Autoreactive B cells in rheumatoid arthritis include mainly activated CXCR3+ memory B cells and plasmablasts.

Many autoimmune diseases (AIDs) are characterized by the persistence of autoreactive B cell responses, which have been directly implicated in disease pathogenesis. How and why these cells are generated or how they are maintained for years is largely unknown. Rheumatoid arthritis (RA) is among the most common AIDs and is characterized by autoantibodies recognizing proteins with posttranslational modifications (PTMs). This PTM-directed autoreactive B cell compartment is ill defined. Here, we visualized the B cell response against the three main types of PTM antigens implicated in RA by spectral flow cytometry. Our results showed extensive cross-reactivity of PTM-directed B cells against all three PTM antigens (citrulline, homocitrulline, and acetyllysine). Unsupervised clustering revealed several distinct memory B cell (mBC) populations. PTM-directed cells clustered with the most recently activated class-switched mBC phenotype, with high CD80, low CD24, and low CD21 expression. Notably, patients also harbored large fractions of PTM-directed plasmablasts (PBs). Both PTM-directed mBCs and PBs showed high expression of CXCR3, a receptor for chemokines present in abundance in arthritic joints. Together, our data provide detailed insight into the biology of B cell autoreactivity and its remarkable, seemingly exhaustless persistence in a prominent human AID.

Anti-carbamylated protein antibodies in systemic sclerosis.

BACKGROUND: To investigate the presence of different isotypes of anti-carbamylated protein (CarP) antibodies in systemic sclerosis (SSc) patients and its association with skin involvement. METHODS: Sera of 194 SSc patients from the Leiden CCISS cohort, fulfilling ACR/EULAR 2013 criteria and a clinical diagnosis of SSc, 83 patients with other connective tissue diseases/Raynaud's Phenomenon, 24 rheumatoid arthritis patients and 98 age and sex-matched healthy controls were tested for the presence of anti-CarP IgG, IgA and IgM, determined by ELISA. Clinical characteristics, that were evaluated in SSc patients, included age, anti-topoisomerase antibodies (ATA), anti-centromere antibodies (ACA) and modified Rodnan Skin Score (mRSS). RESULTS: The SSc patients were 55 (SD:13) years and 155 (80%) were female. Forty-four (23%) patients tested positive for ATA, and 80 (42%) ACA. The median mRSS was 2 (range: 0; 47). Prevalence of anti-CarP IgG was higher in SSc patients than in healthy controls (8% vs 3%, p = 0.007. Prevalence of anti-CarP IgA and IgM and levels of anti-CarP isotypes were comparable between SSc patients and healthy controls. Fifteen (8%) SSc patients tested positive for anti-CarP IgG, 16 (8%) for anti-CarP IgA, and 36 (19%) for anti-CarP IgM. There were no significant correlations between age and levels of anti-CarP isotypes. No correlation between anti-CarP IgG levels and mRSS was found (r = 0.141, p = 0.049), nor for anti-CarP IgM and IgA levels. Anti-CarP IgA levels were higher in ATA compared to ACA positive SSc patients (ATA: 616 aU/ml [359; 1103]; ACA: 424 aU/ml [300; 673], p = 0.015). CONCLUSION: SSc patients can test positive for Anti-CarP IgG, IgA and IgM. We do not observe a relevant clinical association between anti-CarP antibody response and skin involvement in SSc.

Anti-topoisomerase, but not anti-centromere B cell responses in systemic sclerosis display active, Ig-secreting cells associated with lung fibrosis.

