Pathogenesis of lentivirus infections

Absolute quantitation of viremia in human immunodeficiency virus infection by competitive reverse transcription and polymerase chain reaction

A competitive polymerase chain reaction (PCR)-based assay for the quantitative detection of human immunodeficiency virus type 1 (HIV-1) viremia was developed and optimized. This method consists of the reverse transcription and subsequent amplification in the same tube of two similar RNA templates, the wild-type template to be quantified and a known amount of the internally deleted synthetic template, both with identical primer recognition sites. The same strategy also proved to be useful in the quantitative assay of HIV-1-specific cellular transcripts and proviral DNA sequences from peripheral blood mononuclear cells by using competitor DNA. The method might be of interest in the study of the precise level of HIV-1 activity during the different clinical phases of the infection and in the simple, fast, and methodologically correct molecular investigation of patients treated with specific antiviral compounds.

Structural and functional analysis of the visna virus Rev-response element

The distantly related lentiviruses human immunodeficiency virus type 1 (HIV-1) and visna virus each encode a posttranscriptional regulatory protein, termed Rev, that is critical for expression of the viral structural proteins. We genetically mapped the cis-acting target sequence for visna virus Rev, the visna virus Rev-response element or RRE-V, to a complex 176-nucleotide RNA stem-loop structure that coincides with sequences encoding the N terminus of the transmembrane component of envelope. The computer-predicted structure of the RRE-V was validated by in vitro analysis of structure-specific RNase cleavage patterns. The visna virus Rev protein was shown to interact specifically with the genetically defined RRE-V in vitro but was unable to bind the HIV-1 RRE. Similarly, HIV-1 Rev was also unable to bind the RRE-V specifically. We therefore conclude that the HIV-1 and visna virus Rev proteins, while functionally analogous, nevertheless display distinct RNA sequence specificities. These findings provide a biochemical explanation for the observation that these two viral regulatory proteins are functional only in the homologous viral system.

Efficient production of human immunodeficiency virus proteins in transgenic mice

Transgenic mice containing the complete human immunodeficiency virus (HIV) coding sequences fused to the mouse mammary tumor virus long terminal repeat were generated. They were found to produce high levels of authentic gag and env HIV proteins in several tissues known to support mouse mammary tumor virus-driven transcription. HIV proteins were also detected in serum and in body fluids (milk and epididymal secretions) known to be natural sites of retrovirus, and specifically of HIV, production. These results indicate that primary mouse cells from different tissues have the capacity to produce HIV proteins. These mice represent a novel animal model for HIV infection.

Turnover of resident microglia in the normal adult mouse brain

We undertook this study to determine whether the microglia, the resident macrophages of the central nervous system, turn over in the steady-state. The turnover of brain macrophages would lend support to the "Trojan Horse" hypothesis of central nervous system infection, since one origin of replacement cells is the circulating monocyte pool. We combined the immunohistochemical detection of F4/80, a specific macrophage marker, with [3H]thymidine incorporation and autoradiography in normal adult mice. We could detect double-labelled cells in the brains of mice perfused 60 min after isotope administration. Such cells were few in number, randomly scattered throughout the brain and had the morphology of typical resident cells. The labelling index at this survival time was 0.052 +/- 0.003%. Thus resident microglia can synthesise DNA in situ. After longer survival times, we detected larger numbers of double-labelled cells. F4/80+ cells with resident morphology, mitotic figures, pairs of closely apposed (daughter) cells and cells with rounded macrophage-like morphology, all exhibited silver labelling. Twenty-four hours after isotope administration the labelling index was 0.192 +/- 0.052%. From morphologic evidence and comparison of labelling indices at different survival times, we concluded that: (i) resident microglia can synthesise DNA and go on to divide in situ; (ii) cells are recruited from the circulating monocyte pool through an intact blood-brain barrier and rapidly differentiate into resident microglia. We estimate that the two processes contribute almost equally to the steady-state turnover of resident microglia.

Expression of the protease gene of equine infectious anemia virus in Escherichia coli: formation of the mature processed enzyme and specific cleavage of the gag precursor

A 620-bp Bg/II restriction fragment containing the putative protease coding sequence from equine infectious anemia virus (EIAV) proviral DNA was cloned and expressed in E. coli as a Pol precursor protein. In contrast to the 25-kDa fusion protein predicted from the expressed pol sequence, a protein of approximately 10 kDa was generated by apparent autocatalytic processing of the Pol precursor. This mature processed protein was detected in transformed cells using an antisera raised against synthetic peptide from the conserved carboxyl-terminal segment of the predicted EIAV protease coding sequence. Coexpression of this protein with a 35-kDa EIAV Gag-precursor fusion protein resulted in the specific proteolytic processing of the precursor as shown by formation of p26, the major capsid protein of EIAV.

