Serum amyloid P component (SAP) binds to late apoptotic cells and mediates their phagocytosis by macrophages

Background

Serum components, like serum amyloid P component (SAP), together with membrane receptors on phagocytes play essential roles in the phagocytosis of apoptotic cells. Disturbances in one of these factors might reduce phagocytosis and induce autoimmunity. SAP binds to apoptotic cells. SAP deficient mice spontaneously develop autoimmunity. We evaluated SAP binding to early and late apoptotic cells and whether this binding has functional consequences for the phagocytosis of these cells.

Methods

Human peripheral blood monocytes were isolated and cultured for 7 days to obtain monocyte derived macrophages. Jurkat cells were irradiated with UVB to induce apoptosis. After 4 hours 40% of cells stained with annexinV, and were propidiumiodide negative (early apoptotic cells, EA). After 24 hours 65% of cells were annexinV and propidiumiodide positive (late apoptotic cells, LA). EA and LA cells were incubated with FITC labeled SAP in the presence or absence of Ca²⁺ and subsequent binding was measured by flowcytometry. Phagocytosis was performed by incubation of macrophages for 30 minutes with EA or LA cells in the presence of human serum (HS) and depicted as phagocytosis index (PI, number of Jurkatt internalized by 100 macrophages). Experiments were repeated with SAP depleted serum and after reconstitution with different concentrations of SAP.

Results

Sixty percent of LA cells did bind SAP in the presence of Ca²⁺, whereas the EA cells did not. SAP depletion of serum resulted in a 50% decrease of PI for LA cells, and complete restoration of PI could be demonstrated with SAP reconstitution up to 100 μg/ml. SAP depletion had no effect on PI of EA cells.

Conclusion

SAP binds to late apoptotic cells and is involved in the phagocytosis of these cells by human monocyte derived macrophages. This might have consequences for diseases in which phagocytosis of early apoptotic cells is decreased.