Twenty Years of SPARTAN: From Inception to Impact (SPARTAN 2023 Annual Meeting Proceedings).

PURPOSE OF REVIEW: This review takes a look at the past, present, and future of SPARTAN, the Spondyloarthritis Research and Treatment Network, an organization of North American healthcare professionals dedicated to advancing research, education, and patient care in spondyloarthritis. RECENT FINDINGS: In 2022, SPARTAN completed the Classification of Axial SpondyloarthritiS Inception Cohort (CLASSIC) study, a collaboration with the Assessment in SpondyloArthritis International Society (ASAS). CLASSIC aimed to validate the 2009 ASAS classification criteria for axial spondyloarthritis. Other ongoing SPARTAN endeavors include the development of US referral recommendations for axial spondyloarthritis, an update of the 2019 ACR/SAA/SPARTAN treatment recommendations for axial spondyloarthritis and multiple educational initiatives. Twenty years after its inception, SPARTAN continues to grow and broaden its impact, guided by the SPARTAN vision of "a world free of spondyloarthritis through leadership in research and education."

The faecal microbiota is distinct in HLA-B27+ ankylosing spondylitis patients versus HLA-B27+ healthy controls.

OBJECTIVES: Spondyloarthritis (SpA) results from the interplay between genetic and environmental factors. An emerging modifiable factor is the human intestinal microbiota, which multiple studies in children and adults have shown to be abnormal in SpA patients, including enthesitis related arthritis and ankylosing spondylitis (AS). However, HLA-B27 itself appears to impact the contents of the microbiota and is more common in SpA patients versus controls, thus serving as a confounding factor in most comparative studies. METHODS: This was a cross-sectional study that evaluated the contents of the faecal microbiota among 29 patients with HLA-B27+ AS and 43 healthy adults who underwent 16S sequencing and genotyping as part of the TwinsUK Programme. RESULTS: HLA-B27 positive+ patients and healthy controls demonstrated substantial clustering based upon diagnosis. Decreased richness was observed among the AS patients, although measures of evenness were similar. After correction for multiple comparisons, several taxa - including Faecalibacterium prausnitzii and Coprococcus - were elevated in AS patients compared to controls, even when restricted to female subjects, while Bacteroides fragilis, Ruminococcus, and Akkermansia muciniphila were depleted in AS patients. CONCLUSIONS: Consistent with some previous studies, our study demonstrates in patients with AS associations with Coprococcus, Bacteroides, and Ruminococcus. Other findings, including increased Faecalibacterium, are inconsistent with previous studies and thus potentially underscore the necessity of evaluating HLA-B27 positive controls in studies evaluating the impact of the intestinal microbiota on SpA.

Identifying trajectories of radiographic spinal disease in ankylosing spondylitis: a 15-year follow-up study of the PSOAS cohort.

OBJECTIVES: Little is known with certainty about the natural history of spinal disease progression in ankylosing spondylitis (AS). Our objective was to discover if there were distinct patterns of change in vertebral involvement over time and to study associated clinical factors. METHODS: Data were analysed from the Prospective Study of Outcomes in Ankylosing Spondylitis (PSOAS) observational cohort. All patients met modified New York Criteria for AS and had ≥2 sets of radiographs scored by modified Stoke Ankylosing Spondylitis Spinal Score (mSASSS) by two independent readers between 2002 and 2017. Group-based trajectory modelling (GBTM) was used to classify patients into distinct groups of longitudinal mSASSS considering sociodemographic and clinical covariables. The optimal trajectory model and number of trajectories was selected using Nagin's Bayesian information criterion (BIC). RESULTS: A total of 561 patients with 1618 radiographs were analysed. The optimum number of trajectory groups identified was four (BIC -4062). These groups were subsequently categorized as: non-progressors (204 patients), late-progressors (147 patients), early-progressors (107 patients) and rapid-progressors (103 patients). Baseline predictors associated with higher spinal disease burden groups included: baseline mSASSS, male gender, longer disease duration, elevated CRP and smoking history. In addition, time-varying anti-TNF use per year was associated with decreased mSASSS progression only in the rapid-progressor group. CONCLUSIONS: GBTM identified four distinct patterns of spinal disease progression in the PSOAS cohort. Male gender, longer disease duration, elevated CRP and smoking were associated with higher spinal disease groups. Independent confirmation in other AS cohorts is needed to confirm these radiographic patterns.