Macrophage tropism determinants of human immunodeficiency virus type 1 in vivo

Strains of human immunodeficiency virus type 1 differ in their abilities to infect and replicate in primary human macrophages. Chimeric clones were constructed from a provirus unable to infect macrophages (NLHX) and envelope sequences (V3 loop) of viruses derived without cultivation from brain (YU2 and w1-1c1) or spleen (w2-1b4) tissues. The substituted V3 loop sequences in each case were sufficient to confer upon NLHX the ability to infect macrophages. Furthermore, an envelope domain immediately N terminal to the V3 loop also was found to modulate the level of replication in macrophages. These results demonstrate that an envelope determinant derived directly from patients with AIDS confers HIV-1 tropism for macrophages.

Genetic and biological comparisons of pathogenic and nonpathogenic molecular clones of simian immunodeficiency virus (SIVmac)

Simian immunodeficiency virus (SIV) is a designation for a group of related but unique lentiviruses identified in several primate species. A viral isolate from a rhesus macaque (i.e., SIVmac) causes a fatal AIDS-like disease in experimentally infected macaques, and several infectious molecular clones of this virus have been characterized. This report presents the complete nucleotide sequence of molecularly cloned SIVmac1A11, and comparisons are made with the sequence of molecularly cloned SIVmac239. SIVmac1A11 has delayed replication kinetics in lymphoid cells but replicates as well as uncloned SIVmac in macrophage cultures. Macaques infected with virus from the SIVmac1A11 clone develop antiviral antibodies, but virus does not persist in peripheral blood mononuclear cells and no disease signs are observed. SIVmac239 infects lymphoid cells, shows restricted replication in cultured macrophages, and establishes a persistent infection in animals that leads to a fatal AIDS-like disease. Both viruses are about 98% homologous at the nucleotide sequence level. In SIVmac1A11, the vpr gene as well as the transmembrane domain of env are prematurely truncated, whereas the nef gene of SIVmac239 is prematurely truncated. Sequence differences are also noted in variable region 1 (V1) in the surface domain of the env gene. The potential implications of these and other sequence differences are discussed with respect to the phenotypes of both viruses. This animal model is critically important for investigating the roles of specific viral genes in viral/host interactions that cannot be studied in individuals infected with the human immunodeficiency virus (HIV).

Lack of detectable human T-cell lymphotropic virus type I sequences in samples from multiple sclerosis patients

Recently, inconclusive results have followed the early data on the possible association between multiple sclerosis (MS) and human T-cell lymphotropic virus type I (HTLV-I) infection. For this reason, we examined this hypothesis using the polymerase chain reaction (PCR) to study samples of differing origin from Italian MS patients. In particular, we developed a systematic analysis of paraffin-embedded brain white matter from histologically defined lesions of 14 MS patients using PCR and primer sets specific for HTLV-I sequences; additionally, cerebrospinal fluids (CSFs) from 12 patients and peripheral blood mononuclear cells (PBMCs) from subjects at the early and late phase of the disease were investigated for free HTLV-I virions and specific proviral sequences, respectively. In agreement with some groups who reported lack of HTLV-I sequences in PBMCs of MS patients but in clear contrast with others, we failed to detect specific viral sequences using this broad approach.

A simplified method for the detection of maedi-visna virus RNA by in situ hybridization

A simplified in situ hybridization method for the detection of maedi-visna virus (MVV) RNA in cultured cells using 35S-labelled DNA probes is described. The protocol currently used in this laboratory for the in situ detection of MVV RNA involves paraformaldehyde fixation followed by extensive cellular pretreatment prior to hybridization. It was found that substitution of paraformaldehyde fixation with brief acetone treatment and the removal of subsequent pretreatment steps gave a similar level of hybridization signal to that of our standard protocol. Acetone fixed, non-pretreated samples were used to develop a double labelling procedure in which immunocytochemistry and in situ hybridization were combined to allow the simultaneous detection of visna virus antigens and RNA within the same cell.

Antibody to reverse transcriptase of human retroviruses in multiple sclerosis

HTLV-1, HIV-1 and HIV-2 western blot analysis of sera from patients with multiple sclerosis (MS), from patients with other neurological diseases and from blood donors, revealed a rather frequent cross-reactivity with retroviral proteins in the MS group, though no patient was positive with the corresponding specific ELISA serology. Statistical analysis revealed a significant difference between the MS group and the two control groups for HIV-1 and HIV-2 reverse transcriptase fragments and for HTLV-1 p24. The general significance of these observations is discussed in the light of a retroviral hypothesis for the aetiology of MS. It is suggested that, if a retrovirus is present in MS patients, it does not necessarily belong to the HTLV sub-family and could as well be a lentivirus, like Visna virus, the causative agent of a demyelinating disease in sheep which is one--natural--model for MS.

