Masao Matsuoka, Institute for Virus Research, Kyoto University, Kyoto 606-8507, Japan, Retrovirology 2005, 2:27 doi:10.1186/1742-4690-2-27
© 2005 Matsuoka; licensee BioMed Central Ltd., This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

History of humans and Human T-cell Leukemia Virus Type 1

How does Human T-cell Leukemia Virus Type 1 replicate and increase its copy number?

Receptor and transmission of Human T-cell Leukemia Virus type 1

Human T-cell Leukemia Virus Type 1 can infect various types of cells, such as T-lymphocytes, B-lymphocytes, monocytes and fibroblasts [46]. Glucose transporter 1 (GLUT-1) has been identified as a receptor for Human T-cell Leukemia Virus Type 1 and this receptor is ubiquitously expressed on cell surfaces [47]. However, the Human T-cell Leukemia Virus Type 1 provirus is mainly detected in CD4-positive lymphocytes, with about 10% in CD8-positive T-lymphocytes [48]. This situation possibly arises because Tax mainly induces the increase of CD4-positive T-lymphocytes in vivo by enhanced proliferation and suppressed apoptosis.

In Human T-cell Leukemia Virus Type 1-infected individuals, no virions are detected in the serum. In addition, the infectivity of free virions is very poor compared with that of infected cells. These findings suggest that Human T-cell Leukemia Virus Type 1 is spread by cell-to-cell transmission, rather than by free virions. In vitro analyses of Human T-cell Leukemia Virus Type 1-infected cells revealed that HTLV-I-infected cells form "virological synapses" with uninfected cells. Contact between an infected cell and a target cell induces the accumulation of the viral proteins Gag and Env, viral RNA and microtubules, and the viral complex subsequently transfers into the target cell [49]. Human T-cell Leukemia Virus type 1 also spreads in a cell-to-cell manner via such virological synapses in vivo.

Human T-cell Leukemia Virus Type 1 is mainly transmitted via three routes: 1) mother-to-infant transmission (mainly through breast feeding) [50]; 2) sexual transmission (mainly from male-to-female); and 3) parenteral transmission (blood transfusion or intravenous drug use) [12]. In either route, HTLV-I-infected cells are essential for transmission. This was supported by the findings that fresh frozen plasma from carriers did not cause transmission [51] and freeze-thawing of breast milk reduced vertical transmission [52].

Provirus load and transmission

The provirus load varies more than 1000-fold among asymptomatic carriers [53]. Since most infected cells are considered to have one copy of the provirus, the provirus load indicates the percentage of infected cells among lymphocytes. The provirus load is relatively constant during the latent period [53]. Analysis of naive individuals who seroconvert after marrying an Human T-cell Leukemia Virus Type 1-seropositive spouse demonstrated that the proviral gp46 sequences are identical among married couples. This finding confirmed that HTLV-I is transmitted from a seropositive individual to an uninfected spouse. The provirus loads frequently differ between couples despite infection by the same HTLV-I virus, indicating that the number of infected cells is determined by host factors rather than virus itself [54].

Why does Human T-cell Leukemia Virus Type 1 increase the number of infected cells by the pleiotropic actions of Tax? The provirus load in peripheral blood mononuclear cells (PBMCs) is well correlated with that in breast milk, and a higher provirus load in breast milk increases the risk of vertical transmission of HTLV-I [55,56]. Similarly, a higher provirus load in PBMCs may be associated with a higher risk of sexual transmission. Thus, an increase in the number of infected cells by the actions of accessory genes, especially tax, facilitates transmission. Therefore, HTLV-I has strategies that increase the number of HTLV-I-infected cells via the action of accessory gene products, thereby increasing the chance of transmission.

