Production of proinflammatory cytokines by phorbol myristate acetate-treated THP-1 cells and monocyte-derived macrophages after phagocytosis of apoptotic CTLL-2 cells.

Because it is generally believed that apoptosis is not associated with inflammation, we hypothesized that the interaction of phagocytes with apoptotic cells provides a negative or null signal for inflammation. However, we recently found that the interaction led to the production of proinflammatory cytokines but not antiinflammatory cytokines, although the apoptotic cell membranes appeared to be intact. In this study, we examined in detail the relationship among the kinetics of apoptosis, phagocytosis and production of cytokines by macrophages. Among the time points examined, murine CTLL-2 cells became apoptotic in terms of cell size and exposure of phosphatidylserine after 12 h of culture in the absence of IL-2, and at the same time they began to be phagocytosed and lead to proinflammatory cytokine production by PMA-treated THP-1 cells (human macrophages). The phagocytosis of apoptotic cells by macrophages was also confirmed by confocal laser microscopy. The coculturing of human macrophages with murine apoptotic cells led to the production of human proinflammatory cytokines, notably IL-8, at both the mRNA level and the protein level. The coculturing of monocyte-derived macrophages with the apoptotic cells also led to the production of IL-8 protein. Both the phagocytosis and production of the cytokines were suppressed by either phospho-L-serine or RGDS (Arg-Gly-Asp-Ser), but not by RGES (Arg-Gly-Glu-Ser). Thus, the production of proinflammatory cytokines and phagocytosis of apoptotic CTLL-2 cells appear to be closely interrelated.