Autoantibodies to C-reactive protein is a common finding in SLE, but not in primary Sjögren's syndrome, rheumatoid arthritis or inflammatory bowel disease

Mapping autoantibody targets of full-length C-reactive protein in systemic lupus erythematosus: importance for neutrophil function and classical complement activation

C-reactive protein (CRP) is an important pattern recognition molecule of innate immunity. Autoantibodies targeting CRP are common in patients with systemic lupus erythematosus (SLE) and the levels correlate with disease activity. The purpose of this study was to investigate binding sites of IgG autoantibodies on the full linear sequence of CRP and identify potential associations with clinical variables in well-characterized SLE patients; a secondary aim was to investigate the effect of an epitope-based synthesized peptide motif on neutrophil functions. The levels of anti-CRP and SLE-associated antibodies were assessed, and a microarray-based linear epitope mapping was performed to detect binding sites on the full CRP monomer. We observed that anti-CRP antibodies bind to a variety of linear epitopes with a higher prevalence in SLE compared to healthy blood donors. Eleven unique epitopes were identified, of which five were found exclusively in SLE. Furthermore, we show that patients with anticardiolipin IgG and/or anti-β2GPI IgG antibodies have a higher number of positive CRP epitopes, and some CRP autoantibody-specificities associate with antiphospholipid antibodies, disease activity, and classical complement activation. In addition, one identified motif was selected, synthesized, and used for studying neutrophil function. This peptide showed modulatory capacity on neutrophil oxidative burst and chemotaxis, but not on neutrophil extracellular trap formation. Our results implicate a wide variation of anti-CRP autoantibody binding motifs of the linear structure of CRP in SLE patients. Some epitopes have the potential to modify innate host responses of relevance to the pathogenesis of SLE.

The Role of Anti-mCRP Autoantibodies in Lupus Nephritis

Background

Lupus nephritis is characterized by multiple autoantibodies production. However, there are few autoantibodies associated with disease activity and prognosis. CRP exists in at least two conformationally distinct forms: native pentameric C-reactive protein (pCRP) and modified/monomeric CRP (mCRP). Autoantibodies against mCRP are prevalent in sera of patients with lupus nephritis and are reported to be pathogenic.

Summary

The levels of serum anti-mCRP autoantibodies are associated with clinical disease activity, tubulointerstitial lesions, treatment response, and prognosis in patients with lupus nephritis. The key epitope of mCRP was amino acid 35-47. Furthermore, emerging evidence indicated that anti-mCRP autoantibodies could participate in the pathogenesis of lupus nephritis by forming in situ immune complexes or interfering with the biological functions of mCRP, such as binding to complement C1q and factor H.

Key Messages

Here, we review the recent advances in the prevalence, clinical-pathological associations, and potential pathogenesis of anti-mCRP autoantibodies in lupus nephritis, which may provide a promising novel therapeutic strategy for lupus nephritis.

Extracellular vesicles opsonized by monomeric C-reactive protein (CRP) are accessible as autoantigens in patients with systemic lupus erythematosus and associate with autoantibodies against CRP

The pentraxin C-reactive protein (CRP) is a pentameric protein now known to be able to undergo dissociation into a monomeric, modified isoform, referred to as mCRP. In carefully assessing the bioactivities of each isoform, mCRP has strong pro-inflammatory activities while pCRP has mild anti-inflammatory activities. Systemic lupus erythematosus (SLE) is a disease characterized by a vast number of autoantibodies, including anti-CRP autoantibodies which have been associated with SLE disease activity and lupus nephritis. The origin of these autoantibodies is currently unknown. Extracellular vesicles (EVs) have been implicated in SLE pathogenesis as they can expose nuclear antigens on their outside surface, thereby being a potential adjuvant for the generation of autoantibodies. Herein, we studied exposure of both pCRP and mCRP on EVs in SLE plasma and the implications of each in disease activity, organ damage and clinical manifestations. We used flow cytometry to detect CRP isoforms on EV surfaces in 67 well-characterized SLE patients and 60 sex- and age-matched healthy controls. Autoantibodies against mCRP were measured using ELISA. We found an abundance of both pCRP and mCRP on SLE EVs compared to controls. Furthermore, mCRP+ but not pCRP+ EVs were elevated in patients with active disease and in anti-CRP positive patients. The proportions of mCRP+ EVs were lower in patients with acquired organ damage, especially in patients with lupus nephritis (LN), and displayed an inverse relationship with disease duration in LN and patients with active disease. Speculatively, these data suggest EV-bound mCRP as a relevant factor in SLE pathogenesis, which could contribute to development of anti-CRP autoantibodies by stimulating an immune response.

Association of C-reactive protein and complement factor H gene polymorphisms with risk of lupus nephritis in Chinese population

BACKGROUND

Complement overactivation is a major driver of lupus nephritis (LN). Impaired interactions of C-reactive protein (CRP) with complement factor H (CFH) have been shown as a pathogenic mechanism that contributes to the overactivation of complement in LN. However, genetic variations of neither CRP nor CFH show consistent influences on the risk of LN.

AIM

To examine whether genetic variations of CRP and CFH in combination can improve the risk stratification in Chinese population.

METHODS

We genotyped six CRP single nucleotide polymorphisms (SNPs) (rs1205, rs3093062, rs2794521, rs1800947, rs3093077, and rs1130864) and three CFH SNPs (rs482934, rs1061170, and rs1061147) in 270 LN patients and 303 healthy subjects.

RESULTS

No linkage was found among CRP and CFH SNPs, indicating lack of genetic interactions between the two genes. Moreover, CRP and CFH SNPs, neither individually nor in combination, are associated with the risk or clinical manifestations of LN. Given the unambiguous pathogenic roles of the two genes.

CONCLUSION

These findings suggest that the biological effects of most genetic variations of CRP and CFH on their expressions or activities are not sufficient to influence the disease course of LN.

