The SLE review series: working for a better [618740]

The SLE review series: working for a better
standard of care
Pathways leading to an immunological disease:
systemic lupus erythematosus
Olga Zharkova1,2, Teja Celhar1, Petra D. Cravens3, Anne B. Satterthwaite4,5,*,
Anna-Marie Fairhurst1,2,4,* and Laurie S. Davis5,*
Abstract
SLE is a chronic autoimmune disease caused by perturbations of the immune system. The clinical presen-
tation is heterogeneous, largely because of the multiple genetic and environmental factors that contribute todisease initiation and progression. Over the last 60 years, there have been a number of significant leaps inour understanding of the immunological mechanisms driving disease processes. We now know that multipleleucocyte subsets, together with inflammatory cytokines, chemokines and regulatory mediators that arenormally involved in host protection from invading pathogens, contribute to the inflammatory events leadingto tissue destruction and organ failure. In this broad overview, we discuss the main pathways involved in
SLE and highlight new findings. We describe the immunological changes that characterize this form of
autoimmunity. The major leucocytes that are essential for disease progression are discussed, togetherwith key mediators that propagate the immune response and drive the inflammatory response in SLE.
Key words: systemic lupus erythematosus, SLE, SLE pathogenesis, autoantibodies, immunology, tolerance,
inflammation, tissue destruction
Rheumatology key messages
.SLE is a heterogeneous disease influenced by a variety of genetic and environmental factors.
.Innate and adaptive immune responses contribute to the autoimmune dysfunction observed in SLE.
.Autoantibodies form immune complexes that drive target organ inflammation in most individuals with SLE.
Introduction
Autoimmunity affects /C248% of the global population.
However, the incidence is increasing because of anumber of factors, including awareness and improved
clinical diagnoses [1]. Moreover, there is evidence that
autoimmune diseases are a leading cause of death inyoung females within the USA [2]. SLE affects primarilywomen, with a gender bias of 9:1, which has been
attributed in part to oestrogen receptor-1 and to un-
defined immunomodulatory genes on the X chromosome
[3]. There is no prevention or cure, and the mainstay oftreatment is immunosuppressive therapy. Disease aeti-
ology involves genetic predisposition and environmental
factors, with the influence of gender [4]. Over 60 geneticregions have been associated with the development or
severity of human disease [5]. These genetic associations
have directed research towards multiple pathwaysinvolved in innate and adaptive immunity.
Comprehensive discussions of the history of SLE have
been described previously [6, 7]. An important step indetermining that SLE is a disease of the immunesystem, or an immunological disease, was taken in 1948
by Dr Hargraves, who discovered the LE cell or LE body.
The LE cell is a neutrophil or macrophage in the bonemarrow that has a distinct morphology by haematoxylin
staining as a result of the phagocytosis of nuclear debris.
The presence of these cells is indicative of SLE or other1Singapore Immunology Network, 8A Biomedical Grove, Immunos,
2School of Biological Sciences, Nanyang Technological University,
Singapore,3Department of Neurology and Neurotherapeutics,
4Department of Immunology and5The Rheumatic Diseases Division,
Department of Internal Medicine, UT Southwestern Medical Center atDallas, TX, USA
*Anne B. Satterthwaite, Anna-Marie Fairhurst and Laurie S. Davis
contributed equally to this study
Correspondence to: Laurie S. Davis, The Rheumatic Diseases Division,
Department of Internal Medicine, UT Southwestern Medical Center at
Dallas, TX 75390-8884, USA.
E-mail: laurie.davis@utsouthwestern.eduSubmitted 29 July 2016; revised version accepted 20 October 2016
!The Author 2017. Published by Oxford University Press on behalf of the British Society for Rheumatology.
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.or g/licenses/by-nc/4.0/), which permits non-commercial re-use,
distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact journals.permis sions@oup.comRHEUMATOLOGYRheumatology 2017;56:i55 /C150i66
doi:10.1093/rheumatology/kew427
REVIEW

connective tissues disorders. Later, the LE cell phenotype
was defined as an ANA reaction because it could be
reproduced from bone marrow preparations with the add-
ition of SLE serum [7]. This observation and test providedthe key to linking SLE to immunological dysfunction. TheLE test was subsequently replaced by serum ANA assays
[8]. Since these early discoveries, there has been a dra-
matic progression in our understanding of the immunolo-gical pathways driving disease.
