Human T cell lymphotropic virus-1 (HTLV-1) primarily infects CD4+ T cells, causing inflammatory disorders or a T cell malignancy in 5% to 10% of carriers. The cytotoxic T lymphocyte (CTL) response is a key factor that controls the viral load and thus the risk of disease. The ability to detect the viral protein Tax in primary cells has made it possible to estimate the rate at which Tax-expressing infected cells are eliminated by CTLs in persistently infected people. However, most HTLV-1-infected cells are Tax–at a given time, and their immunophenotype is poorly defined. Here, we aimed to identify a cell-surface molecule expressed by both Tax+ and Tax–HTLV-1-infected cells and use it to analyse the CTL response in fresh peripheral blood mononuclear cells. Cell adhesion molecule 1 (CADM1/TSLC1) was the best single marker of HTLV-1 infection, identifying HTLV-1-infected cells with greater sensitivity and specificity than CD25, CCR4 or ICAM-1. CADM1+CD4+ T cells carried a median of 65% of proviral copies in peripheral blood. In a cohort of 23 individuals, we quantified the rate of CTL-mediated killing of Tax+ and Tax−CADM1+ cells. We show that CADM1 expression is associated with enhanced susceptibility of infected cells to CTL lysis: despite the immunodominance of Tax in the CTL response, Tax+CADM1– cells were inefficiently lysed by CTLs. Upregulation of the CADM1 ligand CRTAM on CD8+ T cells correlated with efficient lysis of infected cells. Tax–CADM1+ cells were lysed at a very low rate by autologous CTLs, however, were efficiently killed when loaded with exogenous peptide antigen. High expression of CADM1 on most HTLV-1-infected cells in the face of enhanced CTL counterselection implies that CADM1 confers a strong benefit on the virus.

A role for complement, particularly the classical pathway, in the regulation of immune responses is well documented. Deficiencies in C1q or C4 predispose to autoimmunity, while deficiency in C3 affects the suppression of contact sensitization and generation of oral tolerance. Complement components including C3 have been shown to be required for both B-cell and T-cell priming. The mechanisms whereby complement can mediate these diverse regulatory effects are poorly understood. Our previous work, using the mouse minor histocompatibility (HY) model of skin graft rejection, showed that both C1q and C3 were required for the induction of tolerance following intranasal peptide administration. By comparing tolerance induction in wild-type C57BL/6 and C1q-, C3-, C4- and C5-deficient C57BL/6 female mice, we show here that the classical pathway components including C3 are required for tolerance induction, whereas C5 plays no role. C3-deficient mice failed to generate a functional regulatory T (Treg) –dendritic cell (DC) tolerogenic loop required for tolerance induction. This was related to the inability of C3-deficient DC to up-regulate the arginine-consuming enzyme, inducible nitric oxide synthase (Nos-2), in the presence of antigen-specific Treg cells and peptide, leading to reduced Treg cell generation. Our findings demonstrate that the classical pathway and C3 play a critical role in the peptide-mediated induction of tolerance to HY by modulating DC function.

No abstract

HTLV-1 Tax protein is usually undetectable in freshly isolated peripheral blood mononuclear cells (PBMCs). After in vitro culture, the provirus is reactivated and Tax protein is detectable only in a proportion of infected CD4+ T cells: the percentage of Tax+ cells is always lower than the proviral load (PVL). To identify and further analyse the latently infected cells we have measured the expression of tumour suppressor of lung cancer 1 (TSLC1). TSLC1 is a member of the immunoglobulin super family that is expressed on all cells with the exception of PBMCs. It mediates cell-to-cell adhesion by either homophilic or heterophilic interactions with other members of the immunoglobulin family and also signals to the actin cytoskeleton. TSLC1 has been shown to be expressed on primary adult T cell leukaemia (ATL) cells and ATL cell lines. We assayed the expression of TSLC1 in the PBMCs of 13 asymptomatic carriers (AC) and 13 HAM/TSP patients by flow cytometry. We found that the percentage of TSLC1+ CD4+ T cells was positively correlated with the PVL (P < 0.0001). To test whether TSLC1+ cells themselves were infected, we flow sorted TSLC1+ CD4+ T cells of 3 ACs and 3 HAM/TSP patients. A median of 95% of TSLC1+ CD4+ T cells carried the provirus. Whilst only 20% of infected CD4+ T cells expressed Tax, a further 47% were identified on the basis of TSLC1 expression. As the host cytotoxic T cell (CTL) response is an important protective factor, we are now testing the hypothesis that the presence of TSLC1 on the surface of infected cells affects its recognition and subsequent killing by CTLs.