Comparative biological responses of rabbits infected with human T-lymphotropic virus type I isolates from patients with lymphoproliferative and neurodegenerative disease

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.

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.

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.

Early spatial and temporal events of human T-lymphotropic virus type 1 spread following blood-borne transmission in a rabbit model of infection

Human T-lymphotropic virus type 1 (HTLV-1) infection causes adult T-cell leukemia/lymphoma (ATL) and is associated with a variety of lymphocyte-mediated disorders. HTLV-1 transmission occurs by transmission of infected cells via breast-feeding by infected mothers, sexual intercourse, and contaminated blood products. The route of exposure and early virus replication events are believed to be key determinants of virus-associated spread, antiviral immune responses, and ultimately disease outcomes. The lack of knowledge of early events of HTLV-1 spread following blood-borne transmission of the virus in vivo hinders a more complete understanding of the immunopathogenesis of HTLV-1 infections. Herein, we have used an established animal model of HTLV-1 infection to study early spatial and temporal events of the viral infection. Twelve-week-old rabbits were injected intravenously with cell-associated HTLV-1 (ACH-transformed R49). Blood and tissues were collected at defined intervals throughout the study to test the early spread of the infection. Antibody and hematologic responses were monitored throughout the infection. HTLV-1 intracellular Tax and soluble p19 matrix were tested from ex vivo cultured lymphocytes. Proviral copy numbers were measured by real-time PCR from blood and tissue mononuclear leukocytes. Our data indicate that intravenous infection with cell-associated HTLV-1 targets lymphocytes located in both primary lymphoid and gut-associated lymphoid compartments. A transient lymphocytosis that correlated with peak virus detection parameters was observed by 1 week postinfection before returning to baseline levels. Our data support emerging evidence that HTLV-1 promotes lymphocyte proliferation preceding early viral spread in lymphoid compartments to establish and maintain persistent infection.

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.

Chimeric peptide vaccine composed of B- and T-cell epitopes of human T-cell leukemia virus type 1 induces humoral and cellular immune responses and reduces the proviral load in immunized squirrel monkeys (Saimiri sciureus)

A squirrel monkey model of human T-cell leukemia virus type 1 (HTLV-1) infection was used to evaluate the immunogenicity and protective efficacy of a chimeric peptide vaccine composed of a B-cell epitope from the envelope region (aa 175–218) and three HLA-A*0201-restricted cytotoxic T-lymphocyte epitopes derived from Tax protein (Tri-Tax). These selected Tax peptides induced secretion of gamma interferon (IFN-γ) in peripheral blood mononuclear cells obtained from monkeys chronically infected with HTLV-1. After immunization, a high titre of antibodies and a high frequency of IFN-γ-producing cells were detected against the Env and the Tri-Tax immunogens, but not against the individual Tax peptides. This might indicate that epitope(s) distinct from those recognized by humans are recognized by responder monkeys. After challenge, it was shown by competitive PCR that partial protection against HTLV-1 infection could be raised in immunized animals. Further studies should be developed to determine the duration of this protection.

Reduced cell turnover in lymphocytic monkeys infected by human T-lymphotropic virus type 1

Understanding cell dynamics in animal models have implications for therapeutic strategies elaborated against leukemia in human. Quantification of the cell turnover in closely related primate systems is particularly important for rare and aggressive forms of human cancers, such as adult T-cell leukemia. For this purpose, we have measured the death and proliferation rates of the CD4+ T lymphocyte population in squirrel monkeys (Saimiri sciureus) infected by human T-lymphotropic virus type 1 (HTLV-1). The kinetics of in vivo bromodeoxyuridine labeling revealed no modulation of the cell turnover in HTLV-1-infected monkeys with normal CD4 cell counts. In contrast, a substantial decrease in the proliferation rate of the CD4+ T population was observed in lymphocytic monkeys (e.g. characterized by excessive proportions of CD4+ T lymphocytes and by the presence of abnormal flower-like cells). Unexpectedly, onset of HTLV-associated leukemia thus occurs in the absence of increased CD4+ T-cell proliferation. This dynamics significantly differs from the generalized activation of the T-cell turnover induced by other primate lymphotropic viruses like HIV and SIV.

