Introduction

Despite the fact that new cases of AIDS are declining in thedeveloped world, HIV infection and the spread of the globalAIDS epidemic still present enormous medical, social andeconomic problems. It has been estimated that in 1997 therewere 5.8 million new cases of HIV infection, and that in thesame year 2.3 million people died of AIDS.1 Powerful anti-retroviral therapies have succeeded in increasing the timebetween infection with HIV and the progression to AIDS,although current therapies cannot eliminate the virus frominfected individuals and are, in any case, out of reach of thevast majority of people in the less-developed world whomake up the bulk of newly infected individuals.

The causative agent of AIDS is the human immuno-deficiency virus (HIV). It principally infects Th cells andcells of the monocyte/macrophage lineage, which expressthe CD4 cell surface protein. The gradual and selective loss of the CD4+ subset of T lymphocytes is the centralfeature of the pathogenesis of HIV because it correlateswith the progression from asymptomatic HIV infection toAIDS (review2). Although the precise cause of this loss of CD4+ T cells is unknown, perhaps the most popularhypothesis is that the virus primes the cell for apoptotic celldeath.3

Here, we review the experiments that argue that T cells aredepleted via the induction of apoptosis, and discuss thesefindings in the context of direct versus indirect killing byHIV.

Direct versus indirect depletion of CD4+ T cells in HIV-infected individuals: an overview

Initial measurements of HIV in the blood of infectedindividuals revealed that the number of infected T cells isvery low, typically less than 0.01% of the total peripheralblood mononuclear cells, and of cells in infected lymphnodes.4 These measurements led to the assumption thatinfection of CD4+ T cells by the virus could not account forthe loss of this population and that therefore there must be anindirect mechanism to account for such a loss. This viewgained experimental support in studies of lymph node biop-sies taken from HIV-infected individuals.5,6 It was reasonedthat because the vast majority of T cells reside in lymphoidtissues, and only a small proportion is present in the periph-eral circulation, then these sites are more relevant to studyingthe pathogenesis of HIV infection. In addition these are alsothe primary sites of antigen presentation and lymphocyteactivation, both processes assumed to be important in thepathogenesis of HIV towards T cells. The studies of lymphnode biopsies revealed that cells harbouring viral RNA rarelyshowed morphological or histochemical features of apoptosis(see following). Instead, apoptotic cells were often observednear to cells harbouring virus. This led to the so-calledbystander theory of CD4+ T cell killing in which an HIV-infected cell kills those nearby via an indirect mechanism.

In 1995 two papers were published concurrently whichshowed that there was a very high rate of virus productionthroughout the course of HIV infection, and consequently avery high turnover of virus-producing cells.7,8 Both groupsestimated a CD4+ T cell turnover of ~ 2 × 109 cells/day. Thesedata implied that it is possible for HIV killing of the infectedT cell to account for the eventual depletion of the CD4+ T cellpopulation, even with a very low proportion of cells beinginfected at any one time, and reopened the debate aboutwhether CD4+ T cell depletion is a direct or indirect conse-quence of viral infection.

Immunology and Cell Biology (1999) 77, 90–98

Special Feature

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

ANTHONY JAWOROWSKI and SUZANNE M CROWE

AIDS Pathogenesis Research Unit, Macfarlane Burnet Centre for Medical Research, National Centre for HIVVirology Research, Fairfield, Victoria, Australia

Summary The central pathogenic feature of AIDS is the dramatic loss of CD4+ lymphocytes. Despite more than adecade of intense research, the exact mechanism by which HIV causes this is still not understood. A major model forT cell depletion, proposed originally by Ameison and Capron in a report published in 1991, is that HIV sensitizesCD4+ T cells for activation-induced apoptosis. The apoptotic model of T cell depletion is discussed, and experimentsthat address the questions of whether apoptosis is restricted to infected cells or ‘bystander’T cells, and whether T cellapoptosis requires participation of separate HIV-infected haematopoietic cell populations, are reviewed.

Key words: acquired immunodeficiency syndrome, apoptosis, CD4+ T-lymphocytes, CD95, monocyte/macrophages.

Correspondence: Dr A Jaworowski, Macfarlane Burnet Centre forMedical Research, PO Box 254, Fairfield, Vic. 3078, Australia.Email: <anthonyj@burnet.edu.au>

Received 23 October 1998; accepted 23 October 1998.

Recently, two groups have used reporter constructs todirectly examine the question of whether apoptosis incultured lymphocytes in vitro occurs in infected T cells orbystander cells. Gandhi et al. used an HIV-placental alkalinephosphatase (PLAP) reporter construct to infect both T celllines and purified CD4+ T cells in vitro.9 They showed thatapoptosis was induced primarily in infected cells (i.e. thosecells expressing PLAP). Furthermore, apoptosis did notappear to involve the Fas pathway (see following), nor wasFas expression increased in those cells expressing PLAP.These data are consistent with recent reports in which CD4+

T cells were infected with HIV in vitro.10,11 Herbein et al.have used green fluorescent protein (GFP) technology toshow that apoptosis in cultured PBMC infected with HIVoccurred at a significantly higher rate in cells harbouringvirus.12 In lymphocyte cultures, the apoptosis was predomi-nantly in CD4+ T cells; however, when lymphocytes were co-cultured with monocyte-derived macrophages (MDM),apoptosis was observed in both CD4- and CD8-positivepopulations and dramatically increased the rate of apoptosisin GFP– (i.e. bystander) but not GFP+ (HIV-infected) cells.These results suggest that the lack of apoptosis seen inbystander cells by Gandhi et al. may be due to the fact thatthe experiments were conducted with cultures depleted ofmonocytes. Taken together, these findings strongly suggestthat, in cultures infected in vitro, apoptosis occurs primarilyin infected cells but that in vivo the situation is far morecomplex, and that additional mechanisms exist to causeapoptosis of non-infected T cells.

The question of whether T cell killing occurs via a director indirect mechanism has more than academic significance:it has been pointed out that if the bystander theory is correct,then strategies for interfering with this mechanism could bedesigned which are not targeted towards inhibiting virusreplication.12 Such treatments may circumvent the problem ofthe rapid acquisition of resistance by HIV to drugs thatinhibit its replication.

