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Enhanced neutrophil extracellular trap generation in rheumatoid arthritis:
analysis of underlying signal transduction pathways and potential diagnostic
Arthritis Research & Therapy 2014, 16:R122doi:10.1186/ar4579
Chanchal Sur Chowdhury (firstname.lastname@example.org)
Stavros Giaglis (email@example.com)
Ulrich A Walker (firstname.lastname@example.org)
Andreas Buser (email@example.com)
Sinuhe Hahn (firstname.lastname@example.org)
Paul Hasler (email@example.com)
12 July 2013
21 May 2014
13 June 2014
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Enhanced neutrophil extracellular trap generation
in rheumatoid arthritis: analysis of underlying
signal transduction pathways and potential
Chanchal Sur Chowdhury1,†
Ulrich A Walker3
* Corresponding author
* Corresponding author
1 Department of Biomedicine, University Hospital Basel, Hebelstrasse 20, 4032
2 Department of Rheumatology, Kantonsspital Aarau, Tellstrasse, 5001 Aarau,
3 Department of Rheumatology, University Hospital Basel, Basel, Switzerland
4 Division of Haematology, Department of Internal Medicine, University Hospital
Basel, Blood Transfusion Centre, Swiss Red Cross, Basel, Switzerland
† Equal contributors.
Neutrophil extracellular traps (NETs) have recently been implicated in a number of
autoimmune conditions, including rheumatoid arthritis (RA). We examined the underlying
signalling pathways triggering enhanced NETosis in RA and ascertained whether the
products of NETosis had diagnostic implications or usefulness.
Neutrophils were isolated from RA patients with active disease and controls. Spontaneous
NET formation from RA and control neutrophils was assessed in vitro by microscopy and
enzyme-linked immune assay (ELISA) for NETosis-derived products. The analysis of the
signal transduction cascade included reactive oxygen species (ROS) production,
myeloperoxidase (MPO), neutrophil elastase (NE), peptidyl arginine deiminase 4 (PAD4),
and citrullinated histone 3 (citH3). NET formation was studied in response to serum and
synovial fluid and immunoglobulin G (IgG) depleted and reconstituted serum. Serum was
analysed for NETosis-derived products, for which receiver operator characteristic (ROC)
curves were calculated.
Neutrophils from RA cases exhibited increased spontaneous NET formation in vitro,
associated with elevated ROS production, enhanced NE and MPO expression, nuclear
translocation of PAD4, PAD4 mediated citrullination of H3 and altered nuclear morphology.
NET formation in both anti-citrullinated peptide antibody (ACPA) positive and negative RA
was abolished by IgG depletion, but restored only with ACPA positive IgG. NETosis derived
products in RA serum demonstrated diagnostic potential, the ROC area under the curve for
cell-free nucleosomes being > 97%, with a sensitivity of 91% and a specificity of 92%. No
significant difference was observed between ACPA positive and negative cases.
Signalling elements associated with the extrusion of NETs are significantly enhanced to
promote NETosis in RA compared to healthy controls. NETosis depended on the presence of
ACPA in ACPA positive RA serum. The quantitation of NETosis derived products such as
cell-free nucleosomes in serum may be a useful complementary tool to discriminate between
healthy controls and RA cases.
A novel feature of polymorphonuclear granulocyte (PMN) biology is their ability to generate
neutrophil extracellular traps (NETs)  via a distinct process of cell death termed NETosis
. NETs consist of extruded chromosomal DNA decorated with granular components that
include antimicrobial peptides and proteases. The molecular pathways leading to NETosis
encompass calcium mobilization, generation of reactive oxygen species (ROS), nuclear
delobulation involving the enzymatic activities of myeloperoxidase (MPO) and neutrophil
elastase (NE), and chromatin modification via the citrullination of histones by peptidyl
arginine deiminase 4 (PAD4) [2-6].
A number of studies have implicated NETs in the etiology of auto-inflammatory or
autoimmune conditions such as preeclampsia, Felty’s syndrome, systemic lupus
erythematosus (SLE), multiple sclerosis, and, most recently, rheumatoid arthritis (RA) [7-13].
In the context of RA these findings are especially interesting, as NETs have been proposed to
contribute to the generation of anti-citrullinated protein antibody (ACPA) auto-antigens, and
may also be a target for autoantibodies [13,14]. PMNs isolated from RA patients showed an
increased propensity to undergo spontaneous and LPS-induced NETosis, which was in part
mediated by TNF and IL-17 and could be inhibited by blocking NADPH oxidase or PAD4.
While the citrullinated autoantigens vimentin and α-enolase were expressed on NETs from
RA PMNs, antibodies to the former were able to induce NET formation by healthy control
As we had previously detected significantly increased concentrations of cell-free DNA in the
sera of RA patients compared with healthy controls, we were intrigued whether the
provenience of this material involved NETosis . The premise for the current investigation
was that a link between circulating cell-free DNA levels and NETs has previously been made
in a number of conditions including preeclampsia, sepsis, cancer, thrombosis or even storage
of blood transfusion products [16-19].