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.

Human T-cell lymphotropic virus type 1 open reading frame II-encoded p30II is required for in vivo replication: evidence of in vivo reversion

Human T-cell lymphotropic virus type 1 (HTLV-1) causes adult T-cell leukemia/lymphoma and exhibits high genetic stability in vivo. HTLV-1 contains four open reading frames (ORFs) in its pX region. ORF II encodes two proteins, p30(II) and p13(II), both of which are incompletely characterized. p30(II) localizes to the nucleus or nucleolus and has distant homology to the transcription factors Oct-1, Pit-1, and POU-M1. In vitro studies have demonstrated that at low concentrations, p30(II) differentially regulates cellular and viral promoters through an interaction with CREB binding protein/p300. To determine the in vivo significance of p30(II), we inoculated rabbits with cell lines expressing either a wild-type clone of HTLV-1 (ACH.1) or a clone containing a mutation in ORF II, which eliminated wild-type p30(II) expression (ACH.30.1). ACH.1-inoculated rabbits maintained higher HTLV-1-specific antibody titers than ACH.30.1-inoculated rabbits, and all ACH.1-inoculated rabbits were seropositive for HTLV-1, whereas only two of six ACH.30.1-inoculated rabbits were seropositive. Provirus could be consistently PCR amplified from peripheral blood mononuclear cell (PBMC) DNA in all ACH.1-inoculated rabbits but in only three of six ACH.30.1-inoculated rabbits. Quantitative competitive PCR indicated higher PBMC proviral loads in ACH.1-inoculated rabbits. Interestingly, sequencing of ORF II from PBMC of provirus-positive ACH.30.1-inoculated rabbits revealed a reversion to wild-type sequence with evidence of early coexistence of mutant and wild-type sequence. Our data provide evidence that HTLV-1 must maintain its key accessory genes to survive in vivo and that in vivo pressures select for maintenance of wild-type ORF II gene products during the early course of infection.

Immune responses to HTLV-I(ACH) during acute infection of pig-tailed macaques

Human T cell lymphotropic virus type 1 (HTLV-I) is causally linked to adult T cell leukemia/lymphoma (ATL) and a chronic progressive neurological disease, HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP). A nonhuman primate model that reproduces disease symptoms seen in HTLV-I-infected humans might facilitate identification of initial immune responses to the virus and an understanding of pathogenic mechanisms in HTLV-I-related disease. Previously, we showed that infection of pig-tailed macaques with HTLV-I(ACH) is associated with multiple signs of disease characteristic of both HAM/TSP and ATL. We report here that within the first few weeks after HTLV-I(ACH) infection of pig-tailed macaques, serum concentrations of interferon (IFN)-alpha increased and interleukin-12 decreased transiently, levels of nitric oxide were elevated, and activation of CD4(+) and CD8(+) lymphocytes and CD16(+) natural killer cells in peripheral blood were observed. HTLV-I(ACH) infection elicited virus-specific antibodies in all four animals within 4 to 6 weeks; however, Tax-specific lymphoproliferative responses were not detected until 25-29 weeks after infection in all four macaques. IFN-gamma production by peripheral blood cells stimulated with a Tax or Gag peptide was detected to varying degrees in all four animals by ELISPOT assay. Peripheral blood lymphocytes from one animal that developed only a marginal antigen-specific cellular response were unresponsive to mitogen stimulation during the last few weeks preceding its death from a rapidly progressive disease syndrome associated with HTLV-I(ACH) infection of pig-tailed macaques. The results show that during the first few months after HTLV-I(ACH) infection, activation of both innate and adaptive immunity, limited virus-specific cellular responses, sustained immune system activation, and, in some cases, immunodeficiency were evident. Thus, this animal model might be valuable for understanding early stages of infection and causes of immune system dysregulation in HTLV-I-infected humans.

