Department of Medical Microbiology and Infectious Diseases, University of Manitoba, 539-730 William Avenue, Winnipeg, MB, Canada R3E 0W3
Correspondence
Judie Alimonti
judieali{at}yahoo.com
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ABSTRACT |
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Published ahead of print on 22 April 2003 as DOI 10.1099/vir.0.19110-0
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INTRODUCTION |
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Th cells are thought to be important in controlling HIV infection both in the acute (Copeland & Heeney, 1996; Rosenberg & Walker, 1998
) and in the chronic phases of the disease with a high HIV-specific CD4+ response being noted in long-term non-progressing subjects (Rosenberg et al., 1997
). Primary HIV infection presents with a high HIV titre that is initially controlled by a CD8+ CTL response, along with anti-HIV antibodies (Clark et al., 1991
; Daar et al., 1991
). The virus plasma load reaches a set point that is maintained at a plateau during the asymptomatic phase of 210 years (Fig. 1
). This homeostasis becomes unbalanced, resulting in a gradual decrease in the Th lymphocyte cell number concomitant with an increase in virus load, signalling the onset of AIDS. A hallmark of HIV infection is the progressive loss of Th lymphocytes as HIV disease progresses. When CD4 levels drop from a normal of 1000 cells mm-3 of whole blood to less than 200 mm-3, immune dysregulation and opportunistic infections result. Evidence to date suggests that Th responses play a major role in the control of HIV infection (Rosenberg et al., 1997
; Rosenberg & Walker, 1998
; Phenix & Badley, 2002
). In addition, our studies demonstrating HIV-specific Th responses in highly exposed persistently seronegative individuals suggest this may play a role in the prevention of HIV infection (Fowke et al., 2000
). In HIV disease, the mechanism for deletion of Th cells remains unknown, although several processes probably contribute. As the loss of Th lymphocytes is central to AIDS pathogenesis, we will examine the possible mechanisms of their death and how their loss contributes to disease progression. Although the pathways of CD4+ T cell death in HIV infection are many, they mainly result in programmed cell death (apoptosis).
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The apoptotic pathway involves two families of proteins, the effectors and the regulators (Los et al., 1999; Gross, 2001
; Zimmermann et al., 2001
). Upon receiving a death signal, the effector family of serine proteases called caspases (cysteine-dependent aspartate-specific proteases) are activated to catalyse a cascade of molecular events resulting in the activation or inactivation of numerous cellular proteins (Fig. 2
). This results in the classical apoptotic morphological and biochemical changes such as plasma membrane blebbing, mitochondrial dysfunction and DNA fragmentation. The regulators of the apoptotic pathway are found in a second group of proteins, the Bcl-2 family (Strasser et al., 2000
). These proteins contain anti-apoptotic (Bcl-2, Bcl-XL) and pro-apoptotic (Bax, Bid) members that exert their function primarily at the mitochondrion by either preventing or inducing mitochondrial dysfunction (Korsmeyer et al., 2000
; Alimonti et al., 2001
). Anti-apoptotic molecules localize to the outer mitochondrial membrane whereas the pro-apoptotic molecules are sequestered in the cytoplasm by a variety of mechanisms. Upon receiving a death signal, the pro-apoptotic proteins translocate to the mitochondrion to interact with the anti-apoptotic molecules, thereby promoting mitochondrial dysfunction. The relative level of these proteins is important, as increased amounts of anti-apoptotic proteins are able to override even larger numbers of pro-apoptotic signals. Activation of caspase 3 is the point of no return and results in the initiation of many downstream effector functions leading to the typical apoptotic features such as membrane blebbing and DNA fragmentation.
