Renal Subcapsular Transplantation Research Articles

Establishment and evaluation of a modified mouse model of renal subcapsular transplantation of microvolume cells

The renal subcapsular space provides an easily accessible, nutrition-rich pocket that supports engraftment, and as such, is often used as a site for stem and cancer cell transplantation. Renal capsule transplantation requires high technical requirements, the recipient mice have greater surgical damage, the mouse kidney is small and the kidney capsule is fragile, and the operation is easy to fail. The conventional method is not suitable for microvolume cell transplantation to this site in animals with a small kidney, such as mice, due to high risks of cell loss or dislocation or injury to the capsule. In this study, we developed and validated a modified approach for the mouse model of renal subcapsular transplantation of microvolume mouse skeletal stem cells (SSCs). We used a pipette with a refined tip to separate the capsule from the parenchyma. Moreover, we used cells suspended in Matrigel rather than a liquid carrier for transplantation. Using the modified method, we were able to transplant microvolume mouse SSCs as low as 0.2 μL beneath the mouse renal capsule with excellent reproducibility. After 4 weeks of in vivo culture, the implanted mouse SSCs formed grafts on the surface of the parenchyma at the target site of transplantation. Histological staining of the grafts indicated osteogenic, fibrogenic, and lipogenic differentiation. Micro-CT imaging of the grafts revealed bone formation. This modified model could be used to effectively transplant different types of microvolume cells to the renal subcapsular space when the donor cells are difficult to acquire or the recipient mice have a very small size kidney.

Functional Activity of Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes on a Mouse Renal Subcapsular Xenograft Model.

In the treatment of coronary heart disease, the most promising approach for replacing lost contractile elements involves obtaining cardiomyocytes through cardiac differentiation of pluripotent cells. The objective of this study is to develop a technology for creating a functional layer of cardiomyocytes derived from iPSCs, capable of generating rhythmic activity and synchronous contractions. To expedite the maturation of cardiomyocytes, a renal subcapsular transplantation model was employed in SCID mice. Following explantation, the formation of the cardiomyocyte contractile apparatus was assessed using fluorescence and electron microscopy, while the cytoplasmic oscillation of calcium ions was evaluated through visualization using the fluorescent calcium binding dye Fluo-8. The results demonstrate that transplanted human iPSC-derived cardiomyocyte cell layers, placed under the fibrous capsules of SCID mouse kidneys (for up to 6 weeks), initiate the development of an organized contractile apparatus and retain functional activity along with the ability to generate calcium ion oscillations even after removal from the body.

In Vivo Assessment of Size-Selective Glomerular Sieving in Transplanted Human Induced Pluripotent Stem Cell-Derived Kidney Organoids.

The utility of kidney organoids in regenerative medicine will rely on the functionality of the glomerular and tubular structures in these tissues. Recent studies have demonstrated the vascularization and subsequent maturation of human pluripotent stem cell-derived kidney organoids after renal subcapsular transplantation. This raises the question of whether the glomeruli also become functional upon transplantation. We transplanted kidney organoids under the renal capsule of the left kidney in immunodeficient mice followed by the implantation of a titanium imaging window on top of the kidney organoid. To assess glomerular function in the transplanted human pluripotent stem cell-derived kidney tissue 1, 2, and 3 weeks after transplantation, we applied high-resolution intravital multiphoton imaging through the imaging window during intravenous infusion of fluorescently labeled low and high molecular mass dextran molecules or albumin. After vascularization, glomerular structures in the organoid displayed dextran and albumin size selectivity across their glomerular filtration barrier. We also observed evidence of proximal tubular dextran reuptake. Our results demonstrate that human pluripotent stem cell-derived glomeruli can develop an appropriate barrier function and discriminate between molecules of varying size. These characteristics together with tubular presence of low molecular mass dextran provide clear evidence of functional filtration. This approach to visualizing glomerular filtration function will be instrumental for translation of organoid technology for clinical applications as well as for disease modeling.

