To assess whether abnormal chemokine receptor expression and/or abnormal responsiveness to the cognate ligands might underlie some of the disturbances in B cell homeostasis characteristic of primary Sjögren's syndrome (SS).
To assess whether abnormal chemokine receptor expression and/or abnormal responsiveness to the cognate ligands might underlie some of the disturbances in B cell homeostasis characteristic of primary Sjögren's syndrome (SS).
Chemokine receptor expression by CD27− naive and CD27+ memory B cells from patients with primary SS and healthy control subjects was analyzed using flow cytometry, single-cell reverse transcriptase–polymerase chain reaction (RT-PCR), and migration assays.
In contrast to healthy subjects, significantly higher expression of both surface CXCR4 and CXCR4 messenger RNA (mRNA) was seen in peripheral blood B cells from patients with primary SS. These differences were most prominent in CD27− naive B cells (P ≤ 0.0006). In addition, significantly higher frequencies of CD27− naive B cells from patients with primary SS expressed mRNA for the inhibitory regulator of G protein signaling 13 (P = 0.001). Expression of CXCR5 by peripheral CD27+ memory B cells was moderately diminished in patients with primary SS compared with healthy controls (P = 0.038). No significant differences were noted in the expression of CXCR3, CCR6, CCR7, and CCR9 between B cells from healthy controls and those from patients with primary SS. Transmigration assays of blood B cells from patients with primary SS and healthy controls showed comparable responses of CD27− naive B cells but significantly diminished responses of activated primary SS CD27+ memory B cells to the ligands of CXCR4 and CXCR5, CXCL12 (P = 0.032), and CXCL13 (B lymphocyte chemoattractant; B cell–attracting chemokine 1; P = 0.018), respectively, when compared with those from healthy controls. Finally, compared with controls, peripheral reduction but glandular accumulation of CXCR4+,CXCR5+,CD27+ memory B cells was identified in patients with primary SS.
In primary SS, overexpression of CXCR4 by circulating blood B cells does not translate into enhanced migratory response to the cognate ligand, CXCL12. This migratory response may be modulated by intracellular regulators. Retention of CXCR4+,CXCR5+, CD27+ memory B cells in the inflamed glands seems to contribute to diminished peripheral CD27+ memory B cells in primary SS.
Primary Sjögren's syndrome (SS) is characterized by chronic focal lymphocytic inflammation of the lacrimal and salivary glands, resulting in keratoconjunctivitis sicca and xerostomia. Both interaction of activated glandular epithelial cells with infiltrating lymphoid and dendritic cells and systemic lymphocyte derangement are thought to contribute to the pathogenesis of primary SS (for review, see refs. 1 and2). The lymphoid infiltrates within the inflamed glands often contain germinal center (GC)–like structures consisting of T and B cell aggregates with proliferating lymphocytes and a network of follicular dendritic cells and activated endothelial cells (3, 4). Besides antigen-driven clonal proliferation of B cells (3, 5), analyses of inflamed glandular tissue from patients with primary SS also reveal a polyclonal accumulation of CD27+ memory B cells and CD27high plasma cells (6, 7). Moreover, immunophenotyping studies indicate that there is disturbed B cell homeostasis in patients with primary SS, with diminished frequencies and absolute numbers of peripheral CD27+ memory B cells (6, 8, 9). More recently, a single-cell messenger RNA (mRNA) study showed further abnormalities, especially in the mechanisms of heavy chain switch recombination (10).
Chemokines and their corresponding chemokine receptors play an important role in lymphopoiesis, differentiation, homing, recirculation, and immune responses of lymphocyte subsets under physiologic and pathologic conditions (11–14). The inflamed glands seen in primary SS have been shown to express a unique profile of adhesion molecules, cytokines, and chemokines, including overexpression of CXCL13 (B lymphocyte chemoattractant [BLC]; B cell–attracting chemokine 1 [BCA-1]) mRNA and protein, a central chemokine involved in B cell homing (15–17), as well as of CCL19, CCL18, CXCL9 (monokine induced by interferon-γ), and CXCL10 mRNA (17, 18, 19). Moreover, CXCR5-expressing B cells have been detected in the glandular infiltrates of patients with primary SS (15, 16). Thus, it has been proposed that disturbances in chemokine expression may selectively guide and regulate lymphoid subsets into or within the target tissues as well as the (re)circulation between blood and secondary lymphoid organs of patients with primary SS.
