Intravascular donor monocytes play a central role in lung transplant ischaemia-reperfusion injury

Abstract

RationalePrimary graft dysfunction in lung transplant recipients derives from the initial, largely leukocyte-dependent, ischaemia-reperfusion injury. Intravascular lung-marginated monocytes have been shown to play key roles in experimental acute lung injury, but their contribution to lung ischaemia-reperfusion injury post transplantation is unknown.ObjectiveTo define the role of donor intravascular monocytes in lung transplant-related acute lung injury and primary graft dysfunction.MethodsIsolated perfused C57BL/6 murine lungs were subjected to warm ischaemia (2 hours) and reperfusion (2 hours) under normoxic conditions. Monocyte retention, activation phenotype and the effects of their depletion by intravenous clodronate-liposome treatment on lung inflammation and injury were determined. In human donor lung transplant samples, the presence and activation phenotype of monocytic cells (low side scatter, 27E10+, CD14+, HLA-DR+, CCR2+) were evaluated by flow cytometry and compared with post-implantation lung function.ResultsIn mouse lungs following ischaemia-reperfusion, substantial numbers of lung-marginated monocytes remained within the pulmonary microvasculature, with reduced L-selectin and increased CD86 expression indicating their activation. Monocyte depletion resulted in reductions in lung wet:dry ratios, bronchoalveolar lavage fluid protein, and perfusate levels of RAGE, MIP-2 and KC, while monocyte repletion resulted in a partial restoration of the injury. In human lungs, correlations were observed between pre-implantation donor monocyte numbers/their CD86 and TREM-1 expression and post-implantation lung dysfunction at 48 and 72 hours.ConclusionsThese results indicate that lung-marginated intravascular monocytes are retained as a ‘passenger’ leukocyte population during lung transplantation, and play a key role in the development of transplant-associated ischaemia-reperfusion injury.

Donor intravascular monocyte trafficking: a potential therapeutic target for primary graft dysfunction following lung transplantation?

“…Wet:dry lung ratios and bronchoalevolar lavage protein concentrations, both biomarkers of pulmonary inflammation, were significantly reduced in liposomal clodronate-treated mice following ischaemic reperfusion. Consistent with this, MIP-2 (CXCL2), a murine functional homologue of the powerful neutrophil chemoattractant interleukin 8 (CXCL8)18 and shown to recruit neutrophils in experimental PGD,10 was significantly reduced post reperfusion in the monocyte-depleted group 16. MCP-1 (CCL2) is an important chemokine for monocyte adhesion to vascular endothelium during trafficking to inflammatory sites 19.…”

“…Finally, Tatham and colleagues analysed the presence of donor monocytes and granulocytes in lung tissue from a small cohort of 13 human lungs prior to implantation 16. Consistent with their murine observations, despite anterograde perfusion and retrograde perfusion of 5 L of perfusate, high numbers of donor monocytes and neutrophils were still found in tissue.…”

“…MCP-1 (CCL2) is an important chemokine for monocyte adhesion to vascular endothelium during trafficking to inflammatory sites 19. Consistent with lack of consumption by monocytes,20 this chemokine was significantly increased in the liposomal clodronate group 16. The authors also demonstrated a reduction in the epithelial cell injury marker RAGE in the monocyte-deplete mice, highlighting a potential early role for monocytes in ischaemia reperfusion, and hence PGD development.…”

“…On contact with vascular endothelium, CD62L is downregulated and as part of monocyte maturation/differentiation, markers, including the costimulatory molecule CD86, are increased. Tatham and colleagues demonstrated that following ischaemia reperfusion there was activation of intravascular Ly-6C High monocytes with reduced L-selectin and increased CD86 expression 16. Ly-6C Low monocytes which do not express high levels of L-selectin also demonstrated increased CD86 expression following ischaemia reperfusion.…”

“…Tatham and colleagues then selectively depleted intravascular monocytes using intravenous liposomal clodronate injection 24 hours prior to experimental lung perfusion in order to determine the effect of these vascular passenger cells on ischaemia reperfusion injury 16. This method significantly reduced Ly-6C High and Ly-6C Low monocytes in the vascular compartment, while vascular neutrophils, interstitial macrophages and alveolar macrophage frequencies were unaltered.…”