Programmed cell death and AIDS: significance of T-cell apoptosis in pathogenic and nonpathogenic primate lentiviral infections

Fas/FasL-independent activation-induced cell death of T lymphocytes from HIV-infected individuals occurs without DNA fragmentation

We assessed the effects of activation with phorbol myrystic acetate (PMA) and ionomycin on peripheral blood mononuclear cells (PBMC) from HIV-infected individuals by (51)Cr release, propidium iodide (PI) uptake, electron microscopy, and DNA analysis. Up to 70% (51)Cr release was induced from PBMC of HIV-infected individuals, versus up to 26% (51)Cr release from PBMC of non-HIV-infected volunteers. Flow cytometry identified mostly T cells undergoing activation-induced cell death (AICD). The kinetics of (51)Cr release and the effects of cold target inhibitors were consistent with cell-mediated cytotoxicity. Certain anti-CD3 antibodies or extracellular Ca(2+) chelation prevented AICD, but antagonistic anti-Fas antibodies, caspase inhibitors, and cycloheximide had no effect. The antioxidants thiourea and N-acetylcysteine reduced AICD, indicating a role for oxidative stress. Electron microscopy revealed plasma membrane disruption with nuclear integrity, while DNA analysis showed intact chromosomal DNA. This form of T cell AICD triggered by PMA and ionomycin differs from classical apoptosis in the absence of either caspase involvement or DNA fragmentation.

Exogenous interleukin-2 administration corrects the cell cycle perturbation of lymphocytes from human immunodeficiency virus-infected individuals

Human immunodeficiency virus (HIV)-induced immunodeficiency is characterized by progressive loss of CD4(+) T cells associated with functional abnormalities of the surviving lymphocytes. Increased susceptibility to apoptosis and loss of proper cell cycle control can be observed in lymphocytes from HIV-infected individuals and may contribute to the lymphocyte dysfunction of AIDS patients. To better understand the relation between T-cell activation, apoptosis, and cell cycle perturbation, we studied the effect of exogenous interleukin-2 (IL-2) administration on the intracellular turnover of phase-dependent proteins. Circulating T cells from HIV-infected patients display a marked discrepancy between a metabolic profile typical of G(0) and a pattern of expression of phase-dependent proteins that indicates a more-advanced position within the cell cycle. This discrepancy is enhanced by in vitro activation with ConA and ultimately results in a marked increase of apoptotic events. Conversely, treatment of lymphocytes with IL-2 alone restores the phase-specific pattern of expression of cell cycle-dependent proteins and is associated with low levels of apoptosis. Interestingly, exogenous IL-2 administration normalizes the overall intracellular protein turnover, as measured by protein synthesis, half-life of newly synthesised proteins, and total protein ubiquitination, thus providing a possible explanation for the effect of IL-2 on the intracellular kinetics of cell cycle-dependent proteins. The beneficial effect of IL-2 administration is consistent with the possibility of defective IL-2 function in vivo, which is confirmed by the observation that lymphocytes from HIV-infected patients show abnormal endogenous IL-2 paracrine/autocrine function upon in vitro mitogen stimulation. Overall these results confirm that perturbation of cell cycle control contributes to HIV-related lymphocyte dysfunction and, by showing that IL-2 administration can revert this perturbation, suggest a new mechanism of action of IL-2 therapy in HIV-infected patients.

The human immunodeficiency virus type 1 Vpu protein inhibits NF-kappa B activation by interfering with beta TrCP-mediated degradation of Ikappa B

The human immunodeficiency virus type 1 (HIV-1) Vpu protein binds to the CD4 receptor and induces its degradation by cytosolic proteasomes. This process involves the recruitment of human betaTrCP (TrCP), a key member of the SkpI-Cdc53-F-box E3 ubiquitin ligase complex that specifically interacts with phosphorylated Vpu molecules. Interestingly, Vpu itself, unlike other TrCP-interacting proteins, is not targeted for degradation by proteasomes. We now report that, by virtue of its affinity for TrCP and resistance to degradation, Vpu, but not a phosphorylation mutant unable to interact with TrCP, has a dominant negative effect on TrCP function. As a consequence, expression of Vpu in HIV-infected T cells or in HeLa cells inhibited TNF-alpha-induced degradation of IkappaB-alpha. Vpu did not inhibit TNF-alpha-mediated activation of the IkappaB kinase but instead interfered with the subsequent TrCP-dependent degradation of phosphorylated IkappaB-alpha. This resulted in a pronounced reduction of NF-kappaB activity. We also observed that in cells producing Vpu-defective virus, NF-kappaB activity was significantly increased even in the absence of cytokine stimulation. However, in the presence of Vpu, this HIV-mediated NF-kappaB activation was markedly reduced. These results suggest that Vpu modulates both virus- and cytokine-induced activation of NF-kappaB in HIV-1-infected cells.

Simian immunodeficiency virus replicates to high levels in naturally infected African green monkeys without inducing immunologic or neurologic disease

African green monkeys can maintain long-term persistent infection with simian immunodeficiency viruses (SIVagm) without developing AIDS and thus provide an important model for understanding mechanisms of natural host resistance to disease. This study assessed the levels and anatomic distribution of SIVagm in healthy, naturally infected monkeys. Quantitative competitive reverse transcriptase PCR assays developed to measure SIVagm from two African green monkey subspecies demonstrated high levels of SIV RNA in plasma (>6 x 10(6) RNA copies/ml) in sabaeus and vervet monkeys. Infectious virus was readily recovered from plasma and peripheral blood mononuclear cells and shown to be highly cytopathic in human cell lines and macrophages. SIVagm DNA levels were highest in the gastrointestinal tract, suggesting that the gut is a major site for SIVagm replication in vivo. Appreciable levels of virus were also found within the brain parenchyma and the cerebrospinal fluid (CSF), with lower levels detected in peripheral blood cells and lymph nodes. Virus isolates from the CSF and brain parenchyma readily infected macrophages in culture, whereas lymph node isolates were more restricted to growth in human T-cell lines. Comparison of env V2-C4 sequences showed extensive amino acid diversity between SIVagm recovered from the central nervous system and that recovered from lymphoid tissues. Homology between brain and CSF viruses, macrophage tropism, and active replication suggest compartmentalization in the central nervous system without associated neuropathology in naturally infected monkeys. These studies provide evidence that the nonpathogenic nature of SIVagm in the natural host can be attributed neither to more effective host control over viral replication nor to differences in the tissue and cell tropism from those for human immunodeficiency virus type 1-infected humans or SIV-infected macaques.

HIV, cytokines and programmed cell death. A subtle interplay

HIV infection is marked by the progressive destruction of the CD4 T lymphocyte subset, an essential component of the immune system and a vital source of cytokines required for differentiation of natural killer (NK) and gamma delta T cells, for maturation of B lymphocytes into plasmocytes, and for differentiation of CD8+ T cells into virus-specific cytotoxic T lymphocytes. CD4 T lymphocytes are also a source of chemokines which control migration of lymphocytes to the site of infection and which also inhibit HIV entry into CD4-expressing targets. Continuous production of viral proteins leads to an unbalanced immune activation and to the triggering of apoptotic programs, turning mononuclear cells, including CD4 T cells, CD8 T cells and APC, into effectors of apoptosis, leading to fratricidal destruction of healthy uninfected cells expressing the death receptors. Inappropriate PCD is also responsible for the disappearance of T helper cells primed for type-1 cytokine synthesis, thus contributing to the lack of survival factors which could prevent spontaneous lymphocyte apoptosis. Under potent anti-retroviral therapies, a significant decrease in spontaneous, TCR- and CD95-induced lymphocyte apoptosis is observed, concomitant with a partial quantitative and qualitative restoration of the immune system in treated patients. However, owing to the suppressive effect of anti-retroviral drugs on physiological apoptosis, these therapies are associated with alteration of TNF-alpha-regulated T cell homeostasis, leading to an accumulation in the blood of T cells primed for TNF-alpha synthesis, and contributing to the development of a new syndrome associated with these treatments, the lipodystrophy syndrome.

Rapid CD4(+) T-cell loss induced by human immunodeficiency virus type 1(NC) in uninfected and previously infected chimpanzees

To investigate the pathogenicity of a virus originating in a chimpanzee with AIDS (C499), two chimpanzees were inoculated with a plasma-derived isolate termed human immunodeficiency virus type 1(NC) (HIV-1(NC)). A previously uninfected chimpanzee, C534, experienced rapid peripheral CD4(+) T-cell loss to fewer than 26 cells/microl by 14 weeks after infection. CD4(+) T-cell depletion was associated with high plasma HIV-1 loads but a low virus burden in the peripheral lymph node. The second chimpanzee, C459, infected 13 years previously with HIV-1(LAV), experienced a more protracted course of peripheral CD4(+) T-cell loss after HIV-1(NC) inoculation, resulting in fewer than 200 cells/microl by 96 weeks postinoculation. The quantities of viral RNA in the plasma and peripheral lymph node from C459 were below the lower limits of detection prior to inoculation with HIV-1(NC) but were significantly and persistently increased after superinfection, with HIV-1(NC) representing the predominant viral genotype. These results show that viruses derived from C499 are more pathogenic for chimpanzees than any other HIV-1 isolates described to date.

