Oral administration of human T-cell leukemia virus type 1 induces immune unresponsiveness with persistent infection in adult rats

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.

Milk Transmission of HTLV-1 and the Need for Innovative Prevention Strategies

Breastfeeding is recommended by the World Health Organization for at least 6 months up to 2 years of age, and breast milk protects against several diseases and infections. Intriguingly, few viruses are transmitted via breastfeeding including Human T-cell leukemia virus Type 1 (HTLV-1). HTLV-1 is a highly oncogenic yet neglected retrovirus, which primarily infects CD4+ T-cells in vivo and causes incurable diseases like HTLV-1-associated inflammatory conditions or Adult T-cell leukemia/lymphoma (ATLL) after lifelong viral persistence. Worldwide, at least 5–10 million people are HTLV-1-infected and most of them are unaware of their infection posing the risk of silent transmissions. HTLV-1 is transmitted via cell-containing body fluids such as blood products, semen, and breast milk, which constitutes the major route of mother-to-child transmission (MTCT). Risk of transmission increases with the duration of breastfeeding, however, abstinence from breastfeeding as it is recommended in some endemic countries is not an option in resource-limited settings or underrepresented areas and populations. Despite significant progress in understanding details of HTLV-1 cell-to-cell transmission, it is still not fully understood, which cells in which organs get infected via the oral route, how these cells get infected, how breast milk affects this route of infection and how to inhibit oral transmission despite breastfeeding, which is an urgent need especially in underrepresented areas of the world. Here, we review these questions and provide an outlook how future research could help to uncover prevention strategies that might ultimately allow infants to benefit from breastfeeding while reducing the risk of HTLV-1 transmission.

Short-term cultured autologous peripheral blood mononuclear cells as a potential immunogen to activate Tax-specific CTL response in adult T-cell leukemia patients

Activation of CD8⁺ Tax‐specific CTL is a new therapeutic concept for adult T‐cell leukemia (ATL) caused by HTLV‐1. A recent clinical study of the dendritic cell vaccine pulsed with Tax peptides corresponding to CTL epitopes showed promising outcomes in ATL patients possessing limited human leukocyte antigen (HLA) alleles. In this study, we aimed to develop another immunotherapy to activate Tax‐specific CTL without HLA limitation by using patients’ own HTLV‐1‐infected cells as a vaccine. To examine the potential of HTLV‐1‐infected T‐cells to activate CTL via antigen presenting cells, we established a unique co–culture system. We demonstrated that mitomycin C‐treated HLA‐A2‐negative HTLV‐1‐infected T‐cell lines or short‐term cultured peripheral blood mononuclear cells (PBMC) derived from ATL patients induced cross–presentation of Tax antigen in co–cultured HLA‐A2‐positive antigen presenting cells, resulting in activation of HLA‐A2‐restricted CD8⁺ Tax‐specific CTL. This effect was not inhibited by a reverse transcriptase inhibitor. IL‐12 production and CD86 expression were also induced in antigen presenting cells co–cultured with HTLV‐1‐infected cells at various levels, which were improved by pre–treatment of the infected cells with histone deacetylase inhibitors. Furthermore, monocyte‐derived dendritic cells induced from PBMC of a chronic ATL patient produced IL‐12 and expressed enhanced levels of CD86 when co–cultured with autologous lymphocytes that had been isolated from the same PBMC and cultured for several days. These findings suggest that short‐term cultured autologous PBMC from ATL patients could potentially serve as a vaccine to evoke Tax‐specific CTL responses.

HTLV-1 infection-induced motor dysfunction, memory impairment, depression, and brain tissues oxidative damage in female BALB/c mice

AIMS

The HTLV-1 infection is associated with a neuro-inflammatory disease. In the present study, the behavioral consequences and brain oxidative damages were evaluated in HTLV-1-infected BALB/c mice.