Antigenic diversity thresholds and the development of AIDS

Longitudinal studies of patients infected with HIV-1 reveal a long and variable incubation period between infection and the development of AIDS. Data from a small number of infected patients show temporal changes in the number of genetically distinct strains of the virus throughout the incubation period, with a slow but steady rise in diversity during the progression to disease. A mathematical model of the dynamic interaction between viral diversity and the human immune system suggests the existence of an antigen diversity threshold, below which the immune system is able to regulate viral population growth but above which the virus population induces the collapse of the CD4+ lymphocyte population. The model suggests that antigenic diversity is the cause, not a consequence, of immunodeficiency disease. The model is compared with available data, and is used to assess how the timing of the application of chemotherapy or immunotherapy influences the rate of progress to disease.

The genetic fate of molecularly cloned simian immunodeficiency virus in experimentally infected macaques

We have examined genetic variation of the simian immunodeficiency virus (SIV) in four macaques inoculated with virions derived from molecular clones of proviral DNA. Our data demonstrated that the SIV genome is capable of rapid and extensive genetic variation. This variation was especially large in the env gene, where nucleotide substitution frequencies were as high as 10(-1)/site/year. In some env clones, a high G to A transition rate was observed that accounted for up to 79% of the observed nucleotide substitutions. Moreover, in env clones with a high G to A transition rate, multiple in-frame stop codons were generated exclusively at tryptophan codons. Another interesting observation was the lack of variation in the region analogous to the V3 loop in the HIV-1 Env protein. Considered together, these data have important implications for studies of pathogenesis and vaccine development in the SIV model system.

The topoisomerase I inhibitor, camptothecin, inhibits equine infectious anemia virus replication in chronically infected CF2Th cells

Camptothecin (CPT), a topoisomerase I-specific inhibitor, was found in this study to inhibit the replication of equine infectious anemia virus (EIAV) in chronically infected CF2Th cells (designated CF2Th/EIAV). By measuring viral reverse transcriptase activity in the culture medium, we demonstrated that treatment for 1 h with noncytotoxic doses of this drug inhibited production by 32 to 52%, whereas continuous exposure to this drug resulted in an 85 to 92% inhibition. No effect on the viability or growth rate of the cells was detected in any of these treatments. Indirect immunofluorescence analysis of the CPT-treated CF2Th/EIAV cells with anti-p26 capsid protein antibodies showed 60 to 85% reduction in the immunofluorescence-positive cells following drug treatment, and radioimmunoprecipitation analysis of these cells showed a comparable decrease of the pr55gag precursor protein. These data suggest that CPT acts as an anti-EIAV agent to block virus replication in the chronically infected cells.

Mutational definition of the human immunodeficiency virus type 1 Rev activation domain

Replication of human immunodeficiency virus type 1 requires the functional expression of the virally encoded Rev protein. The binding of this nuclear trans activator to its viral target sequence, the Rev-response element, induces the cytoplasmic expression of unspliced viral mRNAs. Mutation of the activation domain of Rev generates inactive proteins with normal RNA binding capabilities that inhibit wild-type Rev function in a trans-dominant manner. Here, we report that the activation domain comprises a minimum of nine amino acids, four of which are critically spaced leucines. The preservation of this essential sequence in other primate and nonprimate lentivirus Rev proteins indicates that this leucine-rich motif has been highly conserved during evolution. This conclusion, taken together with the observed permissiveness of a variety of eukaryotic cell types for Rev function, suggests that the target for the activation domain of Rev is likely to be a highly conserved cellular protein(s) intrinsic to nuclear mRNA transport or splicing.

Type I conceptus interferons: maternal recognition of pregnancy signals and potential therapeutic agents

In summary, Type I conceptus interferons of sheep, cows, and goats (perhaps all ruminants), which signal maternal recognition of pregnancy, may be useful for enhancing fertility in animal agriculture and may have a role in human and veterinary medicine as well. The Type I conceptus interferons share antiviral, anticellular, and immunosuppressive properties with Type I leukocyte interferons, but lack their cytotoxic effects, which may make them especially useful as therapeutic agents in human and veterinary medicine. The unique N-terminal structure of oTP-1 may account for its distinctive biological properties, including its antiluteolytic activity.

Viral genetic determinants of T-cell killing and immunodeficiency disease induction by the feline leukemia virus FeLV-FAIDS

Within the fatal immunodeficiency disease-inducing strain of feline leukemia virus, FeLV-FAIDS, are viruses which range in pathogenicity from minimally (clone 61E is the prototype) to acutely pathogenic, most of the latter of which are also replication defective (clone 61C is the prototype). Mixtures of 61E and 61C virus and chimeras generated between them, but not 61E alone, killed feline T cells. T-cell killing depended on changes within a 7-amino-acid region near the C terminus of the gp70 env gene or was achieved independently by changes within a 109-amino-acid region encompassing the N terminus of gp70. The carboxy-terminal change was also sufficient for induction of fatal immunodeficiency disease in cats. Other changes within the 61C gp70 gene enhanced T-cell killing, as did changes in the long terminal repeat, the latter of which also enhanced virus replication. T-cell killing correlated with high levels of intracellular unintegrated and proviral DNA, all of which were blocked by treatment of infected cells with sera from 61C-immune cats or with a neutralizing monoclonal antibody. These findings indicate that T-cell killing is a consequence of superinfection and that the mutations in env critical to pathogenicity of the immunosuppressive variant result in a failure to establish superinfection interference in infected cells.

