Front. Immunol. Frontiers in Immunology Front. Immunol. 1664-3224 Frontiers Media S.A. 10.3389/fimmu.2026.1744370 Original Research Presence of autoantibodies targeting the shared epitope in rheumatoid arthritis and psoriatic arthritis GraellEduard 1 2 3 * † Conceptualization Data curation Formal analysis Funding acquisition Investigation Methodology Project administration Resources Visualization Writing – original draft Writing – review & editing DelgadoJuan Francisco 2 3 4 * † Conceptualization Data curation Formal analysis Investigation Methodology Project administration Visualization Writing – original draft Writing – review & editing Domingo GomezAntonio 1 2 3 Data curation Resources Writing – review & editing García-MariqueMaria 1 2 3 Data curation Resources Writing – review & editing RusiñolMenna 1 2 3 Data curation Funding acquisition Resources Writing – review & editing RodrigoAlbert 3 Data curation Formal analysis Methodology Writing – original draft Writing – review & editing Berenguer-LlergoAntoni 3 Data curation Formal analysis Writing – review & editing GratacosJordi 1 2 3 Data curation Methodology Resources Supervision Validation Writing – review & editing CalvetJoan 1 2 3 Data curation Methodology Resources Supervision Validation Writing – review & editing Rheumatology Department, Parc Taulí Hospital Universitari, Sabadell, Spain Institut d’Investigació i Innovació Parc Taulí (I3PT-Centres de Recerca de Catalunya (CERCA)), Universitat Autònoma de Barcelona, Sabadell, Spain Inflammatory Joint Diseases, Bone Metabolism, and Systemic Autoimmune Diseases Research Group, Institut d’Investigació i Innovació Parc Taulí (I3PT-CERCA), Sabadell, Spain Immunology Laboratory, Clinical Laboratories Service, Parc Taulí Hospital Universitari, Sabadell, Spain Correspondence: Eduard Graell, egraell@tauli.cat; Juan Francisco Delgado, jdelgado@tauli.cat

These authors have contributed equally to this work

 03 03 2026 06 03 2026 2026 17 1744370 19 11 2025 09 02 2026 03 02 2026 Copyright © 2026 Graell, Delgado, Domingo Gomez, García-Marique, Rusiñol, Rodrigo, Berenguer-Llergo, Gratacos and Calvet. 2026 Graell, Delgado, Domingo Gomez, García-Marique, Rusiñol, Rodrigo, Berenguer-Llergo, Gratacos and Calvet https://creativecommons.org/licenses/by/4.0/ This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms. Introduction

Rheumatoid arthritis (RA) is an autoimmune disease marked by the production of autoantibodies (AAb) against citrullinated proteins/peptides (ACPA). The shared epitope (SE) in the HLA-DRB1 gene is a major genetic risk factor for RA and has been linked to ACPA production, particularly in individuals exposed to environmental triggers such as smoking. However, the role of the SE itself, especially in its citrullinated form, as a target of humoral immunity in RA remains underexplored.

 Methods

We analyzed autoantibodies against SE-containing peptides (SE-AAb) in 150 RA patients, 62 patients with psoriatic arthritis (PsA), and 204 healthy controls. HLA-DRB1 polymorphisms associated with the SE (QKRAA, QRRAA, RRRAA) were evaluated by PCR-SSO. IgG reactivity against native, citrullinated, and carbamylated SE peptides, in linear and cyclic conformations, was assessed using a custom ELISA.

 Results

SE-AAb were detected in RA patients with frequencies ranging from 26.0% to 45.3%, depending on peptide conformation and post-translational modification, with the highest positivity observed against citrullinated SE peptides. Antibodies against cyclated citrullinated SE peptides were present in 45.3% of RA patients, compared with 21.6% of healthy controls. PsA patients also showed SE-AAb positivity, with frequencies ranging from 17.7% to 35.5%, displaying a pattern largely similar to RA. No significant association was observed between SE-AAb positivity and SE genetic carriage, and SE-AAb presence was not associated with clinical features of RA.

 Conclusion

SE-AAb are present in RA patients, particularly against citrullinated SE peptides, but are not specific to RA, as similar reactivity is observed in PsA. The presence of these autoantibodies is independent of SE genetic carriage, suggesting that inflammatory conditions rather than genetic SE status may contribute to their generation. Further studies are needed to clarify the clinical relevance of SE-AAb in RA pathogenesis.

 anti-citrullinated peptide antibodies autoantibodies inflammatory arthritis rheumatoid arthritis shared epitope The author(s) declared that financial support was received for this work and/or its publication. This work was supported by the 2018 FER Grant from the Spanish Society of Rheumatology for research projects without public funding, and by the CIR 2017/011 Grant from the Parc Taulí University Hospital in Sabadell. section-at-acceptance Autoimmune and Autoinflammatory Disorders : Autoimmune Disorders Introduction

Rheumatoid arthritis (RA) is an autoimmune disease characterized by excessive production of autoantibodies (AAb). This results from dysregulation of the immune response in genetically predisposed individuals exposed to environmental factors such as smoking or periodontitis (1–4).

Among the AAb described in RA, anti-citrullinated protein/peptide antibodies (ACPA) are the most relevant. They are highly specific for RA, form part of the disease classification criteria, and are associated with a more aggressive clinical course (1, 5–7). Studies in population biobanks have shown that ACPAs can be detected decades before the clinical onset of RA (8–10). Initially, IgM is the predominant ACPA isotype. The subsequent appearance of IgA and IgG correlates with clinical onset and established disease, respectively (9, 10). These observations underscore the central role of ACPAs in RA pathophysiology (10, 11).

ACPA production is influenced by genetic factors associated with RA, most notably the shared epitope (SE), the main genetic risk factor for the disease. SE carriage has been strongly associated with ACPA positivity, particularly in individuals exposed to smoking (12, 13). In addition, periodontitis has been linked to ACPA presence, further supporting the interaction between genetic susceptibility and environmental exposures in RA (14).

