Modulation of tumor necrosis factor-alpha gene expression. Desensitization of prostaglandin E2-induced suppression.

PGE2, an immune mediator, is an inhibitor of LPS-stimulated TNF production and gene transcription. In the present study we determined whether pretreatment with PGE2 could desensitize the suppressive function of PGE2 for the production of macrophage (MO)-derived TNF. CFA-elicited MO were incubated with PGE2 or medium only, washed, and then challenged with graded doses of LPS (0.001 to 1000 ng/ml) in the presence or absence of new PGE2. The concomitant addition of PGE2 with LPS shifted the LPS concentration-effect curve 16-fold to the right with a 52% decrease in the maximum LPS response, while MO pretreated with PGE2, washed, and incubated with LPS plus new PGE2 were desensitized to TNF regulation. These latter conditions resulted in a complete loss of the ability of PGE2 to inhibit MO TNF production as demonstrated by no significant change in the EC50 of LPS. In addition, the PGE2 concentration effect curve was shifted to the right after pretreatment of MO, suggesting a desensitized PGE2 receptor system. At the transcriptional level, pretreatment of MO with PGE2 attenuated the ability of new PGE2 to inhibit LPS-dependent TNF mRNA expression. Further studies demonstrated that, although the concomitant addition of the cyclooxygenase inhibitor indomethacin plus LPS could increase TNF production, MO pretreated with indomethacin, washed, and then challenged with LPS demonstrated an inhibition of TNF expression. MO pretreated with indomethacin also demonstrated an increased sensitivity for exogenous PGE2-induced suppression of TNF mRNA and bioactivity. These investigations further support the role of PGE2 as an immunomodulating compound that may effectively regulate the local concentration of specific monokines needed to maintain an inflammatory lesion.