CD8+ effector cells responding to residual class I antigens, with help from CD4+ cells stimulated indirectly, cause rejection of "major histocompatibility complex-deficient" skin grafts

Pathways of Antigen Recognition by T Cells in Allograft Rejection

The adaptive immune response leading to the rejection of allogeneic transplants is initiated and orchestrated by recipient T cells recognizing donor antigens. T-cell allorecognition is mediated via 3 distinct mechanisms: the direct pathway in which T cells recognize allogeneic major histocompatibility complex (MHC) molecules on donor cells, the indirect pathway through which T cells interact with donor peptides bound with self-MHC molecules on recipient antigen-presenting cells, and the recently described semidirect pathway whereby T cells recognize donor MHC proteins on recipient antigen-presenting cells. In this article, we present a description of each of these allorecognition pathways and discuss their role in acute and chronic rejection of allogeneic transplants.

A Short History of Skin Grafting in Burns: From the Gold Standard of Autologous Skin Grafting to the Possibilities of Allogeneic Skin Grafting with Immunomodulatory Approaches

Due to groundbreaking and pioneering developments in the last century, significant improvements in the care of burn patients have been achieved. In addition to the still valid therapeutic standard of autologous split-thickness skin grafting, various commercially available skin substitutes are currently available. Significant progress in the field of tissue engineering has led to the development of promising therapeutic approaches. However, scientific advances in the field of allografting and transplant immunology are of great importance. The achievement of various milestones over the past decades has provided thought-provoking impulses in the field of skin allotransplantation. Thus, biologically viable skin allotransplantation is still not a part of the clinical routine. The purpose of this article is to review the achievements in burn surgery with regards to skin allotransplantation in recent years.

Down-Regulation of MHC Class I Expression in Human Keratinocytes Using Viral Vectors Containing US11 Gene of Human Cytomegalovirus and Cultivation on Bovine Collagen-Elastin Matrix (Matriderm®): Potential Approach for an Immune-Privileged Skin Substitute

Skin transplantation, especially in burn patients, is still challenging because surgeons are faced with limited disposability of autologous donor side material. The in vitro culture of keratinocytes has become an important reconstructive option. However, only non-immunogenic allogenic keratinocytes offer the opportunity to develop a skin graft that can overcome rejection. The purpose of the study was to develop targeted gene modification of keratinocytes in order to reduce immunogenicity for the use as allogenic transplantable skin graft by decreasing the expression of MHC class I. To reduce MHC class I expression, viral vectors containing the US11 gene of human cytomegalovirus were generated and tested on their functionality using Western blotting, indirect immunofluorescence staining, and flow cytometry. Transfected keratinocytes were seeded on commercially available bovine collagen-elastin matrices and further cultured for histological and cell survival assays. Results showed transient down-regulation of MHC class I after 24 h post-transfection, with recovery of MHC class I expression after 48 h. Histological assessments showed long-term cell survival as well as histological patterns comparable to epidermal layers of healthy human skin. The data postulates the potential application of US11 transfected keratinocytes as an approach towards an immune-privileged skin substitute. Nevertheless, further studies and data are needed.

What Is Direct Allorecognition?

Direct allorecognition is the process by which donor-derived major histocompatibility complex (MHC)-peptide complexes, typically presented by donor-derived ‘passenger’ dendritic cells, are recognised directly by recipient T cells. In this review, we discuss the two principle theories which have been proposed to explain why individuals possess a high-precursor frequency of T cells with direct allospecificity and how self-restricted T cells recognise allogeneic MHC-peptide complexes. These theories, both of which are supported by functional and structural data, suggest that T cells recognising allogeneic MHC-peptide complexes focus either on the allopeptides bound to the allo-MHC molecules or the allo-MHC molecules themselves. We discuss how direct alloimmune responses may be sustained long term, the consequences of this for graft outcome and highlight novel strategies which are currently being investigated as a potential means of reducing rejection mediated through this pathway.

Innate immunity and resistance to tolerogenesis in allotransplantation

The development of immunosuppressive drugs to control adaptive immune responses has led to the success of transplantation as a therapy for end-stage organ failure. However, these agents are largely ineffective in suppressing components of the innate immune system. This distinction has gained in clinical significance as mounting evidence now indicates that innate immune responses play important roles in the acute and chronic rejection of whole organ allografts. For instance, whereas clinical interest in natural killer (NK) cells was once largely confined to the field of bone marrow transplantation, recent findings suggest that these cells can also participate in the acute rejection of cardiac allografts and prevent tolerance induction. Stimulation of Toll-like receptors (TLRs), another important component of innate immunity, by endogenous ligands released in response to ischemia/reperfusion is now known to cause an inflammatory milieu favorable to graft rejection and abrogation of tolerance. Emerging data suggest that activation of complement is linked to acute rejection and interferes with tolerance. In summary, the conventional wisdom that the innate immune system is of little importance in whole organ transplantation is no longer tenable. The addition of strategies that target TLRs, NK cells, complement, and other components of the innate immune system will be necessary to eventually achieve long-term tolerance to human allograft recipients.

Immune recognition and rejection of allogeneic skin grafts

The transplantation of allogeneic skin grafts is associated with a potent inflammatory immune response leading to the destruction of donor cells and the rejection of the graft. Shortly after transplantation, skin dendritic cells (DCs) migrate out of the graft through lymphatic vessels and infiltrate the recipient's draining lymph nodes where they present donor antigens via two mechanisms: the direct pathway, in which T cells recognize intact donor MHC antigens on donor DCs; and the indirect pathway, involving T-cell recognition of donor peptides bound to self-MHC molecules on recipient DCs. Some recent studies have suggested that T cells can become activated via recognition of donor MHC molecules transferred on recipient antigen-presenting cells (semidirect pathway). Activation of T cells via direct or indirect allorecognition is sufficient to trigger acute rejection of allogeneic skin grafts. In addition, allospecific antibodies contribute to the rejection process either by killing allogeneic targets in a complement-dependent fashion or by opsonizing donor cells and forming immune complexes. Finally, several studies demonstrate that NK cells, activated due to missing self-MHC class I molecules on allogeneic cells, are involved in allogeneic skin graft rejection via direct killing of donor cells and through the production of proinflammatory cytokines including IFN-γ and TNF-α.

Dual effects of the alloresponse by Th1 and Th2 cells on acute and chronic rejection of allotransplants

The contribution of direct and indirect alloresponses by CD4(+) Th1 and Th2 cells in acute and chronic rejection of allogeneic transplants remains unclear. In the present study, we addressed this question using a transplant model in a single MHC class I-disparate donor-recipient mouse combination. BALB/c-dm2 (dm2) mutant mice do not express MHC class I L(d) molecules and reject acutely L(d+) skin grafts from BALB/c mice. In contrast, BALB/c hearts placed in dm2 mice are permanently accepted in the absence of chronic allograft vasculopathy. In this model, CD4(+) T cells are activated following recognition of a donor MHC class I determinant, L(d) 61-80, presented by MHC Class II A(d) molecules on donor and recipient APC. Pre-transplantation of recipients with L(d) 61-80 peptide emulsified in complete Freund's adjuvant induced a Th1 response, which accelerated the rejection of skin allografts, but it had no effect on cardiac transplants. In contrast, induction of a Th2 response to the same peptide abrogated the CD8(+) cytotoxic T cells response and markedly delayed the rejection of skin allografts while it induced de novo chronic rejection of heart transplants. This shows that Th2 cells activated via indirect allorecognition can exert dual effects on acute and chronic rejection of allogeneic transplants.