ACPAs were initially described as targeting citrullinated filaggrin, an extra-articular protein (15). Subsequently, several intra-articular citrullinated proteins were identified, including fibrin and fibrinogen (16–18). These proteins share peptide sequences with filaggrin (19). Additional autoantibodies against citrullinated peptides and proteins have been described, such as those targeting fibronectin, alpha-enolase, and vimentin. However, their clinical utility is limited due to low sensitivity (9, 18, 20–32).

Currently, the most widely used commercial assay for RA detection is based on cyclic citrullinated peptides (CCP3). This test shows a specificity of 86–99% and a sensitivity of 60–78% (17, 33). However, because CCP3 relies on synthetic peptides, a single human citrullinated protein determinant directly involved in RA pathophysiology has not yet been identified.

The SE is a polymorphism in the HLA-DRB1 gene and represents the genetic factor with the greatest impact on RA susceptibility (34). It encodes a conserved five–amino acid sequence (QKRAA, QRRAA, or RRRAA) located in pocket 4 of the HLA class II molecule. Several studies have shown that this motif confers increased affinity for the presentation of citrullinated peptides (32, 35–37).

The SE sequence contains arginine residues that can be citrullinated by peptidyl arginine deiminase (PAD) enzymes (38). We hypothesized that the SE, particularly in its citrullinated form, may function as an antigen capable of inducing ACPA production in RA. During inflammation, cell death and protein degradation occur alongside PAD activation. Under these conditions, the SE peptide may undergo citrullination. Exposure of this peptide to PAD during inflammatory proteolysis could generate citrullinated sequences capable of eliciting autoantibody responses. Alternatively, citrullination within the HLA class II structure itself could alter molecular conformation and modulate immune recognition.

Despite the strong genetic association between the shared epitope and ACPA production, it remains unclear whether the shared epitope itself represents a direct target of humoral autoimmunity in RA, particularly in its native or post-translationally modified forms. In this study, we therefore investigated the presence of autoantibodies directed against different shared epitope sequences (SE-AAb), in both native and post-translationally modified forms, in patients with RA.

 Materials and methods Study design and participants

This single-center, cross-sectional study analyzed the presence of SE-AAb in a cohort of patients with RA and evaluated its association with the SE and baseline clinical characteristics. Additionally, the prevalence of SE-AAb in the RA cohort was compared with that observed in two control cohorts: one comprising patients with psoriatic arthritis (PsA), and the other consisting of healthy blood donors from the Banc de Sang i Teixits of Barcelona. Donor samples were provided in an anonymized format, and individual-level demographic variables, including age and sex, were not available to the investigators for the control cohort. Given the limited prior data on shared-epitope autoantibodies, this study was designed as an exploratory analysis and no formal sample size calculation was performed.

All RA patients included in the study were recruited from the outpatient clinics of Parc Taulí University Hospital (PTUH). Eligible patients met the 2010 EULAR/ACR classification criteria for rheumatoid arthritis and were over 18 years of age. A systematic retrospective review of medical records was carried out between June and October 2022. When available, data were collected on smoking history, previous and current treatments, radiographic evidence of joint involvement in the hands and feet, and serological markers, including rheumatoid factor (RF), ACPA, and antinuclear antibodies.

All participants, including RA patients, PsA patients, and healthy blood donors, had signed informed consent to participate.

The present study was reviewed and approved by the Ethics and Drug Research Committee of PTUH (Ref.: 2018/534) in accordance with the World Medical Association Declaration of Helsinki.

 HLA typing

HLA-DRB1 polymorphisms were evaluated in RA patients and PsA patients. HLA-DRB1 gene polymorphisms were analyzed using PCR-SSO with the LIFECODES HLA-DRB1 eRES SSO Typing Kit (Immucor, Norcross, GA, USA). We defined the shared epitope alleles in our study as the HLA-DRB1*01:01, HLA-DRB1*01:02, HLA-DRB1*04:01, HLA-DRB1*04:04, HLA-DRB1*04:05, HLA-DRB1*04:08, HLA-DRB1*10:01, HLA-DRB1*14:02 (39).

 Rheumatoid arthritis serological markers

ACPAs were measured using the QUANTA Flash® CCP3 chemiluminescence immunoassay (Inova Diagnostics, San Diego, CA, USA), and RF was assessed using a turbidimetric immunoassay (The Binding Site, Birmingham, UK).

 Autoantibodies against the shared epitope

Serum samples from 150 RA patients, 62 PsA patients, and 204 healthy controls were tested using a custom ELISA with a 13-amino acid peptide sequence from positions 66–78 of HLA-DRB1, containing the three SE sequence variants (native SE, citrullinated SE, and carbamylated SE), synthesized in both linear and cyclic forms, except for the carbamylated peptide, synthesized in its linear form.

Immulon 4 HBX plates (Nunc, Thermo Fisher Scientific, Waltham, MA, USA) were coated with 10 μg/mL of recombinant SE peptides (ProImmune, Oxford, UK) in a 50 mM carbonate-bicarbonate buffer (pH 9.6) and incubated overnight at 4 °C. Subsequently, the plates were blocked with 5% PBS-BSA (Sigma-Aldrich, St. Louis, MO, USA) for 2 hours at room temperature. Serum samples were then diluted 1:100 in PBS-Tween 20® (Sigma-Aldrich, St. Louis, MO, USA) and incubated for 1 hour at room temperature. After washing, HRP-conjugated anti-human IgG (Thermo Fisher Scientific, Waltham, MA, USA) diluted 1:2,000 in PBS-Tween 20® was added and incubated for 1 hour at room temperature. The enzymatic reaction was developed using tetramethylbenzidine (Thermo Fisher Scientific, Waltham, MA, USA) for 30 minutes at room temperature and stopped with 25% sulfuric acid (Merck, Darmstadt, Germany). Absorbance was measured at 450 nm, using 620 nm as the reference filter. The cutoff value was established to ensure 90% specificity among blood donors and was used as the threshold for ELISA reactivity.