OBJECTIVES: Almost all patients with systemic sclerosis (SSc) harbour autoantibodies. Anti-topoisomerase antibodies (ATA) and anti-centromere antibodies (ACA) are most prevalent and associate with distinct clinical phenotypes. B cell responses underlying these phenotypes are ill-defined. To understand how B cell autoreactivity and disease pathology connect, we determined phenotypic and functional characteristics of autoreactive B cells in ATA-positive and ACA-positive patients. METHODS: Levels and isotypes of autoantibodies secreted by ex vivo cultured peripheral blood mononuclear cells from patients with ATA-positive (n=22) and ACA-positive (n=20) SSc were determined. Antibody secreting cells (ASCs) were isolated by cell sorting and cultured separately. Correlations were studied between the degree of spontaneous autoantibody production and the presence and degree of interstitial lung disease (ILD). RESULTS: Circulating B cells secreting either ATA-immunoglobulin G (IgG) or ACA-IgG on stimulation was readily detectable in patients. The ATA response, but not the ACA response, showed additional secretion of autoreactive IgA. ATA-IgG and ATA-IgA were also secreted spontaneously. Additional cell sorting confirmed the presence of ATA-secreting plasmablasts. The degree of spontaneous ATA-secretion was higher in patients with ILD than in those without (p<0.001) and correlated with the degree of pulmonary fibrosis (p<0.001). CONCLUSION: In contrast to ACA-positive patients, ATA-positive patients show signs of recent activation of the B cell response that hallmarks this disease. The degree of activation correlates with the presence and severity of ILD, the most deleterious disease manifestation. This could explain differential responsiveness to B cell depleting therapy. The abundant and spontaneous secretion of ATA-IgG and ATA-IgA may point toward a continuously activating trigger.

Autoreactive B cell responses targeting nuclear antigens in systemic sclerosis: Implications for disease pathogenesis.

A hallmark of disease pathogenesis of systemic sclerosis (SSc) is the presence of autoreactive B cell responses targeting nuclear proteins. Almost all SSc-patients harbour circulating antinuclear autoantibodies of which anti-topoisomerase 1, anti-centromere protein, anti-RNA polymerase III and anti-fibrillarin autoantibodies (ATA, ACA, ARA and AFA, respectively) are the most common and specific for SSc. In clinical practice, autoantibodies serve as diagnostic biomarkers and can aid in the identification of clinical phenotypes of the disease. However, factors driving disease progression in SSc are still poorly understood, and it is difficult to predict disease trajectories in individual patients. Moreover, treatment decisions remain rather empirical, with variable response rates in clinical trials due to patient heterogeneity. Current evidence has indicated that certain patients may benefit from B cell targeting therapies. Hence, it is important to understand the contribution of the antinuclear autoantibodies and their underlying B cell response to the disease pathogenesis of SSc.

Absence of Epstein-Barr virus DNA in anti-citrullinated protein antibody-expressing B cells of patients with rheumatoid arthritis.

OBJECTIVE: Rheumatoid arthritis (RA) is characterized by the presence of disease-specific autoreactive B cell responses, in particular those generating anti-citrullinated protein antibodies (ACPA). For many years, Epstein-Barr virus (EBV) has been implicated in disease pathogenesis, possibly by facilitating the development and persistence of autoreactive B cells. To test this hypothesis, the presence of EBV episomes in ACPA-expressing B cells was analyzed. METHODS: ACPA-expressing B cells derived from peripheral blood (PB) of seven EBV-seropositive RA patients, and synovial fluid (SF) of one additional EBV-seropositive RA patient, were isolated by flow cytometry. PB cells were expanded for 11-12 days, after which supernatant was harvested and analyzed for cyclic citrullinated-peptide (CCP)2 reactivity. SF cells were isolated directly in a lysis buffer. DNA was isolated and qPCR reactions were performed to determine the EBV status of the cells. EBV-immortalized B cell lymphoblastoid-cell lines (EBV blasts) served as standardized controls. RESULTS: Two hundred ninety-six PB and 60 SF ACPA-expressing B cells were isolated and divided over 16 and 3 pools containing 10-20 cells, respectively. Supernatants of all 16 cultured PB pools contained CCP2-Ig. DNA of all pools was used for qPCR analysis. While EBV-blast analysis showed sensitivity to detect EBV DNA in single B cells, no EBV DNA was detected in any of the ACPA-expressing B cell pools. CONCLUSION: ACPA-expressing B cells are not enriched for EBV-DNA-containing clones. These results do not support the hypothesis that EBV infection of autoreactive B cells causes or maintains autoreactive B cell populations in RA. Instead, other mechanisms might explain the association between positive EBV serology and RA.

Benchmarking computational methods for B-cell receptor reconstruction from single-cell RNA-seq data.