Conserved functional organization of the human immunodeficiency virus type 1 and visna virus Rev proteins

Visna virus encodes a posttranscriptional regulatory protein that is functionally analogous to the Rev trans activator of human immunodeficiency virus type 1. Here, we demonstrate that the known functional organization of the human immunodeficiency virus type 1 Rev trans activator is shared by the distantly related visna virus Rev protein. In particular, both Rev proteins contain an N-terminal domain marked by a highly basic core motif that determines RNA sequence specificity, as well as a second C-terminal domain containing an essential leucine-rich motif that functions as an activation domain. Chimeric proteins consisting of the binding domain of one Rev protein fused to the activation domain of the other were fully functional in the viral sequence context cognate for the binding domain. We also describe derivatives of visna virus Rev bearing a defective activation domain that displayed a trans-dominant negative phenotype in transfected cells. These visna virus Rev mutants may prove useful in the derivation of transgenic animals resistant to this agriculturally important retroviral pathogen.

Epitope analysis of capsid and matrix proteins of North American ovine lentivirus field isolates

Monoclonal antibodies (MAbs) directed against two phenotypically distinct ovine lentivirus (OvLV) strains were generated by fusion of BALB/c SP2/0-Ag 14 myeloma cells with spleen cells from mice immunized with purified OvLV. Hybridomas were selected by indirect enzyme-linked immunosorbent assay (ELISA) and analysis of reactivity on immunoblots. The majority (17 of 21) of the MAbs recognized the gag-encoded capsid protein, CA p27, of both strains. Four other MAbs recognized a smaller structural protein, presumably a matrix protein, MA p17. Three distinct epitopes on CA p27 and one on MA p17 were distinguished by the MAbs with competition ELISA. MAbs from each epitope group were able to recognize 17 North American field isolates of OvLV and the closely related caprine arthritis-encephalitis virus (CAEV). Analysis of the data indicated that these epitopes were highly conserved among naturally occurring isolates. A representative MAb from each epitope group of anti-CA p27 MAbs reacted with four field strains of OvLV and CAEV on immunoblots. An anti-MA p17 MAb recognized the same OvLV strains on immunoblots but failed to recognize CAEV. MAbs which recognize conserved epitopes of gag-encoded lentivirus proteins (CA p27 and MA p17) are valuable tools. These MAbs can be used to develop sensitive diagnostic assays and to study the pathogenesis of lentivirus infections in sheep and goats.

Retroviral arthritis: phenotypic analysis of cells in the synovial fluid of sheep with inflammatory synovitis associated with visna virus infection

A phenotypic analysis on synovial fluid cells from the carpal and tarsal joints of sheep with visna virus-induced inflammatory synovitis was performed. The results showed increased representation of cells bearing lymphocyte, macrophage, and dendritic cell markers compared to equivalent synovial fluid cells from normal uninfected age-matched controls. In infected sheep, CD8+ T cells tended to predominate over CD4+ cells, while the numbers of gamma delta T cells varied from being absent in some samples to constituting the major T cell subset in others. B cells were found in relatively smaller numbers. Analysis of the large mononuclear cells showed that they stained with monoclonal antibodies that recognize macrophages and afferent lymph dendritic cells. Major histocompatibility complex (MHC) class II+ macrophage/dendritic cells were found in normal joints, but significantly elevated proportions of such cells were present in the carpal joints of infected sheep. The intensity of MHC class II staining was also significantly elevated in infected animals compared to control animals. A high proportion of these cells also stained for CD1 in both normal and infected animals, but were significantly elevated in number in the carpal joints of infected sheep. The elevated proportion of cells expressing molecules associated with accessary cell function and the increase in the numbers of accessory molecules per cell suggests an enhanced capacity for presenting antigen to a variety of T cell subsets within the joints of infected sheep, which could initiate or perpetuate potentially damaging local synovial inflammatory responses.

