Correspondence to: Soliciting Strategies for Developing Cell-Based Reference Materials to Advance Mesenchymal Stem/Stromal Cell Research and Clinical Translation

Editor: While we applaud the efforts by Viswanathan et al. [1] to address the issues related to definition and comparability of mesenchymal stem/stromal cells (MSCs), we do have several reservations and concerns with their reasoning and suggestions for developing reference materials.

The group suggests the use of a standard or reference MSC source to allow comparison among different cells manufactured for both preclinical and clinical studies. With substantial differences in culture methods (eg, source material, seeding density, base medium, medium supplements, passage number), selecting the optimal reference material becomes difficult to impossible [2]. To date, there is no clear consensus on what the optimal donor for therapeutic use is in MSCs. The cells can vary from donor to donor. The optimal donor may well depend on the route of administration, the target patient population, and the clinical application. Single donor reference material could not be utilized based on inherent donor variability. This variability is similar to what one finds in blood bank components, such as red blood cells (RBC) and fresh frozen plasma (FFP). The donor variability in efficacy can be dependent on multiple factors, such as age, gender, and storage time. However, no consensus has been made on a reference material for RBC or FFP transfusions since the studies have not been conducted. Ideally, there could be a broad-scale study of the efficacy of multiple MSC donors and passage conditions on preclinical and clinical disease outcomes to determine how widely different responses can be to donor and passage condition variability.

The authors suggest other approaches, such as donor pooling, an immortalized line, or self-renewing population. We believe that none of these approaches would be satisfactory. While pooling of plasma for proteins (eg, human serum albumin) makes sense, pooling of MSCs from several individuals would potentially only cloud a comparison of MSCs and introduce potential infections and immune risks. The interactions between MSCs from different donors and the implications on assay output are unknown. The authors suggest, and we agree, that the “perturbations of biology associated with the immortalization event” would render this comparison invalid. A self-renewing population (ie, pluripotent stem cell derivation), similar to the immortalized line, would not likely represent MSCs from adult sources due to in vitro manipulations and concerns regarding conserved biology.

Viswanathan et al. [1] provide several examples of reference materials used for calibration or standardization in laboratory medicine and health care. The examples of flow cytometry beads, protein/DNA ladders, and aluminum for bone mass assessment do not equate to cells as a reference material, as the beads, protein/DNA ladders, and aluminum simply serve as a surrogate to measure a specific physical parameter. The measurements are straightforward and do not approach the complexity involved in the evaluation of a living cell. The viral vector reference is most applicable; however, even that example does not translate well given the multifold greater challenges for analysis of a eukaryotic cell. There are no data to suggest that any of these criteria for defining an MSC are reliable and accurate measures for reference and quality. Assignment of these standards is not evidence based.

Finally, assays are not standardized and can vary from laboratory to laboratory. In addition to laboratories and centers using different types of assays, often dependent on intended use, tests that may be in common have substantial need for both intra- and particularly interlaboratory standardization. Analysis by flow cytometry becomes the closest to being standardized with a major drawback being the lack of a specific marker for the MSC.

Realizing both the need for assessment of MSCs and the clear limitations of an MSC reference line, we suggest a different approach. We recommend a more systematic data-driven approach, in which studies are conducted on the role of donor variability and passage conditions on outcomes for particular therapeutic applications. These studies may reveal that a reference material is not necessary and that the criteria for defining an MSC can be driven by process of production and measures of quality and safety. If these studies reveal that there is significant donor- and passage-related variability in the outcomes studies, then perhaps a reference standard will be required for that particular application. Rather than having a laboratory or a limited number of laboratories that manufacture reference materials, we suggest that laboratories with expertise in MSC assays should serve as centralized testing sites with facilities producing MSCs for preclinical and clinical studies. Involvement would be voluntary, and the data generated could be made available to participants. This approach would be the first step in establishing a measurement of MSCs manufactured at different sites under different methods [3] while allowing a still nascent field to move forward in optimizing cell therapies for the treatment of patients suffering from a variety of diseases.