Multiple methods have recently been developed to reconstruct full-length B-cell receptors (BCRs) from single-cell RNA sequencing (scRNA-seq) data. This need emerged from the expansion of scRNA-seq techniques, the increasing interest in antibody-based drug development and the importance of BCR repertoire changes in cancer and autoimmune disease progression. However, a comprehensive assessment of performance-influencing factors such as the sequencing depth, read length or number of somatic hypermutations (SHMs) as well as guidance regarding the choice of methodology is still lacking. In this work, we evaluated the ability of six available methods to reconstruct full-length BCRs using one simulated and three experimental SMART-seq datasets. In addition, we validated that the BCRs assembled in silico recognize their intended targets when expressed as monoclonal antibodies. We observed that methods such as BALDR, BASIC and BRACER showed the best overall performance across the tested datasets and conditions, whereas only BASIC demonstrated acceptable results on very short read libraries. Furthermore, the de novo assembly-based methods BRACER and BALDR were the most accurate in reconstructing BCRs harboring different degrees of SHMs in the variable domain, while TRUST4, MiXCR and BASIC were the fastest. Finally, we propose guidelines to select the best method based on the given data characteristics.

IgG Anti-Citrullinated Protein Antibody Variable Domain Glycosylation Increases Before the Onset of Rheumatoid Arthritis and Stabilizes Thereafter: A Cross-Sectional Study Encompassing ~1,500 Samples.

OBJECTIVE: The autoimmune response in rheumatoid arthritis (RA) is marked by the presence of anti-citrullinated protein antibodies (ACPAs). A notable feature of IgG ACPA is the abundant expression of N-linked glycans in the variable domain. However, the presence of ACPA variable domain glycosylation (VDG) across disease stages, and its response to therapy, are poorly described. To understand its dynamics, we investigated the abundance of IgG ACPA VDG in 1,498 samples from individuals in different clinical stages. METHODS: Using liquid chromatography, we analyzed IgG ACPA VDG profiles in 7 different cohorts from Japan, Canada, The Netherlands, and Sweden. We assessed 106 healthy individuals, 228 individuals with presymptomatic RA, 277 individuals with arthralgia, 307 patients with new-onset/early RA, and 117 RA patients after prespecified treatment regimens. Additionally, we measured VDG in 234 samples from patients with RA who did or did not achieve long-term drug-free remission (DFR) during up to 16 years follow-up. RESULTS: IgG ACPA VDG significantly increased (P < 0.0001) toward disease onset and was associated with ACPA levels and epitope spreading prior to diagnosis. A slight increase in VDG was observed in patients with established RA, with a moderate influence of treatment (P = 0.007). In patients in whom DFR was later achieved, IgG ACPA VDG was already reduced at the time of RA onset. CONCLUSION: The abundance of IgG ACPA VDG increases toward RA onset and correlates with maturation of the ACPA response. While IgG ACPA VDG levels are fairly stable in established disease, a lower degree of VDG at RA onset correlates with DFR. Although the underlying biologic mechanisms remain elusive, our data support the concept that VDG relates to an expansion of the ACPA response in the pre-disease phase and contributes to disease development.

Sex-specific risk of anti-topoisomerase antibodies on mortality and disease severity in systemic sclerosis: 10-year analysis of the Leiden CCISS and EUSTAR cohorts.

