NF-κB pathway controls mitochondrial dynamics.

The Optic atrophy 1 protein (OPA1) is a key element in the dynamics and morphology of mitochondria. We demonstrated that the absence of IκB kinase-α, which is a key element of the nonclassical NF-κB pathway, has an impact on the mitochondrial network morphology and OPA1 expression. In contrast, the absence of NF-κB essential modulator (NEMO) or IκB kinase-ß, both of which are essential for the canonical NF-κB pathway, has no impact on mitochondrial dynamics. Whereas Parkin has been reported to positively regulate the expression of OPA1 through NEMO, herein we found that PARK2 overexpression did not modify the expression of OPA1. PARK2 expression reduced the levels of Bax, and it prevented stress-induced cell death only in Bak-deficient mouse embryonic fibroblast cells. Collectively, our results point out a role of the nonclassical NF-κB pathway in the regulation of mitochondrial dynamics and OPA1 expression.

Modulation of mammalian apoptotic pathways by intracellular protozoan parasites.

During intracellular parasitic infections, pathogens and host cells take part in a complex web of events that are crucial for the outcome of the infection. Modulation of host cell apoptosis by pathogens attracted the attention of scientists during the last decade. Apoptosis is an efficient mechanism used by the host to control infection and limit pathogen multiplication and dissemination. In order to ensure completion of their complex life cycles and to guarantee transmission between different hosts, intracellular parasites have developed mechanisms to block apoptosis and sustain the viability of their host cells. Here, we review how some of the most prominent intracellular protozoan parasites modulate the main mammalian apoptotic pathways by emphasizing the advances from the last decade, which have begun to dissect this dynamic and complex interaction.

Early divergence in lymphoid tissue apoptosis between pathogenic and nonpathogenic simian immunodeficiency virus infections of nonhuman primates.

The events that contribute to the progression to AIDS during the acute phase of a primate lentiviral infection are still poorly understood. In this study, we used pathogenic and nonpathogenic simian models of simian immunodeficiency virus (SIV) infection of rhesus macaques (RMs) and African green monkeys (AGMs), respectively, to investigate the relationship between apoptosis in lymph nodes and the extent of viral replication, immune activation, and disease outcome. Here, we show that, in SIVmac251-infected RMs, a marked increased in lymphocyte apoptosis is evident during primary infection at the level of lymph nodes. Interestingly, the levels of apoptosis correlated with the extent of viral replication and the rate of disease progression to AIDS, with higher apoptosis in RMs of Indian genetic background than in those of Chinese origin. In stark contrast, no changes in the levels of lymphocyte apoptosis were observed during primary infection in the nonpathogenic model of SIVagm-sab infection of AGMs, despite similarly high rates of viral replication. A further and early divergence between SIV-infected RMs and AGMs was observed in terms of the dynamics of T- and B-cell proliferation in lymph nodes, with RMs showing significantly higher levels of cycling cells (Ki67(+)) in the T-cell zones in association with relatively low levels of Ki67(+) in the B-cell zones, whereas AGMs displayed a low frequency of Ki67(+) in the T-cell area but a high proportion of Ki67(+) cells in the B-cell area. As such, this study suggests that species-specific host factors determine an early immune response to SIV that predominantly involves either cellular or humoral immunity in RMs and AGMs, respectively. Taken together, these data are consistent with the hypotheses that (i) high levels of T-cell activation and lymphocyte apoptosis are key pathogenic factors during pathogenic SIV infection of RMs and (ii) low T-cell activation and apoptosis are determinants of the AIDS resistance of SIVagm-infected AGMs, despite high levels of SIVagm replication.

CD4+ CCR5+ T-cell dynamics during simian immunodeficiency virus infection of Chinese rhesus macaques.

