Biomedical Engineering Reference
In-Depth Information
macrophages. These attach loosely, then adhere more firmly to the vessel walls and
eventually penetrate them, guided by the concentration gradient they move through
the tissue to the site of injury. The migration through the tissue requires the action
of inflammatory proteases that degrade extracellular matrix components and lead to
the loosening of cell-cell interactions. Polymorphonuclear neutrophils (PMNs) are
the first cells that accumulate in the tissue within the first four hours after injury.
These cells can recognize danger signals released from injured tissue via various
pattern recognition receptors (PRRs). The signaling of these receptors can activate
several inflammatory signaling pathways, there by promoting further inflammatory
events and the recruitment of more cells from the blood circulation. Activated PMNs
produce reactive oxygen species and proteases [
19
-
22
]. These reactions are a de-
fense against foreign bodies which, depending on their nature may be degraded and
superficial foreign bodies may be extruded. Any pathogens associated with the in-
trusion of a foreign body are attacked and, if successful, eradicated or at least locally
contained until the adaptive immune system takes over. During excessive inflamma-
tion, these reactions, even lead to cell death and damage of the surrounding tissue.
After a few hours monocytes are attracted from the blood circulation by the action of
small peptides called chemokines that are released from PMNs and to a lesser degree
by other cells. The monocytes mature to macrophages that control subsequent steps
in the tissue, including the wound-healing reaction and the formation of a fibrotic
capsule around the foreign body. Thereby, the inflammation is switched from the
acute to the chronic stage (Fig.
1
)[
19
,
23
]. To establish a reliable testing system
that can evaluate the inflammatory potential of novel implant materials we evaluated
several strategies to visualize biomaterial-associated inflammatory reactions in vivo.
Reactive oxygen species are produced by immune cells in response to inflammation.
The oxidizing potential of these radicals was assessed. Hydrocyanines were used
as chemical sensors which are oxidized to fluorescent cyanines in the presence of
ROS. Cyanines can be visualized after excitation by their fluorescence in the near
infra-red spectrum [
24
]. Alternatively, inflammatory proteases like cathepsins are
produced by activated neutrophils or macrophages. The protease activity can be im-
aged using a specific probe that is hydrolyzed by cathepsins into a fluorescent product
[
25
-
27
]. In a third approach, the activity of the extracellular lipase autotaxin was used
to visualize an inflammatory process. Autotaxin activity generates a lipid signaling
molecule that stimulates cell proliferation, cell migration and cell survival [
29
] and
has been shown to be produced during lung inflammation [
28
]. For imaging, a flu-
orophore linked to a quencher by an autotaxin sensitive substrate was used. Upon
cleavage with autotaxin, the fluorophore and its quencher are separated resulting in
increased fluorescence efficiency. In addition, particularly in the early stages during
the wound healing processes, implants are prone to colonization by bacteria. Bacte-
rial infection induces cytokines like interferon-
. This has originally been
discovered as part of the antiviral response but it is also induced by diverse bacteria
[
30
,
31
]. Bacterial infections could be visualized by using a transgenic mouse model
in which the interferon(IFN)-
β
(IFN-
β
)
gene is replaced by a luciferase reporter gene that
can be used for imaging purposes [
32
]. For imaging, heterozygous mice were used
to allow IFN-
β
β
production from the wild type allele. The suitability of these in vivo