BACKGROUND: We aimed to evaluate sex-specific risk of anti-topoisomerase I antibodies (ATA) on mortality, diffuse cutaneous systemic sclerosis, interstitial lung disease, and pulmonary hypertension in two cohorts of people with systemic sclerosis. METHODS: This study was a 10-year analysis of the prospective Leiden Combined Care in Systemic Sclerosis (CCISS) cohort in the Netherlands and the international European Scleroderma Trials and Research (EUSTAR) cohort. We included participants with systemic sclerosis according to the 2013 American College of Rheumatology-European League Against Rheumatism (ACR-EULAR) classification criteria; available autoantibody status; available skin subtyping; at least one available radiographic assessment of interstitial lung disease; and with a known date of disease onset. People with systemic sclerosis were categorised in six risk groups by sex and autoantibody status (anti-centromere antibody [ACA]-positive female, ACA-positive male, ACA and ATA-negative female, ACA and ATA-negative male, ATA-positive female, and ATA-positive male). We constructed Kaplan-Meier curves and Cox proportional hazard models, accounting for left-truncated survival to prevent bias because the date of disease onset (first non-Raynaud's symptom) preceded the date of cohort entry for all patients. The primary outcome was all-cause mortality and the secondary outcomes were diffuse cutaneous systemic sclerosis, interstitial lung disease, and pulmonary hypertension. FINDINGS: 445 (63%) of 708 participants between April 1, 2009, and Jan 1, 2022, in CCISS (101 [23%] male and 344 [77%] female) and 4263 (50%) of 8590 between June 1, 2004, and March 28, 2018, in EUSTAR (783 [18%] male and 3480 [82%] female) were eligible for this study. In both cohorts, ATA expression occurred significantly more often in males than in females (39 [39%] of 101 males vs 67 [19%] of 344 females in CCISS; p<0·0001 and 381 [49%] of 783 males vs 1323 [38%] of 3480 females in EUSTAR; p<0·0001). According to estimated survival rates, 30% of ATA-positive males versus 12% of ATA-positive females died in the CCISS cohort and 33% versus 15% died in the EUSTAR cohort within 10 years. After adjustment for age, race, and autoantibody status, male sex remained the most important risk factor for all-cause mortality (HR 2·9 [95% CI 1·5-5·5] in CCISS, p=0·0018; and HR 2·6 [2·0-3·4] in EUSTAR, p<0·0001). INTERPRETATION: We show that the association between male sex and increased mortality in systemic sclerosis cannot be explained by higher ATA prevalence. However, additional research on the effect of sex-specific characteristics on people with systemic sclerosis is required. FUNDING: None.

Cross-reactivity of IgM anti-modified protein antibodies in rheumatoid arthritis despite limited mutational load.

BACKGROUND: Anti-modified protein antibodies (AMPA) targeting citrullinated, acetylated and/or carbamylated self-antigens are hallmarks of rheumatoid arthritis (RA). Although AMPA-IgG cross-reactivity to multiple post-translational modifications (PTMs) is evident, it is unknown whether the first responding B cells, expressing IgM, display similar characteristics or if cross-reactivity is crucially dependent on somatic hypermutation (SHM). We now studied the reactivity of (germline) AMPA-IgM to further understand the breach of B cell tolerance and to identify if cross-reactivity depends on extensive SHM. Moreover, we investigated whether AMPA-IgM can efficiently recruit immune effector mechanisms. METHODS: Polyclonal AMPA-IgM were isolated from RA patients and assessed for cross-reactivity towards PTM antigens. AMPA-IgM B cell receptor sequences were obtained by single cell isolation using antigen-specific tetramers. Subsequently, pentameric monoclonal AMPA-IgM, their germline counterparts and monomeric IgG variants were generated. The antibodies were analysed on a panel of PTM antigens and tested for complement activation. RESULTS: Pentameric monoclonal and polyclonal AMPA-IgM displayed cross-reactivity to multiple antigens and different PTMs. PTM antigen recognition was still present, although reduced, after reverting the IgM into germline. Valency of AMPA-IgM was crucial for antigen recognition as PTM-reactivity significantly decreased when AMPA-IgM were expressed as IgG. Furthermore, AMPA-IgM was 15- to 30-fold more potent in complement-activation compared to AMPA-IgG. CONCLUSIONS: We provide first evidence that AMPA-IgM are cross-reactive towards different PTMs, indicating that PTM (cross-)reactivity is not confined to IgG and does not necessarily depend on extensive somatic hypermutation. Moreover, our data indicate that a diverse set of PTM antigens could be involved in the initial tolerance breach in RA and suggest that AMPA-IgM can induce complement-activation and thereby inflammation.

Anticentromere Antibody Levels and Isotypes and the Development of Systemic Sclerosis.

