be observed by all staff occupationally exposed to the virus. 0.9% of all those who reported needle stick injury with contaminated blood have become seropositive. Two laboratory staff working with concentrated virus who reported no penetrating injury have become infected with HIV.27 There is no evidence that HIV can be transmitted by an insect vector.‘*

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Molecular Biology

The HIV virion is loo-130 nM in diameter and contains two identical strands of RNA within a glycoprotein envelope. The 5’ LTR-gag - pol - env 3’ LTR genome and its products are illustrated in Figure 2. The provirus is 9193-9749 bases in length and integrates randomly into the host cell genome. The HIV LTR contains cis acting elements which participate in the integration of the viral DNA and in the regulation of the transcription of viral genes. The gag pol products are synthesised as a fusion protein from full length mRNA. These genes lie in a different translational reading frame and synthesis of the protein is achieved by a ribosomal frame shifting event.29 The env gene product is synthesised from a separate 4.3 Kb RNA. The control of HIV gene expression is complex. The prototype viral genome contains at least 5 accessory open reading frames known as tat III, trs/art, R, sor and 3’ orf. The products of tat III and artltrs are synthesised from a complex array of subgenomic mRNA’s and are essential for viral replication. The tat III product transactivates gene expression by interacting with the transacting response (TAR) element within the viral LTR. tat III acts to increase the levels of both nascently transcribed and steady state mRNA and is required for transcriptional activation.30 art/trs is essential for the synthesis of gag and env proteins from appropriately spliced mRNAs and may have a negative transregulatory role on the rate of viral transcription.30 sor is not required for the production of intact virions but sor negative mutants have a greatly reduced capacity to infect CD4 positive cells.31 The products of 3’ orf down regulate virus replication and 3’ orf mutants are highly cytopathic in vitro.32 The function of the R open reading frame is unknown. These genes interact with each other in both positive and negative regulatory interactions. In addition to its own complex regulatory mechanism, the HIV LTR can be transactivated by a number of factors which may have biological relevance, including cellular transcription factors NF-kB33 as well as the transactivating proteins of eukaryotic DNA viruses.34 HTLV-I tat and T-cell mitogens or phorbol esters3’ can all upregulate transcription from the LTR in vitro. The net effect of each of these transactivating factors is mediated by different positive and negative control elements within the viral LTR.

directly to the CD4 molecule via the envelope glycoprotein Gp120 and the introduction of CD4 into any human cell renders it permissive to HIV infection.36 Although the env region is highly polymorphic, conserved domains have been identified which are critical for receptor binding37 and post-binding events3* The profound immunodeficiency found in AIDS is secondary both to depletion of CD4 positive lymphocytes and to functional impairment of soluble antigen recognition by these cells. The observed decrease in the cytotoxic capacity of NK cells and the diminished chemotaxis of monocyte/macrophages in AIDS patients may be secondary to the loss of CD4 mediated helper/inducer function. Cells of the mononuclear macrophage lineage are non-productively infected by HIV and may have a crucial role in the induction and maintenance of latent infection. The numbers of cytotoxic suppressor cells defined by their OKT-8 (CD8) antigen are increased and may influence HIV expression. These abnormalities lead to a decreased blastogenic response of T-cells to mitogens in vitro and cutaneous anergy to skin test antigens in vivo. Patients with AIDS have evidence of polyclonal Bcell activation manifest by increasing circulating immunoglobulins, detectable immune complexes and enhanced spontaneous proliferation of B-lymphocytes to B-cell growth factors in vitro. Although HIV may be a direct polyclonal activator of these cells, current in vitro studies show that these responses are CD4 lymphocyte dependent. The exaggerated B-cell activity is non-specific and patients with AIDS produce a poor humoral response to neoantigens. In addition, AIDS patients have a high prevalence of auto-antibodies which may be responsible for some features of the disease (see Ref. 39 for a full review). HIV induces a vigorous antibody response to all its structural antigens. The loss of anti-p24 response is predictive of the development of AIDS.40 HIV antigenaemia is characteristically detectable at the time of seroconversion and reappears with the development of clinical symptoms. Antibodies to HIV are relatively weakly neutralising and isolate dependent. Although sera from infected individuals can cross-neutralise different HIV isolates, 41 this property is not shared by antibodies raised to cloned HIV components in vitro. HIV specific cytotoxic T-lymphocytes directed at env gag and the retroviral RT have been identified in seropositive individuals.42 Disease and Treatment

