Sharing of Class I MHC Molecules Between Donor and Host Promotes the Infiltration of Allografts by mHAg‐Reactive CD8+ T cells

Indirect recognition of minor histocompatibility antigens (mHAg) and/or MHC‐encoded allopeptides is an important barrier to long‐term allograft acceptance following solid organ transplantation. Efficient priming of CD8⁺ T cells can occur after allotransplantation as a consequence of cross‐presentation of donor‐derived proteins by the graft recipient's APC. Consistent with this, draining lymph node clonal expansion of OVA‐reactive OT‐I CD8⁺ T cells following placement of OVA‐transgenic skin grafts did not depend on graft expression of K^b. However, OT‐I T cells did accumulate in OVA‐transgenic skin grafts most efficiently only when both the donor and host expressed K^b. OT‐I infiltration of (B6‐OVA × BALB/c)F₁ grafts in B6 recipients was not suppressed by graft expression of H‐2^d. Furthermore, B6 animals transplanted with both B6‐OVA and BALB/c‐OVA skin had more OT‐I T cells infiltrating their B6‐OVA MHC‐matched graft. Therefore, class I MHC matching between donor and host may not always favor an avoidance of alloreactivity within the graft tissue.     

Kidney‐Induced Cardiac Allograft Tolerance in Miniature Swine is Dependent on MHC‐Matching of Donor Cardiac and Renal Parenchyma

Kidney allografts possess the ability to enable a short course of immunosuppression to induce tolerance of themselves and of cardiac allografts across a full‐MHC barrier in miniature swine. However, the renal element(s) responsible for kidney‐induced cardiac allograft tolerance (KICAT) are unknown. Here we investigated whether MHC disparities between parenchyma versus hematopoietic‐derived “passenger” cells of the heart and kidney allografts affected KICAT. Heart and kidney allografts were co‐transplanted into MHC‐mismatched recipients treated with high‐dose tacrolimus for 12 days. Group 1 animals (n = 3) received kidney and heart allografts fully MHC‐mismatched to each other and to the recipient. Group 2 animals (n = 3) received kidney and heart allografts MHC‐matched to each other but MHC‐mismatched to the recipient. Group 3 animals (n = 3) received chimeric kidney allografts whose parenchyma was MHC‐mismatched to the donor heart. Group 4 animals (n = 3) received chimeric kidney allografts whose passenger leukocytes were MHC‐mismatched to the donor heart. Five of six heart allografts in Groups 1 and 3 rejected <40 days. In contrast, heart allografts in Groups 2 and 4 survived >150 days without rejection (p < 0.05). These data demonstrate that KICAT requires MHC‐matching between kidney allograft parenchyma and heart allografts, suggesting that cells intrinsic to the kidney enable cardiac allograft tolerance.     

Alefacept Promotes Immunosuppression‐Free Renal Allograft Survival in Nonhuman Primates via Depletion of Recipient Memory T Cells

Renal allograft tolerance has been achieved in MHC‐mismatched primates via nonmyeloablative conditioning beginning 6 days prior to planned kidney and donor bone marrow transplantation (DBMT). To extend the applicability of this approach to deceased donor transplantation, we recently developed a novel‐conditioning regimen, the “delayed protocol” in which donor bone marrow (DBM) is transplanted several months after kidney transplantation. However, activation/expansion of donor‐reactive CD8⁺ memory T cells (TMEM) occurring during the interval between kidney and DBM transplantation impaired tolerance induction using this strategy. In the current study, we tested whether, Alefacept, a fusion protein which targets LFA‐3/CD2 interactions and selectively depletes CD2^highCD8⁺ effector memory T cells (TEM) could similarly induce long‐term immunosuppression‐free renal allograft survival but avoid the deleterious effects of anti‐CD8 mAb treatment. We found that Alefacept significantly delayed the expansion of CD2^high cells including CD8⁺ TEM while sparing naïve CD8⁺ T and NK cells and achieved mixed chimerism and long‐term immunosuppression‐free renal allograft survival. In conclusion, elimination of CD2^high T cells represents a promising approach to prevent electively the expansion/activation of donor‐reactive TEM and promotes tolerance induction via the delayed protocol mixed chimerism approach.

