In vitro characterization of purified human thymic dendritic cells infected with human immunodeficiency virus type 1

HIV-1 induces DCIR expression in CD4+ T cells

The C-type lectin receptor DCIR, which has been shown very recently to act as an attachment factor for HIV-1 in dendritic cells, is expressed predominantly on antigen-presenting cells. However, this concept was recently challenged by the discovery that DCIR can also be detected in CD4⁺ T cells found in the synovial tissue from rheumatoid arthritis (RA) patients. Given that RA and HIV-1 infections share common features such as a chronic inflammatory condition and polyclonal immune hyperactivation status, we hypothesized that HIV-1 could promote DCIR expression in CD4⁺ T cells. We report here that HIV-1 drives DCIR expression in human primary CD4⁺ T cells isolated from patients (from both aviremic/treated and viremic/treatment naive persons) and cells acutely infected in vitro (seen in both virus-infected and uninfected cells). Soluble factors produced by virus-infected cells are responsible for the noticed DCIR up-regulation on uninfected cells. Infection studies with Vpr- or Nef-deleted viruses revealed that these two viral genes are not contributing to the mechanism of DCIR induction that is seen following acute infection of CD4⁺ T cells with HIV-1. Moreover, we report that DCIR is linked to caspase-dependent (induced by a mitochondria-mediated generation of free radicals) and -independent intrinsic apoptotic pathways (involving the death effector AIF). Finally, we demonstrate that the higher surface expression of DCIR in CD4⁺ T cells is accompanied by an enhancement of virus attachment/entry, replication and transfer. This study shows for the first time that HIV-1 induces DCIR membrane expression in CD4⁺ T cells, a process that might promote virus dissemination throughout the infected organism.

The type II transmembrane protein DCIR belongs to the C-type lectin domain family receptor and is predominantly expressed in cells of the myeloid lineage. However recent evidence suggests that it can also be induced in CD4⁺ T cells placed under an inflammatory condition. We assessed the capacity of HIV-1 to promote DCIR expression in CD4⁺ T cells because the establishment of an inflammatory state is a hallmark of this retroviral infection in humans. We report here that a higher DCIR expression is detected not only in CD4⁺ T cells acutely infected with HIV-1 in vitro but also in clinical cell samples. Additional studies suggest a possible link between DCIR induction and apoptosis through both caspase-dependent and -independent intrinsic pathways. The greater expression of DCIR on the surface of CD4⁺ T cells results in more efficient virus attachment/entry, replication and transfer processes.

Dendritic cells as killers: mechanistic aspects and potential roles

Dendritic cells (DC) are professional APC endowed with the unique capacity to activate naive T cells. DC also have important effector functions during the innate immune response, such as pathogen recognition and cytokine production. In fact, DC represent the crucial link between innate and adaptive immune responses. However, DC are quite heterogeneous and various subsets endowed with specific pathogen recognition mechanisms, locations, phenotypes, and functions have been described both in rodents and in humans. A series of studies indicated that rodent as well as human DC could also mediate another important innate function, i.e., cell-mediated cytotoxicity, mostly toward tumor cells. In this article, we will review the phenotypes of these so-called killer DC, their killing mechanism, and putative implication in the immune response.

Increased thymic output in HIV-negative patients after antiretroviral therapy

OBJECTIVE

To determine the effects of antiretroviral therapy on thymic output independent of HIV infection.

METHODS

Thymic output was evaluated by quantifying signal joint T-cell receptor (TCR) recombination excision circles in peripheral blood lymphocytes from HIV-negative patients undergoing prophylactic antiretroviral therapy. Additionally, effects of the HIV protease inhibitor nelfinavir were assessed in vivo on TCR-induced death of murine double-positive thymocytes.

RESULTS

Five out of seven HIV-negative patients undergoing prophylactic antiretroviral therapy exhibited a dramatic increase (1-3 log10) in recent thymic emigrants containing signal joint TCR recombination excision circles while their peripheral T cell compartments remained relatively unaffected. None of the patients developed subsequent HIV infections. Interestingly, nelfinavir did not have significant effects on TCR-induced apoptosis of murine thymocytes in vivo.

CONCLUSION

Antiretroviral therapy augments thymic output independent of HIV. Furthermore, nelfinavir does not dramatically affect TCR-induced thymocyte death in mice, thus central tolerance remains intact.

