Our faculty research broadens the knowledge of rheumatic disease. 

Cell-Free DNA as a Biomarker in Autoimmune Rheumatic Diseases.

Duvvuri B, Lood C.

Endogenous DNA is primarily found intracellularly in nuclei and mitochondria. However, extracellular, cell-free (cf) DNA, has been observed in several pathological conditions, including autoimmune diseases, prompting the interest of developing cfDNA as a potential biomarker. There is an upsurge in studies considering cfDNA to stratify patients, monitor the treatment response and predict disease progression, thus evaluating the prognostic potential of cfDNA for autoimmune diseases. Since the discovery of elevated cfDNA levels in lupus patients in the 1960s, cfDNA research in autoimmune diseases has mainly focused on the overall quantification of cfDNA and the association with disease activity. However, with recent technological advancements, including genomic and methylomic sequencing, qualitative changes in cfDNA are being explored in autoimmune diseases, similar to the ones used in molecular profiling of cfDNA in cancer patients. Further, the intracellular origin, e.g., if derived from mitochondrial or nuclear source, as well as the complexing with carrier molecules, including LL-37 and HMGB1, has emerged as important factors to consider when analyzing the quality and inflammatory potential of cfDNA. The clinical relevance of cfDNA in autoimmune rheumatic diseases is strengthened by mechanistic insights into the biological processes that result in an enhanced release of DNA into the circulation during autoimmune and inflammatory conditions. Prior work have established an important role of accelerated apoptosis and impaired clearance in leakage of nucleic acids into the extracellular environment. Findings from more recent studies, including our own investigations, have demonstrated that NETosis, a neutrophil cell death process, can result in a selective extrusion of inflammatory mitochondrial DNA; a process which is enhanced in patients with lupus and rheumatoid arthritis. In this review, we will summarize the evolution of cfDNA, both nuclear and mitochondrial DNA, as biomarkers for autoimmune rheumatic diseases and discuss limitations, challenges and implications to establish cfDNA as a biomarker for clinical use. This review will also highlight recent advancements in mechanistic studies demonstrating mitochondrial DNA as a central component of cfDNA in autoimmune rheumatic diseases.

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Low plasma concentrations of apolipoprotein M are associated with disease activity and endothelial dysfunction in systemic lupus erythematosus.

Tydén H, Lood C, Jönsen A, Gullstrand B, Kahn R, Linge P, Kumaraswamy SB, Dahlbäck B, Bengtsson AA.

BACKGROUND:

Apolipoprotein M (apoM) is a 25-kDa apolipoprotein present in 5% of high-density lipoprotein (HDL) particles. It is suggested to be anti-atherogenic and to play a key role in sustaining endothelial barrier integrity. SLE patients have increased cardiovascular disease risk, and we aimed to investigate if apoM levels reflect endothelial function in SLE. Since apoM plasma levels decrease during inflammatory conditions, our aim was also to determine the impact of SLE disease activity on apoM plasma levels.

METHODS:

Plasma concentrations of apoM were measured by ELISA in two patient groups with systemic lupus erythematosus (SLE) and in 79 healthy control individuals. In patient group I (n = 84), evaluation time points were selected with the objective to include a wide range of clinical and laboratory variables reflecting disease activity which was measured as SLEDAI. In patient group II consisting of 140 consecutive patients, endothelial function was measured by a finger plethysmograph. A low Reactive Hyperemia Index (RHI) value indicates endothelial dysfunction.

RESULTS:

SLE patients had decreased levels of apoM compared to healthy controls (p < 0.01), with apoM levels correlating inversely with SLEDAI (r = - 0.31, p < 0.01) as well as with levels of CRP (r = - 0.26, p = 0.02) and positively with levels of C3 (r = 0.29, p < 0.01). ApoM levels were particularly low in patients with active disease from the kidney and skin and in patients with leukopenia or positive anti-dsDNA antibody test (p < 0.05). ApoM levels correlated with RHI values in young SLE patients (r = 0.32, p = 0.01), consistent with the important role of apoM in regulating endothelial integrity.

