Angiogenesis and hemodynamics of microvasculature of transplanted islets of Langerhans

The effect of capsule composition in the reversal of hyperglycemia in diabetic mice transplanted with microencapsulated allogeneic islets

The transplantation of microencapsulated islets may allow reversal of hyperglycemia in the absence of immunosuppression. Poly-L-lysine (PLL) on capsules may potentiate the fibrotic reaction against implanted capsules. The aims of this study were to investigate how the biocompatibility of such capsules affects their function in vivo and to compare their efficacy relative to naked islets after intraperitoneal transplantation to nude or immune competent mice. Alloxan-diabetic C57BL/6 wild-type or nude (nu/nu) mice were transplanted with naked BALB/c islets, empty capsules, or microencapsulated BALB/c islets. Three types of capsules were used, one containing a high guluronic acid (G) alginate and PLL, one with a high mannuronic acid (M) alginate and PLL, and one high M alginate capsule with no PLL. Hyperglycemia in nude mice was reversed after transplantation of naked islets or islets encapsulated in a capsule containing high M alginate. Nude mice transplanted with islets encapsulated in the high G capsules showed only a transient reversal of hyperglycemia. In an allogeneic system, naked BALB/c islets were rejected by day 10 after transplantation, whereas the islets encapsulated in high M capsules continued to function for at least a month. When PLL was excluded from the capsules, the grafts functioned for up to 8 weeks. Islets microencapsulated in high G alginate capsules fail to reverse hyperglycemia for more than a few days in nude mice. However, islets in high M alginate capsules can reverse hyperglycemia in nude and immune competent mice. Islets microencapsulated in PLL-free high M alginate capsules function for 8 weeks in immune competent mice.

Heat shock preconditioning impairs revascularization of freely transplanted pancreatic islets

BACKGROUND

Revascularization of freely transplanted pancreatic islets is essential for appropriate graft function and survival. During the first days after transplantation, however, islet transplants are avascular, and successful engraftment is believed to be markedly hampered by hypoxia-induced tissue injury. Because heat shock has been shown to induce cell resistance against hypoxia, it seems reasonable to stress pancreatic islets by heat before transplantation. In contrast, hypoxia is a major stimulus for angiogenesis, and thus heat shock preconditioning-induced resistance against hypoxia may decrease stimulation of angiogenesis. The authors therefore studied in vivo whether heat shock preconditioning of isolated islets affects angiogenesis and revascularization after free transplantation.

METHODS

After collagenase isolation, heat shock-preconditioned islets (42 degrees C for 30 min) were transplanted syngeneically into nontreated skinfold chambers of Syrian hamsters. In a second group of animals, nontreated islets were transplanted into heat shock-preconditioned chambers. Nontreated islets transplanted into nontreated chambers served as controls. Islet angiogenesis and revascularization were quantitatively analyzed during 14 days after transplantation using intravital fluorescence microscopy. Expression of heat shock proteins (HSP) was confirmed by immunohistochemistry and Western blotting.

RESULTS

Immunohistochemistry revealed expression of HSP32 (heme oxygenase [HO]-1), HSP72, and also intracellular insulin in isolated and transplanted pancreatic islets. Western blot analysis showed enhanced HSP32 but slightly decreased HSP72 expression in heat shock-preconditioned islets when compared with controls. Intravital microscopy revealed appropriate vascularization of control islets within 14 days after transplantation. Heat shock preconditioning of the host tissue (i.e., the skinfold chambers) did not affect islet vascularization when compared with controls. In contrast, heat shock preconditioning of the isolated islets resulted in a significantly (P < 0.05) impaired take rate, a reduced (P < 0.05) size of the newly formed microvascular network, and thus a smaller area (P < 0.05) of microvascularly perfused endocrine tissue.

CONCLUSION

These data suggest that heat shock preconditioning of isolated pancreatic islets before transplantation impairs the process of graft angiogenesis and revascularization. Therefore, transient exposure of isolated islets to heat may not be considered a promising tool to improve the outcome of islet transplantation.

Microdialysis measurements demonstrate a shift to nonoxidative glucose metabolism in rat pancreatic islets transplanted beneath the renal capsule

BACKGROUND

We aimed to use microdialysis to assess, for the first time, the internal milieu of pancreatic islet grafts.

METHODS

One month after transplantation, microdialysis probes were inserted into syngeneic rat islet transplants (500-700 islets) placed beneath the renal capsule of nondiabetic or diabetic recipients. The number of grafted islets was purposely chosen not to cure the diabetic recipients.

RESULTS

During an intravenous glucose challenge, insulin concentrations increased in parallel in serum and in the graft interstitium of nondiabetic animals suggesting the existence of a functionally well-established vascularization. Diabetic recipients had both a lower serum and dialysate insulin concentration than normoglycemic animals. The lactate/pyruvate ratios were determined in the dialysates as a measure of the degree of anaerobic metabolism in the islet grafts. Lactate/pyruvate ratios were between 50 and 100 in grafts of both nondiabetic and diabetic recipients, and they almost doubled in response to the intravenous glucose challenge in the grafts of nondiabetic recipients. In comparison, lactate/pyruvate ratios were approximately 12 in the medium of cultured islets incubated at low glucose (5.6 mmol/L) or high glucose (16.7 mmol/L) concentrations.

CONCLUSIONS

The microdialysis technique has proven to be a valuable tool for evaluating the internal environment of islet transplants. Moreover, the high lactate/pyruvate ratio suggests that islet grafts have an increased anaerobic glucose metabolism.

Histological markers for endothelial cells in endogenous and transplanted rodent pancreatic islets

BACKGROUND/AIMS

To obtain a selective marker to identify endothelial cells is difficult, due to the heterogeneity of these cells. Most described markers perform well in some applications, but fail in others. The aim of this study was to identify a selective and specific marker for rodent microvascular endothelial cells, especially for use in studies on the vascular system of pancreatic islets.

METHODS

A biotin-labelled form of the lectin Bandeiraea or Griffonia simplicifolia in combination with a streptAB-Complex with alkaline phosphatase was used to stain endothelium in paraffin-embedded tissue sections from C57BL/6 mice, Sprague-Dawley or Wistar-Furth rats.

