Bioartificial pancreas: materials, devices, function, and limitations

AN69 Hollow Fiber Membrane will Reduce but Not Abolish the Risk of Transmission of Porcine Endogenous Retroviruses

As the risk of porcine endogenous retrovirus (PERV) infection is a major obstacle to the xenotransplantation of porcine tissue, we investigated whether an AN69 hollow fibre membrane, used for islets of Langerhans transplantation, could prevent the transfer of PERVs and thus reduce the risk of PERV infection. PK15 cells were used as a PERV source. A specific and highly sensitive RCR was used for detection of a PERV provirus DNA (gag region) and a porcine mtDNA. Human U293 cells were incubated in vitro with encapsulated PK15 cells, concentrated encapsulated PK15 supernatant, or concentrated PK15 supernatant as a control. CD1 mice were implanted in vivo with encapsulated PK15 cells or injected with PK15 supernatant. We found no infection in human cells incubated with either encapsulated PK15 supernatant or in 10 out of 11 samples after coincubation with encapsulated PK15 cells. Infection of human cells was, however, detected in 1 out of 11 samples after coincubation with encapsulated PK15 cells. The presence of PERV provirus DNA and porcine mtDNA was detected in all the investigated tissues of the mice injected with PK15 supernatant and in various tissues of the mice implanted with encapsulated PK15 cells. Four weeks after the last injection of PK15 supernatant or a fiber explantation, no mouse showed any presence of PERV provirus DNA or porcine mtDNA. Our results demonstrate that AN69 hollow fiber membrane will reduce but not abolish the risk of PERV infection. Because the real risk of PERV infection still remains unknown, it is necessary to investigate further the real protection that could be provided by hollow fibers to ensure the safety of clinical xenotransplantation.

Retrieval of Microencapsulated Islet Grafts for Post-transplant Evaluation

Microencapsulation of islets is a procedure used to immunoisolate islets in order to obviate the need for immunosuppression of islet transplant recipients. Although microencapsulated islets have routinely been transplanted in the peritoneal cavity, the ideal site for their engraftment remains to be determined. The omentum, a highly vascularized tissue, has been proposed as an alternative site for microencapsulated islet transplantation. An added benefit to the omentum is that implanted microcapsules can be easily retrieved for post-transplant evaluation. This chapter describes a collagenase-based procedure for the retrieval of microencapsulated islets following the harvest of omentum pouch site of transplantation.

Immunoisolation: where regenerative medicine meets solid organ transplantation

Immunoisolation refers to an immunological strategy in which nonself antigens present on an allograft or xenograft are not allowed to come in contact with the host immune system, and it is implemented to prevent allorecognition and avoid immunosuppression. In this setting, the two most promising technologies, encapsulation of pancreatic islets (EPI) and immunocloaking (IC), are used. In the case of EPI, islets are inserted in capsules that, allow exchange of oxygen, nutrients and other molecules. In the case of IC, a natural nanofilm is injected prior to renal transplantation within the vasculature of the graft with the intent to pave the inner surface of the vascular lumen and camouflage the antigens located on the membrane of endothelia cells. Significant progress achieved in experimental models is leading EPI and IC to clinical translation.

Intramedullary cavity as implantation site for bioartifical pancreas: preliminary in vivo study

BACKGROUND

The intramedullary cavity is a widely distributed well-vascularized microenvironment capable of sustaining grafts, and is a potential site for islet transplantation. The bone marrow offers sufficient space that may also be suitable for bioartificial pancreas (BAP) implantation.

OBJECTIVE

To evaluate the feasibility of bone marrow as an implantation site for BAPs.

MATERIALS AND METHODS

A calcium phosphate cement chamber satisfies the criteria for immunoisolation. Mouse insulinoma cells were suspended with agarose gel and enclosed in a calcium phosphate cement chamber to create a BAP, which was implanted in the intramuscular space in diabetic swine or the intramedullary cavity in diabetic dogs. Blood glucose and C-peptide concentrations were determined perioperatively.

RESULTS

In the swine, the mean ± SD blood glucose concentration decreased from 413 ± 24 mg/dL to 285 ± 47 mg/dL, and was maintained in the range of 285 to 336 mg/dL for 15 days. It increased to 368 to 450 mg/dL after the BAPs were implanted in the intramuscular space. In the dogs, the blood glucose concentration decreased from 422 ± 32 mg/dL to 247 ± 52 mg/dL, and was maintained in the range of 247 to 347 mg/dL after the BAPs were implanted in the intramedullary cavity. The C-peptide concentration increased from 6.1 ± 2.8 pmol/L to 104.7 ± 16.4 pmol/L when the BAPs were implanted in the intramedullary cavity.

CONCLUSION

This study indicates superior effectiveness of BAPs implanted in the intramedullary cavity compared with the intramuscular space. This observation may be attributed to the greater oxygen tension in the bone marrow. The BAPs in direct contact with the circulatory system receive sufficient blood flow for function and survival. This preliminary study demonstrates that the intramedullary cavity may be an implantation site for BAP transplantation.