The cellular roles of individual leucocytes, including B
cells, T cells and myeloid cells, in the initiation and pro-
gression of SLE have been recently reviewed elsewhere[9/C15012]. There are several known factors that contribute to
the initiation and progression of autoimmunity in SLE.
These factors include the following: the initial break in tol-
erance and generation of autoantigen-specific effector Band T lymphocytes and the subsequent production ofANAs; defects in cell death or debris clearance pathways
and the continued generation of self-antigens; and tissue
inflammation and deficiencies in immune regulation com-bined with mechanisms that propagate chronicity to drive
lupus immunopathology.
Breaking immune tolerance and the
development of autoimmunity
The loss of tolerance to self and the subsequent elevation
in serum ANA levels is proposed to be a crucial first stepin the development of SLE [13 /C15015]. This observation is
supported by the finding that autoantibodies can be de-
tected before clinical symptoms in most SLE patients [15].
The presence of autoantibodies in patients, including anti-dsDNA, anti-SSA (Ro), anti-SSB (La), anti-Sm and anti-RNPs, suggests that a common mechanism is involved
in the peripheral expansion of autoreactive B cells that
has yet to be delineated fully [15 /C15017].
The majority of B cells generated are self-reactive and
are usually removed by central tolerance mechanisms in
the bone marrow, including receptor editing, deletion orthe induction of anergy, reviewed by Meffre [13] andGoodnow et al. [18]. These are B-cell-intrinsic mechanisms
and are known to be controlled by B-cell antigen receptor
signalling thresholds and regulators of the phosphoinosi-tide 3-kinase pathway [19 /C15021]. Recently, the miR-17 /C15092
family of miRNA has been shown to regulate central B-
cell tolerance by targeting phosphatase and tensin homo-
log [20, 22]. Additional removal of autoreactive B cellsoccurs by selection mechanisms in the periphery, whichare less clear but can involve impaired survival and anergy
[23]. Elevated levels of B-cell activating factor (BAFF, also
known as B lymphocyte stimulator (BLyS) or CD257),which are observed in SLE patients (see below) [24, 25],have been shown in mouse models to promote a breach in
B-cell tolerance and enhance the survival of autoreactive B
cells [26]. Evidence from SLE patients has shown thatthere is failure in both central B-cell checkpoints in thebone marrow and peripheral checkpoints at the transi-
tional-naive B-cell stage [27]. Furthermore, SLE patients
exhibit a defect in anergy of naive B cells [28].Additionally, the generation of de novo autoreactive B
cells occurs after maturation as a result of somatic hyper-mutation in germinal centres (GCs) [29]. It is believed that
the majority of pathogenic autoantibodies are somatically
hypermutated, class-switched IgGs. This class-switchingfrom IgM to IgG occurs primarily, but not solely, in GCs,through interactions of the B cell with antigen and with
CD4
+T follicular helper (Tfh) cells, which are identified by
the markers CD4, inducible T-cell costimulator (ICOS), C-X-C chemokine receptor type 5 (CXCR5), CD57 and pro-grammed cell death protein 1 (PD-1) [12, 30 /C15033]. B cell
activation in GCs is followed by expansion and differenti-
ation into autoreactive plasmablasts and plasma cells thatsecrete high levels of antibodies to autoantigens. The tar-geted deletion of IFN- g, Toll-like receptor 7 (TLR7) and
signal transducer and activator of transcription 1 (STAT1)
in mice results in the disruption of autoreactive GCs andimpaired production of IgG autoantibodies [34 /C15036] (Fig. 1).
Importantly, it has been shown that the IFN- greceptor
(IFN- gR) requirement for the class-switch recombination
of pathogenic autoantibodies and the subsequent devel-opment of systemic autoimmunity is B-cell intrinsic [35,37]. A T-box transcription factor (T-bet) also contributes
to pathogenic autoantibody production [35, 37]. TLR7
within the B cell is also required for spontaneous GC for-mation and antibody production [35, 36, 38, 39].
