microenvironment supportive of germinal center [169, 165].

11.5.5 Adhesion Molecules

In SS molecules in charge of the endothelial cellsÐleukocyte interactions are necessary for the homing and affect the composition of the cellular inÞltrates. Adhesion molecules are controlled in a paracrine manner by locally produced pro-inßammatory cytokines. Intercellular adhesion molecule-1 (ICAM-1), vascular cell adhesion molecule (VCAM-1), and E-selectin are not normally expressed on endothelium, but are signiÞcantly up-regulated by a number of cytokines, including TNF-α, IL-4, and IFN-γ. Activation of endothelial cells is important for the recruitment of T cells, which express CD45RO, L-selectin, and MHC class II, because they migrate preferentially into the lesional tissue [6].

ICAM-1 and E-selectin are increased on endothelium and ICAM-1 on epithelium associated with a strong expression of LFA-1 (integrin αLβ2) and Mac-1 (αMβ2) on the inÞltrating lymphocytes and monocytes. This Þnding suggests a role in the pathogenesis of SS.

11.5.6 Cytokines

Cytokines control the trafÞcking and interactions of cellular components of the immune system, in pSS mostly IL-1, IL-6, IL-10, IL-17, TGF-β, IFN-γ, IFN-α/β, and TNF-α. Reduced levels of TGF-β1 (regarded as an anti-inßammatory cytokine) in SS glands with intense lymphocytic inÞltrates have not only been reported but also refuted [170]. IFN-α/β system is activated in pSS and IFN-α/β is produced by plasmacytoid dendritic cells (pDCs) in tissues [171]. In an already established disease IFN-α/β production by the natural IFN-producing cells is stimulated by intake of anti-SS-A-RNP immune complexes, which are endocytosed by pDC via FcγRIIa. This way the ssRNA hY-RNA molecules get access to Toll-like receptor 7, which stimulates production of IFN-α/β. This is important because hY-RNA

is an adjuvant or a danger signal, whereas the 52 and 60 kDa SS-A protein components can act as autoantigens. It is interesting to study if perhaps membrane-bound microparticles and apoptotic bodies formed from activated and/or apoptotic tubuloacinar epithelial cells can be endocytosed by pDCs already during the early disease stages, before SS autoantibodies have been formed. This could break the autotolerance, stimulate IFN-α/β production, and produce the interferon signature so typical for fully developed salivary glands in SS. Curiously, low-dose oral IFN-α increased production of salivary and diminished the focus score values [172], which may provide a hint that pDCs are not necessarily autoimmune aggressors, but might, for example, via indoleamine 2,3-dioxygenase (IDO) exert immunosuppressive effects. Although effective in many rheumatic diseases, TNF-blocker inßiximab was inefÞcient in a controlled clinical trial in pSS [173] and also effects of rituximab seem to be minor. A more directed use of these drugs might be possible if we could better identify pathogenic subgroups among the patients with SS.

Cytokines have been studied in pSS at three different levels: circulating Th1/Th2 cytokines, cytokine mRNA and proteins in glands, and genetic polymorphism of cytokine genes. Peripherally pSS patients are characterized by a Th2 response, with high levels of circulating IL-6 and IL-10 and cells secreting them. IL-10 levels correlated with IgG1 levels and salivary IL-6 levels have a positive correlation with the focus score. On the other hand, Th1/Th2 cytokines in the SS salivary glands suggest an opposite pattern, with a predominant local Th1 response, although this may have to be revised in favor of Th17 cells (see below) [161]. Therefore, the Th1/Th2 balance shifts in favor of Th1 in exocrine tissues. This shift is most remarkable in glands with the highest focus (inÞltrate) scores. Th1 cytokines IL-2, IL-6, IL-10, TGF-β, and IFN-γ might induce and/or maintain pSS, whereas Th2 cytokines, detected in B and plasma cell-rich cases, might be involved in the progression of the disease [169, 165].

