The tunica adventitia of human arteries and veins as a source of mesenchymal stem cells

Abstract

We previously demonstrated that human pericytes, which encircle capillaries and microvessels, give rise in culture to genuine mesenchymal stem cells (MSCs). This raised the question as to whether all MSC are derived from pericytes. Pericytes and other cells defined on differential expression of CD34, CD31, and CD146 were sorted from the stromal vascular fraction of human white adipose tissue. Besides pericytes, CD34+ CD31- CD146- CD45- cells, which reside in the outmost layer of blood vessels, the tunica adventitia, natively expressed MSC markers and gave rise in culture to clonogenic multipotent progenitors identical to standard bone marrow-derived MSC. Despite common MSC features and developmental properties, adventitial cells and pericytes retain distinct phenotypes and genotypes through culture. However, in the presence of growth factors involved in vascular remodeling, adventitial cells acquire a pericytes-like phenotype. In conclusion, we demonstrate the co-existence of 2 separate perivascular MSC progenitors: pericytes in capillaries and microvessels and adventitial cells around larger vessels.

FIG. 1.

Purification of nonpericyte MSC-like cells from human adipose tissue. (a) After exclusion of DAPI+ dead cells and CD45+ hematopoietic cells, pericytes were isolated as CD146+CD34- cells (gray gate) as previously described. MSC-like cells could also be grown from the mixed population of nonpericyte cells differentially expressing CD34 and CD146 (black gate). (b) The following nonpericyte cell populations were then purified: CD34+CD146-, CD34+CD146+ and cultured in MSC conditions. The outgrowth of MSC-like cells was observed only after culture of CD34+CD146- cells. (c) Analysis of CD31 expression on gated CD34+CD146- cells (left dot plot) and CD34+CD146+ cells (right dot plot). CD31 was detected in only endothelial CD34+CD146+ cells, whereas no endothelial cells were detected within CD34+CD146- cells. (d) RT-PCR analysis was performed to check the purity of sorted CD34+CD31-CD146- cells. CD34 but no CD31, CD144, and CD146 mRNA was detected, thus excluding a possible contamination by endothelial cells or pericytes. C, positive control (unfractionated stromal vascular fraction); S, freshly sorted CD34+CD146- cells. The same results were observed in 3 distinct experiments. MSC, mesenchymal stem cell.

FIG. 2.

CD34+CD146- cells are present in multiple organs. Using the same sorting strategy described for the isolation of MSC-like cells from WAT, CD34+D146- cells were detected in other tissues such as fetal muscle, lung, and bone marrow (from left to right). MSC-like cells were obtained from CD34+CD146- cells regardless of the tissue of origin. WAT, white adipose tissue.

FIG. 3.

Cultured CD34+CD146- cells are clonogenic multipotent progenitors similar to MSC Representative characterization of a clone at late passage of culture (p10). The phenotype and developmental potential are maintained throughout culture with no significant change after long-term culture. (a) Polyclonal cultured CD34+CD146- cells were cloned by limiting dilution. Only a minority of the single-cell clones could be expanded for further characterization. (b) Single cell-derived clones homogeneously express typical MSC markers (CD105, CD44, CD90, and CD73). Red line: unstained negative control sample; blue line: stained sample. (c–e) Single-cell clones can each differentiate into several mesodermal cell lineages when cultured under specific conditions. Oil red O, alizarin red and alcian blue staining reveal differentiation into adipocytes (c), osteocytes (d), and chondrocytes (e), respectively. Color images available online at www.liebertonline.com/scd

FIG. 4.

In situ localization of CD34+CD31-CD146- cells. Immunohistochemistry was performed on hWAT frozen sections. In vessels with a lumen diameter greater than 50 μm, CD31 is specifically expressed by endothelial cells in the tunica intima (a, 20×), whereas CD34 is expressed by 2 distinct populations of differently located cells (b, 20×). As shown by merging the 2 stainings, CD34+CD31+ cells are endothelial, whereas a subset of CD34+CD31- cells is located in the outer layer of the blood vessel (c, 20×). Double staining of CD34 and pericyte/smooth muscle cell markers CD146 (d, 20×) or α-SMA (e, 20×) reveals that nonendothelial CD34+ cells are located in the tunica adventitia surrounding smooth muscle cells. Multi-color staining was performed with antibodies against CD34 (red), CD146 (purple), and α-SMA (green) to identify the 3 subsets of cells constituting the vessel wall (f, 60×). Confocal microscopy confirmed that CD34-positive cells residing in the vascular adventitia do not co-express CD146 or α-SMA. In contrast to adventitial cells, smooth muscle cells surrounding endothelial cells express both CD146 and α-SMA. Endothelial cells are identified by their luminal localization and co-expression of CD34 and CD146 (f, 60×). Color images available online at www.liebertonline.com/scd

FIG. 5.

Adventitial cells express typical MSC markers before ex vivo expansion. (a) Flow cytometry analysis was performed on fresh stromal vascular fraction before culture (n=3). The expression of hallmark MSC markers was detected within the subset of adventitial cells, previously defined as CD34+DAPI-CD45-CD31-CD146- (green box). Adventitial cells homogeneously express all the MSC markers tested. Red histogram line: unstained cells. Blue histogram line: stained cells. (b–e) Serial staining of an adipose tissue artery with antibodies to α-SMA and MSC markers. All markers tested, CD90 (b), CD44 (c), CD73 (d), and CD105 (e), are co-localized in the cell layer surrounding α-SMA+smooth muscle cells, thus confirming that CD34+CD31-CD146- multipotent progenitors reside in the tunica adventitia. As expected, CD105 and CD73 were also expressed in endothelial cells (d, e). Color images available online at www.liebertonline.com/scd

FIG. 6.

In vitro characterization of long-term cultured adventitial cells (a) After 12 weeks of culture, adventitial cells show a significantly lower population doubling time (PDT) compared with pericytes (*P<0.05) (b, c) At difference with pericytes, cultured adventitial cells never express CD146. Red line: unstained negative control sample; blue line: stained sample. (d–q) As previously reported, cultured pericytes express CD146 (d), α-SMA (e), PDGFR-β (f), and NG2 (g). None of these markers is expressed by cultured adventitial cells (i–l). The 2 cell types only share expression of vimentin (h, m). When exposed to 0.1 μM angiopoietin 2 (ANGPT 2) for 72 h, adventitial cells acquired all pericyte markers: CD146, α-SMA, PDGFR-β, and NG2 (n–q). Color images available online at www.liebertonline.com/scd