CONCLUSIONS:

ApoM levels may be regulated by SLE-related inflammatory processes and could be a marker of disease activity and endothelial dysfunction, in particular in young SLE patients. Further studies are needed to investigate the predictive value of apoM in the development of a cardiovascular disease.

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Determinants of Positive Temporal Artery Biopsies in the Veterans Health Administration National Database Cohort.

Chung SH, Morcos MB, Ng B.

OBJECTIVE:

This study sought to determine the effect of temporal artery biopsy (TAB) post-fixation length, laterality, age, and prior prednisone exposure on TAB positivity utilizing the Veterans Health Administration national database.

METHODS:

Subjects with procedure code for TAB between 1999-2017 were queried and pathology reports were reviewed manually. Demographic, laboratory, and prescription data were extracted. Multivariate analyses and logistic regression were run using Stata 13.0.

RESULTS:

A total of 3,057 pathology reports were reviewed; 306 (10%) biopsies were designated positive. The likelihood of a positive TAB significantly correlated with TAB post-fixation length greater than 3.0 cm (OR 1.58, CI 1.06-2.36, p<0.05) as well as with bilateral biopsy in one sitting (OR 1.83, CI 1.29-2.59, p<0.01). Positive TAB also significantly correlated with age greater than 71 years. Prednisone administration up to and beyond 42 days prior to TAB did not influence TAB result.

CONCLUSION:

This retrospective study examines predictors of positive TAB utilizing national data on United States veterans over the span of eighteen years. The results suggest consideration of pursuing initial bilateral TAB or achieving a TAB post-fixation length of at least 3 cm to improve yield. The results also agree with prior studies showing that pre-TAB steroid exposure does not appear to affect yield, even up to and beyond 42 days prior to biopsy. This article is protected by copyright. All rights reserved.

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Sources of Pathogenic Nucleic Acids in Systemic Lupus Erythematosus.

Mustelin T, Lood C, Giltiay NV.

A hallmark of systemic lupus erythematosus (SLE), and several related autoimmune diseases, is the presence of autoantibodies against nucleic acids and nucleic acid-binding proteins, as well as elevated type I interferons (IFNs), which appear to be instrumental in disease pathogenesis. Here we discuss the sources and proposed mechanisms by which a range of cellular RNA and DNA species can become pathogenic and trigger the nucleic acid sensors that drive type I interferon production. Potentially SLE-promoting DNA may originate from pieces of chromatin, from mitochondria, or from reverse-transcribed cellular RNA, while pathogenic RNA may arise from mis-localized, mis-processed, ancient retroviral, or transposable element-derived transcripts. These nucleic acids may leak out from dying cells to be internalized and reacted to by immune cells or they may be generated and remain to be sensed intracellularly in immune or non-immune cells. The presence of aberrant DNA or RNA is normally counteracted by effective counter-mechanisms, the loss of which result in a serious type I IFN-driven disease called Aicardi-Goutières Syndrome. However, in SLE it remains unclear which mechanisms are most critical in precipitating disease: aberrant RNA or DNA, overly sensitive sensor mechanisms, or faulty counter-acting defenses. We propose that the clinical heterogeneity of SLE may be reflected, in part, by heterogeneity in which pathogenic nucleic acid molecules are present and which sensors and pathways they trigger in individual patients. Elucidation of these events may result in the recognition of distinct "endotypes" of SLE, each with its distinct therapeutic choices.

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Pharmacist-managed titration of urate-lowering therapy to streamline gout management.

Huang IJ, Liew JW, Morcos MB, Zuo S, Crawford C, Bays AM.