Other mechanisms that might contribute to ANA pro-
duction include molecular mimicry; for example, autoanti-bodies can be induced during an infection as a result of
activation of lymphocytes that recognize foreign antigens
that cross-react with autoantigens [40]. Alternatively,injury to tissues during infection can induce epitopespreading from immune responses against pathogens to
tissue antigens. Often these self-antigens have undergone
chemical modifications as a result of the inflammatory re-sponse [41, 42]. Autoantibodies in target tissues formimmune complexes (ICs), which, combined with inflam-
matory cytokines from infiltrating leucocytes, perpetuate
organ inflammation and tissue injury.
Similar to B lymphocytes, T cells undergo tolerance
mechanisms to restrict autoreactivity. Multiple autoim-mune-prone strains have demonstrated a requirementfor both B and CD4
+T cells for the production of IgG
autoantibodies, indicating that loss of T-cell tolerance
may play a role in lupus [43]. Tolerance mechanisms in-clude deletion of autoreactive T cells in the thymus duringdevelopment, and peripheral mechanisms such as apop-
tosis, anergy or inhibition by Treg [44]. In SLE, there are
decreased numbers of recent thymic emigrants, suggest-ing that central T-cell tolerance mechanisms are dysregu-lated [45]. A contributing factor could be the upregulated
gene expression of HLA-D region and antigen-presenta-
tion pathways in SLE patients’ dendritic cells (DCs), aswas recently reported [46]. Moreover, elevated expressionof HLA-DR on DCs could impact tolerance of autoreactive
T cells in secondary lymphoid organs. Peripheral auto-
reactive T-cell tolerance can be disrupted by exposureto pathogens and by a variety of other mechanisms [47].In SLE, abnormalities in proximal signalling pathways can
i56 www.rheumatology.oxfordjournals.orgOlga Zharkova et al .

FIG.1Overview of immunological pathways leading to SLE
The development of SLE occurs in three interconnected phases, illustrated by coloured backgrounds. Loss of adaptive
immune tolerance (blue) leads to an increase in autoreactive B cells. Signals from self-antigens, TLR ligands, BAFF/APRILand T-cell-derived cytokines promote the formation of germinal centres and the production of autoantibodies. Innate
immune defects leading to increased availability of self-antigens (pink) include increased NETosis, impaired clearance of
apoptotic debris and reduced phagocytosis. Self-antigens form ICs with autoantibodies, enabling FcR g-mediated uptake
and activation of several downstream pathways. Inflammation and tissue damage (green) is caused by mediators
released by recruited inflammatory cells and IC-induced complement activation. Abs: antibodies; Ags: antigens; APRIL
(CD256): a proliferation-inducing ligand; B: B cell; BAFF (CD257): B-cell-activating factor; BAFF-R: B-cell-activatingfactor receptor; BCMA: B-cell maturation antigen; BCR: B-cell antigen receptor; FcR g: Fc receptor- g; fDC: follicular
dendritic cell; HLA class II: human leucocyte antigen class II; mDC: myeloid dendritic cell; M /C8: macrophage; Mo:
monocyte; NET: neutrophil extracellular trap; ox-mDNA: oxidized mitochondrial DNA; pDC: plasmacytoid dendritic cell;
Stat1: signal transducer and activator of transcription (a transcription factor); T: T cell; TACI (CD267): transmembraneactivator, calcium modulator and cyclophilin ligand interactor; T-bet: a T-box transcription factor; Tfh: T follicular helper;
TLR7/9: Toll-like receptors 7 and 9.
www.rheumatology.oxfordjournals.org i57SLE and immunological pathways

contribute to altered T-cell tolerance and responsiveness
[48]. Dysfunction in autoreactive T-cell anergy can be
mediated by several factors, including genetic variants,
epigenetic modifications or alterations in gene regulation.Such dysfunction can result in hyperactive T-cellresponses, disruption of T-cell trafficking patterns and
alter the balance of T-cell pro-inflammatory and anti-
inflammatory cytokine production [48].