Cytokines can impair secretory function by impairing acetylcholine release [174, 175] and

structural maintenance by stimulating matrix metalloproteinase-mediated destruction of the tubuloalveolar basement membrane [176, 177, 178, 179, 180, 181].

Three cytokines have recently raised interest, IL-18, IL-17, and B-cell-activating factor BAFF. The crucial role for IL-18 in the development of Th1 immune responses has been established since its identiÞcation as a major IFN-γ-inducing factor. IL-18 directly stimulates TNF-α production in macrophages, CD4+, and NK cells, with subsequent release of IL-1β and IL-8, leading to up-regulation of CC and CXC chemokines and adhesion molecules [182].

IL-18, not found in healthy labial salivary glands, is in SS secreted by acinar cells that stimulate them in an autocrine and paracrine mode [182, 183]. IL-18 in periductal inÞltrates was conÞned to CD68+ macrophages [161, 184] being not present in the diffuse cellular inÞltrates, suggesting that a Òcritical massÓ is required for IL-18 expression and that it ampliÞes chronic inßammation [182]. IL-17 was found in ductal epithelial cells, also in healthy controls, and in SS in the inÞltrating T cells [161]. Human parotid gland HSY and salivary acinar AZA3 cell lines express IL-18R and IL-17R on their surface. IL-18, together with IL-17, may activate salivary gland cells and form potential therapeutic targets in SS [161].

IL-17-producing CD4+ T cells are called Th17 cells, which contribute critically to autoimmune diseases. TGF-β induces differentiation of Th17 cells and regulatory T cells from naive T cells, whereas IL-6 and IL-2 act as switch factors for the development of Th17 and regulatory T cells, respectively [161].

Neither IL-17 nor IL-23 (produced by activated antigen-presenting cells) was detected in the submandibular glands of young C57BL/6. NODAec1Aec2 mice, prior to lymphocytic inÞltration, but both were detected in adult mice corresponding to the presence of lymphocyte foci. These cytokines were not observed in SS- non-susceptible C57BL/6 J mice even when leukocyte inÞltrates were present [162]. BAFF (or B-lymphocyte stimulator BLyS) is a transmembrane protein on cell surface of several

cell types [185] and can be proteolytically solubilized. BAFF plays a role in survival, proliferation, and differentiation of B cells. BAFF levels are elevated in systemic autoimmune diseases characterized by autoreactive B cells [5].

The three receptors for BAFF are B-cell maturation antigen (BCMA), BAFF-R, and transmembrane activator and calcium modulator and cyclophilin ligand interactor (TACI). BCMA and BAFF-R are predominantly expressed on B lymphocytes, whereas TACI is mostly found on memory B cells and activated T cells [167].

Increased serum BAFF has been reported in pSS, in some studies associated with increased autoantibody and immunoglobulin levels. BAFF is increased in labial salivary glands, especially among inÞltrating T lymphocytes and increases upon stimulation with IFN-α and IFN-γ [185]. BAFF-mediated survival signals might compromise the ability of autoreactive B cells to die via apoptosis (impaired negative selection). BAFF is required for maintenance, but not initiation of the germinal centers [167].

Two classes of human BAFF antagonist have been developed. BAFF neutralizing fully humanized (human) antibody (belimumab, LymphoStat- B R ), which binds soluble human BAFF and prevents its interaction with the BAFF receptors, is currently in clinical trials in systemic lupus erythematosus and rheumatoid arthritis [186]. BAFF-neutralizing soluble decoy receptor, a fusion protein of TACI with immunoglobulin IgG1-Fc, TACI-Ig (atacicept), is in trials for systemic lupus erythematosus, lupus nephritis, rheumatoid arthritis, multiple sclerosis, and several B-cell malignancies [187]. Finally, intervenous immunoglobulin contains anti-BAFF and

It is an open question if SS is either characterized by improperly functioning lymphocytes attacking innocent acinar targets or, vice versa, by damaged acini attracting an immune aggression mediated by properly functioning lymphocytes [189]. This chapter was written to highlight the last-mentioned side of the coin.