Dendritic Cells, Tolerance Induction and Transplant Outcome

Abstract

Dendritic cells (DC) have been viewed traditionally as ‘foes’ that instigate the rejection process. More recently, they have gained considerable attention as antigen (Ag)‐presenting cells (APC) that are also capable of induction/maintenance of immunologic tolerance (1Banchereau J Steinman RM Dendritic cells and the control of immunity.Nature. 1998; 392: 245-251Crossref PubMed Scopus (12271) Google Scholar, 2Lechler RI Ng WF Steinman RM Dendritic cells in transplantation – friend or foe.Immunity. 2001; 14: 357-368Abstract Full Text Full Text PDF PubMed Scopus (258) Google Scholar, 3Hackstein H Morelli AE Thomson AW Designer dendritic cells for tolerance induction: guided not misguided missiles.Trends Immunol. 2001; 22: 437-442Abstract Full Text Full Text PDF PubMed Scopus (124) Google Scholar). This contemporary view of DC, reflects new insights into how these cells regulate T cell responses. Herein we review accumulating evidence that manipulation of DC can suppress Ag‐specific immune responses and build a case that this may provide a novel approach to therapy of allograft rejection. Observations in rodents, nonhuman primates and humans that support or are consistent with tolerogenic activity of DC are summarized in Table 1 (3Hackstein H Morelli AE Thomson AW Designer dendritic cells for tolerance induction: guided not misguided missiles.Trends Immunol. 2001; 22: 437-442Abstract Full Text Full Text PDF PubMed Scopus (124) Google Scholar, 4Peugh W Austyn J Carter N Wood K Morris P Inability of dendritic cells to prevent the blood transfusion effect in a mouse cardiac allograft.Transplantation. 1987; 44: 706-711Crossref PubMed Scopus (15) Google Scholar, 5Demetris A Murase N Starzl T Donor dendritic cells after liver and heart allotransplantation under short‐term immunosuppression.Lancet. 1992; 339: 1610Abstract Scopus (46) Google Scholar, 6Qian S Demetris A Murase N Rao A Fung J Starzl T Murine liver allograft transplantation: tolerance and donor cell chimerism.Hepatology. 1994; 19: 916-923Crossref PubMed Scopus (387) Google Scholar, 7Lu L Rudert W Qian S et al.Growth of donor‐derived dendritic cells from the bone marrow of murine liver allgraft reccipients in response to granulocyte/macrophage colony stimulating factor.J Exp Med. 1995; 182: 379-387Crossref PubMed Scopus (161) Google Scholar, 8Rastellini C Lu L Ricordi C Starzl TE Rao AS Thomson AW Granulocyte/macrophage colony‐stimulating factor‐stimulated hepatic dendritic cell progenitors prolong pancreatic islet allograft survival.Transplantation. 1995; 60: 1366-1370PubMed Google Scholar, 9Fu F Li Y Qian S et al.Costimulatory molecule‐deficient dendritic cell progenitors (MHC class II+,CD80dim, CD86‐) prolong cardiac allograft survival in nonimmunosuppressed recipients.Transplantation. 1996; 62: 659-665Crossref PubMed Scopus (354) Google Scholar, 10Josien R Heslan M Brouard S Soulillou J‐P Cuturi M‐C Critical requirement for graft passenger leukocytes in allograft tolerance induced by donor blood transfusion...Blood. 1998; 92: 4539-4544Crossref PubMed Google Scholar, 11Garrovillo M Ali A Oluwole S Indirect allorecognition in acquired thymic tolerance: induction of donor‐specific tolerance to rat cardiac allografts by allopeptide‐pulsed host dendritic cells.Transplantation. 1999; 68: 1827-1834Crossref PubMed Scopus (72) Google Scholar, 12Min W‐P Gorczynski R Huang X‐Y et al.Dendritic cells genetically engineered to express Fas Ligand induce donor‐specific hyporesponsiveness and prolong allograft survival.J Immunology. 2000; 164: 161-167Crossref PubMed Scopus (223) Google Scholar, 13Lu L Li W Fu F et al.Blockade of the CD40‐CD40 ligand pathway potentiates the capacity of donor‐derived dendritic cell progenitors to induce long‐term cardiac allograft survival.Transplantation. 