Protection of cynomolgus monkeys against infection by human T-cell leukemia virus type-I by immunization with viral env gene products produced in Escherichia coli

An HTLV-1 envelope mRNA vaccine is immunogenic and protective in New Zealand rabbits

Human T-cell leukemia virus type 1 (HTLV-1) is a retrovirus responsible for adult T-cell leukemia/lymphoma, a severe and fatal CD4+ T-cell malignancy. Additionally, HTLV-1 can lead to a chronic progressive neurodegenerative disease known as HTLV-1-associated myelopathy/tropical spastic paraparesis. Unfortunately, the prognosis for HTLV-1-related diseases is generally poor, and effective treatment options are limited. In this study, we designed and synthesized a codon optimized HTLV-1 envelope (Env) mRNA encapsulated in a lipid nanoparticle (LNP) and evaluated its efficacy as a vaccine candidate in an established rabbit model of HTLV-1 infection and persistence. Immunization regimens included a prime/boost protocol using Env mRNA-LNP or control green fluorescent protein (GFP) mRNA-LNP. After immunization, rabbits were challenged by intravenous injection with irradiated HTLV-1 producing cells. Three rabbits were partially protected and three rabbits were completely protected against HTLV-1 challenge. These rabbits were then rechallenged 15 weeks later, and two rabbits maintained sterilizing immunity. In Env mRNA-LNP immunized rabbits, proviral load and viral gene expression were significantly lower. After viral challenge in the Env mRNA-LNP vaccinated rabbits, an increase in both CD4+/IFN-γ+ and CD8+/IFN-γ+ T-cells was detected when stimulating with overlapping Env peptides. Env mRNA-LNP elicited a detectable anti-Env antibody response after prime/boost vaccination in all animals and significantly higher levels of neutralizing antibody activity. Neutralizing antibody activity was correlated with a reduction in proviral load. These findings hold promise for the development of preventive strategies and therapeutic interventions against HTLV-1 infection and its associated diseases.IMPORTANCEmRNA vaccine technology has proven to be a viable approach for effectively triggering immune responses that protect against or limit viral infections and disease. In our study, we synthesized a codon optimized human T-cell leukemia virus type 1 (HTLV-1) envelope (Env) mRNA that can be delivered in a lipid nanoparticle (LNP) vaccine approach. The HTLV-1 Env mRNA-LNP produced protective immune responses against viral challenge in a preclinical rabbit model. HTLV-1 is primarily transmitted through direct cell-to-cell contact, and the protection offered by mRNA vaccines in our rabbit model could have significant implications for optimizing the development of other viral vaccine candidates. This is particularly important in addressing the challenge of enhancing protection against infections that rely on cell-to-cell transmission.

HTLV-1 Proliferation after CD8+ Cell Depletion by Monoclonal Anti-CD8 Antibody Administration in Latently HTLV-1-Infected Cynomolgus Macaques

Human T-cell leukemia virus type 1 (HTLV-1) induces chronic asymptomatic latent infection with a substantial proviral load but without significant viral replication in vivo. Cumulative studies have indicated involvement of CD8-positive (CD8+) cells, including virus-specific CD8+ T cells in the control of HTLV-1 replication. However, whether HTLV-1 expression from latently infected cells in vivo occurs in the absence of CD8+ cells remains unclear. Here, we examined the impact of CD8+ cell depletion by monoclonal anti-CD8 antibody administration on proviral load in HTLV-1-infected cynomolgus macaques. Five cynomolgus macaques were infected with HTLV-1 by inoculation with HTLV-1-producing cells. Administration of monoclonal anti-CD8 antibody in the chronic phase resulted in complete depletion of peripheral CD8+ T cells for approximately 2 months. All five macaques showed an increase in proviral load following CD8+ cell depletion, which peaked just before the reappearance of peripheral CD8+ T cells. Tax-specific CD8+ T-cell responses were detected in these recovered CD8+ T cells. Importantly, anti-HTLV-1 antibodies also increased after CD8+ cell depletion, indicating HTLV-1 antigen expression. These results provide evidence indicating that HTLV-1 can proliferate from the latent phase in the absence of CD8+ cells and suggest that CD8+ cells are responsible for the control of HTLV-1 replication. IMPORTANCE HTLV-1 can cause serious diseases such as adult T-cell leukemia (ATL) in humans after chronic asymptomatic latent infection with substantial proviral load. Proviruses are detectable in peripheral lymphocytes in HTLV-1 carriers, and the association of a higher proviral load with a higher risk of disease progression has been observed. However, neither substantial viral structural protein expression nor viral replication was detectable in vivo. Cumulative studies have indicated involvement of CD8+ cells, including virus-specific CD8+ T cells in the control of HTLV-1 replication. In the present study, we showed that CD8+ cell depletion by monoclonal anti-CD8 antibody administration results in HTLV-1 expression and an increase in proviral load in HTLV-1-infected cynomolgus macaques. Our results indicate that HTLV-1 can proliferate in the absence of CD8+ cells, suggesting that CD8+ cells are responsible for the control of HTLV-1 replication. This study provides insights into the mechanism of virus-host immune interaction in latent HTLV-1 infection.

Human T-cell lymphotropic virus type 1 (HTLV-1) proposed vaccines: a systematic review of preclinical and clinical studies

BACKGROUND

Numerous vaccination research experiments have been conducted on non-primate hosts to prevent or control HTLV-1 infection. Therefore, reviewing recent advancements for status assessment and strategic planning of future preventative actions to reduce HTLV-1 infection and its consequences would be essential.

METHODS

MEDLINE, Scopus, Web of Science, and Clinicaltrials.gov were searched from each database's inception through March 27, 2022. All original articles focusing on developing an HTLV-1 vaccine candidate were included.

RESULTS

A total of 47 studies were included. They used a variety of approaches to develop the HTLV-1 vaccine, including DNA-based, dendritic-cell-based, peptide/protein-based, and recombinant vaccinia virus approaches. The majority of the research that was included utilized Tax, Glycoprotein (GP), GAG, POL, REX, and HBZ as their main peptides in order to develop the vaccine. The immunization used in dendritic cell-based investigations, which were more recently published, was accomplished by an activated CD-8 T-cell response. Although there hasn't been much attention lately on this form of the vaccine, the initial attempts to develop an HTLV-1 immunization depended on recombinant vaccinia virus, and the majority of results seem positive and effective for this type of vaccine. Few studies were conducted on humans. Most of the studies were experimental studies using animal models. Adenovirus, Cytomegalovirus (CMV), vaccinia, baculovirus, hepatitis B, measles, and pox were the most commonly used vectors.

CONCLUSIONS

This systematic review reported recent progression in the development of HTLV-1 vaccines to identify candidates with the most promising preventive and therapeutic effects.

Vaccination with short-term-cultured autologous PBMCs efficiently activated STLV-1-specific CTLs in naturally STLV-1-infected Japanese monkeys with impaired CTL responses

A small proportion of human T-cell leukemia virus type-1 (HTLV-1)-infected individuals develop adult T-cell leukemia/lymphoma, a chemotherapy-resistant lymphoproliferative disease with a poor prognosis. HTLV-1-specific cytotoxic T lymphocytes (CTLs), potential anti-tumor/virus effectors, are impaired in adult T-cell leukemia/lymphoma patients. Here, using Japanese monkeys naturally infected with simian T-cell leukemia/T-lymphotropic virus type-1 (STLV-1) as a model, we demonstrate that short-term-cultured autologous peripheral blood mononuclear cells (PBMCs) can serve as a therapeutic vaccine to activate such CTLs. In a screening test, STLV-1-specific CTL activity was detectable in 8/10 naturally STLV-1-infected monkeys. We conducted a vaccine study in the remaining two monkeys with impaired CTL responses. The short-term-cultured PBMCs of these monkeys spontaneously expressed viral antigens, in a similar way to PBMCs from human HTLV-1 carriers. The first monkey was subcutaneously inoculated with three-day-cultured and mitomycin C (MMC)-treated autologous PBMCs, and then boosted with MMC-treated autologous STLV-1-infected cell line cells. The second monkey was inoculated with autologous PBMC-vaccine alone twice. In addition, a third monkey that originally showed a weak STLV-1-specific CTL response was inoculated with similar autologous PBMC-vaccines. In all three vaccinated monkeys, marked activation of STLV-1-specific CTLs and a mild reduction in the STLV-1 proviral load were observed. Follow-up analyses on the two monkeys vaccinated with PBMCs alone indicated that STLV-1-specific CTL responses peaked at 3-4 months after vaccination, and then diminished but remained detectable for more than one year. The significant reduction in the proviral load and the control of viral expression were associated with CTL activation but also diminished 6 and 12 months after vaccination, respectively, suggesting the requirement for a booster. The vaccine-induced CTLs in these monkeys recognized epitopes in the STLV-1 Tax and/or Envelope proteins, and efficiently killed autologous STLV-1-infected cells in vitro. These findings indicated that the autologous PBMC-based vaccine could induce functional STLV-1-specific CTLs in vivo.