In view of these findings and reports on the complex involvement of neutrophil NETs in
autoimmunity, we sought to investigate the NETotic response of PMN in RA, with particular
regard to the underlying signal transduction cascade, and whether the products of overt
NETosis could be diagnostically useful.
Materials and methods
All patients fulfilled the American College of Rheumatology classification criteria for RA, or
for systemic lupus erythematosus, respectively. Healthy volunteers, matched for gender and
age, were recruited at the hospitals or at the Blood Bank of the Swiss Red Cross, Basel.
Inclusion criteria for healthy controls were fair general condition, age ≥ 28 and ≤ 70 years
and for blood donors fulfilling national criteria for blood donation. Exclusion criteria were
current or previous systemic autoimmune disease, asthma and reconvalescence after major
illness, surgery, current medication with corticosteroids, immunosuppressive agents and
malignant neoplasia or chemotherapy within 5 years before recruitment for the study. RA
cases had a DAS ≥ 2.6, were from age ≥ 27 to ≤ 70 years and had no other systemic
autoimmune disease, including ankylosing spondylitis and psoriatic arthritis. Exclusion
criteria were corticosteroids ≥ 40 mg equivalent of prednisone daily and those mentioned
above for healthy controls. Informed, written consent was obtained from all subjects in the
study, which was approved by the Cantonal Ethical Review Boards of Aargau-Solothurn and
Preparation of plasma and serum
Plasma and serum was collected and processed as described previously . Samples were
studied immediately or stored at −80°C until analysis.
PMNs were isolated by Dextran-Ficoll density centrifugation . Cell viability was 96–98%,
with a purity of > 95% PMNs. Neutrophils seeded in 24-well plates were allowed to settle for
1 hour at 37°C under 5% CO2 prior to further experimentation.
Cell free DNA isolation and quantification
Cell free DNA extracted from 850 µl plasma or serum using the QIAamp Circulating Nucleic
Acid Kit (Qiagen) was quantified by TaqMan® real-time PCR (StepOne™ Plus Real-Time
PCR System, Applied Biosystems) specific for the glyceraldehyde-3-phosphate
dehydrogenase (GAPDH) gene .
Detection of neutrophil elastase (NE), myeloperoxidase (MPO) and cell-free
The concentrations of neutrophil elastase (NE) and myeloperoxidase (MPO) were measured
by sandwich ELISA (Elastase/a1-PI Complex ELISA Kit, Calbiochem) and the human MPO
ELISA Kit, Hycult Biotech, respectively. Nucleosomes were measured using the Human Cell
Death Detection ELISAPLUS (Roche Diagnostics). Cell culture supernatants were incubated
with DNaseI (10 U for 5 min) (Roche Diagnostics) prior to analysis .
MPO/DNA complex detection
MPO is present on extruded NETs. To detect such structures, NET associated MPO/DNA
complexes were quantified utilizing a modified capture ELISA . In brief, NET associated
MPO in serum or culture supernatant was captured using the coated 96 well plate of the
human MPO ELISA Kit, (Hycult Biotech), following which the NET associated DNA
backbone was detected using the detection antibody of the Human Cell Death Detection
ELISAPLUS (Roche Diagnostics).
PAD4/DNA complex detection
To detect the presence of PAD4 on extruded NETs in culture supernatants following
spontaneous NETosis, cell-free PAD4/DNA complexes were quantified utilizing a modified
capture ELISA, akin to that described for MPO above. In brief, cell-free PAD4 was captured
using the coated 96 well plate of a commercial human PAD4 ELISA (USCN Life Science
Inc) and associated DNA was detected using Human Cell Death Detection ELISAPLUS kit
ROS generation analysis
ROS was measured using a 2′, 7′-dichloro dihydro fluorescein diacetate (DCFH-DA) assay
. 5 × 105 cells in a final volume of 500 µl were incubated for 30 min with 25 µM DCFH-
DA. Fluorescence was measured by flow cytometry (FACSCalibur; BD Biosciences).
Fluorescence and scanning electron microscopy
5 × 104 cells isolated PMN seeded on poly-L-lysine coated coverslips (BD Biosciences) were
stimulated with phorbol-12-myristate-13-acetate (PMA) for 90 minutes and dehydrated with
a graded ethanol series (30%, 50%, 70%, 100%) , coated with 2 nm platinum and analyzed
with a Nova NanoSEM 230 scanning electron microscope (FEI). PMNs were incubated for
10 min with 5 µM Sytox Green dye (Invitrogen Life Technologies) for assessment of NETs
with an Axiovert fluorescence microscope (Carl Zeiss) coupled to a Zeiss AxioCam colour
CCD camera (Carl Zeiss) [8,23].
