Background
B cells are best known for their capacity to produce antibodies. In addition, they also exert a variety of other functions during the immune response, including antigen presentation and production of various cytokines, which are involved in the early and late stages of T-cell-mediated immune responses [
1]. However, B-cell-deficient mice were observed to be susceptible to experimental autoimmune encephalitis (EAE), and to be unable to recover from it [
2]. Furthermore, adoptive transfer of IL-10
+ B cells can suppress inflammation of EAE [
3]. A new population of B cells, regulatory B cells (Bregs), has increasingly gained attention for restraining inflammation [
4,
5]. Bregs can suppress the differentiation of T helper 1 (Th1) and T helper 17 (Th17) cells, and promote regulatory T-cell (Treg) induction [
6,
7]. It was also reported that Bregs support the maintenance of invariant nature killer T (iNKT) cells [
8]. Bregs have been shown to inhibit autoreactive and pathogen-driven immune response mainly through the production of interleukin-10 (IL-10), interleukin-35 (IL-35), and transforming growth factor beta (TGF-β) [
9]. Until now, the production of immune-suppressive cytokine IL-10 was thought to be a hallmark of Breg function [
10]. In some human autoimmune diseases, it has been reported that Breg function is impaired and does not prevent the development of human autoimmune diseases, such as RA [
7], relapsing–remitting multiple sclerosis [
11], systemic lupus erythematosus (SLE) [
12], and so on [
13]. However, the mechanism of impaired Breg function in autoimmune diseases remains unclear.
The heterotrimeric guanine nucleotide-binding proteins (G proteins) are important signal transducers, which when attached to the cell surface plasma membrane receptors, G protein-coupled receptors (GPCRs), can communicate with signals from a large number of hormones, neurotransmitters, chemokines, sensory stimuli, and autocrine and paracrine factors. The heterotrimeric G proteins are composed of three subunits (α, β, and γ subunits) that cycle between inactive and active signaling states in response to guanine nucleotides [
14,
15]. On the basis of downstream signaling targets of α subunits, these α subunits are divided into four classes: Gαi/0, Gαs, Gαq/11, and Gα12/13. Gαq is a member of the Gαq/11 subfamily encoded by GNAQ [
16]. Gαq is ubiquitously expressed in mammalian cells and nearly 40% of all GPCRs rely upon Gαq family members to stimulate inositol lipid signaling [
15]. It is well known that Gαq plays an essential role in the nervous system, endocrine system, and cardiovascular system [
16‐
20]. Many studies have also established the physiological importance of Gαq in the immune system. A previous study showed that Gαq-deficient (
Gnaq−/−) mice exhibited impaired eosinophil recruitment to the lung after antigenic challenge, probably due to an impaired production of granulocyte macrophage colony stimulating factor (GM-CSF) by resident airway leukocytes [
21]. Our previous study reported that
Gnaq−/− dendritic cells were defective in migrating from the skin to draining lymph nodes after fluorescein isothiocyanate sensitization, and
Gnaq−/− monocytes were defective in migrating from the bone marrow into inflamed skin after contact sensitization [
22]. The functional involvement of Gαq in TCR-induced immune responses was also investigated [
23]. In addition,
Gnaq−/− chimeras could spontaneously develop manifestations of systemic autoimmune disease with high titer antinuclear antibody and inflammatory arthritis, which was observed in our previous study [
24]. In humans, our previous work also showed that Gαq mRNA expression was decreased in peripheral blood lymphocyte cells (PBMCs) and T cells from SLE patients compared to that from healthy individuals. What is more, the Gαq expression in T cells from SLE patients was associated with disease severity, the presence of lupus nephritis, and expression of Th1, Th2, and Th17 cytokines [
25]. We also found that B cells from mice lacking the Gαq subunit of trimeric G proteins have an intrinsic survival advantage over normal B cells, suggesting that Gαq is critically important for maintaining control of peripheral B-cell tolerance induction and repressing autoimmunity [
24]. Whether Gαq regulates Breg function is still unknown.
In this study, we found a critical role of Gαq in Breg differentiation and Gnaq−/− Bregs showed an impaired suppressive function on T-cell proliferation. Our human data also showed that the decreased frequency of Bregs showed a significantly positive correlation with Gαq mRNA expression in RA patients. Taken together, our work reveals a novel function of Gαq in regulating Breg function.
