Non-invasive evaluation of the location, the functional integrity and the oxygen supply of implants: 19F nuclear magnetic resonance imaging of perfluorocarbon-loaded Ba2+-alginate beads

A porcine islet-encapsulation device that enables long-term discordant xenotransplantation in immunocompetent diabetic mice

Islet transplantation is an effective treatment for type 1 diabetes (T1D). However, a shortage of donors and the need for immunosuppressants are major issues. The ideal solution is to develop a source of insulin-secreting cells and an immunoprotective method. No bioartificial pancreas (BAP) devices currently meet all of the functions of long-term glycemic control, islet survival, immunoprotection, discordant xenotransplantation feasibility, and biocompatibility. We developed a device in which porcine islets were encapsulated in a highly stable and permeable hydrogel and a biocompatible immunoisolation membrane. Discordant xenotransplantation of the device into diabetic mice improved glycemic control for more than 200 days. Glycemic control was also improved in new diabetic mice "relay-transplanted" with the device after its retrieval. The easily retrieved devices exhibited almost no adhesion or fibrosis and showed sustained insulin secretion even after the two xenotransplantations. This device has the potential to be a useful BAP for T1D.

Interconnected Toroidal Hydrogels for Islet Encapsulation

Islet encapsulation and transplantation promises to improve upon current treatments for type 1 diabetes mellitus, though several limitations remain. Macroscale devices have been designed for in vivo transplantation and retrieval, but traditional geometries do not support clinically adequate mass transfer of nutrients to and insulin from the encapsulated tissue. Microcapsule technologies have improved mass transfer properties, but their clinical translation remains challenging as their complete retrieval is difficult, should the graft become a safety concern. Here, the design, characterization and testing of a novel encapsulation structure, comprised of elastomer-reinforced interconnected toroidal hydrogels is reported. These donut-shaped hydrogels feature a high surface area, higher than conventional spherical capsules of the same volume, bestowing suitable mass transport conditions, while allowing interconnection and reversible deformation for intraperitoneal implantation and retrieval. Diabetes correction up to 12 weeks and complete retrieval is achieved in a diabetic mouse model, providing a proof-of-concept for the potential application as a type 1 diabetes cell replacement therapy.

Simultaneous spatiotemporal tracking and oxygen sensing of transient implants in vivo using hot-spot MRI and machine learning

A varying oxygen environment is known to affect cellular function in disease as well as activity of various therapeutics. For transient structures, whether they are unconstrained therapeutic transplants, migrating cells during tumor metastasis, or cell populations induced by an immunological response, the role of oxygen in their fate and function is known to be pivotal albeit not well understood in vivo. To address such a challenge in the case of generation of a bioartificial pancreas, we have combined fluorine magnetic resonance imaging and unsupervised machine learning to monitor over time the spatial arrangement and the oxygen content of implants encapsulating pancreatic islets that are unconstrained in the intraperitoneal (IP) space of healthy and diabetic mice. Statistically significant trends in the postimplantation temporal dependence of oxygen content between aggregates of 0.5-mm or 1.5-mm alginate microcapsules were identified in vivo by looking at their dispersity as well as arrangement in clusters of different size and estimating oxygen content on a pixel-by-pixel basis from thousands of 2D images. Ultimately, we found that this dependence is stronger for decreased implant capsule size consistent with their tendency to also induce a larger immunological response. Beyond the bioartificial pancreas, this work provides a framework for the simultaneous spatiotemporal tracking and oxygen sensing of other cell populations and biomaterials that change over time to better understand and improve therapeutic design across diverse applications such as cellular transplant therapy, treatments preventing metastatic formation, and modulators for improving immunologic response, for all of which oxygen is a major mechanistic component.

