ReviewThe mesenchymal stromal cells dilemma—does a negative phase III trial of random donor mesenchymal stromal cells in steroid-resistant graft-versus-host disease represent a death knell or a bump in the road?
Introduction
Mesenchymal stromal cells (MSCs) are a cellular product that can be derived from bone marrow and propagated ex vivo using established, clinically applicable methods (1). According to the International Society of Cell Therapy guidelines, MSCs must be plastic-adherent when maintained in standard culture conditions, express major histocompatibility complex (MHC) class I molecules and surface CD antigens CD105, CD90, CD73 and CD44 and lack expression of hematopoietic markers CD45, CD34, CD11b and CD19. They must also retain the ability to differentiate into osteoblasts, adipocytes and chondroblasts in vitro 2, 3. MSCs are also characterized by a unique transcriptome (4). Although MSCs are under intensive investigation for treatment of autoimmune and alloimmune disorders, there is no established consensus on how best to ascertain MSC immune veto potency. Nevertheless, MSCs possess an array of distinctive features rendering them attractive for graft-versus-host disease (GvHD) suppressor cell therapy (5). Culture-expanded human MSCs display robust immune T-, B- and dendritic cell-targeted suppressive properties via expression of indoleamine 2,3-dioxygenase (IDO) and other effector molecules, many of which are augmented by interferon-gamma stimulation (6). MSCs have also been shown to have broad immunomodulatory action on innate and adaptive immune cells and to promote tissue healing (7).
Since the publication of a case report in 2004 describing a durable complete remission of advanced-stage steroid-resistant acute GvHD in a child treated with two transfusions of haploidentical MSCs (8), the use of cryopreserved unmatched allogeneic MSCs for treatment of GvHD has become medical practice in many European jurisdictions. Compelling phase II clinical trial series published in the past few years serve as a rational underpinning to this practice 9, 10, 11, 12. However, a phase III industry-sponsored clinical trial (NCT00366145) conducted in the USA and licensed by the Food and Drug Administration examined the use of an industrial MSC product (Prochymal; Osiris Therapeutics, Inc., Columbia, MD, USA) for treatment of steroid-refractory GvHD and reported negative results. The results of the study were disclosed publicly in a press release in 2009 and in abstract form at the 2010 American Society for Blood and Marrow Transplantation Tandem Meeting (13). It was stated that Prochymal transfused at 2 million cells/kg twice weekly for 4 weeks failed to meet its primary clinical endpoint of achieving an increased GvHD overall complete response rate compared with placebo control. This pivotal phase III study does not support the use of MSCs in this clinical indication, although the published European clinical experience suggests otherwise. The field of MSC therapy is faced with a paradox regarding clinical utility of MSCs for GvHD, with opposite clinical outcomes in the USA and Europe. Although there is no ambiguity regarding the identity of the MSC products used throughout, the body of published scientific reports on MSC biology informs us that there may be meaningful distinctions between the functionality of industrial MSCs and MSCs manufactured by academic centers. These dissimilarities may provide a hypothesis-driven rationale for the disparate outcomes seen in these conflicting trial outcomes. This analytical review reflects on four distinct MSC product variables that may apply to the apparent conundrum at hand: donor variance, epigenetic reprogramming, immunogenicity and cryopreservation.
Section snippets
Donor variance
Bone marrow-derived MSCs used for treatment of acute GvHD are harvested from healthy volunteer donors. After collection and in vitro expansion, MSC identity is typically confirmed using a series of surface markers such as those defined in 2006 by the International Society of Cell Therapy 2, 3. Although useful in confirming identity, these markers have virtually no utility in predicting function or immune suppressive potency. These markers are remarkably uniform in their expression among
Culture expansion, epigenetic reprogramming and senescence
An immediate and apparent distinction between the European and industry-driven experience in MSC use for acute GvHD involves the MSC product itself. In regard to Prochymal, published manufacturing information reveals that marrow harvested from a single volunteer donor is culture expanded in fetal calf serum using proprietary methods, identity of the final product is confirmed by an extended MSC phenotype panel and secretion of tumor necrosis factor receptor TNFR1 is used as a surrogate marker
Immunogenicity
Human marrow-derived MSCs and their culture-expanded counterparts robustly express MHC I molecules on their cell surface but do not express ABO blood group antigens (40). Orthodox blood bank transfusion practice would predict that intravenous infusion of numerous random donor MSCs should not lead to hyperacute rejection (or transfusion reaction). However, it has been found more recently that MSCs are the target of complement-mediated injury and impaired function after exposure to human serum
Cryopreservation
Human clinical trials examining the use of MSCs almost universally use stored, cryopreserved products that are thawed and transfused within no more than a few hours. This logistical approach is convenient and logical and rests on the premise that similar handling of cryopreserved leukapheresis products used in autologous peripheral blood transplantation has good outcomes. However, all pre-clinical studies of MSCs in mouse models of human disease universally involve transfusion, transplantation
Conclusion
The number of clinical trials examining the use of random donor cryopreserved MSCs for treatment of acute, steroid-resistant GvHD and other immune ailments is growing exponentially. Subtle differences in donor source, culture methods, and expansion levels make head-to-head comparisons of the different MSC products challenging when interrogating their utility. However, the negative outcome of the utility of Prochymal in treatment of steroid-resistant GvHD trumps all and may dampen enthusiasm for
Acknowledgments
The author would like to thank Katarina Leblanc, Francesco Dazzi and members of the International Society of Cell Therapy Mesenchymal Stromal Cell Committee for useful conversations. The author is a Georgia Cancer Coalition Distinguished Cancer Scholar.
Disclosure of interest: The author has no commercial, proprietary or financial interest in the products or companies described in this article.
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