Prepared for special issue on thymosins Thymosin 4 and the anti-fibrotic switch

Wound healing involves a rapid response to the injury by circulating cells, followed by inflammation with an influx of inflammatory cells that release various factors. Soon after, cellular proliferation begins to replace the damaged cells and extracellular matrix, and then tissue remodeling restores normal tissue function. Various factors can lead to pathological wound healing when excessive and irreversible connective tissue/extracellular matrix deposition occurs, resulting in fibrosis. The process is initiated when immune cells, such as macrophages, release soluble factors that stimulate fibroblasts. TGF beta is the most well-characterized macrophage derived profibrotic mediator. Other soluble mediators of fibrosis include connective tissue growth factor (CTGF), platelet-derived growth factor (PDGF), and interleukin 10 (IL-10). Thymosin beta 4 (T beta 4) has shown therapeutic benefit in preventing fibrosis/scarring in various animal models of fibrosis/scarring. The mechanism of action of T beta 4 appears related, in part, to a reduction in the inflammatory response, including a reduction in macrophage infiltration, decreased levels of TGF beta and IL-10, and reduced CTGF activation, resulting in both prevention of fibroblast conversion to myofibroblasts and production of normally aligned collagen fibers. The amino N-terminal end of T beta 4, SDKP (serine-aspartate-lysine-proline), appears to contain the majority of anti-fibrotic activity and has shown excellent efficacy in many animal models of fibrosis, including liver, lung, heart, and kidney fibrosis. Ac-SDKP not only prevents fibrosis but can reverse fibrosis. Unanswered questions and future directions will be presented with regard to therapeutic uses alone and in combination with already approved drugs for fibrosis.