Introduction
Although lung transplantation treats otherwise incurable lung diseases, it carries a 5-year mortality of nearly 50%. Reperfusion injury, also known as primary graft dysfunction (PGD), is defined clinically by radiographic lung opacities consistent with edema and by high requirements for supplemental O
2 during the first 72 hours of reperfusion [
1]. PGD affects up to 25% of transplanted lungs and is the major cause of early morbidity and mortality after transplantation. Allograft recipients surviving severe PGD are more likely to be physiologically impaired one year after transplantation and to be more vulnerable to consequences of acute rejection. Moreover, they are more likely to develop bronchiolitis obliterans syndrome (BOS), a manifestation of chronic rejection [
2]. Overall, PGD is a major barrier to success of lung transplantation, and new insights regarding pathogenesis are needed to guide approaches to prevention and therapy [
3]-[
5].
Mast cells have been implicated in the pathogenesis of several types of ischemia-reperfusion injury. In mouse models of ischemia-reperfusion injury to muscle, the extent of tissue damage correlates with mast cell degranulation and is markedly reduced in mice lacking mast cells. Release of mouse mast cell protease-5, an elastolytic protease related to human mast cell chymase, appeared to be critical the development of reperfusion injury in skeletal muscle [
6]. Mast cell-deficient mice also have a less severe phenotype after ischemia-reperfusion injury to myocardium [
7]. Mast cell stabilizers and anti-histamines protect against myocardial ischemia-reperfusion injury [
8].
Mast cells abound at baseline in donor lung airway walls and alveolar interstitia. Their numbers may increase following transplantation and in association with acute rejection and BOS [
9],[
10]. Furthermore, mRNAs encoding mast cell-specific products, such as tryptase, are abundant in transbronchial biopsies of human allografts [
11]. Studies in animals suggest that lung mast cells also can be activated in the setting of ischemia-reperfusion. For example, in rat tracheal allografts, mast cells degranulate and upregulate chemokine ligand expression [
12], and in dog lungs, mast cells appear to be recruited and to degranulate following transient ligation of a pulmonary artery [
13]. Traditional mast cell stabilizers, such as ketotifen and sodium cromoglycate, decrease inflammation following lung reperfusion in rats, as evidenced by decreased levels of ICAM-1 and TNFα and increased NOS-2 [
14],[
15].
There are mechanistic reasons as well to suspect a role for mast cells in PGD. Mast cell products, especially secreted TNFα and proteases (such as tryptases, which are the major secreted proteins of human mast cells), promote neutrophilic inflammation, which is a hallmark of PGD [
16]-[
19]. Also, mast cells express adenosine receptors and are activated by adenosine [
20]-[
22], which accumulates in ischemic tissue prior to re-establishment of perfusion as a by-product of ATP utilization and depletion.
One of the challenges in using mice to model roles of mast cells in human lung pathology is that the numbers and distribution of mast cells differ between laboratory mice and humans. A traditional way to explore the contributions of mast cells to pathology in mice is to compare phenotypes in wild-type mice with those in one of several available strains of mice lacking mast cells due to genetic defects in expression of c-Kit. If differences are seen, then greater certainty about mast cell involvement can be obtained by restoring the wild-type phenotype via adoptive transfer of wild-type bone marrow-derived cultured mast cells (BMCMC). However, the lung is challenging in this regard. While
Kit
W-sh/W-sh
mice lack lung mast cells in all sites, wild-type
Kit
+/+
C57BL/6 mouse lung contain mast cells primarily in the trachea and peribronchial tissues with very few in lung parenchyma. However, parenchymal mast cells can be observed following intravenous injection of BMCMC into mast cell-deficient
Kit
W-sh/W-sh
mice in a C57BL/6 background [
23]-[
25]. The density of the mast cell population in the alveolar interstitium of
Kit
W-sh/W-sh
mice following intravenous injection of BMCMC is substantially higher than in wild-type
Kit
+/+
mice [
23],[
26],[
27]. Interestingly, in humans, alveolar interstitia support perhaps the highest density of mast cells in any normal tissue [
28]. Therefore, these results of adoptive transfer of BMCMC into
Kit
W-sh/W-sh
mice suggested the possibility of “humanizing” mouse lungs with respect to alveolar mast cell density by injecting mice with BMCMC. As a basis for conducting the present study, we hypothesized that mast cell activation and degranulation contribute to the pathogenesis of PGD in lung allografts. We found that mast cells are present and that they degranulate in human PGD and in a mouse model of PGD with alveolar mastocytosis. However, we did not detect differences in the degree of lung injury related to the presence of mast cells in mice, suggesting that they do not make major contributions to ischemia-perfusion injury in lung tissues.