Cell Biology and Immunology HIV belongs to the lentivirus group of retroviruses. It is now clearly established that the T4 (CD4) molecule behaves as a receptor for HIV on human cells. CD4 is a non-polymorphic glycoprotein which defines a subset of lymphocytes known as helper/inducer cells which have antigen recognition and regulatory functions axial to the immune response. HIV binds

The clinical manifestations of infection with HIV vary widely. The Centres for Disease Control recognise three distinct syndromes; progressive generalised lymphadenopathy (PGL); AIDS related complex (ARC) and AIDS. In addition, seroconversion to HIV is associated with a glandular fever-like illness. The opportunistic infections which characterise AIDS are the result of the interaction between the immunocom-

BLOOD REVIEWS Table 2 Diseases moderately indicative of underlying immunodeficiency and seen in AIDS

Table 3

Protozoan infection

Viral

217

Neurological manifestations

Encephalitis

Pneumocystis carinii pneumonitis Toxoplasma gondii encephalitis or disseminated infection Chronic crytosporidium enteritis (> 1 month) Fungal diseases

Candida oral/oesophagitis/bronchitis Cryptococcal meningitis or disseminated infection Histoplasmosis (disseminated) Viruses

Cytomegalovirus of an organ other than a lymph node Progressive multifocal leukoencephalopathy Herpes virus (chronic infection)-simplex or zoster

Bacterial Parasitic Meningitis

Viral Bacterial

HIV or other Mycobacterium IC or TB E. coli TP Other

Fungal

Cryptococcus neoformans Aspergillus fumigatus Histoplasma capsulatum

Bacterial infection

Mycobacterium avian-cellulare or Kawasasii (and disseminated tuberculosis) Legionella sp Nocardia sp Salmonella sp Shigella sp Listeria monocytogenes

HIV Cytomegalovirus (CMV) Herpes simplex I, II and zoster Other Treponema pallidum (TP) Toxoplasmosis gondii Taeni solium

Brain abscess

Bacterial Fungal Paraditis

Mycobacterium or other Candida and others Toxoplasmosis gondic, taenia solium

Neuropathy

promised host and organisms which are commensal or latent within that host and in the environment. The pattern of the syndrome varies geographically and between individuals. The wide range of pathogens which dictate the clinical presentation of AIDS are listed in Table 2. The many forms of HIV related disease are fully discussed elsewhere.43 There are, however, a number of consistent features of AIDS in the West. Pneumocystis carinii is the most consistent pathogen in HIV positive patients. The organism causes a more insidious disease in the context of AIDS than in other immunocompromised hosts. The clinical and radiological signs may be mild but pneumocystis is present in high numbers in bronchoalveolarlavage (BAL) fluid. Treatment with high dose Septrin or Pentamidine is effective but there is a high incidence of relapse after stopping therapy. CMV is widely disseminated in AIDS patients and can cause adrenal insufficiency, colitis and progressive and severe chorioretinitis. CMV is also present in BAL fluid in HIV positive patients but does not appear to be an important respiratory pathogen. Cryptococcus neoformans infections may present as a diffuse pneumonitis, encephalitis or leucopaenia and is associated with intractable diarrhoea. Shigella, salmonella and Giardi lamblia can also cause diarrhea in AIDS patients. HIV can infect and replicate within the central nervous system. Dementia syndromes are well recognised presentation of HIV infection and may occur in the absence of other manifestations of AIDS. The central nervous system manifestations of AIDS are listed in Table 3 and are fully reviewed elsewhere.44 More recently neurotoxicity has been associated with some antiviral agents particularly the dideoxynucleotide dideoxycytione (DCC). AIDS and Malignancy

