In vivo fluctuation of HTLV-I and HTLV-II proviral load in patients receiving antiretroviral drugs

HTLV-1-associated myelopathy in Spain

BACKGROUND

HTLV-1 infection is a neglected disease. Over 10 million people are infected worldwide, with hot spots of high endemicity across all continents. Roughly 5% of HTLV-1 carriers develop HTLV-1-associated myelopathy (HAM), a progressive subacute neurological disabling disease.

METHODS

We report the main features of patients diagnosed with HAM up to date in Spain, a non-endemic country with a relatively high migrant flow from Latin America and Equatorial Africa, where HTLV-1 is endemic.

RESULTS

A total of 451 cases of HTLV-1 had been recorded in Spain until the end of year 2022. HAM had been diagnosed in 58 (12.9%). The current incidence is of 2-3 new cases per year. Women represent 76%. Mean age at diagnosis is 49 years-old. Nearly 60% are Latin Americans. Although sexual transmission is the most likely route of HTLV-1 acquisition, up to 6 individuals had been infected following solid organ transplantation. Rapid onset myelopathy developed in all but one of these transplant recipients from three HTLV-1-positive donors. HTLV-1 subtype 1a transcontinental was the only variant recognized in HAM patients. HTLV-1 proviral load was significantly greater in HAM patients than in asymptomatic HTLV-1 carriers (677 vs 104 HTLV-1 DNA copies/104 PBMC; p = 0.012). Symptom relief medications and physiotherapy have been the only treatment providing some benefit to HAM patients. Neither significant clinical nor virological efficacy was noticed using antiretrovirals in at least 9 HAM patients. Two thirds of HAM patients ended up in a wheelchair and with urinary/fecal sphincter incontinence.

CONCLUSION

HAM is the most frequent clinical manifestation of HTLV-1 infection in Spain, a non-endemic country. Middle aged women migrants from Latin America are the most frequently affected. Two thirds end up in a wheelchair despite using antiretroviral therapy.

Development and Validation of Multiplex Quantitative Real-Time PCR Assays for Simultaneous Detection and Differentiation of HTLV-1 and HTLV-2, Using Different PCR Platforms and Reagent Brands

Brazil currently has the highest number of individuals infected with human T-lymphotropic virus 1- and 2- (HTLV-1 and HTLV-2) globally. At present, neither molecular protocols nor commercial assays are available for HTLV-1/-2 diagnosis or validated by the Brazilian Ministry of Health regulatory agency (ANVISA). We developed and validated two in-house multiplex quantitative real-time PCR for HTLV-1/-2 (mqPCR_HTLV) assays, targeting the pol and tax genes, for the simultaneous identification of HTLV-1, HTLV-2, and the albumin reference gene. The robustness of the assays was evaluated on two platforms using seven commercial master mix formulations. The reactions employed double plasmids (pHTLV1-Alb and pHTLV2-Alb) for the standard curve’s construction and for expressing the detection limit of the assays. They were able to detect 10 and 10 copies of HTLV-1 and 10 and 70 copies of HTLV-2 for the tax and pol targets, respectively. High efficiency was obtained using both the platforms and all the reagents evaluated and were successfully reproduced by other analysts. DNA samples from HTLV-1/-2-infected and non-infected patients and from HIV/HTLV-coinfected patients were evaluated to determine the feasibility of their use in routine diagnosis. The mqPCR_HTLV (pol and tax) assays demonstrated an overall specificity of 100% and a sensitivity of 97.4% when testing samples from patients without HIV infection, and sensitivities of 77.1% (pol) and 74.6% (tax) in samples from HIV/HTLV-coinfected patients. In addition, they resolved the issue of HTLV western blotting (WB) indeterminate and WB-untyped results in 45.5 and 66.7% of cases, respectively. The developed mqPCR_HTLV (pol and tax) assays indicated their feasibility for efficient and reliable HTLV diagnosis in various core facility laboratories under different conditions and supplies.