Apoptotic and non-apoptotic mechanisms for T cell loss

There have been many hypotheses proposed to account forthe T cell loss consequent to HIV infection which involveeither direct effects of the virus on the infected T cells orindirect effects. Early models based on studies of thecytopathology of HIV towards T cell lines proposed that thecytopathic effects of the virus are due to its ability to promotesyncytium formation between cells expressing CD4 and thoseexpressing envelope glycoprotein.13–16 Because HIV-infectedcells did not form syncytia with CD8+ or CD16+ cells, andmany individual CD4+ cells were involved in syncytiumformation, it was felt that this effect could contribute to both the specificity and the stoichiometry of CD4+ celldepletion.16 Other workers, however, showed that HIV wascytopathic towards certain T cell lines in the absence ofsyncytium formation.17,18 Alternatively, the finding that HIVcauses the selective loss of T cells bearing a common set ofVβ regions has led to the suggestion that a virus-encodedsuperantigen may play a role in pathogenesis.19,20 Othermodels of viral pathogenesis have included immuno-supression by conventional cytotoxic T cell-mediatedimmunopathology,21 the removal of T cells by cytotoxic

T lymphocytes, natural killer cell or antibody-dependentcytotoxicity,22 the generation of autoimmune antibodies toclass II β chains which share regions of homology to HIVenvelope protein,23,24 and the chronic down-modulation ofCD4 leading to T cell anergy.25

Perhaps the most widely canvassed and studied potentialmechanism for T cell depletion is that of induction of apoptosis. In 1986, Zagury et al. reported the first long-termcultures of HIV-infected T cells and showed that cell deathoccurred in such cultures following immunological stimu-lation with phytohaemagglutinin.26 They proposed thatactivation in vivo, by antigen, would result in virus produc-tion and cell death of the infected T cells. In 1991, severalgroups reported that loss of HIV-infected T cells wasaccompanied by DNA fragmentation associated with apop-totic cell death, in experiments using both T cells from HIV-infected individuals cultured ex vivo,27,28 CD4+ T cell-enriched PBL29 or PBMC and a T cell line infected in vitro.30

This led to the formulation of the hypothesis that HIV infec-tion results in inappropriate induction of apoptosis in vivo inresponse to antigenic stimulation, such that mature T cellsundergo a similar pathway to that responsible for the removalof autoreactive T cells during thymic selection.3 That apop-tosis may have a direct role in T cell depletion has beeninferred from studies of HIV-2: this virus is associated witha milder clinical course than HIV-1, and also has lesspronounced effects on spontaneous T cell apoptosis.31

Measurement of apoptosis of T cells in vivo during HIVinfection

Apoptosis, or programmed cell death, refers to a form of celldeath characterized morphologically by the condensation ofcytoplasm and nucleus and margination of chromatin in thedying cell.32 Biochemically it is characterized by the activa-tion of aspartate-directed proteases or ‘caspases’, and is oftenaccompanied by internucleosomal cleavage of chromatin toyield a characteristic laddering pattern of cellular DNAfollowing analysis by agarose gel electrophoresis. One of theearliest detectable manifestations of the dying cell is expo-sure of phosphatidylserine residues on the outer leaflet of theplasma membrane which constitutes a recognition signal formacrophages to engulf the dying cell.33 If the cell is notphagocytosed at this stage, its contents are packaged intoapoptotic bodies which can also be ingested by phagocytes.Thus at no stage is the integrity of the plasma membranecompromised, and the most important feature of apoptosis isthat the cellular contents of the dying cell are phagocytosed,usually by macrophages,34 without spilling into the sur-rounding tissues and generating an inflammatory reaction.These features are in sharp contrast to necrosis, where celldeath is accompanied by the loss of integrity of the plasmamembrane, leakage of intracellular contents to the surround-ing tissue and the generation of an inflammatory response.

The efficient phagocytosis of apoptotic cells and remnantsin vivo means that it is difficult to detect the occurrence ofsuch a process, and the role of apoptosis in HIV pathogenesishas usually been inferred from experiments performed invitro. Apoptotic cells have been detected in vivo, followingHIV infection, under special circumstances: in animalmodels in which human thymus is implanted into severe

AIDS and apoptosis 91

combined immunodeficient (SCID) mice, a wave of apop-totic thymocytes was observed following injection of HIVinto the implanted tissue which correlated with sites of activeviral replication.35,36 Su et al. reported that, in this model,apoptosis occurred in thymocytes at various stages of differ-entiation but that most of these cells were uninfected.37

Jamieson et al., however, have recently reported somewhatdifferent findings in this model: using a combination ofTUNEL, annexin V and 7-aminoacridine staining of single-cell suspensions derived from HIV-infected thymuses, theyreport low levels of apoptosis at a time when there is a highrate of CD4+ T cell depletion, and a high proviral burden.38

In agreement with Su et al. they found apoptotic cells at latestages of CD4+ T cell depletion, but these cells were identi-fied as predominantly from the CD4– CD8+ subset. Theypropose that in this model, death of CD4+ cells occurs bydirect killing of infected thymocytes rather than apoptosis,which is predominantly due to indirect effects on CD8+ cells,possibly by destruction of the cytokine network.

Two widely quoted papers have reported histochemicalstudies of lymph nodes from HIV infected individuals.5,6

Finkel et al. analysed lymph node biopsies from four HIV-infected children and a simian immunodeficiency virus(SIV)-infected macaque. They reported the detection of manyapoptotic nuclei using an in situ nick-translation detectionsystem for DNA fragments, but very few of these containedviral RNA. Apoptotic nuclei were often interspersed in areaswithin ‘viral clouds’. The apoptotic cells were identified aspredominantly CD3+ T cells. A correlation was observedbetween the number of apoptotic cells in a given field and thenumber of productively infected cells in that field. The sameworkers also observed a trend towards diminished apoptosiswith anti-retroviral treatment and a correlation betweendisease severity and apoptosis.39 In a similar study, Muro-Cacho et al. studied lymph node biopsies obtained from 29adults infected and 29 adults not infected with HIV.5 Using insitu end-labelling of fragmented DNA (TUNEL), theyobserved a three-to-four-fold increase in the number ofapoptotic cells/mm2 in lymph nodes from HIV-infectedindividuals compared to uninfected individuals. Unfortu-nately, immunohistochemical identification of the apoptoticcells was made only after in vitro culture, where both B andT (CD4+ and CD8+) cells were identified. In contrast to thestudies of Finkel et al., the level of apoptosis in the nodesfrom HIV-infected individuals did not correlate with eitherthe viral burden or the stage of disease; however, a correla-tion with the state of activation, determined histologically asthat of follicular hyperplasia and germinal centre formation,was observed. They proposed that the apoptosis of T cells isa consequence of chronic immune activation and, as such,represents an epiphenomenon rather than the pathogenicmechanism of HIV infection.

These data favour the model that most of the apoptosisobserved is an indirect effect of HIV-infected cells (whichmay involve cell–cell contact or the production of cytotoxicHIV-encoded proteins) on ‘bystander’ cells. From the discus-sion in the previous section it is clear that analysis ofapoptotic cells in models of HIV infection in vivo have so farnot resolved the question of whether apoptosis is the cause of the depletion of CD4+ T cells which is the major patho-genic feature of AIDS. In this context, Jamieson et al. have

calculated that in the SCID mouse model of HIV infectionthe rate of T cell loss, at its peak, is 10% per day.38 At thisrate, the level of apoptosis would be only 2–4% above back-ground: too low to detect experimentally.