1997; 64: 1808-1815Crossref PubMed Scopus (186) Google Scholar, 14Niimi M Shirasugi N Ikeda Y Kan S Takami H Hamano K Operational tolerance induced by pretreatment with donor dendritic cells under blockade of CD40 pathway.Transplantation. 2001; 72: 1556-1562Crossref PubMed Scopus (32) Google Scholar, 15Thomas J Contreras J Jiang X et al.Peritransplant tolerance induction in macaques: early events reflecting the unique synergy between immunotoxin and deoxyspergualin.Transplantation. 1999; 68: 1660-1673Crossref PubMed Scopus (132) Google Scholar, 16Jonuleit L Schmitt E Schuler G Knop J Enk AH Induction of interleukin 10‐producing, nonproliferating CD4+ T cells with regulatory properties by repetitive stimulation with allogeneic immature human dendritic cells.J Exp Med. 2000; 192: 1213-1222Crossref PubMed Scopus (1345) Google Scholar, 17Dhodapkar MV Steinman RM Krasovsky J Munz C Bhardwaj N Antigen‐specific inhibition of effector T cell function in humans after injection of immature dendritic cells.J Exp Med. 2001; 193: 233-238Crossref PubMed Scopus (1212) Google Scholar).Table 1Transplant‐related observations in rodents, humans and nonhuman primates, consistent with tolerogenic properties of DCDonor DC (splenic) can prolong organ allograft survival (mice) (4Peugh W Austyn J Carter N Wood K Morris P Inability of dendritic cells to prevent the blood transfusion effect in a mouse cardiac allograft.Transplantation. 1987; 44: 706-711Crossref PubMed Scopus (15) Google Scholar)Persistence of donor DC in long‐term rodent or human recipients of organ allografts, including stable patients off immunosuppression (5Demetris A Murase N Starzl T Donor dendritic cells after liver and heart allotransplantation under short‐term immunosuppression.Lancet. 1992; 339: 1610Abstract Scopus (46) Google Scholar, 6Qian S Demetris A Murase N Rao A Fung J Starzl T Murine liver allograft transplantation: tolerance and donor cell chimerism.Hepatology. 1994; 19: 916-923Crossref PubMed Scopus (387) Google Scholar)Propagation of donor DC from tolerant but not acutely rejecting organ allograft recipients (mice) (7Lu L Rudert W Qian S et al.Growth of donor‐derived dendritic cells from the bone marrow of murine liver allgraft reccipients in response to granulocyte/macrophage colony stimulating factor.J Exp Med. 1995; 182: 379-387Crossref PubMed Scopus (161) Google Scholar)Infusion of immature donor DC prolongs fully allogeneic graft survival in nonimmunosuppressed recipients (mice) (8Rastellini C Lu L Ricordi C Starzl TE Rao AS Thomson AW Granulocyte/macrophage colony‐stimulating factor‐stimulated hepatic dendritic cell progenitors prolong pancreatic islet allograft survival.Transplantation. 1995; 60: 1366-1370PubMed Google Scholar, 9Fu F Li Y Qian S et al.Costimulatory molecule‐deficient dendritic cell progenitors (MHC class II+,CD80dim, CD86‐) prolong cardiac allograft survival in nonimmunosuppressed recipients.Transplantation. 1996; 62: 659-665Crossref PubMed Scopus (354) Google Scholar)Donor DC are essential for DST‐induced transplant tolerance (rats) (10Josien R Heslan M Brouard S Soulillou J‐P Cuturi M‐C Critical requirement for graft passenger leukocytes in allograft tolerance induced by donor blood transfusion...Blood. 1998; 92: 4539-4544Crossref PubMed Google Scholar)Recipient DC pulsed with donor MHC class I peptide promote organ transplant tolerance (rats) (11Garrovillo M Ali A Oluwole S Indirect allorecognition in acquired thymic tolerance: induction of donor‐specific tolerance to rat cardiac allografts by allopeptide‐pulsed host dendritic cells.Transplantation. 