Apoptosis in T-lymphocyte subsets in human immunodeficiency virus-infected children measured immediately ex vivo and following in vitro activation

Phosphatidylserine molecules are translocated to the outer plasma membrane of lymphocytes undergoing apoptosis and can be detected by the binding of fluorochrome-conjugated annexin V. Using the annexin V assay, we examined CD4 and CD8 T cells from human immunodeficiency virus (HIV)-infected children for apoptosis upon isolation or following in vitro culture. Immediate ex vivo analysis or overnight culture showed significantly higher levels of apoptosis in CD8 cells than in CD4 cells. Following culture with the activating stimulus phytohemagglutinin or anti-CD3 monoclonal antibody, we observed an increase in the percentage of apoptotic CD4 cells, whereas there was no change in the rate of CD8 cell death. These results demonstrate that in HIV-infected children, CD8 apoptosis may occur at a greater rate than CD4 apoptosis in vivo; greater CD4 depletion may be observed due to more efficient mechanisms for peripheral lymphocyte replacement in the CD8 compartment. Furthermore, our data suggest that CD8 lymphocytes may be maximally activated in vivo, a condition which may lead to the exhaustion of CD8-mediated immunity. These findings clarify the differences between the CD4 and CD8 apoptotic responses to HIV.

Induction of CD95 ligand expression on T lymphocytes and B lymphocytes and its contribution to apoptosis of CD95-up-regulated CD4+ T lymphocytes in macaques by infection with a pathogenic simian/human immunodeficiency virus

Using an established SIV/HIV-C2/1-infected cynomolgus monkey model displaying stable CD4+ T cell depletion, the kinetics of apoptosis and the levels of expression of CD95 membrane-associated CD95L on lymphocytes were investigated to test the involvement of the CD95/CD95L system in CD4+ T lymphocyte loss in vivo. Rapid depletion of CD4+ T cells occurred up to 2 weeks after infection, with chronic CD4+ T lymphopenia thereafter. During the initial CD4+ T cell loss, which was accompanied by viraemia, about 90% of the peripheral CD4+ T cell subset underwent spontaneous apoptotic cell death during 24 h of culture. Increased expression of CD95 was observed on both CD4+ and CD8+ T cell subsets, with CD95 expression on CD8+ cells declining rapidly, but high CD95 expression being maintained on CD4+ cells. Since CD95L was expressed on CD8+ T cells, B cells and to a lesser extent on CD4+ T cells, this suggests that CD95-mediated apoptosis might be controlled in an autocrine/paracrine fashion.

Induction of apoptosis by the Vpr protein of human immunodeficiency virus type 1 occurs independently of G(2) arrest of the cell cycle

The HIV-1 accessory gene product Vpr can inhibit cell proliferation via cell cycle arrest at the G(2) phase, and it can induce apoptosis after G(2) arrest. We found recently that C81, a carboxy-terminally truncated form of Vpr, induced apoptosis via G(1) arrest but did not induce G(2) arrest of the cell cycle. Thus, it seemed possible that expression of Vpr in cells might cause apoptosis independently of the ability of Vpr to induce G(2) arrest. We demonstrate here that Vpr-induced apoptosis occurs by a mechanism that does not necessarily require induction of G(2) arrest. First, it was found that the extent of apoptosis reached a maximum even when few cells were arrested at the G(2) phase of the cell cycle and was reduced in inverse proportion to the increased induction of G(2) arrest. Thus, the extent of induction of G(2) arrest was not correlated with the extent of Vpr-induced apoptosis. Furthermore, we replaced the Ile/Leu residues in the leucine zipper-like domain of Vpr with Ala or Pro and used cells that expressed the mutant protein to demonstrate that Vpr caused apoptosis in a manner that was independent of G(2) arrest. Finally, replacement of Ile/Leu by Pro at positions 60, 67, 74, and 81 within the leucine zipper-like domain of wild-type Vpr and C81 revealed that the Ile/Leu residues at positions 60, 67, and 74 in the leucine zipper-like domain were indispensable for induction of apoptosis induced by Vpr and by C81 and confirmed, in addition, that both processes might be regulated by the same pathway. C81 appears to be a useful tool for elucidation of the mechanism of apoptosis induced by expression of Vpr protein.

HIV-1 tat induced apoptosis of T-cells is not mediated by TGF-beta

Recent reports have demonstrated that the HIV-1 transactivator protein, tat, induces apoptosis in T-lymphocyte cell lines, as well as in peripheral blood mononuclear cells, and stimulates a cascade of events resulting in up-regulation of the potent immunosuppressive cytokine, transforming growth factor-beta (TGF-beta). In this study we evaluated the ability of TGF-beta to mediate tat induced apoptosis in T-lymphocyte cell lines. T-cells treated exogenously with either TGF-beta1 or a combination of tat and pan-specific TGF-beta neutralizing antibodies showed little change in the amount of apoptosis. When treated with pan-specific TGF-beta neutralizing antibodies, Jurkat cells that stably express tat protein (Jurkat-tat ) showed only a modest decrease in apoptosis, while CEM-TART cells (CEM T-cells expressing both HIV-1 tat and rev ) demonstrated little change in the amount of apoptosis. In conclusion, we have demonstrated that TGF-beta does not play a significant role in mediating tat induced T-cell apoptosis.

High levels of viral replication during primary simian immunodeficiency virus SIVagm infection are rapidly and strongly controlled in African green monkeys

In contrast to pathogenic human immunodeficiency virus and simian immunodeficiency virus (SIV) infections, chronic SIVagm infections in African green monkeys (AGMs) are characterized by persistently low peripheral and tissue viral loads that correlate with the lack of disease observed in these animals. We report here data on the dynamics of acute SIVagm infection in AGMs that exhibit remarkable similarities with viral replication patterns observed in peripheral blood during the first 2 weeks of pathogenic SIVmac infections. Plasma viremia was evident at day 3 postinfection (p.i.) in AGMs, and rapid viral replication led by days 7 to 10 to peak viremias characterized by high levels of antigenemia (1.2 to 5 ng of p27/ml of plasma), peripheral DNA viral load (10(4) to 10(5) DNA copies/10(6) peripheral blood mononuclear cells [PBMC]), and plasma RNA viral load (2 x 10(6) to 2 x 10(8) RNA copies/ml). The lymph node (LN) RNA and DNA viral load patterns were similar to those in blood, with peaks observed between day 7 and day 14. These values in LNs (ranging from 3 x 10(5) to 3 x 10(6) RNA copies/10(6) LN cell [LNC] and 10(3) to 10(4) DNA copies/10(6) LNC) were at no time point higher than those observed in the blood. Both in LNs and in blood, rapid and significant decreases were observed in all infected animals after this peak of viral replication. Within 3 to 4 weeks p. i., antigenemia was no longer detectable and peripheral viral loads decreased to values similar to those characteristic of the chronic phase of infection (10(2) to 10(3) DNA copies/10(6) PBMC and 2 x 10(3) to 2 x 10(5) RNA copies/ml of plasma). In LNs, viral loads declined to 5 x 10(1) to 10(3) DNA copies and 10(4) to 3 x 10(5) RNA copies per 10(6) LNC at day 28 p.i. and continued to decrease until day 84 p.i. (<10 to 3 x 10(4) RNA copies/10(6) LNC). Despite extensive viremia during primary infection, neither follicular hyperplasia nor CD8(+) cell infiltration into LN germinal centers was detected. Altogether, these results indicate that the nonpathogenic outcome of SIVagm infection in its natural host is associated with a rapidly induced control of viral replication in response to SIVagm infection, rather than with a poorly replicating virus or a constitutive host genetic resistance to virus replication.