Apoptosis of T cells cultured from HIV-infectedindividuals

Most studies have concentrated on determining whether HIVinfection induces apoptosis in T cells infected in vitro orwhether the rate of apoptosis of T cells obtained from HIV-infected blood and cultured in vitro is higher than that of T cells obtained from uninfected donors. Soon after Ameisenand Capron proposed that HIV results in the inappropriateinduction of activation-induced cell death,3 experimentalevidence was obtained supporting this hypothesis. Severallaboratories published data showing that T cells cultured exvivo from HIV-infected donors showed higher levels ofapoptosis compared to those obtained from seronegativeindividuals.40–44 This increase in apoptosis was seen in bothstimulated T cells40,41 (i.e. activation-induced apoptosis), andin unstimulated T cells41–44 (i.e. spontaneous apoptosis),although in one report an increase in spontaneous apoptosiswas not observed.40 Apoptosis was generally found to occurin both CD4+-depleted and CD8+-depleted T cell populations.In the model proposed by Ameisen and Capron, the primingof T cells for programmed cell death may occur followingcrosslinking of CD4 by gp120 immune complexes,3 whichactivates the T cell without appropriate costimulatory signalsprovided by APC. This model was tested directly in earlyexperiments, and it was shown by several laboratories thatCD4 crosslinking could induce apoptosis in CD4+ T cells;42,45

however, others failed to see such an effect.46 These earlyexperiments differed in that Oyaizu et al. observed apoptosisin response to CD4-crosslinking only in PBMC but not inpurified CD4+ T cell populations,42 whereas Banda et al.observed apoptosis in purified CD4+ T cell populationscultured for 3 days, but only on subsequent stimulation of the TCR.45

In conclusion, studies of PBMC isolated from HIV-infected persons generally support the model that HIV primesCD4+ T cells for apoptosis. The fact that increased apoptosiswas observed in cultures derived from patient samples whichcontain only a small proportion of infected cells shows thatthe apoptosis observed does not occur exclusively in HIV-infected cells. Indeed, using electron microscopy to analysethese cultures, Groux et al. reported seeing no viral particleswithin the apoptotic cells.40 The level of CD4+ cell apoptosisappeared to be higher in several of the studies mentioned inthe previous section in unfractionated cultures compared toCD4+ T cell-enriched cultures,40,42 and a role for defectivefunctioning of APC within the culture was postulated (seefollowing section.40

It must be noted that the significance of the afore-mentioned observations to the catastrophic decline in theCD4+ T cell population, which is the major pathogenicfeature of AIDS, has been questioned on two grounds. Firstof all, as discussed earlier, increased apoptosis was observedin both CD4+ and CD8+ T cells from HIV patients; indeed insome studies the proportion of cells dying was higher inCD8+-enriched populations compared to CD4+-enriched

A Jaworowski and SM Crowe92

populations. Second, there is disagreement as to whether astrong correlation exists between the extent of T cell apoptosis in this assay and the clinical progressionof the disease in the patient. Several laboratories have foundno such correlation,47,48 but in an extensive study of spon-taneous and activation-induced apoptosis of various lympho-cyte populations obtained from HIV+ and HIV– donors, aninverse correlation was found between CD4+ T cell apoptosisex vivo and the CD4+ T cell count (a marker of disease pro-gression).49 In this latter study, the apoptotic cells were foundto express markers associated with T cell activation and theauthors conclude that, in agreement with conclusions fromstudies of lymph nodes from infected people (see previoussection5), ‘activation is the primary mechanism responsiblefor the induction of apoptosis in lymphocytes from HIV-infected persons’. These findings have been confirmed in a recent report which also detected a correlation betweenapoptosis and plasma viral load.50 Whether such a correlationis significant or not, its lack would not be surprising becausehigh rates of T cell turnover have been shown throughout thecourse of infection.7 A strong correlation might be evidencethat the sensitivity towards apoptosis is the major determinantin T cell loss as a patient progresses to AIDS, but a lack ofsuch a correlation would only suggest that other processes,such as a dysfunction in T cell replacement, are as important.

The role of Fas/FasL in HIV-induced T cell apoptosis

Given the aforementioned observations that HIV infectionincreases the apoptosis observed following short-term cultureof isolated T cells, interest has centred on the possible mech-anism. By 1995, it had been established that pathwaysstimulated via a member of the tumour necrosis factorreceptor (TNFR) family, Fas (CD95/APO-1), are required foractivation-induced cell death in normal T lymphocytes.51–54

It therefore followed that research would focus on deter-mining the effects of HIV infection on the functioning of theFas pathway in T cells. This aspect of HIV has been recentlyreviewed,55 so it will be treated briefly here. The proportionof lymphocytes expressing Fas was shown to be elevated in HIV-infected individuals.56–59 This has been confirmed insubsequent studies.60–65 Generally, these studies demon-strated that the proportion of Fas-expressing cells increaseswith disease progression, or inversely correlates with CD4+

T cell counts in the blood,58–60,62,63 although a correlationbetween Fas-expressing CD4+ cells and CD4+ T cell counts inblood was not observed in one report.57 The increased Fasexpression was found by some investigators to be in bothCD4+ and CD8+ T cells57,63–65 and, by others, to be restrictedto the CD8+ T cell population.60 T cells isolated from HIV-infected individuals were more sensitive to Fas receptoragonist-induced apoptosis,57,61,63,64,66 and both the CD4+ andCD8+ subsets showed this sensitivity.57,63,64,66

In addition to the observation that Fas expression isincreased in lymphocytes from HIV-infected individuals,several laboratories have reported that the expression of Fasligand (FasL) mRNA is also increased in peripheral bloodmononuclear cells.66,67 However, whether this results inincreased surface levels of FasL has not yet been demon-strated. The levels of FasL mRNA were shown to beincreased by the HIV-encoded protein Tat.68.