1999; 68: 1827-1834Crossref PubMed Scopus (72) Google Scholar)Genetically modified (FasL‐transduced) donor DC prolong organ allograft survival (mice) without adjunctive immunosuppression (12Min W‐P Gorczynski R Huang X‐Y et al.Dendritic cells genetically engineered to express Fas Ligand induce donor‐specific hyporesponsiveness and prolong allograft survival.J Immunology. 2000; 164: 161-167Crossref PubMed Scopus (223) Google Scholar)Immature or mature donor DC combined with anti‐CD154 mAb induce heart transplant tolerance (mice) (13Lu L Li W Fu F et al.Blockade of the CD40‐CD40 ligand pathway potentiates the capacity of donor‐derived dendritic cell progenitors to induce long‐term cardiac allograft survival.Transplantation. 1997; 64: 1808-1815Crossref PubMed Scopus (186) Google Scholar, 14Niimi M Shirasugi N Ikeda Y Kan S Takami H Hamano K Operational tolerance induced by pretreatment with donor dendritic cells under blockade of CD40 pathway.Transplantation. 2001; 72: 1556-1562Crossref PubMed Scopus (32) Google Scholar)Transplant tolerance is associated with in situ inhibition of DC maturation in host lymphoid tissue (rhesus monkey) (15Thomas J Contreras J Jiang X et al.Peritransplant tolerance induction in macaques: early events reflecting the unique synergy between immunotoxin and deoxyspergualin.Transplantation. 1999; 68: 1660-1673Crossref PubMed Scopus (132) Google Scholar)Diversely acting anti‐inflammatory and immunosuppressive drugs inhibit DC maturation (3Hackstein H Morelli AE Thomson AW Designer dendritic cells for tolerance induction: guided not misguided missiles.Trends Immunol. 2001; 22: 437-442Abstract Full Text Full Text PDF PubMed Scopus (124) Google Scholar)In vitro pulsing of allogeneic T cells with immature DC induces T regulatory cells (human) (16Jonuleit L Schmitt E Schuler G Knop J Enk AH Induction of interleukin 10‐producing, nonproliferating CD4+ T cells with regulatory properties by repetitive stimulation with allogeneic immature human dendritic cells.J Exp Med. 2000; 192: 1213-1222Crossref PubMed Scopus (1345) Google Scholar)Ag‐pulsed autologous DC induce Ag‐specific tolerance in normal human volunteers (17Dhodapkar MV Steinman RM Krasovsky J Munz C Bhardwaj N Antigen‐specific inhibition of effector T cell function in humans after injection of immature dendritic cells.J Exp Med. 2001; 193: 233-238Crossref PubMed Scopus (1212) Google Scholar) Open table in a new tab DC are rare, uniquely well‐equipped Ag‐processing and presenting leukocytes resident within virtually all tissues. They play pivotal roles in innate and adaptive immunity. DC constantly sample the local microenvironment by uptake of self and exogenous Ag. In response to infection/inflammation, or following organ transplantation, DC migrate from peripheral sites to T cell areas of secondary lymphoid tissue. This translocation is associated with their maturation, whereby surface expression of major histocompatibility complex (MHC) and costimulatory molecules (e.g. CD40,80,86) necessary for T cell activation is up‐regulated. DC produce the potent T helper (Th) cell‐driving cytokine (IL‐12) and are the only APC that can activate naïve T cells. Apart from their role as the principal instigators of immune reactivity, DC appear to be important in central and peripheral T cell tolerance. Thus, thymic DC are well recognized to delete potentially self‐reactive T cells, whereas immature DC freshly isolated or propagated from nonlymphoid tissues, such as the liver, respiratory tract or intestine, exhibit T cell tolerogenicity. Harnessing and optimizing the potential tolerogenicity of DC may lead to improved outcomes in cell and organ transplantation, including the alleviation of chronic rejection. Hematopoietic growth factors that potently and