Identification of a monocyte-predisposed hierarchy of hematopoietic progenitor cells in the adventitia of postnatal murine aorta

Hox genes are involved in vascular wall-resident multipotent stem cell differentiation into smooth muscle cells

Human vascular wall-resident CD44+ multipotent stem cells (VW-MPSCs) within the vascular adventitia are capable to differentiate into pericytes and smooth muscle cells (SMC). This study demonstrates HOX-dependent differentiation of CD44(+) VW-MPSCs into SMC that involves epigenetic modification of transgelin as a down-stream regulated gene. First, HOXB7, HOXC6 and HOXC8 were identified to be differentially expressed in VW-MPSCs as compared to terminal differentiated human aortic SMC, endothelial cells and undifferentiated pluripotent embryonic stem cells. Silencing these HOX genes in VW-MPSCs significantly reduced their sprouting capacity and increased expression of the SMC markers transgelin and calponin and the histone gene histone H1. Furthermore, the methylation pattern of the TAGLN promoter was altered. In summary, our findings suggest a role for certain HOX genes in regulating differentiation of human VW-MPSC into SMCs that involves epigenetic mechanisms. This is critical for understanding VW-MPSC-dependent vascular disease processes such as neointima formation and tumor vascularization.

JMML patient-derived iPSCs induce new hypotheses

In this issue of Blood, Gandre-Babbe et al have, in part, overcome the obstacle of validating the molecular underpinnings of juvenile myelomonocytic leukemia (JMML) with the generation of induced pluripotent stem cells (iPSCs) from individuals with JMML.

Local proliferation dominates lesional macrophage accumulation in atherosclerosis

During the inflammatory response that drives atherogenesis, macrophages accumulate progressively in the expanding arterial wall^1,2. The observation that circulating monocytes give rise to lesional macrophages^3–9 has reinforced the concept that monocyte infiltration dictates macrophage build-up. Recent work indicates, however, that macrophages do not depend on monocytes in some inflammatory contexts¹⁰. We therefore revisited the mechanism of macrophage accumulation in atherosclerosis. We show that murine atherosclerotic lesions experience a surprisingly rapid, 4-week, cell turnover. Replenishment of macrophages in these experimental atheromata depends predominantly on local macrophage proliferation rather than monocyte influx. The microenvironment orchestrates macrophage proliferation via the involvement of scavenger receptor (SR)-A. Our study reveals macrophage proliferation as a key event in atherosclerosis and identifies macrophage self-renewal as a therapeutic target for cardiovascular disease.

Making a difference: monocyte heterogeneity in cardiovascular disease

Monocytes are frequently described as bone marrow-derived precursors of macrophages. Although many studies support this view, we now appreciate that monocytes neither develop exclusively in the bone marrow nor give rise to all macrophages and dendritic cells. In addition to differentiating to specific leukocyte populations, monocytes, as monocytes, are functionally and ontogenically heterogeneous. In this review we will focus on the development and activity of monocytes and their subsets in mice (Ly-6 C(high/low)) and humans (CD14(+/dim/-) CD16(+/-)) in the context of atherosclerosis and its complications.

Tenascin-C deficiency in apo E-/- mouse increases eotaxin levels: implications for atherosclerosis

AIM

To investigate the potential role of inflammatory cytokines in apo E-/- mouse in response to deletion of Tenascin-C (TNC) gene.

METHODS AND RESULTS

We used antibody array and ELISA to compare the profile of circulating inflammatory cytokines in apo E-/- mice and apo E-/- TNC-/- double knockout mice. In addition, tissue culture studies were performed to investigate the activity of cells from each mouse genotype in vitro. Cytokine array analysis and subsequent ELISA showed that circulating eotaxin levels were selectively and markedly increased in response to TNC gene deletion in apo E-/- mice. In addition, considerable variation was noted in the circulating level of eotaxin among the control apo E-/- mouse group. Inbreeding of apo E-/- mice with high or low levels of plasma eotaxin showed that the level of eotaxin per se determines the extent of atherosclerosis in this mouse genotype. While endothelial cells from apo E-/- mice had low level of eotaxin expression, cells derived from apo E-/- TNC-/- mice expressed a high level of eotaxin. Transient transfection of eotaxin promoter-reporter constructs revealed that eotaxin expression is regulated at the transcriptional level by TNC. Histochemical analysis of aortic sections revealed the massive accumulation of mast cells in the adventitia of double KO mice lesions whereas no such accumulation was detected in the control group. Plasma from the apo E-/- TNC-/- mice markedly stimulated mast cell migration whereas plasma from the apo E-/- mice had no such effect.