Chronic inflammatory back pain commencing late in life: a neglected concept.

Chronic inflammatory back pain (CIBP) occurs in up to one-third of those with chronic back pain. Criteria for diagnosis of inflammatory back pain include an onset below 50 years. Using the US National Health and Nutrition Examination Survey data for 2009-2010, we showed that 3% of adults aged 50-69 years have features of CIBP with onset on or after 50 years. There is little information in the literature on CIBP of late onset. Patients with late onset CIBP may be falling through the cracks.

Efficacy and Safety of Intravenous Golimumab in Ankylosing Spondylitis Patients With Early and Late Disease Through One Year of the GO-ALIVE Study.

BACKGROUND/OBJECTIVE: This post hoc analysis assessed efficacy and safety of intravenous (IV) golimumab in ankylosing spondylitis (AS) patients with early disease (ED) versus late disease (LD). METHODS: The phase 3, double-blind, GO-ALIVE study randomized patients to IV golimumab 2 mg/kg at weeks 0 and 4 and then every 8 weeks through week 52, or placebo at weeks 0, 4, and 12 with crossover to IV golimumab at week 16. Clinical efficacy was assessed by ≥20% improvement in Assessment of Spondyloarthritis International Society response criteria (ASAS20), ≥50% improvement in Bath Ankylosing Spondylitis Disease Activity Index (BASDAI 50), and Ankylosing Spondylitis Disease Activity Score (ASDAS) <1.3 (inactive disease). Using self-reported duration of inflammatory back pain (IBP), patients were grouped into quartiles: first = ED and fourth = LD. Descriptive statistics summarized efficacy and safety findings through 1 year. RESULTS: Early disease patients (n = 60) were ~10 years younger and had shorter median AS (IBP) symptom duration (2-3 years) versus LD patients (n = 52; 21-24 years). At week 16, numerically higher proportions of golimumab- than placebo-treated patients achieved ASAS20 (ED: 71% vs. 32%; LD: 67% vs. 21%), BASDAI 50 (ED: 40% vs. 12%; LD: 33% vs. 7%), and ASDAS <1.3 (ED: 17% vs. 4%; LD 8% vs. 0%) regardless of IBP duration. Efficacy was durable through 1 year of treatment; however, response rates were numerically higher in patients with ED versus LD. Through week 60, adverse events and serious adverse events, respectively, were reported by 46% and 3% of ED patients and 61% and 2% of LD patients. CONCLUSION: Prompt diagnosis of AS and early treatment with IV golimumab may yield more robust improvements in disease activity.

Polygenic Risk Scores have high diagnostic capacity in ankylosing spondylitis.

OBJECTIVE: We sought to test the hypothesis that Polygenic Risk Scores (PRSs) have strong capacity to discriminate cases of ankylosing spondylitis (AS) from healthy controls and individuals in the community with chronic back pain. METHODS: PRSs were developed and validated in individuals of European and East Asian ethnicity, using data from genome-wide association studies in 15 585 AS cases and 20 452 controls. The discriminatory values of PRSs in these populations were compared with other widely used diagnostic tests, including C-reactive protein (CRP), HLA-B27 and sacroiliac MRI. RESULTS: In people of European descent, PRS had high discriminatory capacity with area under the curve (AUC) in receiver operator characteristic analysis of 0.924. This was significantly better than for HLA-B27 testing alone (AUC=0.869), MRI (AUC=0.885) or C-reactive protein (AUC=0.700). PRS developed and validated in individuals of East Asian descent performed similarly (AUC=0.948). Assuming a prior probability of AS of 10% such as in patients with chronic back pain under 45 years of age, compared with HLA-B27 testing alone, PRS provides higher positive values for 35% of patients and negative predictive values for 67.5% of patients. For PRS, in people of European descent, the maximum positive predictive value was 78.2% and negative predictive value was 100%, whereas for HLA-B27, these values were 51.9% and 97.9%, respectively. CONCLUSIONS: PRS have higher discriminatory capacity for AS than CRP, sacroiliac MRI or HLA-B27 status alone. For optimal performance, PRS should be developed for use in the specific ethnic groups to which they are to be applied.