     

Flt3L‐mobilized dendritic cells bearing H2‐Kbm1 apoptotic cells do not induce cross‐tolerance to CD8+ T cells across a class I MHC mismatched barrier

Tolerization of allogeneic CD8⁺ T cells is still a pending issue in the field of transplantation research to achieve long‐term survival. To test whether dendritic cells (DC) bearing allogeneic major histocompatibility complex (MHC) class I mismatched apoptotic cells could induce cross‐tolerance to alloreactive CD8⁺ T cells, the following experimental strategy was devised. Rag2/γ_c KO B6 mice were treated with Fms‐like tyrosine kinase 3 ligand (Flt3L)‐transduced B16 melanoma cells to drive a rapid expansion and mobilization of DC in vivo. Of all DC populations expanded, splenic CD11c⁺CD103⁺CD8α⁺ DC were selectively involved in the process of antigen clearance of X‐ray irradiated apoptotic thymocytes in vivo. Considering that CD11c⁺CD103⁺CD8α⁺ DC selectively take up apoptotic cells and that they are highly specialized in cross‐presenting antigen to CD8⁺ T cells, we investigated whether B6 mice adoptively transferred with Flt3L‐derived DC loaded with donor‐derived apoptotic thymocytes could induce tolerance to bm1 skin allografts. Our findings on host anti‐donor alloresponse, as revealed by skin allograft survival and cytotoxic T lymphocyte assays, indicated that the administration of syngeneic DC presenting K^bm1 donor‐derived allopeptides through the indirect pathway of antigen presentation was not sufficient to induce cross‐tolerance to alloreactive CD8⁺ T cells responding to bm1 alloantigens in a murine model of skin allograft transplantation across an MHC class I mismatched barrier.     

Imaging Tolerance Induction in the Classic Medawar Neonatal Mouse Model: Active Roles of Multiple F1‐Donor Cell Types

The immature immune system is uniquely susceptible to tolerance induction and thus an attractive target for immunomodulation strategies for organ transplantation. Newborn mice injected with adult semi‐allogeneic lymphohematopoietic cells accept transplants without immunosuppressive drugs. Early in vivo/in situ events leading to neonatal tolerance remain poorly understood. Here, we show by whole body/organ imaging that injected cells home to lymphoid organs and liver where various F1‐donor cell types selectively alter neonatal immunity. In host thymus, F1‐donor dendritic cells (DC) interact with developing thymocytes and regulatory T cells suggesting a role in negative selection. In spleen and lymph nodes, F1‐donor regulatory T/B cells associate with host alloreactive cells and by themselves prolong cardiac allograft survival. In liver, F1‐donor cells give rise to albumin‐containing hepatocyte‐like cells. The neonatal immune system is lymphopenic, Th‐2 immunodeviated and contains immature DC, suggesting susceptibility to regulation by adult F1‐donor cells. CD8a T cell inactivation greatly enhances chimerism, suggesting that variable emerging neonatal alloreactivity becomes a barrier to tolerance induction. This comprehensive qualitative imaging study systematically shows contribution of multiple in vivo processes leading simultaneously to robust tolerance. These insights into robust tolerance induction have important implications for development of strategies for clinical application.     

