The involvement of activin A in PPARγ expression in GM-CSF-dependent human macrophages points toward a connection between Smad2/3 activation and PPARG gene expression, whose existence has been already suggested in PAP patients, which exhibit a deficiency in PPARγ and a severe reduction in Activin A expression and secretion (42). The gene discussed is PPARG; the disease is pulmonary alveolar proteinosis.