Physiologic Doses of Bilirubin Contribute to Tolerance of Islet Transplants by Suppressing the Innate Immune Response.

Bilirubin has been recognized as a powerful cytoprotectant when used at physiologic doses and was recently shown to have immunomodulatory effects in islet allograft transplantation, conveying donor-specific tolerance in a murine model. We hypothesized that bilirubin, an antioxidant, acts to suppress the innate immune response to islet allografts through two mechanisms: 1) by suppressing graft release of damage-associated molecular patterns (DAMPs) and inflammatory cytokines, and 2) by producing a tolerogenic phenotype in antigen-presenting cells. Bilirubin was administered intraperitoneally before pancreatic procurement or was added to culture media after islet isolation in AJ mice. Islets were exposed to transplant-associated nutrient deprivation and hypoxia. Bilirubin significantly decreased islet cell death after isolation and hypoxic stress. Bilirubin supplementation of islet media also decreased the release of DAMPs (HMGB1), inflammatory cytokines (IL-1ß and IL-6), and chemokines (MCP-1). Cytoprotection was mediated by the antioxidant effects of bilirubin. Treatment of macrophages with bilirubin induced a regulatory phenotype, with increased expression of PD-L1. Coculture of these macrophages with splenocytes led to expansion of Foxp3+ Tregs. In conclusion, exogenous bilirubin supplementation showed cytoprotective and antioxidant effects in a relevant model of islet isolation and hypoxic stress. Suppression of DAMP release, alterations in cytokine profiles, and tolerogenic effects on macrophages suggest that the use of this natural antioxidant may provide a method of preconditioning to improve outcomes after allograft transplantation.

Role of ITGAE in the development of autoimmune diabetes in non-obese diabetic mice.

There is compelling evidence that autoreactive CD8(+)T cells play a central role in precipitating the development of autoimmune diabetes in non-obese diabetic (NOD) mice, but the underlying mechanisms remain unclear. Given that ITGAE (CD103) recognizes an islet-restricted ligand (E-cadherin), we postulated that its expression is required for initiation of disease. We herein use a mouse model of autoimmune diabetes (NOD/ShiLt mice) to test this hypothesis. We demonstrate that ITGAE is expressed by a discrete subset of CD8(+)T cells that infiltrate pancreatic islets before the development of diabetes. Moreover, we demonstrate that development of diabetes in Itgae-deficient NOD mice is significantly delayed at early but not late time points, indicating that ITGAE is preferentially involved in early diabetes development. To rule out a potential contribution by closely linked loci to this delay, we treated WT NOD mice beginning at 2 weeks of age through 5 weeks of age with a depleting anti-ITGAE mAb and found a decreased incidence of diabetes following anti-ITGAE mAb treatment compared with mice that received isotype control mAbs or non-depleting mAbs to ITGAE. Moreover, a histological examination of the pancreas of treated mice revealed that NOD mice treated with a depleting mAb were resistant to immune destruction. These results indicate that ITGAE(+) cells play a key role in the development of autoimmune diabetes and are consistent with the hypothesis that ITGAE(+)CD8(+)T effectors initiate the disease process.

Integrin αEß7: molecular features and functional significance in the immune system.

Alpha E beta 7 (αEß7) is an α-I domain-containing integrin that is highly expressed by a variety of leukocyte populations at mucosal sites including intraepithelial T cells, dendritic cells, mast cells, and T regulatory cells (Treg). Expression depends largely or solely on transforming growth factor beta (TGF-ß) isoforms. The best characterized ligand for αEß7 is E-cadherin on epithelial cells, though there is evidence of a second ligand in the human system. An exposed acidic residue on the distal aspect of E-cadherin domain 1 interacts with the MIDAS site in the αE α-I domain. By binding to E-cadherin, αEß7 contributes to mucosal specific retention of leukocytes within epithelia. Studies on αE knockout mice have identified an additional important function for this integrin in allograft rejection and have also indicated that it may have a role in immunoregulation. Recent studies point to a multifaceted role for αEß7 in regulating both innate and acquired immune responses to foreign antigen.

Induction of graft-versus-host disease and in vivo T cell monitoring using an MHC-matched murine model.