selectively mobilize DC may assist in reaching this goal. The origin of DC is a controversial issue in immunology. As leukocytes, DC are unique in the sense that they can be produced from either common lymphoid or common myeloid progenitors. Classic ‘myeloid’ DC, identified initially in murine secondary lymphoid tissue, can be isolated from virtually all tissues or propagated from blood or bone marrow with GM‐CSF and IL‐4. In mice, as in humans, several DC subsets have been described, including Langerhans cell‐derived DC, CD8α– (myeloid) and CD8α+ DC, B cell‐like DC and plasmacytoid DC (18Banchereau J Briere F Caux C et al.Immunobiology of dendritic cells.Ann Rev Immunol. 2000; 18: 767-811Crossref PubMed Scopus (5642) Google Scholar, 19Ardavin C Martinez del Hoyo G Martin P et al.Origin and differentiation of dendritic cells.Trends Immunology. 2001; 22: 691-700Abstract Full Text Full Text PDF PubMed Scopus (229) Google Scholar, 20Bjork P Isolation and characterization of plasmacytoid dendritic cells from Flt3 ligand and granulocyte macrophage ‐colony stimulating factor‐treated mice.Blood. 2001; 98: 3520-3526Crossref Scopus (294) Google Scholar). All subsets express CD11c, MHC class II and CD40, but exhibit differences in several other parameters. Initial findings suggested that CD8α+ DC (as opposed to classic CD8α– myeloid DC) were comparatively weak stimulators of CD4+ and CD8+ T cells, and could promote apoptotic death in alloreactive T cells via surface CD95 ligand (Fas ligand; FasL). This lead to speculation that these DC were involved in peripheral tolerance. Both CD8α– and CD8α+DC, however, mature into potent immunostimulatory APC. CD8α+ CD11b– DC secrete IL‐12, prime naive CD4+ T cells to secrete Th1 cytokines, and promote anti‐viral immunity by inducing cytotoxic T lymphocytes (CTL) via secretion of interferon‐γ (IFN‐γ) (21Hochrein H Shortman K Vremec D Scott B Hertzog P O'Keefe M Differential production of IL‐12, IFN‐α, and IFN‐γ by mouse dendritic cell subsets.J Immunol. 2001; 166: 5448-5455Crossref PubMed Scopus (425) Google Scholar). CD8α– CD11b+ DC, on the other hand, are strong stimulators of Th2 cell responses and secrete neither IL‐12 nor IFN‐γ. The murine CD8α+ DC subset has been shown to cross prime T cells (i.e. to take up, process and present exogenous Ag on class I MHC) and, at very low relative concentrations, to inhibit the immunostimulatory capacity of CD8α– DC for delayed‐type hypersensitivity responses (22Den Haan JMM Lehar SM Bevan MJ CD8+ but not CD8– dendritic cells cross prime cytotoxic T cells in vivo.J Exp Med. 1999; 192: 1685-1695Crossref Scopus (1043) Google Scholar). Whether or not this same cell type is capable of cross‐tolerance has yet to be determined. Recent phenotypic and functional analyses show considerable differences between murine and human DC. At least five types of human DC have been described: myeloid or ‘monocytoid’ DC [CD11c+, CD1a+ and CD123– (IL‐3 receptor‐α)], ‘plasmacytoid’ DC that are CD11c–, CD1a– and CD123+, Langerhans cell‐derived DC (CD11c+, CD1a+ and CD123–), B cell‐like DC, and follicular DC‐nonleukocytes that retain immune complexes in B cell follicles and that are believed to play an important role in immunologic memory. All express MHC class II and CD40. Both human monocytoid and plasmacytoid DC (sometimes referred to as DC1 and DC2, respectively) can polarize to either Th1 or Th2 responses. In humans, plasmacytoid DC are the principal IFN‐producing cells of the immune system. Moreover, IL‐12 production is a feature of human DC1 as compared with murine CD8α+ DC. Current understanding of the differentiation, classification and function of murine and human DC subsets has been reviewed comprehensively (19Ardavin C Martinez del Hoyo G Martin P et al.Origin and differentiation of dendritic cells.Trends Immunology. 