Disturbance of cytokine networks in Sjögren's syndrome

The difficulty in predicting the consequences of interactions between different cytokine networks has increased with the expansion of the T helper (Th) cell universe and the discovery of numerous B lymphocyte-derived cytokines. Consequently, it is now difficult to conceptualize a straightforward view of the contribution of these disturbances to the pathogenesis of primary Sjögren's syndrome (SS). Th1 cells, which produce interferon-γ and IL-2, and Th17 cells, which make IL-17 and TNF-α, have been cast in the leading roles of the play. However, the complex role of T-cell subsets in SS is accentuated by the reciprocal effects of Th17 cells and regulatory T cells found in salivary glands of SS patients. Furthermore, B lymphocyte polarization into type-1 B effector (Be1) and Be2 cells and B-cell modulating factors of the TNF family, most notably the B-cell-activating factor (BAFF), and their prominent role in SS are additional complicating factors. Whereas Th17 cells orchestrate autoreactive germinal centers, local BAFF would repress the generation of Th17 cells. Such new insights into interconnected cytokines in primary SS may lead to new treatments for these patients.

the production of cytokines [15]. Accordingly, our current interpretation of cytokine-secreting B-cell subsets stems from the Th cell paradigm. Regulatory B (Breg) cells, recently described in humans [16], do exert regulatory eff ects through the production of cytokines. Furthermore, B-cell activation of the TNF family (for example, by B-cell-activating factor (BAFF), also known as B-lymphocyte stimulator (BLyS), and a proliferationinducing ligand (APRIL)) has further substantiated the concept of a notable role for B-cell cytokines in the pathogenesis of SS [17].
Th e impact of abnormal cytokine production in this disease has attracted considerable attention [18]. Whilst the eff ect of a cytokine on one lymphocyte subset in SS can be discerned, it has become a challenge to understand how the interaction between several interconnected networks of cytokines impact on so many diff erent cell populations. Th e concept that the interplay of cytokineproducing T and B cells shifts the balance towards autoreactive T and B lymphocytes has been questioned. Recent fi ndings on the pathogenesis of SS are benefi cial at a time when cytokine-directed therapies are being tested for the treatment of infl ammatory diseases. However, it remains highly complex to ascribe diff erent symptoms to just a single cytokine.

The polarized Th cell paradigm
Upon T-cell activation, the cytokine milieu dictates Th cell polarization. Th us, IFN-γ and IL-12 engage the T box transcription factor, referred to as Tbet, and the signal transducer and activator of transcription (Stat)-4, to transform naïve CD4+ T cells into Th 1 lymphocytes. Th e latter cells are involved in the response to intracellular pathogens, thus inducing the production of IFN-γ and TNF-α, but not IL-4 and IL-13. In contrast, IL-2 and IL-7 cause the binding of a specifi c transcription factor to the WGATAR nucleotide consensus sequence (GATA-3). Th is promotion polarizes naïve T cells towards Th 2 lymphocytes. Th e latter cells are committed to the elimination of extracellular pathogens, thus favoring the production of IL-4 and IL-13. Undoubtedly, GATA-3 represents the master transcription factor for Th 2 diff erentiation. Although the two groups of cytokines are mutually inhibitory, IFN-γ opposes infl ammation in certain disease settings, and IL-4 enhances IL-12 production by macrophages, which in turn favors Th 1 polarization of naïve Th lymphocytes. Whereas uncon trolled Th 1 cells determine autoimmune states, imbalances in Th 2 cells lead to allergic disorders. However, were this binary paradigm to be as presumed, no autoimmune traits should emerge in a proportion of patients with excessive Th 2 cells [19].
Patients with SS have long been thought to suff er from a Th 1-mediated condition. Such interpretation was supported by high levels of IFN-γ in serum [20] and a predominance of Th 1 over Th 2 cells in blood [21]. In addition, T cells containing mRNA for IFN-γ [22] and Stat-1 have been found in the SGs of patients with SS [23]. In fact, the contribution of each Th subset to SS and their interconnections are more subtle than suggested by the earliest data. In this context, for Th 1 cells to underpin SS pathogenesis, one must verify that the activity of Th 1 cells is decreased in the blood of patients, while increased in their SGs [24]. Furthermore, the cytokine pattern may shift from Th 1 to Th 2 as the immunopathological lesions progress, as postulated by Moutsopoulos' group [25]. Supporting their hypothesis, they made the valuable observation that IFN-γ expression is associated with a high-grade infi ltrate of the SGs, whereas a low-grade infi ltrate is instead accompanied by a type-2 response.

