Biochemical and functional characterization of the interaction between pentraxin 3 and C1q

Abstract

Pentraxin 3 (PTX3) is a recently characterized member of the pentraxin family of acute-phase proteins produced during inflammation. Classical short pentraxins, C-reactive protein, and serum amyloid P component can bind to C1q and thereby activate the classical complement pathway. Since PTX3 can also bind C1q, the present study was designed to define the interaction between PTX3 and C1q and to examine the functional consequences of this interaction. A dose-dependent binding of both C1q and the C1 complex to PTX3 was observed. Experiments with recombinant globular head domains of human C1q A, B, and C chains indicated that C1q interacts with PTX3 via its globular head region. Binding of C1q to immobilized PTX3 induced activation of the classical complement pathway as assessed by C4 deposition. Furthermore, PTX3 enhanced C1q binding and complement activation on apoptotic cells. However, in the fluid-phase, pre-incubation of PTX3 with C1q resulted in inhibition of complement activation by blocking the interaction of C1q with immunoglobulins. These results indicate that PTX3 can both inhibit and activate the classical complement pathway by binding C1q, depending on the way it is presented. PTX3 may therefore be involved in the regulation of the innate immune response.

Evidence That Complement Protein C1q Interacts with C-Reactive Protein through Its Globular Head Region

“…Not only CRP but also PTX3, another member of the pentraxin family, was recently shown to interact with the globular head of C1q (19). Furthermore, competition studies strongly suggested that PTX3, CRP and the pentraxin family member SAP recognize the same domain of C1q (19).…”

“…The following anti-C1q mAbs were used: anti-C1q-2, -42, -52, -85, -89, -124, -130, -171 (also known as mAb 2204; Ref. 19) and anti-C1q-193. F(abЈ) 2 were prepared by incubation of each mAb with pepsin (Sigma-Aldrich) with a mAb-pepsin ratio of 40:1 w/w, in 0.1 M sodium citrate (pH 3.5), at 37°C for 20 h. The reaction was stopped by adding 1 M Tris to adjust the pH to 8.5.…”

“…C1q can bind various activators such as immunoglobulins IgG (16) and IgM (17), as well as non-Ig proteins like CRP (7), SAP (18), PTX3 (19), and ␤-amyloid fibrils (A␤) (20). A common binding site on C1q for all these activators may serve as target for potential inhibitors.…”

Structural Characterization of PTX3 Disulfide Bond Network and Its Multimeric Status in Cumulus Matrix Organization

“…A number of PTX3 ligands have been described so far, suggesting different physiological and/or pathological roles for this protein. PTX3 binds with high affinity to the complement component C1q thus activating the complement system through the classical pathway (10,11). The selective recognition of fibroblast growth factor-2 (FGF-2) by PTX3 inhibits FGF-2 angiogenic activity on endothelial cells (12) and blocks the autocrine and paracrine stimulation exerted by FGF-2 on smooth muscle cells (13).…”

Protective molecules and their cognate antibodies: new players in autoimmunity

“…PTX3 is a member of the superfamily of pentraxins and plays a part in acute inflammatory reactions caused by trauma or infection. PTX3, differently from most other molecules, is produced by many cells, such as macrophages, dendritic cells, fibroblasts and activated endothelial cells, at the inflammatory site [81][82][83]. These cells produce and release PTX3 when stimulated by inflammatory mediators such as lipopolysaccharide, IL-1 and TNF-α, which induce the NFκB promoter which is associated with the gene for PTX3 [81,82].…”