To further assess the specificity of antibody binding observed in ELISA assays using SE–containing peptides, an additional control immunoassay was performed using an unrelated peptide. Sera from patients with RA and control subjects were tested against a peptide derived from alpha-fetoprotein (AFP), corresponding to residues 365–384 (LAVSVILRVAKGYQELLEKC). This peptide does not share sequence homology with HLA-DRB1–derived peptides and does not contain the shared epitope motif. ELISA conditions, serum dilutions, and detection procedures were identical to those used for SE-derived peptides. The optical density values obtained were lower for RA patients than controls with mean ± SD values of 0.041 ± 0.023 and 0.059 ± 0.024, respectively, indicating the absence of reactivity against this unrelated peptide.

Laboratory analyses were performed blinded to clinical data and diagnostic group assignment.

 Statistical analysis

Categorical variables were summarized as absolute frequencies and percentages, while continuous variables were presented as medians and ranges. Differences between patient groups in demographic and clinical characteristics were evaluated using the Mann-Whitney test for continuous variables and Fisher’s exact test for categorical variables. Post-hoc pairwise comparisons were adjusted for multiple comparisons with Holm correction.

As a sensitivity analysis, the discriminative performance of OD values was assessed using ROC analysis (RA vs controls; PsA vs controls), reporting AUC and classification metrics at the optimal threshold (closest-to-(0,1) criterion), with 95% confidence intervals obtained by bootstrap resampling (B = 1000).

Statistical significance was set at 5%. All analyses were performed using R 4.1.2 (R Core Team, 2021. R: A language and environment for statistical computing. R Foundation for Statistica).

 Results Patients and controls

A total of 150 RA patients treated at the outpatient clinics of PTUH were included in the study. The mean age at the time of participation was 63.8 years, with an average disease duration of 5.7 years. The mean age at diagnosis was 53.3 years, and the average time from symptom onset to diagnosis was approximately six months. Women represented 75.3% (n = 113) of the cohort. Overall, 65.4% (87) of RA patients were carriers of the SE, and 11.3% (n = 15) had a double SE load (Supplementary Tables 1, S2).

As an inflammatory joint disease control group, 62 patients with PsA were included. Among them, 37.9% (n = 11) were SE carriers, and 6.9% (n = 2) carried a double SE load (Supplementary Table 2). Additionally, 204 healthy controls were recruited from blood donors at the Banc de Sang i Teixits of Barcelona.

 Association between autoantibodies against the shared epitope and the shared epitope

In RA patients, no association was identified between SE-AAb presence and SE carriage (Table 1). Sequence-specific SE analysis also showed no association between carriage and AAb presence (Supplementary Table 3, S4, and S5). However, SE carriage was significantly associated with RA serological markers, particularly ACPAs (p = 0.0015) (Table 1).

HLA genotype for the shared epitope, presence of autoantibodies against the shared epitope peptide, and autoimmune markers in rheumatoid arthritis patients. The table examines the association between HLA genotype and ELISA positivity across sequences of the shared epitope peptide evaluated in this study (QKRAA, QRRAA, and RRRAA), considering different assay formats (cyclated or linear) and post-translational modifications (citrullination, carbamylation or no modification). The table also includes measurements for classical markers of rheumatic disease (Rheumatoid Factor, ACPA and ANA).

HLA – SE carriers
Autoantibodies AllN=133 YesN=87(65.4%) NoN=46(34.6%) P-value
Cyclated Citrullinated SE peptide Abs 56(42.1%) 40(46.0%) 16(34.8%) 0.2686
Linear Citrullinated SE peptide Abs 42(31.6%) 26(29.9%) 16(34.8%) 0.5634
Linear Carbamylated SE peptide Abs 20(15.0%) 15(17.2%) 5(10.9%) 0.4463
Cyclated SE peptides Abs 34(25.6%) 21(24.1%) 13(28.3%) 0.6773
Linear SE peptides Abs 36(27.1%) 26(29.9%) 10(21.7%) 0.4124
Rheumatoid Factor 86(64.7%) 61(70.1%) 25(54.3%) 0.0869
ACPA* 97(74.0%) 71(83.5%) 26(56.5%) 0.0015
ANA* 42(33.3%) 22(27.5%) 20(43.5%) 0.0791

*For the ACPA analysis, a total of 131 patients were included, of whom 13 were HLA-SE carriers and 118 were non-carriers. For ANA analysis, a total of 126 patients were included, of whom 80 were HLA-SE carriers and 46 were non-carriers.

Bold values correspond to results that are statistically significant (p < 0.05).

In PsA patients, the relationship between the presence of SE-AAb and being a carrier of SE was evaluated. Similar to the RA patients, no significant association was observed (Supplementary Table 6).

 Antibodies against the shared epitope in RA, PsA, and controls

The performance of ELISA assays was evaluated in RA patients, comparing them with healthy blood donors and a cohort of PsA patients, used as a model of non-RA-related inflammatory arthritis. Compared to healthy donors, a significant presence of SE-AAb was observed in RA patients against all citrullinated sequences, both in cyclic and linear conformations, as well as against the carbamylated sequence. For non-citrullinated sequences, significance was only observed for the QRRAA polymorphism and the grouped sequences in its linear conformation (Table 2, Supplementary Tables 7, S8).

Autoantibodies against shared epitope peptides by subject groups. The table displays the rate of positivity corresponding to the QKRAA, QRRAA, and RRRAA sequences, as well as across all of them (All), for linear and cyclated quantifications and considering different post-translational modifications: no-modification and citrullination or carbamylation.

