| dc.description.abstract | Cell therapy of diabetes is today limited by two major obstacles preventing widespread application;
graft rejection by the recipient’s immune system, and scarcity of cells suitable for transplantation.
Microencapsulation has been proposed as a means to protect transplanted islets from rejection by the
host immune system, and alginate has a long history in this field. The work in this thesis addressed a
variety of questions related to the safety and efficacy of alginate beads in cell therapy of diabetes, as
well as their potential as extracellular matrix (ECM) surrogate in stem cell differentiation.
Alginate beads used in cell therapy are often gelled with varying amounts of barium in order to
improve stability of the beads. Barium is toxic to humans, and accumulation in patients due to leakage
from the beads is a safety concern. Leakage of ions from beads gelled with varying amounts of barium
in vitro and accumulation in mice in vivo was measured, and the results were compared to guidelines
for barium exposure provided by the World Health Organization. It was found that in order to reduce
the risk of accumulation of toxic amounts of barium in patients receiving encapsulated cells, alginate
beads gelled predominantly with calcium and a low amount of barium are recommended.
The efficacy of lyase-catalyzed degradation of alginate beads gelled with different amounts of calcium
and barium was tested. It was found that gel degradation worked well for beads crosslinked with
calcium, but it was less effective when barium was added to the bead formulation. Analysis of the
composition of degradation products revealed new information about gelling properties, and in
particular the importance of long G-blocks in maintaining structural integrity of the gel.
Due to a relatively large pore size, alginate beads provide limited permselectivity and have been
shown to allow influx of many components of the immune system. This has been regarded as a
limitation in their capacity to protect against immune rejection, particularly in xenogeneic systems.
The immunoprotective capacity of alginate beads gelled with a combination of calcium and barium
was tested in a xenogeneic mouse model. Human islets encapsulated in alginate beads transplanted to
immunocompetent mice successfully reversed streptozotocin-induced diabetes for several months, and
while the beads protected against xenogeneic rejection, the graft eventually failed due to a foreign
body reaction.
Finally, the capacity of alginate-based three-dimensional matrices with added extracellular matrix
functionality to stimulate maturation of pancreatic progenitors was tested. Human embryonic stem
cell-derived pancreatic progenitors were encapsulated in alginate mixed with relevant basement
membrane or ECM components, and cultured in vitro for up to 46 days. No stimulation of maturation
into insulin-producing β-cells was measured. However, the cells were viable and maintained a stable
morphology throughout the experiment. It was concluded that alginate beads can provide a chemically
defined, xeno-free and easily scalable alternative for culture of pancreatic progenitors. | nb_NO |