Induction of Tolerance by Exosomes and Short-Term Immunosuppression in a Fully MHC-Mismatched Rat Cardiac Allograft Model

Exosomes are MHC‐bearing vesicles secreted by a wide array of cells. We have previously shown that donor‐haplotype exosomes from bone marrow dendritic cells (DCs) injected before transplantation significantly prolong heart allograft survival in congenic and fully MHC‐mismatched Lewis rats. Here we show that donor exosomes administered after transplantation are similarly able to prolong allograft survival, however, without inducing tolerance. We therefore tested the effect of exosomes combined with short‐term LF 15‐0195 (LF) treatment, which blocks the maturation of DCs, so that donor‐MHC antigens from exosomes could be presented in a more tolerogenic environment. LF treatment does not preclude the development of a strong antidonor cellular response, and while LF, but not exosome, treatment inhibits the antidonor humoral response and decreases leukocyte graft infiltration, allografts from LF‐treated recipients were either acutely or strongly chronically rejected. Interestingly, when combined with LF treatment, exosomes induced a donor‐specific allograft tolerance characterized by a strong inhibition of the antidonor proliferative response. This donor‐specific tolerance was transferable to naïve allograft recipients. Moreover, exosomes/LF treatment prevented or considerably delayed the appearance of chronic rejection. These results suggest that under LF treatment, presentation of donor‐MHC antigens (from exosomes) can induce regulatory responses that are able to modulate allograft rejection and to induce donor‐specific allograft tolerance.     

Regulatory T‐Cell Counter‐Regulation by Innate Immunity Is a Barrier to Transplantation Tolerance

Innate immune signals foster adaptive immunity through activation of antigen‐presenting cells. Recent in vitro evidence suggests that innate signaling may also contribute to immunity by countering the effects of regulatory T cells (T‐regs), counter‐regulation. We present in vivo evidence using a transgenic skin allograft model that the function of T‐regs is lost in the setting of acute skin transplantation but remains intact when grafts were transplanted 1 month prior to allow surgery‐induced inflammation to abate. Our findings identify T‐reg counter‐regulation as a naturally occurring process that accompanies transplantation and an important barrier to T‐reg–mediated tolerance. Our finding further highlights the central role of regulatory cell deactivation in the initiation of the immune response.     

The Cell Biology of Cytomegalovirus: Implications for Transplantation

Interpretation of clinical data regarding the impact of cytomegalovirus (CMV) infection on allograft function is complicated by the diversity of viral strains and substantial variability of cellular receptors and viral gene expression in different tissues. Variation also exists in nonspecific (monocytes and dendritic cells) and specific (NK cells, antibodies) responses that augment T cell antiviral activities. Innate immune signaling pathways and expanded pools of memory NK cells and γδ T cells also serve to amplify host responses to infection. The clinical impact of specific memory T cell anti‐CMV responses that cross‐react with graft antigens and alloantigens is uncertain but appears to contribute to graft injury and to the abrogation of allograft tolerance. These responses are modified by diverse immunosuppressive regimens and by underlying host immune deficits. The impact of CMV infection on the transplant recipient reflects cellular changes and corresponding host responses, the convergence of which has been termed the “indirect effects” of CMV infection. Future studies will clarify interactions between CMV infection and allograft injury and will guide interventions that may enhance clinical outcomes in transplantation.     

Recipient Dendritic Cells,But Not B Cells,Are Required Antigen‐Presenting Cells for Peripheral Alloreactive CD8+ T‐Cell Tolerance

Induction of mixed allogeneic chimerism is a promising approach for achieving donor‐specific tolerance, thereby obviating the need for life‐long immunosuppression for solid organ allograft acceptance. In mice receiving a low dose (3Gy) of total body irradiation, allogeneic bone marrow transplantation combined with anti‐CD154 tolerizes peripheral CD4 and CD8 T cells, allowing achievement of mixed chimerism with specific tolerance to donor. With this approach, peripheral CD8 T‐cell tolerance requires recipient MHC class II, CD4 T cells, B cells and DCs. Recipient‐type B cells from chimeras that were tolerant to donor still promoted CD8 T‐cell tolerance, but their role could not be replaced by donor‐type B cells. Using recipients whose B cells or DCs specifically lack MHC class I and/or class II or lack CD80 and CD86, we demonstrate that dendritic cells (DCs) must express CD80/86 and either MHC class I or class II to promote CD8 tolerance. In contrast, B cells, though required, did not need to express MHC class I or class II or CD80/86 to promote CD8 tolerance. Moreover, recipient IDO and IL‐10 were not required. Thus, antigen presentation by recipient DCs and not by B cells is critical for peripheral alloreactive CD8 T cell tolerance.     

