Apoptosis in Liver Transplantation:A Mechanism Contributing to ImmuneModulation, Preservation Injury,Neoplasia, and Viral DiseaseTushar Patel and Gregory J. Gores

A poptosis is a morphologically distinct form ofcell death characterized by cell shrinkage,

chromatin condensation, and ultimately cell frag-mentation into apoptotic bodies.1 The most salientbiochemical features of apoptosis are proteaseactivation and DNA cleavage into oligonucleosomal-sized fragments.2 In health, apoptosis is importantin the deletion of senescent cells, damaged cells,and excess numbers of cells and in the limiting ofimmune reactions.3 Dysregulation of apoptosis playsa central role in many disease processes pertinentto liver transplantation (Fig. 1). Failure of immuno-cyte apoptosis promotes autoimmunity and exagger-ates immune reactions. Lymphotrophic virusesmay sustain infections by inhibiting host cellapoptosis, a process that may contribute to thedevelopment of posttransplantation lymphoprolif-erative disorders. In contrast, excessive apoptosisleads to tissue destruction and organ dysfunctionsuch as occurs in liver preservation injury andhepatic virus infections. In recent years, there havebeen major advances in the understanding of thegenetic and molecular mechanisms involved in theregulation of apoptosis. Increasing knowledge aboutthe involvement and contribution of apoptosis inpathophysiological states and its crucial role in thedevelopment, regulation, and function of the im-mune system are of particular interest in clinicalliver transplantation. In this overview we outlinethe importance, involvement, and role of apoptosisin the pathobiology of liver transplantation.

Cellular Mechanisms of Apoptosis

A wide variety of external and internal cellularstimuli can activate the cellular apoptotic machin-ery (Fig. 2). Intracellular processes contributing toapoptosis that have been elucidated to date includesignaling via the protein kinase C and ceramidepathways, activation of proteases of the caspaseprotease family (formerly known as the interleukin-1b–converting enzyme family) and stimulation ofendonuclease activity.4 A detailed description ofthe cellular mechanisms culminating in cellularapoptosis is provided in several recent excellentreviews and is beyond the scope of this overviewbecause of space constraints.4-6 However, we willbriefly review the concepts that are highly relevantto liver transplantation.

Triggering of apoptosis by the CD95/APO-1 orFas receptor (FasR) is of particular importance inorgan transplantation. FasR is a transmembraneprotein member of the tumor necrosis factor (TNF)/nerve growth factor receptor family. Both FasR andthe TNF receptor induce apoptosis in mammaliancells when bound to their respective ligands, theFas ligand and TNF, respectively.7 These receptorshave a conserved cytoplasmic domain (termed thedeath domain) that mediates the apoptotic signal.7

FasR is expressed in a variety of cells, includingepithelial cells, hematopoietic cells, and both T andB lymphocytes following activation.8,9 The patternof tissue expression of the Fas ligand (FasL) ismore restricted than that of FasR. FasL is expressedon mature CD41 and CD81 lymphocytes andnatural killer (NK) cells after antigen receptor–mediated activation but not on resting T cells.10

Some tissues such as the eye and testis constitu-tively express FasL.11 These tissues are sites ofimmunological privilege presumably because theFasL on the surface of these tissues destroys anyFasR-bearing alloreactive immunocytes by induc-ing apoptosis. Likewise, expression of both FasRand FasL after activation of mature lymphocytes

From the Division of Gastroenterology, Hepatology and LiverTransplantation, Mayo Clinic and Foundation, Rochester, MN.

Supported by grant DK 41876 from the National Institutes ofHealth, by the Gainey Foundation, and by the Mayo Foundation.

Address reprint requests to Gregory J. Gores, MD, Professor ofMedicine, Mayo Clinic, 200 First Street SW, Rochester, MN 55905.

Copyright r 1998 by the American Association for the Study ofLiver Diseases

1074-3022/96/0401-0006$3.00/0

Liver Transplantation and Surgery, Vol 4, No 1 ( January), 1998: pp 42-5042

may represent a mechanism of self-termination tohelp limit immune responses.