The effect of two different polyethylene glycol (PEG) derivatives on the immunological response of PEG grafted pancreatic islets

Islet transplantation is one of the promising ways to treat diabetes. To reduce the immune system response, several methods have been developed, a novel one being the grafting of methoxy polyethylene glycol (mPEG) derivatives onto collagen capsules of islets. In this study, the effects of the first and second generations of activated mPEG on the immunological response of polyethylene glycol (PEG) grafted pancreatic islets were studied. mPEG-Succinimidyl carbonate (mPEG-SC) and mPEG-succinimidyl propionic acid (mPEG-SPA) (with nominal molecular weight 5 kDa), typical of the first and second generations of activated mPEG, were selected, respectively. Both activated mPEGs did not affect the morphology, viability, or functionality of PEGylated islets compared to free islets (naked islets). The amount of IL-2 secreted from lymphocytes co-cultured with mPEG-SPA grafted islets (131.83 ± 15.28 pg/ml) was not significantly different from that with mPEG-SC grafted islets (156.09 ± 27.94 pg/ml). These results indicated that both mPEG-SC and mPEG-SPA had the same effect for camouflaging Langerhans islets, but the former is more suitable due to its easier synthesis process.

The bioartificial pancreas: progress and challenges

Diabetes remains a devastating disease, with tremendous cost in terms of human suffering and healthcare expenditures. A bioartificial pancreas has the potential as a promising approach to preventing or reversing complications associated with this disease. Bioartificial pancreatic constructs are based on encapsulation of islet cells with a semipermeable membrane so that cells can be protected from the host's immune system. Encapsulation of islet cells eliminates the requirement of immunosuppressive drugs, and offers a possible solution to the shortage of donors as it may allow the use of animal islets or insulin-producing cells engineered from stem cells. During the past 2 decades, several major approaches for immunoprotection of islets have been studied. The microencapsulation approach is quite promising because of its improved diffusion capacity, and technical ease of transplantation. It has the potential for providing an effective long-term treatment or cure of Type 1 diabetes.

Toxin-based selection of insulin-producing cells with improved defense properties for islet cell transplantation

Insulin-producing pancreatic beta-cells are known to be extremely susceptible to destruction, primarily by autoimmune mechanisms, infectious agents, and by chemical toxins that cause overt type I diabetes. As development of highly protected insulin-producing cells would be important for successful cell therapy of diabetic patients, gene transfection technique was utilized by several investigators in order to improve the defense properties of transplanted cells. In this article, we summarize other approaches based on a selection strategy that has been developed in our laboratory and by other research groups that engineer pancreatic beta-cells to provide protection against diabetogenic toxins (streptozotocin and alloxan), oxidative stress and cytokines. Selection strategies based on acute repeated or long-term continuous treatment of cell lines with cytotoxic agents have resulted in the selection of highly resistant cell subpopulations. We discuss possible involvement of different expression of cytoprotective genes in the selection of cell subpopulations, which demonstrate a broad spectrum of resistance. Importantly, toxin-based selection did not impair functional activity of the cells as it was shown in vitro. In addition, selected cells preserved their improved metabolic characteristics following encapsulation in alginate and subsequent implantation in diabetic animals. Identifying the mechanisms through which cell defense properties act will help clarify the process responsible for beta-cell regeneration in type I diabetes patients. Such knowledge might be useful in developing strategies focusing on the regeneration of beta-cell resistant populations.

Functional activity of insulinoma cells (INS-1E) and pancreatic islets cultured in agarose cryogel sponges

Here, we describe the preparation, structure, and properties of cryogel sponges, which represent a new type of macroporous biomaterial for tissue engineering. Cryogels were produced through freeze-thawing techniques, either from agarose alone or from agarose with grafted gelatin. The aim of this study was to evaluate agarose cryogel sponges as scaffolds for culturing both isolated pancreatic islets and insulinoma cells (INS-1E). In order to evaluate the effect of cell entrapment in artificial scaffolds, cell function reflected by insulin secretion and content was studied in cells cultivated for a 2-week period either in culture plastic plates or in cryogel sponge disks. Our results show that tumor-derived INS-1E cells grown either on plastic or on cryogels do not differ in their proliferation, morphology, insulin release, and intracellular insulin content. However, isolated pancreatic islets cultivated on cryogels sponge show 15-fold higher basal insulin secretion at 3.0 mM glucose than islets cultivated on plastic plates and fail to respond to stimulation with 16.7 mM glucose. In addition, these islets have about 2-fold lower insulin content compared to those grown in plastic plates. It is possible that the cell dysfunction noted in these in vitro experiments is due to the effect of the limited oxygen supply to the islets cultivated in cryogel sponge. Further in vivo studies are needed to clarify the nature of such an observation since according to previous reports, agarose and gelatin induce new vessel formation supporting enhanced oxygen supply.

Nanoporous microsystems for islet cell replacement

The inadequacy of conventional insulin therapy for the treatment of Type I diabetes has stimulated research on several therapeutic alternatives, including insulin pumps and controlled release systems for insulin. One of the most physiological alternatives to insulin injections is the transplantation of insulin-secreting cells. It is the beta cells of the islets that secrete insulin in response to increasing blood glucose concentrations. Ideally, transplantation of such cells (allografts or xenografts) could restore normoglycemia. However, as with most tissue or cellular transplants, the cellular grafts, particularly xenografts, are subjected to immunorejection in the absence of chronic immunosuppression. Thus, it is of great interest to develop new technologies that may be used for islet cell replacement. This research proposal describes a new approach to cellular delivery based on micro- and nanotechnology. Utilizing this approach, nanoporous biocapsules are bulk and surface micromachined to present uniform and well-controlled pore sizes as small as 7 nm, tailored surface chemistries, and precise microarchitectures, in order to provide immunoisolating microenvironments for cells. Such a design may overcome some of the limitations associated with conventional encapsulation and delivery technologies, including chemical instabilities, material degradation or fracture, and broad membrane pore sizes.