Introduction
Human organs have a limited capacity for repairing themselves. This capacity declines as a function of increasing chronological age, driven by a cocktail of biological, psychological, and sociological stressors that can accelerate organ degeneration. Both transplant-recipient survival and donor organ function are affected by these processes. Novel therapies to tackle this are manifold, but typically limited in effect.
Solid-organ transplants replace diseased organs with biologically newer, healthier whole organs, but this strategy is inherently limited. The requirement for an individual with healthy organs to die or to undergo major surgery in order for an organ to be replaced is the central limiting paradox of whole-organ transplantation. Stem-cell treatments represent perhaps the most exciting and most logical of the many ways in which this clinical problem is being addressed. The isolation and propagation of stem-cell lines promised a more permanent and potent method of repair or regeneration of damaged tissue or organs. Indeed, at the time of James Thompson’s description of the first embryonic stem-cell lines in 1998 [1], solid-organ transplantation had been established for nearly 3 decades, and the step to having perfect, quality-controlled neo-organs on a shelf ready for surgical implantation appeared small. Initial perceptions have seemingly underestimated the quantum leap from single multipotent stem cell to functioning organ.
The holy grail of cell-based tissue-engineering approaches remains the growth of functional (and ideally tolerant) neo-organs that can spontaneously, or surgically, assimilate into the body and fulfill the role of a diseased organ. While pluripotent cell lines of infinite proliferative capacity have reliably been made to form cardiac myocytes, hepatocytes, and many of the different renal-specific cell types, few have been directed into a neo-organ of adequate function to establish a role in clinical practice, and none in the fields currently managed by major abdominal-organ transplants.
Therapeutic applications of novel cell lines are far more advanced in immunomodulation and the augmentation of tissue repair. These protection/repair therapies have already shown clinical benefit and have direct implications for the treatment of age-related disease. Since these approaches are well advanced in clinical trials and therefore likely to find a clinical role in the current abdominal-transplant field, this chapter focuses principally on the potential of cell sources to protect or repair diseased organs. The use of stem cells to grow functional, clinically useful tissue for the treatment of the diseases currently best managed by abdominal-organ transplants remains entirely experimental. Progress and barriers to clinical use are thus also discussed.
Defined stem-cell populations for clinical application
Despite this great promise, the use of regenerative medicine to effect repair of solid organs and tissues is still in its infancy. The type(s) of cell, or cell population, required to effect functional recovery remains to be defined, as do the mechanism, delivery system, and indeed cell numbers to achieve this.
A range of cell types have been touted and tried as candidates for therapeutic use. These include embryo stem cells (ESCs), hematopoietic stem cells (HSCs), multipotent stromal cells (MSCs), endothelial progenitor cells (EPCs), and organ-specific resident stem/progenitor cells, which are known to contribute to solid-organ tissue repair. The individual merits of these cells have been reviewed elsewhere [2]. Currently, their use is limited, but the field is developing rapidly and early clinical trials for solid-organ repair are ongoing.
The main focus is on adult cell sources, since the use of ESCs remains dogged by social and scientific uncertainty, due to moral/ethical issues and basic technical hurdles. The latter include control of the directed differentiation of ESCs and the prevention of neoplasia or tissue dysfunction post-transplant. Most current clinical potential resides with the use of adult cell types, such as MSCs. To date, only MSCs have been applied successfully in both experimental solid-organ transplantation and clinical studies. These are discussed next, with reference to clinical applications in transplantation.
Multipotent stromal cells
MSCs were initially described over 30 years ago by Friedenstein et al. as a bone-marrow-derived mononuclear cell population which exhibited a fibroblast-like morphology when cultured ex vivo on an adherent substrate, such as plastic [3]. MSCs are present in a wide range of adult tissues and exhibit the capacity to be differentiated into multiple specialized cell types from all three germ layers. They also demonstrate immunomodulatory properties, though how this is achieved remains undefined (for a detailed review, see [4]). As such, they are of interest due to their capacity to make cells suitable for transplantation. Their isolation is straightforward, either in tissue culture or by fluorescence-activated cell sorting (FACS), where they can be identified by cytotype.