RESULTS

We were consistently able to selectively stain microvascular endothelial cells in lungs, small intestines, white and brown adipose tissue, pancreas and islets of Langerhans with the lectin Bandeiraea simplicifolia. Furthermore, we were able to visualise the vasculature in syngenically transplanted islets of Langerhans in Wistar-Furth rats and C57BL/6 mice. Attempts to stain rodent endothelial cells with antibodies against CD34, CD31, CD200, Ox43, von Willebrand factor and the lectin Ulex europaeus were not uniformly successful.

CONCLUSION

The lectin Bandeiraea simplicifolia is a versatile marker for rodent endothelial cells, and may be used to study revascularisation after transplantation of pancreatic islet in rodents.

Apoptosis in hypoxic human pancreatic islets correlates with HIF-1alpha expression

To become insulin independent, patients with type 1 diabetes mellitus require transplantation of at least two donor pancreata because of massive beta-cell loss in the early post-transplantation period. Many studies describing the introduction of new immunosuppressive protocols have shown that this loss is due to not only immunological events but also nonimmunological factors. To test to what extent hypoxia may contribute to early graft loss, we analyzed the occurrence of apoptotic events and the expression of hypoxia-inducible factor 1 (HIF-1), a heterodimeric transcription factor consisting of an oxygen-dependent alpha subunit and a constitutive beta subunit. Histological analysis of human and rat islets revealed nuclear pyknosis as early as 6 h after hypoxic exposure (1% O2). Moreover, immunoreactivity to activated caspase-3 was observed in the core region of isolated human islets. Of note, both of these markers of apoptosis topographically overlap with HIF-1alpha immunoreactivity. HIF-1alpha mRNA was detected in islets from human and rat as well as in several murine beta-cell lines. When exposed to hypoxia, mouse insulinoma cells (MIN6) had an increased HIF-1alpha protein level, whereas its mRNA level did not alter. In conclusion, our data provide convincing evidence that reduced oxygenation is an important cause of beta-cell loss and suggest that HIF-1alpha protein level is an indicator for hypoxic regions undergoing apoptotic cell death. These observations suggest that gene expression under the control of HIF-1 represents a potential therapeutic tool for improving engraftment of transplanted islets.

Decreased vascular density in mouse pancreatic islets after transplantation

An adequate revascularization is crucial for islet survival and function after transplantation. Previous studies have suggested that islet revascularization is concluded within 14 days after transplantation. We investigated if the vascular density of transplanted islets and endogenous pancreatic islets differs. Cultured islets were syngeneically transplanted into the kidney, liver, or spleen of C57BL/6 mice. One month later, the graft-bearing organ was removed, and histological specimens were prepared and stained for endothelium with the lectin Bandeiraea simplicifolia. Pancreata from nontransplanted control animals were prepared similarly. Uniform staining of endothelium within the grafts and endogenous islets was obtained. The vascular density was markedly decreased in transplanted islets at all implantation sites, but preferentially in islets implanted into the spleen. The vascular density in the connective tissue surrounding the transplanted islets was very high compared with that of graft intra-islet capillaries. A much lower vascular density was detected in connective tissue surrounding implanted microspheres of a size similar to the islets, which suggests that the islets per se induced blood vessel formation in their vicinity. We conclude that the vascular density in revascularized transplanted islets is markedly decreased compared with endogenous islets. This has potential implications for islet graft metabolism and function.

Molecular detection of circulating beta-cells after islet transplantation

Islet transplantation is a promising treatment for type 1 diabetes. However, islet grafts are submitted to multiple injuries, including immunosuppressive drug toxicity, hyperglycemia, hypoxia, unspecific inflammatory reactions, as well as allo- and autoimmune destruction. Therapeutic approaches to these damage mechanisms require early detection of islet injury, which is currently not feasible because of the lack of efficient markers. Based on the hypothesis of islet dissociation and release of islet cells into the circulation during islet injury, we designed a highly sensitive and specific molecular assay, able to detect two beta-cells per milliliter of venous blood by RT-PCR of insulin mRNA. We report that circulating beta-cells can be demonstrated up to 10 weeks after intraportal islet transplantation, as assessed after six islet grafts in four type 1 diabetic patients. Furthermore, our results suggest that the time during which circulating islet cells can be detected may depend on the graft environment and the immunosuppressive regimen. This test may allow better estimation of islet cell loss and identification of factors involved in islet graft injury.

Gene expression of VEGF and its receptors Flk-1/KDR and Flt-1 in cultured and transplanted rat islets

BACKGROUND

Vascular endothelial growth factor (VEGF) and its two receptor tyrosine kinases, Flk-1/KDR and Flt-1, may play an important role in mediating the revascularization of transplanted pancreatic islets.

METHODS

Using semiquantitative multiplex reverse-transcribed polymerase chain reaction we determined the gene expression of VEGF and its receptors in cultured and transplanted rat islets.

RESULTS

After exposure of islet cells to hypoxia in vitro, increases were found in the gene expression of the VEGF120 and VEGF164 isoforms, with simultaneous increases in VE-cadherin, Flk-1/KDR, and Flt-1. In vivo studies consisted of analysis of islet grafts transplanted into both normal and diabetic recipients. Expression of both VEGF120 and VEGF164 in grafts was up-regulated for the first 2-3 days after transplantation, with the response being more prolonged in the diabetic rats. These increases were followed by reduced expression of VEGF on days 5, 7, and 9. Increases in the expression of VE-cadherin in islet grafts in normal and diabetic recipients tended to parallel VEGF expression, with the increases in both probably being caused by hypoxia. The early increases of VEGF expression were followed by a rise in the expression of VEGF receptors, which probably represents the early stages of angiogenesis. Graft expression of Flk-1/KDR and Flt-1 was enhanced at 3 and 5 days in the normoglycemic recipients, while in the diabetic recipients increases were found later on days 5, 7, and 14.

CONCLUSIONS

The delayed expression of VEGF receptors in the diabetic recipients could reflect impaired angiogenesis caused by the diabetic milieu; this delay could contribute to the less outcomes of grafts transplanted into a hyperglycemic environment.