Ineffective cellular immune response associated with T-cell apoptosis in susceptible Mycobacterium bovis BCG-infected mice

It has previously been reported that inhibition of delayed-type hypersensitivity-mediating functions of T cells during mycobacterial infection in mice is haplotype dependent. In the present study, we show that Mycobacterium bovis BCG infection induced, in susceptible C57BL/6 and BALB/c mice but not in resistant C3H/HeJ and DBA/2 mice, an important splenomegaly. An in vitro defect in T-cell proliferation in response to T-cell receptor (TCR) stimulation with mitogens or anti-CD3 antibodies was associated with enhanced levels of CD4(+) and CD8(+) T-cell apoptosis in susceptible but not in resistant mice 2 weeks after infection. Further investigations of C57BL/6 and C3H/HeJ mice revealed that in vivo splenomegaly was associated with destruction of the lymphoid tissue architecture, liver cellular infiltrates, and increased numbers of apoptotic cells in both spleen and liver tissue sections. Infection of C57BL/6 mice but not of C3H/HeJ mice induced massive production of tumor necrosis factor alpha (TNF-alpha) in serum, as well as an increase in Fas and Fas ligand (FasL) expression in T cells. In vitro addition of neutralizing anti-TNF-alpha antibodies led to a significant reduction in CD3-induced T-cell apoptosis of both CD4(+) and CD8(+) T cells of C57BL/6 mice, while the blockade of Fas-FasL interactions reduced apoptosis only in CD4(+) but not in CD8(+) T cells. Together, these results suggest that TNF-alpha and Fas-FasL interactions play a role in the activation-induced cell death (AICD) process associated with a defect in T-cell proliferation of the susceptible C57BL/6 mice. T-cell death by apoptosis may represent one of the important components of the ineffective immune response against mycobacterium-induced immunopathology in susceptible hosts.

Differential apoptosis effects of primate lentiviral Vpr and Vpx in mammalian cells

The growth inhibitory effects of Vpr and Vpx are species- and cell type-dependent. HIV-1, HIV-2 and SIV Vpr are primarily cytostatic in mammalian cells and HIV-1 Vpr has been reported to induce apoptosis in human cells. Our previous studies have shown that HIV-1, HIV-2 and SIV Vpr and Vpx have differential cytostatic and cytotoxic effects in the yeast cells [Zhang et al.: Virology, 230:103-112; 1997]. Here, we further examined the apoptosis function of HIV-1 Vpr in different species of mammalian cells and investigated if other primate lentiviral Vpr and Vpx exert similar functions. Our results show that none of the primate lentiviral Vpr or Vpx we tested induces apoptosis in nonhuman species of mammalian cells. However, HIV-1 Vpr, but not HIV-2 or SIV Vpr and/or Vpx, induced apoptosis in different types of human cell lines. Further, the apoptotic effect of HIV-1 Vpr can be distinguished from that of the human interferon-gamma, a known proapoptotic protein, that HIV-1 Vpr shows little to no paracrine and/or bystander effect. When coexpressed with Bcl-2 or Bcl-X(L), the apoptotic effect of HIV-1 Vpr became markedly attenuated. These results indicate that the apoptotic effect of HIV-1 Vpr is species-dependent and is intracellularly modulated by the Bcl-2 family of proteins. Our study also suggests that the proapoptotic function of HIV-1 Vpr is developmentally associated with human but not nonhuman primate species.

A carboxy-terminally truncated form of the human immunodeficiency virus type 1 Vpr protein induces apoptosis via G(1) cell cycle arrest

Viral protein R (Vpr) of human immunodeficiency virus type 1 inhibits cell proliferation by arresting the cell cycle at the G(2) phase and inducing to apoptosis after G(2) arrest. We have reported previously that C81, a carboxy-terminally truncated form of Vpr, interferes with cell proliferation via a novel pathway that is distinct from G(2) arrest. However, the mechanism of this effect of C81 is unknown. We demonstrate here that C81 can induce apoptosis via G(1) arrest of the cell cycle. Immunostaining for various markers of stages of the cell cycle and flow cytometry analysis of DNA content showed that most HeLa cells that had been transiently transfected with a C81 expression vector were arrested at the G(1) phase and not at the G(2) or S phase of the cell cycle. Staining for annexin V, which binds phosphatidylserine on the plasma membrane, as an early indicator of apoptosis and measurement of the activity of caspase-3, a signaling molecule in apoptotic pathways, indicated that C81 is a strong inducer of apoptosis. Expression of C81 induced the condensation, fragmentation, and clumping of chromatin that are typical of apoptosis. Furthermore, the kinetics of the C81-induced G(1) arrest were closely correlated with changes in the number of annexin V-positive cells and the activity of caspase-3. Replacement of Ile or Leu residues by Pro at positions 60, 67, 74, and 81 within the leucine zipper-like domain of C81 revealed that Ile60, Leu67, and Ile74 play important roles both in the C81-induced G(1) arrest and in apoptosis. Thus, it appears that C81 induces apoptosis through pathways that are identical to those utilized for G(1) arrest of the cell cycle. It has been reported that Ile60, Leu67, and Ile74 also play an important role in the C81-induced suppression of growth. These results suggest that the suppression of growth induced by C81 result in apoptosis that is independent of G(2) arrest of the cell cycle.

Programmed cell death as a mechanism of CD4 and CD8 T cell deletion in AIDS. Molecular control and effect of highly active anti-retroviral therapy

Infection with human immunodeficiency virus (HIV) results in the progressive destruction of CD4 T lymphocytes, generally associated with progression of the disease. The progressive disappearance of CD4 T lymphocytes leads to the lack of control of HIV replication and to the development of severe immune deficiency responsible for the occurrence of opportunistic infections associated with AIDS. In this review we discuss premature lymphocyte apoptosis in the context of HIV infection as the consequence of the continuous production of viral proteins, leading to an unbalanced immune activation and to the triggering of apoptotic programs. The chronic immune activation induces the continuous expression of death factors which could turn lymphocytes, including CD4 T cells, CD8 CTL or APC, into effectors of apoptosis, leading to the destruction of healthy activated non-infected cells. Thus, programmed cell death would significantly contribute to peripheral T cell depletion in AIDS, particularly if the Th cell renewal is impaired. Under potent anti-retroviral therapies, a complete normalization of lymphocyte apoptosis is observed, concomitant with a partial restoration of the number and the functions of the immune system.

Evolutionary stable strategy: a test for theories of retroviral pathology which are based upon the concept of molecular mimicry

The genetic makeup of animal and plant populations is determined by established principles and concepts. Ecology and evolution provide a basic theoretical framework for understanding how genetic changes occur in populations. Whether these rules can be applied to host retroviral populations is unknown. Individuals infected with the human immunodeficiency virus (HIV) contain within their bodies a viral population. This population is known as a viral quasispecies. Located in the transmembrane protein of HIV-1 is the viral sequence Gly-Thr-Asp-Arg-Val. Previous immunological studies have shown that viral antibody is produced in response to this five-amino-acid sequence. Antibody to this viral sequence also crossreacts and binds to a related peptide sequence found on certain immune cells. This related sequence, Gly-Thr-Glu-Arg-Val, is found on immune cells bearing a structure known as the major histocompatibility complex (MHC). The viral transmembrane sequence, Gly-Thr-Asp-Arg-Val, can be substituted with alanine residues utilizing site-directed mutagenesis. This creates a viral clone devoid of the genetic similarity with the MHC. Chimpanzees progressing to AIDS contain both sequences of interest. Suppression of the chimpanzee quasispecies utilizing anti-retroviral drugs is proposed. This action serves to suppress the presence of the viruses containing the sequence Gly-Thr-Asp-Arg-Val. When viral load has been reduced significantly, a drug resistant, alanine altered clone is to be introduced in large numbers. The concept of evolutionary stable strategy predicts that a viable HIV clone with alanine residues can genetically dominate the viral population. Immune system recognition of the alanine sequence is likely to result in renewed antibody production. Antibodies to the alanine containing viral sequence should not recognize or bind to the MHC. Immunological parameters can then be measured to determine the physiological impact of eliminating a sequence responsible for molecular mimicry between virus and host.

Normal T-cell turnover in sooty mangabeys harboring active simian immunodeficiency virus infection

Sooty mangabeys naturally infected with simian immunodeficiency virus (SIV) remain healthy though they harbor viral loads comparable to those in rhesus macaques that progress to AIDS. To assess the immunologic basis of disease resistance in mangabeys, we compared the effect of SIV infection on T-cell regeneration in both monkey species. Measurement of the proliferation marker Ki-67 by flow cytometry showed that mangabeys harbored proliferating T cells at a level of 3 to 4% in peripheral blood irrespective of their infection status. In contrast, rhesus macaques demonstrated a naturally high fraction of proliferating T cells (7%) that increased two- to threefold following SIV infection. Ki-67(+) T cells were predominantly CD45RA(-), indicating increased proliferation of memory cells in macaques. Quantitation of an episomal DNA product of T-cell receptor alpha rearrangement (termed alpha1 circle) showed that the concentration of recent thymic emigrants in blood decreased with age over a 2-log unit range in both monkey species, consistent with age-related thymic involution. SIV infection caused a limited decrease of alpha1 circle numbers in mangabeys as well as in macaques. Dilution of alpha1 circles by T-cell proliferation likely contributed to this decrease, since alpha1 circle numbers and Ki-67(+) fractions correlated negatively. These findings are compatible with immune exhaustion mediated by abnormal T-cell proliferation, rather than with early thymic failure, in SIV-infected macaques. Normal T-cell turnover in SIV-infected mangabeys provides an explanation for the long-term maintenance of a functional immune system in these hosts.

