Quantitation of (C1INH)2 C1r-C1s complexes in glomerulonephritis as an indicator of C1 activation

Elevated C1s/C1-INH in serum and plasma of myasthenia gravis patients

Myasthenia Gravis (MG) is an autoimmune neuromuscular disorder where acetylcholine receptor (AChR) antibodies induce membrane attack complex formation at the muscle membrane. The C1-inhibitor (C1-INH) regulates the classical pathway and is a promising marker in other autoimmune disorders. Treatment options for AChR antibody MG include complement inhibitors; nevertheless, the early pathway activation in MG remains unclear. Serum and plasma C1s-C1-INH levels were higher in MG patients than in matched healthy controls, supporting early classical pathway activation in most MG patients. These findings allow prospective validation studies of activated C1s as a putative treatment target and potential accompanying biomarker in MG.

sMR and PTX3 levels associate with COVID-19 outcome and survival but not with Long COVID

Biomarkers for monitoring COVID-19 disease course are lacking. Study aim was to identify biomarkers associated with disease severity, survival, long-term outcome, and Long COVID. As excessive macrophages activation is a hallmark of COVID-19 and complement activation is key in this, we selected the following proteins involved in these processes: PTX3, C1q, C1-INH, C1s/C1-INH, and sMR. EDTA-plasma concentrations were measured in 215 patients and 47 controls using ELISA. PTX3, sMR, C1-INH, and C1s/C1-INH levels were associated with disease severity. PTX3 and sMR were also associated with survival and long-term immune recovery. Lastly, sMR levels associate with ICU admittance. sMR (AUC 0.85) and PTX3 (AUC 0.78) are good markers for disease severity, especially when used in combination (AUC 0.88). No association between biomarker levels and Long COVID was observed. sMR has not previously been associated with COVID-19 disease severity, ICU admittance or survival and may serve as marker for disease course.

Distinction of early complement classical and lectin pathway activation via quantification of C1s/C1-INH and MASP-1/C1-INH complexes using novel ELISAs

The most commonly used markers to assess complement activation are split products that are produced through activation of all three pathways and are located downstream of C3. In contrast, C4d derives from the cleavage of C4 and indicates either classical (CP) or lectin pathway (LP) activation. Although C4d is perfectly able to distinguish between CP/LP and alternative pathway (AP) activation, no well-established markers are available to differentiate between early CP and LP activation. Active enzymes of both pathways (C1s/C1r for the CP, MASP-1/MASP-2 for the LP) are regulated by C1 esterase inhibitor (C1-INH) through the formation of covalent complexes. Aim of this study was to develop validated immunoassays detecting C1s/C1-INH and MASP-1/C1-INH complex levels. Measurement of the complexes reveals information about the involvement of the respective pathways in complement-mediated diseases. Two sandwich ELISAs detecting C1s/C1-INH and MASP-1/C1-INH complex were developed and tested thoroughly, and it was investigated whether C1s/C1-INH and MASP-1/C1-INH complexes could serve as markers for either early CP or LP activation. In addition, a reference range for these complexes in healthy adults was defined, and the assays were clinically validated utilizing samples of 414 COVID-19 patients and 96 healthy controls. The immunoassays can reliably measure C1s/C1-INH and MASP-1/C1-INH complex concentrations in EDTA plasma from healthy and diseased individuals. Both complex levels are increased in serum when activated with zymosan, making them suitable markers for early classical and early lectin pathway activation. Furthermore, measurements of C1-INH complexes in 96 healthy adults showed normally distributed C1s/C1-INH complex levels with a physiological concentration of 1846 ± 1060 ng/mL (mean ± 2SD) and right-skewed distribution of MASP-1/C1-INH complex levels with a median concentration of 36.9 (13.18 - 87.89) ng/mL (2.5-97.5 percentile range), while levels of both complexes were increased in COVID-19 patients (p<0.0001). The newly developed assays measure C1-INH complex levels in an accurate way. C1s/C1-INH and MASP-1/C1-INH complexes are suitable markers to assess early classical and lectin pathway activation. An initial reference range was set and first studies showed that these markers have added value for investigating and unraveling complement activation in human disease.