Markedly decreased oxygen tension in transplanted rat pancreatic islets irrespective of the implantation site

In this study, we syngeneically transplanted islets to three different implantation sites of diabetic and nondiabetic rats, then 9-12 weeks later we measured the blood perfusion and compared the tissue partial pressure of oxygen (PO2) levels of these transplanted islets to endogenous islets. Modified Clark microelectrodes (outer tip diameter 2-6 microm) were used for the oxygen tension measurements, and islet transplant blood perfusion was recorded by laser-Doppler flowmetry (probe diameter 0.45 mm). The islet graft blood perfusion was similar in all islet grafts, irrespective of the implantation site. In comparison, the three implantation organs (the kidney cortex, liver, and spleen) differed markedly in their blood perfusion. There were no differences in islet graft blood perfusion between diabetic and nondiabetic recipients. Within native pancreatic islets, the mean PO2 was approximately 40 mmHg; however, all transplanted islets had a mean PO2 of approximately 5 mmHg. The oxygen tension of the grafts did not differ among the implantation sites. In diabetic recipients, an even lower PO2 level was recorded in the islet transplants. We conclude that the choice of implantation site seems less important than intrinsic properties of the transplanted islets with regard to the degree of revascularization and concomitant blood perfusion. Furthermore, the mean PO2 level in islets implanted to the kidney, liver, and spleen was markedly decreased at all three implantation sites when compared with native islets, especially in diabetic recipients. These results are suggestive of an insufficient oxygenization of revascularized transplanted islets, irrespective of the implantation site.

TRANSPLANTE DE ILHOTAS PANCREÁTICAS EM DISPOSITIVOS DE IMUNOISOLAMENTO CELULAR: RESULTADOS INICIAIS

O transplante de pâncreas e de ilhotas pancreáticas vem apresentando grande desenvolvimento nos últimos anos. O isolamento das ilhotas em cápsulas com membrana semi-permeáveis pode ser tratamento de escolha para o diabetes, pois dispensa o uso de imunossupressores. O material ideal para a confecção de uma cápsula para o isolamento celular ainda permanece um sonho. Um novo material a base de látex natural foi implantado no subcutâneo de ratos normais e diabéticos para estudar a biocompatibilidade e a neoformação vascular. A análise após 21 dias de implante mostrou intensa formação capilar na interface membrana-tecido e pouco tecido fibrótico. Estes achados iniciais mostram que o material pode ter algum potencial para a confecção de dispositivos de isolamento celular.

Inducible nitric oxide synthetase is expressed in adult but not fetal pig pancreatic islets

Cytokine-induced expression of inducible nitric oxide synthetase (iNOS) and production of nitric oxide (NO) by pancreatic islet cells has been suggested as one potential mechanism for beta cell destruction. In this study, we investigated the role of iNOS and NO in islet primary non-function. Islets were assessed for their function, viability and expression of iNOS. Adult rat and pig islets isolated by collagenase digestion and fetal pig pancreas (FPP) grafts isolated by collagenase digestion or high oxygen culture were transplanted into C57BL6 mice and nude mice. iNOS protein was detected by immunohistochemistry. iNOS protein was found in normal rat and pig pancreas and adult rat and pig islets that were isolated by collagenase digestion and transplanted into either C57BL6 mice or nude mice. iNOS was not detected in fetal pig islet grafts, regardless of whether collagenase was used in the isolation process. In adult pig islet grafts, the presence of iNOS protein correlated with high levels of islet cell apoptosis and primary non-function. Despite the persistent presence of iNOS in rat islets, there was no evidence that it had a deleterious effect on rat islet viability, or function. Therefore, in isolated adult pig islets, there was a correlation between iNOS expression and apoptosis, suggesting that iNOS activation may be deleterious to the adult pig islets. However, other factors such as the fragility of the islet capsule may be equally important. By contrast, fetal pig islets did not express iNOS and this may be an important reason for their enhanced viability when compared with adult islet tissue.

Local blood flow regulation in transplanted rat pancreatic islets: influence of adenosine, angiotensin II, and nitric oxide inhibition

BACKGROUND

Transplanted islets lack endothelial cells immediately after implantation and therefore depend on an adequate revascularization for their survival and function. However, the functional properties of the newly formed islet graft microvessels are largely unknown. This study aimed to investigate the blood flow regulation of transplanted pancreatic islets.

METHODS

Pancreatic islets were syngeneically transplanted beneath the renal capsule of control and streptozotocin-diabetic rats. Blood flow measurements were performed 4 weeks later using laser-Doppler flowmetry. Adenosine (0.6 mg x kg(-1) x min(-1), angiotensin II (AT II; 0.17 microg x kg(-1) x min(-1)) and the nitric oxide synthase inhibitor NG-nitro-L-arginine (25 mg/ kg) were given to each animal.

RESULTS

An increased basal blood flow and basal vascular conductance in the islet grafts, but not in the renal cortex, were seen in diabetic rats compared with control rats. Adenosine increased, and AT II decreased, the vascular conductance of the islet grafts in both nondiabetic and diabetic animals. A more pronounced circulatory response to AT II was observed in kidneys of diabetic animals, whereas there was no difference in the islet graft blood flow response between nondiabetic and diabetic animals. NG-Nitro-L-arginine decreased islet graft blood flow and vascular conductance in both nondiabetic and diabetic recipients, but the effect was more pronounced in the non-diabetic animals.

CONCLUSIONS

Islet graft blood flow was influenced by adenosine, AT II, and nitric oxide inhibition in all animals. However, diabetic animals were less dependent on nitric oxide to maintain a basal blood flow in the islet graft.

Chronically decreased oxygen tension in rat pancreatic islets transplanted under the kidney capsule

BACKGROUND

A factor of potential importance in the failure of islet grafts is poor or inadequate engraftment of the islets in the implantation organ. This study measured the oxygen tension and blood perfusion in 1-, 2-, and 9-month-old islet grafts.

METHODS

The partial pressure of oxygen was measured in pancreatic islets transplanted beneath the renal capsule of diabetic and nondiabetic recipient rats with a modified Clark electrode (outer tip diameter 2-6 microm). The size of the graft (250 islets) was by purpose not large enough to cure the diabetic recipients. The oxygen tension in islets within the pancreas was also recorded. Blood perfusion was measured with the laser-Doppler technique.

