GAS6 and fibrosis: Intriguingly, fibroblasts from idiopathic pulmonary fibrosis (IPF) lungs have high Axl, coinciding with loss of low-level MerTK.99Axl/Gas6 signaling in fibroblasts has been linked to fibroblast proliferation and the development of fibrosis after bleomycin-induced lung injury.99, 100Importantly, Axl has been shown to require PS along with Gas6 for kinase activation,101suggesting that AC or other dead cells generated during tissue damage may be required for this Axl-fibroblast-fibrosis connection.