Clonal expansion of Human T-cell Leukemia Virus Type 1-infected cells

After Human T-cell Leukemia Virus Type 1 infection, viral proteins such as Tax protein promote the proliferation of infected cells and also inhibit apoptosis by their pleiotropic actions. Since the HTLV-I provirus is randomly integrated into the host genome, the identification of integration sites enables to identify each infected clone, and to trace the kinetics of infected cells in vivo. Analyses using inverse PCR, which can identify the integration sites of the HTLV-I provirus, revealed that the proliferation of infected cells is oligoclonal, and that infected cells persistently survive in vivo [57-59]. Importantly, such clonal expansion in carriers is directly associated with the onset of Adult T-cell Leukemia [60]. Thus, the viral strategies to increase the number of HTLV-I-infected cells work efficiently in most carriers without any adverse effects. However, the increased number of infected cells causes an excess immune reaction, leading to inflammatory diseases, HAM/TSP, infective dermatitis [61] or HTLV-I-associated uveitis [62]. Moreover, such prolonged proliferation of infected CD4-positive T-lymphocytes results in the onset of ATL in some carriers after a long latent period.

Inactivation of Tax expression in Adult T-cell Leukemia (ATL) cells

As mentioned above, Tax expression confers advantages and disadvantages on Human T-cell Leukemia Virus Type 1-infected cells. Although the proliferation of infected cells is promoted by Tax expression, CTLs attack the Tax-expressing cells since Tax is their major target [63]. In HTLV-I-infected cells, Rex, p30 and HBZ suppress Tax expression. On the other hand, loss of Tax expression is frequently observed in leukemic cells. Three mechanisms have been identified for inactivation of Tax expression: 1) genetic changes of the tax gene (nonsense mutations, deletions or insertions) [64,65]; 2) DNA methylation of the 5'-LTR [65,66]; and 3) deletion of the 5'-LTR (Figure 2) [67]. Among fresh leukemic cells isolated from ATL patients, about 60% of cases do not express the tax gene transcript. Interestingly, Adult T-cell Leukemia cells with genetic changes of the tax gene expressed its transcripts, suggesting that ATL cells do not silence the transcription when the tax gene is abortive [65]. Loss of Tax expression gives ATL cells advantage for their survival since they can escape from CTLs.

Longer lifespan of Human T-cell Leukemia Virus type 1-infected cells and cancer

Lymphoid malignancy with a T-cell origin is rare compared with B-cell malignancy. Adult T-cell Leukemia shares hematological, pathological and immunological features with cutaneous T-cell lymphoma (CTCL; Sezary syndrome and Mycosis fungoides). The frequency of CTCL in Japan is estimated to be one/million/year. On the other hand, the frequency of ATL among carriers is estimated to be 1000/million/year. From these data, HTLV-I infection is estimated to increase the risk of T-cell malignancy by up to 1000-fold in carriers.

Human T-cell Leukemia Virus Type 1 infection confers a long lifespan on the infected cells due to the pleiotropic actions of Tax, resulting in increased numbers of infected cells. Such infected cells are essential for the transmission of HTLV-I. This strategy to increase the number of infected cells in vivo is thought to increase the incidence of cancer in T-cells. What is the mechanism for this oncogenesis? DNA methylation is known to be associated with aging. Some genes are hypermethylated in older people, indicating that DNA hypermethylation is a physiological phenomenon in some genes. Under normal circumstances, T-lymphocytes survive for several years, and long-lived T-lymphocytes with disordered methylation should be replaced. However, HTLV-I-infected T-cells are considered to survive and accumulate abnormal methylation. The process of oncogenesis is similar to that of evolution [68]. The infected cells that are suitable for survival should be selected in vivo, and epigenetic and genetic changes of the genome play critical roles in this selection. Accumulating alterations of the host genome transform the HTLV-I-infected cells into ATL cells, and also enable ATL cells to proliferate in the absence of Tax expression (Figure 2). In the provirus, DNA methylation of the 5'-LTR silences viral transcription in leukemic cells, which facilitates the escape of ATL cells from the host immune system [65].

