Complement activating properties of monoreactive and polyreactive IgM rheumatoid factors

Expression of activation-induced cytidine deaminase enhances the clearance of pneumococcal pneumonia: evidence of a subpopulation of protective anti-pneumococcal B1a cells

We describe a protective early acquired immune response to pneumococcal pneumonia that is mediated by a subset of B1a cells. Mice deficient in B1 cells (xid), or activation-induced cytidine deaminase (AID(-/-) ), or invariant natural killer T (iNKT) cells (Jα18(-/-) ), or interleukin-13 (IL-13(-/-) ) had impaired early clearance of pneumococci in the lung, compared with wild-type mice. In contrast, AID(-/-) mice adoptively transferred with AID(+/+) B1a cells, significantly cleared bacteria from the lungs as early as 3 days post infection. We show that this early bacterial clearance corresponds to an allergic contact sensitivity-like cutaneous response, probably due to a subpopulation of initiating B1a cells. In the pneumonia model, these B1a cells were found to secrete higher affinity antigen-specific IgM. In addition, as in contact sensitivity, iNKT cells were required for the anti-pneumococcal B1a cell initiating response, probably through early production of IL-13, given that IL-13(-/-) mice also failed to clear infection. Our study is the first to demonstrate the importance of AID in generating an appropriate B1a cell response to pathogenic bacteria. Given the antibody affinity and pneumonia resistance data, natural IgM produced by conventional B1a cells are not responsible for pneumonia clearance compared with the AID-dependent subset.

Novel mechanisms of action of the biologicals in rheumatic diseases

Biological drugs targeting pro-inflammatory or co-stimulatory molecules or depleting lymphocyte subsets made a revolution in rheumatoid arthritis (RA) treatment. Their comparable efficacy in clinical trials raised the point of the heterogeneity of RA pathogenesis, suggesting that we are dealing with a syndrome rather than with a single disease. Several tumor necrosis factor-alpha (TNF-α) blockers are available, and a burning question is whether they are biosimilar or not. The evidence of diverse biological effects in vitro is in line with the fact that a lack of efficacy to one TNF-α agent does not imply a non-response to another one. As proteins, biologicals are potentially immunogenic. It has been recently raised that anti-drug antibodies (ADA) may affect their bioavailability and eventually the clinical efficacy through local formation of immune complexes and directly by preventing the interaction between the drug and TNF-α. Regular monitoring of drug and ADA levels appears the best way to tailor anti-TNF-α therapies. Owing to the pleiotropic characteristics of the target, anti-TNF-α blockers may affect several mechanisms beyond rheumatoid synovitis. As TNF-α plays a pivotal role in the induction of early atherosclerosis, treatment with TNF-inhibitors may modulate cholesterol handling, in particular, cholesterol efflux from macrophages. Side effects are a major issue because of the systemic TNF-α blocking action. The efficacy of an anti-C5 monoclonal antibody fused to a peptide targeting inflamed synovia in experimental arthritis opened the way for new strategies: Homing to the synovium of molecules neutralizing TNF would allow to maximize the therapeutic action avoiding the side effects.

IgM rheumatoid factor amplifies the inflammatory response of macrophages induced by the rheumatoid arthritis-specific immune complexes containing anticitrullinated protein antibodies

OBJECTIVES

Anticitrullinated protein antibodies (ACPA) are specifically associated with rheumatoid arthritis (RA) and produced in inflamed synovial membranes where citrullinated fibrin, their antigenic target, is abundant. We showed that immune complexes containing IgG ACPA (ACPA-IC) induce FcγR-mediated tumour necrosis factor (TNF)-α secretion in macrophages. Since IgM rheumatoid factor (RF), an autoantibody directed to the Fc fragment of IgG, is also produced and concentrated in the rheumatoid synovial tissue, we evaluated its influence on macrophage stimulation by ACPA-IC.

METHODS

With monocyte-derived macrophages from more than 40 healthy individuals and different human IgM cryoglobulins with RF activity, using a previously developed human in vitro model, we evaluated the effect of the incorporation of IgM RF into ACPA-IC.

RESULTS

IgM RF induced an important amplification of the TNF-α secretion. This effect was not observed in monocytes and depended on an increase in the number of IgG-engaged FcγR. It extended to the secretion of interleukin (IL)-1β and IL-6, was paralleled by IL-8 secretion and was not associated with overwhelming secretion of IL-10 or IL-1Ra. Moreover, the RF-induced increased proinflammatory bioactivity of the cytokine response to ACPA-IC was confirmed by an enhanced, not entirely TNF-dependent, capacity of the secreted cytokine cocktail to prompt IL-6 secretion by RA synoviocytes.