The treat-to-target approach for serum uric acid is the recommended model in gout management according to the 2012 American College of Rheumatology (ACR) guidelines. Adherence to urate-lowering therapy (ULT) can be difficult for patients due to barriers, which include medication burden, financial hardship, and lack of medical literacy. Our aim was to create a pharmacist-managed referral for the titration of ULT to target serum uric acid (sUA) levels in a complex patient population. We utilized a clinical database to query patients seen at a rheumatology clinic over a 12-month period with an ICD-10 diagnosis for gout. The referral criteria were indications for ULT per the 2012 ACR guidelines. Rheumatology providers, consisting of attendings, fellows, and a physician assistant, were asked to refer the identified patients to the pharmacist-managed titration program. The intervention group consisted of 19 referred patients and the control group consisted of 28 non-referred patients. The baseline sUA (median (IQR)) at the time of referral was 8.8 (2) mg/dL for the intervention group and 7.6 (2.8) mg/dL for the control group (p = 0.2). At the end of the study period, the sUA was 6.1 (1.4) mg/dL for the intervention group and 6.8 (3.2) mg/dL for the control group (p = 0.08). At the end of the study period, 6 of 19 (32%) intervention group and 7 of 28 (25%) control group were at goal (p = 0.3). A newly instituted pharmacist-managed titration program was able to achieve lower average sUA levels in referred patients compared to demographically similar individuals who received standard gout management.

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Corrigendum: Increased Binding of Specificity Protein 1 to the IL21R Promoter in B Cells Results in Enhanced B Cell Responses in Rheumatoid Arthritis.

Dam EM, Maier AC, Hocking AM, Carlin J, Ng B, Buckner JH.

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Demographic, Clinical, and Immunologic Correlates among a Cohort of 50 Cocaine Users Demonstrating Antineutrophil Cytoplasmic Antibodies.

Morcos MB, Lood C, Hughes GC.

OBJECTIVE:

Cocaine/levamisole-associated autoimmunity syndrome (CLAAS) is a poorly understood form of drug-induced autoimmunity. Our goals were to better characterize the spectrum of clinical and immunologic features of CLAAS, to identify demographic risk factors, and to generate new hypotheses regarding pathogenesis.

METHODS:

CLAAS subjects were identified between 2001 and 2015 at the University of Washington Medical Center, Harborview Medical Center, and affiliated clinics in Seattle, Washington, USA. Demographic, clinical, and immunologic variables were collected and correlated using contingency and logistic regression analyses. We used similar analyses to compare CLAAS subjects with all individuals exhibiting antineutrophil cytoplasmic antibodies (ANCA+) or cocaine use (Cocaine+) in an associated deidentified clinical data repository.

RESULTS:

We identified 50 CLAAS subjects. Compared to all Cocaine+ individuals (n = 2740), CLAAS subjects were more likely to be female and less likely to self-identify as black/African American. CLAAS subjects showed several ANCA patterns, including anti-MPO (myeloperoxidase)/anti-PR3 (proteinase 3) dual reactivity, a finding that appears to be specific to CLAAS. Hematologic, renal, and skin abnormalities were most frequently reported, including neutropenia and skin purpura. Finally, we observed strong, independent associations between the cytoplasmic ANCA (C-ANCA) pattern and mortality.

CONCLUSION:

We identify sex and race as important risk modifiers in the developing CLAAS among cocaine users. The development of C-ANCA was associated with increased mortality. Moreover, we confirm the enriched presence of anti-MPO/anti-PR3 dual reactivity in CLAAS, further supporting the diagnostic utility of this feature.

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Sources of Pathogenic Nucleic Acids in Systemic Lupus Erythematosus.

Mustelin T, Lood C, Giltiay NV.

A hallmark of systemic lupus erythematosus (SLE), and several related autoimmune diseases, is the presence of autoantibodies against nucleic acids and nucleic acid-binding proteins, as well as elevated type I interferons (IFNs), which appear to be instrumental in disease pathogenesis. Here we discuss the sources and proposed mechanisms by which a range of cellular RNA and DNA species can become pathogenic and trigger the nucleic acid sensors that drive type I interferon production. Potentially SLE-promoting DNA may originate from pieces of chromatin, from mitochondria, or from reverse-transcribed cellular RNA, while pathogenic RNA may arise from mis-localized, mis-processed, ancient retroviral, or transposable element-derived transcripts. These nucleic acids may leak out from dying cells to be internalized and reacted to by immune cells or they may be generated and remain to be sensed intracellularly in immune or non-immune cells. The presence of aberrant DNA or RNA is normally counteracted by effective counter-mechanisms, the loss of which result in a serious type I IFN-driven disease called Aicardi-Goutières Syndrome. However, in SLE it remains unclear which mechanisms are most critical in precipitating disease: aberrant RNA or DNA, overly sensitive sensor mechanisms, or faulty counter-acting defenses. We propose that the clinical heterogeneity of SLE may be reflected, in part, by heterogeneity in which pathogenic nucleic acid molecules are present and which sensors and pathways they trigger in individual patients. Elucidation of these events may result in the recognition of distinct "endotypes" of SLE, each with its distinct therapeutic choices.