Graft-versus-host disease (GVHD) is the limiting barrier to the broad use of bone marrow transplant as a curative therapy for a variety of hematological deficiencies. GVHD is caused by mature alloreactive T cells present in the bone marrow graft that are infused into the recipient and cause damage to host organs. However, in mice, T cells must be added to the bone marrow inoculum to cause GVHD. Although extensive work has been done to characterize T cell responses post transplant, bioluminescent imaging technology is a non-invasive method to monitor T cell trafficking patterns in vivo. Following lethal irradiation, recipient mice are transplanted with bone marrow cells and splenocytes from donor mice. T cell subsets from L2G85.B6 (transgenic mice that constitutively express luciferase) are included in the transplant. By only transplanting certain T cell subsets, one is able to track specific T cell subsets in vivo, and based on their location, develop hypotheses regarding the role of specific T cell subsets in promoting GVHD at various time points. At predetermined intervals post transplant, recipient mice are imaged using a Xenogen IVIS CCD camera. Light intensity can be quantified using Living Image software to generate a pseudo-color image based on photon intensity (red = high intensity, violet = low intensity). Between 4-7 days post transplant, recipient mice begin to show clinical signs of GVHD. Cooke et al. developed a scoring system to quantitate disease progression based on the recipient mice fur texture, skin integrity, activity, weight loss, and posture. Mice are scored daily, and euthanized when they become moribund. Recipient mice generally become moribund 20-30 days post transplant. Murine models are valuable tools for studying the immunology of GVHD. Selectively transplanting particular T cell subsets allows for careful identification of the roles each subset plays. Non-invasively tracking T cell responses in vivo adds another layer of value to murine GVHD models.

Regulation of acute graft-versus-host disease by microRNA-155.

Acute graft-versus-host disease (aGVHD) remains a major complication of allogeneic hematopoietic stem cell transplant (alloHSCT), underscoring the need to further elucidate its mechanisms and develop novel treatments. Based on recent observations that microRNA-155 (miR-155) is up-regulated during T-cell activation, we hypothesized that miR-155 is involved in the modulation of aGVHD. Here we show that miR-155 expression was up-regulated in T cells from mice developing aGVHD after alloHSCT. Mice receiving miR-155-deficient donor lymphocytes had markedly reduced lethal aGVHD, whereas lethal aGVHD developed rapidly in mice recipients of miR-155 overexpressing T cells. Blocking miR-155 expression using a synthetic anti-miR-155 after alloHSCT decreased aGVHD severity and prolonged survival in mice. Finally, miR-155 up-regulation was shown in specimens from patients with pathologic evidence of intestinal aGVHD. Altogether, our data indicate a role for miR-155 in the regulation of GVHD and point to miR-155 as a novel target for therapeutic intervention in this disease.

CD103 deficiency prevents graft-versus-host disease but spares graft-versus-tumor effects mediated by alloreactive CD8 T cells.

BACKGROUND: Graft-versus-host disease (GVHD) remains the main barrier to broader application of allogeneic hematopoietic stem cell transplantation (alloSCT) as a curative therapy for host malignancy. GVHD is mediated by allogeneic T cells directed against histocompatibility antigens expressed by host tissues. Based on previous studies, we postulated that the integrin CD103 is required for CD8-mediated GVHD, but not for graft-versus-tumor effects (GVT). METHODOLOGY/PRINCIPAL FINDINGS: We herein provide evidence in support of this hypothesis. To circumvent the potentially confounding influence of donor CD4 T cells, we developed an alloSCT model in which GVHD mortality is mediated by purified CD8 T cells. In this model, host-reactive CD8 T cells receive CD4 T cell help at the time of initial activation but not in the effector phase in which mature CD8 T effectors migrate into host tissues. We show that donor CD8 T cells from wild-type BALB/c mice primed to host alloantigens induce GVHD pathology and eliminate tumors of host origin in the absence of host CD4 T cells. Importantly, CD103 deficiency dramatically attenuated GVHD mortality, but had no detectable impact on the capacity to eliminate a tumor line of host origin. We provide evidence that CD103 is required for accumulation of donor CD8 T cells in the host intestinal epithelium but not in the tumor or host lymphoid compartments. Consistent with these data, CD103 was preferentially expressed by CD8 T cells infiltrating the host intestinal epithelium but not by those infiltrating the tumor, lamina propria, or lymphoid compartments. We further demonstrate that CD103 expression is not required for classic CD8 effector activities including cytokine production and cytotoxicity. CONCLUSIONS/SIGNIFICANCE: These data indicate that CD103 deficiency inhibits GVHD pathology while sparing anti-tumor effects mediated by CD8 T cells, identifying CD103 blockade as an improved strategy for GVHD prophylaxis.

