Evaluating the differential expression of TAM family receptors and efferocytosis activities in differentiated and polarized THP-1 macrophage

Tissue homeostasis is tightly balanced between cell death and renewal. Each day, as many as 1011 cells die in the human body that need to be removed and replaced. The clearance of apoptotic cells, termed efferocytosis, is crucial to tissue homeostasis. Central to this process is macrophage-mediated efferocytosis. Apart from general phagocytosis, efferocytosis is apoptotic cell-specific, which aids in clearing dead cells and preventing the accumulation of self-antigens released by the apoptotic cells. The TAM family receptor kinases (TYRO3, AXL, MERTK) and ligands (GAS6 and PROS) play important roles in engaging the apoptotic cells to initiate efferocytic engulfment and the downstream cellular responses. Dysregulated efferocytic function in macrophages is associated with human diseases such as atherosclerosis, lupus, lung fibrosis, and cancer. Conversely, understanding the regulation and molecular mechanisms of macrophage efferocytosis can potentially lead to beneficial treatments for the above diseases. Despite numerous efferocytosis studies that use primary and cell line-derived macrophages, there has not been a thorough characterization of a cell line system that can be reliably used for efferocytosis assays. Consequently, many macrophage efferocytosis assays reported do not clearly distinguish efferocytosis from phagocytosis. Here we evaluated the THP-1 cell line as a potential human macrophage cell line system for efferocytosis studies. Consequently, many macrophage efferocytosis assays reported do not clearly distinguish efferocytosis from phagocytosis. Through the study we examined the differential expression of the TAM family receptors and their ligands in the various THP-1 macrophage differentiation and polarization states. We also characterized the THP-1 cell line as a reliable system for performing in vitro efferocytosis studies.