The expanding universe of Th cell subsets Th17 cells
Inevitably, the role of Th 1 and Th 2 cells in SS, gleaned from studies of cultured cells and from observations of SS patients, have become contradictory. Th ese discrepancies were resolved by the discovery of IL-23, after which it was determined that abnormalities fi rst ascribed to Th 1 cells were instead engendered by Th 17 cells, named after their IL-17 cytokine signature [11,[26][27][28][29]. Th 17 cells produce a family of cytokines from IL-17A through IL-17F, and, to a lesser extent, TNF-α and IL-22 [11]. Although IL-17 and IL-22 are structurally similar, they bind to distinct receptors and take part in separate intracellular pathways. Furthermore, in contrast to IL-17, IL-22 exerts minor proinfl ammatory eff ects, and, under certain circum stances, even protects from autoimmune outcomes . Th 17 cells are primed by the association of IL-6 with either IL-1 or IL-21 via the orphan retinoid  nuclear receptor γt, but neither Tbet nor GATA-3. IL-21, a member of the IL-2 family, collaborates with dendritic cell (DC)-derived transforming growth factor (TGF)-β to amplify the tendency to Th 17 cell diff erentiation and induce these lymphocytes to express receptors for IL-23. Th e latter cytokine is required for the maintenance of Th 17 [30,31]. It is interesting that, at least in mice, Th 17 lymphocytes can also function as B-cell helpers [32]. Th ey induce a pronounced antibody response, with preferential immunoglobulin (Ig) class switch to IgG2a and IgG3 for IL-17, and to IgG1 and IgG2b for IL-21. Th ese results establish that Th 17 cells are crucial in GC formation.
In line with the mouse data, high serum [33] and saliva [34] levels of IL-17 have been reported in SS patients. In addition, their SGs exhibit a predominance of IL-17containing cells within the infl ammatory lesions [27], consistent with the production of IL-17 by ductal epithelial cells. Further work on SGs detected TGF-β, IL-6 and IL-23, all requisite promoters of Th 17 diff er entiation [31]. Th ese fi ndings add credence to the view that Th 17 cells are possible drivers of the persistent infl am matory response in the SGs of patients with primary SS.

Regulatory T cells
An exciting aspect of homeostasis of the Th 17 cells is their reciprocal relationship with Treg cells. However, there is as yet no universal consensus on their defi nition. Th ey were originally identifi ed by high membrane levels of CD25. Subsequent studies indicated that this prerequisite for identifying Treg cells did not fi t the observation that CD25-CD4+ T cells exert as many regulatory functions as CD25+CD4+ T cells. Th e Treg cells were subsequently identifi ed by the abundance of the forkhead box protein P3 (Foxp3) transcriptional regulator. Foxp3+ cells develop in the thymus as natural Treg cells, or diff erentiate from naïve T lymphocytes in the presence of TGF-β as immune Treg cells. Natural Treg cells expressing the inducible co-stimulate use IL-10 to suppress DC functions, and TGF-β to restrain T cells. Treg cells that do not express this inducible co-stimulate require TGF-β only [34].
Th e reports are contradictory in that the blood of SS patients contains too many [35] or too few Treg cells [36]. Th e real setting could be that Foxp3+ lymphocytes circulating in the blood correlate inversely with those infi ltrating the SGs [37]. Th e fact that there are fewer Treg cells in advanced than in mild SG infi ltrates supports the view that DC-derived TGF-β induces Foxp3 in naïve T cells and switches T-cell diff erentiation from the defective Treg cell pathway to a Th 17 diff erentiation pathway in the presence of IL-6 [30,31].
Similarly, IL-18, which can be secreted by epithelial cells, has been detected in periductal mononuclear cells (MNCs), and correlated with infi ltrating macrophages and increases in serum IL-18 [26]. Th is supplemental mediator would regulate the Th 1 response and amplify IL-17 synthesis [27]. At the time of its identifi cation, the pathological role of IL-18 in the SGs of SS patients was unclear. Since then, we have learned that IL-18 acts as a chemoattractant for CD4+ T cells and a stimulator for antigen-presenting cells, required for the generation of Th 17 cells ( Figure 2). Furthermore, IL-18 promotes the synthesis of proinfl ammatory cytokines, enhances the secretion of chemokines and worsens tissue damage through cell-mediated cytotoxicity and release of matrix metalloproteinases [28]. Ultimately, a handful of macrophages and DCs can play an IL-18-mediated active role in the SGs and in MNC infi ltration.