TCR-MHC docking orientation: natural selection, or thymic selection?

T cell receptors (TCR) dock on their peptide-major histocompatibility complex (pMHC) targets in a conserved orientation. Since amino acid sidechains are the foundation of specific protein-protein interactions, a simple explanation for the conserved docking orientation is that key amino acids encoded by the TCR and MHC genes have been selected and maintained through evolution in order to preserve TCR/pMHC binding. Expectations that follow from the hypothesis that TCR and MHC evolved to interact are discussed in light of the data that both support and refute them. Finally, an alternative and equally simple explanation for the driving force behind the conserved docking orientation is described.

The indirect alloresponse impairs the induction but not maintenance of tolerance to MHC class I-disparate allografts

We studied the effects of indirect allorecognition on the induction and maintenance phases of tolerance in miniature swine cotransplanted with heart and kidney allografts. MHC class I-mismatched heart and kidney grafts were cotransplanted in recipients receiving CyA for 12 days. Recipients were unimmunized or immunized with a set of donor-derived or control third-party MHC class I peptides either 21 days prior to transplantation or over 100 days after transplantation. T-cell proliferation, delayed type hypersensitivity reaction (DTH) and antibody production were assessed. All animals injected with donor MHC class I peptides developed potent indirect alloresponses specific to the immunizing peptides. While untreated recipients developed stable tolerance, all animals preimmunized with donor allopeptides rejected kidney-heart transplants acutely. In contrast, when peptide immunization was delayed until over 100 days after kidney-heart transplantation, no effects were observed on graft function or in vitro measures of alloimmunity. Donor peptide immunization prevented tolerance when administered to recipients pre transplantation but did not abrogate tolerance when administered to long-term survivors post transplantation. This suggests that the presence of T cells activated via indirect allorecognition represent a barrier to the induction but not the maintenance of tolerance.

The multigenic structure of the MHC locus contributes to positive selection efficiency: a role for MHC class II gene-specific restriction

The study of T cell positive selection in the thymus has long been focused on the specificity of the MHC-TCR interactions, making use of genetically manipulated mice that display TCR specificities or selecting peptides of limited diversity. However, little is known on the role of the MHC molecules irrespective of the peptide specificity and the implications of MHC multigenic structure in thymic positive selection have not been addressed. Here, we investigated the effect of MHC class II genetic configuration on the positive selection efficiency of naturally generated pre-selection repertoires in the mouse thymus. Analysis of positively selected thymocyte populations in MHC-congenic and -transgenic mice revealed that expression of I-E molecule in the thymic cortex increases positive selection efficiency of CD4 cells by approximately 50%. We show that increments in positive selection attributable to either the I-A and I-E genes are not due to increased MHC class II expression in the thymic cortex and are not affected by the number of MHC alleles. Collectively, our findings imply that MHC class II gene-restricted TCR specificities significantly contribute to positive selection efficiency, introducing the notion that multigenic structure of the MHC locus serves to increase selection of non-overlapping TCR repertoires.

Targeted gene disruption of the heat shock protein 72 gene (hsp70.1) in the donor tissue is associated with a prolonged rejection-free survival in the murine skin allograft model

It is known that the expression level of the heat shock protein (HSP) is elevated in allograft tissues, but the specific role of the HSP in the acute rejection has not been elucidated. This study aims to determine how and when the HSP72 molecule works immunologically in the process of acute allograft rejection from a skin graft model in which HSP72 (hsp70.1 gene) knock out (KO) mice were adopted either as a donor or as a recipient. In experiment I, tail skin was grafted from either the HSP72 KO C57BL/6 mice or wild-type C57BL/6 mice--as the allograft control--onto the trunk of B10BR mice. The grafts were observed for any signs of rejection until the 14th day after the graft. The survival of the grafted skin from the two donor groups was analyzed by a log-rank method. The grafted skin was observed for the degree of rejection using light microscopy. In addition, the degree of apoptosis was assessed using a terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL). A mixed lymphocyte reaction (MLR) was observed with HSP72 KO C57BL/6 splenocytes as the stimulator and B10BR mice lymphocytes as the responder cells. The cytokine levels, such as interleukin-2 (IL-2) and interferon-gamma, were measured from an MLR supernatant using enzyme-linked immunosorbent assay (ELISA). In experiment II, the tail skin was grafted in the opposite direction from the B10BR mice to the HSP72 KO or wild-type C57BL/6 mice, and the grafts were observed for any signs of rejection, as defined in experiment I. The absence of HSP72 expression was observed in the HSP72 KO mice lymphocytes after heat stress (42 degrees C) using Western blot analysis. In experiment I, the survival of the skin grafts from the HSP72 KO C57BL/6 mice was 12+/-1.3 days (median+/-S.E.), which was significantly longer than that from the allograft control donor (9+/-0.6 days; p=0.03). This coincided with the microscopic finding of the graft tissues. The MLR response of the CD4+ lymphocytes stimulated by HSP72 KO C57BL/6 splenocytes was lower, and the interferon-gamma concentration from the MLR supernatant was also lower. On Day 9, 3.7+/-1.1(mean+/-S.D.) TUNEL-positive cells per unit high-power field (HPF) were observed from the HSP72 KO C57BL/6 skin, which was significantly lower than that from the control skin (8.7+/-2.1 cells/HPF; p=0.045). In contrast, in experiment II, there was no difference in the survival of the skin graft either from the B10BR to HSP72 KO C57BL/6 mice or from the B10BR to the wild-type C57BL/6 mice. In summary, the survival of the skin grafts from the HSP72 KO C57BL/6 mice was prolonged, and the degree of rejection was lower than that from the allograft control. This means that the presence of HSP72 in the donor tissue is related to the up-regulation of acute rejection from the recipient immune system. The HSP72 KO mice as the donor were shown to have decreased level of antigen presentation to the recipient CD4+ lymphocytes. Therefore, the HSP72 molecule from the donor cells is believed to be related to the induction of the recipient immune reaction via alloantigen presentation.

Chimerism and Tolerance Post-In Utero Transplantation with Embryonic Stem Cells

BACKGROUND

Clinical application of in utero transplantation (IUT) in human fetuses with intact immune systems resulted in a very low level of donor chimerism. In this study, we examined whether the fetal immune system early in the second trimester of pregnancy (13.5 dpc) can initiate immune tolerance for major histocompatibility complex (MHC)-mismatched embryonic stem (ES) cells. We also examined whether immune tolerance mechanisms respond differently to ontogenetically different stem cells.

