**6. Future directions**

432 Advances in Hematopoietic Stem Cell Research

BM-derived cell transplantation compared to placebo. The majority of studies, however, utilized unselected populations of BMCs and these studies provide the longest follow-up of up to 5 years (Assmus *et al.,* 2010; Schachinger *et al.,* 2009). The underling mechanisms leading to the beneficial effect of transplanted BMCs are unclear. The observed benefits of BMCs transplantation is out of proportion of the observed rates of newly formed cardiomyocytes from BMCs' origin (Zuba-Surma *et al.,* 2011). Indeed, recent evidence suggest a primarily paracrine effect of BM-derived stem cells following their transplantation by recruiting and stimulating resident cardiac stem cells (CSCs) (Loffredo *et al.,* 2011). Furthermore, human purified CD34+ cells are a source of several growth factors including VEGF, cytokines and chemokines that may prevent apoptosis of dying cardiomyocytes and promote angiogenesis in damaged myocardium (Majka *et al.,* 2001). Cell membrane derived microvesicles or exosomes that are enriched in S1P may contribute to regeneration of myocardium and its re-vascularization (Baj-Krzyworzeka *et al.,* 2002). Hence, transplanted CD34+ cells may contribute to regeneration of damaged heart by paracrine signals and released microvesicles (Ratajczak *et al.,* 2008) and was recently confirmed by others (Sahoo *et* 

Long-term follow-up studies demonstrated 'catch-up phenomenon' of the placebo treated patients, thus leading to mixed results regarding the sustainability of the BMCs treatment benefit (Assmus*, et al.* 2010; Meyer *et al.,* 2009; Yousef *et al.,* 2009). The benefit of BMCs therapy is less robust among patients with chronic ischemic heart disease (IHD) (Assmus *et al.,* 2006; Strauer *et al.,* 2010). Similarly, smaller studies have demonstrated the antianginal effects of BMCs in patients with non-revascularizable severe coronary artery disease

Selected BM-derived stem cell subpopulations represent an attractive substrate for cellular therapies since they lack the inflammatory cells, which contribute to the ongoing inflammatory response at the site of myocardial infarction, contained in the unselected BMCs populations. Furthermore, highly purified stem cell populations are more likely to induce myocardial regeneration through paracrine effects or by directly differentiating into cardiomyocytes. The largest study utilizing selected BM-derived stem cell population is the REGENT study which compared selected to non-selected populations of BMCs in patients with acute ischemic heart disease and reduced LV function at baseline (Tendera *et al.,* 2009). While there were no significant differences between the groups, patients treated with selected CD34+/CXCR4+ cells showed trends of improvement in LV function when compared to controls. Other studies utilizing primitive populations of BM-SPCs such as CD133+ cells have reported improvement of LV function and perfusion (Bartunek *et al.,*

Nevertheless and despite the disparity in the methodologies of the conducted studies, the overall collective effect of BMCs' transplantation suggests small yet statistically significant benefit in myocardial regeneration (Abdel-Latif*, et al.* 2007; Martin-Rendon *et al.,* 2008). However, these trials have been hampered by their reliance on surrogate endpoints rather than patient important endpoints such as mortality, need for repeat revascularization, recurrent MI or re-hospitalization for congestive heart failure. While surrogate endpoints are important for mechanistic studies, patient-important endpoints are quintessential for a new

*al.,* 2011).

(Losordo *et al.,* 2007; Tse *et al.,* 2007).

2005; Stamm *et al.,* 2004).

therapy to achieve mainstream status.

Growing evidence suggest that a multitude of BM-derived stem and pluripotent stem cells are mobilized in the peripheral blood following AMI. However, the clinical significance and the potential therapeutic use of this mobilization are still not fully understood. Circulating PSCs can be used as markers of ischemic injury in humans or as predictors of myocardial recovery following large ischemic damage. On the other hand, the therapeutic application of VSELs in myocardial regeneration has proven beneficial although the beneficial mechanisms remain elusive and are probably mainly paracrine in nature. Given the pluripotent potential of VSELs, their transplantation at smaller numbers (10,000 cells per mouse) have proven to be more beneficial than larger numbers of the more committed HSPCs (100,000 cells per mouse) indicating their greater therapeutic potential (Dawn *et al.* 2008). Current efforts directed at the ex-vivo expansion and priming of VSELs have proven to be a successful strategy in animal models and their clinical applications are pending (Dawn *et al.* 2008; Zuba-Surma*, et al.* 2011). Nuclear reprogramming has opened the door for creating patientspecific autologous pluripotent stem cells with multiple therapeutic opportunities (Takahashi *et al.,* 2006). Further studies are needed to examine the feasibility as well as the safety of inducible pluripotent stem cells (iPS) particularly their tumorigenicity and immunogenicity before they can be explored in human studies.

On the biotechnology frontier, multiple modifications of the transplanted cells (priming) and the host environment are being tested in humans to improve the efficiency of BM-SPCs' regenerative capacity. Transplanting three dimensional constructs that provide an enriched environment for the transplanted and resident stem cells are attractive modifications to the currently tested protocols [reviewed in (Mooney *et al.,* 2008)]. Similarly, the concept of multiple doses of stem cell to repair the complex process of myocardial remodeling following acute myocardial infarction is gaining traction and is very appealing. While the field of stem cell regenerative therapy for ischemic heart disease is still in its infancy, the accelerated advances in a wide array of biological and biotechnological areas have rapidly propelled the field from the bench to clinical applications.
