Abundant expression of the interleukin (IL)23 subunit p19, but low levels of bioactive IL23 in the rheumatoid synovium: differential expression and Toll-like receptor-(TLR) dependent regulation of the IL23 subunits, p19 and p40, in rheumatoid arthritis

Objective: Interleukin (IL)23, composed of a p19 and a p40 subunit, is suggested to play key roles in rheumatoid arthritis (RA), dependent on the promotion and proliferation of IL17-producing T helper (Th)17 cells. However, previous studies on IL23 expression in human tissues were based on the p19 subunit only. We aimed to study the expression and regulation of IL23 subunits p19 and p40 in RA compared to patients with osteoarthritis (OA). Methods: The expression of p19 and p40 in synovial tissues was analysed by in situ hybridisation and immunohistochemistry. IL23 in RA and OA synovial fluids and sera was determined by ELISA. Toll-like receptor (TLR)-dependent induction of p19, p40 and bioactive IL23 was determined in RA synovial fibroblasts (RASF), monocytes and monocyte-derived dendritic cells (MDDCs) by real-time PCR and reverse transcriptase (RT)-PCR, Western blot and functional assays. Results: The p19 subunit was abundantly expressed in RA but not in OA synovial tissues. p19 was most prominently expressed by RASF in the synovial lining layer and at the site of invasion, but no heterodimeric IL23 was detected at these sites. Correspondingly, soluble IL23 was not detectable or found at very low levels in synovial fluids and sera of patients with RA. By in vitro experiments, we confirmed that TLR-activated RASF expressed p19 but not p40, in contrast to monocytes, which produced IL23 following TLR stimulation. Conclusion: The TLR-dependent induction of p19 but not p40 in RASF and the abundant expression of p19 along with the low or undetectable levels of IL23 in patients with RA provides strong evidence that p19 does not necessarily indicate the presence of IL23, as has been proposed to date.