Evidence that human immunoglobulin M rheumatoid factors can Be derived from the natural autoantibody pool and undergo an antigen driven immune response in which somatically mutated rheumatoid factors have lower affinities for immunoglobulin G Fc than their germline counterparts

Isotypes of autoantibodies against novel differential 4-hydroxy-2-nonenal-modified peptide adducts in serum is associated with rheumatoid arthritis in Taiwanese women

Background

Rheumatoid arthritis (RA) is an autoimmune disorder with systemic inflammation and may be induced by oxidative stress that affects an inflamed joint. Our objectives were to examine isotypes of autoantibodies against 4-hydroxy-2-nonenal (HNE) modifications in RA and associate them with increased levels of autoantibodies in RA patients.

Methods

Serum samples from 155 female patients [60 with RA, 35 with osteoarthritis (OA), and 60 healthy controls (HCs)] were obtained. Four novel differential HNE-modified peptide adducts, complement factor H (CFAH)^1211–1230, haptoglobin (HPT)^78–108, immunoglobulin (Ig) kappa chain C region (IGKC)^2–19, and prothrombin (THRB)^328–345, were re-analyzed using tandem mass spectrometric (MS/MS) spectra (ProteomeXchange: PXD004546) from RA patients vs. HCs. Further, we determined serum protein levels of CFAH, HPT, IGKC and THRB, HNE-protein adducts, and autoantibodies against unmodified and HNE-modified peptides. Significant correlations and odds ratios (ORs) were calculated.

Results

Levels of HPT in RA patients were greatly higher than the levels in HCs. Levels of HNE-protein adducts and autoantibodies in RA patients were significantly greater than those of HCs. IgM anti-HPT⁷⁸⁻¹⁰⁸ HNE, IgM anti-IGKC²⁻¹⁹, and IgM anti-IGKC²⁻¹⁹ HNE may be considered as diagnostic biomarkers for RA. Importantly, elevated levels of IgM anti-HPT⁷⁸⁻¹⁰⁸ HNE, IgM anti-IGKC²⁻¹⁹, and IgG anti-THRB^328−345 were positively correlated with the disease activity score in 28 joints for C-reactive protein (DAS28-CRP). Further, the ORs of RA development through IgM anti-HPT⁷⁸⁻¹⁰⁸ HNE (OR 5.235, p < 0.001), IgM anti-IGKC²⁻¹⁹ (OR 12.655, p < 0.001), and IgG anti-THRB^328−345 (OR 5.761, p < 0.001) showed an increased risk. Lastly, we incorporated three machine learning models to differentiate RA from HC and OA, and performed feature selection to determine discriminative features. Experimental results showed that our proposed method achieved an area under the receiver operating characteristic curve of 0.92, which demonstrated that our selected autoantibodies combined with machine learning can efficiently detect RA.

Conclusions

This study discovered that some IgG- and IgM-NAAs and anti-HNE M-NAAs may be correlated with inflammation and disease activity in RA. Moreover, our findings suggested that IgM anti-HPT⁷⁸⁻¹⁰⁸ HNE, IgM anti-IGKC²⁻¹⁹, and IgG anti-THRB^328−345 may play heavy roles in RA development.

On the origin of rheumatoid factors: Insights from analyses of variable region sequences

OBJECTIVES

Rheumatoid factors (RFs) are thought to play an important role in rheumatoid arthritis (RA), but are also found in healthy donors (HDs). Previous studies examined variable region sequences of these autoantibodies at a time when knowledge of the human germline repertoire was incomplete. Here we collected and analyzed RF sequence data from the literature to elucidate how RFs develop and whether their characteristics differ between RA patients and HDs.

METHODS

A database was built containing nucleotide sequences of RF heavy and light chain variable domains and characteristics including affinity, isotype and specificity, all collected from published papers. Gene usage and mutation frequencies were analyzed using IMGT/HiV-QUEST. Selection strength was assessed with the BASELINe tool.

