Co-culture of synovial fibroblasts and differentiated U937 cells is sufficient for high interleukin-6 but not interleukin-1β or tumour necrosis factor-α release

Inflamed synovium is characterized by high concentrations of cytokines [interleukin (IL)-6, IL-1 beta and tumour necrosis factor (TNF)-alpha] and the abundant presence of infiltrated monocytes, many of which are found adjacent to the resident fibroblast-like synoviocytes. We have used a co-culture of fibroblast-like synoviocytes and differentiated U937 cells to study IL-6, IL-1 beta and TNF-alpha release. After a 3 day co-culture, 35% of the U937 cells had adhered and were fully differentiated towards monocytes, as determined by expression of p47(phox), CD14, MSE-1, Mac-1, collagenase and NADPH oxidase activity. IL-6 release from fibroblast-like synoviocytes was induced 4-fold by co-culture with differentiated U937 cells. However, co-culture of differentiated U937 cells with fibroblast-like synoviocytes failed to release detectable levels of IL-1 beta and TNF-alpha from the U937 cells. Addition of synovial fluid further increased IL-6 release, bur again had no effect on IL-1 beta or TNF-alpha, although U937 cells differentiated by phorbol ester were able to release these two cytokines and, in the case of the co-culture, mRNAs for both cytokines were highly expressed in the U937 cells. We postulate that the influx of monocytes into the synovium is instrumental in the elevation of IL-6 levels, but this is not sufficient to explain high levels of IL-1 beta or TNF-alpha.