HTLV-1 Rex is required for viral spread and persistence in vivo but is dispensable for cellular immortalization in vitro

Human T-cell leukemia virus type 1 (HTLV-1) is associated with leukemia/lymphoma and neurologic disorders. Although the viral transcriptional activator Tax is the critical viral oncoprotein, Rex, which regulates the expression of the viral structural and enzymatic genes, is essential for efficient viral replication. Herein, we investigate the contribution of Rex in HTLV-1 immortalization of primary T cells in vitro and viral survival in an infectious rabbit animal model. A Rex-deficient HTLV-1 (HTLVRex-) was constructed and characterized for viral gene expression, protein production, and immortalization capacity. Cells transiently transfected with the HTLVRex- proviral clone produced low detectable levels of p19 Gag. 729HTLVRex- stable transfectants produced functional Tax, but undetectable levels of Rex or p19 Gag. Coculture of irradiated 729HTLVRex- cells with peripheral blood mononuclear cells (PBMCs) resulted in sustained interleukin-2 (IL-2)-dependent growth of primary T lymphocytes. These cells carried the HTLVRex- genome and expressed tax/rex mRNA but produced no detectable Rex or p19 Gag. Rabbits inoculated with irradiated 729HTLVRex- cells or 729HTLVRex- cells transiently transfected with a Rex cDNA expression plasmid failed to become persistently infected or mount a detectable antibody response to the viral gene products. Together, our results provide the first direct evidence that Rex and its function to modulate viral gene expression and virion production is not required for in vitro immortalization by HTLV-1. However, Rex is critical for efficient infection of cells and persistence in vivo.

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.

Two-step nature of human T-cell leukemia virus type 1 replication in experimentally infected squirrel monkeys (Saimiri sciureus)

After experimental infection of squirrel monkeys (Saimiri sciureus) with human T-cell leukemia virus type 1 (HTLV-1)-infected cells, the virus is transcribed only transiently in circulating blood, spleen, and lymph nodes. Stable disappearance of viral expression occurs at 2 to 3 weeks after inoculation. This coincides with the development of the anti-HTLV-1 immune response and persistent detection of the provirus in peripheral blood mononuclear cells (PBMCs). In this study, the HTLV-1 replication pattern was analyzed over time in PBMCs and various organs from two HTLV-1-infected squirrel monkeys. Real-time quantitative PCR confirmed that PBMCs and lymphoid organs constitute the major reservoirs for HTLV-1. The PCR amplification of HTLV-1 flanking sequences from PBMCs evidenced a pattern of clonal expansion of infected cells identical to that observed in humans. Dissemination of the virus in body compartments appeared to result from cellular transport of the integrated provirus. The circulating proviral burden increased as a function of time in one animal studied over a period of 4 years. The high proviral loads observed in the last samples resulted from the accumulation of infected cells via the extensive proliferation of a restricted number of persistent clones on a background of polyclonally expanded HTLV-1-positive cells. Therefore, HTLV-1 primary infection in squirrel monkeys is a two-step process involving a transient phase of reverse transcription followed by persistent multiplication of infected cells. This suggests that the choice of the target for blocking HTLV-1 replication might depend on the stage of infection.

HTLV type 1 infection in squirrel monkeys (Saimiri sciureus): a promising animal model for HTLV type 1 human infection

We show that the squirrel monkey Saimiri sciureus is susceptible to experimental infection with either syngeneic or allogeneic HTLV-1-immortalized cells. As in humans, such experimental inoculation leads to chronic infection, and HTLV-1 provirus was detected in PBMCs by PCR. Chronically infected monkeys developed high titers of antibodies against the structural proteins of the virus, as do HTLV-1-infected humans. Furthermore, in serially sacrificed squirrel monkeys infected with HTLV-1, proviral DNA was detected at primary phases of infection in PBMCs, spleens, and lymph nodes. Tax/rex mRNA was also detected by RT-PCR in the PBMCs of two monkeys at 12 days after inoculation and in the spleen and lymph nodes of the monkey sacrificed on Day 12. In this animal, scattered HTLV-1-tax/rex mRNA-positive lymphocytes were detected by in situ hybridization in frozen sections of the spleen. These results indicate that PBMCs, spleen, and lymph nodes serve as major reservoirs for HTLV-1 during the early phase of infection. To evaluate the relationship between viral expression and the immune response during infection, humoral and cytotoxic T cell responses (CTL) were studied at various times after inoculation. Antibodies to HTLV-1 were detected 3 weeks after infection and anti-p40Tax and anti-Env CTL activity was detected 2 months after infection and remained detectable thereafter. Our results indicate that the squirrel monkey provides a useful animal model for studying the pathogenesis of HTLV-1 and for evaluating new candidate vaccines.