Advances in preventive vaccine development against HTLV-1 infection: A systematic review of the last 35 years

Introduction

The Human T-lymphotropic virus type 1 (HTLV-1) was the first described human retrovirus. It is currently estimated that around 5 to 10 million people worldwide are infected with this virus. Despite its high prevalence, there is still no preventive vaccine against the HTLV-1 infection. It is known that vaccine development and large-scale immunization play an important role in global public health. To understand the advances in this field we performed a systematic review regarding the current progress in the development of a preventive vaccine against the HTLV-1 infection.

Methods

This review followed the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA®) guidelines and was registered at the International Prospective Register of Systematic Reviews (PROSPERO). The search for articles was performed in PubMed, Lilacs, Embase and SciELO databases. From the 2,485 articles identified, 25 were selected according to the inclusion and exclusion criteria.

Results

The analysis of these articles indicated that potential vaccine designs in development are available, although there is still a paucity of studies in the human clinical trial phase.

Discussion

Although HTLV-1 was discovered almost 40 years ago, it remains a great challenge and a worldwide neglected threat. The scarcity of funding contributes decisively to the inconclusiveness of the vaccine development. The data summarized here intends to highlight the necessity to improve the current knowledge of this neglected retrovirus, encouraging for more studies on vaccine development aiming the to eliminate this human threat.

Systematic review registration

https://www.crd.york.ac.uk/prospero, identifier (CRD42021270412).

A role for an HTLV-1 vaccine?

HTLV-1 is a global infection with 5-20 million infected individuals. Although only a minority of infected individuals develop myelopathy, lymphoproliferative malignancy, or inflammatory disorders, infection is associated with immunosuppression and shorter survival. Transmission of HTLV-1 is through contaminated blood or needles, mother-to-child exposure through breast-feeding, and sexual intercourse. HTLV-1 is a delta retrovirus that expresses immunogenic Gag, Envelope, TAX, and Hbz proteins. Neutralizing antibodies have been identified directed against the surface envelope protein, and cytotoxic T-cell epitopes within TAX have been characterized. Thus far, there have been few investigations of vaccines directed against each of these proteins, with limited responses, thus far. However, with new technologies developed in the last few years, a renewed investigation is warranted in search for a safe and effective HTLV-1 vaccine.

Human T-cell Lymphotropic Viruses: Review and Prospects for Antiviral Therapy

The human T-cell lymphotropic viruses types I and II (HTLV-I, II) pose challenges to researchers and clinicians who seek to unveil mechanisms of viral transformation and pathogenesis. HTLV-I infection in humans is associated with a wide array of primary and secondary diseases ranging from mild immunosuppression to adult T-cell leukaemia/lymphoma and HTLV-associated myelopathy/tropical spastic paraparesis (HAM/TSP), a neurological degenerative syndrome. As retroviruses, HTLV-I and II share similar replicative cycles with human immunodeficiency virus (HIV), the causative agent of acquired immunodeficiency syndrome. However, in contrast to HIV-I which destroys CD4+ T cells, HTLV-I and II can preferentially transform a CD4+ T-cell subset to an unrestricted growth state.

HTLV-I and II, along with simian T-lymphotropic virus (STLV) and bovine leukaemia virus (BLV), form a phylogenetic group which is distinct from ungulate, non-human primate and human lentiviruses such as visna, simian immunodeficiency virus (SIV), and human immunodeficiency viruses types 1 and 2. The proviral genome of HTLV-I is flanked at the 5′ and 3′ ends by long terminal repeats (LTR) and is further subdivided into structural gag and env genes, a pro gene encoding an aspartyl protease, a pol gene which encodes reverse transcriptase and endonuclease, and the regulatory gene elements tax and rex. Regions within the LTR contain recognition sites for cellular proteins and the tax gene product that collectively promote viral expression. Tax-mediated activation of cellular genes involved in growth and differentiation is suspected to play a dominant role in the leukaemogenic process associated with HTLV-I infection. Differential rex-regulated splicing of viral message gives rise to transcripts encoding the polyprotein precursor gag-pro-pol (unspliced), envelope (single spliced), or tax/rex (doubly spliced). The 100nm HTLV virion contains an electron-dense core surrounding a divalent-single stranded DNA genome. This core is in turn enclosed by concentric shells of matrix protein and an outer lipid bilayer, the latter acquired as the virus buds from the surface of the infected cell. Envelope glycoproteins associated with the outside of this lipid bilayer can interact with viral receptors on cells and mediate virus entry. Antiviral strategies have been directed at inhibiting viral entry into cells (sulphated and non-sulphated polysaccharides, vaccines), blocking of viral replication (AZT, suramin), intracellular immunization (transdominant repression of rex), and elimination of virus infected cells (IL-2 receptor-directed toxins). Serological screening of the blood supply and curtailing breast feeding of children by HTLV-I + mothers have likely had a major impact in preventing HTLV-I infection.

The neutralizing function of the anti-HTLV-1 antibody is essential in preventing in vivo transmission of HTLV-1 to human T cells in NOD-SCID/γcnull (NOG) mice

Background

Human T-cell leukemia virus type 1 (HTLV-1) causes both neoplastic and inflammatory diseases, including adult T-cell leukemia and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). Because these life-threatening and disabling diseases are not yet curable, it is important to prevent new HTLV-1 infections.

Findings

In this study, we have established a simple humanized mouse model of HTLV-1 infection for evaluating prophylactic and therapeutic interventions. In this model, HTLV-1-negative normal human peripheral blood mononuclear cells (PBMCs) are transplanted directly into the spleens of severely immunodeficient NOD-SCID/γcnull (NOG) mice, together with mitomycin-treated HTLV-1-producing T cells. Using this model, we tested the efficacy of monoclonal antibodies (mAbs) specific to HTLV-1 as well as human IgG isolated from HAM/TSP patients (HAM-IgG) in preventing HTLV-1-infection. One hour before and 24 h after transplantation of the human cells, each antibody sample was inoculated intraperitoneally. On day 14, human PBMCs isolated from the mouse spleens were tested for HTLV-1 infection. Whereas fresh CD4-positive and CD8-positive T cells isolated from untreated mice or mice treated with isotype control mAb, HTLV-1 non-neutralizing mAbs to envelope gp46, gag p19, and normal human IgG were all infected with HTLV-1; the mice treated with either HTLV-1 neutralizing anti-gp46 mAb or HAM-IgG did not become infected.

Conclusions

Our data indicate that the neutralizing function of the antibody, but not the antigen specificity, is essential for preventing the in vivo transmission of HTLV-1. The present animal model will also be useful for the in vivo evaluation of the efficacy of candidate molecules to be used as prophylactic and therapeutic intervention against HTLV-1 infection.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-014-0074-z) contains supplementary material, which is available to authorized users.