Simian immunodeficiency virus (SIV) infection of rhesus macaques (RMs) provides a reliable model to study the relationship between lentivirus replication, cellular immune responses, and CD4+ T-cell dynamics. Here we investigated, using SIVmac251-infected RMs of a Chinese genetic background (which experience a slower disease progression than Indian RMs), the dynamics of CD4+ CCR5+ T cells, as this subset of memory/activated CD4+ T cells is both a preferential target of virus replication and a marker of immune activation. As expected, we observed that the number of circulating CD4+ CCR5+ T cells decreases transiently at the time of peak viremia. However, at 60 days postinfection, i.e., when set-point viremia is established, the level of CD4+ CCR5+ T cells was increased compared to the baseline level. Interestingly, this increase correlated with faster disease progression, higher plasma viremia, and early loss of CD4+ T-cell function, as measured by CD4+ T-cell count, the fraction of memory CD4+ T cells, and the recall response to purified protein derivative. Taken together, these data show a key difference between the dynamics of the CD4+ CCR5+ T-cell pool (and its relationship with disease progression) in Chinese RMs and those described in previous reports for Indian SIVmac251-infected RMs. As the SIV-associated changes in the CD4+ CCR5+ T-cell pool reflect the opposing forces of SIV replication (which reduces this cellular pool) and immune activation (which increases it), our data suggest that in SIV-infected Chinese RMs the impact of immune activation is more prominent than that of virus replication in determining the size of the pool of CD4+ CCR5+ T cells in the periphery. As progression of HIV infection in humans also is associated with a relative expansion of the level of CD4+ CCR5+ T cells, we propose that SIV infection of Chinese RMs is a very valuable and important animal model for understanding the pathogenesis of human immunodeficiency virus infection.

TGF-beta in intestinal lymphoid organs contributes to the death of armed effector CD8 T cells and is associated with the absence of virus containment in rhesus macaques infected with the simian immunodeficiency virus.

SIV-infected macaques exhibit distinct rates of progression to AIDS and despite significant increases in CD8+ T cells, immune cells fail to control and eradicate SIV in vivo. Here, we investigated the interplay between viral reservoir sites, CD8+ T-cell activation/death and outcome. Our data provide strong evidence that mesenteric (Mes) lymph nodes represent major reservoirs not only for SIV-infected macaques progressing more rapidly toward AIDS but also in controllers. We demonstrate that macaques progressing faster display greater expression of TGF-beta and Indoleamine 2,3 dioxygenase in particular in intestinal tissues associated with a phosphorylation of the p53 protein on serine 15 in CD8+ T cells from Mes lymph nodes. These factors may act as a negative regulator of CD8+ T-cell function by inducing a Bax/Bak/Puma-dependent death pathway of effector/memory CD8+ T cells. Greater T-cell death and viral dissemination was associated with a low level of TIA-1+ expressing cells. Finally, we provide evidence that abrogation of TGF-beta in vitro enhances T-cell proliferation and reduces CD8+ T-cell death. Our data identify a mechanism of T-cell exhaustion in intestinal lymphoid organs and define a potentially effective immunological strategy for the modulation of progression to AIDS.

Inhibiting Drp1-mediated mitochondrial fission selectively prevents the release of cytochrome c during apoptosis.

Most cell death stimuli trigger the mitochondrial release of cytochrome c and other cofactors that induce caspase activation and ensuing apoptosis. Apoptosis is also associated with massive mitochondrial fragmentation and cristae remodeling. Dynamin-related protein 1 (Drp1), a protein of the mitochondrial fission machinery, has been reported to participate in apoptotic mitochondrial fragmentation. Several theories explaining the mechanisms of cytochrome c release have been proposed. One suggests that it relies on the activation of Drp1-mediated mitochondrial fission. Here, we report that downregulation of Drp1 inhibits fragmentation of the mitochondrial network and partially prevents the release of cytochrome c but fails to prevent the release of other mitochondrial factors such as second mitochondria-derived activator of caspase/direct IAP-binding protein with low pI, Omi/HtrA2, adenylate kinase 2 and deafness dystonia peptide/TIMM8a. An explanation for the prevention of cytochrome c release is provided by our observation that inhibiting Drp1-mediated mitochondrial fission prevents the mitochondrial release of soluble OPA1 that was proposed to regulate cristae remodeling and complete cytochrome c release during apoptosis. Finally, we observed that downregulation of Drp1 delays but does not inhibit apoptosis, suggesting that mitochondrial fragmentation is not a prerequisite for apoptosis.

Dissociation of caspase-mediated events and programmed cell death induced via HLA-DR in follicular lymphoma.

