SPP1 and pulmonary fibrosis: Interestingly, FN1+SPP1+ monocyte-derived macrophages are believed to be involved in the pathogenesis of canine idiopathic pulmonary fibrosis (5, 33) and profibrotic SPP1+ monocyte-derived macrophages were also reported in human COVID-19, pulmonary fibrosis and lung cancer (4).