Apoptosis is controlled through the expressionof an increasing number of genes that are con-served in nematodes through to mammals.6 Ofthese, the bcl-2 gene family has received the mostattention.5,6 The proteins of the bcl-2 family areintracellular proteins that can either protect cellsfrom undergoing apoptosis or promote the apop-totic process. Bcl-2, Bcl-XL, Mcl-1, and A1 inhibitand Bcl-XS, Bax, Bak, and Bag promote apopto-sis.12,13

Apoptosis and the Immune System

Apoptosis is involved in the establishment of theperipheral T-cell repertoire as well as in regulatingT-cell function after activation.14,15 Cell death dur-ing development is often referred to as pro-grammed cell death to distinguish it from activation-induced cell death.14 Although the distinctionbetween activation-induced cell death and pro-grammed cell death is helpful in understanding therole of initiating stimuli and specific mechanismsinvolved, the overall end result is identical, namelycellular death by apoptosis. Therefore, in thisoverview we consider these two phenomena to-gether as examples of the importance of apoptosisin the immune system. Apoptosis can be inducedin lymphocytes in a variety of situations dependingon maturational stage (Table 1).

Role of Apoptosis in Development of theImmune System

The importance of apoptotic cell death during thedevelopment of the immune system is emphasizedby the critical involvement of this process at severalsteps in the homeostatic maintenance of lymphoidcell populations. This cell death is highly regulatedand serves to eliminate unwanted or potentiallyself-reactive cells, as well as to select cell popula-tions bearing functional antigen receptors. During

Figure 1. Potential rolesof cell death by apoptosisin the pathophysiology ofliver transplantation.

Figure 2. Schematic representation of the trig-gers and cellular events during apoptosis.

Apoptosis in Liver Transplantation 43

lymphoid development, several selective and highlyregulated cell death pathways are involved, result-ing in selective cell death at specific maturationalstages. Some 95% of T- and B-cell precursors dieduring development, mostly at the pro-B and pre-Bstages (B-cell precursors) and the double-positivestage (T-cell precursors). These cells are rapidlyremoved in situ by phagocytosis. Further selectionbased on the interaction of the T-cell receptor(TCR) and the MHC complex serves to maintain afunctional MHC-restricted T-cell repertoire by theelimination of TCRs that fail to interact with MHCmolecules (benign neglect). During lymphocytematuration, immature CD4/CD8 T cells bearingreceptors to tolerogenic antigens are also elimi-nated (clonal deletion).16-18

Role of Apoptosis in Controlling T-CellResponses After Activation

Under certain circumstances, mature T cells un-dergo apoptosis after activation. Although the initi-ating stimuli and mechanisms involved are quitedifferent from those seen during lymphoid celldevelopment, cells undergoing activation-inducedcell death demonstrate morphological features ofapoptosis.14 The mechanisms by which previouslyactivated T cells undergo apoptosis remain poorlyunderstood. Apoptosis can occur as a result of avariety of stimuli such as signaling via cell surface

receptors (such as FasR), deprivation of growthfactors (such as IL-2), or exposure to hormones(such as glucocorticoids). Because activated T cellsrequire growth factors for survival, any mechanisminfluencing interleukin (IL) 2 release can poten-tially prevent or reduce clonal expansion of T cellsby inducing apoptosis. The threshold for activationof the apoptotic program is dynamically regulatedby the expression and interactions between factorssuch as members of the bcl-2 gene family (e.g.,bcl-2/bcl-XL) and by extrinsic factors, such ascostimulatory signals and cytokines.19

An increasing appreciation of the role of costimu-latory molecules in TCR-mediated pathways showsthat the survival of stimulated T cells depends oncostimulation.20 Costimulatory signals permit animmune response after TCR ligation and are re-quired to prevent cell anergy or apoptosis. In vivo,the CD28 costimulatory molecule rescues T cellsfrom apoptosis after activation; this effect can bereversed by the inhibitor CTLA4Ig.20 Indeed, block-ade of the interaction of CD28 with B7 will preventexperimental transplant rejection, presumably be-cause alloreactive T cells undergo apoptosis with-out the costimulatory signal.21,22 The mechanismsby which costimulation protects from apoptosismay involve augmented lymphokine (e.g., IL-2)production that acts as a cell survival stimulus. Inaddition, CD28 also enhances Bcl-XL production,thereby conferring greater resistance against apop-tosis.23 In contrast, the CD28 homologue, CTLA4,is also expressed after activation but negativelyregulates T-cell activation, possibly by inducingapoptosis.24,25

A cardinal feature of specific immune responsesis self-limitation. This can be achieved by elimina-tion of the inciting antigen or by termination of theimmune response. The mechanisms resulting intermination of the immune response are also usedto maintain tolerance to peripheral self-antigensand to prevent autoimmunity and alloimmunity.Termination of the immune response can occur byT-cell–mediated suppression of effector cells, induc-tion of anergy, or apoptosis of stimulated T cells.Identifying the stimuli and operative mechanismsfor apoptosis will allow us to manipulate andswitch off specific T-cell–dependent immune re-sponses for clinical benefit. Understanding the roleof apoptosis in the limitation of T-cell–dependentimmune responses may ultimately permit the pre-vention or amelioration of unwanted reactionssuch as graft rejection.