Associations of C-reactive protein isoforms with systemic lupus erythematosus phenotypes and disease activity

Background

Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by a large production of autoantibodies and deficient clearance of cellular waste. The disease typically oscillates between episodes of elevated disease activity and quiescent disease. C-reactive protein (CRP) is a pentameric acute-phase protein usually reflecting inflammation and tissue damage. However, despite increased inflammation and elevated interleukin-6, the levels of CRP typically remain low or only slightly raised in SLE. Under certain conditions, pentameric CRP (pCRP) can dissociate into its monomeric isoform (mCRP), which mainly has been ascribed pro-inflammatory properties. The present study aims to investigate the potential relationship between pCRP and mCRP, respectively, with disease activity and clinical features of SLE.

Methods

The levels of pCRP and mCRP were measured, by turbidimetry (high-sensitive) and sandwich enzyme-linked immunosorbent assay (ELISA) respectively, in serum samples from 160 patients with SLE and 30 patients with antineutrophil cytoplasmic antibody-associated vasculitis (AAV). Twenty-two of the SLE cases were selected for analysis at two time-points; quiescent disease and active disease. The two CRP isoforms were evaluated in relation to disease activity and clinical features in the two diseases.

Results

Levels of pCRP and mCRP were significantly lower in SLE than AAV (p < 0.001) and the ratio of mCRP/pCRP was higher in SLE compared to AAV. The mCRP/pCRP ratio was higher for patients in remission and able to significantly separate between active/quiescent disease in paired, but not in non-paired, samples from patients with SLE. Significant correlations were observed with SLICC/ACR damage index for pCRP levels as well as inversely with the mCRP/pCRP ratio. Lower mCRP levels associated with malar rash.

Conclusion

As the interrelationship between the two isoforms appear to (a) discriminate between quiescent and active SLE and (b) differ between SLE and AAV, our data indicates that the two CRP isoforms could exert contrasting immunological effects and/or reflect different milieus. Given the biological effects of mCRP, it is possible that altered levels may indicate increased opsonization of immune complexes and apoptotic debris, and thereby prevent their deposition outside the reticuloendothelial system and manifestations such as lupus nephritis and lupus-related skin disease.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13075-022-02831-9.

Secretory quality control constrains functional selection-associated protein structure innovation

Biophysical models suggest a dominant role of structural over functional constraints in shaping protein evolution. Selection on structural constraints is linked closely to expression levels of proteins, which together with structure-associated activities determine in vivo functions of proteins. Here we show that despite the up to two orders of magnitude differences in levels of C-reactive protein (CRP) in distinct species, the in vivo functions of CRP are paradoxically conserved. Such a pronounced level-function mismatch cannot be explained by activities associated with the conserved native structure, but is coupled to hidden activities associated with the unfolded, activated conformation. This is not the result of selection on structural constraints like foldability and stability, but is achieved by folding determinants-mediated functional selection that keeps a confined carrier structure to pass the stringent eukaryotic quality control on secretion. Further analysis suggests a folding threshold model which may partly explain the mismatch between the vast sequence space and the limited structure space of proteins.

The mismatch in the conserved structure but different expression levels of C-reactive protein (CRP) in distinct species is reconciled by functional selection on hidden activities of unfolded CRPs.

The Complex Role of C-Reactive Protein in Systemic Lupus Erythematosus

C-reactive protein (CRP) is well-known as a sensitive albeit unspecific biomarker of inflammation. In most rheumatic conditions, the level of this evolutionarily highly conserved pattern recognition molecule conveys reliable information regarding the degree of ongoing inflammation, driven mainly by interleukin-6. However, the underlying causes of increased CRP levels are numerous, including both infections and malignancies. In addition, low to moderate increases in CRP predict subsequent cardiovascular events, often occurring years later, in patients with angina and in healthy individuals. However, autoimmune diseases characterized by the Type I interferon gene signature (e.g., systemic lupus erythematosus, primary Sjögren’s syndrome and inflammatory myopathies) represent exceptions to the general rule that the concentrations of CRP correlate with the extent and severity of inflammation. In fact, adequate levels of CRP can be beneficial in autoimmune conditions, in that they contribute to efficient clearance of cell remnants and immune complexes through complement activation/modulation, opsonization and phagocytosis. Furthermore, emerging data indicate that CRP constitutes an autoantigen in systemic lupus erythematosus. At the same time, the increased risks of cardiovascular and cerebrovascular diseases in patients diagnosed with systemic lupus erythematosus and rheumatoid arthritis are well-established, with significant impacts on quality of life, accrual of organ damage, and premature mortality. This review describes CRP-mediated biological effects and the regulation of CRP release in relation to aspects of cardiovascular disease and mechanisms of autoimmunity, with particular focus on systemic lupus erythematosus.

A Review of Anti-C Reactive Protein Antibodies in Systemic Lupus Erythematosus

Systemic lupus erythematosus (SLE), the prototype autoimmune disease, is characterized by the production of a plethora of autoantibodies with various roles in the development of disease-related tissue damage. C-reactive protein (CRP) is an acute phase reactant with a pentameric structure. Under acidic or alkaline conditions, or when urea levels are high and/or calcium levels are low, the pentamer (pCRP) dissociates irreversibly into monomeric CRP (mCRP) and exposes new epitopes (neo-CRP). Importantly, anti-mCRP (but not anti-pCRP) antibodies have been described in patients with SLE, their prevalence varying from 4% to 78% in different cohorts. Numerous studies have investigated the relationship between autoantibodies directed against CRP (anti-CRP) and disease activity as well as their association with lupus nephritis (LN), frequently reporting discrepant findings. The main objective of the present review is to describe the role of anti-mCRP antibodies in SLE according to the currently available data.

Anti-Pentraxin Antibodies in Autoimmune Diseases: Bystanders or Pathophysiological Actors?

Pentraxins are soluble innate immunity receptors involved in sensing danger molecules. They are classified as short (CRP, SAP) and long pentraxin subfamilies, including the prototypic long pentraxin PTX3. Pentraxins act mainly as bridging molecules favoring the clearance of microbes and dead cells. They are also involved in many other biological processes, such as regulation of complement activation, inflammation and tissue homeostasis. Autoantibodies directed against pentraxins have been reported in various autoimmune diseases, especially in systemic lupus erythematosus and ANCA-associated vasculitis. In this review, we review the main biological characteristics and functions of pentraxins and summarize data concerning autoantibodies directed against pentraxins in the context of autoimmune diseases and discuss their potential pathological role.