Materials and methods
Studies on human tissue were performed with written consent from the subject and with approval of the UCSF Human Research Protection Program Committee on Human Research (Protocol #13-10738). All non-human animal studies were approved by the UCSF IACUC in accordance with the NIH Guide for the Care and Use of Laboratory Animals.
Histology
Human lung biopsy tissue sections were stained with hematoxylin and eosin or with c-kit antibody (A4502, Dako, Carpinteria CA). Mouse tissue was fixed in 4% paraformaldehyde and specimens were embedded in paraffin and cut into 5-μm sections and stained with hematoxylin and eosin. To identify mast cells in mice and humans, sections were deparaffinized and stained with 0.5% acidified toluidine blue.
Mouse ischemia reperfusion model
Mast cell–deficient C57BL/6 Kit
W-sh/W-sh (Wsh) and wild-type C57BL/6
Kit
+/+
(+/+) mice were housed as described [
23]. For some experiments, 0.1 mg/kg of LPS (Sigma-Aldrich, St. Louis MO) was injected intraperitoneally 24 hours before surgery. Experiments were performed at 8-12 weeks of age with the exception of adoptive transfer experiments, which were performed at 20-22 weeks to allow mast cell populations to establish and mature in the lung. Mice were anesthetized with ketamine and xylazine, and 0.5 mL of PBS was administered intraperitoneally. The trachea was intubated, and animals were mechanically ventilated (MiniVent, Harvard Apparatus, Holliston MA) with 100% oxygen, respiratory rate of 120/min, tidal volume of 10 ml/kg and positive end-expiratory pressure of 3 cm of H
2O. Anesthesia was maintained with 1% vaporized isoflurane. After left thoracotomy, the left pulmonary hilum was isolated and circumscribed with a suture. Heparin (0.03 U/g of mouse weight) was administered intraperitoneally and the hilar suture was tied using a slipknot to occlude the hilar structures, or left untied in sham surgery. The tidal volume was reduced to 8 ml/kg and the respiratory rate increased to 150/min. The thoracotomy was closed with sutures and the hilar slipknot suture tunneled outside of the chest. Buprenorphine analgesia was administered, the animal was extubated after awakening from anesthesia, and was allowed to recover in a warmed chamber with supplemental O
2 administered at 2 L/min. One hour after hilar occlusion,
125I-labelled albumin was administered intraperitoneally, the slipknot suture was removed, and the animal was transferred to a warmed cage. Four hours after reperfusion, animals were euthanized by anesthetic overdose prior to blood and lung collection. Lungs were homogenized in 1 ml of water. Extravascular lung water was determined by the gravimetric methods and endothelial permeability to
125I-labeled albumin was used to calculate extravascular plasma equivalents using the equations detailed in the Additional file
1[
29].
Mast cell adoptive transfer
BMCMC were derived by culturing bone marrow from C57BL/6 mice for 5-6 weeks in 10 ng/ml of IL-3, as previously described [
23]. Stem cell factor (50 ng/ml) was added to the cultures starting at weeks 3. BMCMC (10
7 in 0.2 ml of PBS) were injected via tail vein into 5-week old mice. Ischemia-reperfusion experiments were performed at least 12 weeks after adoptive transfer.
Statistical analysis
Differences between groups were assessed using Students t-test or one-way ANOVA with Bonferroni-corrected post-test comparisons between pre-specified groups using Prism version 5.0a (GraphPad Software, Inc., La Jolla, CA). Power calculations were performed using R (version 3.0.2, R Foundation for Statistical Computing, Vienna, Austria).
