Therapeutic potential of thymosin ß4 in acute and chronic ischemia

Acute cellular rejection of transplanted hearts has improved within the last years due to amelioration of the immunosuppressive treatment. However, the chronic rejection caused by cardiac allograft vasculopathy still remains a problem, limiting the survival time of the graft [1]. This vasculopathy is characterized by microcirculatory disturbance, caused by proliferation of smooth muscle cell, lymphocytes and macrophages [2]. Increasing vessel density and thereby enhancing micro‐ and macro circulation might be a therapeutic approach. Utilization of pro‐angiogenic factors, such as VEGF, to improve perfusion in these grafts has the disadvantage of increasing vascular permeability. Moreover, VEGF‐A may act as a pro‐inflammatory cytokine [3]. In contrast, an anti‐inflammatory factor providing pro‐angiogenic properties might offer favourable outcome of transplant vasculopathy.Thymosin ß4, an endogenously occurring peptide of 5 kDa, has been demonstrated to be cardioprotective after acute myocardial ischemia, at least in part by reducing postischemic inflammation [4,5]. Furthermore, Thymosin ß4 is essential for coronary vessel development and provides angiogenesis during wound healing of the adult organism [6,7]. These results indicate an anti‐inflammatory effect combined with a strong pro‐angiogenic potential of Thymosin ß4, both required therapeutically, in particular in a transplant setting.To explore the vascular growth potential of Thymosin ß4, we designed an adeno‐associated viral vector (AAV) containing Thymosin ß4 as transgene. The local over expression of Thymosin ß4 in a murine ischemic hindlimb model did not only increase capillarization, but also enhanced blood flow, in a strictly Akt dependent manner. Translation of the gene therapeutic approach to a pre clinical pig model of chronic myocardial ischemia confirmed the pro angiogenic and arteriogenic potential of the Thymosin ß4. Furthermore after 56 days of chronic myocardial ischemia global and regional myocardial as well as regional myocardial blood flow function were enhanced after Thymosin ß4 overexpression.We pursued the potential of Thymosin ß4 to vascularize transplant tissue in 2 models:In a dorsal skinfold chamber, which received engineered heart tissue propagated from neonatal rodent cardiomyocytes, vascularisation was significantly enhanced by exogenously applied Thymosin ß4 (in cooperation with T. Eschenhagen). Secondly, in an allogenic heart transplantation model (pig), where Thymosin ß4 is applied via AAV before transplantation, a better survival of cardiac grafts provided by reduced graft rejection was observed. Taken together, Thymosin ß4 might be a useful factor for chronic graft rejection, by reducing local inflammation and enhancing vascularization in the graft and thereby extent the survival time of the graft.References1. Taylor DO, Edwards LB, Aurora P, et al. Registry of the International Society for Heart and Lung Transplantation: twenty‐fifth official adult heart transplant report‐‐2008. J Heart Lung Transplant 2008; 27: 943–956.2. Pucci AM, Forbes RD, Billingham ME. Pathologic features in long‐term cardiac allografts. J Heart Transplant 1990; 9: 339–345.3. Reinders ME, Sho M, Izawa A, et al. Proinflammatory functions of vascular endothelial growth factor in alloimmunity. J Clin Invest 2003; 112: 1655–1665.4. Bock‐Marquette I, Saxena A, White MD, et al. Thymosin beta4 activates integrin‐linked kinase and promotes cardiac cell migration, survival and cardiac repair. Nature 2004; 432: 466–472.5. Hinkel R, El‐Aouni C, Olson T, et al. Thymosin beta4 is an essential paracrine factor of embryonic endothelial progenitor cell‐mediated cardioprotection. Circulation 2008; 117: 2232–2240.6. Goldstein AL, Hannappel E, Kleinman HK. Thymosin beta4: actin‐sequestering protein moonlights to repair injured tissues. Trends Mol Med 2005; 11: 421–429.7. Smart N, Risebro CA, Melville AA, et al. Thymosin beta‐4 is essential for coronary vessel development and promotes neovascularization via adult epicardium. Ann N Y Acad Sci 2007; 1112: 171–188.