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There are several mechanisms by which HIV can induce cell death directly in the cell it infects. CXCR4-tropic HIV isolates, generally found in the later stages of HIV infection, preferentially infect T cells and induce membrane fusion between cells to form a giant multinucleated cell called a syncytium. Although syncytium formation is not necessary for the progression to AIDS, syncytia have a short lifespan, and the emergence of CXCR4-tropic strains correlates with an increased depletion of T cells (Richman & Bozzette, 1994; Sylwester et al., 1997
; Kimata et al., 1999
). In addition, cell viability can be compromised because the plasma membrane becomes disrupted or more permeable due to the continuous budding of the virion (Fauci, 1988
), or due to specific HIV proteins such as Vpu that can induce membrane permeability (Gonzalez & Carrasco, 2001
). Virus replication in the cell also has terminal consequences as cellular toxicity increases due to a build up of un-integrated linear viral DNA (Shaw et al., 1984
; Levy, 1993
). Also, through cleavage, the HIV protease can inactivate anti-apoptotic Bcl-2 while simultaneously activating pro-apoptotic procaspase 8, making the cell more susceptible to mitochondrial dysfunction in response to internal or external death signals (Korant et al., 1998
; Nie et al., 2002
).
Both CD4+ and CD8+ T lymphocytes are more susceptible to Fas-induced apoptosis in HIV+ individuals, and this is related to the regulation of surface levels of CD95 (Fas) and FasL. Peripheral blood mononuclear cells (PBMC) from HIV+ individuals express higher levels of CD95 (Silvestris et al., 1996), and the proportion of these T lymphocytes increases with disease progression (Aries et al., 1995
; Baumler et al., 1996
; Estaquier et al., 1996
). The HIV proteins Nef (Zauli et al., 1999
), Env (Oyaizu et al., 1994
; Tateyama et al., 2000
) and Tat (Ensoli et al., 1990
; Westendorp et al., 1995
) have also been implicated in increasing CD95 and FasL levels, which presumably enhances susceptibility to Fas-mediated killing. Nef is thought to induce FasL by interacting specifically with the zeta chain of the TCR complex (Xu et al., 1999
). In addition, there are pro-apoptotic influences on other aspects of the Fas death pathway. Tat upregulates initiator caspase 8, resulting in more caspase 8 activity (Bartz & Emerman, 1999
). HIV proteins also act on the central regulators of the death pathway, the Bcl-2 family members, that can either induce or prevent apoptosis. The cell normally has an appropriate balance of pro- versus anti-apoptotic proteins to ensure cell survival but HIV can alter the balance, resulting in the disruption of mitochondrial function. Levels of anti-apoptotic Bcl-2 are significantly lower in HIV-infected individuals (Re et al., 1998
). In particular two HIV proteins, Tat (Sastry et al., 1996
) and HIV protease (Strack et al., 1996
), have been shown to decrease Bcl-2 levels. At the same time, Tat also has the ability to increase pro-apoptotic Bax (Sastry et al., 1996
) and Bim (Chen et al., 2002
). Other evidence demonstrates that deletion of Vpu partially increases survival in response to CD95 cross-linking in Jurkat cells and PBMCs (Casella et al., 1999
). This could be due to the ability of Vpu to suppress NF-
B-dependent expression of anti-apoptotic factors like Bcl-XL (Akari et al., 2001
).
