Since its introduction as a therapeutic concept over 10 years ago, BMMC therapy has been initiated in a variety of formats worldwide. Most clinical trials have yielded mixed results 32, 33
. Two meta-analysis of 18 and 10 trials, respectively, evaluating the effect of intracoronary infusion of BMMCs for the treatment of acute MI have shown marginal benefits at best (≈3–4% increase in LVEF) 34, 35
. Given that autologous cell preparations represent a therapeutic product of far greater complexity than traditional drugs, in-depth study of their in vivo
behavior is required. However, despite the ongoing clinical work, detailed molecular analyses of BMMCs after
transplantation into myocardium remain unknown. In particular, little is known regarding the early biological activity of BMMCs following their delivery into a novel myocardial environment, including the early transcriptional events that might dictate cell survival and therapeutic efficacy.
In the present study, we have attempted to address these issues and provide insight as to how BMMC therapy can be improved for enhanced success in the clinic realm (Supplemental Figure 1
). We utilized molecular techniques and a clinically relevant model of acute ischemic myocardial injury to demonstrate that 1) the ischemic myocardial environment induces a proliferative response in the transplanted cell population, but long-term engraftment is limited, 2) transcriptional profiling of cells transplanted into the heart reveals up-regulation of numerous “house-keeping” and cell proliferation genes, but a marked down-regulation of pathways regulating differentiation and maturation, and 3) transplanted cells induce a mild, transient improvement in cardiac function that does not appear to be long-lasting.
Our molecular imaging results confirm earlier observations from cell therapy studies employing chronic ischemia models that cell survival is, at best, limited to a few weeks 20, 22
. Several other investigators have postulated that limited survival may partially explain why BMMC therapy has not achieved the success suggested by early animal studies 32, 36, 37
. This observation highlights one of the major hurdles facing BMMC therapy in that, even in ideal conditions such as our control animal group (i.e., syngeneic, non-injured myocardium), transplanted cell grafts persisted for a limited time in host tissue. These survival data also raise concern regarding the hypothesis that improvement in cardiac function following cell therapy may be due to secretion of paracrine factors by the transplanted cells 38, 39
. If paracrine signaling is a primary mechanism underlying improved cardiac function, intuitively long-term secretion from a sufficient number of surviving cells would be more beneficial for sustained benefit.
Although long-term survival of BMMCs following acute ischemic injury is limited, early post-transplantation cell behavior appears a dynamic and active process. Utilizing molecular imaging and gene profiling techniques, we were able to discern a differential pattern of early
post-transplant survival and proliferation of BMMCs in the I/R injured hearts as compared to non-ischemic myocardium, confirming previous finding 21
. The implication that the inflammatory nature of the post-infarcted myocardium may affect transplanted cell behavior is rather intuitive and there is evidence to show that the intense inflammatory reaction occurring after myocardial infarction may produce chemokines responsible for lymphocyte tracking and stem cell homing to the damaged myocardium 22, 40, 41
. However, the post-infarct inflammatory milieu also appears to have a dual effect. On the one hand, it serves to activate homing and promote a proliferative response amongst transplanted cells as observed in the present study. On the other hand, it may create a hostile environment, resulting in indiscriminate cellular activation and turning cells away from stable maturation fates 21, 31
. Given the results of the present study, investigations into the optimal BM cell fraction, cell dose, timing, and delivery modality are expected to add much-needed insight into an improved strategy for making cell transplantation an effective therapeutic tool 42
. In addition, understanding the response of transplanted cells to their new host environment will be essential to exploit the possible therapeutic potential of BMMCs. It is with this premise that we chose to examine the early transcriptional events within BMMCs as described in this study.
The gene-profiling findings presented here represent a novel dataset in the growing body of literature examining BMMC therapy for heart disease. To our knowledge, this is the first
study evaluating (on a transcriptional level) the transcriptional response of BMMCs in the ischemic heart. Our gene-ontology analysis shows that the overwhelming cellular response to transplantation is to simply survive in the new environment, rather than activate pathways implicated in engraftment and long-term survival. Specifically, we saw no significant activation of long-term proliferation, differentiation, or maturation pathways in the transplanted BMMCs. Rather, BMMCs activated macromolecule synthesis and cellular machinery genes, suggesting a fraught attempt to mitigate cell death. Of particular interest are the GO terms demonstrating the most robust upregulation, such as “cell cycle”, “cytoskeletal protein binding”, and “establishment of localization”. These groups encompass genes responsible for rapid cell growth and division, confirming the cell behavior observed through BLI and lending credence to the notion of a provocative inflammatory milieu. Of equal importance are the findings of significantly down-regulated groups of genes clustering in the areas of “development”, “cell differentiation”, “pattern specification”, and “cell fate commitment”. These provide transcriptional-level evidence that BMMCs were not able to adopt mature cardiac phenotypes shortly after transplantation in our acute ischemia model 30
. However, the interpretation of the array analysis has some limitations. BMMCs are a heterogeneous cell population representing portions of hematopoietic cells as well as macrophages, granulocytes, and natural killer cells 20
. Therefore, we cannot completely exclude the possibility that the changes observed in the array data may be impacted by changes in the surviving cell population over the time as opposed to transcriptional changes in specific cell populations. Further studies injecting purified cell populations would be needed to tease out the transcriptomic response of specific cell lineages.
In summary, we present a multimodal evaluation of BMMC therapy in a clinically relevant animal model. We have detailed the earliest transcriptional events following transplant, described long-term transplanted cell survival kinetics, and characterized the eventual effects upon cardiac function following transplant. Our findings mirror present clinical experience, which shows limited functional improvement following BMMC therapy 33, 34
. The survival data provided by BLI suggest that therapeutic efficacy may be limited by insufficient cell survival, while our genomic data suggest very active transplanted cellular machinery responding to an acutely inflamed host tissue environment. Successful therapy generating long-term improvement in cardiac function may very well depend on modulation of both the host environment and the transplanted cell transcriptional response to ensure optimal BMMC engraftment and survival.