Role of apoptosis induction in both peripheral lymph nodes and thymus in progressive loss of CD4+ cells in SHIV-infected macaques

To investigate the role of apoptosis in the progressive loss of CD4+ lymphocytes in HIV infection, we have used macaques infected with SHIV, a hybrid virus of HIV and simian immunodeficiency virus (SIV). In the present study, we sequentially analyzed apoptosis induction in the acute phase of SHIV infection. Four macaques infected with a pathogenic SHIV, SHIV89.6P, and four macaques infected with a nonpathogenic SHIV, NM-3rN, were analyzed during the first 2 or 4 weeks postinfection. In the 89.6P-infected macaques the number of peripheral CD4+ cells sharply decreased at 2 weeks postinfection and was maintained below 50/microl until 4 weeks postinfection, while in the NM-3rN-infected macques the number of the CD4+ cells did not change significantly. Plasma viral loads peaked at 2 and 2-3 weeks postinfection, and the peak values were about 1 x 10(9) and 10(6)-10(7) copies/ml in the 89.6P- and the NM-3rN-infected macaques, respectively. In the 89.6P-infected macaques, Fas antigen expression and the extent of apoptosis in PBMCs and peripheral lymph nodes increased at 1-2 weeks postinfection. A high number of apoptotic cells was also observed in thymus sections 2 and 4 weeks postinfection. On the other hand, apoptosis was scarcely induced in the NM-3rN-infected macaques. These results suggest that the extent of apoptosis induction is closely correlated with the pathogenicity of SHIV and that the apoptosis induction in peripheral lymphoid tissues and thymus, where T cell maturation occurs, may play an important role in the depletion of CD4+ lymphocytes in 89.6P infection.

Highly active anti-retroviral therapy (HAART) is associated with a lower level of CD4+ T cell apoptosis in HIV-infected patients

HAART may increase CD4+ T cell counts despite a persistently detectable HIV load. The impact of HAART on apoptosis, which may play a role in the disease process in HIV-infected patients, has not been extensively studied. We performed a study to compare the level of spontaneous T cell apoptosis and anti-retroviral treatments in a cohort of HIV-1-infected patients. Data were obtained from a computerized medical record. Quantification of apoptotic cells was by cytofluorometric technique. From November 1995 to December 1997 we studied T cell apoptosis in 112 HIV-infected patients. Forty patients were classified A, 36 B and 36 C. Thirty patients were naive and 82 received an anti-retroviral treatment, 49 including a protease inhibitor (PI). The median plasma viraemia determined in 63 patients was 3.6 (range 1.3-5.6) log10. The median apoptotic cell count was 22% (range 2-73%) and 12% (range 2-60%) for CD4+ and CD8+ T cells, respectively. We did not observe any correlation between the HIV viraemia and the level of apoptosis of T cell subsets. Patients with HAART showed a lower percentage of apoptotic CD4+ T cells only: 16% (range 2-61%) versus 25% (range 5-73%) for patients receiving two nucleoside analogues (P = 0.02). This effect was significant in stage A patients and remained observable during the whole course of HIV disease. In conclusion, HAART, without any relation to plasma viraemia, is able to reduce apoptosis of CD4+ T cells.

Lymphocyte activation during acute simian/human immunodeficiency virus SHIV(89.6PD) infection in macaques

Host-virus interactions control disease progression in human immunodeficiency virus-infected human beings and in nonhuman primates infected with simian or simian/human immunodeficiency viruses (SHIV). These interactions evolve rapidly during acute infection and are key to the mechanisms of viral persistence and AIDS. SHIV(89.6PD) infection in rhesus macaques can deplete CD4(+) T cells from the peripheral blood, spleen, and lymph nodes within 2 weeks after exposure and is a model for virulent, acute infection. Lymphocytes isolated from blood and tissues during the interval of acute SHIV(89.6PD) infection have lost the capacity to proliferate in response to phytohemagglutinin (PHA). T-cell unresponsiveness to mitogen occurred within 1 week after mucosal inoculation yet prior to massive CD4(+) T-cell depletion and extensive virus dissemination. The lack of mitogen response was due to apoptosis in vitro, and increased activation marker expression on circulating T cells in vivo coincided with the appearance of PHA-induced apoptosis in vitro. Inappropriately high immune stimulation associated with rapid loss of mature CD4(+) T cells suggested that activation-induced cell death is a mechanism for helper T-cell depletion in the brief period before widespread virus dissemination. Elevated levels of lymphocyte activation likely enhance SHIV(89.6PD) replication, thus increasing the loss of CD4(+) T cells and diminishing the levels of virus-specific immunity that remain after acute infection. The level of surviving immunity may dictate the capacity to control virus replication and disease progression. We describe this level of immune competence as the host set point to show its pivotal role in AIDS pathogenesis.

Combined antiviral therapy reduces HIV-1 plasma load and improves CD4 counts but does not interfere with ongoing lymphocyte apoptosis

The progression of HIV-1 disease appears associated with an unregulated Fas-mediated apoptosis of lymphocytes that involves the activation of ICE protease and ceramide generation and antiviral therapy may not be fully effective in the absence of a relevant impact on apoptosis. Six drug-naive HIV-1-infected symptomless patients with advanced immunodeficiency were treated with combined AZT and ddl for 4 months; plasma HIV-1 RNA levels, the counts of CD4 cells, CD4 and CD8 apoptotic lymphocytes, Fas-positive cells and ICE-positive cells, and intracellular ceramide levels were measured at base-line and after 7, 45 and 120 days of treatment. There was a prompt reduction in plasma viremia and a secondary increase in CD4 counts, but the treatment had no impact on apoptotic CD4 and CD8 lymphocytes, Fas-positive cells and ICE-positive cells, and on the intracellular levels of ceramide. A discrepancy exists between the positive impact of combined AZT and ddl treatment on plasma viral load and CD4 counts and the lack of any effect on the process of lymphocyte apoptosis. We suggest to use the measurement of apoptotic lymphocytes as a surrogate marker to predict, in combination with viral load and CD4 counts, a large proportion of the clinical effect of antiviral therapy.

Sequential analysis of apoptosis induction in peripheral blood mononuclear cells and lymph nodes in the early phase of pathogenic and nonpathogenic SIVmac infection

To investigate the role of apoptosis in the early phase of HIV infection, we used macaques infected with simian immunodeficiency virus strain mac (SIVmac) as a primate model and examined sequentially the characteristics of apoptosis of lymphocytes in peripheral blood mononuclear cells (PBMCs) and lymph nodes in the early phase of SIVmac infection. Five macaques infected with a pathogenic strain of SIV, SIVmac239, were analyzed during the first 4 weeks after infection. Peripheral CD4+ and CD8+ cells transiently decreased at 1 week postinfection. The percentage of apoptotic cells in cultured PBMCs increased from about 2 weeks postinfection. The number of apoptotic cells in lymph node sections was higher on days 13 and 28 postinfection than before infection and on day 5 postinfection. Fas antigen expression on peripheral lymphocytes was upregulated from day 8 postinfection. These results indicate that apoptosis is induced about 2 weeks after SIVmac239 infection, following the upregulation of Fas antigen expression on lymphocytes. Since apoptosis was induced about 1 week after the decrease in peripheral CD4+ and CD8+ cell counts, it appears that the apoptosis induction does not play an important role in the transient lymphopenia in the early phase of SIVmac infection. In macaques infected with a nonpathogenic derivative of SIVmac239, SIVmac delta nef, apoptosis of lymphocytes was induced as it was in SIVmac239-infected macaques, but to a lesser degree, suggesting a correlation between the extent of apoptosis induction in lymphocytes in the early phase of SIVmac infection and the pathogenicity of SIVmac.

Effect of didanosine, stavudine, and hydroxyurea therapy on apoptosis in CD45RA+ and CD45RO+ T lymphocyte subpopulations

The effect of aggressive antiretroviral therapy on spontaneous apoptosis (AP) in CD4+ and CD8+ lymphocytes expressing CD45RO (memory cells) and CD45RA (naive cells) and their relationship to cellular activation and viral load were examined. Ten patients receiving simultaneous treatment with d4T, ddI, and HU were evaluated. Flow cytometric analysis showed significant levels of AP (measured by TUNEL assay) among memory and naive T cells and an enhanced expression of CD38 and HLA-DR activation markers. The percentage of apoptotic CD4+CD45RO+ and CD4+CD45RA+ cells decreased, respectively, from 34 +/- 3.3 and 29 +/- 3.6 prior to treatment to 20.5 +/- 4 and 22 +/- 3.8 at week 8 into therapy. The percentage of apoptotic CD8+CD45RO+ and CD8+CD45RA+ cells similarly decreased, respectively, from 20 +/- 2.5 and 24 +/- 3 prior to treatment to 14.5 +/- 2.7 and 16 +/- 3 at week 8 into treatment. The percentage of CD4+ cells expressing the activation markers CD38 and HLA-DR decreased from 27 +/- 6 to 13 +/- 2 and from 26 +/- 4 to 13.5 +/- 3, respectively. The percentage of CD8+ cells expressing either CD38 or HLA-DR fell from 22 +/- 3 to 10 +/- 2 for the former and from 39 +/- 5 to 22 +/- 4 for the latter. This was associated with a significant decrease in viral load (mean, 1.4 log10), and a decline in circulating plasma TNF-alpha and sIL-2R levels from 50.5 +/- 10 to 21 +/- 6 and 92.5 +/- 11 to 68 +/- 9, respectively. These data indicate that short-term therapy with ddI, d4T, and HU in combination diminished AP, immune activation, and partially restored naive and memory T cell subpopulations.