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Aspirin meets cGAS.

Elkon KB.

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The Contribution of PTPN22 to Rheumatic Disease.

Mustelin T, Bottini N, Stanford SM.

One of the unresolved questions in modern medicine is why certain individuals develop a disorder such as rheumatoid arthritis (RA) or lupus, while others do not. Contemporary science indicates that genetics is partly responsible for disease development, while environmental and stochastic factors also play a role. Among the many genes that increase the risk of autoimmune conditions, the risk allele encoding the W620 variant of protein tyrosine phosphatase N22 (PTPN22) is shared between multiple rheumatic diseases, suggesting that it plays a fundamental role in the development of immune dysfunction. Herein, we discuss how the presence of the PTPN22 risk allele may shape the signs and symptoms of these diseases. Besides the emerging clarity regarding how PTPN22 tunes T and B cell antigen receptor signaling, we discuss recent discoveries of important functions of PTPN22 in myeloid cell lineages. Taken together, these new insights reveal important clues to the molecular mechanisms of prevalent diseases like RA and lupus and may open new avenues for the development of personalized therapies that spare the normal function of the immune system.

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Interactions between cadherin-11 and platelet-derived growth factor receptor-alpha signaling link cell adhesion and proliferation.

Madarampalli BWatts GFMPanipinto PMNguygen HNBrenner MBNoss EH.

Cadherins are homophilic cell-to-cell adhesion molecules that help cells respond to environmental changes. Newly formed cadherin junctions are associated with increased cell phosphorylation, but the pathways driving this signaling response are largely unknown. Since cadherins have no intrinsic signaling activity, this phosphorylation must occur through interactions with other signaling molecules. We previously reported that cadherin-11 engagement activates joint synovial fibroblasts, promoting inflammatory and degradative pathways important in rheumatoid arthritis (RA) pathogenesis. Our objective in this study was to discover interacting partners that mediate cadherin-11 signaling. Protein array screening showed that cadherin-11 extracellular binding domains linked to an Fc domain (cad11Fc) induced platelet-derived growth factor (PDGFR)-α phosphorylation in synovial fibroblasts and glioblastoma cells. PDGFRs are growth factor receptor tyrosine kinases that promote cell proliferation, survival, and migration in mesodermally derived cells. Increased PDGFR activity is implicated in RA pathology and associates with poor prognosis in several cancers, including sarcoma and glioblastoma. PDGFRα activation by cadherin-11 signaling promoted fibroblast proliferation, a signaling pathway independent from cadherin-11-stimulated IL-6 or matrix metalloproteinase (MMP)-3 release. PDGFRα phosphorylation mediated most of the cad11Fc-induced phosphatidyl-3-kinase (PI3K)/Akt activation, but only part of the mitogen-activated protein kinase (MAPK) response. PDGFRα-dependent signaling did not require cell cadherin-11 expression. Rather, cad11Fc immunoprecipitated PDGFRα, indicating a direct interaction between cadherin-11 and PDGFRα extracellular domains. This study is the first to report an interaction between cadherin-11 and PDGFRα and adds to our growing understanding that cadherin-growth factor receptor interactions help balance the interplay between tissue growth and adhesion.

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Chronic TLR7 and TLR9 signaling drives anemia via differentiation of specialized hemophagocytes.

Akilesh HMBuechler MBDuggan JMHahn WOMatta BSun XGessay GWhalen EMason MPresnell SRElkon KBLacy-Hulbert ABarnes BJPepper MHamerman JA.