Treg cells appear to play a major role in the maintenance
of peripheral autoreactive T-cell anergy. Although reports
of altered Treg numbers and function in SLE have been
contradictory, Treg defects are observed in some SLE pa-tients [49]. Low-dose IL-2 therapy can correct these de-fects, reduce the relative frequency of Tfh and Th17 cells
and decrease disease activity, suggesting that disruption
of the balance of Treg to Tfh and/or Th17 cells may con-tribute to loss of tolerance in SLE [50, 51]. These observa-tions are also supported by studies in murine models [52].
Tfh cells are important for the activation and selection of
B cells within GCs [53]. Tfh cells are found at increasedfrequencies in the peripheral circulation of SLE patients
undergoing flares, and in LN, Tfh cells can be found in
the kidneys [12]. Elevated numbers of Tfh cells are asso-ciated with increased disease activity and decreasedTreg numbers and/or function in SLE patients and autoim-
mune mouse models [30 /C15033, 54 /C15056]. IFN- gRs i g n a l l i n gi s
also necessary for Tfh cell development, and consistentwith this, excess of IFN- gR signalling leads to an accrual
of Tfh cells, which is associated with elevated ANA titres, a
higher frequency of circulating activated B cells (plasma-
blasts) and disease activity [35, 37, 57] . Interestingly,higher levels of serum IFN- g, along with IL-5 and IL-6,
have been detected >3 years before SLE diagnosis, sug-
gesting their importance in the development of disease [58].
Generation of self-ligand and impaired
clearance mechanisms
It is believed that pathogenic class-switched IgG autoan-
tibodies play a significant role in the inflammatory pro-cesses that lead to tissue destruction. These antibodies
are produced by B cells that have been activated by self-
antigens [59]. Current lines of evidence suggest thatincreased nuclear debris in the periphery of SLE patientsmight provide a source of excess autoantigens that con-
tribute to increased serum ANA levels [59 /C15061]. Nuclear
debris results from increased apoptotic or necrotic cellsor impaired uptake of dying cells through phagocytosis byneutrophils and macrophages [60, 62, 63]. Phagocytosis
is defective in macrophages derived from SLE patients
[64, 65]. Studies in murine models suggest that deficien-cies in cell surface proteins, such as receptor tyrosinekinases Mer and Axl or the combination of milk fat glo-
bule-EGF factor 8 and T-cell immunoglobulin- and mucin-
domain-containing molecule, involved in the clearance ofcirculating cellular debris, may contribute to autoimmunity[66/C15068]. Opsonins, such as CRP, C1q, serum amyloid P,
mannose-binding lectin, pentraxin 3 and other soluble
proteins, bind to apoptotic cells and facilitate theirclearance [64]. In SLE, reduced levels of CRP, which
binds to nuclear proteins and phosphatidylethanolamineon damaged cell membranes, contribute to defects in celldebris clearance [69]. In addition, autoantibodies bindingto opsonins on circulating necrotic cell debris can gener-ate ICs and promote pro-inflammatory responses and
tissue damage, as discussed below [69].
Neutrophils also undergo a unique process ultimately re-
sulting in cell death, known as NETosis, the release of neu-
trophil extracellular traps (NETs), which is enhanced in
paediatric SLE [70, 71]. This specific form of liberating nu-clear contents is believed to contribute to the autoantigensnecessary for the persistent autoreactive inflammatory re-sponse [70] (Fig. 1). In SLE, neutrophils can undergoNETosis after priming by type I IFNs and activation inducedby cytokines (IL-1 b, IL-8, IL-17 and TNF) and anti-RNP anti-
bodies or autoantibodies to cell surface anti-microbial pep-
tides (cathelicidin LL-37, human neutrophil peptide) [70,
71]. The nuclear contents (DNA, RNA, histones, etc.) arereleased in the form of spider web-like NETs [72]. Thereleased DNA and RNA is coated with self-proteins, includ-ing anti-microbial peptides which stabilize the NETs in animmunogenic form [70, 71]. Serum samples from activeSLE patients fail to degrade NETs efficiently because theycontain autoantibodies and C1q, which inhibit DNase-Idegradation of chromatin [73, 74]. Moreover, renal
DNase-I is downregulated in late-stage LN [73, 75]. These
findings are consistent with observations in murine modelsdemonstrating that loss of DNase-I accelerates LN [76].
