Dendritic cells and monocytes as accessory cells in T-cell responses in man. II. Function as antigen-presenting cells

The main antigen-presenting cells (APC) of human blood are reported to be the dendritic cells (DC), whereas monocytes (Mo) are only weakly or not at all capable of inducing T-cell immune responses to the soluble antigen purified protein derivative (PPD). In contrast, we found Mo to have a suppressive effect on the APC function of DC in vitro. Removal of Mo by adherence resulted in an increased APC activity, even though Mo produce more interleukin 1 (Il-1) than DC. Furthermore, addition of the prostaglandin synthesis inhibitor indomethacin gave rise to increased Il-1 production, HLA-class II expression, and a stronger antigen-specific T-cell response to PPD. Taken together, our studies indicate that the superior accessory cell function of DC compared with Mo in in vitro cultures may, at least partly, be attributed to the prostaglandin E2 production by Mo and more stable expression of HLA-class II molecules on DC.