Molecular Mechanisms involved in the Development of Tissue Injury after Ischemia/Reperfusion
Different mechanisms participating in the development of ischemia reperfusion injury will be reviewed in the following section. I/R injury is the result of a prolonged oxygen deprivation in a tissue leading to hypoxia. This results in an ATP-depletion of the cells leading to swelling of mitochondria eventually causing a release of cytochrome c from the mitochondria. Cytochrome c activates an apoptotic signaling cascade involving caspases 1 and 9. These events participate in the induction of an inflammatory response via generation of IL-1β as well as programmed cell death (apoptosis) by activation of different caspases. Moreover, ATP depletion induces a cellular edema that occurs particularly during cold ischemia when Na/K ATPase is inhibited [
2].
A crucial mediator of I/R injury are oxygen derived free radicals [
3]. Particularly hydrogen peroxide, a source of oxygen-derived free radicals after hypoxia, can induce TNF-α by an activation of p38 mitogen activated kinase (MAPK) [
4]. Additionally, a number of intracellular adaptive metabolic responses occur among them an increase in the intracellular Ca
++-concentration with generation of calcium pyrophosphate complexes and the formation of uric acid. Calcium phosphate complexes and uric acid that belong to a group of so called danger signals (DNA fragments, cell membrane fragments, heat shock proteins, etc.) can bind to intracellular protein complexes so called inflammasomes [
5]. The inflammasomes include different adaptor molecules that mediate an increase of the production and secretion of interleukin-1 (IL-1)β. Furthermore also Toll-like receptors are stimulated through danger signals eventually stimulating the secretion of further proinflammatory cytokines/chemokines through an activation of NF-κB [
6].
The transcription factor NF-κB plays a central role in the generation of an inflammatory response as it is activated under conditions of cell stress and inflammation resulting in an activation and formation of other pro-inflammatory factors such as IL-1β, tumor necrosis factor (TNF)-α, or interferon (IFN)-γ and chemokines such as IL-8, MCP-1, or RANTES potentiating the inflammatory response. This is followed by an infiltration of lymphocytes, mononuclear cells/macrophages, and granulocytes into the injured tissue. Here adhesion molecules like the leukocyte function associated antigen-1 (LFA-1) or the intercellular adhesion molecule (ICAM)-1 play an important role. The cellular infiltrate together with the expression of cytokines/chemokines aggravates the interstitial edema of the inflamed tissue.
Apart from the formation of calcium phosphate complexes, the increase of the intracellular calcium concentration also enhances the activation of phospholipases as well as proteases. The latter group includes calpains (cleaving protein kinase c, fodrin, components of the cytoskeleton) and caspases which execute programmed cell death (apoptosis).
An important effect of hypoxia on a tissue is the development of metabolic acidosis. It occurs as a result of hypoxia when anaerobic glycolysis is the only way to generate energy. However, it can induce an inflammatory response when perfusion of the respective tissue is restored after hypoxia as well as hypothermia [
7].
However, not only locally generated inflammatory mediadtors like cytokines/chemokines [
8] resulting from I/R injury but also systemic inflammatory mediators in the donor affect the graft after transplantation. Here brain death profoundly contributes to a systemic inflammatory response through the release of cytokines from the brain. This "cytokine storm" deteriorates organ function resulting in more acute rejection episodes and decreased long-term function [
9‐
11]. The significance of such effects is underlined by experiments demonstrating the deleterious influence of brain death on graft function also over the long-term [
12] even when cold ischemia has been eliminated [
13].
If the inflammation is resolved the tissue can heal without sequale. However, if the inflammatory response is not resolved, for example due to ongoing tissue injury, the inflammation can become chronic, thus, stimulating tissue remodeling that eventually can result in organ fibrosis with a consecutive loss of function and graft failure [
1]. Fibrosis with an accumulation of extracellular matrix is a late non-specific result after I/R injury. However, extracellular matrix breakdown plays also a role in mediating acute tissue injury after I/R as discussed below. Furthermore, components of the innate immune system such as Toll-like receptors or the complement system also participate in the development of I/R injury.
Evidence exists that the temperature during ischemia differentially affects tissue injury as in liver ischemia for example cold ischemia (time when the graft is outside the body during transport from the donor to the recipient) affects more the sinusoidal cells while warm ischemia (time when the graft is in the body during operation before perfusion is reinstalled) affects primarily the hepatocytes [
14,
15].
On the other hand cells do not only produce deleterious factors promoting cell death and inflammation during hypoxia but also form protective factors in order to survive hypoxic episodes. Here the transcription factor HIF (hypoxia inducible factor)-1 plays an important role [
16]. Interestingly, the HIF-1 system may not only be activated under hypoxic conditions but also under inflammatory conditions [
17]. Basically cellular HIF-1 levels are low under normoxic conditions while they increase under hypoxic conditions to increase angiogenesis, erythropoesis, vasomotor control of the vessels and alterate the cellular energy metabolism as well as survival programs in order to protect the cells from the effects of hypoxia. Apart from transcription factors also protective genes like hemogygenase-1, bcl-2 or A20 are induced to protect cells after hypoxia [
18].
Reducing or preventing ischemia/reperfusion injury is a central strategy for an improvement of short-term as well as long-term graft performance after transplantation.
Evidence for this concept is derived from the observations that organs from living donors have a better graft survival and graft performance than organs from cadaveric donors although the HLA mismatch is similar because the ischemia times are significantly shorter in the setting of living donor related transplantation [
19,
20].
Strategies to reduce I/R injury after solid organ transplantation can be divided into pretransplant- and posttransplant strategies. Pretransplant strategies for the reduction of I/R injury include reduction of cold ischemia time through a logistic optimization of graft transport, machine-based perfusion procedures, or optimization of preservation solutions.
A further strategy is preconditioning of the donor with substances that have the capacity to reduce I/R injury. Posttransplant strategies to reduce I/R injury include the administration of substances that interfere with the inflammatory process mainly with the action of chemokines, cytokines, or leukocyte infiltration. On the other hand, strategies interfering with programmed cell death (apoptosis) have been investigated as well in order to reduce cell death and thus, protect the graft from cell loss. An ideal intervention would be a short course of a treatment during the immediate peritransplant period followed by a long-lasting effect in order to spare medications with their potential side effects. One practical problem in the prevention/treatment of I/R injury is that many treatment options were successful in experimental models while only few have been introduced into clinical trials not to mention standard treatment protocols. Here much work has to be done in the future to convert the knowledge derived from experimental approaches into successful clinical practice. In this review we will focus on the different experimental strategies to interfere with I/R damage and discuss strategies that have been already introduced into clinical practice.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
JL, KT, and UH contributed to the writing of this review. All authors read and approved the final manuscript.