Up-regulation of IL-6
Not only does IL-6 participate in the generation of Th 17 cells but it also fosters their proliferation and is associated with multiple eff ects in patients with SS, whose SGs have been shown to contain IL-6. Given that it is also derived from Th 17 cells [38], IL-6 can activate local B cells in an autocrine manner. Th e 80-kDa glycoprotein (gp) receptor for IL-6 associates with a signal-transducing 130-kDa gp chain to shape a membrane-bound aggregate. Th e receptor for IL-6 also exists in a soluble form capable of binding to transmembrane gp130 and facilitating signal transduction through homodimerization of gp130 to the ligand-receptor complex [39]. Th us, IL-6 exerts seemingly opposite eff ects by lending strength to Th 17 cells and exerting polyclonal activation of B cells.

IL-6-related T-and B-cell biology
In the presence of IL-6, Th 17 cells orchestrate the development of GCs dominated by autoreactive lymphocytes [40], such as those that we have described in the SGs of SS patients [41]. Moreover, IL-6 contributes to the expression of recombination-activating genes (Rags). Even though some of the activities of IL-6 proceed via its soluble form, the predominance of complexes of IL-6 and the IL-6 receptor is the therapeutic rationale for targeting the receptor rather than the cytokine. Th e soluble form may retain IL-6 and the complex bound to gp130 on the cell membrane and, thus, engage the receptor to the membrane again.
Th is pivotal cytokine seems to be responsible for abnormal B-cell antigen receptor (BCR)-mediated regula tion of Rag genes in B cells in SS patients. Our own data [42] indicate that, along with BCR engagement, IL-6 signaling results in secondary Ig gene rearrangements, and thereby favors the generation of auto-antibodies. Of further interest is the limiting eff ect of IL-6 on the generation of Treg lymphocytes, and the ultimate suppressive eff ect of the latter cells on B lymphocyte responses.

Dysregulated production of IL-6 by B cells
As described in patients with rheumatoid arthritis and systemic lupus erythematosus, their spontaneous activation can induce B lymphocytes to release copious amounts of IL-6 in primary SS [43]. Furthermore, the IL-6 receptor is preferentially expressed on B cells in patients with active disease, and thereby preferentially stimulates the diff erentiation of autoreactive B lymphocytes.

Polarized B lymphocytes
B cells possess the capacity to produce a range of cytokines. Th ese may be grouped as proinfl ammatory cytokines, such as IL-1, IL-6, TNF-α and lymphotoxin (LT)-α; as immunosuppressive cytokines, such as TGF-β and IL-10; or as hematopoietic growth factors, such as IL-7 and granulocyte/macrophage-colony stimulating factor. Th e third family facilitates Th 1 cell polarization and the production of TNF-α by DCs, and derives from macrophages and endothelial cells in the SGs of patients with SS [44].
In reality, the major breakthrough in determining the potential role of B cells in diseases occurred when two distinct cytokine-secreting subsets were identifi ed through the culture of B cells with eff ector T cells associated with their cognate antigens [15]. B lymphocytes polarized in the presence of Th 1 cells were designated B eff ector (Be)1 cells, based on their signature cytokines, IFN-γ and IL-2, in the expected presence of Tbet. Conversely, Th 2 cells induced naïve B lymphocyte polarization into Be2 cells, which produced IL-4 and IL-6, in the unexpected absence of GATA-3. However, IL-10, LT-β, TGF-β, and TNF-α were similarly expressed in Be1 and Be2 cells, yielding an ever-growing complexity of these B-cell subsets.
Th e kinetics of Be cell generation and the cytokine profi le of B cells raise the possibility that the Th 1 phenotype is imprinted on Be1 cells through IL-2 and that expression of IFN-γ by B cells is sustained through an autocrine loop between IFN-γ and the IFN-γ receptor. However, the diff erentiation of naïve B lymphocytes into IL-4-producing Be2 cells is controlled by T-celldependent signals. Of important note, IL-4 is generated by GC B cells and is necessary for Th 2 polarization [45].

