Implications of the evolution pattern of human T-cell leukemia retroviruses on their pathogenic virulence (Review)

Simian retroviruses pose a serious threat to public health, as two human pathogenic retroviruses, HIV and HTLV, have been already proved to originate from such non-human viruses. Therefore, studying their natural prevalence among wild non-human primates is important for planning strategies to prevent the emergence of additional human retroviral pathogens. This article is focused on tracing the origin and evolution of the human T-cell leukemia viruses HTLV-I and HTLV-II in comparison to that of the simian lymphotropic viruses STLV-I, STLV-II and STLV-L, which are phylo-genically classified into a common group called primate T-lymphotropic viruses (PTLV). Thus, HTLV-I and STLV-I are referred to as PTLV-I and HTLV-II and STLV-II as PTLV-II, whereas STLV-L, which is highly divergent from both HTLV types, comprises a third subgroup called PTLV-L. The phylogeny of PTLV indicates that both, HTLV-I and HTLV-II emerged from a simian origin, but their subsequent evolution continued in different patterns. HTLV-I includes 6 subtypes which evolved from STLV-I through several times of different geographic interspecies transmission between simian and human hosts. These repeated invasions to new primate species are likely to give rise to viral strains with increasing pathogenic potential. On the other hand, HTLV-II includes 4 subtypes which appear to originate from a common human ancestor virus that emerged from only one simian to human transmission, whereas the subsequent evolution of HTLV-II and STLV-II strains continued separately only within the Homo sapiens and Pan paniscus species respectively, without repeated interspecies jumps. Such evolution pattern likely involves less genetic changes and selection of viral strains with low pathogenic virulence that could co-exist with their hosts for long time. These different evolution patterns can explain the much wider implication of HTLV-I with human clinical disorders than HTLV-II. Of note, however, more recently HTLV-II started spreading much more rapidly through intravenous drug users to many geographical regions, with a 150-350 fold higher mutation rate than that of its previous strictly endemic strains. This change in the mode of the virus spread creates a serious risk for emergence of HTLV-II strains with higher virulence.     

Immune recognition of genetically diverse simian T-cell lymphotropic virus type I isolates

Summary Nucleotide sequence analysis of selected regions of the gag, pol, env and pX genes of simian T-cell lymphotropic virus type I (STLV-I) strains indicated that African isolates were more closely related to human T-cell lymphotropic virus type I (HTLV-I) than Asian isolates. Despite these recent comparative studies on nucleotide sequence homology between HTLV-I and STLV-I isolates, only limited information is available regarding the influence of genetic differences on antigen-antibody recognition of distinct STLV-I strains. In this study, we demonstrated that sera from STLV-I-infected yellow baboons (Papio cynocephalus) reacted strongly with env gp62/68 from HTLV-I-infected cell lines MT-2 and C10/MJ. In contrast, sera from Japanese macaques (Macaca fuscata) naturally infected with Asian STLV-I had weak reactivity to env gp62/68 of these prototypic HTLV-I strains. Pst-1 restriction enzyme analysis of proviral DNA indicated that the baboon virus isolates were more closely related to HTLV-I than the Japanese isolates. These results indicate that nucleotide sequence diversity, correlates with variations in proviral restriction enzyme sites and antibody recognition of viral envelope proteins. These differences in immunoreactivity may have important implications for serologic diagnosis, as well as epidemiological and vaccine studies of STLV-I infection.     