Animal Models Utilized in HTLV-1 Research

Since the isolation and discovery of human T-cell leukemia virus type 1 (HTLV-1) over 30 years ago, researchers have utilized animal models to study HTLV-1 transmission, viral persistence, virus-elicited immune responses, and HTLV-1-associated disease development (ATL, HAM/TSP). Non-human primates, rabbits, rats, and mice have all been used to help understand HTLV-1 biology and disease progression. Non-human primates offer a model system that is phylogenetically similar to humans for examining viral persistence. Viral transmission, persistence, and immune responses have been widely studied using New Zealand White rabbits. The advent of molecular clones of HTLV-1 has offered the opportunity to assess the importance of various viral genes in rabbits, non-human primates, and mice. Additionally, over-expression of viral genes using transgenic mice has helped uncover the importance of Tax and Hbz in the induction of lymphoma and other lymphocyte-mediated diseases. HTLV-1 inoculation of certain strains of rats results in histopathological features and clinical symptoms similar to that of humans with HAM/TSP. Transplantation of certain types of ATL cell lines in immunocompromised mice results in lymphoma. Recently, “humanized” mice have been used to model ATL development for the first time. Not all HTLV-1 animal models develop disease and those that do vary in consistency depending on the type of monkey, strain of rat, or even type of ATL cell line used. However, the progress made using animal models cannot be understated as it has led to insights into the mechanisms regulating viral replication, viral persistence, disease development, and, most importantly, model systems to test disease treatments.

Neuroimmunological aspects of human T cell leukemia virus type 1-associated myelopathy/tropical spastic paraparesis

Human T cell leukemia virus type 1 (HTLV-1) is a human retrovirus etiologically associated with adult T cell leukemia/lymphoma and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). Only approximately 0.25-4 % of infected individuals develop HAM/TSP; the majority of infected individuals remain lifelong asymptomatic carriers. Recent data suggest that immunological aspects of host-virus interactions might play an important role in the development and pathogenesis of HAM/TSP. This review outlines and discusses the current understanding, ongoing developments, and future perspectives of HAM/TSP research.

Cellular Factors Involved in HTLV-1 Entry and Pathogenicit

Human T cell leukemia virus type 1 (HTLV-1) is the causative agent of adult T cell leukemia (ATL) and HTLV-1 – associated myelopathy and tropical spastic paraparesis (HAM/TSP). HTLV-1 has a preferential tropism for CD4 T cells in healthy carriers and ATL patients, while both CD4 and CD8 T cells serve as viral reservoirs in HAM/TSP patients. HTLV-1 has also been detected other cell types, including monocytes, endothelial cells, and dendritic cells. In contrast to the limited cell tropism of HTLV-1 in vivo, the HTLV receptor appears to be expressed in almost all human or animal cell lines. It remains to be examined whether this cell tropism is determined by host factors or by HTLV-1 heterogeneity. Unlike most retroviruses, cell-free virions of HTLV-1 are very poorly infectious. The lack of completely HTLV-1-resistant cells and the low infectivity of HTLV-1 have hampered research on the HTLV entry receptor. Entry of HTLV-1 into target cells is thought to involve interactions between the env (Env) glycoproteins, a surface glycoprotein (surface unit), and a transmembrane glycoprotein. Recent studies have shown that glucose transporter GLUT1, heparan sulfate proteoglycans (HSPGs), and neuropilin-1 (NRP-1) are the three proteins important for the entry of HTLV-1. Studies using adherent cell lines have shown that GLUT1 can function as a receptor for HTLV. HSPGs are required for efficient entry of HTLV-1 into primary CD4 T cells. NRP-1 is expressed in most established cell lines. Further studies have shown that these three molecules work together to promote HTLV-1 binding to cells and fusion of viral and cell membranes. The virus could first contact with HSPGs and then form complexes with NRP-1, followed by association with GLUT1. It remains to be determined whether these three molecules can explain HTLV-1 cell tropism. It also remains to be more definitively proven that these molecules are sufficient to permit HTLV-1 entry into completely HTLV-1-resistant cells.

Role of Nucleocytoplasmic RNA Transport during the Life Cycle of Retroviruses

Retroviruses have evolved mechanisms for transporting their intron-containing RNAs (including genomic and messenger RNAs, which encode virion components) from the nucleus to the cytoplasm of the infected cell. Human retroviruses, such as human immunodeficiency virus (HIV) and human T cell leukemia virus type 1 (HTLV-1), encode the regulatory proteins Rev and Rex, which form a bridge between the viral RNA and the export receptor CRM1. Recent studies show that these transport systems are not only involved in RNA export, but also in the encapsidation of genomic RNA; furthermore, they influence subsequent events in the cytoplasm, including the translation of the cognate mRNA, transport of Gag proteins to the plasma membrane, and the formation of virus particles. Moreover, the mode of interaction between the viral and cellular RNA transport machinery underlies the species-specific propagation of HIV-1 and HTLV-1, forming the basis for constructing animal models of infection. This review article discusses recent progress regarding these issues.

Immunopathogenesis of human T-cell leukemia virus type-1-associated myelopathy/tropical spastic paraparesis: recent perspectives

Human T-cell leukemia virus type-1 (HTLV-1) is a replication-competent human retrovirus associated with two distinct types of disease only in a minority of infected individuals: the malignancy known as adult T-cell leukemia (ATL) and a chronic inflammatory central nervous system disease HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). HAM/TSP is a chronic progressive myelopathy characterized by spastic paraparesis, sphincter dysfunction, and mild sensory disturbance in the lower extremities. Although the factors that cause these different manifestations of HTLV-1 infection are not fully understood, accumulating evidence from host population genetics, viral genetics, DNA expression microarrays, and assays of lymphocyte function suggests that complex virus-host interactions and the host immune response play an important role in the pathogenesis of HAM/TSP. Especially, the efficiency of an individual's cytotoxic T-cell (CTL) response to HTLV-1 limits the HTLV-1 proviral load and the risk of HAM/TSP. This paper focuses on the recent advances in HAM/TSP research with the aim to identify the precise mechanisms of disease, in order to develop effective treatment and prevention.

Molecular determinants of human T-lymphotropic virus type 1 transmission and spread

Human T-lymphotrophic virus type-1 (HTLV-1) infects approximately 15 to 20 million people worldwide, with endemic areas in Japan, the Caribbean, and Africa. The virus is spread through contact with bodily fluids containing infected cells, most often from mother to child through breast milk or via blood transfusion. After prolonged latency periods, approximately 3 to 5% of HTLV-1 infected individuals will develop either adult T-cell leukemia/lymphoma (ATL), or other lymphocyte-mediated disorders such as HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). The genome of this complex retrovirus contains typical gag, pol, and env genes, but also unique nonstructural proteins encoded from the pX region. These nonstructural genes encode the Tax and Rex regulatory proteins, as well as novel proteins essential for viral spread in vivo such as, p30, p12, p13 and the antisense encoded HBZ. While progress has been made in the understanding of viral determinants of cell transformation and host immune responses, host and viral determinants of HTLV-1 transmission and spread during the early phases of infection are unclear. Improvements in the molecular tools to test these viral determinants in cellular and animal models have provided new insights into the early events of HTLV-1 infection. This review will focus on studies that test HTLV-1 determinants in context to full length infectious clones of the virus providing insights into the mechanisms of transmission and spread of HTLV-1.

Preventive and therapeutic strategies for bovine leukemia virus: lessons for HTLV

Bovine leukemia virus (BLV) is a retrovirus closely related to the human T-lymphotropic virus type 1 (HTLV-1). BLV is a major animal health problem worldwide causing important economic losses. A series of attempts were developed to reduce prevalence, chiefly by eradication of infected cattle, segregation of BLV-free animals and vaccination. Although having been instrumental in regions such as the EU, these strategies were unsuccessful elsewhere mainly due to economic costs, management restrictions and lack of an efficient vaccine. This review, which summarizes the different attempts previously developed to decrease seroprevalence of BLV, may be informative for management of HTLV-1 infection. We also propose a new approach based on competitive infection with virus deletants aiming at reducing proviral loads.