METHODS

MHC-mismatched ES, fetal liver (FL), and bone-marrow (BM) cells (H-2kd) at 1 x 10(9) cells/kg fetal body weight were injected intraperitoneally into 13.5 dpc BALB/c fetuses (H-2Kd). Peripheral chimerism was determined in blood by flow cytometry (sensitivity< or =0.1%) at monthly intervals. Donor-specific immune responses were determined by cytotoxic lymphocyte (CTL) assay, mixed lymphocyte reaction, and T helper (Th)1 and Th2 cytokine assays. Chimeric mice at the age of 9 months received postnatal boosts (PB) with minimal conditioning of 200 cGy by intravenous injection of 1 x 10(9) of the corresponding cells/kg body weight.

RESULTS

After IUT with ES, FL, or BM cells, the level of peripheral chimerism within the first 9 months of life was 0% to 0.4%. PB with 1 x 10(9)/kg of corresponding cells resulted in a decrease in the peripheral chimerism to 0% within 2 weeks of PB. CTL and cytokine assays before and after PB demonstrated a shift toward immunity.

CONCLUSIONS

Immunologic tolerance was not achieved after IUT of murine fetuses at 13.5 dpc with MHC-mismatched ES cells, and only a low level chimerism was achieved.

Evidence for Tissue-Directed Immune Responses: Analysis of CD4- and CD8-Dependent Alloimmunity

BACKGROUND

Transplant rejection has generally been considered a CD4 T-cell-dependent immune process. CD4-independent, CD8 T-cell rejection pathways have recently gained attention because of their relative resistance to immunosuppression. In the current study, the role of the allograft tissue in activation of these distinct pathways was examined by comparing host-immune responses with allogeneic pancreatic islets or hepatocytes transplanted across the same genetic disparity.

METHODS

To compare activation of CD4-dependent versus CD8-dependent alloimmunity, islets or hepatocytes retrieved from FVB/N (H-2) mice were transplanted into CD8 or CD4 T-cell-reconstituted severe combined immunodeficiency mice, CD4 or CD8 knockout (KO) mice, and anti-CD4 monoclonal antibody (mAb) or anti-CD8 mAb treated C57BL/6 mice (all H-2). The ability to immunomodulate CD4-dependent allograft rejection (in CD8 KO mice) was examined in the context of several mechanistically distinct immunotherapeutic strategies, including anti-CD4 mAb, donor-specific transfusion and anti-CD154 mAb, and anti-lymphocyte function-associated antigen-1 mAb.

RESULTS

The studies demonstrate that, whereas hepatocytes evoke alloreactive CD4-dependent and (CD4-independent) CD8 T-cell immune responses, allogeneic islets only activate CD4-dependent immune pathways. CD4-dependent host-immune responses initiated by pancreatic islet allografts were readily suppressed by a variety of short-term immunotherapies, whereas hepatocyte-initiated CD4-dependent alloimmune responses were not.

CONCLUSIONS

These results demonstrate that immune characteristics of the specific allograft tissue uniquely influence the pattern of host immune responses such that the propensity to activate CD4- or CD8-dependent alloimmune responses can be distinguished. Furthermore, CD4-dependent immune responses activated by different tissues from the same donor strain are distinguished by their susceptibility to specific immunotherapy.

CD8 T cells can reject major histocompatibility complex class I-deficient skin allografts

Following transplantation, recipient T cells can recognize and respond to donor antigens expressed directly on donor cells, and can respond to donor-derived peptides that have been processed and presented in the context of recipient MHC through the indirect pathway. Indirectly primed CD4(+) T cells have been well studied in transplantation, but little information is available regarding whether indirectly primed CD8(+) T cells participate in rejection. To address this, we placed MHC class I-deficient D(b)K(b) knockout skin grafts onto allogeneic H-2 (k) SCID recipients followed by adoptive transfer of purified H-2 (k) CD8(+) T cells. The MHC class I-deficient grafts were rejected and only CD8(+) T cells were detectable in the recipient lymphoid organs and in the skin grafts. Immunohistochemical analysis showed that CD8(+) T cells were found in close proximity to vascular endothelial cells and to recipient infiltrating macrophages, suggesting specific interactions. The data demonstrate that cross-primed polyclonal CD8(+) T cells can function as active participants in the effector phase of rejection. The findings confirm and extend previous studies using a monoclonal TCR transgenic T cell and shed light on mechanisms of acute and chronic graft injury that are potentially relevant to human transplant recipients.

Notch ligation by Delta1 inhibits peripheral immune responses to transplantation antigens by a CD8+ cell-dependent mechanism

Notch signaling plays a fundamental role in determining the outcome of differentiation processes in many tissues. Notch signaling has been implicated in T versus B cell lineage commitment, thymic differentiation, and bone marrow hematopoietic precursor renewal and differentiation. Notch receptors and their ligands are also expressed on the surface of mature lymphocytes and APCs, but the effects of Notch signaling in the peripheral immune system remain poorly defined. The aim of the studies reported here was to investigate the effects of signaling through the Notch receptor using a ligand of the Delta-like family. We show that Notch ligation in the mature immune system markedly decreases responses to transplantation antigens. Constitutive expression of Delta-like 1 on alloantigen-bearing cells renders them nonimmunogenic and able to induce specific unresponsiveness to a challenge with the same alloantigen, even in the form of a cardiac allograft. These effects could be reversed by depletion of CD8+ cells at the time of transplantation. Ligation of Notch on splenic CD8+ cells results in a dramatic decrease in IFN-gamma with a concomitant enhancement of IL-10 production, suggesting that Notch signaling can alter the differentiation potential of CD8+ cells. These data implicate Notch signaling in regulation of peripheral immunity and suggest a novel approach for manipulating deleterious immune responses.

Hierarchical contributions of allorecognition pathways in chronic lung rejection

The role of allorecognition in initiating lung graft rejection is not clearly defined. Using the heterotopic tracheal transplantation model, we examined the contributions of the indirect and direct allorecognition pathways in chronic airway rejection. Fully mismatched, wild-type grafts were transplanted into major histocompatibility complex (MHC) II-/-, class II-like accessory molecule (H2-DMalpha)-/- using MHC I-/- and wild-type allorecipients as control subjects. Similarly, MHC I-/-, MHC II-/-, or MHC I/II-/- allografts were transplanted into wild-type mice with appropriate control subjects. Grafts from nonimmunosuppressed recipients were evaluated at Weeks 2, 4, and 6. Grafts transplanted into MHC II-/- and H2-DMalpha-/- allorecipients showed a more intact epithelium and reduced lumen obliteration compared with grafts transplanted into wild-type or MHC I-/- allorecipients (p < 0.05 for each). These grafts exhibited abundant CD4+ and CD8+ cell infiltrates similar to control allografts. MHC I-/- and MHC I/II-/- but not MHC II-/- allografts placed in wild-type animals demonstrated less severe rejection compared with allograft control subjects (p < 0.05 for each). Although the indirect allorecognition pathway has the strongest influence on rejection, the direct pathway is sufficient to ultimately cause chronic airway rejection. In addition, these results suggest that MHC class I molecules are the principal alloantigens in the mouse heterotopic tracheal model of obliterative bronchiolitis.