Analysis of the transcription pattern and mapping of the putative rev and env splice junctions of bovine immunodeficiency-like virus

The bovine immunodeficiency-like virus (BIV) genome contains the obligatory structural genes of all retroviruses and, in addition, the complex central region of lentiviruses; this novel region may code for at least five nonstructural/regulatory genes in BIV (K.J.Garvey, M.S. Oberste, J.E. Elser, M.J. Braun, and M.A. Gonda, Virology 175:391-409, 1990). As a prelude to determining the function of these novel open reading frames, the transcriptional pattern of BIV was studied by Northern analysis of RNA from BIV-infected cells. Five size classes of BIV-specific RNAs of 8.5, 4.1, 3.8, 1.7, and 1.4 kb were detected. The 8.5-kb RNA contains sequences from all regions of the genome; it is the virion RNA and probably serves as the gag-pol transcript as well. By using gene-specific probes, subgenomic viral RNAs of 3.8, 1.7, and 1.4 kb were tentatively identified as the env, tat, and rev spliced messages, respectively. The 4.1-kb RNA could not be unambiguously identified but may encode vif. The hybridization patterns of the putative tat and rev mRNAs suggest that they are the products of multiple splicing events. Discrete transcripts for the BIV W and Y central region open reading frames were not defined. The characterization of partial cDNA clones has permitted the mapping of the env and putative rev splice junctions.

In situ amplification of visna virus DNA in tissue sections reveals a reservoir of latently infected cells

Maedi and visna are, respectively, the pulmonary and neurological manifestations of slowly progressive infections of sheep caused by retroviruses of the lentivirus subfamily. Lentivirus infections are also persistent infections in which host defenses are generally not successful in eliminating the infectious agent because of restricted viral gene expression in many infected cells. In this report, we describe a method for amplifying and detecting viral DNA in tissue sections which has made it possible to verify experimentally the postulated existence of this reservoir of latently infected cells, as well as to estimate the actual number of cells which harbor viral genomes in infected tissues. In the discussion, we present a simple mathematical model that relates this number to the rate at which inflammatory lesions develop. This model can account for both the slow progression of natural infections and for the rapid accumulation of inflammatory foci in the high dosage experimental system analysed in our studies.

A structural correlation between lentivirus transmembrane proteins and natural cytolytic peptides

Although oncoviruses and lentiviruses replicate by similar mechanisms, they differ fundamentally in the usual fate of the infected host cell during productive natural infections. Oncoviruses typically establish persistent nonlytic infections in natural host cells, while lentivirus infections characteristically result in a variety of cytopathic effects ultimately leading to death of the target cell. Described here is a unique structural motif consisting of a strongly amphipathic and arginine-rich helical peptide segment in the carboxyl end of lentivirus TM proteins that is structurally similar to the family of cytolytic peptides produced as defensive agents by certain insects and amphibians. Also demonstrated is the lytic nature of synthetic peptides constructed from the transmembrane (TM) protein of human and simian immunodeficiency viruses (HIV and SIV). Thus, it appears that the cytopathic properties of lentiviruses may be in part attributed to the presence of lytic peptides within the TM protein, designated lentivirus lytic peptide (LLP) and that variations in this segment could account for some of the differences observed in the cytopathicity among variants of a particular lentivirus.

Detection of human immunodeficiency virus type 1 genomic RNA in plasma samples by reverse-transcription polymerase chain reaction

An application of the polymerase chain reaction (PCR) to the direct detection of human immunodeficiency virus type 1 (HIV-1) viremia is described. The amplification of specific HIV-1 sequences of gag and env viral genes was carried out after the reverse-transcription of plasma samples (plasma RT-PCR) from seropositive subjects. The assay is faster and cheaper than detection of specific HIV-1 transcripts from peripheral blood mononuclear cells by RT-PCR. The data suggest that HIV-1 viremia is detectable by plasma RT-PCR in a large proportion of seropositive individuals.

Modulation of HIV-enhancer activity by heterologous agents: a minireview

The enhancer region of the human immunodeficiency virus (HIV) long terminal repeat (LTR) contains two 10-bp (5'-GGGACTTTCC) repeats (core enhancers) which constitute the binding sites for the ubiquitous inducible cellular transcription factor, NF-kappa B. The NF-kappa B motifs of the LTR play a central role in transcriptional activation of the LTR by several heterologous viral proteins and various external chemical and physical stimuli. Activation of the HIV enhancer by these agents may lead to the onset of HIV gene expression resulting in active viral replication. Viral genes and chemical agents, which interfere with the activity of the enhancers may be useful in inhibiting HIV gene expression, thereby suppressing HIV replication.

Molecular basis of viral persistence: a single amino acid change in the glycoprotein of lymphocytic choriomeningitis virus is associated with suppression of the antiviral cytotoxic T-lymphocyte response and establishment of persistence