The observations of increased Fas expression and Fas-mediated apoptosis in T cells, however, raises severalquestions: (i) is this pathway involved in the increasedapoptosis (either spontaneous or activation-induced) of T cells obtained from HIV-infected individuals; and (ii) whatis the significance of these observations to the pathogenesisof HIV? With respect to the first question, there is disagree-ment in the field, with some investigators reporting that Fasantagonists block the increased activation-induced cell deathseen in T cells isolated from HIV-infected individuals,64,66

whereas others report no effect.69 The latter investigatorsfound that activation-induced cell death was, instead, inhib-ited by antagonists of TNF-related apoptosis-inducing ligand(TRAIL), a member of the TNF family. In an acute in vitroinfection model, activation-induced cell death was alsoshown to be Fas-independent.11 Spontaneous apoptosis ofcultured T cells from HIV-infected individuals has beenreported to not involve a Fas-dependent pathway.66,70

Kaplan and Sieg have argued that while FasL mRNA maybe up-regulated in T cells from HIV-infected persons, thereare indications that FasL activity may be down-regulatedeither by decreased expression of FasL, or an increase in thelevels of soluble FasL, known to be an antagonist of Fasactivity.55 This is supported by studies in which crosslinkingCD4 (as gp120 would do) inhibits CD3-induced apoptosis bydown-modulating the level of FasL expression.71

In conclusion, while it is certain that the level of Fas onthe surface of T cells increases as a result of HIV infectionand with the progression of the disease, and while T cellsfrom HIV-positive individuals show a higher propensitytowards apoptosis when stimulated with Fas agonists, therelevance of the Fas pathway to the higher rate of activation-induced cell death observed in vitro has not been definitivelyproven, nor is the significance of this pathway to the patho-genesis of HIV clear. Indeed, the relevance of this pathway tothe decline in CD4+ T cell numbers has been questioned onthe very basis that Fas-expressing T cells accumulate as thedisease progresses, perhaps the reverse of what would beexpected if such expression precedes the cells’ death.55

The role of monocyte/macrophages in CD4+ T celldepletion

Unlike T cells, monocytes or macrophages do not appear toundergo apoptosis when infected with HIV. Bergamini et al.have shown that human monocyte-derived macrophages(MDM) cultured in the presence of the macrophage-specificgrowth factor macrophage colony stimulating factor (M-CSF) underwent extensive cell death upon HIV infectionin vitro.72 Based on morphological criteria, and the absenceof characteristic features of apoptosis such as DNA ladder-ing, TUNEL or bisbenzimide staining, they later identifiedthis cell death as necrotic rather than apoptotic.73 They alsocorrelated cell death with the formation of multinucleatedgiant cells and failed to detect cell death in mononuclearcells. Data from our laboratory also show that HIV-infectedMDM die via necrosis rather than apoptosis (S Crowe,unpubl. data 1997). Studies using the promonocytic (U937)and myeloblastic (PLB-IIIB) cell lines have shown thatstimulation with agents that elevate NFκβ activity protects these cells from apoptosis.74 The authors conclude that the

AIDS and apoptosis 93

elevation of levels of NFκβ in macrophages that have beenshown to follow HIV infection may be responsible for theprotection aginst apoptosis.

Because macrophages do not readily die following HIVinfection, they may act as relatively long-lived sanctuary sitesfor the virus. In this capacity they may also play a veryimportant role in T cell depletion. Human immunodeficiencyvirus infection of human monocytic cell line U937 and MDMhas been shown to increase the level of FasL mRNA in thesecells, as well to increase FasL activity towards Jurkat cells.75

In this context, however, it should be noted that Sieg et al.reported decreased surface expression and activity of FasL inmonocytes obtained from HIV-seropositive individuals, andsuggested that infection leads to chronic down-modulation.76

In a follow-up paper, Badley et al. showed that HIV-infectedMDM also induce apoptosis in CD4+ T cells obtained fromHIV-seropositive individuals.77 The significance of thisobservation was that the effect was specific for the CD4+

population, required cell–cell contact and was maximal ifboth T cell and macrophage populations were infected. Thismechanism of Fas-mediated killing of T cells differs from thereports of apoptosis in purified T cell populations, which hasgenerally been found to be non-selective for CD4 over CD8(see aforementioned). Recently, the same group has con-firmed that a proportion of CD68+ tissue macrophagespresent in lymph nodes express FasL, and that this numberincreases in HIV-infected individuals.78 The number of FasL-expressing macrophages correlated with the number ofTUNEL-positive, apoptotic cells present in the lymph nodes.Unfortunately, evidence for the identity of these apoptoticcells was not obtained, but, given the data of Muro-Cacho etal.,5 it is likely that they are T cells. Taken together, the resultssuggest that macrophages contribute to T cell depletion inHIV-infected individuals. A role for macrophages in CD4+

T cell killing has also been described in a recent study fromGroux’s laboratory.79 They showed that HIV-infected mono-cytes primed antigen-specific CD4+ T cells for apoptosis onsubsequent stimulation. This priming only occurred withmonocytes pulsed with the same antigen for which the T cellwas specific, which implies that the effect is mediated by theantigen-presenting activity of the monocyte, and requirescell–cell contact. Based on studies using EBV-LCL cellsexpressing different viral proteins, a role for gp120 inpriming was suggested. The authors hypothesize that HIV-infected monocytes, acting as APC, engage both the CD4receptor and the T cell receptor, but the monocyte cannotdeliver the appropriate costimulatory signals, and thereforeapoptosis results. Presumably such a mechanism may act todeplete both infected and uninfected T cells.

Kornbluth has suggested that macrophages may becomeinfected with HIV following phagocytosis of apoptoticbodies derived from HIV-infected T cells.80 Such ‘phago-infected’ macrophages may then infect other T cells. Thesignificance of this pathway in vivo has yet to be demon-strated, but infection of T cells by HIV-infected monocyteswas not a significant cause of the T cell apoptosis observedby Cottrez et al.79

These experiments underline the potential importance ofmonocytes and macrophages in the depletion of CD4+ T cellsand hence in the pathogenesis of HIV. It has been suggestedthat this is supported by observations in chimpanzees where

HIV infects, but does not cause, the catastrophic depletion ofCD4+ T cells, which correlates with a lack of susceptibility ofchimpanzee macrophages to HIV.81 Although the aforemen-tioned experiments reveal a potential role for monocyte/macrophages in CD4+ T cell apoptosis, it must be noted thatother cell types present in mixed cultures may also enhancethe apoptosis seen in HIV-infected T cell populations. Wang et al. have recently demonstrated a role for CD8+

lymphokine-activated killer (LAK) cells in the apoptosisinduced by non-cytopathic HIV isolates.82

The significance of apoptosis in the pathogenesis ofHIV: recapitulation

From the previous section it is clear that there is still noconsensus as to whether T cell apoptosis can account for theselective and catastrophic depletion of CD4+ T cells, and theinvolvement of the Fas pathway in this process. T cellsisolated from HIV-positive patients do show an increasedpropensity towards apoptosis in vitro, especially on activa-tion, and, at least in some studies, in cells obtained frompatients with more advanced disease. However, this probablyis not confined to CD4+ cells, so additional mechanisms haveto be invoked to explain the selective depletion of this popu-lation in the progression to AIDS. One possibility is thatother cell types such as APC and LAK magnify the sensitiv-ity of CD4+ cells to apoptosis in vivo. Alternatively, althoughapoptosis may be occurring throughout infection, destructionof the CD4+ population at late stages of the disease may be a consequence of dysfunction in T cell production or of non-apoptotic mechanisms.