In order to delineate these disturbances in greater detail and to determine whether these abnormalities might contribute to the disturbed B cell homeostasis in patients with primary SS, we analyzed the expression of chemokine receptors known to provide critical positioning clues for B cells and plasma cells during development and/or immune responses, including CXCR3, CXCR4, CXCR5, CCR6, CCR7, and CCR9 (11, 12, 20).
After the local ethics committee granted approval and the patients provided informed consent, heparinized whole blood samples (10 ml) were obtained from 21 patients with primary SS (20 women; mean ± SD age 57.6 ± 14.6 years, age range 25–79 years, 1 man; age 44 years) at the Department of Medicine, University Hospital Charité, Berlin. The mean ± SD disease duration was 7.1 ± 3.8 years (range 1–13 years). The patients fulfilled both the American College of Rheumatology (21) and the revised American–European Consensus Group (22) classification criteria for primary SS. All patients tested positive for antinuclear antibodies (fine speckled pattern) as well as for anti-Ro and/or anti-La antibodies and/or rheumatoid factor. All had focal lymphocytic sialadenitis of the minor salivary glands (focus score >1/4 mm2) and a positive Schirmer I test result. The patients received no glucocorticoids or immunosuppressive drugs. As controls, heparinized blood samples from apparently healthy subjects and patients with systemic lupus erythematosus (SLE), matched by age and sex with the primary SS patients were also analyzed.
Peripheral blood mononuclear cells (PBMCs) were obtained by centrifugation on Ficoll-Paque (Amersham Pharmacia Biotech, Uppsala, Sweden) gradients, as previously described (23). In addition, PBMCs were also analyzed, mononuclear cells were prepared, as previously described (6, 7), from minor salivary gland biopsy samples from 4 patients with primary SS and 1 female patient with nonspecific sialadenitis.
For flow cytometric analysis of chemokine receptor expression on peripheral CD19+,CD27− naive and CD19+,CD27+ memory B cells, PBMCs from 16 patients with primary SS, 10 healthy control subjects, and 12 SLE patients were stained with a fluorescein isothiocyanate (FITC)–conjugated monoclonal antibody (mAb) to CD19 (clone HD37; Dako, Glostrup, Denmark), with a Cy5-labeled mAb to CD27 (clone 2E4; a kind gift from Dr. René van Lier, Department of Immunobiology, Academic Medical Center, Amsterdam, The Netherlands), and with phycoerythrin (PE)–labeled mAb specific for one of the following chemokine receptors: CXCR3 (clone 1C6; BD PharMingen, San Diego, CA), CXCR4 (clone 12G5; BD PharMingen), CXCR5 (FAB 190F; R&D Systems, Minneapolis, MN), CCR6 (clone 11A9; BD PharMingen), CCR7 (FAB 197F; R&D Systems), or CCR9 (FAB 179F; R&D Systems). PE-conjugated IgG2a (clone G155-178; BD PharMingen) and IgG2b (clone 133303; R&D Systems) (as negative controls) were used in conjunction with the respective chemokine receptor–specific antibodies. Incubation with antibodies was performed in phosphate buffered saline (PBS)/0.5% bovine serum albumin (BSA)/5 mM EDTA at 4°C for 15 minutes. Subsequently, cells were washed twice in PBS/2% BSA/4 mM EDTA. Propidium iodide (1 μg/ml; Sigma, Munich, Germany) was added immediately before flow cytometric analysis to exclude dead cells. Flow cytometric analyses were performed using a FACSCalibur and CellQuest software (Becton Dickinson, San Jose, CA). For analysis of CXCR4 and CXCR5 coexpression, streptavidin–peridin chlorophyll protein–labeled/biotinylated anti-CD19 (clone 1D3; BD PharMingen) and FITC-labeled anti-CXR5 (FAB 190F; R&D Systems) mAb were used in combination with anti-CXCR4 and anti-CD27 mAb (shown above).
