Macrophages that have ingested apoptotic neutrophils release tissue inhibitor of matrix metalloproteinase 1

Abstract

Abstract Background Apoptosis of neutrophils (PMNs) in vivo is followed by their rapid uptake into adjacent phagocytic cells, a critical process in tissue remodelling, regulation of the immune response and resolution of inflammation. Tissue inhibitor of matrix metalloproteinase (TIMP) 1 is crucial in regulating the matrix-degrading properties of matrix metalloproteinases released by activated PMNs in inflammatory foci. The aim was to elucidate whether mononuclear-derived phagocytes (MDPs) release TIMP-1 and/or MMP-9 after phagocytosis of apoptotic PMNs. Methods MDPs were prepared from peripheral blood monocytes after isolation by culture in well plates for 7–10 days. PMNs were isolated from similarly healthy volunteers. PMN apoptosis was induced by 10 min of ultraviolet irradiation at 254 nm. After a further 6 h of incubation apoptosis was assessed and was routinely greater than 50 per cent. Apoptotic PMNs were then added to MDPs in specific ratios. After 1 h non-phagocytosed PMNs were washed away. RPMI was added to MDPs in well plates and 18 h later the supernatant was collected. Phagocytosis of opsonized zymosan, exposure to tumour necrosis factor (TNF) α and lipopolysaccharide (LPS), and finally MDPs alone were used as controls. TIMP-1 was quantified by enzyme-linked immunosorbent assay. Results TIMP-1 was released by MDPs after phagocytosis of apoptotic PMNs and opsonized zymosan. MDPs also produced TIMP-1 after stimulation with TNF-α–LPS. Levels of TIMP-1 from these MDPs were significantly different from those in MDPs alone. Conclusion Phagocytosis of apoptotic PMNs is an integral part of the host's attempt to resolve inflammation in altered immune states such as the systemic inflammatory response syndrome in which PMNs persistently release proinflammatory substances damaging the local microenvironment. This study has demonstrated that MDPs release TIMP-1 after phagocytosis of apoptotic PMNs and that this may play a role in the regulation of tissue remodelling and repair in response to inflammation.