Mesenchymal stem cells in drug/gene delivery: implications for cell therapy.

Stem cells have been therapeutically utilized in replacement of hematopoetic cells for decades. This is in contrast to the recent emergence of adult stem cells as, perhaps, safe and beneficial therapeutics for multiple diseases and disorders. In particular, mesenchymal stem cells (MSCs) are currently used in multiple human clinical trials. Although MSCs are ubiquitous, bone marrow, umbilical cord and adipose tissue are the sources where MSCs are isolated for research and clinical application. MSCs were thought to be mesodermal due to the initial reports showing their differentiation into specialized mesodermal cells such as chondrocytes. However, it now appears that MSCs might be neuroectodermal in origin. Thus far, there is no evidence of in vivo transformation of MSCs. However, it is too early to prove or disprove that MSCs can be transformed in vivo in clinical trials. MSCs display immunosuppressive properties when placed in a milieu of inflammatory mediators. This phenotype makes MSCs easily available for therapies as 'off-the-shelf cells. Additionally, MSCs express chemotactic receptors, thereby allowing them to migrate to sites of tissue injury. This latter property has proven useful in the embodiment of MSCs as cellular vehicles to deliver targeted therapeutics to precise regions. The MSCs would typically harbor a prodrug or ectopically express a therapeutic gene to be delivered at a targeted site. This approach has been utilized in a number of different indications requiring precise therapeutic delivery, specifically cancer, cardiovascular disorders and neurodegenerative diseases. Combined with their immune-privileged status, safe clinical profile and low tumorigenicity, MSCs offer vast potential to benefit patients with serious diseases, for which limited treatment options exist.