Immunohistochemical staining and quantification of NETs
5 × 104 isolated PMNs were seeded on poly-L-lysine-coated glass coverslips (BD
Biosciences) in tissue-culture wells and allowed to settle prior to stimulation as described
above. Coverslips were rinsed with ice-cold HBSS and the cells fixed with 4%
paraformaldehyde and blocked overnight (HBSS with 10% goat serum, 1% BSA, 0.1%
Tween20, and 2 mM EDTA) at 4°C. NETs were detected with rabbit anti-NE (Abcam),
rabbit anti-MPO (Dako), two different rabbit anti-PAD 4 (Abcam), mouse anti-PAD4
(Abcam), mouse anti-histone H1 + core proteins (Millipore) and rabbit anti-citrullinated
histone H3 (citH3, Abcam). Secondary antibodies were goat anti-rabbit IgG AF555, goat
anti-rabbit IgG AF488 (Invitrogen) and goat anti-mouse IgG AF647. DNA was stained with
4′,6-diamidino-2-phenylindole (DAPI, Sigma) and NETs were visualized using a Zeiss
Axioplan 2 Imaging fluorescence microscope in conjunction with a Zeiss AxioCam MRm
monochromatic CCD camera and analyzed with Axiovision 4.8.2 software (Carl Zeiss). A
minimum of 20 fields (at least 1000 PMNs) per case was evaluated for MPO/NE and DNA
co-staining; nuclear phenotypes and NETs were counted and expressed as percentage of the
total number of cells in the fields.
RA serum depletion, IgG purification and quantification of NETs
After 3 washes with PBS, 200 µl protein G agarose (Pierce Biotechnology Inc.) was
incubated with 200 µl ACPA + and ACPA- RA or control serum diluted in an equal volume
of phenol red-free RPMI 1640 medium overnight at 4°C. The serum/protein G agarose
mixture was centrifuged at 2500 g for 5 min and the supernatant (IgG depleted serum) was
carefully transferred into a new Eppendorf microcentrifuge tube. The protein G agarose pellet
was gently washed 3 times with 500 µl ddH2O, and the bound antibody was released by the
addition of 50 µl 0.1 M glycine pH 2–3, and immediately equilibrated with 10 µl of 1 M Tris
pH 7,5-9. All protein concentrations were determined by the MN Protein Quantification
Assay (Macherey Nagel) and isolation of IgG was verified with Coomassie staining of SDS-
Neutrophils from healthy donors (n = 3) were isolated and cultured for 2 h in 96-well culture
dishes (Thermo Fischer), supplemented with: serum, depleted serum and purified IgG from
ACPA positive RA patients (n = 3), ACPA negative RA patients (n = 3) and healthy
individuals (n = 3) to a final concentration of 100 µg/ml.
NETs were quantified after IHC staining with mouse anti-human MPO antibody (Abcam)
and rabbit anti-human citH3 antibody (Abcam) or the respective isotype controls, followed
by incubation with goat anti-mouse IgG AF555 and goat anti-rabbit IgG AF488 (Invitrogen).
DNA was counterstained with 4′,6-diamidino-2-phenylindole (DAPI, Sigma). NETs were
visualized using an Olympus IX81 motorized epifluorescence microscope (Olympus) in
conjunction with an Olympus XM10 monochromatic CCD camera (Olympus) and analyzed
with the Olympus CellSens Dimension software (Olympus). A minimum of 20 fields at 10x
magnification (at least 500–1000 PMNs) per case were evaluated for MPO/citH3 and DNA
co-staining through ImageJ analysis software (NIH Image Processing); nuclear phenotypes
and NETs were determined, counted and expressed as percentage of the total area of cells in
the fields .
Protein isolation and western blot analysis
Total protein was isolated by NucleoSpin® TriPrep kit (Macherey-Nagel) from 3 x 106
PMNs. Proteins from the nuclear and cytoplasmic fractions were isolated using the Nuclear
and Cytoplasmic Protein Extraction Kit (Thermo Scientific). Western blotting was performed
using AnykDTM Mini-PROTEAN® TGX Gels (Biorad) and nylon/nitrocellulose membranes
(Biorad). Primary and secondary antibodies utilized were: rabbit anti-PAD4 (Abcam), rabbit
anti-MPO (Cell Signalling), mouse anti-β-Actin (Sigma), goat anti-Mouse and/or anti-Rabbit,
human anti-HRP (Southern Biotech). HRP activity was detected by using SuperSignal® West
Pico Chemiluminescent Substrate (Thermo Scientific). Equal loading was verified using beta-
actin or histone H3, when appropriate. Western blots of citrullinated H3 (citH3) protein were
performed according to Shechter et al. . Densitometric analysis and protein quantification
of the western blots was performed using the ImageJ software.
RNA isolation and quantitative real-time PCR
Total RNA was isolated using RNeasy Mini Kit (Qiagen). TaqMan real-time quantitative RT-
PCR was performed using the Applied Biosystems StepOne PlusTM cycler (Applied
Biosystems) and TaqMan Gene Expression Assay primer/probe sets (Applied Biosystems)
for NE (HS00236952_m1), MPO (HS00924296_m1), PAD4 (HS00202612_m1) and β2-
microglobulin (HS99999907_m1). Data were normalized using the housekeeping gene B2M,
after a selection procedure involving 6 different endogenous reference genes as suggested in
the MIQE guidelines . Relative values were calculated by 2−DDCt analysis .