Genetic fine mapping of systemic lupus erythematosus MHC associations in Europeans and African Americans.

Genetic variation within the major histocompatibility complex (MHC) contributes substantial risk for systemic lupus erythematosus, but high gene density, extreme polymorphism and extensive linkage disequilibrium (LD) have made fine mapping challenging. To address the problem, we compared two association techniques in two ancestrally diverse populations, African Americans (AAs) and Europeans (EURs). We observed a greater number of Human Leucocyte Antigen (HLA) alleles in AA consistent with the elevated level of recombination in this population. In EUR we observed 50 different A-C-B-DRB1-DQA-DQB multilocus haplotype sequences per hundred individuals; in the AA sample, these multilocus haplotypes were twice as common compared to Europeans. We also observed a strong narrow class II signal in AA as opposed to the long-range LD observed in EUR that includes class I alleles. We performed a Bayesian model choice of the classical HLA alleles and a frequentist analysis that combined both single nucleotide polymorphisms (SNPs) and classical HLA alleles. Both analyses converged on a similar subset of risk HLA alleles: in EUR HLA- B*08:01 + B*18:01 + (DRB1*15:01 frequentist only) + DQA*01:02 + DQB*02:01 + DRB3*02 and in AA HLA-C*17:01 + B*08:01 + DRB1*15:03 + (DQA*01:02 frequentist only) + DQA*02:01 + DQA*05:01+ DQA*05:05 + DQB*03:19 + DQB*02:02. We observed two additional independent SNP associations in both populations: EUR rs146903072 and rs501480; AA rs389883 and rs114118665. The DR2 serotype was best explained by DRB1*15:03 + DQA*01:02 in AA and by DRB1*15:01 + DQA*01:02 in EUR. The DR3 serotype was best explained by DQA*05:01 in AA and by DQB*02:01 in EUR. Despite some differences in underlying HLA allele risk models in EUR and AA, SNP signals across the extended MHC showed remarkable similarity and significant concordance in direction of effect for risk-associated variants.

A plausibly causal functional lupus-associated risk variant in the STAT1-STAT4 locus.

Systemic lupus erythematosus (SLE or lupus) (OMIM: 152700) is a chronic autoimmune disease with debilitating inflammation that affects multiple organ systems. The STAT1-STAT4 locus is one of the first and most highly replicated genetic loci associated with lupus risk. We performed a fine-mapping study to identify plausible causal variants within the STAT1-STAT4 locus associated with increased lupus disease risk. Using complementary frequentist and Bayesian approaches in trans-ancestral Discovery and Replication cohorts, we found one variant whose association with lupus risk is supported across ancestries in both the Discovery and Replication cohorts: rs11889341. In B cell lines from patients with lupus and healthy controls, the lupus risk allele of rs11889341 was associated with increased STAT1 expression. We demonstrated that the transcription factor HMGA1, a member of the HMG transcription factor family with an AT-hook DNA-binding domain, has enriched binding to the risk allele compared with the non-risk allele of rs11889341. We identified a genotype-dependent repressive element in the DNA within the intron of STAT4 surrounding rs11889341. Consistent with expression quantitative trait locus (eQTL) analysis, the lupus risk allele of rs11889341 decreased the activity of this putative repressor. Altogether, we present a plausible molecular mechanism for increased lupus risk at the STAT1-STAT4 locus in which the risk allele of rs11889341, the most probable causal variant, leads to elevated STAT1 expression in B cells due to decreased repressor activity mediated by increased binding of HMGA1.

