Inability of Plasmacytoid Dendritic Cells To Directly Lyse HIV-Infected Autologous CD4+ T Cells despite Induction of Tumor Necrosis Factor-Related Apoptosis-Inducing Ligand

The function of plasmacytoid dendritic cells (PDC) in chronic human immunodeficiency virus type 1 (HIV-1) infection remains controversial with regard to its potential for sustained alpha interferon (IFN-α) production and induction of PDC-dependent tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL)-mediated cytotoxicity of HIV-infected cells. We address these areas by a study of chronically HIV-1-infected subjects followed through antiretroviral therapy (ART) interruption and by testing PDC cytolytic function against autologous HIV-infected CD4⁺ T cells. Rebound in viremia induced by therapy interruption showed a positive association between TRAIL and viral load or T-cell activation, but comparable levels of plasma IFN-α/β were found in viremic ART-treated and control subjects. While PDC from HIV-infected subjects expressed less interferon regulator factor 7 (IRF-7) and produced significantly less IFN-α upon Toll-like receptor 7/9 (TLR7/9) engagement than controls, membrane TRAIL expression in PDC from HIV⁺ subjects was increased. Moreover, no significant increase in death receptor 5 (DR5) expression was seen in CD4⁺ T cells from viremic HIV⁺ subjects compared to controls or following in vitro infection/exposure to infectious and noninfectious virus or exogenous IFN-α, respectively. Although activated PDC killed the DR5-expressing HIV-infected Sup-T1 cell line, PDC did not lyse primary autologous HIV⁺ CD4⁺ T cells yet could provide accessory help for NK cells in killing HIV-infected autologous CD4⁺ T cells. Taken together, our data show a lack of sustained high levels of soluble IFN-α in chronic HIV-1 infection in vivo and document a lack of direct PDC cytolytic activity against autologous infected or uninfected CD4⁺ T cells.Human immunodeficiency virus (HIV) infection is associated with chronic immune activation, progressive immune suppression, and deletion of memory adaptive responses, resulting in increased susceptibility to opportunistic infections (23, 51, 52). Loss of CD4⁺ T cells is the hallmark of HIV infection, with multiple mechanisms proposed as contributing to this loss (activation-induced cell death, direct cytopathic effect, immune cells, and death receptor-mediated apoptosis induction) (reviewed in references 33 and 34). One of the most puzzling observations in AIDS pathogenesis has been the progressive depletion of bystander T cells, especially in lymphoid tissues (25, 33, 34, 55). While antiretroviral therapy (ART) initiated in the early stages of HIV infection, when CD4⁺ T-cell counts are high (>500 cells/μl), may prevent the destruction of lymph node (LN) tissue and the massive depletion of CD4⁺ T lymphocytes by decreasing the rate of virally induced apoptosis (20), a persistent, albeit decreased, level of apoptosis of peripheral blood CD4⁺ and CD8⁺ T cells is seen in ART-treated HIV⁺ subjects despite long-term viral suppression (36).A member of the tumor necrosis factor (TNF) family, TNF-related apoptosis-inducing ligand (TRAIL), has been shown to be involved in HIV-1-associated T-cell apoptosis (33, 34). TRAIL (soluble or membrane bound) induces apoptosis upon binding to death receptor 4 (DR4; also named TRAIL-R1) or DR5 (also named TRAIL-R2, TRICK2, or Killer/DR5).On the basis of the in vitro observation that alpha interferon (IFN-α) and interferon regulator factor 7 (IRF-7) are increased in plasmacytoid dendritic cells (PDC) exposed to HIV-1 (40), the hypothesis that PDC activation by HIV-1 is responsible for an increased level of IFN-α throughout chronic disease has been proposed. It has also been proposed that the activation of the PDC compartment by HIV-1 participates in the initial immune activation following acute infection and contributes to CD4⁺ T-cell depletion by inducing, through IFN-α, the production of TRAIL, which mediates apoptosis of DR5-expressing CD4⁺ T cells following HIV-1 infection (37, 38, 40). However, several lines of evidence question the direct involvement of PDC in the loss of T cells during HIV infection, as PDC numbers are depleted during chronic HIV infection and PDC remaining in circulation are functionally impaired (10). Recent data show that circulating PDC in HIV-infected subjects, although unable to secrete IFN-α after Toll-like receptor (TLR)-mediated activation, constitutively express an increased level of IFN-α mRNA, indicating that during HIV infection PDC are activated yet impaired (71). Rodriguez et al. demonstrated the prevention of spontaneous apoptosis of CD4⁺ and CD8⁺ T cells by IFN-α (63), a major product of PDC following HIV-1 stimulation (3, 28). In addition, Audige et al. (2) showed that blockade of IFN-α and IFN-α receptor during in vitro HIV infection of CD4⁺ T cells isolated from human tonsils did not prevent apoptosis or TRAIL production, suggesting a lack of a central link between IFN-α production and apoptosis of tonsillar CD4⁺ T cells in HIV-1 infection. These data are also consistent with the observation that, in the human peripheral blood lymphocyte-transplanted SCID mouse (hu-PBL-SCID) model, IFN-α efficiently increases the survival of CD4⁺ T cells (49). Thus, controversy remains on the role of IFN-α as an indirect or direct inducer of apoptosis of CD4⁺ T cells through PDC/TRAIL induction, whereas the possibility that IFN-α acts as an antiviral agent by controlling HIV-1 replication and thus reducing virally mediated T-cell loss appears to be supported by several studies (reviewed in references 26, 47, and 58). In this regard, endogenous IFN-α produced by PDC has been shown to play an important role in controlling HIV infection in the human thymus (35), upregulating host antiviral factors such as APOBEC (1, 32, 44, 70) and stimulating NK cell-mediated cytotoxic activity against autologous HIV-infected targets (72).In this report, we investigated the in vivo correlates of viremia in chronically infected subjects by studying the relationship between therapy interruption-associated viremia and plasma IFN-α and TRAIL levels. We also tested in vitro the functional outcome of HIV-1-activated PDC in terms of their ability to mediate lysis of primary autologous CD4 T cells (infected or not with HIV-1), compared to indirect PDC-mediated lysis effects on the NK-dependent antiviral cytotoxic response.