**2. The use of ASCs in transplantation — Immunomodulatory and antiinflammatory actions**

Successful transplantation is reliant upon tolerance by the host's immune system. In 2000, human MSCs were transplanted into immunocompetent sheep without significant rejection [52], suggesting that adult stem cells might survive in a xenogeneic environment. Subsequent work with MSCs has described their ability to immunosuppress mixed lymphocyte reactions and to suppress stimulated T cell proliferation [53]-[55]. MSCs are also known to inhibit cytotoxic T lymphocyte toxicity [56], [57] and inhibit B cell proliferation by altering the G0/G1 transition [58]. Likewise ASC-mediated immunosuppression has been confirmed through a series of elegant in vitro experiments that describe the suppression of mixed lymphocyte reactions and/or proliferation of key immune cells like the T cell [59]-[63]. Immunosuppression has also been observed in a variety of in vivo model systems (Table 1). For example, reduced inflammatory infiltration and airspace enlargement results from the systemic administration of human ASCs to murine models of emphysema [64]. Moreover, the ASCs are capable of rescuing the suppressive effects of cigarette smoke on bone marrow hematopoietic progenitor function [64]. Experimental autoimmune hearing loss can be treated in mice through the systemic infusion of human ASCs, resulting in protection of hair cells possibly through the production of the anti-inflammatory cytokine IL10 by splenocytes [65] and decreasing the proliferation of antigen-specific Th1 and Th17 cells. Similar immunosuppression and amelio‐ ration of disease is reported upon injection of ASCs in models of rheumatoid arthritis [66] and IgA nephropathy [67], resulting in decreased inflammatory markers and Th1 cytokine activity, together with the generation of regulatory T cells capable of suppressing T cell responses. Finally significant anti-inflammatory responses are observed upon the transplantation of allogeneic murine ASCs into dystrophin-deficient mice, decreasing markers of oxidative stress and inflammation, including TNFα and IL6, decreasing production of CD3+ T cells, and enhancing the synthesis of anti-inflammatory IL4 and IL10 [68]. While these studies are supportive of the role for ASCs in modulating immune responses, what remains unknown is the mechanism. One theory proposes that cell-cell contact is required [61]. However, others dispute this finding, suggesting that it is the secretion of soluble factors by the ASC that mediates the eventual reaction by the host's immune system [69]. In support of this, inhibition of prostaglandin E2 production in ASCs by indomethacin can abolish the immunosuppressive properties of ASCs. Alternatively, neutralizing leukemia inhibitory factor has had similar effects [70]. Finally, there are those that suggest a role for IL-6 [55].

model systems have reported possible ectodermal and endodermal potentials, describing the repair of nervous and epithelial tissues [42], [43], together with hepatic and pancreatic regeneration [44]-[46]. With these in vivo results, combined with earlier in vitro analysis, it

Despite the in vivo translational studies above suggesting that ASCs are capable of enhancing tissue healing and regeneration, many of these studies cannot confirm the direct differentiation of the ASC into a specific cell type. For example, while bone regeneration is observed upon implantation of ASCs, very few studies report the presence of the ASC within the newly formed bone. Whether this is an oversight by the research team or an indication that the ASC does not directly form part of the new tissue is unclear. It is entirely possible that the ASC does not directly differentiate into the desired regenerating tissue, but simply directs tissue formation "from the sidelines". Tissue development and healing is incredibly complex and the role of paracrine signaling is still not entirely understood. Therefore, it is possible that ASCs may be intimately involved in tissue regeneration and health through their ability to mediate the host's

Two arguments can be made in support of this theory. First, in many translational models, it does not appear that the ASC has any difficulty in surviving within the transplantation region for extended periods of time. In addition, the range of tissues capable of engrafting ASCs appears to be quite broad. Initial studies by Nolta and researchers show that systemic admin‐ istration of human ASCs is followed by multi-organ engraftment in nude mice [47]. In support of this, human ASCs administered via tail vein migrate and home efficiently to multiple tissues (epithelial and endothelial) in irradiated mice [48], [49]. The specific migration of ASCs to injured tissues has also been shown by the Longaker group, who confirm the presence of ASCs specifically in parietal bone defects and their persistence as the defect heals [50]. Second, stem cells like bone marrow MSCs and ASCs are known to secrete numerous factors and cytokines, including VEGF, HGF, NGF, BDNF and multiple interleukins [49], [51]. In fact, Salgado's article calls these factors the "secretome" of ASCs. This secretome may have powerful paracrine effects on the health, repair and function of a tissue and has resulted in an exciting, new theory that proposes the ASC as a mediator of tissue regeneration through the secretion of specific soluble factors. In this regard, the ASC could be used in an incredibly broad range

**2. The use of ASCs in transplantation — Immunomodulatory and anti-**

Successful transplantation is reliant upon tolerance by the host's immune system. In 2000, human MSCs were transplanted into immunocompetent sheep without significant rejection [52], suggesting that adult stem cells might survive in a xenogeneic environment. Subsequent work with MSCs has described their ability to immunosuppress mixed lymphocyte reactions

becomes easier to conclude that the ASC is an adult pluripotent stem cell population.

**1.2. ASC-mediated tissue regeneration: Secretion of soluble factors**

regenerative capacity using paracrine signaling.

52 Regenerative Medicine and Tissue Engineering

of applications. However, the most popular are reviewed below.

**inflammatory actions**

The immunosuppressive properties of ASCs may make it possible to use more xenogeneic transplantation model systems without the fear of significant immune reactions in animal hosts. Such models would allow for a more direct study of human ASCs in vivo, thus allowing researchers to more accurately predict what these cells could do clinically. An excellent review of these models can be found in a recent article by Lin et al. [81]. In this article, they present a detailed table outlining many of the recent xenogeneic model systems, such as one by Paul and colleagues [82], who perform a xenogeneic transplantation of human ASCs into myocar‐ dial infarcts produced in immunocompetent rats. Histology confirms human ASCs in the infarct region after 6 weeks, with no detectable inflammatory reaction even in the absence of immunosuppressive action. Furthermore, these animals show improvement of cardiac function and reduced infarct size, together with significant improvement in myocardial antiinflammatory cytokine levels. The success of such xenogeneic transplantation models may be explained, in part, by the immunogenic profile of the ASC. Immunophenotyping of ASCs has not only provided researchers with a CD antigen profile but has confirmed the absence of the HLA-DR antigen on the ASC surface. Divided into classes such as HLA-A, B and C (or MHC


**Table 1.** Immunosuppressive action of ASCs

class I) and HLA-DP, DM and DR (or MHC class II), HLA receptors display proteins on the cell surface for immune surveillance. Of particular interest is the HLA/MHC class II protein, which is found on the surface of antigen-presenting cells and plays critical roles in immuno‐ tolerance and transplantation (for reviews see [83], [84]). The absence of this class of HLA protein may allow the ASC to evade the host's immune surveillance machinery. Of additional interest is a recent study by DelaRosa et al. [85], who note that human ASCs have lower susceptibility to natural killer (NK) cell-mediated lysis in comparison to bone marrow MSCs. This finding may be part of the reason for xenogeneic tolerance of ASCs in that NK-ASC crosstalk does not result in immediate recognition. Continued research in this area is sure to expand the possible uses of ASCs in translational model systems.
