Prevent Destructive Host Response
The technical requirements for a bioartificial pancreas are have proven very difficult to solve, chiefly because the islets inside typically die of starvation. Most often the surface of the device provokes a foreign body response; the resulting fibrotic reaction walls off the device, so the islets cannot get nutrition. The key requirements for preventing a destructive host response to the implanted bioartificial pancreas are:
Outer surfaces must be totally biocompatible
The device must not provoke any fibrotic response, and the material exposed to body tissues must be completely free of fibrogenic character. A substance that is ignored by the host body is described as “biocompatible”; however, that term means different things in different contexts. For a vascular device, “biocompatible” means that the material induces engraftment; the vascular graft is covered with a layer of collagen fibers permitting overgrowth of endothelial cells. But such overgrowth will starve a cellular implant! For a cellular transplant, “biocompatible” means lacking collagen fibers or any material that will promote cellular adhesion.
The difference is shown in Figure xx. The body’s reaction to material appropriate for a vascular implant (and inappropriate for a bioartificial pancreas) is shown in the left image; it consists of inflammation, granulation and neovascularization followed by collagen deposition and fibrosis. The reaction to material appropriate for a bioartificial pancreas is shown on the right. In both cases the initial interaction is between the material and immune system cells and fibroblasts (blue). For the vast majority of materials, the reaction shown on the left pertains, and cells react to foreign bodies by laying down collagen fibers (green). A bioartificial pancreas made of the material on the left would starve the cells inside because the collagen prevents nutrients from reaching the cells inside.
The ideal bioartificial pancreas provokes only the reaction shown on the right. Very few materials respond in this way. Islet sheets are made from highly purified alginic acids (alginate), which ISM has shown can be made biocompatible, as defined above.
All islets must be completely covered
To prevent immune sensitization, all the islets must be protected. Some membrane configurations do not cover the entire islet surface. If even a small bit of the islet is uncovered, macrophages can infiltrate and destroy the entire islet . The cellular attack and destruction sensitize the immune system, leading to a humoral (antibody) response, which can then destroy even those cells that are completely covered. Thus, complete coverage of all of the islets is required to protect the islet cells from both cellular and humoral immune responses.
Figure xx shows cellular destruction of an islet. First immune cells appear (blue). Three cells protrude outside the right capsule. These immune cells (T cells, macrophages) rapidly identify the foreign islet cells, invade the islet through the hole in the capsule, and destroy the entire islet. The islet on the left is protected by its capsule.
Very few systems have been developed which ensure complete coverage of all islets. None can achieve complete coverage within the dimensional constraints imposed by oxygen requirement. Islet Sheet technology absolutely ensures complete coverage of every islet with a thin, precisely controlled depth of over-coating.
The required extent of exclusion of antibody and complement cannot be predicted. Complement-mediated cell death requires the cooperativity of many large molecules acting in concert. Complete exclusion of antibody and complement is likely to be unnecessary, and would also be deleterious to islet viability because of nutrient exclusion.
Figure xx shows humoral destruction of an islet following cellular destruction of an exposed islet. First immune cells appear (blue) that rapidly identify the foreign islet cells on the right capsule, invade the islet through the hole in the capsule, and destroy the entire islet. Meanwhile, a B cell begins making antibody (red). This combines with circulating complement to destroy the islet through the protecting capsule. Thus the islet on the left is destroyed in spite of its capsule.
By virtue of its retrievability, the Islet Sheet offers the unique ability to resolve the allowable exclusion limit of antibody and complement empirically. By retrieving the intact implant some time after implantation, ISM can study the extent of immune rejection and islet starvation in a realistic and quantitative manner. No other coating technology under development allows complete retrieval of implanted islets, nor the exquisite control of permeability.
Permeability must be controllable
Islet Sheet Medical has studied the permeability of its membrane with encouraging results. For the test they prepared 50-micron-thick sheets using two different alginate preparations to demonstrate their capability to adjust permeability from “tight” to “very open.” The results, as expected, demonstrate a very broad high permeability profile for the XG alginate and a very sharp cutoff for the less permeable MPA. By intermediate modification of the alginate chemistry, ISM can readily tailor both the inflection points and slopes of these profiles to design an optimal coating. In short, they have good control over permeability.
Experimental: Flat sheet alginate membranes were incubated in a two-chamber diffusion cells. The high side of the cell was a protein cocktail containing Vitamin B12, albumin, and IgG in a 0.9% NaCl solution containing 10 mM HEPES, 5 mM CaCl2. Samples were withdrawn from both sides of the diffusion cell, and protein was determined by HPLC. A cast hydrogel membrane prepared from other materials is included in the chart for purposes of comparison. This type of hydrogel membrane has been shown to provide nutritional support and immunoprotection in model xenoprotection studies.
Reflecting the results of testing, the graph (Figure xx) and table below show the permeability of three membranes for molecular species of selected weights.