Environmental and antigen receptor-derived signals support sustained surveillance of the lungs by pathogen-specific cytotoxic T lymphocytes.

Viral infections often gain access to the body of their host by exploiting areas of natural vulnerability, such as the semipermeable surfaces of mucosal tissues which are adapted for adsorption of nutrients and other diffusible molecules. Once the microbes have crossed the epithelial barrier, they can disperse to other tissues where eradication may not be possible. The best opportunity for successful immune intervention is immediately after infection while the pathogen is confined to a localized area of the body. Cytotoxic T lymphocytes (CTL) which reside at the site where the infection begins can make an important contribution to immunity by reducing early dissemination of the infection. Because the lungs provide easy access points for many pathogens to enter the body, they require protection from many complementary mechanisms, including pathogen-specific cytotoxic T cells. In this study we show that an enduring response to pathogen-derived peptide antigens facilitates sustained surveillance of the lungs by pathogen-specific CTL during the recovery from influenza virus infection. Our studies show that these processed peptide antigens reinforce expression of two homing receptors (CD69 and CD103) which help recently activated virus-specific CTL colonize the lungs during a mild inflammatory response. We suggest that this requirement for prolonged antigen presentation to reinforce local CTL responses in the lungs explains why protective cellular immunity quickly declines following influenza virus infection and other viral infections that enter the body via mucosal tissues.

Induction of CD4(+)CD25(+) T regulatory cells with CD103 depletion.

BACKGROUND: M290SAP, a murine CD103 antibody conjugated with the immunotoxin saporin, has been found to induce the indefinite acceptance of transplanted pancreatic islets in mice. We sought to understand the underlying mechanism of this alloacceptance, particularly with respect to the CD4 CD25 T regulatory phenotype. METHODS: In this study, we established the kinetics of M290SAP and evaluated the requirement of alloantigen for the induction and maintenance of CD4 CD25 T regulatory cells (Tregs). Naive C57BL/6 mice were treated with several doses of M290SAP with and without donor-specific blood or splenocytes. Blood and spleens were collected at specific time points and underwent FACS analysis. RESULTS: M290SAP significantly depleted CD103 cells and induced the up-regulation of CD4 CD25 T regulatory population in spleen cell preparations. The combination of alloantigen in the form of donor-specific blood or splenocytes, with M290SAP, further induced the up-regulation of CD4 CD25 Tregs in the spleen compared with either M290SAP alone or alloantigen alone. The generation of CD4 CD25 cells and the depletion of CD103 cells reached a maximum at 7 d and by 3 wk CD103 and CD4 CD25 T regulatory cell populations returned to baseline. When multiple antigenic challenges were administered, the splenic CD4 CD25 cell population was again up-regulated and persisted for 3 wk. CONCLUSION: Our data confirm that M290SAP induces the generation of the CD4 CD25 T regulatory phenotype in spleens of naïve mice. Alloantigen further enhances and rejuvenates the CD4 CD25 cell population in mice treated with M290SAP.

Application of anti-CD103 immunotoxin for saving islet allograft in context of transplantation.

BACKGROUND: Previous studies using knockout mice document a key role for the integrin CD103 in promoting organ allograft rejection and graft-versus-host disease. However, a determination of whether blockade of the CD103 pathway represents a viable therapeutic strategy for intervention in these processes has proven problematic due to the lack of reagents that efficiently deplete CD103(+) cells from wild type hosts. To circumvent this problem, in the present study, we invented an anti-CD103 immunotoxin (M290-SAP). We investigated whether M290-SAP has capacity to eliminate CD103-expressing cells in vivo and protect transplanted islets from destroying by host immune cells. METHODS: Flow cytometry was used to analyze the efficacy of M290-SAP in depleting CD103-expressing cells in vivo. Then using allogenic islet transplantation models as well as NOD mice with recent onset type 1 diabetes, the therapeutic efficacy of CD103-expressing cell depletion was addressed. RESULTS: M290-SAP dramatically reduces the frequency and absolute numbers of CD103-expressing leukocytes in peripheral lymphatic tissues of treated mice. Balb/c islets transplanted into streptozotocin-induced diabetic C57BL/6 mice under single M290-SAP treatment showed an indefinite survival time compared with untreated mice, M290-treated mice and IgG-SAP treated mice (mean survival time, > 100 days vs. < 20 days). C57BL/6 islets transplanted into hyperglycemic NOD mice under single M290-SAP treatment showed a pronounced delay in allograft rejection compared with untreated mice (mean survival time 12 - 13 days vs. < 7 days). Immunological analysis of mice with long-term islet allograft survival revealed an obvious atrophy thymus and severe downregulation of alloimmunity of CD8 subpopulation response to allogenic stimulation. CONCLUSION: Regardless of the underlying mechanisms, these data document that depletion of CD103-expressing cells represents a viable strategy for therapeutic intervention in islet allograft rejection.

