Biomedical Engineering Reference
In-Depth Information
with type 1 diabetes have been clinically treated with islet transplantation. To cure
the disease, a single patient typically requires islets from several donors, due to the
destruction of islets just after transplantation. In the clinical setting, islets are
infused into the liver through the portal vein. Exposure of islets to the blood
activates blood coagulation and complement systems, which induce nonspecific
inflammatory reactions or instant blood-mediated inflammatory reactions (IBMIR).
These host defense mechanisms destroy donor islets because they are considered
foreign bodies [
127
-
129
]. Anticoagulants, including aspirin, heparin, and dextran
sulfate, are typically administered to inhibit blood coagulation. However, systemic
infusion of these drugs increases bleeding. The optimal approach would be to
prevent blood coagulation at the islet. Recent studies have been able to immobilize
various bioactive substances, like heparin, urokinase, thrombomodulin, and the
soluble domain of human complement receptor 1 (sCR1), on islets in attempts
to control local activation of the blood coagulation and complement systems
[
110
,
112
,
115
,
117
-
119
,
121
,
125
].
As an example, we will describe immobilization of the fibrinolytic enzyme,
urokinase (UK), on the islet surface [
115
,
119
]. As shown in Fig.
8
, UK could be
immobilized on islets through ssDNA hybridization of oligo(dT)
20
-PEG-lipid
and oligo(dA)
20
-UK. When the oligo(dT)
20
-PEG-lipid was added to a suspension
of islets, the lipid moiety spontaneously anchored to the lipid bilayer of the
cell membrane through hydrophobic interactions. The oligo(dT)
20
segment was
exposed on the cell surface, which made it accessible for conjugation with the oligo
(dA)
20
on UK (Fig.
8c
).
Figure
10
shows confocal laser-scanning fluorescence images of islets treated
with oligo(dT)
20
-PEG-lipid and oligo(dA)
20
-UK (UK-islets). In these experiments,
lipids with different alkyl chain lengths were attached to the PEG moiety to test the
stability of UK on the islet surface. Both PEG varieties showed clear
fluorescence signals (Fig.
10a
, b) from UK-islets, which indicated that both
facilitated stable UK attachments. On the other hand, fluorescence was nearly
undetectable on unmodified islets and islets treated with oligo(dA)
20
-UK in the
absence of oligo(dT)
20
-PEG-lipids (Fig.
10c
). These results indicated that UK
could be immobilized on islets through DNA hybridization. The retention time of
the oligo(dT)
20
-PEG-lipid on the cell membrane depended on the chain lengths of
the PEG-lipid [
111
]. At 2 days after conjugation, a strong fluorescence signal was
observed for islets treated with oligo(dT)
20
-PEG-lipid (C18), and a weaker fluores-
cence signal was observed for islets treated with oligo(dT)
20
-PEG-lipid (C16). The
longer alkyl chain length conferred longer retention of UK on the islet surface.
UK is a serine protease that activates plasminogen to plasmin. Plasmin dissolves
the fibrin in blood clots. The attachment of UK to the islet surface was expected to
dissolve blood clots that surrounded the islets in the liver; thus, IBMIR could be
inhibited in the initial stages. A fibrin plate-based assay was performed to assess the
