Characterization and Histological Localization of Multipotent Mesenchymal Stromal Cells in the Human Postnatal Thymus

Abstract

The aim of this work was to characterize multipotent mesenchymal stromal cells (MSCs) in the postnatal human thymus and to localize these MSCs in the organ. Adherent cells isolated from thymus samples were characterized by cell-surface antigen expression. This showed that adherent cells have a MSC profile as assessed by the expression of CD73 and CD105 markers and the lack of CD45 expression. These cells are able to differentiate in vitro into adipocytes, osteoblasts, and chondrocytes and to inhibit mixed lymphocyte reaction. This indicates that isolated cells have all of the characteristics of MSC. The fibroblast colony-forming unit (CFU-F) assay was used to determine their frequency in the postnatal thymus. This frequency was 60.9 +/- 14.8 CFU-F per 1 x 10(5) freshly isolated mononuclear cells. Moreover, taking advantage of CD34 and CD105 expression, immunohistological staining allowed us to localize MSC within interlobular trabeculae in close contact with the outer cortex. Polymerase chain reaction experiments indicated that thymic MSC expressed interleukin-7 and stromal cell-derived factor-1 messenger RNA. Overall, these results confirm previous findings of the presence in the adult human thymus of multipotent MSCs with a phenotype similar to adipose-derived adult stem cells. These results also show for the first time a histological localization of MSC in an organ. This suggests a possible role of thymic MSC in intrathymic differentiation.

Is CD34 truly a negative marker for mesenchymal stromal cells?

“…Furthermore, the use of CD34 as a positive MSC marker permitted the identification of MSCs as being intimately associated with the vasculature, and consequently our proposal that MSCs are vascular stem cells (VSCs) (2,3,10). Whether our hypothesis is correct or not, there is little doubt that MSCs are increasingly believed to reside near or within blood vessels, and CD34 is one of the most frequently used markers for this recognition (8,30,54-56). This of course does not exclude the possibilities that certain MSCs are not perivascularly associated and certain subsets of MSCs might indeed be CD34-.…”

“…Second, Fina et al (24) detected CD34 messenger RNA in cultured endothelial cells despite their lack of binding to CD34 antibodies, suggesting that in cell culture the CD34 protein is either downregulated or modified to a form that is nonreactive with CD34 antibodies. In more recent years this cell culture-associated loss of CD34 expression has continued to be observed in endothelial cells (25-27) and in MSCs of various tissue origin (1,7,18,28-30). In support of this notion, a recently accepted manuscript specifically mentions loss of CD34 expression in MSCs upon in vitro cultivation (31).…”

Human Thymus Mesenchymal Stromal Cells Augment Force Production in Self-Organized Cardiac Tissue

“…They found that thymus MSCs were CD73 positive, CD105 positive, and CD45 negative. Mouiseddine and associates [12] also found that 30% of CD73-positive and CD105-positive cells were also CD34 positive but lacked CD11b expression. Both groups of investigators demonstrated the multilineage potential of thymus MSCs into adipocytic, chondrocytic, and osteoblastic lineages as we had done here.…”

“…The presence, isolation, and culture of human thymus MSCs have been described previously [11–15]. Recently, the isolation of thymus MSCs from neonates undergoing cardiac surgery has been described [12, 15]. These two groups of investigators isolated thymus MSCs using collagenase digestion similar to our method and performed a detailed phenotypic characterization.…”

Mesenchymal Stromal Cell Therapy

“…3 Second, MSCs should express specific surface markers, such as CD73, CD90, and CD105 as well as being negative for CD11b, CD14, CD19, CD34, CD45, and HLA-DR. Third, MSCs should be able to exhibit trilineage differentiation into osteoblasts, adipocytes, and chondrocytes. 1,4,5 Contributions from multidisciplinary investigators have focused attention on the differentiation potential of MSCs into differed lineages to be successfully used for tissue regeneration. Mesenchymal stromal cells had originally been shown to differentiate toward cartilage, fat, and bone lineages.…”