Apoptotic Cell Death in the Pathogenesis of Infectious Diseases

Journal of Infection (2001) 42, 227–234 doi:10.1053/jinf.2001.0836, available online at http://www.idealibrary.com on

REVIEW

Apoptotic Cell Death in the Pathogenesis of Infectious Diseases D. H. Dockrell* Division of Genomic Medicine, University of Sheffield Medical School, and Department of Infectious Diseases, Royal Hallamshire Hospital, Sheffield, U.K. Apoptosis is a physiological process critical for tissue homeostasis. It is essential for the regulation of immune responses. A series of molecules transduce apoptoic signals and induce the characteristic morphological appearances of apoptotic cells. Infectious diseases modulate apoptosis and this contributes to disease pathogenesis. Infection with HIV results in enhanced levels of CD4 T-lymphocyte apoptosis in both directly infected cells and in uninfected bystander cells. A variety of HIV proteins including gp120 contribute to this process. A number of different pathways induce HIV-associated CD4 T-lymphocyte apoptosis and apoptosis of uninfected bystander cells is particularly associated with increased susceptibility to Fas. Other viruses including hepatitis viruses and the human herpesviruses also modulate apoptosis. Bacterial infection induces apoptosis which is frequently mediated by the direct activation of caspases in the absence of death receptor ligation. Bacterial induction of apoptosis may either be due to bacterial factors such as the invasin IpaB of Shigella flexneri or be the result of host immune responses which control infection as demonstrated in infections due to Mycobacterium spp. Apoptosis may be modulated by therapeutic strategies, such as antiretroviral therapy, and an improved understanding of infection-associated apoptosis modulation will aid the design of novel therapeutic approaches to control infectious diseases. © 2001 The British Infection Society

Background The term apoptosis, from the Greek word meaning falling off, was first described by Kerr, Wyllie and Currie in 1972.1 Apoptosis, or programmed cell death, is a physiological process involved in tissue homeostasis in multicellular organisms.2 Apoptosis is observed during embryogenesis and hormone- dependent atrophy. In addition, apoptosis plays an important role in regulating immune responses. Central tolerance, which is the term applied to the negativeselection of auto-reactive T-lymphocytes in the thymus, and peripheral tolerance, whereby mature autoreactive T-lymphocytes which escape negative selection in the thymus are deleted in the periphery, require apoptosis.3 Lymphocytes stimulated by antigen proliferate; however, after an initial period of replication cell numbers are regulated by apoptosis. This process is termed activationinduced cell death (AICD) and prevents uncontrolled lymphoproliferation.4 The immune privileged basis of *Please address correspondence to: D. H. Dockrell, Division of Genomic Medicine, University of Sheffield Medical School, and Department of Infectious Diseases, Royal Hallamshire Hospital, Sheffield, U.K. E-mail: [email protected] Accepted for publication 20 April 2001. 0163-4453/01/040227;08 $35.00/0

organs such as the eye and the testis is due to the expression of pro-apoptotic ligands which result in apoptosis of infiltrating lymphocytes.3 Apoptosis is an active energy-dependent process which requires the synthesis of a series of proteins to translate an apoptotic signal into the characteristic features of apoptosis.2 Apoptosis can be differentiated from necrosis by specific features. Apoptotic cells shrink and lose cell–cell contact. Cytoplasmic blebbing, nuclear condensation and DNA cleavage at internucleosomal linker regions result in cellular fragmentation. The mitochondrial inner transmembrane potential (⌬⌿m) is decreased. Phosphatidylserine translocates from the inner surface to the outer surface of cell membranes. Initially cell membranes retain integrity but efflux pumps become less effective. In contrast, necrotic cells swell and cell membrane integrity is lost early in the process. Necrotic cell death is a passive process which is energy independent and results from non-physiological signals. An essential feature of necrotic cell death is that it results in tissue inflammation but, in contrast, phagocytosis of apoptotic cell bodies limits tissue inflammation.5 Macrophages recognize apoptotic bodies which express phophatidylserine and other cell surface molecules using a series of receptors including © 2001 The British Infection Society

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Figure 1. THP-1 cells, a monocytic cell line, infected with Streptococcus pneumoniae undergo apoptosis. Cells stained with 4⬘6⬘diamidino2-phenylindole (DAPI) demonstrate the characteristic morphological features of apoptosis (arrows).

