Changes in bone marrow resident monocyte gene expression upon induction of ischemia

There is considerable interest in the potential of cell-based approaches to mediate therapeutic angiogenesis for acute and chronic vascular syndromes. Using a mouse model of HLI, we showed previously that adoptive transfer of a small number of donor monocytes enhanced revascularization significantly. Herein, we provide data suggesting that the BM resident monocytes sense systemic signals that influence their future functional capacity. Specifically, following induction of distant ischemia, the angiogenic capacity of BM resident monocytes is reduced markedly. We provide evidence that G-CSF and IL-6 represent such conditioning signals. Systemic levels of G-CSF and IL-6 are increased significantly following induction of HLI. Accordingly, BM resident monocytes from ischemic mice exhibited increased pSTAT3 and STAT3 target gene expression. Finally, G-CSFR(-/-) and IL-6(-/-) mice were resistant to the deleterious effects of ischemic conditioning on monocyte angiogenic potential. RNA expression profiling suggested that ischemia-conditioned monocytes in the BM up-regulate the well-described M2 polarization markers Chi3l4 and Lrg1. Consistent with this observation, M2-skewed monocytes from SHIP(-/-) mice also had impaired angiogenic capacity. Collectively, these data show that G-CSF and IL-6 provide signals that determine the angiogenic potential of BM resident monocytes. Bone marrow was harvested from CX3CR1+/GFP heterozygous mice and CX3CR1-GFP+ monocytes were sorted at baseline and at 24 hours post-induction of hindlimb ischemia by femoral artery ligation. Paired samples from three independent experiments were analyzed.