Interconnections between the B-and T-cell cytokine networks
LTs are implicated in establishing and maintaining the organization of normal lymphoid tissues. Mice in which LT-α [46] and/or LT-β [47] signaling is disrupted suff er from disturbances in splenic architecture. Intriguing also is the fi nding that DC networks, conspicuous components of B-cell follicles, are lacking in diff erent LT knockout mice [48]. Gonzalez and colleagues [49] showed that B lymphocytes induce membrane LT-α, and that the transfer of B cells (but not T cells) from membrane LT-αpositive mice (but not membrane LT-α-negative mice) governed the emergence of soluble LT-α in the SGs of IL-14α transgenic mice, a model of primary SS [50]. Th us, signaling through LT-α was necessary to reduce aspects of SS in the SGs of non-obese diabetic mice [51]. Activated Th cells crosstalk with activated B cells to regulate their respective responses. Conversely, Be cells modulate T-cell polarization. Th e factors that aff ect T-cell diff erentiation toward Th 1 cells induce naïve B cells to produce IFN-γ via activation of Stat-3, the phosphorylation of which is initiated by IL-12 [52]. A high level of expression of IL-12 has been found in the SGs of SS patients [53], and IL-12-induced SG dysfunction in IL-12 transgenic mice off ers a new model for primary SS [54]. MNCs infi ltrate their exocrine tissues, suggesting that IL-12 contributed to the circuit involving autoreactive T and B cells in SS. Interestingly, IL-10 produced by B cells suppresses IL-12 production by DCs, thus blocking Th 1 cell responses.
Once B cells have been induced to produce IFN-γ, the presence of Th 1 is no longer required to maintain polarized Be cells. Th is is because antigen-specifi c B lymphocytes take up antigen for presentation to T cells and, by doing so, create a self-sustaining circuit of B and T cells through which other naïve T cells may be recruited.
Aside from promoting Th 1 cell polarization, Be1 cells amplify IFN-γ production by T cells via a TNF-αmediated mechanism. Polarization of B cells may take place at sites of infl ammation, such as aff ected SGs [55]. Although patients with ectopic GCs have lower levels of Be2 cytokines than other SS patients, accumulating evidence supports the view that most of these B-cell clusters do not fulfi ll the requisites for ectopic GCs, but constitute aggregates of immature B cells [36]. However, the high affi nity and class switch of auto-antibodies produced imply a local break of B-cell tolerance.
As suggested above, the proinfl ammatory IL-17, normally considered a T-cell-associated factor, has also been reported to be a central driver of GC-derived autoantibodies. Th is was demonstrated by blocking IL-17 signaling that disrupted the CD4 + T-cell and B-cell interactions required for the formation of GCs [40].
Additionally, memory B cells are markedly reduced in the circulation, possibly due to retention in infl amed SGs [56]. Th eir ensuing accumulation, along with shedding of surface CD27 [57], and altered recirculation of B-cell subsets from these sites may all participate in the disturbed B-cell homeostasis in primary SS [58]. Given that CD27+ memory B cells present with a higher transmigratory capacity to CXCL12, also termed stromal cell-derived factor-1 (SDF-1), and to CXCL13, also termed B-cell-attracting chemokine-1 (BCA-1), than CD27-naïve B cells [59], glandular coexpression of these two chemokines [6,7,60] directs memory B cells preferen tially into infl amed SGs, where they reside [61].

Regulatory circuits The transcription factor Tbet in T and B lymphocytes
Th e fi nding of Tbet in B cells had, in fact, been preceded by its description in T cells. Not only does the binding of IFN-γ to its receptor on the surface of naïve T cells activate and hence translocate Stat-1 into the nucleus, but this interaction also promotes the expression of transcription factors involved in Th 1 development. Th us, Tbet induces the transcription of the IFN-γ gene, as well as the expression of receptors for IL-12. Th e net result is that T cells become responsive to IL-12, and translocate Stat-1 into the nucleus, where IFN-γ expression is induced. In turn, IFN-γ drives T cells along the Th 1 pathway through a positive feedback loop.
Similarly, naïve B cells are equipped with receptors for IFN-γ, and can be induced to release Tbet-triggered IFN-γ in the presence of IL-12. Th en, B-cell-derived IFNγ activates B cells in an autocrine manner, and amplifi es Th 1 responses through a paracrine pathway [55]. Consistent with this view is that Tbet-defi cient murine B cells skew antibody isotypes toward IgG1 and IgE, which are isotypes favored by Be2 cells.

GATA-3 and T-cell diff erentiation
Th e absence of GATA-3 in Be cells raises the question of whether it can be replaced by other transcription factors. By counteracting Tbet in T cells, GATA-3 regulates Th polarization directly and Be cell generation indirectly [62]. Th is transcription factor diverts T-cell diff erentiation towards Th 2 cells by silencing Th 1-cell-specifi c transcription factor, and thereby enabling Th 2 cells to proliferate. Co-culture of naïve B cells with Th 2 cells inhibits Tbet, reduces IFN-γ production and reverses the up-regulation of receptors for IL-12.