All data are presented as mean ± SE. Descriptive statistics for continuous parameters
consisted of median and range, and categorical variables were expressed as percentages.
Comparisons between patients and healthy controls were by the Mann–Whitney U test with a
Welch post-test correction. Statistical significance in multiple comparisons was by one-way
analysis of variance (ANOVA) with a Dunn’s post-test correction. P < 0.05 was considered
statistically significant. Receiver operating characteristic (ROC) curves were calculated and
the area under the curve (AUC) with corresponding standard errors of means was calculated.
Data were processed in GraphPad Prism version 5.0b for MacOSX (GraphPad Software Inc.).
Analysis of covariance (ANCOVA) was conducted with SPSS version 21.0 statistical
software (IBM). Additional professional statistical assistance was provided by A. Schoetzau,
RA-derived PMN exhibit increased spontaneous NETosis
Details of the RA study group and healthy control group are described in Table 1 and
Additional file 1.
Table 1 Demographics and patient population characteristics versus healthy blood
50.34 ± 1.5
Gender (F / M)
Bone erosion (pos / neg)
Serum ACPA (pos / neg)
Serum RF (pos / neg)
Serum ANA (pos / neg)
1961 ± 81.69
3641 ± 149.7
Therapy (yes / no)
DMARDs (yes / no)
Biologics (yes / no)
F: female; M: male; DAS28: disease activity score; n.a.: not applicable; pos: positive; neg:
negative; *: mm/h; **: mg/l; ***: cells/µl; ACPA: anti-citrullinated protein antibodies; RF:
rheumatoid factor; ANA: antinuclear antibodies; ESR: erythrocyte sedimentation rate; CRP:
C-reactive protein; n.d.: not determined; PBMC: peripheral blood mononuclear cells; PMN:
polymorphonuclear leukocytes; DMARDs: disease-modifying anti-rheumatic drugs; n.u.: not
P = 0.214
P < 0.0001
P = 0.021
53.03 ± 1.5
24 / 8
3.07 ± 1.12
22 / 10
20 / 12
19 / 13
21 / 11
16.8 ± 13.1
6.9 ± 5.2
1513 ± 75.90
4575 ± 546.0
31 / 1
27 / 5
30 / 2
24 / 32
Akin to very recent observations , we observed that RA-derived PMNs underwent greater
degrees of NETosis than control PMNs in vitro (data not shown). In order to study this facet
in more detail we examined the kinetics of spontaneous NET extrusion, for which purpose
PMNs isolated from peripheral blood samples were allowed to settle for 1 hour and then
cultured for a period of up to three hours in vitro (Figure 1A), NETs being detected by
immunohistochemistry for neutrophil elastase (NE) and DAPI (4′,6-diamidino-2-
phenylindole) (Figure 1B). In addition, we quantitatively assessed the degree of in vitro
NETosis in these cultures by determining the concentration of cell-free nucleosomes in the
respective supernatants (Figure 1C), specifically their association with myeloperoxidase
(MPO), indicative of the NETotic origin of this material [2,21] (Figure 1D). We also
measured NET-associated MPO enzymatic activity using tetramethylbenzidine (TMB) as a
substrate (data not shown). The results clearly indicate that RA-derived PMNs generated
NETs more rapidly, to a greater magnitude and more extensively than control healthy PMNs,
which was particularly evident at the 3-hour stage of in vitro culture (Figure 1B to D).
Accounting for variances in PMN counts, the difference between the healthy control and RA
subjects remained highly significant in an analysis of covariance (ANCOVA) of the
nucleosome assay (P < 0.001).
Figure 1 RA-derived PMNs exhibited increased spontaneous NETosis and elevated
levels of NET component release. (A) Schematic representation of the time course design
for studying in-vitro spontaneous NET release. (B) Detection of in vitro NETosis by
immunohistochemistry for neutrophil elastase (NE) (green) and DAPI (blue). Magnification:
20x; scale bar: 50 nm. (C) Concentration of cell-free nucleosomes in PMN culture
supernatant by ELISA. (D) Quantification of NET associated MPO/DNA complexes. These
assays indicate that more rapid and extensive progression of NET formation is observed in
RA versus control PMNs. (E) Changes in PMN nuclear morphology during NETosis detected
by immunohistochemistry for NE and DAPI. (F) Steady state (T1) RA-derived PMNs
exhibited a greater proportion of delobulated/diffused cells, and progressed rapidly to a
NETotic spread phenotype during in vitro culture. Data are presented as mean ± SE. *P <
0.05, **P < 0.01, ***P < 0.001, ****P < 0.0001. All data are representative of at least six
During NETosis, the morphology of the PMN nucleus changes from the familiar lobulated to
a diffused and then to a spread phenotype (Figure 1E) . By examining and enumerating
these features, it was observed that at baseline (T1) the nuclei from healthy control PMN
were predominantly lobulated, while in this instance the majority of RA-derived PMN nuclei
exhibited a delobulated or diffused nuclear phenotype (Figure 1F). In RA derived PMN this
delobulated population decreased over time, giving rise to NETotic cells with a spread
phenotype (Figure 1F). In contrast, in normal PMN there was a steady progression in the
proportion of delobulated cells (Figure 1F). The spontaneous progression of nuclei to the
NETotic spread phenotype was more pronounced in RA than in normal PMN, a feature most
evident after 3 hours (T3) (Figure 1F).