Chd4 choreographs self-antigen expression for central immune tolerance

Autoreactive T cells are eliminated in the thymus to prevent autoimmunity by promiscuous expression of tissue-restricted self-antigens in medullary thymic epithelial cells. This expression is dependent on the transcription factor Fezf2, as well as the transcriptional regulator Aire, but the entire picture of the transcriptional program has been obscure. Here, we found that the chromatin remodeler Chd4, also called Mi-2β, plays a key role in the self-antigen expression in medullary thymic epithelial cells. To maximize the diversity of self-antigen expression, Fezf2 and Aire utilized completely distinct transcriptional mechanisms, both of which were under the control of Chd4. Chd4 organized the promoter regions of Fezf2-dependent genes, while contributing to the Aire-mediated induction of self-antigens via super-enhancers. Mice deficient in Chd4 specifically in thymic epithelial cells exhibited autoimmune phenotypes, including T cell infiltration. Thus, Chd4 plays a critical role in integrating Fezf2- and Aire-mediated gene induction to establish central immune tolerance.

The Multiple Faces of C-Reactive Protein-Physiological and Pathophysiological Implications in Cardiovascular Disease

C-reactive protein (CRP) is an intriguing protein which plays a variety of roles in either physiological or pathophysiological states. For years it has been regarded merely as a useful biomarker of infection, tissue injury and inflammation, and it was only in the early 80s that the modified isoforms (mCRP) of native CRP (nCRP) appeared. It soon became clear that the roles of native CRP should be clearly discriminated from those of the modified form and so the impacts of both isoforms were divided to a certain degree between physiological and pathophysiological states. For decades, CRP has been regarded only as a hallmark of inflammation; however, it has since been recognised as a significant predictor of future episodes of cardiovascular disease, independent of other risk factors. The existence of modified CRP isoforms and their possible relevance to various pathophysiological conditions, suggested over thirty years ago, has prompted the search for structural and functional dissimilarities between the pentameric nCRP and monomeric mCRP isoforms. New attempts to identify the possible relevance between the diversity of structures and their opposing functions have initiated a new era of research on C-reactive protein. This review discusses the biochemical aspects of CRP physiology, emphasizing the supposed relevance between the structural biology of CRP isoforms and their differentiated physiological and pathophysiological roles.

Determination of CRP Autoantibodies by SPR Immunoassay

Biosensors based on the principle of surface plasmon resonance (SPR) are surface-sensitive optical devices used for monitoring biomolecular interactions at the sensor surface in real time without any labeling. It is used in a wide variety of areas including proteomics, clinical diagnosis, environmental monitoring, drug discovery, and food analysis. C-reactive protein (CRP) is a marker of inflammation, which undergoes conformation changes in local lesions, leading to the formation of mCRP. Autoantibodies against mCRP are frequently detected in systemic lupus erythematosus (SLE) and associated with disease activity and prognosis. An SPR immunoassay for CRP autoantibodies at complement factor H-CRP interface is described in this chapter.

High Prevalence of Autoantibodies Against Monomeric C Reactive Protein (CRP) in Children with PFAPA Syndrome

PFAPA (periodic fever, aphthous stomatitis, pharyngitis, cervical adenitis) syndrome is an autoinflammatory disorder of unknown etiology. The aim of our study was to evaluate whether the presence of anti-mCRP autoantibodies (anti-mCRP) might possibly contribute to systemic inflammation during PFAPA flares. We carried out anti-mCRP testing (in-house ELISA) in a single-center, prospective cohort of 30 PFAPA patients (12 girls). We found a high prevalence (43.3%) of anti-mCRP antibodies in PFAPA patients during their febrile episodes, which implies the possible involvement of anti-mCRP antibodies in PFAPA pathogenesis.

The relationship between anti-C-reactive protein and disease activity in patients with systemic lupus erythematosus

BACKGROUND/AIMS

Anti-C-reactive protein (CRP) antibody has been introduced as a potential biologic marker in Systemic lupus erythematosus (SLE). The aim of study is to evaluate the level of anti-CRP antibody in patients with SLE.

METHODS

This study investigated the relationship between levels of anti-CRP antibodies and disease activity markers, such as complement, anti-double-stranded DNA antibody, and SLE disease activity index in 34 patients with SLE.

RESULTS

The serum anti-CRP antibody levels of the patients with SLE were significantly higher than those of the healthy controls (11.3 ± 5.6 µg/mL vs. 9.1 ± 2.8 µg/mL). The percentages of the positive anti-CRP antibody were 52.9% in SLE and 27.8% in controls. Disease duration of SLE showed significant correlation with the anti-CRP antibody (r = 0.234, p = 0.026). However no significant relationship was observed between the levels of anti-CRP antibodies and disease activity markers.

CONCLUSIONS

These data show that the anti-CRP antibody levels of the patients with SLE were significantly higher than those of healthy controls. We observed that the presence of the anti-CRP anti-CRP antibody was not associated with disease activity of SLE.

Naturally occurring antibodies against serum amyloid A reduce IL-6 release from peripheral blood mononuclear cells

Serum amyloid A (SAA) is a sensitive inflammatory marker rapidly increased in response to infection, injury or trauma during the acute phase. Resolution of the acute phase and SAA reduction are well documented, however the exact mechanism remains elusive. Two inducible SAA proteins, SAA1 and SAA2, with their variants could contribute to systemic inflammation. While unconjugated human variant SAA1α is already commercially available, the variants of SAA2 are not. Antibodies against SAA have been identified in apparently healthy blood donors (HBDs) in smaller, preliminary studies. So, our objective was to detect anti-SAA and anti-SAA1α autoantibodies in the sera of 300 HBDs using ELISA, characterize their specificity and avidity. Additionally, we aimed to determine the presence of anti-SAA and anti-SAA1α autoantibodies in intravenous immunoglobulin (IVIg) preparations and examine their effects on released IL-6 from SAA/SAA1α-treated peripheral blood mononuclear cells (PBMCs). Autoantibodies against SAA and SAA1α had a median (IQR) absorbance OD (A₄₅₀) of 0.655 (0.262–1.293) and 0.493 (0.284–0.713), respectively. Both anti-SAA and anti-SAA1α exhibited heterogeneous to high avidity and reached peak levels between 41–50 years, then diminished with age in the oldest group (51–67 years). Women consistently exhibited significantly higher levels than men. Good positive correlation was observed between anti-SAA and anti-SAA1α. Both anti-SAA and anti-SAA1α were detected in IVIg, their fractions subsequently isolated, and shown to decrease IL-6 protein levels released from SAA/SAA1α-treated PBMCs. In conclusion, naturally occurring antibodies against SAA and anti-SAA1α could play a physiological role in down-regulating their antigen and proinflammatory cytokines leading to the resolution of the acute phase and could be an important therapeutic option in patients with chronic inflammatory diseases.

