Distinct mechanisms trigger apoptosis in human immunodeficiency virus type 1-infected and in uninfected bystander T lymphocytes

Human immunodeficiency virus type 1 Tat induces apoptosis and increases sensitivity to apoptotic signals by up-regulating FLICE/caspase-8

Apoptosis contributes to the loss of CD4 cells during human immunodeficiency virus type 1 (HIV-1) infection. Although the product of the env gene, gp160/gp120, is known to play a role in cell death mediated by HIV-1, the role of other HIV-1 genes in the process is unclear. We found that HIV-1 lacking the env gene (HIVDeltaenv) still induced apoptosis in T-cell lines and primary CD4 T cells. The ability to induce apoptosis was attributable to Tat, a viral regulatory protein. Tat induction of apoptosis was separate from the transactivation function of Tat, required expression of the second exon of Tat, and was associated with the increased expression and activity of caspase-8 (casp-8), a signaling molecule in apoptotic pathways. Moreover, induction of apoptosis could be prevented by treating cells with an inhibitor of casp-8. In addition, we show that HIV-1Deltaenv infection and Tat expression increased the sensitivity of cells to Fas-mediated apoptosis, an apoptotic pathway that signals via casp-8. The up-regulation of casp-8 by HIV-1 Tat expression may contribute to the increased apoptosis and sensitivity to apoptotic signals observed in the cells of HIV-1-infected persons.

Costimulatory pathways mediate monocyte-dependent lymphocyte apoptosis in HIV

Examination of annexin V binding, an indicator of early apoptosis, on lymphocytes from HIV+ people immediately after isolation showed that both CD4(+) and CD8(+) T cells were apoptotic, whereas B cell apoptosis was induced mainly after incubation. CD8(+) T cell apoptosis correlated with fewer CD4(+) T cells, but not the level of viremia. To determine potential mechanisms for apoptosis, we examined FasL expression, which was dramatically elevated on CD14(+) monocytes; however, antibody to FasL did not reproducibly inhibit apoptosis. Rather, CD8(+) T cell apoptosis was caused by antigen-presenting cells because removal of monocytes or addition of antibodies to CD80 and CD86 reduced apoptosis. B cell apoptosis also involved costimulatory signals delivered by T cells but not monocytes. A unique CD8(bright)CD28(dim) T cell population died after costimulation by monocytes. Because this population was increased in patients with undetectable viremia, abnormal antigen-presenting cells may contribute to continued CD8(+) T cell exhaustion by inducing apoptosis.

Does HIV cause depletion of CD4+ T cells in vivo by the induction of apoptosis?

The central pathogenic feature of AIDS is the dramatic loss of CD4+ lymphocytes. Despite more than a decade of intense research, the exact mechanism by which HIV causes this is still not understood. A major model for T cell depletion, proposed originally by Ameison and Capron in a report published in 1991, is that HIV sensitizes CD4+ T cells for activation-induced apoptosis. The apoptotic model of T cell depletion is discussed, and experiments that address the questions of whether apoptosis is restricted to infected cells or 'bystander' T cells, and whether T cell apoptosis requires participation of separate HIV-infected haematopoietic cell populations, are reviewed.

Mapping of HIV-1 determinants of apoptosis in infected T cells

HIV-1 infection leads to death of CD4(+) T cells in vivo and in vitro, although the mechanisms of this cell death are not well defined. We used flow cytometry to concurrently analyze infection and apoptosis of the CD4(+) CEM T cell line and human peripheral blood mononuclear cells (PBMC). Surprisingly, T cells productively infected with HIV-1 IIIB showed less apoptosis than control, uninfected T cells. This relative paucity of apoptosis was a characteristic of IIIB, since a large number of cells infected with the viral clone, HIV-1 NL4-3, were apoptotic. The nef, vpr, and vpu gene products were not responsible for apoptosis of NL4-3-infected cells, since NL4-3DeltaVprDeltaVpuDeltaNef and HXB-2 (a nef, vpr, and vpu triple mutant derived from IIIB) also killed infected cells. Moreover, only IIIB-infected cells showed a resistance to background levels of apoptosis. Thus, the apoptotic (and antiapoptotic) properties of HIV-1 do not map solely to mutations in nef, vpr, or vpu. We postulate that, in vivo, HIV variants that do not induce rapid apoptosis in the cells they infect may have a selective advantage.

Immunophenotypic Analysis of Peripheral Blood Mononuclear Cells Undergoing In Vitro Apoptosis After Isolation From Human Immunodeficiency Virus–Infected Children

Lymphocytes of human immunodeficiency virus (HIV)-infected individuals undergo accelerated apoptosis in vitro, but the subsets of cells affected have not been clearly defined. This study examined the relationship between lymphocyte phenotype and apoptotic cell death in HIV-infected children by flow cytometry. Direct examination of the phenotype of apoptotic lymphocytes was accomplished using a combination of surface antigen labeling performed simultaneously with the Tdt mediated Utp nick end-labeling (TUNEL) assay. In comparison to live cells, apoptotic lymphocytes displayed an overrepresentation of CD45RO and HLA-DR expressing cells, while CD28 and CD95 expressing cells were underrepresented. Lymphocytes expressing CD4, CD8, and CD38 were equally represented in apoptotic and live populations. When percent lymphocyte apoptosis follow- ing culture was examined independently with lymphocyte subsets in fresh blood, apoptosis was negatively correlated with the percentage of CD4 cells, but not with specific CD4 T-cell subsets. Although not correlated with the percentage of total CD8 cells, apoptosis was positively correlated with specific CD8 T-cell subsets expressing CD45RO and CD95 and negatively correlated for CD8 T cells expressing CD45RA. These results provide direct evidence that a population of activated lymphocytes with the memory phenotype lacking the costimulatory molecule CD28 are especially prone to undergo apoptosis. The findings related to CD95 expression in fresh and apoptotic cells implicate Fas-dependent and Fas-independent pathways of apoptosis in HIV disease in children.