Effects of Intravenous Golimumab on Health-Related Quality of Life in Patients with Ankylosing Spondylitis: 28-Week Results of the GO-ALIVE Trial.

OBJECTIVE: Evaluate the effect of intravenous golimumab on health-related quality of life (HRQoL) in patients with ankylosing spondylitis (AS) through week 28 of the phase III, multicenter, randomized, double-blind, placebo-controlled GO-ALIVE study. METHODS: Adult patients (n = 208) were randomized to IV golimumab 2 mg/kg (n = 105) at weeks 0, 4, and 12 and every 8 weeks or placebo (n = 103) at weeks 0, 4, and 12, with crossover to golimumab 2mg/kg at weeks 16, 20, and every 8 weeks. General HRQoL was evaluated using the Short Form Health Survey (SF-36) Physical Component Summary/Mental Component Summary (PCS/MCS), and the EQ VAS, and AS disease-specific HRQoL was assessed using the Ankylosing Spondylitis Quality of Life (ASQoL) instrument. RESULTS: Mean improvements from baseline in SF-36 PCS were greater in the golimumab group versus the placebo group at weeks 8 and 16 (6.8 vs 2.1 and 8.5 vs 2.9, respectively; P < .001); similar results were observed for SF-36 MCS (5.6 vs 1.7 and 6.5 vs 0.8, respectively; P < .001). Mean improvement in each of 8 subscale scores of the SF-36 were also greater for golimumab-treated patients versus placebo at weeks 8 and 16. Mean improvements in EQ VAS and ASQoL were greater in the golimumab group versus placebo at week 8 and week 16. Greater proportions of golimumab-treated patients had clinically meaningful improvement in SF-36 PCS, SF-36 MCS, EQ VAS, and ASQoL at weeks 8 and 16; improvements in SF-36 PCS/MCS, EQ VAS, and ASQoL were maintained through week 28. CONCLUSIONS: Golimumab-treated patients had greater mean improvements in HRQoL measures compared with placebo through week 16. Clinically meaningful improvements were observed as early as week 8 and continued through week 28.

A latent class based imputation method under Bayesian quantile regression framework using asymmetric Laplace distribution for longitudinal medication usage data with intermittent missing values.

Evaluating the association between diseases and the longitudinal pattern of pharmacological therapy has become increasingly important. However, in many longitudinal studies, self-reported medication usage data collected at patients' follow-up visits could be missing for various reasons. These pieces of missing or inaccurate/untenable information complicate determining the trajectory of medication use and its complete effects for patients. Although longitudinal models can deal with specific types of missing data, inappropriate handling of this issue can lead to a biased estimation of regression parameters especially when missing data mechanisms are complex and depend upon multiple sources of variation. We propose a latent class-based multiple imputation (MI) approach using a Bayesian quantile regression (BQR) that incorporates cluster of unobserved heterogeneity for medication usage data with intermittent missing values. Findings from our simulation study indicate that the proposed method performs better than traditional MI methods under certain scenarios of data distribution. We also demonstrate applications of the proposed method to data from the Prospective Study of Outcomes in Ankylosing Spondylitis (AS) cohort when assessing an association between longitudinal nonsteroidal anti-inflammatory drugs (NSAIDs) usage and radiographic damage in AS, while the longitudinal NSAID index data are intermittently missing.

Longitudinal associations between depressive symptoms and clinical factors in ankylosing spondylitis patients: analysis from an observational cohort.