High risk of heterosexual transmission of human T-cell lymphotropic virus type 1 infection in Brazil

Human T-cell lymphotropic virus type 1 is transmitted primarily either through sexual intercourse or from mother to child. The current study investigated sexual transmission and compared the HTLV-1 proviral load between seroconcordant and serodiscordant couples by examining both men and women among the index partners without using subjective criteria to establish the direction of sexual transmission. Between January 2013 and May 2015, 178 HTLV-1-positive patients had spouses, 107 of which had tested partners, thus increasing the initial sample size (46 men and 61 women). Individuals co-infected with HTLV-2 or human immunodeficiency virus were not included in the analysis. From among the included participants, 26 men and 26 women were paired with each other, resulting in 26 seroconcordant couples; 12 seroconcordant couples were formed from another four men and eight women. Forty-three serodiscordant couples were formed from 16 men and 27 women. The rate of seroconcordance was 46.9%. The HTLV-1 proviral load was compared between 19 and 37 seroconcordant and serodiscondant couples, respectively, and the concordant couples showed higher proviral loads (P = 0.03). There were no differences between the groups according to age, relationship length, having a mother or sibling with HTLV-1, race, ethnicity, nationality, education, history of blood transfusion, HAM/TSP, ALT, or hepatitis C virus status. In multivariate analysis, relationship time was shown associated with ocurrence of seroconcordance status. The apparent association between high circulating levels of provirus and seroconcordance rate among couples suggests that proviral loads contribute markedly to the risk of sexual transmission, regardless of gender index.

SEROPREVALENCE OF HTLV IN A POPULATION OF HIV1-INFECTED PATIENTS IN MIDWESTERN BRAZIL

Human T-cell lymphotropic virus (HTLV) may affect the clinical course of human immunodeficiency virus 1 (HIV1). Both infections are common in endemic areas because these viruses share similar routes of transmission. The aim of this study was to estimate the seroprevalence of HTLV1/2 in a population of HIV1-infected patients in the state of Goiás, Midwestern Brazil. Of the 505 studied patients, four (0.79%) were positive for anti-HTLV1/2 by enzyme-linked immunosorbent assay (ELISA), with HTLV1 infection confirmed by line immunoassay (LIA) and polymerase chain reaction (PCR) in all of the ELISA-positive samples. No cases of HTLV2 infection were observed. The prevalence of HTLV1/HIV1 coinfection was 0.79% (4/505; 95% CI: 0.25-2.16). All the coinfected patients reported sexual risk behaviors and only one reported intravenous drug use. Sequencing of the viral long terminal repeat (LTR) region and phylogenetic analysis revealed that the four HTLV1 isolates belonged to the Transcontinental a subgroup of the Cosmopolitan (1a) subtype, the most frequent subgroup detected in Brazil. This study shows a low prevalence of HTLV1/2 in HIV1-infected patients in Midwestern Brazil.

Short Communication: Current Prevalence and Risk Factors Associated with Human T Lymphotropic Virus Type 1 and Human T Lymphotropic Virus Type 2 Infections Among HIV/AIDS Patients in São Paulo, Brazil

During the 1990s, high prevalences of HIV/human T lymphotropic virus type 1 (HTLV-1) and HIV/human T lymphotropic virus type 2 (HTLV-2) coinfections were detected in São Paulo, Brazil in association with intravenous drug use (IDU). The current prevalences and risk factors for HIV/HTLV-1/-2 were evaluated in 1,608 patients attending the AIDS/STD Reference and Training Center in São Paulo. Blood samples were analyzed for HTLV-1/2-specific antibodies using enzyme immunoassays (EIA Murex HTLV-I+II, Diasorin, and Gold ELISA HTLV-I+II, REM) and immunoblotting (HTLV Blot 2.4, MP Biomedicals and INNO-LIA HTLV-I/II, Innogenetics) and for the pol proviral DNA segments of HTLV-1 and HTLV-2 by "in-house" real-time PCR. These analyses revealed that 50 (3.11%) of the samples were HTLV positive, including 25 (1.55%) that were HTLV-1 positive, 21 (1.31%) that were HTLV-2 positive, and 4 (0.25%) that were HTLV positive (untypeable). The median age of the HIV/HTLV-coinfected individuals was 50 years versus 44 years in the overall population (p=0.000). The risk factors associated with HIV/HTLV-1/-2 coinfections were female gender (OR 3.26, 1.78-5.95), black/pardo color (OR 2.21, 1.21-4.03), infection with hepatitis B virus (HBV) (OR 4.27, 2.32-7.87) or hepatitis C virus (HCV) (OR 24.40, 12.51-48.11), and intravenous drug use (IDU) (OR 30.01, 15.21-59.29). The current low prevalence of HTLV-1/2 in HIV-infected patients in São Paulo could be explained in part by programs providing IDUs with sterile needles and syringes and changes in the drug usage patterns of individuals from injecting cocaine to smoking crack cocaine.

