PF4 and idiopathic pulmonary fibrosis: To investigate whether platelet-derived TGFβ1 has a direct profibrotic role in IPF, a conditional knockout transgenic mouse, in which TGFβ1 is deleted in megakaryocytes and subsequent platelets (TGFβ1fl/fl.PF4-Cre) (29) was used in the bleomycin-induced PF model.