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.

Transmission of human T-cell lymphotropic virus type 1 tax to rabbits by tax-only-positive human cells

The human T-cell lymphrotropic virus type 1 (HTLV-1) is causally related to adult T-cell leukemia and lymphoma and the neurodegenerative diseases tropical spastic paraparesis and HTLV-1-associated myelopathy. In the United States the prevalence of infection has been estimated to range from 0.016 to 0.1% on the basis of serologic tests for antibodies to the viral structural proteins. Blood from donors positive for antibodies to HTLV-1 or HTLV-2 is not used for transfusion. However, patients with the cutaneous T-cell lymphoma mycosis fungoides (MF) are HTLV-1 and -2 seronegative yet harbor proviral sequences identical to those that encode the HTLV-1 transactivating and transforming gene product p40tax in their peripheral blood mononuclear cells (PBMCs), and they usually have antibodies to p40(tax). Moreover, a study of 250 randomly selected blood donors revealed that approximately 8% of these seronegative individuals also had HTLV-1 tax sequences and antibodies to p40(tax), while they lacked sequences and antibodies related to gag, pol, or env. Thus, it seemed important to determine whether the "tax-only" state can be transmitted by transfusion. To this end, PBMCs from HTLV-1 and -2 seronegative tax-only-positive MF patients or from healthy tax-only-positive blood donors were injected into adult rabbits, an established animal model for HTLV-1 infection. The PBMCs of all injected rabbits became tax sequence positive. These observations suggest that HTLV-1 tax can be transmitted by tax-only-positive mononuclear cells.

Functional role of pX open reading frame II of human T-lymphotropic virus type 1 in maintenance of viral loads in vivo

Human T-lymphotropic virus type 1 (HTLV-1) causes adult T-cell leukemia/lymphoma and is associated with a variety of immune-mediated disorders. The role of four open reading frames (ORFs), located between env and the 3' long terminal repeat of HTLV-1, in mediating disease is not entirely clear. By differential splicing, ORF II encodes two proteins, p13(II) and p30(II), both of which have not been functionally defined. p13(II) localizes to mitochondria and may alter the configuration of the tubular network of this cellular organelle. p30(II) localizes to the nucleolus and shares homology with the transcription factors Oct-1 and -2, Pit-1, and POU-M1. Both p13(II) and p30(II) are dispensable for infection and immortalization of primary human and rabbit lymphocytes in vitro. To test the role of ORF II gene products in vivo, we inoculated rabbits with lethally irradiated cell lines expressing the wild-type molecular clone of HTLV-1 (ACH.1) or a clone containing selected mutations in ORF II (ACH.30/13.1). ACH.1-inoculated animals maintained higher HTLV-1-specific antibody titers than animals inoculated with ACH.30/13.1. Viral p19 antigen was transiently detected in ex vivo cultures of peripheral blood mononuclear cells (PBMC) from only two ACH.30/13.1-inoculated rabbits, while PBMC cultures from all ACH.1-inoculated rabbits routinely produced p19 antigen. In only three of six animals exposed to the ACH. p30(II)/p13(II) clone could provirus be consistently PCR amplified from extracted PBMC DNA and quantitative competitive PCR showed the proviral loads in PBMC from ACH.p30(II)/p13(II)-infected rabbits to be dramatically lower than the proviral loads in rabbits exposed to ACH. Our data indicate selected mutations in pX ORF II diminish the ability of HTLV-1 to maintain high viral loads in vivo and suggest an important function for p13(II) and p30(II) in viral pathogenesis.

