Background CRP levels are typically low in active SLE, but not in SLE patients with infections. However, IL-6, which directly leads to CRP increases, has been found increased in active SLE. Membrane-bound IL-6 receptor-α chains (CD126) are exclusively expressed on leukocytes, hepatocytes, and bowel epithelium. In contrast, gp130 (CD130), the second IL-6 receptor chain, is expressed on all cell types. IL-6 bound to soluble IL-6 receptors (sIL-6R), which are mainly derived by shedding CD126, can bind to CD130 on any cell and transmit signals, which is called trans-signalling.
Objectives To analyze IL-6 and IL-6 receptor expression and function in SLE.
Methods We prepared PBMC of 41 SLE patients and 71 healthy individuals (HC). CRP and disease activity (by ECLAM) were recorded. For determining the percentages of CD126 and CD130 positive cells, PBMC were directly stained with PE-labelled or control antibodies. For analyzing IL-6 induced Stat3 phosphorylation, PBMC were stimulated with rhIL-6 (250 ng/ml) for 15 min, fixed with formaldehyde (2%), permeabilized with methanol (80%), and stained with PE-labelled antibodies to phosphorylated Stat3 (pStat3) or control antibodies. For in vitro experiments on the influence on receptor expression, healthy PBMC were incubated for 24 hours with or without the addition of IL-6, IL-10, tumor necrosis factor (TNF), interferon-α (IFNα), or combinations of these cytokines. Stained cells were immediately analyzed on a Becton Dickinson FACSCalibur fluorocytometer, gating for lymphocytes. As a semiquantitative measure of pStat3 contents mean fluorescence intensity (mfi) was used. Serum IL-6 and sIL-6R were measured by ELISA.
Results The serum IL-6 levels of SLE patients were significantly higher than those of HC (median (range) (3.6 (0.69-69.3) vs. 0.9 (0.12-10.5) pg/ml, p<0.0001). IL-6 correlated with SLE disease activity by ECLAM (Spearman r=0.41, p=0.01). CRP was slightly increased in SLE (1.8 (1.0-40.8) vs 0.8 (0.3-4.8) pg/ml for HC, p<0.0001). The percentage of CD126+ lymphocytes was decreased in SLE (mean ± SD 48±16 vs. 61±11% for HC, p<0.0001). Reduced receptor expression was functionally relevant in that the IL-6 induced increase in pStat3 was significantly reduced in SLE (Δmfi 14.2 (-19.91-37.15) vs. 18.8 (-2.2-50.06) in HC, p=0.004). In a mirror image to the membrane receptor, sIL-6R serum levels were increased in SLE (42.1 (24.1-109.6) vs. 38.6 (16.4-80.5) ng/ml in HC, p=0.039). Moreover, sIL-6R was negatively correlated with the percentage of CD126+ lymphocytes (Spearman r=-0.35, p=0.03). In vitro, stimulation of healthy PBMC with combinations of IL-6 with either IFNα or TNF led to a reduction in CD126+ cells (by 39±13 and 16±6%, respectively), mimicking the in vivo situation.
Conclusions In SLE, CD126 is reduced and sIL-6R increased, so that the increased IL-6 will more likely bind to cells only carrying CD130, but not CD126. Only in very high concentrations of IL-6, such as in infections or active serositis, will there be enough IL-6 signalling in cells carrying (reduced) CD126, and thus an increase in CRP. Combinations of IFNα (or TNF) with IL-6, all of which are increased in active SLE, explain this shift from classical IL-6 signalling via CD126 to trans-signalling via sIL-6R.
Disclosure of Interest None declared