CONCLUSION

These observations support the emerging hypothesis that TNC expression controls eotaxin level in apo E-/- mice and that this chemokine plays a key role in the development of atherosclerosis.

Characterization of human adipose tissue-resident hematopoietic cell populations reveals a novel macrophage subpopulation with CD34 expression and mesenchymal multipotency

Adipose tissue (AT) is composed of mature adipocytes and stromal vascular fraction (SVF) cells, including adipose stem/stromal cells (ASCs). We characterized hematopoietic cells residing in human nonobese AT by analyzing the SVF isolated from human lipoaspirates and peripheral blood (PB). Flow cytometry revealed that AT-resident hematopoietic cells consisted of AT-resident macrophages (ATMs) or lymphocytes with a negligible number of granulocytes. AT-resident lymphocytes were composed of helper T cells and natural killer cells. Almost no B cells and few cytotoxic T cells were observed in nonobese AT. More than 90% of ATMs were M2 state CD206(+) macrophages (CD45(+)/CD14(+)) that were located in the periendothelium or interstitial spaces between adipocytes. We also discovered a novel subpopulation of CD34(+)/CD206(+) ATMs (11.1% of CD206(+)ATMs) that localized in the perivascular region. Microarray of noncultured CD34(+)/CD206(+) ATMs, CD34(-)/CD206(+) ATMs, CD45(-)/CD31(-)/CD34(+) ASCs, and PB-derived circulating monocytes revealed that CD34(+)/CD206(+) ATMs shared characteristics with ASCs and circulating monocytes. Unlike CD34(-)/CD206(+) ATMs, CD34(+)/CD206(+) ATMs could grow in adherent culture and were capable of differentiating into multiple mesenchymal (adipogenic, osteogenic, and chondrogenic) lineages, similar to ASCs. CD34(+)/CD206(+) ATMs grew rapidly and lost expression of CD45, CD14, and CD206 by passage 3, which resulted in a similar expression profile to ASCs. Thus, this novel ATM subpopulation (CD45(+)/CD14(+)/CD34(+)/CD206(+)) showed distinct biological properties from other ATMs and circulating monocytes/macrophages. The CD34(+)/CD206(+) ATMs possessed characteristics similar to ASCs, including adherence, localization, morphology, and mesenchymal multipotency. This AT-resident subpopulation may have migrated from the bone marrow and may be important to tissue maintenance and remolding.

The vascular stem cell niche

Stem cells in adult organs reside in specialized niches that regulate their proliferation and differentiation. Investigations during the last few years have unveiled a regulatory role for blood vessels in these microenvironments. Mesenchymal stem cells (MSCs) are located surrounding capillaries in a variety of tissues and have the capacity to differentiate into different mesodermal lineages. Angiogenic progenitor cells have also been found in the adventitial layer of large vessels. In the bone marrow, endothelial cells control hematopoietic stem cell (HSC) release, and in the brain, blood vessels regulate neural stem cell (NSC) self-renewal and neurogenesis. Similarly, perivascular progenitor cells have also been found in the heart. This intimate connection between stem cells and the vasculature contributes to tissue homeostasis and repair. In this review, we focus on the regulation of stem and progenitor cells in different adult niches by blood vessels and the few mechanisms that are known to mediate this interaction.

Myocardial infarction accelerates atherosclerosis

During progression of atherosclerosis, myeloid cells destabilize lipid-rich plaque in the arterial wall and cause its rupture, thus triggering myocardial infarction and stroke. Survivors of acute coronary syndromes have a high risk of recurrent events for unknown reasons. Here we show that the systemic response to ischemic injury aggravates chronic atherosclerosis. After myocardial infarction or stroke, apoE^−/− mice developed larger atherosclerotic lesions with a more advanced morphology. This disease acceleration persisted over many weeks and was associated with markedly increased monocyte recruitment. When seeking the source of surplus monocytes in plaque, we found that myocardial infarction liberated hematopoietic stem and progenitor cells from bone marrow niches via sympathetic nervous system signaling. The progenitors then seeded the spleen yielding a sustained boost in monocyte production. These observations provide new mechanistic insight into atherogenesis and provide a novel therapeutic opportunity to mitigate disease progression.