RESULTS

Within native pancreatic islets, the partial pressure of oxygen was approximately 40 mm Hg (n=8). In islets transplanted to nondiabetic animals, the oxygen tension was approximately 6-7 mm Hg 1, 2, and 9 months posttransplantation. No differences could be seen between the different time points after transplantation. In the diabetic recipients, an even more pronounced decrease in graft tissue oxygen tension was recorded. The mean oxygen tension in the superficial renal cortex surrounding the implanted islets was similar in all groups (approximately 15 mm Hg). Intravenous administration of glucose (0.1 gxkg(-1)x min(-1)) did not affect the oxygen tension in any of the investigated tissues. The islet graft blood flow was similar in all groups, measuring approximately 50% of the blood flow in the kidney cortex.

CONCLUSION

The oxygen tension in islets implanted beneath the kidney capsule is markedly lower than in native islets up to 9 months after transplantation. Moreover, persistent hyperglycemia in the recipient causes an even further decrease in graft oxygen tension, despite similar blood perfusion. To what extent this may contribute to islet graft failure remains to be determined.

Optimal insulin treatment in syngeneic islet transplantation

Insulin-induced normoglycemia has shown to have a beneficial effect on the outcome of pancreatic islets transplanted to diabetic recipients. The aim of the study was to identify the insulin treatment that can maximize its beneficial effect on islet transplants. Six groups of streptozotocin diabetic C57Bl/6 mice were transplanted (Tx) with 100 syngeneic islets, an insufficient beta cell mass to restore normoglycemia, and were treated with insulin as follows: group 1 (n = 9): from day 10 before Tx to day 14 after Tx; group 2 (n = 11): from day 6 before Tx to Tx day; group 3 (n = 11): from Tx day to day 6 after Tx; group 4 (n = 7): from Tx day to day 14 after Tx; group 5 (n = 8): from day 10 to day 24 after Tx; group 6 (n = 18): Tx mice were not treated with insulin. Sixty days after Tx, normoglycemia was achieved in 100% of mice in groups 1, 4, and 5, in 73% of mice in group 2, and in only 45% and 33% of mice in groups 3 and 6, respectively (p < 0.01). Intraperitoneal glucose tolerance, determined only in normoglycemic mice, was similar in groups 1, 2, 4, and normal controls. In contrast, normoglycemic mice from groups 3, 5, and 6, exposed to more severe and prolonged hyperglycemia after Tx, showed higher glucose values after glucose injection, suggesting that hyperglycemia had a long-lasting deleterious effect on transplanted beta cell function. The initially transplanted beta cell mass was maintained in the grafts of normoglycemic mice, but was severely reduced in hyperglycemic mice. Transplanted beta cell mass was similar in normoglycemic groups with normal or impaired glucose tolerance, indicating that impaired glucose tolerance was not due to reduced beta cell mass. In summary, the beneficial effect of insulin-induced normoglycemia on transplanted islets was maximal when insulin treatment was maintained the initial 14 days after transplantation. Exposure to sustained hyperglycemia initially after transplantation had a long-lasting deleterious effect on transplanted islets.

Xenotransplantation

BACKGROUND

Over the past 10 years xenotransplantation has generated much interest in the hope that it will enable us to overcome the current lack of human organ donors. This review examines the evolution and current therapeutic strategies that have been developed to overcome the predominant problem of graft rejection.

METHODS

A literature review was undertaken using a Medline search from January 1966 to August 1999.

RESULTS AND CONCLUSION

Despite the considerable advances that have been made in molecular biological techniques, xenograft rejection cannot be prevented without significant immunosuppression and toxic side-effects. The problem of delayed rejection, in particular, will probably be very difficult to overcome, although some of the difficulties associated with hyperacute rejection have been resolved. The potential risk of porcine endogenous retrovirus transmission has generated much debate recently, but it is likely that some of the important issues relating to xenotransplantation will never be resolved until carefully regulated clinical trials are allowed to begin.

Beneficial effect of prostaglandin E1 on blood flow to the gastric tube after esophagectomy

BACKGROUND

A prospective study on the vasodilatory effect of prostaglandin E1 on blood flow to the gastric tube after esophagectomy is reported.

METHODS

Twelve patients with thoracic esophageal cancer who underwent esophagectomy were enrolled in this study. In all patients, the esophagogastrostomy was performed in the cervical region, and the stomach was used for reconstruction. Immediately after the creation of the gastric tube, baseline blood flow was measured at the oral end, in the center, and at the pyloric ring of the gastric tube using a laser Doppler flowmeter. The prostaglandin E1 group (n = 6) was then infused with prostaglandin E1 until postoperative day 2; the control group (n = 6) received saline. At +5 minutes and +40 minutes after administration, blood flow was again measured at the same three sites.

RESULTS

The control group did not show a significant increase of blood flow to any site over time. For the prostaglandin E1 group, blood flow at +40 minutes increased from the baseline measurements significantly at a rate of 63%, 39%, and 36%, respectively.

CONCLUSIONS

Prostaglandin E1 has a characteristic vasodilating effect on the area of impaired microcirculation of the gastric tube, thereby increasing blood flow to the affected area.

Clinical islet cell transplantation. Are we there yet?

Diabetes mellitus is perhaps the most devastating chronic disease of all time. A brief history of the evolution of treatment modalities is provided, culminating in the rationale for the physiologic replacement of a functioning beta-cell mass by transplantation. Vascularized pancreas transplantation is discussed briefly as an introduction to the transplantation of the isolated islet. A detailed review of the current state of human islet transplantation for the cure of diabetes is then described. Finally, areas for future development are highlighted.

Host-graft circulation and vascular morphology in pancreatic tissue transplants in rats

One of the most determining factors for the survival of tissue grafts is an intact vasculature. This vasculature must, however, be linked to the circulation of the host animals for survival and growth to occur. The mechanism(s) of revascularization of pancreatic tissue grafts is still unclear and more so the process by which the host blood vessels anastomose with those of the graft. The microvasculature and revascularization of neonatal pancreatic tissue fragments transplanted into the anterior eye chamber of rats were investigated using conventional light and electron microscopy as well as the india ink perfusion method. Light microscopy demonstrated that the blood vessels of the host iris revascularized the transplants within 24 h of transplantation. Sinusoidal blood capillaries were observed to invade the peripheral parts of the grafts. The capillary encroachment from the iris into the graft continued through the second day of transplantation. The host-graft anastomosis developed completely and became prominent and conspicuous after the third day of transplantation. Many capillaries were observed to encroach into the graft from the iris. The vasculature of the graft was increased in comparison with that of normal pancreatic tissue. This increase was prominent especially around pancreatic islet and ductal cells which survived after transplantation. Blood cells observed in these vessels of the graft indicated a functional state. The ultrastructure of the intrinsic blood vessels of the graft was intact and showed the peculiar fenestrations normally seen in the blood vessels of endocrine tissues. These observations indicated that pancreatic tissue fragments transplanted into the anterior eye chamber of rats were revascularized within 24 h. The revascularization was completed at the end of the third day of transplantation when definitive blood vessels linked the circulation of the graft with that of the iris.

