In this context, it is interesting to note that IL-18, the secretion of which depends also on inflammasome-induced caspase-1 activation, is not released from activated synoviocytes.13 Taken together with the immunohistological and Western blot data, our results suggest that the main cell types that process and secrete IL-1β (and by inference IL-18) in the arthritic synovium are myeloid cells, endothelial cells and possibly B cells. Synovial fibroblasts do not appear to be a source of mature

secreted IL-1β. Our findings are consistent with previous observations showing that FLS expressed detectable levels of IL-1β mRNA PD98059 upon stimulation with TNF-α or direct T-cell membrane contact, but did not release bioactive IL-1β.14 When we compared and contrasted the expression of Selleckchem Y 27632 different NALPs and inflammasome components between RA and OA synovia, we were surprised that there were few differences in mRNA expression between the two pathologies, nor in the protein expression measured by Western blotting.

Rosengren et al. found increased levels of NALP3 mRNA in RA synovia, but did not perform any Western blot analysis. The only difference we found was a higher concentration of caspase-1 in the synovium as measured by ELISA in RA samples, whereas IL-1β protein levels were similar. As currently available ELISAs do not discriminate between the pro-forms or active forms of caspase-1 and IL-1β, it is impossible to extrapolate from increased caspase-1 levels to increased IL-1β activity. In our study, the higher levels of caspase-1 observed in RA were not associated with increased inflammasome expression, suggesting that its regulation is distinct from that of ASC and NALP3. In this context, it is interesting Ceramide glucosyltransferase to note that as IL-1β plays an important role in murine arthritis, we

investigated the contribution of NALP3, studying the phenotype of NALP3-deficient mice (NALP3−/−) and wild-type (+/+) mice during antigen-induced arthritis (AIA). As expected, IL-1β−/− mice showed reduced severity of AIA. By contrast, NALP3−/− mice did not show any alteration of joint inflammation, indicating that IL-1β activation during AIA is independent of the classical NALP3 inflammasome.15 Taken together, our results on human and experimental arthritis suggest that activation of IL-1β does not seem to occur through the NALP3 inflammasone. Finally, the finding that OA synovial membranes express similar levels of inflammasome components as well as similar IL-1β concentrations compared with RA is interesting, and suggests that synovial IL-1β production does not account for the clear differences in pathology between these two diseases. However, these results should be taken with caution as OA synovial samples were obtained at end-stage disease during joint replacement surgery, where there is often a considerable degree of synovial inflammation reflecting chronic joint injury, and therefore there may not be representative of OA as a whole.