CONCLUSIONS

By showing that it can greatly enhance the proinflammatory cytokine response induced in macrophages by the RA-specific ACPA-IC, these results highlight a previously undescribed, FcγR-dependent strong proinflammatory potential of IgM RF. They clarify the pathophysiological link between the presence of ACPA and IgM RF, and RA severity.

Anticomplementary activity of ergosterol peroxide fromNaematoloma fasciculare and reassignment of NMR data

A very high content (at least 0.23%) of ergosterol peroxide was isolated fromNaematoloma fasciculare Karst. Not only ergosterol peroxide but also ergosterol showed very strong anticomplementary activity on the classical pathway, the IC(50) values being 5.0 muM and 1.0 muM, respectively. The(1)H and(13)C NMR data of ergosterol peroxide were revised and completely assigned by DEPT,(1)H-(1)H COSY, HMQC and HMBC correlations.

Serum COMP-C3b complexes in rheumatic diseases and relation to anti-TNF-α treatment

INTRODUCTION

Cartilage oligomeric matrix protein (COMP) is found at elevated concentrations in sera of patients with joint diseases such as rheumatoid arthritis (RA) and osteoarthritis (OA). We recently showed that COMP activates complement via the alternative pathway and that COMP-C3b complexes are present in sera of RA patients, but not in healthy controls. We now set out to elaborate on the information provided by this marker in a variety of diseases and larger patient cohorts.

METHODS

COMP-C3b levels in sera were measured by using an enzyme-linked immunosorbent assay (ELISA) capturing COMP and detecting C3b. Serum COMP was measured by using ELISA.

RESULTS

COMP-C3b levels were significantly elevated in patients with RA as well as in systemic lupus erythematosus (SLE), compared with healthy controls. SLE patients with arthritis had significantly higher COMP-C3b levels than did those without. COMP-C3b was furthermore elevated in patients with ankylosing spondylitis (AS), psoriatic arthritis (PsA), reactive arthritis, systemic sclerosis, and OA. COMP-C3b did not correlate with COMP in any of the patient groups. COMP-C3b correlated with disease activity in RA, but not in other diseases. COMP-C3b levels in RA patients decreased on treatment with tumor necrosis factor (TNF)-α inhibitors, whereas the levels increased in patients with AS or PsA. The changes of COMP-C3b did not parallel the changes of C-reactive protein (CRP).

CONCLUSIONS

COMP-C3b levels are elevated in several rheumatologic diseases and correlate with inflammatory measures in RA. COMP-C3b levels in RA decrease during TNF-α inhibition differently from those of CRP, suggesting that formation of COMP-C3b relates to disease features not reflected by general inflammation measures.

Evaluation of classical complement pathway activation in rheumatoid arthritis: measurement of C1q-C4 complexes as novel activation products

OBJECTIVE

Novel activation products that are stable and minimally susceptible to in vitro artefacts have recently been described in the classical complement pathway. The present study assessed circulating levels of these products, i.e., covalent complexes between the recognition molecule of the classical pathway (C1q) and activated C4, in plasma samples from patients with rheumatoid arthritis (RA) to establish the relationship between these levels and the clinical and immunologic parameters in these patients.

METHODS

C1q-C4 levels were measured in plasma samples from 41 patients with active RA and 43 patients with inactive RA. These levels were related to other complement activation products and to disease activity according to the Disease Activity Score in 28 joints (DAS28), using Spearman's rank correlations.

RESULTS

C1q-C4 plasma levels were significantly higher in patients with active RA as compared with patients with RA in clinical remission (median 3.3 arbitrary units [AU], range 0.4-13.4 versus 1.7 AU, range 0.2-5.5; P=0.0001), suggesting that activation of the classical complement pathway reflects disease activity. This was supported by a significant correlation between C1q-C4 levels and the DAS28 (r=0.398, P=0.0002). Levels of other complement activation products, such as activated C4 (C4b/c), were also significantly elevated in patients with active disease compared with patients with inactive disease (P=0.03), and were correlated with C1q-C4 levels (r=0.329, P=0.002). Levels of C1q-C4 complexes were higher in synovial fluid samples than in plasma samples from the 4 patients tested.

CONCLUSION

Systemic complement activation via the classical pathway in patients with RA correlates with disease activity. These results indicate that C1q-C4 complexes may be used as a biomarker for RA.