MATERIAL AND METHODS

20 female BALB/c mice were divided into two groups comprising control and HTLV-1-infected. The HTLV-1-infected group was inoculated with a 10⁶ MT-2 HTLV-1-infected cell line. Two months later, the behavioral tests were conducted. Finally, oxidative stress was assessed in the cortex and hippocampus tissues.

KEY FINDINGS

In the HTLV-1-infected group, running time and latency to fall, travel distance and time spent in the peripheral zone, total crossing number and total traveled distance in open field test, the latency of entrance into the dark compartment in the passive avoidance test, the new object exploration percentage, and discrimination ratio were significantly lower than in the control group. The immobility time, time spent in the dark compartment in passive avoidance test, and total exploration time significantly increased in the HTLV-1-infected group compared to the control group. In the cortical tissue of the HTLV-1 group, the malondialdehyde levels were elevated while the total thiol levels decreased in comparison to the control group. The activity of superoxide dismutase in the cortical and hippocampal tissues, and catalase activity in cortical tissue significantly decreased in the HTLV-1 group in comparison to the control group.

SIGNIFICANCE

The HTLV-1 infection seems to induce depression-like behavior, motor dysfunction, disruption in working and fear memory and also oxidative stress in the cortex and hippocampus.

A novel mother-to-child human T-cell leukaemia virus type 1 (HTLV-1) transmission model for investigating the role of maternal anti-HTLV-1 antibodies using orally infected mother rats

Human T-cell leukaemia virus type 1 (HTLV-1) is a human retrovirus that is a causative agent of adult T-cell leukaemia/lymphoma (ATL) and is mainly transmitted from an infected mother to her child via breastfeeding. Such an HTLV-1 infection during childhood is believed to be a risk factor for ATL development. Although it has been suggested that an increased proviral load (PVL), a higher titre of antibody (Ab) in the infected mother and prolonged breastfeeding are associated with an increased risk of mother-to-child transmission (MTCT), the mechanisms underlying MTCT of HTLV-1 remain largely unknown. In this study, we developed an MTCT model using orally HTLV-1-infected rats that have no Ab responses against viral antigens, such as Gag and Env. In this model, HTLV-1 could be transmitted from the infected mother rats to their offspring at a high rate (50-100 %), and the rate of MTCT tended to be correlated with the PVL of the infected mother rats. Furthermore, passive immunization of uninfected adult rats and an infected mother rat with a rat anti-HTLV-1 Env gp46-neutralizing mAb was unable to suppress primary oral HTLV-1 infection to the adult rats and vertical HTLV-1 transmission to the offspring, respectively. Our findings indicate that this MTCT model would be useful to investigate not only the mechanisms of MTCT but also the role of anti-HTLV-1 Ab in MTCT of HTLV-1. They also provide some information on the role of maternal Abs in MTCT, which should be considered when designing a strategy for prevention of MTCT of HTLV-1.

Nation-wide epidemiological study of Japanese patients with rare viral myelopathy using novel registration system (HAM-net)

Background

At least one million people are infected with human T-lymphotropic virus type 1 (HTLV-1) in Japan, a small percentage of whom develop HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) or adult T-cell leukemia/lymphoma (ATLL). Patients with HAM/TSP suffer from progressively worsening myelopathic symptoms, such as motor disability and bladder dysfunction, and may become wheelchair-bound or even bedridden.

Methods

To learn more about this rare, debilitating disease, we established the national registration system “HAM-net” in March 2012. We continuously obtain detailed data from enrolled patients using the registration forms and an annual telephone interview. In this retrospective study, we describe the demographics and clinical histories of 383 registered patients from all over Japan.