OBJECTIVE: Little is known on the disease course of very early systemic sclerosis (SSc). Among the information yet to be elucidated is whether anticentromere antibody (ACA) isotype levels can serve as biomarkers for future SSc development and for organ involvement. This study was undertaken to evaluate whether IgG, IgM, and IgA ACA levels in IgG ACA-positive patients are associated with disease severity and/or progression from very early SSc to definite SSc. METHODS: IgG ACA-positive patients from 5 different cohorts who had very early SSc or SSc fulfilling the American College of Rheumatology (ACR)/European Alliance of Associations for Rheumatology (EULAR) 2013 criteria were included. A diagnosis of very early SSc was based on the presence of IgG ACAs and Raynaud's phenomenon, and/or puffy fingers and/or abnormal nailfold capillaroscopy, but not fulfilling the ACR/EULAR 2013 criteria for SSc. Multivariable regression analyses were performed to determine the association between baseline ACA isotype levels and progression to definite SSc with organ involvement. RESULTS: Six hundred twenty-five IgG ACA-positive patients were included, of whom 138 (22%) fulfilled the criteria for very early SSc and 487 (78%) had definite SSc. Levels of IgG ACAs (odds ratio 2.5 [95% confidence interval 1.8-3.7]) and IgM ACAs (odds ratio 1.8 [95% confidence interval 1.3-2.3]) were significantly higher in patients with definite SSc. Of 115 patients with very early SSc with follow-up, progression to definite SSc occurred within 5 years in 48 (42%). Progression to definite SSc was associated with higher IgG ACA levels at baseline (odds ratio 4.3 [95% confidence interval 1.7-10.7]). CONCLUSION: ACA isotype levels may serve as biomarkers to identify patients with very early SSc who are at risk for disease progression to definite SSc.

Light chain skewing in autoantibodies and B-cell receptors of the citrullinated antigen-binding B-cell response in rheumatoid arthritis.

Rheumatoid arthritis (RA) is a chronic autoimmune disease affecting 1% of the world population. RA is associated with the presence of autoantibodies, of which anti-citrullinated protein antibodies (ACPA) are most prominent. ACPA are produced by citrullinated antigen-binding B cells that have presumably survived tolerance checkpoints. So far, it is unclear how and when such autoreactive B cells emerge. Light chain (LC) rearrangement and mutation rates can be informative with regard to selection steps during B-cell development. Therefore, we studied LC characteristics of ACPA-expressing B cells and secreted ACPA with the aim to better understand the development of this disease-specific, autoreactive B-cell response. Paired ACPA-IgG and ACPA-depleted IgG were isolated from serum (n = 87) and synovial fluid (SF, n = 21) of patients with established RA. We determined the LC composition for each fraction by ELISA using kappa(Igκ)- and lambda(Igλ) LC-specific antibodies. Cellular LC expression was determined using flow cytometry. In addition, we used a B-cell receptor (BCR)-specific PCR to obtain LC variable region sequences of citrullinated antigen- and tetanus toxoid (TT)-binding B cells. In serum, we observed an increased frequency of lambda LC in ACPA-IgG (1.64:1) compared to control IgG (2.03:1) and to the κ/λ ratio reported for healthy individuals (2:1). A similar trend towards higher frequencies of lambda LCs was observed for ACPA-IgG in SF (1.84:1). Additionally, the percentage of Igλ-expressing B cells was higher for citrullinated antigen-binding B cells (51%) compared to TT-specific (43%) and total CD19+CD20+ B cells (36%). Moreover, an increased Igλ percentage was observed in BCR-sequences derived from ACPA-expressing (49%) compared to TT-specific B cells (34%). Taken together, we report an enhanced frequency of lambda LCs in the secreted ACPA-IgG repertoire and, on the cellular level, in BCR sequences of ACPA-expressing B cells compared to control. This skewing in the autoreactive B-cell repertoire could reflect a process of active selection.

Association Between Centromere- and Topoisomerase-specific Immune Responses and the Degree of Microangiopathy in Systemic Sclerosis.