Efficient generation of thymic epithelium from induced pluripotent stem cells that prolongs allograft survival

The thymus plays a significant role in establishing immunological self-tolerance. Previous studies have revealed that host immune reaction to allogeneic transplants could be regulated by thymus transplantation. However, physiological thymus involution hinders the clinical application of these insights. Here, we report an efficient generation of thymic epithelial-like tissue derived from induced pluripotent stem cells (iPSCs) and its potential to regulate immune reaction in allogeneic transplantation. We established an iPSC line which constitutively expresses mouse Foxn1 gene and examined the effect of its expression during in vitro differentiation of thymic epithelial cells (TECs). We found that Foxn1 expression enhances the differentiation induction of cells expressing TEC-related cell surface molecules along with upregulation of endogenous Foxn1. iPSC-derived TECs (iPSC-TECs) generated T cells in nude recipient mice after renal subcapsular transplantation. Moreover, iPSC-TEC transplantation to immuno-competent recipients significantly prolonged the survival of allogeneic skin. Our study provides a novel concept for allogeneic transplantation in the setting of regenerative medicine.

Three-dimensional cell-printing of advanced renal tubular tissue analogue

Despite significant progress in the development of renal tissue, recapitulation of perfusable complex renal tubular tissue with clinically relevant cellular heterogeneity is still remaining a challenge. In this study, using coaxial 3D cell-printing technique, we present microfluidic hollow tubes to realize tubular/vascular renal parenchyma composed of renal tubular epithelial and endothelial cells, respectively. We developed a functional hybrid bioink that inherits microenvironments for vascularized native kidney tissue with rapidly crosslinkable character to optimize cell functionality and retain the predefined hollow tubular structure. In addition, the novel bioink and 3D coaxial cell-printing technique provided a complex tube with tunable feature of monolayer and bilayer structure across the length of printed tube. Through prototyping a vascularized renal proximal tubule-on-a-chip, we showed its applicability to novel microfluidic renal tissue models. The renal subcapsular transplantation of the hollow tubes showed a long-term graft survival with the therapeutic capability of the tubular constructs in in vivo model of renal disease, which serves their applicability in regenerative medicine.

Renal Subcapsular Transplantation of 2'-Deoxyguanosine-Treated Murine Embryonic Thymus in Nude Mice.

The thymus is an important central immune organ, which plays an essential role in the development and differentiation of T cells. Thymus transplantation is an important method for investigating thymic epithelial cell function and T cells maturation in vivo. Here we will describe the experimental methods used within our laboratory to transplant 2'-deoxyguanosine (to deplete donor's lymphocytes) treated embryonic thymus into the renal capsule of an athymic nude mouse. This method is both simple and efficient and does not require special skills or devices. The results obtained via this simple method showed that transplanted thymus can effectively support the recipient's T cells production. Additionally, several key points with regards to the protocol will be further elucidated.

Renal Subcapsular Transplantation of 2'-Deoxyguanosine-Treated Murine Embryonic Thymus in Nude Mice

Renal Subcapsular Transplantation of 2'-Deoxyguanosine-Treated Murine Embryonic Thymus in Nude Mice

Synergistic Effect of Islet Magnetic Resonance Imaging Using Direct Linking of Ferumoxytol to Surface of PEGylated Islet

Purpose: We previously reported the feasibility of islet magnetic resonance imaging (MRI) using ferumoxytol, which is the only clinically-available ultrasmall superparamagnetic iron oxide. However, the labeling efficacy was not sufficient for its clinical use in islet transplantation. We aimed to evaluate the feasibility of islet MRI using direct linking of ferumoxytol to islet surface through PEGylation. Method: Islets were labeled by surface modification with up to 4% PEG-heparin-ferumoxytol. We compared islet function and viability of control islets and ferumoxytol-heparin-PEGylated islets. Efficacy of labeling with ferumoxytol-heparin-PEGylation and ferumoxytol alone was assessed in both ex vivo and in vivo models. Results: Labeling of islets with up to 2% PEG-heparin-ferumoxytol did not derange ex vivo islet viability and function. The T2* relaxation time was optimal when islets were labeled with 1 to 4% PEG-heparin-ferumoxytol. The labeling intensity in the ex vivo MRI of ferumoxytol-heparin-PEGylated islets was stronger than islets labeled with ferumoxytol alone. In syngeneic renal subcapsular islet transplantation, labeling with ferumoxytol-heparin-PEGylation showed better in vivo labeling efficacy than that of islets labeled with ferumoxytol alone. After labeling with ferumoxytol-heparin-PEGylation, there was a correlation between the total area of visualized islets and the transplanted islet mass in syngeneic mouse intraportal islet transplantation, and the visibility was also confirmed in the preliminary analysis of non-human primate intraportal islet transplantation model. Conclusion: Direct linking of ferumoxytol to islet surface through PEGylation improved the labeling efficacy and it could be used for the islet MRI in clinical islet transplantation. Disclosure H. Lee: None. Y. Kwon: None. H. Kim: None. M.R. Haque: None. G. Kim: None. S. Jin: None. M. Lee: None. Y. Byun: None. J. Kim: None.