Kaplan and Sieg have stated that correlating HIV infec-tion to the apoptotic death of the CD4+ T cell may be toosimplistic.55 They point out that apoptosis is a response of avirally infected cell to protect the host, and that viruses havebeen shown in many instances to inhibit apoptosis to allowtheir own production. They suggest that HIV may in factmodulate the apoptotic response depending upon the stage ofthe infection within the cell.55 In this context, HIV-encodedproteins have been shown to either promote (Vpr, Tat andNef) or protect against (Vpr, Tat) apoptosis in various cellculture models in vitro.68,83–93 The concept of a sophisticatedcontrol of apoptosis by HIV is also suggested by a consider-ation of the effect of HIV on the transcription factor NFκβ.Transcription directed by the HIV long terminal repeat (LTR)enhancer is stimulated by NFκβ, a host-derived transcriptionfactor which is required for efficient viral production.94 Con-sequently, our laboratory and others have shown that NFκβlevels are increased in myeloid cell lines and macrophages byHIV stimulation of Iκβ phosphorylation and degrada-tion,95–100 and enhanced levels of NFκβ have been shown toprevent apoptosis induced by some stimuli.101–104

Finally, we should consider whether apoptosis is a hostresponse to viral infection or a result of a viral pathogenicmechanism. In this context, several workers have shown thatinhibiting apoptosis using either inhibitors of caspase activ-ity, viral proteins or overexpression of Bcl-2, actuallyincreases viral production in T cell lines or PBMC frominfected individuals.105–107 These findings have importantconsequences for any plans to use inhibitors of apoptosis innovel therapeutic strategies.

A Jaworowski and SM Crowe94

Acknowledgements

AJ is supported by the Macfarlane Burnet Centre ResearchFund.

References

1 World Health Organization. WHO Report on the globalHIV/AIDS epidemic, June 1998. www.who.int/emc-hiv/global_report/index.html

2 Crowe SM, Carlin JB, Stewart KI, Lucas CR, Hoy JF. Predictivevalue of CD4 lymphocyte numbers for the development ofopportunistic infections and malignancies in HIV-infectedpersons. J. Acquir. Immune Defic. Syndr. 1991; 4: 770–6.

3 Ameisen JC, Capron A. Cell dysfunction and depletion in AIDS:The programmed cell death hypothesis. Immunol. Today 1991;12: 102–5.

4 Harper ME, Marselle LM, Gallo RC, Wong-Staal F. Detection oflymphocytes expressing human T-lymphotropic virus type III inlymph nodes and peripheral blood from infected individuals byin situ hybridization. Proc. Natl Acad. Sci. USA 1986; 83:772–6.

5 Muro-Cacho CA, Pantaleo G, Fauci AS. Analysis of apoptosis inlymph nodes of HIV-infected persons. J. Immunol. 1995; 154:5555–66.

6 Finkel TH, Tudor-Williams G, Banda NK et al. Apoptosis occurspredominantly in bystander cells and not in productivelyinfected cells of HIV- and SIV-infected lymph nodes. NatureMed. 1995; 1: 129–34.

7 Ho DD, Neumann AU, Perelson AS, Chen W, Leonard JM,Markowitz M. Rapid turnover of plasma virions and CD4lymphocytes in HIV-1 infection. Nature 1995; 373: 123–6.

8 Wei X, Ghosh SK, Taylor ME et al. Viral dynamics in humanimmunodeficiency virus type 1 infection. Nature 1995; 373:117–22.

9 Gandhi RT, Chen BK, Straus SE, Dale JK, Lenardo MJ, Balti-more D. HIV-1 directly kills CD4+ T cells by a Fas-independentmechanism. J. Exp. Med. 1998; 187: 1113–22.

10 Noraz N, Gozlan J, Corbeil J, Brunner T, Spector SA. HIV-induced apoptosis of activated primary CD4+ T lymphocytes isnot mediated by Fas–Fas ligand. AIDS 1997; 11: 1671–80.

11 Yagi T, Sugimoto A, Tanaka M et al. Fas/FasL interaction is notinvolved in apoptosis of activated CD4+ T cells upon HIV-1infection in vitro. J. Acquir. Immune Defic. Syndr. Hum. Retro-virol. 1998; 18: 307–15.

12 Herbein G, Van Lint C, Lovett JL, Verdin E. Distinct mecha-nisms trigger apoptosis in human immunodeficiency virus type1-infected and in uninfected bystander T lymphocytes. J. Virol.1998; 72: 660–70.

13 Lifson JD, Reyes GR, McGrath MS, Stein BS, Engleman EG.AIDS retrovirus induced cytopathology: Giant cell forma-tion and involvement of CD4 antigen. Science 1986; 232:1123–7.

14 Lifson JD, Feinberg MB, Reyes GR et al. Induction of CD4-dependent cell fusion by the HTLV-III/LAV envelope glycopro-tein. Nature 1986; 323: 725–8.

15 Sodroski J, Goh WC, Rosen C, Campbell K, Haseltine WA. Roleof the HTLV-III/LAV envelope in syncytium formation andcytopathicity. Nature 1986; 322: 470–4.

16 Yoffe B, Lewis DE, Petrie BL, Noonan CA, Melnick JL,Hollinger FB. Fusion as a mediator of cytolysis in mixtures ofuninfected CD4+ lymphocytes and cells infected by humanimmunodeficiency virus. Proc. Natl Acad. Sci. USA 1987; 84:1429–33.

17 Somasundaran M, Robinson HL. A major mechanism of humanimmunodeficiency virus-induced cell killing does not involvecell fusion. J. Virol. 1987; 61: 3114–19.

18 Stevenson M, Meier C, Mann AM, Chapman N, Wasiak A.Envelope glycoprotein of HIV induces interference and cytoly-sis resistance in CD4+ cells: Mechanism for persistence inAIDS. Cell 1988; 53: 483–96.

19 Janeway C. Immune recognition. Mls: makes a little sense.Nature 1991; 349: 459–61.

20 Imberti L, Sottini A, Bettinardi A, Puoti M, Primi D. Selectivedepletion in HIV infection of T cells that bear specific T cellreceptor V β sequences. Science 1991; 254: 860–2.

21 Zinkernagel RM, Hengartner H. T cell-mediated immuno-pathology versus direct cytolysis by virus: Implications for HIVand AIDS. Immunol. Today 1994; 15: 262–8.

22 Fauci AS, Schnittman SM, Poli G, Koenig S, Pantaleo G. NIHconference. Immunopathogenic mechanisms in human immuno-deficiency virus (HIV) infection. Ann. Intern. Med. 1991; 114:678–93.

23 Golding H, Robey FA, Gates III FT et al. Identification ofhomologous regions in human immunodeficiency virus I gp41and human MHC class II β 1 domain. I. Monoclonal antibodiesagainst the gp41-derived peptide and patients’ sera react withnative HLA class II antigens, suggesting a role for auto-immunity in the pathogenesis of acquired immune deficiencysyndrome. J. Exp. Med. 1988; 167: 914–23.

