No abstract

Objective: Monocytes, which play an important role in arteriogenesis, can build immunologic memory by a functional reprogramming that modifies their response to a second challenge. This process, called trained immunity, is evoked by insults that shift monocyte metabolism, increasing HIF (hypoxia-inducible factor)-1α levels. Since ischemia enhances HIF-1α, we evaluate whether ischemia can lead to a functional reprogramming of monocytes, which would contribute to arteriogenesis after hindlimb ischemia. Methods and Results: Mice exposed to ischemia by 24 hours of femoral artery occlusion (24 hours trained) or sham were subjected to hindlimb ischemia one week later; the 24-hour trained mice showed significant improvement in blood flow recovery and arteriogenesis after hindlimb ischemia. Adoptive transfer using bone marrow-derived monocytes (BM-Mono) from 24-hour trained or sham donor mice, demonstrated that recipients subjected to hindlimb ischemia who received 24 hours ischemic-trained monocytes had remarkable blood flow recovery and arteriogenesis. Further, ischemic-trained BM-Mono had increased HIF-1α and GLUT-1 gene expression during femoral artery occlusion. Circulating cytokines and GLUT-1 were also upregulated during femoral artery occlusion.Transcriptomic analysis and confirmatory qPCR performed in 24 hours trained and sham BM-Mono revealed that among the 15 top differentially expressed genes, 4 were involved in lipid metabolism in the ischemic-trained monocytes. Lipidomic analysis confirmed that ischemia training altered the cholesterol metabolism of these monocytes. Further, several histone-modifying epigenetic enzymes measured by qPCR were altered in mouse BM-Mono exposed to 24 hours hypoxia. Conclusions: Ischemia training in BM-Mono leads to a unique gene profile and improves blood flow and arteriogenesis after hindlimb ischemia.

Introduction: Recent studies have shown that innate response can build immunological memory. This process called “trained immunity” is an epigenetic reprogramming of the monocytes driven by a metabolism shift towards glycolysis. Hypothesis: Since HIF-1a is a major player in monocytes activation and also targets some epigenetic enzymes, we hypothesize that hypoxia/ischemia can lead to “trained immunity” improving arteriogenesis in a mouse model of hindlimb ischemia. Methods: Mice subjected to a unilateral single ischemia insult for 24hours (24h group: femoral artery ligation for 24 h) or by ischemia reperfusion cycles (Cycle group: ischemia-reperfusion in 4 cycles of 5 minutes) or non-ischemia (sham group), were used as donors in the monocyte transfer experiment. Recipient mice underwent to permanent hindlimb ischemia one day prior the tail vein injection of bone marrow (BM) monocytes from the ischemic leg. Laser Doppler assessed blood flow recovery before and after hindlimb ischemia. Arteriogenesis was quantified on recipient mice by assessing the diameter of gracilis collaterals. A 24h group monocytes from ischemia and non-ischemia leg was used as donors to test the systemic vs. local effect of the ischemic-trained monocytes. Lin- cells were used as a control. Cultured human monocytes were subjected to 24h hypoxia and gene expression for epigenetic enzymes was performed by real time rt-PCR Results: Blood flow recovery in recipients who received ischemic-trained monocytes (24h and cycle) improved overtime compared to sham. Arteriogenesis was significant greater in 24h group compared to sham. Surprisingly, the improvement in blood flow recovery was abolished using combined monocytes from ischemic and non-sichemic, suggesting that ischemia training has a local effect in the monocytes. The Lin- experiment confirmed that the ischemia training is monocyte specific. Moreover, hypoxia regulated epigenetic enzymes responsible for histones methylation in human monocytes. Conclusions: Monocytes can be trained by previous ischemia/hypoxia insult and improve arteriogenesis during hindlimb ischemia. The molecular mechanisms that lead the trained immunity by hypoxia still unknown, however epigenetic modifications might play a role in this process.