Establishment of a seronegative human T-cell leukemia virus type 1 (HTLV-1) carrier state in rats inoculated with a syngeneic HTLV-1-immortalized T-cell line preferentially expressing Tax

Human T-cell leukemia virus type 1 (HTLV-1) causes T-cell malignancies in a small percentage of the population infected with the virus after a long carrier state. In the present study, we established a seronegative HTLV-1 carrier state in rats inoculated with a newly established HTLV-1-infected rat T cell line, FPM1. FPM1 originated from rat thymocytes cocultured with a human HTLV-1 producer, MT-2 cells, and expressed rat CD4, CD5, CD25, and HTLV-1 Tax. However, FPM1 scarcely expressed other major HTLV-1 structural proteins and failed to induce typical antibody responses against HTLV-1 in inoculated rats. In contrast, control rats inoculated with MT-2 cells generated significant levels of anti-HTLV-1 antibodies. HTLV-1 proviruses were detected in peripheral blood cells of syngeneic rats inoculated with FPM1 for more than 1 year. Analysis of the flanking region of HTLV-1 provirus integrated into host cells suggested that FPM1 cells remained in these animals over a relatively long period of time. However, a similar seronegative HTLV-1 carrier state was induced in the rats inoculated with mitomycin C-treated FPM1 cells and also in FPM1-inoculated allogeneic rats, suggesting that FPM1 could also transmit HTLV-1 into host cells in vivo. Our findings indicated that (i) HTLV-1-immortalized T cells which preferentially express HTLV-1 Tax persisted in vivo but failed to induce any diseases in immunocompetent syngeneic rats and that (ii) suboptimal levels of HTLV-1 for antibody responses allowed the establishment of persistent HTLV-1 infection.

Quantification of human T-cell lymphotropic virus type 1 proviral load by quantitative competitive polymerase chain reaction

The polymerase chain reaction (PCR) has been established as a highly sensitive technique for detection of viral DNA or RNA. However, due to inherent limitations of PCR the amount of amplified product often does not correlate with the initial amount of template DNA. This is particularly true for PCR detection of viral infections that are characterized by low in vivo viral copy numbers in certain stages of the infection, such as human T-cell lymphotropic virus type 1 (HTLV-1) and simian T-cell lymphotropic virus type 1 (STLV-1). Therefore, we developed a quantitative competitive polymerase chain reaction (qcPCR) for detection of HTLV-1 and STLV-1 proviral DNA. The assay was optimized using an infectious HTLV-1 clone, ACH, HTLV-1 infected cell lines, MT-2.6 and HUT-102 and STLV-1 infected lines Kia and Matsu. Applicability of this system was demonstrated by determining HTLV-1 proviral load in peripheral blood mononuclear cells (PBMC) of human subjects with HTLV-1 associated diseases and an asymptomatic carrier as well as rabbits infected experimentally. This qcPCR method, the first designed specifically for HTLV-1 and STLV-1, will provide an important tool for pathogenesis studies of HTLV-1 and for evaluating the efficacy of antiviral drugs and vaccines against the viral infection using animal models.

Relationship between anti-Tax antibody responses and cocultivatable virus in HTLV-I-infected rabbits

The presence of anti-Tax antibody responses in human T cell leukemia virus type I (HTLV-I)-infected individuals has been correlated with increased proviral load, increased risk of transmitting infection, and increased risk of developing tropical spastic paraparesis/HTLV-I-associated myelopathy (TSP/HAM). In this study, a rabbit model of HTLV-I infection was used to determine whether anti-Tax antibody responses could predict the presence of virus with the potential to replicate. Seven of 14 HTLV-I-infected rabbits developed anti-Tax antibody responses. The onset of Tax reactivity was variable, but once detected remained constant throughout the remainder of the 60-week course of the study. All anti-Tax antibody positive rabbits produced virus as measured by p19 expression upon coculture, while p19 was detected in only one of the Tax antibody negative animals. Thus the presence of an anti-Tax antibody response correlates with p19 expression following cocultivation, and may be a useful predictor of virus replication in HTLV-I infected individuals.

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.