Results

Patients were diagnosed at a median of 53 years old, long after disease onset at 45. Most (55.3 %) were originally from the southernmost regions, Kyushu and Okinawa. The main initial symptoms were difficulty walking (81.9 %), urinary dysfunction (38.5 %), and lower limb sensory disturbances (13.9 %). Many patients reported frequent leg numbness and leg pain, and the vast majority required medical intervention for urinary symptoms and constipation. A median of 8 years elapsed from the onset of motor symptoms to Osame Motor Disability Score (OMDS) 5 (requiring unilateral support), 12.5 years to OMDS 6 (requiring bilateral support), and 18 years to OMDS 9 (unable to walk). Health Assessment Questionnaire - Disability Index (HAQ-DI) tasks related to mobility, as opposed to hand motions, were very difficult for HAM/TSP patients and well-correlated with OMDS. Scores on the MOS 36-Item Short-Form Health Survey (SF-36) indicated that physical functioning was severely impaired in HAM/TSP patients. Patients with a history of blood transfusion (19.1 %) were older and suffered from more severe disability as indicated by their high HAQ-DI scores. Patients with a family history of HAM/TSP (8.4 %) were younger and had relatively mild symptoms given their long disease durations; many (15.6 %) also had a relative with ATLL.

Conclusions

The HAM-net national registration system has been an effective tool for gathering personal and clinical data from HAM/TSP patients scattered throughout Japan. We expect to conduct many retrospective and prospective epidemiological studies using HAM-net in the future.

Mother-to-Child Transmission of HTLV-1 Epidemiological Aspects, Mechanisms and Determinants of Mother-to-Child Transmission

Human T-cell Lymphotropic Virus type 1 (HTLV-1) is a human retrovirus that infects at least 5-10 million people worldwide, and is the etiological agent of a lymphoproliferative malignancy; Adult T-cell Leukemia/Lymphoma (ATLL); and a chronic neuromyelopathy, HTLV-1 Associated Myelopathy/Tropical Spastic Paraparesis (HAM/TSP), as well as other inflammatory diseases such as infective dermatitis and uveitis. Besides sexual intercourse and intravenous transmission, HTLV-1 can also be transmitted from infected mother to child during prolonged breastfeeding. Some characteristics that are linked to mother-to-child transmission (MTCT) of HTLV-1, such as the role of proviral load, antibody titer of the infected mother, and duration of breastfeeding, have been elucidated; however, most of the mechanisms underlying HTLV-1 transmission during breast feeding remain largely unknown, such as the sites of infection and cellular targets as well as the role of milk factors. The present review focuses on the latest findings and current opinions and perspectives on MTCT of HTLV-1.

Stable and transient periodic oscillations in a mathematical model for CTL response to HTLV-I infection

The cytotoxic T lymphocyte (CTL) response to the infection of CD4+ T cells by human T cell leukemia virus type I (HTLV-I) has previously been modelled using standard response functions, with relatively simple dynamical outcomes. In this paper, we investigate the consequences of a more general CTL response and show that a sigmoidal response function gives rise to complex behaviours previously unobserved. Multiple equilibria are shown to exist and none of the equilibria is a global attractor during the chronic infection phase. Coexistence of local attractors with their own basin of attractions is the norm. In addition, both stable and unstable periodic oscillations can be created through Hopf bifurcations. We show that transient periodic oscillations occur when a saddle-type periodic solution exists. As a consequence, transient periodic oscillations can be robust and observable. Implications of our findings to the dynamics of CTL response to HTLV-I infections in vivo and pathogenesis of HAM/TSP are discussed.

Route of primary HTLV-1 infection regulates HTLV-1 distribution in reservoir organs of infected mice

Human T-cell leukemia virus type-1 (HTLV-1) causes adult T-cell leukemia and HTLV-1-associated myelo-pathy/tropical spastic paraparesis. HTLV-1 is mainly transmitted through blood transfusion and breastfeeding, but viral proliferation in the body in vivo shortly after transmission is not well understood. To investigate whether the route of infection influences the early stages of viral proliferation, we inoculated BALB/c mice with MT-2 cells, an HTLV-1-producing human T-cell line, via different routes, and evaluated the proviral load and humoral immune responses. One month after infection, the provirus was detected in most organs of the mice infected intraperitoneally, and substantial proviral loads were detected in the peripheral blood and secondary lymphoid organs. In contrast, the mice infected intravenously and orally showed low proviral loads, and the provirus distribution was limited to the spinal cord among the intravenously inoculated mice and to the liver among the perorally inoculated mice. Mice infected intraperitoneally also exhibited higher interleukin-2 production than the mice infected intravenously or orally, or than the uninfected control mice, while anti-HTLV-1 antibody titers were comparable between the mice infected intraperitoneally and intravenously. These results demonstrate that the route of primary HTLV-1 infection influences the establishment of HTLV-1-infected cell proliferation and the cell reservoir in mice.