Response of Encapsulated Rat Pancreatic Islets to Hypoxia

Hypoxia contributes to encapsulated pancreatic islet graft failure. To gain insight into the mechanisms that lead to hypoxia-induced graft failure, encapsulated islet function, vitality, and cell replication were assessed after 2 and 5 days of hypoxic (1% O₂) and normoxic (20% O₂) culture. The mRNA expression levels of Bcl-2, Bax, inducible nitric oxide synthase (iNOS), and monocyte chemoattractant protein 1 (MCP-1) were assessed, as well as the amount of nitrite and MCP-1 in the culture medium. Hypoxia was associated with loss of encapsulated islet function and vitality, but not with an increase in islet cell replication. Loss of vitality was due to necrosis, and only modestly due to apoptosis. Hypoxia was not associated with changes in the Bcl-2/Bax mRNA ratio, but it did increase the expression of iNOS and MCP-1 mRNA. The increased mRNA levels were, however, not associated with elevated concentrations of nitrite nor with elevated levels of MCP-1 protein. The increased iNOS mRNA levels imply a role for NO in the completion of cell death by hypoxia. The increased MCP-1 mRNA levels suggest that encapsulated islets in vivo contribute to their own graft failure by attracting cytokine-producing macrophages. The discrepancy between iNOS mRNA and nitrite is explained by the longer half-life of NO during hypoxia. MCP-1 protein levels are underestimated as a consequence of the lower number of vital cells in combination with a higher proteolytic activity due to necrosis. Thus, strategies to eliminate hypoxia may not only improve islet function and vitality, but may also reduce the attraction of macrophages by encapsulated islets.

Development and Validation of Noninvasive Magnetic Resonance Relaxometry for the In Vivo Assessment of Tissue-Engineered Graft Oxygenation

Techniques to monitor the oxygen partial pressure (pO2) within implanted tissue-engineered grafts (TEGs) are critically necessary for TEG development, but current methods are invasive and inaccurate. In this study, we developed an accurate and noninvasive technique to monitor TEG pO2 utilizing proton (1H) or fluorine (19F) magnetic resonance spectroscopy (MRS) relaxometry. The value of the spin-lattice relaxation rate constant (R1) of some biocompatible compounds is sensitive to dissolved oxygen (and temperature), while insensitive to other external factors. Through this physical mechanism, MRS can measure the pO2 of implanted TEGs. We evaluated six potential MRS pO2 probes and measured their oxygen and temperature sensitivities and their intrinsic R1 values at 16.4 T. Acellular TEGs were constructed by emulsifying porcine plasma with perfluoro-15-crown-5-ether, injecting the emulsion into a macroencapsulation device, and cross-linking the plasma with a thrombin solution. A multiparametric calibration equation containing R1, pO2, and temperature was empirically generated from MRS data and validated with fiber optic (FO) probes in vitro. TEGs were then implanted in a dorsal subcutaneous pocket in a murine model and evaluated with MRS up to 29 days postimplantation. R1 measurements from the TEGs were converted to pO2 values using the established calibration equation and these in vivo pO2 measurements were simultaneously validated with FO probes. Additionally, MRS was used to detect increased pO2 within implanted TEGs that received supplemental oxygen delivery. Finally, based on a comparison of our MRS data with previously reported data, ultra-high-field (16.4 T) is shown to have an advantage for measuring hypoxia with 19F MRS. Results from this study show MRS relaxometry to be a precise, accurate, and noninvasive technique to monitor TEG pO2 in vitro and in vivo.

Long-Term Function of Alginate-Encapsulated Islets

Human trials have demonstrated the feasibility of alginate-encapsulated islet cells for the treatment of type 1 diabetes. Encapsulated islets can be protected from the host's immune system and remain viable and functional following transplantation. However, the long-term success of these therapies requires that alginate microcapsules maintain their immunoprotective capacity and stability in vivo for sustained periods. In part, as a consequence of different encapsulation strategies, islet encapsulation studies have produced inconsistent results in regard to graft functioning time, stability, and overall metabolic benefits. Alginate composition (proportion of M- and G-blocks), alginate purity, the cross-linking ions (calcium or barium), and the presence or absence of additional polymer coating layers influence the success of cell encapsulation. This review summarizes the outcomes of long-term studies of alginate-encapsulated islet transplants in animals and humans and provides a critical discussion of the graft failure mechanisms, including issues with graft biocompatibility, transplantation site, and integrity of the encapsulated islet grafts. Strategies to improve the mechanical stability of alginate capsules and methods for monitoring graft survival and function in vivo are presented.

Perfluorinated alginate for cellular encapsulation

Molecules of pentadecafluorooctanoyl chloride (PFC) were grafted onto alginate (Alg) using a linear poly(ethylene glycol) linker and amide bonds. The resulting Alg-PFC material was characterized by proton nuclear magnetic resonance and infrared spectroscopies. The degree of PFC functionalization significantly influenced the physical and chemical properties of Alg-PFC, particularly when the resulting polymer was ionically crosslinked into hydrogels. Alg-PFC hydrogel beads fabricated via Ba(2+) crosslinking were found to match the permeability properties of control alginate beads, except upon swelling over time in culture media. When used to encapsulate MIN6 cells, a beta cell line, Alg-PFC beads demonstrated enhanced cell proliferation over alginate control beads. These results indicate that Alg-PFC hydrogels retain some of the PFC's biological-relevant benefits, such as enhancement of mass transport and bioinertness, to enhance cellular viability within alginate three-dimensional hydrogel environments. We envision these functionalized hydrogels to be particularly useful in the encapsulation of cells with a high metabolic demand, such as pancreatic islets.