Discussion
Motivated by the observation of degranulated mast cells in tissue obtained from a patient with PGD, we developed a mouse model of ischemia-reperfusion injury with variation of mast cell density to evaluate the potential role of mast cells in mediating PGD. This model recapitulated the histologic hallmarks and mast cell degranulation and increased mast cell density observed in human pathology. However, we did not observe a significant effect of mast cells across multiple permutations of this experimental model.
Although we did not observe mast cell-dependent effects on lung extravascular water or endothelial permeability, it is likely that ischemia reperfusion injury results in mast cell-dependent effects on other parameters that were not tested. For example, pharmacologic inhibition of mast cells during reperfusion injury has been shown to decrease ICAM-1 and increase cGMP and NOS-2 levels following reperfusion injury [
14]. Further, mast cells are distinct from other cell types in that secondary activation is required for upregulation of Hypoxia-Ischemia Factor Iα in response to ischemia [
34] and hypoxia results in autocrine secretion of IL-6, which promotes mast cell survival [
35]. The unique responses of mast cells to ischemia suggest that they may yet play a role in ischemia reperfusion injury through mechanisms that could not be observed in this model. We selected ischemia and reperfusion times to achieve a relatively severe and reproducible level of injury, but it is possible that varying ischemia and reperfusion times might bring out more subtle differences related to mast cell degranulation.
Prior studies had shown a role for mast cell degranulation in mediating cardiac and skeletal muscle ischemia-reperfusion injury [
6],[
8] and had shown that mast cells are present and active during ischemia-reperfusion injury in tracheal and lung tissue [
12],[
13]. The present data suggest, however, that despite being active, mast cells do not play a non-redundant role in mediating reperfusion injury to the lung. Multiple independent mechanisms are thought to contribute PGD, including oxidative stress, calcium and iron overload, hypercoaguability, cell adhesion molecule upregulation, pro-inflammatory cytokines, membrane lipid remodeling, complement activation, endothelin release, and activation of leukocytes, macrophages, lymphocytes, and neutrophils. Mast cells may contribute to some, but not all of these mechanisms [
36].
Beyond effects related to ischemia-reperfusion injury, studies in mice have suggested that mast cells may help to determine the fates of transplanted organs by effects on immune tolerance [
37]. For example, studies of skin allografts suggested that mast cells promote tolerance [
38], because mast cell-deficient mice could not tolerate allografts. This effect may be due to depression of IL-6 levels in the allograft by the mast cell tryptase mMCP-6 [
39], which can cleave and inactivate IL-6 [
19]. On the other hand, studies in
Kit
W-sh/W-sh
mice undergoing experimental cardiac transplantation showed no significant differences in rejection when compared to wild-type animals as manifested by graft inflammatory cells, cytokine or adhesion molecule expression, or coronary artery disease [
40]. Thus, it is possible that for adaptive immune-mediated rejection, the contribution of mast cells to allograft dysfunction may also be redundant.
This study has limitations. For example, a small effect from mast cell degranulation might not be evident without large study populations, especially given the variability intrinsic to the utilized measures of lung injury. A meaningful effect would likely have been evident in at least one of the described experimental approaches. It is also possible that mast cell degranulation has an important role in ischemia-reperfusion injury but that other cell types can compensate for the absence of mast cells. The lack of observed difference in wild-type animals with and without mast cells, however, suggests that mast cell degranulation beyond what is physiologic in mice does not lead to worsened lung injury. Although we do not have reason to suspect that adoptively transferred mast cells would lose their functionality, we cannot exclude the possibility that the reconstituted mast cells have some dysfunction despite preserved ability to degranulate. Finally, the present model employed warm ischemia, which is physiologically distinct from the cold ischemia present during lung transplantation because of the metabolic changes induced by hypothermia. Similar levels of ischemia-reperfusion injury have been reported independent of hypothermia [
41] and mast cell activation has been reported to be unlinked to hypothermia [
42].
In summary, we found that mast cell frequency did not alter the severity of ischemia-reperfusion injury in mouse lungs. These findings suggest that strategies targeting mast cells for the prevention of PGD may not be effective when used independent of other injury pathways.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
JRG and GHC conceptualized the project and wrote the manuscript. JRG, XX, DMS, and MRL designed and performed experiments in mouse models. KDJ and XX performed and interpreted histologic analyses. All authors read and approved the final manuscript.