Selective Ablation of Human T-Cell Lymphotropic Virus Type 1 p12I Reduces Viral Infectivity In Vivo

Human T-cell lymphotropic virus type 1 (HTLV-1) is the etiologic agent of adult T-cell leukemia and HTLV-1-associated myelopathy. Novel, yet conserved RNA transcripts encoded from open reading frames (ORFs) I and II of the viral pX region are expressed both in vitro and in infected individuals. The ORF I mRNA encodes the protein p12I, which has been shown to localize to cellular endomembranes, cooperate with bovine papillomavirus E5 in transformation, as well as bind to the IL-2 receptor β and γ chains and the H+ vacuolar ATPase. It is unknown what role p12I plays in the viral life cycle. Using an infectious molecular clone of HTLV-1 (ACH) and a derivative clone, ACH.p12I, which fails to produce the p12Imessage, we investigated the importance of p12I in infected primary cells and in a rabbit model of the infection. ACH.p12I was infectious in vitro as shown by viral passage in culture and no qualitative or quantitative differences were noted between ACH and ACH.p12I in posttransfection viral antigen production. However, in contrast to ACH, ACH.p12I failed to establish persistent infection in vivo as indicated by reduced anti-HTLV-1 antibody responses, failure to demonstrate viral p19 antigen production in peripheral blood mononuclear cell (PBMC) cultures, and only transient detection of provirus by polymerase chain reaction in PBMC from ACH.p12I-inoculated rabbits. These results are the first to show the essential role of HTLV-1 p12I in the establishment of persistent viral infection in vivo and suggest potential new targets in antiviral strategies to prevent HTLV-1 infection.

Selective Ablation of Human T-Cell Lymphotropic Virus Type 1 p12I Reduces Viral Infectivity In Vivo

Human T-cell lymphotropic virus type 1 (HTLV-1) is the etiologic agent of adult T-cell leukemia and HTLV-1-associated myelopathy. Novel, yet conserved RNA transcripts encoded from open reading frames (ORFs) I and II of the viral pX region are expressed both in vitro and in infected individuals. The ORF I mRNA encodes the protein p12I, which has been shown to localize to cellular endomembranes, cooperate with bovine papillomavirus E5 in transformation, as well as bind to the IL-2 receptor β and γ chains and the H+ vacuolar ATPase. It is unknown what role p12I plays in the viral life cycle. Using an infectious molecular clone of HTLV-1 (ACH) and a derivative clone, ACH.p12I, which fails to produce the p12Imessage, we investigated the importance of p12I in infected primary cells and in a rabbit model of the infection. ACH.p12I was infectious in vitro as shown by viral passage in culture and no qualitative or quantitative differences were noted between ACH and ACH.p12I in posttransfection viral antigen production. However, in contrast to ACH, ACH.p12I failed to establish persistent infection in vivo as indicated by reduced anti-HTLV-1 antibody responses, failure to demonstrate viral p19 antigen production in peripheral blood mononuclear cell (PBMC) cultures, and only transient detection of provirus by polymerase chain reaction in PBMC from ACH.p12I-inoculated rabbits. These results are the first to show the essential role of HTLV-1 p12I in the establishment of persistent viral infection in vivo and suggest potential new targets in antiviral strategies to prevent HTLV-1 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.

In vitro CD4+ lymphocyte transformation and infection in a rabbit model with a molecular clone of human T-cell lymphotrophic virus type 1

We transfected human and rabbit peripheral blood mononuclear cells (PBMC) with the ACH molecular clone of human T-cell lymphotropic virus type 1 (HTLV-1) to study its in vitro and in vivo properties. PBMC transfected with ACH were shown to transfer infection to naive PBMC. ACH transformed rabbit PBMC, as indicated by interleukin-2-independent proliferation of a transfectant culture. This transformant culture was shown by flow cytometric analysis to be a CD4+ CD25+ T-lymphocyte population containing, as determined by Southern blot analysis, at least three integrated HTLV-1 proviral copies. HTLV-1 infection was produced in rabbits inoculated with ACH-transfected, irradiated PBMC. Inoculated rabbits seroconverted to positivity for antibodies against HTLV-1 and had steady or rising HTLV-1 enzyme-linked immunosorbent assay antibody titers. Western blot (immunoblot) analysis revealed sustained seroconversion of rabbits to positivity for antibodies against all major viral antigenic determinants. Infection of rabbits was further demonstrated by antigen capture assay of p24 in PBMC and lymph node cultures and PCR amplification of proviral sequences from PBMC. These data suggest that ACH, like wild-type HTLV-1, infects and transforms primary CD4+ T lymphocytes and is infectious in vivo. This clone will facilitate investigations into the role of viral genes on biological properties of HTLV-1 in vitro and in vivo.