Angiogenesis in cultured and cryopreserved pancreatic islet grafts

BACKGROUND

The ability of rat pancreatic islets to revascularize after transplantation was examined via in vitro and in vivo imaging of the microvasculature using laser scanning confocal microscopy (LSCM).

METHODS

Cultured or cryoprocessed islets were transplanted at the renal subcapsular site in rats. At various time intervals after transplantation, three-dimensional imaging of the graft was performed by LSCM. In vitro studies were conducted via microvascular corrosion casting of the grafted kidney in situations where it was difficult to obtain in vivo confocal data due to surgical complications. The vascular morphology of the islet grafts was evaluated quantitatively via digital image analysis algorithms to determine the morphology of the neovascular ingrowth and the rate of revascularization.

RESULTS

In cultured islet grafts, the initiation of angiogenesis was observed within 1 week, characterized by the presence of capillary sprouts, tortuous vessels, and blood vessels with blind ends. The revascularization of the graft was typically completed within 2 weeks and could be distinguished as a network of completely perfused blood vessels consisting of intertwining capillaries, with surrounding arterioles and venules. The angiogenesis process in cryopreserved islet grafts required a longer time period to initiate (approximately 2 weeks), and the revascularization was completed in 1 week after the initiation.

CONCLUSIONS

These results successfully demonstrate the potential of the described in vivo and in vitro LSCM techniques to measure the angiogenesis process in pancreatic islet grafts.

RS-61443 prevents microvascular rejection of pancreatic islet xenografts

Inadequate vascularization and microvascular rejection are major limitations for successful free pancreatic islet xenotransplantation. Commonly used immunosuppressive regimens may alter the process of vascularization, and are ineffective at preventing graft rejection. In this study, we investigated, in vivo, the action of the new immunosuppressive agent RS-61443 on angiogenesis and microvascular rejection of rat pancreatic islets after xenogeneic transplantation into the dorsal skinfold of Syrian golden hamsters. In nontreated xenografts, intravital fluorescence microscopy demonstrated a regular process of vascularization during the first 6 days after transplantation. On days 10, 14, and 20, graft rejection was observed, characterized by microvascular leukocyte accumulation (244+/-59 mm(-2)), loss of endothelial integrity, and capillary perfusion failure. Islet xenografts of animals treated with RS-61443 (40 mg/kg per day) demonstrated inhibition of vascularization with the consequence of a markedly reduced size of the grafts' microvascular network (0.05+/-0.007 mm2), when compared with that of nontreated xenografts (0.09+/-0.015 mm2; P< 0.05). However, treatment with RS-61443 effectively prevented microvascular graft rejection, as indicated by the absence of leukocyte accumulation (24+/-9 mm(-2); P<0.01), endothelial damage, and nutritive perfusion failure. Thus, RS-61443 treatment may represent an interesting approach for improving the outcome of pancreatic islet xenotransplantation.

Vascular endothelial growth factor is increased in devascularized rat islets of Langerhans in vitro

Vascular endothelial growth factor (VEGF) is an endothelial cell-specific mitogen with potent angiogenic and vascular permeability-inducing properties, both of which may be important for the function of islets of Langerhans. In this study, we have examined the expression of VEGF and its tyrosine kinase receptors (flt and flk-1) in isolated rat islets of Langerhans in vitro. When analyzed by in situ hybridization, islet tissue showed a significant 4.6-fold increase in VEGF mRNA expression over time in culture from 0 to 7 days. Islet tissue exposed to hypoxic/anoxic conditions for a period of 8 hr showed a 3.7-fold increase in VEGF mRNA when analyzed by Northern blot hybridization. Reverse transcriptase-polymerase chain reaction revealed the presence of both flt and flk-1 in freshly isolated islets, and two VEGF isoforms, namely VEGF120 and VEGF164. Three rodent beta-cell lines derived from insulinomas (RINm5F-2A, INS-1, and MIN6) were also found to express VEGF by Northern blot hybridization. However, neither hypoxia/anoxia nor low (0.3 g/L)- or high (3.0 g/L)-glucose culture conditions modulated their expression of VEGF. VEGF derived from RINm5F-2A cells was bioactive in a three-dimensional in vitro model of angiogenesis, which assays for endothelial cell invasion and capillary morphogenesis. These findings demonstrate, first, that devascularization increases VEGF expression in isolated islet tissue, and they point to VEGF as a potentially important endogenous angiogenic stimulus for subsequent revascularization in vivo. Second, our observations raise the possibility that survival of transplanted islets may be improved by increasing VEGF expression before transplantation.

Discordant xenoislets from a large animal donor undergo accelerated graft failure rather than hyperacute rejection: impact of immunosuppression, islet mass, and transplant site on early outcome

BACKGROUND

It is not known whether discordant, free, nonvascularized xenoislets-akin to discordant, vascularized, solid xenoorgans-are hyperacutely rejected. Quantitative xenoislet requirements and the optimal transplant site also remain to be defined.

METHODS

We studied these questions with a discordant dog-to-diabetic Lewis rat xenoislet model, using (1) functional (cure of streptozotocin-induced diabetes) and (2) histologic (biopsies of intraportal grafts) parameters. WF-to-Lewis alloislet recipients served as histologic controls.

RESULTS

(1) We found that 5000 xenoislet equivalents (IEs) transplanted into the portal veins of nonimmunosuppressed rats never functioned. Peritransplant combination therapy (rapamycin, cyclosporin A, anti-rat lymphocyte serum) significantly prolonged graft survival of 5000 intraportal IEs (median, 3 days) but not of 2500 intraportal or of 5000 intraperitoneal or renal subcapsular IEs. (2) By means of immunofluorescence (at 1 hour after transplantation), we noted immunoglobulin M (IgM) and IgG binding to islets in xenografts but not allografts; we noted complement and fibrinogen binding in both xenografts and allografts. Insulin-positive islet cells within intact xenoislets were demonstrated in nonimmunosuppressed rats up to 48 hours after transplantation. Cellular xenograft infiltration and inflammation, beginning at 6 hours, were observed even in immunosuppressed rats. (3) Thus, in spite of IgM and IgG binding, intraportal discordant xenoislets were not hyperacutely rejected and destroyed. Nevertheless, universal xenoislet nonfunction in nonimmunosuppressed rats was immune mediated. A large xenoislet mass (more than 10,000 IEs/kg), the intraportal site, and combination therapy were absolute prerequisites for immediate function. But even if the prerequisites were all fulfilled, accelerated xenoislet graft failure occurred.