The best algorithm to confirm the diagnosis of HTLV-1 and HTLV-2 in at-risk individuals from São Paulo, Brazil

The ability to confirm the diagnosis of human T-lymphotropic virus types 1 and 2 (HTLV-1 and HTLV-2) in at-risk individuals in São Paulo, Brazil by Western blotting (WB), conventional polymerase chain reaction (tax and pol PCR) and real-time PCR (pol) is compared. Seventy-three blood samples that were reactive in HTLV-1/2 serological screening enzyme immunoassays (EIAs) were evaluated. HTLV-1/2 was confirmed in 53 blood samples: 48 were positive by WB, 41 were positive by PCR and 42 scored positive by real-time PCR assays (37 of 48 WB-positive samples plus five WB-indeterminate samples that were further confirmed by sequencing). Although WB was able to detect more cases of HTLV-1/2 infection, the real-time PCR assay was able to discriminate between these two viruses and confirm an individual HTLV-1/HTLV-2 diagnosis in two HTLV WB-untyped samples and five WB-indeterminate samples. Because of the large number of WB-indeterminate samples and the cost of the WB assay in Brazil, it is proposed an algorithm that employs two EIAs for screening and then real-time PCR to confirm the infection, followed by testing any PCR-negative samples with the WB assay. This strategy reduces costs and improves the accuracy of the diagnosis of HTLV-1/2.

Suppression of human T-cell leukemia virus I gene expression by pokeweed antiviral protein

Human T-cell leukemia virus I (HTLV-I) is a deltaretrovirus that is the causative agent of adult T-cell leukemia and the neurological disorder HTLV-I-associated myelopathy/tropical spastic paraparesis. Currently, no effective antiretroviral treatment options are available to restrict the development of diseases associated with the virus. In this work, we investigated the activity of pokeweed antiviral protein (PAP) on HTLV-I, when expressed from a proviral clone in 293T cells or in an HTLV-I immortalized cell line. PAP is a plant-derived N-glycosidase that exhibits antiviral activity against a number of viruses; however, its mode of action has not been clearly defined. Here, we describe the mechanism by which PAP inhibited production of HTLV-I. We show that PAP depurinated nucleotides within the gag open reading frame and suppressed the synthesis of viral proteins in part by decreasing the translational efficiency of HTLV-I gag/pol mRNA. Observed reduction in levels of viral mRNAs were not due to enhanced degradation; rather, decreased amounts of viral transactivator protein, Tax, led to feed-back inhibition of transcription from the viral promoter. Therefore, PAP efficiently suppressed HTLV-I gene expression at both translational and transcriptional levels, resulting in substantially diminished virus production. Significantly, no changes in viability or rates of cellular transcription or translation were observed in cells expressing PAP, indicating that this protein was not toxic. Antiviral activity, together with the absence of cytotoxicity, supports further investigation of this enzyme as a novel therapeutic agent against the progression of HTLV-I infection.

Upregulation of HTLV-1 and HTLV-2 expression by HIV-1 in vitro

Co-infections with HIV-1 and the human T leukemia virus types 1 and 2 (HTLV-1, HTLV-2) occur frequently, particularly in large metropolitan areas where injection drug use is a shared mode of transmission. Recent evidence suggests that HIV-HTLV co-infections are associated with upregulated HTLV-1/2 virus expression and disease. An in vitro model of HIV-1 and HTLV-1/2 co-infection was utilized to determine if cell free HIV-1 virions or recombinant HIV-1 Tat protein (200-1,000 ng/ml) upregulated HTLV-1/2 expression and infectivity. Exposure to HIV-1 increased the number of HTLV-1 antigen expressing cells, from 6% at baseline to 12% at 24 hr, and 20% at 120 hr (P < 0.05) post-exposure. A similar, although less robust response was observed in HTLV-2 infected cells. HIV-1 co-localized almost exclusively with HTLV-1/2 positive cells. Exposure to HIV-1 Tat protein (1,000 ng/ml) increased HTLV-1 p19 expression almost twofold by 48 hr, and cells co-stimulated with 10 nM phorbol myristate acetate (PMA) showed almost a fourfold increase over baseline. It is concluded that HIV-1 augments HTLV-1/2 infectivity in vitro. The findings also suggest a role for the HIV-1 Tat protein and PMA-inducible cellular factors, in HIV-1 induced HTLV-1/2 antigen expression.