Comparison of CREB- and NF-kappaB-mediated transactivation by human T lymphotropic virus type II (HTLV-II) and type I (HTLV-I) tax proteins

The function of the transactivator protein Tax from HTLV-II subtype A, subtype B, Brazilian subtype C, and African subtype D isolates was compared to that of Tax from an HTLV-I isolate. HTLV-II subtypes A, B, and C were less active in the transactivation of a NF-kappaB reporter compared to HTLV-I Tax in 293T but not Jurkat T cells. In both cell types there were no significant differences between the functions of HTLV-II B, C, and D and HTLV-I Tax proteins on either a full-length HTLV-I LTR or a 21-bp repeat reporter, suggesting that there is equivalent CREB-mediated transactivation between the viruses and these subtypes. In contrast, Tax of some but not all HTLV-II subtype A isolates, including the prototype Mo, had a greatly decreased ability to transactivate, and this could be directly correlated with a decrease in protein expression. Employment of cDNA clones encoding both Rex and Tax demonstrated that Rex was unable to rescue the expression or activity of the IIA Mo isolate. These studies demonstrate that with the exception of some HTLV-IIA subtypes there are no significant differences in Tax transactivation via the CREB and NF-kappaB pathways between the two viruses and suggest that the HTLV-II Tax may have a pathogenic potential equivalent to that of HTLV-I.     

Human T-cell leukemia virus type I isolates from Gabon and Ghana: comparative analysis of proviral genomes

Two isolates of human T-cell leukemia virus type I (HTLV-I) were obtained from lymphocyte cultures of a healthy carrier in Gabon and another in Ghana. Their proviruses were analyzed by Southern blot hybridization and compared with prototypical HTLV-I isolated in Japan and the United States. The provirus genomes of both strains were highly homologous to the prototype HTLV-I along the whole viral genome. The restriction endonuclease sites of the Ghanian isolate were almost identical with those of the prototype HTLV-I, but 10 of 26 sites of the Gabonese isolate were different from those of the prototype. Furthermore, the restriction map of the Gabonese isolate resembled those of a simian T-cell leukemia virus (STLV-I) isolated from a chimpanzee from Sierra Leone and a variant of HTLV-I from Zaire (HTLV-Ib) more closely than those of any other known HTLV-I. These results indicated the existence of some unique strains of HTLV-I transmitted among African people, and the importance of clarifying the origin and transmission of HTLV group viruses.     

Genomic evolution, patterns of global dissemination, and interspecies transmission of human and simian T-cell leukemia/lymphotropic viruses.

Using both env and long terminal repeat (LTR) sequences, with maximal representation of genetic diversity within primate strains, we revise and expand the unique evolutionary history of human and simian T-cell leukemia/lymphotropic viruses (HTLV/STLV). Based on the robust application of three different phylogenetic algorithms of minimum evolution-neighbor joining, maximum parsimony, and maximum likelihood, we address overall levels of genetic diversity, specific rates of mutation within and between different regions of the viral genome, relatedness among viral strains from geographically diverse regions, and estimation of the pattern of divergence of the virus into extant lineages. Despite broad genomic similarities, type I and type II viruses do not share concordant evolutionary histories. HTLV-I/STLV-I are united through distinct phylogeographic patterns, infection of 20 primate species, multiple episodes of interspecies transmission, and exhibition of a range in levels of genetic divergence. In contrast, type II viruses are isolated from only two species (Homo sapiens and Pan paniscus) and are paradoxically endemic to both Amerindian tribes of the New World and human Pygmy villagers in Africa. Furthermore, HTLV-II is spreading rapidly through new host populations of intravenous drug users. Despite such clearly disparate host populations, the resultant HTLV-II/STLV-II phylogeny exhibits little phylogeographic concordance and indicates low levels of transcontinental genetic differentiation. Together, these patterns generate a model of HTLV/STLV emergence marked by an ancient ancestry, differential rates of divergence, and continued global expansion.     