CTLs are responsible for the removal of virus-infected cells from the body, and for inducing apoptosis in HIV-infected CD4+ cells. However, like many other viruses, HIV has developed mechanisms to prevent, or simply delay, apoptosis of the cells it infects. The TCR on CTLs is used to recognize infected cells through the recognition of a non-self viral peptide in conjunction with major histocompatibility complex class I (MHC I). The down-regulation of MHC I surface expression by Nef (Schwartz et al., 1996) and Tat (Howcroft et al., 1993
) avert this recognition and can prevent CTL-mediated killing of the HIV-infected cell. If the cell were to receive a death signal, HIV has a secondary defence already in place. Vpr protein, which is expressed at low levels in the cell, is responsible for the increase in anti-apoptotic Bcl-2, while simultaneously decreasing pro-apoptotic Bax (Conti et al., 1998
). In contrast, other evidence indicates that Vpr induces a prolonged G2 cell cycle delay followed by death in M phase (Watanabe et al., 2000
), and that apoptosis is mediated through interaction of Vpr with the mitochondrion permeability transition pore, which opens the pore causing mitochondrial swelling and release of cytochrome c (Jacotot et al., 2001
; Roumier et al., 2002
). The pro-apoptotic functions of Vpr may be secondary effects dependent upon induction of cell cycle arrest. At the same time, Vpr transactivates the viral promoter, the long terminal repeat (LTR), to increase virus replication (Gummuluru & Emerman, 1999
). The cytoprotective effects of Vpr may play a role during early infection, allowing productive virus replication, but ultimately it promotes apoptosis toward the later stages. Finally, Nef, Vpu and Env all decrease CD4 on the surface of infected cells (Crise et al., 1990
; Willey et al., 1992
; Salghetti et al., 1995
). The fact that three HIV proteins have this function implies a critical role in the survival and replication of the virus. Down-regulation of CD4 would prevent a super infection and Env-induced apoptosis through the CD4 molecule. Overall, HIV has evolved multiple mechanisms to promote survival for long enough to ensure a productive infection and this may be supported by the fact that infected cells do not undergo apoptosis as readily as uninfected bystander cells (Finkel et al., 1995
). However, even infected cells have a shortened lifespan and, therefore, partially contribute to the overall decrease in Th cells (Ho et al., 1995
; Wei et al., 1995
; Perelson et al., 1996
).
Cell death in uninfected cells
Apoptosis plays a major role in killing uninfected cells. Although the Th cells infected by HIV can be directly killed by the virus, or by HIV-specific CTL, there are generally more apoptotic cells than infected cells (Embretson et al., 1993). This is confirmed by direct evidence in lymph nodes where apoptosis was seen primarily in the uninfected bystander cells (Finkel et al., 1995
). There are two mechanisms by which uninfected cells could be killed: either by HIV proteins released from infected cells acting on neighbouring uninfected cells, or by activation-induced cell death (AICD).
Effect of apoptotic HIV proteins on uninfected cells
Inactivated HIV virions (Esser et al., 2001) and HIV proteins released into the extracellular environment can have dramatic effects on uninfected cells. HIV proteins such as gp120, Tat, Nef and Vpu have been shown to induce cell death in uninfected cells.
Soluble gp120 induces apoptosis in uninfected Th cells through cross-linking of the CD4 molecule (Banda et al., 1992). The binding initiates part of the T cell activation pathway; however, in the absence of the TCR being specifically activated, this signal results in apoptosis or inhibition of antigen-induced cell activation (Marschner et al., 2002
). Soluble and membrane-bound gp120 induce, through cell receptors such as CD4, CXCR4 and CCR5, both Fas-dependent (upregulation of Fas/FasL, decreased FLIP) and Fas-independent (increased Bax, decreased Bcl-2) apoptotic pathways (Arthos et al., 2002
). Recently, CCR5-tropic viruses were shown to induce Fas and caspase 8-dependent apoptosis of uninfected Th cells (Algeciras-Schimnich et al., 2002
). Cell surface presentation of gp120 can induce a bystander cell death that requires close cell-to-cell contact and gp41 function (Blanco et al., 2003
). gp120 also has an inhibitory effect when cells at the G0/G1 phase of the cell cycle, such as naive T cells, are most sensitive to gp120-mediated negative signalling, whereas memory T cells are less affected.
