PDCD1 and idiopathic pulmonary fibrosis: In idiopathic pulmonary fibrosis (IPF), an overexpression of mPD-L1 has been reported on different cell types, not only invasive lung fibroblasts but also alveolar macrophages, whereas PD-1 was found to be overexpressed on CD4+ T-cells, and to contribute to the inhibition of their differentiation into Treg cells [82,84,157].