Human leukocyte antigens (HLA) class II antigen-mediated apoptosis has been documented in antigen-presenting cells and B lymphoproliferations. Characteristics of the apoptosis include rapidity and selectivity for mature cells. Follicular lymphomas are particularly refractory to apoptosis. The B-cell lymphoma Ramos shares characteristics of this subgroup and is insensitive to apoptosis via simple HLA-DR engagement. However, oligomerization of HLA-DR antigens induced caspase activation followed by phosphatidylserine externalization, activation of PKC-delta and cleavage of nuclear lamin B. Mitochondrial injury was also detected. However, inhibition of caspase activation simply delayed the apoptotic phenotype but neither protected against cell death nor prevented mitochondrial injury. The data in this report demonstrate that the requirements for the initiating signal (oligomerization versus engagement) as well as the molecular pathways varies between different B lymphoproliferations despite their common expression of HLA-DR. Finally, blockade of caspase activation in parallel with HLA-DR mAb stimulation could provide a potent autovaccination stimulus by leading to necrotic death of B-cell lymphomas.

Apoptosis in SIV infection.

Pathogenic human immunodeficiency virus (HIV)/Simian immunodeficiency virus (SIV) infection is associated with increased T-cell apoptosis. In marked contrast to HIV infection in humans and SIV infection in macaques, the SIV infection of natural host species is typically nonpathogenic despite high levels of viral replication. In these nonpathogenic primate models, no observation of T-cell apoptosis was observed, suggesting that either SIV is less capable of directly inducing apoptosis in natural hosts (likely as a result of coevolution/coadaptation with the host) or, alternatively, that the indirect T-cell apoptosis plays the key role in determining the HIV-associated T-cell depletion and progression to acquired immune deficiency syndrome (AIDS). Understanding the molecular and cellular mechanisms responsible for the disease-free equilibrium in natural hosts for SIV infection, including those determining the absence of high levels of T-cell apoptosis, is likely to provide important clues regarding the mechanisms of AIDS pathogenesis in humans.

A novel mechanism for HIV1-mediated bystander CD4+ T-cell death: neighboring dying cells drive the capacity of HIV1 to kill noncycling primary CD4+ T cells.

CD4+ T-cell death is a crucial feature of AIDS pathogenesis, but the mechanisms involved remain unclear. Here, we present in vitro findings that identify a novel process of HIV1 mediated killing of bystander CD4+ T cells, which does not require productive infection of these cells but depends on the presence of neighboring dying cells. X4-tropic HIV1 strains, which use CD4 and CXCR4 as receptors for cell entry, caused death of unstimulated noncycling primary CD4+ T cells only if the viruses were produced by dying, productively infected T cells, but not by living, chronically infected T cells or by living HIV1-transfected HeLa cells. Inducing cell death in HIV1-transfected HeLa cells was sufficient to obtain viruses that caused CD4+ T-cell death. The addition of supernatants from dying control cells, including primary T cells, allowed viruses produced by living HIV1-transfected cells to cause CD4+ T-cell death. CD4+ T-cell killing required HIV1 fusion and/or entry into these cells, but neither HIV1 envelope-mediated CD4 or CXCR4 signaling nor the presence of the HIV1 Nef protein in the viral particles. Supernatants from dying control cells contained CD95 ligand (CD95L), and antibody-mediated neutralization of CD95L prevented these supernatants from complementing HIV1 in inducing CD4+ T-cell death. Our in vitro findings suggest that the very extent of cell death induced in vivo during HIV1 infection by either virus cytopathic effects or immune activation may by itself provide an amplification loop in AIDS pathogenesis. More generally, they provide a paradigm for pathogen-mediated killing processes in which the extent of cell death occurring in the microenvironment might drive the capacity of the pathogen to induce further cell death.

Caspase-dependent and -independent T-cell death pathways in pathogenic simian immunodeficiency virus infection: relationship to disease progression.

Studies of human immunodeficiency virus (HIV) and nonhuman primate models of pathogenic and nonpathogenic simian immunodeficiency virus (SIV) infections have suggested that enhanced ex vivo CD4 T-cell death is a feature of pathogenic infection in vivo. However, the relative contributions of the extrinsic and intrinsic pathways to programmed T-cell death in SIV infection have not been studied. We report here that the spontaneous death rate of CD4+ T cells from pathogenic SIVmac251-infected rhesus macaques ex vivo is correlated with CD4 T-cell depletion and plasma viral load in vivo. CD4+ T cells from SIVmac251-infected macaques showed upregulation of the death ligand (CD95L) and of the proapoptotic proteins Bim and Bak, but not of Bax. Both CD4+ and CD8+ T cells from SIVmac251-infected macaques underwent caspase-dependent death following CD95 ligation. The spontaneous death of CD4+ and CD8+ T cells was not prevented by a decoy CD95 receptor or by a broad-spectrum caspase inhibitor (zVAD-fmk), suggesting that this form of cell death is independent of CD95/CD95L interaction and caspase activation. IL-2 and IL-15 prevented the spontaneous death of CD4+ and CD8+ T cells, whereas IL-10 prevented only CD8 T-cell death and IL-7 had no effect on T-cell death. Our results indicate that caspase-dependent and caspase-independent pathways are involved in the death of T cells in pathogenic SIVmac251-infected primates.

