TGFB1 and pemphigus foliaceus: The enhancement of PDE1 expression and activity in the lung parenchyma is believed to be one of the crucial mechanisms of PF development and progression by promoting TGFβ1 secretion through cAMP and cGMP depletion, which leads to fibroblast/myofibroblast transformation, myofibroblast activation, and extracellular matrix deposition ultimately ending in PF (11, 43).