RA-derived PMNs demonstrate increased expression of NET-associated
signaling elements, nuclear localization of PAD4 and augmented H3
NETosis has been shown to depend on a number of biochemical signaling elements, among
which are the generation of ROS by nicotinamide adenine dinucleotide phosphate (NADPH)
oxidase, the action of NE in combination with MPO, and histone citrullination by PAD4
[2,3,5]. RA-derived PMNs exhibited increased basal intracellular ROS levels (Figure 2A), as
well as increased levels of NE (Figure 2B) and MPO (Figure 2C and D), as determined by
real-time PCR and/or western blotting.
Figure 2 Increased expression of NET-associated signaling elements and nuclear
localization of PAD4, citrullination of histone H3 in RA-derived PMN and possible
extrusion of PAD4 on NETs. Baseline ROS levels, measured by flow cytometry, were
higher in RA-derived PMNs than control PMNs (A). Quantitative real-time PCR analysis of
NE mRNA (B) and MPO mRNA expression (C), as well as western blot analysis of MPO
protein levels (D), indicated that the levels of these two components required for NETosis
were elevated in RA-derived PMNs. Total PAD4 protein (E), or PAD4 mRNA expression
levels (F) did not indicate any significant difference between control and RA-derived PMNs.
(G) Quantification of PAD4 protein levels in cytoplasmic and nuclear fractions of PMNs
from healthy controls and RA patients. Nuclear levels of PAD4 were significantly increased
in RA patients, whereas the cytoplasmic levels were lower compared to the healthy control
PMNs. (H) Elevated citrullinated histone H3 levels in RA PMN extracts detected by western
blot. (I) Co-localization of citrullinated histone H3 (green) with histone components detected
with a pan-histone antibody (red), spread over the entire NET surface (blue). Magnification:
upper panel 20x, scale bar: 50 nm; lower panel 63x, scale bar: 10 nm. (J) Extracellular
localization of PAD4 (red) on extruded
immunohistochemistry. NET DNA is stained blue (DAPI) and histones (panH) are stained
green. Magnification: 20x, scale bar: 50 nm (K). Detection of PAD4/cell-free DNA
complexes in the culture supernatants of isolated PMN undergoing spontaneous NETosis.
Higher levels of these complexes were detected in RA derived PMN cultures. Time points
correspond to those of Figure 1A. Data are represented as mean ± SE. **P < 0.01, ***P <
0.001, ****P < 0.0001, n.s.: statistically not significant. All data are representative of at least
six independent experiments.
NETs by multi-colour fluorescent
Surprisingly, neither PAD4 mRNA expression nor PAD4 levels in total cellular protein
showed any discernible difference between RA and control PMNs (Figure 2E and F,
respectively). Since PAD4 translocates to the nucleus upon PMN activation [4,29], where it
citrullinates histone proteins, such as H3, we examined its presence in nuclear and
cytoplasmic PMN fractions. When compared to control PMNs, we found that PAD4 was
preferentially located in the nucleus of RA-derived PMNs (Figure 2G). The nuclear presence
of PAD4 was associated with increased citrullinated histone H3 (citH3) levels by western
blot analysis in PMNs from RA cases compared to controls (Figure 2H). Furthermore,
citrullinated histone H3 could be readily detected on NET structures (Figure 2I).
Potential extracellular localization of PAD4 on NETs
Since we observed elevated nuclear translocation of PAD4 in RA PMN, we examined
whether this enzyme is extruded into the extracellular environment during NETosis.
Unfortunately the visualization of such an event by fluorescent immunohistochemistry
proved to be difficult using a variety of commercially available antibodies, and we only
obtained rudimentary evidence for the presence of PAD4 on NETs by this means (refer to
We were, however, able to readily detect PAD4/cell-free DNA complexes in culture
supernatants from isolated PMN, the levels of which were increased in cases with RA (Figure
2K). It is therefore quite probable that PAD4 is associated with NETs structures following
aberrant NETosis in RA.
