Antigen-processing machinery in human dendritic cells: up-regulation by maturation and down-regulation by tumor cells

It has been known for some time that functional properties of dendritic cells (DC), and in particular their ability to process and present Ags to T cells, can be modulated by cytokine-induced maturation and by interactions with tumor cells. However, the molecular basis for these functional changes is unknown. We have investigated whether changes in expression of Ag-processing machinery (APM) components in DC are associated with alterations in their ability to present tumor-derived Ags to T cells. Using a panel of mAbs specific for individual APM components and a quantitative flow cytometry-based method, the level of APM components was measured in DC generated from peripheral blood monocytes of 12 normal donors and of 8 patients with cancer. Immature DC had significantly lower (p < 0.01) expression of MB1, LMP-7, LMP-10, TAP-1, and tapasin than mature DC. However, maturation in the presence of a cytokine mixture up-regulated expression of these components in DC obtained from normal donors and patients with cancer. Immature DC incubated with tumor cells had significantly lower (p < 0.001) expression of MB1, LMP-2, LMP-7, LMP-10, and endoplasmic reticulum p75 than controls. These changes were associated with a decreased ability of DC to present tumor-derived Ags to T cells, as measured in ELISPOT assays and with apoptosis of T cells in DC-T cell cultures. Thus, tumor cells have a significant suppressive effect on DC; however, ex vivo maturation of DC derived from patients with cancer in a polarizing cytokine mix restores normal expression of APM components and Ag-processing capabilities.