OBJECTIVE: Autoreactive antibody responses, including the use of several isotypes of autoantibodies, have been shown to be associated with clinical outcome in several rheumatic autoimmune diseases. The goals of this study were to evaluate whether (1) anticentromere antibody (ACA)- and antitopoisomerase antibody (ATA)-specific isotype expression, and (2) organ involvement are associated with the degree of microangiopathy in systemic sclerosis (SSc). METHODS: ACA and ATA IgG, IgM, and IgA levels were measured in baseline serum samples of ACA IgG-positive (+) and ATA IgG+ patients with SSc. The degree of microangiopathy was determined based on nailfold videocapillaroscopy (NVC) images collected at the same point in time. Logistic regression analyses with autoantibodies, clinical characteristics, isotype expression, and ACA and ATA IgG, IgM, and IgA levels as independent variables, and NVC pattern as the dependent variable were performed. RESULTS: In 164 patients, isotype levels and degree of microangiopathy were evaluated. Logistic regression confirmed the association of the degree of microangiopathy with the presence of digital ulcers (OR 3.07, 95% CI 1.43-6.60), interstitial lung disease (OR 3.41, 95% CI 1.11-10.61), and pulmonary arterial hypertension (OR 5.58, 95% CI 2.05-17.81). ATA positivity was associated with more severe microangiopathy (OR 2.09, 95% CI 1.05-4.13). Patients who expressed solely ACA IgG showed a trend towards less severe microangiopathy compared to patients also expressing ACA IgM and/or IgA. Levels of ACA IgG and ATA IgM were found to be associated with microangiopathy severity. CONCLUSION: We observed an association between ACA and ATA responses and the degree of microangiopathy in SSc. These findings might indicate that the breadth of the autoimmune response, as reflected by autoantibody production and microvascular damage, interacts in the pathophysiology of SSc.

Long-term effects of combined B-cell immunomodulation with rituximab and belimumab in severe, refractory systemic lupus erythematosus: 2-year results.

BACKGROUND: Anti-CD20 B-cell depletion has not shown superior efficacy to standard immunosuppression in patients with systemic lupus erythematosus (SLE). Besides trial design, potential explanations are incomplete B-cell depletion in relation to substantial surges in B-cell-activating factor (BAFF). To improve B-cell targeting strategies, we conducted the first study in SLE patients aimed at investigating immunological effects and feasibility of combining rituximab (RTX; anti-CD20) and belimumab (BLM; anti-BAFF). METHODS: Reported is the long-term follow-up of a Phase 2 proof-of-concept study in 15 patients with SLE including 12 (80%) with lupus nephritis (LN). RESULTS: In 10/15 (67%) patients, a clinical response was observed by achievement of lupus low disease activity state, of which 8 (53%) continued treatment (BLM + ≤7.5 mg prednisolone) for the complete 2 years of follow-up. Five patients (33%) were referred to as 'non-responders' due to persistent LN, major flare or repetitive minor flares. Out of 12 LN patients, 9 (75%) showed a renal response including 8 (67%) complete renal responders. All anti-dsDNA+ patients converted to negative, and both anti-C1q and extractable nuclear antigen autoantibodies showed significant reductions. CD19+ B cells showed a median decrease from baseline of 97% at 24 weeks, with a persistent reduction of 84% up to 104 weeks. When comparing responders with non-responders, CD20+ B cells were depleted significantly less in non-responders and double-negative (DN) B cells repopulated significantly earlier. CONCLUSIONS: Combined B-cell targeted therapy with RTX and BLM prevented full B-cell repopulation including DN B cells, with concomitant specific reduction of SLE-relevant autoantibodies. The observed immunological and clinical benefits in a therapy-refractory SLE population prompt further studies on RTX + BLM.

Persistently activated, proliferative memory autoreactive B cells promote inflammation in rheumatoid arthritis.

Autoreactive B cells mediate autoimmune pathology, but exactly how remains unknown. A hallmark of rheumatoid arthritis (RA), a common autoimmune disease, is the presence of disease-specific anticitrullinated protein antibodies (ACPAs). Here, we showed that ACPA-positive B cells in patients with RA strongly expressed T cell-stimulating ligands, produced abundant proinflammatory cytokines, and were proliferative while escaping inhibitory signals. This activated state was found at different degrees in different stages of disease: highest in patients with recent-onset RA, moderate in patients with established RA, and far less pronounced in ACPA-positive individuals "at risk" for developing disease. The activated autoreactive B cell response persisted in patients who achieved clinical remission with conventional treatment. ACPA-positive B cells in blood and synovial fluid secreted increased amounts of the chemoattractant interleukin-8, which attracted neutrophils, the most abundant immune cell in arthritic joints. Tetanus toxoid-specific B cells from the same patients exhibited properties of memory B cells without the activation and proliferation phenotype, but these cells transiently acquired a similar proliferative phenotype upon booster vaccination. Together, these data indicated that continuous antigenic triggering of autoreactive B cells occurs in human autoimmune disease and support the emerging concept of immunological activity that persists under treatment even in clinical remission, which may revise our current concept of treatment targets for future therapeutic interventions. In addition, our data pointed to a pathogenic role of ACPA-positive B cells in the inflammatory disease process underlying RA and favor approaches that aim at their antigen-specific inactivation or depletion.