CONCLUSIONS

This outcome suggests that the specific binding of IgM and IgG to xenoislets, in conjunction with the binding of complement and fibrinogen, contributed to accelerated graft failure. Thus distinction between discordant and concordant species combinations is important for free, nonvascularized xenoislet transplants. These findings and the steroid-free combination protocol (rapamycin, cyclosporin A, anti-T-cell therapy) warrant further testing in preclinical discordant xenoislet studies.

Blood flow regulation in the transplanted fetal endocrine pancreas. Acquisition of a nitric oxide-dependent glucose-induced increase in blood flow

Islet-like cell clusters (ICCs) were prepared from the fetal porcine pancreas by a culture technique. The ICCs (approximately 500) were implanted under the left renal capsule of nude (nu/nu) C57BL/6J mice. Six weeks, months, 12 months, or 16-24 months later, the animals were anesthetized and the blood flows to the xenogeneic islet graft and the adjacent kidney parenchyma were measured with laser-Doppler flowmetry. After the blood flow measurements, the graft-bearing kidneys were prepared for enzyme and immunohistochemistry. The blood perfusion of the graft was higher than that of the kidney at all times investigated. Intraperitoneal administration of glucose caused only slight and parallel changes in renal and graft blood flows 6 weeks, 6 months, or 12 months after transplantation. However, in all but 1 animal (n=16) transplanted >16 months before the blood flow measurements, glucose caused a marked increase in graft blood flow but did not affect renal blood flow. Injection of 2-deoxy-glucose also increased graft blood perfusion in animals transplanted > 16 months earlier (n=5). Treatment with NG-monomethyl-L-arginine (n=6), an inhibitor of nitric oxide synthase, prevented this glucose-induced flow increase. Nicotinamide adenine dinucleotide phosphate diaphorase histochemistry revealed nitric oxide synthase only in the endothelium and media of graft arterioles in animals in the oldest age group. Thus, with the passage of time after implantation, the grafted xenogeneic ICCs seem to achieve an autonomous blood flow regulation, different from that of the implantation organ. The reactivity to an increment in blood glucose concentration in the graft is similar to that seen in native islets in the pancreas but is not present until >16 months after implantation. The mechanisms for the glucose-induced blood flow increase are obscure but probably depend on local release of nitric oxide within graft arterioles.

In-vivo analysis of angiogenesis and revascularization of transplanted pancreatic islets using confocal microscopy

A technique to measure angiogenesis and revascularization in pancreatic islets transplanted at the renal subcapsular site in the rat has been developed. In-vivo imaging of the microcirculation of transplanted pancreatic islets was conducted using a confocal scanning laser microscope (CSLM) to achieve optical sectioning through the graft in order to perform a computer reconstruction of the three-dimensional neovascular morphology. Individual islets were harvested by enzymatic digestion of excised pancreas from Fischer 344 rats. Isolated islets were cultured for 24 h, and approximately 300-350 islets were transplanted at the renal subcapsular site of the left kidney in an anaesthetized rat. Six to 14 days post-transplantation, the animal was anaesthetized and prepared for in-vivo imaging of the microvasculature on a Zeiss LSM-10. Optical contrast of the microvasculature was enhanced by the administration of fluorescein-labelled dextran into the circulating blood. The transplant site was identified and serial sections were obtained through the vascular bed at varying z-intervals. Complementary fluorescence video images were also obtained via a silicon intensifier tube camera mounted on the CSLM. At completion of the imaging procedure, the kidney was returned into the body cavity, the area was sutured and the animal was allowed to recuperate for subsequent examinations. Image processing algorithms, such as grey-level thresholding, median filtering, skeletonization and template matching, were applied to compute the vessel density and diameters and extrapolated to measure 3-D vessel lengths and the tortousity index of the neovasculature.

Orientation of microvascular blood flow in pancreatic islet isografts

There is evidence that intraislet cellular communication and hormone delivery within the islets of Langerhans is controlled via capillary perfusion directed from the B cell core to the A/D cell mantle (intraislet portal system). To determine whether vascularization of freely transplanted islets repeats this "core-to-mantle" capillary perfusion, hamster islets were isolated by collagenase digestion and transplanted into a skinfold chamber of syngeneic animals (n = 12). 14 d after transplantation, the microvasculature of the islet grafts was analyzed by in vivo fluorescence microscopy. The capillary glomerulum-like network of the islet grafts (n = 109) was found supplied by individual arterioles, which regularly pierced the islet and broke into capillaries within the graft (96/109 [88.1%]), resulting in capillary flow directed from the core to the islet's periphery. Only in 13 of 109 islets (11.9%) arterioles broke into capillaries at the outside margin of the islet and capillary flow was directed simultaneously to vessels located within the core, as well as the periphery of the graft. The islet's capillary network was drained by individual venules and intercapillary anastomoses between the newly formed islet capillaries and the preexisting capillaries of the host muscle tissue. Immunohistochemical staining revealed B cells located within the core, and A and D cells scattered in the periphery of the islets, indicating reestablishment of sequential B-->A/D cellular perfusion of the grafts. Thus, freely transplanted islets develop an intra-islet portal system, similarly to that of pancreatic islets in situ.

