TNF and arthritic joint disease: Lubberts et al. had already shown, in the same patella culture model, that the IL-1β protein levels were still up-regulated in the culture supernatants several days after the induction of arthritis (with concentration values in the same range as those we observed), while the TNF-α protein could only be detected a few hours after the induction, which could be an explanation for our results [28,45].