Table 1. Inducers of Apoptosis in Lymphocytes

Immature lymphocytesFailure to rearrange antigen-receptor genesFailure of positive selectionNegative selection (IgM or TCR signaling) in

thymus or bone marrowGlucocorticoids

Mature T cellsAbsence of antigen, growth factors, or costimula-

tionExposure to self-antigen or repeated high doses

of foreign antigen or superantigensGlucocorticoids

Mature B cellsAbsence of antigen, growth factors, or T-cell inter-

actions or stimulationReduced antigen affinity in germinal centersExtensive Ig crosslinking (in the absence of CD40

ligation, IL-4)Glucocorticoids

Patel and Gores44

The liver has been proposed as a site forextrahepatic T-cell development and plays an impor-tant role in deletion of mature peripheral T cellsdestined for apoptosis after antigenic stimulationof the TCR.26 In lpr mutant mice who have agenetic defect in the T-cell apoptosis mechanism,functional failure of this process results in expan-sion of the intrahepatic T-cell pool. The expansionof this intrahepatic T-cell pool probably contrib-utes to the autoimmunity observed in these ani-mals.

Induction of Apoptosis in Target Cells

Identifying the mechanisms by which immuneeffector cells destroy their targets is of fundamentalimportance in understanding the immune responseto transplanted allografts. Cytotoxic T lympho-cytes induce target cell death by at least twoseparate pathways, both of which induce cell deathby apoptosis (Fig. 3).27 The secretory pathway ismediated by secretion of perforin, which permitspenetration of cosecreted serine proteases (gran-zymes) into the nucleus of the target cell, inducingapoptosis. The nonsecretory pathway involves trig-gering of cell surface receptors by the Fas/FasLpathway. Modulation of target cell apoptosis byinhibition of either the secretory (perforin/gran-zyme) or the nonsecretory (Fas/FasL) pathwaymay thus represent a potential mechanism ofavoiding immune-mediated damage to target cells(such as biliary epithelial cells).

Role of Apoptosis in Tolerance Induction

Tolerance may be defined as allograft- or xenograft-directed immunological nonreactivity while immu-nological reactivity toward other sites is main-tained. The ability to achieve graft tolerance mayavoid the need for nonspecific immunosuppres-sion. There are several potential mechanisms ofinducing donor-specific tolerance, including induc-tion of apoptosis in peripheral and thymic alloreac-tive T cells and ‘‘arming’’ of target cells with Fasligand to induce apoptosis in cytotoxic T lympho-cytes. Compared with other solid organs, the liverappears to have a privileged status in the develop-ment of tolerance. Spontaneous acceptance of ge-netically different liver grafts has been reported inexperimental liver transplantation.28,29 Further-more, experimental and clinical evidence exists tosupport a central role of the liver in the inductionof tolerance to other organ grafts.30 The mecha-nisms by which tolerance is induced in liverallografts remains unknown. Although functionalinactivation of donor-reactive cells (induction ofanergy) may play a role, depletion of donor-reactive cells by apoptosis may also represent apotential mechanism contributing to the inductionof tolerance. Fas/FasL interactions have been impli-cated in this process.31 The eye and testis areimmunologically privileged sites, and both tissuesconstitutively express FasL.11 Reactive CTLs withFas on their surface undergo apoptosis by the FasLon these potential target cells, preventing immuneinjury. Expression of FasL on donor cells is thus anexisting mechanism for induction of tolerance.These observations are of particular importance inxenotransplantation because the expression of FasLby gene transfer technology and genetic manipula-tion may be a potential means of inducing xeno-graft tolerance. However, to date, experimentsusing transgenic expression of FasL as a mecha-nism of inducing tolerance have been disappoint-ing.32 It appears the Fas/FasL system may signal forthe production of IL-8, leading to recruitment ofgranulocytes in the graft. The granulocytic inflam-mation has been associated with graft failure. Analternative mechanism for induction of immuneprivilege may involve soluble HLA class 1 molecule–induced CTL apoptosis.33

Immunosuppressive Drugs

In recent years, increased understanding of themechanisms of allograft rejection has contributed

Figure 3. Involvement of Fas/FasL in target celldamage and limitation of immune responses.Cytotoxic T lymphocytes induce target cell deathby secretion of perforin and granzymes or bytriggering of cell surface receptors such as Fas.Fas/FasL interactions may represent a mecha-nism of limitation of the immune system.