Human T Lymphotropic Virus Type 1 (HTLV-1): Molecular Biology and Oncogenesis

Human T lymphotropic viruses (HTLVs) are complex deltaretroviruses that do not contain a proto-oncogene in their genome, yet are capable of transforming primary T lymphocytes both in vitro and in vivo. There are four known strains of HTLV including HTLV type 1 (HTLV-1), HTLV-2, HTLV-3 and HTLV-4. HTLV-1 is primarily associated with adult T cell leukemia (ATL) and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). HTLV-2 is rarely pathogenic and is sporadically associated with neurological disorders. There have been no diseases associated with HTLV-3 or HTLV-4 to date. Due to the difference in the disease manifestation between HTLV-1 and HTLV-2, a clear understanding of their individual pathobiologies and the role of various viral proteins in transformation should provide insights into better prognosis and prevention strategies. In this review, we aim to summarize the data accumulated so far in the transformation and pathogenesis of HTLV-1, focusing on the viral Tax and HBZ and citing appropriate comparisons to HTLV-2.

Mouse models of human T lymphotropic virus type-1-associated adult T-cell leukemia/lymphoma

Human T-lymphotropic virus type-1 (HTLV-1), the first human retrovirus discovered, is the causative agent of adult T-cell leukemia/lymphoma (ATL) and a number of lymphocyte-mediated inflammatory conditions including HTLV-1-associated myelopathy/tropical spastic paraparesis. Development of animal models to study the pathogenesis of HTLV-1-associated diseases has been problematic. Mechanisms of early infection and cell-to-cell transmission can be studied in rabbits and nonhuman primates, but lesion development and reagents are limited in these species. The mouse provides a cost-effective, highly reproducible model in which to study factors related to lymphoma development and the preclinical efficacy of potential therapies against ATL. The ability to manipulate transgenic mice has provided important insight into viral genes responsible for lymphocyte transformation. Expansion of various strains of immunodeficient mice has accelerated the testing of drugs and targeted therapy against ATL. This review compares various mouse models to illustrate recent advances in the understanding of HTLV-1-associated ATL development and how improvements in these models are critical to the future development of targeted therapies against this aggressive T-cell lymphoma.

Development of a cytotoxic T-cell assay in rabbits to evaluate early immune response to human T-lymphotropic virus type 1 infection

Human T-lymphotropic virus type 1 (HTLV-1) infection causes adult T-cell lymphoma/leukemia (ATL) following a prolonged clinical incubation period, despite a robust adaptive immune response against the virus. Early immune responses that allow establishment of the infection are difficult to study without effective animal models. We have developed a cytotoxic T-lymphocyte (CTL) assay to monitor the early events of HTLV-1 infection in rabbits. Rabbit skin fibroblast cell lines were established by transformation with a plasmid expressing simian virus 40 (SV40) large T antigen and used as autochthonous targets (derived from same individual animal) to measure CTL activity against HTLV-1 infection in rabbits. Recombinant vaccinia virus (rVV) constructs expressing either HTLV-1 envelope surface unit (SU) glycoprotein 46 or Tax proteins were used to infect fibroblast targets in a (51)Cr-release CTL assay. Rabbits inoculated with Jurkat T cells or ACH.2 cells (expressing ACH HTLV-1 molecule clone) were monitored at 0, 2, 4, 6, 8, 13, 21, and 34 wk post-infection. ACH.2-inoculated rabbits were monitored serologically and for viral infected cells following ex vivo culture. Proviral load analysis indicated that rabbits with higher proviral loads had significant CTL activity against HTLV-1 SU as early as 2 wk post-infection, while both low- and high-proviral-load groups had minimal Tax-specific CTL activity throughout the study. This first development of a stringent assay to measure HTLV-1 SU and Tax-specific CTL assay in the rabbit model will enhance immunopathogenesis studies of HTLV-1 infection. Our data suggest that during the early weeks following infection, HTLV-1-specific CTL responses are primarily targeted against Env-SU.

Human T cell lymphotropic virus: necessity for and feasibility of a vaccine

Human T cell lymphotropic virus types I and II (HTLV-I/II) are endemic in certain areas of the world. They cause two life-threatening diseases, adult T cell leukaemia/lymphoma and tropical spastic paraparesis. A vaccine is needed because in developing countries there are no other feasible preventive interventions against these diseases and in Western countries intravenous drug users at high risk for HTLV-I and HTLV-II infections and the health workers in contact with such populations must be protected. We have developed a rat model in which we observed variations of susceptibility to viral infection between inbred strains, the most susceptible being the Fischer F344, and the possibility of viral latency in the nervous system. We have prepared a recombinant adenovirus vector that expresses the HTLV-I envelope glycoprotein env in HeLa cells. A target human population in French Guyana, in which the prevalence rate reaches 5.6% in one ethnic group (Bonis), has been identified for possible intervention.

Enhanced replication of human T-cell leukemia virus type 1 in T cells from transgenic rats expressing human CRM1 that is regulated in a natural manner

Human T-cell leukemia virus type 1 (HTLV-1) is the etiologic agent of adult T-cell leukemia (ATL). To develop a better animal model for the investigation of HTLV-1 infection, we established a transgenic (Tg) rat carrying the human CRM1 (hCRM1) gene, which encodes a viral RNA transporter that is a species-specific restriction factor. At first we found that CRM1 expression is elaborately regulated through a pathway involving protein kinase C during lymphocyte activation, initially by posttranscriptional and subsequently by transcriptional mechanisms. This fact led us to use an hCRM1-containing bacterial artificial chromosome clone, which would harbor the entire regulatory and coding regions of the CRM1 gene. The Tg rats expressed hCRM1 protein in a manner similar to expression of intrinsic rat CRM1 in various organs. HTLV-1-infected T-cell lines derived from these Tg rats produced 100- to 10,000-fold more HTLV-1 than did T cells from wild-type rats, and the absolute levels of HTLV-1 were similar to those produced by human T cells. We also observed enhancement of the dissemination of HTLV-1 to the thymus in the Tg rats after intraperitoneal inoculation, although the proviral loads were low in both wild-type and Tg rats. These results support the essential role of hCRM1 in proper HTLV-1 replication and suggest the importance of this Tg rat as an animal model for HTLV-1.

[Cloning and transmembrane glycoprotein expression of the retrovirus HTLV-1 in mammals' cells]

The retrovirus HTLV-1 is the etiological agent of the adult T-cell leukemia and HTLV-1 associated myelopathy/tropical spastic paraparesis. The proviral genome has 9,032 base pairs, showing regulatory and structural genes. The env gene encodes for the transmembrane glycoprotein gp 21. The development of methodologies for heterologous protein expression, as well as the acquisition of a cellular line that constituently expresses the recombinant, were the main goals of this work. The DNA fragment that encodes for gp 21 was amplified by nested-PCR and cloned into a pCR2.1-TOPO vector. After which, a sub-cloning was realized using the expressing vector pcDNA3.1+. The transfection of mammalian cells HEK 293 was performed transitorily and permanently. Production of the recombinant gp 21 was confirmed by flux cytometry experiments and the cell line producing protein will be used in immunogenicity assays.

Protective efficacy of multiepitope human leukocyte antigen-A*0201 restricted cytotoxic T-lymphocyte peptide construct against challenge with human T-cell lymphotropic virus type 1 Tax recombinant vaccinia virus

Human T-cell lymphotropic virus type 1 (HTLV-1) is the causative agent of adult T-cell leukemia. Multiepitope T-cell vaccines are more likely to generate a broad long-lasting immune response than those composed of single epitopes. We recently reported a novel multivalent cytotoxic T-lymphocyte peptide construct derived from the Tax protein of HTLV-1 separated by arginine spacers that elicited high cellular responses against individual epitopes simultaneously in human leukocyte antigen (HLA)-A*0201 transgenic mice. We now report the effect of epitope orientation on the processing of the multiepitope construct by 20s proteasomes and the effect of the processing rates on the immunogenicity of the intended epitopes. A positive correlation was found between processing rates and the immunogenicity of the intended epitopes. The construct with the highest immunogenicity for each epitope was tested for protective efficacy in a preclinical model of infection using HTLV-1 Tax recombinant vaccinia virus and HLA-A*0201 transgenic mice. Mice vaccinated with the multiepitope construct displayed a statistically significant reduction in viral replication that was dependent on CD8 T cells. Reduction in viral replication was also confirmed to be specific to Tax-vaccinia virus. These results demonstrate the activation of Tax-specific CD8+ T cells by vaccination and are supportive of a multivalent peptide vaccine approach against HTLV-1 infections.