PMNs from RA patients present an enhanced NETotic response to PMA and
normal PMNs are strongly affected by RA serum and synovial fluid
In auto-inflammatory or malignant conditions, such as SLE or cancer, an elevated NETotic
response of PMNs to an external activation signal has been shown [9,13]. In our experiments
it was noted that when RA-derived PMN were treated with PMA, they responded far more
vigorously with regard to NETosis than controls, as detected by SEM and fluorescence
microscopy (Figure 3A and B respectively). In addition, morphometric assessment indicated
that RA-derived PMNs exhibited a larger decrease in cells with a delobulated phenotype and
a greater progression towards a NETotic spread nuclear phenotype than control PMNs
(Figure 3C), a feature accompanied by excessive release of cell-free nucleosomes in culture
supernatants (Figure 3D). PMA appears to activate PAD4, as it enhanced translocation from
the cytoplasm to the nucleus (Figure 3E). The stimulatory effect of PMA on the release of
nucleosomes into the supernatant was abrogated by Cl-amidine, a chemical inhibitor of
PAD4, indicating that PAD4 signaling is necessary for NETosis induced by PMA [4,30]
(Figure 3D). These data indicate that PMNs in RA are susceptible to increased NETosis
following stimulation by a secondary signal, such as that mediated by PMA.
Figure 3 Increased NETotic response of RA-derived PMNs to PMA. (A) Scanning
electron micrographs of NETs induced by PMA (25 nM) indicate the excessive NETotic
response of RA-derived PMN. Scale bar: 20 nm. (B) Assessment of NETs induced by PMA
treatment by fluorescent immunohistochemistry for MPO (red) and DAPI (blue) indicating
the increased response of RA PMN to PMA (25 nM). Magnification: 20x, scale bar: 50 nm.
(C) Analysis of the nuclear phenotype indicated a vast decrease in delobulated/diffused RA
PMN nuclei after treatment with PMA and rapid increase in the NETotic spread phenotype.
(D) Release of cell-free nucleosomes following PMA treatment is abrogated by chloramidine,
a PAD4 inhibitor. (E) Increased nuclear localization and concomitant decrease in cytoplasmic
PAD4 protein levels following PMA treatment, with a clear tendency for an increased
responsiveness to the PMA stimulus by RA PMN. *P < 0.05, **P < 0.01, ***P < 0.001,
****P < 0.0001, n.s.: statistically not significant. All data are representative of at least four
Since SLE sera and RA sera and synovial fluid (SF) have been shown to confer an increased
NETotic response [9,13], we examined whether RA-derived sera or SF exerted similar effects
on normal PMNs. As non-inflammatory controls we used healthy serum or osteoarthritis SF.
Both RA sera and SF induced a pronounced increase in NETosis, which was paralleled by an
increase in the nucleosome content of the supernatant (Figure 4A and B), as well as in ROS
production (Figure 4C) when compared to healthy serum or osteoarthritis SF, respectively.
Figure 4 Influence of RA serum and synovial fluid on normal PMNs. (A) Incubation of
healthy donor PMNS with serum (Se) from healthy donors or from RA patients, synovial
fluid (SF) from patients with non-inflammatory osteoarthritis (OA) or synovial fluid from RA
patients. Immunohistochemical analysis of four main components of NETs (NE, MPO, PAD4
and citH3) revealed that RA-derived serum and SF enhanced NETosis in normal PMN
compared to healthy control serum or non-inflammatory OA SF. PAD4 (white arrowheads)
and citH3 (empty arrowheads) co-localize with unmodified histones on NETs. Magnification:
20x; Scale bars: 50 nm. (B) Release of cell-free nucleosomes during in vitro culture by PMNs
from healthy controls incubated with control serum, RA serum, OA SF or RA SF or PMA.
Data are represented as mean ± SE. *P < 0.05, **P < 0.01. (C) Increased ROS generation
during in vitro culture by PMNs from healthy controls incubated with control serum, RA
serum, OA SF or RA SF or PMA. Data are presented as mean ± SE. (D) Release of cell-free
nucleosomes during in vitro culture by PMNs from healthy controls, ACPA positive and
ACPA negative RA patients incubated with serum (Se +), IgG depleted serum (Se -) or serum
reconstituted with their respective eluted IgG, or with PMA. Data are presented as mean ±
SE. *P < 0.05, ***P < 0.001, ****P < 0.0001, n.s.: statistically not significant. All data are
representative of at least four independent experiments in Figure 4A –C, and 3 in 4D.
To assess whether antibodies participate in the effects of RA serum on normal PMNs, we
depleted IgG from serum of ACPA positive and negative RA patients and healthy controls.
Compared with non-depleted sera, IgG depletion of both ACPA positive and negative sera
markedly reduced NET induction to levels of normal serum (Figure 4D). Whereas the
reconstitution of ACPA negative IgG to serum did not increase NET formation significantly
compared with controls, it was practically reversed to the original value in the ACPA positive
cases. This indicates a prominent role for ACPA in the induction of NETs in ACPA positive
RA, while suggesting that an alternative mechanism is responsible for the increased NETosis
in ACPA negative RA patients.
These data are in accordance with recent findings that RA derived serum and SF induce
NETosis in normal PMNs, and that ACPA and also IgM RF are to a large part responsible for
this effect [13,14].