A dose-effect relationship for deltaretrovirus-dependent leukemogenesis in sheep

Background

Retrovirus-induced tumors develop in a broad range of frequencies and after extremely variable periods of time, from only a few days to several decades, depending mainly on virus type. For hitherto unexplained reasons, deltaretroviruses cause hematological malignancies only in a minority of naturally infected organisms and after a very prolonged period of clinical latency.

Results

Here we demonstrate that the development of malignancies in sheep experimentally infected with the deltaretrovirus bovine leukemia virus (BLV) depends only on the level of BLV replication. Animals were experimentally infected with leukemogenic or attenuated, but infectious, BLV molecular clones and monitored prospectively through 8 months for viral replication. As early as 2 weeks after infection and subsequently at any time during follow-up, leukemogenic viruses produced significantly higher absolute levels of reverse transcription (RT), clonal expansion of infected cells, and circulating proviruses with RT- and somatic-dependent mutations than attenuated viruses. These differences were only quantitative, and both kinds of viruses triggered parallel temporal fluctuations of host lymphoid cells, viral loads, infected cell clonality and proliferation.

Conclusion

Deltaretrovirus-associated leukemogenesis in sheep appears to be a two-hit process over time depending on the amounts of first horizontally and then vertically expanded viruses.

Immunologic control of human T-cell leukemia virus type I and adult T-cell leukemia

Host T-cell responses to human T-cell leukemia virus type I (HTLV-I) control the expansion of HTLV-I-infected cells and are determinants of the equilibrium proviral load in vivo. Insufficient T-cell responses are regarded as an immunologic risk factor for adult T-cell leukemia (ATL) because they allow increased proviral loads, which represent an epidemiologic risk factor for ATL. ATL cells from approximately half of ATL cases retain the ability to express HTLV-I Tax, a major target antigen of HTLV-I-specific cytotoxic T-lymphocytes (CTL), whereas Tax-specific CTL in ATL patients are inactive. Tax-specific CTL responses are strongly activated after hematopoietic stem cell transplantation in some ATL patients in long-term remission, indicating that HTLV-I Tax is expressed in vivo rather than being silent, and that the donor-derived T-cell system can recognize it. These findings strongly suggest that reactivation of Tax-specific CTL by vaccines may be promising for prophylaxis of ATL in the high-risk group of HTLV-I carriers and for therapy of ATL in patients whose tumor cells are capable of expressing Tax.

Leukaemogenic mechanism of human T-cell leukaemia virus type I

Adult T-cell leukaemia (ATL) is a neoplastic disease derived from CD4(+) T-lymphocytes and etiologically associated with human T-cell leukaemia virus type I (HTLV-I). In addition to structural genes, HTLV-I encodes regulatory and accessory genes in the pX region. Among them, Tax is thought to play a central role in leukaemogenesis through its potent transforming activity. However, since Tax is a major target of the host immune system, its expression is often lost in ATL cells, indicating Tax is dispensable in the last phase of leukaemogenesis. The HTLV-I bZIP factor (HBZ), encoded on the HTLV-I minus strand, was recently shown to be expressed in all ATL cells, and to support growth of human T-cell lines. These findings suggest that HBZ is critical to ATL onset. In addition to viral factors and genetic and epigenetic changes in cellular genes, the host immune status and genetic background also function in leukaemogenesis.