2001; 22: 691-700Abstract Full Text Full Text PDF PubMed Scopus (229) Google Scholar, 23Pulendran B Maraskovsky E Banchereau J Maliszewski C Modulating the immune response with dendritic cells and their growth factors.Trends Immunol. 2001; 22: 41-47Abstract Full Text Full Text PDF PubMed Scopus (249) Google Scholar, 24Lotze MT Thomson AW Dendritic Cells: Biology and Clinical Applications.2nd edn. Academic Press, San Diego2001Google Scholar). Although it has been speculated that specialized ‘regulatory’ DC play a role in peripheral tolerance, present evidence is more consistent with the role of DC in tolerance being dependent on their stage of development or maturation (25Mellman I Steinman RM Dendritic cells: specialized and regulated antigen processing machines.Cell. 2001; 106: 255-258Abstract Full Text Full Text PDF PubMed Scopus (1847) Google Scholar). Through their capacity to efficiently ingest exogenous Ag and to present processed Ag to autologous T cells, DC are key instigators of the body's defense system. However, they are not constantly called upon to instigate defense against invading pathogens. In the normal steady state, DC constitute a relatively constant proportion of cells within afferent lymph – trafficking from peripheral sites to draining lymphoid tissue. Important new light has been shed recently on the purpose of this cellular migration, by recognition that immature DC are highly efficient at the uptake of apoptotic bodies in healthy tissues. Although DC may phagocytose necrotic as well as apoptotic bodies, only material derived from the former stimulates T cell immunity (26Sauter B Albert ML Francisco L Larrson M Somersan S Bhardwaj N Consequences of cell death: exposure to necrotic tumor cells, but not primary tissue cells or apoptotic cells, induces the maturation of immunostimulatory dendritic cells.J Exp Med. 2000; 191: 423-433Crossref PubMed Scopus (1220) Google Scholar). This suggests a mechanism by which tolerance to self Ag (as acquired by DC in the form of apoptotic bodies generated by constant cell turnover) may be achieved, and a function for immature tissue‐derived DC at those times when they are not required for active body defense. Evidence in support of this concept was provided by Huang et al. who identified apoptotic bodies of intestinal epithelial cell origin within rat DC trafficking to draining mesenteric lymph nodes (27Huang F‐P Platt N Wykes M et al.A discrete subpopulation of dendritic cells transports apoptotic intestinal epithelial cells to T cell areas of mesenteric lymph nodes.J Exp Med. 2000; 191: 435-443Crossref PubMed Scopus (782) Google Scholar). This putative role for DC in peripheral tolerance may be critical in the maintenance of self‐tolerance to Ags that cannot be presented by thymic DC within the neonatal period (such as mature ovarian or mature breast tissue Ags). Using DC loaded with antigenic peptide via the multilectin receptor DEC (CD) 205, Hawiger et al. have shown that, under steady‐state conditions, such peptide‐loaded DC do not induce Th polarization or either sustained T cell expansion or activation in vivo. Rather they induce Ag‐specific peripheral tolerance (T cell deletion/unresponsiveness) (28Hawiger D Inaba K Dorsett Y et al.Dendritic cells induce peripheral T cell unresponsiveness under steady state conditions in vivo.J Exp Med. 2001; 194: 769-779Crossref PubMed Scopus (1523) Google Scholar). How, precisely, Ags acquired within the periphery and transported to lymphoid tissues by DC are rendered nonimmunogenic is not clear. A possible role for murine CD8α+ DC resident within draining lymph nodes in T cell deletion has