Serum from RA patients shows elevated levels of the principal components of
NETs, indicating enhanced NET extrusion during clot formation, which has
potential clinical utility
As we had previously observed increased levels of cell-free DNA (cfDNA) in RA sera ,
we determined whether this resulted from enhanced NETosis, and whether this could have
diagnostic applications. cfDNA concentrations were indeed significantly higher in serum
samples from RA cases compared to age-matched healthy control sera (Figure 5A). In
parallel, the concentrations of cell-free nucleosomes, NE and MPO were significantly
elevated in RA serum compared to control sera (Figure 5B to D). The association of a
significant fraction of MPO with markedly elevated cell-free nucleosomes in RA serum, that
constitute a main component of NETs (Figure 5E), clearly suggests that NETosis is indeed
the source of nucleosome material present in RA serum .
Figure 5 Elevated serum levels of NET components, in RA patients have potential
clinical utility. (A) Cell-free DNA levels in plasma and serum from healthy matched blood
donors (n = 41) and patients with RA (n = 32) determined by real-time PCR. (B) Cell-free
nucleosome levels in plasma and serum from healthy donor controls and patients with RA,
determined by ELISA. (C) Determination of NE protein concentrations in plasma and serum
from healthy donors and patients with RA as assessed by sandwich ELISA. (D) MPO
concentrations in plasma and serum from healthy donors and patients with RA as determined
by sandwich ELISA. (E) NET-associated MPO/DNA complexes quantified utilizing a
modified capture ELISA. In contrast to the serum levels, none of the plasma levels of these
NET components attained statistical significance (Figure. 5A to E). (F) ROC analysis of cell-
free nucleosomes in serum of patients with RA and healthy controls. (G) Detail of cell-free
nucleosome ROC curve with groups of ACPA + and ACPA- RA cases and (H) scatter box
and whisker plots with individual values for control, ACPA + and ACPA- groups. The ROC
curve analysis of other NET components, cell-free DNA (I), NE (J) and MPO (K), was not
as conclusive as that for cell-free nucleosomes. *P < 0.05, **P < 0.01, ***P < 0.001, ****P <
0.0001, n.s.: statistically not significant.
Since there was no significant elevation of these parameters in simultaneously obtained
plasma samples that were processed immediately, these data demonstrate a propensity for RA
PMN to undergo increased NETosis during coagulation (Figure 5A to E).
To ascertain whether NET associated serum components could be diagnostically useful, we
conducted receiver operating characteristics (ROC) analyses. For serum cell-free
nucleosomes this yielded the surprisingly high AUC value of 0.97 (Additional file 2 and
Figure 5F). Of interest is that there was no significant difference in this value regardless of
whether the RA cases were ACPA positive or not (Figure 5G), although there was a slight
trend for serum nucleosome levels to be higher in ACPA positive cases than ACPA negative
cases (Figure 5H). The AUC for serum nucleosomes was significantly higher than for any of
the other parameters examined (Figure 5I to K). With the cut-off set at 0.78 the ROC AUC
translates into a sensitivity of 91% with a specificity of 92% for differentiating between RA
cases and healthy controls.
In contrast to RA cases, cell-free nucleosome serum values of 14 cases with SLE showed a
slight, but statistically significant increase (Additional file 2). This translated into an ROC
AUC of 0.7639 (Additional file 3), which is below the clinically useful value for diagnostic
Although PMNs figure prominently in the joint effusions and inflamed synovial tissue of RA
patients , the potential roles of NETotic events in the pathophysiology of this disorder
have only recently become a focus of attention [13,14]. These studies indicated that RA-
derived PMNs were more prone to undergo NETosis, and that NETs themselves could
contribute to the generation of auto-antigens (ACPA) or be the target of auto-antibodies
Our studies, performed independently at a similar time as these, corroborate that NETosis is
enhanced in RA, confirming a possible fundamental role of this phenomenon in the
underlying aetiology of RA. In addition, we extended upon these observations by examining
for changes in the underlying signal transduction cascade required for the induction of
NETosis. The results show that the propensity of circulatory PMNs in RA patients to undergo
NETosis is associated with elevations in members of this cascade including increased
intracellular ROS production, enhanced expression of NE and MPO, increased nuclear
translocation of PAD4 and citrullination of histones, notably H3. Consequently, these and
other key NETotic pathway elements  could serve as potential therapeutic targets for
Furthermore, by examining kinetic changes during extended in vitro culture different nuclear
morphometric characteristics were observed in PMNs from RA cases, with a lower
proportion of the classical lobulated phenotype, coupled with a much higher proportion of
delobulated cells at the initial time-point. Unlike in controls, in which an increase in this
population was noted over time, this latter population decreased during in vitro culture in RA
PMNs. RA PMNs also progressed more rapidly and extensively to a NETotic spread
phenotype than controls, a finding confirmed by analysis of culture supernatants for the
products of NETosis.
