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The inflammatory macrophage: a story of Jekyll and Hyde
1Medical Research Council Centre for Inflammation Research, Medical School, University of Edinburgh, Teviot Place, Edinburgh EH8 9AG, Scotland, UK. j.duffield@ed.ac.uk
Abstract:
Recent investigations have highlighted new roles for the macrophage (Mphi) in the biology of inflammation. Selective depletion of Mphis from inflamed sites has confirmed their predominant role in immune-mediated damage. The components of this injury have been dissected. Mphis mediate death of stromal, parenchymal and other immune cells by engaging the death programme, resulting in apoptosis. In addition, Mphis induce destruction of matrix and extracellular structures both directly and indirectly by inducing stromal cells to release matrix metalloproteinases. However, there is another side to the inflammatory Mphi. Evidence is provided that Mphis at the same sites possess the ability to aid cell proliferation, secrete and stabilize new matrix components and induce resident cells to secrete matrix components themselves. Mphi phagocytosis of apoptotic cells brings about a change from the cell-killing matrix-degrading cell to the matrix-generating cell-proliferating tissue-healing cell. Just as both Mphi types are necessary at the inflamed site, the right balance of these two populations is required for healing and resolution. Evidence of excessive inflammation as a manifestation of impaired phagocytosis of apoptotic cells emphasizes that defects in the transition from one Mphi type to another may account for the uncontrolled excessive inflammation seen in disease. Recent insights into the mechanisms by which apoptotic cells signal the change of function to the Mphi offer the prospect of novel targets for manipulation of Mphis in the inflamed tissue.
Insights
Macrophages (Mphi) play dual roles in inflammation, causing damage and promoting healing. Their function shifts from cell killing to tissue repair upon engulfing apoptotic cells, crucial for resolving inflammation.
Area of Science:
- Immunology
- Cell Biology
- Tissue Repair
Background:
- Macrophages (Mphi) are key immune cells involved in inflammation.
- Their roles in immune-mediated damage and tissue destruction are well-established.
- Emerging evidence suggests Mphi also contribute to tissue repair and resolution.
Purpose of the Study:
- To elucidate the dual functions of macrophages in inflammation.
- To investigate the transition of Mphi from a pro-inflammatory to a pro-resolving phenotype.
- To identify mechanisms regulating Mphi function for therapeutic potential.
Main Methods:
- Selective depletion of macrophages from inflamed tissues.
- Analysis of Mphi-mediated cell death and matrix degradation.
- Assessment of Mphi-induced cell proliferation and matrix synthesis.
- Investigation of the role of apoptotic cell phagocytosis in Mphi functional switching.
Main Results:
- Macrophages mediate immune-mediated damage by inducing apoptosis and matrix degradation.
- Engulfment of apoptotic cells transforms Mphi into tissue-healing cells that promote proliferation and matrix stabilization.
- A balance between pro-inflammatory and pro-resolving Mphi populations is essential for effective tissue repair.
- Impaired phagocytosis of apoptotic cells leads to excessive inflammation.
Conclusions:
- Macrophages exhibit distinct, context-dependent functions in inflammation, contributing to both damage and repair.
- The transition of Mphi from a destructive to a regenerative phenotype, triggered by apoptotic cell clearance, is critical for resolving inflammation.
- Dysregulation of this Mphi functional switch, particularly impaired phagocytosis, underlies uncontrolled inflammation and suggests novel therapeutic targets.