24 Habeshaw JA, Dalgleish AG, Bountiff L et al. AIDS patho-genesis: HIV envelope and its interaction with cell proteins.Immunol. Today 1990; 11: 418–25.

25 Mittler RS, Hoffmann MK. Synergism between HIV gp120 andgp120-specific antibody in blocking human T cell activation.Science 1989; 245: 1380–2.

26 Zagury D, Bernard J, Leonard R et al. Long-term cultures ofHTLV-III-infected T cells: A model of cytopathology of T-celldepletion in AIDS. Science 1986; 231: 850–3.

27 Gougeon ML, Olivier R, Garcia S et al. Demonstration of anengagement process towards cell death by apoptosis in lympho-cytes of HIV infected patients. C. R. Acad. Sci. III 1991; 312:529–37 (in French).

28 Groux H, Monte D, Bourrez JM, Capron A, Ameisen JC.Activation of CD4+ T-lymphocytes in asymptomatic HIVinfected patients induce the program action of lymphocyte deathby apoptosis. C. R. Acad. Sci. III 1991; 312: 599–606 (in French).

29 Laurent-Crawford AG, Krust B, Muller S et al. The cytopathiceffect of HIV is associated with apoptosis. Virology 1991; 185:829–39.

30 Terai C, Kornbluth RS, Pauza CD, Richman DD, Carson DA.Apoptosis as a mechanism of cell death in cultured T lympho-blasts acutely infected with HIV-1. J. Clin. Invest. 1991; 87:1710–15.

31 Jaleco AC, Covas MJ, Victorino RM. Analysis of lymphocytecell death and apoptosis in HIV-2-infected patients. Clin. Exp.Immunol. 1994; 98: 185–9.

32 Kerr JFR, Wyllie AH, Currie AR. Apoptosis: A basic biologicalphenomenon with wide ranging implications in tissue kinetics.Br. J. Cancer 1972; 26: 239–57.

33 Fadok VA, Voelker DR, Campbell PA, Cohen JJ, Bratton DL,Henson PM. Exposure of phosphatidylserine on the surface ofapoptotic lymphocytes triggers specific recognition and removalby macrophages. J. Immunol. 1992; 148: 2207–16.

34 Savill J, Fadok V, Henson P, Haslett C. Phagocyte recognition ofcells undergoing apoptosis. Immunol. Today 1993; 14: 131–6.

35 Bonyhadi ML, Rabin L, Salimi S et al. HIV induces thymusdepletion in vivo. Nature 1993; 363: 728–32.

AIDS and apoptosis 95

36 Stanley SK, McCune JM, Kaneshima H et al. Human immuno-deficiency virus infection of the human thymus and disruptionof the thymic microenvironment in the SCID-hu mouse. J. Exp.Med. 1993; 178: 1151–63.

37 Su L, Kaneshima H, Bonyhadi M et al. HIV-1-induced thymo-cyte depletion is associated with indirect cytopathicity andinfection of progenitor cells in vivo. Immunity 1995; 2: 25–36.

38 Jamieson BD, Uittenbogaart CH, Schmid I, Zack JA. High viralburden and rapid CD4+ cell depletion in human immuno-deficiency virus type 1-infected SCID-hu mice suggest directviral killing of thymocytes in vivo. J. Virol. 1997; 71: 8245–53.

39 Cotton MF, Cassella C, Rapaport EL, Tseng PO, Marschner S,Finkel TH. Apoptosis in HIV-1 infection. Behring Inst. Mitt.1996; 97: 220–31.

40 Groux H, Torpier G, Monte D, Mouton Y, Capron A, AmeisenJC. Activation-induced death by apoptosis in CD4+ T cells fromhuman immunodeficiency virus-infected asymptomatic individ-uals. J. Exp. Med. 1992; 175: 331–40.

41 Meyaard L, Otto SA, Jonker RR, Mijnster MJ, Keet RPM,Miedema F. Programmed death of T cells in HIV-1 infection.Science 1992; 257: 217–19.

42 Oyaizu N, McCloskey TW, Coronesi M, Chirmule N, Kalya-naraman VS, Pahwa S. Accelerated apoptosis in peripheralblood mononuclear cells (PBMCs) from human immuno-deficiency virus type-1 infected patients and in CD4 cross-linked PBMCs from normal individuals. Blood 1993; 82:3392–400.

43 Sarin A, Clerici M, Blatt SP, Hendrix CW, Shearer GM, HenkartPA. Inhibition of activation-induced programmed cell death andrestoration of defective immune responses of HIV+ donors bycysteine protease inhibitors. J. Immunol. 1994; 153: 862–72.

44 Lewis DE, Ng-Tang DS, Adu-Oppong A, Schober W, RodgersJR. Anergy and apoptosis in CD8+ T cells from HIV-infectedpersons. J. Immunol. 1994; 153: 412–20.

45 Banda NK, Bernier J, Kurahara DK et al. Crosslinking CD4 by human immunodeficiency virus gp120 primes T cells foractivation-induced apoptosis. J. Exp. Med. 1992; 176:1099–106.

46 Martin SJ, Matear PM, Vyakarnam A. HIV-1 infection of humanCD4+ T cells in vitro. Differential induction of apoptosis inthese cells. J. Immunol. 1994; 152: 330–42.

47 Meyaard L, Otto SA, Keet IPM, Roos MTL, Miedema F. Pro-grammed death of T cells in human immunodeficiency virusinfection. No correlation with progression to disease. J. Clin.Invest. 1994; 93: 982–8.

48 Rothen M, Gratzl S, Hirsch HH, Moroni C. Apoptosis in HIV-infected individuals is an early marker occurring indepen-dently of high viremia. AIDS Res. Hum. Retroviruses 1997; 13:771–9.

49 Gougeon ML, Lecoeur H, Dulioust A et al. Programmed celldeath in peripheral lymphocytes from HIV-infected persons:Increased susceptibility to apoptosis of CD4 and CD8 T cellscorrelates with lymphocyte activation and with disease progres-sion. J. Immunol. 1996; 156: 3509–20.

50 Karmochkine M, Parizot C, Calvez V et al. Susceptibility ofperipheral blood mononuclear cells to apoptosis is correlated toplasma HIV load. J. Acquir. Immune Defic. Syndr. Hum. Retro-virol. 1998; 17: 419–23.

51 Alderson MR, Tough TW, Davis-Smith T et al. Fas ligand medi-ates activation-induced cell death in human T lymphocytes. J.Exp. Med. 1995; 181: 71–7.

52 Ju ST, Panka DJ, Cui H et al. Fas (CD95)/FasL interactionsrequired for programmed cell death after T-cell activation.Nature 1995; 373: 444–8.

