Human L-Ficolin Recognizes Phosphocholine Moieties of Pneumococcal Teichoic Acid

Abstract

International audienceHuman L-ficolin is a soluble protein of the innate immune system able to sense pathogens through its fibrinogen (FBG) recognition domains and to trigger activation of the lectin complement pathway through associated serine proteases. L-Ficolin has been previously shown to recognize pneumococcal clinical isolates, but its ligands and especially its molecular specificity remain to be identified. Using solid-phase binding assays, serum and recombinant L-ficolins were shown to interact with serotype 2 pneumococcal strain D39 and its unencapsulated R6 derivative. Incubation of both strains with serum triggered complement activation, as measured by C4b and C3b deposition, which was decreased by using ficolin-depleted serum. Recombinant L-ficolin and its FBG-like recognition domain bound to isolated pneumococcal cell wall extracts, whereas binding to cell walls depleted of teichoic acid (TA) was decreased. Both proteins were also shown to interact with two synthetic TA compounds, each comprising part structures of the complete lipoteichoic acid molecule with two PCho residues. Competition studies and direct interaction measurements by surface plasmon resonance identified PCho as a novel L-ficolin ligand. Structural analysis of complexes of the FBG domain of L-ficolin and PCho revealed that the phosphate moiety interacts with amino acids previously shown to define an acetyl binding site. Consequently, binding of L-ficolin to immobilized acetylated BSA was inhibited by PCho and synthetic TA. Binding of serum L-ficolin to immobilized synthetic TA and PCho-conjugated BSA triggered activation of the lectin complement pathway, thus further supporting the hypothesis of L-ficolin involvement in host antipneumococcal defense

The Pneumococcal Cell Wall

“…In addition, mice deficient in the mannose-binding protein-associated serine protease 2 are highly susceptible to pneumococcal infection due to a defect in S. pneumoniae opsonization and clearance [119]. L-ficolin is able to interact with the synthetic short-chain analog of LTA and with the synthetic pseudo-pentamer β-D-Glcp-(1-3)α-AATGalp-(1-4)-(6-O-PCho)-α-D-GalpNAc-(1-3)-(6-O-PCho)-β-D-GalpNAc-(1-1)-ribitol [121], a partial structure of pneumococcal TAs [112]. Moreover, it has been shown that mouse ficolin A, human L-ficolin, and CL-K1 recognize the pneumococcal cell surface and trigger activation of the lectin complement pathway [119].…”

Ficolin-2 inhibitors are present in sera after prolonged storage at −80 °C

“…The fibrinogen-like domains of ficolins preferentially bind to acetyl groups (Matsushita, 2013), but ficolin-2 has a fibrinogen-like domain with three additional binding sites relative to the other human ficolins (Endo, Matsushita & Fujita, 2011), and consequently recognizes a larger number of target structures. These include non-acetylated molecular structures such as phosphocholine and heparin (Matsushita, 2013; Vassal-Stermann et al, 2014). Ficolin-2 binds to many bacteria (Matsushita, 2013), fungi (Bidula et al, 2015), and viruses (Liu et al, 2009; Pan et al, 2012).…”

Mannose‐binding lectin and l‐ficolin polymorphisms in patients with community‐acquired pneumonia caused by intracellular pathogens

“…The ligands for MBL have been relatively well defined. The spectrum of ligands for Ficolin 2 includes not only N ‐acetyl groups (including GlucNac) but also sulphate‐ and phosphate‐containing carbohydrates,44 as well as lipoteichoic acid, a cell wall constituent of all Gram‐positive bacteria 45. The balance between MBL‐ and l ‐FCN‐mediated complement activation therefore can have a major effect on elimination or survival of intracellular pathogens.…”