Shared Epitope – AAbs RAN=150(36.1%) PSAN=62(14.9%) ControlN=204(49.0%) OverallP-value Pairwise comparisons p-value
Control –RA Control –PsA RA –PSA
QKRAA Cyclated Citrullinated SE peptides 37 (24.7%) 18 (29.0%) 20 (9.8%) <0.0001 0.0007 0.0012 0.6056
Linear Citrullinated SE peptides 35 (23.3%) 11 (17.7%) 21 (10.3%) 0.0036 0.0033 0.2447 0.4645
Linear Carbamylated SE peptides 28 (18.7%) 7 (11.3%) 21 (10.3%) 0.0699 0.0873 0.8151 0.4516
Cyclated SE peptides 16 (10.7%) 5 (8.1%) 21 (10.3%) 0.9033 >0.9999 >0.9999 >0.9999
Linear SE peptides 16 (10.7%) 7 (11.3%) 21 (10.3%) 0.9757 >0.9999 >0.9999 >0.9999
QRRAA Cyclated Citrullinated SE peptides 36 (24.0%) 20 (32.3%) 21 (10.3%) <0.0001 0.0014 0.0003 0.2331
Linear Citrullinated SE peptides 41 (27.3%) 16 (25.8%) 21 (10.3%) <0.0001 0.0002 0.0108 0.8663
Cyclated SE peptides 20 (13.3%) 7 (11.3%) 22 (10.8%) 0.7808 >0.9999 >0.9999 >0.9999
Linear SE peptides 37 (24.7%) 14 (22.6%) 21 (10.3%) 0.0007 0.0013 0.0353 0.8602
RRRAA Cyclated Citrullinated SE peptides 51 (34.0%) 15 (24.2%) 21 (10.3%) <0.0001 <0.0001 0.0193 0.1930
Linear Citrullinated SE peptides 36 (24.0%) 13 (21.0%) 21 (10.3%) 0.0016 0.0021 0.0961 0.7217
Cyclated SE peptides 18 (12.0%) 5 (8.1%) 20 (9.8%) 0.6868 >0.9999 >0.9999 >0.9999
Linear SE peptides 26 (17.3%) 2 (3.2%) 21 (10.3%) 0.0083 0.1171 0.1192 0.0188
All Cyclated Citrullinated SE peptides 68 (45.3%) 22 (35.5%) 44 (21.6%) <0.0001 <0.0001 0.0601 0.2224
Linear Citrullinated SE peptides 51 (34.0%) 17 (27.4%) 28 (13.7%) <0.0001 <0.0001 0.0381 0.4195
Cyclated SE peptides 39 (26.0%) 11 (17.7%) 43 (21.1%) 0.3582 0.6541 0.7186 0.6541
Linear SE peptides 45 (30.0%) 18 (29.0%) 28 (13.7%) 0.0003 0.0008 0.0148 >0.9999

Bold values correspond to results that are statistically significant (p < 0.05).

Interestingly, PsA patients exhibited a similar pattern to RA patients, with a significant presence of SE-AAb against citrullinated sequences in all three SE peptides, except for the linear conformation of the QKRAA polymorphism. For non-citrullinated sequences, significance was only observed for the QRRAA polymorphism and the grouped sequences in its linear conformation (Table 2).

When comparing the RA group with the PsA group, statistically significant differences were identified only for antibodies against the RRRAA polymorphism in its linear conformation (RA vs. PsA; 17.3% (n = 26) vs. 3.2% (n = 2), p = 0.0063) (Table 2).

 Homology analysis between the shared epitope and the other HLA-DRB1 polymorphisms

Given the presence of SE-AAb in some RA and PsA patients who were not SE carriers, we analyzed the sequence similarity between their HLA-DRB1 alleles and the canonical SE motif. In our cohort, 34 RA and 13 PsA patients lacked SE alleles but tested positive for SE-AAb. Among these 47 patients, a total of 23 different HLA-DRB1 alleles were identified. Sequence homology between these alleles and the SE motif ranged from 60.0% to 93.3%. Notably, only one patient exhibited less than 80% homology between their alleles and the SE reference sequences. (Supplementary Table 9).

 Association between autoantibodies against the shared epitope and clinical variables

The association between the presence of SE-AAb and the main clinical characteristics of RA was analyzed, but no significant relationship was found (Table 3).

Patients’ characteristics by detection of autoantibodies against shared epitope peptides. The table shows the association between ELISA positivity across sequences of the shared epitope evaluated in this study (QKRAA, QRRAA and RRRAA) and patients’ clinical and demographic factors.

Patients’ characteristics N AllN=150 SE-AAb PositiveN=112(74.7%) SE-AAb NegativeN=38(25.3%) P-value
Sex - Female 150 113(75.3%) 87(77.7%) 26(68.4%) 0.2795
Age 150 63.8(23.1, 91.7) 64.7(23.1, 91.7) 61.0(26.8, 88.8) 0.5169
Age at diagnosis 139 53.7(19.2, 86.1) 50.2(19.7, 86.0 54.4(19.2, 86.1) 0.4594
RA duration time (years) 139 5.7(0.0, 46.5) 5.7(0.0, 46.5) 5.8(0.1, 24.3) 0.7594
Time from symptom onset to diagnosis (years) 103 0.5(0.0, 5.0) 0.5(0.0, 5.0) 0.6(0.0, 3.0) 0.6203
Smoking status Never 143 77(53.8%) 61(56.5%) 16(45.7%) 0.0969
Former 46(32.2%) 36(33.3%) 10(28.6%)
Smoker 20(14.0%) 11(10.2%) 9(25.7%)
Joint erosion disease 144 38(26.4%) 30(28.0%) 8(21.6%) 0.5208
Rheumatoid factor 150 95(63.3%) 67(59.8%) 28(73.7%) 0.1724
ACPA 148 108(73.0%) 79(71.8%) 29(76.3%) 0.6751
ANA 142 47(33.1%) 34(32.1%) 13(36.1%) 0.6850
Discussion

This study aimed to analyze the presence of SE-AAb in patients with RA. SE-AAb were detected in 26.0% (n = 39) to 45.3% (n = 68) of cases, depending on the conformation of the SE, with higher positivity observed for citrullinated forms. After confirming that the SE can act as a target for autoantibodies, we assessed whether SE-AAb positivity was associated with SE carriage. No significant association between these variables was observed.