Isolation and functional assessment of mouse skeletal stem cell lineage.

There are limited methods available to study skeletal stem, progenitor, and progeny cell activity in normal and diseased contexts. Most protocols for skeletal stem cell isolation are based on the extent to which cells adhere to plastic or whether they express a limited repertoire of surface markers. Here, we describe a flow cytometry-based approach that does not require in vitro selection and that uses eight surface markers to distinguish and isolate mouse skeletal stem cells (mSSCs); bone, cartilage, and stromal progenitors (mBCSPs); and five downstream differentiated subtypes, including chondroprogenitors, two types of osteoprogenitors, and two types of hematopoiesis-supportive stroma. We provide instructions for the optimal mechanical and chemical digestion of bone and bone marrow, as well as the subsequent flow-cytometry-activated cell sorting (FACS) gating schemes required to maximally yield viable skeletal-lineage cells. We also describe a methodology for renal subcapsular transplantation and in vitro colony-formation assays on the isolated mSSCs. The isolation of mSSCs can be completed in 9 h, with at least 1 h more required for transplantation. Experience with flow cytometry and mouse surgical procedures is recommended before attempting the protocol. Our system has wide applications and has already been used to study skeletal response to fracture, diabetes, and osteoarthritis, as well as hematopoietic stem cell-niche interactions in the bone marrow.

Renal Subcapsular Transplantation of PSC-Derived Kidney Organoids Induces Neo-vasculogenesis and Significant Glomerular and Tubular Maturation InVivo.

SummaryHuman pluripotent stem cell (hPSC)-derived kidney organoids may facilitate disease modeling and the generation of tissue for renal replacement. Long-term application, however, will require transferability between hPSC lines and significant improvements in organ maturation. A key question is whether time or a patent vasculature is required for ongoing morphogenesis. Here, we show that hPSC-derived kidney organoids, derived in fully defined medium conditions and in the absence of any exogenous vascular endothelial growth factor, develop host-derived vascularization. In vivo imaging of organoids under the kidney capsule confirms functional glomerular perfusion as well as connection to pre-existing vascular networks in the organoids. Wide-field electron microscopy demonstrates that transplantation results in formation of a glomerular basement membrane, fenestrated endothelial cells, and podocyte foot processes. Furthermore, compared with non-transplanted organoids, polarization and segmental specialization of tubular epithelium are observed. These data demonstrate that functional vascularization is required for progressive morphogenesis of human kidney organoids.

Cell Therapy Using Human Induced Pluripotent Stem Cell-Derived Renal Progenitors Ameliorates Acute Kidney Injury in Mice.

Acute kidney injury (AKI) is defined as a rapid loss of renal function resulting from various etiologies, with a mortality rate exceeding 60% among intensive care patients. Because conventional treatments have failed to alleviate this condition, the development of regenerative therapies using human induced pluripotent stem cells (hiPSCs) presents a promising new therapeutic option for AKI. We describe our methodology for generating renal progenitors from hiPSCs that show potential in ameliorating AKI. We established a multistep differentiation protocol for inducing hiPSCs into OSR1+SIX2+ renal progenitors capable of reconstituting three-dimensional proximal renal tubule-like structures in vitro and in vivo. Moreover, we found that renal subcapsular transplantation of hiPSC-derived renal progenitors ameliorated the AKI in mice induced by ischemia/reperfusion injury, significantly suppressing the elevation of blood urea nitrogen and serum creatinine levels and attenuating histopathological changes, such as tubular necrosis, tubule dilatation with casts, and interstitial fibrosis. To our knowledge, few reports demonstrating the therapeutic efficacy of cell therapy with renal lineage cells generated from hiPSCs have been published. Our results suggest that regenerative medicine strategies for kidney diseases could be developed using hiPSC-derived renal cells. This report is the first to demonstrate that the transplantation of renal progenitor cells differentiated from human induced pluripotent stem (iPS) cells has therapeutic effectiveness in mouse models of acute kidney injury induced by ischemia/reperfusion injury. In addition, this report clearly demonstrates that the therapeutic benefits come from trophic effects by the renal progenitor cells, and it identifies the renoprotective factors secreted by the progenitors. The results of this study indicate the feasibility of developing regenerative medicine strategy using iPS cells against renal diseases.

