Reduced NO production improves early canine islet xenograft function: a role for nitric oxide in islet xenograft primary nonfunction

Transplantation of Long-term Cultured Porcine Islets in the Rat: Prolonged Graft Survival and Recipient Growth on Reduced Immunosuppression

To evaluate whether further improvement in porcine islet xenotransplantation is feasible, a number of questions were addressed. Earlier we showed significant improvement in the nude mouse of the porcine islets by selection through long-term culture. Now these islets were tested in the stringent pig-to-rat model. Islets were isolated from adult pigs, cultured for 1.5–3 weeks and transplanted to rats. Possible rejection mechanisms were assessed by interference of the cellular response with cyclosporine A (CsA), blocking macrophages with gadolinium chloride (GdCl), and suppressing the humoral response with cyclophosphamide. Modifications in graft size and condition were analyzed. Untreated control recipients showed primary nonfunction (PNF). CsA treatment could fully overcome PNF and resulted in graft survival from 10 to over 134 days. Rejection was the main cause of function loss. Although rejection could not be prevented by intensifying the induction therapy, increased maintenance immunosuppression effectively blocked rejection, albeit at the expense of toxicity. Blocking the humoral response was ineffective; all grafts showed PNF. In contrast, depletion of macrophages fully prevented PNF. Combination of GdCl and CsA gave no additional effect, and grafts were rejected between 57 and 162 days. Generally, graft survivals were similar to those reported in the literature; however, long-term cultured islets required much less maintenance immunosuppression. Cessation of graft function was not always due to rejection; in some cases “islet exhaustion” was found, possibly caused by discrepancy between the graft size and the rapidly growing recipient. Neither the presence of damaged islet tissue in the graft nor the size of the graft exerted any influence on graft survival. On rejection, no real infiltration of the graft was seen; destruction gradually processed from the outside. The good functional capability of the cultured islets was illustrated by disappearance of the clinical symptoms and increase in body weight, which almost doubled in the long-term survivors.

Biological and Biomaterial Approaches for Improved Islet Transplantation

Islet transplantation may be used to treat type I diabetes. Despite tremendous progress in islet isolation, culture, and preservation, the clinical use of this modality of treatment is limited due to post-transplantation challenges to the islets such as the failure to revascularize and immune destruction of the islet graft. In addition, the need for lifelong strong immunosuppressing agents restricts the use of this option to a limited subset of patients, which is further restricted by the unmet need for large numbers of islets. Inadequate islet supply issues are being addressed by regeneration therapy and xenotransplantation. Various strategies are being tried to prevent beta-cell death, including immunoisolation using semipermeable biocompatible polymeric capsules and induction of immune tolerance. Genetic modification of islets promises to complement all these strategies toward the success of islet transplantation. Furthermore, synergistic application of more than one strategy is required for improving the success of islet transplantation. This review will critically address various insights developed in each individual strategy and for multipronged approaches, which will be helpful in achieving better outcomes.

Effect of glucose toxicity on intraportal tilapia islet xenotransplantation in nude mice

BACKGROUND

Discordant xenogeneic islets transplanted intraportally into athymic nude rats experience primary non-function and are rapidly destroyed. Recently, it has been reported that adult porcine islets transplanted intraportally into nude mice are also rapidly destroyed and that this constitutes a new model for instant blood-mediated inflammatory reaction (IBMIR).

METHODS

Tilapia (fish) islets were harvested, mechanically broken into mammalian islet-sized fragments, cultured for 48 h, and transplanted via the portal vein into athymic or euthymic mice.

RESULTS

There were several groups of recipient mice. Streptozotocin-diabetic nude mice received 400 islets via the portal vein (n = 12). Recipients were killed when hyperglycemic (>200 mg/dl); livers and native pancreases were examined histologically. Mean graft survival time, based on function, was 5.4 +/- 1.2 days; at autopsy, histology showed occasional viable islets. We also performed a group of transplants in non-diabetic nude mice (n = 6) and then killed the recipients 2 or 4 weeks later; all had abundant viable, well-granulated islet grafts based on histology. Therefore, the intraportal environs in nude mice are not incompatible with discordant fish islets; rather, it appears as if hyperglycemia adversely affects the intraportal islet grafts (i.e. ''glucose toxicity''). To test this hypothesis, transplants were performed into non-diabetic nude mice and allowed to engraft for either 3 days (n = 6) or 10 days (n = 8) prior to injection of streptozotocin (200 to 220 mg/kg i.v.) to destroy the beta-cells in the recipients' native islets (n.b. tilapia islets are exceedingly resistant to streptozotocin); these recipients were followed for 28 days post-transplantation (or until hyperglycemic) and then killed for histology. Mean graft function exceeded 25 days for both groups and viable well-granulated, tilapia islets grafts were readily identified in all recipients; in all but one, the native pancreases were markedly beta-cell depleted -- confirming that normoglycemia was due to functional fish islet xenografts.

