Factor H-related proteins determine complement-activating surfaces.

Complement factor H-related proteins (FHRs) are strongly associated with different diseases involving complement dysregulation, which suggests a major role for these proteins regulating complement activation. Because FHRs are evolutionarily and structurally related to complement inhibitor factor H (FH), the initial assumption was that the FHRs are also negative complement regulators. Whereas weak complement inhibiting activities were originally reported for these molecules, recent developments indicate that FHRs may enhance complement activation, with important implications for the role of these proteins in health and disease. We review these findings here, and propose that FHRs represent a complex set of surface recognition molecules that, by competing with FH, provide improved discrimination of self and non-self surfaces and play a central role in determining appropriate activation of the complement pathway

Atypical Hemolytic Uremic Syndrome

SummaryHemolytic uremic syndrome (HUS) is a triad of microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure. The atypical form of HUS is a disease characterized by complement overactivation. Inherited defects in complement genes and acquired autoantibodies against complement regulatory proteins have been described. Incomplete penetrance of mutations in all predisposing genes is reported, suggesting that a precipitating event or trigger is required to unmask the complement regulatory deficiency. The underlying genetic defect predicts the prognosis both in native kidneys and after renal transplantation. The successful trials of the complement inhibitor eculizumab in the treatment of atypical HUS will revolutionize disease management

Complement inhibitor C4b-binding protein in primary Sjögren's syndrome and its association with other disease markers.

A subgroup of patients suffering from primary Sjögren's syndrome (pSS) display unexplained low levels of complement components C3 and/or C4 which is associated with increased risk of non-Hodgkin's lymphoma. C4b-binding protein (C4BP) is a major fluid-phase complement inhibitor which can influence C4 and C3 levels. Therefore we analysed C4BP levels in the sera of patients with pSS to better understand the disturbances in complement in pSS. Associations with other disease markers were also investigated to define a possible role of C4BP as marker of high-risk disease course. Plasma levels of C4BP were analysed in pSS patients (n=86) and in controls (n=68) by ELISA. C4BP levels from 49 patients were correlated to disease activity markers and autoantibody profiles. We found that total C4BP plasma levels were significantly higher in pSS patients compared with controls. C4BP levels correlated to the acute phase response, to levels of C4 and C3 as well as to the CD4+/CD8+ T-cell ratio. C4BP levels were inversely related to IgG levels, extent of autoantibody production and global disease activity. C3dg levels, a marker of complement activation, displayed a negative correlation to C4 levels but interestingly not to C4BP levels. In conclusion, C4BP levels are increased in patients suffering from pSS proportional to their acute phase response. However, in the most active cases, with the most widespread autoantibody production, C4BP levels were decreased in parallel with levels of C3 and C4 and CD4+ T cells, suggesting that disturbed complement regulation may contribute to pathogenicity in pSS

A comparison of serum and EDTA plasma in the measurement of glutamic acid decarboxylase autoantibodies (GADA) and autoantibodies to islet antigen-2 (IA-2A) using the RSR radioimmunoassay (RIA) and enzyme linked immunosorbent assay (ELISA) kits.

BACKGROUND: Glutamic acid decarboxylase antibodies (GADA) and tyrosine phosphatase antibodies (islet antigen-2 antibodies; IA-2A) are used in clinical practise to identify type 1 diabetes. METHODS: GADA and IA-2A were measured with RSR-ELISA kits in samples from 76 newly diagnosed type 1 diabetic children and 120 healthy controls. The aim was to evaluate performance of RSR-ELISA kits for GADA and IA-2A when serum and Ca2+ treated plasma were used. RESULTS: GADA achieved high area under the curve (AUC) both for serum 0.95 (95% CI 0.90-0.99) and for Ca2+ treated plasma 0.95 (95% CI 0.91-0.99). At specificity 98%, sensitivity was 84% for serum and 87% for Ca2+ treated plasma. IA-2A achieved AUC 0.92 (95% CI 0.87-0.97) for serum and 0.94 (95% CI 0.90-0.98) for Ca2+ treated plasma. Using the lowest standard (15 WHO-Units/ml) as cut-off, specificity for serum was 100% and for Ca2+ treated plasma 99% with sensitivity 74% in both cases. Sensitivity was higher in ELISA compared to RIA (74%; p = 0.0080) for GADA measurement and similar for ELISA and RIA IA-2A measurements (76%; p = 0.50). CONCLUSION: Both RSR-ELISAs, GADA and IA-2A showed excellent performance for serum as well as for Ca2+ treated plasma

Factor H autoantibodies and deletion of Complement Factor H-Related protein-1 in rheumatic diseases in comparison to atypical hemolytic uremic syndrome

Introduction: Complement activation is involved in rheumatoid arthritis (RA), systemic lupus erythematosus (SLE) and atypical hemolytic uremic syndrome (aHUS). Autoantibodies to complement inhibitor factor H (FH), particularly in association with deletions of the gene coding for FH-related protein 1 (CFHR1), are associated with aHUS. Methods: Autoantibodies against FH, factor I (FI) and C4b-binding protein (C4BP) were measured by ELISA, while CFHR1 homozygous deletion was determined with Western blotting of sera. Epitopes for FH autoantibodies were mapped using recombinant fragments of FH. Results: FH autoantibodies were detected in SLE (6.7%, n = 60, RA patients (16.5%, n = 97 in the Swedish cohort and 9.2%, n = 217 in the Dutch cohort) and thrombosis patients positive for the lupus anticoagulants (LA+) test (9.4%, n = 64) compared with aHUS patients (11.7%, n = 103). In the control groups (n = 354), an average of 4% of individuals were positive for FH autoantibodies. The frequencies observed in both RA cohorts and LA+ patients were statistically significantly higher than in controls. We also found that an average of 15.2% of the FH-autoantibody positive individuals in all studied disease groups had homozygous deficiency of CFHR1 compared with 3.8% of the FH autoantibody negative patients. The levels of FH autoantibodies varied in individual patients over time. FH autoantibodies found in LA+, SLE and RA were directed against several epitopes across FH in contrast to those found in aHUS, which bound mainly to the C-terminus. Autoantibodies against FI and C4BP were detected in some patients and controls but they were not associated with any of the diseases analyzed in this study. Conclusions: Autoantibodies against FH are not specific for aHUS but are present at a significant frequency in rheumatic diseases where they could be involved in pathophysiological mechanisms

Factor H Autoantibodies in Patients with Antiphospholipid Syndrome and Thrombosis.

Autoantibodies to complement factor H (FH) are associated with atypical hemolytic uremic syndrome, but can also be detected in patients with rheumatoid arthritis and in patients positive for lupus anticoagulants and thus potentially antiphospholipid syndrome (APS). To our knowledge, no data are available on the association between the presence of FH autoantibodies in APS and clinical manifestations