53 Brunner T, Mogil RJ, LaFace D et al. Cell-autonomous Fas(CD95) /Fas–ligand interaction mediates activation-inducedapoptosis in T-cell hybridomas. Nature 1995; 373: 441–4.

54 Dhein J, Walczak H, Baumler C, Debatin KM, Krammer PH.Autocrine T-cell suicide mediated by APO-1/ (Fas/CD95).Nature 1995; 373: 438–41.

55 Kaplan D, Sieg S. Role of the Fas/Fas ligand apoptotic pathwayin human immunodeficiency virus type 1 disease. J. Virol. 1998;72: 6279–82.

56 Debatin KM, Fahrig-Faissner A, Enenkel-Stoodt S, Kreuz W,Benner A, Krammer PH. High expression of APO-1 (CD95) onT lymphocytes from human immunodeficiency virus-1-infectedchildren. Blood 1994; 83: 3101–3.

57 Katsikis PD, Wunderlich ES, Smith CA, Herzenberg LA,Herzenberg LA. Fas antigen stimulation induces markedapoptosis of T lymphocytes in human immunodeficiency virus-infected individuals. J. Exp. Med. 1995; 181: 2029–36.

58 Aries SP, Schaaf B, Muller C, Dennin RH, Dalhoff K. Fas(CD95) expression on CD4+ T cells from HIV-infected patientsincreases with disease progression. J. Mol. Med. 1995; 73:591–3.

59 McCloskey TW, Oyaizu N, Kaplan M, Pahwa S. Expression ofthe Fas antigen in patients infected with human immuno-deficiency virus. Cytometry 1995; 22: 111–14.

60 Gehri R, Hahn S, Rothen M, Steuerwald M, Nuesch R, Erb P.The Fas receptor in HIV infection: Expression on peripheralblood lymphocytes and role in the depletion of T cells. Aids1996; 10: 9–16.

61 Silvestris F, Cafforio P, Frassanito MA et al. Overexpression ofFas antigen on T cells in advanced HIV-1 infection: Differentialligation constantly induces apoptosis. Aids 1996; 10: 131–41.

62 Bohler T, Baumler C, Herr I, Groll A, Kurz M, Debatin KM.Activation of the CD95 system increases with disease progres-sion in human immunodeficiency virus type 1-infected childrenand adolescents. Pediatr. Infect. Dis. J. 1997; 16: 754–9.

63 Sloand EM, Young NS, Kumar P, Weichold FF, Sato T,Maciejewski JP. Role of Fas ligand and receptor in the mech-anism of T-cell depletion in acquired immunodeficiencysyndrome: Effect on CD4+ lymphocyte depletion and humanimmunodeficiency virus replication. Blood 1997; 89: 1357–63.

64 Estaquier J, Tanaka M, Suda T, Nagata S, Golstein P, AmeisenJC. Fas-mediated apoptosis of CD4+ and CD8+ T cells fromhuman immunodeficiency virus-infected persons: Differentialin vitro preventive effect of cytokines and protease antagonists.Blood 1996; 87: 4959–66.

65 McCloskey TW, Oyaizu N, Bakshi S, Kowalski R, Kohn N,Pahwa S. CD95 expression and apoptosis during pediatric HIVinfection: Early upregulation of CD95 expression. Clin.Immunol. Immunopathol. 1998; 87: 33–41.

66 Baumler CB, Bohler T, Herr I, Benner A, Krammer PH, DebatinKM. Activation of the CD95 (APO-1/Fas) system in T cells fromhuman immunodeficiency virus type-1-infected children. Blood1996; 88: 1741–6.

67 Mitra D, Steiner M, Lynch DH, Staiano-Coico L, Laurence J.HIV-1 upregulates Fas ligand expression in CD4+ T cells in vitroand in vivo: Association with Fas-mediated apoptosis andmodulation by aurintricarboxylic acid. Immunology 1996; 87:581–5.

68 Westendorp MO, Frank R, Ochsenbauer C et al. Sensitization ofT cells to CD95-mediated apoptosis by HIV-1 Tat and gp120.Nature 1995; 375: 497–500.

69 Katsikis PD, Garcia-Ojeda ME, Torres-Roca JF et al. Inter-leukin-1 b converting enzyme-like protease involvement in Fas-induced and activation-induced peripheral blood T cell

A Jaworowski and SM Crowe96

apoptosis in HIV infection. TNF-related apoptosis-inducingligand can mediate activation-induced T cell death in HIVinfection. J. Exp. Med. 1997; 186: 1365–72.

70 Katsikis PD, Garcia-Ojeda ME, Wunderlich ES et al. Activation-induced peripheral blood T cell apoptosis is Fas independent inHIV-infected individuals. Int. Immunol. 1996; 8: 1311–17.

71 Oberg HH, Sanzenbacher R, Lengl-Janssen B et al. Ligation ofcell surface CD4 inhibits activation-induced death of human T lymphocytes at the level of Fas ligand expression. J.Immunol. 1997; 159: 5742–9.

72 Bergamini A, Capozzi M, Piacentini M. Macrophage-colonystimulating factor (M-CSF) stimulation induces cell death inHIV-infected human monocytes. Immunol. Lett. 1994; 42:35–40.

73 Bergamini A, Dini L, Capozzi M et al. Human immuno-deficiency virus-induced cell death in cytokine-treatedmacrophages can be prevented by compounds that inhibit latestages of viral replication. J. Infect. Dis. 1996; 173: 1367–78.

74 DeLuca C, Kwon H, Pelletier N, Wainberg MA, Hiscott J. NF-kβ protects HIV-1-infected myeloid cells from apoptosis.Virology 1998; 244: 27–38.

75 Badley AD, McElhinny JA, Leibson PJ, Lynch DH, AldersonMR, Paya CV. Upregulation of Fas ligand expression by humanimmunodeficiency virus in human macrophages mediatesapoptosis of uninfected T lymphocytes. J. Virol. 1996; 70:199–206.

76 Sieg S, Smith D, Yildirim Z, Kaplan D. Fas ligand deficiencyin HIV disease. Proc. Natl Acad. Sci. USA 1997; 94: 5860–5.

77 Badley AD, Dockrell D, Simpson M et al. Macrophage-dependent apoptosis of CD4+ T lymphocytes from HIV-infected individuals is mediated by FasL and tumor necrosisfactor. J. Exp. Med. 1997; 185: 55–64.

78 Dockrell DH, Badley AD, Villacian JS et al. The expression ofFas ligand by macrophages and its upregulation by humanimmunodeficiency virus infection. J. Clin. Invest. 1998; 101:2394–405.

79 Cottrez F, Manca F, Dalgleish AG, Arenzana-Seisdedos F,Capron A, Groux H. Priming of human CD4+ antigen-specificT cells to undergo apoptosis by HIV-infected monocytes. Atwo-step mechanism involving the gp120 molecule. J. Clin.Invest. 1997; 99: 257–66.