Apoptosis in Liver Transplantation 45

to the development of a wide range of immunosup-pressive drugs, many of which are currently under-going preclinical and clinical trials. Drugs thatpromote apoptosis of activated T cells could theo-retically be used to inhibit lymphocyte responsesduring allograft rejection. Elucidating the role ofapoptosis in allograft rejection and identification ofthe cellular and biochemical mechanisms involvedin apoptosis may thus ultimately result in thedevelopment of novel immunosuppressive drugs.

Glucocorticoids are potent immunosuppressiveagents that form a cornerstone of combinationimmunosuppressive regimens and are also used asfirst-line therapy for acute allograft rejection. Glu-cocorticoid receptors are present in most mamma-lian tissues and may account for the wide range ofpharmacological effects of glucocorticoids. Theprecise mechanisms of their immunosuppressiveproperties are poorly understood, but potentialmechanisms include reduced synthesis of MHCgene products or cytokines (such as interferon alfaor IL-2) or the modulation of apoptosis. Glucocor-ticoids regulate negative selection of immature Tcells in the thymus by apoptosis. Indeed, glucocor-ticoid-induced apoptosis in immature thymocytesis a classical example of apoptosis, and the biochem-istry and molecular events involved have beenstudied extensively.2 The effects of glucocorticoidson mature T cells differ from those on immaturethymocytes, and glucocorticoids may act as ormodulate signals for clonal deletion during estab-lishment of T-cell repertoire by thymic selection.

Cyclosporine A and tacrolimus are drugs cur-rently used in liver transplantation to suppressimmune responses and prevent allograft rejection.The molecular mechanisms of action of these drugshas been outlined in recent reviews.34 Both drugsbind to immunophilins (cyclophilin and FK-binding proteins, respectively), and are believed toact by inhibiting calcineurin, a calcium/calmodulin-dependent phosphatase.34 The level of calcineurinactivity correlates with the ability of T cells toundergo apoptosis.35 Indeed, tacrolimus augmentsthymic apoptosis induced by in vivo anti-CD3antibody administration in antigen-stimulated thy-mic cells and activation-induced apoptosis in SEB-activated peripheral splenic T lymphocytes. Thusboth cyclosporine and tacrolimus can influenceapoptosis by calcineurin-dependent mechanisms,resulting in disruption of the signal transductionpathways in recipient T cells in response to graft

antigens and thereby contributing to graft toler-ance. However, establishment of tolerance prob-ably involves multiple mechanisms such as inhibi-tion of T-cell activation, interference with cell-to-cell interactions, altered cytokine activity, deficientrecipient APC function, and clonal deletion ofself-reactive B cells. Differences between cyclospo-rine and tacrolimus in the ability to induce toler-ance may thus reflect differences in their effects onmechanisms other than impaired signal transduc-tion that contribute to the induction of tolerance.Cyclosporine, for example, can act in a calcineurin-independent pathway to enhance apoptosis ofstimulated T cells in vitro and can block TCR-mediated apoptosis by inhibiting the DNA bindingactivity of the transcription factor Nur 77 ratherthan by blocking its synthesis.36,37 Further studieson drug-facilitated apoptosis of activated T cellswill contribute to better understanding of theeffects of these drugs on tolerance and lead toimproved immunosuppressive strategies. Immuno-suppressive drugs may have effects on B cells aswell. Both cyclosporine and tacrolimus preventinduction of apoptosis by anti-CD2 monoclonalantibodies and protect B-cell lymphoma cells fromapoptosis caused by ligation of surface immuno-globulin (Ig) M.

