Ischemic cardiovascular disease represents one of the largest epidemics currently facing the aging population. Current literature has illustrated the efficacy of autologous, stem cell therapies as novel strategies for treating these disorders. The CD34+ hematopoetic stem cell has shown significant promise in addressing myocardial ischemia by promoting angiogenesis that helps preserve the functionality of ischemic myocardium. Unfortunately, both viability and angiogenic quality of autologous CD34+ cells decline with advanced age and diminished cardiovascular health.

To offset age- and health-related angiogenic declines in CD34+ cells, we explored whether the therapeutic efficacy of human CD34+ cells could be enhanced by augmenting their secretion of the known angiogenic factor, sonic hedgehog (Shh).

When injected into the border zone of mice after acute myocardial infarction, Shh-modified CD34+ cells (CD34^Shh) protected against ventricular dilation and cardiac functional declines associated with acute myocardial infarction. Treatment with CD34^Shh also reduced infarct size and increased border zone capillary density compared with unmodified CD34 cells or cells transfected with the empty vector. CD34^Shh primarily store and secrete Shh protein in exosomes and this storage process appears to be cell-type specific. In vitro analysis of exosomes derived from CD34^Shh revealed that (1) exosomes transfer Shh protein to other cell types, and (2) exosomal transfer of functional Shh elicits induction of the canonical Shh signaling pathway in recipient cells.

Exosome-mediated delivery of Shh to ischemic myocardium represents a major mechanism explaining the observed preservation of cardiac function in mice treated with CD34^Shh cells.