Transplanted beta cell response to increased metabolic demand. Changes in beta cell replication and mass

We determined the capacity of transplanted beta cells to modify their replication and mass when stimulated by changes in metabolic demand. Five groups of Lewis rats were studied: group 1 (Tx-Px) had a 95% pancreatectomy 14 d after transplantation of 500 islets; group 2 (Px-Tx) had a 95% pancreatectomy 14 d before transplantation of 500 islets; group 3 (Tx) was transplanted with 500 islets; group 4 (Px) had a 95% pancreatectomy; and group 5 (normal) was neither transplanted nor pancreatectomized. Blood glucose was normal in Tx-Px and Tx groups at all times. Px-Tx and Px groups developed severe hyperglycemia after pancreatectomy that was corrected in Px-Tx group in 83% of rats 28 d after transplantation. Replication of transplanted beta cells increased in Tx-Px (1.15 +/- 0.12%) and Px-Tx (0.85 +/- 0.12%) groups, but not in Tx group (0.64 +/- 0.07%) compared with normal pancreatic beta cells (0.38 +/- 0.05%) (P < 0.001). Mean beta cell size increased in Tx-Px (311 +/- 14 microns2) and Px-Tx (328 +/- 13 microns2) groups compared with Tx (252 +/- 12 microns2) and normal (239 +/- 9 microns2) groups (P < 0.001). Transplanted beta cell mass increased in Tx-Px (1.87 +/- 0.51 mg) and Px-Tx (1.55 +/- 0.21 mg) groups compared with Tx group (0.78 +/- 0.17 mg) (P < 0.05). In summary, changes in transplanted beta cells prevented the development of hyperglycemia in Tx-Px rats. Transplanted beta cells responded to increased metabolic demand increasing their beta cell mass.

The quinoline-3-carboxamide linomide inhibits angiogenesis in vivo

Linomide (Roquinimex) has antitumor activity when given in vivo (but not when applied in vitro) that has been attributed to immune host mechanisms. Recent studies, however, suggest that Linomide may also possess antiangiogenic properties. The aim of the present study was to evaluate the antiangiogenic effect of Linomide using an intravital microscopic technique. Syngeneic pancreatic islets were isolated and implanted into the dorsal skinfold chamber of Syrian golden hamsters. This model allows detailed repeated in vivo observations and quantitative analysis of revascularization of pancreatic islet grafts. The neovascularization process of the islets is a highly reproducible phenomenon that is completed within about 2 weeks, resulting in a microvascular network very similar to that of islets in situ. The plasma concentration profile of Linomide following a single oral dose of the compound was determined. The elimination of Linomide was fast, the half-life being 2.6 +/- 0.2 h. Due to the short half-life, the hamsters were given Linomide twice a day. One group of animals (n = 9) was force-fed Linomide (100 mg/kg per day) from the day of implantation throughout the 2-week observation period, and the results were compared with those obtained in a nontreated control group (n = 7). At days 6, 10, and 14 after implantation, the neo-vasculature of the islets was examined. In the control group, 91% +/- 4% (mean +/- SEM) of the islets showed the first signs of angiogenesis at day 6, whereas in the Linomide-treated group the corresponding value was 48% +/- 12%. At days 10 and 14, the "take-rate" in the control group increased to 94% +/- 3% for day 0 and to 94% +/- 4% (n = 6) for day 14, whereas in the treated group the corresponding take-rate was 67% +/- 11% and 72% +/- 12%, respectively. The functional capillary density in the control group at days 6, 10, and 14 was 223 +/- 17,348 +/- 29, and 495 +/- 29 cm-1, respectively, and that in the Linomide treated group was 91 +/- 28, 181 +/- 43, and 229 +/- 47 cm-1, respectively. These results demonstrate that Linomide suppresses the neovascularization of the islet grafts by both delaying the onset of and reducing the percentage of islets displaying angiogenesis as well as by decreasing the rate of proliferation of capillary endothelium of the revascularized islets.

Beta cell mass and growth after syngeneic islet cell transplantation in normal and streptozocin diabetic C57BL/6 mice

In islet transplantation, nonimmunological factors such as limited growth capacity or increased death rate could reduce the beta cell mass in the graft and lead to failure of the transplant. We studied the evolution of beta cell replication and mass after transplantation of insufficient, minimally sufficient, or excessive islet tissue. Streptozocin diabetic C57BL/6 mice received 150 or 300 syngeneic islets under the kidney capsule and normal mice received 300 islets. In streptozocin diabetic mice 300 islets restored normoglycemia; beta cell replication in transplanted islets was similar to replication in normal pancreas and beta cell mass in the graft remained constant. In contrast, 150 islets were insufficient to achieve normoglycemia; beta cell replication was increased initially but not by 18 or 30 d despite persistent hyperglycemia, and beta cell mass fell progressively. When islets were transplanted into normal recipients, beta cell replication remained normal but beta cells underwent atrophy and mass in the graft was substantially reduced. Therefore, with a successful islet transplant, in diabetic mice beta cell replication and mass remain constant. In contrast, when insufficient islet tissue is transplanted an initial increase in beta cell replication can not compensate for a decline in beta cell mass. When excessive islet tissue is transplanted, beta cell mass is reduced despite normal beta cell replication.

Influence of experimental hyperglycemia on microvascular blood perfusion of pancreatic islet isografts

The influence of hyperglycemia on the microvascular blood perfusion of pancreatic islet isografts of Syrian golden hamsters was analyzed by direct visualization of the islet's microvasculature by means of in vivo fluorescence microscopy. The experiments were performed using the hamster dorsal skinfold preparation, which allows for quantitative analysis of the microcirculation of islets grafted on the striated skin muscle. Islets were isolated from inbred hamsters by collagenase digestion and subsequently transplanted in normoglycemic (controls; n = 8) and hyperglycemic (65 mg/kg streptozotocin intravenously; n = 10) recipients. In both groups, revascularization of the islet grafts was completed on day 10 after transplantation. Quantitative analysis of capillary blood perfusion on days 6, 10, and 14 revealed no differences in functional capillary density and capillary red blood cell velocity of islets grafted into normoglycemic as compared to hyperglycemic animals. However, islet capillaries were significantly wider in hyperglycemic recipients (11.9 +/- 1.3 microns, P < 0.01) as compared to normoglycemic controls (8.9 +/- 0.4 microns). The increase of capillary diameters resulted in a significant rise (P < 0.01) of mean capillary blood perfusion from 1.76 +/- 0.39 nl/min in controls to 2.88 +/- 0.63 nl/min in hyperglycemic recipients, indicating an increase in microvascular blood perfusion due to hyperglycemia. From these results it is concluded that hyperglycemia is associated with higher capillary blood perfusion in revascularized islet isografts, similarly as known for pancreatic islets in situ.

