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.
The cartoon 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 we 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.
For the test we prepared 50 micron thick sheets using two different alginate preparations to demonstrate our capability to adjust permeability from "tight" to "very open". The results, as expected, demonstrate a very broad high permeability profile for "XG" and a very sharp cutoff for the less permeable "MPA". By intermediate modification of the alginate chemistry, we can readily tailor both the inflection points and slopes of these profiles to design an optimal coating. In short, we 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 Samples were withdrawn from both sides of the diffusion cell, and protein was determined by HPLC.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.
The following graph and table shows the permeability of three membranes for molecular species of selected weights.
|MPA alginate ("tight")
|XG alginate ("very open")