In vivo noninvasive monitoring of dissolved oxygen concentration within an implanted tissue-engineered pancreatic construct

The function of an implanted tissue-engineered pancreatic construct is influenced by many in vivo factors; however, assessing its function is based primarily on end physiologic effects. As oxygen significantly affects cell function, we established a dual perfluorocarbon method that utilizes (19)F nuclear magnetic resonance spectroscopy, with perfluorocarbons as oxygen concentration markers, to noninvasively monitor dissolved oxygen concentration (DO) in βTC-tet cell-containing alginate beads and at the implantation milieu. Beads were implanted in the peritoneal cavity of normal and streptozotocin-induced diabetic mice. Using this method, the feasibility of acquiring real-time in vivo DO measurements was demonstrated. Results showed that the mouse peritoneal environment is hypoxic and the DO is further reduced when βTC-tet cell constructs were implanted. The DO within cell-containing beads decreased considerably over time and could be correlated with the relative changes in the number of viable encapsulated cells. The reduction of construct DO due to the metabolic activity of the βTC-tet cells was also compatible with the implant therapeutic function, as observed in the reversal of hyperglycemia in diabetic mice. The importance of these findings in assessing implant functionality and host animal physiology is discussed.

Dual perfluorocarbon method to noninvasively monitor dissolved oxygen concentration in tissue engineered constructs in vitro and in vivo

Noninvasive in vivo monitoring of tissue implants provides important correlations between construct function and the observed physiologic effects. As oxygen is a key parameter affecting cell and tissue function, we established a monitoring method that utilizes (19) F nuclear magnetic resonance (NMR) spectroscopy, with perfluorocarbons (PFCs) as oxygen concentration markers, to noninvasively monitor dissolved oxygen concentration (DO) in tissue engineered implants. Specifically, we developed a dual PFC method capable of simultaneously measuring DO within a tissue construct and its surrounding environment, as the latter varies among animals and with physiologic conditions. In vitro studies using an NMR-compatible bioreactor demonstrated the feasibility of this method to monitor the DO within alginate beads containing metabolically active murine insulinoma βTC-tet cells, relative to the DO in the culture medium, under perfusion and static conditions. The DO profiles obtained under static conditions were supported by mathematical simulations of the system. In vivo, the dual PFC method was successful in tracking the oxygenation state of entrapped βTC-tet cells and the surrounding peritoneal DO over 16 days in normal mice. DO measurements correlated well with the extent of cell growth and host cell attachment examined postexplantation. The peritoneal oxygen environment was found to be variable and hypoxic, and significantly lower in the presence of metabolically active cells. The significance of the dual PFC system in providing critical DO measurements for entrapped cells and other tissue constructs, in vitro and in vivo, is discussed.

Enhancing clinical islet transplantation through tissue engineering strategies

Clinical islet transplantation (CIT), the infusion of allogeneic islets within the liver, has the potential to provide precise and sustainable control of blood glucose levels for the treatment of type 1 diabetes. The success and long-term outcomes of CIT, however, are limited by obstacles such as a nonoptimal transplantation site and severe inflammatory and immunological responses to the transplant. Tissue engineering strategies are poised to combat these challenges. In this review, emerging methods for engineering an optimal islet transplantation site, as well as novel approaches for improving islet cell encapsulation, are discussed.

Reversible signal regulation system of 19F NMR by redox reactions using a metal complex as a switching module

We present here the reversible signal regulation of (19)F NMR using the transition metal complex as a switching module. Water-soluble fluorinated dendrimers containing ferrocene were synthesized as a probe, and the signal intensities of (19)F NMR of the probes were monitored by changing the valence state of the ferrocene moiety. By oxidation of the ferrocene moiety, the relaxation of the nearby fluorine atoms was accelerated via the paramagnetic relaxation enhancement, as a result the (19)F NMR signal from the probe was significantly decreased. The reduction of the ferrocenium cation of the probe recovered the signal intensity. Finally, in combination with the difference of the fluorescence quenching ability between ferrocene and ferrocenium cation, we constructed the multimodal (19)F NMR/fluorescence probe based on the redox switching system.