Altogether, 720 single-sorted CD19+, CD27− and CD19+,CD27+ B cells from 4 patients with primary SS (168 CD27− naive cells, 168 CD27+ memory cells) and 4 healthy controls (192 CD27− naive cells, 192 CD27+ memory cells) were analyzed. Individual B cells were sorted (FACSVantage; Becton Dickinson) into single wells containing modified 1× RT-PCR buffer (5 mM dithiothreitol, 400 ng oligo[dT]18, 0.2 mM dNTP, 1% Triton X-100, 10 units RNasin, 40 units avian myoblastosis virus reverse transcriptase), as previously described (10, 24). First-strand complementary DNA (cDNA) was generated at 42°C for 60 minutes. Transcripts for the chemokine receptors CXCR3, CXCR4, CXCR5 splice variant 1, CXCR5 splice variant 2, and the inhibitory regulator of G protein signaling 13 (RGS13) (25) were amplified by specific nested PCR protocols using 5 μl cDNA in the first round and 5-μl aliquots of the external PCR mixtures in the second round. GAPDH-specific transcripts were analyzed as internal controls. The PCR conditions included a 5-minute denaturation at 94°C, followed by 35 cycles of denaturation at 94°C for 1 minute, annealing for 45 seconds, and extension at 72°C for 1 minute. Oligonucleotide sequences are shown in Table 1. The PCR products were separated on 1.2% agarose gel. Following column purification, several PCR products from all primer combinations were directly sequenced using the BigDye Termination Sequencing kit (Perkin Elmer, Emeryville, CA) and analyzed with an automated sequencer (ABI 377; Perkin Elmer). Sequence alignments were performed by BLASTN searches against nucleotide databases (National Center for Biotechnology Information, Bethesda, MD; online at www.ncbi.nlm.nih.gov/blast). To calculate the sensitivity of each specific nested PCR protocol (e.g., for CXCR4, CXCR5, or GAPDH), limiting dilution experiments with purified target DNA were performed, which indicated that as few as 1–10 cDNA copies could be detected with each of the nested protocols used.
|Oligonucleotide||Sequence (5′ → 3′)||NCBI accession no.||Position||Product size, bp|
CD+ peripheral blood B cells were enriched by positive immunomagnetic separation (Miltenyi Biotec, Bergisch Gladbach, Germany) and subsequently incubated overnight at 37°C under 5% CO2–buffered conditions in RPMI 1640 medium (Biochrom, Berlin, Germany) supplemented with 2 mML-glutamine, 10% fetal calf serum, 25 mg/ml penicillin/streptomycin, and 1 μg/ml lipopolysaccharide (LPS) (from Escherichia coli; Sigma). Subsequently, cell migration was examined in wells containing transwell inserts (Costar, Bodenheim, Germany) with a 6.5-mm diameter and 5-μm pores using fibronectin (Invitrogen, Karlsruhe, Germany)–precoated membranes, as previously described (26, 27). Briefly, 5 × 105 B cells per upper well were suspended in RPMI 1640 medium supplemented with 0.5% BSA (Sigma) and then incubated for 90 minutes at 37°C under 5% CO2–buffered conditions. Migrated and nonmigrated cells from each patient were analyzed separately by flow cytometry for the expression of CD19 and CD27. Optimal chemokine concentrations for migration were 50 nM for CXCL12 and 250 nM for CXCL13. In addition, the transmigratory capacity of peripheral B cells was also analyzed without preincubation. B cells from 5 patients with primary SS and 5 healthy controls were analyzed.
Data are expressed as the mean ± SD. Statistical analysis was performed using GraphPad Prism 3.0 software for Windows (GraphPad Software, San Diego, CA). Frequencies of B cells were calculated using CellQuest software, and variations in the chemokine receptor expression on B cells were compared using the nonparametric Mann-Whitney U test. Fisher's exact test was used to compare differences in the frequencies of cells expressing chemokine receptor– or RGS13-specific mRNA transcripts, respectively. P values less than 0.05 were considered significant.