Autoantibodies against C-Reactive Protein Influence Complement Activation and Clinical Course in Lupus Nephritis

Autoantibodies against the major acute-phase reactant C-reactive protein (CRP) are frequently found in patients with lupus nephritis. Further defining the autoimmune epitopes on CRP may not only improve patient stratification but also, hint at mechanisms of CRP action. Herein, we show that amino acids 35-47 constitute the major epitope recognized by anti-CRP autoantibodies in patients with lupus nephritis. Notably, the presence of autoantibodies against amino acids 35-47 associated with more severe renal damage and predicted worse outcome. This epitope is exposed on CRP only after irreversible structure changes, yielding a conformationally altered form termed modified or monomeric CRP (mCRP). ELISA and surface plasmon resonance assays showed that amino acids 35-47 mediate the interaction of mCRP with complement factor H, an inhibitor of alternative pathway activation, and this interaction greatly enhanced the in vitro cofactor activity of complement factor H. In contrast, autoantibodies against amino acids 35-47 inhibited these actions of mCRP. Our results thus provide evidence for the in vivo generation of mCRP in a human disease and suggest that mCRP actively controls the pathogenesis of lupus nephritis by regulating complement activation. Therefore, amino acids 35-47 constitute a functional autoimmune epitope on CRP that can be targeted therapeutically and diagnostically.

Interferon-α coincides with suppressed levels of pentraxin-3 (PTX3) in systemic lupus erythematosus and regulates leucocyte PTX3 in vitro

Dysfunctional elimination of cell debris, and the role of opsonins such as pentraxins, is of interest regarding systemic lupus erythematosus (SLE) pathogenesis. Interferon (IFN)-α is typically elevated during SLE flares, and inhibits hepatocyte production of the pentraxin 'C-reactive protein' (CRP), partly explaining the poor correlation between CRP levels and SLE disease activity. The extrahepatically produced 'pentraxin 3' (PTX3) shares waste disposal functions with CRP, but has not been studied extensively in SLE. We analysed serum PTX3 in SLE, and assessed its interference with IFN-α in vitro. Serum samples from 243 patients with SLE and 100 blood donors were analysed regarding PTX3. Patient sera were analysed for IFN-α, and genotyped for three PTX3 single nucleotide polymorphisms reported previously to associate with PTX3 levels. Stimulated PTX3 release was assessed in the presence or absence of IFN-α in blood donor neutrophils and peripheral blood mononuclear cells (PBMC). Serum PTX3 was 44% lower in patients with SLE compared to blood donors (P < 0·0001) and correlated with leucocyte variables. Patients with undetectable IFN-α had 29% higher median PTX3 level than patients with detectable IFN-α (P = 0·01). PTX3 production by PBMC was inhibited by IFN-α, whereas neutrophil degranulation of PTX3 was increased. No differences in PTX3 levels were observed between the SNPs. In conclusion, median serum PTX3 is lower in SLE (especially when IFN-α is detectable) compared to blood donors. In addition to its potential consumption during waste disposal, it is plausible that IFN-α also attenuates PTX3 by inhibiting synthesis by PBMC and/or exhausting PTX3 storage in neutrophil granules.

Detection of Anti-Pentraxin-3 Autoantibodies in ANCA-Associated Vasculitis

OBJECTIVES

Pentraxin 3 (PTX3), in common with myeloperoxidase and proteinase 3, is stored in human neutrophil granules and is expressed on apoptotic neutrophil surface. We therefore investigated the presence of anti-PTX3 autoantibodies (aAbs) in the sera of antineutrophil cytoplasmic antibodies (ANCA)-associated vasculitis (AAV) patients.

METHODS

Presence of anti-PTX3 autoantibodies was analysed by a specific enzyme-linked immunosorbent assay in sera from 150 patients with microscopic polyangiitis (MPA), granulomatosis with polyangiitis (GPA), and eosinophilic granulomatosis with polyangiitis (EGPA), and in sera of 227 healthy subjects (HS), 40 systemic sclerosis (SSc) patients, and 25 giant cell arteritis patients (GCA). Using indirect immunofluorescence on fixed human neutrophils, we also analyzed the staining pattern associated with the presence of anti-PTX3 aAbs.

RESULTS

Anti-PTX3 aAbs were detected in 56 of 150 (37.3%) of the AAV patients (versus 12 of 227 (5.3%) of HS, p<0.001) and, interestingly, in 7 of 14 MPO and PR3 ANCA negative AAV patients. Moreover, by indirect immunofluorescence on fixed neutrophils, anti-PTX3 aAbs gave rise to a specific cytoplasmic fluorescence pattern distinct from the classical cytoplasmic (c-ANCA), perinuclear (p-ANCA), and atypical (a-ANCA) pattern. Anti-PTX3 aAbs levels were higher in patients with active AAV as compared to patients with inactive disease.

CONCLUSION

Our work suggests that PTX3 is as a novel ANCA antigen. Anti-PTX3 aAbs appear thus as a promising novel biomarker in the diagnosis of AAV, including in patients without detectable MPO and PR3 ANCA.

Prognostic value of anti-CRP antibodies in lupus nephritis in long-term follow-up

Background

Autoantibodies against monomeric C-reactive protein (anti-CRP-Ab) observed in patients with systemic lupus erythematosus (SLE) and lupus nephritis (LN) were suggested to be associated with active LN and a poor response to therapy during short-term follow-up.

The aim of this study was to confirm this finding and to investigate the prognostic value of anti-CRP-Ab in patients with LN during long-term follow-up.

Methods

Sera of 57 SLE patients (47 women, 10 men) with biopsy proven LN and 122 healthy individuals were analyzed for the presence of anti-CRP-Ab by in-house ELISA.

