The Role And Mechanism Of Bone Marrow-derived Ly6C~- Macrophages In AKI-CKD Progression

Background.Macrophages play an important role in renal injury and repair after acute kidney injury（AKI）and the subsequent chronic kidney disease（CKD）that often results.However,as macrophages have a high degree of plasticity and heterogeneity,the function（s）of macrophage subtypes in AKI-to-CKD progression are not fully understood.Methods and Results.We focused on Ly6C^- macrophages,which are derived from the embryonic yolk sac and post-development become resident in the kidneys.We found that C-C chemokine receptor type 2（CCR2）deficiency,which blocks the migration of Ly6C+ macrophages from the bone marrow to the sites of injury,alleviated ischemia-induced AKI in mice.Unexpectedly,though,CCR2 deficiency worsened the subsequent renal fibrosis,which was marked by notable intra-renal infiltration of Ly6C^- macrophages.These Ly6C^- macrophages were greater in number in both the acute and chronic phases after ischemia reperfusion（I/R）in kidneys of wild type（WT）mice,and we showed them to be derived from the bone marrow by bone marrow chimerism.In the second part,we used the macrophage-specific ablation agent-clodronate liposomes（CLs）to deplete Ly6C-macrophages infiltrated in CCR2^-/- mice or in WT mice after I/R.The reduction of fibrosis after CLs injection was consistent with the decrease in Ly6C^-macrophage infiltration.Further,we specifically depleted Ly6C^- macrophages in WT mice using Bardoxolone methyl（BARD）,indicating that depleting Ly6C^- macrophages has a renal protective effect.On the contrary,adoptive transfer of Ly6C^- macrophages from injured kidneys of WT mice into immune-deficient mice was sufficient to induce renal injury and fibrosis.Transcriptome sequencing of Ly6C^- macrophages from injured kidneys revealed that they secreted various cytokines and growth factors,which were associated with the transdifferentiation of fibroblasts into myofibroblasts.This transdifferentiation effect was further supported by in vitro studies showing that Ly6C^- macrophages induced the secretion of extracellular matrix proteins from co-cultured fibroblasts.Conclusion.In this study,we revealed that the intra-renal Ly6C^- macrophages,which primarily originate from the bone marrow and peripheral circulation,were the most abundant subpopulation after I/R,especially in the chronic phase.These Ly6C^- macrophages accelerated kidney injury,activated myofibroblasts and in turn exacerbated the ischemia-induced renal fibrosis.