CONCLUSIONS

Our results suggest that ''glucose toxicity'' plays a role in the immediate demise of intraportal tilapia islet xenografts.

Metallothionein Protects Islets from Hypoxia and Extends Islet Graft Survival by Scavenging Most Kinds of Reactive Oxygen Species

Islet transplantation is a promising therapy for Type 1 diabetes, but many attempts have failed due to early graft hypoxia or immune rejection, which generate reactive oxygen species (ROS). In the current study, we determined that transgenic overexpression of the antioxidant metallothionein (MT) in pancreatic beta cells provided broad resistance to oxidative stress by scavenging most kinds of ROS including H2O2, peroxynitrite radical released from streptozotocin, 3-morpholinosydnonimine (SIN-1), and superoxide radical produced by xanthine/xanthine oxidase. MT also reduced nitric oxide-induced beta cell death. A direct test of hypoxia/reperfusion sensitivity was made by exposing FVB and MT islets to hypoxia (1% O2). MT markedly reduced ROS production and improved islet cell survival. Because MT protected beta cells from a broad spectrum of ROS and from hypoxia, we considered it to be an ideal candidate for improving islet transplantation. We first tested syngeneic transplantation by implanting islets under the kidney capsule of the same strain, FVB mice, thereby eliminating the immune rejection component. Under these conditions, MT islets maintained much greater insulin content than control islets. Allotransplantation was then tested. MT transgenic and normal FVB islets were implanted under the kidney capsule of BALB/c mice that were previously treated with streptozotocin to induce diabetes. We found that MT islets extended the duration of euglycemia 2-fold longer than nontransgenic islets. The benefit of MT was due to protection from ROS since nitrotyrosine staining, an indicator of free radical damage, was much lower in MT grafts than in FVB grafts. The time course of protection suggested that the major mode of MT action may have been protection from hypoxia or hypoxia/reperfusion. These data demonstrate that treatment with a broad spectrum antioxidant protects islets from ROS damage such as that produced during the early phase of islet transplantation.

Fetal pig beta cells are resistant to the toxic effects of human cytokines

BACKGROUND

The cytokine tumour necrosis factor-alpha (TNF-alpha) is thought to be responsible for primary nonfunction of islets when transplanted. This, and two other cytokines, interleukin-1beta (IL-1beta) and interferon-gamma (IFN-gamma) are also implicated in the autoimmune destruction of beta cells. It is unknown if the fetal pig beta cell, which is being transplanted as a treatment for type 1 diabetes, is affected by these cytokines.

METHODS

We compared the effects of the cytokines on the function and viability of adult and fetal pig beta cells. The cells were cultured for up to 3 days in the presence of 2000 pg/ml of human IL-1beta, 1000 U/ml of TNF-alpha, and 1000 U/ml of IFN-gamma, as well as 1000 U/ml of porcine IFN-gamma. Cumulative insulin levels, insulin content, metabolic activity, and viability of these cells were examined. Additionally, nitric oxide production and the activity of antioxidant enzymes in these cells were also determined.

RESULTS

TNF-alpha and the combination of the three human cytokines caused a transient increase in cumulative insulin levels. TNF-alpha alone enhanced insulin content on day 3. There was no effect of these human cytokines on mitochondrial function and viability. In contrast, porcine IFN-gamma inhibited fetal pig beta cell function and also caused their death. Adult pig islets are sensitive to the toxic effects of human TNF-alpha, IL-1beta, the combination of the three cytokines, and porcine IFN-gamma. The activity of the antioxidant enzymes catalase, glutathione peroxidase, and superoxide dismutase were significantly higher in fetal pig beta cells than in adult islets, implying that this may be the reason for the lack of adverse effects of the cytokines on the fetal beta cell.

CONCLUSION

Fetal pig beta cells are resistant to the toxic effect of the human cytokines, TNF-alpha and IL-1beta, in vitro. This resistance suggests that fetal, but not adult beta cells, when transplanted into humans with type 1 diabetes may be protected from primary nonfunction and will be partially protected from autoimmune destruction.