One possible explanation for this finding relates to the methodology used to detect SE-AAb. The ELISA system may detect antibody binding through cross-reactivity, given the high degree of sequence homology among HLA-DRB1 polymorphisms within positions 66 to 78 (40, 41). These variants differ by only 1 to 6 amino acids within the 13–amino acid peptides used in the assay, which could facilitate antibody binding to SE-containing peptides. Consistent with this interpretation, sequence analysis of the HLA-DRB1 region in patients who were not SE carriers but tested positive for SE-AAb revealed a high degree of homology with the canonical SE sequence (41).

An alternative explanation is related to the broad autoreactive capacity observed in RA. Patients with RA can generate autoantibodies against multiple citrullinated proteins and peptides (9). Although SE carriage is associated with ACPA generation, ACPA positivity is also observed in patients who do not carry SE alleles, as citrullinated peptides can still be presented in the P4 pocket of HLA class II molecules (42).

Additional support for cross-reactive immune responses comes from studies using peptide libraries derived from SE-containing sequences. In those analyses, only the shared epitope showed significant homology with microbial proteins from Proteus mirabilis, Escherichia coli, Brucella ovis, Salmonella dublin, and other bacteria (43–47). These findings have led to the proposal that exposure to such microorganisms may induce SE-directed antibodies through mechanisms of molecular mimicry.

To assess the specificity of SE-AAb in relation to RA, a control group of PsA patients was included as a model of non–RA-related joint inflammation. In this group, SE-AAb were detected in 17.7% (n = 11) to 35.5% (n = 22) of cases. When compared with RA patients, no significant differences were observed, except for antibodies directed against the native linear RRRAA sequence.

Analysis of reactivity to native linear SE peptides revealed a progressive effect of arginine content. The presence of a single arginine did not increase reactivity beyond levels observed in healthy controls. In contrast, two arginine residues were associated with an approximately twofold increase in reactivity in both RA and PsA. Interestingly, the presence of a third arginine resulted in divergent patterns between the two diseases. Reactivity decreased mildly in RA but showed a more pronounced reduction in PsA. This may suggest that autoantibodies generated in PsA have a reduced capacity to bind peptides with a high density of positively charged residues. When arginines were citrullinated, peptide charge was altered and antibody binding was restored, reaching reactivity levels higher than those observed in healthy controls.

The relationship between SE-AAb positivity and SE carriage was also evaluated in PsA patients. Similar to the findings in RA, no significant association was observed. The detection of SE-AAb in PsA suggests that joint inflammation, regardless of its underlying cause, may contribute to the breakdown of immune tolerance (48). In this context, HLA-DRB1–derived peptides could act as targets of the autoimmune response.

Although immune dysregulation in PsA is primarily associated with the HLA class I system (49), which likely explains the lower prevalence of autoantibodies such as RF and ACPAs (5, 50, 51), increasing evidence supports a role for the adaptive immune system in this disease. Observations including lymphoid neogenesis and the presence of autoantibodies, particularly those directed against carbamylated peptides, support this concept (52–54). The detection of SE-AAb in our PsA cohort further reinforces this possibility.

Finally, we analyzed the association between SE-AAb positivity and clinical variables in RA using retrospectively collected data. No significant associations were identified. Due to the cross-sectional design of the study and the heterogeneity in disease duration, the influence of SE-AAb on clinical outcomes could not be assessed. An additional limitation is that the SE analyzed in this study corresponds only to the core 70–74 amino acid sequence. Other HLA-DRB1 positions, particularly residue 11, have been shown to independently and strongly influence RA susceptibility (55). Further prospective studies will therefore be required to determine whether SE-AAb have clinical relevance in RA.

In conclusion, SE-AAb are present in RA, particularly against citrullinated shared epitope peptides, but are not disease-specific and occur at similar frequencies in PsA. Their presence is independent of shared epitope genetic carriage, suggesting that inflammatory context rather than genetic risk alone may drive these autoantibody responses.

 Data availability statement

The original contributions presented in the study are included in the article/supplementary material. Further inquiries can be directed to the corresponding authors.

 Ethics statement

The present study was reviewed and approved by the Ethics and Drug Research Committee of PTUH (Ref.: 2018/534) in accordance with the World Medical Association Declaration of Helsinki. The studies were conducted in accordance with the local legislation and institutional requirements. The participants provided their written informed consent to participate in this study.

 Author contributions

EG: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Visualization, Writing – original draft, Writing – review & editing. JD: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Project administration, Visualization, Writing – original draft, Writing – review & editing. AD: Data curation, Resources, Writing – review & editing. MG-M: Data curation, Resources, Writing – review & editing. MR: Data curation, Funding acquisition, Resources, Writing – review & editing. AR: Data curation, Formal analysis, Methodology, Writing – original draft, Writing – review & editing. AB-L: Data curation, Formal analysis, Writing – review & editing. JG: Data curation, Methodology, Resources, Supervision, Validation, Writing – review & editing. JC: Data curation, Methodology, Resources, Supervision, Validation, Writing – review & editing.

 Acknowledgments

We would like to thank Dr. José Luis Rodríguez-Sánchez for his valuable contributions to the conception and design of the study. We acknowledge the CERCA Programme/Generalitat de Catalunya for their support.

 Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

 Correction note

This article has been corrected with minor changes. These changes do not impact the scientific content of the article.

