70
Fig. 5.1 In the presence of their specific antigen, T helper (Th) 1 and Th2 cells polarize naïve B lymphocytes toward B effector (Be)1 and Be2 cells. Conversely, Be1 and Be2 induce naïve T lymphocytes into Th1 and Th2 cells, respectively
|
|
|
J.-O. Pers et al. |
Th1 |
Antigen |
Be1 |
B cell-Type 1 : cytokines |
|
+ |
|
Restimulation |
|
|
|
|
Th2 |
Naïve B cell |
Be2 |
B cell-Type 2 cytokine |
|
|
|
|
|
|
|
B cell-Type 1 : cytokines |
Be1 |
Antigen |
Th1 |
|
|
+ |
|
Restimulation |
|
|
|
|
Be2 |
Naïve B cell |
Th2 |
T cell-Type 2 cytokine |
5.2The Role of B Cells in SjS
5.2.1The Impact of B Cell Cytokines
Acting in synergy with one another, BAFF and APRIL behave as conclusive determinants of the development of autoimmune disorders (reviewed in [27]). These two B cell-specific cytokines have two receptors in common: TACI (“transmembrane- activator, calcium modulator and cytophilin ligand interactor”), and BCMA (“the B-cell maturation antigen”). In addition, BAFF (but not APRIL) binds to BR3, whereas APRIL (but not BAFF) binds to membrane heparin sulfate proteoglycans. Several reviews have touched on this topic (see for example [28]). When overexpressed in transgenic (Tg) mice, BAFF induces autoimmune disorders, with the emergence of an SLE [29] and the subsequent development of a SjS-like pattern [30].
BAFF are elevated in sera [31], saliva [32], and salivary glands [33] of patients with primary SjS. However, a number of conflicting results have cast doubt on the reliability of the enzyme-linked immunosorbent assays (ELISA) presently in use for its quantification. There is also the intriguing issue of why the serum concentrations of BAFF remain within, or below, normal range in a proportion of patients with SLE [34], RA [35], primary SjS [36], or SSc [37].
In addition, estimates of BAFF fluctuate with changes in inflammatory activity, extent of the damages, and classification criteria chosen by the investigators for the diseases. Furthermore, the disease activity correlates better with leukocyte BAFF messenger RNA amounts than with plasma BAFF protein titers [38]. Awareness of so many flaws prompted us to set up our own ELISA for the measurement of BAFF [39]. This in-house test appeared to be satisfactory, based on the finding that the majority of SjS patients display high serum levels of BAFF. We therefore considered the antibodies raised against synthetic peptides and used in certain ELISAs as capture or revealing agents. We reasoned that, the ELISA polyclonal antibodies or monoclonal antibodies recognize the nonglycosylated form of BAFF but not its glycosylated form, a bias might derive from excessive nonglycosylated BAFF, at the expense of its
5 Etiopathologenic Role of B Cells in Primary Sjögren’s Syndrome |
71 |
Fig. 5.2 The whole ontogeny of B lymphocytes. They exit the bone marrow (BM), which is the primary lymphoid organ, as immature B cells, and settle down in the spleen, which is a secondary lymphoid organ. Following the antigen (Ag) encounter, they migrate back to the BM in the form of long-lived plasma cells
Bone marrow |
Spleen |
Blood |
|
|
|
Circulating B cells |
|
PRO-B |
T2 B |
Non-circulating B cells B2 |
|
PRE-B |
T2 B |
MZB |
|
|
|||
|
|
B1 |
|
Long-lived |
Ag |
MATURE |
|
Plasma cells |
B |
||
|
Short-lived
Plasma cells
glycosylated form, or from the presence of intergenic splice variants, such as D3 [40] and D4 BAFF (our unpublished results), or from the predominance of heterotrimers of BAFF and APRIL over homotrimers of BAFF [41] or even from the presence of enormous virus-like structures that contain 60 monomers of BAFF [42].
At first glance, the fact that the Th-17 lymphocyte subpopulation have recently been recognized as dominant within inflammatory tissues [43] does not fit with the B-cell dominance? One may suggest that the Th-17-centric cytokine IL-17 acts in synergy with BAFF to influence B-cell biology in various autoimmune settings. Supporting this hypothesis [44] is the intriguing observation that IL-17 and IL-17 receptor blocker exert opposite effects in mice with SjS.
B lymphocytes are thus stimulated, as reflected by the generation of a vast array of autoantibody-producing cells, including those nicely detected within plasma cells (PCs) in the salivary glands by the Jonsson’s group [45]. Furthermore, IgA class rheumatoid factor (RF), IgA-containing immune-complexes [46], monoclonal Ig [47], and their particular presentation as mixed cryoglobulins [48], are not uncommon in these patients. Moreover, they document the existence of a continuum from benign to malignant lymphomas [49]. In this respect, Southern blot methods may detect B cell clonal expansions in the salivary glands of patients with SjS, without circulating monoclonal Ig [50].