Human Immunodeficiency Virus Type 1 Nef Protein Sensitizes CD4+ T Lymphoid Cells to Apoptosis via Functional Upregulation of the CD95/CD95 Ligand Pathway

Many viruses have evolved genes encoding proteins that regulate cell death by apoptosis. The human immunodeficiency virus type 1 (HIV-1) Nef protein alters T-cell development and signaling and is required for optimal viral replication and pathogenicity in vivo. To analyze the interference of Nef with cell survival, we used both regulated and constitutively expressed nef alleles in stably transfected T-cell lines. Nef-expressing cells were sensitized to cell death by apoptosis, which was specifically exacerbated by an anti-CD95 IgM monoclonal antibody (MoAb). Flow cytometric analysis showed that the surface expression of both CD95 and CD95 ligand (CD95L) was upregulated by endogenous Nef expression. Nef-mediated apoptosis was almost completely suppressed by the addition in culture of an anti-CD95 Fab′ IgG MoAb, which specifically blocks CD95/CD95L interactions. Lastly, mutation of a proline motif in the core region of the nef gene, which disrupts its ability to interact with cellular kinases and reduces HIV-1 replication in vitro, completely abrogated the Nef-mediated induction of apoptosis as well as its ability to upregulate surface CD95 and CD95L. These findings may provide molecular insight into the role of endogenous Nef in the T-cell depletion observed in vivo, particularly HIV-specific cytotoxic CD8+ T cells.

Human Immunodeficiency Virus Type 1 Nef Protein Sensitizes CD4+ T Lymphoid Cells to Apoptosis via Functional Upregulation of the CD95/CD95 Ligand Pathway

Many viruses have evolved genes encoding proteins that regulate cell death by apoptosis. The human immunodeficiency virus type 1 (HIV-1) Nef protein alters T-cell development and signaling and is required for optimal viral replication and pathogenicity in vivo. To analyze the interference of Nef with cell survival, we used both regulated and constitutively expressed nef alleles in stably transfected T-cell lines. Nef-expressing cells were sensitized to cell death by apoptosis, which was specifically exacerbated by an anti-CD95 IgM monoclonal antibody (MoAb). Flow cytometric analysis showed that the surface expression of both CD95 and CD95 ligand (CD95L) was upregulated by endogenous Nef expression. Nef-mediated apoptosis was almost completely suppressed by the addition in culture of an anti-CD95 Fab′ IgG MoAb, which specifically blocks CD95/CD95L interactions. Lastly, mutation of a proline motif in the core region of the nef gene, which disrupts its ability to interact with cellular kinases and reduces HIV-1 replication in vitro, completely abrogated the Nef-mediated induction of apoptosis as well as its ability to upregulate surface CD95 and CD95L. These findings may provide molecular insight into the role of endogenous Nef in the T-cell depletion observed in vivo, particularly HIV-specific cytotoxic CD8+ T cells.

Analysis of nonhuman primate peripheral blood mononuclear cells for susceptibility to HIV-1 infection and HIV coreceptor expression

HIV-1 infection of nonhuman primates does not lead to the acquired immunodeficiency syndrome seen in humans. The basis for this lack of disease progression in these animals is still unknown. In this study, primary nonhuman primate peripheral blood mononuclear cells (PBMC) were tested for their susceptibility to in vitro infection by several different primary HIV-1 isolates representing distinct subtypes or clades. None of the five HIV-1 subtypes tested were able to readily establish an infection in chimpanzee or baboon PBMC, as determined by p24 antigen capture assays. To address the mechanism of in vitro resistance to HIV-1 infection, PBMC were analyzed for HIV coreceptor mRNA expression and cell surface expression. Flow cytometry analysis of the nonhuman primate PBMC demonstrated that they do express CD4, CCR3, CCR5, and CXCR4 on their cell surface. Therefore, the level of restriction in the virus replication cycle does not appear to lie at the point of entry in these cells.

Growth hormone prevents human monocytic cells from Fas-mediated apoptosis by up-regulating Bcl-2 expression

Apoptosis and particularly Fas-mediated apoptosis has been proposed to play a key role in controlling monocyte homeostasis. We and others have documented the regulatory function of human growth hormone (hGH) on monocytic cells, which prompted us to investigate the role of hGH on their response to Fas antigen cross-linking. Using human promonocytic U937 cells constitutively producing hGH upon gene transfer and human primary monocytes cultured in the presence of recombinant hGH, we demonstrated that hGH diminished Fas-mediated cell death by enhancing the expression of the antiapoptotic oncoprotein Bcl-2 as well as the level of bcl-2alpha mRNA. In parallel, we established that overexpression of Bcl-2 through gene transfer into normal U937 cells also diminished Fas-induced apoptosis. Further, as a result of Bcl-2 overexpression, we found that hGH greatly depressed Fas-induced activation of the cysteine protease caspase-3 (CPP32), which in turn affected the cleavage of poly(ADP-ribose) polymerase. Altogether, these data provide evidence that hGH mediates its protective effect through a Bcl-2-dependent pathway, clearly a crucial step in enhanced survival of monocytic cells exposed to Fas-induced death.

Kinetics of lymphocyte apoptosis in macaques infected with different simian immunodeficiency viruses or simian/human immunodeficiency hybrid viruses

Increased rates of T-cell apoptosis have been detected in human immunodeficiency virus (HIV)-infected individuals and in the simian immunodeficiency model (SIV) for AIDS research. We have infected macaques with virulent SIV or SIV/HIV hybrid viruses (SHIV) of different pathogenic potentials to study the early kinetics of apoptosis in this model. Animals infected with SIV showed an increased degree of apoptosis in their peripheral blood mononuclear cells as early as 8 weeks after virus inoculation. Apoptotic cells were detected in the CD4 and CD8 cell populations of infected animals. In contrast, apathogenic SHIV did not lead to increased lymphocyte apoptosis and moderately pathogenic SHIV induced only transient apoptosis. T-cell death was temporally linked to viral replication in vivo. Furthermore, lymphocyte apoptosis in infected macaques was associated with impaired proliferative responses of helper T-cells and with CD4 cell depletion. The monkey model described here provides the opportunity for testing early therapeutic interventions to prevent virus-induced programmed cell death and the subsequent onset of AIDS.

Vpu increases susceptibility of human immunodeficiency virus type 1-infected cells to fas killing

The importance of the Fas death pathway in human immunodeficiency virus (HIV) infection has been the subject of many studies. Missing from these studies is direct measurement of infected cell susceptibility to Fas-induced death. To address this question, we investigated whether T cells infected with HIV are more susceptible to Fas-induced death. We found that Fas cross-linking caused a decrease in the number of HIV-infected Jurkat T cells and CD4(+) peripheral blood leukocytes (PBLs). We confirmed this finding by demonstrating that there were more apoptotic infected than uninfected cells after Fas ligation. The increase in sensitivity of HIV-infected cells to Fas killing mapped to vpu, while nef, vif, vpr, and second exon of tat did not appear to contribute. Furthermore, expression of Vpu in Jurkat T cells rendered them more susceptible to Fas-induced death. These results show that HIV-infected cells are more sensitive to Fas-induced death and that the Vpu protein of HIV contributes to this sensitivity. The increased sensitivity of HIV-infected cells to Fas-induced death might help explain why these cells have such a short in vivo half-life.

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.