Reduction of human T-cell leukemia virus type 1 (HTLV-1) proviral loads in rats orally infected with HTLV-1 by reimmunization with HTLV-1-infected cells

Human T-cell leukemia virus type 1 (HTLV-1) persistently infects humans, and the proviral loads that persist in vivo vary widely among individuals. Elevation in the proviral load is associated with serious HTLV-1-mediated diseases, such as adult T-cell leukemia and HTLV-1-associated myelopathy/tropical spastic paraparesis. However, it remains controversial whether HTLV-1-specific T-cell immunity can control HTLV-1 in vivo. We previously reported that orally HTLV-1-infected rats showed insufficient HTLV-1-specific T-cell immunity that coincided with elevated levels of the HTLV-1 proviral load. In the present study, we found that individual HTLV-1 proviral loads established in low-responding hosts could be reduced by the restoration of HTLV-1-specific T-cell responses. Despite the T-cell unresponsiveness for HTLV-1 in orally infected rats, an allogeneic mixed lymphocyte reaction in the splenocytes and a contact hypersensitivity response in the skin of these rats were comparable with those of naive rats. HTLV-1-specific T-cell response in orally HTLV-1-infected rats could be restored by subcutaneous reimmunization with mitomycin C (MMC)-treated syngeneic HTLV-1-transformed cells. The reimmunized rats exhibited lower proviral loads than untreated orally infected rats. We also confirmed that the proviral loads in orally infected rats decreased after reimmunization in the same hosts. Similar T-cell immune conversion could be reproduced in orally HTLV-1-infected rats by subcutaneous inoculation with MMC-treated primary T cells from syngeneic orally HTLV-1-infected rats. The present results indicate that, although HTLV-1-specific T-cell unresponsiveness is an underlying risk factor for the propagation of HTLV-1-infected cells in vivo, the risk may potentially be reduced by reimmunization, for which autologous HTLV-1-infected cells are a candidate immunogen.

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.

Tumor immunity against adult T-cell leukemia

Human T-cell leukemia virus type-I (HTLV-I) causes adult T-cell leukemia (ATL) in a small population of infected individuals after a long incubation period. Although the process of clonal evolution of ATL cells may involve multiple steps, ATL cells from half of the ATL cases still retain the ability to express HTLV-I Tax, a key molecule of HTLV-I leukemogenesis. A recent finding of reactivation of Tax-specific cytotoxic T lymphocytes (CTL) in ATL patients after hematopoietic stem cell transplantation suggests the presence of Tax expression in vivo and potential contribution of the CTL to antitumor immunity. This is consistent with the results of a series of animal experiments indicating that Tax-specific CTL limit the growth of HTLV-I-infected cells in vivo, although the animal model mimics only an early phase of HTLV-I infection and leukemogenesis. Establishment of an insufficient HTLV-I-specific T-cell response and an increased viral load in orally HTLV-I-infected rats suggests that host HTLV-I-specific T-cell response at a primary HTLV-I infection can be a critical determinant of persistent HTLV-I levels thereafter. These findings indicate that Tax-targeted vaccines may be effective for prophylaxis of ATL in a high-risk group, and also for therapy of ATL in at least half the cases.

[Anti-tumor immunity in adult T-cell leukemia]