Comparative analysis of the HTLV-I Rex and HIV-1 Rev trans-regulatory proteins and their RNA response elements

The Rex proteins of types I and II human T-cell leukemia viruses (HTLV-I, HTLV-II) are required for expression of the viral structural gene products, gag and env and, thus, are essential for the replication of these pathogenic retroviruses. The action of Rex is sequence specific, requiring the presence of a cis-acting Rex response element located in the 3' long terminal repeat. This element corresponds to a predicted RNA secondary structure and functions in an orientation-dependent but position-independent manner. Rex acts through this response element to stimulate the nuclear export of the unspliced or singly spliced viral mRNA species encoding the virion structural proteins that are normally excluded from the cytoplasm. Although the Rex proteins of HTLV-I and HTLV-II can also function via the related Rev response element present in the env gene of the type I human immunodeficiency virus (HIV-1), the analogous HIV-1 Rev protein is unable to act on the HTLV-I Rex response element. This nonreciprocal pattern of genetic complementation by Rex and Rev suggests that these viral trans-regulators may interact directly with their RNA response elements.     

Human T-cell leukemia virus type II: primary structure analysis of the major internal protein, p24 and the nucleic acid binding protein, p15

The 24,000-molecular-weight major internal protein (p24) and the 15,000-molecular-weight nucleic acid binding protein (p15) of human T-cell leukemia virus type II (HTLV-II) were subjected to amino acid composition and amino-terminal amino acid sequence analysis. A comparison of amino acid composition of p24 and p15 of HTLV-II with those of the analogous proteins of HTLV-I revealed that these two proteins share overall similarity. Further, alignment of the amino-terminal amino acid sequence for the first 27 residues of p24 and 34 residues of p15 from HTLV-II showed extensive sequence homology with analogous proteins of HTLV-I. These data suggest that although disease associated with HTLV-I is malignant T-cell leukemia and that associated with HTLV-II is a relatively benign variant of hairy-cell leukemia, HTLV-I and HTLV-II are closely related to each other, at least in their gag-gene-encoded sequences.     

Comparative analysis of nucleotide sequences of the partial envelope gene (5' domain) among human T lymphotropic virus type I (HTLV-I) isolates

Human T-cell lymphotropic virus type I (HTLV-I) is associated with adult T-cell leukemia/lymphoma (ATL) and HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP). The nucleotide sequences of 640 bp of the proviral genome (positions 5158-5797) derived from 11 HTLV-I-infected persons were analyzed using the polymerase chain reaction and M13-based sequencing techniques. Patterns of single nucleotide substitutions were characterized from the extracellular domain of the envelope gene (gp46). Compared with other retroviruses, the nucleotide sequences of the HTLV-I external envelope gene are highly conserved among the genotypes studied. We found no evidence of dual infections with HTLV-II among the seropositive asymptomatic persons or in patients with either ATL or HAM/TSP. No unique sequence differences were observed in the envelope gene of the HTLV-I isolates derived from patients coinfected with human immunodeficiency virus type 1 (HIV-1). However, comparative analysis of these data and other published HTLV-I envelope sequences indicated the presence of four subtypes of HTLV-I in relation to their geographic origin.     

The Origin and Evolution of Human T-Cell Lymphotropic Virus Types I and II

Studies on human T-cell lymphotropic virus types I (HTLV-I) and II (HTLV-II) are briefly reviewed from the viewpoint of molecular evolution, with special reference to the evolutionary rate and evolutionary relationships among these viruses. In particular, it appears that, in contrast to the low level of variability of HTLV-I among different isolates, individual isolates form quasispecies structures. Elucidating the mechanisms connecting these two phenomena will be one of the future problems in the study of the molecular evolution of HTLV-I and HTLV-II.     

Use of a generic polymerase chain reaction assay detecting human T-lymphotropic virus (HTLV) types I,II and divergent simian strains in the evaluation of individuals with indeterminate HTLV serology

