CD4 and idiopathic pulmonary fibrosis: We and others have shown that in IPF, the expression of PD-L1’s cognate receptor PD-1 is increased on the surface of CD4+ T cells, strongly suggesting that the PD-1–PD-L1 (CD274) interaction plays an important functional role in the context of lung fibrosis (Supplemental Figure S1 and [33]).