Expression of the decay-accelerating factor (CD55) in renal transplants--a possible prediction marker of allograft survival.

BACKGROUND: Decay-accelerating factor (CD55) accelerates the decay of C3 and C5 convertases, participating in classical and alternative complement activation pathways. Complement activation plays a major role in antibody-mediated rejection of allografts (AMR); C4d is used as a marker of AMR. Emerging evidence suggests an important role of CD55 in the pathogenesis of AMR. The aim of this study was to investigate the expression of CD55 in renal allografts and to correlate it with the expression of C4d, allograft survival, changes in serum creatinine (SC). METHODS: More than 200 renal allograft biopsies, performed for allograft dysfunction, were assessed for peritubular capillary (PTC) C4d and CD55 expression. RESULTS: We found significant correlation between changes in SC and PTC CD55 staining pattern in patients with no PTC C4d staining. In these patients, SC increased from baseline by 2.2+0.34, 1.7+0.36, and 0.93+0.24 mg/dL in negative, focal, and diffuse PTC CD55 staining subgroups, respectively. Survival of renal allografts was better in diffuse PTC CD55 staining subgroup than in negative PTC CD55 staining subgroup. CONCLUSIONS: These data suggest that CD55 expression has a protective effect on PTC C4d negative renal allografts, and the pattern of PTC CD55 expression may be used as a potential marker of renal allograft survival in patients with no evidence of AMR.

Murine renal transplantation procedure.

Renal orthotopic transplantation in mice is a technically challenging procedure. Although the first kidney transplants in mice were performed by Russell et al over 30 years ago (1) and refined by Zhang et al years later (2), few people in the world have mastered this procedure. In our laboratory we have successfully performed 1200 orthotopic kidney transplantations with > 90% survival rate. The key points for success include stringent control of reperfusion injury, bleeding and thrombosis, both during the procedure and post-transplantation, and use of 10-0 instead of 11-0 suture for anastomoses. Post-operative care and treatment of the recipient is extremely important to transplant success and evaluation. All renal graft recipients receive antibiotics in the form of an injection of penicillin immediately post-transplant and sulfatrim in the drinking water continually. Overall animal health is evaluated daily and whole blood creatinine analyses are performed routinely with a portable I-STAT machine to assess graft function.

Comparison of the portal vein and kidney subcapsule as sites for primate islet autotransplantation.

To date, the portal vein has been the primary site for clinical islet transplantation. Despite success, potential complications such as portal vein thrombosis still exist. The kidney subcapsule has been used successfully in rodent models of islet transplantation. We hypothesized that the kidney subcapsule as a site for islet transplantation in the nonhuman primate model would be as effective as the portal vein. Diabetes was induced in the primate Macaca fascicularis via a total pancreatectomy. Animals were kept under anesthesia during the isolation procedure. Islet isolation was performed using intraductal infusion with Liberase HI and mechanical digestion in the Ricordi chamber, and were purified using a continuous Ficoll gradient. Purified islets were autotransplanted either into the portal vein (n = 6) or the left kidney subcapsule (n = 5) of pancreatectomized animals. Intravenous glucose tolerance tests were performed prior to pancreatectomy and 10 days following transplantation. Three animals underwent pancreatectomy and served as diabetic controls. Of the six animals receiving islets in the portal vein, one developed portal vein thrombosis. All remaining autotransplanted animals in this group remained normoglycemic with glucose-induced insulin secretion that was not different from that prior to pancreatectomy. Of the five animals undergoing transplantation into the kidney subcapsule, only one maintained normoglycemia and elicited insulin secretion in response to glucose stimulation. The other four animals remained hyperglycemic. We conclude that the portal vein is superior to the kidney subcapsule as a site for islet transplantation in nonhuman primates 10 days posttransplantation.

Abrogation of recurrent autoimmunity in the NOD mouse: A critical role for host interleukin 4.