Long-term graft function of adult rat and human islets encapsulated in novel alginate-based microcapsules after transplantation in immunocompetent diabetic mice

We describe the results of the first study to show that adult rat and human islets can be protected against xenogenic rejection in immunocompetent diabetic mice by encapsulating them in a novel alginate-based microcapsule system with no additional permselective membrane. Nonencapsulated islets lost function within 4-8 days after being transplanted into diabetic Balb/c mice, whereas transplanted encapsulated adult rat or human islets resulted in normoglycemia for >7 months. When rat islet grafts were removed 10 and 36 weeks after transplantation, the mice became immediately hyperglycemic, thus demonstrating the efficacy of the encapsulated islets. The explanted capsules showed only a mild cellular reaction on their surface and a viability of >85%, and responded to a glucose stimulus with a 10-fold increase in insulin secretion. Furthermore, transplanted mice showed a slight decrease in the glucose clearance rate in response to intraperitoneal glucose tolerance tests 3-16 weeks after transplantation; after 16 weeks, the rate remained stable. Similar results were obtained for encapsulated human islets. Thus we provide the first evidence of successful transplantation of microencapsulated human islets. In conclusion, we have developed a novel microcapsule system that enables survival and function of adult rat and human islets in immunocompetent mice without immunosuppression for >7 months.

In vivo determination of tumor oxygenation during growth and in response to carbogen breathing using 15C5-loaded alginate capsules as fluorine-19 magnetic resonance imaging oxygen sensors

PURPOSE

The objective was to present a method for the repeated noninvasive measurement of tumor oxygenation (Po(2)) over the whole period of tumor growth.

METHODS AND MATERIALS

A mixture of tumor homogenate (GH3 prolactinoma) and alginate capsules loaded with perfluoro-15-crown-5-ether (15C5) was injected into the flanks of Wistar Furth rats. The temporal behavior of tumor Po(2) was monitored between Day 1 and 26 after injection using fluorine-19 ((19)F) magnetic resonance imaging (MRI). In addition, the response of tumor Po(2) to modifiers of the tumor microenvironment (carbogen [95% O(2)/5% CO(2)], nicotinamide, and hydralazine) was investigated.

RESULTS

An initial increase of tumor Po(2), probably reflecting neovascularization, followed by a decrease after Week 2, probably indicating tumor hypoxia or necrosis, were observed. The minimum and maximum average Po(2) +/- SEM observed were 3.3 +/- 2.0 mm Hg on Day 2 and 25.7 +/- 3.8 mm Hg on Day 13, respectively. Carbogen increased the tumor Po(2), whereas nicotinamide caused no significant change and hydralazine induced a significant decrease in tumor oxygenation.

CONCLUSIONS

A preclinical method for the repeated noninvasive determination of tumor Po(2) was presented. It might help to investigate tumor physiology and the mechanisms of modifiers of the tumor microenvironment and their role in different therapeutic approaches.

Properties of a novel magnetized alginate for magnetic resonance imaging

Implanting recombinant cells encapsulated in alginate microcapsules to secrete therapeutic proteins has been proven clinically effective in treating several murine models of human diseases. However, once implanted, these microcapsules cannot be assessed without invasive surgery. We now report the preparation and characterization of a novel ferrofluid to render these microcapsules visible with magnetic resonance imaging (MRI). The ferrofluid was prepared as a colloidal iron oxide stabilized in water by alginate. The presence of iron particles in the ferrofluid was verified with chemical titration, dynamic light scattering, and magnetization measurement. The microcapsules fabricated with various concentrations of the ferrofluid in the core, or on the surface of alginate microcapsules, or both, all produced microcapsules with smooth surfaces as shown with light and scanning electron microscopy. However, at the nanoscale level, as revealed with atomic force microscopy, the ferrofluid-fabricated microcapsules demonstrated increased granularity, particularly when the ferrofluid was used to laminate the surface. From the force spectroscopy measurements, these modified microcapsules showed increasing surface rigidity in the following order: traditional alginate < ferrofluid in the core < ferrofluid on the surface. Although the mechanical stability of low-concentration ferrofluid (0.1%) microcapsules was reduced, increasing concentrations, up to 20%, were able to improve stability. When these ferrofluid microcapsules were examined with MRI, their T(2) relaxation time was reduced, thereby producing increased contrast readily detectable with MRI, whereas the traditional alginate microcapsules showed no difference when compared with water. In conclusion, such ferrofluid-enhanced alginate is suitable for fabricating microcapsules that offer the potential for in vivo tracking of implanted microcapsules without invasive surgery.