Impaired accessory cell function in a human dendritic cell line after human immunodeficiency virus infection

We generated human dendritic cell (DC) hybridoma cell lines by fusing HGPRT-deficient promonocytic U937 cells with immature DCs obtained by culturing peripheral blood monocytes with interleukin-4 (IL-4; 1,000 U/ml) and granulocyte-macrophage colony-stimulating factor (100 U/ml) for 7 days and mature DCs by treatment with tumor necrosis factor alpha (12.5 microg/ml) for 3 days. Only one fusion with immature DCs was successful and yielded four cell lines--HB-1, HB-2, HB-3, and HB-9--with an overall fusion efficiency of 0.0015%. The cell lines were stable in long-term culture, displayed morphological features typical of DCs, and expressed distinct class I and class II molecules not present on U937 (A*031012, B*51011, Cw*0701, DRB3*01011 52, and DR5*01011). A representative cell line, HB-2, that expressed DC markers including CD83, CD80 and CD86 could be induced to produce IL-12 through CD40 stimulation. After human immunodeficiency virus (HIV) infection, there was impairment of antigen-presenting cell (APC) function, which was manifested by an inability to stimulate allogeneic T-cell responses. There was no change in expression of major histocompatibility complex class I and class II antigens, CD83, CD40, CD4, CD11c, CD80, CD86, CD54, and CD58, or IL-12 production in the HIV-infected HB-2 cells. The HIV-infected HB-2 cells induced T-cell apoptosis in the cocultures. T-cell proliferation could be partially restored by using ddI, indinivir, and blocking anti-gp120 and anti-IL-10 antibodies. Our data suggest that there are multiple mechanisms that DCs use to inhibit T-cell responses in HIV-infected patients. The HB-2 cell line could be a useful model system to study APC function in HIV-infected DCs.

Could Nef and Vpr proteins contribute to disease progression by promoting depletion of bystander cells and prolonged survival of HIV-infected cells?

A growing body of literature suggests that the HIV accessory proteins Nef and Vpr could be involved in depletion of CD4(+) and non-CD4(+) cells and tissue atrophy, and in delaying the death of HIV-infected cells. Cell depletion is likely to be predominantly a bystander effect because the number of cells dying far outnumbers HIV-infected cells and is not confined to CD4(+) cells. The myristylated N-terminal region of Nef has severe membrane disordering properties, and when present in the extracellular medium causes rapid lysis in vitro of a wide range of CD4(+) and non-CD4(+) cells, suggesting a role for extracellular Nef in the depletion of bystander cells. A direct role for HIV-1 Nef in cytopathicity is supported by studies in HIV-infected Hu Liv/Thy SCID mice, in transgenic mice expressing nef gene alone, and in rhesus macaques infected with SIV/HIV chimeric virus containing HIV-1 nef. The N-terminal region of Nef has been directly implicated in development of simian AIDS. Extracellular Vpr and C-terminal fragments of Vpr cause membrane permeabilization and apoptosis of a wide range of CD4(+) and non-CD4(+) cells, and could also contribute to depletion of bystander cells. A direct in vivo role for Vpr in thymocyte depletion, thymic atrophy, and nephropathy is suggested in studies with vpr transgenic mice. Intracellular Nef and Vpr could help HIV-infected cells evade cell death by inhibiting apoptosis of infected cells and by avoiding virus-specific CTL response. Nef and Vpr are potential targets for therapeutic intervention and vaccine development, and strategies that prevent the death of bystander cells while promoting the early death of HIV-infected cells could arrest or retard progression to AIDS.

Thymocyte-thymic epithelial cell interaction leads to high-level replication of human immunodeficiency virus exclusively in mature CD4(+) CD8(-) CD3(+) thymocytes: a critical role for tumor necrosis factor and interleukin-7