In countries with a low prevalence of human T-lymphotropic virus (HTLV) infection, indeterminate HTLV serologies are a major problem in blood bank screening because of the uncertainties about infection in these cases. The recent discovery of two new types of simian T-lymphotropic virus (STLV), which give an HTLV-indeterminate serology, raises the question whether indeterminate serologies in humans may be linked to new types of HTLV. Starting from a Tax sequence alignment of all available primate T-cell lymphotropic virus strains (PTLV), including the two new types STLV-PH969 and STLV-PP1664, we developed generic and type-specific nested polymerase chain reactions (PCRs). The generic PCR proved to be highly sensitive and cross-reactive for all four types of PTLV, while the discriminatory PCRs had a high sensitivity and a specificity of 100%. There was no cross-reactivity with human immunodeficiency virus (HIV), ensuring correct interpretation of results from coinfected patients. Among the 77 serologically indeterminate samples tested, 6 were found to be HTLV-IPCR positive and 1 was HTLV-II PCR positive. Sequencing of one of the HTLV-I PCR positives excluded PCR contamination, and revealed a divergent type of HTLV-I. The majority of the seroindeterminate samples (91%) were however HTLV-PCR negative, and no new types of HTLV were found. This new assay can identify otherwise undetected HTLV-I or HTLV-II infections and is a useful tool of screening for new types of HTLV among seroindeterminate samples. J. Med. Virol. 52:1–7, 1997.© 1997 Wiley-Liss, Inc.     

HTLV-Is in Argentina are phylogenetically similar to those of other South American countries,but different from HTLV-Is in Africa

To understand the origin and past dissemination of human T-cell leukemia/lymphotropic virus type I (HTLV-I) in Latin America, we conducted a phylogenetic study of five new HTLV-I isolates from Argentina. We sequenced partial fragments of long terminal repeats (LTR) of the new HTLV-Is, and then the sequences were subjected to a phylogenetic analysis for comparison with other HTLV-Is of various geographical origins. Our results indicated that all the isolates were members of the Cosmopolitan group. Furthermore, most (four out of five isolates) of the new HTLV-Is belonged to the Transcontinental (A) subgroup, the most widespread subgroup of the four subgroups in the Cosmopolitan group. In this subgroup, they were closely related to HTLV-Is found in other South American countries including those of Amerindians, and were different from those found in Africa. In contrast, the remaining one HTLV-I (ARGMF) did not show any clear similarity to known HTLV-I isolates belonging to the Cosmopolitan group. The close similarity of South American HTLV-Is strongly suggests a common origin of the virus in this continent. Our results do not support the proposed idea of recent introduction of HTLV-I into South America as a consequence of the slave trade from Africa, where phylogenetically different HTLV-Is predominate. J. Med. Virol. 55:152–160, 1998. © 1998 Wiley-Liss, Inc.     

Isolation, cloning, and complete nucleotide sequence of a phenotypically distinct Brazilian isolate of human T-lymphotropic virus type II (HTLV-II)

Analysis of human T-lymphotropic virus type II (HTLV-II) isolates from North America and Europe have demonstrated the existence of two molecular subtypes of the virus, HTLV-IIa and HTLV-IIb. Recently, studies on HTLV-II infections in Brazil have revealed isolates that are related phylogenetically to the HTLV-IIa subtype but have a HTLV-IIb phenotype with respect to the transactivating protein, tax. To more clearly define this relationship, HTLV-II was isolated from peripheral blood of an IVDA from Sao Paulo, Brazil (SP-WV), and the complete provirus was cloned and sequenced. Comparison of HTLV-II(SP-WV) nucleotide sequences to other available complete HTLV-II proviral sequences revealed that HTLV-II(SP-WV) is most closely related to HTLV-II(Mo), the prototypic HTLV-IIa subtype sequence. Phylogenetic analysis of LTR, env, and tax regions unequivocally demonstrated that HTLV-II(SP-WV) and all other Brazilian sequences examined are members of the IIa subtype. The predicted amino acid sequences of the major coding regions of HTLV-II(SP-WV) are also most closely related to HTLV-II(Mo), with the important exception of tax. The tax protein encoded by HTLV-II(SP-WV) is 96-99% identical to the tax of IIb isolates and is similar in that it has an additional 25 amino acids at the carboxy-terminus compared to the HTLV-II(Mo) tax with which it shares 91% identity. Analysis of tax stop codon usage of a number of HTLV-IIa isolates from North American, Europe, and Brazil demonstrated that isolates from the last region appear to be unique in their extended tax phenotype. It could be demonstrated that the extended tax proteins in the HTLV-IIb and Brazilian isolates had equivalent ability to transactivate the viral LTR, and studies with deletion mutants indicated that the extended C-terminus is not essential for transactivation. In contrast, the HTLV-IIa tax was found to have a greatly diminished ability to transactivate the viral LTR, which appeared to be a consequence of reduced expression of the protein. The studies show that although the Brazilian strains do not represent an entirely new subtype based on nucleotide sequence analysis they are a phenotypically unique molecular variant within the HTLV-IIa subtype.     