UVB‐induced depletion of donor‐derived dendritic cells prevents allograft rejection of immune‐privileged hair follicles in humanized mice

Dendritic cells (DCs) are key targets for immunity and tolerance induction; they present donor antigens to recipient T cells by donor‐ and recipient‐derived pathways. Donor‐derived DCs, which are critical during the acute posttransplant period, can be depleted in graft tissue by forced migration via ultraviolet B light (UVB) irradiation. Here, we investigated the tolerogenic potential of donor‐derived DC depletion through in vivo and ex vivo UVB preirradiation (UV) combined with the injection of anti‐CD154 antibody (Ab) into recipients in an MHC‐mismatched hair follicle (HF) allograft model in humanized mice. Surprisingly, human HF allografts achieved long‐term survival with newly growing pigmented hair shafts in both Ab‐treated groups (Ab‐only and UV plus Ab) and in the UV‐only group, whereas the control mice rejected all HF allografts with no hair regrowth. Perifollicular human CD3⁺ T cell and MHC class II⁺ cell infiltration was significantly diminished in the presence of UV and/or Ab treatment. HF allografts in the UV‐only group showed stable maintenance of the immune privilege in the HF epithelium without evidence of antigen‐specific T cell tolerance, which is likely promoted by normal HFs in vivo. This immunomodulatory strategy targeting the donor tissue exhibited novel biological relevance for clinical allogeneic transplantation without generalized immunosuppression.     

Donor apoptotic cell–based therapy for effective inhibition of donor‐specific memory T and B cells to promote long‐term allograft survival in allosensitized recipients

Allosensitization constitutes a major barrier in transplantation. Preexisting donor‐reactive memory T and B cells and preformed donor‐specific antibodies (DSAs) have all been implicated in accelerated allograft rejection in sensitized recipients. Here, we employ a sensitized murine model of islet transplantation to test strategies that promote long‐term immunosuppression‐free allograft survival. We demonstrate that donor‐specific memory T and B cells can be effectively inhibited by peritransplant infusions of donor apoptotic cells in combination with anti‐CD40L and rapamycin, and this treatment leads to significant prolongation of islet allograft survival in allosensitized recipients. We further demonstrate that late graft rejection in recipients treated with this regimen is associated with a breakthrough of B cells and their aggressive graft infiltration. Consequently, additional posttransplant B cell depletion effectively prevents late rejection and promotes permanent acceptance of islet allografts. In contrast, persistent low levels of DSAs do not seem to impair graft outcome in these recipients. We propose that B cells contribute to late rejection as antigen‐presenting cells for intragraft memory T cell expansion but not to alloantibody production and that a therapeutic strategy combining donor apoptotic cells, anti‐CD40L, and rapamycin effectively inhibits proinflammatory B cells and promotes long‐term islet allograft survival in such recipients.     

Allografts Stimulate Cross‐Reactive Virus‐Specific Memory CD8 T Cells with Private Specificity

Viral infections have been associated with the rejection of transplanted allografts in humans and mice, and the induction of tolerance to allogeneic tissues in mice is abrogated by an ongoing viral infection and inhibited in virus‐immune mice. One proposed mechanism for this ‘heterologous immunity’ is the induction of alloreactive T cell responses that cross‐react with virus‐derived antigens. These cross‐reactive CD8 T cells are generated during acute viral infection and survive into memory, but their ability to partake in the immune response to allografts in vivo is not known. We show here that cross‐reactive, virus‐specific memory CD8 T cells from mice infected with LCMV proliferated in response to allografts. CD8 T cells specific to several LCMV epitopes proliferated in response to alloantigens, with the magnitude and hierarchy of epitope‐specific responses varying with the private specificities of the host memory T cell repertoire, as shown by adoptive transfer studies. Last, we show that purified LCMV‐specific CD8 T cells rejected skin allografts in SCID mice. These findings therefore implicate a potential role for heterologous immunity in virus‐induced allograft rejection.     

