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Serum-derived protein S binds to phosphatidylserine and stimulates the phagocytosis of apoptotic cells

Nature Immunologyvolume 4pages8791 (2003) | Download Citation



Rapid phagocytosis of apoptotic cells is thought to limit the development of inflammation and autoimmune disease. Serum enhances macrophage phagocytosis of apoptotic cells. Here we identified protein S as the factor responsible for serum-stimulated phagocytosis of apoptotic cells. Protein S is best known for its anti-thrombotic activity, serving as a cofactor for protein C. Purified protein S was equivalent to serum in its ability to stimulate macrophage phagocytosis of apoptotic lymphoma cells, and immunodepletion of protein S eliminated the prophagocytic activity of serum. Protein S acted by binding to phosphatidylserine expressed on the apoptotic cell surface. Protein S is thus a multifunctional protein that can facilitate clearance of early apoptotic cells in addition to regulating blood coagulation.

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We thank V. Calvert for supplying human monocytes; E. Shores and G. Tosato for critical reading of the manuscript; and E. Petricoin and Y. M. Zhao for expert technical advice.

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Author notes

  1. Howard A. Anderson and Caroline A. Maylock: These authors contributed equally to this work.


  1. Division of Therapeutic Proteins, Laboratory of Biochemistry, Center for Biologics Evaluation and Research, Food and Drug Administration, Bethesda, 20892, MD, USA

    • Howard A. Anderson
    •  & Emily Shacter
  2. Department of Pediatrics, Uniformed Services University of the Health Sciences, Bethesda, 20815, MD, USA

    • Caroline A. Maylock
  3. Experimental Immunology Branch, National Cancer Institute, National Institutes of Health, Bethesda, 20892, MD, USA

    • Joy A. Williams
  4. Laboratory of Pathology, National Cancer Institute, National Institutes of Health, Bethesda, 20892, MD, USA

    • Cloud P. Paweletz
  5. Department of Pharmacology, University of Texas, Southwestern Medical Center, Dallas, 75390, TX, USA

    • Hongjun Shu


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Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Emily Shacter.

Supplementary information

  1. Web Fig. 1.

    Identification of protein S in the prophagocytic serum fraction. (a) Two-dimensional gel of the purified fraction stained with Coomassie blue. Proteins that were identified by tandem mass spectrometry (MS) are indicated (Pr.S, protein S; Kin., kininogen, BSA, bovine serum albumin). (b) MS spectra of tryptic digests of the Coomassie blue-stained protein S spot. CID represents collision-induced dissociation of peptide analyzed in c. (c) MS/MS profile analysis of ion 908.50 identifying protein S from the unique peptide sequence VYFAGVPR. Peptides from kininogen (YSIVFIAR) and BSA (KVPQVSTPTLVEVSR) were analyzed similarly (data not shown). (PDF 162 kb)

  2. Web Fig. 2.

    Immunoanalysis of protein S in enriched phagocytosis fraction. Proteins from serum (25 μg), the purified prophagocytic fraction from serum (5 μg) and human protein S (5 μg) were resolved by SDS-PAGE and stained for protein with Sypro Ruby Protein Gel Stain (Molecular Probes). Immunoblotting of proteins transferred to PVDF membranes was performed using a rabbit anti-protein S IgG primary antibody followed by HRP-conjugated goat anti-rabbit IgG. The three polypeptides seen in the commercial human protein S preparation and the differences in molecular weight observed between bovine and human protein S are most likely attributed to the numerous post-translational modifications that are known to occur to this molecule (for example, proteolysis, glycosylation, γ-glutamyl carboxylation). (PDF 195 kb)

  3. Supplementary information (DOC 11 kb)

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