Date Published: June 30, 2017
Publisher: Public Library of Science
Author(s): Pascal Joly, Thomas Schaus, Andrea Sass, Anke Dienelt, Alexander S. Cheung, Georg N. Duda, David J. Mooney, Maria Cristina Vinci.
The use of autologous cells harvested and subsequently transplanted in an intraoperative environment constitutes a new approach to promote regeneration. Usually cells are isolated by selection methods such as fluorescence- or magnetic- activated cell sorting with residual binding of the antibodies or beads. Thus, cell-based therapies would benefit from the development of new devices for cell isolation that minimally manipulate the target cell population. In the clinic, 5 to 10 percent of fractures do not heal properly and CD31+ cells have been identified as promising candidates to support bone regeneration. The aim of this project was to develop and prototype a simple system to facilitate the enrichment of CD31+ cells from whole blood. After validating the specificity of a commercially available aptamer for CD31, we combined this aptamer with traditional magnetic bead strategies, which led to enrichment of CD31+ cells with a purity of 91±10%. Subsequently, the aptamer was attached to agarose beads (Ø = 100–165 um) that were incorporated into a column-based system to enable capture and subsequent release of the CD31+ enriched cells. Different parameters were investigated to allow a biophysical-based cell release from beads, and a simple mixing was found sufficient to release initially bound cells from the optimized column without the need for any chemicals that promote disassociation. The system led to a significant enrichment of CD31+ cells (initial population: 63±9%, released: 87±3%) with excellent cell viability (released: 97±1%). The composition of the released CD31+ fraction indicated an enrichment of the monocyte population. The angiogenic and osteogenic potential of the released cell population were confirmed in vitro. These results and the simplicity of this system highlight the potential of such approach to enable cell enrichment strategies in intraoperative settings.
In order for cell therapies to be translated from the bench to the clinic, they must follow good manufacturing practice guidelines and be approved by regulatory agencies. In the case of exogenous cell therapies, significant regulatory constraints on cell isolation and in vitro expansion procedures to ensure the quality and safety of the resultant product lead to high costs [1,2]. An alternative approach to obtain a sufficient number of cells is cytokine-based cell mobilization, such as the use of granulocyte colony-stimulating factor for the mobilization of hematopoietic stem cells . However, not only does this approach necessitate several visits to the hospital for the initial injections or to collect the cells, but is also associated with a wide variety of side-effects ranging from flulike symptoms to more severe conditions . In contrast, intraoperative cell therapies, in which cells are harvested from the patient prior or during the initial operation and then re-administered during the same surgical session, represent a new class of exciting approaches that hold promise to overcome the high costs and many of the potential drawbacks associated with ex vivo cell expansion, cytokine-based cell mobilization, and save time for both patients and clinicians (Fig 1). Bone healing may be an ideal candidate to illustrate such a concept: in the US, approximately 7.9 million bone fractures are reported each year with 5 to 10% resulting in an impaired bone-healing situation [5,6]. Predicting patients at risk and initially providing them with additional treatment may significantly reduce the number of non-union cases and decrease the associated costs  and hospital stay .
The goal of this study was to evaluate the potential of a cell sorting strategy that would allow to sort for beneficial or detrimental cells from whole blood. Specifically, we aimed at developing an approach for positive cell isolation that would result in a clean (bead free) cell population enriched for a specified cell surface marker. We prototyped a system to perform biophysically induced reversible binding and validated it using CD31+ cells as an example.