Animal models for human T-lymphotropic virus type 1 (HTLV-1) infection and transformation

Over the past 25 years, animal models of human T-lymphotropic virus type 1 (HTLV-1) infection and transformation have provided critical knowledge about viral and host factors in adult T-cell leukemia/lymphoma (ATL). The virus consistently infects rabbits, some non-human primates, and to a lesser extent rats. In addition to providing fundamental concepts in viral transmission and immune responses against HTLV-1 infection, these models have provided new information about the role of viral proteins in carcinogenesis. Mice and rats, in particular immunodeficient strains, are useful models to assess immunologic parameters mediating tumor outgrowth and therapeutic invention strategies against lymphoma. Genetically altered mice including both transgenic and knockout mice offer important models to test the role of specific viral and host genes in the development of HTLV-1-associated lymphoma. Novel approaches in genetic manipulation of both HTLV-1 and animal models are available to address the complex questions that remain about viral-mediated mechanisms of cell transformation and disease. Current progress in the understanding of the molecular events of HTLV-1 infection and transformation suggests that answers to these questions are approachable using animal models of HTLV-1-associated lymphoma.

New insights in HTLV-I phylogeny by sequencing and analyzing the entire envelope gene

The HTLV-I envelope plays a major role in the process of target cell infection. It is implied in the recognition of the viral receptor(s), penetration of the viral genetic material, and induction of host immunity to the virus. It is thus important to study the genetic variability of the viral env gene as well as its variation in terms of evolution. In a new approach to these features, we sequenced the entire env gene of 65 HTLV-I isolates originating from Gabon, French Guiana, West Indies, and Iran, such isolates representing all major HTLVI phylums but the Australo-Melanesian one. The sequences obtained and all PTLV-I (HTLV-I and STLV-I) env sequences available in the literature were analyzed. Phylogenetic studies using different algorithms (minimum evolution, neighbor joining, maximum parsimony, and maximum likelihood) gave the same clear-cut results. Newly sequenced HTLV-I isolates described in this report allocated in three well-defined subtypes: Cosmopolitan, Central African, and a new distinct one that we termed "Maroni" subtype (present in the Maroni Basin, French Guiana, and West Indies). Clearly, the most divergent PTLV-I strains present in Asia- Australo-Melanesia as well as African and Asian STLV-I derived from the same node in the phylogenetic tree as isolates of the Central African subtype. In addition, we showed that within each PTLV-I subtype, groups of isolates may be characterized by nonrandom and systematically associated mutations.

Association of primate T-cell lymphotropic virus infection of pig-tailed macaques with high mortality

Natural infection of humans with human T-cell lymphotropic virus type I (HTLV-I) and of old world nonhuman primates with the simian counterpart, STLV-I, is associated with development of neoplastic disease in a small percentage of individuals after long latent periods. HTLV-I is also the etiologic agent of a more rapidly progressive neurologic disease, HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP). Macaques have been used experimentally in studies to evaluate HTLV-I candidate vaccines for efficacy, but no evidence of disease was observed. Here we report experimental infection of pig-tailed macaques with STLV-I(sm) and HTLV-I(ACH), both of which were associated with a disease syndrome characterized by rapid onset, hypothermia, lethargy, and death within hours to days. Other pathologic sequelae included diarrhea, rash, bladder dysfunction, weight loss, and, in one animal, arthropathy. Both retroviruses were detected in the central nervous systems of some animals, either by culture or by direct antigen capture for p19 Gag in cerebrospinal fluid. Although virus was recovered throughout infection from peripheral blood mononuclear cells (PBMC), all infected macaques maintained low antiviral antibody titers and stable proviral burdens, which generally ranged between 10 and 100 copies per 10(6) PBMC. However, of 13 macaques infected with HTLV-I(ACH) or STLV-I(sm), seven animals (54%) died between 35 weeks and 412 years after infection. This unexpected high mortality within a relatively short time suggests that infection of pig-tailed macaques might be a useful model for studying immune responses to and pathologic events resulting from HTLV-I infection.

New approach for generation of neutralizing antibody against human T-cell leukaemia virus type-I (HTLV-I) using phage clones

We have screened a phage peptide library to address whether clones binding to a monoclonal antibody (mAb) could be isolated and if the selected phage particles would be able to elicit an in vivo immune response against the original antigen. A phage peptide library, consisting of seven random amino acids inserted in the minor coat protein (pIII), was screened for specific binding to a rat mAb LAT-27, which is capable of neutralizing human T-cell leukaemia virus type-I (HTLV-I) by binding to its envelope gp46 epitope, (amino acids LPHSNL). Total 37 clones were selected from the library and one clone named 4-2-22 was tested for its immunogenicity in three rabbits. The all rabbit immune sera showed strong binding activity to a gp46 peptide carrying the neutralization sequence, stained gp46-expressing cells and neutralized HTLV-I in vitro as determined by cell fusion inhibition assay. These results show that the selected phage clone was capable of mimicking the epitope recognized by a HTLV-I neutralizing mAb, and it can be used as an immunogen to induce protective immune response against HTLV-I. Thus, the present methodology could be one of the approaches to develop vaccines against infectious agents in a simple and inexpensive way.

Molecular biology and pathogenesis of the human T-cell leukaemia/lymphotropic virus Type-1 (HTLV-1)

Retroviruses are associated with a variety of diseases, including immunological and neurological disorders, and various forms of cancer. In humans, the Human T-cell Leukaemia/Lymphotropic virus type 1 (HTLV-1), which belongs to the Oncovirus family, is the aetiological agent of two diverse diseases: Adult T-cell leukaemia/lymphoma (ATLL) (Poiesz et al. 1980; Hinuma et al. 1981; Yoshida et al. 1982), as well as the neurological disorder tropical spastic paraparesis/HTLV-1-associated myelopathy (TSP/HAM) (Gessain et al. 1985; Rodgers-Johnson et al. 1985; Osame et al. 1986). HTLV-1 is the only human retrovirus known to be the aetiological agent of cancer. A genetically related virus, HTLV-2, has been identified and isolated (Kalyanaraman et al. 1982). However, there has been no demonstration of a definitive aetiological role for HTLV-2 in human disease to date. Simian T-cell lymphotropic viruses types 1 and 2 (STLV-1 and -2) and bovine leukaemia virus (BLV) have also been classified in same group, Oncoviridae, based upon their similarities in genetic sequence and structure to HTLV-1 and -2 (Burny et al. 1988; Dekaban et al. 1995; Slattery et al. 1999). This article will focus on HTLV-1, reviewing its discovery, molecular biology, and its role in disease pathogenesis.

Sequence variations in the amino- and carboxy-terminal parts of the surface envelope glycoprotein of HTLV type 1 induce specific neutralizing antibodies

The surface envelope glycoprotein gp46 of the human T cell leukemia virus type 1 elicits a strong immune response. Its protective role against HTLV-1 infection in animal models is well established, suggesting that recombinant envelope glycoproteins or synthetic peptides could be used as an effective vaccine. However, reports have indicated that some variations in envelope sequences may induce incomplete cross-neutralization between HTLV-1 strains. To identify amino acid changes that might be involved in induction of specific neutralizing antibodies, we studied sera from three patients (2085, 2555, and 2709) infected by HTLV-1 with surface glycoprotein gp46 harboring variations in amino acid sequence at positions 39, 72, 265, and 290. Inhibition of syncytia induced by parental, chimeric, or point-mutated envelope proteins indicated that sera 2555 and 2709 primarily recognized neutralizable epitopes located in N- and C-terminal parts of the gp46 glycoprotein. Amino acids changes at positions 39, 265, and 290 greatly impaired recognition of neutralizing epitopes recognized by these two sera. These results demonstrate that amino acid changes in envelope glycoprotein gp46 can induce strain-specific neutralizing antibodies in some patients. On the other hand, the neutralizing activity of serum 2085 was not affected by amino acid changes at positions 39, 265, and 290, suggesting that the neutralizing antibodies present in this serum were directed against epitopes located in other parts of the molecule, possibly those located in the central domain of the molecule, which has the same amino acid sequence in the three viruses.

