ARG1 and pulmonary arterial hypertension: Since MSR1 is the critical maker for M2 macrophages, we next detected other M2 macrophage-related factors in lung tissues from IPF patients and found that the expression of IL-13 and Arg-1 in lung tissues from IPF patients was also significantly increased in IPF patients when compared with PAH patients (Figures 2(e)–2(h)).