Depletion of CD8 Memory T Cells for Induction of Tolerance of a Previously Transplanted Kidney Allograft

Heterologous immunologic memory has been considered a potent barrier to tolerance induction in primates. Induction of such tolerance for a previously transplanted organ may be more difficult, because specific memory cells can be induced and activated by a transplanted organ. In the current study, we attempted to induce tolerance to a previously transplanted kidney allograft in nonhuman primates. The conditioning regimen consisted of low dose total body irradiation, thymic irradiation, antithymocyte globulin, and anti‐CD154 antibody followed by a brief course of a calcineurin inhibitor. This regimen had been shown to induce mixed chimerism and allograft tolerance when kidney transplantation (KTx) and donor bone marrow transplantation (DBMT) were simultaneously performed. However, the same regimen failed to induce mixed chimerism when delayed DBMT was performed after KTx. We found that significant levels of memory T cells remained after conditioning, despite effective depletion of naïve T cells. By adding humanized anti‐CD8 monoclonal antibody (cM‐T807), CD8 memory T cells were effectively depleted and these recipients successfully achieved mixed chimerism and tolerance. The current studies provide ‘proof of principle’ that the mixed chimerism approach can induce renal allograft tolerance, even late after organ transplantation if memory T‐cell function is adequately controlled.     

LFA‐1 Antagonism Inhibits Early Infiltration of Endogenous Memory CD8 T Cells into Cardiac Allografts and Donor‐Reactive T Cell Priming

Alloreactive memory T cells are present in virtually all transplant recipients due to prior sensitization or heterologous immunity and mediate injury undermining graft outcome. In mouse models, endogenous memory CD8 T cells infiltrate MHC‐mismatched cardiac allografts and produce IFN‐γ in response to donor class I MHC within 24 h posttransplant. The current studies analyzed the efficacy of anti‐LFA‐1 mAb to inhibit early CD8 T cell cardiac allograft infiltration and activation. Anti‐LFA‐1 mAb given to C57BL/6 6 (H‐2^b) recipients of A/J (H‐2^a) heart grafts on days –1 and 0 completely inhibited CD8 T cell allograft infiltration, markedly decreased neutrophil infiltration and significantly reduced intragraft expression levels of IFN‐γ‐induced genes. Donor‐specific T cells producing IFN‐γ were at low/undetectable numbers in spleens of anti‐LFA‐1 mAb treated recipients until day 21. These effects combined to promote substantial prolongation (from day 8 to 27) in allograft survival. Delaying anti‐LFA‐1 mAb treatment until days 3 and 4 posttransplant did not inhibit early memory CD8 T cell infiltration and proliferation within the allograft. These data indicate that peritransplant anti‐LFA‐1 mAb inhibits early donor‐reactive memory CD8 T cell allograft infiltration and inflammation suggesting an effective strategy to attenuate the negative effects of heterologous immunity in transplant recipients.     

Immunobiology of rejection and adaptation

Transplantation is an acceptable therapy for failing organs, however, the balance between prevention of acute rejection and immunosuppressant-induced toxicity remains elusive. Organ transplantation from a genetically disparate donor induces an immune response toward donor antigens in the recipient. An uncontrolled cumulative effect of these responses may jeopardize the recipient's life and destroy the grafted tissue. The donor antigen in the form of passenger leukocytes from the allograft migrating to the organized lymphoid collection is a prerequisite for initiation of acute rejection. In the host lymphoid tissue donor-specific dendritic cells primed with donor peptide activate na?ve CD4 helper T cells which in turn activate effector CD8 T-cell clones through the release of cytokines. Activated effector CD8 cells return to the graft and augment destructive activity with the help of adhesive molecules and perforin. This seems to be the mechanism of adaptive immunity to destroy viral pathogens; the pattern of allograft injury is not much different. Adaptation and tolerance are based on the principle of exhaustion of donor-specific immune responses by an activation-deletion-exhaustion pathway.     