Further analysis of the T-cell subsets and pathways of murine cardiac allograft rejection

The present study examined the role of CD4+ and CD8+ T cells in cardiac allograft rejection when either the direct or indirect pathway was eliminated for the CD4+ portion of the response. To study the pathways in vivo, we used genetically altered mouse strains that lack class II antigens as either the donors or recipients for cardiac transplantation. In contrast to earlier published studies, which used different strain combinations, we found that either CD4- or CD8-depletion prolonged cardiac allograft survival moderately, but not indefinitely, in an MHC-mismatched, minor-matched combination. When the CD4+ indirect pathway was eliminated, rapid graft rejection occurred when both T-cell subsets were present and when either CD4+ or CD8+ T cells were depleted. When the CD4+ direct pathway was eliminated, rapid graft rejection occurred when both T-cell subsets were present, there was slow rejection when CD4+ T cells were eliminated, and no rejection was seen for more than 100 days when CD8+ T cells were eliminated. However, the long-surviving allografts on the recipients with only CD4+ cells and an indirect pathway did show evidence of chronic vasculopathy. Thus, either CD4+ or CD8+ T cells can mediate acute cardiac allograft rejection in these experiments when both pathways are available. In addition, CD4+ T cells can provide help for acute rejection through either the direct or indirect pathway. Finally, recipients who have only CD4+ cells and an indirect pathway do not demonstrate acute rejection, but do show evidence of chronic rejection.

Stem cell medicine encounters the immune system

Recent progress in deriving human embryonic stem (hES) cells and defining their capacity to differentiate has inspired hope that they could become a source of replacement cells for damaged or diseased tissues. We review the immunological barriers to transplanting hES cells and consider several potential solutions, including stem-cell banking, modification of the immunogenicity of donor cells and induction of tolerance to the graft. We evaluate the probable efficacy of these approaches with a view to facilitating the use of hES cells in clinical practice.

Cross-primed CD8(+) T cells mediate graft rejection via a distinct effector pathway

To prevent bystander destruction of healthy host tissues, cytotoxic CD8(+) T lymphocytes are fitted with specific receptors that direct their destructive forces specifically against chosen targets. We show here, however, that anti-H-Y monospecific, H-2(b-restricted MataHari CD8(+) T cells reject H-2(k) male skin grafts, with which they cannot directly interact. Such rejection is interferon-gamma-dependent and only occurs if the recipient endothelium expresses H-2(b). The findings suggest an alternate indirect effector pathway that requires processing and presentation of the donor H-Y antigen by recipient endothelium and have implications for both transplantation and autoimmune disease.

Absence of donor MHC antigen expression ameliorates chronic kidney allograft rejection

BACKGROUND

In previous studies, we have demonstrated that a subset of mouse kidney allografts has prolonged survival without any immunosuppressive treatment. Chronic rejection (CR) develops in these long surviving grafts. The pathologic features of CR in this model are similar to CR in human kidney grafts.

METHODS

To explore the role of donor major histocompatibility complex (MHC) antigens in the development of CR, we performed vascularized kidney transplants using kidneys from donor mice that lack expression of both MHC class I and II antigens (MHC-/-).

RESULTS

Survival was significantly improved in recipients of MHC-/- allografts. This enhanced survival was associated with higher glomerular filtration rate (GFR) in MHC-/- allografts (4.92 +/- 0.54 cc/min/kg) compared to controls (2.19 +/- 0.63 cc/min/kg; P = 0.004). The typical histologic features of CR were markedly reduced in MHC-/- allografts. Semiquantitative histopathological scores for MHC-/- grafts (13.3 +/- 2.1) were significantly lower than in control allografts (19.0 +/- 1.0; P = 0.04). Along with this improvement in structural abnormalities, significantly fewer CD4+ T (38.3 cells/mm(2) vs. 75.0 cells/mm(2); P = 0.008), CD8+ T cells (38.7 vs. 96 cells/mm(2), respectively; P = 0.008) and macrophages (60 vs. 134 cells/mm(2), respectively; P = 0.04) infiltrated MHC-/- allografts compared to controls. The levels of intragraft cytokine mRNA expression also were reduced in MHC-/- allografts compared to control allografts. Finally, serum alloantibodies were virtually undetectable in recipients of MHC-/- kidney allografts.

CONCLUSIONS

Cell surface expression of donor MHC antigens promotes the development of CR. Donor antigen expression promotes the accumulation of infiltrating cells in the graft and the development of donor specific alloantibodies. Abrogation of these responses is associated with improved graft survival and reduced CR in MHC-/- grafts.

Transcription factor IRF-1 in kidney transplants mediates resistance to graft necrosis during rejection

In many circumstances kidney transplants remain viable despite extensive inflammation, permitting rejection episodes to be reversed. The mechanisms by which the kidney resists host effector mechanisms are not known. In mouse kidney transplants, resistance requires interferon-gamma (IFN-gamma), which acts on the graft to protect the graft from necrosis during the first days of rejection as well as inducing major histocompatibility complex (MHC) expression. Because some effects of IFN-gamma are mediated by transcription factor IRF-1, the role of IRF-1 in the donor tissue early phases of rejection of mouse kidney allografts was studied. H-2(b) kidneys were transplanted from mice with wild-type IRF-1 genes (WT) or mice with disrupted IRF-1 genes (IRF-1KO) into CBA (H-2(k)) recipients. At day 5 and day 7, IRF-1KO and WT kidneys were functioning despite typical rejection pathology: interstitial infiltration and tubulitis. However, function deteriorated rapidly in rejecting IRF-1KO allografts, associated with widespread epithelial necrosis, peritubular capillary congestion, glomerulitis, and fibrin thrombi in small veins by day 7. At day 21, WT kidneys were viable despite severe tubulitis and arteritis, whereas IRF-1KO kidneys showed massive necrosis of the epithelium despite patent large vessels. Compared with WT kidneys, rejecting IRF-1KO kidneys showed less induction of donor MHC yet had similar mRNA levels of perforin, granzyme B, and Fas ligand and evoked host alloantibody responses. Thus in rejecting kidney transplants, IRF-1 in the graft mediates MHC induction, but it also mediates resistance to necrosis, an effect that could be crucial to permit success in interventions against rejection.

Intrabody-mediated phenotypic knockout of major histocompatibility complex class I expression in human and monkey cell lines and in primary human keratinocytes

Cultured keratinocyte allografts from unrelated donors can be readily grown as sheets in large-scale cell culture and have been used as an immediate skin cover for severely burned patients. Despite the absence of passenger leukocytes and the unlimited amount of material that can be obtained for permanent skin coverage, the allografts are susceptible to rejection. Since MHC class I (MHCI) antigens serve as targets for allograft rejection, we investigated whether 'phenotypic knockout' of human MHCI could be achieved through expression of an ER-directed anti-human MHCI single-chain intrabody (sFvhMHCI) that is directed against a monomorphic, conformational epitope, expressed across species lines, on the MHCI heavy chain. Co-immunoprecipitation of both MHCI heavy chain and beta2-microglobulin occurred in transfected monkey COS-1 cells, while Jurkat T cells stably expressing the ER-directed sFvhMHCI intrabody showed that complete phenotypic knockout of MHCI cell surface expression could be achieved. Infection of several human cell lines of divergent tissue sources and different HLA haplotypes resulted in marked down-regulation of MHCI expression, even under conditions where inflammatory cytokines (eg gamma-IFN) which up-regulate MHCI expression were used. Finally, when adenovirus encoding the anti-human MHCI intrabody was used to transduce primary human keratinocytes, a marked reduction of surface MHCI expression was observed. These in vitro studies set the groundwork for in vivo studies to determine if intrabody-mediated knockout of MHCI can impair alloantigen expression and prolong the survival of keratinocyte allografts.