Anti-CRP-Ab levels were studied in relation to routine laboratory tests, urine analysis, levels of C3, C4, other immunological markers and the overall disease activity as assessed by Systemic Lupus Erythematosus Disease Activity Index (SLEDAI).

The prognostic value of anti-CRP-Ab was tested in a subgroup of 29 newly diagnosed LN patients (median follow-up 5.9 years). Response to therapy at various time points was assessed with respect to baseline anti-CRP-Ab levels. At least partial response in the first/second year of treatment was considered as a “favorable outcome”, while non-response, renal flare or end stage renal disease were considered as “unfavorable outcome”.

Results

Anti-CRP-Ab were only detected in patients with active renal disease and their levels correlated with SLEDAI (r_s = 0.165, p = 0.002). The time to response was shorter in patients being anti-CRP-Ab negative at baseline compared to anti-CRP-Ab positive patients, p = 0.037. In the second year of therapy, baseline anti-CRP-Ab positivity was a significant predictor of “unfavorable outcome” (OR [95 % CI] = 15.6 [1.2-771]; p = 0.021). The predictive value of “baseline anti-CRP positivity” further increased when combined with “non-response to therapy in the first year”. Baseline anti-CRP-Ab positivity was not a predictor of “unfavorable outcome” at the end of follow-up, (OR [95 % CI] = 5.5 [0.6-71.1], p = 0.169).

Conclusions

Baseline serum levels of anti-CRP-Ab seem to be a strong risk factor for a composite outcome of non-response, renal flare or end stage renal disease after two years of standard treatment of LN. The response to therapy seems to be delayed in anti-CRP-Ab positive patients.

Electronic supplementary material

The online version of this article (doi:10.1186/s13075-015-0879-8) contains supplementary material, which is available to authorized users.

Targeting C-Reactive Protein in Inflammatory Disease by Preventing Conformational Changes

C-reactive protein (CRP) is a pentraxin that has long been employed as a marker of inflammation in clinical practice. Recent findings brought up the idea of CRP to be not only a systemic marker but also a mediator of inflammation. New studies focused on structural changes of the plasma protein, revealing the existence of two distinct protein conformations associated with opposed inflammatory properties. Native, pentameric CRP (pCRP) is considered to be the circulating precursor form of monomeric CRP (mCRP) that has been identified to be strongly proinflammatory. Recently, a dissociation mechanism of pCRP has been identified on activated platelets and activated/apoptotic cells associated with the amplification of the proinflammatory potential. Correspondingly, CRP deposits found in inflamed tissues have been identified to exhibit the monomeric conformation by using conformation-specific antibodies. Here we review the current literature on the causal role of the dissociation mechanism of pCRP and the genesis of mCRP for the amplification of the proinflammatory potential in inflammatory reactions such as atherosclerosis and ischemia/reperfusion injury. The chance to prevent the formation of proinflammatory mediators in ubiquitous inflammatory cascades has pushed therapeutic strategies by targeting pCRP dissociation in inflammation. In this respect, the development of clinically applicable derivatives of the palindromic compound 1,6-bis(phosphocholine)-hexane (1,6-bis PC) should be a major focus of future CRP research.

Pentraxins: structure, function, and role in inflammation

The pentraxins are an ancient family of proteins with a unique architecture found as far back in evolution as the Horseshoe crab. In humans the two members of this family are C-reactive protein and serum amyloid P. Pentraxins are defined by their sequence homology, their pentameric structure and their calcium-dependent binding to their ligands. Pentraxins function as soluble pattern recognition molecules and one of the earliest and most important roles for these proteins is host defense primarily against pathogenic bacteria. They function as opsonins for pathogens through activation of the complement pathway and through binding to Fc gamma receptors. Pentraxins also recognize membrane phospholipids and nuclear components exposed on or released by damaged cells. CRP has a specific interaction with small nuclear ribonucleoproteins whereas SAP is a major recognition molecule for DNA, two nuclear autoantigens. Studies in autoimmune and inflammatory disease models suggest that pentraxins interact with macrophage Fc receptors to regulate the inflammatory response. Because CRP is a strong acute phase reactant it is widely used as a marker of inflammation and infection.

Plasma soluble urokinase plasminogen activator receptor (suPAR) levels in systemic lupus erythematosus

OBJECTIVE

Soluble urokinase plasminogen activator receptor (suPAR) is a biomarker of systemic inflammation. We aimed to characterize plasma suPAR levels in SLE patients.

METHODS

We measured plasma suPAR, C reactive protein (CRP) and erythrocyte sedimentation rate (ESR) in 89 SLE patients and 29 healthy controls.

RESULTS

suPAR and ESR values were higher in SLE than in controls, while CRP levels were comparable. ROC analysis of suPAR levels indicated a cut-off value of 5.70 ng/mL to distinguish patients with high disease activity (SLEDAI >8).

CONCLUSION

suPAR might be an objective marker for identifying SLE patients with active disease.

CRP/anti-CRP antibodies assembly on the surfaces of cell remnants switches their phagocytic clearance toward inflammation

Systemic lupus erythematosus (SLE) is a chronic inflammatory disease characterized by the production of autoantibodies, formation of immune complexes (IC), and activation of complement that ultimately fuel acute and/or chronic inflammation. Accumulation in blood and tissues of post-apoptotic remnants is considered of etiological and pathological importance for patients with SLE. Besides receptors directly recognizing apoptotic cells, soluble opsonins of the innate immune system bind apoptotic material dependent on the stage of apoptosis. We describe the binding to the surface of secondary necrotic cells (SNEC) of the serum opsonin CRP and further opsonins. We show that anti-dsDNA and anti-CRP autoantibodies bind and sensitize SNEC. Autoantibody-sensitized SNEC were cleared by macrophages in vitro and induced a pro-inflammatory cytokine response. In conclusion, anti-CRP, CRP, and SNEC form a ternary pyrogen endowed with strong pro-inflammatory capabilities which is able to maintain and perpetuate chronic inflammation.