Molecular cloning and characterization of viruses isolated from chimpanzees with pathogenic human immunodeficiency virus type 1 infections

We have previously described the development of AIDS in a chimpanzee (C499) infected with human immunodeficiency virus type 1 (HIV-1) and the subsequent pathogenic HIV-1 infection in another chimpanzee (C455) transfused with blood from C499 (F. J. Novembre et al., J. Virol. 71:4086-4091, 1997). In the present study, two virus isolates were derived from these animals: HIV-1JC from peripheral blood mononuclear cells (PBMC) of C499, and HIV-1NC from plasma of C455. These virus isolates were used to generate two infectious molecular clones, termed HIV-1JC16 and HIV-1NC7 (JC16 and NC7, respectively). Comparative analyses of the sequences of the two clones showed that they were highly interrelated but distinct. Based on heteroduplex mobility assays, JC16 and NC7 appear to represent dominant viruses in the uncloned stock population. Compared with amino acid sequences of the parental viruses HIV-1SF2, HIV-1LAV-1b, and HIV-1NDK, JC16 and NC7 showed a number of differences, including insertions, deletions, and point mutations spread throughout the genome. However, insertion/deletion footprints in several genes of both JC16 and NC7 suggested that recombination between SF2 and LAV-1b could have occurred, possibly contributing to the generation of a pathogenic virus. Comparative in vitro analyses of the molecular clones and the uncloned stocks of HIV-1JC and HIV-1NC revealed that these viruses had strikingly similar replicative abilities in mitogen-stimulated PBMC and in macrophages. Compared to the SF2 and LAV-1b isolates of HIV-1, HIV-1JC and HIV-1NC isolates were more similar to LAV-1b with respect to the ability to replicate in mitogen-stimulated PBMC and macrophages. These viruses should prove to be useful in mapping determinants of pathogenesis.

In vivo analysis of Fas/FasL interactions in HIV-infected patients

Recent insights into the pharmacological control of HIV replication and the molecular mechanisms of peripheral T cells homeostasis allowed us to investigate in vivo the mechanisms mediating T cell depletion in HIV-infected patients. Before the initiation of highly active antiretroviral therapy (HAART), a high degree of lymphoid tissue apoptosis is present, which is reduced upon HAART initiation (P < 0.001) and directly correlates with reduction of viral load and increases of peripheral T lymphocytes (P < 0.01). Because Fas/FasL interactions play a key role in peripheral T lymphocyte homeostasis, we investigated the susceptibility to Fas-mediated apoptosis in peripheral T lymphocytes and of FasL expression in lymphoid tissue before and during HAART. High levels of Fas-susceptibility found in peripheral CD4 T lymphocytes before HAART were significantly reduced after HAART, coinciding with decreases in viral load (P = 0.018) and increases in peripheral CD4 T lymphocyte counts (P < 0.01). However, the increased FasL expression in the lymphoid tissue of HIV-infected individuals was not reduced after HAART. These results demonstrate that lymphoid tissue apoptosis directly correlates with viral load and peripheral T lymphocyte numbers, and suggest that HIV-induced susceptibility to Fas-dependent apoptosis may play a key role in the regulation of T cell homeostasis in HIV-infected individuals.

A specific T-cell subset with CD4+/CD38- markers derived from HIV-1 carriers induces apoptosis in healthy donor-derived T-lymphocytes

Apoptosis is an important mechanism of human immunodeficiency virus type 1 (HIV-1)-induced T-cell depletion. Our recent findings revealed mitogenic stimulation-dependent apoptosis induction in healthy donor-derived peripheral blood T-lymphocytes after adsorption with defective HIV-1 particles through acquirement by a subset of CD4+/CD38- cells of specific killer function. Based on these in vitro observations, we have extended the significance of this killing activity of CD4+/CD38- cells directly derived from HIV-1 carriers. The CD4+/CD38- cells from HIV-1-positive individuals showed significantly higher cell-killing activities than those from HIV-1-negative donors by co-culture with allogeneic resting T-cells after mitogenic stimulation. Furthermore, most of the samples induced apoptosis in a Fas-dependent manner. Thus, it is suggested that HIV-1 infection-related apoptosis is triggered by inappropriate activation of a certain resting T-cell subset, presumably due to adsorption with HIV-1 particles.

Loss of CD4+ T cells in human immunodeficiency virus type 1-infected chimpanzees is associated with increased lymphocyte apoptosis

Supportive evidence that apoptosis contributes to loss of CD4+ lymphocytes in human immunodeficiency virus type 1 (HIV-1)-infected humans comes from an apparent lack of abnormal apoptosis in apathogenic lentivirus infections of nonhuman primates, including HIV-1 infection of chimpanzees. Two female chimpanzees were inoculated, one cervically and the other intravenously, with HIV-1 derived from the LAI/LAV-1b strain, which was isolated from a chimpanzee infected with the virus for 8 years. Within 6 weeks of infection, both recipient chimpanzees developed a progressive loss of CD4+ T cells which correlated with persistently high viral burdens and increased levels of CD4+ T-cell apoptosis both in vitro and in vivo. Lymph nodes from both animals also revealed evidence of immune hyperactivation. Intermediate levels of T-cell apoptosis in both peripheral blood and lymph nodes were seen in a third chimpanzee that had been infected with the LAI/LAV-1b strain for 9 years; this animal has maintained depressed CD4/CD8 T-cell ratios for the last 3 years. Similar analyses of cells from 4 uninfected animals and 10 other HIV-1-infected chimpanzees without loss of CD4+ cells revealed no difference in levels of apoptosis in these two control groups. These results demonstrate a correlation between immune hyperactivation, T-cell apoptosis, and chronic loss of CD4+ T cells in HIV-1-infected chimpanzees, providing additional evidence that apoptosis is an important factor in T-cell loss in AIDS. Furthermore, the results show that some HIV-1 strains are pathogenic for chimpanzees and that this species is not inherently resistant to HIV-1-induced disease.

The immunomodulating glycoprotein extract from Klebsiella pneumoniae RU 41740 exerts a suppressive effect on human monocyte death by apoptosis

Programmed cell death or apoptosis is a physiological cell suicide process that can be suppressed by survival factors. Monocytes undergo rapid apoptosis in culture, unless signalled by cytokines or the bacterial lipopolysaccharide LPS. We have investigated the effect on monocyte apoptosis of the immunostimulating agent RU 41740 (Biostim), a glycoprotein extract from the Klebsiella pneumoniae K2O1 strain that is used for the prevention of recurrent infections. RU 41740, as LPS, strongly enhanced monocyte survival in vitro, an effect related to apoptosis suppression. RU 41740 at concentration ranging from 1 ng/ml to 10 microg/ml prevented apoptosis induced both by survival factor deprival and by gamma-irradiation. Our observation suggests that enhancement of monocyte survival may represent a component of the reported immunostimulating effect of this compound.

Effect of combination antiretroviral therapy upon rectal mucosal HIV RNA burden and mononuclear cell apoptosis

BACKGROUND

Pathogen-negative diarrhea is common in HIV infection and has been associated with clinical symptoms, histopathology, HIV expression, CD4+ lymphocyte depletion, cytokine mRNA expression, and apoptosis of lamina propria mononuclear cells.

OBJECTIVES AND METHODS

To examine the short-term (7-day) effects of treatment with combination antiretroviral therapies upon gastrointestinal symptoms and rectal mucosa in 15 HIV-infected subjects.

RESULTS

Treatment was associated with significant decreases in the perception of abdominal bloating and cramps. Similar declines in RNA burden and rises in CD4+ lymphocyte counts were found in blood and mucosa. Treatment was also associated with a fall in the number of lamina propria mononuclear cells undergoing apoptosis by in situ labeling, a change that correlated with the change in mucosal viral burden.

CONCLUSIONS

Peripheral blood and mucosal compartments are equally responsive to effective antiretroviral therapies. The detection of significant changes within 7 days of starting antiviral therapy implies that intestinal dysfunction may be a direct result of local HIV infection.

Extensive diversification of human immunodeficiency virus type 1 subtype B strains during dual infection of a chimpanzee that progressed to AIDS

A chimpanzee (C-499) infected for more than 9 years with two subtype B isolates of human immunodeficiency virus type 1 (HIV-1), one (HIV-1(SF2)) that replicates poorly and one (HIV-1(LAV-1b)) that replicates efficiently in chimpanzees, died of AIDS 11 years after initial infection (F. J. Novembre et al., J. Virol. 71:4086-4091, 1997). Nucleotide sequence and phylogenetic analyses of the C2 to V5 region of env (C2-V5env) in proviral DNA from peripheral blood lymphocytes obtained 22 months before death revealed two distinct virus populations. One of these populations appeared to be a recombinant in env, having the V3 loop from HIV-1(SF2) and the V4-V5 region from HIV-1(LAV-1b); the other population had evolved from HIV-1(LAV-1b). In addition to C2-V5env, the entire p17gag and nef genes were sequenced; however, based on nucleotide sequences and phylogeny, whether the progenitor of the p17gag and nef genes was SF2 or LAV-1b could not be determined. Compared to the two original viruses, the divergence of all clones of C2-V5env ranged from 9.37 to 20.2%, that of p17gag ranged from 3.11 to 9.29%, and that of nef ranged from 4.02 to 7.9%. In contrast, compared to the maximum variation of 20.2% in C2-V5env for C-499, the maximum diversities in C2-V5env in proviruses from two chimpanzees infected with HIV-1(LAV-1b) for 9 and 10 years were 9.65 and 2.48%, respectively. These results demonstrate that (i) two distinct HIV-1 populations can coexist and undergo extensive diversification in chimpanzees with progressive HIV-1-induced disease and (ii) recombination between two subtype B strains occurred even though the second strain was inoculated 15 months after the first one. Furthermore, evaluation of env genes from three chimpanzees infected with the same strain suggests that the magnitude of HIV-1 diversification could be related to higher viral burdens, manifestations of disease, and/or dual infection.