The HIV protein Tat, secreted from infected cells (Chang et al., 1997), can be endocytosed by neighbouring cells (Ensoli et al., 1990
; Mann & Frankel, 1991
; Zagury et al., 1998
). Tat upregulates caspase 8 (Bartz & Emerman, 1999
) and FasL (Li-Weber et al., 2000
), and induces apoptosis in neurons (New et al., 1997
) and Th cells (Li et al., 1995
). A Fas-independent Tat-mediated mechanism of bystander T cell death has also been suggested (Zhang et al., 2001
). Tat can upregulate TNF-related apoptosis-inducing ligand (TRAIL) on monocytes which may then interact with uninfected T cells to induce apoptosis (Zhang et al., 2001
). However, Tat also protects T cells from TRAIL-induced apoptosis (Gibellini et al., 2001
). Other anti-apoptotic effects of exogenous Tat include upregulation of Bcl-2, which was observed in the Jurkat cell line and PBMCs (Zauli et al., 1995
).
HIV Nef protein released into the extracellular matrix induces death in neuronal cells (Trillo-Pazos et al., 2000) and a wide range of blood cells by a Fas-independent mechanism (Okada et al., 1997
, 1998
). Much of the toxicity of Nef is likely due to its myristylated N terminus, which can insert into the plasma membrane and induce cell death in uninfected CD4+ and CD4- T cells (Azad, 2000
). It has been suggested that Nef plays a role in allowing HIV-infected cells to evade the immune response by inducing cytotoxic activity in uninfected CD8+ T cells (Silvestris et al., 1999
) and down-regulating CD4 expression in neighbouring CD4+ T cells (Pugliese et al., 1999
). Also, the extracellular addition of Vpu, or its C terminus, can cause membrane disruption and induce cell death in CD4+ and CD4- cells (Azad, 2000
). Clearly a number of extracellular HIV products are capable of inducing apoptosis in uninfected cells.
Activation-induced cell death (AICD) in uninfected cells
One interesting feature of apoptosis is that many of the molecular steps required for apoptosis are shared by cellular activation pathways. For example, the nuclear condensation that occurs in programmed cell death is similar to that which occurs prior to cell proliferation. Also, caspase activation, long known to be associated with apoptosis, occurs when cells become activated and proliferate (Alam et al., 1999; Kennedy et al., 1999
). With these shared similarities it may not be surprising that activation and cell death are closely linked. AICD is a normal multi-step regulatory mechanism that primes a cell for death to limit an activated immune response (Hanabuchi et al., 1994
). Priming can be achieved either by repeated stimulation through CD3/TCR (Kabelitz et al., 1995
), sole stimulation through the CD4 receptor (Banda et al., 1992
) or activation without co-stimulation (Borthwick et al., 2000
). Although not normally expressed at high levels on resting T cells, Fas and FasL can be induced (Suda et al., 1995
). In fact, allo-stimulation induces the expression of Fas and FasL on the surface of T cells (O'Flaherty et al., 2000
). In HIV infection excessive immune activation has been suggested to induce apoptosis through Fas/FasL (Badley et al., 1998
, 1999
; Dockrell et al., 1999
).
The binding of Env to the CD4 molecule renders the CD4+ cell more susceptible to Fas-mediated killing (Algeciras et al., 1998) and also causes an increase in effector caspase 3, 6 and 8 activity, which can be blocked with soluble CD4 (Cicala et al., 1999
, 2000
; Algeciras-Schimnich et al., 2002
). This suggests a CD4-dependent signalling mechanism, which is also known to block antigen-specific activation of primary lymphocytes (Marschner et al., 2002
). T cells acquire an AICD-resistant phenotype when correctly activated by antigen. Engagement of CD4 by Env before TCR signalling prevents the upregulation of the Fas death pathway regulator FLIP, changing T cells from Fas-resistant to Fas-sensitive (Somma et al., 2000
).
There are other HIV proteins that do not increase CD95 expression but still sensitize the cells to Fas-induced death. The Tat protein has the ability to increase pro-apoptotic proteins caspase 8 and Bax, while simultaneously decreasing the anti-apoptotic Bcl-2 (Sastry et al., 1996; Bartz & Emerman, 1999
). Recent data have suggested that elevated levels of AICD in HIV infection may also be the result of altered FasL levels. Tat upregulates FasL expression through Egr transactivation of the FasL promoter resulting in increased AICD (Yang et al., 2002
).