Indirect allorecognition can play an important role in the development of transplant arteriosclerosis

BACKGROUND

Indirect allorecognition has been implicated in the initiation of chronic allograft dysfunction. Our aim was to develop an animal model that allowed the contribution of the direct and indirect pathway of allorecognition in the evolution of transplant arteriosclerosis, the main feature of chronic allograft rejection, to be evaluated.

METHODS

Aortic allografts mismatched for a single MHC class I antigen were transplanted into athymic NUDE or RAG (-/-) mice. Immunodeficient mice were reconstituted with either CD4(+) (indirect) or CD8(+) (direct + indirect) T cells in the presence or absence of depleting antibodies specific for the opposite T-cell subset. Aortic grafts were analyzed by performing morphometry, immunohistochemistry, and quantitative reverse transcriptase-polymerase chain reaction for the detection of cytokine mRNA production. Donor-specific alloantibody production was measured by fluorescence-activated cell sorter analysis.

RESULTS

Reconstitution of athymic nude mice with 4 x 10(7) purified CD4(+) T cells resulted in vascular rejection of MHC class I mismatched aortic grafts. Intimal proliferation was 24+/-8% and did not decrease when nude-derived endogenous CD8(+) T cells were depleted from the nude recipients (intimal proliferation, 21+/-7%). Transplant arteriosclerosis initiated by CD4+ T cells was associated with the presence of intragraft mRNA for interferon-gamma, tumor necrosis factor-alpha, inducible nitric oxide synthase, and interleukin 12. Reconstitution of RAG-1(-/-) mice with 4 x 10(7) purified CD4(+) T cells resulted in a similar degree of transplant arteriosclerosis (intimal proliferation, 20+/-9%) in MHC class I mismatched aortic grafts in the absence of alloantibody production.

CONCLUSION

Indirect recognition of donor MHC class I molecules by CD4(+) T cells can play an important role in the process of transplant arteriosclerosis. CD8(+) T-cell effector function and alloantibody production in this model are dependent on CD4(+) T-cell help after indirect allorecognition.

Embryonic thymic epithelium naturally devoid of APCs is acutely rejected in the absence of indirect recognition

Transplants of tissues depleted of passenger leukocytes are upon in vitro culture usually accepted in allogeneic recipients. Accordingly, fully allogeneic embryonic thymic epithelium was suggested to be poorly immunogenic. However, this tissue is capable of inducing donor-specific tolerance to peripheral tissues, when restoring T cell development in nude mice, through the production of regulatory cells. In the present work, adult immunocompetent allogeneic recipients were grafted with embryonic tissues isolated at stages before hemopoietic colonization or even before the establishment of circulation. Allogeneic thymic epithelium of day 10 embryos and heart primordium of day 8 embryonic donors were always rejected. Acute rejection of the thymic anlagen takes place in less than 12 days, with maximal CD4(+) and CD8(+) T cell infiltrates at 10 days post-transplant. In addition, a significant infiltrate of NK1.1(+) cells is observed, although without any essential role in this process. Furthermore, recipients lacking the indirect pathway of Ag presentation to CD4(+) T cells do not reveal any significant delay in rejection, even when CD8(+) T cells are also eliminated. Thus, our experimental approach reveals acute allograft rejection in the absence of all known pathways of naive T cell activation and therefore unveils a novel graft rejection mechanism that should be mediated by direct recognition of parenchymal cells. Given the importance of dendritic cells in naive T cell activation, it is likely that cross-reactive memory T cells may also drive rejection.

Antigenicity and immunogenicity of allogeneic retinal transplants

The transplantation of neuronal cells and tissues represents a promising approach for the treatment of incurable neurodegenerative diseases. Indeed, it has been reported recently that retinal transplantation can rescue photoreceptor cells and delay age-related changes in various retinal layers in rodents. However, retinal grafts deteriorate progressively after placement in recipients' eyes. Here we investigated whether a host's immune response elicited toward the graft contributes to its deterioration. Using an ELISA spot assay, we measured T cell responses to retinal tissues placed in the vitreous cavity of syngeneic and allogeneic mice. We found that allogeneic retinas induced potent alloimmune responses mediated by T cells secreting type 1 cytokines (IFN-gamma and IL-2). No response was found in mice engrafted with syngeneic retinas. In addition, all syngeneic retinal grafts displayed no signs of tissue damage (at 55 days), while the majority of allogeneic retinas deteriorated as early as 12 days after placement. Next, we showed that anti-donor responses occurred within two phenotypically and functionally distinct T cell subsets: CD4+ T cells secreting IL-2 and CD8+ T cells producing IFN-gamma. Importantly, CD4+ T cells were necessary and sufficient to cause graft deterioration, while CD8+ T cells did not contribute to this process.

The role of the class II transactivator (CIITA) in MHC class I and II regulation and graft rejection in kidney

Class II transactivator (CIITA) induces transcription of MHC class II genes, and induces class I in some cell lines. We examined the effect of CIITA deficiency on class I and II expression in mouse kidney. In CIITA knockout (CIITAKO) mice, basal class II was absent, but class I levels were mildly but significantly increased. Allogeneic stimulation or ischemic injury increased class I and II expression in kidneys of control (wild-type, WT) mice but induced only class I in CIITAKO mice. Thus, in kidney, all basal and induced class II expression was CIITA-dependent, but neither basal nor induced class I was CIITA-dependent. Rejecting kidney allografts from CIITAKO mice in CBA hosts manifested intense induction of donor class I but no donor class II expression. Rejecting kidneys from both WT and CIITAKO donors showed predominantly CD8 T-cell infiltration at days 7 and 21, with increasing tubulitis and arteritis at day 21. CIITAKO kidneys showed fewer infiltrating cells than WT kidneys at day 21. Thus CIITA-deficient kidneys have no basal and induced class II expression but display intense induction of class I expression, and evoke typical rejection lesions, although some indices of infiltration are reduced at day 21.