Complexes between C1q and C3 or C4: novel and specific markers for classical complement pathway activation

Classical pathway activation is often assessed by measuring circulating levels of activated C4. However, this parameter does not discriminate between activation through the classical or the lectin pathway. We hypothesized that during classical pathway activation, complexes are formed between C1q and activated C4 or C3. Using ELISA, we investigated whether such complexes constitute specific markers for classical pathway activation. In vitro, C1q-C3d/C4d complexes were generated upon incubation of normal recalcified plasma with aggregated IgG or an anti-C1q mAb that activates C1 (mAb anti-C1q-130). In contrast, during incubation with C1s or trypsin, C1q-C3d/C4d complexes were not generated, which excludes an innocent bystander effect. Additionally, C1q-C3d/C4d complexes were not generated during activation of the alternative or the lectin pathway. Repeated freezing and thawing did not influence levels of C1q-C3d/C4d complexes in recalcified plasma. To measure C1q-complement complexes in plasma samples, we separated unbound complement proteins from C1q-C3d/C4d complexes in the samples prior to testing with ELISA. In samples from patients undergoing cardiopulmonary bypass surgery or suffering from rheumatoid arthritis, we found higher levels of C1q-C4 complexes than in samples from healthy individuals. We conclude that complexes between C1q and C4 or C3 are specific markers of classical complement pathway activation.

Complement activation in patients with rheumatoid arthritis mediated in part by C-reactive protein

OBJECTIVE

Complement activation in patients with rheumatoid arthritis (RA) is considered to be triggered by immune complexes. Recently, it was shown that C-reactive protein (CRP) can activate the complement system in vivo. We therefore hypothesized that part of the complement activation in RA is due to CRP. The aim of this study was to investigate CRP-mediated complement activation in RA, and to assess its correlation with disease activity.

METHODS

Complexes between CRP and the activated complement components C3d (C3d-CRP) and C4d (C4d-CRP), which reflect CRP-mediated complement activation, as well as the overall levels of activated C3 and C4 were measured in the plasma of 107 patients with active RA and 177 patients with inactive RA. Inactive RA was defined according to the American College of Rheumatology criteria for clinical remission. Disease activity was assessed by the modified Disease Activity Score (DAS28).

RESULTS

Plasma levels of C3d-CRP and C4d-CRP were increased in the majority of the patients, and were significantly higher in patients with active disease versus those with inactive RA (P < 0.001). In patients with active RA, the plasma concentrations of C3d-CRP and C4d-CRP correlated significantly with the DAS28 (Spearman's rho 0.61 and 0.55, respectively; P < 0.001), whereas these correlations were less pronounced in patients with inactive RA (Spearman's rho 0.28 [P < 0.001] and 0.25 [P = 0.001], respectively). Levels of activated C3 and C4 were also increased in the majority of the patients, particularly in patients with active RA.

CONCLUSION

Part of the activation of complement in RA is mediated by CRP and is correlated with disease activity. We suggest that this activation is involved in the pathogenesis of RA.

Interference of rheumatoid factor activity by aspartame, a dipeptide methyl ester

Circulating autoimmune complexes of IgM rheumatoid factors (RF) bound to the Fc portions of normal, polyclonal IgG antibodies are frequently present in humans with rheumatoid arthritis (RA). The sweet tasting methyl ester of L-Asp-L-Phe (aspartame or APM) was found to relieve pain and improve joint mobility in subjects with osteo- and mixed osteo/rheumatoid arthritis [Edmundson, A. B. and Manion, C. V. (1998). Clin. Pharmac. Ther. 63, 580-593]. These clinical observations prompted the testing of the inhibition by APM of the binding interactions of human IgM RFs with IgG Fc regions. The propensity of APM to inhibit IgM RF binding was assessed by competitive enzyme immunoassays with solid-phase human IgG. Ten RA serum samples and three purified monoclonal cryoglobulins, all of which had RF activity, were tested in this system. We found that the presence of APM significantly reduced the binding of IgM RFs. The inhibitory propensity of APM with monoclonal RF cryoglobulins was increased by the addition of CaCl(2) to the binding buffer. Similar inhibition of the binding of RA derived RFs to IgG was observed for Asp-Phe and its amidated derivative, indicating that the methyl ester is not required for APM's interaction with IgM antibodies. A human (Mez) IgM known to bind octameric peptides derived from the Fc portion of a human IgG(1) antibody was tested for binding of dipeptides by the Pepscan method of combinatorial chemistry. The relative binding constants of Asp-Phe and Phe-Asp were ranked among the highest values for 400 possible combinations of the 20 most common amino acids. Possible blocking interactions of APM were explored by computer-assisted docking studies with the model of a complex of an RF Fab with the Fc of a human IgG(4) antibody. Modeling of ternary immune complexes revealed a few key residues, which could act as molecular recognition sites for APM. A structural hypothesis is presented to explain the observed interference with RF reactivity by APM. Extrapolations of the current results suggest that APM may inhibit the binding of IgG in a substantial proportion of IgM RFs. Interference of RF reactivity, especially in RA patients, may alleviate the pain and immobility resulting from chronic inflammation of the joints.

