The HTLV receptor is an early T-cell activation marker whose expression requires de novo protein synthesis

Abstract

The human T-cell leukemia virus type 1 (HTLV) is the first isolated human retrovirus, but its receptor has yet to be identified, in part due to its ubiquitous expression. Here we report that quiescent CD4 and CD8 T lymphocytes do not express this receptor, as monitored with a soluble receptor-binding domain derived from the HTLV envelope. However, HTLV receptor is an early activation marker in neonatal and adult T lymphocytes, detected as early as 4 hours following T-cell-receptor (TCR) stimulation. This induced surface expression of the HTLV receptor requires de novo protein synthesis and results in a wide distribution on the surface of activated lymphocytes. Moreover, the distribution of the HTLV receptor is independent of TCR/CD3-capped membrane structures, as observed by confocal immunofluorescence microscopy. To determine whether HTLV receptor up-regulation specifically requires TCR-mediated signals or, alternatively, is dependent on more generalized cell cycle entry/proliferation signals, its expression was monitored in interleukin 7 (IL-7)-stimulated neonatal and adult T cells.Neonatal, but not adult, lymphocytes proliferate in response to IL-7 and HTLV receptor expression is restricted to the former population. Thus, HTLV receptor expression appears to be an early marker of cell cycle entry. Up IntroductionThe human T-cell leukemia virus type 1 (HTLV-1), the first characterized human retrovirus, 1 is present in all areas of the world as either an endemic or a sporadic infectious agent. 2 In endemic areas, HTLV-1 transmission seems to occur mostly from mother to infant through breast-feeding. 3 The exceptionally broad tropism of HTLV-1 in vitro 4,5 contrasts with the finding that in vivo, HTLV-1 is found primarily in CD4 ϩ lymphocytes and less frequently in other mononuclear blood cells. 6,7 Studies of this apparent discrepancy have been hindered by the high cytotoxicity of HTLV envelopes and their dependence on cell-to-cell contact for infection and spreading. 4,8,9 Moreover, investigations have been limited because the HTLV envelope receptor remains unidentified, even though multiple cell surface components including adhesion molecules, 10,11 matrix-associated proteins, 12 lipids, 13 and lipid rafts, 14 have been implicated in HTLV envelope (Env)-mediated membrane fusion and virus transmission.The HTLV-1 Env receptor-binding determinants are entirely contained within the extracellular surface component (SU). 4,15 Recently, we demonstrated that the amino-terminal 215 amino acids of the SU harbors the receptor-binding domain (RBD) of HTLV Env. 16 The binding specificity of a soluble tagged construct encompassing this region (H RBD ) has been demonstrated by its efficient competition with HTLV Env-mediated cell fusion and infection (F.J.K., E. N. Garrido, N.M., M.S., J.-L.B., manuscript in preparation).Using the RBD of HTLV Env, we have now tracked HTLV receptor expression on T lymphocytes, which have been reported to be a major HTLV-1 reservoir in vivo. Circulating T lymphocytes are almost entirely in th...

Cell Surface Expression of the Bovine Leukemia Virus-Binding Receptor on B and T Lymphocytes Is Induced by Receptor Engagement

“…BLV-binding receptor expression was induced on both B and T lymphocytes under conditions of receptor as well as cytokine stimulation. This induction was independent of DNA synthesis because increased levels of BLV-binding receptor were detected as early as 4 h after receptor engagement while S phase entry requires at least 24 h of stimulation (28). Notably, BLVbinding receptor levels returned to undetectable levels at late time points following lymphocyte stimulation, consistent with their return to a quiescent state.…”

“…The characteristics of BLV expression on lymphocyte subsets are evocative of that previously reported for the HTLV receptor (28,44). Indeed, even before this receptor was identified as the Glut1 glucose transporter, it had been determined that Glut1 expression is closely linked to the metabolic state of the lymphocyte (50 -53).…”

“…Surface up-regulation of the BLV Env-binding receptor was extremely rapid, within 4 h of TCR stimulation (Fig. 5A), similar to that shown for the HTLV Env receptor (28). The kinetics of cell surface BLV-binding receptor induction were slower than that of the CD69 activation marker, which was already induced by 2 h after TCR engagement and more rapid than that of the CD25 IL-2R␣ subunit, which was only detected following 6 -8 h of receptor stimulation (Fig.…”

“…The homology and similar organization of the HTLV and BLV envelopes and their similar cell-type and species distribution made it important to test whether Glut1, the recently identified HTLV receptor, is recognized by the BLV envelope. As expected, transfection of 293T cells with a Glut1 expression vector resulted in a significantly increased level of binding to its HTLV Env SU ligand (H RBD rFc) (28,31). Indeed, the MFI of H RBD rFc binding was ϳ10-fold higher in the Glut1-transfected population as compared with the untransfected cells (Fig.…”

“…This region of the SU, immediately upstream of the so-called proline-rich region (PRR), confers HTLV tropism to heterologous envelopes. Using a soluble-tagged construct encompassing this region (H RBD ), we were able to track HTLV receptor expression in lymphocyte subsets (28) and identified Glut1, the ubiquitous vertebrate glucose as a receptor for HTLV-1 and -2 Env (19, 29 -31). It is of particular interest that Glut1, a deltaretrovirus receptor, like all other identified gammaretrovirus receptors, is a multimembrane-spanning nutrient transporter (19,24,32).…”