Apoptosis in Clinical Transplantation

Graft Rejection

Apoptosis has long been recognized in liver allo-graft rejection. Histological and ultrastructural fea-tures of apoptosis in experimental porcine liverallograft rejection were first noted by Battersby etal38 and Searle et al.39 In an experimental model ofrat liver transplantation, both morphological andbiochemical features of apoptosis correlated withgraft rejection.40 Apoptotic cell death occurred inhepatocytes as well as biliary epithelial cells duringrejection. Studies in patients with chronic rejectionafter liver transplantation show increases in periven-ular apoptotic hepatocytes and evidence of DNAfragmentation noted in mononuclear cells withinportal tracts.41 Apoptosis of biliary epithelial cellshas also been noted in acute hepatic allograftrejection and may involve downregulation of theanti-apoptotic protein bcl-2.42,43 These studies sup-port a role for apoptotic cell death during acuterejection as well as the ductopenia of chronic graftrejection after hepatic transplantation.

Patel and Gores46

Allograft rejection is a T-cell–dependent im-mune response, and cytotoxic T cells are involvedin the immunopathogenesis of acute cellular rejec-tion after liver transplantation. Cytotoxic T cellsinduce target cell apoptosis by perforin and granuleexocytosis or by the Fas/FasL mechanisms (Fig. 3).However, the precise role played by the Fas/FasLsystem in rejection after liver transplantation re-mains controversial. One study showed the ab-sence of FasL in biopsy specimens from patientswith stable noninflamed allografts but the presenceof Fas expression in leukocytes and sinusoidal andendothelial cells during severe acute rejection. Thispattern persisted in chronic rejection.44 These ob-servations support a role for Fas/FasL-mediatedtissue injury in liver allograft rejection. However,in a contrasting study, FasL message but not Faswas upregulated in allogeneic rat liver transplanta-tion and in nonrejecting allografts.45 This studysuggests that Fas/FasL interactions are not essentialmediators of T-cell–induced allograft damage. Fur-ther studies are needed to elucidate the precise roleof Fas/FasL in liver allograft rejection.

Preservation and Reperfusion Injury

The development of storage solutions has permit-ted longer preservation time before liver transplan-tation. However, total ischemia time is predictive ofgraft outcome, and prolonged cold storage can stillresult in injury to the liver. Tissues undergoingtransient ischemia are subject to further damageafter reperfusion. The spectrum of manifestationsof storage injury range from postoperative eleva-tions of transaminase levels and functional chole-stasis to primary graft nonfunction. The durationof cold ischemia time is also related to the develop-ment of nonanastomotic biliary strictures and maypredispose to rejection. Ischemia to the liver causesaccumulation of neutrophils and activation of Kup-pfer cells with the release of soluble toxic media-tors and cytokines and the formation of oxygen-free radicals. Although cell necrosis is the assumedend-result of ischemia-reperfusion injury, cell deathby apoptosis can result from these identified mecha-nisms and has been demonstrated during preserva-tion-induced damage to cultured endothelial cellsas well as after reperfusion following swine livertransplantation.46,47 These observations implicatecellular apoptosis as a mechanism contributing toliver preservation injury.

Neoplasia and PosttransplantationLymphoproliferative Disorders

The balance between cellular proliferation andapoptosis plays a critical role in maintenance ofnormal tissue homeostasis. Dysregulated clonalexpansion characteristic of neoplasia can resultfrom derangement of either enhanced cellularproliferation or failure of cellular apoptosis. Pa-tients who undergo immunosuppression after livertransplantation have an increased frequency oflymphoproliferative disorders, known as posttrans-plant lymphoproliferative disorders (PTLD). Al-though these lymphoproliferative disorders areuncommon, they are often fatal. Development ofPTLD is associated with the intensity of immuno-suppression and is linked to infection with theEpstein-Barr virus (EBV). EBV can exist in a latentform in B lymphocytes or in a replicative form inepithelial cells. Cell-mediated immune responsesare critical in controlling the outgrowth of infectedB lymphocytes, and the proliferation of B lympho-cytes is usually restrained by EBV-specific cyto-toxic T and NK cells. In the presence of immuno-suppression that inhibits T-cell function, virallydriven B-cell lymphoproliferation can go un-checked. EBV is thought to promote the develop-ment of PTLD by inhibiting normal B-cell apopto-sis. EBV-infected cells may resist apoptosis via viralencoded proteins or increasing cellular expressionof Bcl-2.48. BHRF-1 is an EBV-encoded homologueof Bcl-2 that is functionally similar to Bcl-2. How-ever, BHRF-1 does not appear to be upregulated inPTLD, and thus its precise role and functionremain unknown.49 Further investigation of themechanism, role, and involvement of BHRF-1 andBcl-2 in the pathogenesis of PTLD may offer analternative approach to the management of theseneoplasms. Antisense technologies to inhibit expres-sion of these proteins may be useful in the treat-ment of PTLD.