80 Kornbluth RS. Significance of T cell apoptosis formacrophages in HIV infection. J. Leukoc. Biol. 1994; 56:247–56.

81 Schuitemaker H, Meyaard L, Kootstra NA et al. Lack of T celldysfunction and programmed cell death in human immuno-deficiency virus type 1-infected chimpanzees correlates withabsence of monocytotropic variants. J. Infect. Dis. 1993; 168:1140–7.

82 Wang L, Klimpel GR, Planas JM, Li H, Cloyd MW. Apoptotickilling of CD4+ T lymphocytes in HIV-1-infected PHA-stimulated PBL cultures is mediated by CD8+ LAK cells. Virology 1998; 241: 169–80.

83 Jones M, Olafson K, Del Bigio MR, Peeling J, Nath A. Intra-ventricular injection of human immunodeficiency virus type 1(HIV-1) tat protein causes inflammation, gliosis, apoptosis, andventricular enlargement. J. Neuropathol. Exp. Neurol. 1998; 57:563–70.

84 McCloskey TW, Ott M, Tribble E et al. Dual role of HIV Tat inregulation of apoptosis in T cells. J. Immunol. 1997; 158:1014–19.

85 Purvis SF, Jacobberger JW, Sramkoski RM, Patki AH, Leder-man MM. HIV type 1 Tat protein induces apoptosis and deathin Jurkat cells. AIDS Res. Hum. Retroviruses 1995; 11: 443–50.

86 Zauli G, Gibellini D. The human immunodeficiency virus type-1 (HIV-1) Tat protein and Bcl-2 gene expression. Leuk.Lymphoma 1996; 23: 551–60.

87 Zauli G, Gibellini D, Milani D et al. Human immunodeficiencyvirus type 1 Tat protein protects lymphoid, epithelial, and neu-ronal cell lines from death by apoptosis. Cancer Res. 1993; 53:4481–5.

88 Korant BD, Strack P, Frey MW, Rizzo CJ. A cellular anti-apoptosis protein is cleaved by the HIV-1 protease. Adv. Exp.Med. Biol. 1998; 436: 27–9.

89 Ayyavoo V, Mahboubi A, Mahalingam S et al. HIV-1 Vprsuppresses immune activation and apoptosis through regulationof nuclear factor κB. Nature Med. 1997; 3: 1117–23.

90 Conti L, Rainaldi G, Matarrese P et al. The HIV-1 vpr proteinacts as a negative regulator of apoptosis in a human lym-phoblastoid T cell line: Possible implications for the patho-genesis of AIDS. J. Exp. Med. 1998; 187: 403–13.

91 Stewart SA, Poon B, Jowett JBM, Chen ISY. Human immun-odeficiency virus type 1 Vpr induces apoptosis following cellcycle arrest. J. Virol. 1997; 71: 5579–92.

92 Strack PR, Frey MW, Rizzo CJ et al. Apoptosis mediated byHIV protease is preceded by cleavage of Bcl-2. Proc. NatlAcad. Sci. USA 1996; 93: 9571–6.

93 Okada H, Takei R, Tashiro M. HIV-1 Nef protein-induced apop-totic cytolysis of a broad spectrum of uninfected human bloodcells independently of CD95 (Fas). FEBS Lett. 1997; 414: 603–6.

94 Chen BK, Feinberg MB, Baltimore D. The κB sites in thehuman immunodeficiency virus type 1 long terminal repeatenhance virus replication yet are not absolutely required forviral growth. J. Virol. 1997; 71: 5495–504.

95 Bachelerie F, Alcami J, Arenzana-Seisdedos F, Virelizier JL.HIV enhancer activity perpetuated by NFκβ induction on infec-tion of monocytes. Nature 1991; 350: 709–12.

96 Roulston A, D’Addario M, Boulerice F, Caplan S, Wainberg MA,Hiscott J. Induction of monocytic differentiation and NFκβ-likeactivities by human immunodeficiency virus 1 infection ofmyelomonoblastic cells. J. Exp. Med. 1992; 175: 751–63.

97 Roulston A, Beauparlant P, Rice N, Hiscott J. Chronic humanimmunodeficiency virus type 1 infection stimulates distinctNFκβ/rel DNA binding activities in myelomonoblastic cells. J. Virol. 1993; 67: 5235–46.

98 McElhinny JA, MacMorran WS, Bren GD, Ten RM, Israel A,Paya CV. Regulation of IκB alpha and p105 in monocytes andmacrophages persistently infected with human immuno-deficiency virus. J. Virol. 1995; 69: 1500–9.

99 DeLuca C, Roulston A, Koromilas A, Wainberg MA, Hiscott J.Chronic human immunodeficiency virus type 1 infection ofmyeloid cells disrupts the autoregulatory control of theNFκβ/Rel pathway via enhanced IκBalpha degradation. J.Virol. 1996; 70: 5183–93.

100 Lewin SR, Lambert P, Deacon NJ, Mills J, Crowe SM. Consti-tutive expression of p50 homodimer in freshly isolated humanmonocytes decreases with in vitro and in vivo differentiation: A possible mechanism influencing human immunodeficiencyvirus replication in monocytes and mature macrophages. J.Virol. 1997; 71: 2114–19.

101 Wang CY, Mayo MW, Baldwin Jr AS. TNF- and cancertherapy-induced apoptosis: Potentiation by inhibition of NFκβ.Science 1996; 274: 784–7.

102 Van Antwerp DJ, Martin SJ, Kafri T, Green DR, Verma IM.Suppression of TNF-alpha-induced apoptosis by NFκβ. Science1996; 274: 787–9.

103 Beg AA, Baltimore D. An essential role for NFκβ in prevent-ing TNF-alpha-induced cell death. Science 1996; 274: 782–4.

AIDS and apoptosis 97

A Jaworowski and SM Crowe98

104 Mayo MW, Wang CY, Cogswell PC et al. Requirement of NF-κβ activation to suppress p53-independent apoptosisinduced by oncogenic Ras. Science 1997; 278: 1812–15.

105 Antoni BA, Sabbatini P, Rabson AB, White E. Inhibition ofapoptosis in human immunodeficiency virus-infected cellsenhances virus production and facilitates persistent infection. J. Virol. 1995; 69: 2384–92.

106 Sandstrom PA, Pardi D, Goldsmith CS, Chengying D, DiamondAM, Folks TM. bc1–2 expression facilitates human immuno-deficiency virus type-1 mediated cytopathic effects duringacute spreading infections. J. Virol. 1996; 70: 4617–22.

107 Chinnaiyan AM, Woffendin C, Dixit VM, Nabel GJ. Theinhibition of pro-apoptotic ICE-like proteases enhances HIVreplication. Nature Med. 1997; 3: 333–7.