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
Graft engineering encompasses the many approaches currently under development to improve on the outcome for patients given cellular grafts. It is limited for the purpose of this chapter to haematopoietic grafts. Graft engineering can involve the use of gene therapy, as discussed in Section 4.3.3. Grafts can also be altered by the addition of specific cells to the graft to improve outcome, either by facilitating engraftment of the haematopoietic (stem) cells, or by providing additional functionality, such as anti-tumour activity or improved immunocompetence in the post-engraftment period.
Cells that can facilitate the engraftment of HSCs, especially across allogeneic barriers, have been studied for years. DLIs have been used extensively to bolster failing allogeneic HSCT, and can often do so quite well (Tomblyn and Lazarus, 2008; Deol and Lum, 2010; Bar et al., 2013; Chang and Huang, 2013). However, the term ‘facilitating cells’ is more typically used for cell preparations that accompany the original graft and whose function is to improve engraftment. Their use is especially interesting in the context of highly purified HSCs, or HSCs with strict T cell depletion. Facilitating cells can reduce the number of allogeneic HSCs needed for successful engraftment. Several cell types that have this activity have been described, including CD8+ cells that lack a functional T cell receptor. Dendritic cell precursors make up an important facilitator compartment (Gandy et al., 1999; Leventhal et al., 2013b). Recently, facilitator cells have been used successfully clinically in HSCT aimed at inducing tolerance for subsequent organ transplantation (Leventhal et al., 2012).
Other cells may be of interest as adjuvants to HSCs. These include mesenchymal stem cells (MSCs), various types of T cells and myeloid progenitor cells. MSCs, cells that attach and form fibroblast-like colonies when bone marrow is plated on plastic, can be expanded in vitro to generate large numbers of cells. They are of interest for several reasons. They can differentiate into mesenchymal lineage cells of the mesoderm, such as osteocytes, adipocytes and chondrocytes, and can provide support for the growth and differentiation of haematopoietic progenitor cells in the bone marrow environment (Dalal et al., 2012; Leatherman, 2013; Miura et al., 2013). It has also been found, somewhat unexpectedly, that they can inhibit lymphocyte proliferation. They have anti-inflammatory, antiproliferative and immunosuppressive properties, most likely through the release of soluble molecules (Dalal et al., 2012; Miura et al., 2013). While certainly not completely exempt from recognition and rejection by immune cells, MSCs are now used in attempts to modulate undesired immune responses. This includes facilitating engraftment by allogeneic HSCs and reducing GVHD. Long-term systemic engraftment with MSCs has proven elusive, but local administration seems to result in higher engraftment levels.
Lymphocytes, especially T lymphocytes, are of interest in engineered grafts. A full discussion of all the various options and their advantages and disadvantages is well outside the scope of this chapter, but we will list some examples.
One option is complete infusion of donor lymphocytes, harvested from the HSC donor, which, as discussed already, can help in allogeneic engraftment, and can be used to rescue a failing graft. But there are also more defined lymphocyte populations that are of interest. These include regulatory T cells (Treg), classically described by the phenotype CD4+ CD25+ FoxP3+ , which are capable of suppressing immune responses. The development of these cells for clinical use has attracted significant interest, as reviewed in Michael et al. (2013). Another experimental approach that has proven spectacularly successful in a limited number of cases to date in cancer patients is the use of chimeric antigen receptors (CARs), which allows the targeting of large numbers of T cells against a tumour target (solid tumour or haematopoietic malignancy) (Louis et al., 2011; Davila et al., 2014). These cells, however, are typically not given in the context of an HSCT. Cytotoxic T lymphocytes, expanded as reactive to two human viruses, have been successfully used in allogeneic transplants (Leen et al., 2009). Also of potential interest is the addition of non-naпve T cells to an engineered HSCT graft. Non-naпve T cells have responded and been shown to recognize a target other than host tissue (which wasn’t present in the donor). These cells normally do not cause GVHD, but, interestingly, seem to retain GVL activity (Chen et al., 2004, 2006). Of interest, but with no clear path to obtaining clinical useful numbers of cells, is the addition of lymphoid progenitor cells. These cells, which mature and undergo selection in the host, do not cause GVHD and in preclinical models have been shown to be effective in reducing viral infections (Arber et al., 2003).
Myeloid progenitor cells, progenitors capable of differentiating into the myelo-erythroid lineages, are being developed and are undergoing clinical trials as adjuvants. These cells, which can be obtained in clinically useful numbers in short-term defined cultures, will expand significantly and rapidly differentiate into functional myeloid cells (Akashi et al., 2000; Manz et al., 2002). In preclinical models, these cells have been shown to be effective in reducing bacterial and fungal infectious complications in neutropenic animals, and to be able to do so without any need for haplotype matching (Arber et al., 2005; BitMansour et al., 2002). In preclinical models, these cells are also effective in treating acute radiation syndrome (Singh et al., 2012). Clinical trials with these cells in HSCT settings, aimed at reducing infectious complications, are ongoing (Cellerant Therapeutics, 2012).