Transforming growth factor β-at the centre of systemic sclerosis

Transforming growth factor beta (TGF-beta) has long been implicated in fibrotic diseases, including the multisystem fibrotic disease systemic sclerosis (SSc). Expression of TGF-beta-regulated genes in fibrotic skin and lungs of patients with SSc correlates with disease activity, which points to this cytokine as the central mediator of pathogenesis. Patients with SSc often develop pulmonary arterial hypertension (PAH), a particularly lethal complication caused by vascular dysfunction. Several genetic diseases with vascular features related to SSc, such as familial PAH and hereditary haemorrhagic telangiectasia, are caused by mutations in the TGF-beta-sensing ALK-1 signalling pathway. These observations suggest that increased TGF-beta signalling causes both vascular and fibrotic features of SSc. The question of how latent TGF-beta becomes activated in local SSc tissues is, therefore, central to the understanding of SSc. Both TGF-beta 1 and TGF-beta 3 can be activated by integrins alpha(v)beta(6) and alpha(v)beta(8), whose upregulation in bronchial epithelial cells can activate TGF-beta in SSc lungs. Other alpha(v) integrins, thrombospondin-1 or altered TGF-beta sequestration by matrix proteins might be important in other target tissues. How the immune system triggers this process remains unclear, although links between inflammation and TGF-beta activation are emerging. Together, these observations provide an increasingly secure framework for understanding TGF-beta in SSc pathogenesis.