Haematopoietic stem and progenitor cells: Enumeration, phenotypic characterisation, and clinical applications

Over the last few years, the number of clinical applications for autologous and allogeneic stem cell transplantation has expanded considerably. Concurrently, stem cell collections have been obtained from increasingly diverse sources, including bone marrow, peripheral blood and umbilical cord blood. Various ex vivo manipulations have been developed to process stem cell transplants for specific clinical requirements (e.g. positive selection techniques to purify CD34+ cells, purging techniques to remove residual tumour cells from autologous transplants or T lymphocytes from allogeneic transplants). Subsequently, widespread interest developed in clinically useful ex vivo stem cell expansion methodologies and stem cell-based gene therapy protocols. With this increased demand for both unmanipulated and manipulated stem cell products, it is important that flow cytometric techniques are capable of accurately monitor the consequences of such procedures on the quality of stem cell products. Over the last few years, robust flow cytometric protocols have been developed to enumerate viable CD34+ cells accurately in a range of sources of stem cell products, even those of poor quality. Further international standardisation of the flow cytometric techniques used should improve the between-laboratory variation of the enumerations obtained, so that inter-institutional results become more meaningfully comparable. The use of internal quality controls and participation to external quality assessment schemes are indispensable to monitor and improve laboratory practice. These endeavours require stabilized samples with similar characteristics to fresh blood or apheresis products as calibrators and test samples for CD34+ cell enumeration. The emerging clinical interest in the characterisation of subpopulations of CD34+ cells adds to the complexity of this assay. Currently, the number of viable CD34+ cells actually re-infused is clinically the most important variable determining graft success or failure. A systematic evaluation of CD34+ cells from the time of collection to their (re)-transfusion would be ideal to quality assure these products and is relevant for optimal utilisation of resources in the clinical setting of haematopoietic stem and progenitor cell transplants.

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