Alterations in the terminal complement pathway in leukopenic children with malignant diseases during episodes with evidence of infection

Meningococcal surface fibril (Msf) binds to activated vitronectin and inhibits the terminal complement pathway to increase serum resistance

Complement evasion is an important survival strategy of Neisseria meningitidis (Nm) during colonization and infection. Previously, we have shown that Nm Opc binds to serum vitronectin to inhibit complement-mediated killing. In this study, we demonstrate meningococcal interactions with vitronectin via a novel adhesin, Msf (meningococcal surface fibril, previously NhhA or Hsf). As with Opc, Msf binds preferentially to activated vitronectin (aVn), engaging at its N-terminal region but the C-terminal heparin binding domain may also participate. However, unlike Opc, the latter binding is not heparin-mediated. By binding to aVn, Msf or Opc can impart serum resistance, which is further increased in coexpressers, a phenomenon dependent on serum aVn concentrations. The survival fitness of aVn-binding derivatives was evident from mixed population studies, in which msf/opc mutants were preferentially depleted. In addition, using vitronectin peptides to block Msf-aVn interactions, aVn-induced inhibition of lytic C5b-9 formation and of serum killing could be reversed. As Msf-encoding gene is ubiquitous in the meningococcal strains examined and is expressed in vivo, serum resistance via Msf may be of significance to meningococcal pathogenesis. The data imply that vitronectin binding may be an important strategy for the in vivo survival of Nm for which the bacterium has evolved redundant mechanisms.

Low levels of vitronectin and clusterin in acute meningococcal disease are closely associated with formation of the terminal-complement complex and the vitronectin-thrombin-antithrombin complex

Patients with terminal complement deficiencies and thus impaired lytic efficiency have a highly increased likelihood of contracting invasive meningococcal infections but generally experience a mild disease course. Deficiencies of lysis inhibitors might therefore be associated with severe disease. We have quantified the complement lysis inhibitors vitronectin and clusterin, as well as complexes containing the proteins, in plasma from patients with acute meningococcal disease. At hospital admission, the median vitronectin concentrations were 0.10 (range, 0.04 to 0.17) g/liter in 10 septic patients and 0.19 (0.09 to 0.47) g/liter in 14 nonseptic patients (P = 0.001). The corresponding clusterin concentrations were 0.09 (0.01 to 0.13) and 0.14 (0.06 to 0.29) g/liter (P = 0.005). The vitronectin-thrombin-antithrombin complex concentration was 1.8 (0.22 to 35.6) arbitrary units (AU)/ml in septic patients, but the complex was not detectable in most nonseptic patients (< 0.10 to 0.16 AU/ml) (P < 0.0001). The corresponding levels of the terminal complement complex (contains vitronectin and clusterin) were 4.4 (3.6 to 20.1) and 2.6 (1.6 to 4.7) AU/ml (P = 0.0005). We found no evidence of constitutively low levels of vitronectin or clusterin in patients contracting meningococcal disease. The low levels of the proteins may partly be explained by hemodilution, extravasation, and increased consumption due to incorporation into complexes which are quickly removed from circulation.

C3 activation products, C3 containing immune complexes, the terminal complement complex and native C9 in patients with rheumatoid arthritis

Complement activation products, C9 and C3-containing circulating immune complexes (CIC), were evaluated in plasma and synovial fluid (SF) from patients with rheumatoid arthritis (RA) and osteoarthritis. C3 activation products and the fluid phase terminal complement complex were considerably elevated in SF from RA patients reaching levels five- to eighttimes that in plasma, consistant with a local activation of the whole cascade in the joints. The results emphazise the importance of detecting C3 activation by neoepitope expression instead of single fragment determinations. The concentration of native C9 was lower in synovial fluid compared with plasma, consistant with the excessive local complement activation. Increased CIC levels which correlated with the degree of complement activation were also found in the SF from the RA patients.

Quantification of non-activated (native) complement component C9 synthesized by alveolar macrophages from patients with sarcoidosis

Alveolar macrophages (AM) from sarcoidosis patients synthesize the functional alternative and terminal pathways of complement, and increased complement production may be one of multiple factors in the pathogenesis of sarcoidosis. We thus examined whether AM from sarcoidosis patients produced quantitatively more C9 in vitro than AM from healthy controls. AM from 16 patients with active sarcoidosis and seven healthy controls were cultured under serum-free conditions for 6, 12, 24, 48, or 72 h. A quantitative production of C9 was found in the harvested medium in 10 of 16 sarcoidosis patients. There were no detectable levels of C9 in the seven controls. Activated C9 was found in all patients and in the majority of the controls. C9 was quantified by an enzyme immunoassay based on a monoclonal antibody (M1) to non-activated C9. Our results indicate greater production of C9 by sarcoidosis AM than by their healthy counterparts.