Isolates of lymphocytic choriomeningitis virus (LCMV) that elicit a cytotoxic T-lymphocyte response (CTL+) have been compared with isolates that suppress the CTL response (CTL-) in an effort to map this phenotype. A single amino acid change in the glycoprotein of the LCMV Armstrong (ARM) strain is consistently associated with the CTL- trait and the ability of the virus to persist (P+). The CTL+ P- parental strain spontaneously gives rise to CTL- P+ variants within lymphoid tissues of mice persistently infected from birth. To map the structural basis of the phenotype, the complete RNA sequence of LCMV ARM 53b (CTL+) was compared with that of its variant ARM clone 13 (CTL-). Differences in 5 of 10,600 nucleotides were found. Three changes are noted in the large L RNA segment, and two are noted in the small S RNA segment. Only two of the changes distinguishing CTL+ from CTL- isolates affect amino acid coding: lysine to glutamine at amino acid 1079 of the polymerase protein, and phenylalanine to leucine at amino acid 260 of the envelope glycoprotein (GP). We also analyzed two additional CTL- variants and four spontaneous CTL+ revertants. All three CTL- variants differ from the original CTL+ parental strain at GP amino acid 260, indicating that this amino acid change is consistently associated with the CTL- phenotype. By contrast the other four mutations in LCMV are not associated with the CTL- phenotype. Sequence analysis of the coding regions of four CTL+ revertants of ARM clone 13 did not reveal back mutations at the GP 260 locus. This finding indicates that the GP 260 mutation is necessary but not sufficient for a CTL- P+ phenotype and that the reversion to CTL+ P- is likely either due to secondary mutations in other regions of the viral genome or to quasispecies within the revertant population that make significant contributions to the phenotype.

Human and experimental spongiform encephalopathies: recent progress in pathogenesis

The spongiform encephalopathies belong to the group of "slow virus infections" of the nervous system, characterized by a long incubation period, a protracted course and involvement of the nervous system with a lethal outcome. In contrast to the conventional virus infections, such as visna in sheep and progressive multifocal leukoencephalopathy (PML) in humans, the etiological agent for the spongiform encephalopathies has not been clearly defined. The known forms in animals are scrapie in sheep and goats, transmissible mink encephalopathy, and chronic wasting disease of mule deer and elk. In humans, the three known forms are Kuru, now mainly of historical interest, Creutzfeldt-Jakob (CJ) disease and the syndrome of Gerstmann-Straussler-Scheinker (GSS). An important feature of these diseases is the lack of an immune response by the host, which is reflected in the absence of inflammatory infiltrates in the affected tissues. In this editorial the two most important hypotheses on the etiology and pathogenesis of this group of conditions will be discussed. The "prion" hypothesis considers the possibility that a protein, derived from a normal component of the neuronal membranes may have a leading role, not only in the infectivity and transmissibility of these diseases, but in the pathological changes that ensue. A single host gene would code for both the normal and altered proteins. The altered protein would be partially insoluble and would result in the deposition of fibrils and rods which would precipitate in the form of amyloid. Since the involved protein would be coded for by the host, there would be no immune response against it.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurobiology of simian and feline immunodeficiency virus infections

Experimental and clinical evidence indicates that all lentiviruses of animals and humans are neurotropic and potentially neurovirulent. The prototypic animal lentiviruses, visna virus in sheep and caprine arthritis encephalitis virus in goats have been known for decades to induce neurologic disease. More recently, infection of the brain with the human immunodeficiency virus (HIV) has been linked to an associated encephalopathy and cognitive/motor complex. While the visna virus and caprine arthritis encephalitis virus are important models of neurologic disease they are not optimal for the study of HIV encephalitis because immune deficiency is only a minor component of the disease they induce. By contrast, the recently isolated lentiviruses from monkeys and cats, the simian and feline immunodeficiency viruses (SIV and FIV respectively), are profoundly immunosuppressive as well as neurotropic. SIV infection of the central nervous system of macaques now provides the best animal model for HIV infection of the human brain due to the close evolutionary relationship between monkeys and man, the genetic relatedness of their respective lentiviruses, and the similarities in the neuropathology. This chapter will compare and contrast the neurobiology of SIV and FIV with HIV.

Optimal growth schedule of pathogens within a host: switching between lytic and latent cycles

We have studied the optimal growth schedule of a pathogen, which maximizes the total number of transmissions from an infected host to other individuals until host death or recovery. It is assumed that both transmission rate f(N) and host mortality increase with the number of pathogens, N. The model predicts that the optimal growth schedule of pathogens strongly depends on the curvature of f(N): If f(N) increases faster than linearly with N, the pathogens should always reproduce at the maximum speed. By contrast, if f(N) saturates with N, the optimal schedule is composed of (1) a brief initial stage of infection, in which the pathogens proliferate at the maximum speed (productive cycle), (2) followed by the long latent period with the "stationary infection level," N* (latent cycle), (3) which may end when the pathogens start rapid proliferation triggered either by the host's senescence ("programmed break") or by the sudden rise in the host's mortality ("incidental break"). The latter may be caused by the double infection of another strain. We also examine the Nash equilibrium schedule of pathogen growth in the presence of multiple infections.

Proviral sequences detected by polymerase chain reaction in peripheral blood cells of horses with equine infectious anemia lentivirus

Proviral sequences in the peripheral blood mononuclear cells of 3 horses with acute equine infectious anemia virus were monitored using the polymerase chain reaction. Provirus was detected during the initial viremic episode in each horse and during each of 3 relapsing viremic cycles, although the appearance of provirus lagged behind the onset of viremia. Following each viremic episode, provirus levels in the peripheral monocytes decreased to less than 1 copy in 5 x 10(6) cells.

