Fibroblasts are central to wound healing and fibrosis through TGF1-triggered differentiation into contractile, ␣-smooth muscle actin (␣-SMA)-positive myofibroblasts. This is mediated by accumulation of a pericellular matrix of hyaluronan (HA) and the HA-dependent co-localization of CD44 with the epidermal growth factor receptor (EGFR). Interactions of HA with hyaladherins, such as inter-␣-inhibitor (I␣I) and tumor necrosis factor-stimulated gene-6 (TSG-6), are also essential for differentiation. This study investigated the mechanisms involved. TSG-6 and ␣-SMA had different kinetics of induction by TGF1, with TSG-6 peaking before ␣-SMA. Si CD44 or EGFR inhibition prevented differentiation but had no effect on TSG-6 expression. TSG-6 was essential for differentiation, and mAb A38 (preventing I␣I heavy chain (HC) transfer), HA-oligosaccharides, cobalt, or Si bikunin prevented TSG-6 activity, preventing differentiation. A38 also prevented the EGFR/CD44 association. This suggested that TSG-6/I␣I HC interaction was necessary for the effect of TSG-6 and that HC stabilization of HA initiated the CD44/EGFR association. The newly described HC5 was shown to be the principal HC expressed, and its cell surface expression was prevented by siRNA inhibition of TSG-6 or bikunin. HC5 was released by hyaluronidase treatment, confirming its association with cell surface HA. Finally, HC5 knockdown by siRNA confirmed its role in myofibroblast differentiation. The current study describes a novel mechanism linking the TSG-6 transfer of the newly described HC5 to the HA-dependent control of cell phenotype. The interaction of HC5 with cell surface HA was essential for TGF1-dependent differentiation of fibroblasts to myofibroblasts, highlighting its importance as a novel potential therapeutic target.The fibroblast is the most abundant cell type in normal connective tissues. It plays a central role in the synthesis, degradation, and remodeling of extracellular matrix both in health and in disease. At sites of tissue damage and in the context of wound healing, activated fibroblasts, termed myofibroblasts, with a contractile phenotype, characterized by the expression of the smooth muscle isoform of ␣-actin (␣-SMA), 3 are essential for the synthesis of a collagen-rich scar, providing the force for wound contraction (1). During a healing response, myofibroblasts are a transient cell population (2). Myofibroblasts are also the major effectors of fibrosis; their persistent presence has been established as the best marker of numerous types of progressive organ dysfunction, such as that seen in chronic renal failure of various etiologies (3-7), liver disease (8), and pulmonary fibrosis (9).The cytokine TGF1 is a well recognized mediator of progressive tissue fibrosis, and in vitro and in vivo evidence suggests that it is the primary driving force in fibroblast-myofibroblast phenotypic activation (10, 11). The sequence of events leading up to this transformation is being increasingly characterized at a molecular level in an attempt to understan...