RESULTS

Sequences were retrieved for 183 RF clones (87 RA; 67 HDs; 29 other). No biased gene usage was observed for RA and HDs. However, there does appear to be skewed gene usage in RFs from patients with mixed cryoglobulinemia. Mutation frequency varies considerably between RFs, and isotype-switched clones have significantly more mutations. Monospecific RFs carry more mutations than polyspecific RFs; no difference was found for RA- versus HD-derived RFs. Overall, reported affinity is low (median 1 µM), with a non-significant trend toward higher affinity of RA-derived RFs. Mutation frequency and affinity did not appear to be correlated. BASELINe analysis suggests an overall lack of positive selection and less negative selection strength in RA-derived RFs.

CONCLUSIONS

RFs derived from RA patients have similar properties as those derived from HDs. The RF response can be characterized as a moderately matured autoantibody response, with variable levels of somatic hypermutation, but low affinity.

Structure-function analyses of a stereotypic rheumatoid factor unravel the structural basis for germline-encoded antibody autoreactivity

Rheumatoid factors (RFs) are autoantibodies against the fragment-crystallizable (Fc) region of IgG. In individuals with hematological diseases such as cryoglobulinemia and certain B cell lymphoma forms, the RFs derived from specific heavy- and light-chain germline pairs, so-called "stereotypic RFs," are frequently produced in copious amounts and form immune complexes with IgG in serum. Of note, many structural details of the antigen recognition mechanisms in RFs are unclear. Here we report the crystal structure of the RF YES8c derived from the IGHV1-69/IGKV3-20 germline pair, the most common of the stereotypic RFs, in complex with human IgG1-Fc at 2.8 Å resolution. We observed that YES8c binds to the CH2-CH3 elbow in the canonical antigen-binding manner involving a large antigen-antibody interface. On the basis of this observation, combined with mutational analyses, we propose a recognition mechanism common to IGHV1-69/IGKV3-20 RFs: (1) the interaction of the Leu⁴³²-His⁴³⁵ region of Fc enables the highly variable complementarity-determining region (CDR)-H3 to participate in the binding, (2) the hydrophobic tip in the CDR-H2 typical of IGHV1-69 antibodies recognizes the hydrophobic patch on Fc, and (3) the interaction of the highly conserved RF light chain with Fc is important for RF activity. These features may determine the putative epitope common to the IGHV1-69/IGKV3-20 RFs. We also showed that some mutations in the binding site of RF increase the affinity to Fc, which may aggravate hematological diseases. Our findings unravel the structural basis for germline-encoded antibody autoreactivity.

Autoantibodies in systemic autoimmune diseases: specificity and pathogenicity

In this Review we focus on the initiation of autoantibody production and autoantibody pathogenicity, with a special emphasis on the targeted antigens. Release of intracellular antigens due to excessive cell death or to ineffective clearance of apoptotic debris, modification of self-antigens during inflammatory responses, and molecular mimicry contribute to the initiation of autoantibody production. We hypothesize that those autoreactive B cells that survive and produce pathogenic autoantibodies have specificity for self-antigens that are TLR ligands. Such B cells experience both B cell receptor (BCR) activation and TLR engagement, leading to an escape from tolerance. Moreover, the autoantibodies they produce form immune complexes that can activate myeloid cells and thereby establish the proinflammatory milieu that further negates tolerance mechanisms of both B and T cells.

Autoimmune priming, tissue attack and chronic inflammation - the three stages of rheumatoid arthritis

Extensive genome-wide association studies have recently shed some light on the causes of chronic autoimmune diseases and have confirmed a central role of the adaptive immune system. Moreover, better diagnostics using disease-associated autoantibodies have been developed, and treatment has improved through the development of biologicals with precise molecular targets. Here, we use rheumatoid arthritis (RA) as a prototype for chronic autoimmune disease to propose that the pathogenesis of autoimmune diseases could be divided into three discrete stages. First, yet unknown environmental challenges seem to activate innate immunity thereby providing an adjuvant signal for the induction of adaptive immune responses that lead to the production of autoantibodies and determine the subsequent disease development. Second, a joint-specific inflammatory reaction occurs. This inflammatory reaction might be clinically diagnosed as the earliest signs of the disease. Third, inflammation is converted to a chronic process leading to tissue destruction and remodeling. In this review, we discuss the stages involved in RA pathogenesis and the experimental approaches, mainly involving animal models that can be used to investigate each disease stage. Although we focus on RA, it is possible that a similar stepwise development of disease also occurs in other chronic autoimmune settings such as multiple sclerosis (MS), type 1 diabetes, and systemic lupus erythematosus.