Templated insertions at VD and DJ junctions create unique B-cell receptors in the healthy B-cell repertoire.

RAG complexes recognise (cryptic) RSS sites both in and outside immunoglobulin sites. Excision circles may be reinserted into V(D)J rearrangements as long templated insertions to diversify the adaptive immune repertoire. We show that such VDJ with templated insertions are incidentally found in the repertoire of healthy donors.

A reverse translational study on the effect of rituximab, rituximab plus belimumab, or bortezomib on the humoral autoimmune response in SLE.

OBJECTIVES: SLE is a severe autoimmune disease characterized by autoreactive B cells and IC formation, which causes systemic inflammation. B cell-targeted therapy could be a promising treatment strategy in SLE patients; nevertheless, randomized clinical trials have not always been successful. However, some groups have demonstrated beneficial effects in severe SLE patients with off-label rituximab (RTX) with belimumab (BLM), or bortezomib (BTZ), which targeted different B cells subsets. This study assembled sera from SLE cohorts treated with RTX+BLM (n = 15), BTZ (n = 11) and RTX (n = 16) to get an in-depth insight into the immunological effects of these therapies on autoantibodies and IC formation. METHODS: Autoantibodies relevant for IC formation and the avidity of anti-dsDNA were determined by ELISA. IC-mediated inflammation was studied by complement levels and ex vivo serum-induced neutrophil extracellular trap formation. RESULTS: Reductions in autoantibodies were observed after all approaches, but the spectrum differed depending upon the treatment. Specifically, only RTX+BLM significantly decreased anti-C1q. Achieving seronegativity of ≥1 autoantibody, specifically anti-C1q, was associated with lower disease activity. In all SLE patients, the majority of anti-dsDNA autoantibodies had low avidity. RTX+BLM significantly reduced low-, medium- and high-avidity anti-dsDNA, while RTX and BTZ only significantly reduced medium avidity. IC-mediated inflammation, measured by C3 levels and neutrophil extracellular trap formation, improved after RTX+BLM and RTX but less after BTZ. CONCLUSION: This study demonstrated the impact of different B cell-targeted strategies on autoantibodies and IC formation and their potential clinical relevance in SLE.

N-Glycosylation Site Analysis of Citrullinated Antigen-Specific B-Cell Receptors Indicates Alternative Selection Pathways During Autoreactive B-Cell Development.

Many autoimmune diseases are hallmarked by autoreactive B and plasma cell responses that are directly or indirectly involved in disease pathogenesis. These B-cell responses show large variability between diseases, both in terms of the secreted autoantibody repertoire and the dynamics and characteristics of the underlying B-cell responses. Hence, different mechanisms have been proposed to explain the emergence of autoreactive B cells in an otherwise self-tolerant immune system. Notably, most mechanistic insights have been obtained from murine studies using models harboring genetic modifications of B and T cells. Given recent technological advances that have rendered autoreactive human B cells accessible for analysis, we here discuss the phenomenon of extensive N-glycosylation of the B-cell receptor (BCR) variable domain of a prototypic human autoreactive B-cell response and its potential role in the generation of autoimmunity. Anti-citrullinated protein antibodies (ACPA) hallmark the most disease-specific autoimmune response in Rheumatoid Arthritis (RA). Remarkably, ACPA-IgG are heavily N-glycosylated in the variable domain due to somatic mutations that generate abundant N-glycosylation consensus sequences. These sites, obtained from full-length BCR sequences of ACPA-expressing B cells from 12 ACPA-positive RA patients, were here analyzed in detail. Sites that required a single nucleotide mutation to be generated were defined as single somatic hypermutation (s-SHM) sites, whereas sites requiring multiple mutations were defined as m-SHM sites. IgG sequences of 12 healthy donors were used as control. Computational modeling of the germinal center reaction (CLONE algorithm) was used with the germline counterparts of ACPA-IgG heavy chain (HC) sequences to simulate the germinal center response. Our analyses revealed an abundance of N-glycosylation sites in ACPA-IgG HC that frequently required multiple mutations and predominated in specific positions. Based on these data, and taking into account recent insights into the dynamics of the ACPA-response during disease development, we here discuss the hypothesis that N-glycosylation sites in ACPA-IgG variable domains could lead to alternative, possibly antibody affinity-independent selection forces. Presumably, this occurs during germinal center responses allowing these B cells to escape from putative tolerance checkpoints, thereby driving autoreactive B cell development in the pathogenesis of RA.