Cell-Bound Complement Activation Products (CB-CAPs) as a Source of Lupus Biomarkers

Measurement of serum C3 and C4 has been used for several decades to monitor disease activity in patients with SLE. Despite the limited utility and recognized weaknesses of these assays, they have remained the gold standard during an era of unprecedented discovery in the complement field. The current urgent need for lupus biomarkers warrants efforts to mine the complement system for assays superior to serum C3 and C4. Recent studies of soluble and cell-bound complement activation products hold promise for achieving this goal.

Complement components and their activation products in pleural fluid

STUDY OBJECTIVES

The aim of this study was to determine the role of complement components in pleural effusion measured with novel markers of complement activation, to assess which pathway of activation is predominant in different diseases, and to find out whether the analysis of complement components and their activation products could help in diagnostic procedure differentiating the etiologies of pleural effusion.

PATIENTS

The study population consisted of 71 patients who had pleural effusion secondary to tuberculosis (n=23), rheumatic disease (n=10), or malignancy (n=38).

MEASUREMENTS

Complement components and their activation products, including the soluble terminal complex SC5b-9, were measured in plasma and pleural fluid.

RESULTS

In all patients with rheumatic pleurisy, pleural fluid SC5b-9 was higher than 2 AU/mL and in all patients with malignant pleural fluid it was lower than 2 AU/mL. The mean level of SC5b-9 in rheumatic pleural effusion was also significantly higher than in tuberculosis. In addition, the concentrations of pleural fluid C3 and C4 were significantly lower and the ratio C4d/C4 was significantly higher in rheumatic compared with tuberculous or malignant pleurisy. In plasma, both SC5b-9 and C1s-C1r-C1INH-complexes were significantly higher in rheumatic subjects than in other patients. In stepwise multinominal logistic regression analyses, the most significant predictors for rheumatic pleural fluid were high pleural fluid SC5b-9 and low C4.

CONCLUSIONS

These observations indicate that the complement cascade is activated through both the classic and the alternative pathways in rheumatic pleurisy. Determinations of SC5b-9 and C4d/C4 in pleural fluid were the best variables differentiating rheumatic, tuberculous, and malignant effusions.

Improved method for measuring C1-r-C1-s-(C1 inh)2 complexes by an enzyme-linked immunosorbent assay

Measurement of C1-r-C1-s-(C1 inh)2 complexes in serum or plasma by enzyme-linked immunosorbent assay (ELISA) has been proposed as a relatively convenient and sensitive means for assessing C1 activation. However, interference by unactivated C1q (r-s)2 at low serum or plasma dilutions has resulted in estimates that vary widely with the degree of serum or plasma dilution. Precipitating the interfering C1q (r-s)2 with 6% polyethylene glycol has been proposed to resolve this problem, but here it is shown that this procedure also precipitates or coprecipitates some of the C1-r-C1-s-(C1 inh)2 complexes. Satisfactory results have been achieved without PEG precipitation by testing high plasma dilutions under conditions where there is a sufficient excess of anti-C1s coating the microtitration plate wells that removal of C1q (r-s)2 is not necessary. Optimizing conditions for quantitating these complexes at high dilution have been investigated. The mean normal EDTA plasma C1-r-C1-s-(C1 inh)2 complex measurement was 36.6 +/- 7.0 (S.D.) ELISA units with a 95% confidence interval of 19.5-47.6u. Besides providing a sensitive assay for C1 activation, measuring C1-r-C1-s-(C1 inh)2 complexes may help to clarify the pathophysiologic mechanisms resulting from C1 inh deficiency under various conditions.