Viral Infection

Immunosuppression after transplantation facili-tates the development of host infections by nosoco-mial, opportunistic, or reactivation of preexistinginfections. Viral infection can result in cell damageby multiple mechanisms such as direct viral cyto-toxicity, suppression of cellular metabolism, mo-lecular mimicry, or generation of an immune attackon infected cells. Induction of apoptosis contrib-

Apoptosis in Liver Transplantation 47

utes to the cytopathic effects of viral infection, andmorphological features of apoptosis are often seenin cells undergoing direct viral cytotoxicity.

Evidence that viruses can trigger and regulatethe pathways leading to apoptosis is emerging fromseveral studies.50 Regulation of apoptosis by viralgene products can serve as a mechanism for viralsurvival. By inhibiting target cell apoptosis, forexample, viruses may facilitate viral replication,spread, or persistence. On the other hand, byinduction of cell apoptosis rather than cell necro-sis, the immune response against infected cells maybe reduced by preventing the release of viralintracellular antigens. Viral gene products cantrigger apoptosis by various mechanisms. Thegp120 envelope protein of the human immunodefi-ciency virus, for example, can trigger apoptosis byacting as a ligand for the CD4 receptor. Viruses mayinfluence host-cell apoptosis via mechanisms in-volving FasR-FasL interactions or by inhibition ofperforin-granzyme activity by virally encoded pro-tease inhibitors.51

Cytomegalovirus (CMV) infection occurs in asmany as 65% of patients undergoing liver transplan-tation, and prophylactic antiviral therapy to reduceCMV infection is often prescribed. T-cell activationwith elevated CD81 T cells occurs during CMVviremia. CD81 cell expansion is accompanied bydecreased Bcl-2 and increased Fas expression andapoptosis.52 Resolution of viremia is accompaniedby normalization of the CD81 count in immuno-competent hosts. However, persistence of CD81lymphocytes occurs in acute CMV infection aftertransplantation. Cyclosporine A increases Bcl-2expression in vivo and in vitro and may contributeto the prevention of apoptosis of CD81 lymphocy-tosis in CMV-infected allografts.52

The presence of acidophilic or Councilmanbodies in liver biopsy specimens supports a role forapoptosis in hepatocyte death during chronic viralhepatitis.53 Increased Fas antigen expression hasbeen noted in liver biopsy specimens from patientswith chronic hepatitis B virus or hepatitis C virusand correlates with the activity of viral hepatitis inpatients with hepatitis B virus infection.54 Further-more, using the hepatitis B surface antigen trans-genic mouse model, MHC class I–restricted CTLswere shown to induce hepatocyte apoptosis.55

Although the precise mechanisms leading to in-creased Fas expression during viral infection re-main unknown, hepatocyte apoptosis may resultfrom Fas-mediated pathways such as interactions

with soluble FasL or FasL-bearing CTLs. Manipula-tion of the mechanisms by which FasR-FasL inter-actions (such as inhibition of circulating FasL) orinhibition of virally encoded protease inhibitorsthat mediate resistance to apoptosis result in hepa-tocyte apoptosis may provide novel approaches toprevention of cell injury from recurrent viral infec-tion after transplantation.

Summary and Perspectives

Although clinical liver transplantation has becomea reality for the treatment of previously fatalend-stage chronic liver disease or fulminant he-patic failure, there are several limitations to its use.Allograft rejection is prevented only by inductionof an artificial state of immunocompetence in therecipient by the use of nonspecific immunosuppres-sion. This increases the risk of infection andmalignancy. These complications can be avoidedonly if tolerance to specific donor organ antigens isachieved and nonspecific immunosuppression isavoided. Understanding the role of apoptosis in theimmune response to transplantation and study ofthe molecular and biochemical modulation of apop-tosis may provide fertile ground for investigationrelevant to liver transplantation. Such study mayyield novel approaches to (1) the diagnosis, treat-ment, or prevention of rejection after transplanta-tion; (2) therapeutic modulation of apoptosis ineffector and target cells to limit immune-mediateddamage; (3) rational immunosuppressive drug de-sign; (4) strategies to allow development of grafttolerance, e.g., by selective deletion of antigen-specific T-cell populations; and (5) further avenuesfor research into the pathophysiology of conditionsassociated with transplantation such as malig-nancy, infection, preservation injury, and recurrentdisease.

AcknowledgmentThe secretarial assistance of Sara Erickson is gratefullyacknowledged.

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