Influence of cryopreservation on viability and nutritional microcirculation of islets of Langerhans

Isolated pancreatic islets of Syrian golden hamsters were cryopreserved for 1 and 10 weeks, respectively. Following thawing islet mass was found more than 90%. Analysis of viability of these islets by dithizone staining revealed 94 +/- 4% (1 week cryopreservation) and 96 +/- 3% (10 weeks cryopreservation) positively stained islets. After implantation into dorsal skin-fold chambers of syngeneic animals, intravital fluorescence microscopy (5% FITC-dextran, i.v.) showed a regular microvascular network 10 days after implantation. The total diameter of the microvascular network was found similar in cryopreserved islets (day 10: 420.8 +/- 55.2 microns (1 week preservation) and 406.5 +/- 25.2 (10 weeks preservation)) as compared to non-preserved controls (day 10: 411.7 +/- 53.9 microns). Therefore we conclude that cryopreservation of hamster islet isografts for 1 and 10 weeks, respectively, does not alter viability as well as the potential for revascularization. Cryopreservation seems to be an adequate technique for long time storage prior to free transplantation.

Induction of angiogenesis by growth factors: relevance to pancreatic islet transplantation

Biodegradable pellets releasing 20 ng/day of endothelial cell growth factor alpha (alpha ECGF) or a- or b-fibroblast growth factor (FGF) and 90 micrograms/day of heparin were implanted beneath the renal capsule in rats and dogs and the muscularis/serosal border of the pyloric stomach in dogs to test for angiogenesis in a potential pancreatic islet transplant site. These factors were also tested in vitro to determine whether the capillary bed of the isolated islet could be preserved. alpha ECGF was superior to a- or bFGF in promoting endothelial cell growth and capillary formation in isolated islets. Both a- or bFGF and alpha ECGF induced the development of a dense capillary bed in the dog stomach, whereas in the kidney site alpha ECGF was more effective in the rat than was a- or bFGF. Priming the isolated islet as well as the transplant site prior to islet transplantation resulted in islet blood flow being established within 3 days in contrast to 7-14 days in controls.

Microvascular phenomena during pancreatic islet graft rejection

Transplantation of insulin secreting tissue as a free graft has the potential to become a safe and simple procedure to cure diabetes. However, clinical results, i.e. achievement of insulin independency, are poor, in spite of the use of immunosuppressive regimens, which are regularly successful in whole organ transplantation. In contrast to whole organ grafts, which are revascularized immediately after transplantation, free pancreatic islet grafts require the process of revascularization in order to establish a microvascular network, sufficient for the nutritional blood supply. We have demonstrated for the first time in vivo images of the process of revascularization of free islet xenografts including microvascular phenomena during graft rejection. Rat islet xenografts were isolated by collagenase digestion and transplanted into hamster dorsal skinfold chambers. After 6, 10 and 14 days the microvasculature of the islet grafts was analyzed by means of intravital fluorescence microscopy. Xenogeneic grafts were revascularized during the first 6 days similarly compared to syngeneic grafts; however, on day 10 after transplantation a reduction in size of the microvascular network as well as a decrease in functional capillary density and a reduction in capillary red blood cell velocity were observed, accompanied by microvascular rejection phenomena, such as an increase of microvascular permeability, edema formation, capillary widening and intravascular accumulation of white blood cells (WBCs) with concomitant WBC-endothelium interaction in post-capillary venules. Treatment with 2.5 mg/kg/d (+/-)-15-deoxyspergualin could not completely alleviate these microvascular rejection phenomena.(ABSTRACT TRUNCATED AT 250 WORDS)

The use of digital image processing to quantitate angiogenesis induced by basic fibroblast growth factor and transplanted pancreatic islets

We have used digital image processing as a technique to quantify the formation of blood vessels in situ in response to the application of either an angiogenic peptide, basic fibroblast growth factor, or isolated pancreatic islets. The peptide or the islets were placed under the rat kidney capsule. For in vivo microscopy fluorescein-labeled dextran was injected intravenously and video images were made as deemed appropriate. Images to be analyzed were selected manually, digitized, and stored in a computer for processing. The calculation of the total vessel length per area was called the microvascular index. Three weeks after the transplantation of 200 islets, the microvascular index was fairly constant and did not change significantly when the same measurements were performed 1 week later. In another experiment, pellets containing 0.5 micrograms of basic fibroblast growth factor produced an angiogenic response no different from that of a control pellet. At a dose of 1 microgram, however, the response was statistically significant after 1 week. These data indicate that digital image processing may be a useful method to quantitate the angiogenesis induced by local administration of angiogenic stimulants.

Aspects of pancreatic islet transplantation in diabetes mellitus

The justification for pancreatic or islet transplantation in diabetes mellitus is to halt progression of diabetic vascular complications. Although this goal has so far not been achieved, technical progress in transplanting the whole or segmental, vascularized pancreas has been remarkable. However, the operation is complicated and not without risk; immune rejection is furthermore a major problem, and the availability of suitable organs is limited. Transplantation of isolated pancreatic islet grafts, i.e. isolated islets or fetal pancreas, has therefore emerged as an attractive alternative. Iso-, allo- or xenografts of such preparations have been found to reverse diabetes in experimental animals. Only a minor operation is required and islets, although highly immunogenic, may be accessible to immunomodulation in vitro in order to decrease or abolish the allograft rejection and prevent cells from becoming targets for the autoimmune assault. However, problems and difficulties have emerged in this context also. Thus, islets are extremely difficult to isolate from the adult human pancreas. Such glands, be they adult or fetal, are not easily available, and clinical trials have so far remained without documented success. Considerable efforts are being made to overcome the difficulties encountered in islet transplantation. Attempts are being made to improve the techniques for preparation of clean and viable islets and to understand critical factors in the cellular interactions between the graft and the host after transplantation. Factors of importance in this context are the nutritional requirements of the graft and the vascularization process at the site of implantation. In order to provide unlimited access to cells with a high potential for insulin production and adaptive growth in the diabetic recipient, ongoing research is also directed towards methods for preparation of porcine fetal and adult islets suitable for xenotransplantation. The obvious problems involved in immune rejection of the grafted tissue are being investigated with respect both to the possibility of immunomodulation of the graft in vitro, the design of new immunosuppressive drugs, and the possibility of immuno-isolation of the insulin-producing cells with the aid of artificial membranes.