Inflammation resolution is an integral part of liver ischemia reperfusion injury (IRI), which affects not only the severity of tissue damage, but more importantly its long-term outcome. Currently, little is known about the inflammation resolution in liver IRI. In a murine liver partial warm ischemia (90m ischemia in cephalad lobes) model, we dissected the role of macrophages in liver recovery from IRI. H/E staining of ischemic liver sections after various length of reperfusion showed that the repair of necrotic liver parenchyma lasted for at least 7 days despite sALT levels declining rapidly to near baseline levels already after 24h post-reperfusion. Quantitative RT-PCR analysis revealed that the decline of inflammatory gene expression with reperfusion time was accompanied by increased expression of macrophage efferocytosis receptor genes (MerTK and TIM4) in IR-livers. FACS analysis of liver non-parenchymal cells detected a second wave of macrophage infiltration at day 3 post-reperfusion with the Gr1^– phenotype (vs. the first wave of Gr1⁺ infiltration at 6h). Liver inflammation resolution was characterized by the complete clearance of infiltrating neutrophils and macrophages with a recovered KC (Gr1^–) pool at day 7 post-reperfusion. To test the functional role of Gr1^– macrophages in inflammation resolution, we administered clodronate liposomes (CL) at 24h, followed by two more doses at day 3 and 5 post-reperfusion. This regimen allows the development of liver inflammatory injury (peaked at 6h), and eliminated liver macrophages, particularly KCs, only after day +3. As a result, the late upregulation of macrophage efferocytosis receptor genes in IR-livers was abrogated. No de novo tissue damage or inflammation were induced in IR-livers by the CL treatment. However, liver inflammation resolution was severely disrupted: IR livers at day +7 failed to repair necrotic parenchyma (H/E) or clear infiltrating neutrophils and macrophages (FACS). Moreover, liver pro-fibrotic gene (α-SMA, COL1A1/2) expression, which was only transiently induced at day +3 in control mice, was elevated continuously at day +3 and +7 and at significantly higher levels in the CL-treated hosts. Conclusion: liver Gr1^– macrophages are critically important for the resolution of inflammation and restoration of tissue homeostasis after IRI, possibly via efferocytosis.

CITATION INFORMATION: Ni M., Wang H., Busuttil R., Kupiec-Weglinski J., Wang X., Zhai Y. Gr1^– Macrophages Play Critical Roles in Liver Inflammation Resolution after IRI via Efferocytosis Am J Transplant. 2017;17 (suppl 3).

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