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.

Infection of rats with human T-cell leukemia virus type-I: susceptibility of inbred strains, antibody response and provirus location

The susceptibilities of different strains of inbred rats to infection with the human T-cell leukemia virus (HTLV-I) after inoculation of human HTLV-I producer cell lines were compared. The Fisher F344 and Brown Norway strains developed the highest antibody response to HTLV-I, while the Lewis and BB strains were low responders. Antibodies against the HTLV-I gag proteins, and env gp21 but not env gp46, were detected in Western blots with sera from HTLV-I-infected Fischer F344 and Brown Norway rats. These sera were inactive in an in vitro syncytium-formation inhibition test. The HTLV-I provirus was detected by polymerase chain reaction in all Fischer F344, and some Lewis and Brown Norway rats, but not in the BB, which lack CD8+ T lymphocytes. The most frequent locations of the HTLV-I provirus in the Fischer F344, Lewis and Brown Norway rats at 12 weeks after infection were the peripheral blood mononuclear cells (PBMC) and spinal cord. In a second experiment in Brown Norway rats, the provirus was again detected in the PBMC of rats at 12 weeks, but not at 22 weeks, and among the other organs tested at 22 weeks the sympathetic nerve ganglia were positive. It is concluded that HTLV-I infection occurs in adult rats, but is suppressed with time.

Isolation, characterization, and transmission of human T-lymphotropic virus types I and II in culture

Highly sensitive coculture methods were developed both for isolation of human T-lymphotropic virus types I and II (HTLV-1 and HTLV-II) from infected individuals and for productive infection of lymphoid cells. Mitogen-activated peripheral blood mononuclear cells (PBMC) from 13 HTLV-I- and 20 HTLV-II-positive specimens were cocultured with an equal number of mitogen-activated PBMC from HTLV-seronegative individuals, and culture supernatants were tested for the presence of soluble p24gag antigens at weekly intervals for 4 weeks. Eleven of 13 (85%) HTLV-I and 14 of 20 (70%) HTLV-II cultures were positive for p24 antigens. None of the 17 HTLV-seroindeterminate or six HTLV-seronegative specimens were positive for the presence of p24 antigen. The isolation rates for HTLV-I and HTLV-II by an alternative whole-blood lysis procedure were comparable to those obtained by standard PBMC cultures. Furthermore, cocultivation of PHA-stimulated PBMC from healthy donors with lethally irradiated HTLV-I- and HTLV-II-infected cell lines (SP and Mo-T, respectively) resulted in productive viral infection, as reflected by the appearance of p24gag antigens concomitant with specific genomic amplification of HTLV proviral DNA after 3 weeks of cocultivation. Thus, the cocultivation technique provides a highly sensitive and specific procedure both for HTLV isolation and for infection of target cells.

Characterization of a B-cell immunodominant epitope of human T-lymphotropic virus type 1 (HTLV-I) envelope gp46

The immune response elicited by a synthetic peptide derived from an immunodominant external envelope region (Env-5, amino acids 242-257) of human T-lymphotropic virus type 1 (HTLV-I) was tested in a rabbit model of HTLV-I infection. The synthetic peptide elicited a strong antibody response to the HTLV-I envelope protein gp46; however, these antibodies failed to inhibit HTLV-I-mediated cell fusion. Immunized rabbits were not protected from HTLV-I infection as determined by seroconversion to viral core proteins by immunoblot, HTLV-I p24 antigen detection in lymphocyte cultures and polymerase chain reaction for the HTLV-I provirus in lymphocyte DNA. Env-5 peptide immunization failed to induce T-cell lymphocyte proliferative responses in rabbits, but induced antibody responses in T-cell deficient Balb c nu/nu mice suggesting that the antigenic determinant represented by the Env-5 peptide is primarily a B-cell epitope. These results further define an immunodominant epitope of the HTLV-I envelope protein and suggest that potential synthetic peptide vaccines against HTLV-I infection must contain multiple antigens that induce both humoral and cellular immune reactivity.