Alternative Macrophage Programming After Clot Phagocytosis

Abstract

Abstract Repair program(s) in "alternatively activated" macrophages can help with short-term wound responses after sterile damage events. Clot resolution by fibrinolysis and phagocytosis by tissue macrophages can activate anti-inflammatory and pro-fibrotic signals like Transforming Growth Factor- β. It has been hypothesized by the late Andrew Tager and Rachel Chambers that some forms of human pulmonary fibrosis may involve chronic alveolar damage, leakage of blood components, clots, and subsequent pro-fibrotic events that then fail to remodel back to normal alveolar structure. We found evidence of chronic bleeds and hemosiderin-positive macrophages in the lungs of Platelet Endothelial Cell Adhesion Molecule-1 deficient mice that developed a progressive and fatal pulmonary fibrosis. We are testing this hypothesis in vitro using murine broncho-alveolar macrophages exposed to aged clots. Interestingly, fresh blood and clots were not efficiently phagocytosed by the murine myeloid leukemia cell line RAW264 nor by broncho-alveolar macrophages. Clots needed to be aged for two days first. Ingestion also causes the macrophages to become fluorescent, which is a useful way to assess phagocytosis. We are now developing mRNA sequencing protocols to assess the gene programs of macrophages over time.