Distinct cycling CD4(+)- and CD8(+)-T-cell profiles during the asymptomatic phase of simian immunodeficiency virus SIVmac251 infection in rhesus macaques.

Elevated CD4 T-cell turnover may lead to the exhaustion of the immune system during human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus (SIV) infections. However, this hypothesis remains controversial. Most studies of this subject have concerned the blood, and information about the lymph nodes is rare and controversial. We used Ki67 expression to measure cycling T cells in the blood and lymph nodes of uninfected macaques and of macaques infected with a pathogenic SIVmac251 strain or with a nonpathogenic SIVmac251Deltanef clone. During the asymptomatic phase of infection, the number of cycling CD8(+) T cells progressively increased (two- to eightfold) both in the blood and in the lymph nodes of macaques infected with SIVmac251. This increase was correlated with viral replication and the progression to AIDS. In contrast, no increases in the numbers of cycling CD4(+) T cells were found in the blood or lymph nodes of macaques infected with the pathogenic SIVmac251 strain in comparison with SIVmac251Deltanef-infected or healthy macaques during this chronic phase. However, the lymph nodes of pre-AIDS stage SIVmac251-infected macaques contained more cycling CD4(+) T cells (low baseline CD4(+)-T-cell counts in the blood). Taken together, these results show that the profiles of CD4(+)- and CD8(+)-T-cell dynamics are distinct both in the lymph nodes and blood and suggest that higher CD4(+)-T-cell proliferation at the onset of AIDS may lead to the exhaustion of the immune system.

[HIV1-associated CD4 T lymphocyte apoptosis]. / Infection par le VIH1 et apoptose lymphocytaire T CD4.

PURPOSE: Apoptosis during HIV infection has been evoked for ten years. The role of apoptosis during HIV infection have be confirmed by several authors but the exact relationships between viral replication, apoptosis and lymphocyte depletion remain to be clarified. CURRENT KNOWLEDGE AND KEY POINTS: HIV may induce apoptosis of infected but also of uninfected bystander CD4+ lymphocytes. Those two types of HIV induced apoptosis lie on different pathways. While Fas and FasL are involved in apoptosis of bystander cells, mitochondrial pathway is required for apoptosis of infected cells. Cytokines but also anti HIV drugs may modulate HIV-induced lymphocyte apoptosis. Morever while protease inhibitor influence HIV replication and then secondary apotosis of infected cells, they can also interfere with spontaneous apoptosis of lymphocyte beside the context of HIV infection. FUTURES AND PROJECTS: Apoptosis is thought to be one of the mechanism involved in CD4 T lymphocyte cell death during HIV infection. However relationships between apoptosis and HIV replication may be more complex. In fact it has been recently reported that while HIV replication induced lymphocyte apoptosis, apoptosis may in turn induced HIV replication in a loop amplification pathway

On the evolution of programmed cell death: apoptosis of the unicellular eukaryote Leishmania major involves cysteine proteinase activation and mitochondrion permeabilization.

Leishmania major is a protozoan parasite from one of the most ancient phylogenic branches of unicellular eukaryotes, and containing only one giant mitochondrion. Here we report that staurosporine, that induces apoptosis in all mammalian nucleated cells, also induces in L. major a death process with several cytoplasmic and nuclear features of apoptosis, including cell shrinkage, phosphatidyl serine exposure, maintenance of plasma membrane integrity, mitochondrial transmembrane potential (DeltaPsim) loss and cytochrome c release, nuclear chromatin condensation and fragmentation, and DNA degradation. Nuclear apoptosis-like features were prevented by cysteine proteinase inhibitors, and cell free assays using dying L. major cytoplasmic extracts indicated that the cysteine proteinases involved (i) also induced nuclear apoptosis-like features in isolated mammalian nuclei, and (ii) shared at least two nuclear substrates, but no cleavage site preference, with human effector caspases. Finally, isolated L. major mitochondria released cytochrome c and cysteine proteinases with nuclear pro-apoptotic activity when incubated with human recombinant Bax, even (although much less efficiently) when Bax was deleted of its transmembrane domain required for insertion in mitochondrial outermembranes, implying that L. major mitochondrion may express proteins able to interact with Bax. The recruitment of cysteine proteinases and mitochondria to the cell death machinery may be of very ancient evolutionary origin. Alternately, host/parasite interactions may have exerted selective pressures on the cell death phenotype of kinetoplastid parasites, resulting in the more recent emergence of an apoptotic machinery through a process of convergent evolution.

