The induction of dendritic cell (DC) maturation is critical for the induction of Ag-specific T lymphocyte responses and may be essential for the development of human vaccines relying on T cell immunity. In this study, we have investigated the effects of monophosphoryl lipid A (MPL) on human monocyte-derived DC as well as peripheral blood T cells. Calcium mobilization, mitogen-activated protein kinase activation, and the NF-κB transcription factor were induced after MPL stimulation of DC and required high doses of MPL (100 μg/ml). Maturation parameters such as production of IL-12 and increases in cell surface expression of HLA-DR, CD80, CD86, CD40, and CD83 were observed following DC treatment with MPL. However, lower levels of IL-12 were induced by MPL when compared with lipopolysaccharide. This is likely to be related to differences in the kinetics of extracellular signal-related kinase 1/2 and p-38 phosphorylation induced by both molecules. Although maturation induced by MPL was weaker when compared with lipopolysaccharide, it appeared to be sufficient to support optimal activation of allogeneic naive CD45RA+ T cell and anti-tetanus toxoid CD4 T cells. MPL at low doses (5 μg/ml) had no impact on DC maturation, while its addition to DC-T cell cocultures induced full T cell activation. The observed effect was related to the fact that MPL also acts directly on T cells, likely through their Toll-like receptors, by increasing their intracellular calcium and up-regulating their CD40 ligand expression. Together, these data support a model where MPL enhances T cell responses by having an impact on DC and T cells.

Despite limited clinical efficacy in large trials, dendritic cells (DC)-based immunization has yielded impressive responses in some patients. Key questions remain to be solved in order to optimize this therapeutic vaccine. Among them, the nature of the DC used and its state of maturation are pivotal. Besides myeloid DCs which are the cells essentially used in clinical trials, a new type of DC has been described resulting from the differentiation of monocytes in the presence of type I IFNs. In the present study, we analyzed the features of clinical grade type I IFN DC generated in the presence of either IL-3 (IL-3- DCs) or GM-CSF (GM-CSF-DCs) and compared their capacity to respond to poly I:C, a double-stranded RNA analog that mimics viral infection. The two DC types disclosed a similar immunophenotype characterized by high levels of costimulatory molecules, CCR7, HLA-class-I and class-II molecules. After poly I:C maturation, both DC types displayed a marked upregulation of CD80, CD83 and CD86. In addition, poly I:C stimulated them to secrete IFN-α IL-12 p70, IL-12 p40 and IL-6. Both DC types elicited potent allogeneic reactions. Priming of autologous T cells by IL-3-DCs or GM-CSF-DCs pulsed with an HLA-A2 restricted melan-A derived peptide, lead to expansion of melan-A specific CTL secreting high levels of IFN-γ and displaying a phenotype of memory cells. We conclude that mature clinical grade IL-3-DCs and GM-CSF-DCs share similar phenotype and functional properties including the capacity to prime Ag-specific CTL.