Cytokine responses in HIV disease and their effect on apoptosis
It is not surprising, given the death of Th lymphocytes in HIV infection, that there is a significant dysregulation of cytokine responses, which likely influences Th cell apoptosis susceptibility. It has been hypothesized that as HIV disease progresses there is a shift in the cytokine response from a predominantly type-1 cellular immune response (IFN-, TNF-
, IL-12), to a type-2 humoral response (IL-4, IL-5, IL-10, IL-13) (Clerici & Shearer, 1994
). However, additional studies of cytokine profiles and HIV disease progression fail to completely corroborate these findings (Galli et al., 2001
). Type-1 cytokine responses decrease as HIV disease progresses. While there is no clear evidence that type-2 responses increase, there is no concurrent decrease in type-2 responses. One possible explanation for the decrease in type-1 responses may be the increased susceptibility of cells expressing IFN-
or TNF-
to AICD, presumably due to decreased Bcl-2 expression in these type-1 Th cells (Ledru et al., 1998
). Thus, during HIV disease progression, dysregulation of cytokine responses results in a diminished ability to mount effective type-1 cytokine responses.
Resistance to apoptosis in vitro is associated with a predominant type-1 response (Gougeon & Montagnier, 1993). Therefore, deficiencies in type-1 responses could have drastic effects on apoptotic events regulating normal T cell homeostasis, and on HIV-induced AICD. IL-12 can protect against Fas-mediated apoptosis and AICD in HIV-infected patients (Estaquier et al., 1995
). In addition, AICD in lymphocytes isolated from HIV-infected patients was blocked by the addition of recombinant IL-12 and IFN-
, whereas type-2 cytokines IL-4 and IL-10 were shown to increase susceptibility to AICD (Clerici et al., 1996
). Recent gene expression data demonstrated that IFN-
can strongly induce a number of pro-apoptotic genes in the TNF superfamily (Stylianou et al., 2002
), suggesting that susceptibility to apoptosis may not exactly fit the type-1/-2 paradigm. Regardless of the assignation of these cytokines to a type-1 or type-2 phenotype, these data strongly suggest that cytokine dysregulation plays an important role in HIV-induced apoptosis. As HIV disease progresses, not only is apoptosis of Th lymphocytes enhanced due to the dual effects of pro-apoptotic HIV proteins and increased AICD, but cytokine dysregulation is likely to amplify these effects by providing a pro-apoptotic environment.
Although some of the strongest pro-apoptotic cytokine signals are TNF- and TNF family members, their role in HIV-induced apoptosis is not clear. It is apparent that regulation of TNF is severely dysregulated in HIV-infected patients (Zangerle et al., 1994
), as both HIV proteins and HIV infection can induce strong TNF responses in lymphocytes (Capobianchi, 1996
). In fact, HIV infection of lymphocytes, or monocytes, induces TNF that in turn activates NF-
B, which induces high levels of HIV and TNF transcription (Han et al., 1996
). In addition, serum TNF levels have been shown to be elevated in HIV-symptomatic but not asymptomatic individuals (Hober et al., 1996
). TNF and TNF family members can initiate apoptosis immediately via membrane-bound receptors, and although it is apparent that TNF levels are indeed dysregulated in HIV infection, there is limited evidence for TNF-mediated apoptosis of infected cells directly, or by acting on bystander lymphocytes. Apoptosis of bystander Th cells can be reduced by the addition of soluble TNF receptor decoys (Srivastava et al., 1999
) and can protect against HIV protein-induced apoptosis (Cicala et al., 2000
). However, clinical trials of anti-TNF therapies have failed to show improvement in either immunological or clinical outcome (Walker et al., 1996
), making the role of TNF in HIV-induced apoptosis unclear. This is not surprising considering the multifactorial role of TNF in inducing cell activation and death. It is likely that the role of anti-apoptotic signals that negatively regulate TNF and TNF family member signalling such as caspase 8 may play a more important role in determining whether or not a particular cell undergoes apoptosis. The redundancy in cytokine function makes elucidation of a role in induction of apoptosis difficult.