Inefficient clearance of dying cells in patients with SLE: anti-dsDNA autoantibodies, MFG-E8, HMGB-1 and other players

Systemic lupus erythematosus (SLE) is a complex disease resulting from inflammatory responses of the immune system against several autoantigens. Inflammation is conditioned by the continuous presence of autoantibodies and leaked autoantigens, e.g. from not properly cleared dying and dead cells. Various soluble molecules and biophysical properties of the surface of apoptotic cells play significant roles in the appropriate recognition and further processing of dying and dead cells. We exemplarily discuss how Milk fat globule epidermal growth factor 8 (MFG-E8), biophysical membrane alterations, High mobility group box 1 (HMGB1), C-reactive protein (CRP), and anti-nuclear autoantibodies may contribute to the etiopathogenesis of the disease. Up to date knowledge about these key elements may provide new insights that lead to the development of new treatment strategies of the disease.

Serum levels of autoantibodies against C-reactive protein correlate with renal disease activity and response to therapy in lupus nephritis

Introduction

Serum levels of C-reactive protein (CRP) seldom reflect disease activity in systemic lupus erythematosus (SLE). We have previously shown that autoantibodies against neo-epitopes of CRP often occur in SLE, but that this does not explain the modest CRP response seen in flares. However, we have repeatedly found that anti-CRP levels parallel lupus disease activity, with highest levels in patients with renal involvement; thus, we aimed to study anti-CRP in a material of well-characterized lupus nephritis patients.

Methods

Thirty-eight patients with lupus nephritis were included. Treatment with corticosteroids combined with cyclophosphamide, mycophenolate mofetil or rituximab was started after baseline kidney biopsy. A second biopsy was taken after ≥ 6 months. Serum creatinine, cystatin C, complement, anti-dsDNA, anti-CRP and urinalysis were done on both occasions. Biopsies were evaluated regarding World Health Organisation (WHO) class and indices of activity and chronicity. Renal disease activity was estimated using the British Isles Lupus Assessment Group (BILAG) index.

Results

At baseline, 34/38 patients had renal BILAG-A; 4/38 had BILAG-B. Baseline biopsies showed WHO class III (n = 8), IV (n = 19), III to IV/V (n = 3) or V (n = 8) nephritis. Seventeen out of 38 patients were anti-CRP-positive at baseline, and six at follow-up. Overall, anti-CRP levels had dropped at follow-up (P < 0.0001) and anti-CRP levels correlated with renal BILAG (r = 0.29, P = 0.012). A positive anti-CRP test at baseline was superior to anti-dsDNA and C1q in predicting poor response to therapy as judged by renal BILAG. Baseline anti-CRP levels correlated with renal biopsy activity (r = 0.33, P = 0.045), but not with chronicity index. Anti-CRP levels were positively correlated with anti-dsDNA (fluorescence-enhanced immunoassay: r = 0.63, P = 0.0003; Crithidia luciliae immunofluorescence microscopy test: r = 0.44, P < 0.0001), and inversely with C3 (r = 0.35, P = 0.007) and C4 (r = 0.29, P = 0.02), but not with C1q (r = 0.14, P = 0.24). No associations with urinary components, creatinine, cystatin C or the glomerular filtration rate were found.

Conclusions

In the present study, we demonstrate a statistically significant correlation between anti-CRP levels and histopathological activity in lupus nephritis, whereas a baseline positive anti-CRP test predicted poor response to therapy. Our data also confirm previous findings of associations between anti-CRP and disease activity. This indicates that anti-CRP could be helpful to assess disease activity and response to therapy in SLE nephritis, and highlights the hypothesis of a pathogenetic role for anti-CRP antibodies in lupus nephritis.

Pentraxins, anti-pentraxin antibodies, and atherosclerosis

Atherosclerosis is a disease of the vascular wall, which predominantly affects large and medium-sized arteries. It represents a leading cause of morbidity and mortality in the Western world. In the last few decades, it has been clearly shown that immune system plays a relevant role in atherogenesis. The effectors of both innate and adaptive immunity, including immune cells, cell or soluble receptors, cytokines, chemokines, complement components or coagulation systems, and autoantibodies are able to modulate atherosclerosis. Among proteins belonging to innate immunity, the highly conserved pentraxin family, which encompass C-reactive protein (CRP), serum amyloid P (SAP), and the long pentraxin 3 (PTX3) seems to be directly involved in the induction and progression of atherosclerosis. By immunohistochemical staining, pentraxins were found within the atherosclerotic plaques where they could play a key role interacting with atherogenic-modified lipoproteins, favoring the formation of foam cells, and exerting a proinflammatory action. Pentraxin serum levels have been shown to be associated with clinical and subclinical atherosclerosis in general population. Antibodies against pentraxins have been demonstrated in patients with autoimmune diseases, but their role in atherogenesis is still controversial.

Autoantibodies to Tumor Necrosis Factor in Patients with Rheumatoid Arthritis and Systemic Lupus Erythematosus

Objective.

To detect autoantibodies to tumor necrosis factor (TNF) in patients with rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE), and to determine their clinical correlates.

Methods.

Ninety-two patients with RA and 62 with SLE were studied. Sera were examined for autoantibodies to TNF by enzyme linked immunoassay. Levels of these autoantibodies were analyzed in respect to markers of inflammation such as erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), and joint erosions, as well as other clinical, laboratory, and therapeutic aspects of RA and SLE.

Results.

Anti-TNF levels were higher in those RA patients without erosions, but did not correlate with ESR or CRP.

Conclusion.

These observations suggest that autoantibody anti-TNF may be part of the innate immune system and may contribute to decreased inflammation in patients with RA.

Evidence of systemic inflammation and estimation of coronary artery disease risk: a population perspective

A growing body of evidence indicates that patients with immune-inflammatory diseases experience an increased risk for cardiovascular morbidity and mortality. However, patients with immune-inflammatory diseases do not exhibit a corresponding increase in traditional coronary artery disease (CAD) risk factors that could explain the observed increase in CAD. Chronic inflammation is now accepted as playing a potentially important role in the promotion of atherosclerosis, a main cause of CAD. Evidence is also accumulating to suggest that the chronic systemic inflammation associated with immune-inflammatory diseases results in elevated levels of nontraditional CAD risk factors, such as biomarkers of inflammation, in patients with these conditions. Evaluation of only traditional CAD risk factors in these patients, therefore, may result in the underestimation of their future overall CAD risk. Using the Framingham patient cohort, we found associations between markers of inflammation and CAD risk overall. The contribution of inflammatory biomarkers should be considered along with the status of traditional CAD risk factors to gain a complete picture of the CAD risk in patients with underlying conditions that increase inflammation such as rheumatoid arthritis or systemic lupus erythematosus.

