Tumors form in hearts injected with undifferentiated ES cells (top) but not when ES cells are predifferentiated (bottom).
Directing ES cells to become a particular tissue type in vivo is an inefficient process. Consequently, transplanted ES cell populations may give rise to tumors in recipient hosts. Behfar et al. reasoned that delivery of partially differentiated ES cells may result in safe outcome provided that a modulator of differentiation shuts down troublesome tumor pathways.
When ES cells are delivered in vivo, their tumorigenicity is reduced in mice that suffer heart attacks. This is associated with TNF production. But TNF had not been tested for its ability to direct cardiomyocyte differentiation, although ES cells do express receptors for TNF. So the authors wondered whether TNF-driven signaling would be enough to guide cells into a differentiation pathway and thus prevent tumors in ES cell recipient mice.
The team engineered mice to express high levels of TNF in their hearts, and found that the transfer of undifferentiated ES cells did not lead to tumor formation. When ES cells were treated with TNF-induced supernatants in vitro, they not only expressed genes necessary for heart function but also down-regulated oncogenes as they differentiated into cardiac progenitor cells. In mice induced to suffer heart-attacks, these cells integrated into heart muscle, repaired scar tissue, and improved heart function.
TNF reduced the risk of tumor development by inducing the secretion of pro-cardiogenic factors including TGFβ. TGFβ also suppresses host immunity and may therefore reduce the likelihood of rejection of ES-derived cells. The effect of this immunosuppression on the weakened heart's ability to fight infections is not yet clear, so the team plans to test autologous stem cells derived from the host's own bone marrow. This option may remove the need for immunosuppression and improve the safety of stem cell therapy. 