The role of T cell growth factors IL-2 and IL-15 in HIV-induced apoptosis is also unclear. IL-2 displays both pro- and anti-apoptotic properties depending on the cellular microenvironment or activation status of a particular cell. Treatment with recombinant IL-2 or IL-15 can increase or decrease a cell's susceptibility to apoptosis. These discrepancies may be due to the activation status of the cells undergoing apoptosis. IL-2 and IL-15 were shown to increase susceptibility to CD95-mediated apoptosis (Naora & Gougeon, 1999) while protecting against spontaneous apoptosis (Adachi et al., 1996
). The latter effect may be due to the ability of IL-2 and IL-15 to upregulate Bcl-2 expression (Akbar et al., 1994
). Clinically, IL-2 has been useful in the treatment of HIV infection under certain conditions. IL-2 can increase Th cell survival independent of HIV replication, presumably by decreasing the apoptosis of bystander Th cells. Also, treatment with recombinant IL-2 can reduce overall levels of apoptosis in HIV-infected, but not uninfected, individuals (Adachi et al., 1996
), further underscoring the importance of cellular activation, and the cytokine environment in the action of these apoptotic mediators. IL-2 and IL-15 share many similarities in their action due to utilization of common receptor elements (Giri et al., 1995
). Of considerable interest is the observation that IL-15 may be a more potent inhibitor of apoptosis, which is likely because IL-15 (unlike IL-2) does not appear to induce HIV replication (Kovacs et al., 2001
). Also, the relative levels of each cytokine in the apoptotic microenviroment are crucial to their ability to induce, or alternately suppress, apoptosis. A mouse model on senescence suggests that Th cells incapable of producing sufficient levels of IL-2 were unable to proliferate and were susceptible to apoptosis, but could be rescued by the addition of exogenous IL-2 (Nishimura et al., 2002
). Thus, the role of IL-2 and IL-15 in apoptosis is likely to be complex due to the immunoregulatory roles for these cytokines in cellular activation. Determination of whether a cell undergoes apoptosis will depend not only on the levels of IL-2 and IL-15, but also on the presence or absence of other appropriate pro- or anti-apoptotic signals such as TNF, other cytokines or Bcl-2 expression. These molecules may act directly through mediation of cellular activation, indirectly through the induction of other anti-apoptotic mediators such as Bcl-2 or even through more downstream regulatory events such as the abilities of these cytokines to induce mediators like IFN-
that regulate apoptosis in a more direct manner.
It is clear that cytokines play a multifactorial role in apoptosis during HIV disease pathogenesis. The loss of anti-apoptotic cytokines IFN- and IL-12 during disease progression and the increase in pro-apoptotic cytokines IL-4 and IL-10 are likely to amplify the role of apoptosis in the loss of Th cells. TNF-
and the TNF family of cytokines can induce apoptosis in a direct manner by inducing the appropriate (or inappropriate) apoptotic signals, while IL-2 and IL-15 appear to act in a more indirect manner via mediation of cell activation. Cytokines play an important role in apoptosis, and this is likely to be even more important during HIV infection when normal cytokine responses become dysregulated.
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CONCLUSION |
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Local environmental factors can also influence cell survival and the effectiveness of the HIV-specific immune response. During HIV infection the switch from the production of some type-1 cytokines to other type-2 cytokines could be very important since, respectively, they are associated with either protection from or enhancement of AICD.
This review has highlighted the many different modes by which HIV induces apoptotic cell death in both infected and uninfected Th cells. One of the ultimate goals of research in this field is to prevent or minimize death in Th cells. If this were achieved it may be possible to convert HIV infection from a progressively immunosuppressive and ultimately fatal disease to a chronic manageable infection.
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ACKNOWLEDGEMENTS |
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