Anticomplementary activity of stilbenes from medicinal plants

The anticomplementary activity of stilbenes from medicinal plants in Korea was investigated in vitro. 3,5-Dihydroxy-4'-methoxystilbene (3) was most potent with IC50 value of 1.5 x 10(-4) M followed by rhapontigenin (4), oxyresverastrol (2), 2,3,4',5-tetrahydroxystilbene-2-O-beta-glucoside (9), rhaponticin (8), resverastrol (1), and piceid (7). The activity was found to be increased by a methylation on a hydroxy group of C-4' of 1, but decreased by further methylation on hydroxy groups of C-3 and C-5 and glucosylation on any hydroxy group of 1. Addition of hydroxy group on C-2' of 1 or C-3' of 3 was little affected on the anticomplementary activity but the activity was increased by O-glucosylation on C-2 of 1.

Anticomplementary activity of ginseng saponins and their degradation products

The anticomplementary activity of ginseng saponins and their degradation products obtained by chemical treatment of Korean red ginseng saponins was investigated. The total saponin and its major components showed strong anticomplementary activity and their structure-activity relationship was evaluated.

Complement-activating properties of IgM rheumatoid factors reacting with IgG subclasses

To estimate the complement-activating property (CAP) of IgM rheumatoid factor (RF), which was purified from synovial fluids of patients with rheumatoid arthritis, in a reaction with each IgG subclass, the activation and binding of C4 in the classical pathway of complement by IgM RF was measured in an enzyme-linked immunosorbent assay using biotinylated F(ab')2 antibody to human C4. The CAP of IgM RF reacting with IgG3 was significantly higher than that of IgM RFs bound to the other IgG subclasses (P < 0.01). These results suggest that IgM RF reacting with IgG3 in synovial fluid could induce a greater degree of complement-dependent inflammation in RA synovium than IgM RF reacting with other IgG subclasses.

Complement-immunoglobulin interactions

Normal circulating immunoglobulin may control complement binding to targets and thereby the manifestations of autoimmune disease. Molecular analysis of IgG and IgM mutants suggests that C1q binding by IgG utilizes a core Glu-X-Lys-X-Lys motif (where X is any amino acid). Additional amino acids, particularly homologous proline residues at position 331 in IgG and 436 in IgM, appear critical for classical pathway initiation. Glycosylation of IgG heavy chain is important in C1q binding, as well as glycosylation of IgA heavy chain for alternative pathway initiation. Additional recent evidence suggests an important role for C3 in antigen presentation. The data also raise the possibility that C3 plays a significant role in the intracellular antigen processing pathway.

IgG3 reactive rheumatoid factor in rheumatoid arthritis: etiologic and pathogenic considerations

Rheumatoid factor (RF) is a polyclonal autoantibody directed against the Fc portion of IgG. Although the role of RF in patients with rheumatoid arthritis (RA) is unclear, immune complexes that form between RF and IgG can activate the classical complement (C) pathway, leading to pathogenic outcomes involving inflammatory events and tissue damage. The specificity of serum RF and RF produced by rheumatoid synovial cells (RSC) is different. Serum RF has specificity for rabbit IgG and human IgG subclasses IgG1, 2, and 4, but binds poorly to IgG3. The affinity of serum RF for IgG Fc is low, having an association constant of 10(4)-10(5) M-1. RSC RF, however, has specificity for human IgG and high avidity for IgG3. Because of this greater specificity and avidity for IgG3, and because RSC RF may be pathogenically more important than serum RF, an important role for IgG3-reactive RF in RA may exist. Binding of RF to IgG may be dependent on the allotype and glycosylation of IgG. Infectious agents present in RA patients may directly or indirectly induce the production of certain RF. In this communication, we review and expand on several observations examining the role of IgG3-reactive RF in RA including: 1) binding differences between RF derived from RSC and serum; 2) glycosylation characteristics of IgG and its interaction with RF; 3) apparent allotype dependent binding of IgG3-reactive RF; and 4) possible relationship between infectious agents and the production of IgG3-reactive RF. Taken together, these observations suggest an important role for IgG3-reactive RF in better understanding the etiology and pathogenesis of RA.