Principles of stem cell biology and cancer: future applications and therapeutics. Edited by T. Regad, T. J. Sayers and R. C. Rees. John Wiley & Sons (2015)
Part I. Stem Cells
HSC therapy has been studied in preclinical settings using HSCs from different sources and with different degrees of purity, up to highly purified, fluorescence-activated cell sorting (FACS)-sorted cells. Figure 4.1 illustrates the different types of ‘stem cells’ used. Bone marrow, mobilized peripheral blood or UCB can be used either as is or following bulk enrichment/ depletion for a single parameter, using either magnetic beads (in the past Isolex, currently e.g. Clinimacs) or depleting antibodies. These methods can result in considerable enrichment, but contaminants remain. In the case of dangerous contaminants, such as malignant cells, this can contribute to an eventual relapse. Multiparameter enrichment, such as FACS, yields highly purified HSCs (orders of magnitude cleaner than those produced using other methods), but has rarely been used clinically, due to technical limitations. Small trials do suggest that highly purified HSCs can have a better outcome in autologous haematopoietic stem cell transplantation (HSCT) in breast cancer patients (Muller et al., 2012). Multiparameter FACS purification is used routinely to study HSCs in research settings.
Figure 4.1. Purification of HSCs. Left panel: HSC preparations directly after harvest consist of a mixture of cells, including various types of myeloid and lymphoid cells. Other cells, including tumour cells, can also be present in the preparation. The types and amounts of cells depend on the source. Middle panel: HSCs can be enriched from the mixture using bulk enrichment for one parameter, typically CD34 expression. In this approach, in which very large numbers of cells can be processed rapidly in a closed system, antibodies with attached magnetic beads bind the HSCs and allow the external magnet to bind and eventually release them, separating them from the cells not recognized by the antibody. While this type of enrichment can be quite impressive, measurable quantities of all sorts of contaminants remain present. Clinical systems based on this approach have been in use for several decades. Right panel: Multiparameter FACS can sort cells based on the presence or absence of 10 or more markers. However, each additional marker, while improving the purity, reduces the yield. In practice, clinical sorts have used two markers, such as CD34 and CD90.
Clinically, it is important to consider the nature of the cell preparation used. While HSCs provide the long-term engraftment, other cells also play important roles. Cell components can adversely affect the transplant outcome, such as malignant cells in autologous grafts and graft-versus-host disease (GVHD)-inducing lymphocytes in allogeneic grafts. Examples of positive roles include graft-versus-leukaemia (GVL) responses and improved engraftment by allogeneic lymphocytes. Donor lymphocyte infusions (DLIs) are sometimes used to aid a failing graft.