been suggested (29Steinman RM Turley S Mellman I Inaba K The induction of tolerance by dendritic cells that have captured apoptotic cells.J Exp Med. 2000; 191: 411-416Crossref PubMed Scopus (1012) Google Scholar). Given this information, the ability to target Ags to DC and control their function has significant implications for the development of therapies for allo‐ and autoimmunity. Rejection of organ allografts is a process associated, traditionally, with the migration of interstitial donor ‘passenger’ leukocytes to recipient lymphoid tissue. At the time of transplantation, various leukocytes (including B lymphocytes, monocytes, DC) and other cells, such as endothelial cells, make up the APC constituency of the graft. Migration of donor‐derived DC into host lymphoid tissue allows direct presentation of highly immunogenic, donor‐derived MHC Ags to recipient naïve T cells. Mature DC are the most potent APC, capable of directly activating naïve, alloAg‐specific cytotoxic CD8+ T cells via CD40 signaling, without CD4+ T cell help (18Banchereau J Briere F Caux C et al.Immunobiology of dendritic cells.Ann Rev Immunol. 2000; 18: 767-811Crossref PubMed Scopus (5642) Google Scholar). With respect to the indirect pathway, by which exogenous Ag is cross‐presented by DC, resulting in generation of MHC class I peptide complexes, there is new evidence that in vitro, mature DC can tolerize CD8+ T cells in the absence of CD4 Th cells, or a stimulus for CD40 (30Albert M Jegathesan M Darnell K Dendritic cell maturation is required for the cross‐tolerization of CD8+ T cells.Nat Immunol. 2001; 2: 1010-1017Crossref PubMed Scopus (346) Google Scholar). While donor DC are clearly important in direct allorecognition, recipient DC also play a significant role in graft rejection via the indirect pathway. Indirect allorecognition occurs when APC of host origin present donor peptides to host T cells in the context of recipient MHC molecules. The relative contribution of direct and indirect allorecognition to murine skin graft rejection has been examined recently (31Benichou G Valujskikh A Heeger PS Contributions of direct and indirect T cell alloreactivity during allograft rejection in mice.J Immunol. 1999; 162: 352-358Crossref PubMed Google Scholar). During acute rejection, < 10% of T cells recognized allopeptides presented indirectly. By contrast, the remaining 90% of responding T cells responded to directly presented donor MHC peptides. This predominant role of the direct pathway provides a rational basis for the manipulation of donor‐derived DC to prevent acute graft rejection and to promote tolerance induction. Indirect allorecognition on the other hand, is thought to be of greater significance in the pathogenesis of chronic rejection (32Auchincloss Jr, H Lee R Shea S Markowitz JS Grusby MJ Glimcher LH The role of ‘indirect’ recognition in initiating rejection of skin grafts from major histocompatibility complex class II‐deficient mice.Proc Natl Acad Sci USA. 1993; 90: 3373-3377Crossref PubMed Scopus (304) Google Scholar). Thus, manipulation of host‐derived DC by pulsing with donor MHC class I peptide to promote their tolerogenicity in organ and pancreatic islet cell transplantation, as described in rats (33Garrovillo M Ali A Depaz HA et al.Induction of transplant tolerance with immunodominant allopeptide‐pulsed host lymphoid and myeloid dendritic cells.Am J Transplant. 