The nuclear material forms ICs with the autoantibodies,
which are then taken up through either the B-cell antigenreceptor on B cells or Fc gR on DCs [77, 78] (Fig. 1). This
can then activate specific intracellular innate receptors,including TLR7 and Toll-like receptor 9 (TLR9), whichdrive cellular activation and the production of pro-inflammatory cytokines, including IL-6, IL-8, IL-1 b, IL-12
and TNF. Additionally, oxidized mitochondrial DNAreleased by SLE neutrophils can stimulate plasmacytoid
DCs (pDCs) to produce IFN [79]. In SLE, neutrophils po-
tentially represent a major reservoir of autoantigens todrive B-cell and DC activation and effector function, re-sulting in propagation of the inflammatory response.
The complement system is a major mechanism of
innate immunity. It plays an important role in the lysis ofinvading bacteria, in the clearance of antibodies found inICs and in the removal of cellular debris [80]. Activation ofthree possible pathways (classical, alternative or lectin)induces an enzymatic cascade of activated and cleavedcomplement proteins. However, genetic deficiencies in
predominantly the classical components, including C1
(C1s-C1r, C1q), C2 and C4 have been associated withthe development of SLE, reviewed by Sturfelt andTruedsson [81]. In most SLE patients, complement activityis reduced during episodes of inflammation.
The classical pathway is activated by the binding of C1q
to clusters of IgG or IgM, CRP or apoptotic cell debris.This triggers the enzymatic cascade through a series ofproteases to cleave C4, C3 and, ultimately, C5. The re-sulting products, C3a, C4a and C5a, are potent
i58 www.rheumatology.oxfordjournals.orgOlga Zharkova et al .

chemoattractive agents, which can recruit inflammatory
cells. The C3b and C4b fragments bind ICs, which then
activate complement receptors on macrophages that
clear them from circulation in the spleen and liver.
The complement system also plays a role in peripheral
tolerance. Genetic ablation of C4 in a murine model
reduced negative selection during the progression from
transitional to mature naive B cells [82]. These findings
are consistent with the high frequency of patients with
deficiencies in early complement components who have
SLE or other autoimmune disorders [81, 83]. Taken to-
gether, these studies demonstrate that while complement
activation can promote inflammation and tissue damage
in SLE, defects in complement-dependent clearance of
ICs and apoptotic debris results in more antigen availabil-
ity. This, in turn, promotes loss of adaptive immune toler-
ance and promotes autoimmunity.
Immune-mediated perpetuation
of inflammation
Positive feedback loops resulting from the loss of adaptive
immune tolerance, the formation of ICs and the activation
of the innate immune system perpetuate inflammatory re-
sponses that may result in target organ injury (Fig. 1) [84].
Below, we discuss key mediators involved in the
immune-mediated perpetuation of the inflammatory re-
sponse. A recent report has described elevations in a
number of serum innate and adaptive cytokines before
SLE disease onset [58]. These include the innate cyto-
kines IL-23 and IL-12p70 and TNF superfamily members
BAFF (CD257) and CD256 (a proliferation-inducing ligand,
APRIL), the IFN-inducible chemokines IP-10 and CXCL9,
the Th1 cytokines IFN- gand IL-2, the Th2 cytokine IL-5
and the Th17-associated cytokine IL-21. In addition, sol-
uble serum TNFRI and TNFRII levels were elevated,
whereas regulatory TGF blevels were decreased [58].
Importantly, the combination of certain mediators
(CXCL9, IFN- g, IL-5, IL-6) with ANA titres and the pres-
ence of anti-Ro and anti-SSA antibodies had a greater
predictive power for the development of SLE than auto-
antibodies alone. These cytokines along with others
demonstrated to be key in the autoimmune response,
such as type I IFNs and IL-6, are discussed below.
These studies further highlight the dysregulation of mul-
tiple innate and adaptive pathways required to perpetuate
autoreactive immune responses by undergoing a series of
amplification steps, each with increasing complexity,
leading to the establishment of full-blown disease.