CD14, scavenger receptors and integrins. The morphological features of apoptosis from the basis of many of the laboratory techniques used to identify apoptosis (Fig. 1). A series of molecules transduce apoptotic signals. Certain types of apoptosis result from the ligation of death receptors.6 These receptors bind specific ligands which are members of the tumour necrosis factor (TNF) superfamily including TNF, Fas ligand (FasL), TNF-related apoptosisinducing ligand (TRAIL)/(APO2L) and TNF-like weak inducer of apoptosis (TWEAK). Fas is the best characterized death receptor and Fas signaling is essential for a variety of immunological processes.7,8 FasL is expressed by lymphocytes, natural killer (NK) cells, monocytes and macrophages.9,10 It exists as a 40-kDa membrane-bound protein which is cleaved by a matrix metalloproteinase (MMP) to a soluble 28-kDA form.11 Fas is widely distributed and found in a diverse number of cell types and tissues.12 Fas expression on the surface of cells can be regulated by the tumour suppressor protein, p53, both by increased transcription and also by translocation from the Golgi apparatus.13,14 Fas signals through an intracellular transduction pathway via an adapter molecule Fas-associated death domain (FADD) which activates FADD-like interleukin-1␤-converting enzyme (FLICE) (Caspase-8).6 Caspase-8 is an upstream member of a family of proteins termed caspases. Caspases are closely related intracellular proteases which recognize specific tetrapeptide recognition motifs.15 Caspases exist in the cytoplasm as zymogens and are activated by apoptotic stimuli. Apoptotic signals differ in the upstream pattern of specific caspases cleaved (initiator caspases). Two main pathways of apoptosis induction have been described. Fas and other death receptors activate caspase-8. In contrast, a separate pathway of apoptosis

induction involves the mitochondrion and results in activation of caspase-9. Specific stimuli such as oxidants, calcium overload or ceramide cause a decrease in ⌬⌿m and result in the release of cytochrome c from the mitrochondrion.16 Cytochrome c forms a complex with apoptosis protease activating factor-1 (Apaf-1) and procaspase-9 which results in activation of caspase 9.15 Mitochondria can also induce apoptosis via a caspase-independent pathway involving apoptosis inducing factor (AIF).17 Caspases form a cascade and regardless of the proximal caspase activated the downstream effects are cleavage of caspase-3 and caspase-7 (effector caspases). This results in inactivation of poly ADP ribose polymerase (PARP) (a DNA repair enzyme) and activation of endonuleases which cleave DNA. The Bcl-2-family of proteins are cytoplasmic regulatorsof-apoptosis. Some, such as Bcl-2, inhibit apoptosis while others induce apoptosis.16 The pro-apoptotic Bcl-2 family members form two groups; Bax and related proteins are closely related to Bcl-2 while the so-called ‘BH3 only’ proteins, such as Bik and Bid, contain only one of the four conserved motifs which characterize Bcl-2 family members. Bcl-2 family members exist as heterodimers in the cytoplasm. Anti-apoptotic Bcl-2 family members such as Bcl-xL inhibit apoptosis by binding Apaf-1. Specific apoptotic stimuli results in the interaction of pro-apoptotic proteins such as Bax or Bik with anti-apoptotic proteins such as Bcl-XL resulting in the release of Apaf-1, which is then free to form a complex with cytochrome c and procaspase-9. Bcl-2 proteins can interact with mitochondria and some, such as Bax, can induce caspase-independent cell death by forming mitochondrial pores which induce a decrease in ⌬␺m.18 One Bcl-2 family member, Bid, appears to link the two main pathways of apoptosis. Caspase-8 cleavage results in activation of Bid which then translocates from the cytoplasm to the mitochondrion and can trigger the mitochondrial death pathway.17 There are a number of inhibitors of apoptosis besides anti-apoptotic Bcl-2 family members. The inhibitors of apoptosis (IAPs) inhibit the auto-cleavage of caspases by inhibition of effector caspases. It is intriguing that viruses such as cowpox virus and baculovirus encode homologues of mammalian IAPs.17 A further inhibitor of apoptosis is the FLICE inhibitory protein (c-FLIP). c-FLIP competes with procaspase-8 for recruitment to FADD. Over expression of certain forms of mammalian c-FLIP inhibits Fas-mediated apoptosis. Viral homologues of FLIP (v-FLIP) are described below. Although apoptosis is a physiological process, it can also result from pathological stimuli. Multiple disease processes modulate apoptosis. Fas mutations can result in a molecule which does not signal. The resulting inhibition of physiological apoptosis is characterized phenotypically