Methods
Patients and controls
Peripheral blood was obtained from 34 RA patients and 24 healthy controls from the inpatient clinic of the Department of Rheumatology, The First Affiliated Hospital of Xiamen University, Xiamen, China. The criteria used for RA diagnosis were based on those of the American Rheumatism Association (1987) [
26] and the new criteria from the ACR/EULAR (2010) [
27]. Gαq mRNA expressions were detected by RT–PCR, the frequency of CD19
+CD24
hiCD38
hi B cells in PBMCs was detected by flow cytometry, and the association of Gαq mRNA expression level and frequency of CD19
+CD24
hiCD38
hi B cells was studied. The clinical characteristics of the RA patients are summarized in Table
1. Informed consent was obtained from all recruits to this study. This study was approved by the Ethics Committee of the First Affiliated Hospital of Xiamen University in accordance with the World Medical Association Declaration of Helsinki.
Table 1
Demographic data and clinical characteristics of RA patients in the study
Age (years), mean (range) | 56 (34–78) | 45 (29–68) |
Sex (n), female/male | 26/8 | 17/7 |
CRP (mg/L), mean (range) | 30.35 (0.5–75.1) | |
ESR (mm/h), mean (range) | 47.29 (7–117) | |
RF (IU/ml), mean (range) | 223.35 (9.69–991) | |
Anti-CCP (RU/ml), mean (range) | 52.50 (1–200) | |
Tender joint count, mean (range) of 68 joints | 6.35 (0–26) | |
Swollen joint count, mean (range) of 68 joints | 4.47 (0–22) | |
Animals
All experimental procedures involving mice were approved by the institutional animal care committee of Xiamen University. C57BL/6 J (B6) mice were purchased from Xiamen University Laboratory Animal Center. C57BL/6 J (B6) mice and Gnaq−/− (n > 8 backcrossed to C57BL/6 J) mice were bred in Xiamen University Laboratory Animal Center. The mice used in this study were 6–8 weeks age.
Cell isolation
The purification of CD4+CD25− T cells from the spleen of mice was performed using the CD4+CD25− T Cell Isolation Kit (Miltenyi Biotec, Bergisch-Gladbach, Germany), LS Columns (Miltenyi Biotec), and MidiMACS™ Separators (Miltenyi Biotec) according to the manufacturer’s instructions. Briefly, 1*107 cells from the mice spleen were stained with biotin-antibody cocktail in buffer (PBS/2 mM EDTA/0.5% BSA) for 5 min at 4 °C. After that, the anti-Biotin MicroBeads and CD44 MicroBeads were sequentially added and incubated for 10 min at 4 °C.Cells were washed, centrifuged, and resuspended in 0.5 ml of buffer, and applied onto the column. The CD4+CD25− T cells in flow-through fluids were collected. The isolation of B cells and Tregs from the spleen of mice was performed using the corresponding Pan B Cell Isolation Kit II (Miltenyi Biotec) and the CD4+CD25+ Regulatory T cell Isolation Kit (Miltenyi Biotec) according to the manufacturer’s instructions. Purity of the target cells was > 90% in all experiments assessed by flow cytometry. CD1dhiCD5+ B cells were isolated using a MoFlo High-Performance Cell Sorter (Beckman Coulter, Fullerton, CA, USA) with purities of 90–95%. Human PBMCs were isolated from 4 ml sodium heparin-treated venous blood samples by Ficoll density-gradient centrifugation using Lymphoprep™ (Axis-Shied PoC AS, Oslo, Norway). Washed and resuspended, the PBMCs were cryopreserved for future real-time polymerase chain reaction.
Cell culture
Purified B cells were planted in complete RPIM 1640 with 2.05 mM l-glutamine (GE Healthcare Life Sciences, Logan, UT, USA) supplement with 10% fetal bovine serum (PAN Seratech, Aidan Bach, Germany) and maintained in standard cell culture environment (95% humidity, 5% CO2 at 37 °C). For Breg induction, B cells were stimulated with LPS (Escherichia coli 0111:B4; Sigma-Aldrich, St. Louis, MO, USA) (10 μg/ml) for 48 h. PMA (50 ng/ml), ionomycin (250 ng/ml) (Sigma-Aldrich), and brefeldin A (10 μg/ml) (BD Biosciences, San Jose, CA, USA) were added for the last 5 h of culture before flow cytometry. For analysis of CD4+ T-cell proliferation and Treg differentiation, purified LPS-induced Bregs from the spleen of WT mice or Gnaq−/− mice and sorted CD4+CD25− T cells from the spleen of WT mice were 1:1 cocultured and activated with Dynabeads™ Mouse T-Activator CD3/CD28 (Life Technologies AS, Oslo, Norway) at a bead-to-cell ratio of 1:2 in 200 μl medium in a 96-well U-bottom plate for 72 h.