Using 4-color flow cytometry, CD19+ B cells were analyzed for the expression of CD27 as a marker of memory B cells and for the expression of chemokine receptor CXCR3, CXCR4, CXCR5, CCR6, CCR7, or CCR9. Dead cells were excluded by propidium iodide staining. Frequencies of positive cells and the geometric mean fluorescence intensity of anti–chemokine receptor staining were calculated according to statistical thresholds set in reference to staining with negative control antibodies. The frequency of peripheral CD27+,CD19+ memory B cells was significantly reduced in patients with primary SS compared with healthy control subjects (mean ± SD 13.3 ± 12.3% versus 25.6 ± 7.2%; P ≤ 0.0014), whereas the frequency of CD19+,CD27− naive B cells was significantly enhanced in patients with primary SS (mean ± SD 86.1 ± 12.0% versus 74.4 ± 7.2%; P ≤ 0.0014) as reported previously (6, 8, 9).
To ensure that these alterations in patients with primary SS did not further influence the analyses, either CD19+,CD27− or CD19+,CD27+ B cells were gated, and the chemokine receptor expression was subsequently analyzed within each subpopulation (Figures 1A and B). Significantly higher percentages of CXCR4-expressing CD27− naive B cells (mean ± SD 95.2 ± 2.9% versus 87.7 ± 4.2%; P = 0.0003) and CXCR4-expressing CD27+ memory B cells (78.5 ± 10.1% versus 63.6 ± 17.8%; P = 0.0251) were found in patients with primary SS compared with healthy controls. Moreover, the geometric mean fluorescence intensity of anti-CXCR4 staining on CD27− naive B cells (mean ± SD 189.5 ± 75.8 versus 95.1 ± 30.4; P = 0.0021) and CD27+ memory B cells (62.6 ± 26.1 versus 28.7 ± 14.6; P = 0.0021) was significantly enhanced in patients with primary SS as compared with healthy controls (Figures 2A and B).
To evaluate whether this alteration is specific for primary SS or is a general feature of systemic autoimmune diseases, peripheral blood B cells from SLE patients were also analyzed for surface expression of CXCR4. The frequency of CXCR4+,CD27− naive B cells (mean ± SD 95.2 ± 2.9 in primary SS versus 84.5 ± 9.5 in SLE; P = 0.0017) and CD27+ memory B cells (78.5 ± 10.1 in primary SS versus 52.0 ± 14.5 in SLE; P = 0.0001) was significantly enhanced in patients with primary SS as compared with SLE patients, whereas there were no significant differences between SLE patients and healthy subjects. The density of CXCR4 expression on CD27− naive B cells (geometric mean fluorescence intensity ±SD 189.5 ± 75.8 in primary SS versus 85.3 ± 78.6 in SLE; P = 0.0015) and CD27+ memory B cells (62.6 ± 26.1 in primary SS versus 19.3 ± 12.9 in SLE; P = 0.0001) of patients with primary SS was found to be significantly enhanced compared with those in patients with SLE. Again, there were no significant differences in CXCR4 expression between SLE patients and healthy controls. Notably, the density of CXCR4 expression was significantly higher on CD27− naive B cells than on CD27+ memory B cells in all 3 groups analyzed (healthy controls, and patients with primary SS and SLE; P ≤ 0.0007 for all comparisons) (Figure 2A).
The frequency of CXCR5-expressing CD27+ memory B cells (mean ± SD 79.6 ± 14.8% in patients versus SD 89.8 ± 4.1% in controls; P = 0.043) (Figure 1A) and the density of CXCR5 expression on CD27+ memory B cells (geometric mean fluorescence intensity ± SD 259.6 ± 159.4 in patients versus 388.9 ± 60.4 in controls; P = 0.038) were significantly diminished in patients with primary SS as compared with healthy controls. No further differences in chemokine receptor expression on blood B cells between patients with primary SS and healthy controls were identified, neither in the CXCR5 expression on CD27− B cells nor in the expression of CXCR3, CCR6, CCR7, and CCR9 on CD27− or CD27+ B cells.