This work aims at identifying the thymocyte subpopulation able to support human immunodeficiency virus (HIV) replication under the biological stimuli of the thymic microenvironment. In this report we demonstrate that interaction with thymic epithelial cells (TEC) induces a high-level replication of the T-tropic primary isolate HIV-1(B-LAIp) exclusively in the mature CD4(+) CD8(-) CD3(+) thymocytes. Tumor necrosis factor (TNF) and interleukin-7 (IL-7), secreted during this interaction, are critical cytokines for HIV long terminal repeat transactivation through NF-kappaB-dependent activation. TNF is the major inducer of NF-kappaB and particularly of the p50-p65 complex, whereas IL-7 acts as a cofactor by sustaining the expression of the p75 TNF receptor. The requirement for TNF is further confirmed by the observation that the inability of the intermediate CD4(+) CD8(-) CD3(-) thymocytes to replicate the virus is associated with a defect in TNF production during their interaction with TEC and correlates with the absence of nuclear NF-kappaB activity in these freshly isolated thymocytes. Addition of exogenous TNF to the intermediate thymocyte cultures induces NF-kappaB activity and is sufficient to promote HIV replication in the cocultures with TEC. The other major subpopulation expressing the CD4 receptor, namely, the double-positive (DP) CD4(+) CD8(+) CD3(+/-) thymocytes, despite the entry of the virus, do not produce a significant level of virus, presumably because they are unresponsive to TNF and IL-7. Together, these data suggest that in vivo, despite an efficient entry of the virus in all the CD4(+) subpopulations, a high viral load may be generated exclusively within the mature CD4(+) CD8(-) CD3(+) subset of thymocytes. However, under conditions of inflammatory response after infection, TNF might also be present in the intermediate thymocyte compartment, leading to efficient HIV replication in these cells.

Apoptosis in dendritic cell biology

Apoptosis or programmed cell death regulates many aspects in immunological homeostasis and, thus, controls the initiation, magnitude, duration, and termination of immune responses. Recent studies on dendritic cells (DC), including Langerhans cells (LC), have reinforced this concept by documenting that these antigen presenting cells express surface receptors and ligands that are known to mediate apoptotic cell death and that they are highly susceptible to apoptotic signals. In this review article, four major topics concerning apoptosis in the biology of DC will be overviewed: (a) molecular mechanisms of apoptosis; (b) DC apoptosis induced by various stimuli; (c) regulation of DC apoptosis; and (d) cross-priming and cross-tolerance induced by DC ingesting apoptotic bodies.

Human immunodeficiency virus type 1 Vpr is a positive regulator of viral transcription and infectivity in primary human macrophages

It is currently well established that HIV-1 Vpr augments viral replication in primary human macrophages. In its virion-associated form, Vpr has been suggested to aid efficient translocation of the proviral DNA into the cell nucleus. Although Vpr growth-arrests dividing T cells, the relevance of this biological activity in nondividing macrophages is unclear. Here we use Vpr-mutants to demonstrate that the molecular determinants involved in G2-arresting T cells are also involved in increasing viral transcription in macrophages, even though these cells are refractive to the diploid DNA status typical of G2 phase. Our results suggest that the two phenotypes, namely the nuclear localization and the G2-arrest activity of the protein, segregate functionally among the late and early functions of Vpr. The nuclear localization property of Vpr correlates with its ability to effectively target the proviral DNA to the cell nucleus early in the infection, whereas the G2-arrest phenotype correlates with its ability to activate viral transcription after establishment of an infection. These two functions may render Vpr's role essential and not accessory under infection conditions that closely mimic the in vivo situation, that is, primary cells being infected at low viral inputs.

Characterization of the cytotoxic factor(s) released from thymic dendritic cells upon human immunodeficiency virus type 1 infection

We previously demonstrated that infection of primary human thymic dendritic cells (DCs) with laboratory strains of HIV leads to the release of soluble factor(s) which induced thymocyte killing. In the present paper, we extend the characterization of this process. Our results reveal that primary HIV-1 isolates are similarly able to induce the production of cytotoxic factor(s) from thymic DCs and that the release of such factor(s) is dependent on viral infection. Interestingly, we observed that CD4+ and CD8+ purified thymocyte subsets, and activated PBMCs are susceptible to the cytotoxic activity, whereas freshly isolated resting PBMCs are resistant to this effect. Cycloheximide treatment prevents the killing of thymocytes exposed to HIV-infected DC supernatant, revealing that this form of cell death is an active biological process requiring protein synthesis. Finally, our data suggest that FasL and TNF alpha could both participate in the killing process. These in vitro observations provide a plausible model, whereby HIV-infected DCs can play a role in vivo in the induction of uninfected thymocyte killing.