Antibodies
Anti-human antibodies included: anti-CD19-FITC from BD Biosciences; anti-CD24-PE (ML5) and anti-CD38-PE /Cy7 (HIT2) from Biolegend (San Diego, CA, USA); and Human FcR Binding Inhibitor from eBioscience (San Diego, CA, USA). Anti-mouse antibodies included: anti-CD4-FITC (RM4–5), anti-CD19-PE/Cy5, anti-CD19-APC (6D5), anti-CD25-APC (3C7), anti-IL-10-PE (JES5-16E3), PE Rat IgG2b, κ Isotype Ctrl (RTK4530), and anti-PD-L1-APC (10.F.9G2) from Biolegend; anti-CD1d-PE (1B1), anti-CD5-FITC (53–7.3), anti-CD16/CD32 (mouse Fc block), anti-Annexin-V-APC, and 7-AAD from BD Biosciences; and anti-CD25-PE/Cy5.5 (PC61.5), anti-Foxp3-PE (NRRF-30), anti-TLR4-Alexa Fluor® 488(UT41), and anti-FasL-FITC (MFL3) from eBioscience. Phosho-p38 MAPK (Thr180/Tyr182) (28B10) mouse mAb, p38 MAPK (D13E1) XP® rabbit mAb, phosho-p44/42 MAPK (Erk1/2) (Thr202/Tyr204) (D13.14.4E) XP® rabbit mAb, p44/42 MAPK (Erk1/2) (137F5) rabbit mAb, phosho-PI3K p85 (Tyr458)/p55 (Tyr199) antibody, PI3K p85α (6G10) mouse mAb, phospho-STAT1 (Thr701) (58D6) rabbit mAb, STAT1 (D1K9Y) rabbit mAb, GAPDH (14C10) rabbit mAb, β-Tubulin (9F3) rabbit mAB, anti-mouse IgG, HRP-linked antibody, anti-rabbit IgG, and HRP-linked antibody were obtained from Cell Signaling Technology (MA, USA); and anti-MyD88 antibody was obtained from Abcam (Cambridge, UK).
Real-time polymerase chain reaction
Total RNA was extracted from PBMCs with TriPure Isolation Reagent (Roche Diagnostics GmbH, Mannheim, Germany) and the concentration of RNA was determined by measuring the absorbance at 260 nm in a UV–Vis spectrophotometer (Quawell, San Jose, CA, USA). Reverse transcription was performed by the Bio-Rad Systems (Bio-Rad, Hercules, CA, USA) according to standard protocols using the Transcriptor First Strand cDNA Synthesis Kit (Roche Diagnostics GmbH). The expression level of Gαq was measured by real-time quantitative PCR. β-actin was simultaneously amplified and used as an internal control. The primer sequences were as follows: β-actin forward, 5′-AGAAAATCTGGCACCACACC-3′; β-actin reverse, 5′-AGAGGCGTACAGGGATAGCA-3′; Gαq forward, 5′-GTTGATGTGGAGAAGGTGTCTG-3′; and Gαq reverse, 5′-GTAGGCAGGTAGGCAGGGT-3′. Amplification was performed with the 7500 Real Time PCR Systems (Applied Biosystems, CA, USA). Gene expression levels were normalized by comparing to β-actin and relative expression was calculated by the2–ΔΔCt method.
Enzyme-linked immunosorbent assay
The concentration of mouse IL-10 (BD Biosciences), IL-35 (Wuhan Huamei, China), TGF-β, IL-23 (Invitrogen, Carlsbad, CA, USA), and IL-6 (R&D, Minneapolis, MN, USA) were measured using commercially available ELISA kits according to the manufacturer’s instructions. Absorbance at 450 nm was measured with an ELISA microplate reader (Molecular Devices, Sunnyvale, CA, USA).
