Culture conditions shape mesenchymal stromal cell phenotype and function.

Mesenchymal stromal cells (MSC) [1,2] are center stage of world-wide developments in regenerative medicine. Although there is much yet to be learned, we recognize that the cells we infuse into patients after cell processing do not necessarily bear the same phenotype or functional capacity of cells in situ. It is now widely accepted that ex vivo-expanded MSC vary depending on the tissue source and processing protocols [3,4]. It follows, then, that we can adapt specific processing protocols, as well as tissue sources, to generate MSC particularly suited to specific clinical indications [5]. This seemingly obvious notion has not yet been widely exploited; however, the new-found importance of MSC in regenerative medicine and immune modulation is beginning to direct MSC research along this pathway. In this issue of the Cytotherapy, two papers illustrate the increasingly recognized importance of cell processing of MSC for specific clinical indications. The report by Samuelsson et al. [6] builds on the ground-breaking work of this group using MSC to ameliorate graft-versus-host disease (GvHD) [7]. MSC seem to reduce the unwanted immune response of GvHD after hematopoietic stem cell transplantation; however, results are variably successful [8]. While many factors influence this outcome, it seems plausible that MSC processing is an important parameter affecting patients’ response. These investigators found that the yield of ex vivo-expanded MSC obtained from freshly harvested bone marrow (BM) mononuclear cells (MNC) was greater than that from cryopreserved MNC. Moreover, MSC from fresh MNC were more suppressive of the lymphocyte response in a mixed lymphocyte culture (MLC) compared with MSC prepared from cryopreserved MNC. In contrast, cryopreservation of MSC does not seem to affect their capacity for lymphocyte suppression in an MLC. These observations are especially important because maintaining stocks of cryopreserved MNC is often more convenient. Thus, this report provides a rationale for isolating MSC from fresh marrow and then cryopreserving the early passage cells. Naghdi et al. [9] report on the differentiation of MSC into a cell with a cholinergic neuronal phenotype, suggesting these cells could be a source for cell therapy of spinal cord injury. These investigators added specific differentiation mediators to the ex vivo culture of rat BM MSC to induce the expression of cholinergic neuron proteins consistent with a neuronal phenotype. The induced cells, and appropriate controls, were then transplanted into the contused spinal cord in a rat injury model. Transplanted cells were identified at 6 weeks after cell therapy, associated with a functional improvement of locomotion. This study nicely exemplifies in vitro differentiation of MSC targeted to a specific clinical indication in a highly relevant animal model. These two exciting papers represent the beginning of what is likely to be a rapidly expanding research arena within cell therapy. Only through research of cell processing will we develop optimal protocols to enhance the outcome of the patients. Once the in vitro and pre-clinical studies are complete, we must rapidly move to the clinic where, ultimately, the advances of cell therapy will be realized.