Role of CD4+ and CD8+ T cells in allorecognition: lessons from corneal transplantation

Corneal transplantation represents an interesting model to investigate the contribution of direct vs indirect Ag recognition pathways to the alloresponse. Corneal allografts are naturally devoid of MHC class II+ APCs. In addition, minor Ag-mismatched corneal grafts are more readily rejected than their MHC-mismatched counterparts. Accordingly, it has been hypothesized that these transplants do not trigger direct T cell alloresponse, but that donor Ags are presented by host APCs, i.e., in an indirect fashion. Here, we have determined the Ag specificity, frequency, and phenotype of T cells activated through direct and indirect pathways in BALB/c mice transplanted orthotopically with fully allogeneic C57BL/6 corneas. In this combination, only 60% of the corneas are rejected, while the remainder enjoy indefinite graft survival. In rejecting mice the T cell response was mediated by two T cell subsets: 1) CD4+ T cells that recognize alloantigens exclusively through indirect pathway and secrete IL-2, and 2) IFN-gamma-producing CD8+ T cells recognizing donor MHC in a direct fashion. Surprisingly, CD8+ T cells activated directly were not required for graft rejection. In nonrejecting mice, no T cell responses were detected. Strikingly, peripheral sensitization to allogeneic MHC molecules in these mice induced acute rejection of corneal grafts. We conclude that only CD4+ T cells activated via indirect allorecognition have the ability to reject allogeneic corneal grafts. Although alloreactive CD8+ T cells are activated via the direct pathway, they are not fully competent and cannot contribute to the rejection unless they receive an additional signal provided by professional APCs in the periphery.

Rejection of mouse cardiac allografts by costimulation in trans

The activation of T cells by B7 costimulation in trans has been demonstrated in vitro, but the in vivo relevance is unknown. To study costimulation in trans of CD4(+) T cells in vivo, we performed cardiac transplants from B7-1/B7-2-deficient mice to recipients that do not express MHC class II molecules on peripheral APCs, but do have functional CD4(+) T cells (II(-)/4(+) mice). This model restricts the B7-dependent activation of CD4(+) T cells to costimulation in trans and excludes any contribution from indirect Ag presentation. We find that II(-)/4(+) recipients reject B7-deficient grafts as rapidly as wild-type grafts, suggesting that costimulation in trans can mediate rejection as potently as costimulation in cis. Treatment of II(-)/4(+) recipients of B7-deficient grafts with depleting Abs to CD4 or CD8 demonstrates that indirect Ag presentation to CD8(+) cells does not significantly contribute to rejection. This is the first demonstration that costimulation in trans can mediate an immune response in vivo and has important therapeutic implications.

Rejection responses to allogeneic hepatocytes by reconstituted SCID mice, CD4, KO, and CD8 KO mice

INTRODUCTION

The purpose of the current study was to investigate the capacity of CD4+, CD8+, or non-T cells to independently initiate acute rejection of allogeneic hepatocytes using reconstituted SCID, CD4 or CD8 knockout (KO) recipient mice.

METHODS

Allogeneic hepatocytes (FVB/N, H-2q) were transplanted into C57BL/6.SCID (H-2b), CD4 KO (H-2b), CD8 KO (H-2b), or beige/beige (H-2b) mice. SCID mice with functioning hepatocellular allografts subsequently received purified non-T cells (NTC), CD4+, or CD8+ splenocytes. Some mice were treated with anti-CD4, anti-CD8, and/or anti-nkl.1 mAb. Recipient mice were also assessed for donor-reactive delayed-type hypersensitivity (DTH) responses and donor-reactive alloantibody production.

RESULTS

Median hepatocellular allograft survival time (MST) was 28 days in CD4+ reconstituted SCID mice and 14 days in CD8+ reconstituted SCID mice. SCID hosts reconstituted with NTC demonstrated indefinite hepatocellular allograft survival (>120 days). MST was 10 days in untreated beige/beige (NK cell deficient) mice. MST was 14 days in untreated, 35 days in anti-CD4 mAb treated, and 10 days in anti-nkl.1 mAb treated CD8 KO mice. MST was 10 days in untreated, 35 days in anti-CD8 mAb treated, and 7 days in anti-nk1.1 mAb treated CD4 KO mice. Donor-reactive DTH responses were not detected in reconstituted SCID mice, were minimal in CD4 KO mice, and were prominent in CD8 KO mice after rejection of allogeneic hepatocytes. Similarly, donor-reactive alloantibody, was not detected in CD4 KO hosts, but was readily detected in CD8 KO hosts.

CONCLUSIONS

These studies show that both CD4+ and CD8+ T cells (but not host NTC) can independently initiate the rejection of allogeneic hepatocytes. While hepatocyte rejection by isolated CD4+ T cells is not surprising, rejection by CD8+ T cells (in the absence of CD4+ T cells) was unusual, and may explain the failure of "standard" immunosuppressive regimens to suppress acute rejection of allogeneic hepatocytes, as noted in prior studies. Furthermore, NK cells do not appear to be required for either CD4+ T cell or CD8+ T cell initiated hepatocyte rejection.

Replacement of graft-resident donor-type antigen presenting cells alters the tempo and pathogenesis of murine cardiac allograft rejection

BACKGROUND

Graft-resident antigen presenting cells (APCs) are potent stimulators of the alloresponse. To test whether replacement of graft-resident donor-type APCs with those of recipient-type alters allorecognition and the pathogenesis of both acute and chronic rejection, we created chimeric hearts for transplantation into naive recipients.

METHODS

To replace donor-type APCs with those of recipient-type, chimeric animals were created by bone marrow transplantation (BMT) in fully allogeneic mouse and rat strain combinations. The degree of APC replacement in chimeric organs was assessed phenotypically and functionally. Chimeric hearts were transplanted heterotopically into untreated recipients.

RESULTS

Flow cytometric and immunohistochemical analysis did not detect residual bone marrow recipient-type APCs in mouse BMT chimeras. Although semi-quantitative reverse transcription polymerase chain reaction detected 0.001-0.01% residual cells, APCs isolated from chimeric organs were functionally unable to stimulate donor-type cells. When transplanted into naive recipients, chimeric mouse hearts had significantly prolonged survival but were nevertheless rejected acutely. Similar results were obtained in the ACI --> LEW rat strain combination. However, in the PVG --> DA rat model, the majority of chimeric hearts survived >100 days and all long-surviving hearts developed cardiac allograft vasculopathy.

CONCLUSIONS

BMT leads to near complete replacement of organ-resident APCs. The virtual absence of donor-type APCs in chimeric hearts delays or prevents acute rejection in a strain-dependent manner. In contrast, this type of graft modification does not prevent cardiac allograft vasculopathy. This suggests that, although the CD4+ direct pathway may play a role in acute rejection, it is not essential for the development of chronic rejection in rodent cardiac allografts.

