IL-17A and IL-6 are highly expressed in SLE SF
First, we wanted to investigate T-cell associated cytokines in SF of SLE patients. Thus, SF from SLE patients (n=17) were tested for cytokine content in comparison to paired serum samples (n=14) and SF from RA patients (n=10), matched for storage time (Supplementary Table S2). In SLE, IL-17A was detected in SF (44.9 pg/ml (10.7 - 405.2)) and matched serum (37.5 pg/ml (22.4 - 94.2)) (Fig. 1A), although no statistically significant difference was shown (p=0.4 ).
IL-6 was significantly higher in SLE-SF (771 pg/ml (108.5-2135)) as compared to corresponding serum samples (1.7 pg/ml (1.2-4.9),p=0.0006 ). Low concentrations of IL-10 could be detected in 41% of SLE SF samples (0 pg/ml (0-3.4)) and was higher in serum (0.29 pg/ml (0.1-0.4)). Overall, the RA SF samples displayed similar ranges of cytokines as SLE SF, with higher median values of IL-6 (3627 pg/ml (334.6-5000), p=0.11) and IL-10 (1.9 pg/ml (0-7.9), p=0.19) and less IL-17A, (0 pg/ml (0-172), p=0.11) however without statistical significance (Fig. 1A). The cytokine levels did not differ between SLE patients with and without secondary osteoarthritis (data not shown).
Next, we explored the relationship of cytokine levels, in and between compartments (Fig. 1B-C). In SLE SF, the levels of IL-6 correlated with those of IL-10 (r=0.84, p=<0.0001 , CI=0.69-0.92) and, weakly, with IL-17A (r=0.39, p=0.03 , CI=0.02-0.66) (Fig. 1B). Also, IL-10 and IL-17A correlated weakly (r=0.43, p=0.01 , CI=0.07-0.69). When comparing serum and SF, levels of IL-6 correlated(r=0.7, p=0.0001) but neither IL-17A nor IL-10 (Fig. 1C).
Cytokine analyses included also IFN-γ, TNF, IL-2 and IL-4. In sera, IL-4 and IL-2 were higher compared to SF, however their concentrations were consistently low (Supplementary Fig. 1A). Very low levels of IFN-γ could be detected in SF of four SLE patients. TNF levels were below the detection limit in all samples (Supplementary Fig. 2B).
Subsequently, we investigated the presence of IgA and IgM RF in serum and SF. These did not correlate with the cytokine levels nor with other autoantibodies (data not shown). Interestingly, the concentration of RF, when present, was mostly higher in serum compared to SF (Fig. 1D).
For one SLE patient, SF samples from six timepoints were available, the first obtained approximately 15 years after disease onset. This patient presented with a recurrent polyarthritis, but no evidence of secondary OA could be tracked in the clinical records. High IL-6 levels were repeatedly measured over a 10 years period, while IL-17A levels fluctuated and IL-10 concentrations were low (Fig. 2A).