Flow cytometry
Fc receptors were blocked with mouse Fc block and the dead cells were detected using Fixable Viability Dye eFlour™ 506 or 510 (eBioscience) before cell surface staining. For Breg staining, CD19-FITC or PE/Cy5, CD24-PE, CD38-PE/Cy7, CD1d-PE, and CD5-FITC mAbs were used. For intracellular IL-10 staining, cells were stained with CD19-PE/Cy5 or APC mAbs. Cells were washed, fixed with IC Fixation Buffer (eBioscience), permeabilized with Permeabilization Buffer (eBioscience), and stained with IL-10-PE. For Treg staining, cells were stained with combinations of CD4-FITC and CD25-PE/Cy5.5 or APC mAbs, fixed and permeabilized with Fixation/Permeabilization solution (eBioscience) and Permeabilization Buffer, and stained for detection of intracellular Foxp3-PE mAbs. For apoptotic cell detection, cells were washed twice with cold PBS and then resuspended in 1× Binding Buffer (BD Biosciences), and then the cells were stained with CD4-FITC, APC Annexin-V, and 7-AAD and incubated for 15 min at RT in the dark. Last, 400 μl of 1× Binding Buffer was added. Data were acquired using Cytomic FC500 or Cytoflex (Beckman Coulter) and analyzed using CXP Analysis and Cytexpert (Beckman Coulter).
Western blot analysis
Single-cell suspensions were lysed after stimulation in cOmplete Lysis-M (Roche Diagnostics GmbH) containing protease inhibitor cocktail (Roche Diagnostics GmbH) and phosphatase inhibitor cocktail (Roche Diagnostics GmbH) for 10 min with gentle shaking. The lysates were centrifuged at 14,000 × g for 15 min, and frozen at − 80 °C until use. For western blotting assays, the protein concentrations were determined using the BCA Protein Assay Kit (Thermo Scientific, Rockford, IL, USA), and equal amounts of protein (20 μg) per lane were separated by 10% SDS-PAGE gels and transferred to PVDF membranes. Membranes were blocked and then probed with primary antibodies against p-p38 MAPK (1:1000), p-Erk1/2 (1:2000), p-PI3K (1:1000), p-STAT1 (1:1000), or MyD88 (1:1000) at 4 °C overnight. After washing, the membranes were incubated with the appropriate HRP-conjugated secondary antibodies for 1 h at room temperature. After extensive washing, signals were visualized using the chemiluminescent HRP substrate system (Millipore, Billerica, MA, USA). Band quantification was performed on the Molecular Imager® ChemiDoc™ XRS+ system with Image Lab™ Software (Bio-Rad, Hercules, CA, USA). Thereafter, membranes were stripped with stripping buffer before reprobing with anti-p38 MAPK (1:1000), Erk1/2 (1:1000), PI3K (1:1000), and STAT1 (1:1000) to ensure equal loading. GAPDH or β-Tubulin was also detected as the loading control. The level of protein phosphorylation was normalized to the loading control (total protein).
Statistical analysis
All data were obtained from at least three independent experiments and shown as mean ± standard deviation (SD). All data were analyzed using GraphPad Prism 5.01 software (GraphPad, San Diego, CA, USA). Statistical significance was determined by Student’s t test and the Mann–Whitney U test. Correlation was analyzed using Spearman’s test. A probability value of p < 0.05 was considered statistically significant.
Discussion
Recent studies have shown that Bregs play a crucial role in autoimmune diseases through suppressing the differentiation of Th1 and Th17 cells, and promoting Treg induction [
9]. However, the mechanism of Breg differentiation still remains unknown. Our previous studies demonstrated that Gαq exerts an important role in immune regulation, including Th1 and Th17 function, while the role of Gαq in Breg regulation is still unclear. Here, we found that the differentiation and immunosuppressive effect of Bregs were inhibited in the
Gnaq−/− mice. In addition, our data demonstrated that the PI3K, Erk1/2, and p38 MAPK signaling pathways were involved in the regulation of Breg function by Gαq. Furthermore, we also showed a decreased frequency of CD19
+CD24
hiCD38
hi Bregs in RA patients, which positively correlated with Gαq mRNA expression. These data suggest that Gαq was involved in the immune tolerance via regulating Breg function.