Antibody clustering helps refine lupus prognosis

OBJECTIVE

Our aim was to investigate a possible association between patterns of anti-dsDNA antibody isotypes (IgG, IgM, and IgA), rheumatoid factor (RF) isotypes (IgG, IgM, IgA), and IgG anti-C reactive protein (CRP) and systemic lupus erythematosus (SLE) disease activity (SLEDAI).

METHODS

Our study group included 98 patients, 86 women and 12 men, with a mean SLEDAI score of 7.9 +/- 4.1. We divided the patients into 4 groups by the serum anti-dsDNA antibody isotype intensity level.

RESULTS

We found that patients in group 1 (IgG > IgM, 42 patients) had a statistically significantly higher SLEDAI score than group 2 (IgG < IgM, 13 patients) (10.57 +/- 4.62 versus 5.6 +/- 4, P = 0.0012), group 3 (IgG = IgM, 8 patients) (10.57 +/- 4.62 versus 6.2 +/- 1.98, P = 0.04), and group 4 (none, 35 patients) (10.57 +/- 4.62 versus 6 +/- 1.5, P = 0.0001). SLE patients with IgG RF or IgM RF isotype present had a significantly higher SLEDAI score compared with those without IgG RF or IgM RF (10.57 +/- 4.8 versus 7.6 +/- 4.1, P = 0.03, 10.6 +/- 5 versus 7.6 +/- 3.9, P = 0.046). The presence of IgA RF isotype was not associated with a higher SLEDAI score. IgG anti-CRP did not correlate differentially with SLEDAI scores.

CONCLUSIONS

A combination of high-titer IgG anti-dsDNA with a positive RF of IgM isotype may serve as a marker for more active SLE.

C-reactive protein, anti-C-reactive protein antibodies and clinical atherosclerosis

Abstract

There has been considerable interest in the relationship between C-reactive protein (CRP) and atherosclerosis. We have previously demonstrated that individuals, especially those with rheumatoid arthritis and systemic lupus erythematosus, may produce antibodies to CRP. This study was therefore undertaken to determine the possible association between anti-CRP antibodies and atherosclerosis. A total of 103 individuals were identified with or without atherosclerosis, and without clinical rheumatic diseases. They were evaluated with respect to cholesterol, HDL, LDL, high-sensitivity (hs)CRP, and anti-CRP antibody levels, as well as use of statin medications. Individuals with atherosclerosis were much more likely to be taking a statin, and thus have lower lipid levels. However, there was no association between hsCRP or anti-CRP antibody levels with atherosclerosis, statin use, or each other. These observations suggest that anti-CRP antibody is not involved in atherosclerosis, and may represent an epiphenomenon.

CRP and the disposal of dying cells: consequences for systemic lupus erythematosus and rheumatoid arthritis

C reactive protein (CRP) levels directly correlate with the disease activity of many inflammatory diseases, e.g. sepsis, infection, and various autoimmunopathies such as rheumatoid arthritis (RA). In contrast, insufficient CRP levels are implicated in the development of systemic lupus erythematosus (SLE). This article reports on the level-depended effects of CRP in various diseases. In detail we show that increased and decreased levels of CRP, as demonstrated in patients with RA and SLE, respectively can contribute to disease progression.

Autoantibodies against Protective Molecules C1q, C-Reactive Protein, Serum Amyloid P, Mannose-Binding Lectin, and Apolipoprotein A1: Prevalence in Systemic Lupus Erythematosus

Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by the production of several autoantibodies. Among the multiple factors involved in SLE development, apoptotic defects and impaired clearance of cellular debris have gained considerable interest, as they contribute to autoantigen overload. Several molecules of the innate immunity, also participate in the removal of damaged and apoptotic cells. Among them are C1q, C-reactive protein (CRP), serum amyloid P protein (SAP), mannose-binding lectin (MBL), and apolipoprotein A1 (APO A1). To evaluate the prevalence of autoantibodies against CRP, SAP, MBL, APO A1, and C1q among SLE patients, and their relationship with disease activity, a total of 150 SLE patients were screened for the presence of elevated antibody titers against C1q, CRP, SAP, MBL, and APO A1, utilizing the enzyme-linked immunosorbent assay (ELISA) method. Disease activity was assessed using the ECLAM or SLEDAI scores. The study population comprised two groups of patients: 100 patients with quiescent disease (median ECLAM score 2) comprised the first group, and 50 patients with active disease (median SLEDAI score 16) comprised group 2. Elevated titers of anti-CRP antibodies were significantly elevated only in group 1 (10% versus 4% of controls). Antibodies against SAP were evaluated only among patients in group 1, and were found at a significant high prevalence (20%). Elevated titers of anti-MBL antibodies were significantly elevated only in group 1 (15% versus 3.6%); and antibodies directed against APO A1 were significantly elevated in 21% of group 1, and 50% of group 2 patients. Elevated titers of anti-C1q were evaluated only in group 2, and were found at a significant prevalence of 66%. Significant correlation with disease activity was found only for anti-APO A1 antibodies, and only in group 1. Several patients harbored more than one of the autoantibodies tested. In patients with SLE, autoantibodies directed against protective molecules, that is, acute-phase proteins involved in the disposal of cellular and nuclear debris, can be detected. These autoantibodies may play a pathogenic role in the development or perpetuation of autoimmunity in SLE.