Serum complement levels before and after the onset of acute post-streptococcal glomerulonephritis. A case report

Low serum C3, properdin, and C5 levels found in the acute stage of acute post-streptococcal glomerulonephritis (APSGN) indicate the presence of aggressive complement activation. We followed serum complement component levels in a child hospitalized with erysipelas who developed APSGN on the 2nd hospital day. Her initial serum sample, obtained prior to the clinical onset of nephritis, had a low properdin level and normal C3 and C5 levels despite the presence of C3 splitting activity. Two days later she developed gross hematuria and subsequent sera contained low C3, properdin, and C5 levels, as is usual in APSGN. These observations suggest that complement activation, predominantly through the alternative pathway, precedes the clinical onset of APSGN.

Cold-dependent activation of complement: recognition, assessment, and mechanism

Cold-dependent activation of complement (CDAC) is a phenomenon characterized by low hemolytic complement activity in chilled serum. Complement component levels are normal when measured immunologically, and there is normal hemolytic activity in EDTA plasma or serum maintained at 37 degrees C. Little attention has been paid to CDAC except in Japan, and current unfamiliarity with it, even by clinical immunologists, can lead to confusion and unnecessary laboratory tests. A 66-year-old patient with a complex medical history is described whose complement tests showed abnormalities characteristic of CDAC. Evidence for classical complement pathway activation in the cold was obtained by CH50 measurements, by hemolytic C4 determinations, by C4a, C3a, and C4d generation, and by quantitating C1s-C1r-(C1 inhibitor)2 complexes. A good correlation was observed among these parameters. Cryoprecipitates were absent. CDAC activity has persisted for over 5 years and is greater at 13 than at 4 degrees C. Activation is ablated by heating at 56 degrees C and restored by the addition of C1 to the heated serum. Adsorption by streptococcal protein G-Sepharose and precipitation by 2.5% polyethylene glycol support the hypothesis that CDAC is caused by aggregated IgG. The CDAC factor(s) also induces complement activation in normal serum but has not interfered with Raji cell or C1q binding tests or with FACS analysis. More limited studies of a second individual experiencing CDAC yielded similar results.

FN-C1q and C1 INH C1r-C1s complexes as indicators of complement activation in patients with chronic lymphocytic leukaemia

We have previously found low levels of C1 and C4 INH in the sera of chronic lymphocytic leukaemia (CLL) patients. Hypocomplementaemia was supposed to be the consequence of a permanent activation of the classical pathway. We have compared the levels of C1 INH-C1rC1s and C1q-FN complexes in the sera of 95 CLL patients and 100 healthy controls, because these complexes are known to be formed in the early stage of classical pathway activation. A significant increase in the level of both types of complexes was found in sera of CLL patients as compared to the controls. These findings support the assumption that the classical complement pathway is activated in the patients with CLL.

Complement profiles in acute post-streptococcal glomerulonephritis

It is well known that the hypocomplementemia of acute post-streptococcal glomerulonephritis (APSGN) is characterized by markedly reduced serum concentrations of C3 and moderately reduced levels of C5 and properdin (P). However, the extent of the activation of the classical pathway is not well defined and only limited data are available concerning serum concentrations of terminal components other than C5. In serial serum specimens from 14 children with APSGN, the presence and extent of C4 activation was directly assessed by measurement by rocket immunoelectrophoresis for C4 and C4 (C4d/C4 ratio). Elevated values for this ratio, indicating C4 activation, were found in 8 of 14 of the initial serum specimens, and in some patients the ratio remained elevated for several weeks. In contrast, the serum C4 level was low in only 1 specimen (the specimen with the highest C4d/C4 ratio). However, in 10 patients C4 concentrations within the normal range rose in serial serum specimens. Serum C2 concentrations were depressed in the initial specimens from 5 patients. The concentrations of 13 other complement component and control proteins were also measured in these specimens. Levels of terminal components, other than C5, in the initial serum specimens were normal except for depressed C8 in 3 of 13 patients and depressed C6 in 1 of 14. Of these 4 individuals, 3 had the lowest C3 levels in the study. It is concluded that the classical complement pathway is frequently activated in patients with APSGN early in the condition and that subtle abnormalities in C6 and C8 levels occasionally occur.