An aggressive form of Kaposi’s sarcoma (KS) is virtually confined to AIDS patients and can be clearly

Viral

Myositis

HIV CMV Herpes simplex or zoster Of unknown origin

Progressive multifocal leukoencephalopathy

Neoplastic

Vascular complications

Brain lymphoma Metastatic including Carcinomatous meningitis and compressive myelopathy lymphomas, kaposis, and plasmacytoma Haemorrhage secondary to ITP Embolic (marantic endocarditis) Vascular compromise secondary to Aspergillus fumigatus

differentiated from the benign endogenous form of the disease. The pathogenesis of KS is still poorly understood and its classification as a true malignancy is uncertain. The presentation of KS in AIDS patients varies from relatively indolent skin lesions to rapidly expanding regions of the gastrointestinal tract which can cause fatal obstructions. It is striking that KS is distributed predominantly among homosexuals with AIDS and appears to be declining as a manifestation of HIV infection. 45 This suggests that KS requires a cofactor or factors which are sexually transmitted, and is therefore influenced by changing patterns of sexual activity. It is likely that angiogenic factors acting on endothelial and fibroblasts play a role in the development of KS. An alternative to a specific viral factor would be the presence of other diseases which stimulate the production of these factors which would normally be controlled by CD4 lymphocytes which are depleted. AIDS associated KS is often refractory to radiotherapy or a single drug therapy. Nevertheless, AIDS patients with KS often have a much better prognosis than those with opportunistic infections. Aggressive chemotherapeutic regimens often lead to further immune suppression and death by opportunistic infection. Reports using single drug regimens such as VP16, vinblastine or a-interferon are encouraging. Non-Hodgkin’s lymphoma and other monoclonal

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and polyclonal B-cell tumours occurs more commonly in HIV infected individuals. These tumours do not respond well to classical chemotherapeutic regimes when present in an AIDS patient, although dramatic responses and improved survival are occasionally seen. AIDS and Blood Products

The manifestations of HIV infection in those who have contracted the disease by transfusion with blood or blood products do not appear to differ from those in other groups. The course of the disease may be influenced by the high prevalence of chronic liver disease in the haemophilic population and the older mean age of those infected by a blood transfusion. Children with AIDS have a higher incidence of lung disease, including interstitial pneumonitis, and bacterial infections than seen in adults, and benefit from intravenous gammaglobulins. The manifestations of AIDS in childhood are fully reviewed elsewhere.” The course of the AIDS epidemic among haemophiliacs does not conform to predictive modelling as the transmission of HIV by blood products should now have ceased. In addition, the incubation period of the virus and the time of infection in most haemophiliacs are uncertain. A small cohort of haemophiliacs infected at a defined time point may provide useful information.46 The consensus of available evidence suggests that the time between HIV seroconversion and at the development of AIDS in haemophiliacs is similar to that in other risk groups.47m49 A study of the incubation period of AIDS following blood transfusion established a mean value of 1.9 years for children aged 04 years, 8.2 years for adults aged 5-59 years and 5.5 years for elderly patients aged more than 60 years. 5o The life expectancy of a hamophiliac with AIDS is very short-a mean of 167 days in the largest reported study. The presence of a low T-cell count, thrombocytopenia and loss of antibody to core proteins are predictive of the development of AIDS in HIV positive haemophiliacs.46*47 Recently genetic factors have been proposed as affecting the susceptibility and course of the disease in haemophiliacs. However, the haplotype HLA Al B8 DR3 that is associated with infection as well as progression to disease in haemophiliacs is not the same as has been seen in other groups. Anti-HIV Therapy

