Femke Mathot

Chapter 2 30 For clinical applications, this media is supplemented with platelet lysate (PL) to obtain zoonotic free clinical grade MSCs. We note that although Fetal Bovine Serum (FBS) suffices for research applications, the cell populations that emerge upon proliferative expansion in PL versus FBS may differ in their molecular properties and these differences may result in functional differences in cell therapy applications. Upon centrifugation, low density adipocytes emerge at the top of the tube, while stromal cells from the vasculature of fat tissue are collected as a pellet. The pellet can be re-suspended in MEM containing growth supplements (e.g., PL or FBS) and antibiotics (e.g., penicillin/streptomycin solution) for subsequent culture as adherent MSCs. 7, 11 Overall, deriving MSCs from adipose tissue is well-described and technically simple to perform making it advantageous for clinical applications. MECHANISM OF ACTION There are two major hypotheses on how MSCs establish tissue regeneration. The first proposes that the exogenously administered MSCs have a structural function in tissue injury and thus differentiate in vivo into tissue that requires repair. Growth factors and other paracrine molecules produced by the surrounding tissue stimulate the MSCs to differentiate into the requisite cell type. Supportive studies at best only infer this mechanism. Orbay and colleagues labeled undifferentiated MSCs and reported that they were still detected after 3 months and expressed Schwann cell proteins in a rat-model. 12 Tomita and colleagues reported in a rat-model that a small fraction of their GFP-labeled MSCs were still present after 8 weeks and expressed myelin protein, suggesting that some trans-differentiation into Schwann cells occurred. 4 In this model, MSCs may be able to both repair and replace injured tissue. However, to date this model for MSC function remains largely untested. While there is no question that cellular differentiation is required for neuronal development, it is not clear whether therapies relying on MSCs replicate the normal differentiation of Schwann cells. The second hypothesis for MSCs has more recently emerged and this concept poses that MSCs have trophic functions that are important for extracellular matrix remodeling and tissue regeneration. 13 At least a subset of MSCs are derived from pericytes, which are released upon tissue damage or disease. The proteins and molecules produced by the MSCs can enhance angiogenesis, inhibit scar formation and stimulate tissue regeneration. 14 In addition to maximizing the intrinsic regenerative capacity of the tissue, MSCs have key immunomodulatory roles. After the initial immunologic response to injury, pro-inflammatory cytokines produced by NK cells and T lymphocytes ‘activate’ the MSCs. MSCs subsequently prevent the inappropriate and overaggressive activation of T lymphocytes and decrease the cytotoxic activity of NK cells through feedback loops. 15, 16 This ‘trophic’ concept has been corroborated by findings in multiple in vitro and in vivo studies of enhanced gene expression and growth factor production after the introduction of MSCs to damaged tissue. 17, 18 Overall, MSCs most likely have a trophic function and their role in enhancing nerve regeneration is to maximize the intrinsic regenerative capacity of the nerve and minimize the inappropriate