Cytopenias are an important clinical problem associated with inflammatory disease and infection. We show that specialized phagocytes that internalize red blood cells develop in Toll-like receptor 7 (TLR7)-driven inflammation. TLR7 signaling caused the development of inflammatory hemophagocytes (iHPCs), which resemble splenic red pulp macrophages but are a distinct population derived from Ly6Chi monocytes. iHPCs were responsible for anemia and thrombocytopenia in TLR7-overexpressing mice, which have a macrophage activation syndrome (MAS)-like disease. Interferon regulatory factor 5 (IRF5), associated with MAS, participated in TLR7-driven iHPC differentiation. We also found iHPCs during experimental malarial anemia, in which they required endosomal TLR and MyD88 signaling for differentiation. Our findings uncover a mechanism by which TLR7 and TLR9 specify monocyte fate and identify a specialized population of phagocytes responsible for anemia and thrombocytopenia associated with inflammation and infection.

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Autoimmunity to bactericidal/permeability-increasing protein in bronchiectasis exhibits a requirement for Pseudomonas aeruginosa IgG response.

Skopelja-Gardner STheprungsirikul JMeagher REBeliveau CMBradley KEAvery MHenkle ESiegel SGifford AHWinthrop KLRigby WFC.

In non-cystic fibrosis bronchiectasis (BE), idiopathic, genetic, and environmental factors alter the airway landscape and immune responses, rendering the patients susceptible to infection, with the gram-negative bacterium (GNB) Pseudomonas aeruginosa as a major contributor to mortality. The high prevalence of P. aeruginosa in BE patients cannot be explained by a single genetic or environmental influence, and immunologic permissiveness of BE airways to this colonization remains unexplained. We ask two critical questions regarding the immunologic interactions that shape the BE airway permissiveness to P. aeruginosa infection in two BE cohorts from the United States: one from Dartmouth Hitchcock Medical Center (DHMC) in Lebanon, New Hampshire (n=16): i) Do BE patients develop autoimmunity to BPI? and ii) What is the relationship between autoreactivity to BPI and chronic infection by P. aeruginosa? BPI autoreactivity was identified at nearly identical frequencies in the DHMC (56%) and OHSU (52%) cohorts. Autoreactivity to BPI in both BE cohorts was strongly associated with the existence of an antibody response to P. aeruginosa and was absent in patients colonized with Nontuberculous Mycobacterium. Retrospective longitudinal study of sera from 34 BE patients demonstrated that the autoimmune response to BPI follows the same temporal pattern as the humoral response to P. aeruginosa. Our study suggests that rather than viewing anti-BPI as a highly linked epiphenomenon of P. aeruginosa infection, these autoantibodies may play a causal role in the perpetuation of infection. In this model, strategies which eliminate anti-BPI reactivity may enhance clearance of P. aeruginosa leading to improved airway function and clinical outcomes. 

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Inhibition of Cyclic GMP-AMP Synthase Using a Novel Antimalarial Drug Derivative in Trex1-Deficient Mice.

An JWoodward JJLai WMinie MSun XTanaka LSnyder JMSasaki TElkon KB.

OBJECTIVE: Type I interferon (IFN) is strongly implicated in the pathogenesis of systemic lupus erythematosus (SLE) as well as rare monogenic interferonopathies such as Aicardi-Goutières syndrome (AGS), a disease attributed to mutations in the DNA exonuclease TREX1. The DNA-activated type I IFN pathway cyclic GMP-AMP (cGAMP) synthase (cGAS) is linked to subsets of AGS and lupus. This study was undertaken to identify inhibitors of the DNA-cGAS interaction, and to test the lead candidate drug, X6, in a mouse model of AGS.

METHODS: Trex1-/- mice were treated orally from birth with either X6 or hydroxychloroquine (HCQ) for 8 weeks. Expression of IFN-stimulated genes (ISGs) was quantified by quantitative polymerase chain reaction. Multiple reaction monitoring by ultra-performance liquid chromatography coupled with tandem mass spectrometry was used to quantify the production of cGAMP and X6 drugconcentrations in the serum and heart tissue of Trex1-/- mice.