Experiments were performed to examine the cellular distribution and chemokine receptor expression by B cells in salivary glands of patients with primary SS. Comparison of peripheral and glandular B cells from 4 patients with primary SS revealed an accumulation of CD27+ memory B cells in minor salivary gland infiltrates. The vast majority of these glandular CD27+ memory B cells expressed both CXCR4 and CXCR5 (an example is shown in Figure 3B). Conversely, analysis of peripheral CD27+ memory B cells from patients with primary SS revealed a markedly diminished proportion of CXCR4+,CXCR5+ cells as compared with healthy controls (Figure 3A). In contrast, there was no reduction of peripheral CXCR4+,CXCR5+,CD27− naive B cells in patients with primary SS compared with healthy controls (data not shown).
The cDNA samples from all individual cells sorted in the current study were tested for their integrity by amplification of the “housekeeping” gene GAPDH. Each of the subsets manifested a comparable high frequency of positive cells (mean ± SD 46.4 ± 2.5% in healthy subjects versus 46.1 ± 7.7% in patients with primary SS). Notably, a significantly enhanced frequency of CD27− naive B cells that expressed CXCR4 transcripts was found in patients with primary SS (60 of 168 cells; 35.7%) compared with healthy controls (26 of 144 cells; 18.1%) (P = 0.0006) (Figures 4A and B). Furthermore, in patients with primary SS, the frequency of CXCR4-transcript–positive B cells was significantly enhanced in CD27− naive B cells (60 of 168 cells; 35.7%) compared with CD27+ memory B cells (37 of 168 cells; 22.0%) (P = 0.0079). A significantly increased percentage of CD27+ memory B cells expressing CXCR4-specific mRNA transcripts was also found in patients with primary SS (37 of 168 cells; 22.0%) compared with healthy controls (33 of 240 cells; 13.8%) (P = 0.033).
Both known CXCR5–mRNA splice variants (variant 1 NM_001716 and variant 2 NM_032966; National Center for Biotechnology Information database [28,29]) were analyzed in healthy controls and in patients with primary SS. It was found that individual peripheral B cells expressed either variant 1 (which encodes a protein that is 45 amino acids longer at the N-terminus than isoform 2) or variant 2. However, it is currently not known whether there is a functional difference between the variants. Importantly, no differences in CXCR5–mRNA expression were found between patients with primary SS and healthy subjects. Finally, when the expression of mRNA transcripts for the chemokine receptor–signaling regulator protein RGS13 (25) was examined, a significantly enhanced percentage of CD27− naive B cells expressing RGS13 transcripts was found in patients with primary SS (28 of 168 cells; 16.7%) compared with healthy controls (7 of 144 cells; 4.9%) (P = 0.001), whereas the portion of CD27+ memory B cells expressing RGS13 mRNA in patients with primary SS was not significantly different from that in healthy controls (Figure 4A).
To demonstrate the functionality of chemokine receptor expression, peripheral CD19+ B cells from 5 patients with primary SS and 5 healthy subjects were analyzed using transmigration assays. No significant differences in response to either CXCL12 or CXCL13 were found between patients with primary SS and healthy controls when unstimulated B cells were analyzed (Figure 5A). However, both in patients with primary SS and in healthy controls, the transmigratory capacity of B cells was significantly enhanced by LPS stimulation (P < 0.0001). After stimulation, significantly higher percentages of CD27+ memory B cells than of CD27− naive B cells migrated in response to CXCL12 and CXCL13 in both groups. Of note, there were significantly diminished responses of CD27+ memory B cells from patients with primary SS to both CXCL12 (mean ± SD 76.8 ± 7.8% versus 86.0 ± 3.8% in controls; P = 0.032) and CXCL13 (76.6 ± 7.2% versus 88.0 ± 1.8% in controls; P = 0.018), respectively, as compared with those from healthy subjects (Figure 5B).
Recent studies have shown disturbances in peripheral B cell populations in primary SS, with significantly enhanced CD27− naive and diminished CD27+ memory B cells (6, 8, 9). This was confirmed in the present study. An accumulation of CD27+ memory B cells in inflamed tissue (6, 10), altered recirculation of B cell subsets from these sites (7), and/or altered B cell differentiation (30) may contribute to these disturbances. The underlying assumption of the present study was that the expression of chemokine receptors on peripheral B cells might reflect a distinct B cell pattern in primary SS, with specific functional consequences. Overall, a differential expression of chemokine receptors by peripheral blood B cells from patients with primary SS was identified.
