Circulating C1q levels in health and disease, more than just a biomarker. (December 2021)

• Record Type: Journal Article
• Authors: van de Bovenkamp, Fleur S.; Dijkstra, Douwe J.; van Kooten, Cees; Gelderman, Kyra A.; Trouw, Leendert A.
• Is Part Of: Molecular immunology. Volume 140(2021)
• Page Start: 206
• Page End: 216
• Publication Date: 2021-12
• Subjects: C1q -- Complement system -- Classical pathway -- Biomarker -- Adaptive immunity -- Tolerance; Immunochemistry -- Periodicals; Molecular biology -- Periodicals; Immunochemistry -- Periodicals; Allergy and Immunology -- Periodicals; Molecular Biology -- Periodicals; Immunochimie -- Périodiques; Biologie moléculaire -- Périodiques; Immunochemistry; Molecular biology; Periodicals; Electronic journals; 571.96
• DOI: 10.1016/j.molimm.2021.10.010
• Languages: English
• ISSNs: 0161-5890

Abstract

Highlights: C1q levels increase with age but are similar between men and women. Decreased C1q levels have been observed in several autoimmune diseases and can be caused by defective production or increased consumption. Increased C1q levels have been reported for infectious and inflammatory diseases and is suggested to inhibit the adaptive immune response. C1q can protect against infections via the classical pathway, and maintain tolerance by regulating adaptive immunity, likely via C1q receptors. Abstract: C1q is the recognition molecule of the classical pathway of the complement system. By binding to its targets, such as antigen-bound immunoglobulins or C-reactive protein, C1q contributes to the innate defense against infections. However, C1q also plays several other roles beyond its traditional role in complement activation. Circulating levels of C1q are determined in routine diagnostics as biomarker in several diseases. Decreased C1q levels are present in several autoimmune conditions. The decreased levels reflect the consumption of C1q by complement activation and serves as a biomarker for disease activity. In contrast, increased C1q levels are present in infectious and inflammatory diseases and may serve as a diagnostic biomarker. The increased levels of C1q are still incompletely understood but are suggested to modulate the adaptive immune response as C1q is known to impact on the maturation status of antigen-presenting cells and C1q impacts directly on T cells leading to decreased T-cell activity in high C1q conditions. In this review, we provide a comprehensive overview of the current literature on circulating levels of C1q in health and disease, and discuss how C1q can both protect against infections as well as maintain tolerance by regulating adaptive immunity.