Adult T-cell leukemia (ATL) occurs in a small population of human T-cell leukemia virus type I (HTLV-I)-infected individuals. It has been noted that ATL is incidentally associated with mother-to-child infection which occurs mainly through breast-feeding, elevated levels of proviral load, and insufficiency in HTLV-I-specific cytotoxic T lymphocyte (CTL) responses. Among these, anti-tumor potentials of HTLV-I-specific CTL have been shown in ex vivo analysis of human HTLV-I-infected individuals and also in vivo experiments by using rat models of HTLV-I-infected lymphomas. In another rat model of HTLV-I-infection, orally infected rats showed significantly higher HTLV-I proviral load but lower HTLV-I-specific cellular immune responses than in intraperitoneally infected rats. As a result, persistent viral load was inversely correlated with levels of virus-specific T-cell responses. HTLV-I-specific T-cell responses in orally infected rats recovered by re-immunization. Conversion of Tax-specific T-cell responses from low to high levels was also observed in an ATL patient who obtained complete remission after hematopoietic stem cell transplantation. These findings suggest that HTLV-I-specific immune unresponsiveness associated with oral HTLV-I infection may be a potential risk factor for development of ATL, allowing expansion of the infected cell reservoir in vivo, and that immunological strategies targeting Tax may potentially reduce the risk of ATL and induce therapeutic effects on ATL.

Immunological risks of adult T-cell leukemia at primary HTLV-I infection

A small percentage of human T-cell leukemia virus type-I (HTLV-I)-infected individuals develop adult T-cell leukemia (ATL). In animal experiments, inoculation of HTLV-I via the oral route, which is the main route of mother-to-child viral transmission in humans as a result of breastfeeding, induced host HTLV-I-specific T-cell unresponsiveness and resulted in increased viral load. This strongly suggested that the known epidemiological risk factors for ATL (i.e. vertical HTLV-I infection and elevated viral load) are linked by an insufficient HTLV-I-specific T-cell response. Recent findings on the anti-tumor effects of Tax-targeted vaccination in rats and the reactivation of Tax-specific T cells in ATL patients as a result of hematopoietic stem cell transplantation imply promising immunological approaches for the prophylaxis and therapy of ATL.

Human T cell leukemia virus type I-induced disease: pathways to cancer and neurodegeneration

Retroviral infection is associated with a number of pathologic abnormalities, including a variety of cancers, immunologic diseases, and neurologic disorders. Shortly after its discovery in 1980, human T cell leukemia virus type I (HTLV-I) was found to be the etiologic agent of both adult T cell leukemia (ATL) and HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP), a neurologic disease characterized by demyelinating lesions in both the brain and the spinal cord. Approximately 5-10% of HTLV-I-infected individuals develop either ATL or HAM/TSP. Interestingly, the two diseases have vastly different pathologies and have rarely been found to occur within the same individual. While a number of host and viral factors including virus strain, viral load, and HLA haplotype have been hypothesized to influence disease outcome associated with HTLV-I infection, the relative contributions of such factors to disease pathogenesis have not been fully established. Recent research has suggested that the route of primary viral infection may dictate the course of disease pathogenesis associated with HTLV-I infection. Specifically, mucosal exposure to HTLV-I has been associated with cases of ATL, while primary viral infection based in the peripheral blood has been correlated with progression to HAM/TSP. However, the cellular and molecular mechanisms regulating disease progression resulting from primary viral invasion remain to be elucidated. Although a variety of factors likely influence these mechanisms, the differential immune response mounted by the host against the incoming virus initiated in either the peripheral blood or the mucosal compartments likely plays a key role in determining the outcome of HTLV-I infection. It has been proposed that the route of infection and size of the initial viral inoculum allows HTLV-I to infect different target cell populations, in turn influencing the breadth of the immune response mounted against HTLV-I and affecting disease pathogenesis. A model of HTLV-I-induced disease progression is presented, integrating information regarding the role of several host and viral factors in the genesis of both neoplasia and neurologic disease induced following HTLV-I infection, focusing specifically on differential viral invasion into the bone marrow (BM) and the influence of this event on the virus-specific CD8(+) cytotoxic T lymphocyte (CTL) response that is initiated following HTLV-I infection.