RESULTS: On the basis of the efficacy-to-toxicity ratio established in vitro, drug X6 was selected as the lead candidate for treatment of Trex1-/- mice. X6 was significantly more effective than HCQ in attenuating ISG expression in mouse spleens (P < 0.01 for Isg15 and Isg20) and hearts (P < 0.05 for Isg15, Mx1, and Ifnb, and P < 0.01 for Cxcl10), and in reducing the production of cGAMP in mouse heart tissue (P < 0.05), thus demonstrating target engagement by the X6 compound. Of note, X6 was also more effective than HCQ in reducing ISG expression in vitro (P < 0.05 for IFI27 and MX1, and P < 0.01 for IFI44L and PKR) in human peripheral blood mononuclear cells from patients with SLE.

CONCLUSION: This study demonstrates that X6 is superior to HCQ for the treatment of an experimental autoimmune myocarditis mediated in vivo by the cGAS/stimulator of IFN genes (cGAS/STING) pathway. The findings suggest that drug X6 could be developed as a novel treatment for AGS and/or lupus to inhibit activation of the cGAS/STING pathway.

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Neutrophil extracellular traps (NETs) are increased in the alveolar spaces of patients with ventilator-associated pneumonia.

Mikacenic CMoore RDmyterko VWest TEAltemeier WALiles WCLood C.

BACKGROUND: Neutrophils release neutrophil extracellular traps (NETs) in response to invading pathogens. Although NETs play an important role in host defense against microbial pathogens, they have also been shown to play a contributing mechanistic role in pathologic inflammation in the absence of infection. Although a role for NETs in bacterial pneumonia and acute respiratory distress syndrome (ARDS) is emerging, a comprehensive evaluation of NETs in the alveolar space of critically ill patients has yet to be reported. In this study, we evaluated whether markers of NET formation in mechanically ventilated patients are associated with ventilator-associated pneumonia (VAP).

METHODS: We collected bronchoalveolar lavage fluid from 100 critically ill patients undergoing bronchoscopy for clinically suspected VAP. Subjects were categorized by the absence or presence of VAP and further stratified by ARDS status. NETs (myeloperoxidase (MPO)-DNA complexes) and the NET-associated markers peroxidase activity and cell-free DNA were analyzed by enzyme-linked immunosorbent assay and colorimetric assays, respectively. Quantitative polymerase chain reaction of nuclear and mitochondrial DNA was used to determine the origin of the extruded DNA. Interleukin (IL)-8 and calprotectin were assayed as measures of alveolarinflammation and neutrophil activation. Correlations between NETs and markers of neutrophil activation were determined using Spearman's correlation. We tested for associations with VAP and bacterial burden by logistic and linear regression, respectively, using log10-transformed NETs.

RESULTS: MPO-DNA concentrations were highly correlated with other measures of NET formation in the alveolar space, including cell-free DNA and peroxidase activity (r = 0.95 and r = 0.87, p < 0.0001, respectively). Alveolar concentrations of MPO-DNA were higher in subjects with VAP and ARDS compared with those with ARDS alone (p < 0.0001), and higher MPO-DNA was associated with increasedodds of VAP (odds ratio 3.03, p < 0.0001). In addition, NET concentrations were associated with bacterial burden (p < 0.0001) and local alveolar inflammation as measured by IL-8 (r = 0.89, p < 0.0001).

CONCLUSIONS: Alveolar NETs measured by MPO-DNA complex are associated with VAP, and markers of NETosis are associated with local inflammation and bacterial burden in the lung. These results suggest that NETs contribute to inflammatory responses involved in the pathogenesis of VAP.

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Association of High Anti-Cyclic Citrullinated Peptide Seropositivity and Lean Mass Index With Low Bone Mineral Density in Rheumatoid Arthritis.

Wysham KDShoback DMImboden JB JrKatz PP.

OBJECTIVE: Osteoporotic fractures are associated with high morbidity and mortality. Persons with rheumatoid arthritis (RA) have twice the risk of osteoporosis-related fracture than age-matched controls, the causes for which remain unknown. We investigated contributions of RA characteristics, medication use, and body composition to low bone mineral density (BMD) in patients with RA.