Apoptotic Cell Death in Infectious Diseases by the autoimmune lymphoproliferative syndromes (ALPS).19 In these syndromes individuals usually present in childhood with a lupus-like syndrome characterized by lymphoproliferation, lymphadenopathy and glomerulonephritis. Another disease process with dysregulation of apoptosis is the syndrome of idiopathic CD4 T-cell lymphopenia, which is characterized by progressive CD4 T-lymphocyte depletion in the absence of any identifiable pathogenic cause and results in the development of opportunistic infections.20 CD4 T-lymphocytes demonstrate enhanced susceptibility to Fas-mediated apoptosis in this condition. Many other disease processes may involve alterations in the levels of apoptosis including systemic lupus erythematosus (SLE), some forms of hepatitis, cardiomyopathy, a variety of neurodegenerative conditions and acute graft-versus-host disease.21–25

Apoptosis in HIV Immunopathogenesis Infection with the human immunodeficiency virus (HIV) is associated with a progressive decrease in CD4 T-lymphocyte numbers.26 Multiple theories have been put forward to explain the mechanism of CD4 T-lymphocyte death and the cause is likely to be multifactorial. However a number of lines of evidence support the concept that lymphocyte apoptosis is central to disease pathogenesis.27–29 CD4 T-lymphocytes from HIV-seropositive individuals demonstrate enhanced spontaneous apoptosis and activation-induced apoptosis ex vivo compared with lymphocytes isolated from HIV-seronegative individuals.30 However, this is not merely an ex vivo phenomenon – as demonstrated by the finding that, in lymph nodes from HIV-seropositive individuals, apoptotic lymphocytes are clearly demonstrated and intriguingly many of the apoptotic cells are not directly infected with HIV.31 Animal studies also support a role for apoptosis in HIV disease pathogenesis, as previously reviewed.27 Chimpanzees infected with HIV demonstrate viral replication. However, they frequently do not develop a progressive syndrome of immunodeficiency with CD4 T-lymphocyte depletion. In contrast, macaques infected with simian immunodeficiency virus (SIV) demonstrate both viral replication and CD4 T-lymphocyte depletion. It is of interest that macaques but not chimpanzees CD4 T-lymphocytes demonstrate markers of activation and apoptosis which occur predominantly in macrophagerich areas of lymphoidal tissue. Animal studies also confirm that the majority of CD4 T-lymphocytes killed are not directly infected with retrovirus. The role of viral proteins in the development of HIVassociated apoptosis has been difficult to elucidate. Results have been conflicting, depending on the cell type studied

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and whether apoptosis is observed in directly infected cells or uninfected bystander cells. HIV-1 gp120 can enhance the susceptibility of CD4 T-lymphocytes to apoptosis.32,33 This is achieved by gp120 cross-linking CD4 and upregulating Fas. In addition, recent evidence supports a role for chemokine receptors in signalling a rapid onset of apoptosis due to gp120.34,35 Both directly infected and uninfected bystander CD4 T-lymphocytes are susceptible to gp120-induced apoptosis. There is controversy concerning the potential mechanism of chemokine receptor related gp120 triggered apoptosis: it appears that CCR5 (co-receptor for M-tropic virus) may signal a caspase-8 dependent form of apoptosis35 while CXCR4 (a coreceptor for T-tropic virus) signals a caspase-independent form of apoptosis.34,35 Other HIV proteins implicated in the induction of apoptosis in vitro include tat, nef, vpr and protease,36–39 although the in vivo relevance of these is unclear. Tat and nef have both been implicated in Fasmediated apoptosis and both can upregulate Fas but tat, nef, vpr and protease have also been implicated in Fasindependent HIV-associated apoptosis of directly infected or uninfected bystander CD4 T-lymphocytes.29 HIV protease can cleave Bcl-239 and vpr can trigger mitochondrial pathways of apoptosis induction.38 The mechanism by which HIV enhances apoptosis in CD4 T-lymphocytes remains controversial and multiple theories exist. Alterations in death-receptors or specific death receptor ligands, caspases and Bcl-2 family members have all been reported.27 However, we and others have suggested enhanced Fas-mediated apoptosis is an important mechanism.40–42 CD4 T-lymphocytes from HIV-seropositive individuals demonstrate enhanced susceptibility to Fasmediated apoptosis in vitro when compared to those of HIV-seronegative controls.40,43 The enhanced susceptibility to Fas-mediated apoptosis observed in CD4 T-lymphocytes from HIV seropositive individuals can be induced in CD4 T-lymphocytes from HIV-seronegative individuals by incubation with gp120 or CD4 cross-linking antibody.44 This suggests that, in HIV-seropositive individuals, CD4 T-lymphocytes can be sensitized to Fas-mediated apoptosis by gp120 which is often encountered as soluble protein or part of incomplete virions and therefore uninfected cells can become apoptotic.27 Although susceptible to apoptosis, Fas-sensitive CD4 T-lymphocytes must encounter ligand in order to undergo Fas-mediated apoptosis. HIV tat can upregulate FasL in lymphocytes and this may be a major source of FasL.45 We have, however, proposed that macrophages represent another potential source of ligand to induce apoptosis.46 Macrophages infected with HIV in vitro kill CD4 T-lymphocytes from HIV seropositive individuals by apoptosis.41 The apoptosis observed is seen in both allogeneic and

