CCR2 and pulmonary fibrosis: Few studies have been conducted on the function of monocytes during pulmonary fibrosis, and their functions are still controversial: some scholars believe that the release of TGF-β1 by monocytes inhibits collagen degradation and exacerbates pulmonary fibrosis [9, 10], and some scholars believe that C–C motif chemokine receptor 2 (CCR2)+ monocytes can inhibit lung fibrosis [11].