Programmed cell death in mature erythrocytes: a model for investigating death effector pathways operating in the absence of mitochondria.

Human mature erythrocytes have been considered as unable to undergo programmed cell death (PCD), due to their lack of mitochondria, nucleus and other organelles, and to the finding that they survive two conditions that induce PCD in vitro in all human nucleated cells, treatment with staurosporine and serum deprivation. Here we report that mature erythrocytes can undergo a rapid self-destruction process sharing several features with apoptosis, including cell shrinkage, plasma membrane microvesiculation, phosphatidylserine externalization, and leading to erythrocyte disintegration, or, in the presence of macrophages, to macrophage ingestion of dying erythrocytes. This regulated form of PCD was induced by Ca(2+) influx, and prevented by cysteine protease inhibitors that allowed erythrocyte survival in vitro and in vivo. The cysteine proteinases involved seem not to be caspases, since (i) proforms of caspase 3, while present in erythrocytes, were not activated during erythrocyte death; (ii) cytochrome c, a critical component of the apoptosome, was lacking; and (iii) cell-free assays did not detect activated effectors of nuclear apoptosis in dying erythrocytes. Our findings provide the first identification that a death program can operate in the absence of mitochondria. They indicate that mature erythrocytes share with all other mammalian cell types the capacity to self-destruct in response to environmental signals, and imply that erythrocyte survival may be modulated by therapeutic intervention.

Role of CD95-activated caspase-1 processing of IL-1beta in TCR-mediated proliferation of HIV-infected CD4(+) T cells.

CD95 plays a critical role in the homeostasis of the immune system, and has been reported to participate in T cell death during HIV infection. Here we report that the response to CD3-TCR stimulation of CD4(+) T cells from HIV-infected individuals and CD4(+) T cells from healthy donors incubated in vitro with HIV-1(Lai) depends on the manner the CD3-TCR complex is engaged. While stimulation by anti-CD3 antibodies in solution induced CD4 T cell apoptosis both in the absence or presence of anti-CD95 antibodies, stimulation by immobilized anti-CD3 antibodies rendered CD4(+) T cells resistant to CD95-mediated death and led to increased CD4 T cell proliferation in response to CD95 ligation. CD95 ligation of CD4(+) T cells led to the activation of caspases, while costimulation induced by anti-CD3 and anti-CD95 mAb prevented the full processing of caspase-3 and caspase-8. Proliferation of CD4(+) T cells induced by CD3-TCR and CD95 costimulation was decreased by treatments with a caspase-1 inhibitor or with neutralizing antibodies to IL-1ss, indicating a requirement for caspase-1-mediated IL-1beta processing and secretion. Our findings suggest a novel mechanism whereby in addition to its role in inducing T cell apoptosis, CD95 signaling during HIV infection may also provide a costimulatory signal leading to an enhancement of CD4 T cell proliferation in response to CD3-TCR complex engagement.

On the evolutionary conservation of the cell death pathway: mitochondrial release of an apoptosis-inducing factor during Dictyostelium discoideum cell death.

Mitochondria play a pivotal role in apoptosis in multicellular organisms by releasing apoptogenic factors such as cytochrome c that activate the caspases effector pathway, and apoptosis-inducing factor (AIF) that is involved in a caspase-independent cell death pathway. Here we report that cell death in the single-celled organism Dictyostelium discoideum involves early disruption of mitochondrial transmembrane potential (DeltaPsim) that precedes the induction of several apoptosis-like features, including exposure of the phosphatidyl residues at the external surface of the plasma membrane, an intense vacuolization, a fragmentation of DNA into large fragments, an autophagy, and the release of apoptotic corpses that are engulfed by neighboring cells. We have cloned a Dictyostelium homolog of mammalian AIF that is localized into mitochondria and is translocated from the mitochondria to the cytoplasm and the nucleus after the onset of cell death. Cytoplasmic extracts from dying Dictyostelium cells trigger the breakdown of isolated mammalian and Dictyostelium nuclei in a cell-free system, and this process is inhibited by a polyclonal antibody specific for Dictyostelium discoideum apoptosis-inducing factor (DdAIF), suggesting that DdAIF is involved in DNA degradation during Dictyostelium cell death. Our findings indicate that the cell death pathway in Dictyostelium involves mitochondria and an AIF homolog, suggesting the evolutionary conservation of at least part of the cell death pathway in unicellular and multicellular organisms.

