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  • Title: Solid-phase classical complement activation by C-reactive protein (CRP) is inhibited by fluid-phase CRP-C1q interaction.
    Author: Sjöwall C, Wetterö J, Bengtsson T, Askendal A, Almroth G, Skogh T, Tengvall P.
    Journal: Biochem Biophys Res Commun; 2007 Jan 05; 352(1):251-8. PubMed ID: 17113035.
    Abstract:
    C-reactive protein (CRP) interacts with phosphorylcholine (PC), Fcgamma receptors, complement factor C1q and cell nuclear constituents, yet its biological roles are insufficiently understood. The aim was to characterize CRP-induced complement activation by ellipsometry. PC conjugated with keyhole limpet hemocyanin (PC-KLH) was immobilized to cross-linked fibrinogen. A low-CRP serum with different amounts of added CRP was exposed to the PC-surfaces. The total serum protein deposition was quantified and deposition of IgG, C1q, C3c, C4, factor H, and CRP detected with polyclonal antibodies. The binding of serum CRP to PC-KLH dose-dependently triggered activation of the classical pathway. Unexpectedly, the activation was efficiently down-regulated at CRP levels > 150 mg/L. Using radial immunodiffusion, CRP-C1q interaction was observed in serum samples with high CRP concentrations. We propose that the underlying mechanism depends on fluid-phase interaction between C1q and CRP. This might constitute another level of complement regulation, which has implications for systemic lupus erythematosus where CRP is often low despite flare-ups.
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