In vitro and in vivo functional analysis of human T cell lymphotropic virus type 1 pX open reading frames I and II

Human T lymphotropic virus type 1 (HTLV-1) is a complex retrovirus containing regulatory and accessory genes encoded in four open reading frames (ORF I-IV) of the pX region. It is not clear what role pX ORFs I and II-encoded proteins have in the pathogenesis of the lymphoproliferative diseases associated with HTLV-1 infection. The conserved ORF I encodes for a hydrophobic 12-kDa protein, p12, (I) that contains four SH3 binding motifs (PXXP) that localizes to cellular endomembranes when overexpressed in cultured cells. Differential splicing of pX ORF II results in the production of two nuclear proteins, p13(II) and p30(II). p13(II) also localizes to mitochondria. p30(II) shares homology with the POU family of transcription factors. We have identified functional roles of pX ORF I and ORF II in establishment and maintenance of infection in a rabbit model. To functionally study p12(I) we have tested a proviral clone with selective ablation of ORF I (ACH.p12(I)) for its ability to infect quiescent peripheral blood mononuclear cells (PBMC). Our data indicate that T cells infected with the wild-type clone of HTLV-1 (ACH) are more efficient than ACH.p12(I) in infecting quiescent PBMC. These findings parallel our animal model data and suggest a role for p12(I) in the activation of quiescent lymphocytes, a prerequisite for effective viral replication in vivo. To test the ability of p30(II) to function as a transcription factor we have constructed p30(II) as a Gal4-fusion protein. When transfected with Gal4-driven luciferase reporter genes, the p30(II)-Gal4-fusion protein induces transcriptional activity up to 50-fold in both 293 and HeLa-Tat cells. These systems will be useful to identify molecular mechanisms that explain the functional role of pX ORF I and ORF II-encoded proteins in HTLV-1 replication.

Expression, purification and biological properties of the carboxyl half part of the HTLV-I surface envelope glycoprotein

The carboxyl half of the surface envelope protein of HTLV-I contains the major immunodominant and neutralizable domains. Using two affinity chromatography steps and a combination of high salt concentration and non-ionic detergent, we purified this part of the envelope protein from Escherichia coli. Analysis of some immmunological and biological properties of this protein indicated that it was folded in a way that preserved the correct structure of this domain of the HTLV-I envelope protein. It could be utilized in structural studies to further understand the mechanisms of HTLV-I entry and to better define the component(s) of an effective vaccine.

Amino acid changes at positions 173 and 187 in the human T-cell leukemia virus type 1 surface glycoprotein induce specific neutralizing antibodies

The nucleotide sequence of human T-cell leukemia virus type 1 (HTLV-1) is highly conserved, most strains sharing at least 95% sequence identity. This sequence conservation is also found in the viral env gene, which codes for the two envelope glycoproteins that play a major role in the induction of a protective immune response against the virus. However, recent reports have indicated that some variations in env sequences may induce incomplete cross-reactivity between HTLV-1 strains. To identify the amino acid changes that might be involved in the antigenicity of neutralizable epitopes, we constructed expression vectors coding for the envelope glycoproteins of two HTLV-1 isolates (2060 and 2072) which induced human antibodies with different neutralization patterns. The amino acid sequences of the envelope glycoproteins differed at four positions. Vectors coding for chimeric or point-mutated envelope proteins were derived from 2060 and 2072 HTLV-1 env genes. Syncytium formation induced by the wild-type or mutated envelope proteins was inhibited by human sera with different neutralizing specificities. We thus identified two amino acid changes, I173-->V and A187-->T, that play an important role in the antigenicity of neutralizable epitopes located in this region of the surface envelope glycoprotein.

Induction of adult T-cell leukemia-like lymphoproliferative disease and its inhibition by adoptive immunotherapy in T-cell-deficient nude rats inoculated with syngeneic human T-cell leukemia virus type 1-immortalized cells

Human T-cell leukemia virus type 1 (HTLV-1) has been shown to be the etiologic agent of adult T-cell leukemia (ATL), but the in vivo mechanism by which the virus causes the malignant transformation is largely unknown. In order to investigate the mechanisms of HTLV-1 leukemogenesis, we developed a rat model system in which ATL-like disease was reproducibly observed, following inoculation of various rat HTLV-1-immortalized cell lines. When previously established cell lines, F344-S1 and TARS-1, but not TART-1 or W7TM-1, were inoculated, systemic multiple tumor development was observed in adult nude (nu/nu) rats. FPM1 cells, newly established from a heterozygous (nu/+) rat syngeneic to nu/nu rats, caused transient tumors only at the injection site in adult nu/nu rats, but could progressively grow in newborn nu/nu rats and metastasize in lymph nodes. The derivative cell line (FPM1-V1AX) serially passed through newborn nu/nu rats acquired the potency to grow in adult nu/nu rats. These results indicated that only some with additional changes but not all of the in vitro HTLV-1-immortalized cell lines possessed in vivo tumorigenicity. Using the syngeneic system, we further showed the inhibition of tumor development by transferring splenic T cells from immunized rats, suggesting the involvement of T cells in the regression of tumors. This novel and reproducible nude rat model of human ATL would be useful for investigation of leukemogenesis and antitumor immune responses in HTLV-1 infection.

Influence of amino acid substitutions on antigenicity of immunodominant regions of the HTLV type I envelope surface gylcoprotein: a study using monoclonal antibodies raised against relevant peptides

By the use of sera of human T cell leukemia virus type I (HTVL-I)-infected individuals it was shown that amino acid substitutions at positions 192 (proline to serine) and 250 (serine to proline) in major immunodominant regions (175-199 and 239-261) of the surface envelope glycoprotein (gp46) of the virus may influence the humoral response. Since human sera are polyclonal in nature, one cannot readily discriminate between an immunoglobulin-specific recognition and multiple bindings of diverse antibodies. To overcome this difficulty we generated murine monoclonal antibodies to synthetic peptides mimicking all or portions of these gp46 regions. The reactivity of some of these antibodies to synthetic peptides harboring (or not harboring) the preceding amino acid substitutions at position 192 or 250, to denatured gp46 by Western blotting, and to live (variously substituted) HTLV-I-infected cells, combined with blocking experiments with various peptides, allow us to conclude that the major epitopes (positions 183-191, 190-197, 190-199, and 246-252) in the two immunodominant regions may elicit different antibody responses according to their sequences. It is worth noting that in a reporter gene inhibition assay, it was found that a neutralizing monoclonal antibody (MF1), the epitope for which is located between residues 190 and 197, had a high level of activity when cells (2060) harboring a gp46 with proline at position 192 were used and had no activity toward cells (1010) with a serine at this position. Therefore our results establish that certain amino acid substitutions of gp46 may drastically affect the antigenicity of the molecule and the biological activity of the antibodies elicited.

Experimental HTLV-I infection and associated myelopathy

HTLV-I infection and associated myelopathy has been reproduced experimentally in vitro and in vivo and these studies have shown the possibility of creating several lines of infective cells and of detecting minor and major clinical expressions of HTLV-I associated myelopathy in rabbits and rats.

