IFN-γ and T-bet Expression in Human Dendritic Cells from Normal Donors and Cancer Patients Is Controlled through Mechanisms Involving ERK-1/2-Dependent and IL-12-Independent Pathways

Dendritic cells (DC) play a major role in priming naive T cells and modulating the immune response. We have previously reported that bryostatin-1, a potent immune modulator with antitumor activity, activates monocytes and lymphocytes to produce cytokines. Studies have shown that tumor-bearing hosts have a Th1/Th2 cytokine pattern that is associated with decreased production of IFN-γ. We investigated the expression of IFN-γ in bryostatin-1-treated human DC. Bryostatin-1 induced both IFN-γ and T-bet mRNA expression in a dose- and time-dependent manner. As little as 1 ng/ml bryostatin-1 induced IFN-γ and T-bet transcripts within 3 h and protein at 12 h. Treatment of DC with cycloheximide revealed that bryostatin-1-induced T-bet expression requires de novo protein synthesis, but bryostatin-1-induced IFN-γ expression is independent of protein synthesis. Furthermore, dexamethasone inhibits bryostatin-1-induced IFN-γ mRNA expression but increases bryostatin-1-induced T-bet mRNA expression. Experiments with ERK-1/2 inhibitors demonstrated that bryostatin-1 induction of IFN- γ and T-bet was ERK-dependent and IL-12-independent. Similar results were obtained from both normal donors and cancer patients. In summary, our results suggest that bryostatin-1-induced IFN-γ expression is T-bet independent. They also suggest for the first time that IFN- γ and T-bet can be induced in human DC through an ERK-dependent pathway. Bryostatin-1-induced IFN- γ may play a crucial role in the initiation of the immune response, before specific recognition by T cells that could be beneficial in the treatment of cancer.