First, there was overexpression of CXCR4 by blood B cells from patients with primary SS that was most prominent in CD27− naive B cells. In particular, significantly higher frequencies of CXCR4-expressing B cells were detected in patients with primary SS compared with healthy controls, both in CD27− naive B cells (P = 0.0003) and in CD27+ memory B cells (P = 0.0251). Moreover, the density of CXCR4 surface expression was significantly enhanced in patients with primary SS as compared with healthy controls (P = 0.0021) for both CD27− naive and CD27+ memory B cells. Remarkably, these differences were also evident when blood B cells from patients with primary SS were compared with those from patients with SLE (P = 0.0015 for CD27− naive cells and P = 0.0001 for CD27+ memory cells), whereas there was no significant difference in CXCR4 expression between healthy subjects and SLE patients.
Thus, this abnormality appeared to be specific to primary SS rather than being common in systemic autoimmunity. Moreover, when individual B cells were analyzed for chemokine receptor mRNA, significantly enhanced frequencies of CD27− naive B cells (P = 0.0006) and CD27+ memory B cells (P = 0.033) expressing CXCR4 transcripts were found in patients with primary SS compared with healthy controls. However, CXCR4 overexpression by blood B cells from patients with primary SS did not translate into an enhanced migratory response to the corresponding chemokine, CXCL12, as compared with those from healthy controls. These results suggest that there was intracellular modulation of the migratory response in primary SS B cells.
To assess the discrepancy between CXCR4 expression and migratory response to the corresponding chemokine (CXCL12) in greater detail, mRNA expression of RGS13 (25, 31) as one potential influencing factor was examined in individual CD27− naive and CD27+ memory B cells. RGS13 belongs to the family of RGS proteins (for review, see refs. 25 and31) that are thought to be responsible for the fine-tuning of the intracellular signaling of G protein–coupled receptors, especially chemokine receptors. Thereby, they establish thresholds for responsiveness, provide stop signals for migration, and/or contribute to receptor desensitization to corresponding chemokines (25, 31). RGS13 has recently been shown to modulate signaling through CXCR4 and CXCR5 in murine and human germinal center B cells possessing one of the most limited patterns of expression of known RGS (25). Moreover, cotransfection with RGS13 inhibited the migrational response of CXCR4-transfected Chinese hamster ovary cells toward CXCL12 in vitro (25). In the present study, significantly enhanced expression of RGS13 mRNA by CD27− naive blood B cells from primary SS patients (P = 0.001) was found. Thus, the combined data suggest that CXCR4 overexpression by blood B cells from patients with primary SS might be partly compensated by up-regulation of the inhibitory regulator protein RGS13 and, thereby, might contribute to the discrepancy between CXCR4 expression and migratory response to its corresponding ligand, CXCL12.
In this context, it is well established that surface expression of chemokine receptors does not necessarily indicate their migratory functionality (32–34). Indeed, the responsiveness of chemokine receptors for their respective ligands is differentially regulated (e.g., by RGS proteins) during the orchestration of the migration of lymphoid subpopulations into anatomic compartments, their development, activation, and immune response (26, 27, 31–36). B cells from different developmental stages, e.g., developing bone marrow B cells (36), B cells leaving GC structures (33), and medullary plasmablasts leaving lymph nodes (34), have been found to express high levels of surface CXCR4 but were unresponsive to CXCL12. In this regard, there is some evidence that CXCR4 might fulfill additional functions besides chemotaxis, e.g., cell growth, proliferation, and transcriptional activation (11, 33, 37, 38). In accordance, CXCL12 treatment has been found to increase NF-κB activity in nuclear extracts from CXCR4-transfected murine pre–B lymphoma cells (37). Moreover, it has been shown that CXCL12–CXCR4 interaction stimulates G protein–mediated activation processes in peripheral T cells (39). Although it is currently unclear whether CXCR4 also fulfills such additional functions in human blood B cells, it might be speculated that CXCR4 and RGS13 (over)expression might contribute to or, alternatively, reflect abnormal B cell stimulation in primary SS, which warrants further studies.