Low DNA HTLV-2 proviral load among women in São Paulo City

BACKGROUND

HTLV-2 infections are almost always asymptomatic, and diseases associated with the infection are rarely reported. Little information is available on the relationship between HTLV-2 proviral load and gender or expression of disease, especially among patients with HIV-1 co-infection.

METHODS

We studied 77 HTLV-2-infected subjects followed in our clinic for the last 9 years; 53 (69%) of them were co-infected with HIV-1. HTLV-2 DNA proviral load (PVL) was measured by real time PCR, a test with a sensitivity of 10 in 10(4) PBMCs.

RESULTS

Six of 53HTLV-2/HIV-1 cases had a myelopathy (all of them had undetectable PVL of HTLV-2). Only 3 of 35 women (2 out of 3 co-infected with HIV) had a detectable PVL, whereas 10 of 42 men had a detectable PVL. Regardless of their HIV status women had significantly lower PVL than men (10 vs. 43 copies/10(4) PBMCs, p<0.05).

CONCLUSIONS

We noticed the occurrence of myelopathy in HTLV-2/HIV-1 co-infected patients, with undetectable HTLV-2 viral load. There was a sex difference in viral load for HTLV-2, what may be the result in mode of transmission or acquisition of the virus.

Difficulties in the diagnosis of HTLV-2 infection in HIV/AIDS patients from Brazil: comparative performances of serologic and molecular assays, and detection of HTLV-2b subtype

The current diagnosis of human T-lymphotropic virus type-2 (HTLV-2) infection is based on the search of specific antibodies; nevertheless, several studies conducted in Brazil pointed deficiencies of the commercially available kits in detecting HTLV-2, mostly in HIV/AIDS patients. This study searched for the presence of HTLV-1 and -2 in 758 HIV/AIDS patients from Londrina, Paraná, Brazil. Serum samples were screened for HTLV-1/2 antibodies using two EIA kits (Vironostika and Murex), and confirmed by WB (HTLV Blot 2.4, Genelabs). The results obtained by EIA disclosed 49 (6.5%) reactive sera: 43 positive by both EIA kits, and six with discordant results. WB confirmed HTLV-1 infection in seven samples (0.9%) and HTLV-2 in 21 sera (2.8%). Negative and indeterminate results were detected in four (0.5%) and 16 (2.1%) sera, respectively. Blood from 47 out of 49 HTLV seroreactive patients were collected and analyzed for the presence of env, LTR and tax genomic segments of HTLVs by PCR. PCR confirmed six cases of HTLV-1 and 37 cases of HTLV-2 infection (14 out of 16 that were found to be WB indeterminate). Restriction analysis of the env PCR products of HTLV-2 disclosed 36 isolates of HTLV-2a/c subtype, and one of HTLV-2b subtype. These results emphasize the need of improving serologic tests for detecting truly HTLV-2 infected patients from Brazil, and confirm the presence of HTLV-2b subtype in the South of this country.

Effect of phosphonated carbocyclic 2'-oxa-3'-aza-nucleoside on human T-cell leukemia virus type 1 infection in vitro

There is currently little research and development of new compounds with specific anti-human T-cell leukemia virus type 1 (HTLV-1) activity. The few antiretrovirals that have been tested against HTLV-1 in vitro have already been developed into anti-human immunodeficiency virus (HIV) drugs. Here, we show the effects of a newly synthesized family of phosphonated nucleoside compounds, phosphonated carbocyclic 2'-oxa-3'-aza-nucleosides (PCOANs), on HTLV-1 infection in vitro. To ascertain the anti-HTLV-1 activity of PCOANs, peripheral blood mononuclear cells from healthy donors were infected in vitro by coculture with an HTLV-1 donor cell line in the presence of three prototype PCOAN compounds. PCOANs were able to completely inhibit HTLV-1 infection in vitro at a concentration of 1 microM, similar to what has been observed for tenofovir and azidothymidine. Treatment with PCOANs was associated with inhibited growth of HTLV-1-infected cells, and their effects were 100 to 200 times more potent than that of tenofovir. The mechanisms involved in the anti-HTLV-1 effects of PCOANs can mainly be ascribed to their capacity to inhibit HTLV-1 reverse transcriptase activity, as ascertained by means of a cell-free assay. PCOANs caused little reduction in proliferation or induction of apoptotic cell death of uninfected cells, showing toxicity levels similar to tenofovir and lower than azidothymidine. Overall, these results indicate that the family of PCOANs includes potential candidate compounds for long-lasting control of HTLV-1 infection.

