Key Points Both immature and mature neutrophils differentiate into a previously unrecognized hybrid population when cultured with GM-CSF. The resulting hybrids exhibit dual phenotype and functionality of both neutrophils and dendritic cells.

Sustained low-grade inflammation mediated by non-resolving inflammatory monocytes has long been suspected in the pathogenesis of atherosclerosis; however, the molecular mechanisms responsible for the sustainment of non-resolving inflammatory monocytes during atherosclerosis are poorly understood. Here we observe that subclinical endotoxemia, often seen in humans with chronic inflammation, aggravates murine atherosclerosis through programming monocytes into a non-resolving inflammatory state with elevated Ly6C, CCR5, MCP-1 and reduced SR-B1. The sustainment of inflammatory monocytes is due to the disruption of homeostatic tolerance through the elevation of miR-24 and reduction of the key negative-feedback regulator IRAK-M. miR-24 reduces the levels of Smad4 required for the expression of IRAK-M and also downregulates key lipid-processing molecule SR-B1. IRAK-M deficiency in turn leads to elevated miR-24 levels, sustains disruption of monocyte homeostasis and aggravates atherosclerosis. Our data define an integrated feedback circuit in monocytes and its disruption may lead to non-resolving low-grade inflammation conducive to atherosclerosis.

Here we show that iNOS-deficient mice display enhanced classically activated M1 macrophage polarization without major effects on alternatively activated M2 macrophages. eNOS and nNOS mutant mice show comparable M1 macrophage polarization compared with wild-type control mice. Addition of N6-(1-iminoethyl)-L-lysine dihydrochloride, an iNOS inhibitor, significantly enhances M1 macrophage polarization while S-nitroso-N-acetylpenicillamine, a NO donor, suppresses M1 macrophage polarization. NO derived from iNOS mediates nitration of tyrosine residues in IRF5 protein, leading to the suppression of IRF5-targeted M1 macrophage signature gene activation. Computational analyses corroborate a circuit that fine-tunes the expression of IL-12 by iNOS in macrophages, potentially enabling versatile responses based on changing microenvironments. Finally, studies of an experimental model of endotoxin shock show that iNOS deficiency results in more severe inflammation with an enhanced M1 macrophage activation phenotype. These results suggest that NO derived from iNOS in activated macrophages suppresses M1 macrophage polarization.

Key Points Neutrophils differentiate into neutrophil–dendritic cell hybrids upon recruitment to sites of inflammation or infection. Hybrids play dual protective roles against bacterial infection by rapidly clearing bacteria and presenting bacterial antigens to T cells.

1,25-Dihydroxyvitamin D3 [1,25(OH)2D3] has many noncalcemic actions that rest on inhibition of proliferation and promotion of differentiation in malignant and normal cell types. 1,25(OH)2D3 stimulates osteoblast differentiation of human marrow stromal cells (hMSCs), but little is known about the effects of 25-hydroxyvitamin D3 [25(OH)D3] on these cells. Recent evidence shows that hMSCs participate in vitamin D metabolism and can activate 25(OH)D3 by CYP27B1/1α-hydroxylase. These studies test the hypothesis that antiproliferative and prodifferentiation effects of 25(OH)D3 in hMSCs depend on CYP27B1. We studied hMSCs that constitutively express high (hMSCshi-1α) or low (hMSCslo-1α) levels of CYP27B1 with equivalent expression of CYP24A1 and vitamin D receptor. In hMSCshi-1α, 25(OH)D3 reduced proliferation, downregulated proliferating cell nuclear antigen (PCNA), upregulated p21Waf1/Cip1, and decreased cyclin D1. Unlike 1,25(OH)2D3, the antiapoptotic effects of 25(OH)D3 on Bax and Bcl-2 were blocked by the P450 inhibitor ketoconazole. The antiproliferative effects of 25(OH)D3 in hMSCshi-1α and of 1,25(OH)2D3 in both samples of hMSCs were explained by cell cycle arrest, not by increased apoptosis. Stimulation of osteoblast differentiation in hMSCshi-1α by 25(OH)D3 was prevented by ketoconazole and upon transfection with CYP27B1 siRNA. These data indicate that CYP27B1 is required for 25(OH)D3's action in hMSCs. Three lines of evidence indicate that CYP27B1 is required for the antiproliferative and prodifferentiation effects of 25(OH)D3 on hMSCs: Those effects were not seen (1) in hMSCs with low constitutive expression of CYP27B1, (2) in hMSCs treated with ketoconazole, and (3) in hMSCs in which CYP27B1 expression was silenced. Osteoblast differentiation and skeletal homeostasis may be regulated by autocrine/paracrine actions of 25(OH)D3 in hMSCs. © 2011 American Society for Bone and Mineral Research.