N-Linked Glycans in the Variable Domain of IgG Anti-Citrullinated Protein Antibodies Predict the Development of Rheumatoid Arthritis.

OBJECTIVE: Anti-citrullinated protein antibodies (ACPAs) are disease-specific biomarkers in rheumatoid arthritis (RA). More than 90% of IgG ACPAs harbor N-linked glycans in the antibody variable (V) domain. The corresponding N-glycosylation sites in ACPA V-region sequences result from somatic hypermutation, a T cell-dependent process. As ample evidence indicates that T cells drive the maturation of the ACPA response prior to arthritis onset, we undertook this study to investigate whether the presence of glycans in IgG ACPA V domains predicts the transition from predisease autoimmunity to overt RA. METHODS: We analyzed 2 independent sets of serum samples obtained from 126 ACPA-positive first-degree relatives (FDRs) of RA patients. Both sets originated from an Indigenous North American population and comprised cross-sectional and longitudinal samples of individuals who did or did not develop inflammatory arthritis. Serum IgG ACPAs were affinity-purified and subjected to ultra high-performance liquid chromatography-based glycan analysis. RESULTS: In both data sets, FDR-derived IgG ACPA displayed markedly lower levels of V domain glycans (<50%) compared to IgG ACPA from RA patients. Notably, FDRs who later developed RA showed extensive V-domain glycosylation before the onset of arthritis. Moreover, IgG ACPA V-domain glycosylation was strongly associated with future development of RA (hazard ratio 6.07 [95% confidence interval 1.46-25.2]; P = 0.013). CONCLUSION: Extensive glycosylation of the IgG ACPA V domain is present in a subset of predisposed FDRs of Indigenous North American RA patients. The presence of this feature substantially increases the risk of RA development. Based on these findings, we propose that glycosylation of the IgG ACPA V domain represents a predictive marker for RA development in ACPA-positive individuals and may serve to better target prevention measures.

Generation and Characterization of Anti-Citrullinated Protein Antibody-Producing B Cell Clones From Rheumatoid Arthritis Patients.

OBJECTIVE: Anti-citrullinated protein antibodies (ACPAs) are a hallmark of rheumatoid arthritis (RA). Aside from autoantibody production, the function of autoantigen-specific B cells remains poorly understood in the context of this disease. This study set out to elucidate autoantigen-specific B cell functions through the isolation and immortalization of unique citrullinated protein/peptide (CP)-reactive B cell clones from RA patients. METHODS: B cell clones from either the blood or synovial fluid of cyclic citrullinated peptide 2 (CCP2) antibody-positive RA patients were immortalized by genetic reprogramming with Bcl-6 and Bcl-xL. Enzyme-linked immunosorbent assay and flow cytometry were used to identify CCP2-reactive clones and to further characterize surface marker and cytokine expression as well as B cell receptor signaling competence. Global gene expression profiles were interrogated by RNA sequencing. RESULTS: Three unique CP-reactive memory B cell clones were generated from the blood or synovial fluid of 2 RA patients. CP-reactive memory B cells did not appear to be broadly cross-reactive, but rather had a fairly restricted epitope recognition profile. These clones were able to secrete both pro- and antiinflammatory cytokines and had a unique surface profile of costimulatory molecules and receptors, including CD40 and C5a receptor type 1, when compared to non-CP-reactive clones from the same patient. In addition, CP-reactive clones bound citrullinated protein, but not native protein, and could mobilize calcium in response to antigen binding. CONCLUSION: CP-reactive memory B cells comprise a rare, seemingly oligoclonal population with restricted epitope specificity and distinct phenotypic and molecular characteristics suggestive of antigen-presenting cells. Cloning by genetic reprogramming opens new avenues to study the function of autoreactive memory B cells, especially in terms of antigen processing, presentation, and subsequent T cell polarization.