Background and Objectives IL-6 plays a definite role in RA and has been found increased in SLE. However, differences in serum CRP suggest differences in signalling. We therefore analysed serum IL-6, IL-6 receptors, and phosphorylated Stat3, as the main signalling molecule involved, in patients with SLE and RA.
Materials and Methods Peripheral blood mononuclear cells (PBMC) of 46 SLE, 20 RF and ACPA positive RA patients (RA+), 14 seronegative RA (RA-) patients, and 68 healthy individuals (HC) were compared. PBMC were immediately prepared from peripheral venous blood. For determining the percentage of IL-6Rα (CD126), gp130 (CD130), PBMC were stained with PE-labelled or control antibodies. For the determination of pStat3, cells were fixed with formaldehyde (2%), permeabilised with methanol (80%), and stained with PE-labelled anti-pStat3 or control antibodies. Cells were analysed on a Becton Dickinson FACSCalibur fluorocytometer, gating for lymphocytes. Mean fluorescence intensity (mfi) was used as a semiquantitative measure of pStat3 contents. Serum levels of IL-6 and soluble IL-6 receptor (sIL-6R) were measured by ELISA.
Results The percentage of CD126+ lymphocytes, as compared to HC (mean ± SD 60 ± 8%) was similarly decreased in SLE (50 ± 17%, p = 0.0002) and RA+ (52 ± 13%, p = 0.01), but not RA- (60 ± 12%, p = 0.98) patients. SLE was significantly (p = 0.02) different from RA-. Likewise, CD130+ lymphocytes were decreased in SLE and RA+ , but not RA- patients (HC 59 ± 10%; SLE 49 ± 19% (p = 0.006); RA+ 53 ± 12% (p = 0.08); RA- 62 ± 9% (p = 0.3)). SLE and RA+ were different from RA- (p = 0.002, and p = 0.003, respectively). In contrast, both serum levels of IL-6 and intracellular pStat3 were increased in all three groups compared to HC: serum IL-6 was a median (range) 0 (0-4.8) pg/ml in HC, 3 (0.5-69.3) pg/ml (p<0.0001) in SLE, 8.6 (2.4-158.5) pg/ml (p<0.0001) in RA+ , and 4.2 (0.9-17.5) pg/ml (p = 0.0001) in RA-. Median pStat3 mfi, as compared to HC (10 (5.4-23)) was 18 (10-70), p<0.0001, in SLE, 17 (9.8-39), p<0.0001, in RA+ , and 13 (10-29), p = 0.008, in RA-. Serum IL-6 levels were higher in RA+ than SLE (p = 0.0004) and RA- (p = 0.009), while pStat3 was similar for all groups. Serum IL-6R levels were increased in all patient groups (HC 38.1 (18.1-57) ng/ml; SLE 45.1 (20.8-109.6) ng/ml, p = 0.02; RA+ 54.3 ± 17.1ng/ml, p = 0.0004; RA- 48.5 ± 14 ng/ml, p = 0.02) without differences between patient groups.
Conclusions While IL-6 and sIL-6R levels were most elevated in RA+, as expected, Stat3 phosphorylation was similarly increased in all patient groups. Intriguingly, regarding both surface IL-6 receptor chains, SLE grouped with RA+ , suggesting an influence of immune complex disease.