Akin to what has been observed in an array of other pathological conditions ranging from
preeclampsia and SLE to cancer and RA [8,9,12,13,17], PMNs from RA patients exhibited an
increased response to further stimulation, for instance by treatment with IL-8, the phorbol
ester PMA or with LPS. This response is in part mediated via the action of PAD4, as the
effect of PMA could be significantly reduced by treatment with Cl-amidine, an inhibitor of
PAD4 . In addition, PMA treatment lead to an increased nuclear localisation of this
enzyme, where it presumably could catalyse a more extensive citrullination of histone
proteins, thereby speeding up the induction of NETosis.
Although our data are preliminary, they do suggest that PAD4 is extruded onto the NETs
during NETosis, as detected by ELISA technology and to a lesser extend by fluorescence
microscopy. Such an occurrence would have important implications for the development of
anti-PAD4 auto-antibodies observed in cases with RA . Since the presence of such
antibodies precede the development of RA, our data provide further support that NETs may
contribute to the underlying aetiology of RA, and may be a relatively early event. As the
presence of such anti-PAD4 antibodies has been shown to enhance the enzymatic activity of
PAD4 in an extracellular environment by reducing the calcium requirement , their
combination with NETs associated PAD4 could lead to prodigious quantities of citrullinated
autoantigens. In addition, the extracellular presence of PAD4 on NETs may further promote
the prodigious generation of citrullinated antigens, since molecular structures involving the
attachment of enzymes to DNA lattices have been shown to increase their catalytic activity
enormously, and thereby form the basis of nano-machines or nano-factories, generating such
Although these findings will need to be verified and it remains to be ascertained whether
extracellular NETs associated PAD4 is active, these data do support and extend upon recent
reports indicating that NETs can be a source for citrullinated autoantigens, and that they react
with ACPA or anti-PAD4 antibodies [13,14,35]. Taken together, these data provide further
evidence concerning a key role for PAD4 in the underlying aetiology of RA, and offer a
potential explanation for the efficacy of PAD4 inhibitor chloramidine in reducing disease
symptoms in collagen-induced rat and murine models for RA .
It has recently been reported that ACPA or IgM RF led to potent increases in NET formation
compared with control IgG . In our IgG depletion experiments on ACPA positive and
negative RA cases we observed a marked reduction of NET induction to control levels in
both cases, whereas reconstitution of serum with IgG gained from depletion almost
completely restored NET induction in the ACPA positive cases. However, in the ACPA
negative cases no significant increase followed reconstitution. These results support the
notion that ACPA are important inducers of NET formation in ACPA positive RA cases, and
indicate that other mechanisms, such as IgG complexes similar to those involved in NET
induction in SLE , are operative in ACPA negative RA. Both mechanisms could lead to a
common distal mechanism of induction of arthritis.
The observation that the coagulation of blood samples from RA patients during serum
preparation triggers extensive NETosis, evident by increased concentrations of cell-free
DNA, nucleosomes or nucleosome/MPO complexes, may have unexpected clinical
ramifications. With a sensitivity of 91% and a specificity of 92%, it is possible that the
assessment of serum cell-free nucleosomes may serve to distinguish suspected RA patients
from healthy controls with a high degree of specificity. It is of interest that this aspect was not
significantly influenced by the ACPA status of the RA patients. As such, this assay may be a
useful complementary test to perform in conjunction with current ACPA or RF assays, not
only to extend diagnostic accuracy, but also to assist in detecting RA in cases that are either
ACPA or RF negative. Similar NET induction by ACPA positive and negative RA sera and
its abrogation by IgG depletion, as discussed above, supports the functional aspect of the
nucleosome measurement in RA serum.
In a preliminary series of SLE sera, there was a small and not statistically significant increase
of cell-free nucleosomes over normal controls, indicating a slightly elevated propensity for
PMNs from SLE patients to undergo NETosis. This was, however, nowhere near the range
seen in RA cases, and failed to reach an ROC AUC considered to be clinically relevant.
These aspects will need to be validated in larger multi-centre studies.
In summary our data reaffirm an intricate relationship between NETosis and the aetiology of
RA, since the signalling elements associated with NET extrusion are significantly enhanced
to promote NETosis in RA compared to healthy controls. Both ACPA positive and negative
serum lost the ability to induce NETosis upon depletion of IgG molecules, but reconstitution
of NET induction was only seen with IgG molecules obtained from ACPA positive serum.
The assessment of NETosis derived products in the sera of suspected RA cases may offer a
novel complementary diagnostic tool.
ACPA, anti-citrullinated peptide antibody; AUC, area under the curve; citH3, citrullinated
histone; DAPI, 4′,6-diamidino-2-phenylindole; DCFH-DA, 2′, 7′-dichloro dihydro
fluorescein diacetate; ELISA, enzyme-linked immune assay; GAPDH, glyceraldehyde-3-
phosphate dehydrogenase; HBSS, Hank’s balanced salt solution; MPO, myeloperoxidase;
NADPH, nicotinamide adenine dinucleotide phosphate; NE, neutrophil elastase; NETs,
neutrophil extracellular traps; PAD4, peptidyl arginine deiminase 4; PMA, phorbol-12-
myristate-13-acetate; PMN, polymorphonuclear granulocyte; RA, rheumatoid arthritis; RF,