2001; 1: 129-138Abstract Full Text Full Text PDF PubMed Scopus (63) Google Scholar), may be an equally important approach to therapy of this process. The critical role that DC may play in determining the balance between transplant tolerance and rejection is indicated by murine liver transplant studies in which the number of potential allostimulatory donor DC within the graft is dramatically increased by treatment of donors with the potent endogenous hematopoietic growth factor, fms‐like tyrosine kinase 3 ligand (Flt3L). When Flt3L is administered to donor mice, liver allografts that would otherwise be accepted without immunosuppression, are rejected acutely. Studies of spontaneously accepted murine liver allografts in the B10 (H2b) to C3H (H2k) strain combination suggest that early apoptosis of alloreactive T cells (day 2–4 post‐transplant) is associated with successful liver engraftment and induction of donor‐specific tolerance. When acute liver rejection is induced by donor treatment with Flt3L (34Qian S Lu L Fu F et al.Donor pretreatment with Flt‐3 ligand augments antidonor cytotoxic T lymphocyte, natural killer cell, and lymphokine‐activated killer cell activities within liver allografts and alters the pattern of intragraft apoptotic activity.Transplantation. 1998; 65: 1590-1598Crossref PubMed Scopus (41) Google Scholar), or by recipient treatment with IL‐12 or IL‐2 (35Qian S Lu L Fu F et al.Apoptosis within spontaneously accepted mouse liver allografts: evidence for deletion of cytotoxic T cells and implications for tolerance induction.J Immunol. 1997; 158: 4654-4661Crossref PubMed Google Scholar), T cell apoptosis within the liver graft is dramatically reduced. This suggests that an active process of donor APC‐induced activation‐induced cell death (AICD) may facilitate the induction of liver transplant tolerance. It also raises the possibility that potentiation of this process might promote allograft survival in various types of organ transplantation. Blockade of costimulatory molecule expression on allogeneic DC promotes their capacity to induce AICD in alloactivated T cells (36Lu L Qian S Hershberger PA Rudert WA Lynch DH Thomson AW Fas ligand (CD95L) and B7 expression on dendritic cells provide counter‐regulatory signals for T cell survival and proliferation.J Immunol. 1997; 158: 5676-5684Crossref PubMed Google Scholar). Moreover, the enhanced tolerogenic potential of immature donor DC when combined with anti‐CD40L (CD154) mAb in heart graft recipients is associated with increased apoptotic death of graft‐infiltrating cells (37Lu L Li W Zhong C et al.Increased apoptosis of immunoreactive host cells and augmented donor leukocyte chimerism, not sustained inhibition of B7 molecule expression are associated with prolonged cardiac allograft survival in mice preconditioned with immature donor dendritic cells plus anti‐CD40L mAb.Transplantation. 1999; 68: 747-757Crossref PubMed Scopus (80) Google Scholar). The importance of AICD in the establishment of peripheral T cell tolerance to alloAgs has been demonstrated recently in liver and pancreatic islet cell transplantation (35Qian S Lu L Fu F et al.Apoptosis within spontaneously accepted mouse liver allografts: evidence for deletion of cytotoxic T cells and implications for tolerance induction.J Immunol. 1997; 158: 4654-4661Crossref PubMed Google Scholar, 38Li Y Li XC Zheng X Wells AD Turka LA Strom TB Blocking of both signal 1 and signal 2 of T‐cell activation prevents apoptosis of alloreactive T cells and induction of peripheral tolerance.Nat Med. 1999; 5: 1298-1302Crossref PubMed Scopus (651) Google Scholar), and also by administration of donor leukocytes in liver transplant models (39Yan Y Shastry S Richard C et al.Posttransplant administration of donor leukocytes induces long‐term acceptance of kidney or liver transplants by an activation‐associated immune mechanism.J Immunol. 