OBJECTIVES: Although cross-sectional studies have shown that ankylosing spondylitis-specific factors correlate with depressive symptom severity, the association of these factors over time is unresolved. We examined the demographic and clinical factors associated with longitudinal depressive symptom severity in AS patients. METHODS: We analyzed sociodemographic, clinical, behavioral and medication data from 991 patients from the Prospective Study of Outcomes in Ankylosing spondylitis cohort, and measured depression severity with the Center for Epidemiological Studies Depression (CES-D) Scale administered at approximately 6-month visit intervals. Multivariable longitudinal negative binomial regression models were conducted using generalized estimating equation modeling to assess the demographic, clinical, and medication-related factors associated with depression severity by CES-D scores over time. RESULTS: The median baseline CES-D score (possible range 0-60) was 10.0 (interquartile range = 5, 17). In longitudinal multivariable analyses, higher CES-D scores were associated with longitudinal smoking, greater functional impairment, greater disease activity, self-reported depression, and poor global health scores. Marital status (e.g., being married) was associated with lower CES-D. Adjusted mean CES-D scores in our model decreased over time, with a significant interaction between time and gender observed. CONCLUSION: This study identified longitudinal clinical factors such as greater disease activity, greater functional impairment, and poor global health to be associated with longitudinal depression severity. These factors are potentially modifiable and may help manage depressive symptoms in AS.

IgG Galactosylation status combined with MYOM2-rs2294066 precisely predicts anti-TNF response in ankylosing spondylitis.

BACKGROUND: Tumor necrosis factor (TNF) blockers have a high efficacy in treating Ankylosing Spondylitis (AS), yet up to 40% of AS patients show poor or even no response to this treatment. In this paper, we aim to build an approach to predict the response prior to clinical treatment. METHODS: AS patients during the active progression were included and treated with TNF blocker for 3 months. Patients who do not fulfill ASASAS40 were considered as poor responders. The Immunoglobulin G galactosylation (IgG-Gal) ratio representing the quantity of IgG galactosylation was calculated and candidate single nucleotide polymorphisms (SNPs) in patients treated with etanercept was obtained. Machine-learning models and cross-validation were conducted to predict responsiveness. RESULTS: Both IgG-Gal ratio at each time point and differential IgG-Gal ratios between week 0 and weeks 2, 4, 8, 12 showed significant difference between responders and poor-responders. Area under curve (AUC) of the IgG-Gal ratio prediction model was 0.8 after cross-validation, significantly higher than current clinical indexes (C-reactive protein (CRP) = 0.65, erythrocyte sedimentation rate (ESR) = 0.59). The SNP MYOM2-rs2294066 was found to be significantly associated with responsiveness of etanercept treatment. A three-stage approach consisting of baseline IgG-Gal ratio, differential IgG-Gal ratio in 2 weeks, and rs2294066 genotype demonstrated the ability to precisely predict the response of anti-TNF therapy (100% for poor-responders, 98% for responders). CONCLUSIONS: Combination of different omics can more precisely to predict the response of TNF blocker and it is potential to be applied clinically in the future.

HLA class I and II alleles in susceptibility to ankylosing spondylitis.

OBJECTIVE: To examine associations of HLA class I and class II alleles with ankylosing spondylitis (AS) in three cohorts of patients of European, Asian and African ancestry. METHODS: HLA-A, HLA-B, HLA-C, HLA-DRB1, HLA-DQB1 and HLA-DPB1 alleles were genotyped in 1948 unrelated white and 67 African-American patients with AS from the Prospective Study of Outcomes in Ankylosing Spondylitis cohort, the North American Spondylitis Consortium and Australo-Anglo-American Spondyloarthritis Consortium, 990 white and 245 African-American Controls and HLA-B alleles in 442 Han Chinese patients with AS and 346 controls from Shanghai and Gansu, China. In addition to the case:control analyses, HLA-B*27-negative patients with AS were analysed separately, and logistic regression and 'relative predispositional effects' (RPE) analyses were carried out to control for the major effect of HLA-B*27 on disease susceptibility. RESULTS: Although numerous associations were seen between HLA alleles and AS in whites, among HLA-B*27-negative patients with AS , positive associations were seen with HLA-A*29, B*38, B*49, B*52, DRB1*11 and DPB1*03:01 and negative associations with HLA-B*07, HLA-B*57, HLA-DRB1*15:01, HLA-DQB1*02:01 and HLA-DQB1*06:02. Additional associations with HLA-B*14 and B*40 (B60) were observed via RPE analysis, which excludes the HLA-B*27 alleles. The increased frequency of HLA-B*40:01 and decreased frequency of HLA-B*07 was also seen in Han Chinese and African-Americans with AS. HLA-B*08 was decreased in whites with acute anterior uveitis. CONCLUSIONS: These data, analysing the largest number of patients with AS examined to date in three ethnic groups, confirm that other HLA class I and II alleles other than HLA-B*27 to be operative in AS predisposition.

