Patient material
SF was obtained from routine large joint aspiration from SLE, SpA and RA patients fulfilling the 1982 ARA/ACR2, ASAS11 and ACR classification criteria12 respectively. Informed consent was given prior to SF and blood sampling, as approved by the local ethics committee.
Once collected, SF was centrifuged and stored as acellular at -80°C. In parallel, paired serum samples were also processed. The mononuclear cell fractions from SF (when present) and peripheral blood were collected following gradient density centrifugation over Ficoll-Hypaque and were cryopreserved in DMSO/FCS in N2(l) until use.
SLE synovial fluid samples, once retrieved, were selected for inclusion in the present study after revision of the electronic clinical charts, in order to exclude biological material from patients with evidence of joint involvement not primarily dependent on SLE. We therefore revised clinical information to exclude primary osteoarthritis, overlap with rheumatoid arthritis, crystal related arthritis, and other comorbidities which could entail potential confounders.
From an initial list of 27 patients, we therefore selected 17 SLE patients with joint involvement, excluding 10 patients (1 with SLE RA overlap, 1 with onset of arthritis after the occurrence of a haematological malignancy, 8 for evidence of primary OA or crystal related arthritis). The clinical characteristics are listed in Table 1 (SLE patients used for the cytokine analysis) and Supplementary Tables S1 (RA patients used for the cytokine analysis) and S2 (patient samples for cell analysis).