CXCR4 and pulmonary fibrosis: Notably, the combination of CXCR4 and COL1 expression can identify fibrocytes in idiopathic pulmonary fibrosis.29 As the disc fibrocytes highly express CXCR4 and CCR1 (Table 2), they might therefore communicate with the ProgNP population via the CXCL12-CXCR4 axis.