Effect of cross-linked hemoglobin on functionality and viability of microencapsulated pancreatic islets

Of many obstacles involved in developing a bioartificial pancreas, which consists of encapsulated and physically immunoprotected islets, for long-term implantation in insulin-dependent diabetic patients, the impaired functionality and decreasing viability of encapsulated islets over time are critical factors in determining the size and longevity of the implant. These factors are closely associated with short oxygen supply to the encaged islets from the implant site. To facilitate oxygen transport to islets in the capsules, we coencapsulated hemoglobin cross-linked with difunctional polyethylene glycol (Hb-conjugate, Hb-C) which is large in size (>100 kDa), thus preventing diffusional loss through the immunoprotecting membrane. The coencapsulation of Hb-C with islets in alginate-poly-L-lysine microcapsules by dissolving Hb-C in an islet-suspended alginate solution at a concentration of 0.25 mM improved the insulin secretion and viability of the islets. At week 0, the islets, coencapsulated with Hb-C, cultured at P(O2) = 40 mmHg (assumed oxygen partial pressure in the most common implant site, the peritoneal cavity), secreted 200% more insulin compared with the control islets without Hb-C at glucose concentrations of both 100 and 300 mg/dL. The Hb-C effect became more significant with time at higher glucose concentrations. After culturing the islets for 8 weeks at 40 mmHg, the insulin secretion was enhanced 200 and 550% at glucose concentrations of 100 and 300 mg/dL as compared with the control, respectively. The results were closely associated with improved viability and suggest that the introduction of Hb-C is an effective approach to maintaining the oxygen supply to encapsulated islets. In addition, Hb-C coencapsulation with pancreatic islets may (1) provide a partial clue to reducing the large size of the biohybrid artificial pancreas, (2) lead to a reduced need for pancreas donation, and (3) prolong the longevity of the biohybrid artificial pancreas in the body.

A novel class of amitogenic alginate microcapsules for long-term immunoisolated transplantation

In the light of results of clinical trials with immunoisolated human parathyroid tissue Ba2+-alginate capsules were developed that meet the requirements for long-term immunoisolated transplantation of (allogeneic and xenogeneic) cells and tissue fragments. Biocompatibility of the capsules was achieved by subjecting high-M alginate extracted from freshly collected brown algae to a simple purification protocol that removes quantitatively mitogenic and cytotoxic impurities without degradation of the alginate polymers. The final ultra-high-viscosity, clinical-grade (UHV/CG) product did not evoke any (significant) foreign body reaction in BB rats or in baboons. Similarly, the very sensitive pERK assay did not reveal any mitogenic impurities. Encapsulated cells also exhibited excellent secretory properties under in vitro conditions. Despite biocompatible material, pericapsular fibrosis is also induced by imperfect capsule surfaces that can favor cell attachment and migration under the release of material traces. This material can interact with free end monomers of the alginate polymers under formation of mitogenic advanced glycation products. Smooth surfaces, and thus topographical biocompatibility of the capsules (visualized by atomic force microscopy), can be generated by appropriate crosslinking of the UHV/CG-alginate with Ba2+ and simultaneous suppression of capsule swelling by incorporation of proteins and/or perfluorocarbons (i.e., medically approved compounds with high oxygen capacity). Perfluorocarbon-loaded alginate capsules allow long-term non-invasive monitoring of the location and the oxygen supply of the transplants by using 19F-MRI. Transplantation studies in rats demonstrated that these capsules were functional over a period of more than two years.

Challenges in small animal noninvasive imaging

The current status and challenges of small animal non-invasive imaging is briefly reviewed. The advantages of non-invasive studies on living animals versus post-mortem studies are evaluated. An argument is advanced that even in post-mortem situations, non-invasive imaging may play an important role in efficiently characterizing small animal phenotypes as well as pathology. Issues of data interpretation under anesthetized conditions in live animal studies are also reviewed. The five imaging technologies covered include CT, PET, ultrasound, MRI and optical imaging. The structural and physiological information content of these different modalities is reviewed along with the ability of these techniques to scale down for use in small mammals such as mice and rats. In general, it was found that most of these technologies scale favorably to the study of small mammals, generally providing more physiological information than when used on the larger human scale. This suggests that these types of small mammal imaging capabilities will play a very significant role in the full utilization of these important animal models in biomedical research.