Polyphosphate is a novel cofactor for regulation of complement by a serpin, C1 inhibitor

• Published in: Blood Vol. 128; no. 13; pp. 1766 - 1776
• Main Authors: Wijeyewickrema, Lakshmi C., Lameignere, Emilie, Hor, Lilian, Duncan, Renee C., Shiba, Toshikazu, Travers, Richard J., Kapopara, Piyushkumar R., Lei, Victor, Smith, Stephanie A., Kim, Hugh, Morrissey, James H., Pike, Robert N., Conway, Edward M.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 29-09-2016; American Society of Hematology
• Subjects: Binding Sites; Blood Platelets - immunology; Blood Platelets - metabolism; Cells, Cultured; Complement C1 Inactivator Proteins - metabolism; Complement C1 Inhibitor Protein; Complement C1s - chemistry; Complement C1s - metabolism; Complement C2 - metabolism; Complement C4 - metabolism; Complement Pathway, Classical; Complement System Proteins - metabolism; Endothelial Cells - immunology; Endothelial Cells - metabolism; Heparin - metabolism; Humans; In Vitro Techniques; Polyphosphates - chemistry; Polyphosphates - metabolism; Thrombosis and Hemostasis
• ISSN: 0006-4971; 1528-0020
• DOI: 10.1182/blood-2016-02-699561

The complement system plays a key role in innate immunity, inflammation, and coagulation. The system is delicately balanced by negative regulatory mechanisms that modulate the host response to pathogen invasion and injury. The serpin, C1-esterase inhibitor (C1-INH), is the only known plasma inhibitor of C1s, the initiating serine protease of the classical pathway of complement. Like other serpin-protease partners, C1-INH interaction with C1s is accelerated by polyanions such as heparin. Polyphosphate (polyP) is a naturally occurring polyanion with effects on coagulation and complement. We recently found that polyP binds to C1-INH, prompting us to consider whether polyP acts as a cofactor for C1-INH interactions with its target proteases. We show that polyP dampens C1s-mediated activation of the classical pathway in a polymer length- and concentration-dependent manner by accelerating C1-INH neutralization of C1s cleavage of C4 and C2. PolyP significantly increases the rate of interaction between C1s and C1-INH, to an extent comparable to heparin, with an exosite on the serine protease domain of the enzyme playing a major role in this interaction. In a serum-based cell culture system, polyP significantly suppressed C4d deposition on endothelial cells, generated via the classical and lectin pathways. Moreover, polyP and C1-INH colocalize in activated platelets, suggesting that their interactions are physiologically relevant. In summary, like heparin, polyP is a naturally occurring cofactor for the C1s:C1-INH interaction and thus an important regulator of complement activation. The findings may provide novel insights into mechanisms underlying inflammatory diseases and the development of new therapies.
•PolyP, like heparin, is a physiologic cofactor for the C1-INH:C1s interaction, and thus a key negative regulator of complement.•Colocalization of polyP with C1-INH in activated platelets suggests that its cofactor function is physiologically relevant.