Somatic hypermutations confer rheumatoid factor activity in hepatitis C virus-associated mixed cryoglobulinemia

OBJECTIVE

Hepatitis C virus (HCV) is the most frequent cause of mixed cryoglobulinemia (MC), which is characterized by endothelial deposition of rheumatoid factor (RF)-containing immune complexes and end-organ vasculitis. MC is a lymphoproliferative disorder in which B cells express RF-like Ig, yet its precise antigenic stimulus is unknown. We have proposed that IgG-HCV immune complexes stimulate B cell expansion and somatic hypermutation (SHM)-induced affinity maturation in part via engagement of an RF-like B cell receptor. This study was undertaken to test the hypothesis that SHM augments RF activity.

METHODS

RFs cloned from single B cells from 4 patients with HCV-associated MC (HCV-MC) were expressed as IgM, IgG, or IgG Fab. Selected Ig were reverted to germline. RF activity of somatically mutated Ig and germline-reverted Ig was determined by enzyme-linked immunosorbent assay.

RESULTS

Ig with SHM had RF activity, with the preference for binding being highest for IgG1, followed by IgG2 and IgG4, and lowest for IgG3, where there was no detectable binding. In contrast, reverted germline IgG exhibited markedly diminished RF activity. Competition with 1 μg/ml of protein A abrogated RF activity, suggesting specificity for IgG Fc. Swapping of mutated heavy-chain pairs and light-chain pairs also abrogated RF activity, suggesting that context-specific pairing of appropriate IgH and Igκ, in addition to SHM, is necessary for RF activity.

CONCLUSION

SHM significantly contributes to RF activity in HCV-MC patients, suggesting that autoreactivity in these patients arises through antigen-dependent SHM, as opposed to nondeletion of autoreactive germline Ig.

Retinal autoantibody profile in early age-related macular degeneration: preliminary findings from the Blue Mountains Eye Study

BACKGROUND

To compare the retinal autoantibody profile in individuals with and without early age-related macular degeneration (AMD) and to determine whether baseline autoantibodies are associated with progression to advanced AMD 5 and 10 years later.

METHODS

Western immunoblotting was used to detect the presence of retinal autoantibodies in the baseline serum of 47 individuals with early AMD and 16 healthy controls from the Blue Mountains Eye Study. Autoantibody isotype, IgG subclass distribution and target antigens in the early AMD group were compared with the control group. The association between baseline autoantibodies and progression to advanced AMD 5 and 10 years later was evaluated using the chi-square test.

RESULTS

Retinal autoantibodies were found in a higher proportion of the early AMD group compared with controls (57%vs. 25%, chi2=5.028, P=0.025). Autoantibodies targeted a range of retinal polypeptides in both AMD cases and controls. The commonest polypeptide antigens occurred between 20-35 kDa and 50-60 kDa. More than one IgG subclass was often present in participants with autoantibodies. Over a 10-year period, 11 participants (23.4%) with early AMD progressed to advanced AMD. No association was found between baseline autoantibodies and the development of advanced AMD over 10 years (chi2=0.16, Fisher's exact P=0.59).

CONCLUSIONS

The retinal autoantibody profile in early AMD is complex, both in terms of antigenic targets and immunoglobulin isotypes. Detection of potential disease-associated autoantibodies will require a larger sample size and full characterization of the retinal antigens involved.