Lupus Risk Variant Increases pSTAT1 Binding and Decreases ETS1 Expression.

Genetic variants at chromosomal region 11q23.3, near the gene ETS1, have been associated with systemic lupus erythematosus (SLE), or lupus, in independent cohorts of Asian ancestry. Several recent studies have implicated ETS1 as a critical driver of immune cell function and differentiation, and mice deficient in ETS1 develop an SLE-like autoimmunity. We performed a fine-mapping study of 14,551 subjects from multi-ancestral cohorts by starting with genotyped variants and imputing to all common variants spanning ETS1. By constructing genetic models via frequentist and Bayesian association methods, we identified 16 variants that are statistically likely to be causal. We functionally assessed each of these variants on the basis of their likelihood of affecting transcription factor binding, miRNA binding, or chromatin state. Of the four variants that we experimentally examined, only rs6590330 differentially binds lysate from B cells. Using mass spectrometry, we found more binding of the transcription factor signal transducer and activator of transcription 1 (STAT1) to DNA near the risk allele of rs6590330 than near the non-risk allele. Immunoblot analysis and chromatin immunoprecipitation of pSTAT1 in B cells heterozygous for rs6590330 confirmed that the risk allele increased binding to the active form of STAT1. Analysis with expression quantitative trait loci indicated that the risk allele of rs6590330 is associated with decreased ETS1 expression in Han Chinese, but not other ancestral cohorts. We propose a model in which the risk allele of rs6590330 is associated with decreased ETS1 expression and increases SLE risk by enhancing the binding of pSTAT1.

A multiple imputation method based on weighted quantile regression models for longitudinal censored biomarker data with missing values at early visits.

BACKGROUND: In patient-based studies, biomarker data are often subject to left censoring due to the detection limits, or to incomplete sample or data collection. In the context of longitudinal regression analysis, inappropriate handling of these issues could lead to biased parameter estimates. We developed a specific multiple imputation (MI) strategy based on weighted censored quantile regression (CQR) that not only accounts for censoring, but also missing data at early visits when longitudinal biomarker data are modeled as a covariate. METHODS: We assessed through simulation studies the performances of developed imputation approach by considering various scenarios of covariance structures of longitudinal data and levels of censoring. We also illustrated the application of the proposed method to the Prospective Study of Outcomes in Ankylosing spondylitis (AS) (PSOAS) data to address the issues of censored or missing C-reactive protein (CRP) level at early visits for a group of patients. RESULTS: Our findings from simulation studies indicated that the proposed method performs better than other MI methods by having a higher relative efficiency. We also found that our approach is not sensitive to the choice of covariance structure as compared to other methods that assume normality of biomarker data. The analysis results of PSOAS data from the imputed CRP levels based on our method suggested that higher CRP is significantly associated with radiographic damage, while those from other methods did not result in a significant association. CONCLUSION: The MI based on weighted CQR offers a more valid statistical approach to evaluate a biomarker of disease in the presence of both issues with censoring and missing data in early visits.

Immunochip analysis identifies multiple susceptibility loci for systemic sclerosis.