Human T-cell leukemia/lymphoma viruses. Life cycle, pathogenicity, epidemiology, and diagnosis.

Human T-cell leukemia/lymphoma virus type I (HTLV-I) was discovered in 1980, and it subsequently was found to be the cause of adult T-cell leukemia/lymphoma. A progressive neurologic disease known as tropical spastic paraparesis, or HTLV-I-associated myelopathy, has also been linked to infection with HTLV-I. A related virus, HTLV type II (HTLV-II), has been isolated from patients with hairy-cell leukemia, but it has not been proved to be the cause of any disease. In late 1988, US blood banks began screening all blood donations for antibodies to HTLV-I/II. This program has resulted in the identification of many unexpectedly seropositive blood donors and provided much information about the prevalence of HTLV-I/II in the United States. In this article, I review the replication of these agents, as well as their pathogenesis, diagnosis, and mechanisms of spread.     

Human T-cell lymphotropic virus types I and II infections in patients with leukaemia/lymphoma and in subjects with sexually transmitted diseases in Nigeria

Summary Serological assays that distinguish antibodies to human T-cell lymphotropic virus types I (HTLV-I) and type II (HTLV-II), and polymerase chain reaction (PCR) tests were used to investigate association of these two human retroviruses with several well-defined clinical conditions in Nigeria. We compared the frequency of HTLV-I and HTLV-II infections among patients with lymphoproliferative disorders (n=65), individuals with various sexually transmitted diseases (n=40), patients with genital candidiasis (n=25) and apparently healthy individuals (n=60). Serological analysis of blood samples from all four groups showed that 10 of the 190 (5.3%) individuals tested were confirmed positive for the presence of antibodies to HTLV-I (6) or HTLV-II (4). Using the PCR technique, specific HTLV-I or HTLV-II sequences were amplified from the genomic DNA of 4 of 6 HTLV-I seropositive and 3 of the 4 HTLV-II seropositive individuals respectively. However, sequences of both viruses were amplified from the genomic DNAs of the remaining 3 seropositive individuals. Since one of the 5 sets of primer pairs [(SK110 (II)/SK III (II)], which is used for specific identification of HTLV-II did not amplify the target sequence from the genomic DNAs of any of the 4 HTLV-II-confirmed seropositive individuals in this study, it suggested sequence diversity of these viruses in Nigeria. The virus-infected individuals identified in this study were one (1.5%) of the 65 patients with leukaemia/lymphoma (HTLV-I), 6 of 40 (15.0%) individuals (HTLV-I=1, HTLV-II=3, HTLV-I/II=2) with sexually transmitted diseases (STD), one of 25(4.0%) subjects with genital candidiasis for HTLV-I, and 2 of 60 (33.3%) healthy individuals (one for HTLV-I and one for HTLV-I/II). There was a significant difference (P<0.025) between the prevalence of HTLV-I/II infections among patients with lymphoma/leukaemia and those who attended STD clinic in Ibadan, Nigeria. This study also suggests that while HTLV-I and HTLV-II may be important sexually transmitted viruses, they may not be specific aetiological agents of the common lymphoproliferative disorders in Nigeria.     