Prolongation of allograft survival by passenger donor regulatory T cells

Tissue resident lymphocytes are present within many organs, and are presumably transferred at transplantation, but their impact on host immunity is unclear. Here, we examine whether transferred donor natural regulatory CD4 T cells (nT‐regs) inhibit host alloimmunity and prolong allograft survival. Transfer of donor‐strain lymphocytes was first assessed by identifying circulating donor‐derived CD4 T cells in 21 consecutive human lung transplant recipients, with 3 patterns of chimerism apparent: transient, intermediate, and persistent (detectable for up to 6 weeks, 6 months, and beyond 1 year, respectively). The potential for transfer of donor nT‐regs was then confirmed by analysis of leukocyte filters recovered from ex vivo normothermic perfusion circuits of human kidneys retrieved for transplantation. Finally, in a murine model of cardiac allograft vasculopathy, depletion of donor CD4 nT‐regs before organ recovery resulted in markedly accelerated heart allograft rejection and augmented host effector antibody responses. Conversely, adoptive transfer or purified donor‐strain nT‐regs inhibited host humoral immunity and prolonged allograft survival, and more effectively so than following administration of recipient nT‐regs. In summary, following transplantation, passenger donor‐strain nT‐regs can inhibit host adaptive immune responses and prolong allograft survival. Isolated donor‐derived nT‐regs may hold potential as a cellular therapy to improve transplant outcomes.     

Allospecific Rejection of MHC Class I‐Deficient Bone Marrow by CD8 T Cells

Avoidance of long‐term immunosuppression is a desired goal in organ transplantation. Mixed chimerism offers a promising approach to tolerance induction, and we have aimed to develop low‐toxicity, nonimmunodepleting approaches to achieve this outcome. In a mouse model achieving fully MHC‐mismatched allogeneic bone marrow engraftment with minimal conditioning (3 Gy total body irradiation followed by anti‐CD154 and T cell–depleted allogeneic bone marrow cells), CD4 T cells in the recipient are required to promote tolerance of preexisting alloreactive recipient CD8 T cells and thereby permit chimerism induction. We now demonstrate that mice devoid of CD4 T cells and NK cells reject MHC Class I‐deficient and Class I/Class II‐deficient marrow in a CD8 T cell–dependent manner. This rejection is specific for donor alloantigens, since recipient hematopoiesis is not affected by donor marrow rejection and MHC Class I‐deficient bone marrow that is syngeneic to the recipient is not rejected. Recipient CD8 T cells are activated and develop cytotoxicity against MHC Class I‐deficient donor cells in association with rejection. These data implicate a novel CD8 T cell–dependent bone marrow rejection pathway, wherein recipient CD8 T cells indirectly activated by donor alloantigens promote direct killing, in a T cell receptor–independent manner, of Class I‐deficient donor cells.     

Regulatory T Cells Are Critical to Tolerance Induction in Presensitized Mouse Transplant Recipients Through Targeting Memory T Cells