Rheumatoid factors induce signaling from B cells, leading to Epstein-Barr virus and B-cell activation

B-cell antigen receptor signaling is initiated upon binding of the antigen to membrane-bound immunoblobulin (Ig), and the anti-Ig antibody (Ab) mimics this signaling. In B cells latently infected with Epstein-Barr virus (EBV), the same signals induce virus activation. We examine here whether rheumatoid factors (RFs), autoantibodies directed against the Fc portion of IgG, induce EBV and B-cell activation. As a source of RFs, RF-producing lymphoblastoid cell line (LCL) clones were isolated from peripheral blood mononuclear cells (PBMC) and synovial cells from patients with rheumatoid arthritis (RA) by EBV transformation. Burkitt's lymphoma-derived Akata cells, which are highly responsive to EBV activation by anti-Ig Abs, were used for the assay of EBV activation. Akata cells expressed IgG3 as membrane-bound Ig. RFs from a synovium-derived LCL were directed to IgG3 and induced EBV activation in 16 to 18% of Akata cells, whereas RFs from another synovium-derived LCL were directed to IgG1 and did not induce EBV activation. Pretreatment of RFs with the purified Fc fragment of human IgG completely abolished EBV activation. Furthermore, B-cell activation was assessed by incorporation of [3H]thymidine. RFs from synovium-derived LCLs efficiently induced B-cell activation, and the addition of CD40 ligand had a synergistic effect. On the other hand, RFs from PBMC-derived LCLs were polyreactive, had a lower affinity to IgG, and did not induce EBV and B-cell activation. The present findings imply a possible role for RFs as EBV and B-cell activators.

Polyreactivity of human IgG Fc-binding phage antibodies constructed from synovial fluid CD38+ B cells of patients with rheumatoid arthritis

Recent data indicate that rheumatoid factors (RFs) that occur in patients with rheumatoid arthritis (RA) are derived from Ig-producing terminally differentiated CD20-, CD38+ plasma cells present in synovial fluids (SFs). Phage antibody display libraries were constructed using CD38+ plasma cells isolated from SFs of two RF-seropositive RA patients. The libraries were enriched for phage antibodies (Phabs) binding to human IgG (HuIgG) Fc fragments and the sequences of their V genes were analysed. These data provided further evidence for an Ag-driven immune response in patients with RA, including expansion of clonally related B cells, selection and isotype switching, all hallmarks of a germinal center reaction. In the present study, the functional characteristics of these HuIgG Fc-binding monoclonal (mo) Phabs were further analysed in order to provide more insight into the specificity of HuIgG Fc-binding Phabs. Remarkably, all HuIgG Fc-binding moPhabs tested (n=48; derived from four different libraries) displayed polyreactivity. Structural analysis of the CDR3 regions revealed characteristic features of polyreactive Igs. Most H chain CDR3 regions harboured tryptophan/tyrosine-rich parts and approximately 60% of the L chain CDR3 regions of both RA patients displayed an identical stretch of amino acids (W/Y-D-S-S). Supportive for a dominant role of VH in specificity, exchange of VL regions with a single VH region yielded moPhabs with similar specificities. All together, the data suggest the presence of an Ag-driven process in the joints of patients with RA, including somatic mutation and clonal selection entailing isotype switching, resulting in the differentiation of B cells into polyreactive RF-secreting plasma cells.

Recognition of IgG-derived peptides by a human IgM with an unusual combining site

A monoclonal immunoglobulin (Ig)M cryoglobulin (Mez) with interesting binding behaviour was isolated from a Waldenström's macroglobulinemia (WM) patient. It demonstrated very strong binding to peptides derived from the sequences of human IgG. However, when tested for binding to intact IgG, this antibody (Ab) did not show any rheumatoid factor (RF) activity. We propose several nonexclusive structural interpretations of the Mez-binding propensities, based on the orientations and solvent accessibilities of ligand residues and the nature of the Ab-binding site. To further characterize the structural features of Mez-peptide binding, IgG-derived octapeptides were docked into the Mez fragment variable (Fv)-binding site, revealing additional reasons for Mez-binding selectivity based on the interactions of the docked peptides with the Mez Fv. The problem was also approached from an immunological perspective. Comparisons of Mez variable region of the light chain (VL)/variable region of the heavy chain (VH) sequences with those of human germlines and known IgM RFs allowed us to provide a possible outline tracing the structural and functional origins of the Mez IgM. Coupled with examinations of interactions in docked complexes, this analysis led us to propose that the potential for RF activity, demonstrated through Mez binding to IgG-derived peptides, was owing to the inherent sequence and structure of the Mez IgM, rather than to somatic mutations. Thus, Mez IgM may occupy an intermediate niche between IgMs with and without RF activity.