Slow release of soluble TNF receptors by monocytes in vitro.

In this study, we investigated the release of soluble(s) TNF-R by PBMC in vitro. T cell activation by mAb anti-CD3, as well as activation with phorbol esters (PMA), enhanced the release of both sTNF-R55 and sTNF-R75 by PBMC. In contrast to shedding of TNF-R by neutrophils upon activation, release of sTNF-R by PBMC proved to be a relatively slow process, reaching a plateau after 2 days of culture. Monocytes appeared to be the main source of the released sTNF-R, whereas activation of purified T cells induced only a minor release of sTNF-R as compared with the whole cell population. To unravel the mechanism, a number of cytokines were added during a 2-day culture of cells. IL-10 enhanced sTNF-R levels with similar kinetics as mAb anti-CD3 and PMA, whereas the other cytokines tested did not affect the release of sTNF-R by PBMC, pure T lymphocytes, or purified monocytes, either activated or not. Conversely, inhibitors of cytokines were added during the activation period to study the effect of endogenously produced cytokines on sTNF-R release. mAb anti-IL-10 and IL-1ra partly sTNF-R release, whereas other inhibitors did not affect the release. The results obtained in vitro may extend our insight in the mechanism via which sTNF-R are enhanced in vivo during inflammatory reactions.