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syngeneic co-cultures and the levels of apoptosis are enhanced by HIV infection of macrophages and by using donor lymphocytes from HIV-seropositive individuals. The killing shows selectivity for CD4 T-lymphocytes and requires cell–cell contact between lymphocytes and macrophages. The cell death observed is mediated by FasL in many donors, but in some donors TNF and other ligands also play a role. Subsequent studies have demonstrated that TRAIL is another potential ligand which induces CD4 T-lymphocyte apoptosis in CD4 T-lymphocytes from HIV-seropositive individuals.47 HIV infection of macrophages in vitro results in upregulation of the membrane bound form of FasL at a transcriptional level.46 Furthermore macrophage associated FasL is upregulated in lymphoid tissue from HIV-seropositive individuals and correlates with enhanced levels of lymphoid tissue apoptosis in vivo. Therefore macrophage expressed FasL can induce apoptosis in CD4 T-lymphocytes which are already sensitized to Fas-mediated apoptosis by prior gp120 engagement of CD4. This mechanism is likely to contribute to the apoptosis of uninfected CD4 T-lymphocytes. Other processes which could induce Fas-mediated apoptosis include bystander killing by monocytes or CD8 T-lymphocytes and AICD. Killing of directly infected CD4 T-lymphocytes has rarely been demonstrated to be Fassensitive and other mechanisms are likely to be involved.29 Ultimately CD4 T-lymphocyte depletion in HIV infection is likely to be multifactorial but apoptosis induction makes a significant contribution. Apoptosis also contributes to other processes during HIV infection, including CD8 T-lymphocyte cell death and neuronal depletion.

(cytomegalovirus (CMV) or human herpesvirus-6 (HHV-6)) results in lymphocytopenia secondary to induction of apoptosis. As with HIV infection, many of the lymphocytes killed are not directly infected. The immune system aims to limit viral replication by killing directly infected cells although uninfected bystander cells are also killed.51 This explains in part how infection with beta-herpesviruses in immunocompromised individuals adds to the net state of immunosuppression. In contrast, the gammaherpesviruses [Epstein-Barr virus (EBV) and human herpesvirus-8 (HHV-8)] inhibit apoptosis via upregulation of Bcl-2 and inhibition of p53 which result in lymphoproliferation or the induction of Kaposi’ sarcoma by HHV-8.53,54 Alternatively, EBV can modulate the immune response to acute infection by killing cytotoxic lymphocytes (CTL) via apoptosis.55 Therefore viruses may inhibit apoptosis in cells they directly infect to facilitate viral replication while enhancing apoptosis in elements of the immune system aiming to control viral replication. The immune response however involves the induction of apoptosis in virally infected cells but can result in apoptosis of uninfected cells which causes enhanced immunosuppression or organ specific toxicity. The mechanisms by which viruses other than HIV modulate apoptosis are less well characterized than those for HIV. Once again FasL is implicated in several examples.48,51,55 Many viral proteins have been identified which modulate apoptosis and in particular HHV-8 encodes analogues of eukaryotic proteins which inhibit apoptosis including a viral homologue of FLICE inhibitory proteins (v-FLIP).56

Apoptosis in Non-viral Infections Apoptosis in other Viral Infections Other viral infections also modulate apoptosis. The hepatocyte cell death induced by hepatitis B virus (HBV) and hepatitis C virus (HCV) is apoptosis.22,48,49 In HBV-associated viral hepatitis it is cytotoxic T-lymphocytes that target HBV-infected hepatotcytes and induce apoptosis.48 This is an example of an immune response to infection causing unwanted end-organ damage by apoptosis. In a minority of individuals acute viral replication is not controlled and chronic infection results which can result in hepatocellular carcinoma. This complication may involve the oncoprotein HBx which inactivates the tumour suppressor p53 and inhibits apoptosis.50 In hepatitis C infection, immunological control of acute infection is usually incomplete and chronic viral infection results in associated ongoing hepatocyte apoptosis.49 The herpesviruses also modulate apoptosis.51,52 In particular, infection with the beta-herpesviruses