Increased induction of apoptosis by a Sendai virus mutant is associated with attenuation of mouse pathogenicity

An avirulent mutant of Sendai virus, Ohita-MVC11 (MVC11), was generated from a highly virulent field strain, Ohita-M1 (M1), through successive passages in LLC-MK2 cell cultures (M. Itoh, Y. Isegawa, H. Hotta, and M. Homma, J. Gen. Virol. 78:3207-3215, 1997). In LLC-MK2 cells, MVC11 induced a high degree of apoptotic cell death that was demonstrated by chromatin condensation of the nucleus and DNA fragmentation, and production of MVC11 declined markedly after prolonged culture. On the other hand, M1 did not induce prominent apoptosis and maintained high virus titers. In primary mouse pulmonary epithelial cell cultures, M1 replicated rather slowly to reach maximum level of virus production at 3 days postinfection, and high levels of virus production were maintained thereafter without causing apoptosis. In contrast, MVC11, which produced 20 times more progeny virus than M1 at 1 day postinfection, induced a high degree of apoptotic cell death before the virus replication cycle was completed. Accordingly, the production of progeny virus was strongly inhibited thereafter. In the lungs of mice infected with MVC11, virus antigens and signals of DNA fragmentation detected by the in situ terminal deoxynucleotidyltransferase-mediated dUTP nick end-labeling technique colocalized in bronchial epithelial cells, clearly demonstrating that infection by MVC11 triggered apoptosis in vivo as well as in vitro. These results suggest the possibility that induction of apoptosis by MVC11 plays an important role in attenuation of mouse pathogenicity by restricting progeny virus production in the lung. The C protein was shown to have the capacity to induce apoptosis, and the increased level of the C protein in MVC11-infected cells was considered to account partly, if not entirely, for the induction of apoptosis.

Altered receptor expression and decreased sensitivity of T-cells to the stimulatory cytokines IL-2, IL-7 and IL-12 in HIV infection

A dysregulated production of regulatory cytokines has been proposed as a determinant in the progression of HIV infection. The sensitivity of T-cells to these cytokines has, however, not fully been investigated. Therefore, the responses of PBMC and T-cell subsets to the stimulatory cytokines IL-2, IL-7 and IL-12 in HIV-infected patients and HIV-negative controls were compared by examining their effect on the production of secondary cytokines (IFNgamma, IL-4 and IL-10), by simultaneous determination of T-cell activation and apoptosis and by measuring cytokine receptor expression. Production of IFNgamma was decreased in PBMC from the patients after stimulation with several combinations of stimulatory cytokines. IL-10 was only induced upon stimulation with IL-2 and IL-12 and tended to be produced more in patients. Expression of the different cytokine receptor chains showed complex alterations in HIV+ patients as compared to controls. The most pronounced changes were decreased expression of both IL-2Ralpha and IL-7Ralpha chain on CD8+ T-cells and an increase of IL-12Rbeta on both T-cell subsets from the patients. Evaluation of CD25 upregulation and blast formation revealed a deficient response to all three stimulatory cytokines in CD8+ but not in CD4+ T-cells from patients as compared to controls. Both CD4+ and CD8+ T-cells from the patients were less sensitive to the anti-apoptotic effect of IL-7 whereas only CD8+ T-cells were less sensitive to the anti-apoptotic effect of IL-2. The present data show that CD8+ T-cells, and to a lesser extent CD4+ T-cells, become less sensitive to IL-2, IL-7 and IL-12 during HIV infection. The decreased capacity of T-cells to respond to these cytokines could contribute to the HIV-related immune dysfunction.

Combination of whole blood culture and a rapid and sensitive cell assay for the determination of the cytopathogenicity of human immunodeficiency virus type-1 isolates

It has been reported that in vitro biological properties of human immunodeficiency virus type 1 (HIV-1) isolates from patients are correlated with the prognosis of HIV-1 infection. A rapid assay was developed to study the phenotype of HIV-1 isolates. The P4 cell line is a HIV-1 infectible Hela CD4 cell carrying the bacterial LacZ gene under the control of the HIV-1 LTR (long terminal repeat). Conventional peripheral blood mononuclear cells (PBMCs) co-culture and heparinized whole blood (HWB) co-culture with normal PBMCs were used for HIV-1 isolated strains from 17 HIV-1-infected patients. The sensitivity of P4 cells was higher than that of MT-2 cells for detecting syncytia induced by HIV-1LAI (lymphadenopathy-associated virus). Like MT-2 cells, P4 cells enable the detection of syncytium inducing strains isolated in peripheral blood mononuclear cells (PBMCs) and HWB cultures. HIV-1 isolates with both culture methods from certain patients induced cytolysis without syncytium in P4 cells but had no cytopathic effect on MT-2 cells. The experiments are in favour of the direct effect of HIV-1 isolates of these patients in the lysis of P4 cells but its mechanism has not been elucidated. It was shown that the combination of whole blood culture for HIV-1 isolation and phenotype study with P4 cell assay is rapid and sensitive and could be used to monitor HIV-1-infected patients.

Increase in Vdelta1+ gammadelta T cells in the peripheral blood and bone marrow as a selective feature of HIV-1 but not other virus infections

Dysregulation of T-cell receptor (TCR) alphabeta bearing lymphocytes and an increase in Vdelta1+ gammadelta T cells are typical features of HIV-1 infection. However, the role of gammadelta T cells remains unclear. Therefore, peripheral blood mononuclear cells (PBMC) of 103 HIV-1-infected patients were investigated with respect to expression of Vdelta1. These results were compared to the Vdelta1 expression of bone marrow mononuclear cells (BMMC). In contrast to healthy controls, Vdelta1+ cells dominated among both PBMC and BMMC in HIV-1-infected patients. Analysis of the coexpression of CD25, CD8, HLA-DR and CD45RO revealed a high prevalence of Vdelta1/CD45RO and Vdelta1/HLA-DR double-positive PBMC only in HIV-1-infected patients but not in healthy donors. Furthermore, analysis of the gammadelta TCR repertoire in patients infected with hepatitis B virus, hepatitis C virus, herpes simplex virus (HSV)-1 and HSV-2 showed that the selective enhancement of Vdelta1+ cells was restricted to HIV infection and not observed in other virus diseases. Our data provide further support for the involvement of gammadelta T cells in immunosuppression and progression of HIV infection.

Apoptosis of CD4+ and CD19+ cells during human immunodeficiency virus type 1 infection--correlation with clinical progression, viral load, and loss of humoral immunity

Enhanced rates of programmed cell death (apoptosis) have been detected in T cells and B cells from human immunodeficiency virus type 1 (HIV-1)-infected individuals. To evaluate the possible relevance of this event to HIV pathogenesis and disease progression, apoptosis in CD4+ T lymphocytes and CD19+ B lymphocytes, viral load, and neutralizing antibody titers were assayed in HIV-1-infected slow progressors and progressors. A correlation was found between progressive disease and apoptosis of CD4+ T cells. The extent of apoptosis in CD4+ cells was similar in slow progressors and seronegative control subjects. By contrast, we found elevated levels of B-cell apoptosis in all HIV-1-infected individuals compared with seronegative control subjects, with a tendency toward increased levels of apoptosis with progressive disease. Apoptosis in CD4+ T cells and CD19+ B cells correlated with viral RNA levels in plasma. Furthermore, higher rates of B-cell apoptosis were observed in individuals with poor neutralizing activity against a panel of six clinical HIV-1 isolates. From these results we conclude that the extent of apoptosis in cultured CD4+ cells and CD19+ cells appears to parallel the decline in CD4 cell counts in infected individuals. The finding of a relation between apoptosis in B cells and poor neutralizing capacity suggests that apoptosis may be related to loss of immune function. A role for apoptosis in the pathogenesis of AIDS is also supported by the strong correlation between viral load and rates of apoptosis in CD4+ T cells.

Mycobacterium bovis Bacillus Calmette Guérin infection prevents apoptosis of resting human monocytes

Apoptosis plays an essential role in the development and homeostasis of multicellular organisms. Some infectious agents interfere with this programmed cell death to their own benefit. Here, we show that infection of resting human monocytes with Mycobacterium bovis Bacillus Calmette Guérin (BCG) increases monocyte viability by preventing them from undergoing apoptosis. Heat-killed BCG also prevented apoptosis, indicating that replication of BCG is not required to prevent cell death. Analysis of BCG-infected monocytes revealed an up-regulation of the A1 mRNA, whereas the bcl-2 mRNA was not up-regulated. Interestingly, preinfection with BCG renders the cells resistant to interleukin (IL)-10-induced apoptosis which may be one of the mechanisms mycobacteria use to modulate immune responses. BCG infection was also accompanied by an impairment of the capacity of monocytes to secrete IL-10 and by an induction of the capacity to secrete tumor necrosis factor-alpha, two cytokines known to induce and prevent human monocyte apoptosis, respectively. Since it has been reported that apoptosis is involved in killing of intracellular mycobacteria, the prevention of apoptosis may represent a strategy for mycobacterial survival in the infected host.