The critical role of mouse CD4+ cells in the rejection of highly disparate xenogeneic pig thymus grafts

Long-term survival of fetal pig thymus (FP THY) grafts and efficient repopulation of mouse CD4+ T cells is achieved in thymectomized (ATX) B6 mice that receive T and NK cell depletion by injection of a cocktail of mAbs (GK1.5, 2.43, 30-H12, and PK136) and fetal pig thymus/liver (FP THY/LIV) grafts. The requirement for each mAb in this conditioning regimen in order to avoid the rejection of FP THY grafts has not yet been defined. In our present studies, CD4 cell-depleted ATX B6 mice and euthymic MHC class II-deficient (IIKO) mice were employed to investigate the role of mouse CD4+ cells in the rejection of FP THY grafts in vivo. After grafting FP THY/LIV to CD4+ cell-depleted ATX B6 mice, efficient repopulation of mouse CD4+ T cells was observed in the periphery. However, only two of four mice had remaining FP THY grafts by 17 weeks post-implantation, and these were of poor quality, whereas four of four T and NK cell-depleted ATX B6 mice had well-developed FP THY grafts. Furthermore, three of four FP THY/LIV-grafted, CD4+ cell-depleted ATX B6 mice rejected donor MHC-matched pig skin grafts. In contrast, three of three FP THY/LIV grafted, T and NK cell-depleted, ATX B6 mice accepted donor MHC-matched pig skin grafts, suggesting that optimal tolerance to xenogeneic pig antigens was not achieved in mice conditioned only with anti-CD4 mAb. ATX B6 mice treated with only anti-CD8 mAb rejected FP THY completely by 6 weeks post-grafting, a time when CD4+ cell-depleted ATX B6 mice had well-vascularized FP THY grafts. In addition, when euthymic IIKO mice were pre-treated with the standard conditioning regimen that includes four different mAbs, FP THY grafts survived and supported the repopulation of mouse CD4+ T cells in the periphery, while high levels of mouse CD8+ T cells developed in host thymi. These studies suggest that mouse CD4+ T cells play a critical role in the acute rejection of xenogeneic FP THY grafts. Without help from CD4+ cells, mouse CD8+ cells, NK, NK/T, and TCR(gamma/delta)+ T cells do not mediate acute rejection of FP THY grafts. Furthermore, our results suggest that other cell subsets besides CD4+ T cells play a role in the delayed rejection of highly disparate xenogeneic FP THY grafts.

CD4+ T cells responsive through the indirect pathway can mediate skin graft rejection in the absence of interferon-gamma

BACKGROUND

T cells responding through the indirect pathway can induce allograft rejection, but mechanisms of rejection are not known. Interferon-y (IFN-gamma) may be an important mediator of rejection under these circumstances.

METHODS

We transferred CD4+ T cells from IFN-gamma-deficient (IFN-gamma-/-) mice into SCID recipients of MHC II-deficient (MHC II-/-) skin grafts. Under these conditions, rejection can only occur via the indirect pathway and cannot be mediated by T-cell production of IFN-gamma.

RESULTS

IFN-gamma-/- CD4+ T cells rejected MHC II -/- skin grafts. Flow cytometry revealed only CD4+ T cells in the recipients. Cytokine enzyme-linked immunosorbent spot assays confirmed only indirect recognition with an associated expansion of an alloreactive population of IL-2-, IL-4-, and IL-5-secreting T cells.

CONCLUSION

CD4+ T cells recognizing alloantigens via the indirect pathway can mediate skin graft rejection in the absence of IFN-gamma.

Gene targeting: applications in transplantation research

Gene targeting, the manipulation of gene in the mouse genome using homologous recombination in embryonic stem cells, is a powerful experimental tool that has been widely utilized in a number of disciplines. The ability to precisely alter genes in this way provides an avenue for investigating the role of a gene product in normal and pathological processes in the intact animal, with a precision and efficacy not possible using pharmacological agents, antibodies or engineered proteins. In transplant research, gene targeting provides a unique tool for discriminating the contributions of gene expression in donor versus recipient tissues. This review focuses on several areas in transplantation research where gene targeting has made useful contributions. These include studies of the role of donor and recipient multiple histocompatibility complex antigens in regulating rejection responses, the role of CD4+ T cell in mediating acute rejection, and the functions of cytokines during rejection and tolerance induction. These studies highlight the unique advantages of gene targeting in studies of complex processes in whole animals and illustrate the contributions of this technique to understanding the pathogenesis of allograft rejection.

Tolerance to Antigen-Presenting Cell-Depleted Islet Allografts Is CD4 T Cell Dependent

Pretreatment of pancreatic islets in 95% oxygen culture depletes graft-associated APCs and leads to indefinite allograft acceptance in immunocompetent recipients. As such, the APC-depleted allograft represents a model of peripheral alloantigen presentation in the absence of donor-derived costimulation. Over time, a state of donor-specific tolerance develops in which recipients are resistant to donor APC-induced graft rejection. Thus, persistence of the graft is sufficient to induce tolerance independent of other immune interventions. Donor-specific tolerance could be adoptively transferred to immune-deficient SCID recipient mice transplanted with fresh immunogenic islet allografts, indicating that the original recipient was not simply “ignorant” of donor antigens. Interestingly, despite the fact that the original islet allograft presented only MHC class I alloantigens, CD8+ T cells obtained from tolerant animals readily collaborated with naive CD4+ T cells to reject donor-type islet grafts. Conversely, tolerant CD4+ T cells failed to collaborate effectively with naive CD8+ T cells for the rejection of donor-type grafts. In conclusion, the MHC class I+, II− islet allograft paradoxically leads to a change in the donor-reactive CD4 T cell subset and not in the CD8 subset. We hypothesize that the tolerant state is not due to direct class I alloantigen presentation to CD8 T cells but, rather, occurs via the indirect pathway of donor Ag presentation to CD4 T cells in the context of host MHC class II molecules.

Direct and indirect recognition: the role of MHC antigens in graft rejection

In graft rejection, T-cell stimulation by donor APCs and self-APCs (presenting peptides of donor origin) has been called 'direct' and 'indirect' recognition, respectively. Here, Dina Gould and Hugh Auchincloss consider the traditional arguments favoring direct recognition and highlight recent findings suggesting the importance of indirect responses, thereby questioning some of our basic concepts of transplantation immunology.

Murine mentors: transgenic and knockout models of surgical disease

OBJECTIVE

Transgenic and knockout technologies have emerged from the "molecular biology revolution" as unprecedented techniques for manipulating gene function in intact mice. The goals of this review are to outline the techniques of creating transgenic and knockout mice, and to demonstrate their use in elucidation of the molecular mechanisms underlying common surgical diseases.

SUMMARY BACKGROUND DATA

Gain of gene function is created by transgenic technology, whereas gene function is ablated using gene knockouts. Each technique has distinctive applications and drawbacks. A unique feature of genetically manipulated mice is that combinatorial genetic experiments can be executed that precisely define the functional contribution of a gene to disease progression. Transgenic and knockout mouse models of wound healing, cardiovascular disease, transplant immunology, gut motility and inflammatory bowel disease, and oncology are beginning to illuminate the precise molecular regulation of these diseases. Transgenic technology has also been extended to larger mammals such as pigs, with the goal of using genetic manipulation of the xenogenic immune response to increase the availability of transplant organs. Continual refinements in gene manipulation technology in mice offer the opportunity to turn genes on or off at precise time intervals and in particular tissues, according to the needs of the investigator. Ultimately, investigation of disease development and progression in genetically manipulated mammals may delineate new molecular targets for drug discovery and provide novel platforms for drug efficacy screens.

CONCLUSIONS

Emulation of human disease and therapy using genetically manipulated mammals fulfills a promise of molecular medicine: fusion of molecular biochemistry with "classical" biology and physiology. Surgeons have unique skills spanning both worlds that can facilitate their success in this expanding arena.