DCs and type I IFNs
DCs play an important role in the initial stages of lympho-
cyte activation, presenting antigen to drive the immune
response. In SLE, debris from apoptotic cells can be pre-
sented as self-antigens to propagate B- and T-cell hyper-
reactivity [62, 85]. Furthermore, previous data have shown
that monocytes cultured with serum from SLE patients
mature into cells with myeloid DC-like morphology and
function [86]. These studies demonstrated thatdifferentiation of healthy control monocytes by SLE
serum was dependent on IFN- aand correlated with SLE
disease activity. This has important implications, as it hasbeen proposed that lymph node myeloid dendritic cells
(mDCs) participate in the maintenance of peripheral toler-
ance by regulating autoreactive T cells. IFN- aupregulates
costimulatory expression and TLR7 mRNA expression in
human mDCs, making them potentially potent antigen-presenting cells for foreign and self-antigens [87]. In
BXSB and Y-autoimmune accelerator (Yaa)-associated
murine models of lupus, a 2-fold increase in TLR7 expres-sion is required for the development of severe disease [34,88/C15091]. Moreover, the increase in DCs, and not B cells, is
crucial for TLR7-induced severe pathology [87, 89].
In SLE, pDCs play an important role through the pro-
duction of IFN- a. ICs activate immature pDCs via the
innate TLRs, TLR7 and TLR9, to produce inflammatory
cytokines, including type I IFNs [92]. Although pDCs are
reduced in the periphery of SLE patients with active dis-ease, they accumulate at inflammatory sites in tissues,including cutaneous lesions and kidneys [86, 93 /C15095].
Type I IFNs serve to propagate autoimmune responses
through activities including the maturation of monocytesinto mDCs (see above) [86], the priming of neutrophils toundergo NETosis in the presence of anti-RNP autoantibo-
dies [70] and the promotion of B-cell responses to TLR7
engagement [96]. Previous data have shown that an ele-vated serum IFN- alevel is a heritable trait that can con-
tribute to SLE disease susceptibility [97, 98]. The majorityof SLE patients have a consistent upregulation in a broadarray of type I IFN-responsive genes compared with con-
trols [99 /C150102]. This IFN signature could be induced by IFN
and correlated with increased serum type I IFN levels[101, 103]. Mixed results were obtained in longitudinalstudies correlating an IFN signature score with diseaseactivity [104 /C150106]. A specific IFN signature, represented
by a select number (of the order of 5 /C15030 mRNAs) of re-
producibly affected expressed genes, has been success-fully used as a diagnostic biomarker or in clinical trials as apharmacodynamic biomarker [107, 108]. For example, instudies of sifalimumab and rontalizumab, mAbs targeting
type I IFN, a dose-dependent suppression of the IFN sig-
nature was correlated with improvement in clinical symp-toms in SLE patients [109, 110]. Thus, the IFN signatureappears to be detectable in most, but perhaps not all,lupus patients’ peripheral blood cells, during increasesin disease activity.
In addition to TLRs, type I IFN responses can be
induced by stimulation of cytosolic DNA or RNA sensors.Mutations or deficiencies in related signalling pathway
members have been shown to alter lupus-like disease sig-
nificantly in humans and in murine models. For example,in humans a constitutively active mutant STING (stimulatorof IFN genes) protein has been shown to be associatedwith elevated serum type I IFN and an IFN signature in
peripheral blood mononuclear cells, resulting in a lupus-
like syndrome [111]. Unexpectedly, the MRL/lpr mousemodel of lupus intercrossed to mice deficient in the keydownstream signalling component STING demonstrated
www.rheumatology.oxfordjournals.org i59SLE and immunological pathways

accelerated lupus pathology, including increased auto-
antibody production and TLR-mediated cytokine produc-tion [112]. These studies suggest that in addition tomediating signalling by cytosolic sensors, STING mightactivate negative feedback mechanisms that control in-flammatory responses. Further studies are warranted togain a full understanding of the role of the cytosolic sen-sors and their signalling pathways in lupus.
