Session Title: Innate Immunity and Rheumatic Disease
Session Type: Abstract Submissions (ACR)
Background/Purpose: Effective clearance of apoptotic cells (AC) is necessary for the maintenance of tolerance to self. MFG-E8 is one of ~20 different opsonins reported to promote AC clearance in mice. Deficiency of MFG-E8 on certain strain backgrounds promotes a lupus-like disease in mice. However, very little is known about the expression and function of MFG-E8 in humans, and its potential role in systemic lupus erythematosus (SLE). The goals of this study were to examine the expression and function of MFG-E8 in human phagocytes and its potential role in SLE.
Methods: Macrophages and dendritic cells (DC) were derived from monocytes by culture with M-CSF or GM-CSF and IL-4 respectively. MFG-E8 expression and secretion were quantified by flow cytometry (FACS) and ELISA respectively. Phagocytosis of AC was quantified by microscopy (phagocytic index) and by FACS using CFSE-labeled AC. Phagocytes were stimulated by LPS (1 ng/ml) or zymosan (50ug/ml) and cytokines quantified by ELISA.
Results: MFG-E8 expression and function was very different in human macrophages and DCs. Unlike murine thioglycollate-elicited macrophages, human macrophages did not express or secrete MFG-E8. However, when MFG-E8 was added to AC, increased cell clearance was observed in a dose responsive manner. Furthermore, specific addition of MFG-E8 to AC resulted in suppression of TNF and IL-1b in response to LPS and zymosan. In striking contrast, human DCs expressed high levels of intracellular and cell surface MFG-E8 but exogenous MFG-E8 addition to AC did not enhance DC-mediated AC uptake. Remarkably, at higher doses MFG-E8 actually exerted an inhibitory effect on AC clearance by DC.
Conclusion: These results suggest that MFG-E8 plays an important role in clearance of AC by macrophages in humans. In addition, coating of AC by MFG-E8 alone enhances the known anti-inflammatory effects of AC by further dampening the production of pro-inflammatory cytokines such as TNF and IL-1b. Surprisingly, human DC did not seem to be functionally impacted by MFG-E8 coated AC despite or, perhaps, because MFG-E8 was expressed at high concentrations on the surface of these cells. We propose that MFG-E8 plays a central role in AC clearance and dampening inflammatory responses in human macrophages. It should therefore be considered as a therapeutic to enhance AC clearance and suppress inflammation in lupus and, possibly, other autoimmune disorders.
K. B. Elkon,
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ACR Meeting Abstracts - https://acrabstracts.org/abstract/expression-and-function-of-human-mfg-e8-in-the-clearance-of-apoptotic-cells-and-suppression-of-inflammation/