Electrofusion of mesenchymal stem cells and islet cells for diabetes therapy: a rat model.

Islet transplantation is a minimally invasive treatment for severe diabetes. However, it often requires multiple donors to accomplish insulin-independence and the long-term results are not yet satisfying. Therefore, novel ways to overcome these problems have been explored. Isolated islets are fragile and susceptible to pro-apoptotic factors and poorly proliferative. In contrast, mesenchymal stem cells (MSCs) are highly proliferative, anti-apoptotic and pluripotent to differentiate toward various cell types, promote angiogenesis and modulate inflammation, thereby studied as an enhancer of islet function and engraftment. Electrofusion is an efficient method of cell fusion and nuclear reprogramming occurs in hybrid cells between different cell types. Therefore, we hypothesized that electrofusion between MSC and islet cells may yield robust islet cells for diabetes therapy. We establish a method of electrofusion between dispersed islet cells and MSCs in rats. The fusion cells maintained glucose-responsive insulin release for 20 days in vitro. Renal subcapsular transplantation of fusion cells prepared from suboptimal islet mass (1,000 islets) that did not correct hyperglycemia even if co-transplanted with MSCs, caused slow but consistent lowering of blood glucose with significant weight gain within the observation period in streptozotocin-induced diabetic rats. In the fusion cells between rat islet cells and mouse MSCs, RT-PCR showed new expression of both rat MSC-related genes and mouse β-cell-related genes, indicating bidirectional reprogramming of both β-cell and MSCs nuclei. Moreover, decreased caspase3 expression and new expression of Ki-67 in the islet cell nuclei suggested alleviated apoptosis and gain of proliferative capability, respectively. These results show that electrofusion between MSCs and islet cells yield special cells with β-cell function and robustness of MSCs and seems feasible for novel therapeutic strategy for diabetes mellitus.

Monitoring transplanted islets by high-frequency ultrasound

Islet transplantation is a cell replacement therapy to improve glycometabolic control in type 1 diabetic patients. Establishing methods to monitor engrafted islets, as well as the islet preparation, is important when performing islet transplantation. Since current imaging techniques are still not available to directly detect transplanted islets, we propose a novel method to visualize transplanted islets using high-frequency ultrasound (HF-US), and to evaluate the correlation between these US findings and metabolic parameters.We transplanted syngeneic (BALB/c mice) and xenogeneic (SD rats) islets into the renal subcapsular space of diabetic mice. The recipient mice were examined by HF-US until post-operative day (POD) 28 and, while syngeneic islets could be detected by HF-US throughout the observational period, the xenogeneic islets had vanished by POD 28. The islet volume calculated by HF-US was correlated with the number of transplanted islets (R2 = 0.31, p = 0.0003) and the metabolic function of islets (blood glucose: R2 = 0.15, p < 0.0001, serum insulin: R2 = 0.22, p < 0.0001).In conclusion, HF-US may be a useful imaging modality for visualizing the islet mass in renal subcapsular transplantation models. It may also be an available modality for clinical settings in the future.

Caspase Inhibitor Therapy Enhances Marginal Mass Islet Graft Survival and Preserves Long-Term Function in Islet Transplantation