[HTLV-1 and HTLV-2 proviral load: a simple method using quantitative real-time PCR]

When the human T cell lymphotropic virus (HTLV) is integrated with the host cell genome (provirus), its proviral DNA is a replication marker. Proviral load appears to be an important factor in the development of diseases related to these retroviruses. In this study, a methodology for absolute quantification of the HTLV-1 and HTLV-2 proviral load using real-time PCR was developed. Fifty-three blood donor samples with positive ELISA test result were subjected to this methodology, which utilized the TaqMan system for three target sequences: HTLV-1, HTLV-2 and albumin. The absolute proviral load was quantified using the relative ratio between the HTLV genome and the host cell genome, taking into consideration the white blood cell count. The method presented is sensitive (215 copies/ml), practical and simple for proviral quantification, and is efficient and appropriate for confirming and discriminating infections according to viral type.

Zidovudine and interferon-alpha treatment induces a high response rate and reduces HTLV-1 proviral load and VEGF plasma levels in patients with adult T-cell leukemia from North East Iran

Human T-cell lymphotropic virus type I (HTLV-I) associated adult T-cell leukemia/lymphoma (ATLL) is endemic in southern Japan, the Caribbean, intertropical Africa, and Brazil. Recently north east Iran, particularly the region of Mashhad, has been recognized as a new endemic region. ATLL is an aggressive T-cell lymphoproliferative disorder. Patients with ATLL have high plasma levels of VEGF that induce angiogenesis. Prognosis of ATLL remains poor because of immunosuppression and intrinsic resistance to chemotherapy. Important advances in the treatment of ATLL were reported with the combination of zidovudine (AZT) and interferon-alpha. We investigated the effect of AZT/IFN treatment on vascular endothelium growth factor (VEGF) plasma levels and HTLV-I proviral load in ATLL patients from the region of Mashhad. We confirmed that AZT/IFN treatment induces a high response rate and prolonged survival with minimal side effects. We also confirmed that VEGF plasma levels and HTLV-I proviral load are higher in ATLL patients than in asymptomatic carriers. We finally showed that AZT/IFN treatment reduced both HTLV-I proviral load and importantly VEGF plasma levels, suggesting a potential antiangiogenic effect of this therapy. These results provide further evidence for the efficacy and the mechanism of action of AZT/IFN therapy for ATLL in a developing country.

Relationship between Human T Lymphotropic Virus (HTLV) Type 1/2 Viral Burden and Clinical and Treatment Parameters among Patients with HIV Type 1 and HTLV-1/2 Coinfection

BACKGROUND

Human T lymphotropic virus types 1 (HTLV-1) and 2 (HTLV-2) are frequent copathogens among individuals infected with human immunodeficiency virus type 1 (HIV-1). The long-term effects of coinfection are unknown, and little information exists regarding how levels of HTLV-1/2 viral burden are affected by antiretroviral medications.

METHODS

Factors associated with HTLV-1/2 viral burden were examined in patients with HIV-HTLV-1/2 coinfection. A total of 72 subjects were evaluated. The variables analyzed included HTLV-1/2 proviral load, HTLV-1/2 tax/rex mRNA expression, HIV load, HTLV-1/2 viral antigen detection in peripheral blood mononuclear cell (PBMC) cultures, T cell subsets, demographic variables (age, race, sex, and reported use of injection drugs), and administration of highly active antiretroviral therapy.

RESULTS

An HTLV-1/2 proviral DNA copy number >20,000 copies/10(6) PBMCs was significantly associated with the following variables: (1) a positive HTLV-1 Western blot test result, (2) a positive HTLV-1/2 PBMC culture result, (3) a positive tax/rex mRNA result, (4) an HIV load <10,000 copies/mL, and (5) higher CD4 cell counts among subjects with HIV-HTLV-1 coinfection. There was no correlation between HTLV-1/2 proviral copy number or HTLV-1/2 tax/rex mRNA detection and administration of antiretroviral therapy.