Conversely, upregulation of IL-4 in Be2 cells depends on both T cells and IL-4. Th is is why B lymphocytes defi cient in the receptor for IL-4 do not transcribe IL-4, and why B cells primed by IL-4-defi cient Th 2 cells substitute IFN-γ for IL-4. Put simply, Tbet (in T cells, but also in B cells) and
GATA-3 (in T cells, but also in B cells) suppress cytokines synthesized by the opposing Th cell subpopulation.

A new generation of ligands and receptors
Two cytokines and their receptors have been demonstrated to be key in B-cell homeostasis: BAFF, which rescues B cells from apoptosis, and APRIL, which participates in B-cell activation [63]. Like most members of the TNF family, BAFF is a transmembrane type I protein that can be cleaved by a furin convertase to produce a 17-kDa soluble form. Th e biologically active form of BAFF is trimeric, but 20 trimers can also associate to form a virus-like 60-mer structure. APRIL and BAFF, referred to as growth factors rather than cytokines by some investigators, have two receptors in common: the B-cell maturation antigen (BCMA) and the transmembrane activator calcium modulator and cyclophilin ligand interactor (TACI). In addition, BAFF binds specifi cally to BAFF receptor 3 (BR3), whereas heparin sulfate proteoglycans are specifi c receptors for APRIL. BAFF receptors are mainly expressed on B cells, but, for each receptor, cell membrane density varies from transitional type-1 (T1) B lymphocytes to plasma cells. In humans, BR3 is present in BT1 cells to memory B cells, but not in plasma cells.
BAFF is critical for B cells to survive in the periphery. It is also involved in B-cell selection by dictating set points for mature primary B-cell numbers and adjusting thresholds for specifi city-based selection during downstream diff erentiation. Th is cytokine has, therefore, aroused much interest because of its association with maintaining and breaching tolerance ( Figure 3). Normally, few immature B cells successfully pass to the T2 stage. Irrespective of the level of receptor expression, BAFF is the dominant agent in the resistance of BT2 cells to apoptosis. In its absence, B-cell maturation is arrested at the T1 cell stage, while BAFF transgenic mice manifest T2 cell hyperplasia in their exocrine glands, which is reminiscent of the B-cell aggregates in the SGs of SS patients. Th e mice, then, develop systemic lupus erythematosus and SS-like disease [64]. Th e explanation is that excess BAFF protects self-reactive B cells from deletion and allows them to move to forbidden follicle or marginal zone (MZ) niches [65].
In the SGs of BAFF transgenic mice, the expanded MZ B-cell compartment comprises self-reactive B cells [40,64,66], in contrast to a splenic architecture in LTα/βdefi cient mice, which lack a structured MZ, preventing MZ B-cell development [67]. Noticeable in this regard is that the progeny of BAFF transgenic mice crossed with LT knockout mice lack MZ B cells and do not develop sialadenitis [68]. Th ese results came as no surprise, while more intriguingly, Treg cell expansion through B-celldependent mechanisms [69] leads to profoundly compromised T-cell responses [70]. Based on these characteristics, BAFF might be regarded as a cytokine rather than a growth factor for B cells.
BAFF is produced by all sorts of macrophages and DCs, and from epithelial cells and activated T lymphocytes. Its mRNA has also been detected in myeloid cells, bone marrow-derived stromal cells, astrocytes, and fi broblastlike synoviocytes in response to proinfl ammatory cytokines. At the protein level, BAFF exists as a membraneassociated molecule, or a cell-free protein, whereas APRIL occurs only in a soluble form.