Immunophenotypic Analysis of Peripheral Blood Mononuclear Cells Undergoing In Vitro Apoptosis After Isolation From Human Immunodeficiency Virus–Infected Children

Lymphocytes of human immunodeficiency virus (HIV)-infected individuals undergo accelerated apoptosis in vitro, but the subsets of cells affected have not been clearly defined. This study examined the relationship between lymphocyte phenotype and apoptotic cell death in HIV-infected children by flow cytometry. Direct examination of the phenotype of apoptotic lymphocytes was accomplished using a combination of surface antigen labeling performed simultaneously with the Tdt mediated Utp nick end-labeling (TUNEL) assay. In comparison to live cells, apoptotic lymphocytes displayed an overrepresentation of CD45RO and HLA-DR expressing cells, while CD28 and CD95 expressing cells were underrepresented. Lymphocytes expressing CD4, CD8, and CD38 were equally represented in apoptotic and live populations. When percent lymphocyte apoptosis follow- ing culture was examined independently with lymphocyte subsets in fresh blood, apoptosis was negatively correlated with the percentage of CD4 cells, but not with specific CD4 T-cell subsets. Although not correlated with the percentage of total CD8 cells, apoptosis was positively correlated with specific CD8 T-cell subsets expressing CD45RO and CD95 and negatively correlated for CD8 T cells expressing CD45RA. These results provide direct evidence that a population of activated lymphocytes with the memory phenotype lacking the costimulatory molecule CD28 are especially prone to undergo apoptosis. The findings related to CD95 expression in fresh and apoptotic cells implicate Fas-dependent and Fas-independent pathways of apoptosis in HIV disease in children.

Apoptosis of CD8+ T cells is mediated by macrophages through interaction of HIV gp120 with chemokine receptor CXCR4

CD8-positive T cells are thought to play an important role in the control of infection by human immunodeficiency virus (HIV) as a result of their cytotoxic activity and by releasing soluble factors. In AIDS patients, the absolute number of CD8+ T lymphocytes is decreased in peripheral blood and their turnover rate is increased, suggesting that there is more cell renewal and cell death occurring. Anti-retroviral therapy raises CD8+ T-cell counts in HIV-infected patients. Here we report that the death rate of CD8+ T cells by apoptosis increased markedly during HIV infection of peripheral blood mononuclear cells in vitro. Apoptosis is induced in a dose-dependent manner by recombinant envelope glycoprotein gp120 from HIV strain X4, or by stromal-derived factor-1 (SDF-1), the physiological ligand of the chemokine receptor CXCR4. Apoptosis is mediated by the interaction between tumour-necrosis factor-alpha bound to the membrane of macrophages (mbTNF) and a receptor on CD8+ T cells (TNF-receptor II, or TNFRII). The expression of both of these cell-surface proteins is upregulated by HIV infection or by treatment with recombinant gp120 or SDF-1. Apoptosis of CD8+ T cells isolated from HIV-infected patients is also mediated by macrophages through the interaction between mbTNF and TNFRII. These results indicate that the increased turnover of CD8+ T cells in HIV-infected subjects is mediated by the HIV envelope protein through the CXCR4 chemokine receptor.

NF-kappaB protects HIV-1-infected myeloid cells from apoptosis

HIV-1 infection of primary monocytic cells and myeloid cell lines results in sustained NF-kappaB activation. Recently, NF-kappaB induction has been shown to play a role in protecting cells from programmed cell death. In the present study, we sought to investigate whether constitutive NF-kappaB activity in chronically HIV-1-infected promonocytic U937 (U9-IIIB) and myeloblastic PLB-985 (PLB-IIIB) cells affects apoptotic signaling. TNFalpha and cycloheximide caused infected cells to undergo apoptosis more rapidly than parental U937 and PLB-985 cells. Inhibition of TNFalpha-induced NF-kappaB activation using the antioxidant N-acetylcysteine (NAC) resulted in increased apoptosis in both U937 and U9-IIIB cells, while preactivation of NF-kappaB with the non-apoptotic inducer IL-1beta caused a relative decrease in apoptosis. Inhibition of constitutive NF-kappaB activity in U9-IIIB and PLB-IIIB cells also induced apoptosis, suggesting that NF-kappaB protects cells from a persistent apoptotic signal. TNFalpha plus NAC treatment resulted in a marked decrease in Bcl-2 protein levels in HIV-1-infected cells, coupled with an increase in Bax protein compared to uninfected cells, suggesting that the difference in susceptibility to TNFalpha-induced apoptosis may relate to the differences in relative levels of Bcl-2 and Bax. The protective role of NF-kappaB in blocking TNFalpha- and HIV-1-induced apoptosis was supported by studies in Jurkat T cells engineered to express IkappaB alpha repressor mutants (TD-IkappaB) under the control of a tetracycline-responsive promoter. Cells underwent apoptosis in response to TNFalpha only when NF-kappaB activation was inhibited by TD-IkappaB expression. As was observed for the U9-IIIB cells, TNFalpha treatment also induced a marked decrease in Bcl-2 protein levels in TD-IkappaB expressing cells. These experiments demonstrate that apoptotic signaling is perturbed in HIV-1-infected U9-IIIB cells and indicate that NF-kappaB activation may play an additional protective role against HIV-1-induced apoptosis in myeloid cells.