IL-6
IL-6 is a potent cytokine produced by innate and adaptivecells. It stimulates B-cell growth and B- and T-cell differ-entiation, as observed by their diminished activity in micedeficient for IL-6 [113, 114]. IL-6 can also contribute totissue damage independent of its role in B- and T-cellactivation [115]. IL-6, like IL-1 b, has a number of regula-
tory homeostatic functions beyond immune regulation andinduces other mediators during the acute phase responseto infections. Deficiency of IL-6 in numerous murine lupusmodels results in amelioration of autoantibody production,inflammation and glomerulonephritis [116 /C150118]. Given the
important role of IL-6 in the innate and adaptive immuneresponses, tocilizumab, an anti-IL-6 receptor antibody,was developed as a potential therapy for autoimmune dis-eases. Similar to murine models, blockade of IL-6 wasshown to decrease B-cell hyperactivity evidenced bydecreased serum anti-dsDNA levels and disease activityin SLE patients [116, 119 /C150121].
IFN-g
IFN-gsignalling in B cells promotes autoreactive GC
formation and autoantibody production [35, 37].Complementary to the findings of Rahman and Rawlingsand colleagues discussed above for B-cell-intrinsic regu-lation of GC formation, genetic ablation of IFN- gin
the MRL lprand B6. Sle1b murine models prevents lupus-
like disease progression, including glomerulonephritis[37, 122]. IFN- gwas shown to amplify macrophage acti-
vation, enhance anti-dsDNA autoantibody production andupregulate the expression of MHC classes I and II on renalcells, which promoted the inflammatory response in thetarget tissue. IFN- g-mediated production of BAFF by mye-
loid cells was also shown to contribute to disease in theLyn-deficient lupus model [123]. Thus, IFN- gcontributes
to the innate and adaptive arms of the immune responseand augments inflammation in target tissues.
IL-21
IL-21 is produced primarily by Tfh cells and is importantfor B-cell expansion in the GC, class switch recombin-ation and the generation of plasma cells [31, 33, 124,125]. More recently, it has been shown that IL-21, to-gether with IL-6, drives Tfh cell expansion in humansand mice [31, 126]. Furthermore, IL-21 also contributesto Th17 cell differentiation and the expansion ofCD8
+suppressor T cells [127]. Despite its role in expan-
sion of regulatory cells, IL-21 was found to play a crucialrole in driving lupus-like disease in the BXSB. Yaamurine
lupus model [127]. Therefore, IL-21 appears to have thepotential to play both positive and negative roles in pro-
motion of disease manifestations.
BAFF (CD257)
BAFF is primarily secreted by myeloid cells, but can alsobe produced by other types of cells. It stimulates the ex-pansion, differentiation and antibody production of B cells
[128, 129], and its overexpression promotes loss of B-cell
tolerance [26]. It can be active in membrane or solubleform and it can bind to three receptors (transmembraneactivator, calcium modulator, and cyclophilin ligand inter-actor (TACI), B cell maturation antigen (BCMA) and BAFF-R) on B cells, whose expression is modulated in SLE withincreased disease activity [24, 25, 130 /C150132]. BAFF is ele-
vated in the circulation of SLE patients and was shown tobe correlated with anti-dsDNA antibody titres. Mixed re-sults were reported for correlations with disease activity,and BAFF levels were decreased in patients treated withCSs [24, 25, 132]. Of note, 17 b-estradiol induced soluble
BAFF, anti-dsDNA and anti-C1q in a murine lupus model[133]. A mAb to soluble BAFF, belimumab, has beenapproved by the US Food and Drug Administration forthe treatment of SLE and has been found to reduce diseaseactivity and the number of lupus flares [134, 135]. Althoughthis is considered a breakthrough in lupus treatment, furtherstudies are required to determine the best regimen incor-porating this treatment for patient subgroups.