5. Somatic alterations in Adult T-cell Leukemia cells

As described, some Adult T-cell Leukemia cells can proliferate without functional Tax protein, suggesting that somatic (genetic and epigenetic) alterations cause transcriptional or functional changes to the host genes. The p53 gene is frequently mutated in various cancers, and these mutations are associated with disease progression and a poor prognosis. The mutation rate of the p53 gene in ATL cells has been reported to be 36% (4/11) and 30% (3/10) [69-71]. The p16 gene is an inhibitor of cyclin-dependent kinase 4/6, and blocks the cell cycle. Genetic changes in this gene (deletion in most cases) have been described in many types of cancer cells. Deletion of the p16 gene has also been reported in ATL cells [72]. Moreover, DNA methylation of the promoter region of the p16 gene is also implicated in the suppression of p16 [73]. In addition, genetic changes in the p27^KIP1, RB1/p105 and RB2/p130 genes have been reported in ATL, although they are relatively rare: 2/42 (4.8%) for the p27^KIP1 gene; 2/40 (5%) for the RB1/p105 gene; and 1/41 (2.4%) for the RB2/p130 gene) [74]. The fact that higher frequencies of genetic changes in these tumor suppressor genes are observed among aggressive forms of ATL suggests that such genetic changes are implicated in disease progression.

Fas antigen was the first identified death receptor. It transduces the death signal by binding of its ligand, Fas ligand (FasL). Adult T-cell Leukemia cells highly express Fas antigen on their cell surface [75], and are highly susceptible to death signals mediated by agonistic antibodies to Fas antigen, such as CH-11. Genetic changes of Fas gene in ATL cells, which confer resistance to the Fas-mediated signal, have been reported [76,77]. Normal activated T-lymphocytes express FasL as well as Fas antigen. Apoptosis induced by autocrine mechanisms is designated activation-induced cell death (AICD) and this controls the immune response [78]. Although ATL cells express Fas antigen, they do not produce FasL, thereby enabling ATL cells to escape from AICD. Attempts to isolate hypermethylated genes from ATL cells identified the EGR3 gene as a hypermethylated gene compared to PBMCs from carriers [79]. EGR3 is a transcriptional factor with a zinc finger domain, that is essential for transcription of the FasL gene [80]. The finding that EGR3 gene transcription is silenced in ATL cells could account for the loss of FasL expression, and the escape of ATL cells from AICD. Thus, alterations of the Fas (genetic) and EGR3 (epigenetic) genes are examples of ATL cell evolution in vivo.

Disordered DNA methylation has been identified in the genome of Adult T-cell Leukemia cells compared with that of PBMCs from carriers: hypomethylation is associated with aberrant expression of the MEL1S gene [81], while hypermethylation silences transcription of the p16 [73], EGR3 and KLF4 genes as well as many others [79]. It is reasonable to consider that other currently unidentified genes are involved in such alterations of the genome in ATL cells, and play roles in leukemogenesis.

Transcriptome analyses using DNA microarrays have revealed transcriptional changes that are specific to Adult T-cell Leukemia cells. Among 192 up-regulated genes, the expressions of the tumor suppressor in lung cancer 1 (TSLC1), caveolin 1 and prostaglandin D2 synthase genes were increased more than 30-fold in fresh ATL cells compared with normal CD4+ and CD4+, CD45RO+ T-cells [82]. TSLC1 is a cell adhesion molecule that acts as a tumor suppressor in lung cancer. Although TSLC1 is not expressed on normal T-lymphocytes, all acute ATL cells show ectopic TSLC1 expression. Enforced expression of TSLC1 enhances both the self-aggregation and adhesion abilities to vascular endothelial cells in ATL cells. Thus, TSLC1 expression is implicated in the adhesion or infiltration of ATL cells. By screening a retrovirus cDNA library from ATL cells, a gene with oncogenic potency was identified in NIH3T3 cells, and designated the Tgat gene [83].

Immune control of Human T-cell Leukemia Virus Type 1 infection

Pathogenesis of Human T-cell Leukemia Virus Type 1 infection

Treatment of Adult T-cell Leukemia - the remaining mission and challenges

Two human retroviruses - Human T-cell Leukemia Virus Type 1 and HIV-1