Bacterial infections also modulate apoptosis, as has recently been reviewed.57 The examples most studied involve the induction of macrophage apoptosis in vitro and in vivo by Shigella flexneri and Salmonella typhimurium.58–60 In the case of S. flexneri, infection bacteria ingested by macrophages induce host cell apoptosis via a direct interaction between an invasin IpaB and caspase-1.59 Inflammation is a prominent part of this form of cell death in contrast to the more usual forms of apoptosis, and this reflects the role of caspase-1 in the production of the pro-inflammatory cytokines IL-1␤ and IL-18. In the case of S. typhmurium a similar invasin SipB induces caspase-1 activation.60 These examples occur rapidly after infection in vitro and are associated with recovery of bacteria from cultures suggesting the apoptosis observed is a bacterial mechanism of evading the immune response to infection.61 Some authors have suggested that the apoptosis associated with in vitro infection of macrophages with S. typhimurium is not typical of apoptotic cell death as it

Apoptotic Cell Death in Infectious Diseases lacks nuclear fragmentation and that it should be regarded as a variant of classic apoptosis.62 A variety of other bacteria are capable of inducing apoptosis. Yersinia spp. induce macrophage apoptosis via a secreted protein YopJ63 and impair activation of NF-␬B with suppression of TNF␣ production and apoptosis generation.64 Legionella pneumophila can induce macrophage and epithelial cell apoptosis from an extracellular location.65 This form of apoptosis is caspase-3-dependent. Pseudomonas aeruginosa can also induce macrophage and epithelial cell apoptosis from an extracellular location.66 A different model of apoptosis in macrophages is provided by infection with Mycobacterium tuberculosis. Apoptosis occurs both in vitro and in vivo but, unlike the examples listed above, apoptosis in this example may be a consequence of an effective immune response to infection which limits intracellular bacterial replication.67,68 In mice the Nramp-1 (natural resistance-associated macrophage protein 1) gene influences susceptibility to infection with intracellular pathogens including mycobacteria, Leishmania spp. and S. typhimurium.69 Mice which are relatively resistant to infection demonstrate enhanced nitric oxide-dependent killing of mycobacteria and enhanced macrophage apoptosis compared to those that are susceptible to infection.67 As shown in Figure 1, Streptococcus pneumoniae can also induce apoptosis in macrophages and, in this in vitro model, infection is associated with intracellular killing of bacteria rather than evasion of the host immune response to infection.70 Furthermore, in a murine model of P. aeruginosa pneumonia apoptosis induction in bronchial epithelium is associated with protection against sepsis and death.71 These examples illustrate that in bacterial infections apoptosis induction may be beneficial to the host and linked with successful killing of micro-organisms. Apoptosis in bacterial infections contributes to organ damage. In animal models of meningitis, bacteria such as group B. streptococci and S. pneumoniae induce neuronal apoptosis, which has a propensity to involve the hippocampus and contributes to the morbidity of infection.72,73 Interestingly, some microbial pathogens inhibit apoptosis in phagocytes or lymphocytes and this may facilitate intracellular survival. Although a controversial finding, some investigators have demonstrated that the neisserial outer membrane protein PorB can inhibit apoptosis in lymphocytes,74 L. pneumophila survival in murine macrophages may be enhanced by the transcriptional upregulation of an IAP family member neuronal apoptosis inhibitory protein (Naip).75 M. tuberculosis enhances its intracellular survival by cleaving TNF-R2, which can then bind TNF hence inhibiting TNF-mediated apoptosis via uncleaved TNF-R1.76 The intracellular parasite Trypanosoma cruzi, which causes