The existence of B cells with a suppressive capacity was initially reported in the study of guinea pigs in the mid-1970s [
33,
34]. In the past 40 years, lots of studies have been focused on regulatory B cells and their mechanisms of action. Mizoguchi et al. [
35] defined the B cells that produce IL-10 as regulatory B cells. Through producing IL-10, IL-35, and TGF-β, Bregs suppress immunopathology by prohibiting the expansion of pathogenic T cells and maintaining the pool of Tregs [
9]. In our study, we also showed that the impaired immunosuppression of
Gnaq−/− Bregs might be due to the decreased production of IL-10 and TGF-β. Bregs have been considered an important immune regulatory cell in many diseases, such as EAE, type 1 diabetes, collagen-induced arthritis, inflammatory bowel diseases, lupus, and so on [
36]. Similarly, CD19
+CD24
hiCD38
hi B cells, which were considered Bregs in human, can limit the differentiation of naïve CD4
+ T cells into Th1 and Th17 populations, and maintain Treg function [
7]. RA patients with active disease have reduced numbers of CD19
+CD24
hiCD38
hi B cells in PBMCs compared with healthy individuals [
7]. Our data also found a remarkable decrease in the frequency of CD19
+CD24
hiCD38
hi Bregs in RA patients. Although the number of CD19
+CD24
hiCD38
hi B cells was increased in SLE patients, they lacked the suppressive capacity due to their failure to produce IL-10 [
12]. Previous studies showed that IL-10 production of human B cells was associated with the activation of STAT3 and ERK [
37]. Our current findings showed that IL-10 production was also impaired in Bregs in the absence of Gαq.
Activation of the ERK pathway is a common requirement for IL-10 expression by T cells, macrophages, and myeloid dendritic cells [
32]. Abrogation of either ERK or p38 activation after TLR stimulation leads to a reduced IL-10 expression, which suggests that these two pathways might cooperate in TLR-induced IL-10 production [
32]. Consistently, inhibition of PI3K, Erk1/2, or p38 MAPK significantly ablates the Breg differentiation in our study here. As expected, we found that the basal levels of phospho-PI3K, phospho-Erk1/2, and phospho-p38 MAPK in response to LPS were lower in
Gnaq−/− B cells than in WT B cells. These data suggest that Gαq was involved in the differentiation of Bregs partly through regulation of PI3K, Erk1/2, or p38 MAPK signaling. That IL-10 can be induced by LPS in many cells has been demonstrated. However, we here observed no marked differences of TLR4 and MyD88 expression between B cells from WT and
Gnaq−/− mice, which further confirms the regulation of Gαq in the PI3K, Erk1/2, and p38 MAPK signaling pathways of Breg function.
In a previous study we demonstrated that
Gnaq−/− chimeras could spontaneously develop manifestations of systemic autoimmune disease with high titer antinuclear antibody and inflammatory arthritis, and B cells from
Gnaq−/− mice have an intrinsic survival advantage over normal B cells, suggesting that Gαq is critically important for maintaining control of peripheral B-cell tolerance induction and repressing autoimmunity [
24]. However, the role of Gαq in Breg regulation remains unknown. Actually, the percentage of Bregs was significantly lower in the spleen of
Gnaq−/− mice. Consistent with the animal experiments, our data here showed a significant positive correlation between the frequency of CD19
+CD24
hiCD38
hi Bregs and the expression of Gαq mRNA in PBMCs from patients with RA and HC. Our current findings showed that Gαq deficiency limited the differentiation of Bregs. Several studies demonstrated that Bregs were important for the generation and maintenance of Tregs [
4]. Bregs could induce the differentiation of type 1 regulatory T (Tr1) cells [
38,
39]. Moreover, Bregs might promote the differentiation of other type of regulatory T-cell subsets [
40]. Consistent with prior studies, purified WT Bregs could convert CD4
+CD25
− T cells into Tregs, but this function of
Gnaq−/− Bregs was impaired. Indeed, we also observed an impaired inhibition of T-cell expansion in
Gnaq−/− Bregs. This might be the reason for impaired suppressive function of
Gnaq−/− Bregs on T-cell proliferation. Some studies suggested that CD40 mAb-stimulated CD1d
hiCD5
+ B cells could not regulate T-cell proliferation in vitro [
41]. TLRs and CD40 activation are well-characterized signals in Breg differentiation [
4,
9]. However, LPS but not CD40 activator can induce IL-10 secretion [
36], which might be the reason for no effect on T-cell proliferation inhibition being observed.