Compared with healthy controls, flow cytometric analysis revealed a moderately diminished frequency of CXCR5+,CD27+ memory B cells (P = 0.0425) combined with a lower density of CXCR5 surface expression on CD27+ memory B cells (P = 0.038) in patients with primary SS. In this context, the CXCL13–CXCR5 pairing has been shown to be critically involved in the homing of B cells into lymphoid follicles, as well as in the development of organized lymphoid follicles (28, 29, 40, 41). The formation of ectopic lymphoid tissue in chronic inflammatory disease, such as primary SS, is a complex process regulated by an array of cytokines, adhesion molecules, and chemokines (4, 13), partly mimicking signals found in normal lymphoid organogenesis (42). Whether expression of CXCL12 and CXCL13 in the target tissues of patients with primary SS is closely associated with the development of GC-like structures or, rather, is a feature of the entire inflammatory process is still controversial (4, 15).
However, it has been suggested that CXCL13 overexpression in the inflamed glands of patients with primary SS plays an active role in the recruitment of lymphoid cells as infiltrating cells, mostly B cells, which express the cognate receptor CXCR5. Thus, in patients with primary SS, overexpression of CXCL13 in inflamed glands with consequent local retention of CXCR5-bearing B cells (15, 16) might also lead to reduced frequencies of peripheral CD27+ memory B cells expressing lower levels of surface CXCR5. This assumption has been supported by recent studies of primary SS indicating accumulation of memory B cells in glandular infiltrates (6, 10). In accordance with this, simultaneous analyses in this study of B cells from peripheral blood and minor salivary gland infiltrates of patients with primary SS also revealed an accumulation of CD27+ memory B cells in the inflamed glands. The vast majority of these infiltrating CD27+ memory B cells coexpressed CXCR5, along with CXCR4. Conversely, diminished frequencies of peripheral blood CD27+ memory B cells coincide with a striking reduction of the peripheral CXCR4+,CXCR5+ memory B cell subpopulation in patients with primary SS. Thus, glandular coexpression of both CXCL12 and CXCL13 (15–18) seems to navigate this subpopulation of peripheral CD27+ memory B cells into the inflamed glands, where it resides. Consistent with this, residual circulating peripheral CD27+ memory B cells from patients with primary SS showed a diminished migratory response to the corresponding ligands of CXCR4 and CXCR5, CXCL12 and CXCL13, respectively, after stimulation. This suggests that memory B cells with less migratory capacity remain in the blood as a result of the selective migration and retention of CXCR4+,CXCR5+ memory B cells into the inflamed glands.
In conclusion, peripheral B cells in primary SS manifest specific abnormalities in chemokine receptor expression and function of both memory and naive subpopulations. The abnormally expressed receptors, CXCR4 and CXCR5, specifically bind the chemokines, CXCL12 and CXCL13 (BLC; BCA-1), respectively, which are important for navigating lymphocytes in lymphoid tissues, and, thereby, for lymphocyte homeostasis (11, 12, 42). Migration/retention of CXCR4+,CXCR5+,CD27+ memory B cells in the inflamed target tissues of patients with primary SS appears to account for the diminished number of these cells in the peripheral blood. However, the increased number of naive B cells in the peripheral blood does not appear to reflect an alteration in chemotaxis. Rather, the increased expression of CXCR4 appears to be offset by intracellular modulation with resultant normal migratory responsiveness. Both differences might reflect an abnormality in activation status of the naive subpopulation. Thus, disturbed B cell differentiation, activation, and/or (re)circulation between immune compartments may contribute to the disturbed B cell homeostasis in primary SS (10, 30). Detailed understanding of the impact of chemokines and their cognate receptors, including their regulation, may allow the development of future therapeutic interventions in primary SS, a disease unresponsive to classic immunosuppression.
We are grateful to Thoralf Kaiser for excellent technical assistance.