 Generative AI statement

The author(s) declared that generative AI was used in the creation of this manuscript. Generative AI and AI-assisted technologies should only be used in the writing process to improve the readability and language of the manuscript.

Any alternative text (alt text) provided alongside figures in this article has been generated by Frontiers with the support of artificial intelligence and reasonable efforts have been made to ensure accuracy, including review by the authors wherever possible. If you identify any issues, please contact us.

 Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

 Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fimmu.2026.1744370/full#supplementary-material

 References Smolen JS Van Der Heijde DMFM St.Clair EW Emery P Bathon JM Keystone E . Predictors of joint damage in patients with early rheumatoid arthritis treated with high-dose methotrexate with or without concomitant infliximab: Results from the ASPIRE trial. Arthritis Rheumatism. (2006) 54:702–10. doi: 10.1002/art.21678, PMID: 16508926 Taylor P Gartemann J Hsieh J Creeden J . A systematic review of serum biomarkers anti-cyclic citrullinated peptide and rheumatoid factor as tests for rheumatoid arthritis. Autoimmune Dis. (2011) 2011:1–18. doi: 10.4061/2011/815038, PMID: 21915375 Weyand CM Goronzy JJ . The immunology of rheumatoid arthritis. Nat Immunol. (2021) 22:10–8. doi: 10.1038/s41590-020-00816-x, PMID: 33257900 Van Delft MAM Huizinga TWJ . An overview of autoantibodies in rheumatoid arthritis. J Autoimmun. (2020) 110:102392. doi: 10.1016/j.jaut.2019.102392, PMID: 31911013 Sanmartí R Graell E Perez ML Ercilla G Viñas O Gómez-Puerta JA . Diagnostic and prognostic value of antibodies against chimeric fibrin/filaggrin citrullinated synthetic peptides in rheumatoid arthritis. Arthritis Res Ther. (2009) 11:R135. doi: 10.1186/ar2802, PMID: 19725967 Sokolova MV Schett G Steffen U . Autoantibodies in rheumatoid arthritis: historical background and novel findings. Clin Rev Allerg Immunol. (2021) 63:138–51. doi: 10.1007/s12016-021-08890-1, PMID: 34495490 Aletaha D Neogi T Silman AJ Funovits J Felson DT Bingham CO . Rheumatoid arthritis classification criteria: An American College of Rheumatology/European League Against Rheumatism collaborative initiative. Arthritis Rheumatism. (2010) 62:2569–81. doi: 10.1002/art.27584, PMID: 20872595 Majka DS Deane KD Parrish LA Lazar AA Baron AE Walker CW . Duration of preclinical rheumatoid arthritis-related autoantibody positivity increases in subjects with older age at time of disease diagnosis. Ann Rheumatic Dis. (2008) 67:801–7. doi: 10.1136/ard.2007.076679, PMID: 17974596 Sokolove J Bromberg R Deane KD Lahey LJ Derber LA Chandra PE . Autoantibody epitope spreading in the pre-clinical phase predicts progression to rheumatoid arthritis. PloS One. (2012) 7:e35296. doi: 10.1371/journal.pone.0035296, PMID: 22662108 Rantapää-Dahlqvist S De Jong BAW Berglin E Hallmans G Wadell G Stenlund H . Antibodies against cyclic citrullinated peptide and IgA rheumatoid factor predict the development of rheumatoid arthritis. Arthritis Rheumatism. (2003) 48:2741–9. doi: 10.1002/art.11223, PMID: 14558078 Verpoort KN Jol-Van Der Zijde CM Papendrecht-Van Der Voort EAM Ioan-Facsinay A Drijfhout JW Van Tol MJD . Isotype distribution of ANTI–CYCLIC citrullinated peptide antibodies in undifferentiated arthritis and rheumatoid arthritis reflects an ongoing immune response. Arthritis Rheumatism. (2006) 54:3799–808. doi: 10.1002/art.22279, PMID: 17133560 Van Der Woude D Houwing-Duistermaat JJ Toes REM Huizinga TWJ Thomson W Worthington J . Quantitative heritability of anti–citrullinated protein antibody–positive and anti–citrullinated protein antibody–negative rheumatoid arthritis. Arthritis Rheumatism. (2009) 60:916–23. doi: 10.1002/art.24385, PMID: 19333951 Padyukov L Silva C Stolt P Alfredsson L Klareskog L . A gene–environment interaction between smoking and shared epitope genes in HLA–DR provides a high risk of seropositive rheumatoid arthritis. Arthritis Rheumatism. (2004) 50:3085–92. doi: 10.1002/art.20553, PMID: 15476204 Bae S-C Lee YH . Association between anti-Porphyromonas gingivalis antibody, anti-citrullinated protein antibodies, and rheumatoid arthritis: A meta-analysis. Z Rheumatol. (2018) 77:522–32. doi: 10.1007/s00393-017-0328-y, PMID: 28555277 Girbal-Neuhauser E Durieux JJ Arnaud M Dalbon P Sebbag M Vincent C . The epitopes targeted by the rheumatoid arthritis-associated antifilaggrin autoantibodies are posttranslationally generated on various sites of (pro)filaggrin by deimination of arginine residues. J Immunol. (1999) 162:585–94. doi: 10.4049/jimmunol.162.1.585, PMID: 34749531 Takizawa Y . Citrullinated fibrinogen detected as a soluble citrullinated autoantigen in rheumatoid arthritis synovial fluids. Ann Rheumatic Dis. (2006) 