Islet transplantation can provide insulin independence in patients with type 1 diabetes, but islets derived from two or more donors are often required. A significant fraction of the functional islet mass is lost to apoptosis in the immediate posttransplant period. The caspase inhibitor N-benzyloxycabonyl-Val-Ala-Asp-fluoromethyl ketone (zVAD-FMK) has been used therapeutically to prevent apoptosis in experimental animal models of ischemic injury, autoimmunity, and degenerative disease. In the current study, zVAD-FMK therapy was examined in a syngeneic islet transplant model to determine whether caspase inhibition could improve survival of transplanted islets. zVAD-FMK therapy significantly improved marginal islet mass function in renal subcapsular transplantation, where 90% of zVAD-FMK-treated mice became euglycemic with 250 islets, versus 27% of the control animals (P < 0.001). The benefit of zVAD-FMK therapy was further demonstrated after intraportal transplantation, where 75% of zVAD-FMK-treated animals established euglycemia with only 500 islets, and all of the controls remained severely diabetic (P < 0.001). zVAD-FMK pretreatment of isolated islets in the absence of systemic therapy resulted in no significant benefit compared with controls. Long-term follow-up of transplanted animals beyond 1 year posttransplant using glucose tolerance tests confirmed that a short course of zVAD-FMK therapy could prevent metabolic dysfunction of islet grafts over time. In addition, short-term zVAD-FMK treatment significantly reduced posttransplant apoptosis in islet grafts and resulted in preservation of graft insulin reserve over time. Our data suggest that caspase inhibitor therapy will reduce the islet mass required in clinical islet transplantation, perhaps to a level that would routinely allow for insulin independence after single-donor infusion.

Small rat islets are superior to large islets in in vitro function and in transplantation outcomes

Barriers to the use of islet transplantation as a practical treatment for diabetes include the limited number of available donor pancreata. This project was designed to determine whether the size of the islet could influence the success rate of islet transplantations in rats. Islets from adult rats were divided into two groups containing small (diameter <125 microm) or large (diameter >150 microm) islets. An average pancreas yielded three times more small islets than large. Smaller islets were approximately 20% more viable, with large islets containing a scattered pattern of necrotic and apoptotic cells or central core cell death. Small islets in culture consumed twice as much oxygen as large islets when normalized for the same islet equivalents. In static incubation, small islets released three times more insulin under basal conditions than did large islets. During exposure to high glucose conditions, the small islets released four times more insulin than the same islet equivalencies of large islets, and five times more insulin was released by the small islets in response to glucose and depolarization with K+. Most importantly, the small islets were far superior to large islets when transplanted into diabetic animals. When marginal islet equivalencies were used for renal subcapsular transplantation, large islets failed to produce euglycemia in any recipient rats, whereas small islets were successful 80% of the time. The results indicate that small islets are superior to large islets in in vitro testing and for transplantation into the kidney capsule of diabetic rats.

Importance of the Gal alpha1-3 Gal antigen in discordant islet xenotransplantation: immunosuppression, which inhibits porcine islet xenograft rejection in ordinary mice, is equally effective in Gal-knockout mice.

Islet xenotransplantation will most likely be performed in diabetic patients treated with immunosuppressive drugs. The importance of the galactosyl alpha(1-3) galactose (Galalpha1-3Gal) antigen in immunosuppressed islet xenograft recipients has not been studied. Fetal porcine islet-like cell clusters (ICCs) were transplanted into the renal subcapsular space of both Gal-knockout mice and ordinary mice. Transplantations were performed in untreated mice and mice immunosuppressed with cyclosporine A (CsA) plus 15-deoxyspergualin (DSG). Studies were also performed in immunosuppressed Gal-knockout mice that had been actively immunized against Galalpha1-3Gal. Evaluation was performed 12 days after transplantation using morphologic techniques. The levels of serum immunoglobulin (Ig)G and IgM to the Galalpha1-3Gal antigen or to the ICCs were determined. No difference in the morphologic appearance could be seen between ordinary mice and Gal-knockout mice. No deposits of IgG, IgM, or C3 could be detected. Almost no difference could be seen between immunosuppressed Gal-knockout mice and immunosuppressed ordinary mice. In immunosuppressed, immunized Gal-knockout mice, the results were similar. In ordinary mice treated with CsA+DSG, the levels of anti-Gal IgM were lower than they were in untreated mice, whereas the levels of anti-Gal IgG were similar. In Gal-knockout mice (including immunized animals) treated with CsA+DSG, the levels of anti-Gal IgG and IgM were lower than they were in untreated Gal-knockout mice. After renal subcapsular transplantation, antibodies against Galalpha1-3Gal have no major influence on islet xenograft rejection in the pig-to-mouse model. Immunosuppression, which inhibits rejection in the pig-to-mouse model, is equally effective when transplantation is performed across the Galalpha1-3Gal barrier.

Anti-CD154 (CD40L) prevents recurrence of diabetes in islet isografts in the DR-BB rat.