Cytokine induction by circulating immune complexes and signs of in-vivo complement activation in systemic lupus erythematosus are associated with the occurrence of anti-Sjögren's syndrome A antibodies

Circulating immune complexes (IC) and levels of IC-induced cytokines have been correlated with complement activation and autoantibody profiles in systemic lupus erythematosus (SLE). SLE sera were analysed concerning levels of immune complexes (IC), classical complement function and different antinuclear and anti-C-reactive protein (CRP) autoantibodies. Blood mononuclear cells from healthy donors were stimulated with isolated IC and production of interleukin (IL)-10, IL-6 and IL-12p40 was measured. Functional experiments revealed that increased levels of IC-induced cytokines were associated with both increased classical complement activation and the occurrence of anti-Sjögren's syndrome A (SSA) and anti-SSB but not other autoantibodies. Biochemical measurement of circulating IC showed that the degree of complement activation and the occurrence of anti-SSA were synergistically associated with levels of circulating IC in SLE sera, as complement activation was a prerequisite for the enhancing effect of anti-SSA. Anti-CRP was associated with complement activation, but not with other autoantibodies. Our results indicate that anti-SSA and possibly anti-SSB antibodies influence IC formation and subsequent IC-induced cytokine induction, and that they thereby participate in the inflammatory process in active SLE.

Pathogenic implications for autoantibodies against C-reactive protein and other acute phase proteins

Systemic lupus erythematosus (SLE) is a systemic rheumatic disease characterized clinically by multiorgan involvement and serologically by the occurrence of antinuclear antibodies. SLE patients may present with multiple autoantibodies to cytoplasmic and cell surface antigens as well as to circulating plasma proteins. Another feature of SLE is that serum levels of C-reactive protein (CRP) often remain low despite high disease activity and despite high levels of other acute phase proteins and interleukin-6, i.e. the main CRP inducing cytokine. Apart from its important role as a laboratory marker of inflammation, CRP attracts increasing interest due to its many intriguing biological functions, one of which is a role as an opsonin contributing to the elimination of apoptotic cell debris, e.g. nucleosomes, thereby preventing immunization against autoantigens. Recently, autoantibodies against CRP and other acute phase proteins have been reported in certain rheumatic conditions, including SLE. Although the presence of anti-CRP autoantibodies does not explain the failed CRP response in SLE, antibodies directed against acute phase proteins have several implications of pathogenetic interest. This paper thus highlights the biological and clinical aspects of native and monomeric CRP and anti-CRP, as well as autoantibodies against mannose-binding lectin, serum amyloid A and serum amyloid P component.

Solid-phase classical complement activation by C-reactive protein (CRP) is inhibited by fluid-phase CRP–C1q interaction

C-reactive protein (CRP) interacts with phosphorylcholine (PC), Fcgamma receptors, complement factor C1q and cell nuclear constituents, yet its biological roles are insufficiently understood. The aim was to characterize CRP-induced complement activation by ellipsometry. PC conjugated with keyhole limpet hemocyanin (PC-KLH) was immobilized to cross-linked fibrinogen. A low-CRP serum with different amounts of added CRP was exposed to the PC-surfaces. The total serum protein deposition was quantified and deposition of IgG, C1q, C3c, C4, factor H, and CRP detected with polyclonal antibodies. The binding of serum CRP to PC-KLH dose-dependently triggered activation of the classical pathway. Unexpectedly, the activation was efficiently down-regulated at CRP levels > 150 mg/L. Using radial immunodiffusion, CRP-C1q interaction was observed in serum samples with high CRP concentrations. We propose that the underlying mechanism depends on fluid-phase interaction between C1q and CRP. This might constitute another level of complement regulation, which has implications for systemic lupus erythematosus where CRP is often low despite flare-ups.

Autoimmunity to protective molecules: is it the perpetuum mobile (vicious cycle) of autoimmune rheumatic diseases?

Apoptotic defects and impaired clearance of cellular debris are considered key events in the development of autoimmunity, as they can contribute to autoantigen overload and might be involved in the initiation of an autoimmune response. The C1q protein and mannose-binding lectin are activators of the complement system. The pentraxins are a group of highly conserved proteins including the short pentraxins, C-reactive protein and serum amyloid P, and the long pentraxin family member, pentraxin 3, all of which are involved in innate immunity and in acute-phase responses. In addition to their role in innate immunity and inflammation, each of these proteins participates in the removal of damaged and apoptotic cells. In this article, we discuss the clinical significance of different levels of these proteins, their role in the induction of or protection against autoimmunity, and the presence of specific autoantibodies against them in various autoimmune diseases.

Utility of procalcitonin concentration in the evaluation of patients with malignant diseases and elevated C-reactive protein plasma concentrations

BACKGROUND

Elevated plasma concentrations of the C-reactive protein (CRP) are frequently found in patients with malignant diseases. Discrimination between infection and noninfectious acute-phase reactions is essential for therapeutic decisions.

METHODS

Because increased procalcitonin (PCT) concentrations have been described predominantly in patients with a systemic infection, PCT plasma concentrations were measured prospectively in 111 patients with a hemato-oncological condition with a CRP concentration >8 mg/L.

RESULTS

Documented cases of infection were identified in 42 patients, 39 patients had unexplained fever, and 30 patients had no signs of infection. Twenty patients in the latter group were classified as having an elevated CRP concentration caused by a high tumor load (tumor group), and 8 had elevated concentrations that were drug related (drug group). Median CRP concentrations did not differ significantly between groups of patients with and without infection. PCT concentrations were higher in patients with an infection than in patients without an infection and were within the normal range in all patients in the drug and tumor groups. As shown by receiver operating characteristic analysis, PCT concentration was a significant discriminator between having and not having infection, having infection and being in the tumor group, and having infection and being in the drug group. In contrast, CRP concentration was only a predictor of being in the drug group, when the cut-off point was set at 85.1 mg/L, which limited its clinical applicability.

CONCLUSIONS

PCT concentration contributes significantly to the differential diagnosis for elevated CRP concentrations in patients with hemato-oncological conditions and facilitates therapeutic decisions.

Antibodies to C reactive protein

BACKGROUND

C reactive protein (CRP) is a known indicator of inflammation. Serum CRP is often raised in patients with inflammatory conditions.

OBJECTIVE

To determine whether individuals make antibodies to CRP and whether this might affect serum CRP concentrations.

METHODS

An enzyme linked immunosorbent assay was developed for the detection of antibodies to CRP. Specificity of the reaction was determined by inhibition of the reaction.

RESULTS

Sera from 413 patients were tested and 25 were found to be positive, particularly in patients with rheumatic diseases. Levels of anti-CRP did not correlate with serum CRP levels.

CONCLUSIONS

The presence of low CRP levels may not reflect the presence of antibodies to CRP.