T cell-mediated xenograft rejection: specific tolerance is probably required for long term xenograft survival

T cell-mediated mechanisms of xenograft rejection appear resistant to standard immunosuppression protocols used to prevent allograft rejection and, consequently, higher doses of immunosuppressive drugs are required to promote xenograft compared to allograft survival. Evidence from recent studies suggests that porcine xenografts may be especially immunogenic in humans because of a prominent and vigorous indirect xenoresponse and because of the ability of porcine endothelium to activate human T cells. This has led to an anxiety that systemic immunosuppressives, used as the mainstay of therapy for clinical xenotransplantation, may not allow the long-term survival of porcine organs transplanted into human recipients. This article will review the biology of T cell xenoresponses, present the case for the development of novel graft-specific immunosuppressive regimes in clinical xenotransplantation, and review recent experimental progress in this area.

T-cell and major histocompatibility complex requirements for obliterative airway disease in heterotopically transplanted murine tracheas

BACKGROUND

One third of human lung allografts develop chronic rejection manifested as obliterative bronchiolitis. Heterotopically transplanted allogeneic murine tracheas develop obliterative airway disease (OAD) leading to a lesion resembling human obliterative bronchiolitis. The purpose of this study was to determine the T-cell and major histocompatibility complex (MHC) molecule requirements of murine OAD.

METHODS

BALB/c allografts and C57BL/6 (B6) isografts were transplanted into B6 severe combined immunodeficient (SCID) and B6 wild-type (WT) recipients. MHC class I-discrepant bm1 grafts, class II-discrepant bm12 grafts, and F1(bm1 x bm12) (F1) grafts also were transplanted into B6 WT recipients. Grafts were harvested between days 5 and 56 following transplantation and evaluated histologically.

RESULTS

Complete MHC-disparate allografts placed in WT recipients had significantly more disease than similar allografts in SCID recipients, and the latter were indistinguishable from isografts in either WT or SCID recipients, indicating a lymphocyte dependence on the disease development. Pathology was significantly more severe in bm1 and F1 allografts than in isografts recovered from B6 recipients, but bm12 allografts appeared no different than isografts. T-cell infiltrates in these bm12 allografts contained only CD4+ cells, whereas infiltrates in the BALB/c, bm1, and F1 allografts manifesting OAD contained both CD4+ and CD8+ cells. No grafts had significant B-cell infiltration.

CONCLUSIONS

These findings suggest that OAD relies on a host T-cell response that includes CD8+ cells, directed against allo-class I-bearing donor cells within the graft.

Immunogenicity of cultured keratinocyte allografts deficient in major histocompatibility complex antigens

BACKGROUND

Full-thickness (FT) and cultured keratinocyte (CK) allografts have been used as temporary skin replacements in patients with massive burns, but these grafts are ultimately rejected after restoration of host immunocompetence. Genetic engineering has permitted the creation of knockout (KO) mice deficient in class I or class II major histocompatibility antigens. This study examines the immunogenicity of such grafts to determine if these genetically modified keratinocytes could be used for permanent wound coverage.

METHODS

Host sensitization to alloantigen was assessed by second-set rejection. CBA mice (n = 111) were primed with flank grafts consisting of FT and CK allografts from normal C57BL/6 donors, FT and CK class I KO allografts, FT and CK class II KO allografts, and CK autografts. Three weeks later, hosts were challenged with normal tail allografts and observed for second-set rejection. Median graft survival was analyzed by chi2 and Wilcoxon rank tests. In the second experiment, cytotoxic T lymphocytes (CTLs) were harvested from CBA mice (n = 28) 3 weeks after flank grafting. CTL effectors were tested on radiolabeled targets at various ratios in a 51Cr release assay. Dilution curves of CTL activity were compared by analysis of variance.

RESULTS

Hosts primed with CK or FT allografts demonstrated accelerated rejection of second-set tail grafts compared with hosts covered with CBA autografts. CK knockout grafts were less immunogenic than FT knockout skin; class II KO allografts were considerably less immunogenic than class I KO allografts. CTL activity against the knockout CK allografts was negligible compared with that of hosts primed with normal allografts or FT knockout allografts.

CONCLUSION

Although full-thickness knockout skin retains substantial immunogenicity, cultured keratinocytes deficient in class II antigens fail to prime for accelerated second-set rejection and do not elicit a CTL response in the graft recipient. This lack of immunogenicity may permit the indefinite survival of allogeneic knockout keratinocytes in patients requiring massive wound excision and coverage.

Identification of an indirectly presented epitope in a mouse model of skin allograft rejection

BACKGROUND

The indirect pathway of allorecognition involves the processing and presentation of donor molecules by recipient antigen-presenting cells to alloreactive CD4+ T cells. Our objective was to assess the occurrence and significance of the indirect presentation of allogeneic major histocompatibility complex molecules in the rejection of major histocompatibility complex class I-disparate skin.

METHODS

A mouse model of allograft rejection was developed in which tail skin from C57.BL/10 (H2b) donors was transplanted onto B10.A(5R) recipients resulting in an allogeneic mismatch at the D locus. T-cell depletion studies were used to characterize T-cell subset involvement in rejection. B10.A(5R) mice were immunized with pools of overlapping peptides spanning the polymorphic region of Db in order to identify Db-derived epitopes involved in rejection. The relevance of these epitopes was assessed through immunization of recipient mice with peptides before skin grafting to observe the effect of presensitization on the kinetics of rejection.

RESULTS

Rejection of Db-disparate skin by B10.A(5R) was delayed by CD4 and CD8+ T-cell depletion, indicating the significance of both cell types in rejection. At least six immunogenic peptides were identified, all of which contained a cryptic T-cell epitope. One peptide, however, was able to accelerate the rejection of Db-disparate skin. Presensitization of B10.A(5R) mice with this peptide also resulted in an increase in alloantibody, indicating the presence of a physiological as well as a cryptic epitope. Presensitization of mice with a peptide containing a distinct cryptic epitope, however, failed to influence rejection.

CONCLUSIONS

These findings demonstrate a significant role for the indirect pathway of antigen presentation in allograft rejection.

The strength of cell-mediated xenograft rejection in the mouse is due to the CD4+ indirect response

Previous studies have shown that CD4+ T cells are responsible for the great strength of cell-mediated xenograft rejection in the mouse. In vitro studies have suggested that this CD4+ response is to xenogeneic antigens that are presented indirectly. The present studies were carried out in order to determine whether the strength of cell-mediated xenograft rejection in vivo is dependent on the CD4+ indirect response. We grafted pig skin onto mice that express class II MHC antigens only on their thymic epithelial cells (II-4+ mice). These mice have normal numbers of functional peripheral CD4+ T cells; however they lack class II MHC expression on their antigen presenting cells and are thus incapable of mounting a CD4+ T cell-mediated indirect response. Xenograft survival was prolonged on these mice. Furthermore, administration of cyclosporine and anti-CD8 monoclonal antibodies to II-4+ recipients prolonged xenograft survival to at least the same extent as allograft survival, demonstrating that the strength of cell-mediated xenograft rejection resides in the CD4+ indirect response. Despite the increased survival time, xenograft rejection still occurred in the absence of the indirect pathway. Depletion of the II-4+ recipients of their CD4+ T cell population prolonged xenograft survival even further, suggesting the presence of a weaker CD4+ direct mechanism which was virtually undetectable in vitro.