CONCLUSIONS

HTLV-1/2 proviral burden was significantly higher among patients with HIV-HTLV-1 coinfection than among patients with HIV-HTLV-2 coinfection. Highly active antiretroviral therapy may be of limited value in controlling virus expression of HTLV-1/2 in patients with HIV-HTLV-1/2 coinfection.

Mechanisms of leukemogenesis induced by bovine leukemia virus: prospects for novel anti-retroviral therapies in human

In 1871, the observation of yellowish nodules in the enlarged spleen of a cow was considered to be the first reported case of bovine leukemia. The etiological agent of this lymphoproliferative disease, bovine leukemia virus (BLV), belongs to the deltaretrovirus genus which also includes the related human T-lymphotropic virus type 1 (HTLV-1). This review summarizes current knowledge of this viral system, which is important as a model for leukemogenesis. Recently, the BLV model has also cast light onto novel prospects for therapies of HTLV induced diseases, for which no satisfactory treatment exists so far.

Molecular footprint of drug-selective pressure in a human immunodeficiency virus transmission chain

Known human immunodeficiency virus (HIV) transmission histories are invaluable models for investigating the evolutionary and transmission dynamics of the virus and to assess the accuracy of phylogenetic reconstructions. Here we have characterized an HIV-1 transmission chain consisting of nine infected patients, almost all of whom were treated with antiviral drugs at later stages of infection. Partial pol and env gp41 regions of the HIV genome were directly sequenced from plasma viral RNA for at least one sample from each patient. Phylogenetic analyses in pol using likelihood methods inferred an evolutionary history not fully compatible with the known transmission history. This could be attributed to parallel evolution of drug resistance mutations resulting in the incorrect clustering of multidrug-resistant virus. On the other hand, a fully compatible phylogenetic tree was reconstructed from the env sequences. We were able to identify and quantify the molecular footprint of drug-selective pressure in pol using maximum likelihood inference under different codon substitution models. An increased fixation rate of mutations in the HIV population of the multidrug-resistant patient was demonstrated using molecular clock modeling. We show that molecular evolutionary analyses, guided by a known transmission history, can reveal the presence of confounding factors like natural selection and caution should be taken when accurate descriptions of HIV evolution are required.

Adult T-cell leukaemia/lymphoma-like human T-cell leukaemia virus-1 replication in infective dermatitis

Adult T-cell leukaemia/lymphoma (ATLL) is a malignant T-cell proliferation that occurs in 3-5% of individuals infected with human T-cell leukaemia virus-1 (HTLV-1). HTLV-1 infection is also linked to the development of infective dermatitis (ID), an exudative dermatitis of children that has been proposed as a cofactor of ATLL. Here, HTLV-1 replication was investigated over time in a girl with ID and multiparasitic infestation including strongyloidiasis, a disease also known to predispose HTLV-1 carriers to ATLL. Quantitative polymerase chain reaction (PCR) revealed extremely high proviral loads. During the 2-year period of the present study, the proportion of circulating infected cells ranged between 12% and 36%. Quadruplicate linker-mediated PCR amplification of HTLV-1 flanking sequences identified a pattern of extensive and persistent oligoclonal expansion of infected lymphocytes. As viral loads, both the number and the degree of infected T-cell expansion were independent of treatment or clinical signs. However, the temporal fluctuation of proviral loads correlated significantly with the degree of infected T-cell expansion, but not with the overall number of detected clones. This pattern of HTLV-1 replication over time is very different from that observed in asymptomatic carriers and reminiscent of that observed in ATLL, a result consistent with the proposal of ID as an ATLL cofactor.

Effect of lamivudine on transmission of human T-cell lymphotropic virus type 1 to adult peripheral blood mononuclear cells in vitro

The effects of lamivudine (3TC) on in vitro infection of peripheral blood mononuclear cells (PBMC) from healthy donors with human T-cell lymphotropic virus type 1 (HTLV-1) were investigated. Direct measures of viral replication (viral DNA, RNA, and protein) all gave similar, very high 50% inhibitory concentrations in comparison with those previously reported for zidovudine. Nevertheless, 3TC inhibited HTLV-1-driven long-term growth of infected PBMC in vitro at concentrations (6.25 micro M) which had poor or no direct antiviral effects, suggesting that another mechanism may be playing a role.