The therapy of AIDS is subdivided into therapy against opportunistic infections or tumours and therapy aimed at the virus itself. Whilst there is no satisfactory cure for AIDS, the only proven effective treatment at the present time is the dideoxynucleotide reverse transcriptase inhibitor known as AZT, Retrovir or Zidovudine, which has been shown to be of moderate benefit in small groups of patients.51 AZT crosses the blood brain barrier and suppresses viral

replication in vitro and in vivo and even people with HIV associated neurological lesions have improved on AZT. Unfortunately AZT is associated with marrow suppression which often requires blood transfusions. More recently, AZT associated myopathy has been noted. Many groups are now trying AZT in combination with other compounds. One of these is acyclovir. It is unclear at present if the alleged improvement reported by some observers of the acyclovir/AZT combination versus AZT alone is due to more than the anti herpes effect of acyclovir. (Herpes viruses can activate dormant HIV in in vitro studies.) Combinations with a-interferon may prove useful especially when interferon is given in very low doses. One of the most effective anti-RT inhibitors is phosphonophormate or Foscarnet, and this has been shown to be effective in vivo against HIV and cytomegalovirus (CMV) infection. An oral form of this drug would be warmly welcomed as alternative methods of treating CMV in AIDS cases, as current treatment with DHPG (the first line drug used against CMV) and AZT is often impractical due to enhanced toxicities. Many promising new drugs are basically AZT variants and the first of these in clinical trials -dideoxycytidine (DCC)-has unfortunately been associated with neurotoxicity. It may be effective when given alternatively with AZT, thus reducing the toxicities of both. Many more such drugs are awaiting assessment. Another strategy which has yet to be tried clinically involves specifically inhibiting the retroviral protease which cleaves the gag pol fusion polypeptide with a new class of antiviral agents.52 A wide variety of other agents show activity against HIV in vitro and many of these are undergoing trials in vivo at the present time. They broadly fall into antiviral agents and immunomodulators. Apart from the areas previously mentioned an interesting new group of compounds are the sulphated polysaccharides; heparin, dextran sulphate, pentosam polysulphate and the carrageens. These compounds inhibit HIV replication at concentrations 1000 to 10 OOO-fold lower than their toxic concentration. They appear to act at a very early stage of replication, and some have been shown to specifically inhibit absorption to the CD4 receptor of host cells. Some of these compounds are already used as anticoagulants but the anti-HIV activity would appear to exist at much lower concentrations than those that affect coagulation. Ampligen is a mismatched nonsense inhibitor of HIV replication in vitro which has shown promise in in vivo studies and further assessment is eagerly awaited.53 A soluble form of the CD4 molecule has recently been made by a number of groups and this appears to inhibit a broad range of HIV isolates in vitro.54 Clinical trials are about to commence. A wide variety of immunomodulators are under investigation. Apart from interferon, the cytokines such as interleukin-2 and interleukin-3 may have a

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role in combination regimens. Many of the drugs such as Imreg-1 and Immunothiol have shown promise in trials but conclusive proof of efficacy is still awaited. A number of other drugs which looked promising in very preliminary studies (e.g. Ribavirin, Fuscidic acid) appear to have no significant effect in in vivo trials. Vaccine Development