BACKGROUND: We previously established a clinically relevant strategy to abrogate recurrent autoimmunity and enable long-term islet graft survival, involving antilymphocyte serum (ALS)-depletion of recipient T cells and intraportal administration of donor pancreatic lymph node cells (PLNCs) along with islet grafts. In this study, we investigated whether Th2 cytokines were required for the tolerizing ability of ALS/PLNC treatment in islet transplantation. METHODS: ALS-treated diabetic NOD recipient mice, and NOD recipient mice deficient in interleukin 4 (IL-4-/-) or 10 (IL-4/10-/-) were transplanted with NOR or NOD.scid islets intraportally along with donor PLNC. Blood glucose levels were monitored to access graft function, sections of graft-bearing livers were histologically examined, and ELISPOT assays were used to assess cytokine profile and frequency of islet-reactive CD4 T cells. RESULTS: We found that ALS/PLNC was not effective in prolonging islet graft survival in diabetic NOD hosts deficient in either IL-4 (NOD.IL-4-/-) or in IL-4 and IL-10 (NOD.IL4-/-/10-/-) (mean survival time, 36 days), contrasting the long-term survival of islet grafts in wild-type NOD mice (mean survival time, > 80 days). In contrast, PLNC deficient in IL-4 promoted long-term graft survival in wild-type NOD hosts similar to that in wild-type PLNC. In wild-type NOD recipients of either wild-type PLNC or IL-4-/- PLNC, the host autoantigen-specific CD4 T cells produced predominately IL-4 coincident with long-term graft survival, whereas, in NOD.IL-4-/- recipients with rejected grafts, the autoreactive T cells produced interferon gamma and low amounts of IL-4. CONCLUSIONS: These data demonstrate that abrogation of recurrent autoimmunity requires host IL-4 and that manipulation of the autoreactive cytokine profile in long-term diabetes may be an effective strategy for islet transplant therapies.

Critical role for CD103+CD8+ effectors in promoting tubular injury following allogeneic renal transplantation.

Immune destruction of the graft renal tubules is an important barrier to the long-term function of clinical renal allografts, but the underlying mechanisms remain obscure. CD103-an integrin conferring specificity for the epithelial cell-restricted ligand, E-cadherin-defines a subset of CD8 effectors that infiltrate the graft tubular epithelium during clinical rejection episodes, predicting a causal role for CD103+CD8+ effectors in tubular injury. In the present study, we used rodent transplant models to directly test this hypothesis. Surprisingly, CD8 cells infiltrating renal allografts undergoing unmodified acute rejection did not express significant levels of CD103. However, we demonstrate that a brief course of cyclosporine A to rat renal allograft recipients promotes progressive accumulation of CD103+CD8+ cells within the graft, concomitant with the development of tubular atrophy and interstitial fibrosis. As in the known clinical scenario, graft-associated CD103+CD8+ cells exhibited a T effector phenotype and were intimately associated with the renal tubular epithelium. Treatment with anti-CD103 mAb dramatically attenuated CD8 infiltration into the renal tubules and tubular injury. Mouse studies documented that CD103 expression is required for efficient destruction of the graft renal tubules by CD8 effectors directed to donor MHC I alloantigens. Taken together, these data document a causal role for CD103+CD8+ effectors in promoting tubular injury following allogeneic renal transplantation and identify novel targets for therapeutic intervention in this important clinical problem.

TGF-{beta}-dependent CD103 expression by CD8(+) T cells promotes selective destruction of the host intestinal epithelium during graft-versus-host disease.

Destruction of the host intestinal epithelium by donor effector T cell populations is a hallmark of graft-versus-host disease (GVHD), but the underlying mechanisms remain obscure. We demonstrate that CD8(+) T cells expressing CD103, an integrin conferring specificity for the epithelial ligand E-cadherin, play a critical role in this process. A TCR transgenic GVHD model was used to demonstrate that CD103 is selectively expressed by host-specific CD8(+) T cell effector populations (CD8 effectors) that accumulate in the host intestinal epithelium during GVHD. Although host-specific CD8 effectors infiltrated a wide range of host compartments, only those infiltrating the intestinal epithelium expressed CD103. Host-specific CD8 effectors expressing a TGF-beta dominant negative type II receptor were defective in CD103 expression on entry into the intestinal epithelium, which indicates local TGF-beta activity as a critical regulating factor. Host-specific CD8 effectors deficient in CD103 expression successfully migrated into the host intestinal epithelium but were retained at this site much less efficiently than wild-type host-specific CD8 effectors. The relevance of these events to GVHD pathogenesis is supported by the finding that CD103-deficient CD8(+) T cells were strikingly defective in transferring intestinal GVHD pathology and mortality. Collectively, these data document a pivotal role for TGF-beta-dependent CD103 expression in dictating the gut tropism, and hence the destructive potential, of CD8(+) T cells during GVHD pathogenesis.