65:1013–20. doi: 10.1136/ard.2005.044743, PMID: 16449316 Masson-Bessière C Sebbag M Girbal-Neuhauser E Nogueira L Vincent C Senshu T . The major synovial targets of the rheumatoid arthritis-specific antifilaggrin autoantibodies are deiminated forms of the α- and β-chains of fibrin. J Immunol. (2001) 166:4177–84. doi: 10.4049/jimmunol.166.6.4177, PMID: 11238669 Tabushi Y Nakanishi T Takeuchi T Nakajima M Ueda K Kotani T . Detection of citrullinated proteins in synovial fluids derived from patients with rheumatoid arthritis by proteomics-based analysis. Ann Clin Biochem. (2008) 45:413–7. doi: 10.1258/acb.2007.007205, PMID: 18583628 Sebbag M Moinard N Auger I Clavel C Arnaud J Nogueira L . Epitopes of human fibrin recognized by the rheumatoid arthritis-specific autoantibodies to citrullinated proteins. Eur J Immunol. (2006) 36:2250–63. doi: 10.1002/eji.200535790, PMID: 16838278 Meng X Ezzati P Smolik I Bernstein CN Hitchon CA El-Gabalawy HS . Characterization of autoantigens targeted by anti-citrullinated protein antibodies in vivo: prominent role for epitopes derived from histone 4 proteins. PloS One. (2016) 11:e0165501. doi: 10.1371/journal.pone.0165501, PMID: 27788229 Lundberg K Kinloch A Fisher BA Wegner N Wait R Charles P . Antibodies to citrullinated α-enolase peptide 1 are specific for rheumatoid arthritis and cross-react with bacterial enolase. Arthritis Rheumatism. (2008) 58:3009–19. doi: 10.1002/art.23936, PMID: 18821669 Goëb V Thomas-L’Otellier M Daveau R Charlionet R Fardellone P Le Loët X . Candidate autoantigens identified by mass spectrometry in early rheumatoid arthritis are chaperones and citrullinated glycolytic enzymes. Arthritis Res Ther. (2009) 11:R38. doi: 10.1186/ar2644, PMID: 19284558 Kinloch A Tatzer V Wait R Peston D Lundberg K Donatien P . Identification of citrullinated α-enolase as a candidate autoantigen in rheumatoid arthritis. Arthritis Res Ther. (2005) 7:R1421. doi: 10.1186/ar1845, PMID: 16277695 Vossenaar ER Després N Lapointe E van der Heijden A Lora M Senshu T . Rheumatoid arthritis specific anti-Sa antibodies target citrullinated vimentin. Arthritis Res Ther. (2004) 6:R142. doi: 10.1186/ar1149, PMID: 15059278 Tilleman K Van Steendam K Cantaert T De Keyser F Elewaut D Deforce D . Synovial detection and autoantibody reactivity of processed citrullinated isoforms of vimentin in inflammatory arthritides. Rheumatology. (2008) 47:597–604. doi: 10.1093/rheumatology/ken077, PMID: 18326534 Burkhardt H Sehnert B Bockermann R Engström Å Kalden JR Holmdahl R . Humoral immune response to citrullinated collagen type II determinants in early rheumatoid arthritis. Eur J Immunol. (2005) 35:1643–52. doi: 10.1002/eji.200526000, PMID: 15832289 Suzuki A Yamada R Ohtake-Yamanaka M Okazaki Y Sawada T Yamamoto K . Anti-citrullinated collagen type I antibody is a target of autoimmunity in rheumatoid arthritis. Biochem Biophys Res Commun. (2005) 333:418–26. doi: 10.1016/j.bbrc.2005.05.137, PMID: 15950180 Schwenzer A Jiang X Mikuls TR Payne JB Sayles HR Quirke A-M . Identification of an immunodominant peptide from citrullinated tenascin-C as a major target for autoantibodies in rheumatoid arthritis. Ann Rheum Dis. (2016) 75:1876–83. doi: 10.1136/annrheumdis-2015-208495, PMID: 26659718 Matsuo K Xiang Y Nakamura H Masuko K Yudoh K Noyori K . Identification of novel citrullinated autoantigens of synovium in rheumatoid arthritis using a proteomic approach. Arthritis Res Ther. (2006) 8:R175. doi: 10.1186/ar2085, PMID: 17125526 Darrah E Rosen A Giles JT Andrade F . Peptidylarginine deiminase 2, 3 and 4 have distinct specificities against cellular substrates: novel insights into autoantigen selection in rheumatoid arthritis. Ann Rheum Dis. (2012) 71:92–8. doi: 10.1136/ard.2011.151712, PMID: 21859690 Okazaki Y Suzuki A Sawada T Ohtake-Yamanaka M Inoue T Hasebe T . Identification of citrullinated eukaryotic translation initiation factor 4G1 as novel autoantigen in rheumatoid arthritis. Biochem Biophys Res Commun. (2006) 341:94–100. doi: 10.1016/j.bbrc.2005.12.160, PMID: 16412378 Harlow L Rosas IO Gochuico BR Mikuls TR Dellaripa PF Oddis CV . Identification of citrullinated hsp90 isoforms as novel autoantigens in rheumatoid arthritis–associated interstitial lung disease. Arthritis Rheumatism. (2013) 65:869–79. doi: 10.1002/art.37881, PMID: 23400887 Nelson HA Banerjee D Novis CL Deane KD Feser ML Nandakumar V . A comparison of anti-cyclic citrullinated peptides (CCP3 and CCP3.1) autoantibody tests in rheumatoid arthritis. Pract Lab Med. (2024) 41:e00420. doi: 10.1016/j.plabm.2024.e00420, PMID: 39161944 Scherer HU Häupl T Burmester GR . The etiology of rheumatoid arthritis. J Autoimmun. (2020) 110:102400. doi: 10.1016/j.jaut.2019.102400, PMID: 31980337 Gonzalez-Gay MA Garcia-Porrua C Hajeer AH . Influence of human leukocyte antigen-DRB1 on the susceptibility and severity