Quest for Circulating Mesenchymal Stem Cells in Man.

Abstract

Mesenchymal stem cells (MSC) are present in the bone marrow, from where they are thought to migrate via the bloodstream to sites of injury. On the other hand, virtually all tissues contain resident MSC that may contribute to local regenerative and immunomodulatory processes, thereby hypothetically pre-empting the need for recruitment of MSC via the bloodstream. Although there is some indication from animal models, the question remains whether there is solid evidence for the mobilization and migration of MSC in human. In the present study we investigated whether circulating MSC were present in the peripheral blood of healthy individuals and patients with organ injury. We were unable to detect MSC in the blood of healthy individuals by flow cytometry and cell culture techniques. We then analyzed the presence of MSC in the blood of patients with end-stage renal disease (n=10), end-stage liver disease (n=10) and in heart transplant patients with biopsy proven rejection (n=8), by culturing of mononuclear cells under MSC-supporting culture conditions. In none of these patients MSC were identified in the blood. In the stromal vascular fraction of adipose tissue and in liver transplant perfusion fluid we were able to detect MSC, indicating that the methods used enabled the detection of MSC. The conclusion of this study is that MSC are not detectable in the circulation in patients with injured solid organs and during aggressive immune responses.