Long-term islet graft survival in NOD mice by abrogation of recurrent autoimmunity.

Islet transplantation has great potential for curing type 1 diabetes; however, long-term islet survival using conventional immunosuppression remains elusive. We present a novel strategy for inducing long-lasting islet graft survival in diabetic NOD mice in the absence of posttransplant immunosuppression by initial treatment with antilymphocyte serum (ALS) followed by coadministration of donor pancreatic lymph node cells (PLNCs). When treated with ALS/PLNC, diabetic NOD mice become normoglycemic and tolerated minor antigen-disparate islet grafts for >100 days and syngeneic islet grafts indefinitely. Donor T-cells are required for graft prolongation, and tolerant hosts have long-term donor T-cell chimerism. Strikingly, host autoreactive T-cells from mice with long-surviving islet grafts predominantly produce interleukin-4, whereas autoreactive T-cells from mice that rejected their islet grafts predominantly produce interferon-gamma. We thus demonstrate a clinically relevant approach for ablation of recurrent autoimmunity in islet transplantation, involving donor lymphocyte-driven alteration of pathogenic autoreactive T-cells.

Regulation of CD103 expression by CD8+ T cells responding to renal allografts.

CD103 is an integrin with specificity for the epithelial cell-specific ligand, E-cadherin. Recent studies indicate that CD103 expression endows peripheral CD8 cells with a unique capacity to access the epithelial compartments of organ allografts. In the present study we used a nonvascularized mouse renal allograft model to 1) define the mechanisms regulating CD103 expression by graft-infiltrating CD8 effector populations, and 2) identify the cellular compartments in which this occurs. We report that CD8 cells responding to donor alloantigens in host lymphoid compartments do not initially express CD103, but dramatically up-regulate CD103 expression to high levels subsequent to migration to the graft site. CD103+CD8+ cells that infiltrated renal allografts exhibited a classic effector phenotype and were selectively localized to the graft site. CD8 cells expressing low levels of CD103 were also present in lymphoid compartments, but three-color analyses revealed that these are almost exclusively of naive phenotype. Adoptive transfer studies using TCR-transgenic CD8 cells demonstrated that donor-specific CD8 cells rapidly and uniformly up-regulate CD103 expression following entry into the graft site. Donor-specific CD8 cells expressing a dominant negative TGF-beta receptor were highly deficient in CD103 expression following migration to the graft, thereby implicating TGF-beta activity as a dominant controlling factor. The relevance of these data to conventional (vascularized) renal transplantation is confirmed. These data support a model in which TGF-beta activity present locally at the graft site plays a critical role in regulating CD103 expression, and hence the epitheliotropism, of CD8 effector populations that infiltrate renal allografts.

CD103 expression is required for destruction of pancreatic islet allografts by CD8(+) T cells.

The mechanisms by which CD8 effector populations interact with epithelial layers is a poorly defined aspect of adaptive immunity. Recognition that CD8 effectors have the capacity to express CD103, an integrin directed to the epithelial cell-specific ligand E-cadherin, potentially provides insight into such interactions. To assess the role of CD103 in promoting CD8-mediated destruction of epithelial layers, we herein examined the capacity of mice with targeted disruption of CD103 to reject pancreatic islet allografts. Wild-type hosts uniformly rejected islet allografts, concomitant with the appearance of CD8(+)CD103(+) effectors at the graft site. In contrast, the majority of islet allografts transplanted into CD103(-/-) hosts survived indefinitely. Transfer of wild-type CD8 cells into CD103(-/-) hosts elicited prompt rejection of long-surviving islet allografts, whereas CD103(-/-) CD8 cells were completely ineffectual, demonstrating that the defect resides at the level of the CD8 cell. CD8 cells in CD103(-/-) hosts exhibited normal effector responses to donor alloantigens in vitro and trafficked normally to the graft site, but strikingly failed to infiltrate the islet allograft itself. These data establish a causal relationship between CD8(+)CD103(+) effectors and destruction of graft epithelial elements and suggest that CD103 critically functions to promote intragraft migration of CD8 effectors into epithelial compartments.