As noted before, B-cell hyperactivity is a major finding in pSS. Although the direct pathophysiological role of B cells in glandular tissue destruction in pSS has not been fully elucidated, B-cell-targeted treatment has been proposed as a therapeutic modality in pSS [34
]. Most B-cell-depleting therapies target CD20, expressed on B cells from the stage of pre-B cells until the stage of activated B cells but not on plasma cells.
An open-label phase II study with the anti-CD20 monoclonal antibody rituximab (four weekly infusions of 375 mg/m2
) in eight patients with early pSS and in seven patients with pSS and mucosa-associated lymphoid tissue lymphoma showed improvement, both subjective and objective, in salivary gland function [35
]. An increase in saliva secretion occurred only in patients with residual saliva production (Figure ). Despite full depletion of CD19-positive B lymphocytes from the peripheral blood, levels of immunoglobulins did not change - but a significant decrease in IgM rheumatoid factor was seen. The percentage and state of activation of T-cell subsets did not change. Peripheral blood B cells had returned after 36 weeks (but were still below baseline) and salivary flow, after initial significant improvement, had declined to just above baseline at 48 weeks [36
Figure 1 Stimulated whole saliva secretion following rituximab treatment in patients with primary Sjögren's syndrome. Stimulated whole saliva secretion (SWS) at baseline and at 5 and 12 weeks following rituximab treatment in 14 patients with primary Sjögren's (more ...)
Retreatment with rituximab resulted in a clinical and biological response fully comparable with that of the initial treatment effect [36
]. In five patients, four of whom showed an increased salivary flow rate following treatment, parotid biopsies were performed before and 12 weeks after treatment [37
]. Histopathological analysis of the biopsies showed a strong reduction of the lymphocytic infiltrate with (partial) disappearance of germinal center-like structures. The B cell/T cell ratio decreased, indicating a higher reduction in B cells than in T cells, but B cells were not completely depleted despite full depletion from the peripheral blood. Intraepithelial lymphocytes in the ducts and the amount and extent of lymphoepithelial lesions decreased, demonstrating reduction in T lymphocytes as well. Most interestingly, cellular proliferation of acinar parenchyma decreased after treatment, sometimes resulting in normal acinar structures (Figure ). These data demonstrate that B-cell depletion via rituximab not only reduces B cells in the diseased glands, but also influences the presence of infiltrated effector T cells - so allowing restoration, at least in part, of the architecture of the ducts and acini. This observation strongly argues for a major role, if not a primary role, of B cells in the pathogenesis of pSS.
Figure 2 Histopathology of parotid gland before and after treatment with rituximab in primary Sjögren's syndrome. Comparison of parotid biopsy specimens obtained from a primary Sjögren's syndrome (pSS) patient before rituximab therapy (A1 to A4) (more ...)
Following these initial studies several, in part controlled, trials - although small in size - have confirmed the efficacy of rituximab in pSS. DevauchellePensec and colleagues treated 16 pSS patients with two infusions of rituximab (375 mg/m2
) and noted a decrease of subjective complaints of dryness, fatigue and arthralgia [38
]. B cells were strongly reduced in the peripheral blood and labial salivary glands but the focus score in the gland did not change and neither did the authors observe an increase in salivary flow, possibly because of the already long history of pSS in these patients. Lack of salivary flow restoration following rituximab treatment was also observed in the study by Pijpe and colleagues in pSS patients with longstanding disease and low levels of salivary flow [35
]. Dass and colleagues performed a controlled study on 17 pSS patients with rituximab (1 g twice, 2 weeks apart) and noted a significant decrease in fatigue persisting for at least 6 months [39
]. Unstimulated salivary flow did not change in this group with longstanding pSS (median disease duration 7.25 years).
Longstanding pSS leads to further decrease in saliva production (Figure ), and residual saliva production, as mentioned before (Figure ), is a prerequisite for an increase in salivary flow following rituximab treatment. B-cell depletion was accompanied by a reduction in rheumatoid factor, but not in levels of immunoglobulins or other autoantibodies. A controlled study on 30 patients with early pSS using two infusions of rituximab (1 g) showed a significant increase in stimulated and unstimulated salivary flow. Again, a decrease in rheumatoid factor but no change in levels of immunoglobulins was noted [40
Figure 3 Relationship between disease duration and salivary flow rates in patients with primary Sjögren's syndrome. The relationship between disease duration (the time from first complaints induced by or related to oral dryness until referral) and mean (more ...)
All of these studies thus report efficacy of rituximab in reducing fatigue and extraglandular symptoms including arthralgia, whereas an increase in salivary flow is dependent on the residual function of the glands that is related to disease duration. Since unpublished data from our group show that rituximab treatment results in decreased serum levels of proinflammatory cytokines, chemokines and adhesion molecules, B cells may play a major role also in the global symptoms and extraglandular manifestations of pSS.
As mentioned above, studying recurrence of B cells after B-cell depletion by rituximab offers an opportunity to analyze the pathogenic events leading to recurrence of symptoms. Lavie and colleagues reported the role of BAFF in B-cell repopulation after rituximab treatment [41
]. They observed an increase of serum BAFF and BAFF mRNA in peripheral blood mononuclear cells. The authors concluded that an increase of serum BAFF is related to disappearance of BAFF receptors after B-cell depletion, and that B cells exert negative feedback on BAFF production by monocytes - explaining the increase of BAFF mRNA in monocytes following B-cell depletion.
The role of BAFF in recruiting (autoimmune) B cells in pSS has been further explored by Pers and colleagues [42
]. They observed that serum BAFF levels were inversely correlated with the duration of B-cell depletion. In some patients repeated labial salivary gland biopsies were performed, showing that partial B-cell depletion in the glands persisted for at least 12 months and B cells had recurred at 24 months. Whereas repopulation of the peripheral blood showed increased numbers of mature naïve B cells (Bm2 cells) and decreased numbers of memory B cells, repopulation of the salivary gland showed memory B cells and transitional type 1 B cells as the first B cells to be identified. These memory B cells were speclated to be autoreactive. We also observed delayed recovery of CD27+
memory B cells in the blood 48 weeks after rituximab treatment, whereas the majority of emerging B cells had a phenotype of transitional B cells [43
A recent study analyzed gene expression profile of labial salivary glands before and after rituximab treatment and related these profiles to the clinical response on rituximab [44
]. Interestingly, the authors found two groups of genes higher expressed in responders than in nonresponders. The first group consisted of genes involved in the B-cell signaling pathway and the second group was related to genes involved in the interferon pathway. These data fit the concept of IFNα-induced BAFF expression resulting in B-cell hyperactivity and prolonged B-cell survival.
One open-label study targets CD22 on B cells [45
]. This molecule has a more or less similar distribution profile to CD20. Treatment of 16 patients with a monoclonal anti-CD22 antibody, epratuzumab, resulted in improvement of unstimulated whole saliva production and a decrease in fatigue in one-half of the patients.
In summary, B cells seem to play a major role in orchestrating the pathological immune response in pSS. Depleting B cells offers a unique possibility to study the immunopathogenesis of pSS. BAFF appears as a strong stimulant for B-cell activation and proliferation and for B-cell survival in pSS.