5.2.2Ontogeny of B Lymphocytes
A prolific approach to unraveling the function of the different B cell subsets has been to analyze their ontogeny. Figure 5.2 depicts the whole ontogeny from the bone marrow (BM), which is the primary lymphoid organ, to the spleen, which is a secondary lymphoid organ (SLO), and, following the antigen encounter, back to the BM in the form of long-lived PCs. On their arrival to the spleen, immature B cells give rise to type-1 (BT1), type-2 (BT2), and possibly type-3 [51] transitional
72 |
J.-O. Pers et al. |
B cells. Only a minor fraction of immature B cells survive the shift from immature to the mature naïve stage, so that the transitional B-cell compartment is widely believed to represent a negative selection checkpoint for autoreactive B cells [52].
BAFF facilitates the maintenance of B cells through this checkpoint, to such an extent that crosstalk between BCR and BR3 emerges as a fundamental mechanism to regulate transitional B-cell survival [53]. The consequence of a local excess of BAFF is that self-reactive B cells are unduly rescued from deletion, and thereby offered the possibility to enter forbidden FO and marginal zone (MZ) niches [54]. Such upregulation of BAFF overcomes the reduced competitiveness of autoantigen-engaged B lymphocytes, due to their increased dependence on BAFF [55]. B lymphocytes differentiate into marginal-zone MZ or into FO B cells [56], depending on the affinity of the BCRs to their antigens [57]. Once entered the FO, the BT2 cells initiate the development of novel germinal centers, or colonize preexisting germinal centers [58].
In the salivary glands of patients with primary SjS, these structures have long been acknowledged as similar to those in SLOs. In reality, numerous MZ-like structures arise as aggregates in the tissue, locate distally to the genuine germinal centers, and position away from the epithelium [59]. Even worse, these aggregates and genuine ectopic germinal centers do not exclude autoreactive B cells as established in primary SjS [21], and SLE [60], through the binding of a 9 G4 monoclonal antibody that recognizes the anti-V4.34-encoded autoantibodies. We have compared the B cells of aggregates with those of ectopic genuine germinal centers. Of interesting note in terms of pathophysiology, the activation-induced cytidine deaminase (AID), required for Ig switch as well as somatic hypermutations [61], was detected [62]. This enzyme was expressed within the DC networks and interfollicular large B cells. The presence of AID suggests that these cell aggregates are functional.
5.2.3Subpopulations of B Cells
Subsequent variations in the expression level of IgD and CD38 have led to a model for Bm homeostasis across the germinal centers. An inference from this proposal is that, once activated, naïve Bm1 lymphocytes become Bm2 cells, and develop into germinal center founder Bm2¢ cells. They differentiate further into centroblastic Bm3 cells, and centrocytic Bm4 cells, ending with early memory Bm5 and memory Bm5 or PCs. Throughout this process, cells representing each subset are released into the peripheral blood [63]. Patients with primary SjS exhibit disturbances in their distribution, mainly involving intrinsic B cell lymphoma (Bcl)-6 and increase in circulating Bm2/Bm2¢ cells [64], and a reciprocal decrease in early Bm5 and Bm5 cells [65]. Thus, not only are the B cells hyperactive in patients with SjS, but their distribution in their peripheral blood is disturbed. In these patients, there is a high ratio of increased percentages of circulating Bm2-plus-Bm2¢ cells to decreased percentages of early Bm5-plus-Bm5 cells that differentiates them (Fig. 5.3) from patients with RA or SLE [66], which is a case for B cells in the pathogenesis of primary SjS. Furthermore, this unique distribution of B cell subsets is a signature for primary SjS, relative to miscellaneous diseases, which might even constitute a diagnostic tool (Fig. 5.3).
|
|
pSS |
|
|
|
Normal control |
|
|
|
|
|
SLE |
|
|
|
|
|
|
RA |
|||
|
|
|
44.0 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
1.8 |
|
0.8 |
|
0.3 |
|
|
2.7 |
2.8 |
|
|
1.4 |
0.2 |
|
|
|
||||||||
CD38 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
2.0 |
|
50.6 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
40.7 |
|
|
|
|
|
9.5 |
|
|
24.6 |
|
20.4 |
|
|
40.3 |
|
10.6 |
|
|
|
|||||
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
1.2 |
|
1.0 |
|
|
|
|
45.0 |
|
15.3 |
|
|
18.3 |
|
|
13.8 |
|
|
|
31.6 |
||
|
|
17.0 |
|
|
|
|
|
|
||||||||||||||
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
IgD
Fig. 5.3 The relative expression of CD38 and IgD of mature B (Bm) lymphocyte defines naïve Bm1, activated Bm2, germinal center founder Bm2¢, centroblasts Bm3, centrocytes Bm4, early memory (eBm5), and memory Bm5 cells. Excess of Bm2/Bm2¢ cells in the circulation characterizes patients with primary Sjögren’s syndrome, relative to normal controls and disease controls with rheumatoid arthritis (RA) or systemic lupus erythematosus (SLE)
Syndrome Sjögren’s Primary in Cells B of Role Etiopathologenic 5
73