Adaptive immune pro-inflammatory mediators
Aside from being essential for ANA production, B cells andT cells play a crucial role in the inflammatory events thatcontribute to disease progression [136]. B cells can func-tion as antigen-presenting cells for memory T-cell re-sponses; they produce an array of cytokines and can
function in a regulatory capacity [137]. Regulatory B-cell
activity has been shown to be impaired in SLE patients[137, 138]. Multiple types of effector T cells have beenimplicated in disease development in both murinemodels and SLE patients. Depletion of CD4
+Th1 cells pre-
vents disease progression in several murine models, andTh1 cells producing IL-2 and IFN- gare required for anti-
body production by autoreactive B cells [9, 35, 37, 139].IFN-g, along with the Th2 cytokines IL4 and IL5, promotes
the recruitment of lymphocytes to lymph nodes. Moreover,some studies have found that Th2 effector cells promotedisease chronicity in target tissues [140, 141].
Immune-mediated end-organ damage
Inflammation of the skin or mucosal membranes or arth-ritis is seen in milder forms of SLE [142]. More commonly,severe disease manifests in neurological symptoms, renaldisease, vasculitis, serositis involving the heart (pericardi-tis) or lungs (pleuritis), or haematological disorders,including leucopenia, lymphopenia, thrombocytopenia,thrombotic thrombocytopenic purpura, myelofibrosis andautoimmune haemolytic anaemia [142 /C150144].
Ultimately, tissue inflammation can result from a variety
of factors. Given the heterogeneous nature of the disease,antibodies can often, but not always, be involved in local
i60 www.rheumatology.oxfordjournals.orgOlga Zharkova et al .

inflammatory responses. Autoantibodies can form ICs, ac-
tivate innate or resident cells and directly induce targetorgan injury. In addition to autoantibodies, defects in cell
death, cell clearance or phagocytic machinery can initiate
complement activation, IC formation with ensuing myeloidcell activation and cytokine production with resultantautoimmune tissue destruction [145]. Infiltrating and resi-
dent renal myeloid cells activated by ICs produce cyto-
kines and chemokines, such as IL-6 and BAFF, thatpromote B-cell activation and differentiation. The chemo-kines CXCL13 and CCL21 direct lymphocyte migration to
lymphoid follicles in lymph nodes and in target tissues.
Specialized tissue-resident pDCs promote the mainten-ance of Treg populations, and this pDC population andTreg cell numbers are decreased during inflammation[146, 147].
As exemplified in LN, which affects up to 60% of SLE
patients [148], mechanisms involving numerous inflamma-
tory cytokines, leucocytes, responses by the resident
tissue cells, along with alterations in vascular endothelialcell function contribute to tissue damage [149]. Each tis-sue’s response to immune-mediated injury is unique and
has been reviewed extensively [84, 142 /C150144, 149 /C150153],
but ultimately, chronic inflammation can result in tissuedysfunction caused by remodelling and fibrosis.
Summary and future directions
The presence of elevated levels of autoantibodies and nu-merous cytokines serve as markers of immune-mediated
pathology in preclinical SLE disease. Most often, the de-
position of ICs combined with inflammatory cytokinesfrom infiltrating leucocytes perpetuate target organ inflam-mation. The unabated inflammatory response eventually
results in autoimmune-mediated tissue destruction.
However, no single mediator consistently serves as adiagnostic marker for patients with SLE. Although mostpatients at the time of diagnosis have elevated levels of
autoantibodies, which appear to contribute to SLE path-
ology, similar disease manifestations can occur in individ-uals without elevated autoantibodies. Likewise, elevationsin circulating type I IFNs have been found in a majority, butnot all patients. Future strategies will incorporate new
biomarkers to evaluate the patient’s disease and immuno-
logical signature based on our continually evolving under-standing of relevant immunological pathways. The era ofimmunologics and biologics has only just begun.
Acknowledgements
O.Z.’s, T.C.’s and A.M.F.’s work is supported by core
funding from A*STAR at the Singapore Immunology
Network, Singapore. A.B.S. is funded by NationalInstitutes of Health grants AR067625, AI122720.
Funding : A.B.S.’s work is funded by National Institutes of
Health (NIH) grants AR067625, AI122720 and an Alliance
for Lupus Research (ALR) Award 257549 (PI, Davis).L.S.D.’s work is funded by an ALR Award 257549 andNIH grants AR067625 (PI, Satterthwaite) and AI122720
(PI, Satterthwaite).
Disclosure statement : The authors have declared no
conflicts of interest.
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