Trafficking of mesenchymal stem cells

In adult life mesenchymal stem cells (MSCs) reside primarily in the bone marrow and are defined according to their ability to self-renew and differentiate into tissues of mesodermal origin. Due to their immuno-modulatory properties and ability to form cartilage and bone, MSCs have clinical potential, for the treatment of autoimmune diseases and tissue repair. This project determines the chemokine receptor profile on murine bone marrow MSCs at early and late passage and on human MSCs derived from a range of fetal tissues including fetal blood, bone marrow, amniotic fluid and placenta. The overwhelming result from this analysis is the consistency across species and tissue source with respect to chemokine receptor profiles. In addition it is clear that expression of specific chemokine receptors defines sub-populations of MSCs. Currently, clinical trials using MSCs have relied on continued in vitro culture in order to obtain sufficient numbers for treatment. Here, MSCs have been shown to lose external chemokine receptor expression and associated chemotactic ability, whilst growing in size upon continued culture. All cultured MSCs investigated in this thesis were shown to be a heterogeneous population of stem cells and progenitors that contained ‘true’ MSCs within its number. This thesis investigates a pharmacological approach to mobilise endogenous MSCs from the bone marrow, increasing their numbers in the blood. It has previously been reported that administration of VEGF-A over 4 days followed by a single dose with a CXCR4 antagonist (AMD3100) causes selective mobilisation of MSCs into blood. The VEGF biology of this response has been interrogated. MSCs were shown to express high levels of VEGFR-1 and lower levels of VEGFR-2 on the cell surface but do not express VEGFR-3. By blocking VEGFR-1 with mAbs during VEGF-A165 treatment, a ten-fold increase in MSC mobilisation in response to AMD3100 was recorded, while treating with VEGFR-2 blocking mAbs had no effect. Using VEGF isoforms specific for VEGFR-1 and VEGFR-2 (PlGF and VEGF-E respectively), it was determined that MSC mobilisation was dependant on activation of VEGFR-2 and not VEGFR-1. PαS cells are a subset of MSCs found in the murine bone marrow that are PDGFRα+, Sca-1+, CD45-, Ter119-. Further characterisation of mobilised mMSCs by flow cytometric analysis of PαS cells, now provides a way to investigate the biology of MSCs, both in their steady state in vivo and in models of injury and inflammation. Molecular mechanisms lying downstream of VEGFR-2 have been explored and it has been shown that MMPs play a critical role in mobilisation. The use of drugs to mobilise MSCs into the blood may provide a cost effective, non-invasive treatment to promote tissue repair