Production of Proinflammatory Cytokines by Resident Tissue Macrophages after Phagocytosis of Apoptotic Cells

It is generally believed that apoptosis is not associated with inflammation. However, we have found that phagocytosis of apoptotic cells by PMA-treated THP-1 cells and human monocyte-derived macrophages led to the production of proinflammatory cytokines, notably IL-8. These macrophages were obtained either by PMA treatment or by M-CSF treatment, possibly affecting the cytokine production after phagocytosis of apoptotic cells. In order to exclude the possibility, we employed resident tissue macrophages such as Kupffer cells and alveolar macrophages in this study and examined the production of cytokines after phagocytosis of apoptotic cells. Kupffer cells produced proinflammatory cytokines MIP-2 and TNF-α at the mRNA level. The MIP-2 protein was also detected by means of ELISA. Alveolar macrophages also produced the MIP-2 protein after phagocytosis of apoptotic cells. Furthermore, apoptotic thymocytes induced a similar response by these macrophages. These findings do support the notion that macrophages are apt to produce proinflammatory cytokines after phagocytosis of apoptotic cells.