Different HTLV-I neutralization patterns among sera of patients infected with cosmopolitan HTLV-I

To determine if sequence variations observed in cosmopolitan HTLV-I interfered with viral recognition by neutralizing antibodies, we evaluated the neutralization potential of sera from persons infected by HTLV-I of this clade selected for amino acid changes in their eny glycoproteins. Each serum was used to neutralize three previously described HTLV-I isolates, 2060, 2072, and 1010, that possess amino acid env sequences differing at several positions, one of them being located in the immunodominant and neutralizable domain (aa 187-199). The results obtained in syncytia and/or reporter gene inhibition assays showed that the neutralization pattern of the sera clearly differed and could be classified in three categories. Five sera completely neutralized the three viruses with an equivalent titer, two sera gave a maximum inhibition, with higher ID50 on the 2072 virus than on the 2060 or 1010 viruses, and three sera had a stronger neutralization potential toward the 1010 virus than toward the 2060 virus. One of these sera partially neutralized the virus produced by 2072 cells, whereas neutralizing antibodies in the other two recognized the neutralizable epitopes on the 1010 or 2072 viruses equally well. Identification of amino acid sequences involved in induction of neutralizing antibodies with different recognition capacities could help identify new neutralizable epitopes of HTLV-I envelope glycoproteins and to better define the component(s) of an effective vaccine.

Identification of exposed epitopes on the envelope glycoproteins of human T-cell lymphotropic virus type I (HTLV-I)

Several lines of evidence underscore the important role of the humoral response specific for HTLV-I envelope protein in the protection against viral infection. One approach to producing efficient immunogens is to synthesize peptides corresponding to the primary amino-acid sequence of neutralizing epitopes found in the external sub-unit gp46. In this study, we have selected synthetic peptides overlapping the major linear neutralizing determinants described earlier and used them as immunogens in rabbits and mice. All rabbit polyclonal anti-sera raised against peptides recognized epitopes in a denaturated context as well as MAbs raised against the HB peptide (aa287-311). By contrast, synthetic peptides O (aa89-110), HH (aa190-209), T (aa190-212) and HB (aa287-311) have generated antibodies efficiently binding their epitopes in a native context, suggesting that these domains are well exposed both at the heterodimer and at the oligomer surface. None of the antibodies induced by synthetic peptides show in vitro neutralizing properties, even those with a good capacity to bind the native form of HTLV-I envelope proteins.

Prophylaxis of experimental HTLV-I infection in cynomolgus monkeys by passive immunization

The protective effect of purified human immunoglobulin against human T-cell leukaemia virus type I (HTLV-I), designated ATLIG on HTLV-I infection was examined in cynomolgus monkeys (Macaca fascicularis) as a preclinical study. Passive immunization of ATLIG 24 h before challenging HTLV-I protected the monkeys from HTLV-I infection. The result suggests that passive immunization of ATLIG could provide safe and sufficient protection against HTLV-I infection in humans.

Neutralizing human monoclonal antibodies to conformational epitopes of human T-cell lymphotropic virus type 1 and 2 gp46

Ten human monoclonal antibodies derived from peripheral B cells of a patient with human T-cell lymphotropic virus (HTLV)-associated myelopathy are described. One monoclonal antibody recognized a linear epitope within the carboxy-terminal 43 amino acids of HTLV gp21, and two monoclonal antibodies recognized linear epitopes within HTLV type 1 (HTLV-1) gp46. The remaining seven monoclonal antibodies recognized denaturation-sensitive epitopes within HTLV-1 gp46 that were expressed on the surfaces of infected cells. Two of these antibodies also bound to viable HTLV-2 infected cells and immunoprecipitated HTLV-2 gp46. Virus neutralization was determined by syncytium inhibition assays. Eight monoclonal antibodies, including all seven that recognized denaturation-sensitive epitopes within HTLV-1 gp46, possessed significant virus neutralization activity. By competitive inhibition analysis it was determined that these antibodies recognized at least four distinct conformational epitopes within HTLV-1 gp46. These findings indicate the importance of conformational epitopes within HTLV-1 gp46 in mediating a neutralizing antibody response to HTLV infection.

HTLV-I infection in squirrel monkeys (Saïmiri sciureus) using autologous, homologous, or heterologous HTLV-I-transformed cell lines

Peripheral blood mononuclear cells (PBMC) from three adult male squirrel monkeys (Saïmiri sciureus) were transformed by human T-cell leukemia/lymphoma virus type I (HTLV-I) by cocultivation with lethally irradiated human MT-2 cells. Three permanent monkey T-cell lines producing HTLV-I were obtained and characterized. Six weeks after inoculation seroconversion was observed in three of three monkeys inoculated with autologous transformed T cells and in two of three monkeys receiving homologous cells. Proviral DNA was detected in their PBMC at various times after inoculation, with the highest proviral load and antibody titers being found in monkeys infected with homologous cells. Monkeys inoculated with heterologous MT-2 cells did not seroconvert, and HTLV-I provirus was detected only transiently in their PBMC. To determine whether in vitro and in vivo HTLV-I infection of squirrel monkey cells led to a selection of monkey-adapted viral mutants, comparative sequencing of the proviral gp21 env between ex vivo monkey HTLV-I-infected PBMC, the inoculum, and MT-2 cells was done and no significant differences were detected. The squirrel monkey, which is naturally free of simian T-cell leukemia/ lymphoma virus, thus appears to be a suitable model for evaluating HTLV-I candidate vaccines and for studying the pathogenesis of HTLV-I.

Expression and immunogenicity in rats of recombinant adenovirus 5 DNA plasmids and vaccinia virus containing the HTLV-I env gene

The complete human T-cell leukemia virus type I (HTLV-I) env gene was inserted into an expression cassette containing the adenovirus 5 major late promoter (Ad5-MLP). Recombinant Ad5-HTLV-I-env was obtained by homologous recombination in 293 cells simultaneously transfected by the expression cassette and the genomic DNA of Ad5. In vitro expression of the HTLV-I-env gene in the recombinant vector was detected by immunofluorescence and Western blotting. Functional expression of HTLV-I-env was confirmed by syncitium formation specifically in HeLa cells infected with Ad5-HTLV-I-env. Two immunization regimens against HTLV-I were tested in WKY and Fischer F-344 rats. The first involved WKY rats primed with Ad5-HTLV-I-env or naked DNA plasmids containing the HTLV-I-env gene and boosted with Ad5 containing the HTLV-I-env gp46 gene or with baculovirus-derived recombinant gp46. No antibody against HTLV-I was detected, while HTLV-I-specific cytotoxic T lymphocytes were recovered from all immunized groups but not from controls. The second approach involved Fischer F-344 rats primed and boosted with recombinant vaccinia virus containing the HTLV-I-env gene. Such rats developed antibodies against the HTLV-I env gp21 and gp46 (non-neutralizing). After challenge with human HTLV-I-producing cells (MT-2), both immunization regimens were found to induce partial protection.

Cervicovaginal synthesis of IgG antibodies to the immunodominant 175-199 domain of the surface glycoprotein gp46 of human T-cell leukemia virus type I

Paired sera, saliva and cervicovaginal secretions from 17 HTLV-I-infected women (19-75 yr) were tested for total IgA and IgG, for IgA and IgG to the immunodominant region gp46/175-Pro-199, for serum IgG to the neutralizing domains gp46/ 190-Pro-199 and gp46/190-Ser-199, or for tax-rex proviral HTLV-DNA. Serum antibodies to gp46/ 175-Pro-199 were detected more frequently in the IgG (13/17) than in the IgA (5/17) isotypes. The majority (8/12) of anti-gp46/175-Pro-199-positive sera reacted also to gp46/190-Pro-199 or to gp46/ 190-Ser-199, demonstrating their neutralizing properties. In saliva, antibodies to gp46/175-Pro-199 were not generally detected. In cervicovaginal secretions, IgG to gp46/175-Pro-199, but not IgA, were detected in 6/15 (40%) patients. The mean specific activity of IgG to gp46/175-Pro-199 showed a trend to be higher in cervicovaginal secretions (218 +/- 109) than in sera (14 +/- 4). Furthermore, in all patients with cervicovaginal IgG to gp46/175-Pro-199, the cervicogaginal/serum ratio (19 +/- 6) of anti-gp46 IgG specific activities were markedly above 1. HTLV-DNA was detected in 4/17 salivas, and in 3/15 cervicovaginal secretions, all from patients demonstrating cervicovaginal synthesis of IgG to gp46/175-Pro-199. In conclusion, IgG to gp46/175-Pro-199 in cervicovaginal secretions, when present, appear to be produced primarily locally because of local HTLV-I excretion. Since anti-gp46/175-Pro-199 antibodies usually support reactivities to neutralizing domains, their presence could be relevant for limiting HTLV-I transmission via cervicovaginal secretions.