In this study, 1,833 systemic sclerosis (SSc) cases and 3,466 controls were genotyped with the Immunochip array. Classical alleles, amino acid residues, and SNPs across the human leukocyte antigen (HLA) region were imputed and tested. These analyses resulted in a model composed of six polymorphic amino acid positions and seven SNPs that explained the observed significant associations in the region. In addition, a replication step comprising 4,017 SSc cases and 5,935 controls was carried out for several selected non-HLA variants, reaching a total of 5,850 cases and 9,401 controls of European ancestry. Following this strategy, we identified and validated three SSc risk loci, including DNASE1L3 at 3p14, the SCHIP1-IL12A locus at 3q25, and ATG5 at 6q21, as well as a suggested association of the TREH-DDX6 locus at 11q23. The associations of several previously reported SSc risk loci were validated and further refined, and the observed peak of association in PXK was related to DNASE1L3. Our study has increased the number of known genetic associations with SSc, provided further insight into the pleiotropic effects of shared autoimmune risk factors, and highlighted the power of dense mapping for detecting previously overlooked susceptibility loci.

Two functional lupus-associated BLK promoter variants control cell-type- and developmental-stage-specific transcription.

Efforts to identify lupus-associated causal variants in the FAM167A/BLK locus on 8p21 are hampered by highly associated noncausal variants. In this report, we used a trans-population mapping and sequencing strategy to identify a common variant (rs922483) in the proximal BLK promoter and a tri-allelic variant (rs1382568) in the upstream alternative BLK promoter as putative causal variants for association with systemic lupus erythematosus. The risk allele (T) at rs922483 reduced proximal promoter activity and modulated alternative promoter usage. Allelic differences at rs1382568 resulted in altered promoter activity in B progenitor cell lines. Thus, our results demonstrated that both lupus-associated functional variants contribute to the autoimmune disease association by modulating transcription of BLK in B cells and thus potentially altering immune responses.

Proteome-wide analysis and CXCL4 as a biomarker in systemic sclerosis.

BACKGROUND: Plasmacytoid dendritic cells have been implicated in the pathogenesis of systemic sclerosis through mechanisms beyond the previously suggested production of type I interferon. METHODS: We isolated plasmacytoid dendritic cells from healthy persons and from patients with systemic sclerosis who had distinct clinical phenotypes. We then performed proteome-wide analysis and validated these observations in five large cohorts of patients with systemic sclerosis. Next, we compared the results with those in patients with systemic lupus erythematosus, ankylosing spondylitis, and hepatic fibrosis. We correlated plasma levels of CXCL4 protein with features of systemic sclerosis and studied the direct effects of CXCL4 in vitro and in vivo. RESULTS: Proteome-wide analysis and validation showed that CXCL4 is the predominant protein secreted by plasmacytoid dendritic cells in systemic sclerosis, both in circulation and in skin. The mean (±SD) level of CXCL4 in patients with systemic sclerosis was 25,624±2652 pg per milliliter, which was significantly higher than the level in controls (92.5±77.9 pg per milliliter) and than the level in patients with systemic lupus erythematosus (1346±1011 pg per milliliter), ankylosing spondylitis (1368±1162 pg per milliliter), or liver fibrosis (1668±1263 pg per milliliter). CXCL4 levels correlated with skin and lung fibrosis and with pulmonary arterial hypertension. Among chemokines, only CXCL4 predicted the risk and progression of systemic sclerosis. In vitro, CXCL4 down-regulated expression of transcription factor FLI1, induced markers of endothelial-cell activation, and potentiated responses of toll-like receptors. In vivo, CXCL4 induced the influx of inflammatory cells and skin transcriptome changes, as in systemic sclerosis. CONCLUSIONS: Levels of CXCL4 were elevated in patients with systemic sclerosis and correlated with the presence and progression of complications, such as lung fibrosis and pulmonary arterial hypertension. (Funded by the Dutch Arthritis Association and others.).