METHODS: Data were from the Arthritis, Body Composition, and Disability Study (n = 138; 82 women, 56 men). Demographic, clinical, laboratory, and functional variables were collected at study visits. Body composition (fat, lean muscle, and BMD) was measured by dual x-ray absorptiometry. Linear regression analyses evaluated the association between predictors and femoral neck BMD.

RESULTS: Average disease duration was 19 years, 70% of patients were rheumatoid factor positive, and 55% were high-positive anti-cycliccitrullinated peptide (anti-CCP). Age and high anti-CCP positivity were negatively associated with BMD after controlling for other variables (β = -0.003 and -0.055, respectively, P < 0.05). Appendicular lean mass index (ALMI) was positively associated with BMD (β = 0.053, P < 0.0001). In high anti-CCP positivity participants, increasing anti-CCP levels were associated with a negative linear trend in BMD (β = -0.011, P = 0.026).

CONCLUSION: High anti-CCP positivity and ALMI were strongly associated with BMD in patients with RA. The linear relationship of anti-CCP levels with lower BMD supports the hypothesis that processes specific to RA negatively impact BMD. In contrast, ALMI was positively associated with BMD, emphasizing the importance of this potentially modifiable risk factor. Our findings highlight the complicated interplay of RA disease-specific and functional factors and their impact on bone mass.

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Evidence for a direct link between PAD4-mediated citrullination and the oxidative burst in humanneutrophils.

Zhou YAn LLChaerkady RMittereder NClarke LCohen TSChen BHess SSims GPMustelin T.

Neutrophils are critical for the defense against pathogens, in part through the extrusion of extracellular DNA traps, phagocytosis, and the production of reactive oxygen species. Neutrophils may also play an important role in the pathogenesis of rheumatoid arthritis (RA) through the activation of protein arginine deiminases (PADs) that citrullinate proteins that subsequently act as autoantigens. We report that PAD4 is physically associated with the cytosolic subunits of the oxidative burst machinery, p47phox (also known as neutrophil cytosol factor 1, NCF1) and p67phox (NCF2). Activation of PAD4 by membranolytic insults that result in high levels of intracellular calcium (higher than physiological neutrophil activation) leads to rapid citrullination of p47phox/NCF1 and p67phox/NCF2, as well as their dissociation from PAD4. This dissociation prevents the assembly of an active NADPH oxidase complex and an oxidative burst in neutrophils stimulated by phorbol-ester or immune complexes. In further support of a substrate-to-inactive enzyme interaction, small-molecule PAD inhibitors also disrupt the PAD4-NCF complex and reduce oxidase activation and phagocytic killing of Staphylococcus aureus. This novel role of PAD4 in the regulation of neutrophil physiology suggests that targeting PAD4 with active site inhibitors for the treatment of RA may have a broader impact on neutrophil biology than just inhibition of citrullination.

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Calpain drives pyroptotic vimentin cleavage, intermediate filament loss, and cell rupture that mediates immunostimulation.

Davis MAFairgrieve MRDen Hartigh AYakovenko ODuvvuri BLood C, Thomas WEFink SLGale M Jr.

Pyroptosis is an inflammatory form of programmed cell death following cellular damage or infection. It is a lytic process driven by gasdermin D-mediated cellular permeabilization and presumed osmotic forces thought to induce swelling and rupture. We found that pyroptotic cells do not spontaneously rupture in culture but lose mechanical resilience. As a result, cells were susceptible to rupture by extrinsic forces, such as shear stress or compression. Cell analyses revealed that all major cytoskeleton components were disrupted during pyroptosis and that sensitivity to rupture was calpain-dependent and linked with cleavage of vimentin and loss of intermediatefilaments. Moreover, while release of lactate dehydrogenase (LDH), HMGB1, and IL-1β occurred without rupture, rupture was required for release of large inflammatory stimuli-ASC specks, mitochondria, nuclei, and bacteria. Importantly, supernatants from ruptured cells were more immunostimulatory than those from nonruptured cells. These observations reveal undiscovered cellular events occurring during pyroptosis, define the mechanisms driving pyroptotic rupture, and highlight the immunologic importance of this event.

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