Identification and mapping of functional domains on human T-cell lymphotropic virus type 1 envelope proteins by using synthetic peptides

To identify the regions that are important in human T-cell leukemia virus type 1 (HTLV-1) envelope function, we synthesized 23 kinds of peptides covering the envelope proteins and examined the inhibitory effect of each peptide on syncytium formation induced by HTLV-1-bearing cells. Of the 23 synthetic peptides, 2, corresponding to amino acids 197 to 216 on gp46 and 400 to 429 on gp21, inhibited syncytium formation induced by HTLV-1-bearing cells but did not affect syncytium formation induced by human immunodeficiency virus type 1-producing cells. The peptide concentrations giving 50% inhibition of syncytium formation for gp46 197 to 216 and gp21 400 to 429 were 14.9 and 6.0 microM, respectively. A syncytium formation assay with overlapping synthetic peptides containing amino acids 175 to 236 and 391 to 448 of the envelope proteins showed that syncytium formation was inhibited by peptides that contained the amino acid sequences 197 to 205 (Asp-His-Ile-Leu-Glu-Pro-Ser-Ile-Pro) and 397 to 406 (Gln-Glu-Gln-Cys-Arg-Phe- Pro-Asn-Ile-Thr). These observations suggest that the two regions corresponding to amino acids 197 to 216 and 400 to 429 are involved] in HTLV-1 envelope function.

Protection of rabbits against HTLV-II infection with a synthetic peptide corresponding to HTLV-II neutralization region

Rabbit immune sera raised against synthetic peptides of the HTLV-II envelope gp46 region were examined for HTLV-II neutralization ability by HTLV-vesicular stomatitis virus (VSV) pseudotype assay and syncytium inhibition assay. HTLV-II neutralization activity was detected in the sera against HTLV-II Env gp46, 80-103 but not in those to HTLV-II Env gp46, 171-196. Three rabbits immunized with the synthetic peptide of HTLV-II Env gp46, 80-103 and three non-immunized rabbits were challenged with intravenous inoculation of an HTLV-II-producing human cell line (MOT, 1 x 10(7) cells). The non-immunized rabbits showed seroconversion for HTLV-II after 2 weeks and maintained persistent infection but the immunized rabbits were protected from HTLV-II infection. Nested or repeated polymerase chain reaction revealed the presence of HTLV-II provirus sequences in the non-immunized rabbits but not in the immunized rabbits. These results suggest that peptide vaccination with a synthetic peptide corresponding to the HTLV-II neutralization region is useful for preventing HTLV-II infection.

Human T-lymphotropic virus type I excretion and specific antibody response in paired saliva and cervicovaginal secretions

Paired sera, salivas, and cervicovaginal secretions from 17 HTLV-I-infected women (10-75 years) were evaluated for total IgA, IgG, IgM, for IgA and IgG to whole HTLV-I lysate, for albumin, and for tax-rex proviral HTLV-DNA. IgG to HTLV-I were constantly detected, with much higher titers in serum (mean titer: 97,800) than in saliva (53) or in cervicovaginal secretions (216). IgA to HTLV-I were detected in only 12 (70%) sera, 6 (35%) salivas, and 8 (53%) cervicovaginal secretions, with higher titers in serum (75) than in saliva (8). Using the relative coefficient of excretion by reference to albumin, as well as the comparison of specific activities, the HTLV-I-specific IgG appeared primarily originating from serum, whereas IgA to HTLV-I were primarily locally produced. Salivary synthesis of IgG to HTLV-I occurred in both patients with a sicca syndrome attesting salivary glands impairment. Local excretions of total IgA, IgG, and IgM evaluated in body fluids were normal. HTLV DNA was detected in 4 (24%) salivas and in 3 (20%) cervicovaginal secretions, always in patients demonstrating local synthesis of HTLV-I-specific IgA or IgG. HTLV-I excretion elicits a weak local immune response to HTLV-I in saliva as well as in cervicovaginal secretions, which could be relevant for HTLV-I transmission via body fluids.

An HTLV-I/II vaccine: from animal models to clinical trials?

A human T-lymphotropic virus type I/II (HTLV-I/II) vaccine is necessary in view of two etiologically related, life-threatening diseases, namely, adult T-cell leukemia/lymphoma and tropical spastic paraparesis/HTLV-I-associated myelopathy. When the risk of developing autoimmune diseases such as uveitis, polymyositis, and arthritis is included, one can estimate the life-long risk of infected individuals to develop an HTLV associated pathology as approximately 10%. The populations at risk are, in a large majority, from developing countries but the epidemic of HTLV-II infection in intravenous drug users (IVDU) represents a possible reservoir for dissemination in the general population. The number of HTLV-I-infected individuals (15 to 25 million), together with the severity of associated disease, justifies the development of a vaccine. Different vaccine preparations have been developed, using mostly recombinant pox and adenoviruses, but DNA plasmid technology will soon become a feasible approach. Various animal models exist for experimental viral infections, involving rats, rabbits, or monkeys, but up to now, neither hematological nor neurological disorders have been induced by HTLV infection in such animal models. For long-term protection from HTLV-I-associated diseases, vaccination should induce both neutralizing antibodies and specific cell-mediated immunity. This will require the incorporation of both env and gag coding sequences in the vaccine preparations. Preventive clinical trials may involve different cohorts of seronegative young girls from endemic areas prior to sexual activity and IVDU in the industrialized world. In parallel, one should consider therapeutic vaccine trials in HTLV-I-positive mothers and IVDU to protect them against disease development. The observed rate of seroconversion in these different cohorts makes such trials feasible.

Isolation and characterization of a new simian T-cell leukemia virus type 1 from naturally infected celebes macaques (Macaca tonkeana): complete nucleotide sequence and phylogenetic relationship with the Australo-Melanesian human T-cell leukemia virus type 1

A study of simian T-cell leukemia virus type 1 (STLV-1) infection in a captive colony of 23 Macaca tonkeana macaques indicated that 17 animals had high human T-cell leukemia virus type 1 (HTLV-1) antibody titers. Genealogical analysis suggested mainly a mother-to-offspring transmission of this STLV-1. Three long-term T-cell lines, established from peripheral blood mononuclear cell cultures from three STLV-1-seropositive monkeys, produced HTLV-1 Gag and Env antigens and retroviral particles. The first complete nucleotide sequence of an STLV-1 (9,025 bp), obtained for one of these isolates, indicated an overall genetic organization similar to that of HTLV-1 but with a nucleotide variability for the structural genes ranging from 7.8 to 13.1% compared with the HTLV-1 ATK and STLV-1 PTM3 Asian prototypes. The Tax and Rex regulatory proteins were well conserved, while the pX region, known to encode new proteins in HTLV-1 (open reading frames I and II), was more divergent than that in the ATK strain. Furthermore, a fragment of 522 bp of the gp21 env gene from uncultured peripheral blood mononuclear cell DNAs from five of the STLV-1-infected monkeys was sequenced. Phylogenetic trees constructed with the long terminal repeat and env (gp46 and gp21) regions demonstrated that this new STLV-1 occupies a unique position within the Asian STLV-1 and HTLV-1 isolates, being, by most analyses, related more to the Australo-Melanesian HTLV-1 topotype than to any other Asian STLV-1. These data raise new hypotheses on the possible interspecies viral transmission between monkeys carrying STLV-1 and early Australoid settlers, ancestors of the present day Australo-Melanesian inhabitants, during their migrations from the Southeast Asian land mass to the greater Australian continent.