of rheumatoid arthritis. Semin Arthritis Rheumatism. (2002) 31:355–60. doi: 10.1053/sarh.2002.32552, PMID: 12077707 Alves CC Lewis J Antunes DA Donadi EA . The role of vimentin peptide citrullination in the structure and dynamics of HLA-DRB1 rheumatoid arthritis risk-associated alleles. Int J Mol Sci. (2024) 26:34. doi: 10.3390/ijms26010034, PMID: 39795892 Ting YT Petersen J Ramarathinam SH Scally SW Loh KL Thomas R . The interplay between citrullination and HLA-DRB1 polymorphism in shaping peptide binding hierarchies in rheumatoid arthritis. J Biol Chem. (2018) 293:3236–51. doi: 10.1074/jbc.RA117.001013, PMID: 29317506 Witalison E Thompson P Hofseth L . Protein arginine deiminases and associated citrullination: physiological functions and diseases associated with dysregulation. CDT. (2015) 16:700–10. doi: 10.2174/1389450116666150202160954, PMID: 25642720 Kampstra ASB Toes REM . HLA class II and rheumatoid arthritis: the bumpy road of revelation. Immunogenetics. (2017) 69:597–603. doi: 10.1007/s00251-017-0987-5, PMID: 28695293 Bondinas GP Moustakas AK Papadopoulos GK . The spectrum of HLA-DQ and HLA-DR alleles, 2006: a listing correlating sequence and structure with function. Immunogenetics. (2007) 59:539–53. doi: 10.1007/s00251-007-0224-8, PMID: 17497145 Trier NH Dam CE Olsen DT Hansen PR Houen G . Contribution of peptide backbone to anti-citrullinated peptide antibody reactivity. PloS One. (2015) 10:e0144707. doi: 10.1371/journal.pone.0144707, PMID: 26657009 Scally SW Petersen J Law SC Dudek NL Nel HJ Loh KL . A molecular basis for the association of the HLA-DRB1 locus, citrullination, and rheumatoid arthritis. J Exp Med. (2013) 210:2569–82. doi: 10.1084/jem.20131241, PMID: 24190431 Auger I Escola JM Gorvel JP Roudier J . HLA-DR4 and HLA-DR10 motifs that carry susceptibility to rheumatoid arthritis bind 70-kD heat shock proteins. Nat Med. (1996) 2:306–10. doi: 10.1038/nm0396-306, PMID: 8612229 Auger I Roudier J . Influence of the QKRAA/QRRAA/RRRAA motifs of the third hypervariable region of HLA-DRB1 in the development of rheumatoid arthritis. J Rheumatol. (1997) 24:227–8. doi: 10.3899/jrheum.24.2.227, PMID: 39921727 Auger I Roudier J . A function for the QKRAA amino acid motif: mediating binding of DnaJ to DnaK. Implications for the association of rheumatoid arthritis with HLA-DR4. J Clin Invest. (1997) 99:1818–22. doi: 10.1172/JCI119348, PMID: 9109425 La Cava A Nelson JL Ollier WE MacGregor A Keystone EC Thorne JC . Genetic bias in immune responses to a cassette shared by different microorganisms in patients with rheumatoid arthritis. J Clin Invest. (1997) 100:658–63. doi: 10.1172/JCI119577, PMID: 9239413 Tiwana H Wilson C Alvarez A Abuknesha R Bansal S Ebringer A . Cross-reactivity between the rheumatoid arthritis-associated motif EQKRAA and structurally related sequences found in Proteus mirabilis. Infect Immun. (1999) 67:2769–75. doi: 10.1128/IAI.67.6.2769-2775.1999, PMID: 10338479 Xiang Y Zhang M Jiang D Su Q Shi J . The role of inflammation in autoimmune disease: a therapeutic target. Front Immunol. (2023) 14:1267091. doi: 10.3389/fimmu.2023.1267091, PMID: 37859999 FitzGerald O Winchester R . Psoriatic arthritis: from pathogenesis to therapy. Arthritis Res Ther. (2009) 11:214. doi: 10.1186/ar2580, PMID: 19232079 Bandinelli F Denaro V Prignano F Collaku L Ciancio G Matucci-Cerinic M . Ultrasonographic wrist and hand abnormalities in early psoriatic arthritis patients: correlation with clinical, dermatological, serological and genetic indices. Clin Exp Rheumatol. (2015) 33:330–5. doi: 10.1136/annrheumdis-2014-206701, PMID: 41686241 Taylor W Gladman D Helliwell P Marchesoni A Mease P Mielants H . Classification criteria for psoriatic arthritis: Development of new criteria from a large international study. Arthritis Rheumatism. (2006) 54:2665–73. doi: 10.1002/art.21972, PMID: 16871531 Chimenti MS Triggianese P Nuccetelli M Terracciano C Crisanti A Guarino MD . Auto-reactions, autoimmunity and psoriatic arthritis. Autoimmun Rev. (2015) 14:1142–6. doi: 10.1016/j.autrev.2015.08.003, PMID: 26254734 Canete JD Santiago B Cantaert T Sanmarti R Palacin A Celis R . Ectopic lymphoid neogenesis in psoriatic arthritis. Ann Rheumatic Dis. (2007) 66:720–6. doi: 10.1136/ard.2006.062042, PMID: 17223654 Koussiouris J Chandran V . Autoantibodies in psoriatic disease. J Appl Lab Med. (2022) 7:281–93. doi: 10.1093/jalm/jfab120, PMID: 34996072 Raychaudhuri S Sandor C Stahl EA Freudenberg J Lee H-S Jia X . Five amino acids in three HLA proteins explain most of the association between MHC and seropositive rheumatoid arthritis. Nat Genet. (2012) 44:291–6. doi: 10.1038/ng.1076, PMID: 22286218

Edited by: Roberto Paganelli, YDA, Institute for Advanced Biologic Therapies, Italy

Reviewed by: Denis Lagutkin, Karolinska Institutet (KI), Sweden

Kanwal Ashiq, Superior University, Pakistan