In this study, we investigated the interactions between DCs and HTLV-1-infected T-cell lines. Entry of HTLV-1 during blood transfusion probably leads to the infection of T cells (CD4+
and, to a lesser extent, CD8+
T cells) according to the relative proportions of T cells and APCs in the peripheral blood and at mucosal surfaces. Alternatively, HTLV-1 entry at mucosal surfaces, during breastfeeding or by sexual transmission, might lead to infection of target APCs, such as DCs (4
), as DCs are directly infected by viruses of various families, including Adenoviridae
(e.g., herpes simplex virus, varicella-zoster virus, Epstein-Barr virus, and cytomegalovirus), Poxviridae
, and Retroviridae
). They might, therefore, be a key target for further spread of HTLV-1 within the host.
In the few papers devoted to the HTLV-1 infection of DCs, contradictory results have been reported. It was shown previously that DCs are susceptible to HTLV-1 infection in vitro (1
), but another report indicated that DCs are not susceptible to HTLV-1 infection (48
), as no evidence of virus uptake was observed after coculture with HTLV-1-infected cell lines. In the present study, we show that, after coculture of HTLV-1-infected cell lines with human DCs, viral particles can be detected within DC vacuoles as early as 3 h after contact and infection of DCs can be observed by immunofluorescence. DC infection is compatible with data obtained in vivo, which demonstrate infection of DCs in HTLV-1-infected asymptomatic carriers (12
) and in TSP/HAM patients (20
). HTLV-1 infection does not generally occur through cell-free virions, but rather by direct cell-cell transfer between lymphocytes (13
), with involvement of cytoskeletal polarization (5
). HTLV-1 disseminates from infected to uninfected cells by the formation of an intimate contact zone, termed the “virological synapse” (13
), also described for HIV (17
), a structure in which exchanges of various factors take place, like those between lymphocytes and APCs (45
). Accordingly, in our experiments, virus transfer was observed only upon cell-cell contact, whereas coculture with HTLV-1-infected cell lines seeded in the upper compartment of Transwell devices was inefficient in transmitting infection. Such cell-cell dissemination in vivo could explain the high proviral load found in infected patients despite good immune responses, as the virus might evade antibody-mediated host defenses.
In this study, virions were detected by electron microscopy within DC vacuoles. This pathway could provide an entry for exogenous presentation of HTLV-1 antigens in DCs. This process has been hypothesized to explain the activation of cytotoxic T lymphocytes in the absence of HIV replication (30
). Moreover, a fraction of HTLV-1 particles might escape degradation and reach the cytosol, leading to productive infection. DCs might provide a reservoir of Tax-producing APCs, resulting in stimulation of a large number of CD8+
T cells, as observed during TSP/HAM (15
). Infected progenitor cells in the bone marrow of TSP/HAM patients could provide a constant pool of infected DCs (14
). Moreover, Tax-induced maturation of DCs has been demonstrated during infection, implicating Tax-specific CTL in the genesis of TSP/HAM (33
In the present study, we demonstrated the involvement of DC-SIGN in efficient syncytium formation. Our results indicate that fusion of infected cells with target cells increases when DC-SIGN is expressed in the target cells. This process can be inhibited by preincubation with anti-DC-SIGN MAbs. The numbers and sizes of syncytia were clearly higher in target cells that expressed this lectin. This was demonstrated by using different cell types expressing DC-SIGN (HeLa and HEK cell lines and human monocyte-derived DCs). As previously shown by some of us, DC-SIGN expression enhances HIV binding and transfer to HeLa CD4+
). The cell-type dependence of DC-SIGN enhancement of HIV transfer was shown to be correlated with the ability of HIV to replicate at a low level in some cells and not in others (36
The data reported in our study are not due to binding of the HTLV-1 envelope glycoprotein to DC-SIGN, in contrast to several viruses, such as Marburg virus (31
), Ebola virus (2
), dengue virus (34
), HIV (8
), cytomegalovirus (11
), and hepatitis C virus (23
). Using target cell lines that either express or do not express the lectin in two different approaches, we did not observe direct binding of HTLV-1 Env glycoproteins to DC-SIGN. Thus, HTLV-1 has particular features that differentiate it from viruses known to bind to DC-SIGN.
DC-SIGN might act as a cofactor, helping to maintain a stable interaction between infected T cells and target cells. HTLV-1 Env gp46 glycoprotein has been reported to interact directly with its target cell receptor, Glut-1 (27
), and with heparan sulfate proteoglycans (38
). This interaction might explain the efficient binding of chimeric envelopes on the different cell lines and primary DCs used in our study. Under our experimental conditions, we cannot exclude the possibility that such interactions could disguise Env binding to DC-SIGN. However, in the case of HIV Env, binding to DC-SIGN can be detected on target cells in the absence, as well as in the presence, of CD4 and chemokine receptors (35
). Furthermore, our data, obtained with primary monocyte-derived DCs, reflect the natural cell environment.
As DC-SIGN is known to interact with ICAM-2 and ICAM-3 adhesion molecules (7
), it might contribute to bringing together DCs and infected cells, increasing the adhesion between them and facilitating their fusion. Nevertheless, syncytium formation takes place only in coculture with infected cells, demonstrating the absolute requirement for HTLV-1 Env glycoprotein for fusion. We observed a large decrease in the number of fusion events when cells were preincubated with anti-ICAM antibodies. As shown by Daenke and colleagues (6
), the fact that syncytium formation in mixed cultures could not be inhibited completely by blocking antibodies (directed to ICAM-1, ICAM-3, and VCAM-1) suggests that other molecules are involved in interactions between DCs and HTLV-1-infected cells.
Our study indicates that HTLV-1-infected cells can use surface adhesion molecules to regulate fusion with target cells, with the involvement of DC-SIGN and its ICAM ligands. This mechanism could have consequences for the regulation of both infection of DCs and dissemination of HTLV-1, but also for immune regulation. Further studies in vivo might allow us to confirm the role of such interactions, especially of the transmigration of DCs across endothelia, which express ICAMs (44
), and might have further implications in inflammatory disorders.