Memory T cells are a significant barrier to induction of transplant tolerance. However, reliable means to target alloreactive memory T cells have remained elusive. In this study, presensitization of BALB/c mice with C57BL/6 skin grafts generated a large number of OX40⁺CD44^hieffector/memory T cells and resulted in rapid rejection of donor heart allografts. Recognizing that anti‐OX40L monoclonal antibody (mAb) (α‐OX40L) monotherapy prolonged graft survival through inhibition and apoptosis of memory T cells in presensitized recipients, α‐OX40L was added to the combined treatment protocol of LF15–0195 (LF) and anti‐CD45RB (α‐CD45RB) mAb—a protocol that induced heart allograft tolerance in non‐presensitized recipients but failed to induce tolerance in presensitized recipients. Interestingly, this triple therapy restored donor‐specific heart allograft tolerance in our presensitized model that was associated with induction of tolerogenic dendritic cells and CD4⁺CD25⁺Foxp3⁺ T regulatory cells (Tregs). Of note, CD25⁺ T cell depletion in triple therapy recipients prevented establishment of allograft tolerance. In addition, adoptive transfer of donor‐primed effector/memory T cells into tolerant recipients markedly reduced levels of Tregs and broke tolerance. Our findings indicated that targeting memory T cells, by blocking OX40 costimulation in presensitized recipients was very important to expansion of Tregs, which proved critical to development of tolerance.     

Comparative study of the role of professional versus semiprofessional or nonprofessional antigen presenting cells in the rejection of vascularized organ allografts

The immune systems of transplant recipients are progressively challenged with exposure to the multiple lineages of donor cells that comprise the vascularized organ allograft. Each lineage of such donor tissue constitutively expresses or can be induced to express varying densities of MHC antigens ranging from no expression of MHC to MHC class I only to both MHC class I and class II. In addition, the cell surface expression of a diverse assortment of costimulatory and cell adhesion molecules also varies in density in a tissue specific fashion within the allograft. The MHC class I/II molecules displayed on the donor cells contain within their clefts a constellation of processed protein antigens in the form of peptides derived from intracellular and to some extent extracellular sources. Therefore, the potential for each cell lineage to induce alloactivation and serve as a target for allospecific immune responses is dependent on the diversity and density of peptide-bearing MHC molecules, constimulatory molecules, and cell adhesion molecules. In addition, the T cell receptor repertoire of the recipient also contributes to the magnitude of the allogeneic response. Consequently, the variety of clinical outcomes following organ transplantation even with the institution of potent immunosuppressive (drug) therapies is not suprising, as it appears reasonable for such therapies to influence the allogeneic response against distinct lineages differentially. Our failure to prevent chronic human allograft rejection may therefore be due to our limited appreciation of the full spectrum of alloactivating experiences encountered by host T cells as they interact with donor cells of diverse tissue lineages.Investigations by our laboratory of the immunopathogenesis of chronic cardiac allograft rejection have revealed an intrinsic inability of human cardiac myocytes to process and present antigens, not only for primary but also for secondary alloimmune responses. One obvious explanation for this phenomenon is the fact that cardiac myocytes do not constitutively express MHC class II molecules and express only low levels of class I molecules. However, this immunological unresponsiveness is maintained even after the induction of MHC class II and upregulation of MHC class I on these cells by interferon-gamma (IFN-γ). Similar results have also been reported for cells of different tissue lineages (e.g. chondrocytes, keratinocytes, neural cells). Until now, cells have been defined as professional or nonprofessional for the purposes of defining their potential for antigen presentation to T cells. Professional antigen presenting cells have been identified as cells that are of haematopoietic origin, that constitutively express MHC class I and class II molecules as well as potent costimulatory molecules, and that are able to induce both primary and secondary immune responses, whereas nonprofessional antigen presenting cells are not bone marrow derived, do not constitutively express MHC class II, but may in some cases initiate primary and secondary immune responses after induction of MHC class II antigen by proinflammatory cytokines (e.g. IFN-γ). The findings of our laboratory and others suggest that cells of certain lineages be considered in the separate class of ‘nonantigen presenting cells’. Indeed, nonprofessional antigen presenting cells can be reclassified into three categories: semiprofessional-, nonprofessional-, or nonantigen presenting cells that are able to present antigen to and activate naive T cells, activated T cells, or no T Cells, respectively.The aim of this review is to identify and (re)examine the antigen presentation characteristics of cells of different tissue lineages in terms of their ability to activate different subsets of T cells. This approach is taken in an attempt to synthesize these concepts into a unified picture of T cell activation in the context of antigen processing and presentation by different cell types.