BAFF overexpression and Sjögren's syndrome
Serum levels of BAFF are increased in association with auto-antibodies in patients with primary SS. Moreover, high levels of BAFF in the serum and saliva of these individuals [71] are associated with anti-sicca syndrome A and anti-sicca syndrome B antibodies and/or rheumatoid factor and/or anti-double-stranded DNA antibody, in some [72,73], but not all [74,75], patients with SS, rheumatoid arthritis or systemic lupus erythematosus. Th ere exists, however, the issue of why serum levels of BAFF remain within, or even below, normal levels in a pro portion of SS patients [76]. In addition, estimates of BAFF fl uctuate with changes in infl ammatory activity. Convinced that such fl uctuations could be due to fl aws or variations in enzyme-linked immunosorbent assays, we developed an in-house assay [77] and detected elevated levels of BAFF in the sera of most SS patients.
BAFF, therefore, is a genuinely promising target for therapy, along with IL-6. Such a combination seems to be in some confl ict, since BAFF promotes B-cell responses whilst IL-6 promotes the Th 17 axis. However, IL-6 is also a prevailing factor in polyclonal activation of B cells, and by rescuing B cells from apoptosis, it promotes their production of IL-6. It is unclear at this stage which of the three cytokines, IL-6, BAFF or IL-17, should be considered the driving force since IL-6-induced B-cell activation also promotes BAFF production [32,38,42,55], and since local BAFF gene silencing suppresses Th 17 cell generation and ameliorates autoimmune arthritis [78]. Th ese data reveal that IL-17 is an eff ector cytokine for BAFF-mediated proinfl ammatory eff ects. Another mouse model, the Act1-knockout mouse, provided information on the signaling pathways induced by BAFF in the development of SS. Act1 is a negative regulator in CD40-and BAFF-mediated B-cell survival [79]. It is relevant that co-stimulation with BAFF rescues Act1-defi cient T1 and T2 B lymphocytes from BCRinduced apoptosis. Consequently, Act1 knockout mice develop autoimmune manifestations similar to SS. Th us, Act1 is negative for B-cell-mediated humoral responses [80], but instead positive for the IL-17 signaling pathway [81].
Th ere have been reports that the aberrant production of these cytokines could be due to excess IFN-α produced by plasmacytoid DCs [82]. A credible candidate for the induction of IFN-α secretion by plasmacytoid DCs is viral infection. Alternatively, IFN-α production in SS may be induced by immune complexes containing nucleic acids. Th e role of this cytokine in SS was recently reviewed by Mavragani and Crow [83]. Th ey highlighted the noted increase in circulating type-1 IFN and an IFN signature in peripheral blood MNCs and minor SGs from SS patients [84]. Altered levels of production of this cytokine may be dependent on genetic and/or epigenetic mechanisms [85], and its blockade therefore is a logical therapeutic target for the treatment of SS.
More importantly, there is good evidence that local production of BAFF contributes to deleterious eff ects of activated B cells by raising their expression of CD19 molecules [4], and ensuring survival of B-cell aggregates, and auto-antibody isotype switching outside and inside GCs [41]. Th is process is sustained by the aberrant expression of BAFF by B lymphocytes infi ltrating the SGs [86,87].

Aberrant production of BAFF by B cells in SS patients
Indeed, due to the dependency of newly formed B cells on BAFF, it is tempting to believe that this cytokine needs to be produced in tissue nearby the cell aggregates. We have demonstrated aberrant expression of BAFF not only in epithelial cells and activated T lymphocytes, but also by single cells isolated from the SGs and by B lympho cytes infi ltrating the SGs of patients with SS [87]. Such might be the reason why rituximab-induced B-cell depletion reduces the Th 17 response [88] in rheumatoid arthritis synovium as well as that of normal Th 17 cells in the absence of B cells in culture. Th is fi nding is also consistent with in vitro and in vivo evidence [89] that activation of B cells induces BAFF and APRIL expression in B cells from normal and autoimmunity-prone mice. Production of BAFF by B lymphocytes is unusual, but malignant B cells produce BAFF [90], which promotes their survival in an autocrine manner. Th is aberrancy is caused by amplifi cation of the BAFF gene in B cells.

Conclusion
Th ere is little doubt that exploring the role of cytokines in SS is a highly promising fi eld of investigation. How the cells and cytokines interact to promote the development of SS is summarized in Figure 4. In general, B-cell depletion has provided clinical benefi ts [91][92][93][94][95]. Some failures might be ascribed [95] to imbalances in Th cell subsets or the depletion of Breg cells. Such striking conceptual advances off er novel perspectives in the treatment of primary SS. Clearly, IL-6, IL-17 and BAFF are major agents in the pathogenesis of SS and, therefore, cytokine targeting would have great therapeutic potential. Nevertheless, B-cell-directed therapies notwithstanding [94], much uncertainty remains as to the best therapeutic

Autoimmune Basis of Rheumatic Diseases
This article is part of a series on Sjögren' s syndrome, edited by Thomas Dörner, which can be found online at http://arthritis-research.com/series/Sjögrens strategy for the treatment of SS. Further development of biotherapies is beyond the scope of this review. However, we can reasonably expect progress in the near future based on the aforementioned new insights into disturbances of the cytokine networks in SS.

Competing interests
The authors declare that they have no competing interests.