Isolation of replication-competent molecular clones of visna virus

Visna virus is the prototypic member of a subfamily of retroviruses responsible for slow infections of animals and humans. As a part of our investigation of the functions of viral gene products in virus replication, we have isolated three infectious molecular clones and determined the complete nucleotide sequences of two of the clones. We have also characterized the progeny of the biologically cloned viral stocks and of the infectious clones and document considerable heterogeneity in plaque size and antigenic phenotype of the former that is reduced to near homogeneity in the progeny of the infectious clones. It thus should now be possible to trace the emergence of antigenic variants of visna virus as well as ascribe defined functions to structural and regulatory genes of the virus in determining neurovirulence and the slow tempo of infection.

Evidence for neurotoxic activity of tat from human immunodeficiency virus type 1

The human immunodeficiency virus (HIV) genome codes for a trans-activating regulatory protein, tat. Using chemically synthesized tat, it was found that 125I-tat and 125I-tat38-86 specifically bound to rat brain synaptosomal membranes with moderate affinity (K0.5 = 3 microM). Interaction of tat with nerve cells was also revealed by flow cytometry, which showed its binding to rat glioma and murine neuroblastoma cells, using both direct fluorescence with fluorescein isothiocyanate-labeled tat and indirect immunofluorescence assays. This interaction was investigated with electrophysiology using isolated excitable frog muscle fibers and cockroach giant interneuron synapses. tat acted on the cell membrane and induced a large depolarization, accompanied by a decrease in membrane resistance, thereby modifying cell permeability. The neurotoxicity of tat was further demonstrated in vitro, on glioma and neuroblastoma cell growth, as well as by a 51Cr release assay in both tumor cell lines. Interestingly, no hemolytic activity of tat for human erythrocytes was found even when tat was tested at its highly neurotoxic concentration. Experiments in vivo showed that synthetic tat is a potent and lethal neurotoxic agent in mice. The use of tat peptide derivatives showed that basic region from 49 to 57 is necessary and sufficient for binding to cell membranes and toxicity.

Intrathecal IgG synthesis and specificity of oligoclonal IgG in patients infected with HIV-1 do not correlate with CNS disease

The CSF/serum immune response to HIV 1 was studied in 24 patients admitted for investigation. The level of antibody to HIV-1 and specificity of oligoclonal IgG were determined in blood and cerebrospinal fluid (CSF). The majority of patients demonstrated elevated levels of intrathecal IgG synthesis, with levels of HIV-1-specific antibody frequently being significantly higher in CSF than in serum. In 16 of 21 patients the CSF/serum antibody ratio indicated active intrathecal synthesis. Oligoclonal banding was present in CSF from all 24 patients. Immunoprinting of serum and CSF demonstrated antigenic specificity (p24, gp 160, RT) of the clonal antibodies in all of 12 patients though the patterns of reactivity in CSF did not necessarily correspond with that of serum. Although a specific association of particular patterns with HIV CNS disease was not found we feel that these markers should be included in longitudinal studies of HIV-related diseases of the CNS. The specificity of oligoclonal antibodies, both in CSF and in serum was demonstrated, and this specificity may be a useful marker for longitudinal studies in HIV-1 antibody-positive asymptomatic patients.

Genetic structure of the pol-env region of the caprine arthritis-encephalitis lentivirus genome

The nucleotide sequence of the pol-env intergenic region of two isolates of caprine arthritis-encephalitis virus (CAEV) was determined. Two open reading frames (orfs) were identified, designated Q and S by homology with visna virus. CAEV orf S is a single exon encoding a deduced 87-amino acid gene product sharing 36 amino acid identities with the visna trans-acting transcriptional activator (Tat). Ten of these identities comprise a conserved CGCRLCNPGW sequence similar to a cysteine-rich domain essential for trans-activation by human immunodeficiency virus Tat. To determine if transcription promoted by the CAEV long terminal repeat (LTR) could be stimulated in CAEV-infected goat synovial membrane cells, a plasmid (pCAE-CAT) expressing bacterial chloramphenicol acetyltransferase (CAT) under control of the CAEV LTR was transfected into uninfected and infected cells. Sixfold enhancement of CAT activity was observed in infected cells using 100 ng of transfected plasmid. To determine if the pol-env region encodes a gene product which trans-activates the CAEV LTR, goat synovial membrane cells were cotransfected with pCAE-CAT and pRSV-1.9, a plasmid expressing the pol-env region under control of the Rous sarcoma virus LTR. Results indicated that the CAEV genome encodes a tat gene product attributable to orf S.