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Chagas’ disease, may survive in an intracellular environment within macrophages by inhibiting nitric oxidedependent killing mechanisms.77 This is facilitated by a novel mechanism involving the ingestion of apoptotic bodies resulting from T. cruzi-induced AICD of lymphocytes by macrophages. The molecular mechanisms of bacterial-associated apoptosis are incompletely delineated although many involve caspase activation.57,59,60 In contrast to viralinduced apoptosis, death receptors have been studied less and when documented have not been found important mechanisms of non-viral induced apoptosis – with the exception of epithelial cell apoptosis in a murine model of P. aeruginosa infection,71 apoptosis induction in peripheral blood monocytes associated with phagocytosis of S. aureus,78 and macrophage apoptosis in an in vitro model of infection with M. tuberculosis,79 all of which are Fas-mediated. Recently, the Toll-like receptors (TLR) have received considerable attention in innate immunity and signalling via TLR-2 appears capable of apoptosis induction.80,81 A complex balance exists: bacteria or parasites can induce apoptosis in immune cells responding to infection to inhibit an immune response, or can inhibit apoptosis in directly infected cells to facilitate intracellular replication or survival. Alternatively, the host can induce apoptosis in directly infected cells to inhibit microbial replication. Host induction of apoptosis may have other advantages to the host organism in addition to controlling intracellular replication of pathogens as tissue damage from inflammation in limited and apoptotic macrophages can be used as a source of antigen for presentation by dendritic cells.5,82 However, non-specific tissue damage may be an unwanted side-effect of this response.

Modulation of Infection Associated Apoptosis as a Therapeutic Strategy As apoptosis is a normal physiological process occurring in all multicellular organisms, any therapeutic strategy which attempts to modulate apoptosis needs to be highly selective and should target specific pathways modulated by a given infection. Our understanding of how infectious diseases modulate these pathways is in its infancy. However, as much redundancy exists in these signalling pathways, it is possible that modulation of pathogen induced apoptosis might have clinical relevance. While therapeutic strategies specifically designed to achieve this goal are only starting to be investigated, a number of therapies already in clinical practice modulate apoptosis indirectly.

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Effective highly active antiretroviral therapy (HAART) decreases HIV viral replication and increases CD4 Tlymphocyte numbers but also decreases CD4 T-lymphocyte apoptosis.43,83 A decrease in plasma HIV-1 viral copy number correlates with decreased susceptibility to Fasmediated apoptosis in CD4 T-lymphocytes from HIVseropositive individuals. Nevertheless the effects are not completely explained by the inhibition of viral replication. Some individuals have a discordant response to HAART and CD4 T-lymphocyte numbers may increase in the absence of inhibition of viral replication.84 It is interesting that the above studies demonstrating apoptosis inhibition by HAART have all used protease inhibitor (PI) containing HAART regimens. PIs decrease levels of CD4 T-lymphocyte apoptosis and caspase activation in vitro in cells derived from HIV-seropositive and HIV-seronegative donors, which suggests inhibition of apoptosis is not exclusively the result of decreased viral replication.85,86 A variety of other therapeutic agents have the potential to modulate apoptosis including antimicrobials, cytokines, vitamin D and prostaglandin inhibitors.77 The utility of exploring the therapeutic modulation of apoptosis is demonstrated by a murine model of sepsis in which the inhibition of lymphocyte apoptosis by a chemical inhibitor of caspase activation or by overexpression of Bcl-2 resulted in improved survial.87 However the manipulation of apoptosis may also contribute to the side-effects of particular regimens.

Conclusions Apoptosis is a physiological process which can be modulated by infection. Often it is not just the fact that modulation occurs but the altered kinetics of cell death which induces disease pathogenesis. Directly infected cells may undergo apoptosis either as the result of induction by microbial products or due to immune-mediated killing. Alternatively the same factors may induce apoptosis in uninfected cells which contributes to the morbidity of infection by adding to the net state of immunosuppression or by inducing end-organ tissue damage. Microbial pathogens may also inhibit apoptosis in directly infected cells as a means of enhancing intracellular survival and replication. Hence a dynamic balance exists between microbial pathogens and the host immune system as it attempts to control infection and this results in modulation of apoptosis. The consequences of this process contribute to the outcome of infection and to disease pathogenesis. An improved understanding of infectionrelated modulation of apoptosis will aid the development of improved therapeutic strategies and immunization programmes.

Acknowledgments The author is indebted to Pauline Whitaker for invaluable secretarial support in the preparation of this manuscript, to Dr Carlos Paya, Dr Rob Read and to all in the Paya, Read or Dockrell laboratories who contributed to work cited. Farzana Ali provided the photograph used in Figure 1. The author s work is sponsored by grants from the Royal Society of Medicine, U.K., Grant No. 19 969 and the Special Trustees for the Former United Sheffield Hospitals Charitable Funds, Grant No. 87 776. This review is based on the text of the Barnett–Christie lecture presented at the Seventh Conference of the Federated Infection Society at Manchester on 30th November 2000.

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