2001; 166: 5258-5264Crossref PubMed Scopus (43) Google Scholar). A mechanism whereby immature donor DC (that deliver signal 1 in the absence of signal 2) could modulate responses to alloAgs within recipient draining lymphoid tissue is by induction of T regulatory (T reg) cells. A subset of CD4+ T reg cells (Tr1 cells) was identified after cloning of alloAg‐activated T cells exposed to IL‐10 (40Groux HO 'Garra A Bigler M et al.T‐cell subset inhibits antigen‐specific T cell responses and prevents colitis.Nature. 1997; 389: 737-741Crossref PubMed Scopus (3159) Google Scholar). These cells secrete IL‐10 and transforming growth factor‐β (TGF‐β), but low levels of IL‐2, and no IL‐4. Under noninflammatory (steady state) conditions, Ag transported to the draining lymphoid tissue via immature DC may be presented to T cells in the absence of costimulatory molecule expression (signal 2) and Tr1‐like cells may be generated (see Figure 1). Adding credence to this hypothesis are recent observations in normal human subjects given a single intravenous (i.v.) injection of immature, autologous, monocyte‐derived DC pulsed with influenza matrix protein. This resulted in the generation of matrix protein‐specific IL‐10‐producing T cells (17Dhodapkar MV Steinman RM Krasovsky J Munz C Bhardwaj N Antigen‐specific inhibition of effector T cell function in humans after injection of immature dendritic cells.J Exp Med. 2001; 193: 233-238Crossref PubMed Scopus (1212) Google Scholar). Furthermore, repeated stimulation of naïve human cord blood T cells with immature allogeneic DC can induce IL‐10‐producing CD4+ T reg cells (16Jonuleit L Schmitt E Schuler G Knop J Enk AH Induction of interleukin 10‐producing, nonproliferating CD4+ T cells with regulatory properties by repetitive stimulation with allogeneic immature human dendritic cells.J Exp Med. 2000; 192: 1213-1222Crossref PubMed Scopus (1345) Google Scholar). The implication for transplantation of these and related animal studies is that immature DC are capable of inducing Ag‐specific T cell hyporesponsiveness in vivo, by what may involve an active regulatory mechanism. Conversely, these observations carry an important caveat for DC therapy of cancer, in that presentation of tumor Ag by immature DC may (unintentionally) inhibit an effective anti‐tumor response (41Roncarlo M‐G Levings MK Traversari C Differentiation of T regulatory cells by immature dendritic cells.J Exp Med. 2001; 193: 5-9Crossref Google Scholar). A murine B lineage‐associated DC derived from liver‐resident progenitors has been shown recently to induce Tr1‐like cells in vitro, and to prolong organ allograft survival (42Lu L Bonham CA Liang X et al.Liver‐derived DEC205+B220+CD19– dendritic cells regulate T cell reponses.J Immunol. 2001; 166: 7042-7052Crossref PubMed Scopus (85) Google Scholar). Using CD4+ T cell lines, human plasmacytoid DC precursors (pDC2) enriched from peripheral blood have been shown to induce T cell anergy in vitro (43Kuwana M Kaburaki J Wright TM Kawakami Y Ikeda Y Induction of antigen‐specific human CD4+T cell anergy by peripheral blood DC2 precursors.Eur J Immunol. 2001; 31: 2547-2557Crossref PubMed Scopus (131) Google Scholar), prompting the authors to suggest that these DC are involved in maintenance of peripheral T cell tolerance and have potential for suppression of allograft rejection. Using DC to skew the immune response from Th1 towards Th2 cell predominance is another theoretical approach to harness DC for anti‐rejection therapy. In a nontransplant setting, prevention of type‐1 autoimmune (insulin‐dependent) diabetes by GM‐CSF/IL‐4 generated autologous DC in nonobese diabetic (NOD) mice was associated with immune deviation (44Feili‐Hariri M Dong X Alber SM Watkins SC Salter RD Morel PA Immunotherapy of NOD mice with bone marrow‐derived dendritic cells.Diabetes. 1999; 48: 2300-2308Crossref PubMed Scopus (152) Google Scholar). Application of this a