Human and ovine lentiviral infections compared

Maedi-visna virus (MVV) of sheep was the first lentivirus to be isolated. The genomic organization of MVV is very similar to that of human immunodeficiency virus (HIV) with several genes regulating the expression of the viral genome. Viral replication is severely restricted in the host and some cells apparently contain the genetic information in a DNA provirus form with little or no expression of viral antigens. This seems to be a major factor in causing the "slowness" of lentiviral infections and the persistence of the virus in the host since the immune system may not recognize the provirus-containing cells. The target cells for HIV and MVV are similar although T4 lymphocytes are not specifically destroyed in maedi-visna. There are also certain similarities in the pathological changes in both diseases, both in the central nervous system, the lungs and the lymphatic system. Although the severe final immunodeficiency state characteristic of AIDS has not been observed in maedi-visna, the basic biological features of the MVV and its interaction with host cells are so similar to HIV infection, that we consider ovine maedi-visna useful animal model for the human lentivirus infections.

Human immunodeficiency virus type 1 infection of U937 cells promotes cell differentiation and a new pathway of viral assembly

The differentiation of U937 monoblastoid cells after human immunodeficiency virus type 1 (HIV-1) infection was studied using the following approaches: reverse transcriptase activity measurement, immunofluorescence labeling, and electron microscopy. For comparison, uninfected U937 cells were induced to differentiate from monocyte to macrophage by phorbol 12-myristate 13-acetate (PMA) or retinoic acid (RA) treatment. Both infected and drug-treated cells showed important and similar ultrastructural cell modifications, with a phenotype that decreased in monocyte specificity and increased in that of macrophages. When U937 cells were induced to differentiate upon HIV-1 infection, a very different pathway of viral production was observed. Production and accumulation of the virus in a vacuolar compartment of intracytoplasmic origin and escape to the antiviral lysosomal activity could explain virus persistence. This makes the cell system a good model with which to study the relationship between HIV-1 production and cell differentiation.

Visna virus encodes a post-transcriptional regulator of viral structural gene expression

Visna virus is an ungulate lentivirus that is distantly related to the primate lentiviruses, including human immunodeficiency virus type 1 (HIV-1). Replication of HIV-1 and of other complex primate retroviruses, including human T-cell leukemia virus type I (HTLV-I), requires the expression in trans of a virally encoded post-transcriptional activator of viral structural gene expression termed Rev (HIV-1) or Rex (HTLV-I). We demonstrate that the previously defined L open reading frame of visna virus encodes a protein, here termed Rev-V, that is required for the cytoplasmic expression of the incompletely spliced RNA that encodes the viral envelope protein. Transactivation by Rev-V was shown to require a cis-acting target sequence that coincides with a predicted RNA secondary structure located within the visna virus env gene. However, Rev-V was unable to function by using the structurally similar RNA target sequences previously defined for Rev or Rex and, therefore, displays a distinct sequence specificity. Remarkably, substitution of this visna virus target sequence in place of the HIV-1 Rev response element permitted the Rev-V protein to efficiently rescue the expression of HIV-1 structural proteins, including Gag, from a Rev- proviral clone. These results suggest that the post-transcriptional regulation of viral structural gene expression may be a characteristic feature of complex retroviruses.

Monocyte-mediated entry of pathogens into the central nervous system

The origin of the microglia has long been a subject of debate. However it is now clear that monocytes enter the normal central nervous system and follow a series of morphological transformations as they differentiate into microglia. Thus, microglia are of monocytic origin. Since monocytes migrate into the normal CNS, they represent potential vehicles for the entry of pathogens into the nervous system and indeed may carry particulate matter into the CNS. Both viruses and bacteria use this 'Trojan horse' mechanism of entry in the pathogenesis of CNS disease.

Macrophage-tropic strains of human immunodeficiency virus type 1 utilize the CD4 receptor

To characterize the role of CD4 in human immunodeficiency virus type 1 (HIV-1) infection of macrophages, we examined the expression of CD4 by primary human monocyte-derived macrophages and studied the effect of recombinant soluble CD4 and anti-CD4 monoclonal antibodies on HIV-1 infection of these cells. Immunofluorescence and Western blot (immunoblot) studies demonstrated that both monocytes and macrophages display low levels of surface CD4, which is identical in mobility to CD4 in lymphocytes. Recombinant soluble CD4 and the anti-CD4 monoclonal antibody Leu3a blocked infection of macrophages by three different macrophage-tropic HIV isolates, and the cytopathic effects of HIV-1 infection were similarly prevented. Dose-response experiments using a prototype isolate which replicates in both macrophages and T lymphocytes showed that recombinant soluble CD4 inhibited infection of macrophages more efficiently than in lymphocytes. These results indicate that CD4 is the dominant entry pathway for HIV-1 infection of macrophages. In addition, recombinant soluble CD4 effectively blocks HIV-1 infection by a variety of macrophage-tropic strains and thus has the potential for therapeutic use in macrophage-dependent pathogenesis in HIV disease.