Sequence variation of functional HTLV-II tax alleles among isolates from an endemic population: lack of evidence for oncogenic determinant in tax.

Human T-cell leukemia-lymphoma virus type II (HTLV-II) has been isolated from patients with hairy cell leukemia (HCL). We previously described a population with longstanding endemic HTLV-II infection, and showed that there is no increased risk for HCL in the affected groups. We thus have direct evidence that the endemic form(s) of HTLV-II cause HCL infrequently, if at all. By comparison, there is reason to suspect that the viruses isolated from patients with HCL had an etiologic role in the disease in those patients. One way to reconcile these conflicting observations is to consider that isolates of HTLV-II might differ in oncogenic potential. To determine whether the structure of the putative oncogenic determinant of HTLV-II, tax2, might differ in the new isolates compared to the tax of the prototype HCL isolate, MO, four new functional tax cDNAs were cloned from new isolates. Sequence analysis showed only minor (0.9-2.0%) amino acid variation compared to the published sequence of MO tax2. Some codons were consistently different from published sequences of the MO virus, but in most cases, such variations were also found in each of two tax2 clones we isolated from the MO T-cell line. These variations rendered the new clones more similar to the tax1 of the pathogenic virus HTLV-I. Thus we find no evidence that pathologic determinants of HTLV-II can be assigned to the tax gene.     

Differential diagnosis of HTLV-I and HTLV-II infections by restriction enzyme analysis of 'nested' PCR products.

A 'nested' polymerase chain reaction (PCR) assay is described which is capable of detecting single copies of human T-cell lymphotropic virus (HTLV) in genomic DNA extracted from peripheral blood mononuclear cells (PBMCs). A single set of 'nested' oligonucleotide primers, based on the highly conserved tax/rex region of the viral genome, was able to detect both HTLV-I and HTLV-II proviral sequences in clinical samples of diverse geographical origins, from the United States, Great Britain, Japan, the Caribbean, Italy, Greece, Iraq and West Africa. Rapid discrimination between HTLV-I and HTLV-II infections was achieved by restriction enzyme analysis of unpurified second-round PCR products, even in those cases in which serological assays had failed to provide a definitive result. Over a 2-year period, a total of 53 HTLV infections (37 HTLV-I and 16 HTLV-II) were identified by this technique and complete concordance with serological typing, available in 41 cases, was observed.     

Evidence in Gabon for an intrafamilial clustering with mother-to-child and sexual transmission of a new molecular variant of human T-lymphotropic virus type-II subtype B

Following the observation of an HTLV-II seropositive 60-year-old woman living in Gabon (Central Africa), a serologic and molecular study of her family members was conducted in an attempt to determine the duration of the HTLV-II infection and the modes of transmission of the virus. Among 41 family members, five were HTLV-I seropositive and 7 exhibited specific HTLV-II antibodies in their sera as demonstrated by high immunofluorescence titers on C19 cells and/or specific Western-blot pattern. The second husband of the index case and two of his sisters were infected by the virus, suggesting the presence of HTLV-II in this family over two generations. Sequence analysis of an amplified fragment of 172 nucleotides within the gp21 of the env region (6469–6640) of four HTLV-II infected individuals revealed a new HTLV-II molecular variant of the subtype b diverging from the prototypes NRA and G12 by seven (4.1%) and five (2.9%) bases substitutions, respectively. Molecular analysis of the total env gene (1462 bp) and fragments of the pol and pX regions confirmed that this new African variant was the most divergent HTLV-II subtype b yet described, exhibiting 2.3% of nucleotide substitutions in the env gene (33 bases) as compared to the two HTLV-II b prototypes. These data demonstrate, for the first time in Africa, intrafamilial both mother-to-child transmission and sexual transmission between spouses of an HTLV-II b molecular variant, and also suggest that this virus has been present in Gabon for a long period of time. © 1996 Wiley-Liss, Inc.