Gamma-interferon induces apoptosis of the B lymphoma WEHI-279 cell line through a CD95/CD95L-independent mechanism.

Gamma-interferon (IFN-gamma) a cytokine produced by CD4+ T helper type 1 cells, CD8+ T cells and natural killer (NK) cells, plays a central role in the development of humoral and cell-mediated immunity. IFN-gamma participates in the maturation and differentiation of B cells, but it has been previously reported that IFN-gamma may inhibit the early stages of B cell activation. We report that the inhibition of the B lymphoma cell WEHI-279-proliferation induced by IFN-gamma, involves the induction of typical features of apoptosis (nuclear chromatin condensation and fragmentation, cell shrinkage, phosphatidyl-serine (PS) exposure and mitochondrial membrane potential (delta psim) loss). IFN-gamma-mediated B cell apoptosis was decreased by the addition of the T helper type 2 cytokine, IL-4. WEHI-279 cells express CD95 and undergo apoptosis after treatment with either an agonistic anti-CD95 Ab or with a soluble recombinant CD95L. However, incubation with CD95-Fc or TRAIL-R1-Fc fusion proteins, did not prevent IFN-gamma-mediated apoptosis, suggesting that IFN-gamma-mediated apoptosis occurs independently of CD95/CD95L and TRAIL-R/TRAIL interactions. IFN-gamma-mediated apoptosis is associated with caspase-3 activation that can be prevented by the addition of the broad caspase inhibitor zVAD-fmk. These data indicate that IFN-gamma may play a major role in the regulation of B cell apoptosis, and suggest the involvement of an alternative pathway which is independent of the death receptors.

Possible mechanism of toxicity of zidovudine by induction of apoptosis of CD4+ and CD8+ T-cells in vivo.

Some HIV-infected patients have a discordant response to highly active antiretroviral therapy with a low virus load and an incomplete restoration of CD4+ T-cell counts. Zidovudine may limit CD4+ restoration by a hematotoxic mechanism. Apoptosis and T-cell counts were assessed in two patients before and after they switched from zidovudine to stavudine. Whereas CD4+ T-cell apoptosis fell from 52% and 66% before the zidovudine switch to 7% and 12%, respectively, after the switch, the patients' CD4+ counts rose gradually to +183 and +150 cells, respectively. It was therefore hypothesized that zidovudine directly induced apoptosis. Zidovudine withdrawal could be tested before immunological interventions such as interleukin-2 therapy are considered.

Early changes in peripheral blood T cells during primary infection of rhesus macaques with a pathogenic SIV.

Primary infection of rhesus macaques with pathogenic strains of simian immunodeficiency virus (SIV) leads to rapid and dynamic changes in both viral load and T cell counts in the peripheral blood. We have performed a sequential analysis of peripheral blood CD4 and CD8 T cells in five macaques during the 8 weeks following SIVmac251 infection. We observed a transient lymphopenia of both CD4 and CD8 T cells during the first 2 weeks, followed by a rebound. The primary phase of infection was associated with changes in the T cells expressing CD25, CD69, or HLA-DR and with a priming of the peripheral blood CD4 and CD8 T cells for a process of apoptosis in vitro that was enhanced by CD95 (Fas) ligation, and was detected in two macaques as early as 7 days after infection. Despite the small numbers of animals studied, the importance of the early transient CD4 and CD8 T lymphopenia was positively correlated with the viral load. No correlation was found, however, between the level of activation markers expressed or of priming for apoptosis in peripheral blood T cells and the viral load. Our findings suggest the possibility that the early activation and priming for apoptosis of CD4 and CD8 T cells may involve indirect, host-related, mechanisms, or alternatively, that the T cells that remain in the peripheral blood during primary infection do not adequately reflect the viral-mediated changes in T cell activation and death that may occur in the lymphoid organs throughout the body.