Islet transplantation for the treatment of autoimmune diabetes is more difficult because of the additional barrier presented by the autoimmunity. We tested the ability of hamster anti-rat CD154 to prevent recurrence of diabetes in renal subcapsular islet isografts in DR-BB (RT1uu) rats with established autoimmune diabetes. Experimental animals with established diabetes received intravenous injections of 15 mg/kg anti-CD154 on a specified schedule starting 2 days before renal subcapsular transplantation of an islet isograft. Control animals received either saline or hamster IgG. Plasma glucose levels >250 mg/dl over 3 days were used to indicate the recurrence of diabetes. Rats that received saline (n = 5) or control antibody (n = 3) had a recurrence of diabetes 6-11 days after transplantation. Histological examination of islet isografts from these rats showed complete destruction of the insulin-producing portion of the isograft with residual cells positive for glucagon. Recipient rats that received anti-CD154 at the 15-mg/kg dosage (n = 6) did not have a recurrence of diabetes for 308-461 days after transplantation. Islet isografts removed from the rats showed low levels of insulin immunoreactivity, high levels of insulin mRNA, and focal infiltration with lymphocytes but no evidence of islet destruction. Mean peak antibody concentration was 266 microg/ml and returned to undetectable levels by 67-88 days after transplantation. Rats that received anti-CD154 starting at 4-7 days after transplantation had a recurrence of diabetes within 11 days of the isotransplantation. Therefore, anti-CD154 as the sole immunomodulator prevented the recurrence of diabetes in islet isografts in rats with established autoimmune diabetes. This suggests that CD40/CD154 blockade is effective in preventing the insulitis or the effector phase of autoimmune diabetes.

Different Histogenesis of Experimental Odontogenic Cysts by the Renal Subcapsular Transplantation of Tooth Germs of Mice.

The tooth germs of first mandibular molars of 17.5-day-old C3H mouse embryos were transplanted into the renal subcapsular spaces of 3-month-old syngenic male mice. Grafts were harvested at timed sequences from 1, 2, 3, 4 weeks and 2, 3, 4, 5 months, and were examined histopathologically by total serial sections. Of 32 cases of grafts, keratocyst-like lesions were formed in 13 cases, and dentigerous cyst-like lesions were formed in 7 cases. None of the cases developed two cysts at the same time. In the keratocyst-like lesions the cavity developed from the enamel epithelium in the early stage after transplantation, and in the dentigerous cyst-like lesion the cavity developed by cystic degeneration of stellate reticular layer of enamel organ in the late stage after transplantation. The present experiment revealed that keratocyst-like lesions and dentigerous cyst-like lesions developed through different histogeneses.

Establishment of islet-specific T-cell lines and clones from islet isografts placed in spontaneously diabetic NOD mice.

One impediment to detailed characterization of islet-specific T cells from the NOD mouse model of diabetes is the difficulty encountered in isolation of such cells. This report describes a method that allows routine isolation of relatively large numbers of T cells highly enriched for reactivity toward islet antigens. The method involves renal subcapsular transplantation of spontaneously diabetic NOD mice with NOD islets. These grafts are rapidly destroyed in a tissue-specific manner and this destruction is accompanied by lymphocytic infiltration. Here we demonstrate that the islet graft infiltrates are an excellent source of islet-specific T cells and that islet-specific T-cell lines and clones can be established from these cells. Islet-specific T-cell clones isolated from a T-cell line established from the islet graft infiltrates were capable of adoptive transfer of diabetes to NOD/LtSz-scid recipients.

The role of histocompatibility antigens in transplantation of isolated islets of Langerhans in the rat.

In a model of congenic and intra-MHC recombinant rat strains, the differential role of various histocompatibility antigens in renal subcapsular transplantation of purified islets of Langerhans is evaluated. Class I MHC antigens of the RT1.A region, expressed on the endocrine cells of the islets themselves, do not induce graft rejection on their own. MHC class I antigens as encoded by the RT1.C region do not induce rejection either. MHC class II antigens as encoded by the RT1.B/D region are not expressed on the endocrine pancreas, not even during rejection. Although interstitial dendritic cells situated within the islets express these antigens, an isolated RT1.B/D incompatibility of islets is associated with prolonged survival in contrast to rapid rejection of fully MHC-mismatched grafts. Unlike other organs, islets matched for all MHC antigens, but incompatible at minor histocompatibility antigens, undergo rejection early after transplantation.