A Role for T-Helper Type-1 and Type-2 Cytokines in the Regulation of Human Monocyte Apoptosis

T-helper type-1 (Th1) and type-2 (Th2) cytokines, respectively, favor T-cell–mediated immunity and defense against intracellular pathogens or antibody-mediated immunity and defense against extracellular pathogens. Here we report that type-1 and type-2 cytokines also exert a regulatory effect on human monocyte survival. Interleukin-12 (IL-12) enhanced survival in long-term (10 days) cultures of adherent monocytes, whereas IL-10 induced death by apoptosis. In short-term cultures (2 days), the Th2 cytokines, IL-10 and IL-4, enhanced apoptosis; however, the Th1 cytokines, IL-12 and IL-2 only showed a reducing effect on monocyte apoptosis in culture conditions that decreased monocyte adhesion leading to increased levels of spontaneous apoptosis; finally, the Th1 cytokine, interferon-γ (IFN-γ), acted in a dose-dependent fashion: At high concentrations, IFN-γ enhanced apoptosis, which is an effect related to IL-10 secretion and reduced by antibodies to IL-10. Th1 cytokines reduced monocyte apoptosis induced by several stimuli: IL-2 reduced apoptosis induced by either IL-10 or high concentrations of IFN-γ, IL-12 reduced apoptosis induced by either the ligation of the Fas (CD95) molecule or γ-irradiation, and IFN-γ (at low doses that did not trigger apoptosis) reduced apoptosis induced by γ-irradiation. These findings suggest that the regulatory role of type-1 and type-2 cytokines on the development of immune responses and inflammatory reactions also involves the regulation of monocyte death by apoptosis.

A Role for T-Helper Type-1 and Type-2 Cytokines in the Regulation of Human Monocyte Apoptosis

T-helper type-1 (Th1) and type-2 (Th2) cytokines, respectively, favor T-cell–mediated immunity and defense against intracellular pathogens or antibody-mediated immunity and defense against extracellular pathogens. Here we report that type-1 and type-2 cytokines also exert a regulatory effect on human monocyte survival. Interleukin-12 (IL-12) enhanced survival in long-term (10 days) cultures of adherent monocytes, whereas IL-10 induced death by apoptosis. In short-term cultures (2 days), the Th2 cytokines, IL-10 and IL-4, enhanced apoptosis; however, the Th1 cytokines, IL-12 and IL-2 only showed a reducing effect on monocyte apoptosis in culture conditions that decreased monocyte adhesion leading to increased levels of spontaneous apoptosis; finally, the Th1 cytokine, interferon-γ (IFN-γ), acted in a dose-dependent fashion: At high concentrations, IFN-γ enhanced apoptosis, which is an effect related to IL-10 secretion and reduced by antibodies to IL-10. Th1 cytokines reduced monocyte apoptosis induced by several stimuli: IL-2 reduced apoptosis induced by either IL-10 or high concentrations of IFN-γ, IL-12 reduced apoptosis induced by either the ligation of the Fas (CD95) molecule or γ-irradiation, and IFN-γ (at low doses that did not trigger apoptosis) reduced apoptosis induced by γ-irradiation. These findings suggest that the regulatory role of type-1 and type-2 cytokines on the development of immune responses and inflammatory reactions also involves the regulation of monocyte death by apoptosis.

Progressive B cell apoptosis and expression of Fas ligand during human immunodeficiency virus type 1 infection

HIV-1 infection causes functional defects in T cells. It also leads to a progressive reduction in numbers of such cells and both CD4+ and CD8+ cells have been reported to undergo apoptosis in culture. A corresponding reduction in B cells has not been described, but these cells are also functionally altered, with reports of polyclonal activation and hyporesponsiveness to antigenic and mitogenic stimuli. Here we investigated B cells from HIV-1-seropositive individuals and found that these cells, which are not the target for virus infection, died of apoptosis on culturing. We could also confirm previous findings that CD4+ cells from HIV-1-infected individuals undergo apoptosis in culture. Apoptosis of both B cells and CD4+ cells correlated inversely with CD4 cell counts. B cells from HIV-1-infected individuals were found to express Fas ligand, and the expression of this protein correlated with the levels of apoptosis in the same cells. Non-B cells, on the other hand, expressed increased levels of Fas but low levels of Fas ligand. These results are in line with suggestions that the Fas/Fas ligand pathway may trigger the increased levels of apoptosis observed in cells from HIV-1-infected individuals.

Evasion of cytotoxic T lymphocyte (CTL) responses by nef-dependent induction of Fas ligand (CD95L) expression on simian immunodeficiency virus-infected cells

Inoculation of macaques with live attenuated SIV strains has been shown to protect against subsequent challenge with wild-type SIV. The protective mechanism(s) remain obscure. To study the effect in more detail, we have investigated the role of virus-specific CTL responses in macaques infected with an attenuated SIV strain (pC8), which has a four-amino acid deletion in the nef gene, as compared with the wild-type SIVmac32H clone (pJ5). Cynomolgus macaques infected with pC8 were protected against subsequent challenge with pJ5 and did not develop any AIDS-like symptoms in the 12 months after infection. The pC8-induced protection was associated with high levels of virus-specific CTL responses to a variety of viral antigens. In contrast, pJ5-infected macaques had little, if any, detectable CTL response to the viral proteins after three months. The latter group of macaques also showed increased Fas expression and apoptotic cell death in both the CD4(+) and CD8(+) populations. In vitro, pJ5 but not pC8 leads to an increase in FasL expression on infected cells. Thus the expression of FasL may protect infected cells from CTL attack, killing viral-specific CTLs in the process, and providing a route for escaping the immune response, leading to the increased pathogenicity of pJ5. pC8, on the other hand does not induce FasL expression, allowing the development of a protective CTL response. Furthermore, interruption of the Fas-FasL interaction allows the regeneration of viral-specific CTL responses in pJ5-infected animals. This observation suggests an additional therapeutic approach to the treatment of AIDS.

Transgenic dissection of HIV genes involved in lymphoid depletion

Transgenic mice carrying an HIV provirus, with selective deletion of all three structural genes, developed extensive lymphoid depletion which was detected not only in the spleen and lymph nodes but also in the thymus. Mice with a high level of HIV gene expression developed acute disease which resulted in premature death, and mice with a low level of viral transcripts developed chronic disease with long-term survival. Neither HIV replication nor the envelope glycoprotein (gp120) was required for T cell depletion. Despite abundant viral gene expression early in life, cell death did not become evident until about the time of full lymphoid maturation, suggesting that thymopoiesis was not affected. The more mature T cells in the peripheral lymphoid organs and in the thymic medulla were less sensitive to the apoptotic process than the immature T cells in the thymic cortex. Gradual depletion of the T cell compartment in the peripheral lymphoid organs was intimately accompanied by the reciprocal expansion of the B cell compartment, resulting in the almost complete replacement of T lymphocytes with B immunoblasts in lymph nodes. Unlike T cells, which showed abundant HIV gene expression, B cells did not. The transgenic approach may help identify the HIV nonstructural gene(s) responsible for immune deficiency and help facilitate dissection of its role in inducing apoptosis.

Induction of CD4+ T cell depletion in mice doubly transgenic for HIV gp120 and human CD4

It has been suggested that loss of uninfected T cells in HIV infection occurs because of lymphocyte activation resulting in cell death by apoptosis. To address the question of whether cross-linking of CD4/HIV gp120 complexes by antibodies were sufficient to induce T cell depletion in vivo, we developed an animal model of continuous interaction between human CD4 (hCD4), gp120 and anti-gp120 antibodies in the absence of other viral factors. Double-transgenic mice have been generated in which T cells express on their membrane hCD4 and secrete HIV gp120. Although these mice have hCD4/gp120 complexes present on the surface of T cells, they do not show gross immunological abnormalities, and they are able to produce anti-gp120 antibodies following immunization with denaturated gp120. However, double-transgenic mice with antibodies to gp120, when immunized with tetanus toxoid, mount an IgG response that is significantly lower than that of double-transgenic mice without antibodies to gp120. Furthermore, the presence of anti-gp120 antibodies leads to CD4+ T cell depletion and immunodeficiency in the absence of HIV infection. Thus, the antibody response to gp120 can lead to CD4+ T cell attrition in vivo.