The development of a successful vaccine against HIV is arguably the most important issue confronting medical science. Unfortunately early studies look far from encouraging. The pattern of antibody response to HIV suggests critical problems in the development of an envelope based vaccine. The major thrust of vaccine development has, however, continued to depend on the presentation of HIV envelope glycoproteins. Programmes are seriously hampered by the lack of a suitable animal model, the difficulty of recruiting enough volunteers willing to be seroconverted, and the necessarily long period of time required to assess the efficacy of any vaccine. There is little reason to doubt that vaccination with envelope products will produce a humoral and cellular immune response, as indeed recently demonstrated in man5’ and chimps.“j The critical issue is whether such immune response confer humoral or cell mediated protection. The current evidence from in vitro studies and primate modelss8 suggests that it will not, although broadening of the humoral response with time is now being seen in some chimpanzees. It is becoming increasingly clear that specific components of the virus will need to be selected for a successful vaccine. Neutralising sites both group and type specific as well as the CD4 binding site have been identified (Fig. 2). These areas may need to be linked to specific T-cell epitopes for efficacy. Even so, what may be good for protecting one isolate has been shown to enhance other isolates and this phenomenon of antibody dependent enhancement may render these approaches impotent. Considerable interest has focused on the gag protein and although it is important in eliciting protective responses in hepatitis B and influenza, it may not be enough to be protective against HIV. However, studies in primates as well as human trials are currently underway. It should be stressed that a successful sub unit vaccine will need to be presented in an appropriate adjuvant and much research is focused on developing these delivery vehicles, such as Tscoms and liposome complexes. Another approach which has been used to protect animal models of infection such as trypanosomiasis, rabies and hepatitis B virus is the use of anti-idiotype vaccines. With regard to HIV a strategy based on the conservation of the CD4 molecule whereby antibodies to CD4 are injected with the intention of eliciting an antibody response which could theoretically mimic the CD4 binding site and compete for binding to the natural CD4 ligand, has been proposed by Dr Ronald Kennedy’s and Peter Beverley’s labora-

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tories. Early studies 57 look encourag in g but the response in mice and primates is so far relatively weak. Studies of the idiotype network, however, has suggested that this approach may be able to ‘tip the balance’ in infected subjects, and a preliminary study using the ‘Leu 3a’ anti-CD4 antibody has commenced at the Northwick Park Hospital, London. Why focus on the infected subject? The median time from infection to disease in HIV infection is about 8 years which suggests that the initial immune response is effective at controlling viral replication. However, it gradually breaks down and it would appear that all components are affected. The main reason it is destroyed initially appeared to be the killing of CD4 lymphocytes. However, the functional capacity of these cells falls off long before significant numbers are killed. It is possible that the interaction between CD4 and its natural ligand the class II HLA molecule is disrupted by virus envelope attachment, and that this would interfere with antigen presentation. Indeed, the CD4 molecule is present on monocytes, macrophages, and dendritic cells as well as many other unexpected cells. Antigen presentation has been shown to be impaired in HIV infected subjects. If this theory should prove to be even partially correct, then HIV envelope based vaccines could be expected to give a milder form of immunosuppression, which would not, however, lead to ‘AIDS’ in the absence of the replicating virus. Apart from the logistics of trying a putative vaccine it would be many years before we would know if such a candidate was successful. In the meantime strategies to induce protection in the well HIV infected subject can theoretically give much quicker answers and as well as the idiotype manipulation approach, (using specific monoclonal antibodies) trials have commenced with killed whole and selected viral components in these patients. In the absence of good news on the therapeutic and vaccine fronts, maximum energies must now be aimed at preventing transmission of the virus by education and other preventive strategies such as the rigorous testing of blood and its products. HIV-II Another pathogenic human retrovirus, termed HIV-II (formerly LAV-II) has recently been isolated from West African patients with AIDS.58 HIV-II causes an immunodeficiency syndrome indistinguishable from that caused by HIV-I. The envelope glycoproteins of HIV-I and HIV-II are 40% similar and immunological cross-reactivity is limited to the core antigens. Assays using synthetic peptides can clearly detect and differentiate between the two forms of HIV.59 HIV-II bears a closer relationship to the simian immunodeficiency viruses than to HIV-I, assessed both by serological cross-reactivity and nucleic acid hybridisation studies. Overall HIV-II does not appear to be associated with disease as consistently as with HIVI, and

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antibody responses to HIV-II will cross protect against HIV-I but not vice versa. This suggests that components of HIV-II may be useful in vaccine development. HIV-II should not be confused with HTLV-IV which is in fact not a human retrovirus but a simian immunodeficiency virus.

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