Expansion of human T-cell leukemia virus type 1 (HTLV-1) reservoir in orally infected rats: inverse correlation with HTLV-1-specific cellular immune response

Adult T-cell leukemia (ATL) occurs in a small population of human T-cell leukemia virus type 1 (HTLV-1)-infected individuals. Although the critical risk factor for ATL development is not clear, it has been noted that ATL is incidentally associated with mother-to-child infection, elevated proviral loads, and weakness in HTLV-1-specific T-cell immune responses. In the present study, using a rat system, we investigated the relationships among the following conditions: primary HTLV-1 infection, a persistent HTLV-1 load, and host HTLV-1-specific immunity. We found that the persistent HTLV-1 load in orally infected rats was significantly greater than that in intraperitoneally infected rats. Even after inoculation with only 50 infected cells, a persistent viral load built up to considerable levels in some orally infected rats but not in intraperitoneally infected rats. In contrast, HTLV-1-specific cellular immune responses were markedly impaired in orally infected rats. As a result, a persistent viral load was inversely correlated with levels of virus-specific T-cell responses in these rats. Otherwise very weak HTLV-1-specific cellular immune responses in orally infected rats were markedly augmented after subcutaneous reimmunization with infected syngeneic rat cells. These findings suggest that HTLV-1-specific immune unresponsiveness associated with oral HTLV-1 infection may be a potential risk factor for development of ATL, allowing expansion of the infected cell reservoir in vivo, but could be overcome with immunological strategies.

Rat CRM1 is responsible for the poor activity of human T-cell leukemia virus type 1 Rex protein in rat cells

Rat models of human T-cell leukemia virus type 1 (HTLV-1)-related diseases such as adult T-cell leukemia and HTLV-1-associated myelopathy/tropical spastic paraparesis have been reported. However, these models do not completely reproduce human diseases partly because HTLV-1 replicates poorly in rats. We investigated here the possible reason for this. We found that the activity of Rex in rat cells is quite low compared to that in human cells. As Rex function depends largely on the CRM1 protein, whose human type (human CRM1 [hCRM1]) directly binds to Rex and exports it from the nucleus to the cytoplasm, we assessed whether rat CRM1 (rCRM1) could act as well as hCRM1 as a cofactor for Rex activity. We first cloned a cDNA encoding rCRM1 and found that both rCRM1 and hCRM1 could bind to and export Rex protein to the cytoplasm with similar efficiencies. However, unlike hCRM1, rCRM1 could hardly support Rex function because of its poor ability in inducing the Rex-Rex interaction required for RNA export into the cytoplasm. These observations suggest that the poor ability of rCRM1 to act as a cofactor for Rex function may be responsible for the poor replication of HTLV-1 in rats.

Immunological aspects of rat models of HTLV type 1-infected T lymphoproliferative disease

The level of host immune responses against human T cell leukemia virus type 1 (HTLV-1) varies among HTLV-1-infected individuals. In the present study, we investigate the role of host immunity on HTLV-1 leukemogenesis in vivo by using animal models. At first, we examined the effect of the routes of HTLV-1 transmission on the host anti-HTLV-1 immune responses. When immune competent adult rats were inoculated with HTLV-1-infected cells, the orally infected rats were persistently infected with HTLV-1 without humoral and cellular immune responses against HTLV-1, whereas all intravenously or intraperitoneally inoculated rats showed significant levels of immune responses. Next, we examined in vivo tumorigenicity of HTLV-1-immortalized cells in the absence of T cell immunity, by using athymic F344/N Jcl-rnu/rnu (nu/nu) rats. When inoculated into nu/nu rats, not all but some HTLV-1-immortalized rat cell lines including syngeneic FPM1-V1AX could grow and form T cell lymphoma in vivo. This syngeneic lymphoma formation was inhibited by adoptively transferred immune T cells. Furthermore, immunocompetent rats allowed in vivo growth of HTLV-1-infected lymphoma, when treated with antibodies that block costimulatory signals for T cell activation. These observations indicated that (1) host anti-HTLV-1 immunity can be affected by the conditions of the primary infection, (2) under the low pressure of anti-HTLV-1 immunity, some HTLV-1-infected cell clones grow in vivo, and (3) T cell immunity is required for in vivo surveillance against these HTLV-1-infected cell clones.

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.

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.