Recent clinical trials have tested the capacity of MSCs to treat cardiac, renal, and liver damage, as noted later in this chapter. What remains unclear, however, is the mode of action of such cells. It is uncertain whether these cells contribute to tissue building via direct differentiation into tissue-specific cells, modulate immune-mediated damage at the site of injury, or even provide trophic support for tissue regeneration [5]. Even the characterization of these cells is contentious.
A basic set of criteria for MSCs has been proposed by the International Society for Cellular Therapy (ISCT) [6]. This appears to function well in practice:
One key question at this juncture is whether the phenotype and properties exhibited by MSCs in vitro are maintained in vivo. MSCs in vitro typically grow as an adherent monolayer, with a distinct immuno phenotype. When grown under nonadherent conditions, this phenotype changes and the cells grow in spherical clusters. This has been proposed to promote intercellular interactions, although that remains to be demonstrated formally [7].
Some findings, however, suggest that MSCs offer exciting therapeutic potential for organ transplantation. Secretory factors derived from MSCs have been demonstrated to have both pro-angiogenic and anti-inflammatory effects, which might be used to assist in solid-organ and cellular transplantation. Furthermore, MSCs grown in the presence of pro-inflammatory cytokines also display enhanced immunosuppressive effects, which might be exploited to aid transplant success [8–10]. The immunomodulatory effect of MSCs appears to be dose-dependent and independent of the major histocompatibility complex (MHC) and mediation by antigen-presenting cells or regulatory T cells [11,12].
MSCs and solid-organ transplantation
Following on the heels of a range of rodent studies demonstrating that transplanted MSCs can improve tissue damage [13], clinical trials are underway. Currently, only three phase-III clinical trials have been concluded. These comprise trials for graft-versus-host disease (GVHD), Crohns disease, and perianal fistula. As such they are not yet directly relevant to abdominal-organ transplantation, and the therapeutic approach is immunomodulatory, rather than building/repairing tissue architecture (i.e. directly replacing damaged tissue in a failing organ).
Early-stage trials for use with solid organs are limited. Initial findings from a safety-and-clinical-feasibility study [14] comprising autologous MSC administration in two subjects receiving living related-donor kidneys showed that 1 year post-transplant the patients had stable graft function and, significantly, an enlargement of the regulatory T-cell (Treg) pool in the peripheral blood, with a concomitant inhibition of memory T cells. This has demonstrated the feasibility of translating beneficial immunomodulatory findings from rodent models into a human clinical setting, though caution, based on the low power of the study, is still advised.
Ongoing trials using MSC to aid outcome in liver–renal transplantation continue at a number of centers, with results awaited. Promising results on deriving liver and biliary cells in vitro using rodent progenitor cells have already been reported [15], though these have yet to translate into clinical practice, as deriving human equivalents has proven problematic.
Recently, a significant technical breakthrough was reported with the identification of adult nephron progenitors capable of kidney regeneration in zebrafish [16]. The authors provided a proof of principal that transplantation of single aggregates comprising 10–30 progenitor cells is sufficient to engraft adults and generate multiple nephrons. The identification of these cells opens up an avenue to isolating or engineering the equivalent cells in humans and developing novel renal regenerative therapies.
Acute hepatic failure and inborn errors of metabolism affecting the liver can also be treated successfully with hepatocyte transplantation alone [17–19]. This technique is feasible for bridging to orthotopic liver transplantation or for long-term correction of underlying metabolic deficiencies. Hepatocytes can be used fresh or cryopreserved, making them readily available for short-notice administration. The main disadvantage of this technique is the limited source of cells which are isolated from unused donor tissue. In addition, these unused organs are of inferior quality, and this is reflected in the function of the isolated hepatocytes. Intraportal injection is the preferred method of delivery, but alternative routes such as splenic intraparenchymal injection and the peritoneal cavity have been considered [20]. Fetal hepatocytes and hepatic stem-cell transplantation are currently experimental but may prove to be an invaluable therapeutic alternative.
How MSCs might work
How MSCs work in clinical trials and animal models is still debated. Any paracrine effect mediated by the secretion of growth factors remains problematic, as the speed of efficacy, duration of immunomodulation, and extent of tissue repair cannot readily be accounted for. This, principally, is due to the transient existence of MSCs following in vivo administration and different syngeneic and allogeneic effects in transplantation models [21,22].
Recent data from Stevenson et al